Guidelines for Optimal Partially Premixed Combustion Operation

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Guidelines for optimal partially premixed combustion operation

Citation for published version (APA): Reijnders, J. J. E. (2017). Guidelines for optimal partially premixed combustion operation. Technische Universiteit Eindhoven.

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Download date: 04. Oct. 2021 Guidelines for Optimal Partially Premixed Combustion Operation Guidelines for Optimal Partially Premixed Combustion Operation by Jos Reijnders Technische Universiteit Eindhoven, 2017

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All rights reserved. No part of the material protected by this copyright notice may be produced or utilised in any form or by any means, electronic or mechanical, including photocopying, recording or by any information storage and retrieval system, without the prior written permission of the author. Guidelines for Optimal Partially Premixed Combustion Operation

PROEFSCHRIFT

ter verkrijging van de graad van doctor aan de Technische Universiteit Eindhoven, op gezag van de rector magniﬁcus, prof.dr.ir. F.P.T. Baaijens, voor een commissie aangewezen door het College voor Promoties, in het openbaar te verdedigen op dinsdag 10 oktober 2017 om 14.00 uur

door

Jos Johannes Egidius Reijnders

geboren te Geldrop Dit proefschrift is goedgekeurd door de promotoren en de samenstelling van de promotiecommissie is als volgt:

voorzitter: prof.dr.ir. M.G.D. Geers 1e promotor: prof.dr. L.P.H. de Goey 2e promotor: prof.dr. B.H. Johansson (King Abdullah University of Science & Technology copromotor: dr.ir. M.D. Boot leden: prof.dr. J. Krahl (Ostwestfalen-Lippe University of Applied Sciences) prof.dr. C. Mounaim - Rousselle (University of Orl´eans) prof.dr.ir. E.J.M. Hensen adviseur: dr.ir C.C.M. Luijten (ASML)

Het onderzoek dat in dit proefschrift wordt beschreven is uitgevoerd in overeen- stemming met de TU/e Gedragscode Wetenschapsbeoefening. To my daughter

Abstract

Despite all the negative news coverage, diesel-powered vehicles remain very popular, particularly in Europe, owing mainly to their superior fuel economy. Diesel engines, however, are still notorious for their soot and NOx emissions, in spite of the mandated cuts that have been realized over the past decades (95- 98% for Euro VI vs. I). In addition to improving local air quality, governments increasingly aim to reduce their reliance on fossil fuels as well. Assuming that at least part of the resulting energy gap will be replaced by renewable, liquid fuels, it is important to consider which type of fuel, from a molecular point of view, might be best placed to deal with the issue of sustainability and the aforementioned hazardous emissions.

In Chapter 2, some key fundamentals on soot and particulate matter (PM) formation will be presented, as well as a detailed discussion on the working principles of the various analytic techniques employed in this study. Further- more, a summary from a toxicity study on particulate matter from diﬀerent sources (both engine and non-engine applications) is given. The results demonstrate that non-engine-related particles can be even more harmful to human health than engine-related particles. Obviously, particle emissions from all sources need to be reduced even further.

One way to reduce emissions is by adjusting the combustion concept, whereby both the fuel and engine conditions are modified concurrently, such that the optimal synergetic outcome is achieved. This is typically the objective of so- called “partially premixed combustion”. In Chapter 3, the influence of two important fuel properties (cetane number and aromaticity) on soot emissions is investigated independently of one another. The main conclusions illustrate that the effects are quite different for conventional combustion (i.e., diffusion or hot combustion) and the partially premixed combustion (cold combustion) variant. To distinct between these two combustion concepts, a new parameter Π is introduced. In the case of hot combustion (Π >1), the cetane number appears to have only minor effects on emissions, while in the case of cold com-

i bustion (Π <1), a low cetane fuel yields markedly better results. Pertaining to aromatic content, the data shows a high fuel aromaticity to be detrimental for the soot-NOx trade-oﬀ, irrespective of combustion mode.

In Chapter 4, aforementioned properties are now varied in tandem. This is achieved by blending in gasoline, which has a low cetane number and high aromaticity, to synthetic diesel, for which the opposite is true. The results clearly show that the emissions trade-oﬀ improves for higher concentrations of gasoline, thereby suggesting that aromaticity might not be such a bad thing, given their function of a cetane number depressant. Furthermore, this chapter investigates how to best achieve cold combustion. As it happens, a combination of a low cetane number fuel (<50) and moderate amounts of exhaust gas recirculation (EGR) (<35%) deliver the best performance with respect to emissions and eﬃciency.

In the previous two chapters, soot emissions were studied only in a gravimetric sense. Alternatively, Chapter 5 looks at the inﬂuence of both fuel properties and engine operating conditions on the particle count and size distribution. From the analysis it becomes clear that herein too a trade-oﬀ can be observed; this time not between soot and NOx, but between smaller (<23 nm) and larger particles (>23 nm). In the European Union, legislation considers only particle diameters greater than 23 nanometer (nm). This would imply that, when low amounts of PM mass and larger particles are measured, chances are that this coincides with a high number of the smallest (non-legislated) particles.

Manipulating engine operating conditions can ensure that when only a small number of larger particles is formed, nearly no small particles are found in the exhaust anymore. In that case, larger particles might act as a sponge for smaller ones. Executed correctly, this strategy could lead to a recipe for a low particle count at both the small and larger end of the spectrum, thereby minimizing any adverse health eﬀects.

In the future, cooler combustion holds the promise of even cleaner engines. The optimal fuel for such a concept is one with low amounts of soot precursors and a low cetane number. Hereby, the latter property appears to be the dominant one. Interestingly, aromatics, otherwise being notorious soot precursors, no longer appear to negatively impact soot emissions when a functional oxygen group is attached to the aromatic ring. Given their intrinsically low cetane number, the resulting aromatic oxygenates adhere to both aforementioned optimal fuel prerequisites.

ii Samenvatting

Ondanks alle negatieve berichtgeving de laatste tijd over dieselvoertuigen, blijft de dieselmotor in Europa zeer populair, vooral door het hoge rendement. Echter, met name de roet en NOx emissies blijven een punt van aandacht voor deze compressie-ontstekingsmotoren, ook al zijn deze drastisch afgenomen in de laatste decennia (95-98% van Euro I naar Euro VI). Ook de herkomst van brandstoffen is een actueel onderwerp, waarbij de wens bestaat om de hoeveelheid gebruikte aardolie flink te verminderen. Maar als er dan naar alternatie- ve brandstoffen wordt gekeken, welke brandstofeigenschappen be¨ınvloeden het verbrandingsproces dusdanig dat de schadelijke uitstoot wordt verminderd, en hoe moeten we deze eigenschappen vertalen naar de moleculaire schaal?

In hoofdstuk 2 worden de grondbeginselen van roetvorming en -oxidatie beschreven. Hierbij wordt ook gekeken naar de roetdeeltjesgrootteverdeling. Daarnaast worden de meetprincipes van de gebruikte meetapparatuur beschreven. Ook wordt er in dit hoofdstuk een samenvatting gegeven van een studie waarin de schadelijkheid van deeltjes afkomstig van verschillende bronnen is onderzocht. Hieruit blijkt dat de uitstoot van niet-motor gerelateerde deeltjes schadelijker kunnen zijn dan motor gerelateerde deeltjes. Echter, alle uitstoot van deeltjes zal verder teruggedrongen moeten worden.

Een manier om de emissies terug te dringen is door het aanpassen van het verbrandingsconcept, waarbij de brandstof en motorcondities tegelijkertijd gewij- zigd worden zodat beide eﬀecten elkaar kunnen versterken. Dit is typisch het doel van “partially premixed combustion”(gedeeltelijk voorgemengde verbranding). In hoofdstuk 3 is onderzocht wanneer er voldaan wordt aan gedeeltelijk voorgemengde verbranding en wat de invloed is van twee belangrijke brandstofeigenschappen (cetaangetal en aromatengehalte), wanneer alle andere eigenschappen constant worden gehouden. Hiertoe is een nieuwe parameter Π ge¨ıntroduceerd, die aangeeft welke van de voorgenoemde brandstofeigenschappen het meest belangrijk is voor de uitstoot van roet en NOx, afhankelijk van het verbrandingsconcept. Hieruit blijkt dat er een verschil gemaakt kan wor-

iii den tussen conventionele verbranding (zogenaamde hete verbranding, veelal nog toegepast bij huidige dieselmotoren) en de meer voorgemengde varianten (koude verbrandingsconcepten, toekomstige technieken). Bij hete verbranding (Π >1) blijkt dat de invloed van het cetaangetal (eigenschap voor zelfontbran- dingskwaliteit) klein is, terwijl bij “koude”verbranding (Π <1) een laag cetaangetal gewenst is. De hoeveelheid aanwezige aromaten in de brandstof (bij gelijk cetaangetal!) heeft bij zowel hete als koude verbranding een negatieve invloed op de gecombineerde roet-NOx uitstoot.

In hoofdstuk 4 is vervolgens gekeken naar de invloed van het verlagen van het cetaangetal door het toevoegen van aromaten. Dit is bewerkstelligd door benzine te mengen met een synthetische (diesel) brandstof. Naar mate er meer benzine wordt toegevoegd verbetert de roet-NOx “trade-oﬀ”, ondanks de grotere hoeveelheid aromaten. Hieruit blijkt dat aromaten toevoegen kan zorgen voor het terugdringen van schadelijke emissies, mits gelijktijdig het cetaangetal hierdoor wordt verlaagd. Ook is er in dit hoofdstuk gekeken naar de beste uitvoering van zogeheten koude verbranding. De resultaten laten zien dat een laag cetaangetal (<50), in combinatie met een gematigde hoeveelheid uitlaatgas recirculatie (EGR) (<35%), de beste motorprestaties oplevert.

In de twee voorgaande hoofdstukken is gekeken naar de roet en NOx uitstoot in termen van massa. In hoofdstuk 5 is daarbij ook de invloed van de brandstof en motorcondities bestudeerd op de deeltjesgrootteverdeling. Uit alle motor- resultaten en de bestudeerde literatuur, blijkt dat er bijna altijd ofwel kleine deeltjes (<23 nm) ofwel grotere deeltjes (>23 nm) worden uitgestoten; een combinatie treedt maar zelden op. In Europese wetgevingen worden alleen de deeltjes boven de 23 nanometer (nm) meegenomen. Dit betekent dat in het geval er voor de wetgeving weinig deeltjes gemeten worden, de kans groot is dat er veel kleine deeltjes uitgestoten worden. Door de motorcondities aan te passen, kan er gezorgd worden voor een paar grotere deeltjes die als een soort spons fungeren voor de kleine deeltjes, waardoor de schadelijkheid van laatst genoemde deeltjes voor de mens beperkt wordt.

Kortom, voor toekomstige, nog schonere dieselmotoren, zal meer gebruik gemaakt gaan worden van koude verbranding. De optimale brandstof daarvoor is er ´e´enmet een laag gehalte aan aromaten en laag cetaangetal, waarbij de laatste eigenschap het meest belangrijk blijkt. Een belangrijke nevenconclu- sie is dat wanneer er een functionele zuurstofgroep wordt gekoppeld aan een aromaat, de resulteerde geoxygeneerde aromaat niet meer fungeert als roet- bouwsteen, maar juist zorgt voor een verlaging in de roetuitstoot.

iv Contents

Abstract i

Samenvatting iii

1 Introduction 1 1.1 Background ...... 1 1.1.1 Energy transition ...... 1 1.1.2 Conventional ICE concepts ...... 2 1.1.3 Future ICE concepts ...... 5 1.2 Added value and objectives ...... 6 1.3 Scope and approach ...... 7

2 Soot formation fundamentals and measurement techniques 9 2.1 Soot particle evolution ...... 10 2.2 Soot particle size distribution ...... 12 2.3 Soot measurement techniques ...... 13 2.3.1 AVL 415S ...... 13 2.3.2 Testo ViPR (Volatile Particle Remover) ...... 13 2.3.3 TSI Engine Exhaust Particle Sizer (EEPS) ...... 15 2.4 Health eﬀects ...... 17 2.4.1 Methodology ...... 17 2.4.2 Results ...... 18 2.4.3 Discussion and conclusions ...... 20

3 Impact of aromaticity and cetane number on the soot-NOx trade-oﬀ in conventional and low temperature combustion 21 3.1 Abstract ...... 21 3.2 Introduction ...... 22 3.3 Methodology ...... 25 3.3.1 Experimental setup ...... 25 3.3.2 Fuel matrix ...... 25

v 3.3.3 Design of experiments ...... 26 3.3.4 Experimental procedure ...... 27 3.4 Results ...... 29 3.4.1 MODE 1 ...... 30 3.4.2 MODE 2 ...... 33 3.4.3 MODE 3 ...... 34 3.4.4 MODE 4 ...... 36 3.5 Discussion ...... 37 3.5.1 Aromaticity ...... 37 3.5.2 Aromaticity versus cetane number ...... 38 3.6 Conclusions ...... 40

4 GTLine - Gasoline as a potential CN suppressant for GTL 43 4.1 Abstract ...... 43 4.2 Introduction ...... 44 4.3 Methodology ...... 46 4.3.1 Experimental setup ...... 46 4.3.2 Fuel matrix ...... 47 4.3.3 Experimental procedure ...... 48 4.4 Results ...... 49 4.4.1 Rate of heat release ...... 49 4.4.2 Ignition dwell ...... 50 4.4.3 Eﬃciency ...... 51 4.4.4 Soot - NOx emissions ...... 52 4.5 Discussion ...... 53 4.5.1 PPC threshold ...... 53 4.5.2 Fuel reactivity and aromaticity ...... 53 4.5.3 φ - T trace ...... 54 4.5.4 Indicated thermal eﬃciency ...... 56 4.6 Conclusions ...... 57

5 Particle accumulation vs. nucleation: A second diesel dilemma? 59 5.1 Abstract ...... 59 5.2 Introduction ...... 60 5.3 Methodology ...... 61 5.3.1 Experimental setup ...... 61 5.3.2 Fuel matrix ...... 63 5.3.3 Experimental procedure ...... 65 5.4 Results and discussion ...... 66 5.4.1 PM vs. NOx ...... 67 5.4.2 PM accumulation vs. nucleation ...... 70

vi 5.4.3 Correlations between trade-oﬀs ...... 72 5.4.4 Impact of molecule structure on fuel properties . . . . . 76 5.5 Conclusions ...... 78

6 Summary and conclusions 79

Appendices 82 Appendices Chapter 2 ...... 82 Appendices Chapter 3 ...... 83 Appendices Chapter 4 ...... 91 Appendices Chapter 5 ...... 95

Nomenclature 96

Dankwoord 105

Curriculum Vitae 107

List of Publications 108

vii this page is intentionally left blank

viii Chapter 1

Introduction

1.1 Background

1.1.1 Energy transition The global energy consumption is continuously growing and according to the International Energy Agency (IEA) and many others, it is expected to increase much further in the coming decades (Figure 1.1). Mainly the growing world’s population and growing industries in the developing countries are the reasons for this. On the other hand, the availability of fossil fuels (coal, gas, and oil) is decreasing. Another issue is the increasing level of CO2 and the temperature rise attributed to it. All these trends need a huge reversal otherwise the next generations (our children) will have a serious problem.

Energy consumpon 1990-2035 20 Renewables 18 Hydroelectricity 16 Nuclear Energy 14 Coal 12 Natural Gas 10 Oil 8

Billion tonnes oil equivalent oil tonnes Billion

0 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010 2015 2020 2025 2030 2035 Year

Figure 1.1: Outlook for global energy landscape [1]

1 GUIDELINES FOR OPTIMAL PPC OPERATION

For several decades, universities and companies have expected this future shortage of fossil fuels, so they started investigating alternatives. Nowadays, approximately 2.5% of the energy is produced by renewables like wind and solar energy [1]. It is expected that this percentage will grow further once these energy sources become economically viable, although the full transition to renewables may still take many decades to be completed. Production of energy from renewables is mostly achieved via electricity. For industrial and residential use this is quite feasible. For the transportation sector, responsible for one quarter of global energy consumption [2], however, electricity as sole energy carrier is likely not feasible for all modalities. People who drive only 100 kilometers a day can in principle opt for an electric car. The range of these cars is limited, but quite suﬃcient for most daily commutes. There is a well-known psychological eﬀect, so-called range anxiety, which arises from fear that one may not always reach their respective destinations. Another issue with electrical cars is their price, owing to rare metals us- age in most battery types. Although the battery prices are forecast to fall, electric cars will outprice their combustion engine driven counterparts for the foreseeable future. Finally, one must consider CO2 emissions. Current legislation in the EU and elsewhere mistakenly suggests that electric cars emit no CO2. While true on the road, detailed well-to-wheel calculations [3] demonstrate that true CO2 emissions from full EV vehicles mostly exceed those emitted by diesel powered equivalents in case the European energy mix is used.

Viewed holistically, thus, electric propulsion is not necessarily, at least in the short term, the panacea media and governments alike would like us to believe. This is particularly true for commercial transport involving heavy duty trucks, ships and planes, which is characterized by high energy consumption over long distances. For these applications, even the most fervent electric vehicle (EV) proponent will admit the internal combustion engine still has a role to play.

1.1.2 Conventional ICE concepts

Since their advent in the late 19th century, the spark ignition (SI, Otto) and compression ignition (CI, Diesel) engines still dominate the road and marine transport sectors. The fundamentals of both engine types are markedly diﬀer- ent from one another, as will be discussed below and as has been summarized in Table (1.1).

2 CHAPTER 1. INTRODUCTION

Table 1.1: Typical hallmarks of main engine types

Spark ignition Compression ignition Flame type premixed diffusion Mixture strength stoichiometric lean Mixture preparation DI*at low temp./pres. DI*at high temp./pres. Means of engine load control air flow (throttle) fuel flow Means of combustion phasing control spark timing injection timing Fuel reactivity low high Hazardous emissions low high Efficiency low high * Direct injection

Spark ignition (SI) The principle of the SI engine is that a mixture of fuel and air is well mixed at the stoichiometric ratio and is subsequently compressed and ignited by a spark plug. Crucially, proper operation depends on the fuel not auto-igniting, an undesired phenomenon more commonly referred to as knock. Consequently, SI engine fuels have historically had a particularly strong resistance to auto-ignition. On the engine side, knock is suppressed by cap- ping combustion temperatures, which is achieved in practice by measures as a late spark timing, low compression ratio and effective inter-cooling. However, thermal efficiency benefits from high compression ratios and early spark timings. This irrevocably leads to a trade-off between the risk of knock and fuel economy. Relatively low compression ratios, combined with stoichiometric operation, which calls for throttling at part load, contribute to the low (typically only 20-25%) overall efficiency of SI engines. The major advantage of SI over CI engines is found in its near-zero tail pipe emissions; other than CO2 and H2O of course. For a part, particularly when it comes to soot, this can be attributed to the premixed vs. diffusion flame character of the combustion event. Negligible levels of NOx, unburnt HCs and CO emissions, however, are only possible in tandem with a three-way catalyst, which has been commonplace downstream of nearly all SI engines since the mid 1980’s. Given that such cat- alysts only function when combustion occurs stoichiometrically, alternative, arguably less effective aftertreatment is needed for CI engines.

Compression ignition (DI) The CI principle distinguishes itself from the SI variant by the nature of the combustion process, which is now of the diffusive sort in lieu of premixed. Not only is the type of flame different, also the mixture ratios and process thereof are quite different. Whereas fuel is mixed stoichiometrically, already early in the intake or compression stroke in SI engines, CI combustion is typically very

3 GUIDELINES FOR OPTIMAL PPC OPERATION lean and fuel is introduced only at the end of the compression stroke near top dead center (TDC).

Another major distinction is that the onset of combustion is controlled by the injection timing in CI engines against the timing of the spark in case of SI. To this end, the cylinder is filled only with air and, owing to compression, the pressure and temperature increases. Just prior to TDC, a (typically) highly reactive fuel is injected, which subsequently ignites more or less instantaneous. As combustion proceeds whilst the injection process is still in progress, multiple diffusion flames set in for each of the respective injection nozzle hole born sprays. Given that rapid auto-ignition chemistry is now desired, far higher compression ratios can be tolerated than in the case of SI engines. Compres- sion ratios, now typically 15-20, are limited only by the maximum allowable thermal and pressure loading of the engine. As a consequence, thermal efficiency soars to levels nowadays even in excess of 50%. While excelling in terms of fuel economy, CI engines suffer from a major drawback intrinsic to the diffusive nature of the combustion process. As mentioned earlier, imme- diately following the auto-ignition event, multiple diffusion flames are formed, one for each nozzle hole. Herein, as is schematically illustrated in Figure 1.2, two main burning zones can be identified. The richer, more upstream part of the two yields high concentrations of soot precursors as well as the primary soot particles (more details to follow in Chapter 2).

Liquid Fuel Thermal NO Production Zone Rich Fuel/Air Mixture Diffusion Flame Fuel-Rich Premixed Combustion PM Oxidation Zone Initial PM Formation

0 10 20 Low High Scale (mm) PM Concentration Figure 1.2: Conceptual model of a diﬀusion ﬂame in CI engines [4]

Fortunately, most soot particles will not survive the far hotter, downstream diffusion flame, but some inevitably find their way into the exhaust system. Soot, unlike gaseous emissions, is a far more difficult challenge for aftertreatment systems. To complicate matters further, the lean nature of the CI com-

4 CHAPTER 1. INTRODUCTION bustion process does not lend itself for the particularly effective three-way catalyst seen in nearly all SI powered vehicles. This means that NOx, too, is more challenging to curb for CI engines. Accordingly, it would appear that the principal engines types on offer today are either efficient or clean.

1.1.3 Future ICE concepts The main challenge for future engine developers is thus to circumvent this trade-off. Hereby, the CI engine is arguably the better starting point, given that stoichiometric combustion and spark-ignition seem to irrevocably rule out any chance of a high thermal efficiency. As discussed in the previous section, CI engines emit high emissions on account of the formation of diffusion flames. The objective of redesigning the CI engine should therefore be to facilitate premixed combustion, whilst still maintaining the high efficiency attributes of lean combustion and compression ignition. Moreover, the ideal premixed compression ignition event should also occur at lower temperatures, so as to not only yield a high efficiency and low soot, but negligible NOx emissions as well. To this end, several (partially) premixed compression ignition concepts have been put forward in the literature over the years. In order of increasing premixedness, the premixed concepts most cited in literature are partially premixed combustion (PPC), premixed charged compression ignition (PCCI) and homogeneous charge compression ignition (HCCI). Note that dual fuel approaches, such as reactivity controlled compression ignition (RCCI), will not be discussed here. An important common denominator for all modes is an ignition delay that exceeds the injection duration, i.e. no overlap between injection and heat release events. Another commonality is that in most cases a low reactive environment in the combustion chamber is created by a combination of low reactive fuel and low reactive charge. The latter feature is typically achieved by high dilution ratios of air and/or EGR.

The principal distinctions amongst the afore mentioned premixed CI concepts are twofold. On the engine side, higher levels of premixedness are achieved by applying a lower compression ratio and earlier injecting timings. In the case of PPC, this means a slightly lower CR and a slightly earlier injection timing compared to a conventional CI engine and in the case of HCCI, both factors are modiﬁed much more extensively. On the fuel side greater premixing is achieved by a combination of low reactivity and high volatility, whereby PPC uses values which are slightly lower and higher than conventional diesel, respectively and HCCI uses values which are again much more extreme.

5 GUIDELINES FOR OPTIMAL PPC OPERATION

From a practical stance, particularly when considering controllability of the engine, PPC seems the most promising candidate, having already been proven to yield combined near-zero emissions of NOx and soot and high eﬃciency over a wide load range [5]. This study will therefore focus on PPC as a premixing strategy.

1.2 Added value and objectives

While it has been well established that PPC allows for CI engines to operate at near-zero soot and NOx levels, this achievement, though promising, is a quantitative one. What is still lacking in the scientific community is insight into the qualitative nature of PPC emissions, most notably soot. There is currently much debate on the impact of soot particle size on the toxicity and carcino- genicity of exhaust gas from CI engines. A more in-depth understanding of how premixed CI affects both the soot particle count and size distribution, as provided by this thesis, therefore promises to be a timely contribution to the scientific community.

The objective of this thesis is to design a recipe, pertaining to both fuel properties and engine operating conditions, for optimal overall PPC performance, pertaining to eﬃciency, as well as to quantitative and qualitative emissions. To this end, three apparent blind spots in the PPC literature will be addressed in this thesis by answering the following three research questions.

What is PPC? Current definitions of PPC in literature are quite vague at best and conflicting at worst. This uncertainty arises from a lack of quantitative metrics by which PPC can be identified experimentally and later replicated by others.

How is PPC best achieved? The second research question concerns how best to realize a given degree of premixedness with respect to emissions and fuel economy. For example, PPC can be realized by running the engine on a high cetane number (CN, highly reactive) fuel in combination with high EGR or, alternatively, by opting for a low CN fuel with only low-to-moderate concentrations of EGR.

How does PPC impact soot quality? Finally, the third research question relates to the quality of emissions, spe- cially the total particle size count and distribution thereof, as a function of

6 CHAPTER 1. INTRODUCTION premixedness. After all, achieving near-zero soot in gravimetric terms, may not be desirable if this coincides with the emission of many ultra-ﬁne particles.

1.3 Scope and approach

With the exception of the toxicological effects related to PM emissions (Chap- ter 2), the scope of this thesis is confined to what goes into and comes directly out of the engine (Figure 1.3). The techno-economic feasibility (TEF) and life cycle assessments (LCA) of the respective fuels and emissions thus fall outside the scope of this thesis. Furthermore, a study into the efficacy of aftertreatment devices such as diesel particulate filters (DPF) and selective catalytic reduction (SCR) will also be omitted here.

Figure 1.3: Scope of the thesis (red: in scope, black: outside scope)

After a description of soot formation fundamentals presented in Chapter 2, the ﬁrst question is to be addressed in Chapter 3. Herein, a new quantitative parameter Π is introduced to discern between the conventional and PPC CI modes. The transition between modes is realized by varying fuel chemistry (CN and aromaticity) and the thermochemical evolution of the charge (λ, EGR, compression ratio, intake pressure). These variations also help to address the second research question, which is dealt with in a more practical manner in Chapter 4 by considering only blends of commercially available fuels (gasoline and GTL). The third and ﬁnal research question is answered in Chapter 5 by analyzing a vast in-house data set, spanning a wide range in both fuel properties and engine operating conditions, from which several important patterns will be distilled. This thesis, in short, has been designed to read like a how to manual for future PPC engine calibrators and fuel formulators.

7 GUIDELINES FOR OPTIMAL PPC OPERATION

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8 Chapter 2

Soot formation fundamentals and measurement techniques

CI engines are known for their high particulate matter (PM) emissions. PM is best visualized as the black smoke (also called soot) coming from mainly older diesel powered vehicles. It is created as a result of bad mixing and therefore incomplete combustion of hydrocarbon-based fuels. Gravimetrically speaking (i.e., in terms g/kWh), PM emissions have been successfully decimated over the past decades (Figure 2.1).

European emission norms − heavy duty vehicles 0.4

0.3

0.2 PM [g/kWh] 0.1

0 I II III IV V VI Euro norms

Figure 2.1: European heavy-duty PM emission standards

Notwithstanding a gravimetric cut in PM emissions of over 97% from Euro I to Euro VI, there are still a number of questions that have to be addressed. Is it practical to reduce the PM further? And, if so, will this negatively impact some overlooked facet of PM on a qualitative level? To answer these questions, we must ﬁrst review the production steps in the soot formation process which is done in Section 2.1. In Section 2.2 the particles size distribution is treated

9 GUIDELINES FOR OPTIMAL PPC OPERATION and the measurement techniques which are used to measure soot are explained in 2.3. Finally, a summary of a health study from diﬀerent PM sources is given in 2.4.

2.1 Soot particle evolution

Particulate matter is a generic term for solids emitted from combustion processes, which may include un- or partially burnt fuel and lubricant oil, water, metals and sulfates [6]. In conventional (i.e., diffusion combustion dominated) compression ignition engines, the formation of PM is a complicated process, whereby unburnt fuel derivatives nucleate from the vapor into the solid phase in the fuel-rich core of the diffusion flame. This conversion can be described by various sequential processes, schematically drawn in Figure 2.2. This figure, as well as the explanation that follows is a digest of various sources [7–11].

Precursors Nuclei Primary Grown Agglomerates particles particles

Pyrolysis Nucleation Coalescense Growth Agglomeration Fuel

Figure 2.2: Schematic representation of the particulate matter formation process in diﬀusion ﬂame-dominated compression ignition combustion processes

1. Fuel pyrolysis Pyrolysis is the process through which fuel molecules decompose to smaller ones (e.g., C2H2) and subsequently recombine to soot precursors (e.g., benzene, PAH) under oxygen deprived and high temperature conditions.

2. Nucleation Nucleation is the process where the first noticeable particulates (called nuclei) come into existence by formation of carbon lamella and crystal- lization of the PAHs. Nuclei itself do not contribute significantly to the total soot mass, although they have an important influence on the mass later due to their high surface area for surface growth.

3. Coalescence Coalescence (also called coagulation) is a process whereby nuclei collide and merge. Accordingly, the number of particulates decreases, while the mass remains more or less the same.

10 CHAPTER 2. BACKGROUND AND METHODOLOGY

4. Surface growth While the nuclei themselves do not contribute signiﬁcantly to total soot mass, their presence allows still gaseous PAH and other soot precursors to condense. In fact, such surface growth, while not adding to the total particle count, is responsible for the greater part of emitted particle mass.

5. Agglomeration Agglomeration is also a secondary collision (coalescence) process, which yields chain structures or clusters of particulates.

6. Oxidation Oxidation may occur during any one or more of the aforementioned processes. The oxidation process converts carbon or hydrocarbons to combustion products as CO, CO2,H2O. When carbon is completely oxidized to CO2 or even partially oxidized to CO, it will not evolve into a soot particulate anymore [7]. Importantly, nearly all of the formed particles are oxidated at one point or other, with only a small part slipping through and ending up in the tailpipe.

In the next section, a typical size distribution of the particulates is presented, including a discussion on the eﬀects of the nucleation and accumulation modes on the distribution curve.

11 GUIDELINES FOR OPTIMAL PPC OPERATION

2.2 Soot particle size distribution

Figure 2.3 shows a typical diesel-engine-based particle size distribution (PSD) as can be found in literature [8]. On the horizontal axis, the particle size is plotted on a logarithmic scale because of the typical log-normal nature of the curves in question. On the vertical axis, the particle normalized concentration is plotted, which allows for comparison of results obtained from diﬀerent types of analytical devices (e.g., with varying resolution, channels) [12].

7 x 10 12 Nucleation 10 Accumulation Bimodal ] 3 23 nm 8

4 dN/dlog Dp [#/cm Particle concentration 2

0 5 10 23 100 500 Particle size [nm]

Figure 2.3: Typical CI engine out PSDs for diﬀerent modes.

The left and right peaks are related to the nucleation and accumulation modes, respectively. The former mode comprises nuclei with a number median diameter (NMD = Dp(N/2)) below 20 nm. These particles are normally formed in the premixed combustion zone of a CI diﬀusion ﬂame (Figure 1.2) or throughout the premixed combustion phase and mainly consist of volatiles (e.g., light HCs and sulfuric acids), but also of solids (e.g., metals and heavy hydrocarbons) [6]. Logically, the volatile nano-droplets are quite sensitive to sampling temperature and diluter settings (explained later in Section 2.3). The accumulation mode on the other hand tends to comprise particles with an NMD ranging from 50 to 200 nm. These particles are typically formed in the rich combustion zone in Figure 1.2 and consist mainly of carbon in the form of PAH. These particles, as explained earlier, start out as nuclei and gradually grow in several steps to larger diameters.

12 CHAPTER 2. BACKGROUND AND METHODOLOGY

2.3 Soot measurement techniques

2.3.1 AVL 415S Gravimetric particulate emissions will be measured by the AVL 415S smoke meter using the well known filter paper method. The measurement principle of this smoke meter is as follows. A part of the exhaust gas flow is sampled by means of a probe in the exhaust pipe and drawn through filter paper. The resultant blackening of this filter is measured by a reflectometer and translated into a so-called filter smoke number (FSN). This number can subsequently be used to calculate the soot mass [mg/m3] by applying the following empirical formula [13]:

(0.38·FSN) PMmass = 4.95/0.405 · FSN · e . (2.1) Given that the formula dates back to Euro I era, Northrop et al. [14] reeval- uated its validity under more modern operating conditions in 2011 and found it to still be quite accurate, even in the lower range (0.02-1.5 FSN).

2.3.2 Testo ViPR (Volatile Particle Remover) There is a growing interest in particle count and size distributions. The device used to evaluate these qualitative aspects of soot emissions in this study is the engine exhaust particle sizer (EEPS) (Section 2.3.3). Before the PSD can be measured, the exhaust flow needs to be conditioned. The conditioning apparatus used here is the Testo volatile particle remover (ViPR) which consists of two diluters (MD19-3E & ASET15-1). A schematic representation of the ViPR setup is shown in the appendix (Figure A2.1). There are two reasons to use this device. First, the particle density in the exhaust flow is too high to be measured directly by the EEPS, which makes it necessary to dilute the flow. Second, as discussed earlier, the volatile particles are too sensitive for thermal conditions [15] and therefore need to be removed.

This prompts the question whether this removal still represents the real case. Fushimi et al. [16] performed a study on this topic and found that in the atmosphere, almost no volatile particles are measured and attribute this to likely evaporation of the small particles. Accordingly, it would appear to be fair to remove the volatile particles from the exhaust as well when measuring the PSD.

13 GUIDELINES FOR OPTIMAL PPC OPERATION

The ViPR is a two stage diluter, the ﬁrst stage is a rotating disc and the second stage a thermo-diluter. In Figure 2.4, a schematic graph is shown which explains the basic idea behind the ViPR. The rotating disc ensures a primary dilution (factor ranging from 15 to 300) at a temperature in the range of 20-150 ◦C (path A → B). For this work, 150 ◦C is used which prevents water to condense in the dilution head and associated tubing. However, some other components (e.g. heavier hydrocarbons) can still condensate.

condensation evaporation

] A 3 prim. dilution

B C heating

sec. (hot) dilution volatile mass concentration [µg/m E D cooling down 100 200 300 gas temperature T [°C]

Figure 2.4: Volatile mass diagram of the Testo ViPR [Reproduced with permission of Testo GmbH]

The flow is then transported to the thermo-diluter, where it is first heated up to 300 ◦C (B → C) and subsequently diluted by a factor ranging from 1 to 11 (C → D). Finally, the flow is cooled down again (path D → E). This secondary dilution process ensures that the nano-droplets which already exist in the gas sample (e.g. heavier HCs) at point B, evaporate again. During the route (C → D → E) the compound remains in the vapor phase. Without using the thermo-diluter (B → E), the number concentration of the droplets will be reduced, but the nano-droplets formed at B are unable to evaporate, because of a hysteresis effect between condensation and evaporation. Once the volatile particles are removed and the particle concentration is in the correct range, the sample can be transported to the EEPS.

14 CHAPTER 2. BACKGROUND AND METHODOLOGY

The settings of the diluters used in this work can be found in Table 2.1.

Table 2.1: Settings of the diluters

Primary diluter: Temperature [ ◦C] 150 Dilution ratio [-] 50 Secondary diluter: Temperature [ ◦C] 300 Dilution ratio [-] 6.7

2.3.3 TSI Engine Exhaust Particle Sizer (EEPS) After the exhaust sample is conditioned and diluted by the ViPR, the PSD can be determined by the EEPS. This instrument draws a sample of the exhaust flow into the inlet continuously (Figure 2.5). Particles are positively charged using a corona charger. Charged particles are then introduced to the measurement region near the center of a high-voltage electrode column and transported down the column surrounded by filtered sheath air. A positive voltage is then applied to the electrode and this creates an electric field that repels the positively charged particles outward, according to their electrical mobility. As a result, charged particles strike the respective electrometers, to which they transfer their charge. Herein, a particle with higher electrical mobility (i.e., smaller particles) strikes an electrometer near the inflow. Alternatively, a particle with lower electrical mobility (i.e., larger particles) strikes an electrometer further in the stack. The measured current, in turn, is a measure for the amount of particles within a specific size range. The EEPS consists of 22 electrometers, which can measure the full PSD concurrently.

The detection range of the EEPS for the particle concentration is dependent on the particle size. In Figure 2.6, the minimum and maximum concentration limits are shown. For all the measurements preformed in this study, the concentration was well between the two limits. The accuracy of this equipment is close to that of a scanning mobility particle sizer (SMPS) when the “soot” matrix is used [17].

15 GUIDELINES FOR OPTIMAL PPC OPERATION

Figure 2.5: Schematic overview of the EEPS [Reproduced with permission of TSI Inc.]

TSI EEPS (Model 3090) Concentration range

8 10 ] 3

6 10 Maximum detection limit Minimum detection limit 4 10 dN/dlog Dp [#/cm Particle concentration

2 10

5 10 100 500 Particle size [nm]

Figure 2.6: Concentration limits of the particle number of the EEPS.

The afore mentioned equipment will be used in Chapter 3 and 4 (AVL 415S) and Chapter 5 (AVL 415S, ViPR and EEPS).

16 CHAPTER 2. BACKGROUND AND METHODOLOGY

2.4 Health eﬀects

Since air pollution, and in particular road traffic emission, is linked to adverse health effects many efforts have been taken to diminish these emissions. This fact has resulted in a considerable reduction of engine emissions in recent years [18]. However, particulate matter (PM, Table 2.2) from road traffic does not only come from the combustion engines, as it also comprises compounds originating from the road itself, as well as from the tires and brakes. Moreover, the contribution of engine exhaust and non-exhaust sources to PM10 traffic- related emissions is roughly equal in magnitude [19].

Table 2.2: PM deﬁnitions

PM10 coarse particles between 2.5 and 10 µm PM2.5 ﬁne particles between 0.1 and 2.5 µm; PM0.1 ultraﬁne particles smaller than 0.1 µm (100nm)

Reduction in exhaust emissions will continue, for example due to stricter legislations for light and heavy duty transport, and thereby the contribution of non-exhaust PM emissions will become even more important over time. The switch to battery-powered vehicles too will increase the impact of PM from brakes and tires because of the incurred weight penalty of the battery pack. A study on PM composition and associated health effects suggest that non- exhaust born PM emissions can be harmful as well [20]. PM traced back to brakes, for instance, has been linked to adverse health effects, owing to both the materials used (e.g. copper) [21] and the relatively small size of the particles (<100 nm, ultra-fines) [22]. Especially these ultra-fines, are suspected to pose a higher risk given their adverse health effects [23].

Besides PM emissions from traﬃc, other sources, such as wood combustion or farming activities, will inﬂuence the composition and consequently the toxicity of the resulting PM mix. From a health perspective, it might be more logical to adopt legislation for all harmful PM sources and not only for combustion born PM, as is the case at present. Since not much is known about the relative toxicity of non-exhaust born PM emissions in relation to other PM sources, the toxic potency of PM from various sources is examined by means of an inhalation study in mice.

2.4.1 Methodology In order to determine the inﬂuence of the toxicity of a particular PM compound, the measurements are performed at diﬀerent locations directly at the

17 GUIDELINES FOR OPTIMAL PPC OPERATION source (Table 2.3). The measurements relating to combustion in road transport are performed at the Eindhoven University of Technology on a heavy- duty research engine. The particles are captured directly into ﬂuid with a dedicated instrument which is called “VACES” [24]. The study was coordi- nated by RIVM (Gerlofs-Nijland et al.) and a collaboration between diﬀerent companies and institutes.

Table 2.3: PM sources and speciﬁcations

PM2.5 source Sample name Details Semi-metallic brake pads Brake wear 1 54% Fe, 3% Cu Semi metallic brake pads without copper Brake wear 2 55% Fe, 0% Cu NOA organic brake pads Brake wear 3 16% Fe, 13% Cu Ceramic brake pads Brake wear 4 24% Fe, 19% Cu Winter tires and Tarmac road pavement Tire/road wear 70 km/h constant speed Modern stove Modern stove dried beech wood Old Stove Old stove dried beech wood Diesel engine emissions Diesel engine mainly Euro III emissions Poultry farm Poultry farm 70.000 chickens

2.4.2 Results The potency to induce inflammatory responses were different for the various PM2.5 samples as shown by the counted neutrophil1 numbers and the induc- tion of pro-inflammatory KC2 markers in the lungs (Figure 2.7). An influx of neutrophils was mainly observed after exposure to high PM2.5 dose of ceramic brake pads (brake wear 4), the three combustion sources (modern stove, old stove and diesel combustion) and PM2.5 of the poultry farm. Exposure to some of the PM2.5 samples resulted in a dose-response (PM2.5 of the poultry farm as well as diesel combustion), while with PM2.5 emitted by an old stove a maximum response was already reached at the lowest PM dose. The KC values show a different trend however, most of the significant values are in alignment with the neutrophil values.

1Neutrophils are a type of white blood cell which are responsible for ﬁghting infections or inﬂammations. A large amount of neutrophils is thus a sign of acute attack to the immune system. 2KC is a neutrophil recruiting chemokine

18 CHAPTER 2. BACKGROUND AND METHODOLOGY

350

300

250

200 control

150 low middle 100 high 50 signiﬁcant

Counted neutrophils/400 cells neutrophils/400 Counted

250

200

150 control low 100 KC pg/mLKC middle high 50 signiﬁcant

Figure 2.7: Inflammatory parameters in bronchoalveolar lavage fluid 24h post- exposure to PM2.5 from different sources (preliminary outcomes).

19 GUIDELINES FOR OPTIMAL PPC OPERATION

2.4.3 Discussion and conclusions In this study, we examined the toxic potential of PM2.5 collected from various PM sources in mice by means of inhalation exposure. Preliminary outcomes suggest that lung inflammation was induced by most PM2.5 samples, including those derived from diesel and wood combustion, the poultry farm and wear emissions of the NAO organic (brake wear 3) and ceramic brake pads (brake wear 4). No significant inflammatory response was noticed after exposure to brake wear PM2.5 of semi-metallic pads and tire/road wear compared to clean air exposure. Dose (mass)-response relationships were assessed revealing the following order for potency to induce an influx of neutrophils to the lungs: tire/road wear, and brake wear 1 and 2 are probably less potent compared to old stoves, diesel engines, modern stoves, and brake wear 3 and 4. All of which are, in turn, less potent compared to poultry farm PM2.5.

The main outcome of this work is that the risk of causing health eﬀects due to exposure to ambient PM will vary based upon the relative contribution of various PM sources rather than on PM mass alone. Regarding diesel tailpipe emissions, it can be concluded that (when operating at Euro III settings) the toxic potency of the resulting PM is similar to ceramic and organic brake pads PM. These conclusions support further reduction of harmful diesel emissions, but at the same time also highlight that other PM sources, which will grow in relative importance as engines become even cleaner, should be included in future PM legislation.

20 Chapter 3

Impact of aromaticity and cetane number on the soot-NOx trade-oﬀ in conventional and low temperature combustion

3.1 Abstract

This paper* investigates whether or not two persistent diesel dogmas, namely “the higher the cetane number (CN) the better” and “the lower the aromaticity the better”, still holds when a compression ignition engine is operated in the low temperature combustion (LTC) regime. The transition from conventional, high temperature combustion (HTC) to LTC is realized in a step-wise approach by increasing the level of exhaust gas recirculation, reducing the compression ratio and by lowering intake pressure. The fuel matrix spans a range of both aromaticity and CN. All experiments are conducted on a modified DAF heavy-duty compression ignition engine. Two main conclusions can be drawn from the results. First, at equal aromaticity, there is no discernible benefit of a high CN with respect to the soot- NOx trade-off in the HTC mode. In fact, when operating in the LTC regime,

* The content of this chapter has been taken from: J.J.E. Reijnders, M.D. Boot and L.P.H. de Goey, Impact of aromaticity and cetane number on the soot-NOx trade-oﬀ in conventional and low temperature combustion, Published in Fuel, DOI: 10.1016/j.fuel.2016.08.009, 2016. Minor edits have been made to streamline the layout of the thesis.

21 GUIDELINES FOR OPTIMAL PPC OPERATION a high CN even results in a penalty in aforementioned trade-off. Second, at equal CN, increased aromatic content always has a negative impact on the soot-NOx trade-off, irrespective of combustion mode. Accordingly, our results demonstrate that, with respect to the soot-NOx trade-off, the first dogma does not hold and the second dogma is valid in both of the combustion modes.

3.2 Introduction

When the investment decision was made to construct gas-to-liquid (GTL) plants, the conventional wisdom held at the time was the higher the cetane number (CN) the better. For most of the compression ignition’s (CI) history, this has proven to be correct. The underlying causality, however, is a bit more complex. As refineries invested heavily in cutting aromatic fractions, soot emissions dropped and engines ran smoother. These phenomena coincided with a rise in CN, a collateral result of removing the intrinsically low reactive aromatics from the cut. Given that this positive correlation between CN and overall engine performance was long accepted as a causality in the combustion community, it is not surprising that later GTL plants - as is frequently emphasized in associated marketing campaigns - were designed to produce as high CN as possible, limited only by a maximum paraffinic chain length owing to cold flow (e.g., wax forming) considerations. Pertaining to the virtues of high CN, contradictory results are found in literature. On the one hand, some researchers argue that the CN should be as high as possible [25–28], while others report that a low CN is preferable [29–35]. To complicate matters further, earlier in-house research [36–38] demonstrated that lowering the CN by adding oxygenated aromatics to the fuel can improve the soot-NOx trade-off, particularly when operating in LTC mode. Other researchers also found that CN and aromaticity can have either a positive or negative impact on soot and NOx emissions. Wang et al. [39], who modeled soot emissions for a variety of fuels and engine operating conditions, concluded that aromatics can have both a positive and negative impact on soot emissions. On the one hand, the authors report that toluene greatly enhances poly aromatic hydrocarbon (PAH) and soot production. Conversely, improved mixing owing to longer ignition delays (ID) intrinsic to the lower CN of the toluene blends has a mitigating impact on soot. More evidence of this duality is found in a review paper on GTL by Gill et al. [40], wherein the authors noted that while 79 of the reviewed studies reported a positive impact of low aromatics and high CN on soot, in 16 there was no such benefit found and in 5 even increased soot emissions were observed. The

22 CHAPTER 3. AROMATICITY AND CETANE NUMBER authors attributed the poor performance in the latter two sets of studies to less time available for mixing ahead of the combustion event as a result of the relatively short ID incurred by high CN fuels in general. The impact of CN and aromaticity on the soot-NOx trade-off is presented in Table 3.1 for various engine studies [25–27, 29–31, 36, 41–47], which are grouped according to the combustion regime in question, from high (HTC) to low (LTC) temperature. From the summarized results, two main conclusions may be drawn. First, irrespective of combustion regime and given a constant CN, a higher aromaticity in all cases has a negative impact on aforementioned trade-off. Second, while beneficial under HTC operating conditions, high CN yields di- minishing and even negative emission returns as combustion temperatures fall. Importantly, this trend appears to hold regardless of compression ratio (CR), engine size or fuel matrix. As may be inferred from Table 3.1, the trends in the reviewed literature are predominantly qualitative in nature. The goal of this paper is to introduce a new, dimensionless parameter Π that holds distinct values for the various combustion modes and can thus predict either a positive, neutral or negative impact of high CN and low aromaticity on the soot-NOx trade-off based on a given set of engine operating conditions. To this end, engine tests will be conducted for a fuel matrix, spanning a wide range of CN and aromaticity, under HTC, transitional regime and LTC operating conditions on a modified DAF heavy-duty CI engine.

23 GUIDELINES FOR OPTIMAL PPC OPERATION

Table 3.1: Overview literature ﬁndings

Ref. # Engine Combustion Fuels CN Aromatic High CN Low mode range content beneficialb aromaticity beneficialc [25] HD, HTC Diesel, 53, 17, + + CR=17.5 GTL 75 0 [26] HD, HTC Diesel, 54, 24, + + CR=15.5 GTL 79 0 [27]a LD, HTC Diesel, 54, ?, + + CR=16.5 GTL 89 0 [41] HD, HTC Taylor 43-62, 0-38, 0 + CR=17 made fuels 0 [42]a LD, HTC-LTC Diesel1, 54, 24, 0 + CR=15.5 Diesel2 61 24 DS/GTL 61 17 [43] LD, HTC-LTC Taylor 28-54, 19-45, 0 + CR=16.2 made fuels [44] LD, LTC Taylor 35-59, 20-55, 0 + CR=16.6 made fuels [45] LD, LTC Taylor 42-53, 3-26, 0 + CR=15.1 made fuels [29] HD, LTC Diesel1, 39, 34, - + CR=14 Diesel2, 30, 50, MK1, 54, 3, Gasoline 29 29 [30]a LD, LTC Diesel, 52, 15, - + CR=16.5 GTL 74 0 [31] LD, LTC Taylor 28-54 20-50 - + CR=17.5 made fuels [36]a HD, LTC Diesel, 56, 20-25, - + CR=16 MK1, 53, 5-10, GTL 75 0 a Oxygenated fuels investigated in the cited studies have been excluded from this table since fuel oxygen has a large impact on soot and NOx emissions. b with respect to soot-NOx trade-off at constant aromaticity. c with respect to soot-NOx trade-off at constant CN.

24 CHAPTER 3. AROMATICITY AND CETANE NUMBER

3.3 Methodology

3.3.1 Experimental setup The heavy-duty direct injection (HDDI) diesel engine used for the measurements is based on a DAF engine. A schematic representation of the test setup is shown in Figure 3.1. It is a 12.6 L, inline 6-cylinder engine (Table 3.2). More detailed information can be found in [38].

Mass !ow meters

Valves Heater Fuel pump Air compressor Injectors

1 3 4 5 6 Brake Surge tanks

Back pressure valve Horiba 7100 DEGRHoriba AVL 415S TSI EEPS

EGR cooler

Figure 3.1: Schematic view of the setup based on a DAF 6-cylinder HDDI engine.

Cylinder 1 is a dedicated test cylinder, the second and third cylinder have no function, and the remaining 3 cylinders are used to rotate the crankshaft to the desired RPM. Table 3.2: Test engine speciﬁcations

Engine type 6-cylinder HDDI Engine model XE355C Cylinders 6 Capacity [l] 12.6 Bore [mm] 130 Stroke [mm] 158

3.3.2 Fuel matrix Standard diesel (EN590, DS52) is compared to both a high and low CN GTL fuel (Table 3.3). Note that the abbreviation of these fuels, FT, refers to Fischer-Tropsch, the process via which these fuels have been produced from

25 GUIDELINES FOR OPTIMAL PPC OPERATION natural gas. The most important difference, with respect to composition, between GTL and diesel is the absence of aromatics in the former. The main variable amongst the GTL fuels is the CN, varying from a diesel-like CN of 52 to a value as high as 72. The spread in CN can be traced back to differences in molecular size and structure. FT52 comprises mainly relatively low CN C12-C17 iso-paraffins, whereas predominantly high CN C8-C22 n-paraffins are found in FT72. More details on the composition of the tested GTL fuels, obtained by means of gas chromatography mass spectrometry (GC-MS), can be found in Appendix 3A.

Table 3.3: Fuel properties as measured by ASG-Analytik GmbH Germany

Property Unit Test Fuels DS 52 FT 52 FT 72 CN (IQT) [-] 51.6 52.1 71.8 HHV [MJ/kg] 46.5 47.4 47.5 LHV [MJ/kg] 43.5 44.1 44.6 Density [kg/m3] 835.4 798.4 777.3 C [wt.%] 86.6 84.9 84.6 H [wt.%] 13.2 14.8 15.1 PAH content [% m/m] 1.8 <0.1 <0.1 Mono aromatics [% m/m] 12.7 <0.1 <0.1 Di aromatics [% m/m] 1.6 <0.1 <0.1 Tri + aromatics [% m/m] 0.2 <0.1 <0.1 AFRstoich [-] 14.5 14.8 14.9

3.3.3 Design of experiments Engines running without exhaust gas recirculation (EGR) in a so-called hot combustion mode, or MODE 1 in this paper, can have local peak temperatures in excess of 2700 K [31, 47]. Today, most engine manufacturers, however, apply moderate to high levels of EGR to decrease otherwise high NOx levels (MODE 2). In-cylinder temperatures can be reduced even further by lowering the intake air pressure (MODE 3). This is possible by maintaining a constant equivalence ratio that translates into lower loads for reduced intake pressures. Lower loads are expected to manifest in lower wall temperatures and, ultimately, lower in-cylinder temperatures. Finally, in MODE 4, the CR is reduced to lower in-cylinder temperatures further still. In Figure 3.2, the key variables in terms of operating conditions (e.g., in- cylinder temperature) and fuel properties (e.g., aromatic content and CN) are plotted schematically in a design of experiments or DoE graph.

26 CHAPTER 3. AROMATICITY AND CETANE NUMBER

Aromatics [%] ----> Mode 1

Mode 2

Mode 3 In-cylinder [K] -----> temperature

Mode 4 Cetane Number [-] ---->

Figure 3.2: Design of experiments (DoE) graph

3.3.4 Experimental procedure

To mimic highway cruising at a constant 80 km/h, taking into consideration a typical transmission ratio, the engine speed is fixed at 1200 RPM. Prior to each experimental cycle, the engine has to run stable at steady state conditions, requiring engine lubrication and coolant water temperatures of 90 and 85 ◦C, respectively. A single injection per cycle is applied at a pressure of 1500 bar. The exhaust gas is cooled to approximately 30 ◦C in the EGR experiments. In MODE 1, all operating conditions are kept constant, save for the injection timing and injection duration. While operating in MODES 2 - 4, the injection duration is fixed and lambda is now allowed to vary between 1 and 2 as a function of EGR. In all MODES, the start of injection (SOI) is adjusted so as to maintain a constant crank angle at which 50% of the fuel is consumed (CA50) at 10 crank angle degrees after top dead center (CAD aTDC) to keep efficiency approximately constant. ID is defined for all MODES as the time, expressed in CAD, between the start of injection (SOI) and start of combustion (SOC). Hereby, SOI is the CAD at which the fuel first enters the combustion chamber and SOC as the moment when the heat release rate first rises above the threshold of 50 J/CAD.

27 GUIDELINES FOR OPTIMAL PPC OPERATION

An overview of the operating conditions in all four MODES is presented in Table 3.4. Associated data on accuracy is summarized in Table 3.5 and in Appendix 3E the repeatability is described.

Table 3.4: Engine operating conditions

Parameter Unit MODE 1 MODE 2 MODE 3 MODE 4 Engine speed [RPM] 1200 1200 1200 1200 Injection pressure [bar] 1500 1500 1500 1500 CA50 [CAD aTDC] 10 10 10 10 Intake pressure [bar] 1.5 1.5 1 1 Compression ratio [-] 15.7 15.7 15.7 14.9 Target lambda (λ) [-] 3 - 1 2 - 1 2 - 1 2 - 1 Target IMEP [bar] 5 - 11 8 6 6 Injection duration [µs] 600 - 1700 960 750 750 EGR [wt.%] 0 0 - 40 0 - 40 0 - 40

The transition from HTC to LTC is realized in a step-wise approach. By adding EGR, the conversion is made from MODE 1 to 2. In MODES 1 and 2, the CR and intake pressure are 15.7 and 1.5 bar, respectively. In order to realize the transition to MODE 3, the intake pressure is lowered to 1 bar, while maintaining the same lambda as is the case for MODE 2. In MODE 4, the lower intake pressure is maintained and LTC is further facilitated by a lowering the CR to 14.9. The graphs with PM as function of NOx are interpolated over NOx which was necessary to normalize the PM emissions in Paragraph 3.5.

Table 3.5: Data acquisition accuracies

Data type Brand Type Unit Accuracy Temperatures Omega k-type thermocouple ◦C ± 1.1 ◦C Pressures GE PMP 1400 Pa ± 0.25% of rate Air mass ﬂow Micro Motion Coriolis CMF200 g/s ± 0.35% of rate Diesel mass ﬂow Micro Motion Coriolis CMF010 g/s ± 0.2% of rate In-cylinder pressure AVL GU21C bar < ± 0.3 % FS (250 bar) Gaseous analyzer HORIBA 7100 DEGR %, ppm ± 0.5% FS Smoke meter AVL 415S / Filter method FSN ± 0.01 FSN (< 6 FSN)

28 CHAPTER 3. AROMATICITY AND CETANE NUMBER

3.4 Results

Before the results from MODES 1-4 are explained separately, ﬁrst the rate of heat release (RoHR) is analyzed for the two most extreme modes (MODE 1 & 4). MODE 1 aims to represent HTC (without EGR), MODE 4 represents LTC with high EGR rates and lower CR and intake pressure. In Figure 3.3, the RoHRs are shown for these two combustion modes.

500 500 DS 52 DS 52 400 FT 52 400 FT 52 FT 72 FT 72 300 300

200 200

RoHR [J/CAD] 100 RoHR [J/CAD] 100

0 0

−5 0 5 10 15 20 −5 0 5 10 15 20 Crank angle degree Crank angle degree (a) MODE 1, IMEP=8 bar, no EGR, (b) MODE 4, IMEP=6 bar, 30% EGR, CA50=10 CAD aTDC. CA50=10 CAD aTDC.

Figure 3.3: Rate of heat release progress as a function of CAD for the hottest and coolest MODE. Arrows indicate the consequences of an increase in CN.

Three phenomena in these figures warrant a further explanation: 1. In both figures, the injection event starts approximately at the same time for all fuels (1-2 CAD bTDC). However, the rise in heat release varies for the different fuels because of the different ignition delays. This important parameter will be analyzed for all the modes together with its consequences.

2. The left peak in the combustion process is noticeably less pronounced for the high CN GTL fuel when compared to DS52 and FT52 (Figure 3.3a & 3.3b, arrows pointing downwards). This peak is closely related to the magnitude of the ID and is indicative of premixed combustion. After the start of injection, fuel mixes with air and when critical (cumulative) auto-ignition limits are reached, the fuel combusts rapidly. Both the ID and premixed peak are higher in MODE 4, which is indicative of a more premixed combustion.

3. Following this premixed phase, the combustion mode switches to mixing- controlled or diﬀusive combustion, characterized by the right peak in Fig-

29 GUIDELINES FOR OPTIMAL PPC OPERATION

ure 3.3a. This diﬀusive combustion phase has completely disappeared for the lower CN fuels in Figure 3.3b. For FT 72 there is still a (small) period, at the plateau around 7 CAD, wherein mixing-controlled combustion takes place.

In the following section, the results per mode will be discussed in greater detail. The discussion will focus on the soot-NOx trade-off. Corresponding data on efficiency and CO/HC emissions can be found in Appendix 3B and Appendix 3C, respectively. Appendix 3D provides an overview of the impact of MODE on the soot-NOx trade-off.

3.4.1 MODE 1 From the RoHR graphs presented earlier is clear that the ID has a pronounced impact on the combustion process and, as a result, on the emission formation process as well. Typically, soot emissions increase with a decreasing ID, owing to a lower degree of premixing. The relation between NOx and ID is less straightforward. A lower premixed peak (e.g., short ID via high CN) generally translates into decreased NOx emissions. However, the ensuing diffusion combustion phase can nullify this effect depending on engine configuration and fuel specifications [48].

4 6 bar IMEP 8 bar IMEP 3.5 10 bar IMEP

3 ID [CAD] 2.5

2 50 55 60 65 70 75 CN [−]

Figure 3.4: ID as a function of load and CN in MODE 1 (Black=DS52, blue=FT52, and red=FT72).

Figure 3.4 shows the ID as a function of both load or indicated mean eﬀec- tive pressure (IMEP) and CN. As might be expected based on their similar CN, DS52 and FT52 have a comparable ID at all loads, which in all cases is signiﬁcantly longer than those observed for the high CN GTL fuel.

30 CHAPTER 3. AROMATICITY AND CETANE NUMBER

0.03 6 bar IMEP 0.025 8 bar IMEP 10 bar IMEP 0.02

0.015

PM [g/kWh] 0.01

0.005

0 50 55 60 65 70 75 CN [−]

Figure 3.5: PM emissions as a function of load and CN in MODE 1 (Black=DS52, blue=FT52, and red=FT72).

Pertaining to soot (measured as (indicated specific) PM), the trend in Figure 3.5 shows an increase with load as lambda approaches 1 (i.e., stoichiometric combustion). When comparing the GTL fuels, no significant differences are observed. DS52, however, generates markedly higher emissions, particularly at higher loads.

12 6 bar IMEP 11 8 bar IMEP 10 bar IMEP 10

9 [g/kWh] x 8 NO

6 50 55 60 65 70 75 CN [−]

Figure 3.6: NOx emissions as a function of load and CN in MODE 1 (Black=DS52, blue=FT52, and red=FT72).

NOx emissions as a function of load and CN are plotted for MODE 1 in Fig- ure 3.6. Note that NOx in all cases is well above legislated limits (0.4 g/kWh for Euro VI), owing, at least to a large extent, to the omission of EGR. Diﬀer- ences amongst the fuels are relatively small, with only FT72 showing a slightly

31 GUIDELINES FOR OPTIMAL PPC OPERATION

higher NOx output that can be traced back, in part, to a higher diﬀusion combustion peak in Figure 3.3a.

Characteristic to diesel engines is the so-called diesel-dilemma or soot-NOx trade-oﬀ. The origin of this trade-oﬀ is quite straightforward. As prevailing combustion conditions are more conducive for soot oxidation (i.e., hotter), more atmospheric nitrogen is burnt to NOx. Naturally, the opposite trend holds for cooler combustion conditions.

0.4 DS 52 FT 52 0.3 FT 72

0.2 Load increase ISPM [g/kWh] 0.1

0 4 6 8 10 ISNO [g/kWh] x

Figure 3.7: Soot-NOx trade-oﬀ as a function of load in MODE 1.

Shown in Figure 3.7, this trade-oﬀ is plotted for MODE 1, with the arrow pointing into the direction of higher loads. Comparing the fuels in Figure 3.7, the low aromaticity GTL fuels outperform conventional diesel, particularly at the highest loads where the equivalence ratio approaches unity. As is clear from the normalized data in Figure 3.8, the relative diﬀerences amongst the fuels remain nearly constant over the whole load range.

32 CHAPTER 3. AROMATICITY AND CETANE NUMBER

2 DS 52 FT 52 1.5 Load increase FT 72

PM [normalized] 0.5

0 4 5 6 7 8 ISNO [g/kWh] x

Figure 3.8: Normalized (to DS52) PM emissions as a function of NOx and load in MODE 1.

3.4.2 MODE 2 As stated earlier, EGR is required to realize the transition from MODE 1 to MODE 2. The results of this MODE will be conﬁned to the ID and normalized soot-NOx trade-oﬀ (similar to Figure 3.8).

6 0% EGR 15% EGR 5 30% EGR

4 ID [CAD] 3

2 50 55 60 65 70 75 CN [−]

Figure 3.9: ID as a function of EGR and CN in MODE 2 (Black=DS52, blue=FT52, and red=FT72).

Figure 3.9 depicts the ID as a function of CN and EGR. With low amounts of EGR, both low CN fuels yield a similar ID, as was the case earlier in MODE 1. As EGR is increased, however, the ID of DS52 becomes longer compared to FT52. In Figure 3.10, the primary purpose of EGR is clearly visible, with

33 GUIDELINES FOR OPTIMAL PPC OPERATION

2 DS 52 FT 52 1.5 FT 72 EGR increase

PM [normalized] 0.5

0 0 2 4 6 8 ISNO [g/kWh] x

Figure 3.10: Normalized (to DS52) PM emissions as a function of NOx and EGR in MODE 2.

NOx emissions dropping as more EGR is applied. In MODE 2, more or less the same ranking is observed as was seen earlier in MODE 1, with both GTL fuels outperforming DS52 with respect to the soot-NOx trade-oﬀ. As was true in MODE 1, this beneﬁt holds for both GTL fuels.

3.4.3 MODE 3 As can be seen in Figure 3.9, the ID spread amongst the GTL fuels is relatively small (1 CAD at 30% EGR). It is believed that fuel identity with respect to ID under equal operating conditions will become more apparent as the combustion mode becomes even more premixed, owing to longer IDs. To this end, in MODE 3, the intake pressure, while maintaining an equal lambda as used in MODE 2, is lowered to roughly 1 bar.

34 CHAPTER 3. AROMATICITY AND CETANE NUMBER

10 0% EGR 15% EGR 8 30% EGR

6 ID [CAD] 4

2 50 55 60 65 70 75 CN [−]

Figure 3.11: ID as a function of EGR and CN in MODE 3 (Black=DS52, blue=FT52, and red=FT72).

Figure 3.11 shows an increase in ID compared to values observed for MODE 2 (Figure 3.9). This is particularly the case for the low CN fuels at 30 wt.% EGR, for which conditions the premixed phase is most pronounced.

2 DS 52 FT 52 1.5 FT 72 EGR increase

PM [normalized] 0.5

0 0 2 4 6 8 ISNO [g/kWh] x

Figure 3.12: Normalized (to DS52) PM emissions as a function of NOx and EGR in MODE 3 .

When comparing the relative soot-NOx performance of all fuels in MODE 2 (Figure 3.10) and MODE 3 (Figure 3.12), it is clear that the cooler conditions of the latter MODE impact the ranking. Whereas in MODE 2 both GTLs outperform diesel at all NOx levels, now this is only the case for the lower CN GTL. There are clearly two competing eﬀects in play here, with low CN improving mixing conditions, thereby suppressing soot, while the aromatics

35 GUIDELINES FOR OPTIMAL PPC OPERATION in DS52 still tend to act as soot precursors. Interestingly, these eﬀects now appear to cancel each other out, accounting for the comparable performance of DS52 (low CN with aromatics) and FT72 (high CN without aromatics). By the same token, the obvious winner in this MODE, FT52, having both low CN and no aromatics, shows ever more pronounced beneﬁts over the other fuels as conditions become less reactive still as EGR levels increase further.

3.4.4 MODE 4

To increase the ID further still, in-cylinder-temperatures should be lowered even more. To this end, the compression-ratio is decreased from 15.7 to 14.9 in MODE 4.

10 0% EGR 15% EGR 8 30% EGR

6 ID [CAD] 4

2 50 55 60 65 70 75 CN [−]

Figure 3.13: ID as a function of EGR and CN in MODE 4 (Black=DS52, blue=FT52, and red=FT72).

In Figure 3.13, the ID is plotted against CN and EGR. When compared to Figure 3.11, the average ID has indeed become even longer. Again, ID increases with EGR, translating once more into lower soot emissions for all fuels. However, for the ﬁrst time, the use of high CN GTL results in a penalty in the soot-NOx trade-oﬀ when compared to DS52, notwithstanding the presence of aromatics in the latter fuel (Figure 3.14). On average, FT52 shows a similar level of performance improvement over DS52 as was observed in MODE3 (Figure 3.12).

36 CHAPTER 3. AROMATICITY AND CETANE NUMBER

3 DS 52 2.5 FT 52 FT 72 2

1.5

1 PM [normalized] 0.5 EGR increase 0 0 2 4 6 8 ISNO [g/kWh] x

Figure 3.14: Normalized (to DS52) PM emissions as a function of NOx and EGR in MODE 4.

3.5 Discussion

3.5.1 Aromaticity

In all non-EGR cases (MODE 1 and 0% EGR in MODES 2-4), the ranking in ID corresponds well with the measured CN (Table 3.3). At higher EGR rates (>15%), however, the ID of DS52 is marginally longer compared to the similar CN fuel, FT52. It should be noted here that all CN values in Table 3.3 have been determined by means of an ignition quality tester (IQT) in accordance with ASTM D6890.The data suggests that the absence of EGR in the IQT protocol and the presence of EGR in MODES 2 - 4, combined with the diﬀerent chemical compositions of the two low CN fuels, somehow gives rise to increased reactivity of the FT52 relative to the DS52 fuel. Although not determined here, it is reasonable to assert that the presence of highly sensitive (i.e., of auto-ignition chemistry to temperature [49]) aromatics in DS52 (Table 3.3) and the low sensitivity of the wholly paraﬃnic FT52 (Table 3.3), might account for the aforementioned discrepancy in ID behavior. Indeed, when switching from MODE 3 to 4 (compare Figures 3.11 and 3.13) the ID increases less for the insensitive FT52 than is the case for the sensitive DS52. For example, at 30 wt.% EGR, the ID increases from roughly 7.8 to 8 CAD and 8.9 to 9.8 CAD for FT52 and DS52, respectively, when switching from MODE 3 to 4.

37 GUIDELINES FOR OPTIMAL PPC OPERATION

3.5.2 Aromaticity versus cetane number

In MODES 1 and 2, the variation with respect to soot-NOx performance amongst FT52 and FT72 was marginal, while that of these GTL fuels relative to DS52 was in fact quite pronounced (Figures 3.8 and 3.10). Note that in both MODES FT52 and FT72 demonstrated better soot-NOx trade-oﬀs over conventional diesel. Given the absence of aromatics in FT52 and FT72 (Appendix 3A) and the wide range in CN spanned by the two fuels, it is fair to assume that aromaticity is the dominant of the two fuel properties with respect to the prevailing soot-NOx trade-oﬀ.

2.5

2 DS 52 Delta_PM_CN FT 52 1.5 FT 72 1 PM [normalized] 0.5 Delta_PM_AROM 0 0 0.5 1 1.5 2 ISNO [g/kWh] x Figure 3.15: Example for evaluating the relative contribution of aromaticity and CN with respect to the soot-NOx trade-oﬀ (MODE 4 at 0.7 g/kWh NOx, zoom-in of Figure 3.14).

To make this assumption more tangible, the diﬀerences in soot emissions between fuels, referred to here as Delta PM (Figure 3.15), is calculated for the low CN fuels DS52 and FT52 (Delta PM AROM) and for the GTL fuels (Delta PM CN). Delta PM AROM covers a spread in aromaticity (0-14.5, Table 3.3) at an equal CN of 52. Delta PM CN covers a spread in CN (52-72, Table 3.3) for a constant (zero) aromaticity.

Delta PM AROM Π = (3.1) Delta PM CN The ratio of the two Delta PM values, denoted here as Π (3.1), is plotted in Figure 3.16 for all fuels and NOx levels. Note that a value above unity suggests that aromaticity plays a dominant role. Conversely, a value lower than 1 implies that the role of CN outweighs that of aromaticity. In Figure 3.16, it can be seen that, for any given NOx level, as average in-cylinder gas

38 CHAPTER 3. AROMATICITY AND CETANE NUMBER temperatures fall with increasing MODE #, owing to chieﬂy EGR and a lower CR, the aforementioned ratio drops from well above unity to near zero.

8 MODE 1 MODE 2 6 MODE 3 MODE 4

[−] 4 Π

2 Aromatics CN 0 0 2 4 6 8 ISNO [g/kWh] x

Figure 3.16: Π plotted for all fuels in all MODES as a function of NOx.

A general observation from Figure 3.16 is that Π values tend to increase with NOx, with this trend being more pronounced in the hot MODES 1 and 2.

MODES 1 and 2 In MODE 1, no EGR is applied and therefore the minimum NOx level achieved is still quite high at 4 g/kWh. In MODE 2, as EGR is added, both lower NOx and Π values are found. However, in both cases the Π values are substan- tially higher than 1, implying that soot-NOx behavior correlates better with aromaticity than with CN. The data suggests that in MODES 1 and 2, both characteristic to HTC, the added value in using a GTL lies not in its higher CN, but in its lower aromaticity.

MODE 3 In MODE 3, the intake pressure is lowered to reach even cooler in-cylinder conditions. With these settings, the variation in PM emissions between the GTL fuels is similar to the variation between DS52 and FT52 (Figure 3.12). This finding implies that aromaticity and CN have a comparable influence over the soot-NOx trade-off. Probably, because of this longer ID, the local air/fuel- ratio is sufficiently high to combust most of the aromatics, which accordingly no longer tend to act as soot precursors [25]. In MODE 3, the optimal fuel is thus one with a low CN and without aromatics.

39 GUIDELINES FOR OPTIMAL PPC OPERATION

MODE 4 In MODE 4, the CR is lowered to reach even cooler in-cylinder conditions. Under these circumstances, CN clearly correlates better with the soot-NOx trade-oﬀ than aromaticity. Compare for instance FT52 and FT72 in Figure 3.14, wherein it can be observed that both wholly paraﬃnic fuels have diver- gent soot emissions at equal NOx levels, with the magnitude of said spread growing with increasing EGR levels. In fact, this divergence is so great that the high CN FT72 fuel now produces more PM than conventional diesel fuel, despite the latter fuel being comprised of nearly 15% aromatics (Table 3.3).

3.6 Conclusions

The main results of this study are summarized in Table 3.6. The symbols “+” (green), “-” (red) and “o” (yellow) indicate the relative ranking of the fuels with respect to the soot-NOx trade-oﬀ. Furthermore, a new parameter Π is introduced to discern between HTC and LTC.

Table 3.6: Summary of main results

<——– CN ↓ ——— Π DS 52 FT 52 FT 72 MODE 1 6-8 (HTC) - + + MODE 2 2-6 (HTC) - + + MODE 3 ≈1 (HTC-LTC) 0 + 0 MODE 4 0-1 (LTC) 0 + - —- Aromaticity ↓ —->

HTC regime (Π >1) When Π is higher than one (MODES 1,2) the best soot-NOx trade-off is realized by a fuel with low aromaticity (FT52 and FT72). It is assumed that Π > 1 corresponds to HTC or diffusion combustion as a negative impact of aromaticity is observed and there is no discernible benefit of high CN at equal aromaticity. In HTC mode, premixing is almost absent, regardless of CN. Moreover, in diffusion flames, aromatics tend to behave as soot precursors, thereby negatively impacting soot emissions.

Transitional regime (Π ≈1) At unity (MODE 3), CN may be lowered, without resulting in any adverse consequences, by increasing the aromatic content as far as aforementioned trade-oﬀ is concerned. Here, it is assumed that both the diﬀusion and premixed combustion phases are important. It is clear when comparing FT72

40 CHAPTER 3. AROMATICITY AND CETANE NUMBER and DS52 that the positive impact of aromatics in LTC (more premixing due to lower CN) is held in check by their negative attribute in HTC mode (soot precursors). The best results, however, are achieved for a fuel that has both low CN and low aromaticity.

LTC regime (Π <1) Once Π dips below unity, higher CN (FT72) manifests in an even greater handicap than aromaticity (DS52) with respect to the soot-NOx trade-oﬀ, suggesting that regardless of how a reduction in CN is achieved, be it via aromatics (DS52) or isomerization (FT52), the outcome is always positive. Still, the best results are achieved for the fuel with both low CN and low aromaticity.

The goal of this paper to discern in a quantitative sense between the HTC and LTC regimes has been achieved by means of the introduction of a new parameter Π, which effectively evaluates if aromaticity and CN impact the soot- NOx trade-off in a manner characteristic of either diffusion (HTC → Π >1) or premixed flames (LTC → Π <1). Irrespective of Π, the best trade-off is realized at low aromaticity. At equal aromaticity, there is no discernible benefit of a high CN. In fact, when Π <1, a high CN is even more detrimental, with respect to the soot-NOx trade-off, than is the presence of aromatics.

Accordingly, two main conclusions can be drawn from the results. First, at equal aromaticity, there is no discernible benefit of a high CN with respect to the soot-NOx trade-off in the HTC mode. In fact, when operating in the LTC regime, a high CN even results in a penalty in aforementioned trade-off. Second, at equal CN, increased aromatic content always has a negative impact on the soot-NOx trade-off, irrespective of combustion mode. Our results demonstrate that, with respect to the soot-NOx trade-off, the first dogma (the higher the cetane number, the better the overall engine performance) does not hold and the second dogma (the lower the aromaticity, the better the overall engine performance) is valid in both of combustion modes.

The added value of this work is to provide design rules on requisite fuel properties (e.g., aromaticity, CN) for various scenarios with respect to the direction of future diesel engine technology, which might be taken into account when designing future fuel production facilities.

41 GUIDELINES FOR OPTIMAL PPC OPERATION

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42 Chapter 4

GTLine - Gasoline as a potential CN suppressant for GTL

4.1 Abstract

The main driver to investigate partially premixed combustion (PPC) is the promise of low soot and NOx emissions. A critical prerequisite for PPC is a temporal separation between the injection and combustion events. In practice, EGR is applied in order to sufficiently extend the ignition delay into PPC territory, whereby higher EGR rates are necessary for fuels with higher cetane numbers. Against this background, the objective of this paper is to investigate the efficacy, with respect to soot-NOx trade-off, of gasoline as a potential GTL CN suppressant in various dosages. The performance of the resulting GTLine will be evaluated under PPC operating conditions in a heavy-duty direct- injected diesel engine. Setting aside for a moment any potential practical issues (e.g., flash point, vapor pressure) that fall outside the scope of this study, our data suggest that blending gasoline to otherwise high CN GTL appears to be a promising route to improve not only the efficiency, but also emissions when operating in PPC mode, notwithstanding the high aromaticity of the gasoline. Given the dieselization trend and associated surplus of gasoline in many regions, along with the fact that the cost price of gasoline is significantly lower than that of GTL, the proposed GTLine approach promises to be a cost

The content of this chapter has been taken from: J.J.E. Reijnders, M.D. Boot, B. Johansson and L.P.H. de Goey, GTLine - Gasoline as a potential CN suppressant for GTL, Under review (Fuel). Minor edits have been made to streamline the layout of the thesis.

43 GUIDELINES FOR OPTIMAL PPC OPERATION eﬀective way to accommodate GTL in a world wherein LTC engine concepts really take oﬀ.

4.2 Introduction

Low temperature combustion (LTC) concepts such as homogeneous charge compression ignition, premixed charge compression ignition and partially premixed combustion (PPC) are seen as promising routes to achieve both low emissions and high efficiency in future compression ignition engines. For practical reasons including controllability of the combustion phasing and thermal loading of the engine, fully premixed combustion, as is the case for the former two concepts, is considered here to be less promising in real-life applications. This study will thus focus on the latter combustion principle, PPC. PPC can be defined as a combustion process in a compression ignition engine, whereby a temporal separation exists between the fuel injection and combustion events, thereby implying that a long ignition delay (ID) is the single most important prerequisite [29, 50–58]. A shared conclusion found in all these studies is that PPC is most readily realized with a low CN fuel, else prohibitively high EGR rates are necessary to adequately extend the ID (Table 4.1). Other key findings from the before mentioned studies, along with associated data on engine specifications, operating conditions and fuel matrices, are presented in Table 4.2.

Table 4.1: Minimal EGR rate to achieve PPC

Source Fuel CN EGR (wt%) [29] Gasoline ± 15 0 [53] Gasoline ± 15 5 [34] Diesel ± 56 25 [50] Diesel ± 42 30 [58] Diesel ± 43 40 [35] Diesel ± 54 48

In our previous work [59], we evaluated the performance of two Fischer- Tropsch (FT) GTL diesel fuels with CNs of 52 and 72 in various combustion modes from high (i.e., conventional diffusion) to low (i.e., PPC) temperature regimes. The results demonstrated that, at equal fuel aromaticity, a high CN yields no discernible benefit in terms of soot-NOx trade-off in high temperature combustion modes and even becomes a serious handicap when transiting to the low temperature regime. While GTL is best known for its very high CN, it is possible to produce GTL

44 CHAPTER 4. GTLINE with lower, more diesel-like values by means of a combination of shortening and branching the paraﬃnic chains intrinsic to the FT process [59]. While future plant owners might take notice of aforementioned insights, producers of high CN GTL diesel must necessarily add low CN compounds in order to decrease the CN. Earlier, we investigated the use of ethanol [60] and oxygenated aromatics [61] as potential CN suppressants for GTL. Shell too acknowledged that for some engine operation conditions the otherwise high CN of their GTL product should be lowered and therefore proposed to blend in oxygenated and nitrated aromatics [62]. A well known approach to lower the CN of diesel fuel in general is to blend it with low CN fossil or biofuels [34, 63–69].

Table 4.2: Summary of literature review on PPC

Ref. Engine Operating Fuels Key findings # specifica- conditions tions [29] HD, 4-9 bar IMEP, Diesel, Engine can run on gasoline in PPC mode with CR=14 0-25% EGR Gasoline a much higher IMEP while maintaining low NOx and smoke. [50] HD, 4 bar IMEP, 5- Diesel The LTC models show a longer liquid-fuel CR=16.1 60% EGR penetration and longer ID for PPC. Moreover, reduced and altered soot formation regions are observed. [51] LD, 5 bar IMEP, 0- Diesel, Soot emissions are strongly related to injec- CR=16.1 65% EGR Gasoline, tion timing. n-Heptane, Higher intake pressures counteract EGR soot n-Butanol penalty. [52] LD, 5 bar IMEP, Diesel, NOx and particulate matter (PM) emissions CR=15.5 20-35% EGR Propane are reduced with early split injection strategy using high propane ratios. [53] LD, 3-7 bar IMEP, Gasoline Double injection improves PPC spark assisted CR=14.7 5-15% EGR combustion efficiency. Lower pilot fuel mass injection reduces CO and UHC emissions. [54] LD, 1-5 bar IMEP, Diesel, Combustion mode moves from two-stage to CR=16.5 0% EGR Ethanol three-stage increasing the premixed ratio. [55] HD, 4-12 bar Diesel, Start of injection (SOI) significantly affects CR=17.1 IMEP, Methane emissions in LTC mode. 0% EGR Heat release shifts from two-stage to Gaussian profile when advancing SOI. [56] LD, 6 bar IMEP, DME Dual injection strategy shows extremely low CR=15.0 0% EGR NOx emissions while maintaining moderate engine output. [57] LD, 8 bar IMEP, 0- Diesel, The joint action of a longer ignition delay and CR=17.5 20% EGR n-Butanol a better fuel volatility reduces smoke at moderate injection pressures. [58] LD, 4 bar IMEP, Diesel Theoretical analysis of the reaction rates sug- CR=16.0 40-45% EGR gests that soot formation is insensitive to equivalence ratio at temperatures below 1500 K.

45 GUIDELINES FOR OPTIMAL PPC OPERATION

When reviewing Table 4.2, one practical and relatively cheap fuel looks particularly promising, gasoline. Coined Dieseline, this mix of fossil fuels already has a proven track record as a PPC fuel. What is more, there exists, in Europe at least, a surplus of gasoline, thereby partially compensating for both the limited availability of GTL as well as its high cost price. As no earlier engine results on gasoline-GTL blends could be found in literature, the objective of this study is to investigate the eﬃcacy, with respect to optimizing the soot-NOx trade-oﬀ, of gasoline as a GTL CN suppressant. The performance of the resulting GTLine blends will be evaluated under both conventional and PPC operating conditions.

4.3 Methodology

4.3.1 Experimental setup All experiments are carried out on a modiﬁed 12.6 liter HDDI DAF XE 355C in-line six diesel engine (see Table 4.3 and [38, 59]).

Table 4.3: Test engine speciﬁcations

Engine type HDDI Engine model XE355C Cylinders 6, with 1 isolated for research purposes Capacity [l] 12.6 Bore [mm] 130 Stroke [mm] 158 Compression ratio [-] 15.7

The first cylinder is used as a test cylinder and is equipped with a dedicated in- and outlet manifold, EGR circuit and fuel injection system. Cylinders 2 and 3 have no function, while numbers 4-6, operating on the stock ECU and conventional fuel, are responsible only for maintaining the desired engine speed. For optimal EGR dosage, temperature control and mixing with fresh air, several surge tanks and coolers have been incorporated into the design. A schematic overview of the test engine is shown in Figure 4.1. Fresh air (up to 5 bar) for the test cylinder is provided by an external compressor, the mass flow of which is measured by a Coriolis mass flow meter. The fuel injection system consists of a Resato double-acting air driven fuel pump (up to 2500 bar) and a prototype common rail injector with eight 0.151 mm diameter holes. Gaseous emissions and filter smoke numbers (FSN) are measured by a

46 CHAPTER 4. GTLINE

Mass !ow meters

Valves Heater Fuel pump Air compressor Injectors

1 3 4 5 6 Brake Surge tanks

Back pressure valve Horiba 7100 DEGRHoriba AVL 415S TSI EEPS

EGR cooler

Figure 4.1: Test engine rig

Horiba Mexa 7100 DEGR and an AVL smoke meter (415S). Gaseous emissions, along with all common pressure, temperature and mass ﬂow sensors are logged for 40 seconds at 20 Hz for each operating point. Data acquisition for in-cylinder pressure and heat-release analyses uses a SMETEC Combi crank angle resolved data acquisition system, which logs in steps of 0.1 crank angle degree for 50 continuous cycles. Variables measured by this system are cylinder pressure (using an AVL GU21C), intake pressure, fuel pressure and injector current. For each operating point, the smoke level is averaged over three measurements.

4.3.2 Fuel matrix In total, ﬁve fuels will be evaluated in the engine tests: neat GTL, three GTLine blends (10, 20 and 50 vol.-% gasoline) and EN590 diesel as reference fuel. Fuel properties, as measured by ASG-Analytik GmbH Germany, can be found in Table 4.4).

47 GUIDELINES FOR OPTIMAL PPC OPERATION

Table 4.4: Fuel properties

Property Unit Fuels 100/0 90/10 80/20 50/50 Ref. (EN590) GTL [%V/V] 100 90 80 50 0 Gasoline [%V/V] 0 10 20 50 0 CN (IQT) [-] 79.8 68.2 62.6 45.5 51.6 LHV ∗ [MJ/kg] 44.6 44.4 44.3 43.7 42.5 Density ∗ [kg/m3] 777.3 773.5 769.7 758.3 835.4 C ∗ [wt.-%] 84.6 84.6 84.5 84.4 85.1 H ∗ [wt.-%] 15.1 14.9 14.7 14.1 13.1 Aromatic content ∗ [%V/V] <0.1 2.6 5.1 12.5 16.3 AFRstoich ∗ [-] 14.9 14.8 14.7 14.5 14.2 Sulfur ∗ [mg/kg] 0.5 0.9 1.3 2.4 14.2 * Blend values calculated from those measured for neat compounds

4.3.3 Experimental procedure All tests are conducted with a single injection per cycle, at a ﬁxed engine speed of 1200 rpm, and constant engine oil and coolant temperatures of 90 and 85 ◦C, respectively. For all fuels, EGR rates are varied from 0-50%. Hereby, the CA50 is kept constant at 10◦CA after top dead center by adjusting the start of injection (SOI). An overview of the operating conditions is provided in Table 4.5. Associated data on accuracy is summarized in Table 3.5.

Table 4.5: Engine operating conditions

Parameter Unit Value Engine speed [RPM] 1200 IMEP [bar] 6 CA50 [CAD aTDC] 10 Intake pressure [bar] 1 Exhaust pressure [bar] 1.3 Intake temperature [ ◦C] 40 EGR [wt.%, CO2/CO2] 0 - 50 Lambda (λ) [-] 2 - 1 Injection pressure [bar] 1500 Injection duration [µs] / ([CAD]) 950 / (6.9)

48 CHAPTER 4. GTLINE

4.4 Results

4.4.1 Rate of heat release

Figure 4.2 shows the RoHR for all fuels at 15 wt.-% EGR. As might be expected, when maintaining a constant CA50, lower CN fuels yield a longer ID1, higher peak rate and shorter combustion duration. The RoHR signal can be unimodal (as 50/50 blend, steep rise and steep fall) and more bimodal like (as 100/0, steep rise and slow fall or in more extreme cases even with a plateau), whereby the ﬁrst and second modes represent the contribution of premixed and subsequent diﬀusion combustion, respectively.

Indeed, wider RoHR curves are indicative of a more dominant role for diffusion combustion, which tends to be slower than its premixed counterpart. A greater importance of diﬀusion combustion, in turn, is to be expected for high CN fuels, owing to their relatively short IDs. As the CN drops, however, the premixed phase begins to play a more dominant role in the heat-release event. In fact, particularly for the GTLine fuels with the highest blend ratios of gasoline, the very nature of the RoHR curve changes from bi- to unimodal as the contribution of diﬀusion combustion wanes and the combustion type gradually moves into PPC territory.

700 100/0 CN ↓ 600 90/10 500 80/20 50/50 400 Ref. 300 200 RoHR [J/CAD] 100 0

−10 0 10 20 30 Crank angle [°]

Figure 4.2: RoHR as a function of CAD at 15% EGR

1Time expressed in CA between SOI and SOC, whereby SOI and SOC are defined as the CAD at which the fuel first enters the combustion chamber, evidenced by RoHR first going negative as a result evaporative cooling, and the moment at which the RoHR first rises above the 50 J/CAD threshold, respectively.

49 GUIDELINES FOR OPTIMAL PPC OPERATION

4.4.2 Ignition dwell An important drawback of diffusion combustion is the so-called diesel dilemma or soot-NOx trade-off. Measures to decrease diffusion flame temperature (e.g., by using EGR) tend to curb NOx and promote soot emissions, and vice versa. LTC concepts therefore aim to switch to a new paradigm, wherein this trade- offs no longer exist. To this end, measures are undertaken to increase the role of premixed over diffusion combustion. The rationale for this is quite straightforward; as fuel is more widely dispersed in the combustion chamber, local equivalence ratios drop and combustion heat is released more homogeneous throughout the cylinder. Both phenomena, in turn, manifest in lower soot and NOx formation rates. In order to measure the relative importance of both combustion modes we will evaluate the so-called ignition dwell (IDw) of the combustion event. The IDw is defined as the difference (expressed in CA) between the end of injection (EOI) and the start of combustion (SOC) (Eq. 4.1). Hereby, a negative and positive value is signifies classical (i.e., lifted diffusion flame) and PPC diesel combustion, respectively [70].

IDw = CA(SOC) − CA(EOI) (4.1)

15 100/0 90/10 10 80/20 50/50 Ref. 5

0 Ignition Dwell [CAD]

−5 0 10 20 30 40 50 EGR [%]

Figure 4.3: Ignition dwell as a function of EGR

Figure 4.3 shows the IDw as a function of EGR. Moving further down the dashed line, diﬀusion combustion is becoming evermore dominant. Conversely, data points positioned above this line will most likely be void of this combustion mode and are considered to be of the PPC type. As discussed earlier when reviewing the PPC literature and further evidenced in Figure 4.3 any fuel can be pushed into PPC territory by means of EGR, irrespective of its

50 CHAPTER 4. GTLINE

CN. Nevertheless, as found in literature (Table 4.1), the data in Figure 4.3 demonstrate that far less EGR is required for lower CN fuels (higher ratio of gasoline). As can be seen in Figure 4.4, a minimum ID (7-8 CAD) is required to reach this threshold, with associated EGR levels falling with CN. Note that for the remainder of this paper, only measurements with IDw values greater than zero will be considered. Complete EGR curves are still provided in the appendices (Appendix 4A - 4E).

20 100/0 90/10 80/20 15 50/50 Ref. CA]

° 10 ID [

0 0 10 20 30 40 50 EGR [%]

Figure 4.4: ID as a function of EGR

4.4.3 Efficiency The penalty of heavy EGR use is made quite tangible by Figure 4.5. As might be expected, EGR weighs on thermal efficiency. Particularly for rates in excess of 35-40 wt.-%, efficiency drops rapidly irrespective of fuel in the face of near stoichiometric combustion. Accordingly, the higher the CN, the higher the drop in efficiency when operating in PPC mode. Note that the authors have not attempted to further optimize the efficiency by, for example, modifying the CA50 or intake pressure. Interestingly, comparable efficiencies are found for all fuels at any given EGR dosage.

51 GUIDELINES FOR OPTIMAL PPC OPERATION

39 [%] th

η 38

37 100/0 90/10 80/20 36 50/50 Ref. 35 0 10 20 30 40 50 EGR [%]

Figure 4.5: Thermal eﬃciency as a function of EGR

4.4.4 Soot - NOx emissions Notwithstanding taking into account only higher than zero IDw data, aforementioned soot-NOx trade-off is still present in varying degrees for all fuels (Figure 4.6). Nevertheless, striking differences can still be observed, with generally more favorable curves found for lower CN fuels. In fact only the two lowest CN fuels, EN590 and 50/50 GTLine, are capable of achieving combined Euro V engine out emissions for soot and NOx. The number of fuels drops to one for Euro VI. Interestingly, this happens to be the 50/50 GTLine fuel. For all fuels, there appears to exist a tipping point whereby the emissions trade-off no longer holds and even goes into reverse. The significance of this tipping point with respect to the validity of the aforementioned IDw boundary between classical and PPC diesel combustion will be discussed in the next section.

52 CHAPTER 4. GTLINE

0.2 100/0 90/10 80/20 50/50 0.15 Ref. IDw=0 Euro V Euro VI

0.1 ISPM [g/kWh]

0.05 EGR increase

0 0 1 2 3 4 ISNOx [g/kWh]

Figure 4.6: Soot-NOx trade-oﬀ in PPC regime

4.5 Discussion

4.5.1 PPC threshold The PPC threshold of zero in the IDw equation has been defined in accordance with the literature consensus with respect to the definition of this combustion concept. Judging from the soot-NOx emissions data (Figure 4.6), however, a classical trade-off is still clearly observable for many data points for which the aforementioned IDw criterion has been met. As the PPC concept is praised by the combustion community for the very absence of this trade-off, its existence here suggests that diffusion combustion is still pervasive well after the EOI. Although detailed optical engine experiments would be required in order to fully comprehend this discrepancy, a possible contributing factor might be attributable to the last fraction of fuel injected near the EOI still burning as a lifted diffusion flame. Given that the onset of auto-ignition is generally a local event in the spray, this assumption would appear to be a promising candidate for further scrutiny in optical experiments.

4.5.2 Fuel reactivity and aromaticity Going against conventional wisdom, the two best performing fuels, both with respect to eﬃciency and emissions, are those with the higher aromaticity. This is in line with the conclusions of our previous work [59], which stated that both high CN and aromaticity have a negative impact on aforementioned parameters when operating in PPC mode, although the former fuel characteristic

53 GUIDELINES FOR OPTIMAL PPC OPERATION appears even more detrimental than the latter. In other words, purposefully increasing aromaticity with the objective to lower CN appears to be a sound approach for this particular LTC concept. Looking at the soot-NOx trade-offs shown in Figure A4.7 (Appendix 4E), it would appear that neat GTL performs worst of all fuels. This trade-off improves with higher concentrations of gasoline and thus lower CN. Interest- ingly, however, the 80/20 blend and reference fuel show comparable behavior, notwithstanding a lower CN for the latter fuel (63 vs. 52, Table 4.4). A plausible explanation for this performance parity, achieved here at dis- similar CN values, might be found in the associated aromaticities of the fuels in question. As it happens, the lower CN, reference diesel fuel contains more than three times the amount of aromatics than the 80/20 GTLine blend (16.3 vs. 5.1%, Table 4.4). As concluded in a earlier study [59], given an equal CN, the fuel with the lowest aromaticity will generally yield the more favorable soot-NOx trade-off. It would appear here that a higher aromaticity can be offset by a lower CN, whereby the respective negative and positive impacts on the trade-off cancel each other out, as the results in Figure A4.7 would suggest is the case for the reference diesel fuel and the 80/20 GTLine blend.

4.5.3 φ - T trace

Prevalence of engine-out soot and NOx emissions is often discussed in the context of a φ-T diagram. Herein, φ and T denote the local fuel/air ratio and temperature, respectively. While useful for modeling purposes and analysis of optical engine experiments, the usefulness of such ﬁgures diminishes for full metal engine research. Nevertheless, when it is assumed that, keeping all other variables constant, the impact of CN and EGR on local φ-T conditions is qualitatively similar to that measured for the charge as a whole1, some insights might emerge that could help explain aforementioned soot-NOx behavior (Figure 4.6, IDw=0). To this end, the temperature trace as a function of CAD is visualized in Figure 4.7. The bulk temperature has been estimated, by means of the ideal gas law, from the measured in-cylinder pressure and computed combustion chamber volume trace2. After the auto-ignition event, the temperature increases rapidly untill a certain maximum value is reached. Afterwards, the temperature decreases again, owing to a combination of physical expansion of the combustion chamber and decrease in the RoHR.

1as determined via air/mass ﬂow meters and in-cylinder pressure sensor 2derived from engine geometry and crank angle position sensor

54 CHAPTER 4. GTLINE

1500

1400

1300

1200

1100

1000

Temperature [K] 900 100/0 800 90/10 80/20 700 50/50 Ref. 600 0 10 20 30 40 50 Crank angle degree

Figure 4.7: Temperature trace during combustion cycle at IDw = 0.

In Figure 4.8, the bulk equivalence ratio is plotted against the bulk gas temperature trace as shown above.

1 100/0 0.95 90/10 80/20 0.9 50/50 Ref. 0.85 Gasoline increase

[−] 0.8 φ 0.75 EGR increase 0.7

0.65

0.6 600 800 1000 1200 1400 1600 Temperature [K]

Figure 4.8: Bulk temperature and equivalence ratio trace during combustion at IDw = 0. Squares and dots indicate the start of combustion and average gas temperature, respectively

From Figure 4.7 and 4.8, it becomes clear that temperatures at the SOC are all similar and slightly below 700 K. Moreover, it can be learnt from Figure 4.8 that there exist obvious diﬀerences amongst the fuels, as well as a clear trend with gasoline content (and thus CN). As discussed earlier, the higher the CN of the fuel, the higher the amount of EGR required to breach the zero

55 GUIDELINES FOR OPTIMAL PPC OPERATION

IDw threshold (Figures 4.3 and 4.9). Elevated EGR concentrations, in turn, manifests in higher equivalence ratios. EGR, which has been cooled to approximately ambient conditions in our experiments, so as not to impact the charge intake temperature, has a cooling eﬀect on the combustion process. This owes much to the relatively high speciﬁc heats of CO2 and H2O. Moreover, given that EGR suppresses oxygen-rich fresh air, the equivalence ratio naturally increases with its dosage. Combining Figures 4.6 and 4.8, it is fair to assume that the high soot and NOx values seen at IDw=0 for the high and low CN fuels can at least in part be traced back to an adverse equivalence ratio and temperature, respectively.

4.5.4 Indicated thermal eﬃciency

The main drawback of using high CN fuels in combination with PPC is most evident from Figure 4.10. Here, the indicated thermal efficiency is plotted for the various fuels at IDw=0. A sharp drop in efficiency can be observed for the two fuels with the highest CN (>60). Note that the penalty is even more pronounced for IDw values in excess of the zero threshold; a fact which, given the previous observation that a higher ignition dwell in excess of 0 is necessary to facilitate proper PPC, appears all the more relevant now. To complicate matters even further, recall that the emissions trade-offs for the higher CN fuels are far less favorable than those measured for the less reactive fuels (Figure 4.6).

EGR [%] 20

100/0 10 90/10 80/20 50/50 Ref. 0 40 50 60 70 80 CN [−]

Figure 4.9: EGR as a function of cetane number for IDw=0 conditions

56 CHAPTER 4. GTLINE

40.5

40 [−] th η 39.5

100/0 39 90/10 80/20 50/50 Ref. 38.5 40 50 60 70 80 CN [−]

Figure 4.10: Thermal eﬃciency as a function of EGR for IDw=0 conditions.

4.6 Conclusions

The objective of this study was to investigate the efficacy, with respect to soot- NOx trade-off, of gasoline as a GTL CN suppressant in various dosages under PPC (i.e., premixed) operating conditions. In order to ascertain whether or not we were operating in PPC mode the ignition dwell (IDw) has been evaluated. Values below and above zero were expected to be indicative of conventional and PPC combustion, respectively. After carefully analyzing the experimental data in PPC mode, a number of specific conclusions may be drawn:

• The higher the CN, the higher the amount of EGR required to suﬃciently extend the ID (Figure 4.4) into PPC territory (Figure 4.3).

• A CN higher than 60 cq. EGR rates above 35 wt.% have a negative impact on indicated thermal eﬃciency (Figure 4.5).

• In general, less favorable soot-NOx curves were found for the higher CN fuels cq. at higher EGR levels (Figure 4.6).

• Discrepancies amongst the fuels with respect to soot-NOx behavior are well explained by qualitative analysis of in-cylinder φ - T history (Figure 4.8)

57 GUIDELINES FOR OPTIMAL PPC OPERATION

More general conclusions include:

• The current PPC consensus in literature that the injection and combustion events are temporally isolated from one another should be more narrowly defined. Simply checking if the ID extends beyond the injection duration appears not to be a sufficiently stringent check. A possible explanation is that some fuel still burns diffusively after the end of injection given the local nature of this phenomenon.

• Setting aside for a moment any potential practical issues (e.g., ﬂash point, vapor pressure) that fall outside the scope of this study, blending gasoline to otherwise high CN GTL appears to be a promising route to improve not only eﬃciency, but also emissions when operating in PPC mode, notwithstanding the high aromaticity of the former fuel.

• Given the dieselization trend and associated surplus of gasoline in many regions, along with the fact that the cost price of gasoline is significantly lower than that of GTL, the proposed GTLine approach promises to be a cost effective way to accommodate GTL in world wherein LTC engine concepts really take off.

58 Chapter 5

Particle accumulation vs. nucleation: A second diesel dilemma?

5.1 Abstract

Historically, regulators of diesel emissions have formulated their legislation in gravimetric terms (i.e., in g/km or g/kWh). This quantitative approach, while arguably valid for gaseous emissions such as NOx, does not appreciate the complexity of particulate matter. More recently, much research, along with new legislation, views particulate matter in a more qualitative way and focuses on such metric as the total particle count and size distribution. While the trade-off in gravimetric particular matter and NOx is well known as the “diesel dilemma”, this study, for the first time, demonstrates that there is also a trade-off in particulate size modes (accumulation-nucleation). To this end, a wide range of fuel properties and engine operating conditions are tested on a dedicated heavy-duty test rig. The main message that can be drawn from the results is that the optima for both the classical and newly presented accumulation-nucleation trade-off appear to be concurrent in nature. This implies that when the position on the curve of any given data point is known for one trade-off, the associated location on the other can be qualitatively predicted. Given the breadth of the operation conditions and fuel properties tested in this study, it would appear that this concurrence, which could help

The content of this chapter has been taken from: J.J.E. Reijnders, M.D. Boot and L.P.H. de Goey, Particle accumulation vs. nucleation: A second diesel dilemma?, Under review (Journal of Aerosol Science). Minor edits have been made to streamline the layout of the thesis.

59 GUIDELINES FOR OPTIMAL PPC OPERATION guide engine calibrators and aftertreatment specialists, is quite robust indeed.

5.2 Introduction

For decades there have been serious health concerns about diesel engine generated particulate matter (PM). As a result, the relationship between particle size and toxicology has become a key subject of investigation for many researchers [71–74]. The advent of commercially available exhaust mounted particle sizers has been an important catalyst for such studies. In literature, researchers typically report on total particle concentration, along with the associated so-called count median diameter (CMD) to characterize the particle size distribution.

Needless to say, both the chemical composition of the fuel and the phys- iochemical conditions in the combustion chamber will leave their respective marks on the particle size distribution. It is the objective of this paper to examine these relationships both qualitatively and quantitatively for a wide range of fuels and engine operating conditions. Hereby, the scope is conﬁned to what goes into and comes directly out of the engine as explained in Chapter 1 (Figure 1.3).

Until now, engine studies on this topic have been generally confined to one particular fuel property or engine operating variable. The added value of this study is that even five operating variables are considered (load, exhaust gas recirculation (EGR), combustion phasing, injection pressure and engine speed). Moreover, these variables will be pushed to extreme levels, so as to evaluate the effects on particle size distribution in both conventional diesel combustion and so-called low temperature or partially premixed compression ignition modes. The fuel matrix, too, is quite expansive, comprising 18 different fuel blends, spanning a large range in aromaticity (0-30 wt-%), cetane number (CN) (15-80) and oxygen concentration (0-8 wt-%).

60 CHAPTER 5. A SECOND DIESEL DILEMMA?

5.3 Methodology

5.3.1 Experimental setup The experiments are performed on a 12.6 litre heavy duty (HD) DAF XE 355C in-line six cylinder CI, the speciﬁcations of which can be found in Table 5.1), with more detailed information available elsewhere [59]. Figure 5.1 shows a schematic overview of the test engine. Importantly, the ﬁrst cylinder, used as test cylinder, is equipped with dedicated in- and outlet manifolds, EGR circuit and fuel injection system.

Table 5.1: Engine speciﬁcations

Type 6-cylinder HDDI CI Model DAF XE355C Cylinders 6, with 1 isolated for research Capacity [l] 12.6 Bore [mm] 130 Stroke [mm] 158 Compression ratio [-] 15.7

Air for the test cylinder is provided using an external air compressor (up to 5 bar). Air mass flow is measured by a Coriolis mass flow meter. The fuel injection system comprises a Resato double-acting air-driven fuel pump (up to 2500 bar) and a prototype common rail injector with 8 holes of 0.151 mm. The first cylinder thus operates autonomously from the rest of the otherwise stock engine, save for its shared cam- and crankshafts, and lubricating and cooling systems.

Several surge tanks have been installed to allow for more homogeneous mixing of air and EGR. The EGR system itself is equipped with a cooler to bring down the gas temperature to approximately the ambient level of 300 K. Con- cerning the other cylinders in this conﬁguration, numbers 2 and 3 are not ﬁred, and the remaining ones are operated by the stock engine control unit (ECU) and are responsible for setting and maintaining the desired engine speed.

Emissions are recorded by a total of three systems. First, a Horiba Mexa 7100 DEGR is used for gaseous exhaust emissions, namely O2, NOx, HC, CO, CO2 (intake) and CO2 (exhaust). Second, smoke is measured using an AVL smoke meter (415S) and expressed in ﬁlter smoke number (FSN). Finally, the particle size distribution, along with total particle mass is measured with a

61 GUIDELINES FOR OPTIMAL PPC OPERATION

Mass !ow meters

Valves Heater Fuel pump Air compressor Injectors

1 3 4 5 6 Brake Surge tanks

Back pressure valve Horiba 7100 DEGRHoriba AVL 415S TSI EEPS

EGR cooler

Figure 5.1: Schematic overview of the test engine

TSI engine exhaust particle sizer (EEPS) 3090. More details on the working principle and methodology of the EEPS can be found in Section 2.3.3. For each operating point, exhaust smoke level is averaged over three measurements and the particle size distribution is measured at 10 Hz and averaged over 60 seconds.

Prior to measurement, the exhaust flow into the EEPS is first conditioned and diluted by a Testo volatile particle remover (ViPR). These additional steps are motivated by two factors. First, the particle density in a typical exhaust flow is too high for it to be measured directly by the EEPS. Sec- ond, the production of volatile particles is very sensitive to thermal conditions [15]. A more detailed explanation of the ViPR is provided in Section 2.3.2. The settings of the EEPS and diluters for this study can be found in Table 5.2.

Table 5.2: EEPS and ViPR settings

EEPS: Matrix [-] Soot Duration [s] 60 Primary diluter: Temperature [ ◦C] 150 Dilution ratio [-] 50 Secondary diluter: Temperature [ ◦C] 300 Dilution ratio [-] 6.7

62 CHAPTER 5. A SECOND DIESEL DILEMMA?

Aforementioned gaseous emissions, along with all common pressure, temperature and mass ﬂow sensors are each logged for 40 seconds at 20 Hz at each operating point. Conditions in the test cylinder are monitored by a SMETEC Combi crank angle (CA) resolved data acquisition system. This data is logged, using a resolution of 0.1 CA degree over a period 50 consecutive cycles. Vari- ables recorded by this system include in-cylinder (AVL GU21C), intake and fuel rail pressure, as well as injector current.

5.3.2 Fuel matrix The diverse nature of the fuel matrix used in this study results from the fact that it has been built up from various smaller fuel matrices from earlier projects conducted on the same engine setup. Grouped into three main categories, as itemized below, the 18 fuel blends considered here were selected such in order to span a wide range in aromaticity (0-30 wt-%), CN (15-80) and fuel oxygen (0-8 wt-%) (Table 5.3).

• Diesel blends: diesel + 10 vol-% ML / EL / FEE / ETE (see Table 5.3 for meaning abbreviations).

• Heptane blends: heptane + toluene (5 vol-%) / anisole or methoxyben- zene (5, 10, 20 vol-%) / veratrole or 1,2-dimethoxybenzene (5, 10, 20 vol-%).

• GTL/gasoline blends: 100/0, 90/10, 80/20, 50/50 and 0/100 vol-%.

63 GUIDELINES FOR OPTIMAL PPC OPERATION 2.0 1.0 0.4 0.2 0.0 0.0 0.0 0.0 0.0 24.9 0.0 6.6 12.2 3.5 4.9 1.8 0.0 2.5 4.0 15.9 28.5 0.1 2.1 1.1 45.5 15.1 7.7 62.6 0.0 0.0 26.8 0.0 68.2 739 0.0 14.0 7.2 79.8 758 0.0 0.0 0.0 770 0.0 0.0 0.0 42.8 773 0.0 43.7 41.6 777 0.0 44.3 46.9 4.0 6.3 49.8 44.4 2.9 16.3 53.1 37.4 4.5 44.6 760 44.5 48.6 0.0 720 700 2.7 0.0 Gasoline 49.1 51.6 680 743 0.0 40.3 Gasoline 711 42.3 Gasoline 695 14.4 43.4 0.0 44.6 Gasoline 14.4 41.6 689 834 Gasoline 14.3 43.0 43.8 42.0 4 44.3 14.5 42.5 44.4 3 GTL Veratrole 40.9 2 GTL Veratrole Veratrole 852 1 GTL 54.0 GTL0Gas100 Anisole 849 GTL GTL50Gas50 Anisole - 856 GTL Anisole 40.0 GTL80Gas20 Toluene 40.8 n-Heptane GTL90Gas10 843 (IQT) 39.6 CN n-Heptane GTL100Gas0 - n-Heptane Density Hep80Ver20 40.3 n-Heptane Hep90Ver10 Levulinate Ether Ethyl LHV Ethyl n-Heptane Hep95Ver5Furfuryl n-Heptane Ether Tetrahydrofurfuryl Levulinate Hep80Ani20 Ethyl Methyl n-Heptane n-Heptane Hep90Ani10 Hep95Ani5 Diesel Hep95Tol5 Diesel fuel Diesel Secondary Hep100 Diesel Diesel Die90ETE10 Die90FEE10 Die90EL10 fuel Die90ML10 Primary Die100 Name Property xgncneti h fuel the group in oxygen content functional Oxygen a group (IQT)) with oxygen content tester functional Aromatic quality a ignition without a content by Aromatic measured (as number Cetane \ nt[Jk][kg/m [MJ/kg] Unit al .:Fe rprisa esrdb S-nltkGb Germany GmbH ASG-Analytik by measured as properties Fuel 5.3: Table 3 - w-][t% [wt-%] [wt-%] [wt-%] [-] ] 1 ArC-O 2 ArC+O 3 OxC 4

64 CHAPTER 5. A SECOND DIESEL DILEMMA?

5.3.3 Experimental procedure To mimic constant HD highway cruising at 80 km/h, the engine speed has been ﬁxed at 1200 RPM. Prior to each experimental cycle, the engine has to run stable at steady-state conditions, characterized here by lubrication oil and coolant temperatures of 90 and 85 ◦C, respectively. In all cases, a single injection per cycle is used.

Safeguarding for a fair comparison between the various experiments, combustion phasing, deﬁned here as CA50, the crank angle where 50% heat is released, is held constant as well at 10 ◦CA after top dead centre (TDC). This ﬁxation is realized in practice by varying the start of injection (SOI). A summary of the base and varied values of the operation conditions is shown in Table 5.4. Note that pure gasoline will not ignite under these conditions and has therefore been omitted from this study. Associated data on accuracy is summarized in Table 3.5. Table 5.4: Operating conditions

Parameter Unit Base case Sweeps IMEPg [bar] 6 4-11 EGR [wt.%, CO2/CO2] 0 0 - 50 Injection pressure [bar] 1500 400 - 1800 CA50 [CAD aTDC] 10 0 - 18 Engine speed [RPM] 1200 800-1800 Intake pressure [bar] 1 Exhaust pressure [bar] 1.3 Intake temperature [ ◦C] 40 Lambda (λ) [-] 1.2 - 3 5 5 varies with load and EGR levels

For the IMEP, EGR and injection pressure sweeps, variation of a single parameter was suﬃcient for a transition from near-zero (in g/kWh) to excessive PM levels. The same wide range in PM emissions could not be achieved by varying CA50 or engine speed alone. To nevertheless still accomplish aforementioned transition for these parameters, some additional engine settings had to be changed concurrently in accordance with Tables 5.5 and 5.6.

65 GUIDELINES FOR OPTIMAL PPC OPERATION

Table 5.5: Operating conditions CA50 sweep

# IMEPg [bar] EGR [wt.%] 1 6 0 2 6 30 3 10 30

Table 5.6: Operating conditions speed sweep

# IMEPg [bar] EGR [wt.%] 1 4 0 2 6 0 3 8 0 4 8 15 5 8 30

5.4 Results and discussion

From Chapter 2, it is known that there is a relationship between PM nucleation and accumulation modes, typically characterized by particles smaller and larger than 23 nm, respectively. In Figure 5.2, the accumulation mode is plotted against the nucleation mode for all experiments. Data in this ﬁg- ure comprises results from 18 diﬀerent fuels and over 500 individual operating points, with varying load, EGR, injection pressure, SOI and engine speed.

The well-known soot-NOx trade-off is now joined by one balancing nucleation and accumulation particles. Moreover, Figure 5.2 would suggest that this new dilemma is equally challenging, as there appears no combination of fuel and operating conditions manifested in concurrent near-zero values of both PM types. In order to acquire a more firm understanding of this trade-off, the various data sets underpinning Figure 5.2 will be scrutinized in the following sections. First, associated PM-NOx trade-offs are discussed, followed up by a treatise on the accumulation-nucleation trade-offs.

66 CHAPTER 5. A SECOND DIESEL DILEMMA?

14 x 10 2

1.5

1 Particle number 0.5 Accumulation mode [#/kWh]

0 0 0.5 1 1.5 2 14 Particle number x 10 Nucleation mode [#/kWh]

Figure 5.2: Accumulation mode vs. nucleation mode for all experiments

5.4.1 PM vs. NOx

Figure 5.3 depicts the PM-NOx trade-offs collected for all fuels (Table 5.3) and obtained from conducting various sweeps in operating conditions (Table 5.4 - 5.6). As is apparent for all curves in Figure 5.3, there exists a pronounced classical CI trade-off between soot and NOx emissions. A general observation that can be distilled from these figures, irrespective of fuel identity, is that when mixing conditions become less optimal, as might be expected when decreasing lambda (higher load, EGR), inhibiting air entrainment (lower injection pressures) and allowing less time for mixing (higher engine speeds), soot emissions tend to rise while NOx falls.

In the associated legends, each individual fuel is denoted by a unique combination of marker color and symbol. The arrows presented in the graphs are indicative of the direction of the sweeps. Where data is available for all fuels (load, EGR and fuel pressure sweeps), a clear and consistent ranking amongst the three aforementioned fuel groups emerges. In all cases, diesel and GTL blends, which show comparable behavior, are outperformed by those having heptane as a base fuel. Furthermore, fuel identity is not equally manifested in all sweeps. For instance, fuel divergence appears more pronounced for the load sweep than in the EGR and injection pressure sweeps.

67 GUIDELINES FOR OPTIMAL PPC OPERATION

0.5 0.2 Die100 Die100 0.45 Die90ML10 0.18 Die90ML10 Die90EL10 Die90EL10 Die90FEE10 Die90FEE10 0.4 Die90ETE10 0.16 Die90ETE10 Hep100 Hep100 0.35 Hep95Tol5 0.14 Hep95Tol5 Hep95Ani5 Hep95Ani5 0.3 Hep90Ani10 0.12 Hep90Ani10 Hep80Ani20 Hep80Ani20 Hep95Ver5 Hep95Ver5 0.25 Hep90Ver10 0.1 Hep90Ver10 Hep80Ver20 Hep80Ver20 0.2 Gtl100Gas0 0.08 GTL100Gas0 ISPM [g/kWh] Gtl90Gas10 ISPM [g/kWh] GTL90Gas10 0.15 Gtl80Gas20 0.06 GTL80Gas20 Gtl50Gas50 GTL50Gas50 Gtl0Gas100 GTL0Gas100 0.1 0.04 EGR increase Load increase 0.05 0.02

0 0 0 2 4 6 8 10 12 14 16 18 0 2 4 6 8 10 12 14 16 18 ISNOx [g/kWh] ISNOx [g/kWh] (a) Load (4-11 bar IMEP) (b) EGR (0 - 50 wt.%)

0.7 0.6 Die100 Die90ML10 Die100 0.6 Die90EL10 GTL100 Die90FEE10 0.5 Die90ETE10 Hep100 0.5 Hep95Tol5 Hep95Ani5 0.4 Hep90Ani10 0.4 Hep80Ani20 Hep95Ver5 Hep90Ver10 0.3 0.3 Hep80Ver20 GTL100Gas0 ISPM [g/kWh]

GTL90Gas10 ISPM [g/kWh] GTL80Gas20 0.2 0.2 GTL50Gas50 GTL0Gas100 CA50 variation 0.1 0.1 Pfuel decrease

0 0 0 2 4 6 8 10 12 14 16 18 0 5 10 15 20 ISNOx [g/kWh] ISNOx [g/kWh] (c) Injection pressure (400 - 1800 bar) (d) CA50 (0-18 CADaTDC)

0.4 Die100 0.35

0.3

0.25

0.2

0.15 ISPM [g/kWh]

0.1 Speed increase 0.05

0 0 5 10 15 20 ISNOx [g/kWh] (e) Engine speed (800-1800 RPM)

Figure 5.3: PM-NOx trade-oﬀs for various fuels and sweeps in operating conditions

As discussed at length in an earlier study [59], for any given fuel, keeping all other variables constant, the proximity of its PM-NOx curve to the origin weighs heavily on the prevailing ID and concentration of fuel borne PM precursors. The ID and concentration of PM precursors, in turn, is a function of fuel CN and aromaticity, respectively. The most favorable trade-oﬀs are to be expected for those fuels having both a low CN and aromaticity [59]. Note

68 CHAPTER 5. A SECOND DIESEL DILEMMA? that aromaticity is conﬁned here to non-oxygenated species alone, as earlier research on the same engine has demonstrated markedly lower PM emissions for oxygenate aromatics such as those found in various heptane blends (anisole and veratrole) [75].

8 20 Die100 Die100 18 Die90ML10 7 Die90ML10 Die90EL10 Die90EL10 Die90FEE10 16 Die90ETE10 Die90FEE10 6 Hep100 Die90ETE10 14 Hep95Tol5 Hep100 Hep95Ani5 Hep95Tol5 12 Hep90Ani10 5 Hep95Ani5 Hep80Ani20

CA] Hep95Ver5

Hep90Ani10 CA] °

° 10 Hep80Ani20 Hep90Ver10

ID [ Hep80Ver20 4 Hep95Ver5 ID [ 8 GTL100Gas0 Hep90Ver10 GTL90Gas10 Hep80Ver20 3 6 GTL80Gas20 Gtl100Gas0 GTL50Gas50 Gtl90Gas10 Gtl80Gas20 4 2 Gtl50Gas50 2

1 0 3 4 5 6 7 8 9 10 11 12 0 10 20 30 40 50 60 70 IMEP [Bar] g EGR [%] (a) ID vs. load (b) ID vs. EGR

Figure 5.4: IDs for various fuels during load and EGR sweep.

Figures 5.4a and 5.4b show ID for all fuels as a function of load and EGR, respectively. Notwithstanding comparable CN levels (Table 5.3), the heptane blends yield markedly longer delays than are seen for the diesel blends in both the load and EGR sweeps. As the CN data in Table 5.3 would suggest, the GTL blends have IDs that are even shorter. What is more, heptane blends also have, by far, the lowest concentration of non-oxygenated aromatics of the three fuel clusters. The highest levels of these compounds are found in the diesel blends. The GTL blends rank in the middle with respect to non-oxygenated aromaticity. Assuming the above rationale, one would thus expect the best results for the heptane blends as this class is characterized by both a long ID and low aromaticity. For the diesel and GTL blends this relation is not that trivial. The former has low CN but high aromaticity, and the latter has high CN and low aromaticity. Looking now at the actual PM-NOx results in Figure 5.3, it is clear that the heptane blends, whenever included in the sweep, indeed outperform the other two fuel groups. The remaining two fuel groups, based on diesel and GTL, yield comparable curves, with the exception of load, which suggests that the drawback of high aromaticity of the former fuel is balanced by that of the high CN of latter. This observation is in agreement with the conclusions of Chapter 3 and 4.

69 GUIDELINES FOR OPTIMAL PPC OPERATION

5.4.2 PM accumulation vs. nucleation The PM accumulation-nucleation trade-offs, obtained during these same se- ries of experiments as were conducted for the soot-NOx plots in the previous section, are plotted in Figure 5.5. Recall that all data in this figure falls more or less on the same trend line, as shown earlier in Figure 5.2. As is apparent for all curves in Figure 5.5, there exists pronounced trade-offs between the PM modes. It was reported earlier, when discussing the trends in Figure 5.3, that less optimal mixing conditions tend to manifest in higher soot emissions and lower NOx. From Figure 5.5 can be observed that when mixing is poor, the accumulation mode tends to rise while the nucleation mode falls. Accordingly, it would appear that a dominant accumulation mode coincides with high and low gravimetric PM and NOx emissions, respectively. The opposite tends to hold whenever the nucleation mode is the more pronounced of the two. Interestingly, the same weak, yet consistent ranking amongst the three fuel types seen in Figure 5.3 holds for Figure 5.5 as well, whereby the data for the heptane blends is again nearest the origin. Unlike what was seen in the PM-NOx trade-offs in Figure 5.3, however, no discernible advantage is visible for the GTL over diesel blends.

70 CHAPTER 5. A SECOND DIESEL DILEMMA?

14 14 x 10 x 10 2 2 Die100 Die100 Die90ML10 1.8 Die90ML10 1.8 Die90EL10 Die90EL10 Die90FEE10 Die90FEE10 1.6 Die90ETE10 1.6 Die90ETE10 Hep100 Hep100 1.4 Hep95Tol5 1.4 Hep95Tol5 Hep95Ani5 Hep95Ani5 Hep90Ani10 1.2 Hep90Ani10 1.2 Hep80Ani20 Hep80Ani20 Hep95Ver5 Hep95Ver5 Hep90Ver10 1 1 Hep90Ver10 Hep80Ver20 Hep80Ver20 Gtl100Gas0 GTL100Gas0 0.8 Gtl90Gas10 0.8

Particle number GTL90Gas10 Particle number Gtl80Gas20 GTL80Gas20 Gtl50Gas50 0.6 0.6 Gtl0Gas100 GTL50Gas50 Accumulation mode [#/kWh] Load increase Accumulation mode [#/kWh] EGR increase GTL0Gas100 0.4 0.4

0.2 0.2

0 0 0 0.5 1 1.5 2 0 1 2 3 4 5 6 7 14 Particle number 13 Particle number x 10 x 10 Nucleation mode [#/kWh] Nucleation mode [#/kWh] (a) Load (4-11 bar IMEP) (b) EGR (0 - 50 wt.%)

14 14 x 10 x 10 2 2 Die100 Die100 1.8 Die90ML10 1.8 GTL100 Die90EL10 Die90FEE10 1.6 Die90ETE10 1.6 Hep100 1.4 Hep95Tol5 1.4 Hep95Ani5 1.2 Hep90Ani10 1.2 Hep80Ani20 Hep95Ver5 1 Hep90Ver10 1 Hep80Ver20 CA50 variation 0.8 GTL100Gas0 0.8 Particle number GTL90Gas10 Particle number 0.6 GTL80Gas20 0.6 GTL50Gas50

Accumulation mode [#/kWh] P decrease fuel GTL0Gas100 Accumulation mode [#/kWh] 0.4 0.4

0.2 0.2

0 0 0 1 2 3 4 5 6 7 0 0.5 1 1.5 2 2.5 3 13 Particle number 13 Particle number x 10 x 10 Nucleation mode [#/kWh] Nucleation mode [#/kWh] (c) Injection pressure (400 - 1800 bar) (d) CA50 (0-18 CADaTDC) 14 x 10 2 Die100

1.5

Speed increase Particle number 0.5 Accumulation mode [#/kWh] 0 0 2 4 6 8 10 13 Particle number x 10 Nucleation mode [#/kWh] (e) Engine speed (800-1800 RPM)

Figure 5.5: PM nucleation-accumulation trade-oﬀs for various fuels and sweeps in operating conditions. NB x-axis range is not constant.

71 GUIDELINES FOR OPTIMAL PPC OPERATION

5.4.3 Correlations between trade-oﬀs

In previous paragraph it became clear that the well-known PM-NOx trade-off shows some correlation with a second emissions trade-off, namely one between accumulation and nucleations PM. In this paragraph, a possible causality between these two trade-offs will be investigated. This is accomplished by over- laying the gravimetric values of both PM and NOx to each data point in the accumulation-nucleation figure in the expression of Euro norms (Table 5.7).

Table 5.7: Values Euro-norms PM and NOx

Euro PM values NOx values norm [g/kWh] [g/kWh] I 0.15-0.36 7-8 II 0.10-0.15 5-7 III 0.02-0.10 3.5-5 IV 0.01-0.02 2-3.5 V 0.01-0.02 0.4-2 VI < 0.01 < 0.4

Gravimetric PM vs. nucleation-accumulation modes From Figure 5.6 may be inferred that, irrespective of engine operation conditions or fuel properties, the position of any arbitrary data point on the PM accumulation-nucleation trade-oﬀ can be qualitatively predicted quite well by the associated gravimetric PM Euro norm.

Indeed, compliance with the lower and higher Euro norms tends to coincide with negligible amounts of nucleation and accumulation PM modes, respectively. Unfortunately, as the word trade-oﬀ would imply, negligible amounts of either mode coincides with excesses in the other. Importantly, most results nearest the origin, i.e., concurrently low emissions of both accumulation and nucleation mode PM, are found for those data points which also yielded the lowest (i.e., Euro VI) levels of gravimetric PM.

While it is true that the optimum in the trade-oﬀ in Figure 5.6 occurs at Euro VI PM levels, the worst results, pertaining to nucleation mode PM, are found at this norm as well. Indeed, the position on the trade-oﬀ curve gradually shifts from a peak in accumulation (at Euro I & II) to one in nucleation mode (at Euro VI) with tighter gravimetric PM legislation. Given the wide range of fuel properties and operating conditions represented by our data, the

72 CHAPTER 5. A SECOND DIESEL DILEMMA? validity of this trend is assumed to be quite robust.

An important implication of Figure 5.6 is thus that a further cut in gravimetric PM levels could lead to an unacceptably high number of the ultra-ﬁne nucleation mode particles. Accordingly, while PM legislation has had a positive eﬀect on both the quantitative as well the qualitative PM thus far, even stricter norms would probably result in an increase in nucleation mode particles. More details on the relation between PM mass and number is given in Appendix 5.

14 x 10 PM EURO−norms 2 EURO I & II 1.8 EURO III EURO IV & V 1.6 EURO VI

1.4

1.2

0.8 Particle number

0.6

Accumulation mode [#/kWh] Gravimetric PM decrease 0.4

0.2

0 0 0.5 1 1.5 2 14 Particle number x 10 Nucleation mode [#/kWh]

Figure 5.6: PM nucleation-accumulation trade-oﬀ data shown together with associated PM Euro norms

73 GUIDELINES FOR OPTIMAL PPC OPERATION

NOx vs. nucleation-accumulation modes

Turning now to NOx, Figure 5.7 depicts the associated emission norms of the PM accumulation-nucleation trade-oﬀ data. It is evident that nearly all nucleation mode PM coincides with high NOx emissions (Euro I & II). Besides that, there does not appear to be any further NOx correlation in the data.

14 NO EURO−norms x 10 x 2 EURO I & II 1.8 EURO III EURO IV 1.6 EURO V EURO VI 1.4

1.2

0.8 Particle number

0.6 Accumulation mode [#/kWh] 0.4

0.2

0 0 0.5 1 1.5 2 14 Particle number x 10 Nucleation mode [#/kWh]

Figure 5.7: PM nucleation-accumulation trade-oﬀ data shown together with associated NOx Euro norms

74 CHAPTER 5. A SECOND DIESEL DILEMMA?

Gravimetric PM-NOx vs. nucleation-accumulation modes To continue, Figure 5.8 shows the combined Euro level for both gravimetric PM and NOx for each of the data points on the accumulation-nucleation curve. An important observation here is that the worst excesses, pertaining to both accumulation and nucleation mode PM, are found for the lower norms (Euro I - III).

14 x 10 EURO−norms 2 EURO I & II 1.8 EURO III EURO IV 1.6 EURO V EURO VI 1.4

1.2

0.8 Particle number

0.6 Accumulation mode [#/kWh] 0.4

0.2

0 0 0.5 1 1.5 2 14 Particle number x 10 Nucleation mode [#/kWh]

Figure 5.8: Accumulation mode as function of nucleation mode. Colors indi- cates the Euro-norms (both PM and NOx).

Another important conclusion that may be drawn here is that those data points, which satisfy Euro VI requirements, also yield concurrent low nucleation and accumulation mode particles.

75 GUIDELINES FOR OPTIMAL PPC OPERATION

5.4.4 Impact of molecule structure on fuel properties To study this ﬁnding in more detail, Figure 5.9 zooms in on the relevant area of Figure 5.8, thereby focussing only on those fuels meeting Euro VI targets.

13 x 10 EURO−norms 10 EURO VI 9

7 Hep95Ani5 Hep95Ver5 Hep90Ani10 6 Hep90Ani10 Hep90Ver10 Hep90Ver10 5 Hep90Ver10

4 Particle number

3 Hep80Ver20 Accumulation mode [#/kWh] Hep80Ver20 2 Hep80Ani20 Hep80Ani20 1 Hep80Ani20 0 0 2 4 6 8 10 12 Particle number x 10 Nucleation mode [#/kWh]

Figure 5.9: Zoom-in of acc-nucl trade-oﬀ (Figure 5.8). All measurement points satisfy Euro VI norms for PM and NOx.

It is evident from Figure 5.9 that all the fuels that satisfy Euro VI norms are based on n-heptane, albeit with varying concentrations of distinct aromatic oxygenates. What is further striking here is that neat n-heptane does not meet the Euro VI threshold. A ﬁnal observation concerns the apparent grouping of aromatic oxygenates blended to a speciﬁc ratio. In other words, irrespective of type of aromatic oxygenate, those n-heptane blends containing 5, 10 or 20 volume percent of either aromatic compound are clustered in accordance with their respective blend aromaticity.

With respect to the operating conditions, it is noteworthy to mention that all (Euro VI) measurement points are achieved in combination with EGR; this for the simple reason that (engine-out) NOx limits could not be met by any other means. The EGR levels range from 30-45%. The combustion phasing (CA50) is mainly responsible for the eﬃciency, with the highest values generated at 5-10 CAD aTDC. Concerning fuel pressure, the location on the accumulation-nucleation mode trade-oﬀ can be well regulated by this parameter, whereby higher pressure manifest in more nucleation mode particles.

76 CHAPTER 5. A SECOND DIESEL DILEMMA?

Fuel properties 30

20 Die100 Die90ML10 Die90EL10 Die90FEE10 15 Die90ETE10 Hep100 Hep95Tol5 Hep95Ani5 Hep90Ani10 10 Hep80Ani20 Aromatic content [wt−%] Hep95Ver5 Hep90Ver10 Hep80Ver20 5 GTL100Gas0 GTL90Gas10 GTL80Gas20 GTL50Gas50 GTL0100Gas100 0 0 10 20 30 40 50 60 70 80 Cetane number [−]

Figure 5.10: Aromatic content versus cetane number for all the fuels studied in this work. The solid markers correspond to those data points satisfying the Euro VI norms.

Figure 5.10 depicts the most important fuel parameters studied in this work. Note that only the solid blue markers satisfy the Euro VI norms. At a ﬁrst glance, there appears to be no CN-aromaticity rationale for the better performance of these blends. Notwithstanding an equal aromaticity and CN, for example Die90FEE10 and Hep90Ani10, Hep95Tol5 and Hep95Ani5, or, Hep95Tol5 and Hep95Ver5, blends containing n-heptane and aromatic oxygenates consistently outperform all other fuels. A possible explanation for this distinctive behavior may be found in the nature of the aromaticity in question. Aromaticity in the underperforming blends is non-oxygenated, while that in the best performing blends is oxygenated. It is well-known from earlier in-house research [38] that, contrary to what is the case for non-oxygenated aromatics, the oxygenated variant tend to no longer act as soot precursors. However, other fuels void of any soot precursors (Hep100 and GTL100Gas0) do not meet the Euro VI emission limits either. Arguably, here the high CN appears to be the handicap. Accordingly, optimal performance in both trade- oﬀs is secured only by those fuels having both a low amount of soot precursors and a CN below, according to this data at least, 51. Whether or not there exists a lower limit of the CN cannot be concluded from these results.

77 GUIDELINES FOR OPTIMAL PPC OPERATION

5.5 Conclusions

Our main conclusion is that optima for both the classical PM-NOx and newly formulated accumulation-nucleation trade-off appear to occur concurrently. In other words, any effort to improve the gravimetric trade-off by means of engine calibration and/or fuel tailoring, is likely to yield dividends on the qualitative trade-off as well. Given the sheer breadth of operation conditions and fuel properties evaluated in this study, it would appear that this coincidence of optima, which might serve to guide engine calibrators and aftertreatment specialists towards the least harmful exhaust gas composition, is quite robust indeed.

Apart from this general observation, some more detailed conclusions may be drawn as well:

• For Euro I-V, there is a strong correlation between gravimetric PM emissions and the relative position on the accumulation-nucleation trade-oﬀ.

• For Euro VI, this apparent relationship is more ambiguous, given that near-zero levels in both modes are observed, but also excessively high levels of nucleation mode particles.

• Although NOx does not correlate well with the general accumulation- nucleation trade-oﬀ, the aforementioned nucleation mode excesses are observed only at the highest NOx levels (Euro I & II). Conversely, at lower NOx levels, mainly accumulation particles are measured. An important limitation of the above conclusions is that they hold only for engine out emissions. A practical implication of this caveat is that compliance with Euro VI at the tailpipe level does not necessarily translate into a low (nucleation mode) particle count, as this will depend greatly on the NOx reduction strategy of choice. Low engine out NOx emissions, as are common for engines equipped with EGR, generally coincide with negligible amounts of nucleation mode PM. Excessively high nucleation mode PM, however, are to be expected for vehicles that rely heavily on NOx aftertreatment (e.g., selective catalytic reduction), as the associated engines tend to operate at particularly high NOx levels, in part motivated by fuel economy considerations.

Overall, the results of this chapter suggest that the optimal fuel, with respect to both the PM-NOx and accumulation-nucleation mode trade-oﬀ, appears to be one characterized by a low cetane number and a minimal amount of soot precursors.

78 Chapter 6

Summary and conclusions

During the last decades, reducing harmful exhaust emissions from road transport has attracted a lot of attention. As a result, combustion engines became very clean by reducing both NOx and PM emissions up to 98% relative to their respective 1992 levels. Reducing NOx even further will undoubtedly have positive eﬀects on public health and the environment. The goal of this thesis is to establish whether or not the same holds for PM in compression ignition engines, operating in either conventional or so-called partially premixed combustion (PPC) mode, by evaluating the impact of a wide range of fuel properties and engine operating conditions on quantitative (i.e., gravimetric) as well as qualitative (i.e., particle count and size distribution) PM emissions. Chapter 1 and 2 serve mainly to provide the reader with some necessary background information regarding the motivation for and methodology used in the research, respectively. The associated research goal is broken down into the following three research questions, each of which is treated separately in Chapter 3 thru 5.

Chapter 3: What is PPC? A new parameter Π is introduced to discern between high temperature diﬀu- sion (HTC) (Π >1) and low temperature premixed (LTC or PPC) combustion (Π <1). The shift in modes is accomplished by creating ever-milder chemical boundary conditions, such as higher levels of exhaust gas recirculation (= less oxygen) and lower compression ratios (= lower temperatures).

During this transition, throughout which the ignition delay becomes pro- gressively longer as a result of these measures, the impact of two important fuel properties, cetane number and aromaticity, on engine out soot-NOx emissions is evaluated. The objective of this exercise is to validate whether or not the

79 GUIDELINES FOR OPTIMAL PPC OPERATION conventional wisdom that a compression ignition engine should ideally run on a fuel with a high cetane number - for smooth running - and low aromaticity - for low PM emissions - holds irrespective of engine operating conditions.

Two main conclusions may be drawn from the results. First, at equal fuel aromaticity, there is no noticeable benefit of a high CN with respect to the soot-NOx trade-off in the HTC mode (Π >1). In fact, when operating in the LTC regime (Π <1), a high CN even results in a penalty in aforementioned trade-off. Second, at equal CN, increased aromatic content always has a negative impact on the soot-NOx trade-off, irrespective of Π. However, at LTC, a high CN is even more harmful, with respect to the soot-NOx trade-off, than the presence of aromatics.

Chapter 4: How is PPC best achieved? In Chapter 3, a step wise approach, involving many engine operating variables was used to transit from conventional high temperature combustion to PPC. In Chapter 4, PPC is realized by increasing the EGR level alone. Various fuel blends were prepared, comprising diﬀerent ratios of gas-to-liquid (GTL) diesel relative to gasoline, with CN falling and aromaticity increasing as the contribution of the latter fuel was more pronounced.

The first conclusion, building further on the first research question, is that the current definition of PPC found in literature, which merely holds that injection and combustion events are to be temporally isolated, should be more narrowly defined. Simply establishing whether or not the ignition dwell is positive does not guarantee the near-zero PM and NOx emissions promised by the concept. A possible explanation is that insufficiently temporally isolated events will leave some fuel, in the tail of the fuel spray, still burning diffusively well after the end of injection. To resolve this, a certain minimum spacing is required, narrowing down somewhat aforementioned broad definition of PPC still generally accepted in literature.

Another finding, now pertaining more to PPC fuel requirements, is that a higher CN will require a greater amount of EGR to sufficiently extend the ID deep enough into PPC territory to realize near-zero PM and NOx emissions. Higher EGR levels, however, generally manifest in a decrease in thermal efficiency, thereby favoring PPC operation with lower CN fuels.

In short, PPC is best achieved when some time is allowed between the end of injection and start of combustion, thereby increasing the likelihood that

80 CHAPTER 6. SUMMARY AND CONCLUSIONS the PPC promise of low emissions is also realized. Given that aforementioned events properly phased, PPC is ideally realized with a low CN fuel, requiring a minimal amount EGR, thereby maximizing thermal eﬃciency.

Chapter 5: How does PPC impact soot quality? So far, we have confined the work to gravimetric or quantitative PM emissions research. In this final chapter, we considered PPC in the context of qualitative PM emissions, characterized here by a particle count and size distribution, as well. The relationship between PM quantity and quality was studied for 18 fuels, spread over 500 unique measurement points in the same engine as used in the foregoing two chapters. From these experiments became clear that the classical PM-NOx trade-off is no longer the only existing diesel dilemma.

This research exposed a robust, new second diesel dilemma. Herein, either large accumulation PM are emitted, or, alternatively, the smaller nucleation mode variant. For any given data point, the relative position on this qualitative PM trade-oﬀ curve was found to correlate quite well with the associated quantitative (gravimetric) PM level.

Another observation of note was that aforementioned two trade-offs also correlated with each other. Specifically, those data points nearest the origin in the gravimetric PM-NOx curve were also found to be closest to the origin in the qualitative PM trade-off. In other words, engine-out compliance with the most stringent gravimetric emissions norm evaluated here (Euro VI), coincided with minimal emissions of both PM modes.

Overall, the aggregate results in this chapter suggests that fuel identity, notably cetane number and soot precursor concentration, determines the proximity of both trade-oﬀs to the origin. An interesting side note here is that those oxygenated aromatics investigated in this thesis appear not to behave like soot precursors. Concerning engine operating conditions, it is noteworthy to mention that Euro VI NOx compliance is possible only in combination with EGR (e.g., 30-45%). Eﬃciency is governed mainly by the combustion phasing (CA50), whereby the highest values are typically found around 5-10 CAD aTDC.

81 Appendices

Appendix Chapter 2

Sample out (to EEPS)

Thermo diluter (secondary diluter)

Sample in (from exhaust)

Rotating disc diluter (primary diluter)

Figure A2.1: Schematic diagram of the Testo ViPR [Reproduced with permission of Testo GmbH]

82 Appendices Chapter 3

Appendix 3A: GC-MS results for fuels FT52 and FT72 See Figures A3.1 and A3.2 and Tables A3.1 and A3.2

10.0 11.0 12.0 13.0 14.0 15.0 16.0 17.0 18.0 19.0 20 Time [min]

Figure A3.1: GC-MS chromatogram of FT52.

Table A3.1: GC-MS results for FT52 fuel

Peak# Retention time Name Carbon # 1 9,488 di-methyl decane 12 2 10,182 5-methyl undecane 12 3 10,26 4-methyl undecane 12 4 10,459 3-methyl undecane 12 5 10,974 n-dodecane 12 6 11,264 di-methyl undecane 13 7 11,96 5-methyl dodecane 13 8 12,242 3-methyl docecane 13 9 13,049 di-methyl dodecane 14 10 13,609 6-methyl tridecane 14 11 14,688 tri-methyl dodecane 15 12 15,205 6-methyl tetradecane 16 13 16,148 di-methyl tetradecane 17 14 16,704 7-methyl pentadecane 16 15 17,484 n-hexadecane 16 16 18,142 7-methyl hexadecane 17

83 3.0 10.0 20.0 28.0 Time [min]

Figure A3.2: GC-MS chromatogram of FT72.

Table A3.2: GC-MS results for FT72 fuel

Peak# Retention time Name Carbon # 1 4,022 n-octane 8 2 5,511 n-nonane 9 3 7,279 n-decane 10 4 9,139 n-undecane 11 5 10,975 n-dodecane 12 6 12,734 n-tridecane 13 7 14,403 n-tetradecane 14 8 15,985 n-pentadecane 15 9 17,481 n-hexadecane 16 10 18,896 n-heptadecane 17 11 20,242 n-octodecane 18 12 21,522 n-nonadecane 19 13 22,745 n-eicosane 20 14 23,915 n-heneicosane 21 15 25,03 n-docosane 22

84 Appendix 3B: Thermal eﬃciency MODE 1

Figure A3.3 shows that at low loads the indicated thermal efficiency is approximately 44%. At higher loads, this efficiency decreases. In part, this is because of the higher CO emissions (Appendix 3C), however, the main reason is the longer combustion duration, which results in a less optimal combustion phasing. Overall, the differences in efficiency amongst the fuels are small.

45 6 bar IMEP 8 bar IMEP 44 10 bar IMEP

42 Thermal efficiency [%]

41 50 55 60 65 70 75 CN [−]

Figure A3.3: Indicated thermal eﬃciency for all fuels over a variety of load points in MODE 1 (Black=DS52, blue=FT52, and red=FT72)

MODE 2 The eﬃciencies remain in the 42-44% range, with indicated thermal eﬃciency decreasing slightly at higher EGR levels (Figure A3.4). This phenomenon is mainly attributable to higher CO emissions, particularly as lambda approaches one.

85 45 0% EGR 15% EGR 44 30% EGR

42 Thermal efficiency [%]

41 50 55 60 65 70 75 CN [−]

Figure A3.4: Indicated thermal eﬃciency for all fuels over a variety of EGR points in MODE 2 (Black=DS52, blue=FT52, and red=FT72).

MODE 3 The intake pressure is lowered from MODE 2 to MODE 3. No clear diﬀerences are observed (Figure A3.5).

45 0% EGR 15% EGR 44 30% EGR

42 Thermal efficiency [%]

41 50 55 60 65 70 75 CN [−]

Figure A3.5: Indicated thermal eﬃciency for all fuels over a variety of EGR points in MODE 3 (Black=DS52, blue=FT52, and red=FT72).

86 MODE 4 Figure A3.6 depicts the indicated thermal efficiency for MODE 4 wherein the CR was lowered compared to the other modes. As might be expected in light of the reduced CR, the efficiencies are lower for all fuels. Again, the differences amongst the fuels are relatively small.

42 0% EGR 41.5 15% EGR 30% EGR 41

40.5

Thermal efficiency [%] 39.5

39 50 55 60 65 70 75 CN [−]

Figure A3.6: Indicated thermal eﬃciency for all fuels over a variety of EGR points in MODE 4 (Black=DS52, blue=FT52, and red=FT72).

87 Appendix 3C: CO and HC emissions MODE 1 The trend of increasing CO levels for higher loads observed in Figure A3.7, can be explained to a large extent by the fact that lambda approaches its stoichiometric value.

2.5 6 bar IMEP 8 bar IMEP 2 10 bar IMEP

1.5

1 CO [g/kWh]

0.5

0 50 55 60 65 70 75 CN [−]

Figure A3.7: CO emissions for all fuels over a variety of load points in MODE 1 (Black=DS52, blue=FT52, and red=FT72).

In Figure A3.8, the HC emissions are presented as function of load, whereby all fuels perform below Euro V levels (Table 5.7) and HC decreases for higher loads. This behavior is most likely due to the high overall temperatures in the cylinder whereby mixing conditions in nearly all locations in the chamber are within the ﬂammability limits. Finally, especially at low loads, the high CN fuel yields lower HC emissions. The reason is most likely the shorter ignition delay, for this aﬀects the fraction of fuel that can reach the crevices prior to auto-ignition.

88 0.3 6 bar IMEP 0.25 8 bar IMEP 10 bar IMEP 0.2

0.15

HC [g/kWh] 0.1

0.05

0 50 55 60 65 70 75 CN [−]

Figure A3.8: HC emissions for all fuels over a variety of load points in MODE 1 (Black=DS52, blue=FT52, and red=FT72).

Appendix 3D: Absolute soot-NOx trade-oﬀs for DS52

0.4 MODE1 MODE2 0.3 MODE3 MODE4

0.2

ISPM [g/kWh] 0.1

0 0 2 4 6 8 10 ISNOx [g/kWh]

Figure A3.9: Absolute soot-NOx trade-oﬀs for DS52.

In the main text of this paper, the relative soot-NOx trade-offs are shown. Figure A3.9 presents the absolute values for this trade-off for diesel fuel (DS52) in all MODES. It is clear that this trade-off improves when switching from MODE 1 to the cooler burning, more premixed MODES.

89 Appendix 3E: Repeatability During all the performed measurements, points have been re-measured on a regular basis in order to check the repeatability. First the complete set has been measured whereafter some points have been repeated. In all cases, the results were in good agreement. In MODE 3, ﬁrst the complete set with diesel has been measured, then the GTL fuels and ﬁnally the complete set with diesel is repeated (1 week later). Figure A3.10 shows the results thereof.

0.4 DS52 #1 DS52 #2 0.3

0.2

ISPM [g/kWh] 0.1

0 0 2 4 6 8 10 ISNOx [g/kWh]

Figure A3.10: Repeatability measurements for soot-NOx trade-oﬀ in MODE 3.

90 Appendices Chapter 4

Appendix 4A: Ignition delay

20 100/0 90/10 80/20 15 50/50 Ref. CA]

° 10 ID [

0 0 10 20 30 40 50 EGR [%]

Figure A4.1: ID as function of EGR

Appendix 4B: Thermal eﬃciency

39 [%] th

η 38

37 100/0 90/10 80/20 36 50/50 Ref. 35 0 10 20 30 40 50 EGR [%]

Figure A4.2: Indicated thermal eﬃciency as function of EGR

91 Appendix 4C: HC and CO emissions

6 100/0 90/10 5 80/20 50/50 Ref. 4

ISHC [g/kWh] 2

0 0 10 20 30 40 50 EGR [%]

Figure A4.3: Unburnt hydrocarbon emissions as function of EGR

20 100/0 90/10 80/20 15 50/50 Ref.

10 ISCO [g/kWh]

0 0 10 20 30 40 50 EGR [%]

Figure A4.4: Carbon monoxide emissions as function of EGR

92 Appendix 4D: Soot and NOx emissions

0.2 100/0 90/10 80/20 0.15 50/50 Ref.

0.1 ISPM [g/kWh]

0.05

0 0 10 20 30 40 50 EGR [%]

Figure A4.5: Soot emissions as function of EGR

20 100/0 90/10 80/20 15 50/50 Ref.

10 ISNOx [g/kWh] 5

0 0 10 20 30 40 50 EGR [%]

Figure A4.6: NOx emissions as function of EGR

93 Appendix 4E: Soot-NOx trade-oﬀ

0.2 100/0 90/10 80/20 50/50 0.15 Ref. Euro V Euro VI

0.1 ISPM [g/kWh]

0.05

0 0 1 2 3 4 5 6 ISNOx [g/kWh]

Figure A4.7: Soot-NOx trade-oﬀ

94 Appendix Chapter 5: particle matter correlations

In Chapter 5, a correlation was observed between the PM Euro norm of any given data point and its associated position in the accumulation-nucleation trade-oﬀ (Figure 5.6).

13 x 10 PM correlation between mass and numbers 12 >0.025 g/kWh <0.025 g/kWh 10 <0.020 g/kWh <0.015 g/kWh 8 <0.010 g/kWh <0.005 g/kWh

6 Gravimetric PM decrease

Particle number 4

Accumulation mode [#/kWh] 2

0 0 1 2 3 4 5 6 13 Particle number x 10 Nucleation mode [#/kWh] Figure A5.1: PM mass and number correlations

In Figure A5.1 more details are shown on the relation of PM with respect to the mass and number concentrations. With decreasing mass, the accumulation mode numbers decrease as well. Below 0.010 g/kWh (Euro VI norm), this number still decreases, but the number in nucleation mode starts to increase, as indicated by the red markers (below 0.005 g/kWh).

95 Nomenclature

φ equivalence ratio a after AMP accumulation mode particles b before CA crank angle CA50 50% of total heat released CAD crank angle degree CN cetane number CO carbon monoxide CR compression ratio DI direct injection DS diesel (EN590) EGR exhaust gas recirculation EL ethyl Levulinate ETE ethyl tetrahydrofurfuryl ether EV electric vehicle FEE furfuryl ethyl ether FT Fischer-Tropsch g gram g/kWh gram per kiloWatthour GTL gas-to-liquid GTLine blend of GTL and gasoline HC hydrocarbons HCCI homogeneous charge compression ignition HD heavy-duty HDDI HD direct-injection HTC high temperature combustion ID ignition delay IDw ignition dwell IQT ignition quality tester

96 J Joule kWh gram per kiloWatthour LD light duty LTC low temperature combustion ML methyl levulinate nm nano meter NMP nucleation mode particles NOx nitrogen oxide(s) PAH poly aromatic hydrocarbons PCCI premixed charge compression ignition PM particulate matter PPC partially premixed combustion RoHR rate of heat release rpm revolutions per minute SCR selective catalytic reduction SOC start of combustion SOI start of injection T temperature TDC top dead center

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104 Dankwoord

Het was nooit de bedoeling om een promotie traject te volgen, maar achteraf ben ik blij dat ik het gedaan heb. Ik heb er veel van geleerd en het is een leuke periode geweest. Ik had dit echter nooit kunnen doen zonder de steun van velen om me heen. Ik wil daarom bij deze iedereen bedanken die op welke wijze dan ook heeft bijgedragen aan dit proefschrift.

Graag wil ik een aantal mensen in het bijzonder bedanken, waarbij als eerste mijn (co)promotoren Philip, Bengt en Michael. Jullie hebben er mede voor gezorgd dat ik aan dit traject ben begonnen en door alle besprekingen hebben jullie het voor elkaar gekregen dat er een mooie rode draad is ontstaan in mijn werk. Ook wil ik Carlo hierbij betrekken; jij hebt altijd gezorgd voor net wat extra motivatie.

Daarnaast wil ik een woord van dank richten aan mijn (ex)collega’s, waarmee ik dagelijks contact had/heb (Niels, Cemil, Ron, Ulas, Noud, Nard, PC, Rob- bert, Michel, Robin, Mo en Joris), voor de vele leerzame, leuke en ontspan- nende momenten. Aan onze technici (Theo, Gerard, Hans, Martin, Marc, Jan, Bart, Henry, Jaap, Frank en Wietse), zonder jullie had ik al mijn experimenten niet kunnen uitvoeren. De wetenschappelijke staf (Nico, Niels, Bart, Jeroen, Rob en Harry) heeft uiteraard ook mijn dank. Een tweetal studenten die mij enorm hebben geholpen bij mijn metingen wil ik graag bij naam noemen, namelijk Robin en Thomas. Marjan en Linda, jullie zorgden voor een goede sfeer in de groep en af en toe een luisterend oor was heel erg ﬁjn.

Er zijn een aantal bedrijven en instituten die op uitlopende wijze hebben bijgedragen aan mijn promotie. Hiervoor wil ik RIVM, DAF Trucks, Shell en TNO bedanken. Dan wil ik ook mijn werkgever Progression Industry bedanken dat ik deze promotie grotendeels in jullie tijd heb mogen uitvoeren. Vanaf nu kan ik weer fulltime gaan werken ;-).

105 Ook wil ik graag mijn vrienden bedanken die voor de nodige ontspanning hebben gezorgd, maar ook voor de goede gesprekken die op welke manier dan ook bijgedragen hebben aan dit werk. Een speciale dankjewel voor Ron, voor het ontwerpen van de kaft.

En uiteraard zal ik mijn drie belangrijkste dames niet vergeten, namelijk mijn moeder, mijn vriendin en mijn dochtertje. Mama, bedankt voor je on- voorwaardelijke steun die je mijn hele leven al hebt gegeven. Carol, jij stond altijd achter mij, als het goed ging, maar vooral ook in mindere tijden. Heel erg bedankt daarvoor. Bo, alleen al door jouw lach was mijn dag goed en kon ik er weer vol enthousiasme tegenaan. En natuurlijk ook mijn zus, broer, Jac en schoonfamilie voor alle mooie momenten die we afgelopen jaren hebben beleefd.

Iedereen heel erg bedankt, en natuurlijk braaf zijn! ;-)

106 Curriculum Vitae

Jos Reijnders was born on November 26th, 1980, in Geldrop, the Netherlands. He started his higher education at the MAVO (Jan van Brabant college, Hel- mond) and then moved on to obtain a BSc. degree in Automotive Sciences, after completing both the MTS (ROC, Eindhoven) and HTS (Hogeschool van Arnhem en Nijmegen). Finally, he received his MSc. degree in Mechanical Engineering (Combustion Technology) from Eindhoven University of Technol- ogy (TU/e). For his master thesis, he invented and investigated a novel fuel injector to suppress soot emissions; a technology he later continued to work on in his professional career.

This career started in 2009, when he worked as a research engineer at the TU/e. Here, he investigated various measures to decrease engine emissions, both via improved hardware, as well as by means of alternative (bio)fuels. In 2011, he continued this line of research in a TU/e spin-oﬀ company, Progression Industry BV, where he still works to date. In the period 2013-2017, Jos worked part-time on a PhD project, closely related to his earlier work at Progression- Industry and the TU/e, under the supervision of Philip de Goey and Michael Boot.

107 List of Publications

Scientiﬁc peer reviewed papers: • Jos Reijnders, Micheal D. Boot and Philip de Goey. Impact of aromaticity and cetane number on the soot-NOx trade-oﬀ in conventional and low temperature combustion. Fuel (2016) DOI:10.1016/j.fuel.2016.08.009. • Miao Tian, Robin van Haaren, Jos Reijnders and Michael D. Boot. Lignin derivatives as potential octane boosters. SAE International Journal of Fuels and Lubricants (2015) DOI:10.4271/2015-01-0963. • Lei Zhou, Michael D. Boot, Bengt Johansson and Jos Reijnders. Performance of lignin derived aromatic oxygenates in a heavy-duty diesel engine. Fuel (2014) DOI:10.1016/j.fuel.2013.07.047 • Jos Reijnders, Michael D. Boot, Philip de Goey, Maurizio Bosi and Lucio Postri- oti. Spray analysis of the PFAMEN injector. SAE Technical Paper (2013) DOI:10.4271/2013-24-0036 • Jos Reijnders, Michael D. Boot, Philip de Goey and Bengt Johansson. Styro- foam precursors as drop-in diesel fuel. SAE International Journal of Engines (2013) DOI:10.4271/2013-24-0108 • Noud Maes, Jos Reijnders, Micheal D. Boot, Carlo Luijten, Philip de Goey and Marcel Dhaenens. Spray and failure analysis of porous injection nozzles. SAE International Journal of Engines (2012) DOI:10.4271/2012-01-1654 • Jos Reijnders, Michael D. Boot, Carlo Luijten, Peter Frijters and Philip de Goey. Porous fuel air mixing enhancing nozzle (PFAMEN). SAE International Journal of Engines (2009) DOI:10.4271/2009-24-0028 • Jos Reijnders, Michael D. Boot, Carlo Luijten and Philip de Goey. Investigation of direct-injection via micro-porous injector. Proceedings of the 4th European Combustion Meeting (2009), Vienna, Austria (pp. 1-6). Patent: • Michael D. Boot and Jos Reijnders. (2010). Liquid injector for a combustion engine. Patent No. WO2010041928.

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