Alcohol Fuels for Spark-Ignition Engines

Special issue article

Proc IMechE Part D: J Automobile Engineering 2018, Vol. 232(1) 36–56 Alcohol fuels for spark-ignition engines: Ó IMechE 2018 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav Performance, efficiency and emission DOI: 10.1177/0954407017752832 effects at mid to high blend rates for journals.sagepub.com/home/pid binary mixtures and pure components

James WG Turner1, Andrew GJ Lewis1, Sam Akehurst1,ChrisJBrace1, Sebastian Verhelst2 , Jeroen Vancoillie2, Louis Sileghem2, Felix Leach3 and Peter P Edwards3

Abstract The paper evaluates the results of tests performed using mid- and high-level blends of the low-carbon alcohols, methanol and ethanol, in admixture with gasoline, conducted in a variety of test engines to investigate octane response, efficiency and exhaust emissions, including those for particulate matter. In addition, pure alcohols are tested in two of the engines, to show the maximum response that can be expected in terms of knock limit and efficiency as a result of the beneficial properties of the two alcohols investigated. All of the test work has been conducted with blending of the alcohols and gasoline taking place outside the combustion system, that is, the two components are mixed homogeneously before introduction to the fuel system, and so the results represent what would happen if the alcohols were introduced into the fuel pool through a conventional single-fuel-pump (dispenser) approach. While much has been written on the effect of blending the light alcohols with gasoline in this way, the results present significant new findings with regard to the effect of the enthalpy of vaporization of the alcohols in terms of particulate exhaust emissions. Also, in one of the tests, two mid-level blends are tested in a highly downsized prototype engine – these blends being matched for stoichiometry, enthalpy of vaporization and volumetric energy content. The consequences of this are discussed and the results show that this approach to blend formulation creates fuels which behave in the same manner in a given combustion system; the reasons for this are discussed. One set of tests using pure methanol alternately with cooled exhaust gas recirculation and with excess air shows a significant increase in thermal efficiency that can be expected as the blend level is increased. The effect on nitrous oxide emissions is shown to be similarly beneficial, this being primarily a result of the enthalpy of vaporization of the alcohol cooling the charge coupled with the lower adiabatic flame temperature of the alcohol. Whereas it is normally a beneficial characteristic in spark-ignition combustion systems, one disadvantage of a high value of enthalpy of vaporization is shown in another series of tests in that, in admixture with gasoline, it is a driver on flash boiling of the hydrocarbon component in the blend in direct injection combustion systems. In turn, this causes the particulate emissions of the engine to increase quite markedly over those of ethanol–gasoline blends at the same stoichiometry. The paper shows for the first time the dichotomy of the potential efficiency improvement with the challenge of particulate control. This effect poses a challenge for the future introduction and use of such high blend rate fuels in engines without particulate filters. Although it must be stated that the overall particulate emissions of the methanol– gasoline blend are lower than for gasoline, the effect is only present at extremely low load, and that there is a likelihood that particulate filters will be adopted in production vehicle anyway, nullifying this issue.

Keywords Alcohol, methanol, ethanol, downsized, fuel efficiency/economy

Date received: 29 July 2016; accepted: 23 March 2017 1University of Bath, Bath, UK 2Ghent University, Ghent, Belgium 3 Introduction University of Oxford, Oxford, UK Corresponding author: The two light alcohols, methanol and ethanol, have James WG Turner, University of Bath, Claverton Down, Bath BA2 7AY, been investigated by numerous authors either as pure UK. fuels for internal combustion engines (ICEs) or in Email: [email protected] Turner et al. 37 admixture with gasoline. As pure fuels, they were pro- of this paper being blends created outside the engine’s posed as an alternative to gasoline for reasons of energy fuel system, requiring only one such handling system. security at least as long ago as 1907.1 Most commonly However, many of the results presented in those studies they can be configured as binary mixtures of one of the by other workers do use blends of the alcohols with a alcohols with gasoline, or sometimes jointly together as low-octane base fuel for initial testing or derive an ternary mixtures. This paper discusses the case of the equivalent in-cylinder octane number after the fuels pure alcohols and binary blends at mid and high levels have been introduced;10 consequently, much can be and uses a variety of engines to investigate the response deduced from these studies in terms of how the alco- of these fuels in terms of knock limit, efficiency and hols increase the knock resistance of pure hydrocar- emissions. bons, especially when the alcohol is port-fuel-injected Modern gasoline commonly contains 5% ethanol and the effect of its enthalpy of vaporization is reduced (so-called ‘E5’), in a bid to improve energy security and (see also the discussion on ‘direct injection octane num- reduce the fossil CO2 impact of the fuel. (In this paper, ber’ (DON) in section ‘Experiments into efficiency the term ‘EXX’ refers to a binary mixture of ethanol in improvements and engine load control utilizing pure gasoline of volume percentage XX ethanol, and ‘MXX’ methanol’).6 is used as the corresponding case for a binary mixture Ethanol is the commonest alcohol used in SI fuels. of methanol in gasoline.) In addition, E10 is now being It can be made from biological sources which give rise introduced in Europe and the United States, to extend to energy security and net greenhouse gas (GHG) these advantages, and there are waivers in place to per- reduction advantages; growing plants for fuel use mit the use of up to 15% ethanol in gasoline (agreed by within a nation’s borders addresses the former, while 2,3 the US Environmental Protection Agency). the airborne CO2 absorbed and trapped within a part Furthermore, in Europe, up to 3% methanol is permit- of the fuel lifecycle addresses the latter. Ethanol is also ted as a blend component in gasoline, within an overall an extremely good SI fuel: it has very high octane num- oxygen concentration limit of 3.7% by mass.4 These bers, high octane sensitivity (S = RON 2 MON, where low blend rate levels have been adopted as part of the S is sensitivity, RON and MON are the research and worldwide fuel charter and are consequently considered motor octane number, respectively), a high laminar to be fully commercialized and hence will not be stud- burning velocity (LBV) and low adiabatic flame tem- ied in detail in this work. Instead, mid- and high-level perature.11 Consequently, it can support high compres- blends are investigated, ‘mid-level’ being defined herein sion ratios (CRs) and high levels of boost pressure for to be 15–30% of the alcohol by volume and ‘high-level’ high Otto cycle efficiency12 and can also operate very anything above that volumetric percentage, up to and efficiently with higher rates of cooled EGR than con- including the pure alcohol. In reality, blend rates of ventional hydrocarbons can support. These advantages 85% have successfully been used for both the low-car- increase in a DI combustion system, where its very high bon-number alcohols in specific markets and trials enthalpy of vaporization in conjunction with its low (including Brazil, which has traditionally used varying stoichiometric air–fuel ratio (AFR) generates signifi- rates of ethanol in its gasoline, up to and including cant charge cooling which can be used to increase volu- hydrous pure ethanol), but this work seeks to clarify metric efficiency or knock resistance, or both. certain responses of the fuels in modern spark-ignition Furthermore, since it has only two carbon atoms and a (SI) combustion systems utilizing exhaust gas recircula- single carbon–carbon bond, it does not directly form tion (EGR), lean combustion operation and/or direct particulate matter (PM) easily; experiments relating to injection (DI). this will be explained in section ‘Particulate emissions All of the results discussed in this work relate to the from high-blend alcohol fuels in a DISI engine’. use of alcohol blend fuels in a single fuel delivery sys- All of these advantages apply to methanol too, only tem. Dual-fuelling, so-called ‘octane-on-demand’, more so (with certain provisos which will be discussed where a low-octane base fuel is used for most operating later), it having a higher enthalpy of vaporization, a conditions, with a second, high-octane fuel being intro- lower stoichiometric AFR and, being the single-carbon duced as and when necessary to suppress knock, has alcohol, no carbon–carbon bonds. Methanol can also been investigated with various fuel introduction meth- be made from biological sources (as ‘wood alcohol’), ods.5–8 The light alcohols are generally favoured in but importantly it is very easily synthesized from any such advanced combustion systems, and these carbonaceous feed stock generally via a gasification-to- approaches can extend the benefit of the alcohol and syngas approach. It can also be made directly from displace a proportionally larger amount of fossil fuel. CO2 and hydrogen, opening up the eventual possibility They also permit the reduction of fuel processing for of carbon-free energy being used to extract CO2 from the hydrocarbon component, with naphtha being one air and this being used to fix hydrogen in a fully renew- possible choice for this, and with this comes the poten- able liquid fuel, without the feed stock limitation con- tial to significantly reduce the energy used in refining cerns of ethanol production with its biomass limit, for and thus the well-to-tank carbon intensity of the fuel.9 example.13–18 Recently, researchers have shown the fea- Such approaches will not be discussed here, the focus sibility of its formation directly from air in a combined 38 Proc IMechE Part D: J Automobile Engineering 232(1) carbon-capture-and-synthesis process, which brings the denaturant. In addition, small amounts of additives can possibility of a truly practical liquid renewable fuel even be used to denature them, with one – benzoate, or closer.19 ‘Bitrex’, the bitterest material known – capable of mak- As a result of the hydrogen bonding characteristic ing them so bitter as to be unbearable at concentrations provided by their OH characteristic group, both metha- of as low as 10 ppm (parts per million).28,29 However, it nol and ethanol are infinitely soluble in water; as a con- is interesting to note that regardless of its toxicity, the sequence, historical research has been undertaken on US Environmental Protection Agency conducted signif- various alcohol–water blends.20 This has also been sug- icant research into methanol safety and concluded that gested as one approach to octane-on-demand.21 Such if the whole US light-duty fleet switched from gasoline an approach could be seen as a potential extension to to this alcohol, then deaths and property loss attributa- simple water injection as is being commercialized by ble to the fuel would reduce by at least one order of BMW.22 The use of an alcohol mixed into the water, or magnitude.30,31 This is primarily due to the low flamm- metered in some manner if the water is harvested on ability index of methanol in its liquid form, and the board, again as proposed in several forms by BMW,23 opposite problem that gasoline has with unintended presents some advantages in terms of avoiding the ignition due to its high flammability index. freezing of water and bioorganism suppression. As As discussed above, many researchers have investi- such, some of the complication of tank heating could gated synthesizing methanol using CO2, with the poten- be avoided and the knock-suppressant system could be tial to extract this from the atmosphere and combine it made to work immediately at low temperatures; a 50– with hydrogen from water electrolysis. When all of the 50% (by volume) mixture of methanol and water has a process energy is carbon-free, this results in a non-fos- freezing point of 240°C, while a 30–70 mixture has a sil-carbon liquid energy carrier made from limitless feed freezing point of 220°C.24 Varying the concentration stocks.13–18 Audi32 have recently promoted a similar of methanol in such ‘anti-detonant injection’ fluids, as process with methane, which they term e-gas; Specht et they were termed when used at take-off in piston aero al.33 have also shown how this permits storage of engines,25 could be controlled automatically either in renewable energy in the gas grid, which for some coun- separate-fuel octane-on-demand applications or in sys- tries is, in terms of energy capacity, significantly greater tems where water is mixed with gasoline just before than the daily usage amount. The approach therefore injection. The infinite miscibility of the alcohol in water decouples renewable energy from peaks and troughs of is a clear enabler for this. While further investigation of generation and usage. this approach is beyond the scope of this paper, it does Pearson et al.17 discussed how the approach of suggest an interesting potential direction of research. Specht et al. could be extended by making liquid fuels However, there are disadvantages to the alcohols, so that renewable energy can then be stored and used many born of their OH characteristic group. They have in the liquid as well as the in gaseous fuel distribution low vapour pressures in their pure form, but, when they system. This opportunity has driven the blending and are mixed with hydrocarbons, the fact that the resulting pure fuel investigations discussed here. As a further mixtures do not follow Raoult’s Law means that they potential, methanol can also be part of a significant have very high vapour pressures. They possess hydro- pathway to the synthesis of higher hydrocarbons, albeit philic tendencies, which particularly at low blend rates with a concomitant reduction in the energy carried, can lead to phase separation when relatively small opening up the opportunity to decarbonize transport by amounts of water are absorbed into a mixture. Thus, providing a blend component which is of similarly low care must be taken to ensure that any alcohol used for carbon intensity. Such approaches have recently been blending with gasoline is fully dried, and that water is reviewed by France et al.34 and are shown in Figure 1. kept out of the fuel supply and storage system. In the However, the reduction in process efficiency arising case of ethanol, because it forms an azeotrope with from the onward synthesis of methanol to hydrocar- water (at 4.4% water by volume), this gives rise to high bons, and thus the resulting increase in the amount of process energy requirements during its manufacture. primary renewable energy required to provide the same Conversely, the very low vapour pressures of the pure amount of delivered fuel energy, combined with the loss alcohols can give rise to startability issues in this case, of potential engine efficiency entailed using a hydrocar- although DI has been known to be capable of mitigat- bon fuel versus the original alcohol (see later) makes ing this problem for sometime.26 They are also more investigation into methanol combustion important; the aggressive to fuel system materials than hydrocarbons, same observation can be made for ethanol against the but countermeasures have been known and understood backdrop of the biomass limit on the manufacture of for this for sometime also.27 this fuel. More contentious than the above is the fact that The paper will show that significant improvements methanol is toxic, although the level of its toxicity is in thermal efficiency are possible when alcohols are similar to gasoline and diesel, albeit manifesting in a used in SI combustion engines compared to using different form. As well as being an intoxicant, ethanol hydrocarbon fuels with octane numbers in the range is toxic in large amounts too, and for this reason both currently used in most non-premium fuels, including have habitually been mixed with gasoline as a when such an engine is configured with a CR Turner et al. 39

Figure 1. Potential pathways to decarbonized liquid transport.34 significantly higher than can be normally tolerated by a for gasoline and the other a modification of a diesel hydrocarbon gasoline-type fuel, or when it is highly engine to SI and high-EGR operation. The purpose of boosted. This will be shown by tests in a variety of this section is to show how much extra efficiency can be research engines using pure methanol and ethanol and achieved when an engine is configured to better match various mixtures of these alcohols in binary blends with what methanol is capable of delivering. gasoline at the mid level (ca. 15–30% v/v) or high level Section ‘Performance and emissions of binary gasoline– (up to ca. 85% v/v). The paper will also discuss the alcohol blends in a production SI engine’ describes tests effect of gasoline–alcohol mixtures on particulate emis- of two equal-stoichiometry blends in a production SI sions from DI combustion systems. Emissions are not engine and shows that again such fuels effectively per- speciated to aldehydes, although this is a significant form identically. It also describes knock tests showing issue in terms of the pollutant-forming mechanisms of the performance of the equal-stoichiometry blends and alcohol combustion, and likely to become more so. also that methanol itself has the highest knock limit of While not wanting to ignore this issue, it is believed the pure fuel blend stocks tested. that changes in catalyst formulation will be sufficient Section ‘Particulate emissions from high-blend alcohol to address it, since oxidation of these molecules is easier fuels in a DISI engine’ describes engine-out emissions than the reduction of oxides of nitrogen (NOx), and tests in a DI engine in which some difference is discov- one of the advantages of alcohol combustion is lower ered in the particulate performance of the equal- NOx output than for hydrocarbon fuels. stoichiometry blends. This is shown to be dependent on Thus, to orient the reader, the remainder of this load, and a hypothesis is put forward as to what may paper comprises four main sections and conclusion: be causing this. It is acknowledged here that the use of a gasoline particulate filter will effectively eliminate this Section ‘Binary gasoline–alcohol blend tests in the issue, although the phenomenon is one which would Ultraboost extreme downsizing demonstrator engine’ undoubtedly benefit from extra study. focuses on tests of a highly downsized multi-cylinder engine optimized for gasoline in which the perfor- Binary gasoline–alcohol blend tests in the mance- and efficiency-enhancing attributes of the alco- Ultraboost extreme downsizing hols are investigated. This section also shows that when demonstrator engine configured for equivalent stoichiometry, alcohol– gasoline blends can perform fundamentally identically To investigate some of the potential of alcohol–gasoline in the same combustion system when operated under mid- and high-level blends, a series of tests were con- the same conditions. ducted in the Ultraboost extreme downsizing demon- Section ‘Experiments into efficiency improvements and strator engine. These utilized an external boosting engine load control utilizing pure methanol’ details tests system with and without cooled EGR to accurately of pure methanol in two different SI combustion sys- control the intake charge conditions delivered to the tems: one is a conventional engine originally designed engine. 40 Proc IMechE Part D: J Automobile Engineering 232(1)

The ‘Ultra Boost for Economy’ project was a colla- ethanol derived from biomass in Europe and the borative project part-funded by the UK Technology United States, given concerns about food supply and Strategy Board (now Innovate UK) with eight part- indirect land use change (ILUC), and M15 represents a ners: Land Rover was the lead, the others comprising blend which is in common use in China, the methanol Lotus Engineering, GE Precision Engineering, Shell generally being made from coal as a means of improv- Global Solutions, CD-adapco, the University of Bath, ing the nation’s energy security. However, Table 2 Imperial College and the University of Leeds. It is dis- shows that these two blends are coincidentally matched cussed in depth elsewhere,35 but in brief, the engine is a for AFR and also for volumetric lower heating values 2.0 L in-line four-cylinder unit designed to reproduce (LHVs). This phenomenon has been discussed by sev- the torque curve of the Jaguar Land Rover AJ133 5.0 L eral different researchers39–43 and will be returned to in naturally aspirated (NA) V8 engine while operating on a later publication discussing further tests conducted commercial 95 RON gasoline. A successful target of with three-component gasoline–ethanol–methanol the project was to demonstrate a vehicle-level CO2 (GEM) blends matched for stoichiometry. reduction of 35% (by using dynamometer test results in Results of spark loops for E20 and M15 at the same a vehicle simulation package). The final engine was boost and speed conditions are shown in Figures 2 and supercharged and turbocharged, but an intermediate- 3, compared to that of the base fuel. For Figure 2, the level engine was operated with a facilitated charging conditions provided are an engine speed of 2000 r/min system at the University of Bath, where all of the proj- and intake plenum conditions of 40°C and 130 kPa ect test work was conducted. Table 1 presents brief gauge, whereas for Figure 3, the corresponding values specifications of the engine in this externally boosted are 3000 r/min, 60°C and 140 kPa gauge with 10% (or ‘UB100’) specification. cooled EGR. Both conditions (referred to as test points During the main programme, the engine was used for A and D later in this paper) therefore represent high an extensive series of fuel tests, the results of which, for load conditions, as shown by brake mean effective pres- all but the mid- and high-level alcohol blends, are dis- sure (BMEP) evident in the figures. cussed in detail in a series of other publications.36–38 In Immediately apparent from these sets of data is that the work reported here, the alcohol blends shown in the two alcohol blend fuels perform identically. Part of Table 2 are tested. The results will be compared to those the reason for the comparable performance will of conventional 95 and 98 RON gasoline, and with the undoubtedly be that the two mid-level blends (which Ultraboost control ‘base fuel’. This is a commercial gaso- are defined on a volumetric basis, as discussed above) line sold as 95 RON fuel, but with some octane giveaway actually contain the same molar concentration of the (see Table 2). While the 98 RON fuel did not contain respective alcohol in the mixture – approximately 40%. ethanol, both of the 95 RON fuels were E5 blends; how- This is obviously a much higher number than the volu- ever, all three were production fuels of EN228 standard, metric blend rate and illustrates the effect of the light and so for the purposes of this paper, they are considered alcohol molecules in the much heavier bulk hydrocar- gasolines and not gasoline–alcohol blend fuels. bon, as has been discussed by Pearson et al.44 The pure The two mid-level blends, E20 and M15, were cho- alcohols possess similar octane numbers and thus in the sen for different reasons: E20 represents a maximum gaseous state, as found in a combustion system, they blend in the region of what could be made using would be expected to perform in the same manner.

Table 1. Specifications of the Ultraboost extreme downsizing demonstrator engine in externally boosted configuration, as used for fuels testing.35

General architecture Four-cylinder in-line with four valves per cylinder and double overhead camshafts Construction All-aluminium Jaguar Land Rover AJ133 cylinder block converted to single-bank operation on the A Bank (right-hand side) Siamesed liner pack to facilitate reduced bore diameter Dedicated cylinder head Firing order 1-3-4-2 Bore 83 mm Stroke 92 mm Swept volume 1991 cm3 Combustion system Pent-roof combustion chamber with asymmetric central direct injection and spark plug High-tumble intake ports Auxiliary port-fuel injection Possible second spark plug position in an under-intake-port location Compression ratio 9.0:1 Valve gear Chain-driven double overhead camshafts with fast-acting dual continuously variable camshaft phasers (DCVCP) Cam profile switching (CPS) tappets on inlet and exhaust Engine management system Lotus ‘T6’ engine control unit Turner et al. 41

Table 2. Specifications of the gasoline and binary alcohol–gasoline fuels tested in the Ultraboost extreme downsizing demonstrator engine.

Fuel name RON MON S Volumetric LHV (MJ/L) Stoichiometric air–fuel ratio (:1)

Base 97 85.3 11.7 31.29 14.18 95 RON pump 95.1 85 10.1 31.51 14.19 98 RON pump 98.7 86.5 12.2 31.57 14.45 M15 99.8 86.1 13.7 29.56 13.25 E20 99.6 85.7 13.9 29.81 13.3 E85 107.4 89.5 17.9 22.65 9.68

RON: research octane number; MON: motor octane number; S: sensitivity; LHV: lower heating value.

advance, and since fixed plenum pressure and temperature were used, the improvement in efficiency is due to an increase in volumetric efficiency due to the greater enthalpy of vaporization of the alcohols (together with minor benefits due to their reduced heat transfer and an increased molar expansion ratio). Since the curves end at the knock-limited spark advance (KLSA), the improvement due to octane number is additive to this effect. Calculating the enthalpy of vaporization based on a volume basis (as discussed by Chupka et al.45) shows that the two blends have near-identical values in the region of 311.6–314.0 kJ/L (M15 being 0.7% higher) although the value for the mixture is also dependent on Figure 2. Spark loops for E20 and M15 compared with the the composition of the gasoline (for which 280 kJ/kg base gasoline fuel, all at matched engine speed and plenum has been used here). (Note that there is an alternative conditions (2000 r/min, 40°C and 130 kPa gauge in the plenum, view that mass-based enthalpy of vaporization should 46 no cooled EGR). Note that BMEP was allowed to vary with be used, as proposed by Zhou et al. Adopting this spark advance, the speed and plenum conditions being fixed approach, E20 would have an enthalpy of vaporization throughout the test. of 417.4 kJ/kg and M15 421.9 kJ/kg; that is, using this BMEP: brake mean effective pressure; BTDC: before top dead centre. calculation approach that of M15 is 1.1% higher. Note also that these values are approximately 50% higher than the value of 280 kJ/kg used for the gasoline.) Since this is a DI engine, matching the enthalpy of vaporiza- tions would also be expected to equalize the performance between the blends, which appears to be the case here. It is acknowledged that the enthalpy of vaporization varies with temperature, as discussed by Chen and Stone,47 and that detailed liquid–vapour equilibria and distillation data are required to understand the evaporation of the whole mixture properly. Nevertheless, since the plenum pressure and temperature conditions were held constant during these tests, comparison between the fuels is valid. Further discussion on the enthalpy of vaporization of the components and the resulting effect on particle emissions is made in section Figure 3. Spark loops for E20 and M15 compared with the ‘Particulate emissions from high-blend alcohol fuels in base gasoline fuel, all at matched engine speed and plenum a DISI engine’. conditions (3000 r/min, 60°C and 140 kPa gauge in the plenum, During this sequence of tests to compare the fuels, 10% cooled EGR). Note that BMEP was allowed to vary with four operating points were determined that were repre- spark advance, the speed and plenum conditions being fixed sentative of different operating regimes. They were throughout the test. selected so that all of the fuels in the full fuel test regime BMEP: brake mean effective pressure; BTDC: before top dead centre. that the Ultraboost engine was performing within the main project could be operated so that the advantages Also apparent in the data is that the two alcohols of higher octane values could be ascertained. The other perform markedly better than the base fuel (measured test results have been reported elsewhere.36–38 The test at 97 RON, as shown in Table 2). At the same spark points used are shown in Table 3. 42 Proc IMechE Part D: J Automobile Engineering 232(1)

Table 3. Test points for Ultraboost engine testing.

Test point Engine Load EGR (%) Boost pressure Plenum Exhaust back Notes speed (r/min) condition (bar) temperature (°C) pressure (bar)

A 2000 High 0 1.3 40 0.45 B 2000 High 10 1.5 40 0.45 Boost pressure increased to compensate for the presence of EGR C 3000 Mid–high 10 1.4 60 0.7 Mimics supercharged operation D 3000 Mid–high 10 1.4 60 1.2 Mimics turbocharged operation

EGR: exhaust gas recirculation.

that the plenum pressure was increased so that the rate at which oxygen was ingested by the engine was the same for points A and B; effectively, the same BMEP was targeted at KLSA. Point C targeted a back pressure at the exhaust manifold outlet typical of a supercharged engine, while point D targeted a back pressure representative of a turbocharged one. The higher exhaust back pressure of test point D was intended to reduce the knock limit of the engine with respect to point C. For more information on the rationale behind choosing these test conditions, see Remmert and colleagues.36–38 As described above at each point, a spark loop was conducted with the relative AFR held at l = 1 (i.e. the Figure 4. Results for torque for two standard gasolines and stoichiometric AFR) and the boost conditions were three binary alcohol blends, at fixed intake conditions. held constant (i.e. pressure, temperature and EGR rate Ultraboost research engine, operated at MBT ignition timing. were not varied), but torque and therefore brake spe- E20 and M15 have matched stoichiometry, enthalpy of cific fuel consumption (BSFC) and brake thermal effi- vaporization and octane numbers, and essentially perform ciency (BTE) could change. identically. RON: research octane number; MBT: maximum brake torque. The results for torque from the tests with the base fuel, 95 RON pump gasoline and the two mid- level blends (E20 and M15) are shown in Figure 4. E85 is also shown and will be discussed later. What is apparent in these results is that in terms of torque output for the fixed intake conditions, both of the mid-level blends perform better than the gasolines, which might be expected given their higher octane values shown in Table 2. It can also be seen that within experimental error, they essentially perform identically, in line with the blending rules established in Turner and colleagues.39,44 Thus, mid-level blends in which the octane numbers are increased over those readily achievable with conventional gasoline (in the manner discussed by Anderson et al.48) offer the potential to improve engine efficiency while permit- ting renewable energy to be introduced into the fuel Figure 5. Results for brake thermal efficiency for two standard gasolines and three binary alcohol blends, at fixed intake pool. conditions. Ultraboost research engine, operated at MBT BSFC and BTE data are shown in Figures 5 and 6. ignition timing. The BSFC results show the expected trend: because of RON: research octane number; MBT: maximum brake torque. their lower LHVs, the alcohols record significantly higher BSFCs, but in terms of efficiency they beat the two gasolines shown. In Table 3, the phrase ‘boost pressure increased to In Figure 5, the slight differences between the BTEs compensate for the presence of EGR’ refers to the fact for the fuels when producing the same torque with the Turner et al. 43

Table 4. Test engine specifications.

Engine type Volvo 1.8 l Audi/NSU

Cylinders 4 in-line 1 Valves 16 2 Valvetrain Double Overhead overhead camshaft camshaft Bore 83 mm 77.5 mm Stroke 82.4 mm 86.4 mm Displacement 1783 cm3 407.3 cm3 Compression 10.3:1 10.17:1 (results in this ratio (CR) section) 13.1:1 (results in section ‘Performance and emissions of binary Figure 6. Results for brake specific fuel consumption (BSFC) gasoline–alcohol blends in a for two standard gasolines and three binary alcohol blends, at production SI engine’) fixed intake conditions. Ultraboost research engine, operated at Fuel system PFI PFI MBT ignition timing. Aspiration NA NA RON: research octane number; MBT: maximum brake torque. Engine control MoTeC M800 MoTeC M4 unit (ECU)

PFI: port-fuel injection; NA: naturally aspirated. same LHV is considered to be within experimental error. Since this engine had a relatively low CR of 9.0, albeit capable of being very heavily boosted and habitually operated at 35 bar BMEP on gasoline,35 the data for this fuel suggest that significant improvements in BTE could be realized for mid- and high-alcohol blends and pure components if an engine with significantly higher CR could be used. Such a situation is discussed in the next section.

Experiments into efficiency improvements and engine load control utilizing pure methanol Experiments with SI base engines Figure 7. NA single-cylinder Audi engine: maximum torque when operated on gasoline (open symbols) and methanol As discussed above, thanks to a variety of interesting (closed symbols). properties the light alcohols have the potential to NA: naturally aspirated. increase the power and efficiency of SI engines. While the previous section investigated blends of the alcohols with conventional gasoline, Ghent University on gasoline, and as such have relatively low CRs has operated several engines on M100 as part of a appropriate for using this fuel.) The specifications of research stream into utilizing the desirable character- these engines are given in Table 4. istics of methanol combustion in optimized engines The use of the Volvo production engine in this work while also investigating different strategies for load was partly to demonstrate the ease with which wide- control that are enabled by those properties as well. spread automotive technology can be modified to Other studies using methanol in SI engines have accept high alcohol blends, thus easing any potential shown that the potential improvement depends on the evolution of the transport system in this direction. degree to which an engine can be biased towards A well-known advantage of using methanol in SI alcohol-only operation while still maintaining engines is the increase in maximum achievable engine flex-fuel functionality. To investigate the potential load. In PFI engines, this is mainly a consequence of improvement, operation on a commercial 95 RON the elevated knock resistance of methanol, but the gasoline and pure methanol was compared in two NA high degree of charge cooling as the fuel is injected port-fuel injection (PFI) engines: a production-type leads to increased volumetric efficiency. These factors four-cylinder Volvo engine and a research-type sin- provide a relative power increase of more than 10%, as gle-cylinder Audi engine.49 (Note that both of the results obtained with the Audi engine shown in these engines were originally designed for operation Figure 7 demonstrate. 44 Proc IMechE Part D: J Automobile Engineering 232(1)

Table 5. Properties of alcohol fuels and gasoline.

Fuel Methanol Ethanol Propanol Butanol Gasoline

a Heat of vaporization (kJ/kgfuel) 1170 930 693 583 440 Stoichiometric AFR 6.5 9 10.5 11.3 14.4 Heat of vaporization (kJ/kgstoichiometric mixture) 180.0 103.3 66.0 51.6 30.6 Heat of vaporization per unit mass of stoichiometric 5.9 3.4 2.2 1.7 1 mixture relative to gasoline (:1) Research octane number (RON)b 106 107 104 98 95 Motor octane number (MON)b 90 91 89 83 85 Direct-injection octane number (DON)b52 119 122 108 98 95c

Source: Data taken from Luef et al.52 and Turner et al.54 aMid-point of values given in Luef et al.52 for Eurosuper 95 RON E0, with 10% ethyl tert-butyl ether (i.e. no ethanol was present). bFor the alcohols, derived from Figure 6 of Luef et al.52 cDerived from Figure 7 of Luef et al.52 and an assumed MON of 85 (i.e. the European minimum value).

In addition to its potential to increase performance and provide a route to greater efficiency through increased engine downsizing, methanol also enables relative BTE gains of up to 10% (see Figure 8) even with an engine which was originally optimized for operation on gasoline. This is partly explained by the higher LBV of methanol; the reduced in-cylinder heat transfer losses compared to gasoline operation are another con- tributing factor. Not only does the charge cooling due to methanol evaporation reduce the unburned mixture temperature, but the high heat capacity of methanol’s combustion products also reduces the flame and exhaust gas temperatures compared to gasoline; these results mirror those for E85 reported in Turner et al.54 At high loads, the knock resistance of methanol ensures that optimal values for ignition timing can be retained, which is also helpful for efficiency. It is readily appar- Figure 8. Brake thermal efficiency (BTE) at part load for the ent that optimization of an engine to make better use NA 4-cylinder Volvo engine. Constant brake torque of 20 N m of the improved qualities of alcohols over a conven- (diamonds), 40 N m (triangles) and 80 N m (circles). Open symbols for gasoline and closed symbols for methanol. tional non-premium gasoline would improve this situa- Stoichiometric fuelling, throttled load control. tion further; principal among the factors which could NA: naturally aspirated. be adjusted would be CR. As mentioned above, the lower peak combustion temperature of methanol leads to a reduction in engine- In DI engines, a combination of the extremely high out NOx. Figure 9 shows that the values measured on enthalpy of vaporization of alcohols together with their the single-cylinder engine were consistently 5–10 g/ relatively low stoichiometric AFRs means that the pos- kW h lower on methanol compared to gasoline. No sig- sibility to reduce charge temperature at full load pro- nificant changes in carbon monoxide (CO) emissions vides a significant potential development stream in line were observed, although some authors report that the with the megatrend in the automotive industry towards oxygenated nature of alcohols can cause more complete downsized, DISI engines.35 The degree to which alcohols combustion and thus reduced CO emissions,55 due provide increased knock resistance in such combustion partly to the fact that they can be described as being systems is the subject of much research at present.50–53 already partially oxidized as a result of the oxygen Table 5 quantifies the potential charge cooling effect of atom present in the alcoholic OH group defining this several alcohols and also lists a ’direct-injection octane family of chemicals. 52 number’ (DON) as defined by other researchers. In terms of specific CO2 emissions, a reduction of In Table 5, the values shown for propanol and buta- more than 10% was observed on methanol, as shown in nol are those for the normal alcohols (i.e. propan-1-ol Figure 10 for the four-cylinder Volvo engine. In addi- and n-butan-1-ol, respectively). The values for DON, as tion to the improvement in performance of the combus- reported in Luef et al.,52 are derived from the data given tion system when operating on the alcohol, this is partly in that work and included to show the outright poten- due to the fact that lower alcohols produce less CO2 per tial of the alcohols in DI combustion systems. unit of energy contained (68.4 g/MJ for methanol when Turner et al. 45

Figure 9. Engine-out NOx for the single-cylinder Audi engine. Constant brake torque of 10 N m (diamonds) and 20 N m (triangles). Open symbols for gasoline and closed symbols for Figure 11. Indicator diagram for operation with different load methanol. Stoichiometric fuelling, throttled load control. control strategies at 1500 r/min and 25 N m for the Audi engine. Dotted line is conventional throttled case, dashed line is WOT EGR case and solid line is WOT lean case. WOT EGR: wide-open throttle exhaust gas recirculation.

and variable amounts of external EGR to control load while the throttle position was fixed (and preferably wide open to the benefit of minimum pumping work). Clearly, there would be varying demands on the exhaust after treatment (EAT) system for any engine adopting these strategies, but they are interesting for a number of reasons, the main ones being the reduction of throttling losses, which is beneficial for part load efficiency, and a reduction in heat transfer and endother- mic dissociation losses thanks to the lower in-cylinder temperatures associated with dilution. Unfortunately, the application of excess air or EGR also renders the combustion increasingly unstable and less isochoric due to reduced burning rates. A fuel with wide flammability limits and high LBV (in turn providing elevated EGR Figure 10. Specific CO2 (in g/kW h) for the four-cylinder Volvo engine. Constant brake torque of 20 N m (diamonds) and tolerance) such as methanol will be less prone to the 80 N m (circles). Open symbols for gasoline and closed symbols negative side-effects of any charge dilution strategy. for methanol. Stoichiometric fuelling, throttled load control. Measurements on the single-cylinder engine indicated that EGR levels up to 30% could be applied before the coefficient of variance (CoV) of indicated mean effec- compared to a typical value of 74 g/MJ for gasoline, or tive pressure (IMEP) exceeded the threshold value of more than 7% less). Another reason is the higher BTE 10% used, whereas for gasoline operation, this limit on when using methanol. Emissions of unburned hydro- CoV of IMEP was found to be at only 10% EGR. For carbons are not reported since flame ionization detec- the lean-burn strategy, the combustion stability limits tors (as used in this work) are reported to have a slow were l = 1.5 and l = 1.25 on methanol and gasoline, response time to oxygenated species.56,57 respectively.58 The elevated LBV and wide flammability limits of Figure 11 illustrates some of the key differences methanol offer some alternative options for load con- between the load control strategies under consider- trol, again beyond what can be achieved with straight ation. The pumping loops for the lean burn and WOT hydrocarbon fuels. Two such strategies are wide-open EGR strategies are considerably smaller than for con- throttle (WOT) lean-burn operation and WOT EGR ventional load control via the throttle due to the operation. These were compared against the conven- absence of those throttling losses. Also visible on the tional throttled stoichiometric operation on the single- diagram is the adverse influence of air and EGR dilu- cylinder engine.58 These strategies, respectively, used tion: the combustion itself is less isochoric, the effect variable mixture strength or stoichiometric mixtures possibly being greater for EGR operation than for lean 46 Proc IMechE Part D: J Automobile Engineering 232(1)

Figure 12. Indicated thermal efficiency for a fixed brake torque of 25 N m for the Audi Engine. Diamonds for throttled case, Figure 13. Brake thermal efficiency in percent as a function of squares for WOT EGR case and triangles for WOT lean case. engine speed and BMEP for WOT EGR operation on a light-duty WOT EGR: wide-open throttle exhaust gas recirculation. diesel engine converted to SI operation and fuelled with methanol. BMEP: brake mean effective pressure; WOT EGR: wide-open throttle burn. Figure 12 shows that both alternative strategies exhaust gas recirculation; SI: spark-ignition. lead to relative improvements in indicated thermal efficiency (ITE) of the order of 5%, thanks to reduced throttling, cooling and dissociation losses. These are in For this work, the engine was mapped from 1000 to addition to any improvements due to operating the 4000 r/min in 500 r/min increments, and at each engine on methanol over its performance on gasoline. speed in steps of 25 N m up to the full load point. This resulted in 62 test points, to provide a fine grid of test data to produce performance maps, as shown in Experiments with a light-duty diesel engine Figure 13. Here, it can be seen that the high CR helped converted to SI to produce a peak BTE of 42%. At the same time, the specification rendered full-load operation on standard After this series of investigations using a single-cylinder commercial 95 RON gasoline impossible due to heavy SI engine operating on pure methanol, the strategy knock. Arguably, the revised configuration of the employing cooled EGR at WOT was further investi- engine was closer to that which would be arrived at if gated on a turbocharged four-cylinder engine. This was full optimization of a SI engine was undertaken to take done because the fact that stoichiometric operation full benefit of the properties of light alcohols, although coupled with EGR dilution supports the use of a simple it could still be argued that its CR was too high. The and robust three-way catalyst makes this load control advantage of this was shown at part load, though the strategy very attractive. This four-cylinder research absence of throttling losses and the benefits associated engine was based closely on a Volkswagen Type AXR with lower in-cylinder temperatures enable relative effi- 1.9 L turbocharged diesel engine to provide the high ciency improvements of up to 20% compared to con- swirl rate that was desired. ventional throttled operation. In converting the engine, a similar approach was The high levels of EGR dilution at these loads (up to taken to that reported in Brusstar et al.59 It retained its 50%) also reduced the engine-out NO emissions to CR piston of 19.5:1, because the crank train of the x extremely low levels (see Figure 14). These results engine was not changed, and also retained the high- demonstrate that methanol can be used in dedicated SI swirl intake port and diesel piston bowl optimized for engines to yield diesel-like efficiencies with emission lev- diesel combustion. These features were considered els comparable to, or lower than, those achieved when more appropriate for EGR dilution59 because of its ele- using gasoline. The primary reason for the very low vated levels of mixture turbulence. They increased the NO results in Figure 14 in comparison to Figure 9 is burn rates sufficiently to allow throttleless operation x because of the ability of the engine to operate with very with the EGR strategy down to 3 bar BMEP without high levels of EGR dilution due to the use of methanol, excessive cyclic variability. For comparison, when oper- the high CR and high tumble. Thus, the development ating on methanol with the single-cylinder Audi engine, of a fully optimized engine could be expected to pro- the corresponding lower load limit on cyclic variability duce higher efficiencies due to lower friction from crank was 6 bar BMEP. A standard ignition system was used train components suitable for the lower cylinder pres- together with PFI; more details on this engine are to be sures than the diesel base engine reaches, and reduced found in Brusstar et al.59 Turner et al. 47

Table 6. Properties of the iso-stoichiometric binary blends tested in a four-cylinder 1.8 L PFI production engine.

Fuel blend Blend A Blend D

Component ratios (%) G15 G43 E85 E0 M0 M57 Oxygen content (% m/m) 23.34 22.54 Gravimetric LHV (MJ/kg) 29.22 29.53 Volumetric LHV (MJ/L) 22.82 22.59 AFRstoich (kg/kg) 9.72 9.73 DHvap (kJ/kg) 762.4 770.8 a (H/C in fuel) 2.83 2.9 Specific CO2-emissions (gCO2/MJ) 71.69 71.41

PFI: port-fuel injection; LHV: lower heating value; AFR: air–fuel ratio.

electric vehicles or, on its take up, molecular hydrogen Figure 14. Engine-out NOx in ppm as a function of engine fuel) during the transition to high-alcohol fuels. The speed and BMEP for the results shown in Figure 13; light-duty miscibility of alcohols with gasoline in the same fuel diesel engine converted to SI operation and fuelled with system means that the vehicle on-cost will be small to methanol. provide this complete flexibility and reassurance for the BMEP: brake mean effective pressure; SI: spark-ignition. customer.

Performance and emissions of binary gasoline–alcohol blends in a production SI engine In this piece of work, the performance and engine-out emissions of two binary gasoline–alcohol blends corresponding to the value of stoichiometry of E85 were examined in a four-cylinder 1.8 L PFI production engine. In parallel, a single-cylinder engine with high CR was also used for a preliminary study of the knock behaviour of these blends. The measurement results are compared with those when operating on commercial gasoline and neat methanol and ethanol to demonstrate Figure 15. Brake thermal efficiency (BTE) of Blend A (E85) and the potential of these blends as a fossil fuel alternative. Blend D (M57) as a function of engine speed for a fixed brake The composition and properties of the two blends torque of 40 N m (2.82 bar BMEP) when tested in a four- cylinder 1.8 L PFI production engine. are shown in Table 6. Here, ‘Blend A’ represents idea- BMEP: brake mean effective pressure; PFI: port-fuel injection. lized ‘normal’ E85, whereas the equivalent stoichiometry methanol–gasoline ‘Blend D’ contains 57% v/v methanol. These are therefore considered to be iso- heat losses from what would likely be a lower CR than stoichiometric blends, and they were configured in the 19.5:1, both coupled with a more-favourable surface same way as discussed in previous sections. Blend D area-to-volume ratio from a combustion chamber (M57) has 1% v/v methanol more than in the study of developed for SI combustion rather than for diesel Turner et al.60 because of a difference in the stoichio- (where very high swirl rates are a dominant feature). metric AFR of the gasoline used for blending in this The technologies used in achieving the improvement section; nevertheless, the same blending rules were used in efficiency demonstrated when optimizing for a high- to determine it. blend or pure alcohol fuel are essentially the same as Stoichiometric operation was used throughout these those used in production gasoline engines now. The tests to maximize the pollutant conversion rate of a importance of this is that when combined with flex-fuel three-way catalyst. engine control strategies, calibrations can be developed which still permit the use of gasoline (with its much lower octane numbers and enthalpy of vaporization) in Performance the vehicle, albeit with lower performance. This obvi- Figure 15 shows the BTE for all blends at a constant ates any issues of range anxiety (such as is the case with load of 40 N m (2.82 bar BMEP) for a range of engine 48 Proc IMechE Part D: J Automobile Engineering 232(1)

Figure 16. Brake thermal efficiency (BTE) when operating on Figure 18. Brake specific fuel consumption (BSFC) as a various fuels as a function of engine speed for different fixed function of engine speed for different fixed brake torques of brake torques of 40 N m (2.82 bar BMEP) (dashed lines) and 40 N m (dashed lines) and 80 N m (solid lines) when tested in a 80 N m (5.64 bar BMEP) (solid lines) when tested in a four- four-cylinder 1.8 L PFI production engine. cylinder 1.8 L PFI production engine. GEM: gasoline–ethanol–methanol; BMEP: brake mean effective pressure; GEM: gasoline–ethanol–methanol; BMEP: brake mean effective pressure; PFI: port-fuel injection. PFI: port-fuel injection.

LHV of the binary blends. Again, this might be expected from the discussion on BTE. While the binary blends display similar BTE to pure ethanol, their BSFCs are less penalized than pure methanol. This is shown in Figure 18 in which a comparison of the BSFCs of gasoline, methanol, ethanol and a mean value for the binary blends for 40 N m and 80 N m is shown. Despite the better efficiency on binary blends, this engine will consume ;32% more fuel blend than when it is running on gasoline on a volume basis. This difference will be smaller for modern highly downsized and pressure-charged engines with DI in which the properties of alcohols will have greater benefits; similarly, the situation would be further improved Figure 17. Brake specific fuel consumption (BSFC) of the with a full optimization of SI engines to suit the blends as a function of engine speed for a fixed brake torque of improved fuel characteristics. 40 N m (2.82 bar BMEP) when tested in a four-cylinder 1.8 L PFI The in-cylinder pressure measurements can be used production engine. to obtain information regarding the combustion pro- BMEP: brake mean effective pressure; PFI: port-fuel injection. cess in the engine, in terms of heat release and mass fraction burned (MFB). In Figure 19, the heat release speeds. The hypothesis that all iso-stoichiometric rates of the iso-stoichiometric binary blends are shown, blends have similar BTEs is confirmed here as all the as calculated from the in-cylinder pressure measure- values fall within the range of experimental uncertainty. ments. As can be seen in the figure, the heat release Furthermore, this statement is valid for all loads tested rates are close to each other, again as might be here. expected. As a result, the MFBs for the different blends Figure 16 displays the comparison with gasoline, are very similar. The ignition delay (defined here as 0– methanol, ethanol and a mean value for the two binary 2% MFB), the 0–10% and 0–50% MFB durations are blends for 40 N m and 80 N m, labelled ‘GEM mean’. plotted in Figure 20 as a function of engine speed, When compared to gasoline, it is clear that the binary showing the similar burning velocities of the two blends blends show significant efficiency gains. The mean at these operating conditions. value for the binary blends is similar to the BTE of pure ethanol. Pure methanol clearly still has superior performance, which might be expected from the results Emissions already discussed in section ‘Experiments into effi- In this section, NOx emissions are shown and discussed ciency improvements and engine load control utilizing for the two high-blend fuels tested. Figure 21 compares pure methanol’. the engine-out NOx emissions for the two high-blend BSFC does not depend on mixture composition as fuels, gasoline and pure methanol, at a load of 40 N m. shown in Figure 17, primarily because of the near-identical As can be seen, the highest NOx emissions were found Turner et al. 49

Figure 21. NOx emissions as a function of engine speed for gasoline, methanol and two iso-stoichiometric gasoline–alcohol blends as a function of engine speed for a fixed brake torque of Figure 19. Heat release rates of two iso-stoichiometric 40 N m (2.82 bar BMEP) when tested in a four-cylinder 1.8 L PFI gasoline–alcohol blends for an engine speed of 2000 r/min and a production engine. fixed brake torque of 40 N m (2.82 bar BMEP) when tested in a BMEP: brake mean effective pressure; PFI: port-fuel injection. four-cylinder 1.8 L PFI production engine. BMEP: brake mean effective pressure; PFI: port-fuel injection. seen that there is an increase in NOx emission with increased gasoline content in the mixture. This variation with gasoline content and thus total alcohol concentration for all other measurements could be ascribed to a slight variation in flame temperature. It is considered that further research in this area would be warranted.

Knock behaviour For the Volvo four-cylinder engine at the low loads tested, no knock occurred during the measurements on all of the fuels and thus MBT timing could be set for every load and engine speed. From previous work, it is expected that the two binary blends would have very similar knock behavior.39,40,44 To investigate the hypothesis that such blends exhi- bit quasi-constant RON and MON via their knock Figure 20. Ignition delay (0–2% burned), 0–10% and 0–50% resistance, Blends A and D and their individual compo- burn duration of two iso-stoichiometric gasoline–alcohol blends nents (pure methanol, ethanol and gasoline) were tested as a function of engine speed for a fixed brake torque of 40 N m in a single-cylinder research engine with a high CR. (2.82 bar BMEP) when tested in a four-cylinder 1.8 L PFI The properties of this test engine are listed in Table 3 production engine. BMEP: brake mean effective pressure; PFI: port-fuel injection. (i.e. which are used for the work reported in section ‘Experiments into efficiency improvements and engine load control utilizing pure methanol’). At an operating when operating on gasoline and the lowest NOx emis- point of 25 N m (BMEP = 7.71 bar) and 2000 r/min, sions were operating on methanol. All of the NOx emis- ignition timing was advanced until an intermediate sions for high-blend fuels are somewhere between knocking condition was obtained. In addition to the 95 gasoline and methanol. The lower combustion tem- RON gasoline (which was used as the gasoline compo- perature of the alcohol fuels is responsible for the lower nent to formulate the blends), a 98 RON gasoline was NOx emissions since most NOx is produced by the ther- also tested. The results of this experiment are shown in mal mechanism which is dependent on the maximum Figure 22. As can be seen, the blends have similar temperature reached. The lower NOx emissions at knocking behaviour, resulting in the same ignition tim- lower engine speeds are hypothesized to be the result of ing at incipient knock. Furthermore, the blends display elevated levels of internal EGR since the vacuum in the the same ignition timing as pure ethanol. As discussed, intake due to the use of conventional throttling for load the M57 blend was configured to be equivalent to E85 control is considerable at this low engine load of and so the fact that their performance is so close to that 40 N m. For the two alcohol–gasoline blends, it can be of pure ethanol is to be expected because the molar 50 Proc IMechE Part D: J Automobile Engineering 232(1)

Table 7. Composition of the iso-stoichiometric fuel blends tested in the single-cylinder DI engine.

Fuel Gasoline Ethanol Methanol Predicted (%v/v) (%v/v) (%v/v) DVPEa (kPa)

Gasoline 100 0 0 62 E85 15 85 0 39 M56 44 0 56 87

DI: direct injection; DVPE: dry vapour pressure equivalent. aPrediction derived from a UNIFAC model.61

Table 8. Composition of the base CEC RF-02-03 gasoline test fuel.

Parameter Unit Result Figure 22. Knock-limited spark advance of the blends and individual blend components. RON 97.0 BTDC: before top dead centre. MON 87.7 Density at 15°C kg/L 0.7439 DVPE kPa 60.0 Aromatics %v/v 29.0 concentrations of the alcohol in each blend are similar. Olefins %v/v 1.8 (By mole fraction, E85 is approximately 94% ethanol Saturates %v/v 69.2 and M57 is approximately 84% methanol.) Having Carbon %m/m 86.74 said this, pure methanol still displays superior knock Hydrogen %m/m 13.26 resistance to all of the other fuels. Gross calorific value MJ/kg 46.19 Net calorific value MJ/kg 43.38 IBP °C 28.8 T90 °C 166.6 Particulate emissions from high-blend FBP °C 200.8 alcohol fuels in a DISI engine RON: research octane number; MON: motor octane number; DVPE: Despite the fact that significant activity has been under- dry vapour pressure equivalent; IBP: Initial boiling point; FBP: Final taken investigating the performance of iso- boiling point. stoichiometric blends in engines and vehicles, to date particle number (PN) emissions from these blends have emissions compared with straight gasoline).63 This not been evaluated in a DI engine. In this section, parti- reduction is thought to be due to the fact that the pres- culate emissions from a single-cylinder DI engine oper- ence of oxygen in the fuel molecule reduces the concen- ating on E85 and M56 fuels and a base gasoline were tration of intermediate species that are important to measured. the formation of precursors to soot.64 As methanol is added to the fuel, its vapour pressure increases, as can be seen by the change in predicted Fuels tested DVPE in Table 7. High vapour pressures can result in For this experiment, E85 and M56 were splash-blended the flash-boiling of the fuel on injection, leading to very with a base gasoline, their composition being shown in poor mixture preparation. This poor mixture prepara- Table 7, together with their predicted dry vapour pres- tion would be expected to lead to high levels of particu- sure equivalent (DVPE). late emissions being observed when methanol is The base gasoline for these blends was a CEC RF- blended with gasoline, as is the case for M56. 02-03 specification gasoline supplied by BP. This gaso- Flash boiling occurs when the fuel evaporates so line has no oxygenate components present in it, and its quickly on injection that it essentially does so as it properties are shown in Table 8. leaves the injector in an ‘onion skin’ manner, with the outer layers evaporating first, regardless of their composition. This is known to occur in DI engines65 and is Expected fuel effects on PN emissions the result of rapid boiling of a liquid (in this case fuel), It has previously been noted62 that stoichiometric occurring as a liquid jet is injected into a volume with a AFRs increase particulate emissions. This is true of pressure lower than the saturation vapour pressure of ethanol, but the increase in stoichiometric PM emis- the liquid. In this case, the liquid is rapidly depres- sions for ethanol is very small, and compared to gaso- surised, and the liquid becomes superheated and conse- line is effectively a dramatic reduction (E10 showing a quently thermodynamically unstable, regaining its greater than order of magnitude reduction in PM stability by flash boiling. Turner et al. 51

Table 9. Single-cylinder DI engine parameters.

Bore (mm) 89 Stroke (mm) 90.3 Cylinder capacity (cm3) 562 Compression ratio 11.1 Fuel injection pressure (bar) 150

DI: direct injection; PN: particle number. PN emissions were measured using a Cambustion DMS500, which is fully described by Reavell et al.67 A Wiebe filter was used to mimic the effect of the PMP legislatively compliant protocol68 for measuring PN emissions, and this method has been shown to be representative of legislative tests,69 although this was not a legislatively compliant test.

Table 10. Engine operating conditions for model fuels. Figure 23. PN emissions from gasoline, E85 and M56 fuels at 2.6 bar IMEP for two different fuelling conditions (l = 1.01 and IMEP (bar) 2.6 and 4.9 0.9). Error bars correspond to 6s. Inlet air temperature (°C) 40 PN: particle number; IMEP: indicated mean effective pressure. Coolant temperature (°C) 60 AFR (l) 0.9 and 1.01 Engine speed (r/min) 1500 Start of ignition (degrees BTDC) 35 Start of injection (degrees BTDC) 280

IMEP: indicated mean effective pressure; AFR: air–fuel ratio; BTDC: before top dead centre.

Test engine and particulate measurements The engine for these tests was a single-cylinder engine with a central spray-guided DI combustion system based on the Jaguar Land Rover AJ133 5.0 L V8 engine.66 This research engine is described more fully by Leach et al.,61 and it represents a modern DISI configuration. The engine was fitted with a titanium piston Figure 24. PN emissions from gasoline, E85 and M56 fuels at with a steel blanking plate, and the relevant engine 4.9 bar IMEP for two different fuelling conditions (l = 1.01 and specifications are shown in Table 9. 0.9). Error bars correspond to 6s. PN: particle number; IMEP: indicated mean effective pressure.

Results E85 gives essentially particle free combustion at light For these experiments, the engine was run at the fixed load regardless of relative AFR, which is significant. operating point shown as in Table 10. Here, the engine One repeat (that of M56) was managed in the experi- was run with fixed injection timing, so it should be mental sequence (which was done in a random order), noted that injection timing among other parameters and it can be seen that M56 gives repeatable PN emis- can have a big impact on PN emissions. Some work sions, leading to confidence in these results. In general, looking at the effect of injection timing on PN emis- other testing on this engine shows good repeatability sions on this engine has been previously done by with PN emissions. Murad et al.70 The engine was run with open-loop It is also seen that the levels of PN emissions from AFR control with l = 0.9 and 1.01, and at low load the base gasoline are higher than E85, but lower than and medium load (2.6 and 4.9 bar IMEP). l = 1.01 M56, suggesting that the presence of ethanol at this was chosen to avoid any small rich mixture excursions load condition reduces PN, but methanol acts to which are known to have a large effect on the PM emis- increase PN. This could be explained by the flash boil- sions from non-oxygenated fuels.62 Because of this, ing phenomenon described above, where the methanol these rich excursions tend to dominate drive cycle PM is flashing, leading to a poorly prepared mixture and results, hence the relevance of testing at l = 0.9. hence an increase in PN emissions, despite a higher The particulate emissions from the fuels at light load level of oxygenates. Alternatively, at rich conditions, (2.6 bar IMEP) are shown in Figure 23. It can be seen large amounts of fuel will need to be injected at high from these results that the trend predicted above is oxygenate content levels to maintain l = 0.9, and look- clearly demonstrated here – with a significant increase ing at the DHvap of methanol and ethanol (159 and in particulate emissions from E85 to gasoline to M56. 92 kJ/kg stoichiometric mixture, respectively), large 52 Proc IMechE Part D: J Automobile Engineering 232(1) amounts of local cooling would be expected upon injec- PN emissions may be observed with blend fuels with tion even without flash boiling. This may lead to high methanol content, and this could be due to metha- incomplete mixture evaporation and hence worse mix- nol promoting flash boiling, alternatively the very high ture preparation and higher PN emissions. This is sup- latent heat of vaporisation might be leading to poor ported by the larger standard deviation of the M56 local mixture formation on injection. As the load results (compared to E85), suggesting a less homoge- increases, the results show that these methanol blend neous mixture. It must be noted that the volume percen- fuels will give lower particulate emissions than gasoline, tages of the two alcohols in the respective mixtures are which is thought to be due to the effect of reduction in not the same, though. It could be that M85 would have flash boiling; the presence of oxygen in the molecules low PN emissions, or that as methanol is removed from then reduces the number of particulate precursors the fuel, the effect is similarly eliminated, and the factors formed in combustion. At both light and intermediate that cause oxygenate fuels (in general) to reduce PN load, high ethanol content in the fuels results in signifi- emissions discussed above become dominant with only cantly lower PN emissions compared to the reference ethanol present, with the PN consequently being reduced gasoline, again because the presence of oxygen in the relative to the gasoline levels. In addition, it is acknowl- molecules reduces the number of particulate precursors edged that the particulate emissions of different gasoline formed during combustion. Of course, PN emissions formulations can vary widely. Overall, this suggests that are not solely a function of oxygenate content, but are more work would be justified to gather the change in PN also significantly influenced by a wide range of other emissions as methanol proportion is changed. parameters including base gasoline composition.61,71 Of particular interest is that the effect of l shows a Here, the engine has been run fully warm and the significant increase in PN emissions from gasoline (as widely documented effect of cold start dominating PN would be expected) and M56, but there is almost no dif- emissions has not been captured.69 A more comprehen- ference in PN emission between E85 at l = 1.01 and sive study would be needed to validate this more l = 0.9. The presence of oxygen in the fuel molecule is thoroughly. clearly having a significant effect here, despite the need to inject more fuel on a volumetric basis, which might Conclusion lead to worse mixture preparation and hence higher PN emissions. This shows that from a PN perspective, E85 This paper has shown that the light alcohols, methanol is a very promising fuel, and even rich mixture excur- and ethanol, can be used in mid- and high-blend fuels sions, which are inevitable on any drive cycle (with to improve the efficiency of SI engines and thus can more ‘aggressive’ drive cycles having more such excur- help to move towards a more decarbonized transport sions), do not have a significant effect on PN emissions network. Furthermore, pathways exist to enable fully from E85. However, a similar study of ethanol propor- decarbonized hydrocarbon fuels to be synthesized from tion in gasoline would be justified, since in theory in methanol (albeit with an associated energy loss). While flex-fuel vehicles, any ethanol concentration (up to the ethanol production using biological sources is restricted maximum permitted in the blend) could be present in by a biomass limit which varies from region to region, the tank. no such restriction exists for methanol production Figure 24 shows the PN emissions from E85, M56 because it can be manufactured straightforwardly from and gasoline at a higher load (4.9 bar IMEP). Here, the any carbonaceous feed stocks using chemical processes. trends shown at light load are repeated, although now As far as the use of these alcohols in binary blends M56’s PN emissions are lower than that of the base gas- with gasoline at mid- and high-level is concerned, the oline. This may be because at higher cylinder pressures, paper has shown the following: either less flash boiling occurs or the higher pressures promote spray evaporation more generally, leading to 1. When configured to have the same stoichiometry, the factors which cause oxygenate fuels to reduce parti- blends of gasoline and ethanol and of gasoline and culate emissions in ethanol to dominate in methanol methanol behave in an identical manner in SI com- too. Again the effect of l on E85 is minimal for the bustion systems. This is due to fixing the stoichio- same reasons as discussed above. The same observation metric AFR resulting in matched LHVs, heats of with regard to the range of particulate emissions from vaporization and molar concentrations in the different gasoline compositions that is made above charge, in turn leading to matched octane numbers must also be made, together with the fact that the wide- due to the extremely close RON and MON of the spread adoption of gasoline particulate filters will ren- two alcohols. der any issues discovered here largely null anyway. 2. The two high-blend matched stoichiometry fuels, Overall, these results show that using oxygenated E85 and M57, effectively exhibited the same knock fuels will not significantly impact the PN emissions wit- performance as pure ethanol in a high CR single nessed from gasoline direct injection (GDI) engines, cylinder test engine. This is due to the very high and some of them can significantly decrease the PN molar concentration of the alcohols in their respec- emissions observed. At light load, a slight increase in tive blends, being approximately 90% and thus Turner et al. 53

dominating any negative effects of the hydrocar- excess air to a much greater degree than is possible bon components present. using gasoline. This is due to the higher LBV of 3. The use of the light alcohols in SI combustion sys- the alcohol. At the same time, significantly lower tems permits significant improvements in thermal engine out NOx can be shown, which is due to the efficiency which, due to the fact that they can be lower adiabatic flame temperature coupled to the manufactured using biological or zero-carbon pro- high enthalpy of vaporization reducing the overall cesses, will have benefits in terms of the demands cycle temperature. placed on such future energy systems. 4. The use of identical technologies for pure alcohols 4. In DI combustion systems, methanol blends have a and gasoline means that engines can be optimized tendency to increase particulate number emissions, for the high-efficiency alcohol fuel while being pro- whereas those with ethanol do not. This is a new tected to still be able to operate on conventional finding. It only occurs at light loads, and increasing non-premium gasoline. While there would be load reduces the methanol blend particulate num- expected to be a concomitant reduction in perfor- ber results to values less than gasoline. It is sur- mance when using the hydrocarbon fuel, this mised that this is a result of the very high enthalpy removes the issues of range anxiety prevalent with of vaporization and lower boiling point of the electric vehicles (and likely to be the case with methanol component causing flash boiling and hydrogen fuel as and when it is first introduced). thus incomplete combustion of the hydrocarbon This will permit the gradual evolution of the trans- components. Increasing methanol concentration port system as discussed above, with significant may also reduce the effect; the expected widespread benefits in terms of the viability of the approach. adoption of particulate filters because of the general challenges of gasoline fuels will, of course, nul- lify it. The particulate-forming characteristics of Acknowledgements different gasoline compositions will also affect any The support and encouragement of Ben Iosefa at results. Overall, more research is warranted in this Methanex; Eelco Dekker, Greg Dolan and Matt area. Roberts at The Methanol Institute; and Kenth 5. The miscibility of the alcohols with gasoline, and Johansson and Kjell ac Bergstro¨m of Saab Automobile the results shown here, present a method by which Powertrain AB are gratefully acknowledged. The vehicles and fuels can co-evolve towards the com- Ultraboost project was part-funded by Innovate UK, plete decarbonization of transport via the use of and all of the partners involved express their gratitude liquid energy carriers; the current economic viabi- for the support shown for the project. Thanks are due lity of the personal transport system is based on to Bob Head, Sarah Remmert and Roger Cracknell of the ease with which liquids can be stored and dis- Shel Global Solutions for the supply of the alcohol- tributed. This potentially has profound implica- based fuels used to generate the results discussed in sections for future energy and transport policy, since tion ‘Binary gasoline–alcohol blend tests in the it does not require a revolution to be undertaken Ultraboost extreme downsizing demonstrator engine’ by any of the major stakeholders in the area. of this paper. The authors would also like to acknowledge the insights gained from discussions with, among When used in their pure forms, the paper has shown others, the following people: Tony Marmont and Peter the following: Harrison of Air Fuel Synthesis; Richard Pearson of the University of Bath (formerly of Lotus Engineering); 1. The alcohols provide much higher knock limits Matt Eisaman of Brookhaven National Laboratory; than that of conventional non-premium gasoline, Paul Wuebben of Carbon Recycling International; which in turn increases the thermal efficiency of Steve Brueckner of Chrysler (formerly of Lotus engines in which they are used. This has been Engineering Inc.); Leon di Marco of FSK shown in test engines with PFI systems, which Technologies; Gordon Taylor of GT-Systems; Keith were also suitable for gasoline use. Waldron and David Wilson of the Institute of Food 2. When an SI engine can be configured to make Research; Tim Fox of the Institution of Mechanical more complete use of the beneficial characteristics Engineers; Leslie Bromberg and Daniel Cohn of MIT; of the alcohols in SI combustion systems, thermal Frank Zeman of New York Institute of Technology; efficiencies higher than those that can typically be Martin Davy and Richard Stone of the University of achieved by current diesel engines can be realized. Oxford; Gareth Floweday and Andre Swarts of Sasol More-optimal development of such engines is Technology; George Olah, Surya Prakash and Alain expected to yield further benefits, leading to the Goeppert of the University of Southern California; possibility to extend the use of renewable energy in Karl Littau of Stanford University; and Matt Brusstar transportation because of increased gasoline of the United States Environmental Protection Agency. displacement. J.V. and L.S. gratefully acknowledge PhD fellowships 3. When operated on methanol, such high-CR of the Research Foundation – Flanders (FWO09/ASP/ engines allow load control using cooled EGR or 030 and FWO11/ASP/056). 54 Proc IMechE Part D: J Automobile Engineering 232(1)

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