energies

Article Influence of Combustion Characteristics and Fuel Composition on Exhaust PAHs in a Compression Ignition Engine

Hamisu Adamu Dandajeh 1,2,*, Midhat Talibi 2, Nicos Ladommatos 2 and Paul Hellier 2

1 Department of Mechanical Engineering, Ahmadu Bello University, Zaria PMB 1045, Nigeria 2 Department of Mechanical Engineering, University College London, Torrington Place, London WC1E 7JE, UK * Correspondence: [email protected] or [email protected]



Received: 29 May 2019; Accepted: 23 June 2019; Published: 4 July 2019


Abstract: This paper reports an experimental investigation into the effects of fuel composition on the exhaust emission of toxic polycyclic aromatic hydrocarbons (PAHs) from a diesel engine, operated at both constant fuel injection and constant fuel ignition modes. The paper quantifies the US EPA (United State Environmental Protection Agency) 16 priority PAHs produced from combustion of fossil diesel fuel and several model fuel blends of n-heptane, toluene and methyl decanoate in a single-cylinder diesel research engine based on a commercial light duty automotive engine. It was found that the level of total PAHs emitted by the various fuel blends decreased with increasing fuel ignition delay and premixed burn fraction, however, where the ignition delay of a fuel blend was decreased with use of an ignition improving additive the level of particulate phase PAH also decreased. Increasing the level of toluene present in the fuel blends decreased levels of low toxicity of two to four ring PAH, while displacing n-heptane with methyl decanoate increased particulate phase adsorbed PAH. Overall, the composition of the fuels investigated was found to have more influence on the concentration of exhaust PAHs formed than that of combustion characteristics, including ignition delay, peak heat release rate and the extent of the premixed burn fractions.

Keywords: exhaust PAH; fuel composition; compression ignition engine; premixed phase; ignition delay; ignition improver

1. Introduction While diesel engines continue to be used due to their fuel economy and potential for high power output, they still suffer from high level of NOx and particulate emissions [1–3]. Even though the advent of EURO 6 emission legislation has so far aided the reduction of NOx and particulate levels via the incorporation of diesel particulate filters (DPF) and other exhaust after treatment systems in diesel engines, particulate emissions are still of great concern in terms of health effects [4–6]. The health effect of diesel particulates is principally owing to the polycyclic aromatic hydrocarbons (PAHs) associated with them. Concurrent with greater awareness of the need to address the adverse impacts of diesel exhaust emissions on urban air quality and public health, is the continuing need to reduce greenhouse gas emissions from the transport sector [7]. In addition to improvements to both vehicle and powertrain efficiency, the use of renewable fuels for transport is now mandated by legislation in many countries; for example, the recently updated Renewable Energy Directive sets a new minimum of 14% average renewable fuels for road transport in all EU member states [8]. While this presents an opportunity to increase the range of biofuels utilized in transport, for example biogas from anaerobic digestions

Energies 2019, 12, 2575; doi:10.3390/en12132575 www.mdpi.com/journal/energies Energies 2019, 12, 2575 2 of 19 of organic wastes [9], biodiesel (fatty acid esters of vegetable and animal oils [10]) continues to be most commonly utilized for the displacement of fossil diesel. While biodiesels from a wide variety of possible renewable oil feedstocks have been considered [11–14], all differ from the components of fossil diesel in the oxygen content of the ester functional group. PAHs are not directly regulated by diesel emissions regulations and are known to be human carcinogens [3,15]. They are precursors to soot particle formation and abundant in atmospheric aerosols [16]. Coarse soot particles of diameters (Dp) > 1 µm could potentially be deposited in the superior respiratory airways, while inhalable particles at sub-micron levels, bearing carcinogenic-PAHs, could reach into the human lung pulmonary alveoli causing various health complications [17]. PAHs can be found as constituents of fossil fuels (petro-genic PAHs) [18,19] and can also be generated during combustion (pyrogenic PAHs) [20,21]. Evidence has long been available on the survival of petro-genic PAHs during combustion and their tendency of being carried over to the engine exhaust [1,2,22]. This mostly happens to fossil fuels such as diesel which contains significant proportions of aromatic compounds, including PAHs. In the case of pyrogenic PAHs, they can be formed from a wide range of molecules in diesel fuel and their formation involves a complex series of reactions. These often begin with the formation of the first aromatic ring as a result of fuel breakdown and culminate in the formation of fused aromatic ring structures and PAHs through various reaction pathways [23,24]. Emission quantity of pyrogenic PAHs depend on the engine operating conditions [6,25] and the type of fuel used [17,26]. Fossil fuels with high poly-aromatic content have been reported to promote PAH formation [18,27]. However, no definitive conclusions have been reached, yet, regarding the effect of fuel mono-aromatic content on the formation of PAHs. There are divergent views in the literature on the effect of total aromatic content of fuels on PAH emissions, as outlined below. Barbella et al. [28] investigated the effect of fuel aromatic content on PAH and soot emissions from a single cylinder direct injection diesel engine fueled with n-tetradecane, a mixture of 70% n-tetradecane + 30% toluene, and two diesel fuel oils of dissimilar aromatic contents. They observed that the concentration of the exhaust PAHs was dependent on the composition of the fuel tested. Barbella et al. also found higher PAH mass from combusting 70% n-tetradecane + 30% toluene as compared to combusting pure n-tetradecane. Diesel fuel oils produced the highest concentration of PAHs among all the fuels tested, and most of these PAHs were believed to have emanated from the unburned PAHs in the diesel fuel oils. Mi et al. [27] studied the effect of total aromatic and poly-aromatic contents of fuels on the exhaust PAH emission from a heavy-duty diesel engine under steady state conditions. They added 10% of single ring toluene and 5% of the three ring PAH fluorene (by volume basis) to a base diesel fuel. The proportions of toluene and fluorene in the diesel blend represented mono-aromatic and poly-aromatic hydrocarbons respectively. Mi et al. reported that adding PAH fluorene to the diesel fuel increased the emission of total PAHs as well as the PAH fluorene itself. However, no significant change in PAH emission was observed by the authors when adding toluene to the diesel fuel. Borrás et al.[6] investigated PAH emissions from different reformulated diesel fuels in a light duty diesel engine. Their results showed that adding 5%–25% of mono-aromatics to low sulphur diesel and reference diesel fuels had no considerable effect on the exhaust PAH concentrations when compared with the concentration of exhaust PAHs from combusting pure reference diesel. However, the biodiesel rape oil methyl ester produced a greater range of PAHs but at considerably lower concentrations (~50% lower) than the PAHs in the reformulated diesel. Research published in the literature leads to the conclusion that the poly-aromatic content of fuels has a greater effect on the exhaust PAHs than the mono-aromatics, but the extent to which the mono-aromatic content in the fuel affects PAH formation is not fully established. It is still not clear from the literature whether the mono-aromatics in the fuel influences the Group B2 PAHs (PAHs that are possible human carcinogens). 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GroupPAH NA—Not B2 areGroupCarcinogenicity ‘posavailable.sibly carcinogenic FactorToxicity to humans’MolecularWeight while ofNumber Rings Group DStructure are Sn classified‘unclassifiable PAHsby US as toEPA carcinogenicity’ (1993).Abbreviation GroupPAH NA—Not B2 areCarcinogenicity ‘posavailable.Groupsibly carcinogenic ToxicityFactor to (ghumans’Molecular/mole)Weight while ofNumber RingsGroup DStructure are Sn ‘unclassifiableclassified byPAHs US as toEPA carcinogenicity’ (1993).Abbreviation GroupPAH NA—Not B2 areCarcinogenicity ‘posavailable.Groupsibly carcinogenic ToxicityFactor to humans’Molecular(g/mole)Weight while ofNumber RingsGroup DStructure are Sn ‘unclassifiablePAHs as to carcinogenicity’AbbreviationPAH NA—Not Carcinogenicity available.Group ToxicityFactor Molecular(g/mole)Weight ofNumber Rings Structure Sn ‘unclassifiablePAHs as to carcinogenicity’AbbreviationPAH NA—Not Carcinogenicity available.Group ToxicityFactor Molecular(g/mole)Weight ofNumber Rings Structure 1Sn1 ‘unclassifiable NaphthaleneNaphthalenePAHs as to carcinogenicity’ NPHAbbreviationPAHNPH NA—Not Carcinogenicity D available.GroupD 0.001ToxicityFactor0.001 Molecular 128(g/mole)Weight128 Numberof 2Rings2 Structure Sn1 NaphthalenePAHs AbbreviationNPHPAH CarcinogenicityGroupD ToxicityFactor0.001 Molecular(g/mole)Weight128 Numberof Rings2 Structure Sn1 NaphthalenePAHs AbbreviationPAHNPH CarcinogenicityGroupD ToxicityFactor0.001 Molecular(g/mole)Weight128 ofNumber Rings2 Structure Sn12 AcenaphthyleneNaphthalenePAHs AbbreviationPAHNPHACY CarcinogenicityGroupD ToxicityFactor0.001 Molecular(g/mole)Weight128152 Numberof Rings23 Structure 2Sn21 AcenaphthyleneAcenaphthyleneNaphthalenePAHs ACYAbbreviationNPHPAHACY Carcinogenicity DGroupD 0.001ToxicityFactor0.001 Molecular(g/mole) 152Weight152128 Numberof 3Rings32 Structure Sn12 AcenaphthyleneNaphthalenePAHs AbbreviationPAHNPHACY CarcinogenicityGroupD ToxicityFactor0.001 (g/mole)Weight128152 ofNumber Rings23 Structure Sn123 AcenaphthyleneAcenaphtheneNaphthalenePAHs AbbreviationNPHACNACY GroupNAD Factor0.001 (g/mole)Weight128152154 of Rings23 Structure 132 AcenaphthyleneAcenaphtheneNaphthalene AbbreviationACNNPHACY GroupNAD Factor0.001 (g/mole)128154152 of Rings23 3312 AcenaphtheneAcenaphthyleneAcenaphtheneNaphthalene ACNAbbreviationACNNPHACY NAGroupNAD 0.001Factor0.001 154(g/mole)154128152 of 3Rings32 1234 AcenaphthyleneAcenaphtheneNaphthaleneFluorene NPHACNACYFLU NAD 0.001 (g/mole)128152154166 23 2413 AcenaphthyleneAcenaphtheneNaphthaleneFluorene ACNNPHACYFLU NAD 0.001 152166128154 32 2143 AcenaphthyleneAcenaphtheneNaphthaleneFluorene NPHACNACYFLU NAD 0.001 152128166154 32 42345 FluoreneAcenaphthyleneAcenaphthenePhenanthreneFluorene FLUACNPHNACYFLU DNAD 0.0010.001 166152154166178 3 3 3524 AcenaphthyleneAcenaphthenePhenanthreneFluorene ACNPHNACYFLU NAD 0.001 154178152166 3 2534 AcenaphthyleneAcenaphthenePhenanthreneFluorene ACNPHNACYFLU NAD 0.001 152178154166 3 53456 PhenanthreneAcenaphthenePhenanthreneAnthraceneFluorene PHNACNPHNATRFLU DNAD 0.0010.0010.01 178154166178 3 3 4653 AcenaphthenePhenanthreneAnthraceneFluorene ACNPHNATRFLU NAD 0.0010.01 166178154 3 3465 AcenaphthenePhenanthreneAnthraceneFluorene ACNPHNATRFLU NAD 0.0010.01 154166178 3 4567 PhenanthreneFluorantheneAnthraceneFluorene PHNATRFLUFLT D 0.0010.01 166178202 34 65764 AnthracenePhenanthreneFluorantheneAnthraceneFluorene ATRPHNATRFLUFLT DD 0.010.0010.01 178178202166 3 34 4567 PhenanthreneFluorantheneAnthraceneFluorene PHNATRFLUFLT D 0.0010.01 166178202 34 567 PhenanthreneFluorantheneAnthracene PHNATRFLT D 0.0010.01 178202 34 6857 PhenanthreneFluorantheneAnthracenePyrene PHNATRPYRFLT NAD 0.0010.01 178202 34 78567 FluoranthenePhenanthreneFluorantheneAnthracenePyrene FLTPHNATRPYRFLT DNAD 0.0010.0010.01 202202178 4 43 6758 PhenanthreneFluorantheneAnthracenePyrene PHNATRPYRFLT NAD 0.0010.01 178202 34 786 FluorantheneAnthracenePyrene ATRPYRFLT NAD 0.0010.01 202178 43 6897 Benzo[a]anthraceneFluorantheneAnthracenePyrene B[a]AATRPYRFLT NAB2D 0.0010.010.1 178202228 34 8798 Benzo[a]anthracene PyreneFluoranthenePyrene PYRB[a]APYRFLT NANAB2D 0.0010.0010.1 202202228 4 4 879 Benzo[a]anthraceneFluoranthenePyrene B[a]APYRFLT NAB2D 0.0010.1 202228 4 10789 Benzo[a]anthraceneFluorantheneChrysenePyrene B[a]ACRYPYRFLT NAB2D 0.0010.010.1 202228 4 1089 Benzo[a]anthraceneChrysenePyrene B[a]ACRYPYR NAB2 0.0010.010.1 202228 4 1089 Benzo[a]anthraceneChrysenePyrene B[a]APYRCRY NAB2 0.0010.010.1 202228 4 91098 Benzo[a]anthraceneBenzo[a]anthraceneChrysenePyrene B[a]AB[a]ACRYPYR B2NAB2 0.10.0010.010.1 228228202 4 4 111089 Benzo[b]FluorantheneBenzo[a]anthraceneChrysenePyrene B[a]AB[b]FCRYPYR NAB2 0.0010.010.1 202228252 45 11109 Benzo[b]FluorantheneBenzo[a]anthraceneChrysene B[a]AB[b]FCRY B2 0.010.1 228252 45 10119 Benzo[b]FluorantheneBenzo[a]anthraceneChrysene B[a]AB[b]FCRY B2 0.010.1 228252 45 1010119 Benzo[b]Fluoranthene ChryseneBenzo[a]anthraceneChrysene CRYB[a]AB[b]FCRY B2B2 0.010.010.1 228228252 4 45 1210119 Benzo[k]FluorantheneBenzo[b]FluorantheneBenzo[a]anthraceneChrysene B[a]AB[k]FB[b]FCRY B2 0.010.1 228252 45 101211 Benzo[k]FluorantheneBenzo[b]FluorantheneChrysene B[k]FB[b]FCRY B2 0.010.1 228252 45 111012 Benzo[b]FluorantheneBenzo[k]FluorantheneChrysene B[b]FB[k]FCRY B2 0.010.1 252228 54 111012 Benzo[b]FluorantheneBenzo[k]FluorantheneChrysene B[b]FB[k]FCRY B2 0.010.1 252228 54 11 11101312Benzo[b]Fluoranthene Benzo[b]FluorantheneBenzo[k]FluorantheneBenzo(a)pyreneChrysene B[b]FB[b]FB[a]PB[k]FCRY B2B2 0.10.010.11.0 252252228 5 54 111312 Benzo[b]FluorantheneBenzo[k]FluorantheneBenzo(a)pyrene B[b]FB[a]PB[k]F B2 0.11.0 252 5 121113 Benzo[k]FluorantheneBenzo[b]FluorantheneBenzo(a)pyrene B[k]FB[b]FB[a]P B2 0.11.0 252 5 111312 Benzo[b]FluorantheneBenzo[k]FluorantheneBenzo(a)pyrene B[b]FB[a]PB[k]F B2 0.11.0 252 5 12 121413Benzo[k]Fluoranthene Indeno[1,2,3-cd]pyreneBenzo[k]FluorantheneBenzo(a)pyrene B[k]FI[123cd]PB[k]FB[a]P B2B2 0.10.11.0 252252276 5 56 121413 Indeno[1,2,3-cd]pyreneBenzo[k]FluorantheneBenzo(a)pyrene I[123cd]PB[k]FB[a]P B2 0.11.0 252276 56 131214 Indeno[1,2,3-cd]pyreneBenzo[k]FluorantheneBenzo(a)pyrene I[123cd]PB[a]PB[k]F B2 1.00.1 252276 56 121314 Indeno[1,2,3-cd]pyreneBenzo[k]FluorantheneBenzo(a)pyrene I[123cd]PB[k]FB[a]P B2 0.11.0 252276 56 1314 Indeno[1,2,3-cd]pyreneBenzo(a)pyrene I[123cd]PB[a]P B2 1.00.1 252276 56 13131514 Benzo(a)pyreneDibenzo[a,h]anthraceneIndeno[1,2,3-cd]pyreneBenzo(a)pyrene B[a]PI[123cd]PD[ah]AB[a]P B2B2 1.01.00.1 252252278276 5 56 151314 Dibenzo[a,h]anthraceneIndeno[1,2,3-cd]pyreneBenzo(a)pyrene I[123cd]PD[ah]AB[a]P B2 1.00.1 278252276 56 141315 Dibenzo[a,h]anthraceneIndeno[1,2,3-cd]pyreneBenzo(a)pyrene I[123cd]PD[ah]AB[a]P B2 0.11.0 276252278 65 1514 Dibenzo[a,h]anthraceneIndeno[1,2,3-cd]pyrene I[123cd]PD[ah]A B2 1.00.1 278276 56 1415 Dibenzo[a,h]anthraceneIndeno[1,2,3-cd]pyrene I[123cd]PD[ah]A B2 0.11.0 276278 65 14 161514Indeno[1,2,3-cd]pyrene Dibenzo[a,h]anthraceneIndeno[1,2,3-cd]pyreneBenzo[g,h,i]perylene I[123cd]PI[123cd]PB[ghi]PD[ah]A B2B2D 0.10.011.00.1 276276278 6 65 151416 Dibenzo[a,h]anthraceneIndeno[1,2,3-cd]pyreneBenzo[g,h,i]perylene I[123cd]PD[ah]AB[ghi]P B2D 0.011.00.1 278276 56 1516 Dibenzo[a,h]anthraceneBenzo[g,h,i]perylene D[ah]AB[ghi]P B2D 0.011.0 278276 56 1516 Dibenzo[a,h]anthraceneBenzo[g,h,i]perylene D[ah]AB[ghi]P B2D 0.011.0 278276 56 1516 Dibenzo[a,h]anthraceneBenzo[g,h,i]perylene D[ah]AB[ghi]P B2D 0.011.0 278276 56 15 1615Dibenzo[a,h]anthracene Dibenzo[a,h]anthraceneBenzo[g,h,i]perylene D[ah]AB[ghi]PD[ah]A B2B2D 1.00.011.0 278276278 5 65 2.1516 Experimental Dibenzo[a,h]anthraceneBenzo[g,h,i]perylene Approach D[ah]AB[ghi]P B2D 0.011.0 278276 56 2.16 Experimental Benzo[g,h,i]perylene Approach B[ghi]P D 0.01 276 6 2.16 Experimental Benzo[g,h,i]perylene Approach B[ghi]P D 0.01 276 6 2.16 Experimental Benzo[g,h,i]perylene Approach B[ghi]P D 0.01 276 6 162.16 Experimental Benzo[g,h,i]perylene Benzo[g,h,i]perylene Approach B[ghi]PB[ghi]P DD 0.010.01 276276 6 6 2.1.2. Experimental Test Fuels Approach 2.2.1. Experimental Test Fuels Approach 2.2.1. Experimental Test Fuels Approach 2.2.1. Experimental Test Fuels Approach 2.2.1. Experimental TestThe Fuels test fuels Approach included reference fossil diesel (Halter-mann Carless Ltd, UK), heptane, toluene and 2.1.2. Experimental TestThe Fuels test fuels Approach included reference fossil diesel (Halter-mann Carless Ltd, UK), heptane, toluene and 2. Experimental2.1.methyl-decanoate TestThe Fuels test Approach fuels included(Sigma Aldrich, reference UK). fossil The diesel fossil (H dieselalter-mann fuel had Carless 22.2% Ltd, total UK), aromatics heptane, and toluene 3.4% and by 2.1.methyl-decanoate TestThe Fuels test fuels included(Sigma Aldrich, reference UK). fossil The diesel fossil (H dieselalter-mann fuel had Carless 22.2% Ltd, total UK), aromatics heptane, and toluene 3.4% and by 2.1.methyl-decanoatemass TestThe of poly-aromaticFuels test fuels included(Sigma contents. Aldrich, reference Table UK). fossil 2 showsThe diesel fossil the (H propdieselalter-mannerties fuel ofhad Carless the 22.2% test Ltd, fuels. total UK), Inaromatics heptane,view of and thetoluene complex3.4% and by 2.1.massmethyl-decanoate TestThe of poly-aromaticFuels test fuels included(Sigma contents. Aldrich, reference Table UK). fossil 2 showsThe diesel fossil the (H propdieselalter-mannerties fuel ofhad Carless the 22.2% test Ltd, fuels. total UK), Inaromatics heptane,view of and thetoluene complex3.4% and by 2.1. 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Energies 2019, 12, 2575 4 of 19 a model to several of these aromatics; therefore, heptane/toluene blends have been recommended as convenient model fuels for compression ignition engines [30,31]. Methyl decanoate was chosen as a fatty acid methyl ester frequently studied as a surrogate for biodiesel. Qualitative PAH analysis of the diesel fuel and lubricating oil (Castrol Magnetic, SAE 10W-40) used in this work was carried-out using GC-MS (Gas Chromatography- Mass Spectrometry) prior to the analysis of the pyrogenic exhaust PAHs. Two PAHs (naphthalene and fluorene) were identified in the diesel fuel but no PAH was found in the fresh and used lubricating oil.

Table 2. Properties of test fuels. Point of 50% (v/v) recovery obtained according ASTM D86 [32–34].

Fuel Properties Diesel Heptane Toluene Methyl Decanoate Assay (%) - 99 >99.8 (GC) >99 Fuel nomenclature Diesel H T MD H/C ratio 1.771 2.28 1.143 2 PAH content (% mass) 3.4 - - - a Boiling point (◦C) 271.0 98.3 110.6 224 Density (g/mL, 20 ◦C) 0.835 0.684 0.867 0.871 Cetane number 52.7 54.4 7.4 51.6 Lower heating value (MJ/kg) 43.14 44.5 40.6 36.7

2.2. Experimental Layout and Conditions Mixtures of single/binary/tertiary fuels were formulated by blending toluene and methyl-decanoate into heptane. The proportions of toluene and methyl-decanoate (on volume basis) added to heptane are shown in Table3. The highest proportion of toluene added to the fuel mixtures was 22.5% and is similar to the percentage of mono-aromatics found in commercial diesel fuel [18]. Methyl decanoate was added to at a level of 20% vol/vol to a blend of 65% vol/vol n-heptane and 15% vol/vol toluene, intended to be representative of the highest level biodiesel and fossil diesel levels currently under consideration for commercial implementation. Fuel mixtures were combusted in the diesel engine and their ignition delays were noted. For all tests, the duration of ignition delay was taken as the period between the start of fuel injection and the first appearance of positive apparent net heat release rate. To assess the influence of ignition delay on exhaust PAHs, equalization of ignition delay durations for different blends was achieved by applying a given amount (in ppm) of an ignition improving additive (2-ethylhexylnitrate—2-EHN) to the fuel mixtures. The ignition improver was added at ppm levels in order not to change the bulk fuel composition. For example, it can be seen from Table3 that 850 and 1500 ppm of 2-EHN were added to 77.5%H22.5%T to reach ignition delays of 9.8 and 9.4 Crank Angle Degrees (CAD) respectively. The equalized ignition delays only differed by approximately the resolution of the engine shaft encoder (0.2 CAD). The necessary dosages of the ignition improver that was added to the heptane/toluene mixtures to equalize ignition delay were found using an iterative procedure. Two sets of experiments were conducted at a constant indicated mean effective pressure (IMEP) of 7 bar, a fixed engine speed of 1200 rpm and a constant fuel injection pressure of 450 bar:

1. Constant injection timing tests were conducted on the engine with start of fuel injection at 10 crank-angle-degrees (CAD) before-top-dead-centre (BTDC).

2. Constant ignition timing tests were conducted by maintaining the 10◦ CAD BTDC injection timing while also adding 2-EHN to the fuels to equalize the ignition delays.

The molecular structures of the fuels shown in Table1 and the experimental conditions outlined in Table3 allowed the following objectives to be achieved:

Characterize the particulates produced from the combustion of the fuel mixtures in terms of mass, • number and size using a DMS 500 instrument. Energies 2019, 12, 2575 5 of 19

Highlight the PAH distributions of fuel mixtures in (i) from the diesel engine and understand the • influence of combustion characteristics of the fuels on PAH emissions. Determine the effect of ignition improver on the concentrations of gas phase and particulate • phase PAHs. Assess the influence of ignition delays and premixed burn fraction on the total PAH concentrations. •

Table 3. Fuel blend composition and experimental matrix for the test fuels run in a compression ignition engine at a constant indicated mean effective pressure (IMEP) of 7 bar and a fixed speed of 1200 rpm.

Blend Composition Ignition Ignition Delay Blend 2-EHN Fuel/Fuel Blend Methyl Delay after Addition Number n-heptane Toluene (% Dosage Decanoate (CAD) of 2-EHN (% vol/vol) vol/vol) (ppm) (% vol/vol) (CAD) 0 Diesel start - 9.4 - - 1 100%H 100 0 0 9.4 - - 2 85%H15%T 85 15 0 9.8 400 9.4 3A 77.5%H22.5%T 77.5 22.5 0 10.8 850 9.8 3B 77.5%H22.5%T 77.5 22.5 0 10.8 1500 9.4 4 65%H15%T20%MD 65 15 20 9.8 - - 0 Diesel finish - 9.4 - -

2.3. Experimental Set-Up The tests reported in this paper were conducted on a single cylinder, 4-stroke, compression-ignition engine. The schematic of the engine assembly is shown in Figure1. Table4 lists the specifications for the engine assembly. The cylinder head (including intake and exhaust valves), piston and connecting road, were taken from a 2.0 liter four-cylinder Ford Duratorq donor engine (a direct injection diesel engine utilized extensively in EURO IV compliant applications) and were mounted on a single cylinder Ricardo Hydra crankcase. The in-cylinder gas pressure was measured to a resolution of 0.2 CAD using a Kistler 6056A piezoelectric pressure transducer in conjunction with a Kistler 5018 charge amplifier, Energies1800 pulse2019, 12 per, x FOR revolution PEER REVIEW (ppr) shaft encoder and a digital data acquisition system. 6 of 21

FigureFigure 1. 1. SchematicsSchematics of of the the engine engine sampling sampling system: system: (1) (1) hi highgh pressure pressure needle valve (2)(2) O-ringO-ring (3)(3) freefree movingmoving piston. piston.

Table 4. Engine specification.

Description Specification Bore 86 mm Stroke 86 mm Swept volume 499.56 cm3 Compression ratio (geometric) 18.3:1 Maximum in-cylinder pressure 150 bar Piston design Central ω–bowl in piston Fuel injection pump Delphi single-cam radial-piston pump High pressure common rail Delphi solenoid controlled, 1600 bar max. Diesel fuel injector Delphi DFI 1.3 six-hole solenoid valve Electronic fuel injection system 1 µs duration control Crank shaft encoder 1800 ppr, 0.2 CAD resolution Oil and coolant temperature 80 ± 2.5 °C

A conventional diesel fuel system was used as a hydraulic system to deliver pressurized diesel fuel to chamber A of the fuel vessel. The pressurized diesel fuel acted as a hydraulic fluid which created pressure in the test fuel via the moveable free piston, as shown in Figure 1. The O-rings (2), installed circumferentially on the surface of the piston, ensured that there was no mixing of test fuel and the diesel fuel. The diesel fuel was pressurized to a precise level using the Emtronix EC-GEN 500 engine control system (ECU) and the free moving piston always communicated this pressure to the test fuel. A high-pressure filter was installed between the fuel vessel and the fuel injector to prevent any damage to the injector by potential debris in the test fuel. High pressure needle valves (1) were used to switch between the fossil diesel fuel and the test-fuels, with the diesel fuel providing baseline data and also used for flushing the fuel line and injector. Pressures and temperatures were monitored in real-time and logged onto PCs using National Instruments (NI) data acquisition systems (DAQ). A NI LabVIEW program, developed in-house, evaluated the real-time in-cylinder pressure data to determine the net apparent heat release rates and IMEP. A Delphi DFI 1.3 six-hole, servo-hydraulic solenoid valve fuel injector was used to inject the liquid test and diesel fuels directly into the combustion chamber. The Emtronix engine control system

Energies 2019, 12, x; doi: FOR PEER REVIEW www.mdpi.com/journal/energies

Energies 2019, 12, 2575 6 of 19

Table 4. Engine specification.

Description Specification Bore 86 mm Stroke 86 mm Swept volume 499.56 cm3 Compression ratio (geometric) 18.3:1 Maximum in-cylinder pressure 150 bar Piston design Central ω–bowl in piston Fuel injection pump Delphi single-cam radial-piston pump High pressure common rail Delphi solenoid controlled, 1600 bar max. Diesel fuel injector Delphi DFI 1.3 six-hole solenoid valve Electronic fuel injection system 1 µs duration control Crank shaft encoder 1800 ppr, 0.2 CAD resolution Oil and coolant temperature 80 2.5 C ± ◦

The experimental set-up consisted of an existing special low volume, high pressure, fuel system which was used to deliver test fuels to the fuel injector at a common rail injection pressure previously described by Talibi et al. [35]. The fuel system facilitated the direct injection of the test fuels, which had low lubricity. Without the use of this low volume system, lubricity additives would have been required to avoid damage to the conventional common rail fuel circuit. Similarly, the use of this custom-made system, which required only low fuel volumes, reduced the cost of the fuel tests. Figure1 shows the fuel system which consists of a stainless-steel vessel with capped ends and a free moving piston (3) with circumferential O-rings (2). The circumferential O-rings divided the high-pressure fuel vessel into two chambers (A and B). Chamber ‘A’ contained diesel fuel, while ‘B’ contained the test fuel. A conventional diesel fuel system was used as a hydraulic system to deliver pressurized diesel fuel to chamber A of the fuel vessel. The pressurized diesel fuel acted as a hydraulic fluid which created pressure in the test fuel via the moveable free piston, as shown in Figure1. The O-rings (2), installed circumferentially on the surface of the piston, ensured that there was no mixing of test fuel and the diesel fuel. The diesel fuel was pressurized to a precise level using the Emtronix EC-GEN 500 engine control system (ECU) and the free moving piston always communicated this pressure to the test fuel. A high-pressure filter was installed between the fuel vessel and the fuel injector to prevent any damage to the injector by potential debris in the test fuel. High pressure needle valves (1) were used to switch between the fossil diesel fuel and the test-fuels, with the diesel fuel providing baseline data and also used for flushing the fuel line and injector. Pressures and temperatures were monitored in real-time and logged onto PCs using National Instruments (NI) data acquisition systems (DAQ). A NI LabVIEW program, developed in-house, evaluated the real-time in-cylinder pressure data to determine the net apparent heat release rates and IMEP. A Delphi DFI 1.3 six-hole, servo-hydraulic solenoid valve fuel injector was used to inject the liquid test and diesel fuels directly into the combustion chamber. The Emtronix engine control system was used to control the injection pressure, injection timing and duration of injection. The intake air flow rate was measured using a Romet G65 positive displacement volumetric air flow meter.

2.4. Generation and Sampling of Particulate and Gas Phase PAHs Particulates and gas phase PAHs generated within the engine cylinder were sampled from the engine exhaust. Particulates phase (PP) and gas phase (GP) PAHs were sampled about 1000 mm downstream of the engine exhaust by means of a 12 mm diameter stainless steel tube connected to a vacuum pump, which provided a flow rate of 40 L/min through a particulate and gaseous PAH collection system (see Figure1). To avoid condensation of gas phase PAHs and water vapor during sampling, the sampling probe was thermally insulated. The temperature of the soot sampling stainless steel probe that led to the filter housing was measured by K-type thermocouple and was always found be > 100 ◦C. Particulate filters and XAD-2 resin were employed in series, as previously used in a Energies 2019, 12, 2575 7 of 19 tube reactor reported in Dandajeh et al. [36], to collect the particulate and gaseous PAHs respectively. The cumulative volume of gas (Vg) that passed through the particulate filter and the XAD-2 resin was measured, at each test condition, using a volumetric gas meter (Bell flow Systems UK). The meter was at atmospheric pressure and temperature which were also monitored, allowing calculation of gas volume at STP (Standard Temperature and Pressure). The particulate samples were collected on a glass micro fiber filter (70 mm diameter and 0.7 µm pore size) (Fisher Scientific UK). Before sampling, the filter was dried in a desiccator for 12 h and weighed using a high precision mass balance ( 0.001 mg). After sampling, the filter was re-dried ± in the desiccator for the same duration and re-weighed. The particulate mass collected on the filter (Ms) was recorded. The gravimetric filter mass measurements (Ms, mg) from the engine exhaust and 3 calculated soot mass concentration (Ms/Vg, mg/ m ) of all the fuels tested are shown in Tables5 and6, 3 where Vg (m ) is the cumulative volume of gas that passed through the particulate and resin measured at each test condition using the cumulative volumetric gas meter. To avoid deterioration of the filter in the high temperature pulsating flow of the engine exhaust, the filter was supported by sandwiching it between two stainless steel wire meshes (0.026 mm aperture, 0.025 mm wire diameter) (The Mesh Company, UK), which were cut to the same diameter as the filter. Particulates in the diesel engine exhaust were characterized at sub-micron levels using a fast differential mobility spectrometer (Cambustion DMS-500). The DMS-500 was used to determine real time outputs of the size, mass and number distributions of particulates in the range of 5–1000 nm. The exhaust gas was sampled about 300 mm downstream of the engine exhaust valves and conveyed to the particulate analyzer (DMS500) via a heated line maintained at 80 ◦C. Tables5 and6 show the summary of the soot particle mean diameter and particle surface area as measured by the DMS 500 instrument. Engine test and subsequent PAH analysis for each test fuel was repeated twice and the average value of the test values is reported in this paper. A reference fossil diesel fuel was tested at the start and end of each daily test. These repeats produced a record of daily variabilities in engine performance and instrumentation as well as provision of baseline data for comparison with the results of the fuel blends. The cumulative standard deviations (δ) of the parameters investigated were thus generated from results of the daily fossil diesel fuel repeats.

Table 5. Filter soot mass measurements without 2-EHN ignition improver.

Soot Particle Mean Soot Mass (Ms) Soot Surface Area Fuel Concentration Diameter (Dp) 2 3 (mg) 3 (µm /cm ) (Ms/Vg) (mg/m ) (nm) 100%H 23.0 48.0 176 2,720,005 85%H15%T 19.3 40.0 178 1,865,106 77.5%H22.5%T 23.5 45.0 181 2,367,265 65%H15%T20%MD 6.90 13.7 160 1,210,826 Reference 28.7 58.0 177 1,846,044 diesel

Table 6. Filter soot mass measurements with 2-EHN ignition improver.

Soot Particle Mean Soot Mass Soot Surface Area Fuel + 2-EHN (ppm) Concentration Diameter (Dp) 2 3 (Ms) (mg) 3 (µm /cm ) (Ms/Vg) (mg/m ) (nm) 77.5%H22.5%T + 1500 ppm 13.7 28.9 171 1,889,521 85%H15%T + 400 ppm 16.6 34.3 171 1,833,473 77.5%H22.5%T + 850 ppm 14.8 30.8 178 1,841,702

2.5. Sample Preparation/GC-MS Analysis The sample preparation (sample extraction and solvent evaporation) and GC-MS analysis has been described in detail previously in Dandajeh et al. [36,37]. The extraction of the PAHs from the Energies 2019, 12, 2575 8 of 19 soot particles and XAD-2 resin samples was carried out using accelerated solvent extraction (ASE) in dichloromethane as a solvent. The solvent was evaporated by bubbling nitrogen stream into the PAH extracts. The PAHs from the extracts were then identified using gas chromatography (GC) and thereafter analyzed using mass spectrometry (MS).

3. Results and Discussion

3.1. Soot Formation Characteristics at Constant Injection and Ignition Timings Energies 2019, 12, x FOR PEER REVIEW 8 of 21 The profiles of soot particle number concentrations for the test fuels, sampled using the DMS500 particle3.1. Soot spectrometer formation characteristics are shown at constant in Figure injection2. Figureand ignition2a,b timings presents the distributions of soot particle numberThe concentrations profiles of soot particle for the numb fuelser without concentrations and with for the 2-EHN, test fuels, respectively. sampled using Figure the DMS5002a shows that the sootparticle particle spectrometer sizes for are all theshown fuels in Figure ranged 2. fromFigure 30 2a,b to 360presents nm withoutthe distri 2-EHNbutions of and, soot similarly, particle the range wasnumber 13–316 concentrations nm when 2-EHNfor the fuels was without added toand the with fuel 2-EHN, blends. respectively. Figure 2a shows that the soot particle sizes for all the fuels ranged from 30 to 360 nm without 2-EHN and, similarly, the range µ 2 3 wasThe 13–316 mean nm sootwhen particle 2-EHN was size added and theto the soot fuel volume-normalized blends. surface area ( m /cm ) of each test fuel withoutThe mean and soot with particle 2-EHN size areand alsothe soot shown volume-normalized Tables5 and6 respectively.surface area (µm2/cm3) of each test fuelIt without is apparent and with from 2-EHN Table are5 thatalso shown toluene Tables addition 5 and to 6 respectively. heptane slightly increased the mean size of the soot particlesIt is apparent generated; from Table from 5 that 176 toluene nm for addition 100%H to to heptane 178 and slightly 181 nm increased for 85%H15%T the mean size and of 77.5%H22.5%T, the respectively.soot particles The generated; correlation from 176 coe nmffi cientfor 100%H (r) for to 178 the and percentage 181 nm for of 85%H15%T toluene addition and 77.5%H22.5%T, as a function of soot respectively. The correlation coefficient (r) for the percentage of toluene addition as a function of soot particle mean size was high (r = 0.95), which suggests a strong statistical relationship. Increasing soot particle mean size was high (r = 0.95), which suggests a strong statistical relationship. Increasing soot meanmean particle particle sizessizes subsequent subsequent to totoluene toluene addition addition to heptane to heptane was also was reported also reportedby Wei et byal. [31] Wei for et al. [31] for in-cylinderin-cylinder sootsoot generated from from toluene/n-heptane toluene/n-heptane combustion combustion in a diesel in aengine. diesel Table engine. 5 also Table shows5 also shows thethe gravimetric gravimetric filterfilter measur measurementsements of soot of soot masses masses (Ms) (Mands )calculated and calculated soot concentrations soot concentrations (Ms/Vg) of (Ms/Vg) of the test fuels as sampled from the engine exhaust. Vg is the cumulative volume of gas that passed the test fuels as sampled from the engine exhaust. Vg is the cumulative volume of gas that passed throughthrough the the particulate particulate filter filter and and the the XAD-2 XAD-2 resin resin in series. in series. It is pertinent It is pertinent to note here to notethat the here trend that of the trend of the soot mass concentration based on the gravimetric filter soot mass measurements is consistent with the soot mass concentration based on the gravimetric filter soot mass measurements is consistent with the trend of soot mass concentration measured using the DMS 500 instrument. the trend of soot mass concentration measured using the DMS 500 instrument.

3 FigureFigure 2.2. DistributionDistribution of of soot soot particle particle numb numberer concentrations concentrations (dN/dLogDp (dN/dLogDp (cm-3)) for (cm the− fuel)) for blends the fuel blends andand reference reference diesel diesel fuel fuel (a) ( awithout) without 2-ethylhexylnitrate 2-ethylhexylnitrate (2-EHN) (2-EHN) (b) with 2-EHN. (b) with 2-EHN.

Table5 showsTable that 5. theFilter soot soot mass mass measurements concentration without of 2-EHN heptane ignition/toluene improver. blend increased from 40 to 3 45 mg/m when the proportionSoot of tolueneSoot in heptane was increased from 15% to 22.5%. The soot Particle mean Soot surface concentration of 48 mg/m3 formass 100%H wasconcentration unexpectedly high and should be interpreted with caution. Fuel diameter area (Ms) (Ms/Vg) Table5 shows a drastic reduction in soot concentration(D byp) (nm) 66%, mean(µm particle2/cm3) size by 10% and (mg) (mg/m3) soot surface area by 35%, when the 20% (by volume) of a C7 heptane in 85%H15%T blend was 100%H 23.0 48.0 176 2,720,005 replaced with 20% (by volume) of a C methyl-deconoate (an ester molecule). This result was as 85%H15%T 19.3 11 40.0 178 1,865,106 expected since77.5%H22.5%T oxygenated fuels,23.5 such as 45.0alcohols and esters, 181 were reported 2,367,265 to have substantially 65%H15%T20%MD 6.90 13.7 160 1,210,826 Reference diesel 28.7 58.0 177 1,846,044

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Energies 2019, 12, 2575 9 of 19

lower particulate emissions when burned in internal combustion engines, compared with their alkanes/aromatic counterparts [38,39]. Generally, pyrolysis of n-heptane leads to generation of large quantities of C2-C6 species such as acetylene and propargyl radicals which are highly instrumental in the formation of the first aromatic ring [15,40] and subsequent PAH/soot surface growth. Additional support for this finding comes from the study of Alexiou and Williams [41] using reflected shock-tube pyrolysis, who reported a decrease in the sooting propensity of toluene, when heptane was blended into it. Toluene is a prolific soot producer owing to its nucleation rate increasing exponentially with temperature [42]. In diffusion flames, Ladommatos et al. [43] reported the soot propensity of toluene being substantially higher than heptane, with toluene having a threshold sooting index of 18.5 times higher than that of heptane. Table6 shows that adding 2-EHN to the heptane /toluene fuel blend decreased the mean soot particle diameter, soot surface area and, surprisingly, soot mass concentration. The decreasing trend of soot concentration after adding ignition improver is contrary to what was reported in earlier works [43,44]. This was partly because the experimental conditions employed in this work were different from those of Kidoguchi et al. [44] and Ladommatos et al. [45], who reported an increase in soot mass concentration when ignition improver was added to the fuel.

3.2. PAH Distributions from Diesel Engine Exhaust at Constant Injection Timing This section focuses on the analysis of the EPA 16 priority PAHs collected from the diesel engine exhaust at constant fuel injection timing of 10 CAD before top dead centre (BTDC), speed of 1200 rpm and load of 7 bar. Figure3 shows the distributions of exhaust PAHs following combustion of the fuels studied. Measured PAHs in the exhaust included both those in the gaseous phase as well as those on the particulate surfaces. The gaseous PAHs shown in Figure3, were those extracted from the XAD-2 resin, and are designated as gas phase (GP) PAHs, whereas those PAHs extracted from the soot particles are designated as particle phase (PP) PAHs. Energies 2019, 12, x FOR PEER REVIEW 10 of 21

Figure Figure3. The distributions 3. The distributions of exhaust of PAHs exhaust in diesel PAHs engine: in diesel (a) Gas engine: Phase (a )(GP, Gas µg Phase of PAH/m (GP, µ3 gof of gas), PAH (b/)m 3 of gas), Particle( bPhase) Particle (PP, µg Phase of PAH/m (PP, µ3 ofg ofgas), PAH (c) /Particlem3 of gas),Phase ( c(ng) Particle of PAH/mg Phase of soot) (ng of and PAH (d) /totalmg ofPAHs soot) (GP and + (d) total PP, µg ofPAHs PAH/m (GP3 of+ gas).PP, µ g of PAH/m3 of gas).

In Figure 3a, the most abundant exhaust PAHs found in the gas phase (GP) had two and three rings, irrespective of the fuel combusted. These PAHs included NPH, ACY, ACN, FLU, PHN and ATR. It is interesting to note the six most abundant PAHs listed above are group D members and, therefore considered to be non-mutagenic. The total sum of these six PAHs in the GP accounted for the bulk of the total PAHs in the case of all the fuels. For example, 91% in the case of the fuel 100%H, 94% in the case of the fuel 85%H15%T, 91% in the case of the fuel 77.5%H22.5%T, 80% in the case of the fuel 65%H15%T20%MD and 83% in the case of the fossil diesel fuel. These results of high proportions of the six PAHs (NPH, ACY, ACN, FLU, PHN and ATR) are consistent with those observed by He et al. [46] in a diesel engine fueled with biodiesel and diesel. It is interesting to note that in Figure 3a, only four heavier PAHs having four and five rings (CRY, B(b)F, B(k)F and B(a)P) were detected in the GP in low concentrations, and these four PAHs are highly mutagenic (Group B2 members). It can be further observed in Figure 3a that the GP PAH concentrations decreased to near-zero when the number of PAH rings increased from five to six regardless of the fuel investigated. The observation of decreasing PAH concentrations in the GP with increasing molecular weight of the PAHs (increasing number rings) resembled those observed by Dandajeh et al. [37] in a tube reactor at a temperature of 1050 °C and under oxygen free pyrolysis conditions. Figure 3b shows that the six smaller PAHs with two to three rings (NPH, ACY, ACN, FLU, PHN and ATR) detected earlier in the gas phase at high concentrations (see Figure 3a) were now identified in the particle phase at very low concentrations (lower by several orders of magnitude). These results suggest that the six lighter PAHs might have been consumed in the growth of larger PAHs and subsequently formed soot. Conversely, Figure 3b shows that the concentrations of PP PAHs increased with increasing molecular weight of the PAH regardless of the fuel combusted. This result also matched with observations in tube reactors in [36,47]. Figure 3c presents the concentration of PAHs adsorbed on the soot particles per unit mass of soot. It can be seen from Figure 3c that the PP PAH mass increased with increasing proportions of toluene

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Energies 2019, 12, 2575 10 of 19

The mass of gas phase PAHs was normalized with the volume of gas (Vg) passed through both the filter and the XAD-2 resin (positioned in series with the filter) and is shown in Figure3a. The mass of PAH extracted from the soot particles was also normalized with volume of gas (Vg) passed through the filter and the XAD-2 resin and is shown in Figure3b. Figure3c shows the PAH mass extracted from the soot particles but this time normalized with the soot particle mass (Ms) (see Table5). Lastly, Figure3d shows the total PAH (sum of gas and particle phase) and this was normalized by the volume of gas (Vg) passed through the filter and the XAD-2 resin. In Figure3a, the most abundant exhaust PAHs found in the gas phase (GP) had two and three rings, irrespective of the fuel combusted. These PAHs included NPH, ACY, ACN, FLU, PHN and ATR. It is interesting to note the six most abundant PAHs listed above are group D members and, therefore considered to be non-mutagenic. The total sum of these six PAHs in the GP accounted for the bulk of the total PAHs in the case of all the fuels. For example, 91% in the case of the fuel 100%H, 94% in the case of the fuel 85%H15%T, 91% in the case of the fuel 77.5%H22.5%T, 80% in the case of the fuel 65%H15%T20%MD and 83% in the case of the fossil diesel fuel. These results of high proportions of the six PAHs (NPH, ACY, ACN, FLU, PHN and ATR) are consistent with those observed by He et al. [46] in a diesel engine fueled with biodiesel and diesel. It is interesting to note that in Figure3a, only four heavier PAHs having four and five rings (CRY, B(b)F, B(k)F and B(a)P) were detected in the GP in low concentrations, and these four PAHs are highly mutagenic (Group B2 members). It can be further observed in Figure3a that the GP PAH concentrations decreased to near-zero when the number of PAH rings increased from five to six regardless of the fuel investigated. The observation of decreasing PAH concentrations in the GP with increasing molecular weight of the PAHs (increasing number rings) resembled those observed by Dandajeh et al. [37] in a tube reactor at a temperature of 1050 ◦C and under oxygen free pyrolysis conditions. Figure3b shows that the six smaller PAHs with two to three rings (NPH, ACY, ACN, FLU, PHN and ATR) detected earlier in the gas phase at high concentrations (see Figure3a) were now identified in the particle phase at very low concentrations (lower by several orders of magnitude). These results suggest that the six lighter PAHs might have been consumed in the growth of larger PAHs and subsequently formed soot. Conversely, Figure3b shows that the concentrations of PP PAHs increased with increasing molecular weight of the PAH regardless of the fuel combusted. This result also matched with observations in tube reactors in [36,47]. Figure3c presents the concentration of PAHs adsorbed on the soot particles per unit mass of soot. It can be seen from Figure3c that the PP PAH mass increased with increasing proportions of toluene added to heptane. This result can be anticipated since adding toluene to heptane could create more PAH formation routes which aid PAH growth. The trend in Figure3c further exhibits a substantial increase in the PAH mass produced from diesel fuel combustion. Abundance of PAHs produced from diesel combustion is not surprising, since the fossil diesel used in this work had up to 3.4% PAHs (including NPH, FLU and other alkylated PAHs). It is likely that the PAHs in the diesel fuel contributed to the pyrogenically formed exhaust PAHs. Some evidence to support this suggestion is the fact that the total NPH and FLU concentrations out of the total PAHs identified from diesel fuel combustion were unusually high; 34% in the case of NPH and 10% in the case of FLU. The implication of the above findings could be that some of the naphthalene and fluorene originally present in the fossil diesel could have survived combustion. Several studies have highlighted the possibility of petrogenic PAHs in a fuel surviving combustion. For example, Rhead and Pemberton [48] observed that 23.8% of the NPH detected in the exhaust of a direct injection diesel engine originated from the fuel combusted. Similarly, Tancell et al. [2] also reported high PAH survival rates of FLU (0.87%) and NPH (0.47%); and the survival rate of each PAH was found to proportionally increase with the energy level of the lowest un-occupied molecular orbital. The concentrations of total PAH per unit volume of gas is shown in Figure3d, and the figure corresponds to the sum of the gas phase PAHs in Figure3a and the particle phase PAHs in Figure3b. It can be seen from Figure3d that the trend of the total PAH mass resembles to a significant extent, Energies 2019, 12, 2575 11 of 19 those of the GP PAHs in Figure3a regardless of the fuel tested. It is also apparent from Figure3d that the six PAHs (NPH, ACY, ACN, FLU, PHN and ATR), which were most abundant in the gas phase, also dominated the total PAH distribution. Four out of these six PAHs (NPH, FLU, PHN and ATR) constituted over 75% of the PAHs identified in the diesel combustion and, interestingly, this finding corresponds with those observed by Jin et al. [49] who reported that 50% to 92% of the exhaust PAHs of a heavy-duty diesel engine had two to three rings. The observation that two to three ring PAHs dominated the exhaust PAHs is also in agreement with the findings reported by Wang et al. [50], who researched in-cylinder and exhaust PAHs in a diesel engine fueled with heptane and heptane/toluene blends.

3.3. Total PAH Analysis at Constant Injection Timing Figure4 shows the total PAHs obtained by summing up all the EPA 16 priority PAHs produced for each fuel. The error bars denote standard deviations. Figure4a shows the concentrations of total GP PAHs. It can be seen from the figure that the GP PAH mass for the 100%H was 1906 µg/m3. The PAH mass concentration decreased linearly to 1336 and 1067 µg/m3 when 15% and 22.5% of toluene were, respectively,Energies 2019, blended12, x FOR PEER into heptane.REVIEW 12 of 21

Figure 4. Normalized total PAH concentrations in diesel engine: (a) Gas Phase (µg of PAH/m3 of gas), (bFigure) Particle 4. PhaseNormalized (µg of total PAH /PAHm3 of concentrations gas), (c) Particle in Phasediesel (ngengine: of PAH (a)/ Gasmg ofPhase soot) (µg and of ( dPAH/m) total PAH3 of gas), (µ(gb of) Particle PAH/m Phase3 of gas). (µg of PAH/m3 of gas), (c) Particle Phase (ng of PAH/ mg of soot) and (d) total PAH (µg of PAH/m3 of gas). The total PP PAHs in Figure3b shows increasing PP PAH concentrations with increasing proportion of tolueneThe addedtotal PP into PAHs heptane. in Figure A comparison 3b shows between increasing Figure PP4 a,bPAH suggest concentrations that toluene with addition increasing suppressesproportion the of formationtoluene added of exhaust into heptane. gaseous A PAHs, comparison and at between the same Figure time promotes4a,b suggest the that PAH toluene to proceedingaddition suppresses onto particulate. the formation In order words,of exhaust the GPgaseou PAHss PAHs, were more and rapidlyat the same consumed time promotes and converted the PAH toto soot, proceeding following onto toluene particulate. addition toIn heptane.order words, For example, the GP thePAHs PP PAHwere concentration more rapidly was consumed 60 µg/m 3and inconverted the case of to 100%H,soot, following and it later toluene increased addition by to 48% heptane. and 67% For respectively, example, the when PP PAH 15% concentration and 22.5% of was toluene60 µg/m were3 in the respectively case of 100%H, blended and into it later heptane. increased by 48% and 67% respectively, when 15% and 22.5% of tolueneOne unanticipated were respectively finding blended that emerged into heptane. in Figure 4a,b is that the fuel blends of 77.5%H22.5%T and 65%H15%T20%MDOne unanticipated exhibited finding that roughly emer similarged in concentrationsFigure 4a,b is that of GP the and fuel PP blends PAHs of on 77.5%H22.5%T volume of and 65%H15%T20%MD exhibited roughly similar concentrations of GP and PP PAHs on volume of gas basis, considering the standard deviations. It is difficult to explain these results, but it might be related to the resulting opposing effects of both increasing the alkyl chain length from seven to 10 when displacing heptane with methyl decanoate in the heptane/toluene blend while also introducing oxygen to the fuel blend. The particle phase PAH mass from diesel fuel combustion shown in Figure 4b was found to be higher than those from 100%H and in fact, this was the case for all the blends. The result for diesel PAH mass is consistent with observations from previous studies [28]. The propensity of the fossil diesel fuel used to form both soot and PP PAHs is believed to be attributable to the fossil diesel fuel having 22.2% total aromatic and 3.4% PAH content, which included several single ring aromatics (benzene and toluene) and alkylated PAHs (methyl-naphthalenes). The aromatics, originally from the diesel fuel, could have facilitated the formation of particle phase PAHs and soot, by serving as precursors to soot formation processes. Comparing Figure 4b and Table 5, the particle phase PAH concentrations, regardless of the fuel combusted, are strongly positively correlated with the soot particle sizes onto which the PAHs were attached. Further comparison shows a negative correlation between the PP PAHs Figure in 4b and the soot surface area (µm2/cm3) (see Table 5) on to which the PAHs were deposited. These results suggest that more PP PAHs were present when particle surface area decreased, possibly due to decrease in soot

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Energies 2019, 12, 2575 12 of 19 gas basis, considering the standard deviations. It is difficult to explain these results, but it might be related to the resulting opposing effects of both increasing the alkyl chain length from seven to 10 when displacing heptane with methyl decanoate in the heptane/toluene blend while also introducing oxygen to the fuel blend. The particle phase PAH mass from diesel fuel combustion shown in Figure4b was found to be higher than those from 100%H and in fact, this was the case for all the blends. The result for diesel PAH mass is consistent with observations from previous studies [28]. The propensity of the fossil diesel fuel used to form both soot and PP PAHs is believed to be attributable to the fossil diesel fuel having 22.2% total aromatic and 3.4% PAH content, which included several single ring aromatics (benzene and toluene) and alkylated PAHs (methyl-naphthalenes). The aromatics, originally from the diesel fuel, could have facilitated the formation of particle phase PAHs and soot, by serving as precursors to soot formation processes. Comparing Figure4b and Table5, the particle phase PAH concentrations, regardless of the fuel combusted, are strongly positively correlated with the soot particle sizes onto which the PAHs were attached. Further comparison shows a negative correlation between the PP PAHs in Figure4b and the soot surface area (µm2/cm3) (see Table5) on to which the PAHs were deposited. These results suggest that more PP PAHs were present when particle surface area decreased, possibly due to decrease in soot mass concentration or an increase in mean particle diameter. Another possibility is that PAHs proceeded to soot rapidly, reducing their concentration in the gas phase for condensation onto soot particles. Figure4c shows the mass of PAHs (mostly group B2) extracted from the soot particles normalized with the mass of soot particles (see Table5). The trends in the Figure4c appear to correlate positively with those of peak in-cylinder pressures and peak heat release rates. However, this trend does not seem to apply in the case of 65%H15%T20%MD blend. The total PAH concentrations shown in Figure4d are the result of summing-up the GP and PP PAH concentrations. Comparing Figure4a,b,d, suggests that the GP PAHs dominated the total PAHs in the diesel engine, regardless of the fuel combusted. For example, the proportion of GP PAHs generated during combustion of 100%H was 97%, while this proportion decreased to 94% and 91% when 15% and 22.5% of toluene were, respectively, blended into heptane, suggesting greater propensity of toluene towards PP PAHs. Combustion of fossil diesel generated only 88% of the GP PAHs. Similar proportions of GP PAHs were reported by He et al. [46] in a diesel engine fueled with diesel and biodiesel, as well as by Jin et al. [49] in a heavy duty diesel engine. Looking at Figure4d, a clear decreasing trend of PAH concentrations can be seen with increasing proportions of toluene added to heptane. This result is in line with previous studies [50].

3.4. Effect of Ignition Delay and Premixed Phase on Total PAH Emissions Figure5a,b shows the variations of total PAHs with ignition delay and percentage premixed phase respectively. It is apparent from Figure5a that increase in ignition delay due to corresponding increase in the proportion of toluene added to heptane was associated with decreased total PAH mass concentration. For example, the total PAH concentration for 100%H was 1966 µg/m3 and it decreased by 27% when the ignition delay was increased from 9.4 to 9.8 CAD in the case of 85%H15%T. This trend saw a further reduction by 41% in the total PAH emissions when the ignition delay was increased from 9.8 to 10.8 CAD in the case of 77.5%H22.5%T. The finding that the total PAH emission decreased with increasing ignition delay could be explained by the fact that the maximum average in-cylinder temperature was also observed to increase with increasing ignition delay [38]. This could be expected, since the total PAHs were reported to decrease with increasing temperatures [36,47]. Further extrapolation from these results would suggest that the relatively low in-cylinder temperature inhibits the consumption of PAHs. It is somewhat surprising that combusting 65%H15%T20%MD does not seem to affect the total PAH concentration significantly from those of 77.5%H22.5%T, even though the ignition delay was increased by 0.4 CAD. Energies 2019, 12, x FOR PEER REVIEW 13 of 21 mass concentration or an increase in mean particle diameter. Another possibility is that PAHs proceeded to soot rapidly, reducing their concentration in the gas phase for condensation onto soot particles. Figure 4c shows the mass of PAHs (mostly group B2) extracted from the soot particles normalized with the mass of soot particles (see Table 5). The trends in the Figure 4c appear to correlate positively with those of peak in-cylinder pressures and peak heat release rates. However, this trend does not seem to apply in the case of 65%H15%T20%MD blend. The total PAH concentrations shown in Figure 4d are the result of summing-up the GP and PP PAH concentrations. Comparing Figure 4a,b,d, suggests that the GP PAHs dominated the total PAHs in the diesel engine, regardless of the fuel combusted. For example, the proportion of GP PAHs generated during combustion of 100%H was 97%, while this proportion decreased to 94% and 91% when 15% and 22.5% of toluene were, respectively, blended into heptane, suggesting greater propensity of toluene towards PP PAHs. Combustion of fossil diesel generated only 88% of the GP PAHs. Similar proportions of GP PAHs were reported by He et al. [46] in a diesel engine fueled with diesel and biodiesel, as well as by Jin et al. [49] in a heavy duty diesel engine. Looking at Figure 4d, a clear decreasing trend of PAH concentrations can be seen with increasing proportions of toluene added to heptane. This result is in line with previous studies [50].

3.4. Effect of Ignition delay and premixed phase on total PAH emissions Figure 5a,b shows the variations of total PAHs with ignition delay and percentage premixed phase respectively. It is apparent from Figure 5a that increase in ignition delay due to corresponding increase in the proportion of toluene added to heptane was associated with decreased total PAH mass concentration. For example, the total PAH concentration for 100%H was 1966 µg/m3 and it decreased by 27% when the ignition delay was increased from 9.4 to 9.8 CAD in the case of 85%H15%T. This trendEnergies saw2019 a ,further12, 2575 reduction by 41% in the total PAH emissions when the ignition delay was 13 of 19 increased from 9.8 to 10.8 CAD in the case of 77.5%H22.5%T.

Figure 5. Correlation of total PAH (µg PAH/m3 gas) with (a) ignition delay (Crank Angle Degrees, FigureCAD) 5. and Correlation (b) premixed of total phase PAH (%).(µg PAH/m3 gas) with (a) ignition delay (Crank Angle Degrees, CAD) and (b) premixed phase (%). In addition, also apparent from Figure5b is the decreasing trend of total PAH concentration with increasingThe finding percentage that the of premixedtotal PAH phase. emission For example,decreased it with can beincreasing observed ignition from Figure delay5b thatcould at be lower explainedpremixed by burnt the fact fraction that the of maximum 61% for example, average in-c combustionylinder temperature of 100%H producedwas also observed the highest to increase PAH mass, withwith increasing the PAHs ignition presumably delay made [38]. fromThis significantcould be expected, concentration since ofthe free total radicals PAHsgenerated were reported via heptane to decreasepyrolysis with at relativelyincreasing low temperatures in-cylinder [36,47]. temperatures. Further Asextrapolation the proportion from ofthese the premixedresults would phase suggest increased that the relatively low in-cylinder temperature inhibits the consumption of PAHs. It is somewhat and the diffusion phase decreased, it can be seen from Figure5b that the total PAH concentration also surprising that combusting 65%H15%T20%MD does not seem to affect the total PAH concentration decreased. This result matched those observed by Wang et al. [50]. Wang et al. reported a decrease in significantly from those of 77.5%H22.5%T, even though the ignition delay was increased by 0.4 CAD. total in-cylinder PAH mass while the combustion mode shifted from premixed to diffusion and finally In addition, also apparent from Figure 5b is the decreasing trend of total PAH concentration with to late combustion phase, as crank angle progressed after TDC from 0 to 40 CAD. increasing percentage of premixed phase. For example, it can be observed from Figure 5b that at lower Figure6 presents the profiles of individual total concentrations of group D PAHs (PAHs with low Energiestoxicity 2019 factors), 12, x; doi: with FOR percentage PEER REVIEW premixed phase. It can be seen from thewww.mdpi.com/journal/energies figure that the concentration of the individual PAHs decreased with increasing percentage premixed burn fraction. Premixed phase is characterized by high temperature regime, where fuel and air react in a pre-mixed near-stoichiometric fuel air mixture [3]. Lower premixed percentage therefore signifies pockets of richer mixture and higher PAHs formed, since the concentration of PAH increases with increasing fuel concentration [51]. Closer inspection of Figure6 shows progressive reduction in the PAH mass while increasing the number of PAH rings (from two to four rings) regardless of the premixed phase percentage. Figure7 shows the profiles of individual total concentrations of the group B2 PAHs (PAHs with high toxicity factors) with percentage premixed phase. The figure shows a decreasing trend of PAH mass with increasing percentage premixed phase, but at markedly lower concentrations (by a factor of 16) as compared with those in Figure6 for group D PAHs. One striking feature of PAH concentrations at premixed phase of 70% is the low PAH mass in Figure6 and comparatively higher PAH mass in Figure7. This point is not likely to be an outlier, as it corresponds to a fuel blend with a slightly different molecular configuration due to addition of 20% methyl-decanoate to the heptane/toluene blend. The effects of the low and high PAHs concentrations at premixed phase of 70% in Figures6 and7 respectively, were o ffset in the total PAH concentrations shown in Figure5b. Energies 2019, 12, x FOR PEER REVIEW 14 of 21 premixed burnt fraction of 61% for example, combustion of 100%H produced the highest PAH mass, with the PAHs presumably made from significant concentration of free radicals generated via heptane pyrolysis at relatively low in-cylinder temperatures. As the proportion of the premixed phase increased and the diffusion phase decreased, it can be seen from Figure 5b that the total PAH concentration also decreased. This result matched those observed by Wang et al. [50]. Wang et al. reported a decrease in totalEnergies in-cylinder2019, 12, 2575 PAH mass while the combustion mode shifted from premixed to diffusion and14 finally of 19 to late combustion phase, as crank angle progressed after TDC from 0 to 40 CAD.

Figure 6. Profiles of Group D individual total PAH (µg PAH/m3 gas) with premixed phase (%) Figure(a) naphthalene, 6. Profiles (bof) acenaphthene,Group D individual (c) fluoranthene total PAH and (µg (d PAH/m) pyrene.3 gas) with premixed phase (%) (a) Energiesnaphthalene, 2019, 12, x FOR (b) PEER acenaphthene, REVIEW (c) fluoranthene and (d) pyrene. 15 of 21

Figure 6 presents the profiles of individual total concentrations of group D PAHs (PAHs with low toxicity factors) with percentage premixed phase. It can be seen from the figure that the concentration of the individual PAHs decreased with increasing percentage premixed burn fraction. Premixed phase is characterized by high temperature regime, where fuel and air react in a pre-mixed near- stoichiometric fuel air mixture [3]. Lower premixed percentage therefore signifies pockets of richer mixture and higher PAHs formed, since the concentration of PAH increases with increasing fuel concentration [51]. Closer inspection of Figure 6 shows progressive reduction in the PAH mass while increasing the number of PAH rings (from two to four rings) regardless of the premixed phase percentage.

3 EnergiesFigure 2019, 12 7., x; Profilesdoi: FOR ofPEER Group REVIEW B2 individual total PAH (µg PAH/m gas) withwww.mdpi.com/journal/energies premixed phase (%) 3 Figure(a) chrysene, 7. Profiles (b) benzo(a)anthracene,of Group B2 individual (c) benzo(k)fluoranthene total PAH (µg PAH/m and (gas)d) benzo(a)pyrene. with premixed phase (%) (a) chrysene, (b) benzo(a)anthracene, (c) benzo(k)fluoranthene and (d) benzo(a)pyrene. 3.5. PAH Distribution at Constant Ignition Timing and Constant Ignition Delay FigureFigure 78 showspresents the the profiles total PAHof individual distribution total at concentrations constant ignition of the delay. group Figure B2 PAHs8a,b shows(PAHs the with hightotal toxicity PAH distribution factors) with at percentage ignition delays premixed of 9.4 ph andase. 9.8 The CAD, figure respectively. shows a decreasing It can be seen trend from of PAH the massfigure with that increasing equalizing percentage the ignition premixed delays forphase, the but heptane at markedly/toluene lower blends concentrations did not result (by in a identical factor of 16) as compared with those in Figure 6 for group D PAHs. One striking feature of PAH concentrations at premixed phase of 70% is the low PAH mass in Figure 6 and comparatively higher PAH mass in Figure 7. This point is not likely to be an outlier, as it corresponds to a fuel blend with a slightly different molecular configuration due to addition of 20% methyl-decanoate to the heptane/toluene blend. The effects of the low and high PAHs concentrations at premixed phase of 70% in Figures 6 and 7 respectively, were offset in the total PAH concentrations shown in Figure 5b.

3.5. PAH distribution at constant ignition timing and constant ignition delay Figure 8 presents the total PAH distribution at constant ignition delay. Figure 8a,b shows the total PAH distribution at ignition delays of 9.4 and 9.8 CAD, respectively. It can be seen from the figure that equalizing the ignition delays for the heptane/toluene blends did not result in identical PAH concentrations as intuitively anticipated. Instead it reduced the mass of individual PAHs due to addition of ignition improving additive (2-EHN). For example, the total PAH concentration of naphthalene in Figure 3d for the 85%H15%T was 500 µg/m3 and it decreased by 20% in Figure 8a when 85%H15%T + 400 ppm of 2-EHN was combusted. A similar correlation applies to the naphthalene concentration of 77.5%H22.5%T fuel blend, as can be seen in Figure 3d.

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PAH concentrations as intuitively anticipated. Instead it reduced the mass of individual PAHs due to addition of ignition improving additive (2-EHN). For example, the total PAH concentration of naphthalene in Figure3d for the 85%H15%T was 500 µg/m3 and it decreased by 20% in Figure8a when 85%H15%T + 400 ppm of 2-EHN was combusted. A similar correlation applies to the naphthalene concentration of 77.5%H22.5%T fuel blend, as can be seen in Figure3d. Energies 2019, 12, x FOR PEER REVIEW 16 of 21

Figure 8. Distributions of total exhaust PAHs (GP + PP) (µg of PAH/m3 of gas) at constant ignition Figure 8. Distributions of total exhaust PAHs (GP + PP) (µg of PAH/m3 of gas) at constant ignition delay delay(a) ignition (a) ignition delay delay (ID) (ID)= 9.4 =CAD9.4 CADand (b and) ID( =b )9.8 ID CAD.= 9.8 CAD.

Similarly,Similarly, the the total total PAH PAH concentrationconcentration of of naphth naphthalenealene in in Figure Figure 6d6 dfor for 77.5%H22.5%T 77.5%H22.5%T was was 500 µ 3 500µg/mg/m3, and, and it itdecreased decreased by by20% 20% when when 850 850 ppm ppm of 2- ofEHN 2-EHN was was added added to 77.5%H22.5%T to 77.5%H22.5%T and and52% 52%for forwhen when 1500 1500 ppm ppm of of2-EHN 2-EHN was was added added to 77.5%H22.5%T to 77.5%H22.5%T as shown as shown in Figure inFigure 8a,b, respectively.8a,b, respectively. Closer Closerinspection inspection of Figure of Figure 8a,b shows8a,b shows a decline a decline in PAH in co PAHncentration concentration of most ofof mostthe 16 of PAHs the 16 due PAHs to addition due to additionof 2-EHN of 2-EHNto fuel blends to fuel at blends both ignition at both ignitiondelays of delays 9.4 and of 9.8 9.4 CAD. and 9.8 CAD.

3.6.3.6. Total Total PAH PAH Analysis analysis at at Constant constant IgnitionIgnition timing Timing and and constant Constant ignition Ignition delay Delay TheThe total total PAH PAH concentration concentration at at constant constant ignition ignition timingtiming isis shownshown inin FigureFigure9 9 and and waswas foundfound byby summingsumming up up all all the the EPA EPA 16 16 priority priority PAHs PAHs at at both both gas gas and and particulate particulate phases. phases. WhileWhile Figure 88 showsshows reductionreduction in in concentration concentration of of the the individual individual total total PAH PAH due due to to addition addition ofof 2-EHN,2-EHN, FigureFigure9 9 rather rather showsshows thethe cumulative cumulative reductions reductions in in PAH PAH concentration concentration afterafter addingadding 2-EHN for for each each fuel. fuel. Comparing together Figure9a–d, one can observe that adding 400 ppm of 2-EHN to 85%H15%T decreased the GP PAH concentration by 23% (Figure9a,b) and the PP PAH concentration by 82% (Figure9c,d). A similar observation can be seen when Figure9e–h are compared. The PP PAH concentration on soot mass basis decreased by 95% (Figure9e,f) while the total PAH concentration decreased by 26% (Figure9g,h) when 400 ppm of 2-EHN was added to the 85%H15%T blend. The implication of this observation is that the addition of 2-EHN to the fuel blends results in early ignition reaction with high amount of energy liberated, and the PAHs associated with exhaust particulates and the gaseous species are considerably abated. However, it is interesting to note that addition of 2 EHN to any of the fuel blends appears, in most cases, to reduce PAH emissions, which could suggest a secondary effect on PAH formation. This is either through the presence of 2 EHN influencing, directly, the rates of PAH formation and growth reactions, or through the influence of 2 EHN on ignition delay. Lastly, it can be observed when comparing Figures5 and9 that, since equalizing, the ignition delays for the fuel blends did not result in identical PAH concentrations. It can be said, therefore, that fuel composition seems to have more influence on the propensity/identity of PAHs formed than the combustion characteristics such as ignition delay, heat release rates and premixed burnt fraction of the fuel.

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Figure 9. Figure 9. Normalized total PAH concentrations at constant ignition delays: (a) Gas Phase Figure 9. Normalized total PAH concentrations at constant ignition delays: (a) Gas Phase PAH at ID = PAH at ID = 9.4, (b) Gas Phase PAH at ID = 9.8, (c) Particle Phase PAH at ID = 9.4, (d) Particle Phase 9.4, (b) Gas Phase PAH at ID = 9.8, (c) Particle Phase PAH at ID = 9.4, (d) Particle Phase PAH at ID = 9.8, PAH at ID = 9.8, (e) Particle Phase at ID = 9.4, (f) Particle Phase PAH at ID = 9.8, (g) total PAH at (e) Particle Phase at ID = 9.4, (f) Particle Phase PAH at ID = 9.8, (g) total PAH at ID = 9.4 and (h) total ID = 9.4 and (h) total PAH at ID = 9.8. PAH at ID = 9.8. 4. Conclusions Comparing together Figure 9a–d, one can observe that adding 400 ppm of 2-EHN to 85%H15%T The results presented for gas-phase and particle-phase PAHs generated from combustion of decreased the GP PAH concentration by 23% (Figure 9a,b) and the PP PAH concentration by 82% several fuel blends can be summarized as follows: (Figure 9c,d). A similar observation can be seen when Figure 9e–h are compared. The PP PAH concentrationIncreasing on toluene soot mass proportion basis decreased in heptane by/toluene 95% (Figure mixture 9e,f) resulted while in the an increasetotal PAH in theconcentration mean soot • decreasedparticle by sizes26% and(Figure decrease 9g,h) in when overall 400 soot ppm surface of 2-EHN area. Anwas opposite added decreasingto the 85%H15%T trend of meanblend. soot The implicationparticle of size this and observation increasing is sootthat surfacethe addition area was of 2-EHN observed to whenthe fuel ignition blends improving results in additiveearly ignition was reactionadded with to high the amount heptane /oftoluene energy blend liberated, so as an tod keep the ignitionPAHs associated delay constant. with exhaust particulates and the gaseousThe lower species toxicity are considerably Group D PAHsabated. (naphthalene, However, it is acenaphthylene, interesting to note acenaphthene, that addition fluorene,of 2 EHN • to anyphenanthrene of the fuel blends and anthracene) appears, in dominated most cases, the to GPreduce PAHs PAH as well emissions, as the total which PAH could distributions suggest a secondary(>90%) effect regardless on PAH of formation. composition This of is the either fuel tested.through The the fraction presence of of group 2 EHN D PAHs influencing, in both directly, the gas the ratesphase of PAH and totalformation PAHs and decreased growth with reactions, increase orin through the proportion the influence of toluene of 2 EHN blended on ignition into heptane. delay. Lastly,Fossil dieselit can hadbe observed the lowest when proportion comparing of gas Figures phase 5 PAHs and 9 at that, 88% since and, equalizing, invariably, thethe highestignition delaysparticle for the phasefuel blends PAHs. did not result in identical PAH concentrations. It can be said, therefore, that fuel compositionThe total PAHs seems decreased to have with more increase influence in the on ignitionthe propensity/identity delays and proportions of PAHs of premixedformed than phase the • combustionof the variouscharacteristics heptane such/toluene as ignition blends. delay, This suggestsheat release an influencerates and onpremixed PAHs of burnt the corresponding fraction of the fuel.

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shift in combustion phasing from premixed to diffusion modes and subsequent changes in in-cylinder conditions. The apparent heat release rates of the heptane/toluene blends from diesel engine combustion may • have an influence on the concentrations of the higher toxicity Group B2 PAHs deposited onto the soot particles, and their corresponding weighted carcinogenicities. The particle phase PAHs were observed to be strongly positively correlated with the soot particle • sizes from which the PAHs were extracted, regardless of the fuel combusted. The total PAH concentration decreased with increase in the amount of ignition improver • (2-ethylhexyl nitrate) added to the heptane/toluene fuel blend. Ignition improving additive impacted more significantly on the reduction of particle bound PAHs than those PAHs in the gas phase. Fuel composition seems to have more influence on the concentration of PAHs formed than the • combustion characteristics (ignition delay, heat release rates and premixed burn fractions).

Author Contributions: Conceptualization, H.D, M.T, N.L. and P.H.; methodology, H.D, M.T, N.L. and P.H.; formal analysis, H.D.; investigation, H.D. and M.T.; writing—original draft preparation, H.D. and N.L.; writing—review and editing, H.D, M.T, N.L. and P.H.; supervision, N.L. and P.H.; project administration, H.D. and N.L.; funding acquisition, H.D., P.H. and N.L. Funding: This research was funded by the UK Engineering and Physical Sciences Research Council (EPSRC), grant numbers EP/M007960/2 and EP/M009424/1. Acknowledgments: The first author, H.D., wishes to gratefully acknowledge the Petroleum Technology Development Fund (PTDF) for sponsoring his research studies at University College London (UCL). The authors would like to express sincere gratitude to the Researcher Links programme of the British Council for the financial support of this publication. Conflicts of Interest: The authors declare no conflict of interest.

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