Impact of Fuel Composition and Intake Pressure on Lean Autoignition

Article pubs.acs.org/EF

Impact of Fuel Composition and Intake Pressure on Lean Autoignition of Surrogate Gasoline Fuels in a CFR Engine Vickey Kalaskar,† Dongil Kang,‡ and André L. Boehman*,§ †

The EMS Energy Institute Department of Energy and Mineral Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, United States ‡ Department of Chemical Engineering, The University of Michigan, Ann Arbor, Michigan 48109, United States § Department of Mechanical Engineering, The University of Michigan, Ann Arbor, Michigan 48109, United States ABSTRACT: The critical compression ratio (CCR) criterion (defined as the minimum compression ratio at which the fuel shows initial signs of autoignition) was examined for various gasoline surrogate fuels in a motored engine. This investigation builds on the concept of CCR which is a good indicator of a fuel’s autoignition characteristics, to study the fuel compositional effects with increasing intake manifold pressure. The blends consisted of binary and ternary mixtures of n-heptane and/or isooctane, and a fuel of interest. These fuels of interest were higher octane components; toluene, ethanol, and iso-butanol. A lean condition (Φ = 0.25) with varying intake pressure (atmospheric to 3 bar, abs) and at a constant intake temperature of 155 °C was used to investigate the ignition behavior of all the blends. Two sets of blends consisted of varying percentages of fuels of interest, formulated to approximately have research octane numbers (RON) at 80 and 100. For comparison, neat iso-octane was selected as the representative RON 100 fuel, and (Primary Reference Fuel) PRF 80 blend (20% n-heptane, 80% iso-octane, %v/ v) was selected as the representative RON 80 fuel. The results were deduced based on engine-indicated data and exhaust emissions. It was observed that the blends with a higher percentage of n-heptane showed a stronger tendency to autoignite at lower intake pressures. However, as the intake pressure was increased, the lower reactivity components (in this study the highoctane components toluene, ethanol, and iso-butanol) hindered the radical formation in the low-temperature regime and/or delayed the onset of high-temperature heat release. The heat release analysis revealed that the higher-octane components in the blends reduced the low-temperature reactivity of n-heptane and iso-octane as the intake pressure was increased. In addition, distinctively different low-temperature heat release patterns were observed for blends consisting of alcohols and toluene as the intake pressure was increased, confirming distinctively different reaction mechanisms as well as inter component interactions in the blends.

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INTRODUCTION Compression ignition of gasoline-like fuels has been a subject of research for several decades1,2 as this offers efficiency benefits compared to traditional spark ignition (SI). With the recent trend of downsizing and boosting of SI engines, autoignition of fuels before the conventional spark timing in engines also has become an issue, especially at low speeds and highly boosted conditions, which is referred to as low-speed preignition (LSPI)3−6 and “super knock”.7 Lean autoignition is of interest for combustion modes like gasoline compression ignition (GCI) and homogeneous charge compression ignition (HCCI). Many fuels have been characterized previously for the compression ignition processes mentioned above. Some of these combustion systems provide a more controlled combustion environment such as jet-stirred reactors, rapid compression machines (RCMs), shock tubes, etc. However, in practical systems such as internal combustion (IC) engines, the combustion behavior and the control of volumetric heat release during such strategies is very difficult and cycle-to-cycle variability makes it difficult to study the combustion chemistry. Heat transfer also plays an important role in IC engines and often is difficult to model. This investigation tries to bridge the gap between the bench-scale devices and practical IC engines by using a Cooperative Fuels Research (CFR) variable compression ratio engine to study the underlying chemistry © XXXX American Chemical Society

and physical effects of different temperature/pressure histories during autoignition of gasoline-like fuels with similar research octane number (RON) and significantly different chemical compositions investigated under lean conditions. The current work is performed to test the following hypothesis: “Fuels with similar research octane number but with varying aromatic/ alcohol content and corresponding fuel octane sensitivities will exhibit significant fuel specific differences under lean, boosted, compression-ignition conditions”. Octane rating, which is typically defined to characterize knock, can also be used to define the behavior of homogeneous ignition of fuels. In fact, the whole premise of octane index (OI) developed by Kaghatgi and co-workers is based on correlating the autoignition behavior of gasoline-like fuels with their octane rating for stoichiometric SI as well as lean homogeneous combustion.8 The choice of the lean condition is justified based on prior HCCI studies9,10 where high-pressure rise rates are often a limiting factor. Octane numbers are typically used to characterize the autoignition behavior of gasoline fuels under SI relevant conditions, where equivalence ratio is slightly richer than stoichiometric while the intake Received: April 23, 2017 Revised: August 28, 2017 Published: August 29, 2017 A

DOI: 10.1021/acs.energyfuels.7b01157 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 1. Schematic of the CFR engine setup. Reproduced with permission from ref 30. Copyright 2016 Elsevier.

pressures and temperatures are SI engine relevant.11,12 The choice of octane rating for comparing different fuel blends in this study is justified since these ratings are universally used for characterizing gasoline fuels. In this study, the equivalence ratio employed was much leaner compared to SI engines. The ability to control the compression ratio (CR) on the fly using this setup gives broad control over the end of compression temperatures and pressures, simultaneously, permitting unique investigations of ignition behavior for the model and full boiling range fuels. Granted that the heat transfer to the walls in a CFR engine also poses similar challenges as discussed above, the experimental conditions are much more repeatable using this engine while operating it at a low engine speed of 600 rpm and in a well-premixed configuration. One might argue that well-premixed conditions in the combustion chamber may not represent a more realistic set of conditions. Especially, the latent heat of vaporization differences between fuels may offset the observations from this study. However, well-premixed configuration gives the ability to investigate the autoignition behavior of fuels solely induced by chemical interactions neglecting the physical aspects of individual fuel components making the comparison more feasible from the point of view of this study. Investigation of physical effects such as latent heat of vaporization could be a focus of future study. Gasoline typically is a complex mixture of a quasi-continuous spectrum of hydrocarbons depending on the crude oil source and fuel refinement techniques. While it would be enlightening to study the chemistry that occurs for such a practical fuel under compression ignition conditions, it is important to note that it would be very difficult to express with certainty which fuel component is affecting what aspect of the compression ignition process. Also, modeling the combustion chemistry for such a complex fuel is computationally expensive and presently untenable. Hence to understand the underlying chemistry, surrogate fuels are often used. Several publications discuss the autoignition mechanisms of gasoline surrogates, particularly the fuels presented here. Surrogate fuels reduce the complexity observed in a real fuel and can be used to enhance the fundamental understanding of the combustion process. A very simplistic gasoline surrogate would consist of only one fuel component. Generally, iso-octane is used to simulate gasoline

fuels. Pitz et al.13 in their detailed review on surrogates for gasoline indicated that there are three necessary components required to define a gasoline surrogate adequately. They are nheptane (n-alkane/n-paraffin representative), iso-octane (isoalkane/iso-paraffin representative), and toluene (aromatic representative). As a result, these fuels have been studied in depth, and detailed kinetic models have been developed for these fuels (n-heptane,14,15 iso-octane,15,16 and toluene17−21). Additionally, recent emphasis on the usage of bioalcohols has also changed the fuel scenario, especially across the U.S. The octane number and composition of a fuel also plays an important role when existing legacy SI engines are considered which could be a decade or two old. The foreseeable way to achieve this would be to start blending lower RON fuel streams with very high RON streams or blending components, which are frequently alcohols. Autoignition behavior of such fuels under lean conditions usually observed in HCCI and partially premixed combustion (PPC) modes would differ significantly compared to the existing fuel streams and deserves some discussion. This further demands the study of fuel chemistry under compression ignition conditions for fuels with significantly different autoignition behaviors and yet having similar octane numbers. There have been numerous studies that have focused on fuel composition and its effect on HCCI8,22−26 and PPC.27−29 For HCCI, there has been strong research emphasis on the effects of the fuel composition, engine operability, development of kinetic models that are valid for lean autoignition of various fuel surrogates, etc. Similarly, for PPC, the focus has been on finding larger differences in engine operation with changing fuel compositions, especially the effect of changing octane numbers. However, in this study, the focus is on similar octane blends with different fuel compositions and how that impacts the lean autoignition characteristics. As such, this study highlights the shortcomings of the octane measurement methodology which has not been updated for decades, conceptually. The emphasis is put on the impact of varying the intake pressure on gasoline surrogate blends with varying percentages of toluene, ethanol, and iso-butanol (fuels of interest) in mixtures of n-heptane and iso-octane. The blends are limited to RON of 80 and 100 given the current and near future expected gasoline fuel scenarios. B


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EXPERIMENTAL SECTION

Table 1. CFR Engine Specifications

A modified Cooperative Fuels Research (CFR) octane rating engine is used in this study. This engine allowed for varying the CR precisely (a resolution of 0.01 CR can be achieved) from 4 to 15.6 on the fly. This engine is motored at a constant speed of 600 rpm throughout this study. Figure 1 shows the engine setup schematically. Originally the CFR engine was equipped with a carbureted fueling system which was replaced with a heated intake system in conjunction with a gasoline direct injection (GDI) fuel injector.31 Note that the injector is not used for doing the direct injection in the combustion chamber, but rather it is used as a means of injecting fuel in the intake manifold sufficiently upstream of the intake valve such that fuel is wellpremixed before entering the combustion chamber. Additionally, the intake system on this engine can deliver dry intake air up to 3 bar (abs) boost pressure. Also, shown in the figure is a two-way valve which allowed for changing the intake air to atmospheric pressure, if desired. The heated intake consists of a series of electric heaters followed by tape heaters to ensure that the charge stayed at the desired set point as it entered the engine. The charge could be heated to a maximum temperature of 260 °C under naturally aspirated as well as boosted conditions. This also ensured that the injected fuel was fully vaporized as it was inducted into the engine. Zhang32 did an analysis to confirm that the injected fuel was being fully vaporized using this fuel injection and heated intake setup. It was shown in Zhang’s analysis that the vaporization was complete when the partial pressure of the given fuel at intake temperature was below the saturation vapor pressure of the given fuel for a given equivalence ratio (Φ) being investigated. This capability of the fuel injection and heating system thus enables examination of different fuels with a wide range of boiling points. Importantly, heating the intake charge to a set temperature eliminates the effect of the heat of vaporization while studying the autoignition phenomenon, thus enabling a more direct comparison between fuel autoignition tendency, which is driven by chemical kinetics rather than due to the differences in charge temperature and pressure conditions at intake valve closing (IVC). While not the focus of this study, investigating the effects of direct injection and heat of vaporization may add valuable information to the existing data in this publication and may even improve some of the observed trends in data. This will, however, require a carefully designed experimental setup which can investigate both the scenarios, fully premixed and a more realistic injection condition accounting for the heat of vaporization effects. The fuel was injected using a GDI injector which was controlled using a triggered signal, and the fuel flow rate was controlled by varying the duration of the injection pulse while the frequency of injection was fixed at 10 Hz. Since this injector did not account for fuel physical properties like density and viscosity, the injector had to be precalibrated for each tested fuel. This injector required the upstream fuel pressure to be maintained at 700 psi using an inert gas, in this case, helium. The intake air flow rate was measured using a hot wire mass air flow (MAF) sensor, which was calibrated for the expected flow range. It must be pointed out that a detailed analysis of residual gas fraction was performed for this engine by previous researchers.32,33 It was shown that the error introduced in Φ due to accounting for residual gas fraction in this engine was minimal. It is expected that operation at higher intake pressure with exhaust always near atmospheric pressure will reduce the residual gas fraction, especially at higher compression ratios. As such, all the results are reported here based on Φ derived from air and fuel mass flow rates. To measure the indicated data, a Kistler 6052B piezoelectric pressure transducer is incorporated in place of the standard knock meter on this engine. The signal from this pressure transducer is amplified using a Kistler 5010 dual mode amplifier. This transducer in conjunction with an AccuCoder angular encoder facilitated the acquisition of cylinder pressure data at a resolution of 0.1 °CA. The low-speed data which consisted of temperatures, pressures, and emissions measurements were acquired every 10 s under steady-state operating conditions. Details of the CFR engine are presented in Table 1.

number of cylinders

1

bore (cm) stroke (cm) connection rod (cm) swept volume (cm3) compression ratio number of overhead valves engine speed (RPM) intake valve opening (°CA after TDC) intake valve closing (°CA after BDC) exhaust valve opening (°CA before BDC) exhaust valve closing (°CA after TDC)

8.26 11.43 25.4 611.7 4−15.7 2 600 28 14 27 0

To enhance the steady-state operation, a few modifications were made to the cooling jacket circuit of the engine. The evaporative steam cooling system used to maintain water jacket temperatures in the standard CFR engine design is replaced with a 1000 W refrigerated/ heating coolant circulator. In conjunction with the circulator, the coolant is passed through a series of radiators. This ensures a constant jacket temperature (90 °C ± 5 °C) during motored as well as fired conditions. An additional 1100 W refrigeration/heating system is used to stabilize the custom-made GDI injector jacket’s temperature at 90 °C ± 1 °C. This ensured constant temperature operation during calibration and actual engine operation. For the current study, an intake temperature of 155 °C and Φ = 0.25 is selected for all the fuels. The temperature of 155 °C is selected such that the fuels under current study vaporized completely and did not autoignite at the lowest CR of 4 and at the highest intake pressure investigated (3 bar abs). This ensured that precombustion reactions could be observed even at the highest intake pressure. The intake pressure was swept from atmospheric to 3 bar (abs). The main intake heater can heat the intake air to 155 °C up to 2 bar (abs) intake pressure, the supplemental heater is used at and above 2 bar (abs) intake pressures. The supplemental heater is set at 120 °C. As the intake pressure was increased the fuel flow rate was adjusted to maintain a constant Φ, which was monitored and recorded throughout the study. Exhaust Analysis. Emissions were measured via a custom emissions bench incorporating california analytical instruments (CAI) analyzers. Emissions of CO (NDIR), CO2 (NDIR), and O2 (paramagnetic) were measured. During most of these experiments, either fuel did not ignite or complete combustion was never achieved, and hence, the exhaust stream had very high concentrations of unburned hydrocarbons. To avoid the condensation of these in the analyzer circuit, a series of chillers are used to condense the heavier hydrocarbons. Hence all the emissions reported here are on a dry basis as moisture from the exhaust is also removed during condensation of the hydrocarbons. No hydrocarbon measurements are reported as most of the heavier total hydrocarbons (THCs) condensed in the chillers, and the lighter components were still in large enough concentrations to easily saturate the THC emissions analyzer. In-Cylinder Pressure Data Analysis. In-cylinder pressure data are acquired at a resolution of 0.1 °CA using a piezoelectric transducer as mentioned above. These data are then processed through a custom LabVIEW based data acquisition program, which aids in acquiring pressure for 40 individual engine cycles. Using these data, an average pressure trace is computed, which is then filtered using appropriate band-pass filters, and smoothened using a cubic spline algorithm. Then this pressure trace is used to compute apparent heat release rate (AHRR) using a zero-dimensional and single-zone model as described by Heywood.34 The term apparent is used to indicate that the model used here does not account for heat losses or heat transfer that occurs in the engine. The instantaneous bulk cylinder temperature as a function of crank angle is also computed using the pressure data and the ideal gas law. The detailed explanation of the combustion analysis procedure is presented by Zhang.32 C


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Energy & Fuels Critical Compression Ratio Criterion. Several ideal reactors such as rapid compression machines (RCMs), constant volume combustion chambers, shock tubes, flow reactors, etc. are traditionally used to study hydrocarbon oxidation. One of the advantages of using such systems is the control of initial conditions such as intake temperature, pressure, and charge composition. Additionally, these systems can be modified to provide optical access as required, which enables detailed in situ diagnostic measurements of temperature, chemical species, etc. However, these processes in such ideal systems are inherently different from an actual engine, where the pressure and temperature of the fuelair mixture change dynamically. As mentioned earlier, the modified CFR engine gives the flexibility to control the maximum pressures and temperatures within the combustion chamber dynamically such that preignition chemistry can be probed. Another benefit is that a motored engine method provides relatively convenient means for product analysis from the engine exhaust, and the repeating cycles can generate sufficient amounts of sample for analysis under steady-state conditions, prior to ignition. Such a setup has been extensively used to study hydrocarbon oxidation for various fuels. A few of the relevant studies are cited here.32,35−39 A similar approach is used in this study where the motored engine is essentially used as a reactor to compress the fuel-air charge to study the effect of various pressure and temperature conditions on the amount of heat release and exhaust products. It can be thought of as an HCCI engine, but the CR determines where the autoignition occurs and preignition chemistry can be studied. The method adopted here is to start at the lowest CR of 4, where little or no reaction is expected as the charge undergoes compression in the engine. The CR is then increased in steps until autoignition is reached. While sweeping the CR, the amount of CO is monitored, and typically it is observed that the CO stays low until a certain point beyond which it increases exponentially, indicating that the charge is close to autoignition. The CR at which the CO starts to fall off from a maximum is defined as CCR. In other words, reduction in the amount of CO occurs as the fuel combustion becomes more complete, which is also indicated by a simultaneous increase in CO2 emission. The higher the CCR, the higher is the resistance of the fuel to autoignite, indicating lower reactivity in comparison to a fuel with a lower CCR. Figure 2 shows a representative CCR sweep for iso-octane at 3 bar (abs) intake pressure, intake temperature of 155 °C, and Φ = 0.25.

Fuels. Fuels with different octane numbers have been widely studied for both HCCI and PPC combustion processes. However, octane number becomes important when a new fuel is being commercially marketed, the reason being that all the SI engines are calibrated for a certain minimum octane number. Also, it is well-known that for low temperature combustion (LTC) strategies, fuel chemistry can affect the autoignition phenomenon. For example, a fuel with higher aromatic content but similar octane number will exhibit some differences in its autoignition behavior.40 These effects may be exacerbated with changing intake conditions like the charge equivalence ratio, temperature, and pressure. In this study, two PRFs were chosen as base fuels iso-octane (PRF 100) with a RON of 100, and a 20/80 (% v/v) mixture of n-heptane and iso-octane (PRF 80) with a RON of 80. In addition, three fuels of interest are studied here: toluene, ethanol, and iso-butanol were mixed in varying proportions with n-heptane and iso-octane to approximately match the RON values of 80 and 100. The selection is based on the fact that toluene is one of the most abundantly found aromatic species in gasoline fuels, and it exhibits a reaction inhibiting effect for compression ignition,39,41 and hence it is necessary to establish the autoignition behavior of a blend with varying percentages of toluene. Additionally, the interest in ethanol as a renewable fuel justifies the selection of this fuel as it is already being employed in current gasoline blends, and future fuels may involve higher percentages of ethanol.42 iso-Butanol is chosen as one of the representative biobutanol for this study. iso-Butanol is more compatible with the current fueling systems than ethanol, including being compatible with pipelines because it does not separate into an aqueous phase in the presence of water.43 iso-Butanol offers similar volumetric energy density as that of gasoline, higher octane number, and has a high latent heat of vaporization, although the latent heat of vaporization is not as high as that of ethanol. Relevant characteristics of these fuels are shown in Table 2. Three blend sets were investigated. The blends comprised of nheptane, iso-octane, and a fuel of interest (toluene, ethanol, or isobutanol). For each blend, the volume percentage fraction of the fuel of interest was fixed (20, 35, or 50% v/v) and the remainder was some combination of n-heptane and iso-octane such that the RON values of each blend were close to the intended target values of 80 and 100. For toluene blends, an octane prediction model by Ghosh et al.46 was used. Additionally, for toluene blends, the RON values were measured following the ASTM standard and more information is presented in Table 3. For the alcohol blends, a separate model by Anderson et al.47 was utilized for octane number prediction. The corresponding blend matrices are presented in Table 4 and Table 5. It must be noted that RON and MON predictions for alcohol blends (ethanol blends in this case) are significantly different than the measured values as observed in Table 4. Moreover, further analysis of the data and trends in the data presented in this publication will be subject to some bias based on the choice of RON and MON values (measured vs predicted). However, it is expected that even the largest differences in predicted and measured values will not affect the observed trends in the data presented in this publication. It is worthwhile to mention that measured values were given precedence over predicted values of both RON and MON wherever applicable in this matrix. A ternary plot of the complete blend matrix is shown in Figure 3. One peculiar thing to notice is that for a similar RON value, ethanol blends have a larger fraction of n-heptane compared to the other fuels of interest. This can be attributed to the fact that volumetric addition of ethanol to a mixture of PRF blends results in octane number that

Figure 2. CO expressed as a function of compression ratio sweep for iso-octane at intake pressure of 3 bar (abs), intake temperature 155 °C, and Φ = 0.25. Maximum CO observed at CR of 7.35 and CCR at 7.4 (red ---).

Table 2. List of Pure Fuel Properties fuel 3

density (g/cm ) RON, ASTM D2699 (−) MON, ASTM D2700 (−) lower heating value (LHV) (MJ/kg)

n-heptane 44

0.6795 0 0 44.5544

iso-octane 44

0.692 100 100 44.3444 D

toluene 44

0.8719 11846 103.546 40.5244

ethanol 44

0.789 10934 9034 26.8244

iso-butanol 0.80245 11345 9445 33.1


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Table 3. Toluene Blend Matrix Formulation Expressed in Terms of Volumetric Fractions of n-Heptane, iso-Octane, and Toluene, and Corresponding RON and MON Valuesa predicted values46

expressed as a volume fraction T1 T2 T3 T4 T5 T6 a

n-heptane

iso-octane

toluene

RON

MON

S(RON‑MON)

measured RON ASTM (D2699)11

0.28 0.35 0.41 0.06 0.10 0.15

0.52 0.30 0.09 0.74 0.55 0.35

0.20 0.35 0.50 0.20 0.35 0.50

80 80 80 100 100 100

79.10 74.73 70.36 96.50 93.91 91.34

0.90 5.27 9.64 3.50 6.09 8.66

77.7 76.7 74.4 98.1 97.1 97.0

Sensitivity S is calculated using predicted values.

Table 4. Ethanol Blend Matrix Formulation Expressed in Terms of Volumetric Fractions of n-Heptane, iso-Octane, and Ethanol and Corresponding RON and MON Valuesa expressed as a volume fraction

E1 E2 E3 E4 E5 E6

predicted values47

nheptane

isooctane

ethanol

RON

MON

S(RON‑MON)

actual RON48

0.35 0.45 0.50 0.10 0.2 0.25

0.45 0.2 0.00 0.70 0.45 0.25

0.20 0.35 0.50 0.20 0.35 0.50

77.44 76.48 78.08 96.32 92.88 92.91

69.81 65.38 64.47 88.58 81.58 79.09

7.63 11.10 13.61 7.80 11.30 13.81

80b 80b 83.8c 102b 100b 101b

a

Sensitivity S is calculated using predicted values. bRON derived based on actual measurements in ref 48. cActual measured value of RON.

Table 5. iso-Butanol Blend Matrix Formulation Expressed in Terms of Volumetric Fractions of n-Heptane, iso-Octane, and Ethanol, and Corresponding RON and MON Valuesa expressed as a volume fraction I1 I2 I3 I4 I5 I6 a

Figure 3. Ternary plot of the blend matrix. RON 100 blends shown by filled symbols and RON 80 blends shown by open symbols.

predicted values47

n-heptane

iso-octane

iso-butanol

RON

MON

S(RON‑MON)

0.30 0.35 0.40 0.05 0.15 0.15

0.50 0.30 0.10 0.75 0.50 0.35

0.20 0.35 0.50 0.20 0.35 0.50

77.62 77.88 77.64 99.66 94.36 97.21

71.93 68.86 65.86 93.82 85.17 85.19

5.68 9.01 11.77 5.83 9.18 12.02

Table 6. Summary of CCR for All Investigated Fuels and Intake Pressures CCRs at intake pressures

Sensitivity S is calculated using predicted values.

follows a highly nonlinear trend as a function of volumetric substitution by ethanol.47

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RESULTS Global CCR versus Fuel Octane Number. A summary of critical compression ratio for all the fuel blends investigated in this study is presented in Table 6 for intake pressures of 1−3 bar (abs) in a stepwise manner of 0.5 bar increments. It is important to note that for a few fuel blends, intake pressures of 1.25 and 1.75 bar were also investigated but not shown here as there were no trend-wise differences. However, these results were used during statistical correlational analysis. A few randomly selected blends were rerun, and a maximum repeatability error was determined to be 0.15 in terms of CR. The CCR of all fuels is also shown in Table 6 segregated by intake pressures. In general, the CCR decreases as intake pressure is increased. At each intake pressure, a linear trend can be observed which is expected as the CCR increased for less reactive fuels. Hence, for higher octane blends, the CCR tends

fuel

RON

MON

1 bar

1.5 bar

2 bar

2.5 bar

3 bar

PRF 80 T1 T2 T3 E1 E2 E3 I1 I2 I3 PRF 100 T4 T5 T6 E4 E5 E6 I4 I5 I6

80 80 80 80 77.44 76.49 78.08 77.62 77.88 77.64 100 100 100 100 96.32 92.88 92.91 99.66 94.36 97.21

80 79.10 74.73 70.36 69.81 65.38 64.47 71.93 68.87 65.87 100 96.50 93.91 91.34 88.58 81.58 79.09 93.82 85.17 85.19

10.7 10.58 10.3 10.55 11 11 11.2 10.75 11.43 11.4 14.1 13.4 13.55 13.3 13.3 12.88 12.6 13.25 12.75 12.8

8.35 8.65 8.4 8.6 8.57 8.73 9 8.58 9.05 9.1 11.6 11.1 11.3 11 11.4 10.95 10.95 11.35 10.75 11.05

7.23 7.55 7.4 7.5 7.33 7.35 7.5 7.35 7.65 7.75 9.8 9.55 9.58 9.43 10.1 9.75 9.75 10 9.6 9.85

6.4 6.7 6.6 6.73 6.45 6.48 6.55 6.58 6.75 6.88 8.43 8.27 8.45 8.45 9 8.65 8.8 9 8.45 8.85

5.8 6.13 6.1 6.18 5.88 5.85 6 5.98 6.1 6.25 7.4 7.38 7.6 7.73 8.05 7.8 7.9 8.15 7.55 8.05

to be higher. Important to note is the spread in the data at a constant RON value, which suggests that fuel composition also plays an important role in the determination of the autoignition E


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low and intermediate temperature regimes.49 Typically, CO is produced in the localized regions of the combustion chamber where low-temperature reaction pathways are active. However, for the homogeneous system described here, CO will form globally within the chamber. One of the pathways for CO formation is through consecutive hydrogen abstraction steps from formaldehyde (CH2O), which is formed during the low temperature heat release (LTHR).50 By tracing the CO emission, the whole reaction process can be classified into three regimes: low-temperature, intermediate temperature also referred to as the negative temperature coefficient (NTC) regime, and high temperature.51 It can be observed in these figures that the CCR decreased with increasing intake pressures for each fuel, promoting the ignition process. It is certainly welldocumented that the window for NTC regimes where the CO emissions either remain constant or decreased slightly gets narrower and the CO emissions produced are higher as pressure increases, suggesting increasing reactivity in the lowtemperature regime as a function of intake pressure. As expected, the CCR was lower for the more reactive PRF 80 blend compared at the same intake pressure, which is also evident in Figure 6. Additionally, it can also be observed that at

limit which was also confirmed in a recent study by Lilik and Boehman.33 In their study, the metric used for comparing fuels was a derived cetane number (DCN) rather than the octane number and the fuels under discussion were pure n-heptane, a 61/39 (%v/v) blend of n-dodecane and toluene, and a 50/50 (%v/v) blend of n-dodecane and iso-octane. While the DCNs for the above-mentioned fuels were very similar, the critical equivalence ratios (defined as the minimum equivalence ratio at a constant compression ratio at which fuels can autoignite) were distinctively different. Additionally, with increasing percentage of simulated exhaust gas recirculation (EGR), the difference in the critical Φ was exacerbated. Primary Reference Fuels Results. Figure 4 and Figure 5 show CO emissions as a function of CR at various intake

Figure 4. CO emission expressed as a function of compression ratio for iso-octane at various intake pressures (red ■ 1 bar, ▲1.5 bar, redoutlined □ 2 bar, blue-outlined ○ 2.5 bar, and green-outlined ◇ 3 bar).

Figure 6. Critical compression ratio expressed as a function of intake pressure for PRF 80 (red ◆) and iso-octane (●).

lower CRs, PRF 80 produced higher amounts of CO emission compared to iso-octane in low and intermediate temperature regimes, since n-heptane acts as a radical donor to facilitate more active reaction in low-temperature regime. Thus, PRF 80 blends display more low-temperature reactivity and fuel conversion at a given intake pressure and constant CR. A global plot showing the comparison of these two PRF blends is shown in Figure 6. Comparisons of low-temperature reactivity in terms of apparent heat release rate (AHRR) for iso-octane at a CR of 6 and PRF 80 at a CR of 5 and at different intake pressures are presented in Figure 7 and Figure 8, respectively. Noticeable is the higher percentage of LTHR for PRF 80 fuel at any intake pressure despite being at a lower CR compared to iso-octane. PRF 80 exhibits two-stage behavior at lower intake pressures compared to iso-octane, while for iso-octane the LTHR appeared at intake pressure of 2 bar (abs). In fact, for conventional engines involving ambient to moderate boost levels, often the LTHR for iso-octane would be suppressed in the preignition region. Such a behavior of iso-octane is however not very surprising and has been reported previously in a highpressure shock tube study by Fieweger et al.52 The AHRR results here clearly show a transition for iso-octane from a single-stage to a two-stage ignition fuel, which might be of importance for compression ignition engines utilizing higher

Figure 5. CO emissions expressed as a function of compression ratio for PRF 80 at various intake pressures (red ■ 1 bar, blue ◆ 1.25 bar, ▲ 1.5 bar, green ● 1.75 bar, red-outlined □ 2 bar, blue-outlined ○ 2.5 bar, and green-outlined ◇ 3 bar).

pressures for iso-octane and PRF 80. As explained earlier, the point where CO starts to fall off from a maximum during a CR sweep is the point where autoignition is achieved and is termed as the CCR. Depending on a fuel’s tendency to autoignite, the fuel may not achieve complete combustion at this instance; however, this point is considered as the initiation of autoignition and is used as the basis of comparison for all fuels and blends. Emission of CO is known as a good indicator used for tracking the reaction progress of the ignition process in F


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Blend Results. The next set of figures show comparisons off CCR versus intake pressure for blends having the maximum level (50 vol %) of the high-octane blend component. Figure 9

Figure 7. Apparent low-temperature heat release rate profiles for isooctane at a compression ratio of 6 and intake pressures, 1 bar (), 1.5 bar (red ---), 2 bar (blue dash), 2.5 bar (pink dots), and 3 bar (blue dash-dot-dot-dash).

Figure 9. Comparison of critical compression ratios for RON 100 blends; iso-octane (●), T6 (red-outlined □), E6 (green-outlined ○), and I6 (blue-outlined ◇) expressed as a function of intake pressure.

shows the comparison of RON 100 fuel iso-octane with RON 100 blends T6, E6, and I6. One important thing to notice is that as the slope of the CCR decrease is steeper for iso-octane than for these blends. This indicates that theses blends tend to slow down the autoignition process as the intake pressure is increased compared to neat iso-octane, despite the presence of small fractions of reactive n-heptane. Also, note that the RON for each of the blends is lower than iso-octane. It appears that as the intake pressure increases, the RON metric itself is not sufficient to describe the autoignition of a blend which also suggests that there exists a relationship between the CCR and the in-cylinder temperature and pressure profiles. However, when RON 80 blends are compared (Figure 10), it can be Figure 8. Apparent low-temperature heat release rate profiles for PRF 80 blends at a compression ratio of 5 and intake pressures, 1 bar (), 1.5 bar (red ---), 2 bar (blue dash), 2.5 bar (pink dots), and 3 bar (blue dash-dot-dot-dash).

octane blends and employing a high amount of boost and/or a higher compression ratio (gasoline compression ignition has been demonstrated on diesel engines with compression ratios typically higher than gasoline engines). Further, with the recent trends of downsizing and boosting of SI engines, a higher tendency of abnormal ignition or LSPI exists especially at low speed, high load conditions. Literature suggests many possible mechanisms that lead to LSPI.5,53,54 Significant fuel effects have been observed; however, no direct relation between the octane rating of the fuel and LSPI has been established. As such, the transition from a single-stage burn to a noticeable two-stage behavior of branched alkanes may pose a significant challenge in designing fuels for near future engines. Important to note that such a transition may occur earlier with increasing equivalence ratio and may be realizable with moderate levels of boost that could be extracted from an engine employing lowtemperature combustion strategies with limited enthalpies to drive the turbocharger. This also challenges the existing method of correlating the autoignition behavior of a fuel to the respective octane numbers which are obtained under atmospheric intake conditions which may not represent the actual operating conditions in a modern, boosted, and downsized, SI engine.

Figure 10. Comparison of critical compression ratios for PRF 80 (●) and blends T3 (red-outlined □), E3 (green-outlined ○), and, I3 (blueoutlined ◇) expressed as a function of intake pressure.

observed that the CCR for alcohol blends are higher compared to PRF 80 blend at all intake pressures, suggesting reduced LTHR for alcohol blends (E3, I3) compared to PRF 80. However, toluene blend, T3, seems to match the CCR at atmospheric intake pressure and exhibits greater CCR than PRF 80 at higher intake pressure. The different behavior of alcohol and toluene blends suggests a difference in autoignition mechanism. This suggests that, while there exists a relationship between CCR and the intake pressure, this information is not sufficient to explain the behavior of the RON 80 blends. In addition, it seems that relative percentage of the more reactive versus less reactive components in the blend also plays a G


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Energy & Fuels significant role in determining the autoignition reactivity of the mixture. From the data presented in Table 6, similar plots can be made for various combinations of RON 80 and RON 100 blends; however, the trends are very similar and hence not discussed here. This relationship between CCR and n-heptane content in a fuel blend is analogous to the sensitivity of the critical equivalence ratio to the n-alkane content in a fuel. Heat Release Rates. While the overall trends of CCR versus intake pressure draw an outline explaining the different aspects that affect the reactivity of a blend under lean boosted conditions, the heat release data can shed some light on the low-temperature reactivity differences of the blends with different compositions. Figure 11 shows a comparison of iso-

Figure 12. Apparent heat release rate profiles at autoignition condition for iso-octane (, CR 7.4) and E4 (red ---, CR 8.05), E5 (blue ---, CR 7.8), and E6 (green dash-dot-dash, CR 7.9) blends expressed as a function of crank angle, intake pressure of 3 bar.

co-oxidation pathways during the low-temperature regime for ethanol and toluene blends. Co-oxidation refers to a process for a mixture in which the overall oxidation rate is determined by either synergistic or antagonistic interaction within the fuel components in a mixture. For RON 100 blends at 1 bar intake pressure, the LTHR was completely suppressed for all the fuels, suggesting that the observed differences in CCRs for these blends correlate with the RON values. This also suggests that the low-temperature reactivity does not play a significant role at lower intake pressures but becomes more important as the intake pressure is increased for gasoline-like fuels. Now the question posed here is how does one account for this change in behavior based on the intake pressure for a given fuel composition and equivalence ratio? Before answering that question, consideration must also be given to the RON 80 blends. The biggest difference in RON 80 and RON 100 blends is the difference in n-heptane/isooctane content in the blend that is required to bring the octane number down to 80 while maintaining the same volume percentage of the fuel of interest. The distinction within the RON 80 blends is similar as seen for RON 100 blends where volume percentage of n-heptane is much higher for ethanol blends compared to the other fuels of interest. In Figure 13, it can be observed that at a higher intake pressure of 3 bar, as the volume percentage of iso-butanol increases the amount of LTHR reduces and is also delayed, while the PRF 80 blend at CR of 5.8 had already ignited. These results are like what was observed in the previous figures for RON 100 blends. However, at ambient intake pressure, the CCR trends do not reverse for RON 80 blends (Figure 10) as observed for RON 100 blends (Figure 9). Figure 14 shows the comparison of ethanol blends with PRF 80 at 1 bar intake pressure. Again, it can be observed that LTHR under lean conditions and ambient intake pressure is mostly suppressed for all blends, but it is important to note that ethanol blends have a significantly higher percentage of n-heptane compared to the PRF 80 blend. In fact, blend E3 was a binary, 50:50 (vol %) mixture of n-heptane and ethanol, and yet the two-stage behavior of n-heptane was completely suppressed suggesting that ethanol has an antagonistic interaction under ambient conditions on n-heptane suppressing the LTHR completely and igniting at a higher compression ratio as compared to the other blends. Essentially, there are two competing effects here: one being the interaction between fuel components (reactive versus

Figure 11. Apparent heat release rate profiles for iso-octane () and T4 (red ---), T5 (blue ---), and T6 (green dash-dot-dash) blends expressed as a function of crank angle, intake pressure of 3 bar, and CR 7.2.

octane with RON 100 toluene blends (T4-T6) at a CR of 7.2 and intake pressure of 3 bar. Recall from Table 6 that iso-octane ignited at a CCR of 7.4, lower than for most toluene blends (T4 with only 20% toluene had similar CCR compared with isooctane). In this figure, it is observed that the LTHR is similar for all fuels; however, iso-octane also shows a distinctive second peak indicating the start of high temperature heat release (HTHR). Blend T4 also shows some two-stage behavior, but this effect is suppressed for blends T5 and T6, suggesting that the higher percentage of toluene in the blend delays the onset of HTHR but does not hinder the low-temperature chemistry as suggested by the similar LTHR. Figure 12 shows the AHRR profiles for RON 100 ethanol blends compared with iso-octane at their respective CCRs for an intake pressure of 3 bar. The impact of the addition of ethanol is that the two-stage behavior is completely suppressed for all the blends (E4-E6) even at the highest of the intake pressures and at a condition where the fuel is ignited. Similar observations are made for iso-butanol; hence, they are not discussed here. Furthermore, if we look at the blend matrix data for RON 100 fuels, the ethanol blends have higher percentages of n-heptane as compared to the corresponding toluene blends. One might, therefore, expect that these ethanol blends would more readily create a large radical pool and consequently result in the earlier consumption of the high-octane blend components. But the AHRR results indicate that despite the higher percentage of n-heptane, ethanol seems to suppress the low-temperature activity more effectively compared to toluene blends. This suggests that there are separate mechanisms and H


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observed this behavior for 1-butanol/n-heptane blends where radical pool generated by n-heptane aided in consumption of 1butanol. However, that study was conducted under atmospheric conditions. As the intake pressure increases, neat iso-octane’s reactivity increases. But the fuels of interest in the blends, especially the alcohols, are mostly nonreactive until autoignition occurs and no difference in reactivity is expected with the change in intake pressure. In fact, Sjöberg and Dec56 recently showed that ethanol did not exhibit two-stage ignition even under boosted conditions. Gasoline, on the other hand, showed a trend wise increase in intermediate temperature heat release (ITHR) as a function of intake pressure. Also, significant LTHR was observed at the highest of the investigated intake pressures. Furthermore, for RON 80 blends, the AHRR profiles are very similar, where the CCR increases as a function of increasing percentage of fuel of interest as compared with a PRF 80 blend. Unlike RON 100 blends, all RON 80 blends have a significant amount of n-heptane in them and the CCR is controlled by the amount of the less reactive fuels of interest despite the differences in the amount of n-heptane. This is especially true for toluene blends for which actual RON measurements were made and all the blends were closely matched. Regression Analysis. The CCR data presented earlier in Table 6 is used for the following regression analysis. It is important to note that since the equivalence ratio and temperature were kept constant throughout this study, these parameters were not included in the regression analysis. However, it would be interesting to see the effect of equivalence ratio and intake temperature while doing such an analysis. Moreover, the RON and MON values that are used for both ethanol and iso-butanol blends are inherently subject to prediction bias as compared to measured values. Three figures are shown here where the number of correlating factors is increased in steps. First, the effect of MON and intake pressure is considered (Figure 15). The adjusted R2 value for the plot

Figure 13. Apparent heat release rate profiles for PRF 80 () and I1 (red ---), I2 (blue ---), and I3 (green dash-dot-dash-dot) blends expressed as a function of crank angle, intake pressure of 3 bar, and CR 5.8.

Figure 14. Apparent heat release rate profiles at autoignition condition for PRF 80 (, CR 10.7) and E1 (red ---, CR 11.0), E2 (blue ---, CR 11.0), and E3 (green dotted line, CR 11.2) blends expressed as a function of crank angle, intake pressure of 1 bar.

less reactive, higher octane blend components), and the effect of increasing intake pressure on the overall reactivity for each of the blend components. It is conjectured at this point that the CCR sensitivity of a given fuel component to increasing intake pressure is different and is also exhibited from the AHRR traces for respective blends. In summary, the AHRR profiles indicate that for RON 100 blends, iso-octane is less reactive compared to the blends of toluene, ethanol, and iso-butanol at lower intake pressures. This can be explained in two ways. First, the predicted RON values for the blends with the fuel of interest are slightly lower compared to iso-octane. This is especially true for toluene blends for which RON were measured and confirmed. However, a difference of 2−3 RON points does not seem significant in the greater context. Second, this can also be explained based on fuel composition and the resulting reactivity of the blend compared with iso-octane. As such, with increasing intake pressure, the reactivity of iso-octane relatively increases, causing it to ignite at earlier CCRs compared to the toluene, ethanol, and iso-butanol blends. It is conjectured at this point that at lower intake pressure, the blends with a higher percentage of fuel of interest also have a significant amount of n-heptane, which aids in creating a radical pool subsequently consuming the less reactive iso-octane and the fuels of interest at higher pressures and temperatures. Zhang and Boehman55

Figure 15. Linear regression fit of CCR data for all the investigated blends as a function of MON and intake pressure (bar). Adjusted R2 = 0.8735.

suggests a loose correlation between CCR, MON, and intake pressure. MON is measured for stoichiometric mixture at higher speed (900 rpm) and high intake temperature (100 °C); however, the ignition condition in this study is different from the MON test condition. It appears that the MON metric is not good for defining the mixture behavior for lean autoignition conditions. On the other hand, RON and intake pressure correlated well with CCR as shown in Figure 16. Furthermore, I


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Figure 16. Linear regression fit of the CCR data for all the investigated blends as a function of RON and intake pressure (bar). Adjusted R2 = 0.9377.

Figure 18. Linear regression of CCR data for all investigated blends expressed as a function of RON, MON, intake pressure (bar) and a second-order term for intake pressure (bar). Adjusted R2 = 0.9742.

as the effect of MON and the corresponding blend sensitivity is considered, the adjusted R2 values shows improvement as can be observed in Figure 17, although the improvement in the fit is

under autoignition conditions investigated here and extended to more realistic engine operation conditions. Discussion of Reaction Pathway Differences. The detailed ethanol oxidation mechanism by Marinov57 discusses the possible reaction pathways for ethanol consumption and is used here to explain how ethanol may be consumed. It appears that ethanol provides a similar path for H atom abstraction as that for an alkane. However, ethanol after H atom abstraction produces short chain species, which is stable at low-temperature and pressure conditions. Also, according to Marinov, ethanol is primarily consumed by C2H5OH + OH ↔ products and C2H5OH + H ↔ products, at lean conditions.57 The key steps may involve removing a methyl group or the hydroxyl group from ethanol which is difficult at low-temperature and pressure conditions based on the experimental studies discussed by Marinov.57 Species like acetaldehyde, methane, formaldehyde, formic acid, ethane, and alkenes, etc. are typically observed during ethanol oxidation in smaller fractions. However, smaller chain length inhibits alkyl peroxy chemistry and the subsequent internal isomerization reactions. Due to these stable species, ignition delays reported for ethanol, mostly from ideal reactor systems are much longer compared to n-heptane, iso-octane, and toluene. Also, ethanol does not exhibit a two-stage behavior even under high-pressure conditions, as is observed by Sjöberg and Dec in a recent engine study.56 It is likely that the more reactive components in the mixture create a radical pool from which hydrogen abstraction of the alcohols becomes possible. Since the radical pool is generated at relatively lower temperatures and pressures, for n-heptane and iso-octane compared to neat ethanol, ethanol consumes these radicals relatively early. However, the stable intermediates from the ethanol oxidation at low temperatures and pressures stay relatively inert until just before the onset of HTHR. Typically, during low-temperature oxidation, HȮ 2 and OḢ radicals provide many different pathways for chain propagation. It appears that at higher intake pressures and lower temperatures, alcohols, especially ethanol, consume OḢ and HȮ 2 radicals, thereby significantly reducing the chain propagation pathways. Similar observations are made by Haas et al.58 in their modeling study of ethanol and ethanol-PRF blends oxidation. Notable among the observations was the fact that the competition for HȮ 2 and OḢ radicals between ethanol (and its intermediates) and n-heptane results in slower regeneration of OḢ radicals, which subsequently results in a net reduction in the reactive radical pool relative to the neat n-heptane. Also, alkylhydroper-

Figure 17. Linear regression of CCR data for all investigated blends expressed as a function of RON, MON, and intake pressure (bar). Adjusted R2 = 0.9452.

marginal. Additionally, since both RON and MON are being considered here, the effect of fuel sensitivity (RON − MON) is indirectly considered. On the basis of the limited data presented here, it can be inferred that fuel sensitivity does not play a significant role in altering the autoignition behavior under lean conditions and varying boost levels. In Figure 17, it appears that there exists a slight curvature in the data trend. For the next iteration (Figure 18), cross-term second-order interaction between RON, MON, and intake pressure were considered and it was found that including the second-order intake pressure term resulted in a more linear distribution of the data. This also led to an improvement in the correlation adjusted R2 value. Any further attempts to improve the correlation by considering fuel composition did not result in significant increase in adjusted R2 value. Such correlations often do not explain the differences in LTHR chemistry well but are wellsuited for correlating the behavior for the end use applications such as engines. For instance, a similar correlation parameter, the octane index (OI), developed by Kalghatgi et al.8 tries to find a solution to this issue by correlating RON, MON, and the engine operation condition. It remains to be seen whether such correlations can be used to predict the behavior of a blend J


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heptane content in all the blends that were required to match the RON, which overcame differences in the reactivity of the individual fuels of interest in this study. Nonetheless, based on the heat release rate data, especially for RON 100 blends, it was observed that the effect of alcohol added to the blends was to inhibit/suppress the n-heptane and iso-octane from forming the intermediates that eventually take part during HTHR. It is conjectured that at lower temperatures, the less reactive alcohols, ethanol and iso-butanol, scavenge the radicals from the radical pool, thus delaying the conversion of more reactive components in the mixture. Conversely, toluene, for a fixed RON, did not affect the low-temperature chemistry much. Hence it is observed that the overall oxidation mechanisms for toluene and alcohol blends are distinctly different where alcohols seem to inhibit the low as well as high-temperature, preignition chemistries, while toluene delays the onset of HTHR. It is interesting to note that relative changes in reactivity between different blends may also play an important role for SI engines operating at stoichiometric and highly boosted conditions. Especially, the appearance of LTHR in the prespark regime at higher intake pressures, even for high RON blends may affect the knock phenomenon in these engines. Further careful investigation is required to understand such a behavior. The engine was run at MON-like high-temperature condition; however, the equivalence ratio was distinctively different. Nonetheless, the correlation between intake pressure, MON values, and CCR yielded a weak correlation, suggesting that the definition of the MON metric is limited to certain engine operation conditions. It was also shown that a combination of RON, MON, and intake pressure was sufficient to establish a good correlation with CCR, which was further improved by considering a second-order intake pressure term. It must also be pointed out that the effect of intake pressure by itself is very important as the relative reactivity of these fuel blends changed with respect to each other with changing intake pressures, especially true for higher octane blends with low volumetric concentrations of n-heptane. Such correlations, however, will not be able to distinguish the differences in lowtemperature reactivity between fuels as observed in this study and should only be used for coarse blend designs. Authors also acknowledge the fact that a higher equivalence ratio comparison of these blends would have added more information to this publication; however, autoignition of premixed fuel under highly boosted conditions at equivalence ratios higher than 0.5 could have damage the engine. This will certainly be the focus of future publications.

oxide from the parent fuel molecule typically decomposes to provide alkyl radical and additional OḢ radicals for chain propagation. However, ethanol does not form an alkylhydroperoxide, thus shutting that pathway for chain propagation during the low-temperature regime. This subsequently affects the more reactive components in the blend as the deficiency of such radicals is experienced, especially for blends with very high percentages of ethanol. Toluene differs from ethanol, as it provides a pathway for regeneration of chain propagating radicals during low-temperature regime, especially for higher intake pressure condition where LTHR is observed for iso-octane and all the RON 100 toluene blends. During low-temperature oxidation, ring structure of toluene is preserved due to high bond dissociation energies of the C−H bonds in the ring (465 kJ mol−1).59 The only available site for hydrogen abstraction and oxygen addition is thus at the methyl group attached to the benzene ring. The reaction sequence under low-temperature conditions for toluene are well-summarized by Vanhove et al.60 At relatively, low-temperatures, toluene is converted to stable benzyl radicals and benzaldehyde. During this process, toluene thus participates in radical pool generation under low-temperature conditions via the methyl group attached to the benzene. According to Andrae et al.,18 the benzyl radical thus formed is thermally stable due to electron delocalization and is less susceptible to oxidation. However, these stable species can take part in radical−radical reactions and can attack n-heptane and iso-octane to create toluene and much more reactive radicals.18,59 Such reactions are termed as co-oxidation reactions by Andrae et al.18 As a result, the low-temperature consumption of n-heptane and iso-octane is continued despite the presence of lesser reactive species during low-temperature regime resulting in LTHR (for ≥2 bar intake pressure in this study). Vanhove et al.60 also state that the autoignition chemistry remains dominated by n-heptane in binary mixtures of n-heptane and toluene. This thus explains the similarity between the toluene blends and PRF blends in terms of their low-temperature reactivity as is indicated by similar phasing and % of LTHR. However, the cyclic resonant structure of toluene is conserved, resulting in an overall delay of HTHR of the blend.

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CONCLUSIONS The focus of this study was on probing the low-temperature combustion characteristics of high-octane surrogate fuels under lean boosted conditions. To investigate the effect of fuel composition on autoignition under boosted conditions (up to 3 bar), a modified CFR engine was utilized at intake temperature of 155 °C and equivalence ratio of 0.25 through all test conditions. Fuels tested in this study consisted of 2 base fuels (iso-octane, a PRF 100 fuel, and a PRF 80 blend) and 18 blends of n-heptane, iso-octane, and a fuel of interest (toluene, ethanol, and iso-butanol), while their RONs were targeted to 80 and 100. The following conclusions are drawn from this study. The reactivity of all the blends investigated increased as a function of intake pressure. Also, for iso-octane, two-stage ignition behavior was observed at higher intake pressures even for the lean conditions investigated here. For RON 100 blends, increasing the pressure led to a relative reduction in blend reactivity compared to neat iso-octane. For RON 80 blends, increasing the intake pressure lead to relatively small changes in CCR and, hence, the overall reactivity, compared to the base PRF 80 blend. This is attributed to higher percentages of n-

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

André L. Boehman: 0000-0002-0965-9288 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was sponsored by Volvo Technology of America and the U.S. Department of Energy under DOE Award DEEE0004232. The authors are particularly grateful for the support and guidance of Sam McLaughlin, John Gibble, Dick Morton, Arne Anderssen, and Pascal Amar of Volvo and for the K


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valuable collaboration with Dan Haworth and Jun Han at Penn State University.

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