Effect of octane number and thermodynamic conditions on combustion process of spark ignition to compression ignition through a rapid compression machine

Fuel 262 (2020) 116480

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Full Length Article

Effect of octane number and thermodynamic conditions on combustion process of spark ignition to compression ignition through a rapid compression machine

T

Qinhao Fan, Yunliang Qi, Zhi Wang

⁎

State Key Laboratory of Automotive Safety and Energy, Tsinghua University, Beijing 100084, China

ARTICLE INFO

ABSTRACT

Keywords: Spark ignition to compression ignition Octane number Ethanol blends Transition temperature Control authority Knock intensity

Spark assistance in homogeneous charge compression ignition (HCCI) is a promising method to improve combustion stability. Fundamental experiments were carried out in a rapid compression machine along with chemical kinetics analysis to investigate the complete combustion process of spark ignition to compression ignition (SICI) using ethanol-blended fuels. Five fuels, consisting of n-heptane, iso-octane and ethanol with different fractions, are divided into two groups. The fuels with different research octane number (RON) and motor octane number (MON) but identical octane sensitivity (S) are in the same group. The equivalence ratio is fixed at 0.5, and the experimental pressure covers the engine-relevant conditions (10–35 bar) while the target temperature ranges from 735 K to 860 K, overlapping most regions with negative temperature coefficient (NTC) of n-heptane and iso-octane. Results show that octane sensitivity with low RON has poor ability to evaluate fuel reactivity especially in the vicinity of “beyond MON” area due to low-temperature oxidation acceleration of ethanol. The influence of fuel reactivity, auto-ignition heat release amount and flame compression effect on knock intensity enhancement decreases in turn. The lower transition temperature and pressure at the time of auto-ignition is observed in the fuel with lower RON regardless of S, resulted from stronger LTHR and greater temperature rise from cool flame. The fuel with medium RON and S based on ethanol blending is more suitable for SICI combustion since it can make a better balance among knock intensity, dilution tolerance and control authority from flame in the conditions studied, which gives an insight into the effect of ethanol blends on combustion process and provides a reference for fuel design aimed at lean SICI combustion.

1. Introduction Energy saving and environmental protection are the main driving forces to develop internal combustion engines (ICEs) with higher thermal efficiency and lower emissions. It is known that high compression ratio (CR) along with oxygenated fuels [1,2], lean burn [3], cooled exhaust gas recirculation (EGR) [4], and fuel-engine co-optima [5,6] are well-accepted methods to achieve aforementioned goals. Ethanol as the promising and widely sourced alternative fuel has been applied in transportation in several countries around the world [7]. With the consideration of energy safety and policy promotion, gasoline blended with 10% ethanol by volume (E10) will be extensively used in China [8]. However, more fundamental investigation related to oxygenated fuels including ethanol should be further conducted [9] especially for lean mixture combustion at higher thermodynamic conditions, which can provide a guidance for advanced engine applications.

⁎

Both spark ignition (SI) engine and compression ignition (CI) engine can benefit from ethanol blends. In the co-optimization between fuels and modern SI engine, ethanol with high research octane number (RON), octane sensitivity (S), and latent heat of vaporization (HOV) [10] is regarded as a fuel with strong resistance to engine knock. When it is blended with commercial gasoline fuel, the blended fuel permits the engine boosted to operate in the “beyond RON” area [11,12]. In this area, S and octane index (OI = RON – K·S and S = RON – MON, put forward by Kalghatgi [13,14]) reflect a stronger ability to evaluate fuel’s anti-knock performance. As for CI engines, moderate ethanol added into diesel can decrease soot emissions and achieve the trade-off between thermal efficiency and nitric oxides (NOx) [15]. Moreover, in homogeneous charge compression ignition (HCCI) combustion, ethanol normally plays a role in decreasing reactivity to control low temperature heat release (LTHR) and maximum pressure rise rate [16]. To effectively control the combustion phase and extend the engine

Corresponding author. E-mail address: [email protected] (Z. Wang).

https://doi.org/10.1016/j.fuel.2019.116480 Received 2 September 2019; Received in revised form 20 October 2019; Accepted 22 October 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.

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Nomenclature AHRA CI CR EGR EOC HCCI HOV HRR HTHR ICEs IHRR IMEP KI LHV

LTHR MON NTC NVO OID PRF RCM RON ROP S NOx SACI SI SICI TDC TPRF

auto-ignition heat release amount compression ignition compression ratio exhaust gas recirculation end of compression homogenous charge compression ignition heat of vaporization heat release rate high temperature heat release internal combustion engines integrated heat release rate indicated mean effective pressure knock intensity lower heating value

load conditions, spark assistance is introduced into HCCI, termed as spark-assisted compression ignition (SACI). This combustion mode takes the advantage of HCCI combustion [17–21], especially when the fuels are blended with ethanol [22,23]. Wang et al. [17] reported that using commercial gasoline with RON of 94.4, spark ignition can smooth the transition fluctuations in SI/HCCI hybrid combustion engine and improves the stability of HCCI combustion at HCCI critical status where misfire easily occurs in the end-gas. Combining advanced spark timing with EGR, indicated mean effective pressure (IMEP) was extended from 4 bar to 8 bar at stoichiometric condition [18]. Manofsky et al. [19] found that spark assistance could enhance (or improve) the control of the combustion phasing in a HCCI combustion engine and decrease ringing intensity under high engine loads when the commercial gasoline with RON of 90.5 was used. Olesky et al. [20,21] also presented the promotion of flame propagation on bulk ignition and further investigated the influence of spark assistance under conditions at different intake temperature, EGR rates and equivalence ratios. Their results showed that spark timing and intake temperature were the primary factors affecting SACI combustion based on the research grade gasoline with RON of 87, and that spark assistance became almost useless in the vicinity of lean flammability limits. Ethanol blended fuels have also been studied on SACI engines. Sjöberg et al. [22] compared the performance between E30 (RON 104 and MON 91) and gasoline (RON 96.6 and MON 88.7) in lean and diluted conditions and found that E30 had less ability to adjust the combustion process in lean mixture even though it diminished cycle-to-cycle variation. Gentz et al. [23] observed that E10 (RON 92 and MON 84.9) could decrease the requirement of intake temperature compared with standard Tier II certification gasoline (RON 96 and MON 88.3). However, there is lack of systematic research focused on the impact of octane number on the complete combustion process of spark ignition to compression ignition (SICI) especially when the fuel is blended with ethanol, and it is difficult to carry out a study decoupling RON/MON and S (i.e. fuels with varied RON/MON but identical S and vice versa) on the engine test bench. Therefore, fundamental experiment is more suitable for investigating characteristics of SICI combustion and the role of ethanol in gasoline surrogate fuels under engine-relevant conditions. Fundamental research on interaction between the flame propagation and auto-ignition has also been performed in the past decade but only with non-oxygenated gasoline surrogate fuels such as n-heptane and iso-octane (primary reference fuel, PRF). Assanis et al. [24] quantified the lean flammability limits under diluted combustion at low pressure but high temperature state. The effect of flame propagation on overall ignition delay (OID) reduction was observed based on iso-octane/O2/N2 lean mixture. Ma et al. [25] used high speed imaging in an optical engine to study the flame-induced reaction front propagation, and the combustion mode changed from normal flame propagation to

low temperature heat release motor octane number negative temperature coefficient negative valve overlap overall ignition delay primary reference fuel rapid compression machine research octane number rate of production octane sensitivity nitric oxides spark-assisted compression ignition spark ignition spark ignition to compression ignition top dead center toluene primary reference fuel

subsonic reaction front at 795 K for n-heptane. The temperature for isooctane was about 1100 K which was obtained from a 1-D simulation by Martz et al. [26]. Strozzi et al. [27] utilized PLIF and high-speed chemiluminescence in a rapid compression machine (RCM) and found that SICI combustion showed a more volumetric but subsonic formaldehyde consumption compared with fast fronts in HCCI mode. However, only a few studies investigated the effect of octane number of ethanol blends on SICI combustion fundamentally under lean-burn conditions, and the trend of transition temperature from SI to CI for fuels with different octane numbers is still unclear. To further improve our understanding of the impact of conventional fuel metrics (e.g. RON/MON/S) on SICI combustion based on the previous work [28], this study is to investigate impact of octane number and provide a comprehensive analysis for fuel design in SICI combustion. Five fuels comprising of n-heptane, iso-octane and ethanol with different fractions are used and RON/MON values for these fuels are dedicatedly designed through adjusting the ratio of PRF to ethanol. These five fuels are divided into two groups, and the fuels in the same group have the same S but varied RON. All of the experiments are conducted in an RCM at the equivalence of 0.5 with and without spark assistance, then chemical kinetics analyses are carried out to understand the effect of fuel reactivity on SICI combustion. OIDs of fuels are firstly measured and the change of ethanol reactivity is investigated by analyzing the rate of production (ROP) and reaction pathway of OH radical. After that the impact of RON value on knock intensity (KI) and control authority from flame in combustion process are examined, and non-knock area (KI < 1) for each fuel is identified. Then, the influence of RON to dilution tolerance is compared and sensitivity analysis of flame speed to critical reactions is performed. Finally, transition temperature affected by the octane number is discussed and the distribution of SI/SICI/SACI mode is compared among the different fuels. Based on the experimental and computational simulation results, the requirements of fuel properties and fuel metrics for SICI combustion are examined, contributing a reference in fuel design for lean-burn SICI engines. 2. Research methods 2.1. Experimental setup and computational method The experiments about OID measurement and SICI combustion investigation were carried out in a rapid compression machine at the Tsinghua University (TU-RCM). Detailed information of this RCM and experimental setup can be found in Ref. [28] and Ref. [29]. A high speed camera was used to record the combustion process and determine the auto-ignition timing (tauto) jointly with the heat release rate (HRR) profile derived from the cylinder pressure data [20,28]. 2

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Computational simulation was performed using CHEMKIN-PRO [30]. 0-D, closed homogeneous reactor and premixed laminar speed simulator were utilized for chemical kinetics analysis and sensitivity analysis of flame speed, respectively. The mechanism [31] comprising of 679 species and 3479 reactions from LLNL was used, and the accuracy of this mechanism to study n-heptane/iso-octane/ethanol ternary mixtures has been verified both in stoichiometric [32] and lean mixtures [28]. To take into account the heat loss in ignition delay, simulation using a closed homogeneous reactor and a non-reactive shot for each test case was run, in which O2 was replaced by N2 to extract equivalent volume history from the pressure profile [33]. The rationality to use laminar speed simulator for sensitivity analysis is also considered based on the fact that flame propagation of lean mixture in RCM is in the magnitude of laminar flame speed under the circumstance with and without extra dilution [28].

2.3. Definitions of characteristic parameters and uncertainty analysis To accurately describe SICI combustion process, characteristic parameters will be introduced and the corresponding uncertainty analysis will be discussed. The ignition delay time for auto-ignition with and without spark assistance is abbreviated as τspark and τig, respectively. Both τspark and τig are the interval between the time of EOC and the time of maximum pressure gradient, which is identical to those in Ref. [24]. τspark_delay is the interval from the spark timing to the time with minimum pressure during ignition delay, and spark timing is held near EOC. Specific definition of combustion characteristic time is depicted in Fig. 1. The primary uncertainty of peff is from initial pressure (p0) and the maximum variation of p0 is 0.1 bar which results in 1.5% maximum uncertainty of peff calculated by Eq. (2). The uncertainty of Teff mainly comes from two causes: (a) fluctuation of initial temperature (dT1) and (b) variation of initial pressure (dT2). Based on the instrument specifications and the type of buffer gas, the total uncertainty of Teff is 4.8 K in this study according to Eq. (4).

2.2. Fuel composition and test conditions Five fuels comprising n-heptane, iso-octane and ethanol were divided into two groups with sensitivity of 3 and 8, respectively. In each group, the fuels had different RON in order to investigate the effect of octane number on SICI combustion process. Ethanol was the only S booster in the ternary mixture. As seen in Table 1, for the cases with a fixed S, the ethanol contents are not changed while the concentration of n-heptane increases as RON decreases. All of the five fuels were formulated by the blending role used in our previous work [32]. This blending role takes the singularity and synergistic effect into consideration for ethanol and can be extended to formulate toluene primary reference fuel (TPRF). Synergistic effect occurs when ethanol is blended with PRF due to the large difference in molar weight [34,35] and this effect does not exist in TPRF. Singularity effect is mainly attributed to the form of Eq. (1) and should be emphatically taken into account when the parameter values of additive and baseline are close.

Target value - Base value Normalized objective parameter = Additive's value - Base value

Knock intensity was obtained from Eq. (5), which is the integral of filtered pressure over time. High-pass filter with cut-off frequency of 8 kHz is assigned. The natural frequency of pressure transducer (KISTLER 6125C) is about 75 kHz and the first-order oscillation frequency [37] is about 10 kHz based on the diameter of the combustion chamber. Meanwhile, oscillation energy is mainly shown on the frequency band of 10–35 kHz. Therefore, the influence of transducer resonance induced by the pressure wave can be ignored and pressure oscillation characteristics can be fully captured. Based on repeated tests for each thermodynamic condition, the maximum standard deviation of KI is 1.9 bar·ms. The region with KI less than 1.0 is defined as non-knock area [38,39].

KI =

(1)

1 t pmin

Teff T0

1

t pmin

t pEOC

t pEOC

d ln T = ln

pdt

t2 t1

|pfilter |·dt

Inert index =

inert index

(2)

peff ·

ig

(inert index ) peff

=

(6)

KI + 1 2

·

2 peff

(inert index )

+

ig

2

·

2 ig

+

(inert index ) KI

2

·

2 KI

(7)

peff p0

(5)

Inert index and normalized time (τspark/τig) which were used in previous work [28] are adopted to evaluate the knock propensity and control authority of combustion process from flame, respectively. Inert index is calculated by Eq. (6) and the deviations of inert index and normalized time are calculated based on root mean square method [40] as shown in Eq. (7) and (8).

Effective thermodynamic parameters rather than the state at end of compression (EOC) were adopted in this study since longer ignition delay in lean burn causes more obvious heat loss and stronger boundary layer effect [36]. Effective temperature (Teff) and pressure (peff) can be calculated by Eq. (2) and (3) in accordance with adiabatic core hypothesis. Teff and peff are used in all the tests including spark assisted test and OID measurement. The uncertainty analysis on Teff and peff will be introduced in the following section.

peff =

(4)

(dT1 ) 2 + (dT2 ) 2

dTtotal =

(3)

normalized time

where p0 and T0 represent initial pressure and temperature, peff and Teff mean effective pressure and temperature, respectively, and γ represents specific heat ratio. Target temperature in the experiment ranges from 735 to 860 K and pressure is close to the in-cylinder pressure at top dead center (TDC). Ultra-high purity grade nitrogen (> 99.999%), oxygen (> 99.995%), argon (> 99.999%) and carbon dioxide (> 99.995%) were used to prepare the gaseous mixture. Equivalence ratio was calculated in terms of O2 content. N2 and Ar were the diluent gases, and CO2 was used for extra dilution conditions. Higher effective temperatures were obtained through replacing a part of N2 with Ar in consequence of the higher specific heat ratio of Ar. Detailed mixture compositions for the five fuels are listed in Table 2 and slightly adjusted according to the change of ambient temperature and specific heat ratio. It is worth mentioning that Fuel 1 and Fuel 4 in this study are consistent with Fuel 2 and Fuel 3 in the previous research [28], respectively.

=

(normalized time ) ig

2

·

2 ig

+

(normalized time ) spark

2

·

2 spark

(8) In this study, OID (i.e. τig) is used in the experiments without spark assistance and τig of 50 ms is regarded as the criterion to distinguish the Table 1 The formulating target and specific composition of fuels used in this study.

3

Fuel Type

Target for RON

Target for MON

Target for S

n-heptane (Vol %)

isoOctane (Vol %)

Ethanol (Vol %)

1 2 3 4 5

100 93 87 100 93

97 90 84 92 85

3 3 3 8 8

4.69 12.23 18.82 16.44 29.29

89.40 81.86 75.27 47.11 34.26

5.91 5.91 5.91 36.45 36.45

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Table 2 Mixture composition and related test conditions of the experiment. Fuel type

Gas composition in molar basis (%)

CR

Target value

Calculated thermodynamic conditions*

T (K)

T (K)

p (bar)

Fuel vapor

O2

N2

Ar

1

0.95 0.95 0.95

20.81 20.81 20.81

78.24 58.68 35.21

0 19.56 43.03

16

735 785 860

728.4–738.7 777.8–791.7 858.8–863.1

12.3–37.3 12.3–32.6 12.6–25.5

2

0.95 0.95 0.95

20.81 20.81 20.81

78.24 17.99 41.47

0 60.25 36.77

16

735 785 860

733.0–742.1 774.9–789.8 853.2–860.8

11.7–30.1 10.5–30.9 11.9–25.7

3

0.96 0.96 0.96

20.81 20.81 20.81

78.23 60.63 37.55

0 17.60 40.68

16

735 785 860

734.8–743.5 777.4–794.1 853.5–862.4

11.5–30.3 11.4–30.3 11.8–25.5

4

1.45 1.45 1.45

20.70 20.70 20.70

77.85 54.49 31.14

0 23.35 46.71

16

735 785 860

724.1–729.4 780.1–787.5 854.4–864.5

20.9–37.6 14.9–37.2 11.2–25.3

5

1.60 1.60 1.60

20.67 20.67 20.67

77.73 59.85 36.53

0 17.88 41.19

16

735 785 860

733.4–743.1 779.6–794.7 854.9–862.4

11.7–30.0 11.8–29.8 11.7–25.7

this study is 4.2 ms corresponding to the uncertainty of 12%, which is acceptable for the result analysis. It is well recognized that as temperature increases more and more intense HTHR and pressure oscillation occur, which is closely related to the enhanced fuel reactivity in the unburned mixture and higher concentration of H radical in the flame structure [42]. The effects of fuel reactivity and thermodynamic conditions on KI will be further discussed in the following parts. 3.2. Validation of OID without spark and chemical kinetics analysis 3.2.1. OID verification in the studied thermodynamic conditions Predicted OIDs for the five fuels were validated by experimental results at the targeted temperature from 735 K to 860 K and the tested effective pressure range. The predicted OIDs can well capture the trend of experimental results and the maximum relative difference is 18.6% at low pressures, as shown in Fig. 3. It is well recognized that OID decreases with decreased RON regardless of S but the degree of reduction drops as temperature rises. Moreover, regardless of RON 100 and RON 93, the fuel of S = 8 with more ethanol contents has more significant reduction in OID than the fuel of S = 3 does when temperature rises from 785 K to 860 K. This can be attributed to oxidation acceleration of ethanol over 800 K [28,43]. It is worth noting that Fuel 2 (RON 93 S = 3) and Fuel 5 (RON 93 S = 8) have quite similar OID under all the test conditions, indicating that the ability to predict fuel

Fig. 1. Typical pressure profiles of experiment with and without spark assistance under similar thermodynamic conditions (Teff = 789.1 K, peff = 25.4 bar with spark and Teff = 784.7 K, peff = 24.8 bar without spark).

SACI from the SICI combustion mode. For SICI mode (τig more than 50 ms), if there is no flame propagation in combustion chamber misfire will occur. In other words, flame propagation is indispensable in SICI combustion [41]. As for SACI mode, auto-ignition can spontaneously occur whether or not spark assistance exists. This means that the amount of flame-based heat release ahead of bulk ignition in SACI mode is much smaller than that in SICI mode. The whole combustion process of SACI/SICI has two stages, flame propagation, and bulk ignition. The heat release amount due to auto-ignition at tauto (AHRA) should be concerned since it affects KI value and reflects the relative intensity of heat release from flame propagation (high temperature heat release, HTHR) and bulk ignition in the unburned mixture. 3. Results and discussions 3.1. Repeatability of measurements with spark assistance Three repeated tests were performed in each engine operating condition to improve the result robustness and to minimize the standard deviations of τig, τspark and KI along with inert index and normalized time. Fig. 2 shows that in the same thermodynamic condition, the difference in pressure traces (marked by Δτ) is increased with the increase of τspark. However, the maximum standard deviation of τspark in

Fig. 2. Repeated pressure profiles of three fuels used in the current experiments (Repeatability of Fuel 1 and Fuel 4 was inspected in the previous work [28]). 4

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(a) Target temperature = 735 K

(b) Target temperature = 785 K

(c) Target temperature = 860 K

(d) Fuel composition by molar basis in the experiment

Fig. 3. Overall ignition delay verification and specific fuel composition for five fuels involved in this study.

reactivity based on S is substantially weakened. This phenomenon usually occurs in the “beyond MON” region and high reactivity system where high temperature and low pressure cause the similar OID for fuels with different S [44]. To further demonstrate the change of fuel reactivity and factors influencing this change, the contours of OID for Fuel 2, Fuel 3, and Fuel 5 are depicted in Fig. 4 along with RON and MON standard test conditions. The pink short dot line represents the reactivity demarcation line for iso-octane and ethanol at the equivalence of 0.5 [28], which well distinguishes the reactivity of Fuel 2 and Fuel 5 at high pressure. As shown in Fig. 4(a)-(c), higher S fuel (Fuel 5) corresponds to stronger anti-knock ability at the left side of this line, which is consistent with the result predicted by OI = RON – K·S. However, when pressure is under 20 bar Fuel 5 is more reactive and this phenomenon is mainly attributed to two parts. On one hand, more n-heptane in Fuel 5 causes enhanced low-temperature oxidation of ethanol. On the other hand, OID of iso-octane is more sensitive to pressure compared to ethanol, which means a larger increase in OID will occur with pressure decreased [32,45]. The orange line in Fig. 4(f) is obtained by connecting cross points in Fig. 4(a)-(e) and represents the reactivity demarcation line for Fuel 3 and Fuel 5. Both MON value and n-heptane in mole basis are quite close for Fuel 3 and Fuel 5. Compared with the reactivity demarcation presented by the pink line, the thermal state (presented by the orange line) shows that the reactivity of iso-octane is over that of ethanol in the more reactive system (more n-heptane). It is reasonable that the orange line shifts to high-T but low-p region compared with the pink line since OID of ethanol is more sensitive to temperature but less

sensitive to pressure in comparison to alkanes [45]. Consequently, this moving direction can enlarge the reactivity difference between ethanol and iso-octane and consequently result in higher reactivity of Fuel 5 than Fuel 2 at higher temperature but lower pressure state. Moreover, the equation of OI = RON – K·S can be rewritten as Eq. (9). As mentioned above, Fuel 3 and Fuel 5 have almost identical MON and OI values along the orange line but with varied S, so it can be inferred that the orange line should be in the vicinity of the real line for “K = 1”. The theoretical line of “K = 1” is T-p trajectory in standard MON test, which requires an inlet mixture temperature of 149 °C (cooling effect of liquid fuels has been removed) [46]. Consistent with standard MON test, the gaseous mixture is prepared in advance for RCM measurement, and HOV effect from ethanol can be ignored. Therefore, this shift of the line for “K = 1” based on the experiments in RCM indicates chemical influence of ethanol rather than HOV effect on the measurement of octane number. Similar phenomenon was also observed in the assumed line of “K = 0” (standard RON test line) [47] while more prominent shift occurred since both fuel chemistry and charge cooling simultaneously affected [48].

OI = RON

K ·S = MON + (1

K )· S

(9)

3.2.2. Chemical kinetics analysis on ethanol oxidation acceleration In this section, low-temperature oxidation of ethanol enhanced by nheptane will be fundamentally analyzed to investigate the component interaction in species pool. Fig. 5 shows the auto-ignition process of Fuel 4 (RON 100 and 5

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(a) OID = 15 ms

(b) OID = 25 ms

(c) OID = 35 ms

(d) OID = 50 ms

(e) OID = 65 ms

(f) Reactivity comparison

Fig. 4. Comparison of OID contours in studied conditions for three fuels. Red circle means Fuel 2 with the same RON value as Fuel 5 marked by green star and Fuel 3 marked by blue triangle has the nominally same MON value as Fuel 5. The pink short dot line represents the reactivity demarcation line between iso-octane and ethanol.

attaining 1 × 10−9 is selected for aforementioned analysis, as depicted in Fig. 5. Fig. 6 shows that with the increase of n-heptane, OH radical consumed by itself is significantly enhanced. Meanwhile, the OH consumption rate of ethanol also increases especially at 735 K and 785 K. The increased concentration of characteristic species including CH2O, CO, and C2H4 at the same OH mole fraction indicates low-temperature oxidation acceleration of ethanol [49]. At 860 K, the difference in OH radical ROP between Fuel 4 and Fuel 5 is reduced because ethanol undergoes more β-H abstraction to produce OH radical and C2H4 instead of acetaldehyde (mainly generated from α-H abstraction), consequently increases the reactivity in the species pool [50]. However, there is little change even slight suppression in OH consumption from

S = 8) and Fuel 5 (RON 93 and S = 8) from 735 K to 860 K at 22.5 bar. Reaction time for each temperature in Fig. 5 (a) and (b) is normalized by the τig at 735 K. It is clear that Fuel 4 presents greater OID changes as temperature increases, which is mainly ascribed to chemical inertness of ethanol at low temperature. For ethanol, relatively stable acetaldehyde generated from H-abstraction reactions rather than OH radical generated from chain branching reactions results in rather low reaction rate comparing with alkanes [32]. Fuel 5 demonstrates stronger LTHR and smaller difference in OID with the temperature increase since greater n-heptane fraction leads to more OH radical. The specific influence of reactivity in species pool on ethanol low-temperature oxidation is revealed through OH radical ROP analysis and reaction pathway analysis. The time of OH radical mole fraction

Fig. 5. Effect of temperature on auto-ignition process for described by the evolution of temperature and characteristic radicals. 6

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and ethanol are not changed, as no obvious fuel-to-fuel interaction occurs in the n-heptane/iso-octane/ethanol ternary mixture. This is consistent with the observation in Ref. [52] based on the speciation sampling and model prediction. The small reactive radicals occurring in the respective reaction pathway will be shared in the species pool. In general, the smaller change in OID with the temperature increase for Fuel 5 is mainly attributed to low-temperature oxidation acceleration of ethanol and has little to do with iso-octane. 3.3. Effect of RON on knock intensity and control authority of ignition KI value is mainly affected by fuel’s reactivity and contained energy (i.e. AHRA) in the unburned mixture when the fuels have the same RON but different S and reactivity plays a more significant role at lower temperature [28]. To extend aforementioned conclusions, the effect of RON value will be investigated to reveal the influence from RON on KI with varied S. Moreover, non-knock area for each fuel overlapped on the corresponding OID contours will be identified. Fig. 8 shows AHRA and KI variation with peff at three target temperatures for Fuel 1–3 with different RON values but the same S. As shown in Fig. 8, for a given pressure with the decrease of RON, AHRA increases and SI mode has less potential to be achieved under low pressures, which reflects the shorter timescale of τspark than that of flame propagation. In other words, flame of lower RON fuel will consume less unburned mixture ahead of bulk auto-ignition. AHRA is nearly positively correlated with KI either in a fixed fuel or among three fuels at a given pressure except for 735 K. It is worth noting that at 735 K, Fuel 1 with lower AHRA and lower reactivity has the KI larger than Fuel 2 and Fuel 3 do, as shown in Fig. 8 (a). This phenomenon is attributed to the flame compression effect when the reactivity of the end-gas is low. In this situation, the enhancement of KI is resulted from the flame compression rather than the fuel reactivity and AHRA even though the flame propagation usually shows the effect of knock mitigation. One common trend shown at three temperatures is that the KI value is prone to similar levels at high pressure (> 24 bar), but AHRA is not for different fuels. This means that the fuel reactivity has a more decisive influence on KI than AHRA does. This can be further supported by the results shown in Fig. 9(a) in which AHRA for Fuel 4 and Fuel 5 are nearly the same but KI of Fuel 4 is significantly lower than that of Fuel 5. Therefore, the influence of fuel reactivity to KI is strongest, AHRA second and flame compression the weakest. Fig. 9 shows the performance of high-S fuel with varied RON. As shown in Fig. 9, Fuel 5 has larger AHRA than Fuel 4 does and even larger than Fuel 3 (with the lowest RON in the five fuels studied) does at 860 K. The larger AHRA implies the weaker control authority from flame in combustion process, which means that the improvement of laminar flame speed by high ethanol concentration cannot be manifested in a more reactive system. Moreover, Fuel 2 and Fuel 5 have different S with the same RON but results in similar AHRA and KI. This indicates that S may not be used to describe the end-gas reactivity. Similar observations were reported in simulation work [44] and engine research [10]. It requires that new fuel metrics different from RON/ MON and S should be proposed to evaluate reactivity especially in “beyond MON” area. To further evaluate fuels’ anti-knock performance that determines the amount of EGR and the selection of operating conditions for real engine application, KI contours and non-knock region combined with OID contours are plotted in Fig. 10. As shown in Fig. 10, the fuel with higher RON has a larger non-knock area. Even though high-S and highRON fuel (Fuel 4) has the largest knock-free region, significant change in reactivity results in the control authority from flame too weak to adjust combustion process at 735 K and severe pressure oscillation at 860 K [28], which is the main drawback for high-S fuel. For the fuel group with S = 3, there is no knock-free region belonging to SACI mode for Fuel 1, although it has the largest non-knock area among three fuels. Based on Fig. 10 (c) and Fig. 8, Fuel 3 shows the strongest pressure

(a) 735 K

(b) 785 K

(c) 860 K Fig. 6. Comparison of OH radical rate of production between Fuel 4 and 5 under three studied temperatures at the time of OH radical attaining 1 × 10−9 mole fraction.

iso-octane with RON decreased and lower H-abstraction reaction rate means that the reactivity of iso-octane is decreased. The analysis of reaction pathway is used to further reveal the fundamental behind this phenomenon, as shown in Fig. 7. As shown in Fig. 7, for low-temperature oxidation of iso-octane, the most significant distinction between Fuel 4 (marked by black) and Fuel 5 (marked by red) is that Fuel 5 has lower fraction in the pathway of second O2 addition followed by chain branching to produce OH radical and release heat [51]. Because the second O2 addition along with isomerization is the main cause of LTHR, the lower fraction of reaction participation clearly indicates a suppression on low-temperature oxidation of iso-octane shown in Fuel 5. As for ethanol, a slightly larger fraction in the pathway of β-H abstraction can be observed in Fuel 5, which is further conducive to improving ethanol oxidation under low temperatures. However, the main reaction pathways for both iso-octane 7

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(a) iso-Octane

(b) Ethanol

Fig. 7. Low temperature reaction pathways analysis for iso-octane and ethanol in Fuel 4 (marked by black) and Fuel 5 (marked by red) at 785 K, 22.5 bar and the time of OH radical attaining 1 × 10−9 mole fraction.

oscillation and the lowest control authority from flame. However, high reactivity of Fuel 3 is conducive to diminishing the possibility of misfire under extra dilution conditions, which will be analyzed later.

(e.g. H, OH) generation and temperature rise in the mixture because of the stronger LTHR. Dilution ratio =

3.4. Effect of RON on dilution tolerance

nextra_buffer gas pextra_buffer gas = × 100% n fresh mixture + nextra_buffer gas p0 + pextra_buffer gas

(10)

EGR has been widely applied in ICEs to control NOx emissions and reduce pumping loss for efficiency improvement [18,53,54]. In HCCI and SACI combustion mode, internal EGR through negative valve overlap (NVO) is a common method to increase temperature in cylinder at TDC for steady combustion and simultaneously suppressing maximum pressure rise rate [20]. Dilution tolerance [55] is a meaningful parameter to evaluate the possibility of misfire and EGR limit in lean SICI combustion. In this section, the condition of peff = 25.5 bar and Teff = 860 K is selected as baseline because fuels at this thermal condition have the largest pressure oscillation and suitable for investigating combustion characteristics and dilution tolerance. Heat release rates under different dilution ratios are compared in Fig. 11 in which the zero timing means the spark timing. The mixtures are diluted by CO2 only, and the dilution ratio is defined by Eq. (10). It is clear that the fuel with higher RON has the larger amount of HTHR, but HTHR is significantly suppressed with the increase of dilution ratio manifested as misfire and longer τspark. Although no misfire occurs for Fuel 2, Fuel 3 and Fuel 5, the smooth and slowly increased profiles of integrated heat release rate (IHRR) of these three fuels indicate that the flame makes less contribution to the whole combustion process (i.e. less flame-based heat release), compared with other two fuels. Fig. 12 further quantifies the impact of dilution ratio on flame propagation in the diluted mixture with a trend that τspark_delay reduces as dilution ratio increases. This can be attributed to more and more reactive radicals

As well known, EGR can provide thermal dilution, chemical dilution and reactivity dilution to retard combustion process. However, a little drop in τspark can be observed for Fuel 3 and Fuel 5 when the dilution ratio increases from zero to 5%, as shown in Fig. 12. Note that these two fuels have the strongest reactivity at 860 K and 25.5 bar based on Fig. 4, indicating that low dilution ratio (5%) cannot suppress the intensity of auto-ignition but enhance it in high-reactivity fuels. Meanwhile, the effect of extra dilution on reactivity prohibition is more prominent for high-RON fuel in ultra-lean mixture, which is consistent with simulation result [44]. Even though misfire is not easy to occur in low-RON/ MON fuel, the control authority from flame in combustion process is weak as demonstrated by AHRA more than 0.9. This means that spark assistance has little effect on SICI combustion. It is worth noting that the change of AHRA is reduced with decreased RON in S = 3 and 8. AHRA can partially reflect the intensity of HTHR, and HTHR has a positive correlation with flame speed and lower heating value (LHV) of mixtures. For fuels with the same S but varied RON, i.e. the nearly identical ethanol concentration, LHV in the mixture for each fuel is similar. In this situation, flame speed can reflect HTHR directly. Based on the fact that measured flame speed (SFlame) is in the same order of laminar flame speed (SL) due to weak turbulence in RCM and low density ratio in two sides of flame [28], sensitivity analysis was adopted to analyze the variation of AHRA among different fuels. As mentioned above, the concentration of H radical and O2 shows a 8

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(a) Target temperature = 735 K

(b) Target temperature = 785 K

(c) Target temperature = 860 K Fig. 8. Comparison of AHRA and KI for three fuels with S = 3 but varied RON under three temperatures.

dominant influence on laminar flame speed and lean flammability limits, which is mainly affected by critical reactions such as the chain branching reaction of H + O2 < = > O + OH (R1) and chain carrying reaction of CO + OH < = > CO2 + H (R2) and HCO + M < = > H + CO + M (R3) [56]. Taking R1 and R2 as examples, sensitivity of SL to these two reactions were analyzed for Fuels 1, 2 and 3, as plotted in Fig. 13 with doubled pre-factor A for R1 and R2, respectively. As shown in Fig. 13, the sensitivity index reduces with the

increasing dilution ratio in two reactions for all the three fuels, while the most significant decrease of sensitivity index occurs in Fuel 1 which has the highest RON among the three fuels. The degree of change in the sensitivity indexes is consistent with that of AHRA as shown in Fig. 12 (a), further confirming that the flame of high-RON fuel is more prominently affected by chemically inert species under extra dilution conditions and the possibility of misfire is greatly increased.

9

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(a) Target temperature = 735 K

(b) Target temperature = 785 K

(c) Target temperature = 860 K Fig. 9. Comparison of AHRA and KI of fuels with S = 3 and 8 but varied RON under three temperatures.

3.5. Effect of RON on transition temperature and combustion mode

experimental results for the five fuels are utilized to calculate the change of temperature in unburned mixture during flame propagation, and temperature at tauto is extracted for each fuel. Fig. 14(a)-(e) demonstrates transition temperature and pressure of the cases with auto-ignition for each fuel and Fig. 14(f) summarizes and compares these temperatures and pressures for S = 3 and 8, respectively. Transition temperature and pressure are defined as the T and p in the unburned mixture at tauto. If there is no bulk auto-ignition in the

It has been studied that the transition temperature of n-heptane from SI to CI is 795 K [25], and the transition temperature for iso-octane is 1100 K [26] at 20 bar with equivalence ratio of 0.45. It seems that the transition temperature will decrease when RON value rises at a given octane sensitivity but little work has been done to investigate the effect of RON on this transition temperature. In this section, all the 10

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(a) Fuel 1

(b) Fuel 2

(d) Fuel 4

(e) Fuel 5

(c) Fuel 3

(f) Comparison of KI in the test conditions

Fig. 10. Comparison of KI and distribution of non-knock area for five fuels under the whole thermodynamic conditions.

combustion process (i.e. SI mode), the corresponding temperature and pressure represent the maximum T and p in the unburned mixture before the flame approaches, as marked by red color in Fig. 14. SI-only mode exists in four fuels except Fuel 3. Normally the maximum temperature of the end gas for SI-only mode is in the vicinity of 950 K below 25 bar, which is higher than the model prediction of 800 K at

40 bar for iso-octane [57] and will be more comparable when pressure decreases from 40 to 25 bar. It is primarily attributed to the weaker flame compression effect and shorter OID at higher initial pressure. Moreover, it can be well recognized from Fig. 14(f) that as RON drops, the transition temperature and pressure decrease in regardless of S. The reduction of temperature is more significant when the RON value

(a) S = 3

(b) S = 8

Fig. 11. Comparison of integrated heat release rate profiles under different dilution ratios using CO2. 11

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(a) S = 3

(b) S = 8

Fig. 12. Effect of dilution ratio on AHRA and spark delay for fuels with different RON values.

(a) H + O2 < = > O + OH

(b) CO + OH < = > CO2 + H

Fig. 13. Sensitivity analysis of laminar flame speed to pre-factor A of two most representative reactions under varied dilution ratio.

decreases from 100 to 93 than that when RON decreases from 93 to 87. To have a deeper understanding in the aforementioned observation, Fig. 15 shows the T-p trajectories of SACI/SICI/SI mode overlapped on OID contours at 785 K for Fuels 1, 2 and 3. As shown in Fig. 15, both T-p trajectories under non-reaction and reaction conditions are plotted to investigate the intensity of LTHR in the unburned mixture for fuels with different RON at S = 3. For the simulation of non-reaction case marked by “NR” in Fig. 15, O2 is completely replaced by N2 because O2 and N2 have similar heat

capacity values and the temperature and pressure rise in a non-reactive run is only attributed to adiabatic compression of flame. Based on the criterion (τig = 50 ms) to distinguish SACI from SICI mode, three pressure levels (i.e. 30 bar, 15 bar and 12 bar) are selected. As shown in Fig. 15, all the three fuels have NTC behavior while the OID for Fuel 1 (RON 100 and S = 3) is retarded more than that of other fuels due to the iso-octane concentration of Fuel 1 is greater than that of the other fuels [32]. In SACI mode, T-p trajectories of the three fuels passing through NTC regions lead to stronger LTHR and larger temperature rise 12

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(a) Fuel 1

(b) Fuel 2

(d) Fuel 4

(e) Fuel 5

(c) Fuel 3

(f) Comparison of thermal conditions at tauto

Fig. 14. Effect of RON value on thermal dynamic conditions at tauto for five fuels.

deviation source when single-stage Arrhenius expression is used for knock prediction [59]. In both SACI and SICI, the decreased transition temperature from high-RON to low-RON fuel is mainly resulted from the enhanced LTHR, and in turn shortens the period for flame compression. The close reactivity (τig) between Fuel 2 and Fuel 3 means the small difference in the intensity of LTHR and further causes the small drop in transition temperature as presented in Fig. 14(f) and Fig. 15. Fig. 16 shows the inert index varied with normalized ignition delay to summarize all the experimental results and comprehensively evaluate the fuel impact on different combustion modes. The data of τig = 50 ms at three temperatures in Fig. 16(a)-(e) are interpolated based on OID measurement in RCM. Their standard deviations are calculated with Eqs. (7) and (8). As shown in Fig. 16, the control authority of combustion process from flame is manifested by τspark/τig. In the case with high AHRA, τspark/τig tends towards 1.0. The propensity of knock can be measured by the inert index, and the test points of the fuel with higher reactivity are distributed at the relative lower inert index. The high and low temperature boundaries (735 K and 860 K) of each fuel are depicted in Fig. 16(f). As shown in Fig. 16(f), for fuels with S = 8 and decreased RON, the control authority from flame at low temperature is substantially enhanced but simultaneously weakened at high temperature, which restricts the range of acceptable thermodynamic conditions and the causes are explained by the results shown in Fig. 9 and Fig. 12(b). Moreover, for fuels with S = 3, the lower RON value is, the severer pressure oscillation occurs at 860 K, resulting in smaller inert index. However, this trend is not followed at 735 K. One special phenomenon shown in Fig. 16 is that Fuel 2 has the largest inert index at a given τspark/τig. This phenomenon and the strongest knock resistance of Fuel 2 shown in Fig. 10(f) indicate that the flame compression effect of Fuel 1 and the highest reactivity of Fuel 3 result in their lower inert indexes than Fuel 2 does. In other words, Fuel

Fig. 15. Typical T-p trajectories of three combustion modes (SACI/SICI/SI) overlapped on OID contours for fuels with different RON values and same S at Teff = 785 K.

than in SICI mode. Under SICI condition, thermodynamic state of the unburned mixture of Fuel 1 directly develops into high temperature regime, so almost no LTHR can be observed. LTHR for Fuel 2 (RON 93 and S = 3) and Fuel 3 (RON 87 and S = 3) are also weakened and result in smaller difference between reactive and non-reactive lines, compared with those in SACI mode. Furthermore, low-RON fuel has strong LTHR and large temperature rise by cool flame due to prominent low-temperature reactivity of n-heptane, which can in advance meet the requirement of Livengood-Wu integral [58]. This is also the main 13

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(a) RON = 100 S = 3

(b) RON = 93 S = 3

(d) RON = 100 S = 8

(e) RON = 93

S=8

(c) RON = 87 S = 3

(f) Summary of five fuels

Fig. 16. Distribution of SI/SICI/SACU combustion modes for five fuels in studied thermodynamic conditions.

4. Conclusions

2 performs better than the other two fuels at 735 K under engine-relevant pressures, demonstrating its greater potential to be applied in lean SICI combustion engines.

Experimental study along with chemical kinetics analysis on fuel reactivity and flame speed was performed under lean-burn and enginerelevant operating conditions. OIDs of five fuels divided into two groups were measured and validated. Fuels in each group had the same octane sensitivity but varied RON to decouple the effect of octane number from octane sensitivity on SICI combustion. The transition of ethanol reactivity is analyzed and the impact of ethanol on octane number measurement is investigated. Based on knock intensity, control authority of flame propagation and dilution tolerance, requirements for the fuel in lean SICI combustion are proposed and the suitable fuel is identified. Conclusions can be drawn as follows.

3.6. Brief discussion on the fuel requirement for SICI combustion Two stage combustion including flame propagation and bulk autoignition is a significant characteristic shown in SICI mode. Therefore, strong combustion control authority, moderate HRR during auto-ignition and flame stability under heavily diluted conditions are three main requirements for robust and controllable SICI combustion. Therefore the suitable fuel for SICI should have the following characteristics: (a) strong HTHR along with high flame speed, (b) relatively low reactivity, (c) high dilution tolerance. Meeting the above requirements, ethanol is a rational additive and also conducive to reducing emission. According to our previous research about the impact of octane sensitivity on SICI combustion [28], at RON = 100, the fuel with intermediate S (S = 3) is more preferable for SICI mode as it avoids the heavy knock associated with low-S fuel and weak HTHR associated with high-S fuel at low temperature. Furthermore, the fuel with S of 3 and intermediate RON (RON = 93) is more suitable for SICI mode because high-RON fuel shows low dilution tolerance and low-RON fuel presents poor combustion control authority from flame. In conclusion, the fuel with medium RON and S is the best choice for SICI combustion and has the potential to be applied in real engines. It should be noted that this result is obtained from ternary gasoline surrogate fuels (n-heptane/iso-octane/ethanol) in which aromatics and olefins in commercial gasoline are not included. That is to say, the preferable value of fuel metric (i.e. octane number and octane sensitivity) may be varied. However, this study in association with that reported in [28] has fundamentally investigated the ethanol fuel’s effect on SICI combustion. The ethanol blend ratio studied in this paper can be referred in real fuel design. The performance of commercial gasoline blended with ethanol in SICI combustion will be investigated in the future.

1. With reactivity enhanced due to more n-heptane in reaction system, the conventional fuel metrics, i.e. RON/MON and S, have the weaker ability to distinguish fuel reactivity due to low temperature oxidation acceleration of ethanol caused by the increase of reactive radicals such as OH produced by n-heptane while LTHR of iso-octane is slightly suppressed. The increase of n-heptane concentration has a limited influence on reaction pathway of ethanol and iso-octane, and no obvious fuel interaction among these three components is observed. 2. For low-RON fuels, S is not suitable to predict the knock intensity. KI increases with the effective pressure and is simultaneously affected by the fuel reactivity, AHRA and flame compression effect. Among them, the influence of fuel reactivity to KI is strongest, AHRA second and flame compression the weakest. More concretely, fuel reactivity is dominant regardless of AHRA and flame compression effect when fuel reactivity varies greatly while AHRA plays a significant part in the reaction systems with comparable reactivity. 3. Control authority from flame in combustion process gets weak and weaker with the decreased RON and keeps at a low level for lowRON fuel especially under extra dilution conditions. Moreover, the intensity of LTHR increases with the increase of RON. More 14

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prominent temperature rise in the first-stage ignition delay and inadequate time for flame compression contribute to lower transition temperature and pressure from SI to CI mode for fuels with lower RON. This trend is independent of octane sensitivity. 4. On one hand, flame has little effect on adjusting combustion process, and heavy pressure oscillation will be generated for low-RON fuel. Meanwhile, flame propagation of high-RON fuels is significantly suppressed by extra dilution, and misfire is easy to occur. On the other hand, high-S fuels with medium RON are more sensitive to extra dilution and thermodynamic conditions than the fuels with low RON. This high sensitivity increases the control difficulty in real engine applications and restricts the range of operable thermal states. Therefore, the fuel with medium RON and S is more suitable for lean SICI combustion.

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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by National Natural Science Foundation of China (Grant No. 91541206), National Key Research and Development Program of China (Grant No. 2017YFE0102800) and the Key program of State Key Laboratory of Automotive Safety and Energy of China (Grant No. ZZ2019-031). The assistance of Professor Guang Hong of the University of Technology Sydney with improving language of the paper and Shandong Chambroad Petrochemicals Co., Ltd with providing chemical reagents for experiments are also gratefully acknowledged. References [1] Wang Z, Liu H, Ma X, Wang J, Shuai S, Reitz RD. Homogeneous charge compression ignition (HCCI) combustion of polyoxymethylene dimethyl ethers (PODE). Fuel 2016;183:206–13. [2] Liu H, Wang Z, Zhang J, Wang J, Shuai S. Study on combustion and emission characteristics of polyoxymethylene dimethyl ethers/diesel blends in light-duty and heavy-duty diesel engines. Appl Energy 2017;185:1393–402. [3] Gentz G, Dernotte J, Ji C, et al. Spark Assist for CA50 Control and Improved Robustness in a Premixed LTGC Engine-Effects of Equivalence Ratio and Intake Boost. SAE Technical Paper 2018-01-1252, 2018. [4] Nakata K, Nogawa S, Takahashi D, et al. Engine Technologies for Achieving 45% Thermal Efficiency of S.I. Engine[J]. SAE Int. J. Engines 9(1):2016. [5] Department of Energy, “Co-Optimization of Fuels & Engines.” Department of Energy, Web. 12 Oct. 2016. [6] The use of HCCI combustion in a CFR engine to understand fuel behavior. SAE Cooptimization of Fuels and Engines Symposium, October 9-10, 2018. [7] D'Agosto M d A. Chapter 5 - Energy sources for transportation, in: M.d.A. D'Agosto (Ed.) Transportation, Energy Use and Environmental Impacts, Elsevier, 2019, pp. 177–225. [8] Lu H. Chapter 9 – China’s Fuel Ethanol Market, in: S.L.M. Salles-Filho, L.A.B. Cortez, J.M.F.J. da Silveira, S.C. Trindade, M.d.G.D. Fonseca (Eds.) Global Bioethanol, Academic Press, 2016, pp. 197–208. [9] Sarathy SM, Farooq A, Kalghatgi GT. Recent progress in gasoline surrogate fuels. Prog Energy Combust Sci 2018;65:67–108. [10] Stein R, Polovina D, Roth K, et al. Effect of heat of vaporization, chemical octane, and sensitivity on knock limit for ethanol – gasoline blends. SAE Int J Fuels Lubr 2012;5(2):823–43. [11] Amer A, Babiker H, Chang J, et al. Fuel effects on knock in a highly boosted direct injection spark ignition engine. SAE Int J Fuels Lubr 2012;5(3):1048–65. [12] Szybist J, Wagnon S, Splitter D, et al. The reduced effectiveness of EGR to mitigate knock at high loads in boosted SI engines. SAE Int J Engines 2017;10(5):2305–18. [13] Kalghatgi GT. Fuel Anti-Knock Quality – Part I. Engine Studies. SAE Technical Paper 2001-01-3584, 2001. [14] Kalghatgi GT. Fuel Anti-Knock Quality-Part II. Vehicle Studies – How Relevant is Motor Octane Number (MON) in Modern Engines?: SAE Technical Paper 2001-013585, 2001. [15] Belgiorno G, Di Blasio G, Shamun S, Beatrice C, Tunestål P, Tunér M. Performance and emissions of diesel-gasoline-ethanol blends in a light duty compression ignition engine. Fuel 2018;217:78–90. [16] Vuilleumier D, Kozarac D, Mehl M, et al. Intermediate temperature heat release in an HCCI engine fueled by ethanol/n-heptane mixtures: an experimental and modeling study. Combust Flame 2014;161(3):680–95.

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