On pre-ignition heat release of fuels with various octane sensitivities under compression ignition conditions

Accepted Manuscript On pre-ignition heat release of fuels with various octane sensitivities under compression ignition conditions Haiqiao Wei, Feng Liu, Jiaying Pan, Lei Zhou, Zhen Hu, Mingzhang Pan PII: DOI: Article Number: Reference:

S1359-4311(19)30131-0 https://doi.org/10.1016/j.applthermaleng.2019.113953 113953 ATE 113953

To appear in:

Applied Thermal Engineering

Received Date: Revised Date: Accepted Date:

7 January 2019 6 May 2019 13 June 2019

Please cite this article as: H. Wei, F. Liu, J. Pan, L. Zhou, Z. Hu, M. Pan, On pre-ignition heat release of fuels with various octane sensitivities under compression ignition conditions, Applied Thermal Engineering (2019), doi: https://doi.org/10.1016/j.applthermaleng.2019.113953

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On pre-ignition heat release of fuels with various octane sensitivities under compression ignition conditions Haiqiao Wei1, Feng Liu1, Jiaying Pan1,*, Lei Zhou1, Zhen Hu1, Mingzhang Pan2 1 State

Key Laboratory of Engines, Tianjin University, Tianjin 300072, China

2 College

of Mechanical Engineering, Guangxi University, Nanning 530004, China

*Corresponding author: Haiqiao Wei, Jiaying Pan E-mail: [email protected], [email protected]

Abstract Fuel and engine interactions are the essential issues for advanced compression ignition engines, and low-octane gasoline is considered as prospective fuel under compression ignition conditions. However, with enhanced low-temperature chemistry, low-octane gasoline may exhibit distinctive characteristics affecting ignition phenomena. In this study, the ignition phenomena of low-octane gasoline was investigated under gasoline compression ignition conditions using validated chemical kinetic mechanism. The toluene primary reference fuel blends were employed to obtain the same RON = 75 with various octane sensitivity. With addressing ignition characteristics and pre-ignition heat release, the role of fuel sensitivity, inlet conditions, and exhaust gas recirculation on fuel reactivity was clearly demonstrated. The results show that the fuel sensitivity is positively correlated with fuel reactivity in hightemperature low-pressure regime, whereas there is a negative correlation in low-temperature high-pressure regime. The opposite correlation between fuel sensitivity and fuel reactivity is closely related to the pre-ignition heat release, with decreasing low-temperature heat release 1

at high fuel sensitivity. Finally, the effects of exhaust gas recirculation on low-temperature heat release were investigated. It shows that affected by low-temperature heat release, distinct characteristics in combustion phasing are observed with the addition of the exhaust gas recirculation. Keyword: Gasoline compression ignition; Octane sensitivity; Low-temperature heat release; Negative temperature coefficient; Exhaust gas recirculation

Nomenclature ATDC

after top dead center

BMEP

brake mean effective pressure

CA

crank angle

CA50

crank angle of 50% heat release

CV

mole-specific heat capacity

CI

compression ignition

EGR

exhaust gas recirculation

GCI

gasoline compression ignition

HCCI

homogeneous charge compression ignition

HRR

heat release rate

HTC

high temperature chemistry

IMEP

indicated mean effective pressure

ITHR

intermediate temperature heat release

LTC

low-temperature combustion 2

LTHR

low-temperature heat release

MON

motor octane number

NTC

negative-temperature coefficient

PCCI

premixed charge compression ignition

PRF

primary reference fuel

PODE

polyoxymethylene dimethyl ethers

RON

research octane number

S

octane sensitivity

SOI

start of injection

TPRF

toluene primary reference fuel

3

1. Introduction In order to meet stringent emission regulations and improve fuel economy, the internal combustion engine is developing towards higher thermal efficiency and lower emissions. Advanced compression ignition combustion strategies such as homogeneous charge compression ignition (HCCI) [1] and premixed charge compression ignition (PCCI) [2] have received extensive attentions. Both of them have the abilities to achieve diesel-like efficiency while maintaining low NOx and soot formation due to low-temperature combustion (LTC). The main challenge of HCCI is the uncontrollability of the mixture autoignition controlled by chemical kinetics [3,4]. In addition, the tendency of knock [5,6] caused by rapid heat release rate also limits the operation range. When diesel-like fuel is used in PCCI mode, high levels of cooled exhaust gas recirculation (EGR) are usually used to control autoignition timing, which limits the range of clean combustion conditions and power density at high load [7]. Kalghatgi et al. [8] proposed a concept of gasoline compression ignition (GCI) by taking advantage of higher volatility and more resistance to autoignition of gasoline compared with diesel. Under GCI conditions, a longer mixing period results in increased charge premixing and mixture homogeneity, which helps reduce NOX and PM emissions [9,10]. Meanwhile, GCI has the advantage of controllable combustion phasing because its combustion phasing is closely coupled with the fuel injection quantities and timings [11,12]. Despite the above benefits, GCI combustion is generally limited to mid-load operating conditions. The large explosive pressure rise rate, resulting in heavy combustion noise and even engine knock, is the main issue under high-load conditions, whereas the low reactivity of gasoline fuel increases the instability of combustion and even leads to misfire [13] under 4

low-load conditions. To realize practical application, this approach must be able to operate over the entire engine operating map. The adjustment of fuel composition and reactivity is considered as an effective means to extend the GCI load. A recent study by Liu et al. [14] found that adding Polyoxymethylene dimethyl ethers (PODE) to gasoline can reduce the soot emission through PODE’s high oxygen content; and meanwhile, low explosive pressure rise rate can also be obtained with single injection strategy under high-load (BMEP = 16 bar) operating condition. Wang et al. [15] investigated the effects of fuel reactivity on GCI engine performance under low-load condition by blending different proportions of gasoline and diesel mixture. The minimum load to 0.07 MPa IMEP can be achieved with G90 (90vol% gasoline and 10vol% diesel) when the injection pressure is 20 MPa with the intake pressure of 0.14 MPa. The above studies demonstrate the significant role of fuel and engine cooptimization in optimizing engine performance. Generally, the most intuitive indicator describing gasoline fuel performance is research octane number (RON), motor octane number (MON) and sensitivity (S=RON-MON). In conventional SI engines, RON and MON and S are usually used to describe knocking propensity. When gasoline is used under compression ignition (CI) conditions, high-octane gasoline fuels become more problematic under low load condition due to poor autoignition quality. Therefore, low-octane gasoline is considered as the prospective fuel under compression ignition conditions. And many studies have shown that GCI engines can operate optimally with fuels have low-octane numbers [16]. Patrick et al. [17] investigated the low load limitations of the gasoline fuels with RON 69 and RON 87. It shows that the RON 69 gasoline can run at a lower load without a significant fraction of trapped hot residual gases. It 5

should be noted that with enhanced low-temperature chemistry, low-octane gasoline may show some distinctive characteristics (e.g. two-stage autoignition), which further affects ignition phenomena and combustion phasing [18]. The autoignition process of the GCI engine is controlled by chemical kinetics, where low-temperature heat release (LTHR) is a very important stage. Because LTHR phenomenon is closely related to combustion phasing [19]. Several studies have shown that fuels with twostage autoignition have significant advantages in controlling combustion phasing and reducing the peak heat release rate (HRR) [20, 21]. Two-stage autoignition refers to the existence of a first stage ignition (or a “cool flame”) due to low-temperature chemistry before the high-temperature chemistry (HTC) region. Stratification significantly and rapidly changes the combustion phasing of fuels with obvious LTHR phenomenon but has little effect on fuels without LTHR [20]. Meanwhile, a staged combustion event achieved by fuel stratification can increase combustion duration and reduce peak HRR [21]. Although two-stage ignition fuels have these potential advantages, controlling the significant LTHR in practical engines is still challenging. This is because that the amount of LTHR is not only a fuel characteristics, but also depends on operating conditions, i.e. temperature and pressure trajectories. Experimental and supporting simulation work by Vuilleumier et al. [22] studied the LTHR for different primary reference fuel (PRF) mixtures. The results illustrate that the magnitude of LTHR increased as the PRF number decreased. Leppard demonstrated that the chemical origin of octane sensitivity of different classes of hydrocarbons is induced by their different extent of negative temperature coefficient (NTC) [23]. For alkanes, there is a two-stage ignition process: LTHR followed by a NTC region. In contrast, neither aromatics nor olefins exhibited 6

the two-stage ignition behavior. A recent work by Han et al. [24] found that the shape feature of the heat release rate is determined by start of injection (SOI), and LTHR occurs only when SOI is sufficiently advanced. To control LTHR, it is very meaningful to have a comprehensive understanding of the variation of LTHR with the fuel properties and operating conditions. The direct properties that influence pre-ignition heat release is not only RON and fuel type, but the fuel sensitivity is also a key factor. In this study, using the well-validated surrogate and kinetic models, we attempt to comprehensively investigate the effect of fuel sensitivity on pre-ignition heat release under different operating conditions, which further explains why fuel with different sensitivity has different reactivity. In addition, the preignition heat release affected by inlet conditions and exhaust gas recirculation has also been comprehensively studied. Overall fuel reactivity is indicated by the combustion phasing CA50 (the crank angle corresponding to 50% of total heat release), and iso-contours of CA50 (curve consisting of the same CA50) are plotted under different inlet conditions. By observing CA50 iso-contours, the sensitivity of fuel to inlet conditions can be evaluated by the change of CA50 with the operating conditions. We believe that these mechanisms can be used to qualitatively predict the sensitivity of combustion phasing and pre-ignition heat release to changes in fuel properties and operating conditions. The results can provide useful guidance for regulating of pre-ignition behavior to take advantage of the potential advantages of twostage ignition fuels. 2. Models and Methodology 2.1. TPRF surrogate formulation 7

Gasoline fuels are regarded as multi-component and there are many studies demonstrating that gasoline surrogate models containing a limited number of components can show high consistency with gasoline fuel engine combustion performance [25,26]. Because gasoline fuels contain some aromatic compounds, the aromatic compounds should be considered when developing surrogate models. For aromatic compounds, toluene is a good choice and it is often applied as the gasoline surrogate mixtures [27,28]. These studies [29,30] have proven that toluene primary reference fuel blends (TPRF) can well meet the needs of simulating actual engine performance. Various compositions of TPRF are obtained through the approach proposed by Kalghatgi et al. [31]. This approach has proven to be accurate enough to predict the RON and MON of the TPRF fuels. Taking Refs. [32,33] for reference, a low-octane-number gasoline fuel with RON=75 has been selected. The fuel sensitivity is provided since the sensitivity of both n-heptane and iso-octane is zero by definition. Herein the fuel sensitivity varies between 0 and 7.1 by modifying the addition of toluene. Besides, the mole fraction of n-heptane is increased slightly with increasing sensitivity, which is then used to balance the elevated octane number due to the addition of toluene. Compositions of four TPRFs with matched RON = 75 but different octane sensitivity are shown in Table 1. Table 1 TPRF compositions of the RON = 75 fuel with different octane sensitivity. Composition (mole fraction %) Fuel

C-atoms

H-atoms Iso-octane

n-Heptane

Toluene

S=0

7.727

17.454

72.7

27.3

0

S=3.6

7.449

14.85

44.9

29.5

25.6

S=5.3

7.31

13.604

31

31.3

37.7

8

S=7.1

7.164

12.288

16.4

33.1

50.5

2.2. Mechanism verification There are several representative kinetic mechanisms of TPRF [26,29,34,35]. Among them, the mechanisms developed by Mehl et al. [34] and Andrae [35] have been widely used in many studies [36,37], and they have proven to be the suitable mechanisms for the simulation study of TPRF blends. To ensure the effectiveness of the mechanism, the representative kinetic mechanisms [34,35] are validated against the experimental ignition delay time in shock tube [38], as shown in Table 2. Table 2 Validation targets for different TPRF blends. Gasoline

Composition (mole fraction %) T (K)

surrogates

Iso-octane

n-Heptane

Toluene

TPRF 70[28]

36.5

35.1

28.4

TPRF 80[28]

32.9

26.6

40.5

P (atm)

φ

20;40

0.5; 1

765-1220 729-1183

Figure 1 shows a comparison of simulated ignition delay time for TPRF blends with the experimental data from Javed et al. [38] at different initial pressures and equivalence ratios. Data points of relative error between experimental results and simulation results of each mechanism are also plotted. The mean absolute values of the relative error of the mechanism developed by Andrae [35] and Mehl et al. [34] are 19.26% and 20.90% respectively. From Fig. 1(b), we can see that the two mechanisms have a similar relative error when the temperature is higher than 1000K; and the mechanism developed by Mehl et al. [34] has larger errors when the temperature is less than 1000K, meaning that further optimization is 9

needed in low temperature regime. Due to the smaller mean absolute value of relative error and better performance, the mechanism developed by Andrae [35] is eventually selected to calculate combustion phasing and pre-ignition behavior in this study. This validated mechanism is sufficient to demonstrate how the combustion process changes with fuel property and operating conditions.

10

Fig. 1. (a) Experimental and simulated ignition delay times for TPRF blends with different pressure and equivalence ratios. (b) Relative errors between experimental results and simulation results. Negative errors correspond to underprediction of ignition delay. 2.3 Model specifications The simplified approach to model GCI is zero-dimensional adiabatic modeling, and the IC engine module is available in CHEMKIN-PRO [39]. The zero-dimensional single-zone model combined with chemical kinetics is based on the simulation of gas-phase chemical reactions in a well-mixed reactor. Table 3 shows the configuration of the GCI engine with corresponding operation conditions. Under the operating conditions, the CA50 values are obtained at the intervals of 20 K in intake temperature and 0.2 atm in intake pressure. Based on the simulation results, iso-counters of CA50 are plotted according to the inlet conditions, and the pre-ignition heat release is also plotted on a crank angle degree basis. Through the 11

current approach with reducing time consumption and costs, we investigated a wide range of operating conditions and a series of different sensitivity fuels to qualitatively evaluate their impact on the combustion process. Table 3 The configuration of engine and operating conditions. Parameter

Values

Bore×Stroke (cm)

12×14

Compression ratio

16

Engine speed (rpm)

1000

Cylinder clearance volume (cm3)

103.3

Starting crank angle (ATDC)

-142°

Equivalence ratio

0.3

Rod to crank ratio

3.714

Intake temperature (K)

350-500

Intake pressure (atm)

0.5-4.0

2.4 EGR addition simulation The effects of EGR addition and its individual constituents were studied in the present study. The simulated EGR is a mixture of air and three major complete combustion products (i.e. CO2, H2O, and N2). The relative proportions of these components vary with fuel sensitivity and can be calculated based on the average number of C and H atoms, as listed in Table 1. For TPRF with zero sensitivity, complete stoichiometric combustion can be written as: C7.727H17.454 + 12.09 (O2 + 3.773 N2) = 7.727 CO2 + 8.727 H2O + 45.62 N2 12

(1)

In simulation, the gas composition (excluding fuel) of intake air is 20.95% O2 and 79.05% N2. For complete combustion with φ = 1, the exhaust compositions are 12.45% CO2, 14.06% H2O and 73.49% N2. The ratio of EGR is the ratio of the intake air replaced by the complete combustion products. For TPRF with zero sensitivity, the mole fraction of the intake gas compositions at different EGR ratios are listed in Table 4. Table 4 Mole fraction of intake gas compositions with different EGR ratios. Mole fraction (%) EGR ratio (%) O2

N2

H2O

CO2

0

20.95

79.05

0

0

10

18.855

78.494

1.406

1.245

20

16.76

77.938

2.812

2.49

30

14.665

77.382

4.218

3.735

3. Results and discussion 3.1 Effects of intake conditions and fuel sensitivity Fig. 2 shows the ignition delay time under adiabatic constant volume conditions for each fuel. In high-temperature regime, all the TPRFs exhibit an approximately same linear decrease in ignition delay time with increasing temperature. In the low temperature and NTCaffected regimes, the ignition delay time of different sensitivity fuels begins to show significant differences. For a given pressure, the non-monotonicity of ignition delay time due to the NTC effect can be gradually eliminated with the decrease of fuel sensitivity. This phenomenon can be explained by the conclusion Leppard drew in [23], who demonstrated that the chemical origin of octane sensitivity of different classes of hydrocarbons is induced 13

by their different extent of NTC. A higher S value actually implies is less pronounced low temperature chemistry and NTC effect. On the right side of the cutting line AA’ in Fig. 2, with increasing fuel sensitivity, the ignition delay time is prolonged due to the weakening of NTC behavior for a given pressure. In Fig. 2, we can also see that increasing pressure from 1atm to 100atm enlarges the temperature range over which NTC behavior occurs. The simulation results are useful to illuminate the effect of fuel sensitivity on the shape of the ignition delay time curve. The impact of different S values is mainly reflected in the low to intermediate temperature regime.

Fig. 2. Ignition delay time of TPRFs with RON = 75 and different S values. Taking into account the fuel sensitivity effect on ignition delay, the iso-contours of CA50 in the entire GCI operating regime for TPRFs with the same RON = 75 are further compared in Fig. 3. Temperature and pressure are critical factors influencing the CA50. Increasing intake temperature and pressure are both beneficial to advance the CA50. By the 14

compromise between intake temperature and intake pressure, the same CA50 can be achieved (i.e. under high-temperature low-pressure condition or low-temperature high-pressure condition can have the same CA50). In the high-temperature low-pressure regime (i.e. the region above the cutting line BB’ in Fig. 3), under the same intake pressure, a lower intake temperature is needed for a high sensitivity TPRF to maintain the same CA50, indicating that the fuel reactivity increases with decreasing S values in this operating regime. The isocontours of CA50 for TPRFs with S = 3.6, 5.3 and 7.1 are very close and show negligible differences, indicating that they have similar actual reactivity. However, in high-pressure lowtemperature regime (i.e. the region below the cutting line BB’ in Fig. 3), the correlation between CA50 and S shows an opposite trend compared to the above. To maintain the same CA50, a higher intake temperature is needed for a high sensitivity TPRF at the same intake pressure. In this operating regime, the fuel reactivity is increasing as S value decreases. In addition, different sensitivity fuels can achieve a similar CA50 in an intermediate temperature/pressure regime.

15

Fig. 3. CA50 iso-contours of the TPRFs with RON = 75 and different S values. 3.2 Role of fuel sensitivity in pre-ignition heat release Because in high-temperature low-pressure, intermediate temperature pressure and lowtemperature high-pressure regions, there are different correlations between fuel reactivity and sensitivity. In order to figure out this phenomenon, so in 3.2, along the iso-contours of CA50 equal to -10°CA ATDC, we selected three representative operating conditions, namely, Condition C (i.e. high-temperature low-pressure), Condition B (i.e. intermediate temperature pressure), and Condition A (i.e. low-temperature high-pressure) to provide more details on engine combustion process. The temperature histories of the selected conditions are shown in Fig. 4. Both thermodynamic and chemical effects from fuel composition have an effect on combustion phasing. Due to the different proportions of the TPRF compositions, it may affect the specific heat ratio and further change the thermodynamic process before autoignition, so both reactive and non-reactive temperature traces are shown in Fig. 4. The overlap of the non16

reactive temperature traces in Fig. 4 illustrates that the specific heat ratio of these mixtures is almost unchanged, and thus the thermodynamic states during the compression stroke are not affected by the mixture compositions. The difference in ignition chemistry is the only reason for the combustion phasing difference. For Condition A in Fig. 4(a), the combustion phasing advances with the decrease in S value, which indicates that there is a negative correlation between fuel reactivity and fuel sensitivity. In addition, with decreasing fuel sensitivity, greater pre-ignition heat release can be observed. For Condition B in Fig. 4(b), fuels with different sensitivities have similar combustion phasing and pre-ignition heat release. For Condition C in Fig. 4(c), the correlation between fuel reactivity and fuel sensitivity is positive, and the pre-ignition heat release is increasing with the increase of fuel sensitivity. Through the above comparison, we can find that fuel sensitivity has significant influences on preignition heat release, especially in low-temperature high-pressure regime.

17

Fig. 4. Temperature history of the TPRFs with RON = 75 under (a) Condition A, (b) Condition B, and (c) Condition C. To further demonstrate the role of fuel sensitivity, Fig. 5 presents the pre-ignition heat 18

release of different sensitivity TPRFs under the selected conditions mentioned above (i.e. Condition A, Condition B, and Condition C). As for Condition A in Fig. 5(a), all TPRFs have shown prominent LTHR with NTC behavior. The emergence of LTHR helps accelerate temperature rise, which can be clearly observed in Fig. 4(a). In addition, the magnitude of LTHR decreases with increasing fuel sensitivity. The presence of LTHR can increase the temperature of the gas mixture and accelerate the chemical reactions during the period between the LTHR and high-temperature heat release (HTHR), which is beneficial to rapid auto-ignition. H2O2 is accumulated during this period and broken down into OH radicals. The temperature rises due to LTHR and the rate of H2O2 accumulation apparently triggers the start of HTHR. Therefore, a prominent LTHR is beneficial to advance combustion phasing. So the combustion phasing is advanced with the decrease of fuel sensitivity in Condition A. For Condition B in Fig. 5(b), all TPRFs with different sensitivities also show distinct LTHR phenomena. Although the sensitivity of the fuels is different, it shows that they have an approximately same magnitude of LTHR and combustion phasing. In Fig. 5(c), preignition heat release for different sensitivity TPRFs only demonstrate as ITHR. As fuel sensitivity increases, the magnitude of ITHR also increases. Therefore, the combustion phasing is advanced as the fuel sensitivity increases.

19

20

Fig. 5. HRR history of TPRFs with the same RON = 75. (a) Condition A, (b) Condition B, and (c) Condition C. 3.3 Pre-ignition heat release affected by intake conditions The pre-ignition heat release exhibits different types in different temperature-pressure regimes, which indicates that temperature and pressure have an important impact on it. To have a clear understanding of such impact, we have compared the effects of intake pressure and intake temperature on pre-ignition heat release. In Section 3.2, the fuel sensitivity effect on pre-ignition heat release in different temperature and pressure regimes has been clearly demonstrated, and its effect is mainly reflected in the magnitude. Therefore, only the effect of intake temperature and pressure on pre-ignition heat release for fuel with S=0 is investigated in this section. To investigate the variation of pre-ignition heat release with intake pressure, an intermediate intake temperature (400K) within the range of operating condition has been chosen, and the simulated operation conditions are shown in Table 5. A shown in Fig. 6, a 21

direct observation is that the increase in intake pressure enhances the pre-ignition heat release at a given intake temperature. The increase in intake pressure implies an increase in the amount of fuel/air mixture charged into the cylinder, volume efficiency is increased and more air and fuel are burned in engine chamber, which is conducive to complete combustion, thereby leads to an increase in peak of pre-ignition heat release and an advance of combustion phasing. By comparing the type of pre-ignition heat release, we can find ITHR continuous growth and gradual transition into a distinct LTHR with NTC behavior as the intake pressure increases. Table 5 Operation conditions for studying the effect of intake pressure on pre-ignition heat release Operation conditions

Values

Compression ration

16

Equivalence ratio

0.3

Engine speed (rpm)

1000

Intake temperature (K)

400

Intake pressure (atm)

0.5-3.7

22

Fig. 6. Pre-ignition heat release of TPRF with S = 0 at the same 400K intake temperature with different intake pressures. GCI combustion is controlled by chemical kinetics and is very sensitive to temperature. Similar to the study on the effects of intake pressure, an intermediate intake pressure (1.5atm) has been chosen to investigate the pre-ignition heat release at a series of different intake temperature, and the simulated operation conditions are shown in Table 6. As shown in Fig. 7, the phenomenon of ITHR continuous growth and gradual transition into a distinct LTHR with NTC behavior can also be observed. The magnitude of LTHR increases with decreasing intake temperature, but the combustion phasing is advanced as the intake temperature increases. It shows that although intensive low-temperature reactions are beneficial to autoignition, the increase in intake temperature has a more important influence on combustion phasing. Table 6 Operation conditions for studying the effect of intake temperature on pre23

ignition heat release. Operation conditions

Values

Compression ration

16

Equivalence ratio

0.3

Engine speed (rpm)

1000

Intake temperature (K)

350-490

Intake pressure (atm)

1.5

Fig. 7. Pre-ignition heat release of TPRF with S=0 under the same 1.5atm intake pressure at different intake temperature. To further explain why the ITHR will transition into LTHR as the intake pressure increases or the intake temperature decreases, the pressure-temperature trajectories of the

24

operating conditions mentioned in Figs. 6 and 7 are plotted over the iso-contours of ignition delay time under constant volume conditions including the low temperature, NTC-affected, and high temperature segments as shown in Figs. 8 and 9. These maps are used to understand the chemical kinetic regimes covered by these operating conditions. As shown in Fig. 6, when the intake pressure is less than 0.9atm, the pre-ignition heat release is displayed as ITHR. By observing their pressure-temperature trajectories in Fig. 8, we can find that the pressuretemperature trajectories almost go through the high-temperature chemistry regime. The operating conditions are dominated by high temperature chemistry, the LTHR stage will not occur due to the temperature is higher than the temperature at which the cool flame reaction occur. However, under the operating conditions with intake pressure above 1.3atm as shown in Fig. 6, they all demonstrate a distinct LTHR behavior followed by NTC. And their pressure-temperature trajectories mainly go through the NTC-affected and low-temperature chemistry regimes where are suitable for the cool flame reactions. By comparing the distribution of their pressure-temperature trajectories in Fig. 8, we can observe that as the intake pressure increases, the pressure-temperature trajectory gradually shifts from high temperature chemistry regime to NTC-affected and low temperature chemistry regimes. Therefore, we observe the transition from ITHR to LTHR. As for Fig. 9, the decrease in intake temperature moves the pressure-temperature trajectory into an area where NTC reaction pathways are active, also lead to a transition from ITHR to LTHR.

25

Fig. 8. Pressure-temperature trajectory for TPRF with S=0 at the same intake temperature under different pressure conditions.

Fig. 9. Pressure-temperature trajectory for TPRF with S=0 with the same intake 26

pressure under different temperature conditions. 3.4 Effects of adding EGR on LTHR The main objective of this section is to demonstrate the effect of EGR addition on ignition delay time and LTHR. Since EGR is an effective way to manage LTHR for two-stage fuel [40], it is therefore possible to exploit the advantages of LTHR for combustion phasing and HRR control over a wide range of operating conditions. Similarly, EGR can be used to control changes in LTHR caused by changes in intake pressure and temperature. In this study, EGR is simulated by adding a complete stoichiometric product (i.e. CO2, H2O, and N2) in an amount proportional to the complete combustion of the fuel blends. Fig. 10 compares the effect of different proportions of EGR on ignition delay time. As the EGR ratio increases, ignition delay time shows an overall increase as expected. A direct observation is that ignition delay time shows an approximately proportionally increase with the increase of EGR ratio in high-temperature regime. Major deviations in ignition delay time are seen in low temperature and NTC-affected regimes, which is related to the addition of different proportions of EGR. Another observation is that with increasing EGR ratio, the nonmonotonicity in ignition delay time due to the NTC effect could also be gradually eliminated at a given pressure, resulting in the prolonged ignition delay time.

27

Fig. 10. Adiabatic ignition delay time of TPRF with S=0 with different amounts of EGR added. The delay effect of EGR can be divided into thermodynamic and chemical effects. Before analyzing how these two ways lead to the autoignition timing delay, we need to understand the thermodynamic effect of EGR as a basis. The thermodynamic effect of EGR is mainly reflected in the temperature of the compressed gas and this is mainly affected by the mole-specific heat capacity (Cv) of the compressed gas. For a higher Cv, the more energy (compression work) is required to raise a certain amount of temperature. Furthermore, Fig. 11 shows Cv as a function of temperature for EGR constituents and air. It can be seen that CO2 has the highest Cv, which means that CO2 has the strongest cooling effect. The Cv of N2 is slightly lower than that of air, which means that replacing air with N2 will result in an increase in the temperature of compressed gas. The thermodynamic effect of EGR can be approximately attributed to the addition of high Cv gas, and the chemical effect can be 28

approximately attributed to the variation of O2 concentration.

Fig. 11. Mole-specific heat capacity as a function of temperature for air and EGR constituents. Figure 12 shows the delay effect of the addition of N2 and CO2 to replace O2 on CA50 for TPRF with S=0 under Condition A (intake temperature=370K, intake pressure=2.5atm). The CA50 is equal to -11.99 °CA for operation with air dilution only (i.e. with O2 mole fraction equal to 21%), and then N2 and CO2 are used to replace part of O2. Although the addition of N2 can slightly increase the compressed gas temperature, it can be observed in Fig. 12 that the CA50 is still delayed. The only reasonable explanation is that the CA50 is sensitive to the variation of O2 concentration. It is assumed that the O2 concentration reduction effect affects all gas mixtures equally. Since the Cv of CO2 is higher than that of N2, which results in a lower compressed gas temperature. Therefore, in the case of adding CO2, CA50 has a larger delay compared with the case of adding N2. CA50 is delayed by 12.3 °CA 29

and 6.9 °CA respectively when CO2 and N2 are used to replaced part of O2 until the mole fraction of O2 is reduced to 13%.

Fig. 12. Effect of addition of N2 and CO2 on CA50 under Condition A (intake temperature=370K, intake pressure=2.5atm). Table 7 Operation conditions and four cases of intake compositions used to study the EGR effects. Cases

Intake compositions (mole fraction %)

Intake condition

φ

Tin: 370K 1

O2: 21%, N2:79% Pin: 2.5atm Tin: 370K

2

O2: 12%, N2:88% Pin: 2.5atm

3

Tin: 370K

O2: 21%, CO2: 6%, N2:73% 30

0.3

Pin: 2.5atm

Tin: 370K 4

O2: 15%, CO2: 3%, N2:82% Pin: 2.5atm

For a better understanding, Fig. 13 demonstrates how the thermodynamic-cooling effect and chemical effect lead to the different amount of combustion phasing delay under Condition A (intake temperature=370K, intake pressure=2.5atm). We designed different intake compositions to simulate the thermodynamic-cooling effect and chemical effect of EGR respectively, and then investigated their influence on the combustion process. Four cases of intake compositions used to study the EGR effects and operation conditions are shown in Table 7. The temperature curve with the most advanced combustion phasing (i.e. Case 1) is the baseline with only air dilution. Other temperature curves show two different ways in which the combustion phasing can be delayed. From the comparison of Case 1 with Case 3 in Fig. 13, the amount of O2 is the same but 6% of N2 is replaced by CO2, and the combustion phasing of Case 3 is delayed by approximately 3 °CA. By observing their temperature curves, it can be found that the compressed gas temperature in Case 3 is lower than that in Case 1. Moreover, Fig. 11 has shown that the Cv of CO2 is higher than that of N2. The delay of combustion phasing in Case 3 is caused by thermodynamic-cooling effect and such delay effect is similar to lowering the intake temperature. In Case 4, the combustion phasing is delayed even though there is no significant change in the temperature of compressed gas compared with the baseline. By further comparing their HRR curves, it can be found that the

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decrease in O2 mole fraction leads to a reduction in the amount of LTHR. This illustrates the delay of combustion phasing caused by chemical effects. As for Case 2, the mole fraction of O2 is further reduced to 12%, and the corresponding mole fraction of N2 is increased to 88%. Although the compressed gas temperature has slightly increased, there is still a significant decrease in the amount of LTHR, which eventually leads to a delay of combustion phasing. The advance effect on combustion phasing caused by the increase of compressed gas temperature is less than the retardation effect due to chemical effect caused by the decrease of mole fraction of O2. The insights obtained from above comparisons are meaningful. The thermodynamic-cooling effect of EGR has little effect on LTHR, and the amount of LTHR shows negligible differences despite the decrease in compressed gas temperature. However, the chemical effect of EGR has significant effect on LTHR, which is an effective way to manage LTHR. The decrease in O2 mole fraction leads to a significant reduction in the amount of LTHR.

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Fig.13. Effect of addition of CO2 and N2 on temperature and heat release rate for TPRF with S=0 under Condition A.

4. Conclusions In this work, the effects of inlet condition, fuel sensitivity and EGR on combustion phasing and pre-ignition heat release were studied based on well-validated surrogate and kinetic models under a typical GCI condition. By comparing these combustion phasing and HRR maps, the following conclusions can be drawn: (1) For the fuels with identical RON but different sensitivity, it is found that fuel sensitivity has a significant impact on the shape of ignition delay curve in the low to immediate temperature regime where the low-temperature reactions are the most active, and NTC behavior becomes less prominent as fuel sensitivity increases. (2) An intensive pre-ignition heat release is beneficial to autoignition. In hightemperature low-pressure regime, the pre-ignition heat release only manifests as ITHR, and its magnitude increases with the increase of fuel sensitivity. Therefore, there is a positive correlation between fuel reactivity and fuel sensitivity in this regime. However, the preignition heat release is transformed into LTHR in low-temperature regime, and its magnitude increase with the decrease of fuel sensitivity, resulting in a negative correlation between the fuel reactivity and fuel sensitivity. (3) Both the thermodynamic-cooling effect and chemical effect of EGR can lead to the delay of combustion phasing, and it also has an important effect on the LTHR. The chemical effect caused by the decrease of oxygen mole fraction has a significant effect on the amount 33

of LTHR. The amount of LTHR decreases as the oxygen concentration decreases, which inhibits the transition to main combustion. However, the thermodynamic-cooling effect due to the reduction of compressed gas temperature has negligible effect on LTHR. The current study only fundamentally investigated the behavior of pre-ignition heat release under compression ignition conditions relevant to GCI engines, and the influence from thermal and chemical kinetics interactions was addressed, especially under high-temperature and low-pressure and high-pressure and low-temperature conditions. Further experimental validations on realistic GCI engines will considered to elucidate the role of pre-ignition heat release in combustion performance in the future. Acknowledgement This work was supported by the National Natural Science Foundation of China (51706152, 51825603, 91641203), National Key R&D Program of China (2017YFE0102800), and Tianjin Natural Science Foundation (17JCZDJC31500, 18JCQNJC07500). References [1] Y. Yang, J. Dec, M. Sjöberg, C. Ji. Understanding fuel anti-knock performances in modern SI engines using fundamental HCCI experiments, Combust Flame, 162 (2015) 4008–4015. [2] A. Jain, A.P. Singh, A.K. Agarwal, Effect of fuel injection parameters on combustion stability and emissions of a mineral diesel fueled partially premixed charge compression ignition (PCCI) engine, Applied Energy, 190 (2017) 658-669. [3] C. Zhang, C. Zhang, L. Xue, Y. Li, Combustion characteristics and operation range of a RCCI combustion engine fueled with direct injection n-heptane and pipe injection n34

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Highlights ► Ignition characteristics of fuels with various octane sensitivities are numerically

studied. ► Fuel sensitivity shows distinct correlations with fuel reactivity under different

conditions. ► Correlations between fuel sensitivity and reactivity is related to pre-ignition

phenomena. ► Inert EGR addition inhibits pre-ignition heat release and transition to main combustion.

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