Title:

Effect of dilution strategies and direct injection ratios on Stratified Flame Ignition (SFI)

hybrid combustion in a PFI/DI gasoline engine

Author names and affiliations:

Xinyan Wang a, Hua Zhao a, b, Hui Xie a,*

a State Key Laboratory of Engines, Tianjin University, Weijin Road 92, Nankai District,

Tianjin 300072, PR China.

b Centre for Advanced Powertrain and Fuels, Brunel University London, Uxbridge UB8 3PH,

United Kingdom.

* Corresponding author: Tel/Fax: +86 22 27406842 8009, Email: xiehui@tju.edu.cn.

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Abstract:

Three-dimensional (3-D) computational fluid dynamics (CFD) simulations were used to

investigate and obtain a fundamental understanding of the effect of dilution strategies and

direct injection ratios on the stratified flame ignition (SFI) hybrid combustion. The

combination of port fuel injection (PFI) and direct injection (DI) was used to form the

homogeneous lean/diluted mixture and stratified charge respectively. Studies were carried out

on effects of dilution strategies with different combinations of fuel/air equivalence ratio (ϕair)

and fuel/dilution equivalence ratio (ϕdilution) with negative valve overlap (NVO). Compared to

the stoichiometric SFI hybrid combustion, the air-diluted SFI hybrid combustion optimizes

the early flame propagation process because of the avoidance of over-rich mixture around

spark plug. In order to explore the potential of SFI hybrid combustion under a high

compression ratio (14:1) operation, the lean boosted dilution strategy with additional intake

air and internal residual gas was proposed to address the trade-off between indicated mean

effective pressure (IMEP) and maximum pressure rise rate (PRRmax) in air-diluted SFI hybrid

combustion. Furthermore, the effect of direct injection ratio (rDI) was investigated as a means

to optimize the fuel/air equivalence ratio distribution as well as the air-diluted SFI hybrid

combustion performance. It is found that the optimal SFI hybrid combustion with rDI of 0.16

can be used to both achieve higher IMEP for a given amount of fuel and moderate the rate of

heat release. Finally, three different combustion regimes, including pure flame propagation

zone, hybrid combustion zone and pure auto-ignition zone, are proposed to understand the

effect of typical fuel/air equivalence ratio distribution patterns on the air-diluted SFI hybrid

combustion characteristics and performances. In order to obtain optimal hybrid combustion

with high IMEP and low PRRmax, the in-cylinder stratified mixture should avoid over-rich

condition around spark plug and over-lean condition at outer region. In addition, the internal

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residual gas in the dilution strategy should be carefully controlled to maintain sufficient

thermal condition and ensure the stable auto-ignition of the lean mixture at outer region.

Keywords: computational fluid dynamics, hybrid combustion, stratified mixture, controlled

auto-ignition, diluted combustion

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1. Introduction

Controlled auto-ignition (CAI) combustion, which is characterized by multi-site auto-ignition

process, can lead to ultra-low NOx emissions and increase the fuel conversion efficiency [1].

However, the high sensitivity of CAI combustion to the boundary conditions and its narrow

operation range [1] have prevented it from being adopted in production engines. Further

research and development of this combustion concept are needed in order to adapt it to the

practical applications. In recent years, spark ignition (SI) has been introduced into the CAI

combustion concept to assist the control of auto-ignition [2, 3]. The SI-CAI hybrid

combustion, also known as spark assisted compression ignition (SACI), obtains higher

thermal efficiency and lower NOx emission compared to the traditional SI combustion, while

achieves lower maximum pressure rise rate (PRRmax) and wider load operation range

compared to the pure CAI combustion [4] at some operating conditions. Meanwhile, this

hybrid combustion concept facilitates the smooth transitions between pure SI mode and CAI

mode [5-9].

The SI-CAI hybrid combustion compromises two different combustion modes and results in

complex interactions between the early flame propagation and subsequent auto-ignition

process. Different control strategies, including spark timing [10], intake temperature [10-12],

wall temperature [12], in-cylinder flow [13] and dilution composition [10, 14], have been

studied to understand their effects on hybrid combustion process. However, the optimal high

load operation would still be limited by the severe knock or unacceptable pressure rise rate

for the SACI combustion with the homogeneous charge [8, 14-16].

The fuel stratification achieved by direct injection can enrich the central region while leave

diluted mixture in the peripheral region, which provides a natural resistance to severe auto-

ignitions [17, 18]. In order to expand the operation range and enhance the control of SI-CAI

hybrid combustion through the fuel stratification, the stratified flame ignition (SFI) hybrid

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combustion was proposed and investigated in a PFI/DI gasoline engine with stoichiometric

charge operations [19]. In the SFI hybrid combustion concept, the central rich mixture around

spark plug offered by direct injection can be used to enhance the initiation and formation of a

flame kernel, which then promotes the auto-ignition of the premixed diluted lean mixture

away from the spark plug. As a result, the lean or dilute burn limit can be extended by the

auto-ignition combustion and the heat release rate can also be moderated by the delayed auto-

ignition combustion. It was found the piston shape, direct injection timing and direct injection

ratio play important roles in controlling in-cylinder fuel stratification patterns and heat release

process in the stoichiometric SFI hybrid combustion. However, it was also pointed out that

the overall stoichiometric condition results in high sensitivity of SFI hybrid combustion to the

degree of the fuel stratification because the strong coupling of the mixture conditions in the

central region and outer region. Specifically, the over-rich mixture in central region achieved

by higher direct injection ratio definitely leads to a very lean mixture in the outer region,

which would increase the unburned hydrocarbon (uHC) emissions. Although the reduced

direct injection ratio can create more appropriate conditions for better combustion

performance, the reduced fuel stratification would lead to high PRRmax.

In some other studies, split injections were employed to reduce the heat lease rate at high

load. Similar to findings in [19], there is a trade-off between the fuel conversion efficiency

and excessive rate of heat release between the first injection (premixed) and the second

injection (stratified charge) operation, when the split injection was used as the principal

means to create stratified charge [16] in the direct injection gasoline CI engine. In addition,

Olesky et.al [20] investigated the effects of diluent composition on heat release rates of the

SACI combustion in a gasoline research engine with early injection in the intake stroke. It

was found that the thermal efficiency of SACI combustion gradually increased and ringing

intensity showed decreasing trend as the charge molar O2 fraction increased. Zigler et.al [21]

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studied the effects of a range of fuel/air equivalence ratio (0.38–0.62) conditions on spark

assisted HCCI combustion in a single-cylinder research engine with early port fuel injection.

It was found the spark assist showed enhanced effect on heat release process with the fuel/air

equivalence ratio increasing.

In order to further enhance the control of the hybrid combustion, some studies also applied

stratified mixture through later direct injection with overall lean air conditions. Persson et al.

[22] investigated the effect of fuel stratification on SACI with overall diluted ethanol/air

mixture (lambda 1.4) and found a clear decrease in heat release rate as well as in accumulated

heat release rate for the cases with a DI ratio of 30 and 60 %. This was thought to be the

result of high stratification in the combustion chamber perimeter with possible wall-wetting

as well as overly rich mixtures giving rise to partial burn. In order to expand the high load

operation range of gasoline HCCI combustion, a two-step combustion concept with separate

heat release from SI and auto-ignition was proposed [23] and validated with the stratified lean

mixture (overall air/fuel ratio around 26:1) by split injection in a gasoline engine with

compression ratio of 15:1. On the other hand, Berntsson et.al [24] applied stratified lean

mixture (overall lambda around 1.4) to control the combustion phasing of spark assisted

HCCI combustion and successfully expanded the operational range towards lower loads to

1.5 bar IMEP without sacrificing indicated fuel consumption.

It can be inferred from the above literature reviews that the stoichiometric hybrid combustion

with stratified mixture would be limited by the trade-off between IMEP and PRRmax when

expanding to higher load operations, while the lean air mixture shows promising effect on

controlling the hybrid combustion process with both homogeneous operations and stratified

operations. However, these studies were performed with either homogeneous conditions [20,

21] or fixed dilution conditions [22-24]. The effect of different dilution levels on SFI hybrid

combustion with stratified fuel/air mixture is still unclear. Most importantly, the dilution

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criterion and the optimal fuel/air equivalence ratio distribution to achieve efficient hybrid

combustion with acceptable PRRmax have not been demonstrated. In this study, effects of

different dilution strategies and stratified charge by the late DI injection on in-cylinder

fuel/air equivalence ratio distribution and SFI hybrid combustion are systematically

investigated by three dimensional (3-D) computational fluid dynamics (CFD) simulations.

The impact of the fuel/air equivalence ratio distribution on IMEP and PRRmax of SFI hybrid

combustion is revealed through the detailed analysis of simulation results.

In the first part of paper, in order to decouple the fuel/air equivalence ratio in the central

region and outer region in the stoichiometric operation, the air dilution is introduced through

different dilution strategies to regulate the fuel/air equivalence ratio distributions with a

compression ratio (CR) of 10.66:1. Then performances of lean SFI hybrid combustion at a

higher CR of 14:1 are presented and discussed in Section 2. In addition, the fine tuning of

direct injection ratio (rDI) is performed in Section 3 to obtain the optimal SFI hybrid

combustion performance with higher IMEP and lower PRRmax. In the last part of the paper,

the effect of in-cylinder fuel/air equivalence ratio distribution patterns on air-diluted SFI

hybrid combustion is analyzed.

2. Methodology

2.1 SFI hybrid combustion modelling

The 3-D CFD simulations were performed in STAR-CD software. Reynolds-averaged Navier

Stokes (RANS) equations with the RNG k-ε model were used to model flows and turbulence.

The energy equation of the fluid mixture was solved through the general form of the enthalpy

conservation equation. [25]. The wall heat transfer was calculated with Angelberger wall

function [26]. The Pressure-implicit with splitting of operators (PISO) algorithm was used to

solve the discretized equations. In order to simulate the physical process of fuel injection,

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spark ignition, flame propagation and auto-ignition in SFI hybrid combustion, a set of models

were adopted and validated as follows.

2.1.1 Spray modeling

In this study, a multi-hole injector was used as the direct injector [19]. MPI2 nozzle model

[27] was employed to calculate the velocity of the liquid fuel as it exits the nozzle and enters

the combustion chamber. The atomization and break-up of the liquid droplets were simulated

with Reitz-Diwakar model [28]. O’ Rourke model [29] was adopted to consider collisions

between fuel droplets. Bai model [30] was applied to consider the wall impingement. Model

parameters were well tuned and the simulation results of the spray process showed good

agreement with the corresponding optical visualizations. The detailed spray modelling and

validation results can be found in the previous works [19, 31].

2.1.2 Hybrid combustion modeling

The SFI hybrid combustion comprises both early flame propagation and subsequent auto-

ignition process. A set of models for the premixed flame propagation and auto-ignition

combustion was employed to cover both the turbulent mixing effects and chemical kinetics in

the hybrid combustion. The three-zones extended coherent flame model (ECFM3Z) [32],

which can consider premixed flame propagation, diffusion flame propagation and auto-

ignition combustion, was adopted as the framework of the hybrid combustion model. The gas

state in ECFM3Z is represented by a pure fuel zone, a pure air plus possible residual gas zone

and a mixed zone. The flame surface density equation was used to describe the flame

propagation process. The average flame surface density is defined as the local area of flame

per unit of volume (m−1), which is used to describe the intensity of flame propagation. The

tabulated chemistry approach [33] was adopted to predict the auto-ignition of the unburned

charge. With the tabulated chemistry approach, the transportation equation of affected by

the fuel tracer and local auto-ignition delay in a cell was solved to monitor the auto-

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ignition progress. Auto-ignition occurs when exceeds the fuel tracer . The auto-

ignition tendency , which ranges between 0 (no tendency to auto-ignition) and 1 (auto-

ignition), was defined to explicitly describe the close degree of fresh mixture from auto-

ignition in each cell. The definition of is shown as following:

(1)

Chemical kinetic calculations under various thermodynamic and dilution conditions were

performed with a reduced gasoline surrogate mechanism [34] to construct the tabulated

database used for the tabulated chemistry approach.

During the calculation, the reaction regime of each cell is determined by the average flame

surface density and the auto-ignition tendency. The available fuel/air mixture in a cell will be

consumed by the flame propagation according to the flame surface density equation when the

local average flame surface density of the cell is greater than 0. By contrast, the available

fuel/air mixture in a cell will be consumed by auto-ignition combustion according to the

tabulated chemistry approach if the auto-ignition tendency of the cell achieves 1. The

application of above models enables the prediction of the stratified flame ignition (SFI)

hybrid combustion. The detailed modelling and validation of the hybrid combustion model

can be found in a previous paper [35].

2.2 Experimental engine

The engine experiment was carried out on a single cylinder gasoline engine to validate the

SFI hybrid combustion model. A specially designed cylinder head equipped with a 4-variable

valve actuation system (4VVAS) was mounted on a Ricardo Hydra single cylinder block to

enable the continuous adjustment of intake/exhaust valve lift and the valve timing. Table 1

shows the basic engine specifications.

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An electric dynamometer was coupled with the engine to maintain constant engine speed

during experiments. A linear oxygen sensor with the ±1.5% accuracy was mounted in the

exhaust pipe to control the air/fuel ratio. The Kistler 6125B piezoelectric transducer coupled

with 5011B charge amplifier was used to monitor the in-cylinder pressure. A laminar flow

meter with the ±1% accuracy was used to measure the amount of airflow. The coolant and

lubricant oil temperatures were maintained at 80 ± 1 ºC and 55 ± 1 ºC respectively.

Table 1 Engine specifications.

Bore 86 mm

Stroke 86 mm

Displacement 0.5 L

Compression ratio 10.66:1

Combustion chamber Pent roof / 4 valves

Fuel injection PFI/DI

Fuel Gasoline 93 RON

Intake pressure Naturally aspirated

Throttle WOT

Table 2 Operation conditions.

IMEP 3.6 bar

Engine speed 1500 r/min

Piston shape Flat piston

Exhaust valve open (EVO) 177 °CA aTDC a

Exhaust valve close (EVC) 254 °CA aTDC a

Exhaust valve lift (EL) 1.9 mm

Intake valve open (IVO) 226 °CA bTDC a

Intake valve close (IVC) 117 °CA bTDC a

Intake valve lift (IL) 5.0 mm

Spark Timing 35 ºCA bTDC a

Fuel injection PFI

Fueling rate 13.4 mg/cycle

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Fuel/air equivalence ratio 1

eEGR 0.08

iEGR 0.36a 0 ºCA is defined as the combustion top dead centre (TDC) in this study.

A typical part-load operation point at IMEP 3.6 bar was selected as the baseline case in the

numerical study. The operation conditions are shown in Table 2. Both internal exhaust gas

recirculation (iEGR) and external exhaust gas recirculation (eEGR) were used to achieve the

stable hybrid combustion. The iEGR was obtained by the negative valve overlap (NVO)

strategy. The other experimental details could be found in [7, 8].

2.3 Simulations setup and validation

In this study, the 3D CFD simulations with different dilution strategies and direct injection

ratios were performed to understand the effect of in-cylinder dilution level and fuel/air

equivalence ratio distribution on SFI hybrid combustion. The fueling rate in CFD simulations

was the same with experiment and fixed at 13.4 mg/cycle. The fuel/air equivalence ratio (ϕair)

and fuel/dilution equivalence ratio (ϕdilution) are used to indicate the lean and dilution

conditions in the simulations. The equations are shown as following [20]:

(2)

(3)

Details of simulation cases are given in Table 3. Case 1 is the baseline case with pure PFI and

the original flat piston in the engine. Port fuel injection (PFI) and late direct injection (DI)

were implemented to produce the premixed lean/diluted mixture and the stratified charge

respectively. rDI in Table 3 is defined as the fuel mass ratio of the direct injection. The direct

injection timing was fixed at 60 ºCA bTDC for all simulations. In all the other cases (except

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Case 1), a shallow bowl was incorporated into the piston top [31], in order to facilitate the

formation of stratified flame by the late direct injection.

Table 3 Simulation cases

rDI eEGR iEGR ϕair ϕdilution

Baseline case(CR=10.66:1) Case 1 0 0.08 0.36 1 0.58

Group 1(CR=10.66:1)

Case 2 0.28 0.08 0.36 1 0.58

Case 3 0.28 0 0.36 0.92 0.58

Case 4 0.28 0 0.28 0.83 0.58

Group 2(CR=14:1)

Case 5 0.28 0.08 0.36 1 0.58

Case 6 0.28 0 0.36 0.92 0.58

Case 7 0.28 0 0.28 0.83 0.58

Case 8 0.28 0 0.36 0.83 0.55

Case 9 0.28 0 0.36 0.7 0.49

Group 3(CR=14:1)

Case 10 0.50 0 0.36 0.83 0.55

Case 11 0.16 0 0.36 0.83 0.55

Case 12 0 0 0.36 0.83 0.55

In Group 1, the effect of different dilution strategies was investigated at a compression ratio

of 10.66:1. The direct injection ratio was set as 0.28. Case 2 used the same dilution strategies

with the baseline case, while Case 3 replaced the external exhaust gas with the fresh air and

obtained lower ϕair. In order to further increase the air dilution level, a part of the internal

residual gas was replaced with fresh air in Case 4, which further reduced the ϕair. Because the

total dilution charge was fixed with the above dilution strategies, the ϕdilution in Case 2-4 was

kept the same as the baseline case. For the sake of brevity, the SFI hybrid combustion with

overall ϕair below 1 is termed as air-diluted SFI hybrid combustion although the exhaust gas is

also involved in the hybrid combustion.

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In Group 2, the compression ratio was increased to 14:1 to evaluate the potential of SFI

hybrid combustion in a high compression ratio engine. The dilution strategies for Case 2-4

were applied for Case 5-7 in sequence. In addition, a lean boosted dilution strategy was tried

in Case 8 and Case 9. The ϕair in Case 8 was reduced to 0.83 by increasing additional intake

air charge while maintaining the internal residual gas. With the same method, ϕair in Case 9

was further reduced to 0.7. Because this dilution strategy increased the total dilution charge,

the ϕdilution in Case 8 and Case 9 was reduced to 0.55 and 0.49, respectively.

In order to further optimize the air-diluted SFI hybrid combustion and understand the effect

of fuel/air equivalence ratio distribution on the air-diluted SFI hybrid combustion, the direct

injection ratio (rDI) was reduced from 0.5 in Case 10 to 0 in Case 12 to regulate the in-

cylinder fuel/air equivalence ratio distribution in Group 3.

In this study, the moving meshes for the baseline flat piston and bowl piston were generated

in ES-ICE using the mapping method. The engine mesh with bowl piston is shown in Fig. 1

as an example. The grid size for the meshes is around 0.8 mm. The simulations were carried

out from the intake valve opening (IVO) timing to the end of combustion. In the simulations,

the initial and boundary conditions were obtained from the validated one-dimensional (1D)

engine simulations in GT-Power [36]. The wall temperature for the cylinder head, piston

head and cylinder liner in 3D CFD simulations were 400 K, 442 K and 371 K, respectively.

The initial exhaust temperature and pressure at IVO were fixed at 773 K and 1.02 bar. In the

baseline case (Case 1), the initial intake temperature and pressure were 355 K and 0.99 bar,

and the initial in-cylinder temperature and pressure were 571 K and 0.49 bar, respectively. In

the simulations, the iEGR and eEGR were controlled by setting up the initial and boundary

conditions of the mixture components in the cylinder and intake port. Specifically, the initial

mixture in the cylinder at IVO was set as the pure residual burnt gas and the mass can be

controlled by the adjustment of initial temperature and pressure in the cylinder according to

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ideal gas equation of state. The initial and boundary conditions of the mixture components at

intake port were set as the fuel/air/exhaust gas mixture and the mass fraction between fuel/air

mixture and exhaust gas was adjusted to control the eEGR. The fuel/air mixture prepared by

the port fuel injection (PFI) in the intake port was set as the homogeneous fuel/air mixture.

The intake pressure was adjusted to control the total amount of the intake mixture in the

simulations. The sweep of the spark timing (ST) was performed for all the cases.

Fig. 1. Engine mesh with the bowl piston.

Fig. 2 compares the predicted and measured in-cylinder pressure and heat release rate profiles

of the baseline case. The experimental pressure profile was calculated from the averaged

pressure data of over 200 successive cycles. As shown in Fig. 2, the adopted simulation

models can reproduce the hybrid combustion process and shows good agreement with the

experimental data.

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Fig. 2. The in-cylinder pressure and heat release rate of the baseline case (Case 1) from

experiment and simulation.

3. Results and discussions

3.1 Effect of dilution strategies on SFI hybrid combustion with a CR of 10.66:1

Fig. 3 shows the effect of ϕair on the combustion phasing measured by CA50 (the crank angle

of 50% total fuel mass burned), IMEP and PRRmax of the SFI hybrid combustion with

different dilution strategies. As shown in Fig. 3, the spark timing has a direct impact on the

SFI hybrid combustion. With the retarded spark timing, CA50 is gradually delayed and IMEP

decreases. The impact of the ϕair on SFI hybrid combustion is complicated. The combustion

phasing gradually advances with decreased ϕair when the spark timing is earlier than 40 ºCA

bTDC. With late spark timing, combustion can still be advanced in Case 3, while the

combustion is slowed down significantly by lower ϕair in Case 4. However, it is noted that

both the PRRmax and IMEP in Case 3 is significantly increased at different spark timings. In

addition, the PRRmax shows no sensitivity to the spark timing, which means that the delay of

spark timing is no longer effective to reduce PRRmax for this dilution strategy. In comparison,

the over-diluted mixture in Case 4 results in both lower IMEP and PPRmax.

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Fig. 3. Effect of ϕair on combustion phasing (CA50), IMEP and PRRmax of SFI hybrid

combustion with a CR of 10.66:1

It is also noted in Fig. 3 that the differences between Case 3 and Case 4 gradually increase

with the delaying of the spark timing because of the increasing sensitivity of the auto-ignition

on the in-cylinder conditions with delayed spark timing. In Case 3, the in-cylinder dilution

and thermal conditions are suitable for both flame propagation and auto-ignition. Therefore,

the combustion phasing can be significantly advanced even with very late spark timing. In

addition, the peak PRRmax occurred during auto-ignition process shows less sensitivity on

spark timing. Because of the over-diluted mixture in Case 4, the subsequent auto-ignition

process is very dependent on the early flame propagation process, thus leading to higher

sensitivity on spark timing. As indicated in Fig. 3, the earlier spark timing is, the higher the

IMEP is in Case 4. This can be attributed to the enhanced auto-ignition by early flame

propagation.

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The overall performance of the SFI hybrid combustion, as shown in Fig. 3, indicates the

significant impact of dilution conditions on hybrid combustion process. In order to

demonstrate the effect of dilution conditions on the SFI hybrid combustion process, the in-

cylinder conditions are analysed. Fig. 4 compares the distributions of fuel/air equivalence

ratio at 36 ºCA bTDC. The spark timing is fixed at 35 ºCA bTDC. As shown in Fig. 4, the

area of the rich mixture (ϕair >1) in the central region gradually shrinks with the ϕair reduced

from Case 2 to Case 3. Meanwhile, the fuel/air equivalence ratio in the outer region also

shows decreasing trend. Fig. 5 shows the averaged fuel/air equivalence ratio at different

regions to quantify the in-cylinder ϕair stratification. The whole cylinder volume is divided

into seven cylindrical zones. It can be seen that with ϕair reduced from Case 2 to Case 4, the

local ϕair in Zone 1 where the flame propagation mainly takes place gradually approaches 1.1

which is the most favourable ϕair to achieve the highest laminar flame speed [34]. The local

ϕair of the outer zones gradually reduces from Case 2 to Case 4. Specifically, the local ϕair of

Zone 7 in Case 4 is as low as 0.6 and the fuel/dilution equivalence ratio (ϕdilution) is only 0.43,

which would significantly inhibit the auto-ignition process.

Fig. 4. Section views of the fuel/air equivalence ratio distributions at 36 ºCA bTDC with

different dilution strategies.

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Fig. 5. The average fuel/air equivalence ratio of the mixture in different zones at 40 ºCA

bTDC.

Fig. 6 compares the average pressure and the flame surface density of the hybrid combustion

with different dilution strategies. The spark timing is fixed at 35 ºCA bTDC. The flame front

in Fig. 6 (b) is denoted by the iso-surface of 80% of the maximum flame surface density.

Compared to the baseline Case 1, the stoichiometric SFI hybrid combustion in Case 2 shows

slower flame propagation process and leads to lower pressure at the early stage of the

combustion process. The reason is attributed to the over-rich mixture in the central region

around spark plug, as indicated in Fig. 5.

When the external exhaust gas recirculation is replaced by the fresh air in Case 3, the

appropriate local ϕair in the central region leads to enhanced flame propagation, as indicated

by the higher average flame surface density trace and larger visualized flame front at 10 ºCA

bTDC in Fig. 6 (b). The pressure profile of the early stage in Case 3 almost overlaps with that

of the baseline Case 1. However, it is found that the subsequent auto-ignition process in Case

3 is advanced because of the enhanced flame propagation and the reduced auto-ignition delay

of the slightly rich mixture near the flame front in the central region compared to baseline

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Case 1 with homogeneous mixture. In Case 4, the local fuel/air equivalence ratio in the

central region approaches 1.1, which results in a much faster flame speed, as indicated in Fig.

6(b). However, the over-diluted mixture in the outer region slows down the auto-ignition

process. Therefore, the transition from flame propagation to auto-ignition in Case 4 is less

obvious on the pressure profile in Fig. 6 (a). The significantly weakened auto-ignition in Case

4 would finally lead to large amount of unburned mixture and reduce the IMEP significantly,

as shown in Fig. 3.

Fig. 6. (a) In-cylinder pressure traces and (b) average flame area density of the SFI

combustion with different dilution strategies. The spark timing is fixed at 35 ºCA bTDC.

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The in-cylinder pressure traces, IMEP and PRRmax of the SFI hybrid combustion with the

combustion phasing around 7.5 ºCA aTDC are compared in Fig. 7. In Case 2, the spark

timing has to be advanced to maintain the combustion phasing because of the slower flame

propagation process of the over-rich mixture in the central region. In case 3, the spark timing

is delayed to maintain combustion phasing because of stronger flame propagation and

subsequent auto-ignition process due to the more favourable fuel/air equivalence ratio

distribution. In Case 4, the later auto-ignition combustion is relatively weak due to the over-

diluted mixture in outer region, although the early flame propagation process is enhanced. As

a result, the spark timing is slightly delayed to maintain the combustion phasing in Case 4.

Although the IMEP in Case 3 is the highest, the corresponding PRRmax is also dramatically

increased. In Case 4, the IMEP is significantly deteriorated, as shown in Fig. 7 (b), although

the PRRmax can be effectively reduced with this dilution strategy.

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Fig. 7. (a) in-cylinder pressure traces and (b) IMEP and PRRmax for the SFI combustion with

different dilution strategies. The CA50 is fixed around 7.5 ºCA aTDC.

Fig. 8 compares the flame propagation dominated combustion duration D1, auto-ignition

dominated combustion duration D2 and the ratio of the accumulated heat released (RCAT) at

CAT. CAT is defined as the Crank Angle corresponding to the mode Transition from SI to

CAI. D1 is the duration between CA10 and CAT, and D2 is the duration between CAT and

CA90. In Case 3, the slightly fuel rich mixture (ϕair =1~1.1) gradually moves to the central

region, as shown in Fig. 5, and the corresponding auto-ignition delay can be significantly

shortened with the heating effect by the flame front. As a consequence, the mode transition

from SI to CAI occurs quickly after the flame propagation. As shown in Fig. 8, the flame

propagation dominated combustion duration D1 is shortest and RCAT is also the lowest.

Therefore, the dilution strategy adopted in Case 3 can enhance both the early flame

propagation and the later auto-ignition because of the appropriate fuel/air equivalence ratio

distribution. In Case 4, the flame propagation process occurs mostly in the leaner stratified

charge region. In addition, the reduced hot internal residual gas in Case 4 further lowers the

thermal condition, inhibiting the occurrence and development of auto-ignition. As a result,

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the auto-ignition shows great reliance on the early flame propagation, leading to increased D1

and RCAT. The over-lean mixture of the mixture in outer region and poor thermal condition in

Case 4 result in the longest combustion duration of the auto-ignition stage (D2).

Fig. 8. The flame propagation dominated combustion duration (D1), the auto-ignition dominated combustion duration (D2) and the ratio of the accumulated heat released (RCAT) at

CAT.

Fig. 9 shows the average auto-ignition tendency of the mixture in the whole combustion

chamber and its variation among different zones. It can be seen that the profiles of the

average auto-ignition tendency in the whole combustion chamber almost overlap in Case 2

and Case 3 at the early stage of the combustion process and gradually deviate from each other

after 5 ºCA aTDC. It can be inferred that the in-cylinder dilution conditions in Case 3 are

quite beneficial for promoting the auto-ignition because of the less heat release from flame

propagation in Case 3, as shown in Fig. 8. With the development of the combustion process,

the leaner mixture in the outer region shows longer auto-ignition delay and reduces the

average auto-ignition tendency to a slight extent in Case 3. This can be verified by the

dramatically increased difference of the average auto-ignition tendency in Zone 7 between

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Case 2 and Case 3 with the development of combustion process. Although the early flame

propagation in Case 4 is enhanced because of the higher fuel/air equivalence ratio distribution

in the central region, its positive impact on the later auto-ignition is not sufficient to

compensate for the negative impact on auto-ignition brought by the over-lean mixture and

lower thermal conditions, leading to lowest auto-ignition tendency traces in Fig. 9.

Therefore, both in-cylinder thermal and component conditions show essential impact on SFI

hybrid combustion. In order to optimize SFI hybrid combustion, the adopted dilution

strategies should not only improve the early flame propagation, but also benefit the later auto-

ignition process because auto-ignition is more sensitive to the dilution and thermal

conditions.

Fig.9. The average auto-ignition tendency of the mixture in the whole combustion chamber and its variation among zones for the SFI combustion with different dilution strategies.

3.2 Effect of dilution strategies on SFI hybrid combustion with a CR of 14:1

As discussed in Section 3.1, the proposed hybrid SFI combustion concept could effectively

control the PPRmax with appropriate dilution strategies, which indicates the potential to

accommodate a higher compression ratio to further improve thermal efficiency. In this

section, the effect of dilution strategies on the SFI hybrid combustion with a higher

compression ratio of 14:1 is investigated.

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Fig. 10 shows the effect of dilution strategies on CA50, IMEP and PRRmax of the SFI hybrid

combustion at the higher compression ratio. Case 5 with the same dilution strategy as Case 2

produces greater output and hence leads to higher efficiency than Case 2 because of the

increased compression ratio. However, the PRRmax in Case 5 increases more significantly,

which exceeds 5 bar/ ºCA at all ignition timings. The air dilution strategy adopted in Case 6

enhances the early flame propagation process and elevates both IMEP and PRRmax. The

replacement of the in-cylinder residual gas with the fresh intake air in Case 7 reduces the

PRRmax at the expense of lower IMEP. Therefore, the trade-off between IMEP and PRRmax

still exists with the above dilution strategies in a high compression ratio engine.

In Case 8, a new dilution strategy with increased intake fresh air at a constant concentration

of internal residual gas was studied. The ϕair in Case 8 was kept the same as Case 7, and

correspondingly the ϕdilution was decreased to 0.55 because of the increased total dilution mass.

In this case, the central flame propagation would be enhanced as that in Case 7, while the

subsequent auto-ignition would not be dramatically inhibited because of the maintained

thermal conditions brought by sufficient internal residual gas. In general, the combustion

phasing in Case 8 is delayed compared to that in Case 6 and comparable to that in Case 5.

Compared to Case 7, the combustion phasing in Case 8 is more advanced at the retarded

spark timings because of the improved thermal conditions that can guarantee the stable auto-

ignition even with late spark ignition. As a result, the IMEP values in Case 8 are slightly

lower than those in Case 5 and 6 and relatively higher than Case 7. In the meantime, the

PRRmax values in Case 8 are reduced below 5 bar/ºCA at all spark timings. However, it is

noted that further dilution with the intake fresh air in Case 9 would significantly inhibit the

combustion process and reduce IMEP dramatically as shown in Fig. 10.

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Fig. 10. Effect of ϕair and ϕdilution on CA50, IMEP and PRRmax of SFI hybrid combustion with a

CR of 14:1.

Fig. 11 (a) compares the in-cylinder pressure traces with different ϕair and ϕdilution with CA50

around 2.6 ºCA bTDC. Under such condition, Case 5 produces the maximum IMEP. In Case

6, the spark timing is delayed in order to maintain the combustion phasing, leading to

relatively weak flame propagation. However, the higher compression ratio increases the

charge pressure and temperature, leading to dramatically increased PRRmax during the auto-

ignition combustion process, as shown in Fig. 11 (b). In Case 7, the additional air facilitates

the early flame propagation but results in lower IMEP, similar to that in Case 4. In Case 8,

the dilution strategy adopted can effectively lower the PRRmax with marginal decrease in

IMEP compared to that of Case 5. As indicated by the pressure traces, the optimized fuel/air

equivalence ratio distribution in Case 8 enhances the early flame propagation process.

However, the auto-ignition combustion in Case 8 is less affected by the increased air dilution

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when the amount of residual gas remains constant. Therefore, the IMEP only shows slight

decrease compared to that of Case 5. In Case 9, more intake fresh air is introduced and leads

to higher in-cylinder pressure even before the spark ignition because of the increased total in-

cylinder charge. However, the over-diluted condition leads to weak auto-ignition process and

it is hard to observe the transition from SI to CAI combustion from the pressure trace.

Correspondingly, the IMEP is significantly reduced, as shown in Fig. 11 (b).

The above results have shown that the dilution strategies can have significant impact on SFI

hybrid combustion under high compression ratio operations. The dilution strategy in Case 8

with additional air charge enhances SFI hybrid combustion performance with acceptable

PRRmax. However, too much intake air would lead to over-diluted mixture and deteriorate the

SFI hybrid combustion performance, as shown in Case 9.

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Fig. 11. (a) in-cylinder pressure traces and (b) IMEP and PRRmax for the SFI combustion with

different dilution strategies. The CA50 is fixed around 2.6 ºCA bTDC.

In order to demonstrate the SFI hybrid combustion with different dilution strategies, the in-

cylinder dilution and thermal conditions are analysed. Fig. 12 compares the average fuel/air

equivalence ratio (ϕair) and temperature in different zones. As expected, the local fuel/air

equivalence ratio in each zone shows decreasing trend with the overall ϕair decreasing from

Case 5 to Case 7. The temperature in Case 6 is slightly higher than that in Case 5 because of

the increased specific heat ratio of the in-cylinder charge. In Case 7, the average temperature

in each zone is significantly reduced because of the reduction of the internal residual gas. The

addition of intake fresh air without sacrificing internal residual gas in Case 8 can lead to

higher in-cylinder temperature because of the heating effect from hot residual gas and

increased total dilution mass. The overall fuel/air equivalence ratio in Case 8 is kept the same

as Case 7. This leads to similar local fuel/air equivalence ratio in different zones between

Case 8 and Case 7. The addition of further intake air in Case 9 lowers the overall ϕair and

local ϕair in different zones and meanwhile increases the in-cylinder temperature slightly

because of the increased dilution mass.

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Fig. 12. In-cylinder local fuel/air equivalence ratio (ϕair) and temperature in different zones at

40 ºCA bTDC.

Fig. 13 and Fig. 14 compare the mass burned fraction (MFB) traces of the SFI hybrid

combustion and the auto-ignition tendency of the mixture in the representative outer region

(Zone 7) with different dilution strategies, respectively. The auto-ignition in Case 6 is

significantly enhanced because of appropriate fuel/air equivalent ratio in central region

although the early flame propagation is weakened because of the delayed spark timing. In

Case 7, the local fuel/air equivalence ratio in central region is closer to 1.1, which

significantly enhances the early flame propagation process. However, the significantly

decreased in-cylinder temperature, as shown in Fig. 12, leads to slower subsequent auto-

ignition combustion.

The addition of intake air in Case 8 leads to a similar distribution of fuel/air equivalence ratio

to Case 7 and results in a stronger flame propagation than Case 5. Although the early flame

propagation in Case 8 is weakened slightly compared to that in Case 7 because of the

increased total dilution mass, the subsequent auto-ignition process is enhanced because of the

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elevated charge temperature by the presence of the hot residual gas. In Case 9, the fuel/air

equivalence ratio of the mixture in the central region is around 1.1 because of the further

dilution by additional intake air, which further enhances the early flame propagation.

However, the additional intake air also leads to over-diluted mixture in the outer region,

which significantly deteriorates the subsequent auto-ignition process, as indicated in Fig. 13

and 14.

Therefore, both the in-cylinder thermal and dilution condition are vital to achieve better SFI

hybrid combustion performance. The thermal condition in Case 7 is not sufficient while the

dilution condition is not appropriate in Case 9, which both led to poor combustion

performance. Comparatively, the thermal and dilution conditions in Case 8 are suitable to

achieve better combustion performance.

Fig. 13. The mass fraction burned (MFB) traces of the SFI hybrid combustion with different

dilution strategies, and the CA50 is fixed around 2.6 ºCA bTDC.

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Fig.14. The average auto-ignition tendency of the mixture in Zone 7 for the SFI hybrid combustion with different dilution strategies.

Fig. 15 shows the flame propagation dominated combustion duration (D1), the auto-ignition

dominated combustion duration (D2) and the ratio of the accumulated heat released (RCAT) at

the transition point CAT. The comparison between Fig. 8 and 15 indicates that the effect of

dilution strategies in Case 2/5, Case 3/6 and Case 4/7 on the combustion duration and RCAT

shows similar trends under different compression ratio operations. However, the increased

compression ratio leads to shorter combustion duration under different dilution strategies.

Compared to Case 7, the lean boosted dilution in Case 8 elevates thermal condition and

decreases the dependency of auto-ignition on the early flame propagation, leading to lower

RCAT (19.76%) and shorter flame propagation dominated combustion duration (D1).

Meanwhile, the later auto-ignition process is also enhanced, leading to shorter auto-ignition

dominated combustion duration (D2). But it should be noted that the combustion duration in

Case 8 is still longer than that in Case 5, which is responsible for the lower PRRmax. The

additional air dilution in Case 9 leads to increased dependency of subsequent auto-ignition on

early flame propagation because of the over-diluted condition in the outer region. This in turn

increases the RCAT and flame propagation dominated combustion duration (D1) and auto-

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ignition dominated combustion duration (D2). The prolonged combustion duration in Case 9

leads to incomplete combustion and deteriorate IMEP dramatically.

Fig. 15. The flame propagation dominated combustion duration (D1), the auto-ignition dominated combustion duration (D2) and the ratio of the accumulated heat released (RCAT) at

CAT.

Fig. 16 compares the peak IMEP and the corresponding PRRmax of the SFI hybrid combustion

with different dilution strategies and compression ratios. The SFI hybrid combustion with a

lower compression ratio (CR=10.66:1) shows lower IMEP and PRRmax than the baseline Case

1 in which the stoichiometric homogenous charge is combusted by traditional SI-CAI hybrid

combustion. Both IMEP and PRRmax become higher with the increased compression ratio. By

replacing a part of residual gas by fresh air in Case 7, both PRRmax and IMEP are lowered

notably. In comparison, by adding the air to the cylinder charge with the same amount of

residual gas in Case 8, there is a 17.5% decrease in PRRmax and slight decrease (3.66%) in

IMEP compared to the baseline Case 1. The over-diluted mixture in Case 9 leads to the

lowest IMEP although the PRRmax is significantly reduced. The above results indicate that the

SFI hybrid combustion with the proposed dilution strategy in Case 8 shows better combustion

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performance. The significantly reduced PRRmax in Case 8 also indicates the potential to

optimize the hybrid combustion performance through adjusting the direct injection ratio.

Fig.16. The peak IMEP and the corresponding PRRmax for the SFI hybrid combustion with

different dilution conditions and compression ratios.

3.3. Optimization of SFI hybrid combustion by the direct injection ratio

In this section, the effect of direct injection ratio (rDI) is analysed for the higher compression

ratio operations as the percentage of direct injection varied from 50% to 0%. As shown in

Fig. 17, the homogeneous hybrid combustion (Case 12) is characterised with both the highest

IMEP and PRRmax. The SFI hybrid combustion with direct injection reduces PRRmax. With rDI

=0.16, the PRRmax of the SFI hybrid combustion is significantly reduced to around 2.2

bar/ºCA and the IMEP values show slight reduction. As the direct injection ratio is increased

further to 28% and 50%, the enriched central mixture around spark plug advances the

combustion phasing but slows down the auto-ignition combustion of the leaner premixed

mixture, leading to reduced IMEP. The above results indicate the existence of the optimal rDI

to achieve the air-diluted SFI hybrid combustion with both higher IMEP and lower PRRmax. It

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is found in this study that a small quantity of direct injection (i.e. rDI =0.16) is preferred to

obtain the optimal SFI hybrid combustion.

Fig. 17. Effect of direct injection ratio (rDI) on CA50, IMEP and PRRmax.

Fig. 18 directly compares the peak IMEP and the corresponding PRRmax of the SFI hybrid

combustion with different rDI. The homogeneous hybrid combustion can obtain highest IMEP

of 3.66 bar, which is 11.59% higher than that of the baseline Case 1. However, the PRRmax of

the homogeneous hybrid combustion is 10.39 bar/ºCA, which is much higher than the

acceptable limit of 5 bar/ºCA for a practical engine. With rDI of 0.16, the peak IMEP can

achieve 3.41 bar, which is 3.96% higher than that of baseline Case 1. Meanwhile, the

corresponding PRRmax is as low as 2.11 bar/ ºCA. The relative low PRRmax indicates the

potential to further elevate IMEP with a lower rDI (<0.16).

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Fig. 18. Peak IMEP and the corresponding PRRmax with different rDI.

Fig. 19 shows the fuel/air equivalence ratio distributions at 36 ºCA bTDC for a fixed spark

timing at 35 ºCA bTDC. As shown in the figure, the local fuel/air equivalence ratio of the

mixture in the central region gradually increases with the rDI. The local ϕair in Zone 1 has

exceeded 1.5 in Case 10, and consequently leads to slower flame propagation process, as

shown by the MFB traces in Fig. 20. The local ϕair in central region is closest to 1.1 in Case 8

and leads to the fastest flame propagation process. In Case 11, the mixture in the central

region is a little leaner for the flame propagation and leads to moderate flame propagation

process among three cases.

On the other hand, the local ϕair in the outer region gradually decreases with rDI. The local ϕair

in the outer region in Case 10 with highest rDI is as lean as 0.4 and leads to highest in-cylinder

fuel stratification from central to outer region. As a consequence, the over-lean condition in

outer region deteriorates the auto-ignition and leads to incomplete combustion and lowest

IMEP in Case 10, as indicated in Fig. 17 and 18.

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Fig. 19. In-cylinder fuel/air equivalence ratio (ϕair) at 36 ºCA bTDC. The spark timing is

fixed at 35 ºCA bTDC.

Fig. 20. The mass fraction burned (MFB) traces of the SFI hybrid combustion with different

dilution strategies. The spark timing is fixed at 35 ºCA bTDC.

The previous study on stoichiometric SFI combustion [19] has shown that the higher IMEP is

always accompanied with higher PRRmax when rDI is reduced to obtain a more homogeneous

SFI combustion. Although the trade-off between higher IMEP and lower PRRmax can also be

observed when rDI is reduced from 0.5 to 0.28 with the lean boosted dilution strategy, both

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higher IMEP and lower PRRmax can be obtained with an rDI of 0.16 as shown in Fig. 18. In

order to explain the inherent reason for the higher IMEP and lower PRRmax with rDI of 0.16,

the detailed analysis of the SFI hybrid combustion with different rDI is performed. The

combustion phasing of all cases analysed is fixed around 0.8 ºCA bTDC where Case 8

obtains peak IMEP.

Fig. 21 shows the MFB traces of the SFI hybrid combustion with different rDI. The crank

angles with mode transitions (CAT) and maximum PRR (CAPRRmax) are also marked in the

figure. In order to maintain the same combustion phasing, the spark timing has to be delayed

to 20 ºCA bTDC in Case 10 because of the relatively higher heat release rate during the early

stage of the auto-ignition combustion. However, the auto-ignition is gradually weakened at

the later stage of the auto-ignition in Case 10 because of the gradually diluted mixture in the

outer region. On the contrary, the heat release rate of the auto-ignition combustion in Case 11

is moderate and the spark timing has to be advanced to 42 ºCA bTDC in Case 11, which can

be observed in Fig. 21.

Fig. 21. The mass fraction burned (MFB) traces of the SFI hybrid combustion with different

dilution strategies. The CA50 is fixed around 0.8 ºCA bTDC.

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It is noted in Fig. 21 that the peak of PRR normally occurs just after CAT, i.e. at the early

stage of the auto-ignition stage. Therefore, the control of the early stage of the auto-ignition

combustion is essential to control the PRR in SFI hybrid combustion. Fig. 22 shows the iso-

surface with the local fuel/air equivalence ratio (ϕair) of 1 and the early auto-ignited sites after

CAT. In Case 10 (rDI =0.5), the diameter of the iso-surface with local ϕair of 1 is significantly

larger, which can also be inferred from Fig. 19. As shown in Fig. 22, the early auto-ignition

sites in Case 10 are surrounded and far from the iso-surface with local ϕair of 1, indicating the

early auto-ignition takes place in the region with richer mixture. With the rDI decreasing, the

iso-surface with local ϕair of 1 gradually shrinks, and the auto-ignition sites are closer to the

iso-surface with local ϕair of 1.

Fig. 22. Iso-surface with the fuel/air equivalence ratio of 1 and the auto-ignition sites after

mode transition.

The relationship between the auto-ignition sites and the iso-surface with local ϕair of 1, as

shown in Fig. 22, indicates the early stage auto-ignition behaviour. In Case 10, the early auto-

ignition mainly occurs in the fuel-rich region with larger charge cooling and stratification,

leading to slower auto-ignition process, reflected by the lower auto-ignition tendency in Fig.

23. On the other hand, the auto-ignition in the outer region, e.g. Zone 7, is also slowed down

in Case 10 because of the leaner mixture in these regions. Actually, the over-lean mixture is

hard to auto-ignite, leading to incomplete combustion and lower IMEP in Case 10. In Case

11, the smallest rDI leads to more homogeneous mixture with least charge cooling effect,

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leading to faster development of auto-ignition process, reflected by the highest auto-ignition

tendency in Fig. 23. In theory, the auto-ignition tendency in Case 8 (rDI = 0.28) should locate

between that in Case 10 and Case 11. However, it is interesting to find that in Case 8 the

auto-ignition process in Zone 2 is comparable to that in Case 11. As indicated in Fig. 19, the

average ϕair of the mixture in Zone 2 in Case 8 is around 0.87 and slightly higher than that in

Case 11, which in turn leads to higher auto-ignition tendency.

Fig. 23. The traces of the average auto-ignition tendency in different zones.

In addition to the evolution of the auto-ignition tendency, the available fuel/air mixture in

these auto-ignited cells also plays an important role on the heat release process of auto-

ignition in the SFI hybrid combustion. The fuel/air equivalence ratio distribution brought by

different rDI actually changes the balance of the competition between the flame propagation

and early auto-ignition process in the central region. Fig. 24 shows the distribution of the

ratio of the fuel consumed by flame propagation (rSI) in these earliest auto-ignited cells (5%

of total cell number at TDC). In Case 10, the over-rich mixture in the central region leads to

slower flame propagation process, leading to lower rSI in these auto-ignited cells. Therefore,

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the accumulated heat release rate of the early auto-ignition in the fuel-rich mixture would be

very significant once the auto-ignition occurs although the auto-ignition tendency is lowest

(Fig. 23). This explains the delayed spark timing in Case 10 to maintain combustion phasing.

In Case 11, the fuel/air equivalence ratio in central region is around 1.05, which enhances the

early flame propagation process. This leads to significantly higher rSI in these early auto-

ignited cells, indicating lower heat release from auto-ignition. The overwhelming flame

propagation over auto-ignition in central region resulted from the slightly rich mixture

explains the slower heat release process in Case 10, as shown in Fig. 21, although the

corresponding auto-ignition tendency is highest in Fig. 23.

The rSI of the early auto-ignited cells in Case 8 is obvious lower than that in Case 11 because

the mixture is a little richer (ϕair =1.2 in Zone 1) for fast flame propagation. This would leads

to increased heat release from auto-ignition. On the other hand, the increased auto-ignition

tendency in Zone 2, as shown in Fig. 23, also contributes to the increased heat release rate in

Case 8. This finally leads to the highest instantaneous heat release rate as shown in Fig. 21,

and hence the highest PRRmax in Fig. 17.

Fig. 24. Distribution of the ratio of the fuel consumed by flame propagation (rSI) in the early

auto-ignited cells (5% of total cell number at TDC).

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3.4. Discussion of the effect of thermal and dilution conditions on controlling SFI hybrid

combustion

The SI-CAI hybrid combustion is characterized with early spark ignited flame propagation

and subsequent auto-ignition process. The competition between flame propagation and auto-

ignition dominates the behaviour of SI-CAI hybrid combustion. The introduction of a

stratified mixture through the direct injection enables the control of subsequent auto-ignition

by the stratified flame. This combustion mode was termed as stratified flame ignition (SFI)

hybrid combustion. In this study, it is found the in-cylinder thermal and dilution conditions

show significant impact on SFI hybrid combustion.

First, the auto-ignition process in SFI hybrid combustion shows high sensitivity to the in-

cylinder thermal conditions. The key issue in SFI hybrid combustion is the adjustment of the

quantity and the thermal condition of the dilution components simultaneously. However,

different dilution components, i.e. fresh air, external exhaust gas and internal residual gas,

show different thermal properties and dilution effects on combustion process. The fresh air

can be used to optimize the fuel/air equivalence ratio distribution, which is very effective for

the improvement of the flame propagation process, as shown in Case 3 and 6. The internal

residual gas is favorable to enhance the auto-ignition because of its heating effect. Therefore,

the combustion process would be deteriorated with lower internal residual gas in Case 4 and

7. The external exhaust gas is a pure dilution medium which shows no direct impact on

air/fuel equivalence ratio and thermal conditions. As a result, the SFI hybrid combustion with

constant dilution mass shows high sensitivity to these dilution strategies that it obtains either

high PRRmax or low IMEP.

Therefore, the optimal dilution strategy should meet the basic demand of the thermal

condition to achieve stable auto-ignition combustion, especially for the SFI hybrid

combustion with leaner mixture in the outer region. In this study, the NVO strategy was used

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to achieve SFI hybrid combustion. Therefore, the internal residual gas fraction is responsible

to maintain appropriate thermal conditions and achieve efficient SFI hybrid combustion.

Otherwise, the incomplete combustion would be occurred and IMEP is deteriorated

dramatically, although the early flame propagation is enhanced with the dilution strategy, as

shown in Case 4 and 7.

Secondly, the SFI hybrid combustion process shows high sensitivity to the in-cylinder

dilution condition, especially to the fuel/air equivalence ratio distribution. As indicated

above, the later combustion process in SFI hybrid combustion, mainly characterized by the

auto-ignition process, is slowed down in the leaner mixture (local ϕair <1) in the outer region.

Therefore, the regulation of early combustion process is essential to control the PRRmax in SFI

hybrid combustion. It can be inferred from this study that the dilution conditions of the

central mixture controls the early heat release of SFI hybrid combustion through the

adjustment of the balance between flame propagation and auto-ignition. Once the mixture in

central region is too rich, the auto-ignition overwhelms the flame propagation, leading to

higher accumulated heat release from the auto-ignition of the fuel-rich mixture. However, the

increased fuel stratification would slightly slow down the heat release rate. When the mixture

in central region is slightly richer than the stoichiometry, the flame propagation overwhelms

the auto-ignition, leading to less contribution of fast auto-ignition to the heat release rate.

These results reveal the inherent mechanism of lower PRRmax for the SFI hybrid combustion

with rDI of 0.5 and 0.16. That is to say, both the degree of fuel stratification (or homogeneity)

and the specific local fuel/air equivalence ratio distribution dominate the heat release process

of SFI hybrid combustion. The former mainly controls the auto-ignition process itself, and

the latter mainly controls the competition between flame propagation and auto-ignition.

However, the higher rDI (e.g. 0.5) would inevitably leads to over-lean mixture in outer region

and deteriorates later auto-ignition process and IMEP. Therefore, the optimal rDI of the air-

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diluted SFI combustion for the selected engine operation condition is 0.16 in this study,

which can achieve both higher IMEP and lower PRRmax.

Table 4 has been compiled to summarise the findings based on four typical fuel/air

equivalence ratio distribution patterns. In order to facilitate the description, three different

combustion regimes are proposed, including pure flame propagation zone, hybrid combustion

zone and pure auto-ignition zone. The symbols “+”, “○”, “–” and their combinations are used

to qualitatively indicate local fuel/air equivalence ratio and their impact on heat release rates

and combustion performances. It should be noted that “○” represents the stoichiometric

condition of the fuel/air equivalence ratio distribution characteristics in the first part of the

table. With this method, Pattern 1 indicates a strong stratification with over-rich mixture in

flame propagation zone while over-lean mixture in the auto-ignition zone. Pattern 2 indicates

a moderate stratification while Pattern 3 indicates a slight stratification. At last, Pattern 4

indicates the homogeneous lean mixture.

As shown in Table 4, Pattern 1 slows down both flame propagation and auto-ignition process

and deteriorates IMEP, as in Case 10. The moderate stratification, as revealed by Pattern 2 in

the table, leads to slightly weaker flame propagation and auto-ignition process. The slight

stratification in Pattern 3 with slightly richer mixture in flame propagation zone and slightly

leaner mixture in auto-ignition zone ensure a relatively stronger flame propagation and auto-

ignition process, maintaining the IMEP. On the other hand, the heat release rate in the hybrid

combustion zone is suppressed because a larger amount of mixture is consumed by strong

flame propagation, which ensures a lower PRRmax. The homogeneous lean mixture in Pattern

4 would enhance the auto-ignition process because of the lack of stratification although the

flame propagation is slightly weakened, which finally leads to higher PRRmax, as indicated in

Case 12.

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Table 4 Typical fuel/air equivalence ratio distribution patterns and their impact on the air-

diluted SFI hybrid combustion.

1. Fuel/air equivalence ratio distribution characteristics

Pattern 1 Pattern 2 Pattern 3 Pattern 4

flame propagation zone + + + + + + –

hybrid combustion zone + + + ○ –

auto-ignition zone – – – – – – –

2. Combustion characteristics (heat release rate)

Flame propagation – – – + –

Hybrid combustion + + + ○ ○

Auto-ignition – – – ○ +

3. Combustion performances

IMEP – – – ○ +

PRRmax – + ○ +

Because of the competition between flame propagation and auto-ignition combustion under

stratified conditions, Pattern 3 shows promising potential to achieve optimal performance of

air-diluted SFI hybrid combustion. In this case, the in-cylinder stratified mixture avoids over-

rich mixture in the central region around spark plug to achieve both higher IMEP and lower

PRRmax. Meanwhile, the mixture in outer region is not too lean to achieve complete auto-

ignition combustion at outer region.

4. Summary and conclusions

In this paper, results by the validated 3D CFD simulations are presented and discussed of

the effect of dilution strategies and direct injection ratios on the stratified flame ignition (SFI)

hybrid combustion. The combination of port fuel injection (PFI) and direct injection (DI) was

used to form the premixed lean/diluted mixture and a stratified charge, respectively. Effects

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of dilution strategies with different combinations of fuel/air equivalence ratio (ϕair) and

fuel/dilution equivalence ratio (ϕdilution) were studied at two engine compression ratios. Then

the effect of direct injection ratio (rDI) was investigated to optimize the fuel/air equivalence

ratio distribution as well as the air-diluted SFI hybrid combustion performance. The main

findings can be summarized as follows:

(1) The dilution strategy shows significant impact on in-cylinder fuel/air equivalence ratio

distribute and thermal condition. Compared to the stoichiometric SFI hybrid combustion, the

air-diluted SFI hybrid combustion optimizes the early flame propagation process because of

the avoidance of over-rich mixture around spark plug. However, the hybrid combustion with

fixed dilution mass can hardly achieve both higher IMEP and lower PRRmax simultaneously

when replacing a part of external exhaust gas (Case 3/6) or internal residual gas (Case 4/7) to

achieve the air-diluted SFI hybrid combustion, which is more apparent under high

compression ratio operation.

(2) The lean boosted dilution strategy with additional intake air and sufficient internal

exhaust gas recirculation (iEGR) was proposed in Case 8 to address the trade-off between

IMEP and PRRmax in air-diluted SFI hybrid combustion. In this strategy, the slightly richer

mixture around spark plug enhances the early flame propagation, and the sufficient hot

residual gas ensures the auto-ignition of end-gas, which leads to relatively higher IMEP.

Meanwhile, the increased dilution mass and fuel stratification from central region to the outer

region effectively suppress the PRRmax. However, the quantity of the additional intake air

mass needs to be controlled as too much intake air would lead to over-diluted mixture and

deteriorate the SFI hybrid combustion performance, as shown in Case 9.

(3) The direct injection ratio (rDI) can directly regulate the in-cylinder fuel/air equivalence

ratio distribution and in turn affect the air-diluted SFI hybrid combustion. It is found that the

optimal SFI hybrid combustion with rDI of 0.16 in Case 11 can lead to simultaneous high

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IMEP and low PRRmax. Too much stratified fuel charge (Case 10) would leads to deteriorated

IMEP because of the over-lean mixture at outer region, while the further decrease of rDI (Case

12) would leads to unacceptable PRRmax because of more homogeneous mixture.

(4) The auto-ignition combustion in the air-diluted SFI hybrid combustion shows high

sensitivity to the in-cylinder thermal conditions. In order to achieve efficient air-diluted SFI

hybrid combustion, the internal residual gas fraction in the dilution strategy should be

carefully managed to maintain sufficient thermal conditions and ensure stable auto-ignition

combustion.

(5) The in-cylinder fuel/air equivalence ratio distribution pattern dominates the balance of the

competition between flame propagation and auto-ignition in the air-diluted SFI hybrid

combustion. Three different combustion regimes, including pure flame propagation zone,

hybrid combustion zone and pure auto-ignition zone, are proposed to understand effect of

typical fuel/air equivalence ratio distribution patterns on the air-diluted SFI hybrid

combustion characteristics and performances. In order to obtain optimal hybrid combustion

with high IMEP and low PRRmax, the in-cylinder stratified mixture should avoid over-rich

mixture around spark plug. Meanwhile, the mixture in outer region should avoid over-lean

conditions to reduce the deterioration of auto-ignition combustion at outer region.

Funding

The study is a part of the State Key Project of Fundamental Research Plan (Grant

2013CB228403) supported by the Ministry of Science and Technology of China.

Nomenclature

3-D three-dimensional

CFD computational fluid dynamics

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SFI stratified flame ignition

NVO negative valve overlap

PFI port fuel injection

DI direct injection

ϕair fuel/air equivalence ratio

ϕdilution fuel/charge equivalence ratio

IMEP indicated mean effective pressure

PRRmax maximum pressure rise rate

rDI direct injection ratio

CAI controlled auto-ignition

SI spark ignition

SACI spark assisted compression ignition

CR compression ratio

ECFM3Z three-zones extended coherent flame model

aTDC after top dead centre

bTDC before top dead centre

iEGR internal exhaust gas recirculation

eEGR external exhaust gas recirculation

ST spark timing

CA50 crank angle of 50% total heat release

D1 flame propagation dominated combustion duration

D2 auto-ignition dominated combustion duration

RCAT the ratio of the accumulated heat released

CAT crank angle corresponding to the mode transition from SI to CAI

MFB mass burned fraction

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CAPRRmax crank angles with maximum PRR

rSI ratio of the fuel consumed by flame propagation in a certain cell

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Table captions:

Table 1 Engine specifications.

Table 2 Operation conditions.

Table 3 Simulation cases.

Table 4 Typical fuel/air equivalence ratio distribution patterns and their impact on the air-

diluted SFI hybrid combustion.

Figure captions:

Fig. 1. Engine mesh with the bowl piston.

Fig. 2. The in-cylinder pressure and heat release rate of the baseline case (Case 1) from

experiment and simulation.

Fig. 3. Effect of ϕair on combustion phasing (CA50), IMEP and PRRmax of SFI hybrid

combustion with a CR of 10.66:1.

Fig. 4. Section views of the fuel/air equivalence ratio distributions at 36 ºCA bTDC with

different dilution strategies.

Fig. 5. The average fuel/air equivalence ratio of the mixture in different zones at 40 ºCA

bTDC.

Fig. 6. (a) In-cylinder pressure traces and (b) average flame area density of the SFI

combustion with different dilution strategies. The spark timing is fixed at 35 ºCA bTDC.

Fig. 7. (a) in-cylinder pressure traces and (b) IMEP and PRRmax for the SFI combustion with

different dilution strategies. The CA50 is fixed around 7.5 ºCA aTDC.

Fig. 8. The flame propagation dominated combustion duration (D1), the auto-ignition

dominated combustion duration (D2) and the ratio of the accumulated heat released (RCAT) at

CAT.

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Fig.9. The average auto-ignition tendency of the mixture in the whole combustion chamber

and its variation among zones for the SFI combustion with different dilution strategies.

Fig. 10. Effect of ϕair and ϕdilution on CA50, IMEP and PRRmax of SFI hybrid combustion with a

CR of 14:1.

Fig. 11. (a) in-cylinder pressure traces and (b) IMEP and PRRmax for the SFI combustion

with different dilution strategies. The CA50 is fixed around 2.6 ºCA bTDC.

Fig. 12. In-cylinder local fuel/air equivalence ratio (ϕair) and temperature in different zones

at 40 ºCA bTDC.

Fig. 13. The mass fraction burned (MFB) traces of the SFI hybrid combustion with different

dilution strategies, and the CA50 is fixed around 2.6 ºCA bTDC.

Fig.14. The average auto-ignition tendency of the mixture in Zone 7 for the SFI hybrid

combustion with different dilution strategies.

Fig. 15. The flame propagation dominated combustion duration (D1), the auto-ignition

dominated combustion duration (D2) and the ratio of the accumulated heat released (RCAT) at

CAT.

Fig.16. The peak IMEP and the corresponding PRRmax for the SFI hybrid combustion with

different dilution conditions and compression ratios.

Fig. 17. Effect of direct injection ratio (rDI) on CA50, IMEP and PRRmax.

Fig. 18. Peak IMEP and the corresponding PRRmax with different rDI.

Fig. 19. In-cylinder fuel/air equivalence ratio (ϕair) at 36 ºCA bTDC. The spark timing is

fixed at 35 ºCA bTDC.

Fig. 20. The mass fraction burned (MFB) traces of the SFI hybrid combustion with different

dilution strategies. The spark timing is fixed at 35 ºCA bTDC.

Fig. 21. The mass fraction burned (MFB) traces of the SFI hybrid combustion with different

dilution strategies. The CA50 is fixed around 0.8 ºCA bTDC.

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Fig. 22. Iso-surface with the fuel/air equivalence ratio of 1 and the auto-ignition sites after

mode transition.

Fig. 23. The traces of the average auto-ignition tendency in different zones.

Fig. 24. Distribution of the ratio of the fuel consumed by flame propagation (rSI) in the early

auto-ignited cells (5% of total cell number at TDC).

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