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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: [email protected].
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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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