Stratified Charge Compression Ignition Research Articles

Exergy loss characteristics of DME/air and ethanol/air mixtures with temperature and concentration fluctuations under HCCI/SCCI conditions: A DNS study

The exergy loss characteristics of combustion processes under homogeneous-charge compression ignition (HCCI) and stratified-charge compression ignition (SCCI) conditions are numerically investigated by analyzing two-dimensional (2-D) direct numerical simulation (DNS) data. Two fuels, dimethyl ether and ethanol, together with the initial conditions of different mean temperatures, and levels of temperature and concentration fluctuations relevant to HCCI/SCCI conditions were investigated. It is found that the prevalent deflagration mode significantly decreases the maximum exergy loss rates and spreads out the exergy loss rate for all the cases regardless of fuel types, temperature regimes, and temperature and/or concentration fluctuations. The primary irreversible sources of exergy loss are also identified. The chemical reaction is found to be the primary contributor to the total exergy loss, followed by heat conduction and mass diffusion, regardless of the fluctuation levels. It is also found that the relative change of exergy loss due to chemical reactions, ELchemrel, correlates strongly with the heat release fraction by deflagration. The maximum ELchemrel is found to be less than 10%. Chemical pathway analysis reveals that the exergy loss induced by low-temperature reactions, represented by the decomposition of hydroperoxy–alkylperoxy and the H-abstraction reactions of the fuel molecule, is much lower under the SCCI conditions than that under the HCCI conditions. Generally, the dominant reactions contributing to the exergy loss in the high-temperature regime are nearly identical for the HCCI and SCCI combustion. Key reactions, including the H2O2 loop reactions, the reactions of the H2–O2 mechanism, and the conversion reaction of CO to CO2, CO+OH=CO2+H, are found to contribute more than 50% of the total exergy loss. Due to locally higher reactivities by temperature and concentration fluctuations inducing deflagration dominance, these reactions occur at a relatively higher temperature (1600 K–1900 K) compared with the homogeneous zero-dimensional cases (∼1400 K), resulting in a net reduction in exergy loss.

Effect of Stratified Charge Compression Ignition on Emission and Power Output- A Review

The emissions from the vehicles causes for severe air pollution which resulted the development of stringent regulatory norms. Therefore there is a need of improvement in combustion phenomenon to decrease the NOx and PM, as they are directly dependent on combustion. To improve environmental conditions which are directly affecting human health, all automobile companies are endeavouring to satisfy strict emission standards. In this context stratified charge compression ignition (SCCI) engines seems promising and developing its application by incorporating the different types of combustion mode with the help of direct fuel injection and by ranging its capacity to full load condition. Alongside SCCI, a new concept of multi stage induction and Exhaust Gas Recirculation (EGR) can be incorporated in engine to reduce emissions, increase power output and efficiency. Various parameters have been considered to understand the combustion process for developing an engine to meet the requirements. The effect of different variables influencing the combustion process is highlighted.

A Conditional Moment Closure Study of Chemical Reaction Source Terms in SCCI Combustion

The objective of this study is to evaluate conditional moment closure (CMC) approaches to model chemical reaction rates in compositionally stratified, autoigniting mixtures, in thermochemical conditions relevant to stratified charge compression ignition (SCCI) engines. First-order closure, second-order closure and double conditioning are evaluated and contrasted as options in comparison to a series of direct numerical simulations (DNSs). The two-dimensional (2D) DNS cases simulate ignitions in SCCI-like thermochemical conditions with compositionally stratified n-heptane/air mixtures in a constant volume. The cases feature two different levels of stratification with three mean temperatures in the negative-temperature coefficient (NTC) regime of ignition delay times. The first-order closure approach for reaction rates is first assessed using hybrid DNS-CMC a posteriori tests when implemented in an open source computational fluid dynamics (CFD) package known as OpenFOAM $^{{\circledR }}$ . The hybrid DNS-CMC a posteriori tests are not a full CMC but a DNS-CMC hybrid in that they compute the scalar and velocity fields at the DNS resolution, thus isolating the first-order reaction rate closure model as the main source of modelling error (as opposed to turbulence model, scalar probability density function model, and scalar dissipation rate model). The hybrid DNS-CMC a posteriori test reveals an excellent agreement between the model and DNS for the cases with low levels of stratification, whereas deviations from the DNS are observed in cases which exhibit high level of stratifications. The a priori analysis reveals that the reason for disagreement is failure of the first-order closure hypothesis in the model due to the high level of conditional fluctuations. Second-order and double conditioning approaches are then evaluated in a priori tests to determine the most promising path forwards in addressing higher levels of stratification. The a priori tests use the DNS data to compute the model terms, thus directly evaluating the model assumptions. It is shown that in the cases with a high level of stratification, even the second-order estimation of the reaction rate source term cannot provide a reasonably accurate closure. Double conditioning using mixture-fraction and sensible enthalpy, however, provides an accurate first-order closure to the reaction rate source term.

Development of fuel metering techniques for spark ignition engines

This chapter starts by providing an overview of fuel-metering techniques developed for spark ignition (SI) engines, following by a case study on liquid versus gaseous phase liquified petroleum gas (LPG) port injection in a single-cylinder SI engine. Fuel-metering approaches, from the least to the most modern, include carburettor, throttle body injection (TBI), manifold port injection (MPI or sometime called PFI, port fuel injection), and gasoline direct injection (GDI). Based on the mixture formation process, GDI engines can be categorised into air-guided, wall-guided, and spray-guided methods while based on the ignition modes they can be homogeneous spark ignition - (HCI), stratified spark ignition (SPI) and homogeneous charge compression ignition (HCCI) engines. The comparative study shows significant variations in cylinder to cylinder mixture distribution for both liquid and gaseous LPG phases. The experimental data confirms the power and efficiency advantages of liquid phase injection over gaseous phase injection. Analysis shows the charge state at the end of compression is the major contributor to the performance difference. Secondary differences such as the mixture homogeneity also have an important impact. The lower temperature combustion with liquid phase LPG also delivers substantial NOx emission reduction. Suggestions are made for overcoming the deficiency of gaseous phase injection which may be preferred as its application avoids the high cost of the high pressure in-tank LPG pump needed for liquid phase injection.

Simulation and experiments of advanced gasoline engine combustion modes from spark ignition to compression ignition

This paper reviews some advanced gasoline engine combustion modes, including direct injection spark ignition (DISI), homogeneous charge compression ignition (HCCI), stratified charge compression ignition (SCCI), and multiple premixed compression ignition (MPCI). To reveal the engine knock mechanism of DISI combustion, the flame propagation and pressure oscillation in a boosted gasoline engine were numerically analysed. To further improve thermal efficiency and reduce emissions, the effects of injection strategies on fuel economy, emissions, and heat release in HCCI combustion were studied. To solve the challenges of ignition control and rapid combustion of HCCI, SCCI and MPCI combustion modes were adopted to achieve high efficiency, low emissions, and low acoustic noise simultaneously.

A thermodynamic model for homogeneous charge compression ignition combustion with recompression valve events and direct injection: Part I — Adiabatic core ignition model

This two-part article presents a model for boosted and moderately stratified homogeneous charge compression ignition combustion for use in thermodynamic engine cycle simulations. The model consists of two components: one an ignition model for the prediction of auto-ignition onset and the other an empirical combustion rate model. This article focuses on the development and validation of the homogeneous charge compression ignition model for use under a broad range of operating conditions. Using computational fluid dynamics simulations of the negative valve overlap valve events typical of homogeneous charge compression ignition operation, it is shown that there is no noticeable reaction progress from low-temperature heat release, and that ignition is within the high-temperature regime ( T > 1000 K), starting within the highest temperature cells of the computational fluid dynamics domain. Additional parametric sweeps from the computational fluid dynamics simulations, including sweeps of speed, load, intake manifold pressures and temperature, dilution level and valve and direct injection timings, showed that the assumption of a homogeneous charge (equivalence ratio and residuals) is appropriate for ignition modelling under the conditions studied, considering the strong sensitivity of ignition timing to temperature and its weak compositional dependence. Use of the adiabatic core temperature predicted from the adiabatic core model resulted in temperatures within ±1% of the peak temperatures of the computational fluid dynamics domain near the time of ignition. Thus, the adiabatic core temperature can be used within an auto-ignition integral as a simple and effective method for estimating the onset of homogeneous charge compression ignition auto-ignition. The ignition model is then validated with an experimental 92.6 anti-knock index gasoline-fuelled homogeneous charge compression ignition dataset consisting of 290 data points covering a wide range of operating conditions. The tuned ignition model predictions of [Formula: see text] have a root mean square error of 1.7° crank angle and R2 = 0.63 compared to the experiments.

Ignition of a lean PRF/air mixture under RCCI/SCCI conditions: A comparative DNS study

The ignition characteristics of a lean primary reference fuel (PRF)/air mixture under reactivity controlled compression ignition (RCCI) and stratified charge compression ignition (SCCI) conditions are investigated using 2-D direct numerical simulations (DNSs) with a 116-species reduced mechanism of PRF oxidation. For RCCI combustion, n-heptane and iso-octane are used as two different reactivity fuels and the corresponding global PRF number is PRF50 which is also used as a single fuel for SCCI combustion. The 2-D DNSs of RCCI/SCCI combustion are performed by varying degree of fuel stratification, r, and turbulence intensity, u′, at different initial mean temperature, T0, with negatively-correlated T–r fields. It is found that in the low- and intermediate-temperature regimes, the overall combustion of RCCI cases occurs earlier and its mean heat release rate (HRR) is more distributed over time than those of the corresponding SCCI cases. This is because PRF number stratification, PRF′, plays a dominant role and T′ has a negligible effect on the overall combustion within the negative temperature coefficient (NTC) regime. In the high-temperature regime, however, the difference between RCCI and SCCI combustion becomes marginal because the ignition of the PRF/air mixture is highly-sensitive to T′ rather than PRF′ and ϕ′. The Damköhler number analysis verifies that the mean HRR is more distributed over time with increasing r because the portion of deflagration mode of combustion becomes larger with increasing fuel stratification. Finally, it is found that the overall combustion of both RCCI and SCCI cases becomes more like the 0-D ignition with increasing u′ due to the homogenization of initial mixture by turbulent mixing.

Ignition of a lean PRF/air mixture under RCCI/SCCI conditions: Chemical aspects

Chemical aspects of the ignition of a primary reference fuel (PRF)/air mixture under reactivity controlled compression ignition (RCCI) and stratified charge compression ignition (SCCI) conditions are investigated by analyzing two-dimensional direct numerical simulation (DNS) data with chemical explosive mode (CEM) analysis. CEMA is adopted to provide fundamental insights into the ignition process by identifying controlling species and elementary reactions at different locations and times. It is found that at the first ignition delay, low-temperature chemistry (LTC) represented by the isomerization of alkylperoxy radical, chain branching reactions of keto-hydroperoxide, and H-atom abstraction of n-heptane is predominant for both RCCI and SCCI combustion. In addition, explosion index and participation index analyses together with conditional means on temperature verify that low-temperature heat release (LTHR) from local mixtures with relatively-high n-heptane concentration occurs more intensively in RCCI combustion than in SCCI combustion, which ultimately advances the overall RCCI combustion and distributes its heat release rate over time. It is also found that at the onset of the main combustion, high-temperature heat release (HTHR) occurs primarily in thin deflagrations where temperature, CO, and OH are found to be the most important species for the combustion. The conversion reaction of CO to CO2 and hydrogen chemistry are identified as important reactions for HTHR. The overall RCCI/SCCI combustion can be understood by mapping the variation of 2-D RCCI/SCCI combustion in temperature space onto the temporal evolution of 0-D ignition.

Thermodynamic model for homogeneous charge compression ignition combustion with recompression valve events and direct injection: Part II—Combustion model and evaluation against transient experiments

This two-part article presents a combustion model for boosted and moderately stratified homogeneous charge compression ignition combustion for use in thermodynamic engine cycle simulations. The model consists of two parts: one an ignition model for the prediction of auto-ignition onset and the other an empirical combustion rate model. This article focuses on the development of the combustion model which is algebraic in form and is based on the key physical variables affecting the combustion process. The model is fit with experimental data collected from 290 discrete automotive homogeneous charge compression ignition operating conditions with moderate stratification resulting from both the direct injection and negative valve overlap valve events. Both the ignition model from part 1 and the combustion model from this article are implemented in GT-Power and validated against experimental homogeneous charge compression ignition data under steady-state and transient conditions. The ignition and combustion model are then exercised to identify the dominant variables affecting the homogeneous charge compression ignition and combustion processes. Sensitivity analysis reveals that ignition timing is primarily a function of the charge temperature, and that combustion duration is largely a function of ignition timing.

Assessment of flamelet versus multi-zone combustion modeling approaches for stratified-charge compression ignition engines

The spray-interactive flamelet and extended multi-zone combustion models coupled with multi-dimensional computational fluid dynamics are applied to investigate the effects of charge stratification in a direct-injection compression ignition engine under low load conditions. A parametric study was carried out in order to compare the two approaches for early and late fuel injection timings. Comparison of numerical results with available experimental data shows that for early fuel injection, both models predict the auto-ignition and combustion characteristics with comparable fidelity. As the fuel injection timing is delayed, however, the spray-interactive flamelet model is found to capture the onset of combustion and subsequent heat release with greater accuracy. Further investigation reveals that the better performance of the spray-interactive flamelet model over a wider range of mixture-stratified conditions is mainly attributed to its ability to capture the diffusive transport resulting from small-scale mixing and turbulence–chemistry interaction, which becomes more important when significant mixture inhomogeneities exist in the engine cylinder.

Homogeneous Charge Compression Ignition (HCCI) Combustion Engine- A Review

At present, it is highly required from the automobile sector to develop clean technologies with lower fuel consumption for ambient air quality improvement, green house gas reduction and energy security. Furthermore, due to continuously stringent emission legislation and the fast depletion of the primary energy resources, the development of new highly efficient and environment friendly combustion systems becomes of paramount importance and hence research need to be done in this area. One such combustion system is Homogeneous Charge Compression Ignition (HCCI) technology, which has the potential to reduce oxides of nitrogen (NOx) and particulate matter (PM) simultaneously maintaining the thermal efficiency at par with that of conventional diesel engine combustion. However, some issues such as combustion phasing control, controlled auto-ignition, operating range, homogeneous charge preparation, cold start, pressure rise rate and noise and emissions of unburned hydro carbon (UHC), and carbon monoxide (CO) need to be solved for successful operation and therefore commercial application of HCCI engine. Other similar combustion concepts, which can be considered as the extension of HCCI, are Low Temperature Combustion (LTC) and Stratified Charge Compression Ignition (SCCI). This paper reviews all the three advanced combustion concepts along with their merits and demerits.

Computational study of charge stratification in early-injection SCCI engines under light-load conditions

Under light-load conditions in early-injection stratified-charge compression-ignition (SCCI) engines, excessive premixing can lead to undesirable levels of unburned hydrocarbons (UHC) and carbon monoxide (CO) emissions. Optimal stratification can reduce these emissions. In this work, the effects of changes in swirl, injection pressure, injector hole-size and number of holes, injection timing, and piston geometry on stratification are computationally investigated. It is shown that these parameters affect the stratification through their influence on the rate of spray penetration, drop vaporization, and fuel/air mixing. The outcome is characterized by examining the evolution of the spatial distribution of the fuel vapor in the chamber and its mass-based distribution function. All other parameters remaining the same, decreasing drop size leads to faster vaporization and richer mixtures. Increasing penetration leads to greater spreading and leaner mixtures. Increasing spray included-angle leads to greater spreading and leaner mixtures. Increasing injection pressure leads to increased mixing and leaner mixtures. Increasing injector hole-size leads to richer mixtures at lighter loads because the duration of injection is reduced and the fuel is confined closer to the axis. Increasing swirl leads to faster breakup of the head-vortex and confinement of the fuel closer to the axis, and hence richer mixture.

Controlled three-stage heat release of stratified charge compression ignition (SCCI) combustion with a two-stage primary reference fuel supply

This paper discusses the heat release mode and its effect on combustion characteristics of stratified charge compression ignition (SCCI) combustion with a two-stage fuel supply. To create and control the fuel concentration stratification, composition stratification, and temperature stratification, primary reference fuels or their mixtures were supplied from the intake port, while n-heptane was directly injected into the cylinder near the top dead center of the compression stroke. To achieve a controllable staged heat release and to optimize the thermal efficiency and emissions, important factors, including premixed fuel properties, direct injection timing, the overall equivalence ratio, and the premixed ratio were tuned to modulate the heat release pattern. The experimental results revealed that, with the port fuel injection of a two-stage reaction fuel, the heat release curve of the SCCI combustion exhibits a three-stage heat release pattern. The in-cylinder fuel delivery advance angle plays an important role in the indicated thermal efficiency, and the earlier fuel delivery angle has a positive effect on the indicated thermal efficiency. It should be noted that an excessively advanced fuel delivery angle will lead to a sharp increase of NOx emissions. With the port fuel injection of PRF50, both fuel efficiency and ultra-low NOx emissions were obtained over wide ranges of the premixed ratio and the equivalence ratio. Moreover, the experimental results suggest that a higher premixed ratio for low-to-medium equivalence ratios and a smaller premixed ratio for larger equivalence ratios are preferred. The maximum thermal efficiency was observed at the zone with the earlier CA50 but with shorter burn duration. NOx levels were determined not only by CA50 and burn duration but also by the heat release mode. One-stage SCCI combustion, which was dominated by the diffusion burn, exhausted considerable NOx emissions, compared to the staged heat release mode.

Incorporation of Detailed Chemical Mechanisms in Reactive Flow Simulations Using Element-Flux Analysis

An on-the-fly mechanism reduction approach for coupling complex chemistry and computational fluid dynamics (CFD) is proposed in this paper. The approach consists of element flux analysis and identification of active species and reactions based on flux magnitudes. A reduced mechanism involving the active species and reactions is generated to describe the local chemistry. The approach is applied dynamically in the CFD calculation by generating a locally accurate reduced mechanism for every computational cell and time step, enabling on-the-fly reduction. The emphasis of this work is on the numerical study of stratified homogeneous charge compression ignition (HCCI) combustion with detailed chemistry by using the proposed on-the-fly reduction scheme. A mechanism of n-heptane combustion with 161 species and 1540 reactions is used as the detailed mechanism in the simulation. KIVA-3V and CHEMKIN are used as the computational platforms. On-the-fly reduction predictions of species concentrations, temperature, and ...

Influence of Fuel Supply Timing and Mixture Preparation on the Characteristics of Stratified Charge Compression Ignition Combustion with N-Heptane Fuel

Stratified charge compression ignition (SCCI) combustion was investigated on a single-cylinder, four-stroke diesel engine with n-heptane. A premixed atmosphere was realized by the early injection at the intake port, while the rest of fuel was directly injected into the cylinder. Premixed ratio (defined as the ratio of fuel amount in port injection divided by the total fuel amount in the charge) and fuel supply timing of direct injection were altered to explore their influences on SCCI combustion, emission, and engine performance characteristics. The results show that increased premixed ratio and earlier fuel supply timing can remarkably enhance the peak cylinder temperature and pressure while shortening the main combustion duration. NOx is strongly dependent on fuel supply timing, whereas premixed ratio and equivalence ratio play a secondary role. CO can be effectively reduced by increasing premixed ratio and advancing fuel supply timing. HC rises with increasing premixed ratio but is insensitive to the change of fuel supply timing. Finally, indicated thermal efficiency (ITE) drops with retarded fuel supply timing, and the maximum indicated mean effective pressure (IMEP) can be achieved by optimizing the premixed ratio and fuel supply timing.

Experimental study on the effects of mixture homogeneity on stratified-charge compression ignition combustion

In this paper, an investigation of the n-heptane stratified-charge compression ignition (SCCI) combustion on a single-cylinder engine using port fuel injection combined with in-cylinder direct injection is reported. The effects of mixture homogeneity, overall equivalence ratio, and fuel delivery advance angle of directly injected fuel on the SCCI combustion characteristics and emissions were evaluated. The experimental results revealed that the heat release curve of SCCI combustion exhibited a three-stage combustion mode, which includes low-temperature reaction (LTR), high-temperature reaction (HTR), and diffusion burn. The operating ranges can be significantly broadened by mixture stratification, owing to the smooth heat release. As the mixture homogeneity increases, the peak value of the heat release in the HTR increased but the maximum heat release in the third stage decreased. For a fixed overall equivalence ratio, with an increase in the mixture homogeneity, the nitrogen oxide (NO x) emissions from SCCI combustion started to decrease initially and attained the lowest level, but carbon monoxide (CO) emissions increased initially and achieved the highest level. Once the mixture homogeneity further increased, NO x emissions began to increase but CO emission decreased. Compared with the traditional direct-injection compression ignition combustion mode, NO x emissions from SCCI combustion can be reduced markedly. Also, hydrocarbon emissions from SCCI combustion were much lower than from neat homogeneous charge compression ignition combustion. The fuel delivery advance angle also had an important influence on SCCI combustion and the emissions. In particular, NO x emissions can be further significantly reduced using the late fuel injection strategy.

Numerical Simulation of Gasoline Stratified Charge Compression Ignition Using 3D-CFD Coupled with Detailed Chemistry

In this paper, the detailed chemical kinetics was coupled with the three-dimensional CFD code to study the combustion process in HCCI engines. A detailed hydrocarbon oxidation reaction mechanism (89 species, 413 reactions) for high octane fuel was constructed and then used to simulate the chemical process of the ignition, combustion and pollutant formation under HCCI conditions. The three-dimensional FIRE/CHEMKIN model was validated using the experimental data from a Rapid Compression Machine. The simulation results show good agreements with experiments. Finally, the improved three-dimensional CFD code was employed to simulate the intake, spray, combustion and pollutant formation process of the gasoline direct injection HCCI engine with two-stage injection strategy. The models account for intake flow structure, spray atomization, spray/wall interaction, droplet evaporation and gas phase chemistry in complex multi-dimensional geometries. The calculated results show the periphery of fuel-rich zone formed by the second injection which ignited first and worked as an initiation to ignite the surrounding lean mixture zone formed by the first injection. The results provide a detailed insight into the processes governing combustion and pollutant formation in the HCCI engine.

Evaluation of SCCI Potentials in Comparison to HCCI and Conventional DICI Combustion Using n-Heptane

This work investigates the potential of stratified charge compression ignition (SCCI) to extend the high-load limits of homogeneous charge compression ignition (HCCI) engine. Engine experiments were conducted on a single-cylinder diesel engine equipped with an electrical port fuel injection system. When the n-heptane fuel amount of port injection and in-cylinder direct injection was calibrated, partial fuel stratification was created and three different combustion modes as SCCI, HCCI, and direct injection compression ignition (DICI) were achieved. The premixed ratio (rp) was used to indicate the partial fuel stratification, and the effects of premixed ratio on SCCI combustion were examined. It was found that SCCI could achieve a staged combustion event of reduced pressure-rise rates, thus offering good potential to expand the operating range of the HCCI engine. Also, the engine performance and emission characteristics of SCCI were evaluated, focusing on the comparison with the HCCI and conventional DICI combustion. The results show that, with suitable partial fuel stratification, SCCI has the capability to achieve lower CO and HC emissions in comparison to HCCI and keep the NOx emissions on a low level within 50 ppm while maintaining good efficiency equivalent to DICI.

An Experimental Study on the Combustion and Emission Characteristics of a Stratified Charge Compression Ignition (SCCI) Engine

Stratified charge compression ignition (SCCI) combustion, which is developed to extend the homogeneous charge compression ignition operating range, has a good potential to improve fuel economy and reduce emissions. In this study, SCCI combustion and emissions characteristics were investigated in a single-cylinder gasoline engine with a direct-injection system. From the experimental results, we elucidated the effects of air -fuel ratio, intake temperature, and injection timing including early and late injection on the attainable SCCI combustion region. Injection timing during the intake process was found to be an important parameter that affects the extension of the SCCI operating region. The effects of stratified mixture formation can be utilized to extend the operating range for suitable SCCI combustion under different engine speeds and compression ratio conditions. In this study, we found that the higher intake temperature is very effective to evaporate the injected fuel, and this effect helps to enhance SCCI combustion. The intake temperature can be controlled by the compression ratio. Finally, the injection timing and stratified mixture formation played an important role in achieving stable combustion and extending the operating range.

An experimental study of the combustion characteristics in SCCI and CAI based on direct-injection gasoline engine

Emissions remain a critical issue affecting engine design and operation, while energy conservation is becoming increasingly important. One approach to favorably address these issues is to achieve homogeneous charge combustion and stratified charge combustion at lower peak temperatures with a variable compression ratio, a variable intake temperature and a trapped rate of the EGR using NVO (negative valve overlap). This experiment was attempted to investigate the origins of these lower temperature auto-ignition phenomena with SCCI and CAI using gasoline fuel. In case of SCCI, the combustion and emission characteristics of gasoline-fueled stratified-charge compression ignition (SCCI) engine according to intake temperature and compression ratio was examined. We investigated the effects of air–fuel ratio, residual EGR rate and injection timing on the CAI combustion area. In addition, the effect of injection timing on combustion factors such as the start of combustion, its duration and its heat release rate was also investigated.