Stratified Charge Compression Ignition Combustion 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.

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.

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.

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.

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.

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.

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 on the Combustion Characteristics Using the Stratified Charge Compression Ignition in a Direction Injection Gasoline Engine

The combustion and emission characteristics of gasoline-fueled stratified-charge compression ignition (SCCI) engine according to intake temperature and compression ratio was examined. The fuel was injected directly to the cylinder under the high temperature condition resulting from heating the intake port. With this injection strategy, the SCCI combustion region was expanded dramatically without any increase in NOx emissions, which were seen in the case of compression stroke injection. Injection timing during the intake temperature was found to be an important parameter that affects the SCCI region width. The mixture stratification and the fuel reformation can be utilized to reduce the required intake temperature for suitable SCCI combustion under each set of engine speed and compression ratio conditions.