Stratified Charge Combustion Research Articles

Influence of heat release and turbulence on scalar dissipation rate in autoigniting n-heptane/air mixtures

Scalar dissipation rate χ is an important parameter in stratified charge combustion because it is a measure of the extent of fuel/air mixing in the flow field. In this work, the influence of heat release on the evolution of χ in laminar and turbulent autoigniting stratified n-heptane/air mixtures is studied. Multi-step kinetics is employed to model n-heptane oxidation. The pressure and temperature conditions selected are relevant to compression-ignited engines. Ignition is initiated in fuel-rich regions. Subsequently, an ignition front is observed to propagate from the initiation location to the final flame stabilization location. The influence of heat release on scalar dissipation rate is observed to be controlled by a balance of two competing effects: expansion of burned gases which decreases χ and increased laminar diffusivity due to combustion which increases χ. Behind the propagating ignition front, decrease in χ due to expansion is greater than its increase due to increased laminar diffusivity; therefore, the net effect of heat release on χ is to decrease it. The magnitude of decrease in χ due to expansion is observed to increase with increasing compositional gradients in the flow field. Ahead of the ignition front, the increased diffusivity effect is more pronounced than the expansion effect which results in an increase in χ. Turbulence, as expected, modifies the initially specified scalar dissipation rate to generate a range of scalar dissipation rates around the initial value. Turbulent mixing creates well-mixed spots with low χ, and turbulent strain creates regions with high χ. Ignition is observed in the regions with low χ, and the expansion effect due to heat release further decreases the scalar dissipation rate at such locations.

A Comparison of Spray-Guided Stratified-Charge Combustion Performance with Outwardly-Opening Piezo and Multi-Hole Solenoid Injectors

A Comparison of Spray-Guided Stratified-Charge Combustion Performance with Outwardly-Opening Piezo and Multi-Hole Solenoid Injectors

Experimental Study on the Stratified and Homogeneous Charge of a Direct-Injection Spark Ignition Methanol Engine

Based on the gasoline direct-injection spark ignition (DISI) engine technologies and methanol fuel characteristics, a complex-guided multi-hole spray coupled with induction swirl and chamber-wall-guided stratified-charge combustion system for a DISI methanol engine was developed. It was retrofitted from a four-cylinder diesel engine with a compression ratio of 14. It was developed to run on methanol in stratified-charge and homogeneous-charge combustion modes; under part-load operating conditions, stratified-charge combustion is adopted to improve fuel economy while, under high- and full-load conditions, the homogeneous-charge combustion mode is adopted for the purpose of a higher power output. Under both combustion modes, the key parameters of the combustion system were matched and optimized respectively. The operating map for both combustion modes was obtained. Then, the engine power performance, emissions, and combustion characteristics were studied. The power output of the developed methanol engine is higher than that of the original diesel engine. By using the stratified combustion mode under low-load conditions, the engine fuel economy (the brake thermal efficiency) is improved; the stratified-charge combustion of methanol performed relatively well with acceptable values of the coefficient of the cyclic variation in the indicated mean effective pressure. The smoke and nitrogen oxide emissions from the DISI methanol engine are both reduced simultaneously and significantly.

Extending the Lean Limit of Natural-Gas Engines

Two different methods to improve the thermal efficiency and reduce the emissions from lean-burn natural-gas fueled engines have been developed and are described in this paper. One method used a “squish-jet” combustion chamber designed specifically to enhance turbulence generation, while the second method provided a partially stratified-charge mixture near the spark plug in order to enhance the ignition of lean mixtures of natural gas and air. The squish-jet combustion chamber was found to reduce brake specific fuel consumption by up to 4.8% in a Ricardo Hydra engine, while the NOx efficiency trade-off was greatly improved in a Cummins L-10 engine. The partially stratified-charge combustion system extended the lean limit of operation in the Ricardo Hydra by some 10%, resulting in a 64% reduction in NOx emissions at the lean limit of operation. Both techniques were also shown to be effective in increasing the stability of combustion, thereby reducing cyclic variations in cylinder pressure.

Development of a direct-injection stratified-charge methanol engine

On the basis of the wall-guided, spray-guided, and air-guided technologies related to gasoline direct-injection spark-ignition (DISI) engines, a complex-guided stratified-charge combustion system for a methanol DISI engine was developed. The test engine was a retrofitted four-cylinder diesel engine. The key parameters were optimized numerically and experimentally, such as the location of the swirl deflector, the spatial location of spray, the swirl ratio, the injection and ignition timings, and the needle valve opening pressure. The results show that the direct-injection stratified-charge (DISC) methanol engine can work with an excessive air ratio λ as high as 2.23, and its brake thermal efficiency reaches 29.7 per cent at a speed of 1500r/min, and a brake mean effective pressure of 0.45MPa. The DISC methanol engine exhibits relatively good performance with little cyclic variations, although it is sensitive to induction swirl. The test results indicate that the matching principle is successful. The developed DISC methanol engine can run under variable induction swirl to meet the requirement of stratification combustion under different engine speed operating conditions.

Measures to Overcome Low-Load Misfiring of 2-Stroke Cycle Spark Ignition Engine by Direct Injection Stratified Charge Combustion

2-stroke cycle spark ignition engine with direct injection was studied to avoid inherent misfiring in low load region by using stratified charge combustion. The three mixture formation concepts of spray-guided, wall-guided and air-guided were introduced to achieve stratified charge combustion and the stable combustion was obtained in three concepts. The most stable ignitability was achieved by spray-guided concept and the second was wall-guided concept. In the case of air-guided concept stable combustion was obtained with swirl motion that was generated by the closure of half part of scavenging port. Decreasing delivery ratio within no smoke and stable combustion regions was found to cause the reduction of NOx emission.

LPG-DME 성층혼합 압축착화 엔진

The combustion characteristics of a liquefied petroleum gas-di-methyl ether (LPG-DME) compression ignition engine was investigated under homogeneous charge and stratified charge conditions. LPG was used as the main fuel and injected into the combustion chamber directly. DME was used as an ignition promoter and injected into the intake port. Different LPG injection timings were tested to verify the combustion characteristics of the LPG-DME compression ignition engine. The combustion was divided into three region which are homogeneous charge, stratified charge, and diffusion flame region according to the injection timing of LPG. The hydrocarbon emission of stratified charge combustion was lower than that of homogeneous charge combustion. However, the carbon monoxide and nitrogen oxide emission of stratified charge combustion were slightly higher than those of the homogeneous charge region. The indicated mean effective pressure was reduced at stratified charge region, while it was almost same level as the homogeneous charge combustion region at diffusion combustion region. The start of combustion timing of the stratified charge combustion and diffusion combustion region were advanced compared to the homogeneous charge combustion. It attributed to the higher cetane number and mixture temperature distribution which locally stratified. However, the knock intensity was varied as the homogeneity of charge was increased.

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.

Effect of the Ratio between the Connecting-Rod Length and the Crank Radius on Thermal Efficiency

In reciprocating internal combustion engines, Otto cycle indicates the best thermal efficiency under the same compression ratio among ideal cycles. To achieve this, combustion must take place instantaneously at top dead center, but it is actually impossible. Meanwhile, if a slower piston motion around top dead center was allowed, both the in-cylinder pressure and degree of constant volume would increase, leading to higher thermal efficiency. In order to verify this idea, an engine with a slow piston motion by adopting a large ratio between the connecting-rod length and the crank radius was tested. As expected, while degree of constant volume was increased, thermal efficiency was not improved due to increased heat loss. Further experiments were carried out using a direct injection stratified charge combustion system which allows selective reduction of heat loss, and high thermal efficiency was attained. On the contrary, an engine with a faster piston motion by adopting a smaller ratio between the connecting-rod length and the crank radius attained high thermal efficiency under the quick burn pre-mixed spark ignition combustion.

Modeling Mixture Formation in a Gasoline Direct Injection Engine

Mixture formation and combustion in a gasoline direct injection (GDI) engine were studied. A swirl-type nozzle, with an inwardly opening pintle, was used to inject the fuel directly in a four stroke, four cylinder, four valves per cylinder engine. The atomization of the hollow cone fuel spray was modeled by using a hybrid approach. The most important obstacle in the development of GDI engines is that the control of the stratified-charge combustion over the entire operating range is very difficult. Since the location of the ignition source is fixed in SI engines the mixture cloud must be controlled both temporally and spatially for a wide range of operating conditions. Results show that the volume of the spark must be considered when discretizing the computational domain because it highly influences the flow field in the combustion chamber. This is because the volume occupied by the plug cannot be neglected since it is much bigger than the ones used in port fuel injection engines. The development of a successful combustion system depends on the design of the fuel injection system and the matching with the in-cylinder flow field: the stratification at part load appears to be the most crucial and critical step, and if the air motion is not well coupled with the fuel spray it would lead to an increase of unburned hydrocarbon emission and fuel consumption

Gasoline direct injection spray simulation

In this paper the problems related to mixture formation in a GDI engine are analyzed. The atomization of a hollow cone fuel spray generated by a high pressure swirl injector is studied by means of a numerical technique. The model distinguishes between primary atomization and secondary breakup. The latter was modeled, as done in a previous work on Diesel atomization, using different mechanisms as the droplet Weber number changes. At first the spray atomization in a quiescent chamber, at ambient pressure and temperature, was considered. The validation of the model was made comparing the numerical penetration and spray morphology with experimental results. Combustion simulations were also performed comparing numerical results with experimental data of a GDI (Gasoline Direct Injection), 4 stroke, 4 cylinder, 4 valves per cylinder engine. Such simulations were made to analyze and understand the mixture formation mechanism in both stoichiometric and stratified operation mode. The results show how, the interaction between the air motion and the fuel spray, leading factor in spray atomization, is fundamental to realize an efficient mixture formation and combustion locally very lean, typical of stratified charge combustion.

정적연소기내에서의 분위기 온도 및 압력에 따른 혼합기 분포에 관한 성층화 정도 특성

It is well known that a lean bum engine caused by stratified mixture formation has many kinds of advantages to combustion characteristics, such as higher thermal efficiency and lower CO, NOx levels than conventional homogeneous mixture combustion. Although this combustion can achieve low fuel consumption technology, It produces much unburned hydrocarbon and soot because of heterogeneous equivalence ratio in the combustion chamber Therefore, the stratified mixture formation technology is very important to obtain the stable lean combustion In this paper, fundamental studies for stratified combustion were earned out using a constant volume combustion chamber The local effect of mixture formation according to control air-fuel distribution in the chamber was examined experimentally In addition, the effect of turbulence on stratified charge combustion process was observed by schlieren photography. From this study, we found that the flame propagation speed Increase With swirl flow and the swirl promotes the formation of fuel and au mixture<br/>

Stratified-charge combustion: modeling and imaging of a spray-guided direct-injection spark-ignition engine

Spray-guided spark-ignited direct-injection (SG-SIDI) engines are likely to be the next generation of gasoline engines. Compared to current wall-guided (WG-SIDI) engines, they offer improved fuel economy and substantially reduced hydrocarbon and soot emissions. Two stages of stratified combustion (partially premixed flame propagation and mixing-controlled combustion of rich products left from partial combustion of initially fuel-rich mixtures) are investigated here in an experimental SG-SIDI engine using detailed CFD modeling, high-speed spectrally resolved combustion luminosity imaging, and cylinder pressure analysis. The CFD model reproduces the spatial and temporal distribution of both combustion stages. Correlation of CFD calculations and experimental combustion measurements demonstrates that optimum ignition timing occurs for somewhat rich equivalence ratios (0.9–1.6) and relatively low flow velocities (<15 m/s) at the spark location. Both model and experiment show: (1) for fixed injection time, increased rich burning occurs for earlier ignition times and (2) higher in-cylinder swirl leads to asymmetric flame propagation and increased rich burning. Calculated and measured overall heat-release rates agree quite well over a wide range of engine operating conditions. These experiment-model comparisons provide a detailed picture of combustion processes in spray-guided SIDI engines and a sound scientific basis for engine design optimization.

A Study on the Effect of Stratified Mixture Formation on Combustion Characteristics in a Constant Volume Combustion Chamber

It is well known that a lean burn engine caused by stratified mixture formation has many kinds of advantages to combustion characteristics, such as higher thermal efficiency and lower CO, NOx levels than conventional homogeneous mixture combustion. Although this combustion can achieve low fuel consumption, it produces much unburned hydrocarbon and soot because of inhomogeneity of the charge mixture in the combustion chamber. Therefore, it is necessary to investigate the effect of mixture formation on combustion characteristics in order to obtain the stable lean combustion. In this paper, fundamental studies for stratified combustion were carried out using a constant volume combustion chamber. The effect of mixture formation on the combustion characteristics in the chamber was examined experimentally. In addition, the effect of turbulence on stratified charge combustion process was observed by schlieren photography. From this study, as the swirl intensity increases, (Sv)max is rapidly enhanced and the period of combustion is shortened. We also find that the stratification degree can be quantified by using burning velocity and it was controlled by induced air pressure and turbulent intensity.

Simulation for high efficiency liquefied petroleum gas engine development

It has been recognized that alternative fuels, such as liquid petroleum gas (LPG), have less polluting combustion characteristics than diesel fuel. Furthermore, when LPG direct-injection, stratified-charge combustion can be adopted in the spark-ignition engine in place of the conventional premixed spark-ignition combustion, it will be possible to achieve less polluting and more efficient combustion characteristics. However, the physical properties of LPG, such as boiling point (—40-0°C) and viscosity, differ largely in comparison with current fuels. Therefore, there are many unknown phenomena in the behaviour of spray, mixture formation, and combustion of LPG in the cylinder. These issues make the development of a high-efficiency LPG engine difficult. Thus, in this study, a numerical method has been adjusted to predict the mixture formation and combustion of LPG. KIVA-3 code was used for this purpose, and tuning factors were adjusted by comparing them with the measurements. In the combustion model, the eddy-dissipation combustion model proposed by Magnussen and Hjertager [1] is adopted. The predicted results show favourable agreement with the measurements and the possibility of useful methods to improve engine performance.

411 成層燃焼場での燃焼特性に及ぼす液滴粒径と燃料性状の影響について(熱工学II)

411 成層燃焼場での燃焼特性に及ぼす液滴粒径と燃料性状の影響について(熱工学II)

Enhancement of Stratified Charge for DISI Engines through Split Injection : Effect and Its Mechanism(S.I. Engines, Stratified-Charge Combustion)

The effect of split injection on the mixture characteristics of DISI (Direct Injection Spark Ignition) engines was investigated firstly by the Laser Absorption Scattering (LAS) technique. Through splitting the fuel injection process, two possible benefits were found: 1) High density liquid phase spray piling up at the leading edge of the spray can be circumvented, subsequently the reduction of the spray tip penetration; 2) The quantity of "over lean" (φ_v<0.7) in the spray can be significantly reduced. These are believed to contribute to the reduction of the engine-out smoke and HC emissions. In order to clarify the mechanism behind the effect of the split injection, the spray-induced ambient air motion was investigated by the LIF-PIV technique. The strong ambient air entrainment into the tail region of the spray and a counter-vortex structure were found in both the single and split injections. In the case of the single injection, the spray developed in extending its length, subsequently a larger volume resulted and thus it was diluted to "over lean" by the air entrainment. In contrast, in the case of split injection. The second spray was injected into the tail region of the first spray and it distributed some distance from the leading edge of the first spray and the evaporation of the second spray was promoted by the ambient air motion. Hence the replenishment of the liquid phase second spray on the leading edge of the first spray would be reduced. As a consequence, the high density liquid phase spray piling up at the leading edge was avoided. In addition, the ambient air motion played a positive role on evaporating the spray into an "more combustible" (0.7<φv<l.3) mixture. This was especially true in the tail region of the spray and the region where the counter-vortex motion was occurring.

Quantitative Analysis of Mixture Preparation Processes in New Direct-Injection Spark Ignition Engines(S.I. Engines, Stratified-Charge Combustion)

Visualization plays an effective role in the establishment of a new combustion concept by helping to find the optimal results quickly among many different parameters and contributing to a shorter development period. Laser-induced fluorescence, Raman scattering and infrared absorption were used to measure the air/fuel ratio quantitatively in a three generation direct-injection gasoline (DIG) engine including a spray-guided mixture formation process and comparisons were made with the mixture formation concepts of the first- and second-generation DIG engines. The optimum combination of fuel spray, gas flow and combustion chamber configuration was found to be different for the three generations of DIG engines. The characteristics of the stable combustion region for obtaining higher thermal efficiency and cleaner exhaust emissions differed among the three mixture formation concepts.

Charge Stratification in a Strong Tumble SI Engine(S.I. Engines, Stratified-Charge Combustion)

Charge Stratification in a Strong Tumble SI Engine(S.I. Engines, Stratified-Charge Combustion)

04/00419 New concept of a direct injection SI gasoline engine: a study of stratified charge combustion characteristics by radical luminescence measurement: Nakashima, T. et al. JSAE Review, 2003, 24, (1), 17–23

04/00419 New concept of a direct injection SI gasoline engine: a study of stratified charge combustion characteristics by radical luminescence measurement: Nakashima, T. et al. JSAE Review, 2003, 24, (1), 17–23