Stratified Charge Spark Ignition Engine Research Articles

Development of a three-zone combustion model for stratified-charge spark-ignition engine

A thermodynamic model for calculating the operating process in the cylinder of a spark-ignition engine with internal mixture formation and stratified air-fuel charge based on the volume balance method was developed. The model takes into account the change in the working fluid volume during the piston movement in the cylinder. The equation of volume balance of internal mixture formation processes during direct fuel injection into the engine cylinder was compiled. The equation takes into account the adiabatic change in the volume of the stratified air-fuel charge, consisting of fuel-air mixture volume and air volume. From the heat balance equation, the change in the fuel-air mixture volume during gasoline evaporation in the fuel stream and from the surface of the fuel film due to external heat transfer was determined. Basic equations of combustion-expansion processes of the stratified air-fuel charge were derived, taking into account three zones corresponding to combustion products, fuel-air mixture and air volumes. The equation takes into account the change in the working fluid volume due to heat transfer and heat exchange between the zones and the walls of the above-piston volume. Dependences for determining the temperature in the three considered zones and pressure in the cylinder were obtained. Graphs of changes in the volumes of the combustion products, fuel-air mixture and air zones with the change of the above-piston volume in partial load modes (n=3,000 rpm) were plotted. With increasing load from bmep=0.144 MPa to bmep=0.322 MPa, at the moment of fuel ignition, the volume of the fuel-air mixture increases from 70 % to 92 % of the above-piston volume. At the same time, the air volume decreases from 30 % to 8 %. Analysis of theoretical and experimental indicator diagrams showed that discrepancies in the maximum combustion pressure do not exceed 5 %

Ability of Particulate Matter Index to describe sooting tendency of various gasoline formulations in a stratified-charge spark-ignition engine

This study investigates the ability of Particulate Matter Index (PMI) to describe the sooting behavior of various gasoline formulations in a stratified-charge (SC) spark-ignition engine. The engine was operated at 2000 rpm with an intake pressure of 130 kPa where soot formation is known to primarily occur in the bulk gases. Exhaust soot emissions were measured for nine test fuels at various exhaust gas recirculation levels. A comparison between measured soot levels and PMI shows that PMI is a relatively poor predictor of the sooting tendency of the tested fuels under lean SC combustion. Among the fuels, three fuels, namely the di-isobutylene blend, High Olefin, and E30 fuels exhibit measured soot behavior opposite of that predicted by PMI. Optical diagnostics were utilized to further investigate the in-cylinder phenomena for these three fuels. Analysis of natural luminosity and diffused back-illumination extinction imaging suggests that fuel-induced differences in the amount of soot formed are responsible for a majority of the discrepancy in measured versus predicted sooting tendency. Fuel-induced differences in soot oxidation and spray development seem to play minor roles. Because the combustion and air-fuel mixing processes for lean SC combustion are different from conventional stoichiometric operation it is hypothesized that the PMI correlation needs to be modified to account for differences in stoichiometric air-fuel ratio and level of oxygenation between fuels. Furthermore, the role of fuel volatility in PMI possibly needs to be de-emphasized for SC operation with fuel injection into compression-heated gases.

Thermodynamic modelling of a stratified charge spark ignition engine

Combustion of a charge with spatially and temporally varying equivalence ratio in a spark ignition engine was modelled using the Leeds University Spark Ignition Engine quasi-dimensional thermodynamic code. New sub-models have been integrated into Leeds University Spark Ignition Engine that simulate the effect of burnt gas expansion and turbulent mixing on an initial equivalence ratio distribution. Realistic distribution functions were used to model the radially varying equivalence ratio. The new stratified fuel model was validated against experimental data, showing reasonable agreement for both the pressure trace and percentage heat released. Including the effect of turbulent mixing was found to be important to reproduce the trend in the differences between the stratified and homogeneous simulations.

Cyclic variations of fuel-droplet distribution during the early intake stroke of a lean-burn stratified-charge spark-ignition engine

Lean-burn spark-ignition engines exhibit higher efficiency and lower specific emissions in comparison with stoichiometrically charged engines. However, as the air-to-fuel (A/F) ratio of the mixture is made leaner than stoichiometric, cycle-by-cycle variations in the early stages of in-cylinder combustion, and subsequent indicated mean effective pressure (IMEP), become more pronounced and limit the range of lean-burn operation. Viable lean-burn engines promote charge stratification, the mixture near the spark plug being richer than the cylinder volume averaged value. Recent work has shown that cycle-by-cycle variations in the early stages of combustion in a stratified-charge engine can be associated with variations in both the local value of A/F ratio near the spark plug around ignition timing, as well as in the volume averaged value of the A/F ratio. The objective of the current work was to identify possible sources of such variability in A/F ratio by studying the in-cylinder field of fuel-droplet distribution during the early intake stroke. This field was visualised in an optical single-cylinder 4-valve pentroof-type spark-ignition engine by means of laser-sheet illumination in planes parallel to the cylinder head gasket 6 and 10 mm below the spark plug. The engine was run with port-injected isooctane at 1500 rpm with 30% volumetric efficiency and air-to-fuel ratio corresponding to both stoichiometric firing (A/F=15, Φ =1.0) and mixture strength close to the lean limit of stable operation (A/F=22, Φ =0.68). Images of Mie intensity scattered by the cloud of fuel droplets were acquired on a cycle-by-cycle basis. These were studied in order to establish possible correlations between the cyclic variations in size, location and scattered-light intensity of the cloud of droplets with the respective variations in IMEP. Because of the low level of Mie intensity scattered by the droplets and because of problems related to elastic scattering on the walls of the combustion chamber, as well as problems related to engine “rocking” at the operating conditions close to the misfire limit, the acquired images were processed for background subtraction by using a PIV-based data correction algorithm. After this processing, the arrival and leaving timings of fuel droplets into the illuminated plane were found not to vary significantly on a cycle-by-cycle basis but the recorded cycle-by-cycle variations in Mie intensity suggested that the amount of fuel in the cylinder could have been 6–26% greater for the “strong” cycles with IMEP 115% higher than the average IMEP, than the ones imaged for “weak” cycles at less than 85% the average IMEP. This would correspond to a maximum cyclic variability in the in-cylinder equivalence ratio Φ of the order of 0.17.

05/01735 Flame chemiluminescence studies of cyclic combustion variations and air-to-fuel ratio of the reacting mixture in a lean-burn stratified-charge spark-ignition engine

05/01735 Flame chemiluminescence studies of cyclic combustion variations and air-to-fuel ratio of the reacting mixture in a lean-burn stratified-charge spark-ignition engine

The relative effects of fuel concentration, residual-gas fraction, gas motion, spark energy and heat losses to the electrodes on flame-kernel development in a lean-burn spark ignition engine

The potential of lean combustion for the reduction in exhaust emissions and fuel consumption in spark ignition engines has long been established. However, the operating range of lean-burn spark ignition engines is limited by the level of cyclic variability in the early-flame development stage that typically corresponds to the 0-5 per cent mass fraction burned duration. In the current study, the cyclic variations in early flame development were investigated in an optical stratified-charge spark ignition engine at conditions close to stoichiometry [air-to-fuel ratio (A = 15] and to the lean limit of stable operation (A/F = 22). Flame images were acquired through either a pentroof window (‘tumble plane’ of view) or the piston crown (‘swirl plane’ of view) and these were processed to calculate the intra-cycle flame-kernel radius evolution. In order to quantify the relative effects of local fuel concentration, gas motion, spark-energy release and heat losses to the electrodes on the flame-kernel growth rate, a zero-dimensional flame-kernel growth model, in conjunction with a one-dimensional spark ignition model, was employed. Comparison of the calculated flame-radius evolutions with the experimental data suggested that a variation in A/F around the spark plug of Φ(A/F) ≈ 4 or, in terms of equivalence ratio Φ, a variation in Φ ≈ 0.15 at most was large enough to account for 100 per cent of the observed cyclic variability in flame-kernel radius. A variation in the residual-gas fraction of about 20 per cent around the mean was found to account for up to 30 per cent of the variability in flame-kernel radius at the timing of 5 per cent mass fraction burned. The individual effect of 20 per cent variations in the ‘mean’ in-cylinder velocity at the spark plug at ignition timing was found to account for no more than 20 per cent of the measured cyclic variability in flame kernel radius. An individual effect of similar level was attributed to the heat losses to the electrodes, and a much lower effect than that, namely 5-10 per cent at most, could be attributed to cyclic variations in spark-energy release traces. Even when the variations in mean velocity were coupled to both the effects of heat losses to the electrodes and spark-energy release variations, the combined effect was found to account for no more than 40-50 per cent of the observed cyclic variability in flame-kernel radius at the 5 per cent mass-fraction burned timing of 40 ° crank angle after ignition timing.

329 層状給気火花点火エンジンの燃焼モデリング(OS-2 ガソリン機関シミュレーション)

329 層状給気火花点火エンジンの燃焼モデリング(OS-2 ガソリン機関シミュレーション)

The nature of early flame development in a lean-burn stratified-charge spark-ignition engine

The operating range of lean-burn SI engines is limited by the level of cycle-by-cycle variability in the early flame development, which typically corresponds to the 0–5% mass fraction burned. An experimental investigation was undertaken to study this flame variability in an optical, stratified-charge, SI engine close to the lean limit of stable operation (A/F=22). Double-exposed flame images acquired through either a pentroof window (“tumble plane” of view) or the piston crown (“swirl plane” of view) were processed to calculate the intra-cycle flame growth and convection rates under 1500 RPM low-load conditions. Projected flame-boundary analysis was also performed to investigate the effect of flame shape/wrinkling on the subsequent timing of 5% mass fraction burned on a cycle-by-cycle basis. The images showed that the flame always preserved its shape while growing in size (even when it had been initiated with a highly convoluted shape); image processing demonstrated the manner with which the flame-growth speed varied as the flame propagated and approached the pentroof and piston-crown walls for slow, “typical” or fast burning cycles. It was found that it was beneficial to have a high convection velocity in the swirl plane of flow during the first 10° CA after ignition timing (corresponding to less than 0.1% mass fraction burned), but after this stage it was beneficial to have a moderate convection velocity for the flame. However, on the tumble plane of flow, a high convection velocity was preferable up to 30° CA after ignition timing (corresponding, typically, to 1% mass fraction burned). Slow development of a flame was associated with higher stretch rates for the same flame radius than fast-developing flames during the period of growth from 3 to 6 mm in radius (about 0.1–1% mass fraction burned). Extended analysis of the projected flame front's shape and its wrinkling showed that the fastest lean-condition flames had contour characteristics similar to those of the flames recorded for stoichiometric conditions. This suggested that the fastest lean flames on a cycle-by-cycle basis might have been richer than the average in the vicinity of the spark plug at ignition.

Flame chemiluminescence studies of cyclic combustion variations and air-to-fuel ratio of the reacting mixture in a lean-burn stratified-charge spark-ignition engine

The operating range of lean-burn spark-ignition engines is limited by the level of cyclic variability in the early flame development that typically corresponds to the 0–5% mass fraction burned duration. An experimental investigation was undertaken to study the levels of flame chemiluminescence in an optical stratified-charge spark-ignition engine, using a Cassegrain optical system with high spatial resolution. Measurements of OH and CH-radical intensities were simultaneously acquired with double flame images per cycle for a range of air-to-fuel ratios (A/F=12–22). These signals of chemiluminescence were used to evaluate the in-cylinder equivalence ratio of the reacting mixture and to further examine its contribution to the flame growth speed and the cyclic variability in the crank angle by which 5% mass fraction was burned (θXb5%). Specifically, the ratio of the OH/CH chemiluminescence signals was calibrated in the engine and tested extensively for different injection strategies and spark advances, to measure the “global” and “local” in-cylinder A/F ratio around the spark plug. The complications encountered towards this goal are discussed in detail. The results showed that the equivalence ratio exhibited large variations on a cycle-by-cycle basis and consistently produced negative correlation coefficients with θXb5%, especially for lean-set operating conditions (A/F=20–22). Particularly, for open-valve injection strategy that yielded a stratified mixture, the degree of this correlation lied in the range ≈−0.4 to −0.8, being lower for the locally measured A/F ratio and higher for the globally evaluated one. Some issues related to the opposite gradients of the calibration curves deduced for the measurement of the global and local in-cylinder A/F ratios need to be examined outside the engine using a combustion facility with controlled conditions of pressure, temperature, turbulence intensity, and dilution by combustion residuals.

Modelling of NO emissions from homogeneous and stratified charge spark ignition engines

The basis for modelling NO formation in spark ignition (SI) engines by the so-called thermal mechanism is reviewed, along with a comparison of the coefficients that have been recommended for use in the rate equations over the last 25 years. The importance of considering heat transfer, and a multizone representation of the burned gas, is demonstrated by reference to modelling NO in a homogeneous charge SI engine. The model has then been extended to a stratified charge SI engine, in order to investigate the influence of overall equivalence ratio and degree of stratification on the NO emissions and the engine brake specific fuel consumption. For fixed throttle operation, it is concluded that the best trade-off is with an overall weak mixture that is close to homogeneous. For maximum power output using a slightly rich stoichiometric mixture, the mixture should also be close to homogeneous. However, if the engine is constrained to operate with an overall stoichiometric mixture, then the trade-off between NO emissions and brake specific fuel consumption is with a stratified mixture that is rich at the spark plug.

Development of an engine simulation program and its application to stratified charge SI engines

The paper presents the development of an engine simulation program and its application to the analysis of a spark ignition (SI) engine with stratified charge. The engine simulation program has been developed to predict engine combustion, auto-ignition, performance and emissions. Full NOx formation is included in the model with a refined method to calculate the equilibrium burnt gas composition. To predict the onset of knocking combustion, an auto-ignition model involving 31 different chemical reactions is included. The thermodynamic and auto-ignition models are coupled through cylinder pressure and unburned gas temperature. The inclusion of a geometry sub-model permits the modelling of SI engines with complicated cylinder head geometry. The model was then applied to a SI engine with stratified fuel or diluent. The effects of the charge stratification on engines combustion, performance and emissions are discussed.

Modeling Turbulent Combustion and Pollutant Formation in Stratified Charge Si Engines

A conditional averaging approach is applied to model combustion and pollutant formation in stratified charge spark-ignition engines. A set of diagnostic equations follows the local composition and temperature of the unburned gases during combustion. The Coherent Flame Model is chosen to simulate combustion. It is extended to rich or diluted combustion. NO and CO reaction rates are computed conditionally based on the burned gas temperature with two contributions : a source term function of the flamelet density and unburned gas composition and temperature, and another source term including equilibrium and kinetic mechanisms in the burned gas. The coupled combustion and pollutant models are implemented in the Kiva-2 code and tested on a simple engine geometry for a set of operating conditions including variation of equivalence ratio, dilution by residual gases and fuel stratification. The ability of the model to reproduce observed trends for combustion rate is demonstrated. The influence of the local laminar flame on the burn rate is pointed out. A much larger production rate of NO and CO during combustion resulting in higher computed NO levels for most of the operating conditions, and similar final CO levels after combustion completion is observed for the conditional averaging as compared to the usual ensemble averaging procedures. The predicted levels of emissions are comparable to measured engine values.

Modeling turbulent combustion and pollutant formation in stratified charge SI engines

A conditional averaging approach is used to model combustion and pollutant formation in stratified charge spark-ignition (SI) engines. A set of diagnostic equations follows the local composition and temperature of the unburned gases during combustion. The coherent flame model is chosen to simulate combustion and extended to rich or diluted conditions. NO and CO reaction rates are computed conditionally based on the burned gas temperature with two contributions: a source-term function of the flamelet density and unburned gas composition and temperature, and another source term including equilibrium and kinetic mechanisms in the burned gas. The coupled combustion and pollutant models are implemented in the Kiva-2 code and tested on a simple engine geometry for a set of operating conditions including variation of equivalence ratio, dilution by residual gases, and fuel stratification. The ability of the model to reproduce observed trends for combustion rate is demonstrated. The influence of the local laminar flame on the burn rate is pointed out. A much larger production rate of NO and CO during combustion, resulting in higher computed NO levels for most of the operating conditions and similar final CO levels after combustion completion, is observed for the conditional averaging as compared to the usual ensemble averaging procedures. The predicted levels of emissions are comparable to measured engine values.

Heat loss and heat release on premixture combustion in a stratified charge spark ignition engine with an auxiliary combustion chamber.

高効率・低公害化機関を目的とした層状給気機関の一つである副室付き火花点火機関において,燃焼機構と熱損失を実験的に調査した.特にこの形式の機関の特徴である連絡孔からの火炎噴流と熱損失および熱発生との因果関係を熱流束と燃焼圧力とから求め,この燃焼方式の機関における熱伝達率を推定する実験式を得た.さらに熱発生率の形が連絡孔径だけではなく熱損失によっても変化することを明らかにした.