Multi-Dimensional Modeling of Mixing and Combustion of Direct Injection Spark Ignition Engines

Fan, L. Multi-Dimensional Modeling of Mixing and Combustion of Direct Injection Spark Ignition Engines. University of Wisconsin-Madison, 2000.

A numerical study is carried out of Direct-Injection Spark-Injection (DISI) engine in-cylinder processes using multi-dimensional models. An ignition model, called DIPK, is developed. The model describes ignition flame kernel growth using a Lagrangian method. Spark plug heat transfer and drag effects were also modeled. The characteristic time scale combustion model was improved for stratified charge combustion. Intake flow, spray, and mixing are validated using available state-of-art experimental results.

Due to its Lagrangian nature, the present ignition model is shown to be less sensitive to the numerical mesh size than previous ignition models. Computation of a quiescent bomb case suggests that the growth rate of the flame kernel is affected by the spark plug heat transfer, the ignition power, and the total spark energy. Heat transfer from the spark plug accounts for as much as 60% of the total heat deposited. The developed models were validated using two homogeneous charge engines with variations of spark timings, loads, and air-fuel ratios.

The present models were first applied to a conceptual DISI engine late-injection stratified-charge case. Computations demonstrate that gas flow tumble ratios and injector mounting angles are important to the ignition and combustion events. Furthermore, the results show that the air-fuel ratio of the mixture at the spark location is important to the combustion. The results demonstrate that, for this conceptual engine the spray orientation angle, piston deflection and gas tumble are important factors for the fuel preparation. These factors cause different in-cylinder fuel stratification details.

As the final application of the models, medium load cases with an air-fuel ratio of 30:1 were studied for a Mercury Marine two-stroke DISI engine. The gas flow details, .including the entire scavenging process, the spray, and the combustion processes were simulated. Three injection timings, 271, 291 and 306 ATDC were selected to investigate the effects of the injection timing on mixture formation, ignition and combustion. The results indicate that at this particular load condition, an earlier injection timing allows more fuel to evaporate. However, because the fuel penetrates further toward the piston, a leaner mixture is created near the spark plug; thus, a slower ignition process with a weaker ignition kernel was found for the SOI 271 ATDC case. Later injection results in higher turbulence intensity and earlier injection is more significant in reducing the in-cylinder charge temperature. The measured and computed combustion results such as average in-cylinder pressure and NOx are in good agreements. The later injection case produces lower NOx emission and higher CO emission; this is due to poor mixing and is in agreement with experimental measurements.