engleski

NEW PHYSICALLY BASED SUB-MODELS FOR THE CYCLE- SIMULATION OF SPARK-IGNITION ENGINE

The research and study presented in this thesis are related to the modeling of turbulence, ignition and combustion phenomena of the cycle- simulation of SI engines. The developed sub- models for the turbulence, ignition and combustion process of SI engine are integrated into the cycle-simulation code AVL BOOST and the cycle-simulation results of several SI engines are compared with the available 3-D CFD and experimental data. Modeling of the in- cylinder turbulence was performed with the single and two zone k-ԑ turbulence model applicable to the 0-D approach. First, the single zone k-ԑ turbulence model was developed and applied during the high pressure cycle with initial results of the in- cylinder turbulence at the intake valve closure specified from the 3-D CFD results. A developed two zone turbulence model was applied during the combustion period with the cylinder mixture divided into a burned and unburned zone. The calculation of combustion process was performed using the quasi-dimensional combustion model based on the fractal theory. The application of the two zone turbulence model, with unburned zone turbulence quantities used for the definition of flame front propagation properties showed significantly better, easier and physically more accurate prediction of the combustion process of SI engine at different engine operating conditions. To eliminate the model dependency on the initial conditions that had to be specified either from 3-D CFD or from the experimental data, the full k-ԑ turbulence model for the entire engine cycle was developed. In order to avoid the manual tuning of ignition delay and the transition from laminar to fully developed turbulent flame from one operating point to another, the new quasi- dimensional ignition model was developed and a modified calculation of transition time was proposed and applied in the fractal combustion model. After that, the parameterization of the turbulence model constants and the parameterization of the upper limit of fractal dimension are introduced into the cycle- simulation model. In order to find a single set of turbulence model constants for one engine, an optimization process for calculation of model constants was applied. The validation of the cycle-simulation model that had a single set of constants related to turbulence and combustion sub-models of one engine showed a good agreement with the 3-D CFD results at several different operating conditions of SI engine. The modeling of cycle-to-cycle variations (CCV) in combustion that occur in SI engine was performed by random variation of intake turbulence model parameter that was set to follow the Gaussian distribution around the mean value. The cycle-simulation model was also extended so that the random variation of flow angle at the spark plug location from cycle- to-cycle can be considered. The cycle- simulation results obtained with the single set of constants and with the variation of in- cylinder turbulence level and flow angle showed very good agreement with the experimental data of Waukesha Cooperative Fuel Research (CFR) F4 engine at considered operating points that represent low and high CCV in combustion. The presented cycle-simulation model including the developed sub-models for modeling of turbulence, ignition and combustion represents simple, fast and well-promising solution for modeling of engine output performances by using a simulation of mean cycle or by using CCV if necessary.

spark-ignition engine; combustion; cycle-simulation; turbulence; ignition; cycle-to-cycle variations

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