engleski

Modelling the multi cylinder SI engine performance using cycle-simulation model

In order to meet the required performance (power and torque), and to simultaneously reduce the fuel consumption and exhaust gas emissions, modern spark-ignition (SI) engines are usually turbocharged and "downsized". High intake pressure at the cylinder inlet increases the tendency of knocking combustion. The presence of knock reduces the overall engine efficiency, combustion stability and structural durability. The applications of different fuels and their blends, different combustion chamber designs, lean stratified mixtures, different levels of exhaust gas recirculation (EGR), together with the spark-timing settings enables the reduction of knock occurrence. On the other hand, high EGR ratios increase the cyclic variability in combustion which leads to unstable engine operation. In order to minimize cyclic variability and knock occurrence it is necessary to find the compromise between the engine operating parameters. In this paper the numerical analysis of a four cylinder SI engine performance is made, with the emphasis on the knock occurrence and the cyclic variability. For the validation of ignition, turbulence and combustion sub-model constants, the experimental results of multi cylinder SI engine were used as the averaged results over four engine cylinders. The ignition, turbulence and combustion sub-model constants were optimized at the different operating conditions with a single cylinder (reduced) engine model. After validation of the simulation model constants on the reduced single cylinder model, the full engine cycle-simulation model is made representing the modern multi cylinder SI engine. The applied cycle-simulation model includes modelling of the early flame kernel growth, mixture stratification, turbulent combustion, in-cylinder turbulence, knock and cyclic variability over 300 cycles. For several operating points of the four cylinder SI engine, the optimum operating parameters (such as spark advance, EGR ratio, intake pressure) were found to meet the imposed requirements in terms of fuel consumption, percentage of knocking cycles and variation of indicated mean effective pressure (IMEP). The presented methodology represents the fast and well promising tool for a definition of engine operating conditions and performance even in the conceptual development phase.

SI Engine; Cycle-simulation; Turbocharged; Cyclic variability

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