A Three-Zone Heat Release Model for Combustion Analysis in a Natural Gas SI Engine. -Effects of Crevices and Cyclic Variations on UHC Emissions

A thermodynamic analysis based on a pressure-time history measured during the combustion in a SI engine is a commonly used tool used for analyzing the combustion process. Both one-zone and two-zone models have been applied for this purpose.

One of the major sources of the emission of unburned hydrocarbons from SI engines is the presence of crevices in the combustion chamber where a part of the unburned fuel-air mixture is trapped during the compression and the combustion.

In this paper a three-zone heat release model including the effect of crevices is presented. The model is based on a thermodynamic analysis of three connected zones consisting of burned gas, unburned gas and gas trapped in crevices.

Engine experiments have been carried out on a natural gas SI engine. The results from these experiments have been analyzed by the model. This analysis showed that approximately 7% of the unburned fuel-air mixture are trapped in the crevices at the end of the primary combustion on this engine where the crevice volumes amount to 3% of the compression volume. The model also indicate that one underestimates the combustion rate by neglecting the effect of crevices and that the presence of crevices has a cooling effect on the unburned gas.

The developed heat release model has been used to examine the cyclic variation of the engine. This investigation showed that the maximum mass fraction burned for an average combustion cycle decreases with increasing excess air causing higher UHC emissions.