Multidimensional Calculation of Combustion in an Idealised Homogeneous Charge Engine: a Progress Report 810151

This paper is concerned with the calculation of combustion in an idealised homogenous-charge spark-ignited engine having a disc-shaped combustion chamber equipped with a central spark plug and inlet/exhaust valve.

The purpose of the study is to assess the ability of a class of turbulence combustion models, first developed for steady flows, to simulate the major features and trends of reciprocating engine combustion. The models are based on the supposition that the time scales of the turbulence energy dissipation will be the controlling rates at which reactions can take place. Experimental evidence lends support to this supposition. Hitherto, in multi-dimensional engine prediction methods, combustion predictions have almost invariably been based on chemical-kinetics reaction models. In the model evaluated here the reaction rate is related to the local species concentrations and the dissipation time scale, which is deduced from the predicted levels of turbulence energy and its dissipation rate.

Calculations are performed for a range of engine speeds, ignition timings and fuel-air ratios. The results reported reveal that, subject to uncertainties about the empirical treatment employed for the ignition process, and the need to make a once-for-all adjustment of the burning rate coefficients to procure the correct flame speed at one set of conditions, the model yields the correct trends in flame speed, heat release and pressure variations. In addition, examination of the details of the flow and combustion fields, show that, contrary to the predictions of Rapid Distortion Theory, the flame appears not to provoke substantial changes in the turbulence levels of the unburned gases.