Further Aspects of Combustion Modelling in Spark Ignition Engines

An existing “phenomenological” computer model of the spark ignition engine combustion process has been used to reveal further information on flame development in such engines. A detailed flame map and pressure-time diagram ( from Ref ( 6 ) ) has been analysed to determine more precisely the 3-dimensional nature of the flame development across the CFR engine cylindrical disc combustion chamber. The conventional spherical flame assumption (centred at the spark plug) is found to be inaccurate with the extent of the deviation from sphericity varying with flame radius and compression ratio in both the vertical and horizontal (plan view) planes. The flame map provides input data in the form of enflamed volumes, heat transfer surface areas etc for the subsequent evaluation of turbulent burning velocities, mass burn rates, pressure-time diagrams etc. This is achieved by an iterative technique involving the mass burnt during any crank angle interval so that the burnt volume is correctly predicted (i.e. coincides with the known, measured, enflamed volume). A “thin flame” assumption appears valid across the major part of the chamber but, during the final “termination period”, combustion calculations involving finite burn-up rates in “thick” turbulent flames need to be incorporated. The transitional point coincides with a rapid escalation in the surface:volume ratio of the “end gas” and, possibly also, with the appearance of enhanced low frequency flow velocity fluctuations induced by piston motion on the expansion stroke. An exponential expression is used to portray mass burn rates within the entrained charge during flame propagation at this time. Pressure-time diagrams using the 'thick' flame model are compared with 'thin' flame predictions and measured values.