Abstract
“Phenomenological” computer models of the spark ignition engine combustion process are predominantly used in a “predictive” mode i.e. the modeller develops the computer program by utilising the best current understanding of phenomena associated with the process. Thus, suitable expressions are incorporated to depict turbulent burning velocity, heat exchange with the surroundings, flame pattern development, exhaust emissions etc.. The accuracy of the model is then tested by subsequent comparison with pertinent “global” engine performance parameters such as pressure-time diagrams, average flame speeds and exhaust emissions. The “predictiveness” of the model arises from the inclusion of the turbulent burning velocity expression and assumed flame pattern development. These are notoriously difficult to specify precisely and resort is often made to generalised approximations. Thus, turbulent burning velocity is frequently accounted for by the use of expressions developed from experiments on dissimilar engine designs or even “out-of-engine” flames. Accurate flame pattern development is an equally unknown quantity since it is influenced by turbulence and swirl levels, combustion chamber surface temperatures and design, mixture homogeneity etc.. These difficulties are invariably overcome by subsuming them in a general inaccurate assumption that the flame development is spherical in nature and centred at the spark plug. Even though the “global” computer model predictions may be acceptably accurate with such assumptions, it is highly unlikely that the detailed flame progression across the chamber is modelled correctly. To overcome such criticisms, the computer model can be used in a different operational mode (1) with the ultimate aim of more precise specifications of turbulent burning velocity and how it varies across the combustion chamber.
The effect of swirl on spark-ignition engine combustion.
