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A parametric study of SI engine efficiency was carried out using a two-zone cycle simulation. The simulation models are first validated and then used to study the effects of different engine function parameters on the engine efficiency and on energy and availability losses. The following parameters are considered in this study: combustion duration, spark timing, spark plug position, combustion chamber heat isolation and compression ratio. The availability balance has been developed with the purpose of recovering the energy lost. It indicates the percentage of energy that could be recovered out of the total energy lost (in the exhaust system, for instance). This study requires detailed information on the gas state (temperature, pressure) and on the temperatures of the combustion chamber walls and of the cylinder head. Some of these temperatures are almost impossible to measure experimentally with the material available at this time. That is why we resort to cycle simulation.