In this paper an improved mathematical model of spark ignition engines is presented. The model is derived based on a combination of thermodynamically relations and dynamical characteristics of engines. The geometrical parameters of the engine, valves and spark timing, air to fuel ratio, specific fuel consumption and engine operational conditions are considered during all four strokes of the engines. The engine is divided into some related subsystems namely; throttle body, intake manifold, fuel delivery system, intake and exhaust valves, E.G.R., torque production system, rotational dynamics and emission system. Each subsystem is modeled and the inter-relation between them has been considered. A new computer code is developed in Visual C++6 based on derived relations. This code may be considered as an appropriate means to investigate the performance of a given SI engine and to derive the emission results. The computer code is able to evaluate twelve combustion production species in a full thermodynamic equilibrium. Other engine performance quantities such as power, torque, thermal efficiency, work done by piston, pressure and temperature time histories are also evaluated in this simulation.

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