ABSTRACT: The application of ion current signals is one of the most recent approaches in engine management systems. By applying a small constant DC voltage across the electrodes of the spark plug and measuring the current the electrode gap, the state of gas may be measured and investigated. In this paper a computer code is developed in order to analyze the state of gas during the combustion period. It is shown that there is a strong correlation between the peak pressure position and the maximum current position. It is also shown that among all combustion products, the NO has the most contribution in generating electrical current. The two zones model is used for calculating the cylinder pressure and temperature.