This paper describes the development and results of a mathematical model for four stroke diesel engine in Direct-Injection Diesel engine. Studies of combustion performance were involves many factors. Discuses were make of factors of combustion cycle, such as physical properties of fuel,shapes of combustion chamber, and operating of engine cycle. This paper attempt was made to simulate the process in the combustion chamber by using a numerical analysis to find the combustion performance factor in diesel engine. The combustion performance factor, cylinder pressure, gas temperature, rate of heat release and rate of pressure rise can eb found easily by using the result of the simulation. The results from the simulation were compared with the orher author results.