Four-stroke air cooled, direct injection diesel engine Torpedo BT4L912 during rotational speed variation was investigated in this paper. By using the measurement results obtained at engine brake was calculated several main operating parameters in each engine rotational speed. Rotational speed was varied from 1000 rpm to 2300 rpm. Calculated engine operating parameters are engine torque, effective power, engine brake mean effective pressure, specific effective fuel consumption and volume fuel consumption per engine process. The highest obtained engine torque amounts 338.8 Nm and the highest engine effective power amounts 71.76 kW. Specific effective fuel consumption has the lowest value of 197.42 g/kWh at engine rotational speed of 1800 rpm. Regarding several calculated engine operating parameters, optimal operating point of the analyzed engine is at 1800 rpm.