Worsening of economic and environmental parameters of a dual fuel engine at partial engine loads results from protracted combustion of leaned gas air mixture. This leads to reduction of overall efficiency and growth of CO and THC emissions. Such problems are difficult to solve in traction engines due to theirfrequent operation at partial loads and rapid changes of engine load and rotational speed. Negative effects of leaning of gaseous mixture can be partially diminished through division of initial dose and activation of combustion of gaseous mixture through delayed injection of additional portion of liquid fuel. In the paper are presented test results of experimental SB3.1 engine run on CNG with divided dose of the Diesel oil. There were analyzed overall efficiency and selected parameters of combustion process. Division of the dose influenced advantageously on parameters of the engine. The overall efficiency increased, while maximal pressure pmx, heat release rate (dQ/da)muxe and pressure growth rate (dp/da)max decreased. The engine with divided dose operated more smoothly with less noise.