Abstract The paper deals with the influence of selected component of synthesis gas on internal parameters of combustion engine that is planned to be used in micro-cogeneration unit. The aim is to better understand the mechanism of combustion of carbon monoxide mixed with methane and as a follow-up to optimize the operation of the Lombardini LGW 702 engine on change of fuel composition. Generally, an increasing proportion of carbon monoxide in methane mixture leads to a decrease in engine performance (mean indicated pressure) and the hourly fuel consumption in each of the operating modes of the engine increases. With growing proportion of CO in mixture with CH₄, the maximum pressure in the cylinder increases together with pressure rise rate up to approximately 10 % vol. of CH₄. With further increasing proportion of CH₄, there is a significant decrease of the before-mentioned engine parameters. The optimum ignition angle for pure methane, or carbon monoxide, does not change significantly and it is about 27° CA BTDC.