Due to diminishing liquid fuel resources comparing with gaseous fuel as well as difference in their price one can observe a tendency to substitute liquid fuel with gaseous one. The reduction in the emission of toxic ingredients of fumes is also of extreme importance. However, together with the change of the fuel new problems have occurred. One of them is the increase in heat load that shortens the life of an engine. Therefore, studies on the chosen type of an engine were carried out. A heat flux in representative points of the engine’s head was measured. The engine was firstly fed with petrol, later with LPG. For both types of fuel the obtained power was similar. Studies showed a significant growth in heat load on the surface of the engine’s head after changing the fuel from petrol to LPG.