NOVELTY - The heat engine has an expansion device (11) having inflow device (11a) with variably controllable flow cross section (A). An electronic control device (13) is provided with function module (F1) for controlling inflow device. The control device detects the inlet pressure (p) of inflow device and compares with tolerance band to preset point pressure. The pressure-reducing step of flow cross-section is switched, when pressure exceeds upper limit of tolerance band. The pressure increasing step of flow cross-section is switched when pressure falls below the lower limit of tolerance band.    USE - Heat engine for motor car e.g. passenger car.    ADVANTAGE - The vapor pressure can be maintained within a predetermined range by opening and closing of individual nozzles. Since the system pressure dependant on the system volume can rise and/or fall according to delay time reacted on the change of nozzle cross-section, the upper and the lower pressure limits for preventing unnecessary fast sequential change-over processes, which can make the system unstable, can be moved upward and/or downward according to shift operation for defined time. The permissible pressure range widens temporarily and another switching occurs only if the pressure continues to rise steadily. The mobile waste heat recovery system with low volumes is desired because of space limitations, and thus rapid pressure changes are favored. The pressure control for waste heat utilization system with impulse turbine can be realized by the suggested measures as expansion machine, although pressure is highly variable in the application of heat in vehicle entry, so widely varying steam mass flow is exposed. The waiting times and short-term extension of the permissible pressure nozzle bands after switching unnecessarily fast sequential switching, which can make the system unstable, can be effectively avoided. The formation of droplets and the excessive rise in temperature at the outlet of evaporator can be prevented, and the damage to evaporator and turbine are avoided so that the lifespan is prolonged. The volumes of steam is kept low in the construction of the system, leading to reduced heat losses, faster heating after cold start, lesser weight, lower cost and reduced space requirements.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of the heat engine.    Expansion device (11)    Inflow device (11a)    Electronic control device (13)    Flow cross-section (A)    Function module (F1)    Inlet pressure of inflow device (p)