The interaction between a combustion engine and a Foettinger clutch is illustrated in the figure.
The pump characteristic represents the torque delivered by the pump wheel. Since the torque at the pump is determined by the combustion engine, the specific operating point can be obtained by the intersection of the pump characteristic and engine torque. The speed nP is hence defined. According to the instantaneous characteristic of driving resistance, the torque at the pump can be converted into a driving velocity and hence into a turbine speed. Since the pump characteristic changes depending on the pressure ratio between the turbine and pump, the operating point also moves on the curve of the engine torque.
When the vehicle is stationary, the clutch transfers a moment that results from the point of intersection of pump characteristic "0" and the engine characteristic. When the vehicle accelerates, the turbine or output speed nT increases. As a result, the k-factor changes resulting in a change in the slope of the pump characteristic. The new point of intersection provides the new clutch torque. This process takes place continuously until the coupling is achieved and nT/nP is constant.
Moreover, the slip of a Foettinger clutch cannot become zero since otherwise no torque would then be transferred. The maximum efficiency is 98 %. With the aid of mechanical converter lockup clutches efficiencies up to 100 % can be achieved.