Controlling a CVT-Equipped Hybrid Car

In order to achieve maximum fuel efficiency, the SI engine of the new CVT-equipped hybrid car developed at the Swiss Federal Institute of Technology (ETH) is operated in a high power regime (such as highway driving at speeds above 120 km/h) with its throttle in its 100-percent open position. Whenever an engine power which exceeds 11 kWs is demanded, there exists an equilibrium point between the engine torque and the torque induced by the drag. Any regulation of the vehicle speed has to be performed by altering the gear ratio of the CVT. If any acceleration is required, it is necessary to increase the engine speed. This requires that the vehicle has to be slowed down for a certain short period of time. If this characteristic behaviour of the car (which is typical for a non-minimum-phase system) is not accepted by a driver who demands and expects immediate acceleration, it might lead to critical situations. Therefore three different approaches have been investigated in order to find an appropriate control strategy. While the first is based on a rather simple interpretation of the driver's intention, approaches two and three consist of a control concept using the feedback linearization technique. All three concepts are compared in a special driving cycle with the behaviour of a conventional car equipped with a manually shifted gear.

It is demonstrated with simulations using a simple nonlinear dynamic model of the plant and a linear dynamic model of the behaviour of the driver that this vehicle can easily be handled in critical traffic situations when being conducted by a conventional driver who is obliged to adapt his behaviour to the car only slightly.