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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.

The potential of Duty Cycle Operation (DCO) of a Spark Ignited (SI) engine on part load has been investigated. DCO keeps an engine running at full throttle in a stop and go mode to speed up a flywheel as a short time energy storage device. So the actual power demand is covered by the flywheel instead of the convenient direct power transfer from the engine. This work includes the calculation of the theoretical potential and preliminary results of a test setup. The results show a clear advantage of fuel consumption at the engine's low power output. The potential of DCO has proved to be higher than that of variable intake valve timing.