Driveability Assessment of a CVT-Powertrain with Mechanical Torque Assist

Due to rotating inertias within the engine and transmission, the response of a vehicle during large and/or fast engine speed shifts may appear reluctant or even counteractive. Reminiscent of comparable behaviour seen in aircraft jet-propulsion, this phenomenon is also referred to as ‘jet-start’. To overcome this behaviour, a CVT powertrain is augmented with a powersplitting planetary gear stage and compact steel flywheel. The new transmission—coded ‘Zero Inertia (ZI) powertrain’— seamlessly combines two contradictive features: the driveability in terms of the pedal-to-wheel response is greatly improved and a large leap towards optimal fuel economy can be made. This is achieved by cruising the vehicle at extremely low engine speeds as of the large ratio-coverage of the CVT. The flywheel acts as a ‘peak shaver’ during engine speed shifts: it delivers power during (semi-) pedal kick down and absorbs kinetic energy of the engine at pedal back-out. This paper presents results from simulations of, and field experiments with a test vehicle incorporating the ZI powertrain. The evaluation concentrates especially on longitudinal driveability and is compared with that of basic CVT and 4-gear automatic powertrains. Results on the substantially improved fuel economy are summarized at the end of the paper. The driveability has an accurately controllable character, i.e., the accuracy with which desired drive shaft torque excursions can be realized by the powertrain. The subjective driveability depends highly on how drive pedal deflections are translated into desired drive shaft torque excursions. Evaluation by experts and non-experts in general determines the quality of this translation. A non-rigorous attempt to formalize the translation and use it in a powertrain control strategy is made. Preliminary conclusions on possible acceptance by public are drawn. From this, directions for ongoing research are given.