Performance analysis of the impulse shift CVT

Due to rotating inertias within the engine and transmission, the response of a vehicle during large engine speed shifts may appear reluctant or even counteractive. To overcome this behaviour, a CVT drivetrain was augmented with a powersplitting planetary gear stage and steel flywheel in the so-called Zero Inertia (ZI) powertrain [1]. This transmission concept managed to combine two contradictive goals: · the driveability in terms of the pedal-to-wheel response is greatly improved; · and a large leap towards optimal fuel economy can be made. These goals were achieved by cruising the vehicle at extremely low engine speeds, enabled by the large ratiocoverage of the CVT. The flywheel acts as a 'peakshaver' during engine speed transients: it delivers power during (semi-) pedal kickdown and absorbs the engine's kinetic energy at pedal back-out. The current paper presents an improved concept with respect to the Zero Inertia drivetrain, which enables a reduced flywheel size while enhancing the pedal-towheel response even further. This concept is designated the Impulse Shift CVT (IS-CVT). The IS-CVT incorporates a slightly modified drive clutch and uses it to interrupt the power flow through the CVT during kickdown accelerations. Disengaging the drive clutch directs the entire engine power flow through the parallel (flywheel and planetary gear) branch, resulting in an immediate flywheel assist while the engine speeds up rapidly. When the engine has reached its desired speed, the drive clutch is re-engaged and the power flow through the CVT is reinstated. The presented IS-CVT concept enables much faster speed transients, independent of the CVT shift rate, resulting in a more effective use of the flywheel energy. Hence the flywheel can be reduced, and the ZI effect can be exploited to the fullest. Furthermore, the demands on the variator actuators can be lower. Simulations of the performance of the IS-CVT are presented and compared to those of CVT and ZI equipped vehicles.