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Performance Analysis of the Impulse Shift CVT
Technical Paper
2004-40-0023
Sector:
Language:
English
Abstract
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.