Environmental Engineering Reference
In-Depth Information
What the gear shift event does in a power split transmission is to cause the sun
gear speed to toggle from clockwise rotation to counter-clockwise rotation while
remaining well away from zero speed. This ensures higher operating efficiency and
no stalled operation of the S/A (i.e. as it would be, had its speed been commanded
to zero while holding torque level).
A second rationale for providing a gear shift to a power split is to implement a
high/low range feature. With the added gear ratio active, the driveline is essentially
given a different, much shorter final drive. For example, if the inserted ratio is 1.4:1
and the final drive, FD, was 3.5 then the new, equivalent final drive will be 4.9.
This much higher final drive is typical of towing applications and provides the
vehicle with a low range function. When engaged, the vehicle has much higher
launch traction for grade climb, deploying or launching a boat, or for driving in deep
snow for example (see also Chapter 11 for towing example). Disengaged, the final
drive reverts back to its normal setting, or the equivalent of high range transmission.
The terminology of shorter and longer final drive has been used in the above
discussion without explanation. It may be clearer if this terminology is defined in
the context of driveline revolution counts per mile. Vehicle speedometers and
odometers rely on a signal taken from the transmission output shaft that delivers a
pulse per wheel revolution. This means, for example, that a vehicle having a pro-
duction tyre will turn a prescribed number of revolutions per mile of travel, typi-
cally 850 for the production final drive ratio. Now, if the customer changed tyres to
a different aspect ratio, but the same rim size, the count would be off and so would
the speedometer and odometer readings. For illustration, suppose the final drive
ratio is changed from 3.5 to 3.7. With this increased ratio, the propeller shaft to rear
wheel drive or half-shaft speed in the case of front wheel drive will spin 5.7% faster
for the same distance travelled. The final drive is thus said to be shorter because
each revolution of the propeller shaft results in a proportionally shorter distance
travelled. Had the final drive ratio been decreased from 3.5 to perhaps 3.2, then the
propeller shaft would only turn 91.4% of a revolution to traverse the same distance,
in effect a longer final drive. Longer final drive means the engine speed is lower for
a given vehicle speed. As a consequence,
the engine exhibits more lugging
behaviour.
The disadvantage of shifting a power split transmission is the increased control
complexity of blending torque from three sources plus the need for a driveline
clutch. A clutch in the driveline always introduces a torque hole in propulsion while
the clutch disengages, the component speeds re-establish themselves to new equi-
librium points and the clutch ceases to slip. The resulting loss of transmitted torque,
or a torque hole, may persist for 150-300 ms. In addition to interruption of tractive
effort, a clutch event introduces power loss and contributes to driveline shudder and
potentially to driveline oscillations if left unchecked.
Toyota Motor Company incorporated a gear shifting function to their single
mode eCVT with the introduction of the GS450h in 2006. The reason for the gear
shift was to equip this luxury vehicle with autobahn cruising speed capability. The
single mode eCVT discussed to this point is restricted in WOT speed by the upper
limit of its main traction M/G, MG1, angular speed. In the GS450h the 14,000 rpm
