8113 Electronicallycontrolled Continuouslyvariable Transmission

While in series hybrid-drives batteries and IC engine typically power motor/generators, to drive the road wheels, in parallel configured, hybrid-electric/IC-engine drive vehicles there is mechanical drive between IC engine and road wheels, usually via continuously variable transmission, Fig. 8.21. A widely accepted CVT is the variable-pitch pulley and belt-drive type originating in the Van Doorne design; this transmission heralded the steel drive belt having separate tension and thrust members which considerably increased torque capacity. Axial force applied to the pulley sheaves tensioned the belt; then, the combination of lateral and radial force on the blocks was sufficient to transmit drive from pulley to belt. Maximum speed ratio was 6:1 - limited by the size of the pulleys involved - and input torque capacity 90 lb ft (122 Nm). To give a ratio spread of either 12:1 or 16:1 in the car, a fully automatic range-change was incorporated. Transmission efficiency of about 90% was claimed and gear ratios varied from 2.31 to 0.58:1. Unit weight complete with clutch was 60 kg, including variator mechanism.

VDT have developed 24 and 30 mm width-type belts. The 24 mm type is used in lower and medium torque applications, the 30 mm type in higher torque applications. In higher torque conditions, prestressing forces will be higher, and belt running radii shall be larger than in low torque applications. When the running radius of the belt increases, belt speed will increase as well. From this it may be clear that a 30 mm belt element, used for high torque application, cannot be seen as a scaled-up version of a 24 mm one. Its high belt speed capabilities must be better than those of 24 mm elements, in a comparable application (=engine speed). Transmission design affects mainly belt length, centre distance and element width. Because element width does no have to vary over a large range of specifications, the same transmission type can be used over a large range of engine type/vehicle combinations.

A CVT design concept (P884) extending applications into the 1.9-3.3 litres range has also been developed by the company, as illustrated. The transmission offers a choice of driving style - either comfort or sporting mode. In sporting mode there is lock up at low engine speed. Up to 17% better fuel economy is claimed, compared with an electronically controlled four step automatic. A hydraulic wet-plate clutch is used, with a torque converter for lock-up at start-off. The transmission uses a reduced number of belts by using wider, 30 mm rings. This leads to less internal friction and reduced production costs.

Fig. 8.21 Continuously variable transmission; the VDT P884 CVT.
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