Electronically Controlled Continuously Variable Transmission (ECVT) (Automobile)


Electronically Controlled Continuously Variable Transmission (ECVT)

An interesting feature of the electronic control system is that it observes driver’s behavior and accordingly optimizes the interaction of the engine and transmission to provide best performance and economy. This gives a fuel saving of about 10% than a conventional four speed automatic transmission. The ECVT uses a selector lever very similar to that of a conventional automatic transmission. When D is engaged the driver can accelerate away, with the transmis­sion operating at the optimum ratio for the desired engine speed. ECVT cars are generally much faster from a standing start, than their manual counterparts.
During running the ECVT offers a very wide ratio span, roughly equivalent to that of a six-speed manual transmission. Theoretically, this should provide exceptional fuel economy if the engine is at the optimum speed and load point for a given road speed. Unfortunately, frictional losses in the transmission consume a considerable amount of energy and therefore in practice ECVT cars are about 5% less fuel efficient than their equivalent manual transmission counterparts.

Subaru ECVT

The Subaru ECVT, the world’s first practical electronically controlled CVT, was developed jointly by Fuji Heavy Industries in Japan (Subaru’s parent company) and VDT in Holland. The
Van Doorne steel thrust belt (Subaru).
Fig. 25.73. Van Doorne steel thrust belt (Subaru).
ECTV is consisted of an electromagnetic powder clutch and electronic control unit developed by Subaru, and steel thrust belt and pulleys (Fig. 25.73) developed by VDT.
ECVT was first installed in Japan in February, 1987 on the Subaru Justy and has since been used on several other small vehicles, specifically the Nissan Micra with considerable success. The microcomputer based transmission control system always ensures the operation of the engine in its most efficient speed range, thereby reducing emissions and improving fuel economy.


An electromagnetic powder clutch transmits en­gine torque to the primary pulley, which, in turn, drives the steel thrust belt to rotate the secondary pulley. Each pulley has two patered sides, called sheaves. One sheave is secured to a specific transmission shaft and the other can move under hydraulic pressure. The pressures are control-
led to widen or narrow the groove widths be­tween the sheaves of each pulley in inverse proportion to each other (Fig. 25.74), so that the belt is clamped and a stepless variation in ratio is provided from 2.503:1 to 0.497:1. About 280 wedge-shaped blocks, each precision-ground from high-friction steel constitute the drive belt. The blocks transmit thrust by pressing against each other, and are guided between the pulleys by two thin steel rings.

Sheave clutches to effect ratio changes (Subaru)
Fig. 25.74. Sheave clutches to effect ratio changes (Subaru).
Figure 25.75 illustrates the important com­ponents of the electromagnetic powder clutch. For operation of the clutch, the ECVT microcom­puter permits flow of an energizing current to the exciting coil to magnetise the metal powder. The magnetised powder progressively binds together. This action locks the external driving member to the internal driven member, so that smooth transfer of engine torque to the primary pulley takes place.

Control System.

The electro-hydraulic control system, il­lustrated in Fig. 25.76, uses an 8-bit microcom­puter to control both the clutch and the hydraulic system. The control system, during operation
Electromagnetic powder clutch. (Sabaru).
Fig. 25.75. Electromagnetic powder clutch. (Sabaru).
ECVT control system (Subaru).
Fig. 25.76. ECVT control system (Subaru).
uses inputs from various sensors indicating the vehicle operating mode.
A shift lever position switch prevents engagement of the clutch when the transmission is in the P or N position. A brake pedal switch signals the controller that the vehicle is slowing down so that the controller disengages the electromagnetic clutch to prevent a stall. An accelerator pedal position switch alerts the control unit about the pressing of the pedal by the driver to drive away the vehicle. An energizing current is then fed to the clutch electromagnet, causing gradual engagement of drive. The controller judges vehicle speed and maximises this current when the vehicle attains normal road speeds.
Besides performing the normal activities like stopping, starting and smooth ratio changing, the microcomputer provides additional features, which include,
(i) Prevention of fierce clutch engagement when the engine is running at a fast ideal
during cold weather starting. (ii) A self-diagnosis and back up system, which safeguards the transmission against
damage in the event of a control failure. (Hi) Provision of a small clutch energizing current to prevent the vehicle from rolling backwards during a hill-start.

Pressure Control Valve Systems.

The electro-hydraulic valve unit is positioned in the body of the transmission. For the control of the gear ratio, the primary pressure is applied to the Drimary pulley servo cylinder. This forces the primary pulley to the desired running diameter. Since the steel belt is of a fixed length it, in turn, forces the secondary pulley, against line pressure to a running diameter, which is reciprocal to that of the primary. Therefore, the line pressure opposes the primary pressure and controls the clamping force on the belt. The clamping force must be such to eliminate damaging slip as well as avoid excessive loads.
A solenoid valve switches the line pressure between two values to improve drivability. When the engine torque sensor signals the ECVT microcomputer that output torque is below 60% of maximum, the solenoid is switched on to give LOW line pressure. As a result the pulley clamping force applied to the belt is reduced, thereby cushioning the drive. The transmission operates more smoothly in stop-go driving eliminating jolts and jerks. When the engine output torque is above 60% of maximum, the line pressure control solenoid is switched on to give HIGH line pressure, about 50% increase over the LOW setting. This provides firm clamping of the pulleys to the belt, eliminating any possibility of slip so that maximum power transfer is ensured.

ZF Ecotronic ECVT

This ECVT has been designed by the ZF Company of Germany suitable for use on medium size cars, in contrast to the Subaru ECVT, designed for use on small cars. The ZF Ecotronic operates on the same principle as the Subaru ECVT, but uses a wider 30 mm steel thrust belt to provide a higher torque capacity of 210 Nm, making the transmission suitable for the engines of up to 2.5 litres displacement. The transmission ratio varies from 2.44:1 to 0.46:1 and a lockup torque converter, rather than electromagnetic clutch, is used to transfer power from the engine.

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