Continuously variable transmission control

A large gain in automobile fuel economy has been made by a continuously variable transmission under microcomputer control. The transmission, of toroid and roller type, has a very wide range of ratios and the controller makes full use of this range to minimise the fuel consumption of the vehicle. The controller also has to ensure that the vehicle responds rapidly and predictably to changes in the driver's demand. These gains in fuel economy have been possible because only about 10 to 40 per cent of the installed power is needed for medium speed cruising and that amount of power can be obtained for much less fuel than usual if the engine is run at very low speeds. The internal losses of the engine are largely speed related and not much affected by the higher torque needed to maintain the power output. To develop the controller a special test rig was built with mechanical simulation of all the major elements in the driveline. The controller was designed to use information from speed sensors and the carburettor butterfly angle to minimise fuel consumption while conforming smoothly to the driver's demands for performance. After tuning of the feedback loops and initial proving of the controller on the test rig the transmission was installed in a car. Some further development was needed to improve driveability. Fuel savings were very close to the predicted values.