As the struggle to meet increasingly stringent fuel-economy and exhaust-emission regulations continues, engineers are turning to transmissions for help. The theory is simple: Keep the engine operating at its most efficient point.
Much like the different speeds on a bicycle, transmissions are used to multiply the torque produced by the engine. Lower gears provide more torque multiplication for acceleration but limit vehicle speed; higher gears allow greater vehicle speeds but little torque multiplication and less acceleration.
The fewer the gears in a transmission, the further the range of engine speeds. For example, a three-speed might require an engine to go from 1,000 to 4,000 rpm in first gear, dropping to 2,000 when put in second and again accelerating to 4,000 before the shift to third. A five-speed transmission might allow it to go from 1,000 to 3,000 rpm in first and from 2,000 to 3,000 in second through fourth, before being placed in fifth on the highway with revs around the 2,000 point, keeping the engine close to its optimum range.
Transmissions allow an engine to operate as closely as possible to optimum speed for a given throttle position regardless of vehicle speed and loads. And the manual transmission continues to be the model of efficiency in terms of power loss and expense. It also continues to sink into oblivion in terms of popularity among North American consumers.
Automatic transmissions have grown in popularity with five- and even six-speeds becoming more common. But the disadvantages are still there: power loss, complexity and cost.
Another method of shifting gears is resurfacing after more than a century in relative oblivion: the continuously variable transmission (CVT).
First invented by German engineers Gottlieb Daimler and Karl Benz in 1886, the CVT was based on a rubber V-belt riding between two angled cones. Not much has changed in the intervening 118 years, but CVTs are once again becoming popular as they allow an engine to operate more efficiently by varying gear ratios instead of engine speed.
The CVT is a very simple device, consisting of pulleys and a belt. The attendant shafts and related controls are already in place in the vehicle. No gears, plates, valves, converters or fluids; rather a pair of cone-shaped pulleys, facing each other, on a shaft connected by a belt to a second pair of similarly-shaped pulleys on another shaft.
One end is connected to the engine and the other to the drive wheels. As the pulleys at each end of the belt move closer together or further apart, the belt rides lower or higher along the walls of the pulley, thereby changing the gear ratio.
The pulleys are controlled by the engine control unit, a computer that uses information regarding throttle, engine speed, vehicle speed and other conditions to move the pulleys via oil pressure. If the throttle is depressed for acceleration, for example, the system widens the driving pulleys and narrows the driven pulleys -- creating a lower gear ratio. The beauty is the infinite number of ratios available and the simplicity of the system.
The engine can be set to operate at peak efficiency and kept in that range while the belt moves between the wide and narrow portion of the pulleys, altering ratios over a huge range and thus, final drive and vehicle speeds without a single shift.
If, for example, an engine's peak operating efficiency is 2,758 rpm, the CVT allows engineers to maintain that speed regardless of vehicle speed or load.
Although CVT's have been around for more than a century, used mostly to run power tools and other low-load devices, they have seen limited use in automobiles.
Subaru was the first to offer one in a production car, the 1.2-litre Justy in 1989. It remained in production until 1993, but neither the car nor the CVT captured the buying public's attention.
The problem with CVT's has been their inability to handle torque. Belt slippage caused when engine torque exceeded grip is a major concern. Traditional belts deteriorate very quickly in these situations.
Advances in metallurgy and electronic control have allowed engineers to renew their interest in CVTs. Audi and Nissan, for example, are using multilink steel chains and all current CVT systems (Audi, Honda, Ford, Nissan, Saturn and Toyota) are taking advantage of ECUs with vastly more computing power than what was available on Apollo-program spacecraft.
As the battle for increased fuel mileage and reduced emissions continues, expect further development and use of the CVT. But don't look for one hooked up to a heavy duty pickup or 400-horsepower sports car any time soon.
