CVT: a simple alternative to the automatic transmission

Simplicity rules in a CVT ... one end is connected to the engine, the other to the drive wheels

Published: March 22, 2012, 3:00 PM
Updated: August 4, 2015, 3:17 AM

As bicyclists know well, gears are needed to provide the torque multiplication necessary to move a device efficiently. In an automatic transmission, that is done through use of a torque converter plus a toothed gearset, which comes at the expense of increased cost and reduced efficiency and fuel economy.

In the auto industry we’ve seen everything from two-speed automatics to 18-speed manuals for the big rigs.

The manual transmission continues to be the model of efficiency in terms of power loss and expense. It also continues to sink slowly into oblivion, at least in the minds of North American consumers, fewer than 10% of whom chose it when purchasing a new vehicle, and we suspect the vast majority of even those would prefer an automatic if it didn’t cost $1,000 more.

Automatic transmissions have grown in popularity with five and even six speeds becoming more common in the search for that last bit of efficiency.

But the disadvantages are still there: power loss, efficiency, complexity and cost.

Another method of "shifting" gears is resurfacing after more than a century in relative oblivion: the CVT or Continuously Variable Transmission. First invented by German engineers Gotlieb 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 115 years.

In the constant drive for better fuel mileage and lower emissions CVTs are once again gaining favour because they allow an engine to operate more efficiently by varying gear ratios instead of engine speed. If, for example an engine’s peak operating efficiency is 2,758 rpm the CVT allows the engineers to maintain that speed regardless of vehicle speed or load.

Another advantages are simplicity and a

s a low-cost alternative to conventional transmissions.

The CVT is a very simple device, consisting of only pulleys and a belt. The attendant shafts and related controls are already in place in the vehicle.

Disadvantages include the inability to handle larger power inputs, size and weight.

This is one of those cases where simplicity rules. 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 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 operated by oil pressure controlled by the ECU or engine control unit.

The computer 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, rather than only a few and the simplicity of the system. The problem is that of the engine producing more torque than the belt can handle.

Think of it as constant speed drive using an infinite number of ratios, from bull low to overdrive. 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, all without a single shift.

Although CVTs have been around for more than a century, used mostly to run power tools and other low-load devices (like snowmobiles), they have seen limited use in automobiles. Subaru was the first to offer one in a production car, in 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 CVTs has been their inability to handle torque. Belt slippage caused when engine torque exceeds 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. Some companies are using multi-link steel chains and all current CVT systems take advantage of ECUs with vastly more computing power than available previously.

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 in a heavy-duty pickup anytime soon.