Continuously Variable Transmissions
by Jim Kerr
Continuously Variable Transmissions, or CVT's, as they are called in the trade, are used in snowmobiles, some all terrain vehicles (ATV's), and a few automobiles. A continuously variable transmission works just like it sounds. It continuously varies the gear ratios between the engine and the final drive. There is no need to select different gears: just place the shifter in forward and away you go.
Why use a CVT rather than a conventional automatic or manual transmission? The answer is efficiency; engine efficiency. With a CVT, the engine goes from an idle to a pre-programmed rpm immediately so the engine input is constant, and the transmission varies the output speed for smooth, seamless acceleration. Keeping the engine at a constant rpm allows the engineers to optimise ignition timing, camshaft design, and manifold tuning for excellent volumetric efficiency and low emissions.
Other reasons for using CVT's include simplicity of design, and smooth power application to the ground. The simplicity part comes by using fewer parts. Vehicles with reverse gear obviously have more parts than a simple forward drive, but there are still fewer parts and gears to manufacture.
The smooth power application of CVT's is useful in off-road vehicles. Power can be applied without any jerks or surges that could cause the vehicle to lose traction on steep climbs or loose terrain.
In this cutaway view of a General Motors CVT, you can see the hydraulically controlled variable width pulleys and the metal drive chain.
So how do they work? The two most common systems use a belt or a chain. Let's look at snowmobiles first. They use a belt. On a snowmobile there are two pulleys connected by a rubber drive belt. Both pulleys are "V" shaped and the width of the "V" can be varied. The drive pulley is connected to the engine and the width of the pulley is controlled by engine speed. At idle, the pulley is wide and the belt is not gripped. Push on the throttle, and the governor causes the pulley sides to move together. When engine speed reaches it's maximum, the pulley continues to become narrower as vehicle speed increases, causing the drive belt to climb up and keeping the engine at its peak power.
The driven pulley also varies its width, but its main job is to keep the slack out of the belt. The driven pulley halves are spring loaded to force them together, and as the drive pulley closes up, the belt climbs up the drive pulley causing the belt to tighten, forcing the driven pulley apart. The variable width of the pulleys and the variable distance of the belt from the centre of the pulleys provide the continuously variable drive.
Automobiles with CVT's use a drive chain instead of a rubber belt. The chain is stronger and more durable on a much heavier vehicle. Although much more complex, automotive CVT's use variable width pulleys just like snowmobiles. Hydraulics and computer controls are used to vary the width of the drive pulley.
They say you can't push a chain, but that is exactly what happens inside automotive CVT's. The flexible "chain" looks more like a squashed slinky toy than a conventional chain. It bends easily around the pulleys but locks into a solid drive when torque is applied to it on the drive side of the pulley. The chain "pushes" the car! This special chain is the magic part of CVT operation.
Several manufacturers offer CVT's, but they are not commonly found in Canada. Honda, Subaru, General Motors, and Nissan are some manufacturers that build automotive CVT's.
This is the Honda Rubicon's computer controlled variable angle plate that varies the output of the hydraulic motor and therefore the speed of the machine. The dimpled plate is supported by a bearing and turns with the motor.
The Rubicon CVT swash plate is turned by the motor to operate the pump plungers.
The output end of the Rubicon CVT is a variable displacement hydraulic motor with 9 plungers. Seven pump plungers are located on the opposite end of the assembly. The control valves can be seen in the background.
Recently, I ran across a new type of CVT. Designed by Mr. Honda himself, it was offered in Japan on a small Honda motorcycle in the early 1960's. It was ahead of its time. Seven years ago, the Honda engineers pulled it back off the shelf and applied modern technology to put it back into production. Now it is featured on a Honda ATV: the Rubicon.
This design uses no belts, chains, or pulleys. It operates by hydraulics and is so small you can hold the complete assembly in one hand. After seeing it apart, it still took me a couple hours to figure out exactly how it works, but the general principles are easy.
Both the input and output of the transmission are on the same shaft. When the motor runs, an angled swash plate driven by the motor causes plungers in the input side of the CVT to pump oil. The oil flow is controlled by eccentric piston-type valves that direct the flow to the output side of the CVT. The output side is essentially a hydraulic motor and oil pressure acts on plungers to push them outward against an angled plate. As the plungers push outward, they try to move "down" or to the lowest part of the plate, causing the plungers and pump housing to rotate.
The variable part of this transmission is the angle of the plate that the hydraulic motor plungers push against. A computer uses several sensors to determine the best angle for the plate and then operates a small electric motor that changes the plate angle.
In simple terms, this CVT uses the motor to drive a pump, which in turn drives a variable displacement motor. The neat part about it is how compact everything is!
The Honda hydraulic CVT is only available on the Rubicon ATV right now, but it wouldn't surprise me to see it show up on a small Honda car in the future. For many years, continuously variable transmissions were a novelty. Modern electronic controls and new metallurgy now enable them to become viable alternatives for older "shifty" transmissions.
Jim Kerr is a master automotive mechanic and teaches automotive technology. He has been writing automotive articles for fifteen years for newspapers and magazines in Canada and the United States, and is a member of the Automotive Journalist's Association of Canada (AJAC).