CVT Common Operating Characteristics

CVT Operation

A Continuously Variable Transmission (CVT) is an automatic transmission that doesn’t use gears, instead relying on a chain and pulleys to transfer power to the driven wheels. As it has no physical gears or fixed gear ratios it provides an advantages in terms of acceleration, smoothness, fuel efficiency and eco-friendliness. The Lineartronic^® CVT has proven so well suited to the SUBARU BOXER engine and Subaru symmetrical full-time All-Wheel Drive, it has replaced most traditional automatic transmissions in the Subaru line-up.

To understand the operating characteristics of a CVT we need to understand why CVTs were developed and what they are designed to accomplish. The following sections are meant to describe the relationship between an engines power output, gear ratios, and their effect on fuel efficiency.

Engine Power, Torque & Efficiency

Power is the speed at which energy (Gasoline) is converted to mechanical forces necessary to propel the vehicle. In an engine such as the Subaru Boxer Engine the power from the rotational force of the engine’s crankshaft is calculated as a function of the engine torque and engine speed. In simple terms, torque describes how much work an engine can do and horsepower describes how quickly that work can be done.

Engines are designed to work over a wide range of engine speeds (RPM) however the torque and power output do not remain constant. Within this wide range of RPM, maximum torque is achieved at the RPM the engine is able to move the maximum amount of air and fuel in and out of the engine. This is also the point at which the combustion of air and fuel is most efficient, resulting in lower exhaust emissions.

Because power is the speed at which energy is converted, more power or higher engine RPM generally means higher fuel consumption, with some exceptions in a vehicle equipped with a CVT.

Traditional Transmission Design

When accelerating from a stop, a vehicle’s transmission uses lower gear ratio’s to allow for acceleration and once a steady speed is achieved, the transmission uses a gear with a much higher ratio to keep the vehicle in motion.

There is a compromise between the lowest gear ratio which improves acceleration and the highest gear ratio which improves fuel economy.

An ideal transmission would have a very low first gear ratio and a very high final gear ratio. However, the further apart the first gear ratio is from the final gear ratio the less responsive the vehicle is at all speeds in between.

It is possible to increase the number of gear ratios to overcome these design limitations, however in doing so the transmission becomes more complex, heavier, and contains more components which also increases opportunities for a component to fail.

Traditional Automatic Transmissions

Under acceleration the engine on a vehicle equipped with a conventional automatic transmission will rise in RPM starting below the range of maximum torque and then pass above it. When upshifting to subsequent gears the same will occur once more. There is a compromise between the lowest gear ratio which improves acceleration and the highest gear ratio which improves fuel economy, in an attempt to keep the engine RPM in this range of efficiency.

When the transmission computer determines a new gear ratio is required the engine computer reduces the power output of the engine to reduce “Shift Shock” and possible damage to the transmission and resumes once the gear change is complete. This downshifting can be both heard and felt.

The reduced power output and delay actually delays acceleration, but we have become accustomed to these sensations and equate them with increased acceleration.

Automatic Transmission Torque Converters

A manual transmission has a clutch that mechanically locks the engine output power to the transmission, while a traditional automatic transmission has a torque converter which uses hydraulic fluid to transfer power. The torque converter is not capable of transferring all of the input power and may lose up to 10% which increases fuel consumption along with transmission fluid temperatures, requiring the fluid to be replaced more often.

To overcome these losses torque converters are equipped with a lock up clutch that can mechanically lock the engine output to the transmission under certain conditions. The lock up clutch is not able to withstand the forces associated with shifting gears and generally does not lock up until the transmission is in the highest gear and above 60 km/h. This is why fuel efficiency is usually higher on a similarly equipped vehicle with a manual transmission.

Subaru Continuously Variable Transmission

CVTs differ from traditional automatic transmissions in that they do not have gears that provide "steps" between low and high speed operation. Instead, the CVT uses a pair of variable diameter, cone-shaped pulleys connected by a steel chain, which transfers all engine power. The variable diameter cones in a Subaru CVT are referred to as Variator’s with one considered the input and the other the output. To change the effective gear ratio, the width of these pulleys changes, causing the chain to ride higher or lower resulting in a different gear ratio.

Because of this design simplicity, CVTs offer a number of advantages over traditional automatic transmissions. In addition to fuel economy and reduced weight, they offer steady acceleration, smooth operation, and the ability to adapt to varying road conditions and power demands to provide a smoother overall ride. With the ability to continuously vary the gear ratio, the CVT is able to have a lower gear ratio for quicker acceleration and a higher final gear ratio for fuel economy when compared to a traditional automatic transmission.

When under moderate to heavy acceleration the transmission control module continuously adjusts the gear ratio to maintain an efficient engine RPM. Once a steady speed is met, the transmission control module adjusts to the highest gear ratio possible to reduce engine RPM resulting in reduced fuel consumption and emissions. Also, without a delay for shifting gears, a CVT equipped vehicle is able to steadily accelerate without interruption.