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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.

CVT Torque Converter

Due to the linear nature of the CVT, the torque converter is able to partially lock-up at approximately 9 km/h then lock up completely at 24 km/h, and remain locked as the gear ratio changes.

It is not unusual for a CVT equipped vehicle to have better fuel efficiency than similar vehicle equipped with a manual transmission.

Common Operating Characteristics

1. Droning Noise

Droning Noise

As an engine rises in RPM the sound produced varies in frequency. When at a steady higher RPM during acceleration, the frequency remains constant and may resonate.

2. Whining Noise at Certain Speeds

Whining Noise at Certain Speeds

All engine power is transferred through small pins on the side of the chain and squeezed between the Variator’s at over 145,000 psi. With the small contact area, high pressure, and somewhat bell shaped Variator’s an unfortunate high pitched whining noise may be generated.

A traditional automatic transmission has heavy gears with a large contact area. Noise is created by these gears, however at a lower frequency.

3. Engine Revving High – Moderate to Heavy Acceleration

Engine Revving High – Moderate to Heavy Acceleration

When under moderate to heavy acceleration the Transmission Control Module (TCM) continuously adjusts the gear ratio to maintain an engine RPM that produces the most torque and efficiency, and may maintain this RPM for quite some time. Once a steady speed is met, the TCM adjusts to the highest gear ratio possible to reduce fuel consumption and emissions.

This characteristic takes some time to become accustomed to and may lead you to believe the transmission is slipping or that the engine RPM does not change in direct relation to accelerator input.

4. Engine Revving Higher When Cold

4. Engine Revving Higher When Cold

The optimal operating temperature for power output, fuel efficiency, and reduced emissions for Boxer Engines ranges from 84 °C to 100 °C.

The Catalytic Converter which is the main exhaust emissions control device also require a minimum temperature of 300 °C before it can operate efficiently.

The Engine Control Module (ECM) and Transmission Control Module (TCM) work together to bring the engine and catalytic converter up to operating temperature as quickly as possible by using a lower gear ratio to increase engine RPM.

The Climate Control Module also transmits the desired interior temperature to the ECM, which may also increase the engine RPM to meet customer demands.

Note: You may become concerned the engine is revving too high when cold based upon past experiences. The ECM is in constant control of this RPM and ensures it is suitable based upon temperature conditions.

5. Perceived Lack of Acceleration

5. Perceived Lack of Acceleration

If you are accustomed to conventional automatic transmissions, you may equate acceleration with the sound or vibrations from the rise and fall of engine RPM and the shock felt while the transmission upshifts.

A Subaru equipped with a CVT will generally raise the engine RPM to a certain level based upon accelerator input and engine load. The steady engine RPM and lack of shift shock are contrary to what you may have been accustomed to and will take some time to adjust to.

The design of the CVT also allows for a lower gear ratio than conventional automatic transmissions and can therefore accelerate quicker than a comparable vehicle equipped with a conventional automatic transmission, although it may not feel this way.

6. Perceived Lack of Acceleration When Overtaking Another Vehicle

6. Perceived Lack of Acceleration When Overtaking Another Vehicle

For vehicles equipped with a conventional automatic transmission upon pressing the accelerator the transmission control module will request a reduction of engine torque from the engine control module before shifting to a lower gear. Upon shifting to the lower gear the torque reduction request is cancelled and upon resuming a shock can be felt and the engine RPM will begin to rise within about 1 second. The driver will physically feel the downshift and hear the change in engine RPM. These sensations are what we have become accustomed to in both manual and automatic transmissions.

A CVT is able to transition from the highest gear ratio to the lowest gear ratio within 5 milliseconds. A CVT does not require a reduction in engine torque. It will simply change the gear ratio to match the accelerator input, the engine RPM will increase accordingly and no shift shock will be felt.

Without the physical perception of the downshift, a CVT driver may perceive this as a lack of acceleration, when in fact the CVT equipped vehicle is accelerating without any delay.

7. Shift Shock Heavy Acceleration

7. Shift Shock Heavy Acceleration

To impart a sportier feel on certain vehicles equipped with a CVT, under heavier acceleration the transmission control module will simulate specific gear ratios and a feeling of shift shock, similar to using the manual shift controls available on a CVT equipped Subaru.

8. Shift Shock Upon Deceleration

8. Shift Shock Upon Deceleration

When decelerating over a longer distance such as a highway off ramp the CVT will keep the torque converter locked-up as long as possible to provide engine braking similar to a manual transmission equipped vehicle. When the torque converter unlocks it may feel similar to a conventional automatic transmission down shifting.

9. Hissing or Squealing Sound

9. Hissing or Squealing Sound

You may hear hissing or squealing type sound for a few seconds after shifting into any gear, the noise is caused by the transmission fluid inside of the CVT and is a normal operating characteristic.

This noise is most noticeable when outside the vehicle alongside of the right or left front fender in colder temperatures. The CVT requires fluid pressures up to 1,200 psi to operate and it must pass through small passageways, which may result in a momentary hiss or squeal sound.

This noise does not affect transmission performance, function or reliability.

10. Delay from Reverse to Drive/Drive to Reverse

10. Delay from Reverse to Drive/Drive to Reverse

It is not uncommon for a vehicle operator to place the vehicle into Drive while the vehicle is still moving backwards. In a traditional automatic transmission, the shock of this directional change is passed through solid gears and absorbed by the torque converter.

For a CVT equipped vehicle, the Chain is not solidly locked to the Variator’s, and a sudden shock such as this could possibly cause it to slip, damaging both it and the Variator’s, so the vehicle must come to a complete stop before a change in direction can be achieved.