A torque converter takes the place of a clutch in cars with an automatic transmission. Unlike a manual clutch, it uses fluid to transmit mechanical power from the engine to the transmission. While a manual transmission has a flywheel and clutch discs, automatic transmissions have a flexplate and torque converter. The torque converter is located between the flexplate and the transmission itself.
What Does a Torque Converter Do?
A torque converter uses a fluid coupling to transfer engine power to the transmission. Torque is also multiplied to avoid the loss of power that comes with fluid transfer. There isn’t a mechanical interruption, unlike the clutch system in a manual transmission. This means seamless gear changes that don’t require any input from the user.
To keep the power output of the engine in the optimal range, the torque converter multiplies torque at low RPM. This is required to have enough power for acceleration.
While a car is in operation, the engine must always be running. However, when the vehicle stops, the wheels, axles, and driveshaft are no longer spinning. The engine’s crankshaft must be allowed to continue to spin while the driveshaft is stationary. A torque converter allows the engine and transmission to rotate independently of one another. This keeps the vehicle from stalling when it's at a stop.
Torque Converter Components
The major components of a torque converter are the impeller, the turbine, and the stator. There’s also the fluid itself, and often a lock-up clutch. All are contained within the housing of the torque converter, which connects to the engine’s flexplate (sometimes called the flywheel).
Impeller - The component that is connected to the engine. The engine’s crankshaft turns the flexplate, which turns the entire torque converter housing. The impeller is welded to the torque converter housing. Therefore, the impeller always rotates at the same speed as the engine when it’s running. The impeller is also known as the pump.
Turbine - The turbine receives the force created by the impeller and mirrors its movement. When the turbine spins, it drives the input shaft of the transmission. Unlike the impeller, the turbine can spin freely at any speed. It doesn’t spin at all when the vehicle is stopped.
Stator - The stator sits between the impeller and turbine. It’s the component that actually makes the device a torque converter. Otherwise, it would just be a regular fluid coupling.
The stator receives fluid from the turbine and flings it back towards the impeller, multiplying the amount of torque produced. The torque multiplication is especially powerful when the impeller and turbine are spinning at very different speeds.
Fluid - The torque converter housing is filled with transmission fluid. This fluid is what transfers the energy between the impeller, turbine, and stator.
Lock-Up Clutch - A lock-up clutch locks the impeller and turbine together at high speeds so they spin at the same rate. This makes the system more efficient by getting rid of the natural loss of torque that happens with a fluid coupling. When the vehicle slows down again, the lock-up clutch will disengage. The system goes back to being a fluid coupling only, preventing the engine from stalling.
How Does a Torque Converter Work?
To understand how a torque converter works, start by picturing two fans sitting face-to-face. One fan, the impeller, is powered because it’s connected to the engine. The other fan, the turbine, is not plugged in because it’s connected to the transmission. When the plugged-in fan spins, the other fan rotates as well. The same effect is happening inside a torque converter, but it’s the hydraulic fluid that moves the fans instead of air. A simple step-by-step overview of this complex process is outlined below.
- The engine turns the crankshaft. The crankshaft is always spinning when the engine is on.
- The crankshaft spins the flexplate, which is attached to the torque converter housing.
- The impeller spins. Because the impeller is welded to the torque converter housing, it spins as well.
- The impeller distributes hydraulic fluid across the housing to the turbine. It does so using centrifugal force. Just like a carnival ride that spins and pushes you against the outside wall, the impeller spreads fluid to the outsides of the turbine. This increases the effectiveness of the rotational force.
- The turbine begins to spin. The turbine is connected to the driveshaft, which turns the axles and wheels.
- The turbine sends fluid back towards the impeller as it spins.
- The fluid passes through the stator in the center of the torque converter. The stator multiplies torque as it returns to the impeller.
- The impeller spins faster as it receives fluid force from the stator. The entire cycle is repeated as the engine RPM increases, allowing for acceleration.
How Does a Torque Converter Get Power to the Wheels?
Once the turbine is spinning, the power is sent through the transmission to the wheels. When the transmission upshifts, the torque necessary to keep the gears moving increases. In other words, as the gears go up, the turbine gets “heavier” to move. The torque converter helps handle this discrepancy by multiplying torque.
When the turbine (transmission) catches up with the impeller (engine) speed, the lock-up clutch is engaged. This keeps them spinning at the same rate, increasing efficiency.
When the vehicle comes to a stop, the engine is still running and the impeller still spins. However, with the brakes applied, the turbine is held in place so it doesn’t spin. This keeps the engine from stalling while the car is at a stop.
The impeller is always spinning while the transmission is in “Drive.” For this reason, taking your foot off the brake in an automatic car will cause it to “creep” forward, even without pressing the gas. Some modern vehicles prevent this by automatically applying brakes until the gas pedal is pressed.
Do All Automatic Cars Use a Torque Converter?
No, not all cars known as “automatics” use a torque converter. That’s because some transmissions, such as dual-clutch automatics, are mechanically closer to manual transmissions. All dual-clutch automatics use physical clutches instead of a torque converter. The exception is Honda’s 8-speed DCT, which uses both a torque converter and a clutch.
CVTs, or continuously variable transmissions, generally use a torque converter. But some types do use a clutch instead.
What Causes a Torque Converter to Fail?
A torque converter failure is often the product of bad transmission fluid or a fluid leak. The transmission fluid should naturally flow through the torque converter. If it doesn’t, power can’t be transferred efficiently from the engine to the transmission.
Common symptoms of a bad torque converter can include shuddering, overheating, or slipping of gears. It may take a trained mechanic to properly isolate and diagnose the issue.
How Much Does It Cost to Replace a Torque Converter?
A torque converter can cost around $300-$600 to replace. The total amount will be closer to $1,000 if labor is included.
Performance torque converters designed to handle race car amounts of power will be closer to $1,000 just for parts.
The Future of Torque Converters
Though torque converters are an incredible invention and important for many automatic cars, manufacturers are shifting to dual-clutch transmissions. These have become cheaper over time and provide quicker shifts. Even non-performance economy cars now frequently feature dual-clutch automatics.
Even though automatic transmissions are no longer the much-derided “slush boxes” of the past, torque converters may still be on the way out. Manufacturers like BMW have switched out torque converters for electric motors in some cases. Plus, electric cars don’t have gears at all. While they’re not extinct by any means, it’ll be interesting to see how the technology changes in the next decade.
Sources: SAE | AAMCO | Learn Engineering | Car and Driver