What is a Torque Converter [and How Does it Function]?

Have you ever noticed that your car goes from being able to make it a week on a tank of gas to barely making it through two days? Has your car ever broken down and been able to go anywhere at all, despite the engine seeming to run fine? During any of these problems, has your mechanic ever brought up a torque converter?

The torque converter is what makes automatic transmission in cars or trucks move. And while they’re an integral part of an automatic vehicle, many people don’t understand how they work. Read on to learn what a torque converter is and how it keeps you rolling down the road. 

What Is Torque

Before we get into the idea of a torque converter, let’s take a quick look at what torque is. In simplest terms, torque is the potential energy you create when you twist something up. The wind-up toys you used to play with as a kid and the cars that roll forward after you pull them back both work on torque force.

In cars, the turning of the engine’s crankshaft produces torque. This is what allows you to accelerate your car. The more torque your engine produces, the faster it goes.

Importance of a Torque Converter

The torque converter is what transmits that torque from the engine to a rotating driven load. In an automatic transmission car, the torque converter connects the power source to the load.

Anatomy

Torque converters are comprised of five main components: the impeller, the turbine, the stator, a clutch, and the fluid. The stator is what makes a torque converter a torque converter; without the stator, it’s just the fluid coupling.

The impeller is a piece with tilted blades that look somewhat like a fan. This piece is turned mechanically by the engine. As it spins, the impeller pushes transmission fluid through its blades; the faster it goes, the faster the fluid moves.

When the fluid leaves the impeller, it moves into the turbine, a nearly identical bladed piece that sits opposite the impeller. The fluid hitting the angled blades of the turbine causes the turbine to begin turning, which turns the transmission shaft and pump in your car. The fluid is redirected through the center of the turbine, where it hits the impeller again.

This is where the stator comes in; the stator sits in the center of the torque converter. This is another series of fan type blades that are angled so that when the transmission fluid flows into them, it reverses direction again. The stator keeps the transmission fluid, which is turning in the opposite direction of the engine, from hitting the converter housing and slowing it down.

A torque converter also has a housing that is attached to the engine along with the impeller. Most torque converters also use a lock-up clutch that locks the impeller and the turbine together at high speeds to increase vehicle fuel efficiency.

Phases

A torque converter works in three phases: stall, acceleration, and coupling. 

During stall, the engine is still turning, as is the impeller. But the turbine cannot turn, so the car does not move. This is what happens when you have your car's engine running, the transmission is in gear, and you have your foot on the brakes so the car isn’t moving.

Acceleration is when that torque multiplication power comes into play. As engine RPM increases, the impeller starts moving faster, which makes the turbine start to move faster. But the impeller is still moving more quickly than the turbine at this point. 

Coupling is what happens when you drive at high speeds. The impeller and turbine speeds are nearly identical at this stage, and this is when some models lock the two together with a friction clutch to increase efficiency. The stator actually mostly stays out of this process, since at high enough speeds, the fluid will be moving in a way that will not risk it hitting the converter housing. 

Efficiency

One of the stator’s biggest jobs is to make a torque converter more efficient. By redirecting the fluid coming off the turbine, the stator can harvest that kinetic energy and put it back into the cycle. This allows torque convertors to multiply torque for greater acceleration.

But torque convertors cannot be 100 percent efficient until lock-up occurs; there are friction and some kinetic energy loss involved in the process. Torque converters are most efficient at very low speeds. Although companies like Buick have played with adding additional turbines to their torque couplers, these models have never been as efficient as the traditional three-part models and have been discontinued.

Common Problems 

There are several common ways a torque convertor may break down, some of which can be dangerous. Continuous high levels of slippage in the converter can cause overheating, which can damage the elastomer seals that keep the transmission fluid in the converter. The fluid will start to leak out, and when the system runs out of fluid, it may stop functioning altogether.

Your stator clutch may also seize or break. During seizure, the inner and outer elements of the clutch can become permanently locked, causing a huge drop in fuel efficiency. If the stator clutch breaks altogether, the stator will free spin and your car may not be able to move under its own power at all.

You may see blade deformation and fragmentation in some cases. In most cases, this will cause the torque converter not to work as efficiently, driving down your gas mileage. In some extreme cases, the convertor may violently self-destruct.

There is a lot of pressure and hot fluid moving around inside the housing of a torque convertor. In some cases, that pressure can get too high and cause the housing to balloon or even burst. If the housing ruptures, you’re going to be in danger of flying bits of shrapnel and hot oil. 

Learn More About How Your Car Works

The torque converter is one of the most important and overlooked parts of the car. It’s what allows automatic transmission cars to work, and it’s a big part of what determines your fuel efficiency. Knowing a little about how these pieces work can help you diagnose problems that might otherwise be attributed to the transmission, saving you a lot of money in repairs.