Torque converter cutaway view

More oil is thrown out by centrifugal force, turning the turbine. As a result, the transmission input shaft and vehicle starts to move, but with some slippage. Figure At cruising speeds, the impeller and turbine spin at almost the same speed with very little slippage. When the impeller is spun fast enough, centrifugal force throws oil out hard enough to almost lock the impeller and turbine.

After the oil has imparted its force to the turbine, the oil follows the contour of the turbine shell and blades so that it leaves the center section of the turbine spinning counterclockwise. Because the turbine has absorbed the force required to reverse the direction of the clockwise spinning of the oil, it now has greater force than is being delivered by the engine. The process of multiplying engine torque has begun. Torque multiplication refers to the ability of a torque converter to increase the amount of engine torque applied to the transmission input shaft.

Torque multiplication occurs when the impeller is spinning faster than the turbine fig. For example, if the engine is accelerated quickly, the engine and impeller rpm might increase rapidly while the turbine is almost stationary. This is known as stall speed. Stall speed of a torque converter occurs when the impeller is at maximum speed without rotation of the turbine.

This condition causes the transmission fluid to be thrown off the stator vanes at tremendous speeds. The greatest torque multiplication occurs at stall speed. When the turbine speed nears impeller speed, torque multiplication drops off. Torque is increased in the converter by sacrificing motion.

The turbine spins slower than the impeller during torque multiplication. If the counterclockwise oil were allowed to continue to the center section of the impeller, the oil would strike the blades of the pump in a direction that would hinder its rotation and cancel any gains in torque.

As the oil strikes the turbine blades, it imparts a force to the turbine, causing it to turn. Figure shows the torque converter in the coupling stage. When the engine is idling and the converter is not spinning fast, the force of the oil is not great enough to turn the turbine with any efficiency. This allows the vehicle to stand in gear with the engine idling.

As the throttle is opened and the pump speed is increased, the force of the oil increases and the engine power is more efficiently transmitted to the turbine member and the gear train. After the oil has imparted its force to the turbine, the oil follows the contour of the turbine shell and blades so that it leaves the center section of the turbine spinning counterclock-wise.

As shown in the figure below, there are four components inside the very strong housing of the torque converter:.

The housing of the torque converter is bolted to the flywheel of the engine, so it turns at whatever speed the engine is running at. The fins that make up the pump of the torque converter are attached to the housing, so they also turn at the same speed as the engine. The cutaway below shows how everything is connected inside the torque converter. The pump inside a torque converter is a type of centrifugal pump. As it spins, fluid is flung to the outside, much as the spin cycle of a washing machine flings water and clothes to the outside of the wash tub.

As fluid is flung to the outside, a vacuum is created that draws more fluid in at the center. The fluid then enters the blades of the turbine , which is connected to the transmission. The turbine causes the transmission to spin, which basically moves your car.

You can see in the graphic below that the blades of the turbine are curved. This means that the fluid, which enters the turbine from the outside, has to change direction before it exits the center of the turbine. It is this directional change that causes the turbine to spin.

In order to change the direction of a moving object, you must apply a force to that object -- it doesn't matter if the object is a car or a drop of fluid. And whatever applies the force that causes the object to turn must also feel that force, but in the opposite direction. So as the turbine causes the fluid to change direction, the fluid causes the turbine to spin.

The fluid exits the turbine at the center, moving in a different direction than when it entered. If you look at the arrows in the figure above, you can see that the fluid exits the turbine moving opposite the direction that the pump and engine are turning. If the fluid were allowed to hit the pump, it would slow the engine down, wasting power.

This is why a torque converter has a stator. The stator resides in the very center of the torque converter. Its job is to redirect the fluid returning from the turbine before it hits the pump again. This dramatically increases the efficiency of the torque converter. The stator has a very aggressive blade design that almost completely reverses the direction of the fluid. A one-way clutch inside the stator connects the stator to a fixed shaft in the transmission the direction that the clutch allows the stator to spin is noted in the figure above.

Because of this arrangement, the stator cannot spin with the fluid -- it can spin only in the opposite direction, forcing the fluid to change direction as it hits the stator blades. Something a little bit tricky happens when the car gets moving. There is a point, around 40 mph 64 kph , at which both the pump and the turbine are spinning at almost the same speed the pump always spins slightly faster.

At this point, the fluid returns from the turbine, entering the pump already moving in the same direction as the pump, so the stator is not needed. Even though the turbine changes the direction of the fluid and flings it out the back, the fluid still ends up moving in the direction that the turbine is spinning because the turbine is spinning faster in one direction than the fluid is being pumped in the other direction.