Torque converter engineering

Just like a manual transmission, cars with automatic transmissions also need a way to let the engine turn while its wheels and gears in transmission halt. For this purpose, manual transmission cars use a clutch, which completely disconnects the engine from the transmission. Since Automatic transmission cars do not have a transmission, they use a torque converter.

A torque converter is a fluid coupling device, which enables the engine to spin somewhat independently of the transmission. When the engine is turning slowly, such as idling at a red light, the amount of torque passed through the torque converter is very small; therefore, to keep the car still only a little pressure on the brake pedal is required.

If you step on the gas pedal while the car is still, you would have to press harder on the brake pedal in order to keep the car from moving. As you step more on the gas, the engine speeds up and starts pumping more fluid into the torque converter, which in turn causes more torque to be transmitted to the wheels.

These devices are sealed units. Inside the torque converter, there are two series of opposite facing curved blades. There is a impeller or fan present which is connected to the engine.

The impeller combined with the front cover forms the outer covering of the convertor. The other fan, or the turbine, is connected to the transmission input shaft. Any motion of the turbine will move the vehicle unless the transmission is in neutral or park. Instead of using air, the space inside the torque converter is normally full of an incompressible transmission fluid, which assists in transmitting the power generated from the engine to the transmission. Not exactly! The torque converter is efficient since its blades are precisely engineered to maximize the energy transmission reducing heat build-up or turbulence.

We can understand the insides of a torque converter by a simple example. Imagine having two fans facing each other. Turning on one of the fans will blow air over the blades of the second fan, causing it to spin as well.

But if you are to hold the second fan still, the first fan will keep spinning nevertheless. The impeller, also called a pump is filled up with fluid, which rotates with the engine crankshaft. A torque converter, like any fluid coupling , is a sealed chamber filled with hydraulic fluid typically light oil and containing a pump or impeller driven by the engine s and a turbine connected to an output shaft.

The impeller is a toroid disc connected to the engine's crankshaft or output shaft of the motor or other power source.

A torque converter differs from a simple fluid coupling by the addition of a stator , a disc with fan-like blades connected to the transmission via a fixed shaft with a one-way clutch that allows it to rotate only in the opposite direction of the fluid's radial motion. Without the stator, fluid leaving the turbine would strike the impeller with a radial motion opposite its rotation, causing a braking effect. With the stator, the returning fluid strikes the stator blades, which reverses the radial direction of the fluid's motion so that it is moving the same direction as the impeller when it reenters the impeller chambers.

This reversal of direction greatly increases the efficiency of the impeller, and the force of the fluid striking the stator blades also exerts torque on the turbine output shaft, providing additional torque multiplication equivalent to a higher numerical gear ratio.

When the engine or power source is operating it turns the impeller at the same speed. The rotation of the radial chambers on the inner surface of the pump imparts a centrifugal radial flow to the fluid in the converter, which causes hydraulic fluid to strike the outer edges of the turbine.

The radial chambers on the surface of the turbine transmit the angular momentum of the fluid centripetally , reversing its direction and exerting a twisting force torque on the turbine disc that causes it to rotate in the same direction as the impeller.

The fluid exits the center of the turbine and returns to the impeller to begin the cycle again. Because some of the kinetic energy imparted to the fluid is lost to friction raising the temperature of the fluid rarther than causing motion within it , the turbine always slips rotates slower than the impeller , particularly at very low speeds.

If the speed of the impeller is very low, such as at idle speed for an automobile engine, the torque exerted on the turbine output shaft will not be enough to overcome the shaft's inertia, allowing the shaft to remain stationary without stalling the engine and eliminating the need for de-clutching. As engine speed increases the speed of the impeller and the turbine become nearly the same reaching their point of minimum slippage.

Because the turbine is spinning faster than the fluid can exit its radial chambers, the net angular momentum of the exiting fluid is in the same direction as the turbine's rotation, rather than opposite it. As the impeller approaches this speed, the torque multiplication provided by the stator decreases. In this way, the torque that is transmitted to the transmission via the input shafts can be measured. The torque converter housing is connected to the flywheel and rotates at the same speed as the crankshaft in the turbine housing.

The impeller or centrifugal pump effectively hurls the transmission fluid into the ribs of the turbine, which in turn rotates or transmits the torque to the transmission. The stator is the barrier that directs the fluid back into the turbine instead of the pump, thereby increasing the efficiency of the system. When the vehicle is idling, the speed at which the transmission oil pumps into the turbine is slow, which means that very little torque is getting through the transmission to the engine.

As the crankshaft spins faster and more momentum spins the flywheel, the fluid moves faster from the pump to the turbine, forcing the turbine to spin faster, allowing more torque through the transmission. It is important to note that the insides of the torque converter are still a mystery. The basic mechanics may be understandable, but the complicated calculations and engineering behind it is best understood with someone with an advanced understanding of fluid mechanics.

Some of the signs of a malfunctioning torque converter are: shudders, dirty fluid, shifting gears at high revs, and strange noises like clicking or purring. Because a torque converter is responsible for converting the engine torque into the hydraulic pressure required to shift gears in the transmission, a damaged rib or bearing can cause the transmission to delay a shift or slip out of gear.

Slippage can also be caused by insufficient or excessive fluid in the transmission. Be sure to check your fluid level before you take your car to a store. If the temperature gauge shows that your car is overheating , it could be a sign that the fluid pressure has dropped and there is a problem with your torque converter.

If a converter overheats, it cannot transfer power from the engine to the transmission. This leads to poor throttle response and excessive wear and tear on the internal functioning of the transmission. Low fluid levels or a defective magnet can also cause the gear unit to overheat.

When the lock-up clutch in the torque converter begins to fail, there may be a shudder at around mph. The feeling is very noticeable and usually feels like you are driving over a bumpy road with lots of small bumps.

When the converter switches to direct drive, a worn lock-up clutch can make the transition difficult, leading to this feeling. The feeling can start and stop abruptly and not last long.