Most of the modern vehicles are equipped with internal combustion engines (ICE). One disadvantage of the ICE, compared with the electric motor, is that it can not start under load and it needs an external starting device (electric starter). Therefore, to avoid engine stopping while the vehicle is stationary, we need to disengage the engine from the wheels.
On a vehicle with manual transmission (MT), the engine disengagement can be done in two ways:
- by pressing the clutch pedal
- by selecting Neutral with the gear-shift lever
On a vehicle with automatic transmission (AT), the engine disengagement from the transmission is done automatically, without the intervention of the driver. This is possible thanks to the torque converter‘s work principle.
The torque converter is placed between the internal combustion engine and the gearbox. An automatic transmission, inside its case, contains three main parts: the torque converter, the epicyclic (planetary) gearbox and the electro-hydraulic control module.
The ICE crankshaft is mechanically connected to the torque converter. Inside the torque converter, the engine power is transferred to the gearbox hydrodynamically. When the torque converter is not locked, there is no mechanical connection between the input (engine) and the output (gearbox).
In order to understand better how a torque converter works, let’s discuss on the following example. If you have two electric desktop fans, placed on in front of the other (like in the image below), with one of them powered, what is going to happen?
The left fan is powered with electric current from the grid. While spinning it will generate an axial flow of air. The air flow will hit the right fan (not powered), which will start to rotate. Power is transferred from the left fan to the right fan through a working fluid (in this case is air). Obviously, the efficiency of this system is very low since a lot of air will be dissipated around the blades of the right fan.
The same principle applies to the torque converter, with some differences. In the case of the torque converter, both “fans” are very close together, in order to minimize the power loss. The working fluid is a liquid (AT oil). Also, between the two “fans”, there is another component, which redirects the fluid flow in order to minimize the losses and amplifies the transferred torque.
The “fan” which is generating power is called impeller and it’s connected mechanically to the engine crankshaft. The “fan” receiving the hydraulic power is called turbine and it’s connected mechanically to the input shaft of the gearbox. Between the impeller and the turbine is a stator, which redirects the oil flow. The volume created by these components is filled with oil.
When the ICE is idling, the impeller rotation is “throwing” the oil into the turbine. Since the engine speed is low, the kinetic energy of the moving oil is not enough to power the vehicle. There is a small amount of torque being transferred, this torque being called drag torque.
The drag torque increases if the oil viscosity increases (at low temperature). The drag torque makes the vehicle to “creep“. This meas that, when the shift selector is in drive (D), with both accelerator and brake pedal released, the drag torque is slightly moving the vehicle. If the driver presses the brake pedal, the vehicle will stop because the drag torque is insignificant compared to the braking torque at the wheels.
When the driver pushes the accelerator pedal, the engine speed will increase. The impeller will rotate faster and increase the kinetic energy of the oil. The turbine will receive more energy which will result in a higher torque transferred to the gearbox.
In the schematic above we can distinguish easily between the components of the torque converter. The impeller (green) is connected to the engine and the turbine (yellow) to the gearbox input shaft. The stator (blue) as the name suggests is most of the time static (fixed).
The oil flow movement in the torque converter has two components:
- revolution, around the center axis together with the impeller and turbine
- rotation (red arrows), around the radial center of the torque converter
The rotation movement is the transition of the fluid from impeller to turbine, to stator and back to impeller.
Between the impeller and the turbine there is a permanent slip. This means that they rotate with different speeds. The ratio between the speed of the turbine and the speed of the impeller is called the speed ratio of the torque converter. The speed ratio is 0 when the turbine is static and the impeller rotates, and 1 when both rotate at the same speed.
The torque converter has also a torque ratio. This is the ratio with which the input (engine) torque is multiplied before being transferred to the gearbox. The maximum value of the torque ratio (around 2.3 – 3.0) when the speed ratio is 0.0 and the minimum (1.0) when the speed ratio is above 0.85 – 0.9.
The stator is fixed as long as there is significant slip between impeller and turbine. When the speeds are close to each other, when the speed ratio is around 0.85 – 0.9, the direction of the fluid changes and the stator begins to rotate as well. This is possible because the stator is mounter on a on-way mechanism.
The torque converter has also an efficiency, which is quite low. Because it has permanent slip, there is a lot of friction between the working fluid (oil) and mechanical components (impeller, turbine and stator).The efficiency is minimum (below 10%) when the speed ratio is near 0, and peaks 85 – 90 % when the speed ratio is around 0.85.
In order to improve the efficiency of the torque converter, when the slip between the impeller and the turbine is relatively small, the torque converter is locked. This is possible by using a lock-up clutch, which links mechanically the impeller with the turbine. This way there is no more friction between the oil and components, and engine power is mechanically transferred to the gearbox.
The torque converter is locked usually in the higher gears (above 2nd), or when the vehicle speed is above 20 kph. When the gearbox is performing a gearshift, the lock-up clutch is put in a slipping state to help damp the driveline oscillations.
The torque converter is the default coupling device in most of the epicycloidal automatic transmissions (AT) as well as in some continuously variable transmissions (CVT). The main characteristics of the torque converter being automatic engine disconnection from the driveline at low engine speeds, torque amplification and vibration damping (due to hydrodynamic power transfer).
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