How does a torque convertor transmission work in automobiles? There is a combustion in the engine, a crankshaft, that converts it into a rotary moment, so where does this rotary moment go and how are the shifts changing? Obviously, ATF running somewhere, along where? Doing what?
In: Engineering
Imagine you have an electric fan. The blades spin around and around, pushing air.
Now imaging you have a toy pinwheel. What would happen if you put the toy pinwheel in front of the fan? It would turn.
Now imaging that the toy pinwheel, instead of just spinning around, was connected via complicated system of gears and shafts to some wheels. Now, when the pinwheel turns, the wheels will turn.
But, and this is the important bit, if the pinwheel doesn’t turn, the electric fan can continue to turn.
Now imagine that instead of an electric fan, your fan is connected to a gasoline motor. And imagine that instead of pushing air, it is pushing automatic transmission fluid. And the complicated system of gears and shafts is the automatic transmission.
There are complications, of course. Some automatic transmissions don’t have a complicated system of gears and shafts, but a complicated system of belts and cones, and some have a complicated system of gears and electric motors. And so on.
The crank shaft is connected to a turbine in the torque converter, which is a donut-shaped metal case filled with ATF. The drive shaft is connected to a second turbine in the torque converter, but the two aren’t physically connected. At low RPM the engine output just spins freely in the fluid, but at higher RPM the drive shaft turbine gets driven by the spinning of the transmission fluid.
Torque converter is not a transmission, it replaces the clutch in a manual transmission.
In a manual transmission you disengage the clutch before changing gears so the input and output shaft of the gearbox downstream of the clutch is not loaded and gears can change smoothly without grinding against each other. The clutch also provides a gradual friction engagement to not stall the engine when the car starts to move.
To achieve the same functions, the torque converter has a pump wheel driven directly by the engine, and a turbine wheel driven by fluids agitated by the pump wheel. The turbine wheel turns the input shaft of the downstream gearbox.
The shell of the torque converter also has a spline or key to go into the transmission to turn the transmission oil pump, in this way the transmission is always ready for use as long as the engine is turning.
When the vehicle is in neutral or park, the input and output of the transmission isn’t connected at all, so the engine turns the converter pump wheel, the fluid agitated by the pump wheel turns the turbine wheel, the turbine wheel turns the input shaft of the transmission, which turns nothing.
When the gear engages in Reverse or Drive, the transmission engages in gear, the input shaft turns the gears and drives the output to the wheels. At this point the car isn’t rolling yet, so everything that is eventually connected to the wheels STOP, including the turbine wheel, but the turbine wheel wants to move because the pump wheel keeps agitating fluids and tries to turn the turbine wheel, the fluid is, well, fluid, it isn’t solid so it allows speed mismatch between the pump and the turbine. That’s why the car tries to move when you shift into drive, but can stop without stalling the engine.
There is also a guide vane stator in between the pump and the turbine, this stator redirects the agitated fluid so it drives the turbine better. When the turbine is stopped, the reaction of the fluids causes the stator to turn backwards, which is stopped by the one-way clutch in the stator, in this way the stator acts as a fixed “fulcrum“ for the fluids, increasing efficiency during startups.
Once the turbine starts spinning up to speed, the stator would cause drag if it does not follow up the forward spinning speed, so the one way clutch releases and allows the stator to spin forward freely, reducing drag.
Once the car is up to cruising speeds and shifted into higher gear, the engine outputs significant power, the losses due to the torque converter being non-solid during power transfer is also significant, maybe 10%-20%. To improve efficiency, a lock-up clutch rigidly connects the turbine and the pump, so the torque converter becomes a full passthrough, transmitting basically 100% of the power without any slip losses.
In older cars the converter lock up clutches only engage at 3rd or 4th gear for a 4AT and above 40-50mph, because these older gearboxes shift very rough at lower gears, so they use the converter to absorb the rough shifting. Modern gearboxes are much smarter and smoother when shifting, so the converter can engage lock-up as low as about 15mph, making it highly efficient almost comparable to a regular manual gearbox.
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