# Physical intuition behind torque converter

A torque converter (also here) is a device used in some cars. It uses several "fans" coupled through a liquid (transmission fluid) in order to perform the function of a clutch, but more importantly it acts as a liquid gear in the sense that it multiplies the torque going from the engine to the wheels.

Is there an intuitive way to explain what is happening in the liquid? In particular, is it possible to explain the torque multiplication effect without resorting to numerical analysis?

• Have you checked out if one of the manufacturers of torque converters provides a better explanation? Those links seem mediocre at best... – Solar Mike May 16 at 18:41
• @solarmike, you want to furnish the answer, or let me take a crack at it? – niels nielsen May 17 at 3:54

A torque converter contains a propeller that is spun by the engine and another close by which is connected to the rest of the transmission. these two propellers face each other in a chamber filled with oil, so that when the engine-driven prop spins, it pumps oil through the second prop and causes it to spin as well.

Between these two props is a disc with blades in it which catches the oil after it has passed through the second prop, and diverts its flow back around to the engine-driven prop which then picks up that return oil and pumps it into the second prop again.

This disc is cleverly designed so the return oil flow strikes the engine-driven prop blades at such an angle that the energy contained in the return flow adds to the energy being pumped into the flow by the engine-driven prop, in an amount proportional to the difference in rotating speed (called the slippage) between the engine-driven propeller and the second prop.

Recovering the energy in the return flow multiplies the torque force exerted on the second prop by as much as ~3X in the high-slip limit where the second propeller is standing still and the engine is running at speed (which is called the stall torque condition)- as they would be when you put your foot on the gas from a standstill.

In the limit where both props are running at roughly the same speed (the no-slip condition), the torque multiplication factor falls to ~1X.

The action of that extra disc of blades is the key element in getting torque multiplication during the slip condition and is the hardest thing to grasp in the operation of a torque converter!

• Just to complement your answer (plus 1 btw), here is a link to the video by Allison : floridatorqueconverters.com/torque-video.html – Solar Mike May 17 at 6:18
• Thank you @solar mike! – niels nielsen May 17 at 6:24
• I guess we agree that the velocity triangles won't help here :) – Solar Mike May 17 at 6:26
• Hi guys, real cool stuff, thanks for the explanation here and the link to the video. However, I am still not getting what is going on in the liquid. I can but agree that "The action of that extra disc of blades is the key element in getting torque multiplication during the slip condition and is the hardest thing to grasp in the operation of a torque converter!". I hoped for some physical intuition, eg, in terms of expanding Euler equations or something, or what happens to the flow lines in the stall torque situation. – ffc May 17 at 8:59
• Am I right in understanding that what the torque converter is doing is storing energy (which was not transferred to the output fan) in the momentum of the moving liquid, and when this re-strikes the fan, it provides an additional force on the input fan on top of the torque applied by the input shaft? So it is, in a manner, a sort of liquid flywheel, in which any slip between the fans causes energy to be transferred into the momentum of the circulating transmission fluid, and this then discharges again by striking the input fan a second time on its opposite side? – Steve May 19 at 19:33