Torque Converter, Single Disc

How does a torque converter work?

Let’s start with two wall fans facing each other. If we turn one fan on, the wind from this fan will start make the other fan turn, although at a much slower rate than the "drive" fan.

In the case of a torque converter, the drive fan is connected to the engine, and the fan being “driven” is connected to the input shaft of the transmission. In addition, oil is used to transmit the energy between the two fans, as opposed to air in the example scenario.

When a vehicle is stationary, such as at a stop sign, with the transmission in gear and engine at idle, the drive fan that is connected to engine is spinning so slow that it will not transfer enough oil to turn the driven fan. As the vehicle’s brakes are let off and the throttle is held, the engine speed increases, and the drive fan blows more oil at the driven fan. The driven fan then starts to turn and move the vehicle. This important concept is commonly referred to as fluid coupling.

The drive fan will always turn slightly faster than the driven fan, just like the wall fans. If you were to stick a feather into the blades of the driven wall fan, the driven fan will slow down, but not the drive fan. In a real application, this is just like pulling a heavier trailer. The feather in the driven wall fan is essentially like adding a load to the vehicle.


What determines stall speed?

The two major things that determine the stall speed are engine torque and the torque converter.

This is one way to think about it: You’re on a stationary exercise bike that uses a fan for its pedal resistance. The smaller the fan is, the faster you can pedal. You may be able to pedal up to 200 RPM on the bike with a small fan, but only be able to pedal up to 50 RPM if you fit a bigger fan on the bike. The small fan was able to stall you at 200 RPM and the bigger fan stalled you at 50 RPM. Two different fans, two different stall speeds.

Now a professional athlete hops on the bike. With the big fan, they may be able to pedal 100 RPM, as opposed to your 50 RPM.

To compare this to the engine torque and torque converter, you must remember that a torque converter is nothing more than two fans. One fan is connected to the engine and the other fan is connected to the transmission. The fan connected to the engine blows oil at the fan connected to the transmission. Once the engine fan blows enough oil at the transmission fan, the transmission fan will start to rotate. The vehicle will then start to move.

Just like the exercise bike analogy, if you install a torque converter with bigger fan blades into a vehicle with the same engine torque, the stall speed will be lower. If you add more horsepower to the engine, just like the athlete hopping on the exercise bike, the stall speed will increase.

Now let’s talk about air. It's all about the oxygen. You’ll be able to pedal harder on the bike at sea level than on the top of a mountain because you’ll be able to breathe better, since there is more oxygen in the air. Similar to your lungs, an engine is nothing more than an air pump that uses fuel. The fuel an engine uses must react with oxygen in order to push the piston down and turn the crankshaft.

The higher the elevation, the less oxygen the engine has to work with, and the less fuel that can be injected to react with the oxygen. By using less fuel, it means the power input into the converter will be lower, resulting in a lower stall speed. Normally, the higher the elevation, the higher the stall speed that is needed in order to help offset the lack of oxygen that is present at sea level.

The engine turbo also can make a huge difference. Like the converter, the turbo is also a set of fans. The drive fan is located in the exhaust of the engine and it’s the exhaust flow makes this fan spin.

The driven fan of the turbo is connected to the drive fan and sits on the intake side of the engine. The driven forces oxygen into the engine so it can burn more fuel, resulting in more power.

A turbo with bigger fans takes longer to spool up and start to blow more air into the engine. Because of this, it takes longer to get oxygen into the engine cylinders to burn fuel. This is called turbo lag.


What is true, flash, and breakaway stall speed?

To explain stall speed, let’s start with a true full stall speed.

True stall speed is tested by putting the vehicle in drive and holding the throttle wide open while simultaneously holding the brake, so the vehicle remains stationary. When this happens, the torque converter will stall the engine at a certain RPM. When stalled, the engine will not be able to increase RPM until the vehicle is allowed to move. This is a true full stall. Do not test for true stall, as it can damage transmission shafts and overheat the torque converter. We have specialized equipment we use to perform this test.

The next stall speed is generally known as breakaway stall speed. A vehicle in drive is stopped on an incline and the throttle was held enough to hold the vehicle in place, but not enough to move the vehicle uphill. Once the throttle is held enough to start creeping the vehicle uphill, breakaway stall speed has been reached.

For example, if the engine RPM required to hold the truck is 1100 RPM, and an increase of 25 RPM started to move the truck, the breakaway stall speed would be 1125 RPM.

The last stall speed is generally known as flash stall speed. The flash stall speed takes effect under hard acceleration. A vehicle idling in drive and the throttle is then held wide open. The engine starts to accelerate quickly, then temporarily pause at an RPM as it starts to pull the vehicle.

If the engine took 1.5 seconds to get from an idle to 1500 RPM when floored, then took another two seconds to get from 1500 RPM to 2000 RPM, this would mean the flash stall speed would be 1500 RPM.

When we lower the stall of a converter, it can also lower the breakaway stall speed as well as the flash stall speed. Lowering the stall makes the engine work at a lower RPM for any given road speed and in most cases, will increase fuel mileage.


Lock Up & Speed Ratio

Once a vehicle gets up to speed there is a mechanism called a lock up clutch that locks the driven fan of the torque converter to its front cover, which is bolted to the engine. When this occurs, the drive fan and driven fan turn at the same RPM, with no loss of power in fluid coupling.

When the drive and driven fan are not locked together, heat is generated inside the converter. The greater the RPM difference between the drive and driven fan, the higher the temperature will reach. This heat is essentially lost power which results in a lower transmission life, performance, and fuel economy.

The loss of energy in this process can be calculated. A converter’s drive fan (impeller) may be turning at 2500 RPM and the driven fan (turbine) may be turning at 1800 RPM. By dividing the turbine RPM by the impeller RPM, we can find the efficiency of the converter, which is 72% at the given speeds of the impeller and turbine. This efficiency rate is also known as a converter’s speed ratio. When the vehicle gets up to speed and the lock up clutch engages, the engine and impeller RPM match the RPM of the turbine.

Low stall converters will be able to achieve a higher speed ratio before lock up. When low stall converters lock up, their RPM drop is substantially less, therefore easier on the converter’s clutch lining and will reduce clutch glazing. Because the fluid coupling of a low stall converter is more efficient, they are able to transfer more power, create less heat, and deliver better fuel economy than high stall converters.