A torque converter is the fluid coupling that lets an automatic transmission connect to a running engine without a manual clutch. It transfers engine torque to the transmission using moving automatic transmission fluid instead of direct friction plates during normal driving. This matters because it allows the car to idle in gear, start smoothly from rest, and multiply torque when extra pulling force is needed.
Key Facts
- Torque multiplication ratio = turbine torque / pump torque, often about 2:1 at stall in many passenger vehicles.
- Power = torque x angular speed, so P = Tω.
- The pump is driven by the engine and throws fluid outward by centrifugal effect.
- The turbine is driven by the moving fluid and turns the transmission input shaft.
- The stator redirects returning fluid to increase torque at low vehicle speed.
- A lockup clutch can connect the engine and transmission directly to reduce slip and improve fuel economy.
Vocabulary
- Torque converter
- A sealed fluid coupling in an automatic transmission that transfers and can multiply engine torque.
- Pump
- The impeller connected to the engine side of the converter that accelerates transmission fluid outward.
- Turbine
- The bladed wheel connected to the transmission input shaft that is spun by moving fluid.
- Stator
- The bladed part between the pump and turbine that redirects fluid flow to improve torque multiplication.
- Lockup clutch
- A clutch inside the converter that mechanically links the engine to the transmission to reduce energy loss from fluid slip.
Common Mistakes to Avoid
- Thinking the torque converter is just a simple clutch. It is wrong because it uses fluid flow to transfer torque and can multiply torque, especially during launch.
- Assuming the pump and turbine always spin at the same speed. They usually have some slip unless the lockup clutch is engaged.
- Forgetting the role of the stator. Without the stator redirecting fluid, the converter would act more like a basic fluid coupling with less torque multiplication.
- Treating slip as always bad. Some slip is necessary for smooth idle and takeoff, but too much slip wastes energy as heat.
Practice Questions
- 1 At stall, an engine sends 160 N m of torque to a converter with a torque multiplication ratio of 2.1:1. What torque is delivered to the turbine?
- 2 A converter turbine spins at 1800 rpm while the pump spins at 2000 rpm. What is the speed difference in rpm, and what percent of pump speed is the slip?
- 3 Explain why an automatic car can sit stopped in Drive with the engine running, but a manual car would stall if the clutch stayed fully engaged.