Library / Driveline and transmission / DRV-12
Driveline and transmission
Torque converter power loss
Power turned into heat inside a torque converter: the power going in at the pump minus the power coming out at the turbine.
- Ref
- DRV-12
- Rev
- v1
- Field
- Driveline and transmission
- Result
- Ploss [kW]
The power into the converter is the pump torque times pump speed, Tpωp, and the power out is the turbine torque times turbine speed, Ttωt. The difference is lost as heat in the fluid, Ploss = Tpωp − Ttωt. With torque in N m and speed in rad/s the product is in W; the calculator divides by 1000 to give kW. At stall ωt = 0 and all the input power is lost. Dividing the required span by the stall torque ratio is only a launch estimate.
Power turned into heat inside a torque converter: the power going in at the pump minus the power coming out at the turbine. At stall all the input power is lost as heat.
Calculator
- Input power, Tpωp80 kW
- Output power, Ttωt72 kW
- Efficiency, Ttωt/Tpωp0.9
What drives the result
Each variable is moved across its typical range while the others stay at the values above. The longest bar is the key driver.
- With 80 kW going in and 72 kW coming out, 8 kW is lost as heat: an efficiency of 90 %.
- At stall (ωt = 0) the entire 80 kW input is lost. That heat is why holding a vehicle on a slope with the throttle against the brake, or repeated launches with a heavy tow, overheat the transmission.
Chart data as tables
Ranked variables
| Variable | Typical range | Lowest result | Highest result | Swing (kW) | Local step | Change in result |
|---|---|---|---|---|---|---|
| Tt Turbine torque | 50 to 4,000 N m | −400 | 74 | 474 | +10% | −90 % |
| Tp Pump torque | 50 to 2,000 N m | −62 | 328 | 390 | +10% | +100 % |
| ωt Turbine speed | 0 to 500 rad/s | −220 | 80 | 300 | +10% | −90 % |
| ωp Pump speed | 50 to 500 rad/s | −52 | 128 | 180 | +10% | +100 % |
Sweep of Turbine speed
| Turbine speed (rad/s) | Converter power loss (kW) |
|---|---|
| 0 | 80 |
| 62.5 | 42.5 |
| 125 | 5 |
| 187.5 | −32.5 |
| 250 | −70 |
| 312.5 | −107.5 |
| 375 | −145 |
| 437.5 | −182.5 |
| 500 | −220 |
Sweep one variable
Variables and how each one contributes
Converter power loss
What the equation gives you. Everything else feeds it.
Pump torque
The torque at the converter’s pump (input) side, in N m. Raises the loss in proportion to the input power. Raising it by 10 % (to 440) moves the result by +100 %, from 8 to 16 kW.
Pump speed
The pump (input) angular speed, in rad/s. 200 rad/s is about 1900 rpm. Raises the loss in proportion to the input power. Raising it by 10 % (to 220) moves the result by +100 %, from 8 to 16 kW.
Turbine torque
The torque at the turbine (output) side, in N m. Lowers the loss: more output power means less is wasted. Raising it by 10 % (to 660) moves the result by −90 %, from 8 to 0.8 kW.
Turbine speed
The turbine (output) angular speed, in rad/s. Lowers the loss: more output power means less is wasted. Raising it by 10 % (to 132) moves the result by −90 %, from 8 to 0.8 kW.
Worked example with the values above
- Input power: 400 × 200 = 80,000 W. Output power: 600 × 120 = 72,000 W.
- Difference: 80,000 − 72,000 = 8,000 W.
- Ploss = 8 kW.
Assumptions and limits
- Steady operation at one point, so no energy is being stored in the rotating parts.
- Torques and speeds measured at the same point and in the same units.
Common mistakes
- Using rpm instead of rad/s in the power products.
- Assuming stall multiplication holds in cruise.
Statistics and uncertainty
The loss is a small difference of two large powers, so a 2 % error in each power gives a much larger relative error in the loss: here ±2 % of 80 kW is ±1.6 kW on the input and ±1.4 kW on the output, which can swing the 8 kW loss by about 27 %. Use direct temperature measurement for cooling sizing.
Where it is used
- Sizing the transmission cooler for launch and towing.
- Judging the heat load of an automatic transmission over a drive cycle.
Maths you need
- Power: torque times angular speed
- Efficiency: output over input