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Driveline and transmission
Torque converter efficiency
Efficiency of a torque converter: the torque ratio multiplied by the speed ratio between turbine and pump.
- Ref
- DRV-11
- Rev
- v1
- Field
- Driveline and transmission
- Result
- ηTC
A torque converter has a torque ratio TR = Tt/Tp (turbine torque over pump torque) and a speed ratio SR = ωt/ωp. Its efficiency is the power out over the power in, which is the product ηTC = TR × SR. At stall the turbine does not turn, so SR = 0 and the efficiency is zero however large the torque multiplication. Stall torque multiplication is a launch aid only; it cannot be counted on during sustained motion. A lock-up clutch or a coupling phase has TR = 1 and efficiency equal to the speed ratio.
Efficiency of a torque converter: the torque ratio multiplied by the speed ratio between turbine and pump. At stall the speed ratio is zero, so the efficiency is zero however large the torque multiplication.
Calculator
- Power loss as a fraction of input0.1
- Slip, 1 − SR0.4
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.
- At a speed ratio of 0.6 and a torque ratio of 1.5 the converter is 90 % efficient. At stall (SR = 0) it is 0 % however large TR is, and all the input power is lost as heat.
- At the coupling point TR = 1 and the efficiency equals SR, so 90 % at SR = 0.9. A lock-up clutch removes the loss entirely by connecting pump and turbine.
Chart data as tables
Ranked variables
| Variable | Typical range | Lowest result | Highest result | Swing () | Local step | Change in result |
|---|---|---|---|---|---|---|
| SR Speed ratio, turbine over pump | 0 to 1 | 0 | 1.5 | 1.5 | +10% | +10 % |
| TR Torque ratio, turbine over pump | 0.8 to 3 | 0.48 | 1.8 | 1.32 | +10% | +10 % |
Sweep of Speed ratio, turbine over pump
| Speed ratio, turbine over pump | Converter efficiency |
|---|---|
| 0 | 0 |
| 0.125 | 0.1875 |
| 0.25 | 0.375 |
| 0.375 | 0.5625 |
| 0.5 | 0.75 |
| 0.625 | 0.9375 |
| 0.75 | 1.125 |
| 0.875 | 1.313 |
| 1 | 1.5 |
Sweep one variable
Variables and how each one contributes
Converter efficiency
What the equation gives you. Everything else feeds it.
Torque ratio, turbine over pump
The torque ratio: turbine torque divided by pump torque. About 2 to 3 at stall, falling to 1 at the coupling point. Raises the efficiency in proportion, at the same speed ratio. Raising it by 10 % (to 1.65) moves the result by +10 %, from 0.9 to 0.99.
Speed ratio, turbine over pump
The speed ratio: turbine speed divided by pump speed, from 0 (stall) to about 1 (locked). Raises the efficiency in proportion, at the same torque ratio. Raising it by 0.1 (to 0.7) moves the result by +16.7 %, from 0.9 to 1.05.
Worked example with the values above
- Multiply the two ratios: TR × SR = 1.5 × 0.6.
- ηTC = 0.9.
- So 90 % of the input power reaches the turbine and the rest becomes heat.
Assumptions and limits
- Steady operation at one point on the converter characteristic.
- Torque ratio and speed ratio measured at that same point.
Common mistakes
- Assuming stall torque multiplication is available in sustained motion.
- Reading TR as efficiency. A ratio above 1 does not mean a gain in power.
Statistics and uncertainty
TR and SR come from a measured converter characteristic and vary with the load; ±3 % on each gives about ±4 % on the efficiency. Efficiencies above 1 are not possible, so a calculated value above 1 signals inconsistent inputs.
Where it is used
- Judging how much engine power a converter passes on at a given speed ratio.
- Deciding when a lock-up clutch pays off.
Maths you need
- Torque and power: power equals torque times angular speed
- Efficiency: output over input