# Maximum power and maximum torque

1. Jan 30, 2014

### timetravelller

why maximum power and maximum torque is not produced at the same rpm in an engine?

2. Jan 30, 2014

### SteamKing

Staff Emeritus
For an IC engine, the amount of torque produced is dependent on other factors besides RPM. The power output, on the other hand, is directly proportional to the product of torque and RPM. The torque curves of most IC engines are relatively flat over the RPM range of the engine, which implies that the max. HP will occur at a higher RPM than the max. torque.

Different types of engines have different torque-speed characteristics. For example, steam and gas turbines produce torque roughly proportional to the speed of the turbine, so the speed at which max torque and max power occur will be the same.

3. Jan 31, 2014

### Kozy

Power and torque are the same at 5252rpm. For the power to peak at 5252rpm, the torque would have to drop faster than the RPM increases past that point.

Peak torque is never typically followed immediately by a sharp drop like that, the rpm will almost always increase more than the torque drops for a reasonable range after peak torque, ergo peak power is never at the same RPM as peak torque.

4. Feb 7, 2014

### sgb27

That completely depends what units you use for power and torque - it's pretty meaningless to say they are "the same" as they measure different quantities. If you use Nm and HP then they are the same at about 7100rpm, if you use Nm and kW it's 9550 rpm, if you use Nm and W it's 9.5 rpm etc.

5. Feb 7, 2014

### Kozy

You, and everyone reading this, knows what I mean by that.

6. Feb 7, 2014

### xxChrisxx

Pedants.

Pedants everywhere.

7. Feb 7, 2014

### sgb27

Sorry your point doesn't make sense. Torque doesn't have to "sharply drop" after the max power rpm, torque is already dropping after the max *torque* rpm (which is lower), all that happens at the max power rpm is that the gradient of the torque curve becomes steep enough to stop the power rising further, no sharp drop required, it can be a nice smooth curve. Plenty of engines have max power below 5252rpm, I'm sure if you looked hard enough you could find one with it between 5200 and 5300 rpm. My point is that 5252rpm isn't some magic RPM, it's pretty meaningless.

8. Feb 7, 2014

### Kozy

I didn't say it dropped sharply after max power rpm, I said "For the power to peak at 5252rpm, the torque would have to drop faster than the RPM increases past that point"

Ignore 5252rpm. I chose 5252rpm as an arbitrary point because in imperial units that is where they are the same value. It could be any RPM, but for the peak power to be at the same RPM as peak torque, the torque would have to drop off faster than the RPM increased past that point.

Find me one engine where that is the case.

The torque never drops off that quickly immediately after the peak unless it is artificially restricted.

9. Feb 8, 2014

### sgb27

Sorry I misunderstood what you wrote originally (the 5252rpm threw me), I guess you meant "For the power *and torque* to peak at 5252 rpm..." that makes more sense.

10. Feb 8, 2014

### Kozy

Yea sorry, I was just chosing that point because people are familiar with them being the same at that point (when measured in lbft and bhp anyway).