Quote:
|
Originally Posted by blue2000s
Wow. Don't confuse torque with power, they're directly related but totally different and can't be used like this.
If you go back to the fundamentals, force=mass*acceleration, torque is a twisting force. It can be directly calculated to force by the wheel/tire radius, therefore it is the property that defines acceleration.
|
Hi,
Yes, you are correct in pure physics, but in Autodom, and especially in the context of this thread, the term acceleration is, strictly speaking, misused and does
imply a time component and work being done. You can generate 300 ft.-lbs. of Torque at 4000 RPM the instant the Clutch is released (from a dead stop), but the Car isn't yet doing any work.
There is really only Torque. Torque is the only thing that a driver feels, and Horsepower is just sort of an esoteric measurement in that context but adding in a time component.
300 foot-pounds of torque will accelerate you just as hard at 2000 rpm as it would if you were making that torque at 4000 rpm in the same gear, yet, per the formula, the horsepower would
double at 4000 rpm. Therefore, horsepower isn't particularly meaningful from a driver's perspective, and the two numbers only get friendly at 5252 rpm, where Horsepower and Torque always come out the same.
In contrast to a Torque curve, Horsepower rises rapidly with RPM, especially when Torque values are also climbing. Horsepower will continue to climb, however, until well past the Torque peak, and will continue to rise as engine speed climbs, until the Torque curve really begins to plummet, faster than engine rpm is rising. However, as I said, horsepower has nothing to do with what a driver
feels.
It is better to make torque at high rpm than at low rpm, because you can take advantage of
gearing.
An extreme example of this, imagine a waterwheel. A pretty massive wheel (let's say 4 Tons), rotating lazily on a shaft. You determine that the wheel typically generated about 2600 foot-pounds of Torque, and it rotated at about 12 RPM. If you hooked that wheel to the drive wheels of a car, that car would go from zero to twelve RPM in a flash. But, 12 RPM at the drive wheels is around 1 MPH for the average car. In order to go faster, you'd need to gear it up. To get to 60 mph would require gearing the wheel up enough so that it would be effectively making a little over 43 foot-pounds of Torque at the output, which is not only a relatively small amount, it's less than what the average car would need in order to actually get to 60. Applying the conversion formula 12 RPM * 2600/5252 = 6 HP. While it's clearly true that the water wheel can exert a lot of Force, its Power (ability to do work over time) is severely limited.
Happy Motoring!... Jim'99