bill ellison -torque vs. (?) horsepower  November 17, 2009 01:24 PM
Why don't people simply go to the equations? "Torque means nothing without horsepower" might better be translated ...."torque means nothing without engine RPM."

It's not torque vs. horsepower, because the two are related ...indeed:

Torque is "work" ...and Horsepower is "rate of work." It really is that simple, i.e.: horsepower is rate of torque.

So, this statement: "but what is the horsepower or the ability to do work?" ....is problematic, because horsepower is the ability to do RATE of work.

When someone says, "the engine has great torque" what they really mean is that is has higher average HP over the torque vs. RPM curve
(higher than a "peaky" engine with less torque at low RPM, or relatively constant torque over the RPM band). I say this, because the engine is "talking" through a geartrain, to the drive wheels.

All of this comes from the simple equation defining HP:

Torque X RPM
Horsepower = -----------------
5252

The definition of HP makes it clear that it is rate of work, because it is 550 lbs-ft. (torque).... PER SECOND (rate). Pi (3.1416) gets in there via the constant 5252 in order to translate rotational torque to linear force.

What matters - in terms of vehicle dynamics - is tractive effort (linear force at the contact patch) at the drive wheels (and gearing is a part of that) vs. total drag (force required at the drive wheels) as a function of speed (aero + rolling). Excessive tractive effort - at a level above the sum of rolling resistance and aero resistance - at a given speed means the vehicle will accelerate.

The total "system" looks at torque (yes) but torque over the RPM range (average power), and gearing that keeps the engine at the appropriate speed relative to vehicle speed. Tall gearing (for fuel economy) means that top speed is often achieved in lower (numerically higher) gears.

If you look at an electric motor which has peak torque at zero RPM and torque which tapers off with speed, since HP is a function of speed (RPM) and torque, the motor will have a relatively flat power curve. It will accelerate off the line, but taper off as total drag increases with speed.

Anyway, understanding the terms helps one avoid the newscasters and police spokesman's absurd verbiage, "the vehicle left the road at a high rate of speed!" Might happen, but highly unlikely and not what they mean. It may have left the road at high speed, or at a high rate of distance (speed is rate of distance, eh?), but - in all probability - it likely was leaving the road at a low rate of speed (low acceleration rate)! Rate of speed is acceleration. Now, a Top Fuel dragster DOES leave the gate at a high rate of speed, even at 30 mph.

Striving for clarity, and understanding... not that the above is error-free, b

eric -piaggio usb  November 17, 2009 12:11 PM
Attn Piaggio, if you read this - I want one.......make this available to consumers 155 mpg is awesome. Ps That model is a knockout and the scooter is nice too......

themountain -well...  November 12, 2009 07:07 PM
...hard for us to believe that now, but this IS the future!!