Peter wrote:
> I've asked several people about the differences/similarities between
> horsepower and torque, and each time the vague response is prefaced by head
> scratching and " Well, I'm not sure but"..
> Both apparently refer to the ability to do a certain amount of work within a
> specific time, and they seem to max out at different RPMs, but other than
> these two sophomoric opinions, I'm stumped. Anyone really know??
> Peter Firla
The Mechanical Engineer from MIT pipes up:
Well if only the whole world would use consistent units, I could just
say "Power is equal to torque times rotation rate". But that's only
true for consistent sets of units, such as watts, newton-meters, and
radians per second. For non-consistent units, there's a proportionality
fudge-factor.
As other posters have pointed out, torque is a measure of the
strength of the twisting effect that the engine produces (or that
the driveshaft is transmitting, or that the transmission outputs,
etc). Note that the torque value will be different at different points
in the drivetrain due to gearing and dissipation.
Power is the rate of work that the engine is performing. Work is
a force applied while traversing a distance. In SI units, the unit
of work is the Joule, defined as the work performed while applying
one Newton of force and traversing one meter of distance in the same
direction as the force. The units of force are Newtons, defined as the
amount of net force required to cause one kilogram of mass to accelerate
at a rate of one meter per second per second (ms^-2). So force is "how hard
it's pushing", torque is "how hard it's twisting", and power is "how much
work it's doing".
In case anyone is interested, here's how the relationship between torque,
power, and rotation rate is derived.
For a shaft rotating at Omega radians per second, with torque T,
the amount of force that would be required to oppose the torque
would be T Newtons applied at one meter from the axis. The distance
travelled by a point one meter from the axis would be 2PI times one
meter times the rotations per second, which is the same as Omega times
one meter, which has the same value as Omega. So the amount of work
done by moving a force T through the distance Omega meters (which is
how far the force T would move in one second) is T * Omega. Work per
second is the same as power - so
Power (in watts) = T (Newton meters) * Omega (radians per second)
Converting this to "regular British car units" (rpm, foot-pounds, HP)
requires a fudge factor. A foot-pound is about 1.35 Newton meters,
a radian per second is 9.55 rpm, and a Horsepower is 750 Watts. So we get
T(foot pounds) * rpm
Horsepower = --------------------
5305(fudges)
PS: no need to crack a textbook on this stuff, it's too basic.
--
---
John R. Lupien
lupienj@wal.hp.com
WARNING: This product exerts an attractive force on every
other object in the universe, including those
objects which are the products of our competitors.
|