| There are two main factors that contribute to the power
of a rotary style engine (internal combustion engine,
electric, turbine, etc). These are torque and
maximum RPM. These two together determine the horse
power of the engine.
One horse power is defined to be the power necessary
to lift 33,000 pounds one foot in one minute. This can be
done with a REALLY high torque engine lifting the weight
directly, or a high RPM engine using a series of gears or
pulleys to obtain the power. The formula for horse power is:
hp = (2pi * torque * RPM) / 33000
simplified:
hp = (torque * RPM) / 5252.268
Based on this formula, its easy to see that an engine
with high RPMs and low torque can accomplish the same
work as an engine with high torque and low RPMs.
Horse power is "work" (work = force * distance).
Torque is the leveraged force. If you put a wrench on a
rusty bolt and pull on it, but it doesn't break lose, you
are applying a force to the bolt. Now, the length of the
wrench gives leverage. The longer the wrench, the less
force you will need to apply to break the bolt lose. The formula for torque is:
torque = force * radius
The "force" is the amount of force the engine can put on
the shaft of the engine. The "radius" is the distance from
the center of the shaft where the force is applied.
If you have ever seen (a picture of) a crank shaft, they
tend to look like:
_ _ _ _
- |_| |_| |_| |_-
This is to place the point where the force is applied
farther away from the center of the shaft. Thus, increasing
the radius which increases the overall torque, which
increases the horse power of the engine.
It would make sense to put the point of force as far
out as possible to get the maximum torque you can, but
there is point of diminishing returns. Think of a
figure skater spinning. If she pulls in her arms, she
spins faster. If she lets them out, she slows down. If
the point of force is too far out, it takes longer for
the engine to reach the higher RPM (more mass, farther out) and ultimately limits the max RPM. So you need to find
a balance.
When designing an engine, the engineers can calculate how
much force the explosion, or magnetic pull will generate.
Then they need to figure out how far away from the center
of the shaft the point of force will be. Once they do that,
they can (theoretically) calculate the power of the engine
at any given RPM.
The max RPM of an engine is a factor of several things.
It basically boils down to how well the engine was designed,
how well the parts are manufactured, and overall physical
limitation. If you think about an internal combustion
engine, it needs to take fuel into the cylinder, explode
it, release the fumes, and bring in fresh air for the next
go around. Springs and valves control all of these functions. If the engine is spinning faster than the springs
and values can retract, it is beyond its capabilities and
will start to backfire and malfunction.
In terms of engine performance, the horse power
determines the top speed of the vehicle (given by horse
power divided by the weight and aerodynamic drag of the
vehicle). While Torque determines the rate of
acceleration.
Brute force torque
is usually obtained by having engines that consume
large amounts of fuel. Commercial semi-trucks have large
diesel engines that have insane amounts of torque. This
torque is needed to move the heavy load that they are
pulling. Sure, you could pull it if you had less torque
and more RPMs (same power), but think of it like the rusty
bolt. What would be safer? Breaking it loose with a wrench
and a lot of torque, or trying to slam a socket wrench on
it that happens to be spinning at 10,000 RPMs?
Whew. Man, don't I feel like white trash now. I have
a sudden urge to go out and buy a Camaro and start wearing wife beaters soaked in engine oil. Please feel free to vote for this node for the redneck
node of the day.
If I've made any mistakes or wrong info, let me know
and I will correct it. I've been learning about engines so
that I may someday know how to fix my motorcycle if it
ever breaks down. | 
