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
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.