Thanks for the replies everyone!
Tom, I definitely see what you are saying there, but I have to point out my one disagreement. I’m a heavy equipment mechanic by trade so we actually had to learn about this when I went to school. You said horsepower is not a measure of a machines ability to do work, this is not right, that is exactly what horsepower is. Horsepower is a calculation of work completed in a specific time. There is actually a formula for it, torque x rpm / 5252. Now we actually know that one horsepower is not actually equivalent to the power of one horse, but rather it was there best guess at the time. It’s measured by the ability for a (engine, person, motor, water wheel) to move 550lbs one foot in one second, and any variant thereof (more weight, faster speed, farther distance) will have an effect on that number.
Like you said most engines will produce full torque around half rpm rating, the actually reason for this is because at half rpm the engine is actually at its best “breathing” speed. The faster you rev an engine after the full torque speed, the engine cannot actually suck in the equivalent amount of air, sure it may be close, but at that specific rpm because of aerodynamics within the engine, drags and a bunch of other things some very smart people figured out, that max torque rpm actually gets more air in the cylinder per cycle or revolution than at a higher rpm. It also has to do with how quickly and effectively the combustion and expansion of the gases can take place. The engine needs adequate time for that expansion to take place to extract a good working pressure during the power stroke, and that expansion we want captured between 0 and 90 degrees crank throw, which is where the greatest force will be able to be properly utilized. As engine speed increases after the full torque speed it loses its ability to extract the most potential from the combustion process due to the shorter period of time given to do that at a higher speed, which will result in a lower torque figure. Remember torque is just a pushing or pulling force on a rotating lever.
As you pointed out also, your horsepower as viewed on a dyno graph will continue to climb, and especially for a gas engine the max hp rating will usually fall somewhere in the top of its rpm limit. Now because horsepower is a measure of work done within a specific time, our torque may drop, but as rpm increases we can still in fact achieve more work completed due to the rise in rotational speed. Eventually as you will see in the same dyno graphs the horsepower will once again begin to fall off if engine speed increases from there, because at a certain point that continuing drop in torque is going to have an affect on how much work we can complete.
Horsepower may seem like a very abstract idea, but it is very much in fact measurable. The one thing I found interesting when I was completing my schooling is when we learned that horsepower cannot be changed by means of gearing, pulleys, or multipliers. That’s because total work done will always remain the same (assuming constant horsepower output, better example of this would be an electric motor). Torque and speed can be changed with gears, but at the end of the day those have to change together, torque down equals speed up, and vice versa. So no matter what you do with transmissions and gearboxes those two numbers in our equation will not differ. We either produce ample torque at a slow speed, or little torque at a high speed, but the amount of work completed will be equivalent. If you have the right gearing system you could make a 500hp engine have so little torque that it couldn’t turn a door handle, but with no load the end shaft would be turning millions of rpms. This is the same way you could stack up enough gears to make a 5hp Honda put out 500,000 lb/ft of torque, but the end shaft will take 9.5 million years to make one revolution, so at the end of the day even though you’ve made 500,000 lb/ft of torque you still only have 5hp, that number cannot be changed. What’s that old saying, give me a long enough lever and I could turn the earth itself?
One of my instructors actually put it very well, he said imagine you have 6 foot wrench, you can put it on a bolt and put as much effort as you want to pull on that wrench. You could pull on it, putting 500 lb/ft of torque on the bolt, but if that bolt doesn’t move, you didn’t put out any horsepower. If it moves slightly, you put out horsepower, you have done work. That’s because in our formula, anything multiplied by or divided by 0, is zero. So zero turning speed means 0 horsepower. In the same way, if a hydraulic motor is stalled so it won’t turn, sure it’s putting out mountains of torque, but it’s jot putting out any horsepower because it isn’t accomplishing any work.
On but off topic, I’ve worked on diesel electric mining dump trucks that, in a manner of speaking, have an onboard dyno. At a standstill you can press a button and tell the truck to load up the generator with the resistor grids used for dynamic braking, and at rated speed the computer will calculate the rpm, current and voltage and give you a live reading of how much horsepower the engine is putting out. It’s very cool and makes trouble shooting a low power engine much easier. It’s also cool to see how much power a Diesel engine makes when it’s cold vs operating temp. From a cold run you can actually lose (on those big engines) up to 500 hp just by the engine being cold, and as it warms up you can see the hp slowly increasing until it eventually reaches its rated output.
Anyhow sorry for the long rant, but I absolutely love this kind of stuff and find it fascinating so I tend to go a little overboard.
