Why HP?

[GaryCrowell]

I agree torque is force (T = Distance * Weight), and I feel that is power. Basically I see Force and Power being the same.

I can not agree HP is power because it's just a number derived from torque and engine speed. Your example of the tanks is really deceiving to me because, logically, it seems gearing can give you speed. And no matter how much engine speed you have, if you don't have enough "Force" to get the weight moving, it ain't moving, regardless of the engine speed.
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BTW how did they ever come up with using 5252 to get HP, why not use 5000 to make it seem as if they were giving more horses?

Torque and horsepower have strict definitions in physics. When you state that torque is power, strictly speaking by the physics book you're wrong, it just isn't so. Horsepower is one way power is measured, another way is by the Watt. Think of the 100 watt light bulb in your house, that's how much power it consumes. An engine's output could just as easily be expressed in Kilowatts (1000 watts) by juggling the units around. If you look on the ID tag of most diesel engines they'll have the power stated in Kilowatts also. Horsepower is gradually being phased out in favor of Watts since horsepower is considered kind of an archaic unit. As for where the term "horsepower" came from, here's what wikipedia says:

"The term "horsepower" was coined by the engineer James Watt (1736 to 1819) in 1782 while working in the performance of steam engines. This occurred while using a mine pony to lift coal out of a coal mine. He conceived the idea of defining the power exerted by these animals to accomplish this work. He found that, on the average, a mine pony could pull (lift by means of a pulley) 22,000 foot-pounds per minute. Rather than call this "pony" power, he increased these test results by 50 percent, and called it horsepower i.e. 33,000 foot-pounds of work per minute."


The number 5252 is a conversion factor used to convert the rotational speed of an engine (RPM) to straight line speed. Since Mr. Watt decided that a unit of horsepower is 33,000 lb. ft/min, to get the conversion factor you take 33,000 and divide it by 2 times the numerical value of Pi, which is 3.1416. The result is 5252. It's just a conversion number like dividing by 3 if you want to know how many yards are in 100 ft.

Years ago, someone explained it to me like this. Torque is power because it's used to move weight, and HP was speed; assuming you had enough torque to move it in the 1st place.

That's the old hot rodder's explanation of why a big block motor will typically get off the line quicker than a small block, they justified it by the big block's having more torque. It's wrong, but that doesn't seem to get in the way of a good story. The real reason is because the big block produces more Horsepower at lower engine speeds (because it has more torque, since torque is a component of horsepower), but torque in and of itself can't, by definition, do anything. Only when there's movement associated with the torque can it do work. You have to have torque to have horsepower, if you have 0 torque you also have 0 horsepower, just as if you have 0 RPM you have 0 horsepower.

It gets confusing because so much of the common perception about torque and horsepower is wrong. Many old timers have drawn conclusions based upon their perceptions rather than what's really happening, i.e. "My tractor's not got much horsepower but a bunch of torque and it can pull a house, so torque must mean more power". In reality it doesn't take much power to pull a house, it just takes a lot of mechanical advantage (leverage), which can be multiplied through gearing, and a means of maintaining traction. You won't pull the house very quickly without a lot of horsepower, but to just get it to move you could use a hamster in a wheel as long as you run the power through the proper gears and get some traction.


Give me a lever long enough and a fulcrum on which to place it, and I shall move the world. -Archimedes

 

[Rara Avis]

I'm just wondering out loud here....

QUOTE]

Well it's because if the fuel runs out that is what we will be back to using...HORSE POWER...

 

[wkpoor]

I read somewhere what the opposite ends of the spectrum are for torque in a machine.
A water wheel produces the most torque for its RPM
A turbine engine produces the least. Thats not to say a water wheel has more torque, just that the ratio of torque to RPM.
Since HP is derived from torque and RPM obviously some engines will apply more force at lower speeds than others. Steam power produces alot of torque but isn't practical. Diesels produce more torque for a givin RPM therefore make better powerplants for tractors. It seems that an engine with greater torque at lower speeds is more efficient for moving objects.