flytoboat
Well Known Member
It's the power of an old nag walking in a circle <g>.
Shouldn't that be called horse torque????
It's the power of an old nag walking in a circle <g>.
I appreciate this discussion. It really allows me to use my brain.
If the spinning flywheel on an IC engine is held in place and not allowed to spin but the engine continues to run there will be fuel being expended. That fuel is being converted from chemical energy to some mechanical energy. There will be some measure of "power" being transmitted regardless of whether the flywheel spins. That "power" may very well be displayed as an explosive catastrophic failure of a rod, bearing, gasket or something. It may not be represented by the "spinning" of a flywheel, or prop, but there will still be power being exerted.
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Paul, keep in mind though that while work and torque have the same dimensions (force times distance)...
It takes a force to make something move. Horsepower is measure what that force accomplished. The classic example is a jet aircraft. engines roaring at full throttle, brakes locked and the aircraft is not moving means that there is zero horsepower (ignoring accessory PTOs). Once the brakes are released and the aircraft starts moving (doing work) then horsepower can be calculated.
* * *
I was trying to refute some of the earlier assertions that it takes horsepower to make something move. It does not, only a force can make movement.
...snip
Watt's wrong with that?
I was trying to refute some of the earlier assertions that it takes horsepower to make something move. It does not, only a force can make movement. When the throttle is moved, what is adjusted is the amount of force generated by the new level of fuel/air consumed.
It takes a force to make something move.
It's a small thing, but actually you have done work in lifting your body up onto the wrench. the amount of power used depends on how quickly you climbed up there.
Then Einstein thought something else again, and I hope someday to have an RV that is fast enough so that makes a difference.
A difference relative to what inertial reference frame?
I can't stand the thought of relatavistic mass increase making my bird heavier.
Works fine. Your VW TDI makes 54hp at 1900rpm and 118 lb/ft at 4000. Hard to compare turbocharged engines directly to atmo engines. Very few engines have torque and power peaks at the same rpm.
Gary, the formula is correct... it relates torque to hp, at a given rpm.
Taking that Jetta engine example:
90 bhp @ 4000 rpm means it is making 118 lb-ft of torque at that rpm.
149 lb-ft of torque @ 1900 rpm means it is making 54 bhp at that rpm.
If you look at the complete torque and hp curves vs. rpm for an engine, you can see the relationship holds at every rpm. In fact, you only need one of the curves... you can compute the other one using the formula. And again in fact, that's the way it is done in practice; a dynamometer just measures torque at different rpms, and bhp is computed from that using the formula.
--Paul
With apologies to the Bard, "There are more things in heaven and earth, Horatio, Than are dreamt of in your philosophy.
To produce 90hp @ 4000rpm the engine is producing about 118 lb/ft of torque, @ 4000rpm. That is a very believable value, given that the engine is operating twice as fast as it's torque peak. It is typical for an IC engine's torque to fall off in the upper rpm range of the engine.
You didn't think that the 149 lb/ft of torque was maintained forever, did you?
A major goal of IC engine design is to have the torque peak as early as possible in the rpm band, and maintain that torque as late as possible. For several reasons related to simple physics, this is a difficult task.
Thanks everyone for proving that the formula is not correct. The formula may be correct for one and only one RPM as you point out but cannot be used for all RPM and HP values.
Thanks everyone for proving that the formula is not correct.
With apologies to Charles Babbage: "I am not able rightly to apprehend the kind of confusion of ideas that could provoke such a statement".
Gary,
Exactly what leads you to believe the horsepower formula is incorrect? You're disagreeing with an awful lot of engineering textbooks and theory with that statement and I'm trying to understand exactly what leads you to that conclusion.
Automobile manuals typically give two values for engine performance; Maximum horsepower and maximum torque. These two peak values typically occur at different engine speeds, which is the reason you see peak torque specified at one engine speed and peak horsepower specified at another. Please note that in the examples you gave the engine speed is always specified for the given torque or horsepower value. That is the speed that each of those values occur, and nowhere else. If you can find somewhere a plot or table of horsepower and torque vs. engine speed, the formula horsepower = (torque*rpm)/5252 will hold true at any given engine speed. Incidentally, you will also find that horsepower and torque are ALWAYS equal at 5252 RPM (the plots cross each other at that point), at least for engines capable of running that speed. Stated another way, horsepower and torque at any specific speed (RPM) are directly related by the above formula, but if you compare horsepower at one speed to torque at another the formula will not work (and of course if it did, it would violate the definition of a mathmatical function).
Skylor
It takes a force to make something accelerate.--Paul
http://www.team-integra.net/sections/articles/showArticle.asp?ArticleID=467If you can find somewhere a plot or table of horsepower and torque vs. engine speed, the formula horsepower = (torque*rpm)/5252 will hold true at any given engine speed.
UNDERSTEER is when you hit the mountain with the front of the car.
OVERSTEER is when you hit the mountain with the rear of the car.
HORSEPOWER is how fast you hit the mountain.
TORQUE is how far you move the mountain when you hit it.
...the PT-6 in my Air Tractor turns 37,500 RPM on takeoff, yup, thirty seven thousand, five hundred, but the prop is driven by a double planetary gearbox that reduces it by 17 times for a redline of 2200.
So the normally rated 680 SHP engine developes 1633 lbs of torque during all this.
Best,
.....! I knew it! There's magic in numbers. That 5252 comes from 550X60/(2Xpi), and everyone knows that the value "pi" is a magical number, and was first was discovered in the writings of Merlin the Magician! Now it all makes sense!
Paul,
And I hope the gearbox examples, especially Pierre's PT-6, make it clear that you can get any torque value you want out of any engine with the correct gear ratio. Of course the horsepower of the engine cannot be increased by the gearbox, in fact, it is decreased by the frictional losses (heat and noise) inside the gearbox.
And, as it's very apparent, torque must be the reason our Lycomings are getting off the runway faster than the Subies. The Subie just has to wind up to those high RPM's, while the Lyc doesn't. And the six cylinder Subie even has a higher horsepower rating.
L.Adamson --- RV6A
This was fun, and I hope we all learned a lot about the relationship of these values through the various examples and arguments presented. And I think that now we can put it to bed! Agree?