Torque versus Horsepower

[Trev]

Someone over on the Lawn and Garden Tractor forum pointed me to a good article about torque and horsepower, but I have to confess that I'm still not clear about the relationship.

Here's the article: <A target="_blank" HREF=http://vettenet.org/torquehp.html>http://vettenet.org/torquehp.html</A>

I'm trying to apply the concepts there to tractors.. and not having a whole lot of success. /w3tcompact/icons/eyes.gif

I'm coming from a rather naive place in all of this. Seems to me that a certain amount of energy is needed to do a certain amount of work. The interesting formulas, which use 5252 as a constant, seem to make sense.. but I don't think I quite "get it" yet.

Would anybody be intersted in discussing this?

I've always had the kind of vague idea that "lots of power" at low RPM, i.e. the ability to keep pulling as engine speed drops, is called torque, and "lots of power" at high RPM is called horsepower. I know that's incredibly simplistic, but it has always seemed that way to me. I liked the guy's explanation in the article I noted.. but I have to confess that he lost me at some point.

I'll understand if nobody wants to try to tackle this thorny subject with me, believe me! /w3tcompact/icons/laugh.gif I come from a psychology and computer background.. and engineering doesn't come easily. Anyone want to talk about this torque thing?

Bob

 

[kevincook]

I'll give it a shot.

Torque is a measure of the force that you can apply, therefore, the work that you can do.

Horsepower measures how FAST you can apply that torque.

For Example, if you are pulling a heavy load with your tractor and you start to bog down the engine you need to down shift into a lower gear. You are increasing the amount of TORQUE at the wheels by doing this but you aren't changing the HORSEPOWER of your tractor at all. You are able to pull the heavy load, you just have to do it more slowly.

Now if you want to move that same load faster you need more HORSEPOWER. If instead of using a 25hp tractor you were using a 50hp tractor you wouldn't need to downshift because you have enough horsepower to apply that torque at the speed you where traveling.

Does that help at all?

 

[karmakanic]

Here's the way i've always thought of the concepts:

Torque is the ability to do work by twisting or turning a shaft.
It is a force, and the only force, that turns the wheel, or drives the hydraulic pump.

The amount of torque an engine produces is a result of it's design and it's size. It is a measured force, we measure it on a dynometer.

Horsepower is the rate at which torque can be applied. It is a function of the torque the engine produces and the rate (rpm) at which the engine can turn or twist the shaft. It is a derived figure, we don't measure it directly, we get it by measuring torque and then multiplying torque by rpm.

Torque is the force that does all the work. Because it is a force, we can multiply it with a transmission. (we also use transmissions to maintain internal combustion engines in the rpm range where they most efficiently produce torque). We can't multiply horsepower with a transmission because horsepower is a construct.

All in all, horsepower is just a convenient term to describe the rate at which a machine can apply torgue. It is used in addition to torque because it better describes the amount of work we can get from the machine. The water wheel example illustrates this. Even though the water wheel had a large amount of torque, it's rpm was low, so the resulting horsepower was low also. With a higher rpm, you can always gear down to increase torque, so rpm range of the engine is as important as torque in determining how many horses it would take to duplicate the work you're doing with your machine. That's what the term horsepower is trying to describe....but it's still torque that does the work.

HTH,
Dave

 

[BigDave]

Bob,

That was a good article. It was geared towards cars & not tractors. So I guess you want the tractor version? I can't do that, but can I give you my thoughts.

The car wants a flat torque curve to provide constant force upon the car for acceleration. The longer & flatter the torque curve, the more useful the power is over a larger rpm range, thus less shifting that is required. Time spent shifting slows down the overall acceleration.

What does a tractor want? It wants plenty of torque to pull implements and drive PTO shafts. My tractor makes 33hp at 2800rpm, and maybe 90lbs of torque at about 1700rpm. A small car engine has as much torque as my tractor. So why not put a 90hp car engine with a 90lbs torque at 5000rpm in my tractor? Engines in this type of car typically have steeply rising horsepower curves, thus the torque curve also rises but at slower rate until it starts to level off near the rated rpm. Lets assume I put one of these car engines in my tractor. I run the engine at 5000rpm while I am running my chipper on the rear PTO. I throw in long piece of hardwood into the chipper. The load is too much and it starts to lose rpm. If it loses rpm, it loses torque, so it loses more rpm, and it loses more torque, and it quickly stalls. Now, if I have my tractor engine re-installed and throw the same log. (I glued it back together.) I will still lose rpms, but I move into a region of more torque, and it helps to increase the rpms due to the increased torque. Basically the tractor engine is self correcting under load. I have posted in the past that I have stalled my tractor while using my chipper, how can this be? I submitted such a large load on the chipper that my tractor was driven below it's max torque at 1700rpm, and it stalled like the car engine above. That's why I now watch my rpm when I am feeding large items into the chipper. So why not design a relatively high rpm engine with a low torque peak? It's possible; just look at your pick-ups & SUVs. Higher rpms cause more wear on the moving parts. Higher rpms engines are more costly to build...basically trying to overcome the increased wear. Remember from the article, the 12rpm water wheel was a couple of hundred years old. If that wheel spun at 5000rpm, how long do you think it would have lasted?

Here's my question; Is a live PTO is better than an independent PTO as far as torque is concerned. My live PTO drops in rpm as load increases, thus realizing higher torque in proportion to engines increased torque. Does an independent PTO maintain speed when the engine RPMs drop? If so, it doesn't realize the increase in engine torque, just the drop in horsepower and thus PTO torque. Am I confused?

 

[EdKing]

<font color=blue>Does an independent PTO maintain speed when the engine RPMs drop?</font color=blue>

PTO speed, weather nonlive, live, or indepenant, is directly controled by engine RPM's. The difference in the 3 PTO types is how they are connected to the engine. Non-live is connected using the main clutch, live has a two stage clutch, and indepenant has independant clutch.

 

[Richard]

My basic understanding:

Torque is what GETS you going
Horsepower is what KEEPS you going

/w3tcompact/icons/smile.gif

 

[Trev]

Thanks for the great posts, folks! I am going to need some time to digest them and ponder (my brain isn't running at the speed of the latest Pentium) but I think it's slowly sinking in.

This reminds me of when I was a kid. I was trying to get a handle on "centrifugal force." When the light finally came on, I realized that they had made up a term for which there was no need. We already had a perfectly good term for this phenomena; "inertia!" I'm starting to wonder if this "horsepower" thing isn't another example of the same thing. What's that famous saying ... when the mind is lost at sea, a new word provides a life raft. /w3tcompact/icons/smile.gif

But I think I'm still missing something in the physics of this.. I think it's the torque band... how wide it is. This seems to be central to the thoughts I've heard expressed so far.

Is it perhaps time we stopped measuring output in reference to horses? /w3tcompact/icons/smile.gif

<font color=blue>Here's my question; Is a live PTO is better than an independent PTO as far as torque is concerned. My live PTO drops in rpm as load increases, thus realizing higher torque in proportion to engines increased torque. Does an independent PTO maintain speed when the engine RPMs drop? If so, it doesn't realize the increase in engine torque, just the drop in horsepower and thus PTO torque. Am I confused?</font color=blue>

The way I've understood it, which may of course not be the correct way of understanding it, is that the only difference is whether pushing in the clutch disengages the PTO or just the tranny/driveshaft. Independent; the PTO keeps running ... live; the PTO clutch disengages too. If I'm right, it shouldn't make any difference at all.

 

[budman72]

I've heard it said that you talk horsepower, but you drive torque.

Torque is measured on a prony brake - a clutch connected to a lever that has the other end sitting on a scale. Start the engine, engage the clutch and read the force on the scale. Torque = force X length of the lever.

For horsepower, HP = 2X Pi X rpm X torque, so HP is porportional to torque X rpm.

Diesels give torque at low rpm, modern engines give less torque but rev higher. A small, high revving engine with low torque can have the same HP as a high torque, low revving engine, but don't expect it to pull a plow at 540 rpm /w3tcompact/icons/laugh.gif

 

[Anonymous Poster]

Hi Trev and all,

The way I look at it, when it is all over, is that it is torque that makes the motion happen.

I can’t claim to understand why we need to give both torque and HP ratings for engines. Others can explain that. If I knew why in the past I forgot.

But, I do know a high HP, high RPM gasoline engine, and a high torque, low RPM diesel, can both do the same work. That is, if the gas engine is connected to a transmission that is geared low enough to make that happen.

Given the same load variations, the higher RPM engine would probably experience wider absolute RPM variations, but perhaps the percentage change would be about the same.

If an engine did not output torque, we would not be able to move our tractor wheels. Hp is energy, just like heat is energy. You can convert between HP and KW and BTU and other energy units. But a flame rated in HP, rather than BTU will not make a vehicle move…unless you see it and do the job yourself /w3tcompact/icons/smile.gif.

There is a trend, by the way, to rate electric motors in KW rather than HP these days…not sure why…

I guess what I am getting at is that both torque and HP are measures of engine performance and can be used for comparison purposes. But it is the torque that the engine outputs at the end of the crankshaft that actually makes something move.

Low torque, high RPM can be converted to low RPM high torque with the proper gear box.

BUT, if the crankshaft of the engine was somehow just not connected at the output end [stripped splined shaft, or for whatever reason] that engine, if it still operated, could provide heat to the garage it was in, and that heat could be stated in term of HP, BTU or KW…while no usable output torque was produced. The tractor would not move but the room would be warm…/w3tcompact/icons/smile.gif and the heater would have a standard HP rating!

Hope these sound like more than random thoughts…

Bill in Pgh, PA

 

[BigDave]

/w3tcompact/icons/blush.gif Thanks for clearing that up. I was wondering what made it better. I guess you can engage an independent PTO while the tractor remains in gear and moving. Or am I still confused?

 

[hazmat]

<font color=blue>Low torque, high RPM can be converted to low RPM high torque with the proper gear box.</font color=blue>

True, but you actually lose torque & power due to driveline friction etc. Better to start with the appropriate motor for the application.

Ever play with the lego sets as a kid? I remember using every gear I could find to make a hand cranked windmill. well I geared it up so much that I wasn't able to turn it. The friction force was greater than my 10 year old hand could produce.

 

[Anonymous Poster]

Is this torque or horse power

 

[Trev]

<font color=blue>Diesels give torque at low rpm, modern engines give less torque but rev higher. A small, high revving engine with low torque can have the same HP as a high torque, low revving engine, but don't expect it to pull a plow at 540 rpm...</font color=blue>

Well, if you geared it down low enough (say, about one inch per hour forward speed) it might pull a plow, no? /w3tcompact/icons/smile.gif

Seriously, I keep coming back to the feeling that there is something fundamental in what I am missing here. When I feel that way, generally I try to go back to basics and start from the beginning ... hoping to spot where I'm going wrong. In this case, that approach may or may not work:

Let's say we start with the idea that a given amount of "energy" is needed to do a given amount of work. You can dole the energy out slowly, and accomplish the job slowly, or you can dole it out fast and get the job done quickly. But the amount of energy needed will be a constant. (Let's forget for the moment about incidental losses in the drivetrain, gears, and so on.)

Some jobs need to be done quickly, like drag racing.. while some other hypothetical job isn't time-critical (you leave for vacation and when you return it's completed.)

Hmm.. I think this isn't leading in a useful direction. All of the above is probably either obvious or simply incorrect. /w3tcompact/icons/smile.gif

Let's try again:

An engine needs to output energy in a way that's useful for the given application. For pulling stumps, all you need to do is find the right gearing and almost any engine can do it. For drag racing, you need to avoid the wasted time of shifting ... so what you want is an engine that can maintain its energy output over a wide RPM range.

Could it be that "horsepower" really reflects how wide this RPM range is for a given energy output? For example, if we need 50 foot pounds of torque, an engine that can generate this much between 3000 and 4000 RPM would be seen as having a lot less horsepower than one that can generate it between 1000 and 10000 RPM?

Okay, I've displayed enough ignorance for one post. /w3tcompact/icons/blush.gif

Thanks to everyone who's replied!

 

[Trev]

<font color=blue>Is this torque or horse power</font color=blue>

Actually, that's an excellent question! We keep thinking in terms of twisting force exerted on driveshafts and wheels, but here is a form of energy that doesn't lend itself to that kind of analysis at all. /w3tcompact/icons/smile.gif

 

[BigDave]

O.K. it took two different Physics text books /w3tcompact/icons/clever.gif, and the great inputs from my fellow TBNers who contributed to this post, but I think I got it. /w3tcompact/icons/grin.gif

In plain simple english,

Torque is the amount of force that a rotating shaft can apply to an object.
Horsepower is the speed at which it can supply that force.

Remember from the original article:

HP = (Torque * RPM) / 5050

Rearrange it so that it makes sense to tractor folks:

Torque = (5050 * HP) / RPM

If the rpms are higher, the horsepower must be higher to maintain the same amount of torque. Lower the RPM's, and you lower the horsepower requirement.

This is why manufacturers don't specify HP alone, they always specfy HP at a particular RPM. My tractor is rated at 33HP at 2800RPM, but that's car talk. What really matters is what comes out of the PTO. It delivers 27 PTO HP at 540 rpm, thus from the equation it makes about 250 lbs of torque at the PTO, or to clarify, 250 pounds of force in the direction of rotation.

 

[ddl]

Torque is rotational force, t=Fl, where t= torque. time is not part of the equation. we increase torque by increasing the size of shaft or raduis of the gear. If we apply a torque and rotate a shaft a distance, work has been accomplished. Horsepower is a measure of work per time. thus we have HP= (torque*radians)/time and since we are dealing with rotational work, as the size of shafts or gears change horsepower developed at that point change. engine horsepower vs PTO vs wheel horsepower.
Dan L

 

[RobS]

Here's a slightly different way to look at it Bob. Not sure it will help, but what the heck...

Grab your handy dandy torque wrench. Put your biggest socket on it and find a matching large nut on your tractor. Make like you're tightening the nut and you'll see torque in action. The key is that you can apply a lot of torque without even moving the nut. If it's like the blade bolts on my bush hog, you can apply enormous amounts of torque without budging a thing. Now consider all that work you thought you did trying to budge a frozen nut... lots of torque but no movement. Your torque wrench was moving at ZERO RPM! Thus, no horsepower was developed and in spite of your sweating and swearing, us engineer types would say you did no work! Again, lots of torque but no power as you didn't actually move anything.

Now take the same example but this time the torque wrench is moving. Voila, you've got some horsepower. Not much, but a little.

Now to engines. Engines don't like to operate at zero rpm. They need a little speed just to keep running. If you hooked your engine up to a torque wrench with a slip clutch you could easily measure the torque (someone else mentioned the Prony Brake method). The engine speed while it's doing that work is what gives you the horsepower. The torque/power curves you see are generated on a dyno at max output conditions. Rarely would any of us operate our tractors/vehicles like that.

Hope this helps, it is a confusing subject /w3tcompact/icons/smile.gif

 

[Trev]

Hi Rob,

<font color=blue>Now take the same example but this time the torque wrench is moving. Voila, you've got some horsepower. Not much, but a little.</font color=blue>

Actually, that did help!

In a way it's kind of like earlier posters explained, but I was still stuck in a dead-end train of thought at that point so it didn't sink in.

The concept was a hard one for me.. and still needs a tad more thought.. but this nudge in the right direction helped.. so thanks again.

Bob

 

[Anonymous Poster]

These explanations all appear to be technically correct, and make a great deal of sense, but you have to wonder sometimes how they apply to the real world. Looking at the specs for the new John Deere 4x10 series tractors shows some strange numbers. If you compare the 4310 thru 4710, all the engines are rated at 2600RPM, and of course the PTO is rated at 540RPM, so you would expect to find a linear relationship between engine torque and PTO horsepower using the gear transmissions in each case:

4310 - 73.1 ft lb, 27 PTO HP (3 cylinder)
4410 - 81.0 ft lb, 29 PTO HP
4510 - 73.7 ft lb, 33 PTO HP (4 cylinder)
4610 - 78.7 ft lb, 37 PTO HP
4710 - 86.3 ft lb, 41 PTO HP

The 4410 is the torque champ (for the money), but sure doesn't show the PTO HP you would expect. It's hard to believe that its gear transmission is that much less efficient than a very similar transmission in the larger tractors.