Is Horsepower the whole story?

[N80]

Where is a tractor's HP measured from, flywheel or rear tires? In car and light truck specs it is at the flywheel, which provides very little useful information to the buyer but sounds real good. The HP rating at the rear wheel (or front in a front wheel drive car) doesn't sound as impressive but sure would help when making comparisons.

 

[carpenter383]

Given the formula posted, it then means that torque and HP are always equal at 5252 RPM. Interesting.

In therory yes, but I'm pretty sure I've seen dyno charts where they didn't cross at 5252.

 

[Bill_C]

In therory yes, but I'm pretty sure I've seen dyno charts where they didn't cross at 5252.

Yep, in reality internal combustion enginestorque/horsepower curves are full of spikes and squigles... they are not constant horsepower or constant torque. A motor may make 100 hp at one rpm but a few rpm lower, it's got nothing (a 2 stroke dirt bike motor for example). A constant horsepower motor would be 100 hp for example throughout its entire RPM range, with high torque at low RPM's and little torque at high RPM's.


The torque/horsepower discussions seem to devolve into heated debates. Some folks don't seem to realize that torque, hp, and rpm numbers for a particular motor are all integrally linked--know any two, the other third is mathematically fixed. Others wrongly seem to treat engines as constant horsepower (or constant torque) not understanding that a few factors can change the curves drastically, maybe adding a spike here or there but perhaps introducing a big dip in power (or torque, take your pick) at another particular RPM range.

Oh, and the 5252 is just a conversion factor, use different units (kw instead of HP) and the number is different; use inch-lbs instead of ft-lbs and the curves will never cross! The units determine where the two plots happen to intersect (or not).

Back to the practical side: my diesel tractor is rated a lowly 13 hp. It pulls a 5 ft rear finishing mower through fairly tall grass without hardly breaking a sweat. My 18 hp gas garden tractor, with only a 42 inch mowing deck, can stall in the same grass.

 

[Orange 08]

I will go with Horse Power as the measure of work.

I read somewhere about torque that if I had a lug nut torqued to 51 pounds (pounds foot) and came back with the torque wrench set at 50 the nut won't turn. If the nut won't turn then no work is happening; even though torque is happening.

If work is happening then it can be measured in horsepower.

If the example used if both units have hydrostatic transmissions and both can be set at their max horsepower RPM the one with more horse power will win. It shouldn't matter what torque or speed the engine makes to get the horse power.

 

[GaryCrowell]

Lots of misunderstanding here as to the relationship of torque to horsepower.

In the end, horsepower is the really important number.

What many don't understand is that torque is a component of horsepower. Torque is a force and without it you can't have horsepower. There is no "Horsepower vs. torque", they're not competitors, one is a part of the other. It's kind of like saying "what should I buy, a car or an engine?", without an engine you have no car.

There are two components to horsepower, torque and RPM, it's a measurement of the rate of applying torque. You can either apply a large amount of torque at a slow rate, or a small amount of torque at a high rate, either one will get you to the same horsepower.

People get hung up on the question of why their 20 HP lawnmower is capable of a lot less work than their 20 HP tractor and they try and simplify it in their minds by saying "the tractor has a lot more torque, so the torque must really be what's doing to work and HP doesn't matter", well that's not really correct. First off, what's being quoted as horsepower is actually the peak horsepower of the engine, which will occur at a certain RPM. A 20 HP lawnmower engine running at it's rated RPM will do exactly the same amount of work (work of the physics definition) as the 20 HP tractor engine running at it's rated RPM. If you had them both hooked to an equally efficient constantly variable transmission which allowed each to operate at it's own peak HP RPM, they would do the same amount of work if you swapped them between the tractor and lawn mower. Problem is, that's not real world, we don't have extremely efficient CVT's and even if we did, a 20 HP gas lawnmower engine isn't going to hold together very long running at it's rated RPM, a tractor engine will do it all day long. There's the duty cycle of the engine, the lawnmower engine will produce 20 HP, for about 10 minutes till it self destructs, it's not designed to put that power out continuously. The tractor engine being much heavier and robust will run all day long putting out that power. Then there's the fact that we rarely run engines at their rated RPM. Look at the HP for the lawnmower engine, it might produce 20 HP at 5000 RPM, but drop the rpm's to 2000 and it will produce almost no horsepower, the HP curve is a spike. Look at the tractor HP curve, it's going to be a lot more flat and it's 20 HP might come at 2000 rpm, but drop the RPM to 1600 and it's still probably putting out 15 or more. And yes, that's because the torque output is much greater from the tractor than the lawnmower, and torque is a part of horsepower remember. In other words, if you take that lawnmower and run it off of it's rated HP RPM, it's going to be very weak because it doesn't put out much HP at that RPM, whereas the tractor engine will still be producing good HP all across it's RPM range.

At the end of the day, what determines the amount of work an engine will do is it's Horsepower, that's why you see all heavy equipment rated in horsepower and somewhere buried deep in the specs page it might tell you the engine's peak torque output. Yes, a 20 HP lawnmower engine is a VERY different animal than a 20 HP tractor engine, but the reasons are a bit different than most people think.

 

[N80]

Another problem is using the strict definition of work. For most of us that is meaningless. For us, work is not part of an equation. Work is the things we get done whether it is pulling a plow, pushing a tree over or mowing with a belly finish mower or moving a pile of dirt.

Someone who doesn't understand torque and hp and 'work' might be confused by the 20 hp riding mower vs the 20 hp CUT. But, intuitively they will understand that the CUT can do what the riding mower can plus an infinite number of things that the riding mower can never do. Torque, as a concept, does help you understand that. Gearing, as a concept, helps also.

But, I must say, I don't really agree the HP is the really important number. Each number is important. Its level of importance depends on how it is being applied.

 

[dodge man]

I agree, in a tractor, the HP number is way less important than the torque number. My BX2350 is listed as 23 hp at something like 3200 rpm's. They also make a B2320 that is listed at 23 hp also, but at a lower RPM. This engine is also bigger in displacement. I'm almost sure the 23 hp engine in the B makes more torque than the 23 hp in the BX, but you can't tell because you have to look at the torque numbers, and they don't even list them.

On the other hand, some thing like a motorcycle, which may redline at over 10,000 rpms, horsepower is more important.

 

[RoyJackson]

Boy...this thread is as bad as a "Gear vs. Hydro" thread.

 

[_RaT_]

Here's how I keep Torque and HP in perspective:
Bear with me on this ...

Torque is a unit of Work: It's a force acting through a distance.
I like to think of this as carrying a 100 pound box up a 10 ft ladder.
100 pounds = force (force of gravity is pulling it down)
10 ft = distance (distance you have to work against gravity)
If you dont have enough torque, you will stall out before you reach the top of the ladder and not get anything done.

Horsepower is a unit of Power : Power is the time rate of doing work.
HP tells you how many times you can carry that 100 pound weight up that 10 ft ladder in an hour. So power is important, too, if you want to finish before sundown.

So you need enough torque to get the job done at any speed you choose. As was mentioned by others, most Diesels have lots of torque over a wide range of speed, so they can usually get the job done over a range of lower speeds. Gas engines (at least most big box lawnmower engines) usually develop their claimed HP at some high rev speed, and only at that speed. Through the rest of their lower speed range they can be very weak. So you have to keep them pegged to get any heavy work done. They usually don't last very long when run that way - something breaks.

-Jim

Thanks, good explanation. By the way, I have been running my 1987 Snapper 14Hp Kohler riding lawnmower every year and even now it still starts great, runs great and has never needed a repair. Whats that 23 years? Out here in my part of the country you run your mower every month that includes December because that's when we are still sucking up falling leaves. Right now the grass is really starting to take off so its once a week mowing again.

 

[ModMech]

You can have a world of Tq and produce no work (HP) - if you don't MOVE anything. A 200# person hanging from a 10' long tree limb is exerting 2000 lb-ft of Tq, but producing zero HP.

5252 is a unitless "constant", it doesn't convert anything to or from anything else; it is part of a mathematical equasion just like pi is, nothing more nothing less. Yes, HP and Tq ARE ALWAYS EQUAL at 5252 RPM, assuming of course the engine is capable of making that many revolutions

Tractors are rated in three common ways, in terms of their power:
- Drawbar HP
- PTO HP
- Engine (crankshaft) HP

Of those, only PTO and drawbar are of any real value when comparing one tractor to another in terms of how much work it can do.

 

[Bill_C]

Well, torque and horsepower would be equal at 5252 RPM...if the units of torque were ft-lbs. This means that 1 horsepower would produce 1 ft-lb of torque at a rotational speed of 5252 rpm. Or 100 hp would produce 100 ft-lbs of torque at the 5252 rpm.

If the units of torque were inch-lbs, then they'd be equal at 63024 RPM. 1 hp produces 1 in-lb of torque at 62024 rpm.

And if instead of horsepower we were using kilowatts, the number is something else. And something else if torque were Newton-meters.

It's a constant--but it's dependent on the units being used, so that's why I sometimes call it a conversion factor. (The 5252 can be derived, I've done it, but it's late and I've got a migraine so I don't feel like getting out my textbooks)

 

[ShenandoahJoe]

... and now I know why my tachometer has a mark indicating the RPMs I should use to run an attachment on the PTO.

 

[ModMech]

Well, torque and horsepower would be equal at 5252 RPM...if the units of torque were ft-lbs. This means that 1 horsepower would produce 1 ft-lb of torque at a rotational speed of 5252 rpm. Or 100 hp would produce 100 ft-lbs of torque at the 5252 rpm.

Correct, at 5252 RPM HP = Tq, period.

If the units of torque were inch-lbs, then they'd be equal at 63024 RPM. 1 hp produces 1 in-lb of torque at 62024 rpm.

Um., no.

If the units of Tq were "In-lbs", 1 HP @ 5252 RPM = 12 lb-in (instead of 1 lb-ft). Changing the units of measure does not change the RPM at which Tq and HP are equal. Ergs, dyns, Watts, N-M, it doesn't matter, HP = Tq at 5252 RPM.

 

[Bill_C]

I'm not sure why or how you are disputing this.

First, let's assume we are discussing a theoretical constant-horsepower motor. It's rated 1 hp from 1 rpm to 100,000 rpm for example.

If torque is specified in inch-lbs, as it often is for fractional horsepower motors such as servo motors and stepper motors, then the formula is merely converted by multiplying the 5252 by 12. The curves then cross at 63000 rpm. This formula is just as common, and just as valid, as the formula using 5252. At 63000 rpm, the theoretical 1HP motor produces 1 inch-lb of torque. At 63000 rpm, if we want to use different units, the same 1 hp motor produces 1/12 ft-lbs of torque. This is indisputable.

Torque is always "equal" to Hp times a constant for a given RPM. The constant merely changes depending on the units. I can shift the curves anywhere I want merely by changing the units. There is nothing magic with respect to the 5252 rpm.

From the old physics books: Torque equals Force times Distance, or T=FxD

Work equals Torque times delta Theta, where Theta is the angular displacement in Radians. Power is the work done in a unit of time, which means Power=W/t. Substituting backwards, this equates to P=Tw where w is the angular velocity, in radians per sec for example. We can convert radians/sec back to RPM using the appropriate conversion factors. Then we decide whether torque will be in ft-lbs or inch-lbs or n-m, plug in the appropriate conversion factors...and we arive at 5252 for the constant. Or 63024. Or whatever, it depends on the units.

 

[SPYDERLK]

Yay Bill C. :thumbsup:
larry

 

[Typhoon]

I always had trouble understanding torque vs. hp, until I read this little exerpt in a physics book, and it cleared it up to me. Just remember this...... A windmill (the great big wooden ones that they used back in the day to mill flour, etc) exerts approximately 5 horsepower, with about 1,200 foot pounds of torque!!!!

 

[rphinney]

Gary has it right. The lawnmower engines advertise "maximum output" horsepower numbers. If you look at the output curves the engine suppliers put out there will be 2: "maximum output" and "recommended maximum output". The recommended maximum output will typically be much less. If you tried to mow at maximum output you wouldn't be mowing for long.