Does HP alone influence to the "power" of a tractor?

   / Does HP alone influence to the "power" of a tractor? #21  
Dan, you seem to well informed in diesel theory, and maybe you can help us understand this theory.
In the motorcycle (gasoline powered) world, horsepower is a calculated number based on the torque at 5250 rpm. Now, we know most diesel engines won't turn 5250 rpm, but the graph is true at any given rpm. Can we calculate torque based on the gasoline calculation chart?
My 2 cents comment on this excellent topic is "Why don't manufacturers publish the torque of our tractor engines." Torque is where it's at, not horsepower...
 
   / Does HP alone influence to the "power" of a tractor? #22  
Its a mathmatical calculation to compute horsepower.

Equation: P=TN/5250


Where:

P = Power, hp
N = Rotational shaft speed, rpm
T = Torque, lb-ft

Peak power would not always be at 5250 rpm. I'm just guessing here, but it must just be a number they show the h.p. at. Alot of motorcycles would make peak power at a high RPM.

Motor cycle engines are a good comparision to a diesel engines in the fact that they are almost exact opposites. For example alot of motorcycle engines have very light weight internals and rev very high, often over 10,000 rpms. They are moving fairly light loads and make alot of h.p. for the size of the engine but very little torque. The one disadvantage of a diesel is the internals are very heavy. They have to be because of the very high compression ratios. They don't rev very well, but make alot of torque for the displacement, but low hp. Thats why torque is where its at for diesels, but they still list hp as the standard among manufactorers, probably becasue that is the way it has always been.
 
   / Does HP alone influence to the "power" of a tractor? #23  
Dan, you seem to well informed in diesel theory, and maybe you can help us understand this theory.
In the motorcycle (gasoline powered) world, horsepower is a calculated number based on the torque at 5250 rpm. Now, we know most diesel engines won't turn 5250 rpm, but the graph is true at any given rpm. Can we calculate torque based on the gasoline calculation chart?
My 2 cents comment on this excellent topic is "Why don't manufacturers publish the torque of our tractor engines." Torque is where it's at, not horsepower...

I think you misunderstand the (Torque * RPM)/5252

5250 (5252) is a constant, not the speed at which HP is measured. HP can be specified at any speed, as long as you know the Torque Produced (measured or calculated) and the RPM's

There are torque/hp curves published for many tractor models.

Andy
 
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   / Does HP alone influence to the "power" of a tractor? #24  
Its a mathmatical calculation to compute horsepower.

Equation: P=TN/5250


Where:

P = Power, hp
N = Rotational shaft speed, rpm
T = Torque, lb-ft

Peak power would not always be at 5250 rpm. I'm just guessing here, but it must just be a number they show the h.p. at. Alot of motorcycles would make peak power at a high RPM.

Motor cycle engines are a good comparision to a diesel engines in the fact that they are almost exact opposites. For example alot of motorcycle engines have very light weight internals and rev very high, often over 10,000 rpms. They are moving fairly light loads and make alot of h.p. for the size of the engine but very little torque. The one disadvantage of a diesel is the internals are very heavy. They have to be because of the very high compression ratios. They don't rev very well, but make alot of torque for the displacement, but low hp. Thats why torque is where its at for diesels, but they still list hp as the standard among manufactorers, probably becasue that is the way it has always been
.[/QUOTE][/SIZE]


Thanks, Dodgman, for the formula. I thought I had it in my database, but never could find it. But... I'm not all that bright. IF you took a 34 hp Kubota for instance, what would the torque be @ 3000 rpm?

Max torque and hp do not always (actually - seldom) match though. For instance, my motorcycle puts out 95 hp max, but 114# of torque. Max torque is at a much lower rpm than max hp, naturally, as would a tractor diesel be. (Heck! Aren't Harley's pretty much tractor motors anyway?:)) These numbers are verified on a SuperFlo dyno.

Hopefully, I have not diverted the original question; if so, please accept my apology. But since torque is the real number that we tractor owners are interested in, I would hope that the manufacturers would at least post those figures in the specs.

Thanks
.
 
   / Does HP alone influence to the "power" of a tractor?
  • Thread Starter
#25  
Reg, yes I have all the time in the world because my living is made else where. If it's not obvious from my questions, I'm new to this. :)

"Drawbar force DOES matter, but that can be achieved with a very small engine, very low gearing and very high weight - as long as you have the TIME"

Drawbar force is the force that the tractor is able to pull. Is that correct? Is drawbar force a number that can be calculated based on other numbers that can be found in the tractor's manual?
 
   / Does HP alone influence to the "power" of a tractor? #26  
Andy MA

This is great conversation isn't it?
Thanks for the question Mr. Dodgman, to solve for torque, the formula is:
Torque = HP x 5252
RPM
Your 34 HP diesel at governed speed (revving flat out, has reached the max speed the governor will permit) 3000RPM, the engine will produce 59.52 Ft Lbs of Torque. Interestingly, this is not the engine's peak torque value. In other words, this is not where the engine is at its strongest. At rated speed, (the RPM point for published HP) which is likely 2600RPM on your engine, the torque is 68.8 Ft Lbs.

An engine rated at 2600 RPM might have a peak torque set point of maybe 1800 or 2000 RPM. I can't verify this because I've never seen a Kubota HP or toque curve, let alone a BSFC (brake specific fuel consumption) curve, so it's hard to tell. Kubota engines are somewhat similar to other diesel engines (actually better than others) so this example may work; say you are working your engine hard, the engine is at full throttle and the tractor plowing on hard ground, and you lug down to maybe 2000 RPM (if that's the engine's peak torque RPM), HP would have dropped off slightly to say 28 or 30 HP at 2000 RPM, then the engine would be producing 78.8 Ft Lbs torque.

You may have all heard the term torque rise? This term defines the difference (as a percentage) between torque available at and engine's rated speed versus torque available at peak torque RPM. For example, if your engine is rated 34HP @ 2600 (slight drop-off to 30HP@ 2000RPM, at full throttle and full load) the math looks like this (78.8 peak torque divided by 68.7 torque at rated -1 x100 = 14.7% torque rise, in other words, your engine has gotten stronger as RPM dropped off (under full throttle at full load and lug down).

The trick to fully understanding torque vs. HP is hidden in the reason behind why an engine gets stronger as it lugs down. We all say such things as "HP will get you there fast but it is torque that turns the wheels" etc, and this is very true.
The reason lies in the engine"s speed governor. Under full load conditions and under full throttle application, the engine's governor tries to maintain the speed it was designed to maintain at full throttle. (Remember all governors are speed limiting devises, they govern an engine's max speed and minimum speed at idle). As we lug down an engine the governor sees full throttle demand by the operator, yet the engine is slowing down, so the governor begins (through the fuel pump's control rack or other such devices on electronic controlled engines) to deliver more fuel in an attempt to regain the engine's governed top speed. The governor will continue to increase fuel to the injectors until a maximum set fuel delivery point is reached (this is usually designed to coincide with the engine's peak torque set point.

Good God, I am long winded .... sorry folks. More later if requested.

Dan
 
   / Does HP alone influence to the "power" of a tractor? #27  
The older engines had heavier crank assemblies, was harder to pull the engine down under load, hence better sustained torque under load.
 
   / Does HP alone influence to the "power" of a tractor? #28  
I have a good grasp on the hp and torque concept but it is hard to fully understand the differences in my ZTR and tractor.

The ZTR is 21 hp and is made to cut grass at 3600 rpm's. Top speed is around 8 mph. I bet 2 guys could grab the rear bumper and keep it from moving anywhere while it is given full throttle. The unit weighs probably 500 lbs.

The tractor is only 17 hp at around 2800 rpm's. Top speed is around 10 mph. It would easily pull my truck around the yard and does well with a box blade fully dug into the ground. The tractor weighs about 2000 lbs.

The ZTR is gas and the tractor is diesel, but it is still hard to understand the differences in power applied and usable power.

ZTR is designed to transfer most of engine power to the mowing deck. It needs only fraction of the available power to move. If you look at the opening of the throttle you might see that the engine doesn't run at full power when stopped with spining wheels. Engage the deck and the throttle opens far more.
Tractors are designed to transfer all the available power to the ground. There is a relationship between mass (more mass equals more traction) and pulling power. If you have enough mass to have infinite traction you can pull the earth even with small tractor. Provided that you have fixed place in the space and gear low enough.

Maybe to understand it better we need to add another term: Work. Work = force over distance. If you have two tractors pulling the same load (but with different HP) running at top power the tractor with more HP could move faster and thus go longer distance. Or pull larger load the same distance. Stronger tractor could do more work and that what matters.
 
   / Does HP alone influence to the "power" of a tractor? #29  
The older engines had heavier crank assemblies, was harder to pull the engine down under load, hence better sustained torque under load.

Heavy flywheels help only for a very short period, though long enough that if you know what you are doing and "listen" to what is happening you can back off.
A sustained load will eventually load an engine down.

To the O/P's question;
It is MOSTLY about "Pulling" force and the most limiting factor to that is usually tractor WEIGHT, more accurately load on the driving tires.
Tire ballast is a very effective and low cost way of increasing this, adding as much as 25% (maybe more) to the weight and it is LOW, so also helps stability.
Just about any tractor you can buy will be geared low enough in it's lowest gears to spin it's tires at very low engine speed (some even on black top), so "FORCE" is available, but it isn't effective until it is coupled to the ground.

Why "Horse Power" as a unit of measure ?
Well, for tractors it might have made marketing sense toward the end of the horse drawn era - today ? I suppose it depends on whether you relate to automobile horse power.
I am told (though it was "before my time") that tractors used to be rated by their plow capacity and that was a more meaningful measure of farm work when figuring if they could replace horses. There were "One bottom" and "Two bottom" tractors, meaning they could pull one or two shares (through some uncalibrated soil type to some unspecified depth and width).

In REAL HORSE terms; I am told by a friend that the Amish near her have a nine horse hitch. I have never seen it, since I have never been there when they are plowing.
The THOUGHT of nine draft horses plowing is kinda awesome (to ME) and a reminder that many agricultural tools have evolved from horse drawn versions.
e.g. the design of small plows has evolved from plows used at walking speeds.
Not to get too carried away with this, but nine Belgians have traction largely because of the WEIGHT they put down and the very small area that they put it down on, i.e. PRESSURE, they DIG IN !
 
   / Does HP alone influence to the "power" of a tractor? #30  
"Actual production requires both the potential to produce and the opportunity to produce."

This is a profound statement regarding dairy genetics, but it is an important statement in nearly any endeavor.

Horsepower of an engine provides the potential horsepower of a tractor.
But just as a running engine on a pallet is not a tractor, engine horsepower is only the potential horsepower of a tractor. Designers must provide the opportunity to apply that horsepower to a load.

If you want a puller, it must be heavy, with aggressive tires, and in my opinion, the tractor likely must be long. Proportionately shorter tractors can be build if all the wheels are full size tractor tires, or if you go to tracks instead. Otherwise, you have to add a great deal of weight to the front end, sometimes to the point of challenging its structural strength.

If you see a case where the tractor has plenty of power, but is otherwise light and short, they mean for you to take the power out as pto power, or hydraulic power.

If you see a case where there is no possible way to get the power out to the real world of work, then it is either a gimmick, or an engine longevity play.
Engines running below capacity are likely to have a longer service life, but often at a lesser fuel efficiency. HP inflation on a riding mower will likely allow it to last longer, but if it gets out of hand, it will likely guzzle fuel.
 

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