Does any one know how the horse power of a tractor engine is figured compared to other engines?

 For example,my lawn tractor has a 22 hp engine,compared to say an Allis G rated at 10 horsepower.

LIke the old saying goes, there is NO REPLACEMENT FOR DISPLACEMENT...   BIG MOTORs LUG at low RPM........... Small motors need HIGH RPM to get the HP.

I dial in my scopes and shoot using minutes of angle, not milliradians.  

Torque is setting on your Red Ryder wagon at the top of a hill, potential work capability, depending on how high the hill is. HP is going down that hill, and how steep the hill is. The work will be the same, but the steeper the hill (RPM) the faster it will get done.

The lawn tractor's ENGINE is advertised as 22hp.  The TRACTOR is rated at 10hp.  There's a BIG difference.

Let's do a basic analysis of the circumstance:

Your lawn tractor engine says 22hp on the top of it's shroud, and if it performs as advertised, at the speed it's advertised to do so, under the atmospheric conditions that it was designed and set up for, the crankshaft will feed that 22hp to whatever it's connected, for as long as the engine's designed performance lifespan was intended.

When you engage the blades of your mower, the engine slows slightly, and the throttle opens.  This happens because the blades are thrashing through the air, and it takes horsepower to do it, so your horsepower at the edge of the blade will be a little less.

In the same realm, engaging the driveline includes driveline losses, so there'll be a little less horsepower available at the drive too.

Let's arbitrarily say that blade drag consumes about 4hp, and that driveline losses consume about 3hp... that means you've got 22-7= (did everyone get 15?)

Now let's look at how that power is actually used...

The machine needs to move around the yard.  At the reduction levels the lawn tractor uses to generate a walking pace, with my 215lb weight, the driveline will need practically no power (mebbie 1.5) on flat ground... only about 4hp to climb a 5% grade.  Bump it to 15%, and you'll need closer to 8hp, but after that, it won't matter, as the machine's turf tire will not generate enough adhesion to successfully climb.

The deck needs to cut grass.  Let's say the deck is a 46" three-blader, with good flow characteristics and sharp edges on good (meaning, better than mine) dry grass.  It will require about 10hp to run that deck through modest fescue at a fast walking pace.  Thicker grass at same will require 14-15hp instead.

How much power did we have left?  Ah... 15hp.  Which makes perfect sense- if you ask it to climb, and cut really thick grass, fast, it'll bog... so you 1) slow down  2) cut going downhill, and 3) make a half-cut pass going uphill.

Now for practical limitations:  With this lawn tractor, you will NOT be able to get 22hp of PULLING power.  Why?  First off... agricultural traction is a function of tire diameter (rolling resistance) and a ratio of weight on driven vs. undriven wheels.  With no rider, there's about 120lbs on the front wheels, and mebbie 80lbs on the REAR.  With agricultural traction, a soil traction tire, on soil, yields around 18-25% traction coefficient... meaning... drawbar FORCE will only be about 18-25% of the DRIVE TIRE weight, LESS whatever drag occurs with the FRONT wheels (not driven).  With only 80lbs on the back tires, you'll generate what... 30lbs of tractive effort at the drawbar?  Not much.  Sit on the tractor, and now you have 215+80= 295 on the rear wheels, and if you place your drawbar high, it'll lift the front wheels, and you'll have 295+120=415lbs so about 120lbs of drawbar pull... IF the tires don't start slipping on wet grass.

But wait- that's not all...

The transaxle in your lawn tractor... if it came out of a '60's through '80's IH Cub Cadet, it'll be strong enough to transmit enough torque to the tires to provide all that (and more), but if it's a die-cast zinc belt-driven 'module' held to the sheet metal frame, it'll litter your yard with sharp glitter and promptly stop, before getting anywhere NEAR that capacity, so...

You won't be getting 22hp worth of power pulling anything.

Now let's look at how an Agricultural Tractor is rated:

NOBODY 'rates' a tractor ENGINE.  A 'power unit' yes, but a 'tractor' ENGINE, no.

Tractors do work, and that work is done either with a belt, a PTO, or a drawbar.  Hence, their ratings are based on those loads:

Belt HP  - Measured by a dynomometer at the BELT PULLEY.

PTO HP - Measured by a dynomometer at the PTO SHAFT.

Drawbar HP - Measured by a drag sled, with strain gauge, distance measuring apparatus, and a TIMER. 

Of course, there's some obvious limitations here... if the BELT used for the FLYWHEEL test is inappropriate, insufficient length, or not drawn out properly, it might not be able to transmit all the tractor's power to the dyno.  In the same way, the tractor's TIRES may not be able to provide enough tractive effort to make use of the engine's power potential, so it 'spins out' before getting to a respectable load level.  The latter can be improved upon by adding weight (agricultural traction is highly dependant upon weight, remember?  If not, search for, and read the Wismer-Luth Agricultural Traction Prediction white paper published decades ago...).  Like the lawn-tractor, if the back wheels spin, but the tractor doesn't move, that means there's engine capacity left, but the tires can't put it to the ground.

NONE of these measurements have anything to do with ENGINE rating... because regardless of HOW the engine is tested, it MUST be able to generate MORE flywheel horsepower, to accomodate ALL losses (like gearbox friction, power steering pump, alternator drag) as well as accomodate environmental conditions (like fuel quality, ambient temperature, pressure, and humidity) as well as DUTY CYCLE... and perform that way within the manufacturer's advertised Mean Time Between Overhaul (MTBO).

The engine in your Allis G is a Continental 69CI flathead.  Using quality gasoline, It will generate around 22-28 hp at the flywheel under average circumstances.

Placing it under extreme conditions, it EITHER engine might be able to do as well, but frequently, extremes will limit the machine's ability.

OFTENTIMES, the engine's limitations will be found not to be in crankshaft horsepower availibility, but instead, on DUTY CYCLE... meaning, it'll work at some level HIGHER than rated horsepower, but for a LIMITED duration of time. 

Hint- Liquid coold motors are frequently MUCH more capable of running above their 'continuous duty' limits, for longer, than an air-cooled equivalent.

So the key to realize, is that if you're buying a big-box 'lawn tractor', the 22hp might mean 15hp worth of deck, comparing it to an Allis G will be a case of comparing apples to deer pellets.  The Allis G will pull, AND run PTO, and climb through things a 'lawn tractor' will simply not... and it all goes back to a basic situation:  Agricultural tractors were built to replace HORSES, so a 2-horse plow would be best matched to a 2-horse team, or a 2-horse tractor.

And from another perspective, if you want to determine how many horses it takes to trim your lawn, just put up a fence and let them graze...  but... watch your step.

Thanks for the explanations.

 You would think the tractor manufactures would rate them by torque instead.

The term HORSE POWER was used as WORK FOR A DAY to compare to an average horse at the time.. The motor does not get tired after 8 hours... the horse does.

Torque isn't the whole story, either.

Work = force * distance

Power = Force * distance * time.

I could take a 3hp Briggs engine, put enough gear reduction behind it to develop a half-million foot-pounds of torque, but it'd still only be a 3hp output... because of TIME.

And furthermore... a half-million foot-pounds of torque, going into a 300lb chassis with 16.6.50-8 tires, will NOT have any substantial value in drawbar force... it's just not heavy enough to create tractive effort.

Duty cycle!  Let's say a kid takes his imported squash-bug car, and puts twin turbos on it, and can get it to squirt a chassis dyno to 350hp.

Tie the car down really tight, have him bring it up to boost, dial up the dyno to 350hp, and walk away.

How LONG will that engine run, before it becomes a baptismal font of hot aluminum all over the shop floor?  Three hours?  An hour?  20 minutes?

Now take a big Cummins tractor engine that's advertised as a 350hp powerplant... put it on a dyno at 350hp, and go on a 10 week vacation.  When you get back, it will STILL be churning out 350hp. 

The industrial/agricultural rating assumes a 100% duty-cycle circumstance... it will not overheat or self-destruct at that power level.

Ag/industrial ratings are not 'under-specified'... they're spot-on.  Automotive horsepower ratings, however, are mis-stated, because the DUTY CYCLE is intentionally left out.  Why?  Because it takes you 8 seconds to pass somone on a 2-lane road, and after that, you're driving a little over the speed limit for the next 18 miles....

Shetland pony, myself

Funny you should mention Shetlands, Lou-  James Watt based his power unit description on ponies used in mines of England and Scotland... they were energetic little fellers, but not in the same realm as my grandfather's Percherons...

Yesterday, as I was climbing the south driveway with a load on the forks, I realized I'd left off an important point that is alluded to in other's posts, but an excellent example:

That 22hp lawn tractor engine, in order to produce that 22hp, had to be running at it's governed speed.  In order to climb my hill with the load I had on my back forks, it would take considerable gearing-down of that air-cooled motor.

My D17, although not generating it's full horsepower, I had it throttled way down, in first and high-range, to ascend gently...

Because the D17's front wheels were barely touching the ground.

But it climbed with no drama.  The engine was probably generating 20hp at best, but at idle speed, thus, it was all very placid, smooth torque.

When running a tractor, it is not a common thing to use ALL of it's output potential.  It is very often that the tractor needs to run at some less aggressive speed, in order to stay under control of the task-at-hand.  This is what is referred to as 'tractible power'... you don't have to fidget or fight with the machine to get the job done, the machine is more than capable of doing it at slower crankshaft speed, thus, flexible to the circumstances at-hand.

 

A 13hp repowered Massey Harris 33 plowing.

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