Is Horsepower the whole story?

[ShenandoahJoe]

Ads in the paper this morning for riding mowers at the big box store. Fairly common to see 20 HP (+/- 1) engines on them. Now, I've used one like that before. The manufacturers seem pretty proud of that horsepower rating, but no way could that engine drag a 4-ft rotary cutter up a 3:1 slope like my little Kubota's 16 HP engine does, without complaint.

Is there something besides horsepower that I should be looking at, to figure out how much work a tractor can do?

 

[RobertN]

Torque curve. If you look at the curve on those lawn tractors, the peak power is at a narrow power/rpm band.

Your Kubota has a wide torque band. The peak hp might not be high, but it makes torque over a wide range.

 

[aczlan]

Ads in the paper this morning for riding mowers at the big box store. Fairly common to see 20 HP (+/- 1) engines on them. Now, I've used one like that before. The manufacturers seem pretty proud of that horsepower rating, but no way could that engine drag a 4-ft rotary cutter up a 3:1 slope like my little Kubota's 16 HP engine does, without complaint.

Is there something besides horsepower that I should be looking at, to figure out how much work a tractor can do?

Torque (not that they will publish that on a big box LGT), but Torque is what makes the difference between the two. See http://vettenet.org/torquehp.html for more info.

Aaron Z

 

[bontai_Joe]

Max torque and where it occurs RPMwise is more important than just raw HP numbers. The HP number advertised is from a test motor run in a lab that may not have had a muffler on it, may not have had an air filter on it and has no load on it. In other words, it's a bit of hype to get a big number in a situation totally different than the real world.

BUT...

You also need to look at the rest of the tractor and how it's built because there is more to a tractor than just the motor. If I need to do some serious work (besides just cutting lawn) I"ll take the 16 HP diesel tractor that weighs over 1000 pounds over the 25 HP air cooled gas motor tractor that weighs 500 pounds. The lighter tractor will have less strength in the frame, axles, and driveline. Kinda comes down to what you need, if it's a tractor to cut grass on an acre and a half of flat ground, the LT with the 25 HP motor will probably work great. If you plan on plowing a garden, heavy snow removal, pulling a plug aerator, pulling a rototiller, then the heavier stronger machine is better. You can look at it this way also, there is a market for a 300 HP two seat sports car and a market for a 200 HP pickup truck. They are designed for different kinds of "work" but both have their palce.

 

[RalphVa]

My 18.5 hp 4010 is a beast. Yeah, its hydraulics bypass and keep it from bogging down every so often, but it has done everything I've wanted it to. It was awesome this winter with our 56 inches of record snow.

People want a mowing machine that lets them whack 8 inch high grass down to 2 inches or less. Need hp to do that, even though they're killing the grass (weakening it anyway) by doing this.

Ralph

 

[ModMech]

My "inFamous" saying from another forum....

"Without Tq, you have no HP."

In other words, HP gets the credit, but Tq gets the work done. This is why in most cases, a diesel will do a job better, with less apparent strain, than a gasoline engine - they produce more Tq.

The formula for HP is: HP = (( Tq x RPM) / 5252 ) => Measured Tq times the RPM at that Tq divided by 5252 = HP produced at the measured RPM.

For example, two engines, (G) = Gas, (D) = Diesel with the same HP rating....

G: HP = 20 @ 3600 rpm, so what's the TQ? Tq = (( HP x 5252) / 3600 ) = 29.177 lb-ft @ 3500 rpm.

D: HP = 20 @ 2400 rpm, so the Tq is... Tq = (( HP x 5252) / 2400 ) = 43.766 lb-ft @ 2400 rpm.

Now, if you wanted to move something heavy, would you have an esier time using 30 lb-ft of of Tq or 44 lb-ft?

 

[magicheater]

Nice post

 

[orezok]

The formula for HP is: HP = (( Tq x RPM) / 5252 ) => Measured Tq times the RPM at that Tq divided by 5252 = HP produced at the measured RPM.

I've seen this formula many times, and in theory it is probably correct, but I have a problem understanding how a "fixed" formula results in different curves for HP and torque when run on a dyno. Typically both curves are very different and not linearaly related.

 

[Baby Grand]

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

 

[RoyJackson]

Torque is important (more so then engine HP, IMHO), but don't forget gearing. It's that low gearing (combined with the torque) that allows ShenandoahJoe's 16 HP engine to pull that 4' cutter up a steep slope.

HP ratings are pretty deceptive. Most are gross HP measured with no load on the engine. Some are "estimated" and are what the engine should develop (in theory). For tractors, drawbar and PTO HP ratings are a much better indicator of what the machine can do.