What does horsepower actually mean?

[TimNY]

I can't seem to find a good online answer and asking a mechanically minded friend only confused me more. So here goes a dumb question...


What does horsepower actually mean? Not the definition according to James Watt (how much a horse can move) but something that explains what it actually measures.

My confusion comes from having a john deere lawn tractor with 23 horsepower (that actually doesn't have the ability to really pull something) and a kubota tractor with 21 horsepower that can actually do relatively heavy work. Obviously horsepower doesn't explain the difference between these two machines.

thanks -- tim

 

[murphy1244]

1 HP= 33,000 foot lbs per minute. foot lbs = amount of energy required to raise 1 lb weight a distance of 1 foot.

 

[murphy1244]

I can't seem to find a good online answer and asking a mechanically minded friend only confused me more. So here goes a dumb question...


What does horsepower actually mean? Not the definition according to James Watt (how much a horse can move) but something that explains what it actually measures.

My confusion comes from having a john deere lawn tractor with 23 horsepower (that actually doesn't have the ability to really pull something) and a kubota tractor with 21 horsepower that can actually do relatively heavy work. Obviously horsepower doesn't explain the difference between these two machines.

thanks -- tim

The weight of the machines is where the torque can be applied.

 

[dodge man]

The Kubota tractor has more torque, is why is seems to have more power.

I'll give two extremes of engine design. Take a 1000 cc motercycle engine that is very high performance, its probably 4 cylinders, and makes something over 100 horsepower. Take a 1000 cc engine in a Kubota tractor. It probably makes something like 23 to 25 horsepower. The difference is how it makes that horsepower. The motorcycle engine makes it peak power at probably something close to 10,000 rpms, the tractor engine at something like 2800 rpms.

Can you picture a engine in a tractor that you have to crank over 8000 rpms? In theory, it would work, but I don't think it would last long. How about a 3 cylinder diesel in a motorcycle? It would work, but not well.

The truth is, torque in a high load, low rpm engine is more improtant, but they still rate just about everything in horsepower.

 

[hosspuller]

I can't seem to find a good online answer and asking a mechanically minded friend only confused me more. So here goes a dumb question...


What does horsepower actually mean? Not the definition according to James Watt (how much a horse can move) but something that explains what it actually measures.

My confusion comes from having a john deere lawn tractor with 23 horsepower (that actually doesn't have the ability to really pull something) and a kubota tractor with 21 horsepower that can actually do relatively heavy work. Obviously horsepower doesn't explain the difference between these two machines.

thanks -- tim

Let me try ... The deere and kubota are rated at X horsepower at the output shaft of their engine. The HP is a calculated number based on the speed of the shaft and the torque produced. example... Deere motor turns at 33,000 RPM but only produces 1 foot pound of torque. ( a foot pound is the force applied on a lever ) So it's rated at one horse power.

The kubota motor turns at 1 rpm but produces 33,000 foot pounds of torque. It too is rated at one horsepower.

Pulling ability requires traction and the ability to convert the turning of the engine shaft to the drive wheels. ( I'll wager that the kubota is heavier and has lower gearing to the wheels.) After that, more horsepower just lets you go faster.

 

[schoolsout]

Torque gets loads moving
Horsepower keeps loads moving

simple answer we use in the trucking industry

 

[ironnut]

Horsepower is a measure of power in the mechanical world much like watts is a measure of power in the electrical world. Horsepower is the product of torque and rpm. Torque is the twisting force and the rpm, how rapidly you can perform this twisting force, determines the power (horsepower). If you lift 550 pounds 1 foot in 1 second you have produced 1 horsepower. If you have a model airplane engine that turns at 10,000 rpm and with suitable gearing it is coupled to a winch and it manages to raise 550 lbs, 1 foot, in 1 second it produced at least 1 horsepower (more, unless you have a no friction perfect gearbox on your winch). A 1725 rpm electrical motor again with suitable gearing connected to the winch lifting 550 lbs, 1 foot, in 1 second also produces 1 horsepower. Obviously torque from the model airplane engine is much lower than the torque on the electric motor, much like the difference between the 20+ hp lawn tractor engine and the 20+ hp compact tractor.

Then there are muddling factors like gross hp ratings vs net hp ratings. The garden tractor manufacturer may have quoted a gross hp output at the flywheel, where as the FEL may have quoted net hp at the draw bar or PTO. It was this marketing nonsense and liberties that created the Nebraska testing laboratory in the early days of ag tractor manufacturing. Add to that the durability factor i.e. how long can that engine generate that 20+ hp and not self destruct, and things get rather confusing.

As a kid growing up we would see that the Chevy 283 V8 was rated at a 195 hp. Yet my dad's 400 cubic inch IH 856 was only 100 hp. What's up with that? One thing, the rpm at which those horsepower outputs had occurred, another how long can you keep that up?

Yes I am sure that the 283 V8 installed in a tractor suitably geared to the transmission would indeed pull 100 possibly 150 horsepower but I daresay that a few days of pulling the field cultivator would leave you with a smoking ruin of an engine, as sustaining the power output to put a 100+ draw bar horsepower on the field cultivator would overstress pistons, valves, etc. to point of failure. If you ballasted the 20+ hp garden tractor sufficiently you could pull that scaled down land plane almost as well as the 20+ hp Kabota. But the Kabota will be doing it long after the garden tractor shucked a rod or blew a transmission.

gordon

 

[Verticaltrx]

Its actually a very simple measurement.

HP = (Torque x RPM)/5252


So your lawn tractor may only have 34lb-ft of torque since it's rated at 3600rpms.

A 21hp tractor rated at say, 2200 may have roughly 50lb-ft. As related to work ability, not only does the tractor have more torque but also more gearing and weight.

 

[Bob77064]

Torque is power or twisting effort. Horse Power is more like speed.

 

[J F]

Kudos to the guys explaining it...I was having the same "wonderings" the last couple of days upon discovering this site "My riding mower has suchandsuch HP...".

Didn't quit make sense in my uneducated mind, so thanks for the explanations, and to the op for asking. :thumbsup:

 

[90cummins]

Torque gets the load moving down the road or field;
HP determines how fast you can go with it!

 

[J F]

What the heck...how did my above post end up with a link in it? Just darn.

 

[strum456]

Duplicate post dumb smart phone!

 

[strum456]

1st calculate work by multiplying force (pounds) by distance (feet) 2nd divide work by the time it took to do the work to get ft.lbs/sec 3rd divide ft.lbs/sec by 550 to get horsepower.

The above formula proves that a small force at a high speed or a large force at a low speed could produce the same amount of horsepower.

 

[bcp]

HP determines how fast you can go with it!

If one horse can run 30 mph, how fast can two horses run?



Bruce

 

[doall]

The gearing is also related to the tire sizes . The large back tires on a tractor is adding lower gears to already low gears. My toyota Tundra has 381 horse power, my new holland has 40. My tundra weighs more than the tractor, but I bet the tractor would pull the truck all over the place because of gearing and a larger foot print.

 

[scrappy isb67]

The gearing is also related to the tire sizes . The large back tires on a tractor is adding lower gears to already low gears. My toyota Tundra has 381 horse power, my new holland has 40. My tundra weighs more than the tractor, but I bet the tractor would pull the truck all over the place because of gearing and a larger foot print.

Larger tires REDUCE the mechanical advantage of the gearing. they will give better traction but hurt the power by raising(lower numerically) the final drive ratio. the only reason the taller tires on the tractor don't hurt it is because of the super low gearing that the tractor has to begin with. the tractor's gearing is 3-4X lower at least. if you put the same size tires on your truck it would have plenty of traction but no power due to the super high final drive ratio.
case in point I have a ~450hp 4wd diesel pickup, the stock tires are 32's. the axles have 3.73 gears. It had 37's put on it but the gears are still 3.73s.
the bigger tires effectively LOWERED my gear ratio to ~3.08. The truck would be more efficient and pull better with the stock tires as long as it could get traction. it has been proven on the dyno that larger tires hurt power if the gearing isn't adjusted to compensate for them.

 

[Monster5601]

Torque is power or twisting effort. Horse Power is more like speed.

Torque gets the load moving down the road or field;
HP determines how fast you can go with it!

Spot on but just to clarify, HP is how fast the torque can be applied.

 

[LD1]

Its actually a very simple measurement.

HP = (Torque x RPM)/5252


So your lawn tractor may only have 34lb-ft of torque since it's rated at 3600rpms.

A 21hp tractor rated at say, 2200 may have roughly 50lb-ft. As related to work ability, not only does the tractor have more torque but also more gearing and weight.

:thumbsup::thumbsup: I couldnt have said it better myself:thumbsup:

As to the mower vs tractor comparison......well to compare it depens on what you are doing. With the mower, its about HP. Because HP determins how well you can maintain a FAST blade speed and NOT bog down in heavier grass.

With a tractor, most of the time it is about torque being applied to the wheels to get traction. If you have the torque to spin the tires, more HP will only make them spin faster.

Also, back to the comparision, since some believe that these "small" engines are over rated on HP. Look at zero-turns. I have one that has a 25HP motor on it. You mention your tractor is ~21 or something. Put a 61" cutterdeck on it, and try to mow thick grass as heavy and fast as said zero-turn and see which engine bogs down quicker?? Because that is what HP does for you.


Another note, changing speeds (gearing) like PTO gearboxes or ground speeds does NOT change HP. Horsepower remains constant through out. Torque is the only thing that changes with a speed change.

IE: Slower =more torque
Faster = Less torque

Calculating it is easy with the formula I quoted from V...

 

[Baby Grand]

Fun fact: Torque multiplies; HP doesn't.

What this means is that you can change the Torque being delivered just by changing the speed (gear ratio). Regardless what gear ratio you are running, an engine producing 20 HP will deliver 20 HP to the final drive (minus frictional losses), even though the Torque has been increased or decreased by the transmission.

Heavy duty tractor engines can deliver high Torque over a broad range of engine RPMs. This allows flexible use of engine speed to the task.

Light duty lawnmower engines, as previously observed, only produce high torque at high engine RPM, so the HP drops off quickly below high engine RPM (HP is the product of both Torque and RPM). This results in an engine that must be "pegged" to get much done.