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  1. #21
    Platinum Member High Compression's Avatar
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    Default Re: What does horsepower actually mean?

    Simply horsepower = Work done over time. (Work Done/Time)
    "It is better to be thought a fool, than to open your mouth and prove it."

  2. #22
    Veteran Member Jerry/MT's Avatar
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    Default Re: What does horsepower actually mean?

    Quote Originally Posted by TimNY View Post
    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
    Work is defined as a force moving through a distance. Horse power is the rate of doing work, i.e a force moving through a distance per unit time. As an example, if you took a one pound weight and lifted in 1 ft. You would have done one ft-lb of work. Now if you took the same weight and lifted it 1 ft in 1 sec you would have expended 1 ft-lb/sec of power. Since 1 horsepower is defined as 550 ft-lb/sec, you would have produce 1/550 hp or 0.00181 hp. If you were really fast in lifting that 1 lb weight , 1 ft in 1/1000 of a second, you'd produce 1.81 horsepower. Again, horsepower is the rate of doing work.

    Your comparison of the JD lawn tractor and the Kubota with the same hosepower rating's has to do with how you utilize horsepower. There is another issue about how engines are rated for HP but lets not complicate the discussion with that issue to get at what I think is at the heart of your question.

    Horsepower is horsepower. If I take the JD engine rated at 23 hp and adapt it into the Kubota chasis and make appropriate changes to the drive gears and then ballast the tractor to the same equivalent weight that engine will be almost 2 hp better than the Kubota engine. (I say almost because the extra gears would cause losses to reduce the 2 hp advantage on the input shaft.) Those are a lot of if's. And we are not talking about durability performance across the speed range , etc. I'm just trying to get you to understand that horsepower is horsepower.

    The reason we have those if's is in one case we have a 2 cylinder Briggs & Stratton air cooled engine developing 23 hp at 3600 rpm on gas weighing say 90 lbs and in the other case we have a three cyinder liquid cooled diesel running at 2500 rpm developing 21 hp with an engine weight of say 400 lbs.( I'm making up the engine numbers but the relative levels are probably close to being correct and this is just an illustrative example. I'm not trying to make a case for putting a B&S engine in a Kubota!


    Horsepower = K x Torque x RPM , so we have slow the output of the B&S engine to compare the installed performance with the much slower Kubota engine. Since the HP is constant, as we reduce the driveline speed to match the speed the of the Kubota, the B&S torque increases by a factor of 3600/2500 = 1.44.

    KTk =21/2500=.0084;KTb&s= 23/3600 =0.0069 X1.44=0.0099 so the ratio of torques is

    Tb&s/Tk =0.0099/0.0084 =1.179.

    The B&S has more torque, ideally,when geared down to match the Kubota engine's speed. Again this is ideal because the losses in the additional gearing are not accounted for. But I think it illustrates the point.

    The reason the lawn tractor can't "out pull" the ag tractor is it can't put the same tractive force on the ground due the big differnce in weight between a 500 lb lawn tractor and a 2500 lb ag tractor( even if you geared it down, you have to have the addtional weight to transfer the wheel toque to the ground), so it can't utilize the power for tractive force and that should not be surprising because that's not what it was designed to do! However you see that the lawn tractor engine engine could potentially outperform the tractor engine if it were installed in the ag tractor because it has more output power.

    Now before everyone starts shouting about the Kubota's rpms be slightly different etc , remember I offered this as an illustrative example to answer the original poster's question. Go run your own numbers and you'll come to the same conclusions.
    Last edited by Jerry/MT; 05-23-2012 at 03:14 PM.

  3. #23
    Platinum Member
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    Default Re: What does horsepower actually mean?

    Quote Originally Posted by scrappy isb67 View Post
    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.
    Yeah, that was stupid thinking. And I know this. Thanks for showing my stupid. lol. But I bet my tractor will still pull my truck. lol
    NH 3040 fel 6' cutter
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  4. #24
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    Default Re: What does horsepower actually mean?

    Quote Originally Posted by doall View Post
    Yeah, that was stupid thinking. And I know this. Thanks for showing my stupid. lol. But I bet my tractor will still pull my truck. lol
    not trying to call you out but the info wasn't 100% correct. the effects of taller tires on gear ratios was wrong but you are correct about footprint. Your tractor is not all that much lighter than your truck and has a larger footprint and more agressive tires than the truck so it should be able to pull it. horsepower and gearing do nothing if you can't get the power to the ground.
    2010 JD 2320, 200cx, Frontier 1060R RFM, 2048 BB, LR1072 rake, IMatch, 5 suitcase weights.
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  5. #25
    Platinum Member zing's Avatar
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    Default Re: What does horsepower actually mean?

    Quote Originally Posted by bcp View Post
    HP determines how fast you can go with it!
    If one horse can run 30 mph, how fast can two horses run?



    Bruce
    The same speed, but they can pull twice the load while they do it.

    Good point because it defines the fact that speed is balanced against load moved over time in the rating of HP.

    Think of two rocks tied to two ropes, one big rock and one tiny rock. A 2 HP motor will pull the string with the small rock at incredible speed, but two horses will pull the big rock much more slowly. In the end both "engines" are doing the same amount of work. Theoretically if you took the little two HP engine and applied enough gear reductions to it, it would then be able to pull the big rock too. All of the gears would mean that even though it was spinning at high RPM, it was now pulling the rock as slowly as the horses do.

    HP is a measure of power. Power = work/time. Work = force x distance

    In other words HP = (force x distance)/time If you play with different numbers in that formula you will come to see the changes that you can do to "fiddle" with HP. As you increase the force (a bigger rock to move) you can decrease the distance moved per second and your HP will stay the same. If you want to move a larger distance per amount of time (go faster) you have to decrease the force required (pick a smaller rock to move). If you want things to happen faster with the same HP, you have to either reduce the distance travelled or mass of the object you move.

    That is ignoring real world things like friction,air resistance, and manufacturers who fudge their numbers, which makes a mess of theoretical calculations.

    All of this is to say that if your two tractors had the same HP, they should be able to do the same work. The difference is how they manufacturers have designed the HP to be applied to the machine. Your lawn tractor is geared to move something fast (the mower blade at high RPM) and your diesel tractor is designed to use the HP for moving something slower (a PTO shaft at 540 rpm). There are so many other things that come into play when you compare the two machines that it is easy to get confused. Things like tire grip based on mass of the vehicle and friction grip of the tires is going to add confusion.

    All the talk about torque is just adding in a different factor, because torque is basically force, except it is applied by turning instead of in a straight line. With torque, you can change the power in the same way you do for force, by changing the time or distance. If you have a little wheel turning really fast or a big wheel turning slow, they might have the same torque if they move you across the ground at the same speed.

    Here is a silly but comparable question (yup, here comes the bad analogy! ) "If you had to choose one, what would you rather get hit by? A 2000 lb car travelling at 100 mph, or a 40,000 pound dumptruck travelling at 5 mph? Most people would choose the truck, but the truth is that they have the same power. If your back was against a wall and you couldn't move anywhere, they both would kill you. The only way to change the result is to change one of the factors involved. If you were not against a wall and you could move backward with the truck, then you could eventually slow it down and survive. By increasing the distance, you have decreased the force. Manufactures do that with machines. The power might be the same, but they change the application of that power by altering the force, the time or the distance (with levers and gears and angular momentum, and everything else under the sun), and that gives you a different result.
    Case IH 1390, 64 PTO HP
    Loader with bucket, 6' back blade, Woods bushhog, 2 furrow plow and a couple of very old, rusty disk harrows

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  6. #26
    Silver Member
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    Default Re: What does horsepower actually mean?

    I agree with the several previous posters that assert horspeower is horsepower. The 100 hp motorcycle engine will indeed, pull just as much as a 100 hp kubota diesel and pull it just as fast if the gearing is appropriate for each engine.
    The difference in your JD lawn tractor and your Kubota is likely just the result of exaggerated horspower claims on the mower engine. There are a bunch of class action lawsuits floating around on that very subject right now. Notice that most new small gas engines now show a "Torque" rating rather than hp.

  7. #27
    Platinum Member Pooh_Bear's Avatar
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    Default Re: What does horsepower actually mean?

    There was a very similar discussion 5 years ago about this topic.

    http://www.tractorbynet.com/forums/o...e-power-2.html

    Here is my reply from back then.

    Everyone has provided a lot of great info about horsepower and torque,
    but nothing as where it is derived from in an engine.
    It's actually quite simple. From a 19th century steam engine book:

    (P*L*A*N)/33000

    P = Pressure (Mean Effective Pressure, MEP)
    L = Length of Stroke
    A = Surface Area of Face of Piston
    N = Number of (Power Strokes) per minute

    The formula may have originally been for steam engines
    but it also works for internal combustion engines.

    Some additional info:
    Mean Effective Pressure (MEP).
    At the top of the stroke a pressurized gas is introduced into the cylinder.
    The pressurized gas can be steam, or it can be pressurized by sealing an
    air/fuel mixture in the cylinder and igniting it. If steam is used, a valve is
    opened momentarily to admit steam, then closed. The steam pushes the
    piston and expands the volume of the cylinder. The piston pushes the
    crank rod which pushes the crank which rotates because of the torque
    produced by the crank angle and the force of the crank rod.
    If an air/fuel mixture is used in an internal combustion engine the same thing
    happens but the pressure is generated inside the cylinder by the burning
    fuel. I'll get to flame speed in a bit as it relates to gas or diesel engines.
    If you inject 100psi steam into a cylinder and then close the valve, the
    piston will move and the volume inside the cylinder will increase. As the
    volume increases the pressure will drop. MEP is sort of an average pressure
    as the piston moves from the top to the bottom. MEP is also affected by
    the crank angle as the crank rotates it changes the moment arm between
    the crank rod and the crank shaft. This also affects torque in the stroke.
    It is the same thing in an I.C engine. The fuel ignites and expands to create
    pressure. Then this pressure does the work the same as in a steam engine.
    Now this is also where flame speed comes in. Someone once said that time
    is nature's way of keeping everything from happening at once. This is so true
    in the timing of an engine. Everything has to happen in its proper time.
    In a steam engine, the crank reaches top dead center (TDC) and a valve
    opens to admit steam. But even as the steam is being admitted the piston
    is already moving down the shaft increasing the volume. If steam is
    admitted at a rate so that the MEP is constant thru the stroke then the only
    H.P and Torque being created is being used to overcome friction losses.
    You have to admit steam fast enough to keep cylinder pressure up so that
    there will be extra H.P and Torque to do useful work from the engine.
    In I.C engines where the pressure is generated inside of the cylinder
    The fuel will burn at a certain rate (flame speed) until it is consumed.
    As it burns the gaseous vapors will be expanding (creating pressure) and
    the piston will be moving down the cylinder. When the fuel is consumed the
    resulting gaseous vapor will be at its maximum volume and the rest is up to
    MEP in the cylinder. Gasoline has a high flame speed so most of the
    combustion takes place at the top of the stroke where the moment arm is
    smallest. The rest of the stroke is dependent on MEP so torque is lower and
    must be produced at higher RPMs burning more fuel. With diesel engines the
    flame speed is slower and the pressure is generated thru a great portion of
    the stroke. At half stroke the moment arm is greatest and the most torque is
    produced. Because of the slower flame speed the torque curve is shifted into
    this section of the stroke giving the diesel engine a higher torque rating and
    at a lower RPM, making it more fuel efficient.

    Horsepower is a unit in the English Standard system and is defined as
    33000 foot-pounds per minute, or 550 foot-pounds per second.
    Any permutation of these numbers can be used. 1100 ft-lbs/2sec = one H.P.
    16500 foot-pounds per 30 seconds = one horse power.
    It is a derived unit based on amount of work done per unit of time.
    Around 1780 James Watt defined the unit arbitrarily by watching horses
    power a water pump. Four horses were hitched to a device that walked
    them in a circular path. Their walking rotated the device and the rotative
    energy was used to lift water from a deep well. Watt studied this process
    and kept notes of the volume of water lifted and the speed of the horses.
    From this data he calculated one average horse could lift 33000 pounds of
    water a distance of one foot in one minute. Or 550 pounds in one second.

    Since it is a unit of power per unit time it can be converted to other similar
    units. It may sound strange to hear it but my 23hp 8N tractor could also be
    said to be a 17151.097 watt 8N tractor. A 200 watt light bulb =
    .268204418 horsepower. And one HP = 745.699872 watts.
    A watt is a unit in the S.I system (metric) named in honor of James Watt.
    Units of power can also be expressed as joules per second, or it can be
    (((kilogram*meter/second^2))*meter)/minute, or newton*meter/second, etc.

    And horsepower don't mean nothing if you can't put it to the ground.
    You can have 3000HP but if you just sit there and spin your tires in the mud
    then it don't mean anything. Putting the power to work is what counts.
    You can put your power in traction, or operating a hay baler, or brush hog,
    or basically anything that requires energy input to receive a desired action.
    How you can use it is what counts. Just like the man that couldn't pull the
    heavy boat out of his barn with the 300 hp truck, but the 20 hp tractor
    could do it, it's all in how you put the power to use.

    And the reason you see the inflated HP ratings on things today is horsepower
    sells tractors. Salesmen are counting on you to not know what it means.
    Then you get into real horsepower vs developed horsepower.
    Developed HP is an imaginary number and basically can be ignored.
    It's what you can actually use that is what counts.

    Pooh Bear (aka Fluff for Brains)

  8. #28
    Veteran Member RaydaKub's Avatar
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    Default Re: What does horsepower actually mean?

    Pooh Bear (Fluff for Brains):
    Maybe so, but this is pretty smart fluff...
    ==>
    Gasoline has a high flame speed so most of the combustion takes place at the top of the stroke where the moment arm is smallest. The rest of the stroke is dependent on MEP so torque is lower and must be produced at higher RPMs burning more fuel. With diesel engines the flame speed is slower and the pressure is generated thru a great portion of the stroke. At half stroke the moment arm is greatest and the most torque is produced. Because of the slower flame speed the torque curve is shifted into this section of the stroke giving the diesel engine a higher torque rating and at a lower RPM, making it more fuel efficient.
    <==

  9. #29
    Super Star Member Egon's Avatar
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    Default Re: What does horsepower actually mean?

    Somewhere from the distant libraries of my old mind something says that a piston steam engine has maximum torque at zero rpm! As the engine speeds up the torque drops. The calculations were much to difficult for my comprehension.
    Egon
    50 years behind the times
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  10. #30
    Super Star Member LD1's Avatar
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    Default Re: What does horsepower actually mean?

    Quote Originally Posted by Egon View Post
    Somewhere from the distant libraries of my old mind something says that a piston steam engine has maximum torque at zero rpm! As the engine speeds up the torque drops. The calculations were much to difficult for my comprehension.
    I believe electric motors are the same way. Locked Rotor Torque is its max. Above that, it drops off. If you ever do the calculations, electric motors up to speed dont really produce much torque.

    For example, a 5HP 3400RPM motor like found on most air compressors is only generating~7.5 ft lbs of torque heck, most torque wrenches dont even go that low. 7.5ft lbs isnt even enough to break a lousy 1/4" drive china ratchet.

    But look at how much torque they have at start-up. Probabally more than enough to snap a good 3/4" drive tool

    Its all based on the formula of: torque = (HP x 5252)/RPM So the lower the RPM, torque goes up BIG TIME. At least until the winding burn up
    ".........there is only one way to find out."
    "Ok, hold my beer and watch this.........."


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