Re: Re: Re: What kind of horses are tractor horsepower measured in?

Subject: Re: Re: Re: Re: What kind of horses are tractor horsepower measured in?

Comments:
: : As mentioned, one HP = 550 ft-lb/sec, originally derived from the amount of work a real horse could produce over a measured time period, though the details of that derivation are unknown to me. : : In any case, the issue here is how the engine is designed (by virtue of cam timinmg, compression ratio, etc. and whether gas or diesel) : : to produce its power. Recall that all power and torque ratings are specified as available at a given engine speed. (Power = rate of doing work.) In a typical automobile engine, : : the horsepower figures are quite high (relative to the displacement, and especially so for motorcycles) but the power is produced at high RPM. It is : : characteristic of a diesel engine, due to it's high compression (typ. ~17:1) and long stroke that the moving parts are heavy so high RPM are not possible. However, it produces : : enormous torque at modest RPM, an ideal situation for trucks/tractors. Gasoline engines can be optimized for this low end lugging behavior using the same tricks (in reverse) that racers use. : : The camshaft specs, carburetor setup, and combustion chamber design can be tuned to produce a slow turning, high torque engine. In both gas and diesel engines, a long stroke vs the bore dimension also : : gives great low end "stump pulling" characteristics. A natural trade-off of such design is to produce usable power/torque over a wide RPM band. : : I have a Cummins diesel truck and although only producing 160 HP, it produces over 400 lb-ft of torque at 1250 RPM. In other words, the power : : it does produce is at an engine speed that is sustainable indefinitely. A gas engine produces its max torque (and power) : : at engine speeds that would greatly shorten its life (not to mention the poor fuel consumption and noise) if operated at that speed. : : Edwin Whitelaw, P.E. - BSME, VPI '76 :-) : I didn't realize when I asked the question how many ways it could be answered. I'm getting a good education here. I looked up the definition of a horsepower in "Marks Standard Handbook for Mechanical Engineers" since I remembered it incorrectly. Mr. Whitelaw is correct. : It also gives some information about the work that a HORSE can do. It states: "A 1,500 to 1,900 horse can work continuously for up to 10 hr a day at a rate of 1 hp, or equivalently pull 10 percent of its body weight for a total of 20 miles per day." In short term work (5-30 minutes) it can produce four times that, and in brief maximal output 25 horsepower! An average man for comparison can put out about .1 horsepower over an 8-hour day. Muscle power appears to be relatively closely correlated with body mass. : Of course my original question wasn't about horses, or even Horsepower, but about the testing and calculating techniques and comparisons to automobiles. The discussions on torque, rpm and reliability are very relevent.

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