Re: Re: What kind of horses are tractor horsepower measured in?
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Posted by Edwin Whitelaw on June 16, 1998 at 18:25:40:
In Reply to: Re: What kind of horses are tractor horsepower measured in? posted by Charlie English Jr on June 16, 1998 at 15:29:29:
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. Having said all that, I was reading this list hoping to find pointers on what a good used tractor along the lines of a Ford 2000/3000 should cost. Any suggestions? Regards, Edwin Whitelaw, P.E. - BSME, VPI '76 :-)
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