Re: Re: Re: What kind of horses are tractor horsepower measured in?
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Posted by Steamer on June 17, 1998 at 14:57:11:
In Reply to: Re: Re: What kind of horses are tractor horsepower measured in? posted by Edwin Whitelaw on June 16, 1998 at 18:25:40:
Automotive engines produce their peak horsepower at high RPM's (6000-8000). HP goes up with the RPM. My Legends race car has a 1200 CC gas engine that operates at 10,000 RPM and produces 100-110 HP. It is short stroke engine, with high compression, double overhead camshafts and double overhead valves. There is one carb and exhaust per each of the four cylinders. The power output is measured at the crankshaft.
A tractor has an engine with a long stroke, and much simpler fuel, exhaust and valve systems. It does not run very fast (2,000 RPM or so), so the peak HP will be much lower than my 10,000 RPM race motor. Also, the tractor HP is measured at either the PTO shaft (post transmission) or at the drawbar (post complete driveline). Once the HP is actually measured, there is a 'correction factor' applied to a certain air density, pressure, humidity, etc. This gives you the HP rating. The rating is designed to allow you to compare machines HP based on, basically, their lowest output.
Automobiles are still NOT rated at their wheels! The change in HP figures years ago came from actually measuring the engines on water-brake dynamometers, rather than on a calculation based on fuel, mixture, gas-flow and cylinder displacement. The 'figuring' was always stretched into a 'best case' number for marketing reasons.
Keep in mind, also, the operational differences between cars and tractors. A car needs a high peak HP rating for acceleration (especially when pulling out into traffic). A tractor needs far less peak HP, but greater steady torque for the very different work load of pulling a piece of metal through the hard dirt.
The exception in the automotive world is 'tow vehicles'. They need both high torque and HP to pull those trailers loaded with old tractors. :-) Stan Disbrow - BSEE, Auto Racer, Old tractor fan
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