Re: How to measure horsepower
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Posted by Milan H. Dilworth on June 20, 1998 at 23:25:32:
In Reply to: How to measure horsepower posted by Nate on June 19, 1998 at 10:49:38:
: A couple days ago there was a discussion about how to measure horsepower. This is how it's done. A variable braking device (often a water pump or a generator) is attached to the powered output shaft ...
: Nate
-------------------------- Dynamometer is the name of the variable braking device used to measure torque at RPM's and thereby calculate horsepower.
Engine speed / resisting force vary; thereby, the actual output of horsepower is constantly varying. Tractors with heavier built engines are made to handle greater torque and greater capacity to dissipate resident heat.
Most tractors have fuel systems set to produce a max. engine speed, or HIGH RPM, and also a
LOW IDLE RPM. Torque is measured when the resisting brake mechanism loads the engine and the RPM's are pulled down to the speed at which the engine HP is rated.
Most tractor engines are built with fuel systems which continue to supply more fuel to the cylinders as the load on the engine increases and pulls down the speed even further. This will generate a noted raise in the torque, but decrease in RPM's and therefore the HP be adjusted. This "torque rise" is usally set for an engine as a % of the maximum torque produced at rated HP.
The fuel keeps supplying more fuel for more torque until the engine speed is slowed to the speed at which the fuel system is built to automatically cut-back the fuel and prevent the engine from destroying itself. Measured torque at the cutoff engine speed should be the "torque rise amount over the torque at eng. rated speed.
You can learn quite a bit about different tractors by studying the specs. for the engines. In the big shop where I've worked for 20 years, most engines overhauled are "broken in" by a number of hours running on the dyno. Then the HP can be checked. On diesel engines the torque rise is sometimes increased to produce more ability to pull through a large increase in load without need for shifting down. Larger tractors often get fuel turned up to increase the max. HP.
Gasoline engines don't fare well against diesels in overall fuel efficiency because diesel fuel has more BTU's and will produce more heat. Milan
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