Posted by Indiana Ken on November 06, 2012 at 06:12:43 from (66.249.234.243):
In Reply to: True Horse Power posted by David G on November 05, 2012 at 21:40:15:
Horsepower is a calculated value: RPM and torque are measured on a dyno and horsepower is calculated from the following formual:
HP = (RPM x Torque) / 5252
HP is a measure of the amount of work an engine can do per unit time and holds true regardless of the combination of RPM and torque. Note that for a low speed engine the torque must be high and conversersely for a low torque engine the RPM must be high for equal HP.
Engine torque is a function displacement and cylinder pressure. For a given engine displacement, increasing cylinder pressure will result in an increase in torque. The average cylinder pressure for any engine can be estimated by the folllowing formula:
MEP = (HP x 792,000) / (Displacement x RPM) MEP = Mean Effective Pressure
Increasing engine displacement will result in an increase in engine torque. The increase in displacement can be from boring or stroking - the torque increase will be similar.
For low speed applications a longer stroke is typically used since large valves are not required to flow air at high RPM. Also, longer stroke engines have smaller pistons so the torque comes with a lower rod and crank bearing load. In this case low speed is a mis-nomer since piston speed can be very high in these engines.
If very high HP is the goal then the engine is typically designed with a larger bore to allow room for large valves. The large valves allow the engine to breathe at high RPM and the shorter stroke reduces piston speed.
Long stroke, short stroke, high torque, low torque, high speed, low speed; HP is HP. However, for this to be true the engine needs to be coupled to the load with the appropriate gearing.
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