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Horsepower refers to a unit of measurement of power, or the rate at which work is done, usually about the output of engines or motors.

In the metric system, the engine power output is measured in kilowatts, This is the amount of electricity an electric generator produces when driven by the engine. One horsepower is equal to 0.746kW. One kW is equivalent to 1.34hp. You can calculate the horsepower if you know the engine torque and speed in rpm. The formula is hp= torque x from  = 5252. 

Indicated Horsepower is the power developed inside the combustion chambers during combustion. It is determined by measuring the pressures in the engine cylinder. Indicated horsepower is always higher than brake horsepower BPH, Some of the power developed in the cylinder is lost in overcoming the internal friction in the engine.

Friction Horsepower is the power required to overcome the friction of the internal moving parts. A major cause of friction loss is piston ring friction, It can account for 75 percent of all friction losses in the engine. Pistons in the short-stroke oversquare engine do not move as far. This lowers ring friction. Some manufacturers further reduce friction by installing thinner and barrel faced piston rings,