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Exactly what is horsepower? Horsepower is defined as the amount of energy or work required to raise a weight of 33,000 pounds a height of one foot in one minute of time or to overcome or create a force which is equivalent to doing that amount of work. Therefore in simplified terms, a horsepower is 33,000 foot pounds of work done in one minute. However, horse power may be required at various power points of the engine and the calculation must be accordingly. For instance, the horse power generated by an engine, as indicated by the power in the cylinders, may not be the actual power which can be utilized for work as there is the friction of the engine to be considered. Then there is "brake" horsepower which is the actual horsepower delivered to the engine shaft and which can be used for work, and the "effective" horsepower, etc. The following definitions should add some clarity:
This represents the power developed in the engine cylinder as obtained from the pressure in the cylinders. This is pressure obtained from an indicator which shows cylinder pressures. Indicated horsepower does not represent the actual useful power delivered by the engine.
This is the actual horse power delivered by the engine to the shaft. It is equal to the "indicated horsepower" less the friction of the engine. It is useful horsepower and is also called the "shaft horsepower".
This is the final horse power delivered to equipment. An engine may be operating compressors, pumps and auxiliary equipment as part of its own power production needs, and the remaining power is therefore the effective power for drive. The difference between indicated horsepower and effective horsepower may be as much as 25%.