Before there were engines, the only power available in the Western world, besides brute human strength, was the power of a horse.Engineer James Watt coined the term when making improvements to the steam engine in the 1760s.

Watt needed to be able to place a value on the energy produced by the mechanical steam engine.The only way that made any sense was by comparing this energy to the energy produced by a team of horses doing the same job, which in this case was pumping water out of coal mines.

He hooked up nine horses, one by one, and determined that an average horse could lift 150 pounds of water 220 feet in one minute.This he expressed as 1/33,000 foot-pounds per minute, he multiplied the 150 by the 220, or “one horsepower.”

Watt needed to be able to place a value on the energy produced by the mechanical steam engine.The only way that made any sense was by comparing this energy to the energy produced by a team of horses doing the same job, which in this case was pumping water out of coal mines.

He hooked up nine horses, one by one, and determined that an average horse could lift 150 pounds of water 220 feet in one minute.This he expressed as 1/33,000 foot-pounds per minute, he multiplied the 150 by the 220, or “one horsepower.”

Then the rate at which the engine can do work is measured in horse power (HP). One HP is equivalent to 4500 kg m per min. The various methods of defining horsepower are described below :-

**Indicated Horse Power :-**

The amount of power that can be measured on the flywheel is always less than the power generated in the engine on account of expansion of the combusted fuel. The power that is actually developed in the cylinder is called indicated horse power and is given by:

P - means effective pressure in kg/cm2

L - stroke length in m

A - area of cylinder in cm2

N - power stroke per min (for a four stroke engine N = rpm/2 and for a two-stroke engine N = rpm).

**Brake Horse Power :-**

It is the horsepower available on the crankshaft and is measured by a suitable dynamometer.

**SAE Horse Power (Taxable Horse Power) :-**

The SAE (Society of Automotive Engineers) horse power rating is used to compare engines on a

uniform basis, usually for tax purposes. The formula is

where

D - diameter of cylinder in inches

N- number of cylinders

**Belt Horse Power :-**

It is the power of the engine. measured at the end of a suitable belt, receiving drive from the PTO

shaft of the tractor.

**Power Take Off Horse Power :-**

It is the power delivered by a tractor through its PTO shaft. In general the belt and PTO horse power

of a tractor will approximately be the same and is measured by either a hydraulic or an electrical

dynamometer.

**Drawbar Horse Power :-**

It is the power of a tractor measured at the end of the drawbar. It is that power which is available to

pull loads.

**Maximum Horse Power And Net Horse Power :-**

The maximum HP is measured at the engine flywheel without any of the power consuming

accessories being attached. This is not a practical rating as it does not represent "usable" HP.

Net HP is measured at the engine flywheel in the same manner as the maximum HP. The difference

in the two is because the engine is equipped with accessories. Net HP is the basis for rating the HP

of industrial and farm tractors.

**Effect of Environment on Horse power :-**

HP is affected by barometric pressure and atmospheric temperature. Therefore, whatever actual

horsepower is observed on the dynamometer is called the observed HP, whereas the corrected HP is

the observed HP corrected to standard atmospheric conditions.

**The standard operating conditions are:**

1. Mean barometric pressure of 736 mm of mercury corresponding to an altitude of 300 m above

mean sea level.

2. Water vapour pressure of 27.4 mm of mercury corresponding to a relative humidity of 65 per

cent at 35°C.

3. Intake air temperature of 35°C.

For decreases in the atmospheric pressure, a deduction from the rated output of the engine shall

be made at the rate of 1.4 per cent per 100 m of altitude above 300 m. This de-rating is valid up to

an altitude of 2500 m.

Also, for any increase of the intake air temperature above 35°C, a further deduction shall be

made at the rate of 0.25 per cent per 0C.