Horsepower

Horsepower (hp) is a unit of measurement for power, typically used to quantify the output of engines or motors. It was originally developed in the late 18th century by Scottish engineer James Watt to compare the output of steam engines with the power of draft horses[1].

Definition and Types

There are several definitions of horsepower, including:

• Imperial horsepower: Approximately 745.7 watts
• Metric horsepower: Approximately 735.5 watts

History and Development

The concept of horsepower emerged during the Industrial Revolution when steam engines began replacing horses for various tasks. James Watt, seeking to market his improved steam engine, conducted experiments to determine the average power output of a horse[1].

Watt’s calculations were based on the following assumptions:

• A horse could turn a mill wheel 144 times in an hour
• The wheel had a 12-foot radius
• The horse could pull with a force of 180 pounds-force

Using these parameters, Watt arrived at the figure of 32,572 ft·lbf/min, which was later rounded to 33,000 ft·lbf/min[1].

Calculating Power

Horsepower can be calculated using different formulas depending on the available measurements:

1. Using torque (T) in pound-feet and rotational speed (N) in rpm:
   $$P_{hp} = \frac{T_{ft·lbf} \times N_{rpm}}{5252}$$
2. Using torque (T) in inch-pounds:
   $$P_{hp} = \frac{T_{in·lbf} \times N_{rpm}}{63,025}$$

Types of Horsepower

1. Imperial Horsepower: The standard definition, equivalent to about 745.7 watts[1].
2. Metric Horsepower: Also known as PS, CV, or ch in various languages, it’s defined as the power to raise 75 kilograms by 1 meter in 1 second, equivalent to about 735.5 watts[1].
3. Tax Horsepower: A non-linear rating used for tax purposes, originally related to engine size but now often based on CO2 emissions[1].
4. Electrical Horsepower: Used for electrical motors, defined as exactly 746 watts in the United States[1].
5. Hydraulic Horsepower: Represents power in hydraulic machinery or drilling rigs[1].
6. Boiler Horsepower: Measures a boiler’s capacity to deliver steam, equal to the thermal energy rate required to evaporate 34.5 pounds of fresh water at 212°F in one hour[1].
7. Drawbar Power: Measures the pulling power of locomotives or tractors[1].
8. RAC Horsepower: A calculated figure used in early 20th-century British cars for tax purposes[1].

Comparison to Human Power

For context:

• A healthy human can briefly produce about 1.2 hp
• A trained athlete can sustain about 0.35 hp for several hours
• Usain Bolt produced a maximum of 3.5 hp during his world record 100-meter sprint[1]

Modern Usage

While many countries now use the SI unit watt for power measurement, horsepower remains a common unit in several contexts:

• Automotive industry (especially for marketing purposes)
• Aviation
• Marine applications
• Small engine specifications

Conversion Table

In conclusion, horsepower remains a widely recognized unit of power measurement, particularly in the automotive and machinery industries. Its various definitions and applications reflect its historical development and adaptation to different technological contexts over time[1].

Horsepower (Wikipedia)

For other uses, see Horsepower (disambiguation).

Horsepower (hp) is a unit of measurement of power, or the rate at which work is done, usually in reference to the output of engines or motors. There are many different standards and types of horsepower. Two common definitions used today are the imperial horsepower, which is about 745.7 watts, and the metric horsepower, which is approximately 735.5 watts.

Horsepower
One imperial horsepower lifts 550 pounds (250 kg) by 1 foot (30 cm) in 1 second.
General information
Unit of	power
Symbol	hp

The term was adopted in the late 18th century by Scottish engineer James Watt to compare the output of steam engines with the power of draft horses. It was later expanded to include the output power of other types of piston engines, as well as turbines, electric motors and other machinery. The definition of the unit varied among geographical regions. Most countries now use the SI unit watt for measurement of power. With the implementation of the EU Directive 80/181/EEC on 1 January 2010, the use of horsepower in the EU is permitted only as a supplementary unit.