Measuring Engine Power

Power is a word used in many different contexts. One common setting for its use is the automotive industry, where horsepower is its main unit. Horsepower is the quantity of force or work an engine can produce over time. Because engines vary considerably in size, application and fuel type, horsepower provides a useful comparison between them. Whether measuring diesel performance in a freight truck or the high performance of race car engines, horsepower is a common denominator.

How Does an Engine Produce Power?

Internal combustion engines convert the potential energy contained in a combustible fuel into mechanical energy to turn the wheels of a vehicle. This is accomplished by a rapid coordinated series of controlled explosions within a set of closed cylinders. At one end of this tube is a movable piston that is linked to the drive shaft of the vehicle. When the proper fuel and air mixture is delivered to the cylinder, an ignition source, the spark plug, sets off the explosion that pushes the piston and turns the drive shaft.

What Is Horsepower?

A Scottish engineer named James Watt determined that the work a pony could do was equal to 33,000 foot-pounds per minute. This poor pony didn’t actually carry 33,000 pounds. It more likely carried 500 pounds 66 feet in a minute or something similar. This odd measurement was derived to help him sell his newly designed steam engine. It gained popularity in many engineering equations and has endured as a standard Imperial unit of measurement.

How Is Horsepower Calculated?

Engines used for transportation require rotational force, known as torque, to turn the wheels. Converting this torque into horsepower requires a simple equation. Horsepower is the product of torque, usually expressed in pound-feet, and rotational speed of the drive shaft, normally revolutions per minute. This product is then divided by 5252. The division step simply converts the rotational speed of revolutions per minute (RPM) into radians per second.

Understanding how engines work and the ways in which they are compared is not only an interesting tale but can be useful when considering different vehicles. 

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