Three fundamental concepts of physics are required to understand the relationship between BMEP and output horsepower of an engine: A small engine with a BMEP of 100psi is working four times harder than a large engine with only a BMEP of 25psi even though the larger engine may develop more horsepower because its displacement is much larger than the smaller engine. The same pressure acting on a larger piston area and through a longer stroke will result in a larger power output proportional to the larger displacement.īMEP values can also be used as an indicator of how hard the engine is working. By using the concept of BMEP, the power output of different engines can be compared on a basis independent of size or speed.Ĭonsider a large and a small engine operating with the same BMEP. To state this in a simpler way, an engine can be considered an air motor, where constant pressure input results in constant power output. BMEP is that theoretical constant pressure being exerted on the piston during each power stroke will produce the same brake power output as the varying pressure of the engine’s real operation. This movement is converted into a rotary motion of the crankshaft by a connecting rod. As the fuel burns, it generates a high pressure that forces the piston downward. An engine’s power is created by the burning of fuel in the confined space of the cylinder. We have to start with the concept of brake mean effective pressure, or BMEP, which is measured in pounds per square inch (psi). Some mathematics are absolutely necessary, but we will try to keep it to a minimum. Friction and other losses are real but do not have to be considered to calculate brake horsepower. This is the nameplate power available at the flywheel for useful work, and it would be the power measured by a brake or dynamometer. We will only talk about brake horsepower. We would like to try to explain how horsepower, torque, and mean piston speed are calculated. Working for an engine manufacturer for many years has allowed us to become familiar with the subject. There certainly seem to be a lot of questions and confusion about such a basic concept to our hobby. We have been reading Gas Engine Magazine for years and have seen many questions about engine power and how it is measured or calculated. Our horsepower calculator can help you calculate power in horsepowers.In March of 1987, Marcus Comes wrote in to Gas Engine Magazines wondering about the horsepower rating for his Maytag engine. Our hp to amps calculator can help you with this concept better.Īir Horsepower, ahp: This equals 944 Watts, and is used mainly in air conditioning machines.īoiler Horsepower, hp(S): This denotes a boiler's capacity to deliver steam to a steam engine, which equals approximately 9,809.5 Watts. This definition is mainly used for electrical motors. Metric Horsepower, hp(M): A more scientific designation, approximately 735.50 Watts.Įlectrical Horsepower, hp(E): This equals 746 Watts. Mechanical Horsepower, hp(I): The most common horsepower definition, mechanical horsepower is equal to 745.70 Watts. However, there are many different types of horsepower, such as When people are talk about horsepower, they generally mean mechanical horsepower. Horsepower is generally used to identify an engine's or motors' efficiency and was first defined in the late 18th century by Scottish engineer James Watt, when he wished to compare an engine's efficiency with a horse. The horse will move this wheel a certain distance over a second, which is known as a horsepower - the rate of work that is done. Imagine a horse attached to a wheel the wheel starts at rest, but as soon as the horse starts pulling, it moves.
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