Hello From the Frozen Tundra of Wisconsin!


| July/August 1997



SI W25765 North view Road Waukesha, Wisconsin 53188 and Bob Stadowicz New Berlin Wisconsin 53146

We have been reading Gas Engine Magazine for years and have seen many questions about engine power and how it is measured or calculated, most recently in the January, 1997 issue. There certainly seem to be a lot of questions and confusion about this basic concept in our hobby. Working for an engine manufacturer for many years has allowed us to become familiar with the subject. We would like to try to explain how horsepower, torque, and mean piston speed are calculated. We will only talk about brake horsepower. This is the nameplate power available at the flywheel for useful work, and would be the power measured by a brake or dynamometer. Friction and other losses are real, but do not have to be considered to calculate brake horsepower. Some mathematics are absolutely necessary but we will try to keep it to a minimum.

We have to start with the concept of Brake, Mean, Effective Pressure, or BMEP, which is measured in pounds per square inch (psi). Power is generated by an engine by the burning of a fuel in the confined space of the cylinder. As the fuel burns, it generates a high pressure, which forces the piston downward. This movement is converted into a rotary motion of the crankshaft by the connecting rod. BMEP is that theoretical constant pressure which can be imagined as being exerted on the piston during each power stroke of an engine, which will produce the same brake power output as does the varying pressure of the engine's real operation. To state this in a simpler way, an engine can be considered as an air motor, where a constant pressure input results in a constant power output. By using the concept of BMEP, the power output of different engines can be compared on a basis which is independent of size or speed.

Consider a large and small engine operating with the same BMEP. 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.

BMEP values can also be used as an indicator of how hard the engine is working. A small engine with a BMEP of 100 psi is working four times harder than a large engine with only a BMEP of 25 psi, even though the larger engine may develop more horsepower because its displacement is much larger than the smaller engine.

Three fundamental concepts of physics are required to understand the relationship between BMEP and output horsepower of an engine: