Internal Combustion Horsepower Ratings

When designing a belt drive with an internal combustion (I.C.) engine it’s important to understand that an IC engine is not like an electric motor. The horsepower rating for a NEMA motor is not the same as a horsepower rating for an IC engine. There is no conversion factor. HP is a HP is a HP. Engines just have different characteristics. Electric motors may have a hard start whereas IC engine may not. IC engines can’t run at their peak torque for a long time. An IC Engine with a HP rating higher than the power needed should be selected. Altitude and other factors can affect an IC engine’s performance.

Internal combustion engines are typically rated based on brake horsepower (BHP), or maximum BHP.

This BHP rating of an engine usually means the horsepower produced by a test engine in a laboratory.  During the test an engine is ran without a fan, generator and other accessories.  The ambient temperature is corrected to some standard, such as 60°F., and the atmospheric pressure is corrected to some given altitude, such as sea level.  The BHP rating should not be used for design, since the standard production engine, with accessories, cannot reach this output in actual usage.  Gross BHP is the term used for test data without any accessories and net BHP is with all of the standard accessories.  This is the horsepower measured at the crankshaft flywheel.  An I.C. engine spec will not contain any other mention of horsepower.

Several decades ago it was more common to refer to a maximum intermittent HP.  For short durations this was generally 85% of BHP.  Continuous duty or rated BHP was 75%-80% of maximum BHP for long duration service.  These terms are not referred to today in engine specifications.

It is still important to verify the horsepower versus engine rpm curves for those applications where the engine and drive are not intended to run at one speed continuously.  A percent time duty cycle is also helpful in selecting a belt drive.