A simple yet reliable method to evaluate the overall friction power of an internal combustion engine is proposed. The method is based on the measurement of the angular deceleration of the engine speed. The tested engine is coupled to a flywheel having a known high moment of inertia, fired and accelerated to a high engine speed. The system is kept running until steady conditions are attained. Then, the ignition system is shut-off, and the engine speed deceleration vs. time, α, is recorded. The total friction-mean-effective-pressure, tfmep vs. time, may then be evaluated from the fundamental relationship between the two (tfmep=2πIα/Vd, where, Vd is the engine displacement volume, and I is the moment of inertia of the integrated system).
The proposed method, is a modification of the direct motoring method, in which the engine is motored by an external means under conditions as close as possible to firing. In the present method, the inertia method, there is no need for an external means for motoring, the test is performed under realistic piston, cylinder and lubricant temperatures, the total friction mep curve vs. engine speed is readily obtained by a single test, and the upper speed is limited only by the speed limit of the engine itself.