The frequency analysis of the crankshaft's speed variation: A reliable tool for diesel engine diagnosis

The speed variation of the crankshaft may be easily and accurately measured using a shaft encoder and counting the pulses of the internal clock of the data acquisition system. If the crankshaft would be a rigid body, the vari ation of its angular speed could be directly correlated to the total gas-pr essure torque. Actually, the variation of the crank shaft's speed has a com plex nature being influenced by the torsional stiffness of the cranks, the mass moments of inertia of the reciprocating masses and the average speed a nd load of the engine. Analyzing only the lower harmonic orders of the spee d variation spectrum can filter out the distortions produced by the dynamic response of the crankshaft. The information carried by these harmonic orde rs permits to establish correlations between measurements and the average g as pressure torque of the engine, and to detect malfunctions and identify f aulty cylinders.