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 variation of its angular speed could be directly correlated to the total gas-pressure torque. Actually, the variation of the crankshaft’s speed has a complex nature being influenced by the torsional stiffness of the cranks, the mass moments of inertia of the reciprocating masses and the average speed and load of the engine. Analyzing only the lower harmonic orders of the speed variation spectrum can filter out the distortions produced by the dynamic response of the crankshaft. The information carried by these harmonic orders permits to establish correlations between measurements and the average gas pressure torque of the engine, and to detect malfunctions and identify faulty cylinders.

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