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The local variation of the crankshaft's speed in a multicylinder engine is determined by the resultant gas-pressure torque and the torsional deformation of the crankshaft. Under steady-state operation, the crankshaft's speed has a quasi-periodic variation and its harmonic components may be obtained by a Discrete Fourier Transform (DFT). Based on a lumped-mass model of the shafting, correlations are established between the harmonic components of the speed variation and the corresponding components of the engine torque. These correlations are used to calculate the gas-pressure torque or the indicated mean effective pressure (IMEP) from measurements of the crankshaft's speed.

The variation of the crankshaft's speed is influenced by the action of the cylinders and shall reflect the contribution of each cylinder to the total engine output. At the same time, the speed variation is influenced by the torsional stiffness of the cranks, the mass moments of inertia of the reciprocating mechanisms and the average speed and load of the engine. As the result, the variation of angular motion of the crankshaft is complex, each particular influence changing its importance as speed and load are modified. The diagnostic method presented in the paper is based on the analysis of the amplitudes and phases of the lowest harmonic orders of the measured speed and is capable to determine the average Indicated Mean Effective Pressure (IMEP), to detect nonuniformities in cylinder operation and to identify the faulty cylinder(s).

The instantaneous frictional torque (IFT) of the engine and the piston-ring assembly frictional force (PRAFF) were determined during cranking and starting of a direct injection single cylinder diesel engine. The measurements included the cylinder gas pressure, the instantaneous torque of the electric starter, the angular velocity of the crankshaft and the axial force on the connecting rod. The engine and piston friction were determined every crank angle degree for all the cycles from the time the starter was engaged to the time the engine reached the idling speed. The data was analyzed and a comparison was made between the friction in successive cycles.

An experimental method for determining the Instantaneous Frictional Torque (IFT) using pressure transducers on every cylinder and speed measurements at both ends of the crankshaft is presented. The speed variation measured at one end of the crankshaft is distorted by torsional vibrations making it difficult to establish a simple and direct correlation between the acting torque and measured speed. Using a lumped mass model of the crankshaft and modal analysis techniques, the contributions of the different natural modes to the motion along the crankshaft axis are determined. Based on this model a method was devised to combine speed measurements made at both ends of the crankshaft in such a way as to eliminate the influence of torsional vibrations and obtain the equivalent rigid body motion of the crankshaft. This motion, the loading torque and the gas pressure torque are utilized to determine the IFT.