Modelling and experimental study on bending vibration of a diesel engine crankshaft

In recent years, a considerable amount of research effort has gone into the investigation of free and/or forced vibration characteristics of a spinning shaft with attached discs, but there has been less research on the transverse vibration of a high-speed diesel engine crankshaft system under firing conditions. This paper presents a new exact general approach based on a distributed continuous model to study the free and forced bending vibrations of an engine crankshaft. Distributed continuous models of the crankshafts for a four-cylinder (with a capacity of 3.81) and an in-line six-cylinder (with a capacity of 5.81) direct-injection diesel engine were constructed. The general frequency equation and mode shapes for the bending vibration of the total crankshaft under operating conditions were obtained using the coefficients transfer matrix method. Modelling and analysis procedures were applied to the crankshaft systems of two diesel engines, and the inherent frequencies of the crankshaft within overall engine speeds were calculated. Then, equations to simulate the steady state vibration amplitudes when the gas forces, rotating masses and reciprocating masses exerted at the crankshaft were deduced. The bending amplitudes at the front free end of the crankshaft were calculated using this method. To verify the modelling and the results of the calculations, the bending vibrations were measured at the front end of the crankshaft using a newly designed device. It is found that there was good agreement between the analytical results and the experimental results.