The noise emission from direct-injection diesel engines depends to a large extent not only on the speed, but also on the load. In this connection, direct excitation of the combustion chamber walls through the combustion process (“direct combustion noise”) is normally the prime source of noise at mean effective pressures of approx. 1 to 6 bar. However, at no-load condition and at mean effective pressures in excess of 6 bar, the motion through clearances in the main running gear, in the timing gears and in the injection pump is the prime source of noise excitation. The latter depends to some extent on the peak pressure (“indirect combustion noise”) and the hydraulic forces in the injection pump, and increases with increasing load, while the time history of cylinder pressure above 6 bar becomes “more gentle” and, with increasing load, less critical as a source of noise excitation.
Excitation through the main running gear, the timing gears and the injection pump is being systematically analyzed, using several engine series and a varying number of cylinders. Determined are above all the noise components of the timing gears, piston slap, injection pumps and main bearings as a function of speed and load. Clearances in the timing gears and in the main running gear, mass moments of inertia, stiffness and damping effects of the shafts, tooth profiles, piston configurations and the injection pump engine block connection are being varied. The results obtained permit approaches to acoustic optimization of the main running gear, the timing gears and the injection pump, and appraisal of the noise reduction potential.