Effect of Engine Structure on Noise of Diesel Engines

Abstract

The objectionable noise of a diesel engine or ‘diesel knock’ is produced by the abrupt rise of cylinder pressure resulting from initial inflammation of the injected fuel. Worth-while reductions of this noise have been achieved by the control of the fuel injection characteristics producing a smoother pressure rise, but these are limited by performance considerations. Further reductions can only be obtained by structural changes of the engine and these are considered here. This has led to the construction of a number of structure research engines using two basically different principles for controlling vibration of the outer surfaces, namely, control by damping and control by stiffness. In each case conventional cylinder heads, pistons, connecting rods and crankshafts are used but with different crankcase cylinder-block constructions. The first principle, employing control by damping, entails a skeleton load carrying structure arranged so that it exposes to the sides of the engine only narrow edges which do not radiate much noise because of their small exposed surface area. Highly damped panels, which cannot be set into resonant vibrations, are attached to this load carrying framework. Using the second principle, i.e. control by increased stiffness, an engine with a cast magnesium crankcase having a wall thickness of about 1 1/4 in was constructed. Magnesium was chosen so that the thickness of the crankcase walls could be increased to 5 or 6 times that of a normal cast-iron crankcase of the same weight. In subjective units these engines are about half as noisy as a normal engine using the same working parts. There are distinct differences in the characteristics of the noise of the two engines. The magnesium engine shows a tendency to be less noisy in the low and middle frequency range and is a little noisier from about 4000 c/s upwards.