Prototype model of diesel engine vibration and noise generation

Abstract

Insight into how combustion‐pressure time variations generate external noise results from analysis of a simplified model of a single cylinder four‐cycle engine. The crankcase is a long rectangular “thin‐walled” box that vibrates as a cantilever beam. Oscillations are excited by forces exerted on the main bearings, these forces being transmitted from the combustion cavity through the piston and connecting rod. The oil pan is a thin plate whose oscillations are driven by displacements along its perimeter, these displacements resulting from the crankcase beam oscillation. The engine head is assumed rigid. Damping results from the sliding of the piston relative to the block. Analysis leads to a detailed description of the outer surface vibrations. Sound generation is bracketed by computations based on (1) the multipole matched‐asymptotic‐expansion model (low frequencies) and (2) the geometrical theory of diffraction (high frequencies). The result relates ratio of pressure spectral densities (external to internal) to engine structure dimensions, mass distribution, elastic modulus, piston‐cylinder dashpot constant, and engine speed.