Fundamental vibratory phenomena in internal combustion engines

Powertrain vibrations at low and medium frequencies, i.e. between 20 Hz and 700 Hz, represent a challenge in terms of reliability, comfort and noise inside the passenger compartment. In this frequency band, GMP is the main source of vibrations and noise perceived inside the vehicle, and the main pathways are the points where GMP attaches to the vehicle structure. Controlling these vibrations requires an understanding of the phenomena and an analysis of the most influential factors. This article expands on part of the [BM2773] article, focusing on GMP support vibrations, but it should be borne in mind that GMP vibrations are also transmitted via the drive shafts. This aspect is not dealt with here, but we can consider as a first approximation that the transmissions are excited in displacement imposed by the gearbox differential. Some of the results presented in this article could therefore be used as input data for a possible driveshaft model.

The analyses presented here concern four-stroke engines with a three- or four-cylinder in-line architecture, as these are the most widespread engines in the world and particularly in Europe. However, apart from the specific nature of the excitations relating to these two architectures, the proposed approach of simply modeling the main components of the GMP remains fairly general and can be applied to other architectures.

For the reader unfamiliar with piston engines, the fundamental frequency of the inertia forces developed in each cylinder is the frequency of crankshaft rotation, and the fundamental frequency of the gas torsor of a cylinder is half the frequency of crankshaft rotation in the case of a four-stroke engine, since there is one combustion in a cylinder every two crankshaft revolutions. In the case of two-stroke engines, the fundamental frequency of the gas torsor of a cylinder is equal to that of the inertia forces, since there is combustion in a cylinder every revolution of the crankshaft. Engine manufacturers have adopted the convention of relating the frequency of each periodic phenomenon to the engine's rotation frequency. One consequence of this convention is the existence of non-integer harmonics for engines with an odd number of cylinders. In particular, for a four-stroke engine with three equally-spaced cylinders in line (i.e. the three combustions are evenly spaced), there is one combustion every 240 crankshaft degrees, i.e. three combustions for every two revolutions. The fundamental frequency of gas excitation is therefore 3/2 that of crankshaft rotation. This convention creates a harmonic of order 1.5.