Instantaneous speed fluctuations of thermal engines. Excitations applied to the crankshaft

In general, many mechanical studies of internal combustion engines require knowledge of crankshaft time excitations. These may be low-speed or high-speed acyclism analyses, or stability analyses. These analyses may or may not require a precise description of the excitation forces, or of the crankshaft's elasticity. Depending on the type of analysis, different representations are used:

• excitations applied either to the crankshaft itself, or to the pistons, connecting rods and crankshaft. These excitations are subsequently broken down into an inertial component and a component directly linked to the thrust of the combustion gases on the pistons. Depending on the precision required, in some cases we may be satisfied with a single harmonic excitation component instead of the exact representation given by the Fourier series of the temporal excitation;

• of the model under consideration, which may or may not include the crankshaft and its elasticity. Depending on the requirements of the analysis, the inertia of the mobile coupling components needs to be described exactly, or a simple global inertia of the assembly is sufficient.

The originality of this article is :

• give the exact and approximate formulas for the torque on the crankshaft ;

• to distinguish between the gas and inertia components and their various harmonics;

• show the impact of connecting rod inertia by calculating torque in two different ways;

• show the contribution of angular acceleration depending on the regime, taking into account the second derivative of the regime, which is not usually done because the regime is generally considered to be constant.