Instantaneous angular speed fluctuations of thermal engines - Forces, torques and moments acting on single-cylinder engines

When searching for the meaning of the term « acyclism » in a dictionary of current words or even a technical encyclopaedia, the automotive engineer is surprised not to find one. Conversely, the adjective « acyclical » does exist and qualifies a non-continuous state or one which does not follow a regular cycle.

We are not arguing that engineers are good linguists ! But we know that the phenomena described by the term « acyclism » are systematically studied and optimised during the development of a new engine.

For our part, we consider that acyclism can be defined as the non-uniformity of rotation of the crankshaft during an operating cycle. The study of this phenomenon in reciprocating internal combustion engines consists, on the one hand, of analysing the phenomena and mechanisms which generate or result from the non-uniform rotation of the crankshaft, and, on the other hand, of studying the means intended to limit its effects.

The analysis and reduction of noise and vibration phenomena have become essential concerns for the development of an automotive vehicle powertrain unit, just as for the development of the body shell of the latter. Therefore it is necessary to reduce excitation sources as much as possible.

For these various reasons, it has become important to accurately formulate all the aspects reducing the phenomenon of acyclism, and to do so gradually (concepts of gas pressure, gas and inertial forces, gas moments, inertial torque, etc.).

Therefore this article deals exclusively with the single-cylinder 4-stroke engine in order to lay the foundations on which developments relating to multi-cylinder engines will be based.

The reader will find a glossary and a table of the symbols used at the end of the article.