Instantaneous angular speed fluctuations of thermal engines - Focus on 3-cylinder engine and comparison with 4-cylinder engine

Engine acyclism is one of the phenomena that engine manufacturers keep under control, since noise and vibration are nowadays just as important as performance and fuel consumption. The notion of acyclism is defined in a previous article [BM 2 588] , concerning the single-cylinder engine; we recall that acyclism can be presented as the non-uniformity of crankshaft rotation during an operating cycle. A second article [BM 2 589] looks at the acyclism of the 4-cylinder engine, seen as a combination of single-cylinder engines. While the in-line 4-cylinder engine is still the most widely used in the automotive world today, the acyclism of the in-line 3-cylinder engine should also be studied. Today, this type of engine is increasingly used in entry-level vehicles, in particular to minimize fuel consumption. However, it is not without its drawbacks, particularly in terms of noise and vibration. The acyclism of the 3-cylinder engine is genetically more pronounced than that of the 4-cylinder engine from which it is generally derived.

In this article devoted to the acyclism of the 3-cylinder engine, we remain faithful to our systematic approach, detailing each notion precisely and progressively. Numerical examples are also systematically provided, most often graphically, as acyclism is practically "visualizable" enough.

We can't discuss the acyclism of the 3-cylinder engine without comparing it to its 4-cylinder "big brother" (gasoline and diesel). The main quantities relating to these two types of engine are recalled and put into perspective in the form of tables, the most effective way of synthesizing the whole in the form of a simple, usable tool.

Finally, we explain the dynamic behavior of the connecting rod (a valid approach whatever the type of combustion engine considered: 3-cylinder, 4-cylinder, gasoline or diesel) and provide the reader with orders of magnitude for the main rotary inertias of the components making up the mobile coupling (flywheel, clutch mechanism, crankshaft, timing drive pulley, connecting rod, piston).

At the end of the article, readers will find a glossary and a table of notations and symbols used.