Valve timing

The distribution system groups together all the components that enable the cylinder to communicate with the outside environment during emptying and filling (transfer operations). Its function is to define the evolution of the flow cross-section of burnt and fresh gases as a function of the angle of rotation of the crankshaft (distribution diagram).

Timing obviously plays a decisive role in engine performance. It also plays a secondary role in the formation of pollutant emissions. In conjunction with the intake and exhaust systems, it defines the engine's permeability and, consequently, its air fill as a function of engine speed. It must therefore allow the cylinder to be emptied as completely as possible, and the maximum mass of fresh air to be introduced. Since these two phenomena are highly dependent on engine speed, the characteristics and technological solutions of the timing system must be adapted to the engine's objectives: flexibility, high specific power, low pollution levels, etc.

In addition to its essential active role during the transfer phases, the valve train must ensure a seal between the cylinder and the outside environment during the high-pressure cycle. Some of these components, in direct contact with the combustion process, are subject to considerable thermal stress (temperatures during the cycle can exceed 2,500 K) and high pressures (between 60 and 120 bar, depending on the cycle). In addition, combustion residues are a source of clogging that is detrimental to proper operation.

This article deals with the various problems involved in defining a valve train. It covers in turn :

• functional analysis establishing the relationship between engine permeability and valve geometry and kinematics. Engine filling, involving other factors such as intake and exhaust manifold geometry, is not covered in this article;

• the main sizing criteria for distribution components ;

• valve train architectures used on today's engines. The advantages and disadvantages of each are analyzed according to the criteria defined in the first two parts.

The curves presented in this article are by no means reference charts, but illustrate, in qualitative terms, the evolution of the various quantities studied.