How is the combustion chamber formed in diesel engines? What governs its shape?

In normal engines the combustion chamber is formed in the space between the cylinder cover and the piston crown. The upper part of the cylinder liner usually forms the periphery to the space.
The shape of a combustion chamber may vary between that of a spheroid which will be formed from a concave piston crown and cylinder cover, to that of an inverted saucer, formed from a concave cylinder cover and a slightly convex piston crown. In opposed-piston engines the combustion chamber will be spheroidal. The piston crowns on the upper and lower pistons are usually identical in form. Combustion chambers of the shapes mentioned are referred to as open types.
The shape of a combustion chamber must be such that all parts of the space are accessible to the fuel sprays. If any part is not accessible, the space is wasted and combustion has to take place in a reduced space, which causes further difficulties due to less air being available in the region of the fuel spray. The wasted space is sometimes referred to as parasitic volume. The shape of the various parts must also be satisfactory in respect of their strength as they must be able to withstand the pressures in the cylinder without flexing.
With high-speed engines, open combustion chambers can create problems with very high rates of pressure rise due to the shortness of time available for injection and combustion. To overcome this problem the fuel is injected into a separate chamber which is connected to the main combustion chamber by a restricted passage. The restricted passage is at a high temperature, the fuel spray is long and narrow. Following injection the fuel commences to burn in the separate chamber and issues from the restricted passage at a high velocity due to the pressure rise in the chamber. The fuel enters the main combustion chamber as burning vaporized particles and combustion is then completed. The small chamber is about one-third of the clearance volume and is called a pre¬combustion chamber or antechamber. Its use allows high-speed engines to operate over wide speed ranges without combustion difficulties, and is a necessity in automotive engines. It is met in the marine field when automotive engines are used for electrical generation or other auxiliary purposes.

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