Engines: Engine Components: Cylinder heads
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Diesel combustion chambers

Summary
Direct-injection diesels inject into the combustion chamber formed in the top of the piston. For indirect injection, the combustion chamber is a separate chamber formed in the head.

Diesel combustion chambers come in 2 main types. Direct and indirect injection. Both are designed to promote turbulence, to help the compressed air and injected fuel mix well.

Engines using direct injection have cylinder heads with a flat face. The combustion chamber is formed in the top of the piston.

Sometimes, the rim of the piston provides “squish”, forcing the air to the centre of the combustion chamber. This causes turbulence as fuel is injected into the cylinder.

In indirect injection, the piston is fairly flat, or has a shallow cavity. The main combustion chamber is between the cylinder head and the top of the piston, but a smaller, separate chamber is in the head. Fuel is injected into this smaller chamber. It can have various designs. A swirl chamber is spherical, and connected to the main chamber by an angled passage. Both the injector and glow plug are screwed into the head. The glow plug preheats the air inside to help start the engine.

During compression, the spherical shape makes the air swirl in the chamber. This helps make a better mixture of the air and fuel, which improves combustion. This combustion chamber is divided into a main combustion chamber and an air cell, joined by a throat. The injector is in the throat.

When injection commences, combustion pressure forces the air to flow from the air cell where it mixes with fuel from the injector. The rush of air from the air cell produces a rotary motion of gas in the main chamber which helps make combustion more efficient.

This pre-combustion chamber is screwed into the cylinder head. The injector is mounted in the upper end.

Injection occurs near the top of the compression stroke. Only part of the fuel is burned in the pre-combustion chamber because of the limited amount of air there. The high rise in pressure forces burning fuel into the main chamber. This happens very rapidly, which helps make more efficient combustion.