Combustion chamber design, which includes the shape of the cylinder head, the shape of the top of the piston and the air flow through the inlet ports, is one of the most important factors in efficient operation of the diesel engine. Because of the very short space of time available in a diesel engine in which the fuel and air can mix, various methods have been devised in an attempt to give improved mixing and combustion.
Combustion chambers can be of several designs but all are concerned in creating turbulence to the air during the compression stroke. In the diesel engine, the fuel is in the form of fine particles sprayed into the cylinder after the air has been compressed. To secure complete combustion, each particle of fuel must be surrounded by sufficient air. The mixing of the air and fuel is greatly assisted by the combustion chamber air turbulence.
Some engines have helical inlet ports to provide additional swirl.
Generally, combustion systems can be classified as direct and indirect injection types.
· Direct injection.
· Indirect injection, the two most common types being:
· Turbulence chamber and
· Pre-combustion chamber.
The larger, slow speed engines and medium speed engines do not have the same difficulty in achieving good combustion as small high speed engines.
Direct injection combustion chamber
With direct injection, the fuel is injected directly into the combustion chamber which is usually formed by a cavity in the piston crown.
This cavity is carefully shaped to promote air swirl and the direction of the injector nozzle ensures that rapid mixing of the fuel and air assists complete combustion.
Advantages - It is claimed that direct injection gives higher thermal efficiency with lower fuel consumption. This is bought about by the fact that no heat is lost or power wasted in pumping air through a restricted opening into the separate chamber or in discharging the gases from the chamber. This gives easier starting and generally this type of engine does not require a starting aid device, such as glow plugs.
Disadvantages - This kind of injection is prone to "diesel knock".
The indirect injection or separate chamber system is where a separate small chamber is connected to the main chamber by a narrow passage or orifice.
The pre-combustion chamber and the turbulence chamber (also called a compression swirl chamber) work on the same principle. The main physical difference is the location and size of the connecting passage.
With pre-combustion chambers only about 30% of the combustion air is forced into the chamber, fuel is injected and primary burning takes place in the chamber. This prevents too sudden a rise in pressure which can contribute to the so called 'diesel knock'. The burning mixture of fuel and air is vigorously expelled through the connecting passage into the main combustion chamber or cylinder where an excess of air permits combustion to be completed.
Advantages - lower injection pressures can be used, resulting in less wear of injector nozzles; simpler design of nozzle equipment, which are easier to maintain, and smoother idling of the engine.
Engine manufacturers may in some instances use either design in their range, depending on operating requirements.
Disadvantages - not as efficient as direct injection. It can also be prone to pre-combustion burn-out.