There are several types of combustion chambers in diesel engines, each designed to provide the most efficient vortex flow.
Main types of design of combustion chambers:
- Direct injection combustion chamber;
- Indirect injection combustion chamber.
Direct injection combustion chamber
In a direct injection combustion chamber, fuel is injected directly at the closed end of the cylinder. You can see the open-type combustion chamber layout below.
As you can see in the diagram, in the upper part of the piston there is a recess in which air is located at the moment when the piston is at TDC in close proximity to the cylinder head. To obtain the required compression ratio, an overhead valve mechanism is used. The piston head has shallow recesses to maintain the required clearances with the cylinder head. If the valves are incorrectly adjusted, their knock will be heard. The nozzle delivers atomized fuel at a pressure of 175 bar into the air stream and the resulting air-fuel mixture enters the piston recess (combustion chamber). Whirlwinds form in the vertical and horizontal planes.
When the piston moves up, air enters the recess and moves approximately as shown in the diagram. When the piston is at TDC, this effect is enhanced by the swirl of the piston between the piston and the head. Horizontal or rotating swirling occurs with the help of a swirler on the intake valve. The combination of two vortex flows creates a “circulation” of air in the recess and provides the necessary supply of oxygen to the combustion area.
Indirect injection combustion chamber
With indirect injection, fuel injection is more uniform, so less injection pressure is required. Indirect injection allows the engine to operate over a wide rev range.
The indirect injection chambers have a swirl chamber, which is connected by a channel to the main chamber. Thanks to this, the design allows you to work with higher temperatures.
By the compression stroke, the air is injected through the channel of the vortex chambers. The nozzle delivers fuel to a rapidly moving mass of air, after which it is atomized into tiny particles. After combustion in the swirl chamber, the already burning fuel with unburned fuel enters the main combustion chamber, which is located in the piston crown.
When the injection time is increased to maintain the required engine power, the main part of the fuel injected already at the end of the injection period is thoroughly mixed with air in the main chamber and only then ignites. Due to this, the combustion period can continue for a long time until the fuel does not have enough oxygen for combustion. From this point on, black smog will begin to appear. It indicates the maximum amount of fuel that can be injected to run the engine at maximum power without compromising economy.