GENERAL INFORMATION

When the diesel engine operates, clean air is drawn into the cylinders and compressed to high pressure. The air temperature rises to 700 – 900°C, exceeding the ignition temperature of diesel fuel. Fuel is injected into the cylinder slightly ahead of TDC and ignites. Therefore, spark plugs are not used to ignite the fuel.

Fuel is drawn from the tank directly by the high-pressure fuel pump (HPFP). In the HPFP, the fuel is compressed to the pressure required for injection. Injection follows a sequence similar to the firing order of a gasoline engine. To reduce harmful substances in the exhaust gases, diesel engines have a diesel oxidation catalytic converter. At the same time, the exhaust gas recirculation (EGR) system significantly reduces nitrogen oxides in the exhaust gases. This is achieved by supplying exhaust gas to the intake air, which reduces the oxygen concentration in the air entering the cylinders. This delays ignition and lowers combustion temperature, which ultimately reduces NOx formation. The EGR process must, however, be precisely metered, otherwise soot emissions increase. For this purpose, the amount of intake air is measured by the mass air flow meter, which allows the electronic control unit to control the EGR process.

When the engine is cold, the temperature of the compressed air is insufficient to ignite the fuel. In this case, preheating is required. For this purpose, an electric glow plug is installed in each cylinder. Glow time depends on the outside air temperature and is controlled by a special relay.

Fuel can be injected into the cylinders in three ways: into a pre-chamber, into a swirl chamber, or directly into the combustion chamber.

In pre-chamber injection, diesel fuel is injected into the pre-chamber of the respective cylinder. The hot fuel mixture ignites immediately. However, the oxygen content in the pre-chamber is sufficient to burn only part of the injected fuel. The remaining unburned fuel is forced by combustion pressure into the main combustion chamber, where complete combustion takes place.

In swirl chamber injection, fuel is injected into a chamber separated from the main combustion chamber. The difference from pre-chamber injection lies in the design of the connecting passage between the swirl chamber and the main chamber. Here, strong air swirl develops in the swirl chamber during compression, which promotes good mixing of air with the injected fuel.