In a diesel engine, fuel and air are supplied separately. First, air is sucked into the cylinder, then it is compressed, and only then, at the end of the compression stroke, is fuel injected.
Self-ignition of fuel is accompanied by a sharp, abrupt increase in pressure in the cylinder - this explains the usually noisy, "harsh" operation of a diesel engine. In low-speed diesel engines of large working volume, which are used in trucks, this drawback is manifested to a lesser extent, and it is tolerated. In diesel engines of passenger cars, they try to get rid of it by using a swirl chamber, or pre-chamber, a small section of the combustion chamber into which fuel is injected. There it ignites, partially mixes with air, and then spreads throughout the main volume of the cylinder. This method somewhat reduces the "harshness" of engine operation, but reduces its thermal efficiency and fuel economy. For smoother ignition of the fuel, two-stage injection and a complex electronic control circuit are used.
A characteristic feature of diesel engines is the presence of solid particles in the exhaust gases - soot. For good combustion of diesel fuel, a significant, even excessive, amount of air is required.
The compression ratio of a diesel engine is twice as high as that of a petrol engine. High, at least 14 (reaches 25), the compression ratio is necessary to increase the air temperature in the cylinder to a value sufficient for fuel ignition. Typically, in diesel engines, the compression ratio is 21–22 and is limited only by the strength characteristics of the engine.
Fuel supply devices in diesel engines are much more complex than in petrol engines. Their complexity is determined primarily by the fact that it is necessary to inject very small portions of fuel, only a few milligrams, into a high-pressure environment. Injectors are used to inject and spray the fuel, as well as to distribute its particles throughout the combustion chamber. The high compression ratio requires the use of appropriate fuel pumps: the pressure in the injector nozzle must reach several hundred bars. All this complicates and significantly increases the cost of the fuel supply system and, accordingly, the diesel engine itself.
Fuel from the fuel tank is supplied to the injectors by a high-pressure fuel pump.
To reduce the amount of harmful substances contained in exhaust gases, cars with diesel engines are equipped with an oxidizing filter catalytic converter. The exhaust gas recirculation system is also used for this purpose. Adding exhaust gases to the working mixture to reduce the maximum combustion temperature allows for a reduction in nitrogen oxide (NOx) emissions.
There are three known methods of injecting diesel fuel into engine cylinders: through a pre-chamber, a swirl chamber and direct injection.
When diesel fuel is injected through the pre-chamber, the fuel is sprayed on it and instantly ignites. Due to the small amount of oxygen in the pre-chamber, only part of the fuel burns, the rest is forced out of the pre-chamber into the engine cylinder, where it burns completely.
When fuel is injected through a swirl chamber, the combustion process is the same as when fuel is injected through a pre-chamber. The difference lies in the shape and size of the channel connecting the swirl chamber to the combustion chamber. When fuel is injected into a swirl chamber, it mixes with air much better, and the combustion process is smoother.
With direct injection, the fuel enters the combustion chamber immediately. The fuel pump delivers fuel under pressure of about 900 bar, and the fuel is injected in two stages.
The use of dual-channel fuel injectors allows for the initial injection of a small amount of fuel, resulting in improved fuel combustion and a smoother combustion process. The amount of fuel injected is controlled by the electronic engine management system based on sensor information.
The air flow entering the engine is twisted in a spiral in the intake channels, which improves the combustion process in the engine cylinders. In addition, this makes it easier to start a cold engine, and the engine must be preheated at an air temperature below -10°C.
Before entering the high-pressure fuel pump, the fuel passes through fuel filter, in which water is also separated from it, which must be drained periodically.
The fuel pump is driven by a toothed belt. Its internal moving parts are "lubricated" with diesel fuel.
The vehicle's fuel system consists of a multi-section fuel tank, pipelines, fuel filter, fuel pump, high-pressure fuel pump and injectors.
The fuel tank is located under the bottom at the rear of the car. The fuel tank is ventilated through a closed air removal system. During ventilation, fuel vapors from the tank are captured by a canister with activated carbon - an adsorber.
Warnings: Do not allow diesel fuel to come into contact with the cooling system hoses. Hoses that have been exposed to diesel fuel for a long time must be replaced.
The fuel system of a diesel engine is particularly sensitive to contamination, so when working on the system, maintain maximum cleanliness. Before disconnecting the fuel lines, thoroughly clean them from dirt. When removing system components, cover open areas with a lint-free cloth.
Do not use compressed air to clean engine mounted system components.
Install only clean power supply components.
Remove spare parts from packaging immediately before installation.
When checking the operation of the injectors, never place your hands or any part of your body under the jet of fuel coming out of the injector. Fuel comes out of the injector under high pressure and can penetrate the skin.
Use goggles to protect your eyes from fuel getting into them. If fuel gets on your skin, rinse it with plenty of water.