In a diesel engine, fuel and air are supplied separately. First, air is sucked into the cylinder, then it is compressed, and only then fuel is injected at the end of the compression stroke.
Self-ignition of fuel is accompanied by a sharp, spasmodic increase in pressure in the cylinder - this usually explains the noisy, «tough» diesel engine operation. In low-speed diesel engines of large displacement, which are used in trucks, this disadvantage is manifested to a lesser extent, and it is tolerated. In passenger car diesel engines, they try to get rid of it by using a swirl chamber, or prechamber, a small compartment of the combustion chamber into which fuel is injected. There it ignites, partially mixes with air, and then spreads through the main volume of the cylinder. This method somewhat reduces «rigidity» operation of the engine, but reduces its thermal efficiency and fuel economy. For smoother fuel ignition, two-stage injection and a complex electronic control circuit are used.
A characteristic feature of diesel engines is the presence in the exhaust gases of solid particles - 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 that of a gasoline engine. High, not less than 14 (reaches 25), the compression ratio is necessary to increase the temperature of the air in the cylinder to a value sufficient to ignite the fuel. Usually 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 complicated than in gasoline engines. Their complexity is determined primarily by the fact that it is necessary to inject very small, only a few milligrams, portions of fuel into a high-pressure environment. For injection and atomization of fuel, as well as for the distribution of its particles throughout the volume of the combustion chamber, nozzles are used. A high compression ratio requires the use of suitable fuel pumps: the pressure in the injector nozzle must reach several hundred bar. 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, an oxidizing system is installed on vehicles with diesel engines catalytic converter. An exhaust gas recirculation system is also used for this purpose. The addition of exhaust gases to the working mixture to reduce the limiting combustion temperature makes it possible to reduce emissions of nitrogen oxides in them (NOx).
Three methods of injection of diesel fuel into the engine cylinders are known: through the prechamber, vortex chamber and direct injection.
When diesel fuel is injected through the prechamber, the fuel is sprayed over it and instantly ignites. Due to the small amount of oxygen in the prechamber, only part of the fuel burns out, the rest is displaced from the prechamber into the engine cylinder, where it burns out completely.
When fuel is injected through a vortex chamber, the process of fuel combustion occurs in the same way as when fuel is injected through a prechamber. The difference lies in the shape and size of the channel connecting the vortex chamber with the combustion chamber. When fuel is injected into the swirl chamber, it is much better mixed with air, and the combustion process proceeds more smoothly.
With direct injection, the fuel immediately enters the combustion chamber. The fuel pump delivers fuel at a pressure of about 900 bar, and fuel injection is carried out in two stages.
The use of dual-channel fuel injectors makes it possible to make an initial injection of a small part of the fuel, as a result of which the combustion process of the fuel is improved, and the combustion process itself proceeds more smoothly. The amount of fuel injected is controlled by the electronic engine management system based on information from the sensors.
The air flow entering the engine is twisted in a spiral in the intake ducts, as a result of which the combustion process in the engine cylinders is improved. In addition, this makes it easier to start a cold engine, and pre-heating of the engine must be carried out 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 periodically drained.
The fuel pump is driven by a toothed belt. Its internal moving parts «are lubricated» diesel fuel.
The vehicle power system consists of a multi-section fuel tank, pipelines, a fuel filter, a fuel priming pump, a high pressure fuel pump and injectors.
The fuel tank is located under the bottom at the rear of the vehicle. The fuel tank is vented through a closed air removal system. During ventilation, fuel vapors from the tank are captured by a canister with activated carbon - an adsorber.
Warnings: Avoid getting diesel fuel on the cooling system hoses. Hoses that have been exposed to diesel fuel for a long time must be replaced.
The power supply system of a diesel engine is particularly sensitive to contamination, therefore, when working on the system, maintain maximum cleanliness. Before disconnecting the fuel lines, thoroughly clean them of dirt. When removing system components, cover exposed areas with a lint-free cloth.
Do not use compressed air to clean engine-mounted system components.
Install only clean power elements.
Remove spare parts from the packaging just before installing them.
When checking the operation of the injectors, never put your hands or any part of your body under the jet of fuel coming out of the injector. The fuel exits the nozzle at high pressure and can penetrate through the skin.
Use goggles to protect your eyes from getting fuel in them. If fuel comes into contact with skin, wash with plenty of water.
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