The second important point is the method of fuel supply. In a gasoline engine, the working fluid is a mixture of gasoline and air. The mixture is prepared in advance (in the carburetor) or directly at the moment of its supply to the cylinders (in injection systems) - the main thing is that the fuel is supplied together with air, and a relatively homogeneous air-fuel mixture is ignited and burned.
In a diesel engine, fuel and air are supplied separately. First, air is sucked into the cylinder, then it is compressed, and only after that fuel is injected, so there is no need to talk about a homogeneous air-fuel mixture. The injection is made at the end of the compression stroke, the fuel and air do not actually mix with each other, combustion occurs at the front of the fuel jet injected into the compressed air.
Self-ignition of fuel is accompanied by a sharp, spasmodic increase in pressure in the cylinder - this explains the usually noisy, harsh operation of a diesel engine. In low-speed diesel engines with large displacement, which are used on trucks, this disadvantage is less pronounced, 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, after which the burning mixture spreads through the main volume of the cylinder. This method somewhat reduces the rigidity of the engine, but reduces its thermal efficiency and fuel efficiency. For smoother fuel ignition, two-stage injection and a complex electronic control circuit are used.
A characteristic feature of diesel engines is the presence of particulate matter in the exhaust gases. Due to the heterogeneity of the combustion process, a certain lack of oxygen is always observed on the surface of individual fuel particles, as a result of which, instead of their oxidation, partial thermal decomposition occurs with the formation of solid products - soot. For good combustion of diesel fuel, a significant, even excessive amount of air is required.
Also, the compression ratio of a diesel engine is 2 times higher than that of a gasoline engine. High, not less than 14 (reaches 25), the compression ratio is necessary in order for the air temperature in the cylinder to rise 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. These portions must be very accurately measured - it is the amount of fuel supplied that controls the operation of a diesel engine. This requires fast and accurate nozzles. A high compression ratio requires the use of appropriate 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 is drawn from the fuel tank by the high pressure fuel pump and then delivered under high pressure to the fuel injectors.
To reduce the amount of harmful substances contained in the exhaust gases, an oxidizing catalyst is installed on vehicles with diesel engines. A system is also used to mix exhaust gases with fresh air entering the engine, as a result of which the percentage of oxygen in the air that will be burned in the engine cylinders is reduced. As a result, the combustion temperature of the fuel mixture decreases, resulting in a decrease in the amount of nitrogen oxides formed.
There are three methods of injection of diesel fuel into the engine cylinders: through the prechamber, through the vortex chamber and direct injection.
When diesel fuel is injected through the prechamber, the fuel is sprayed over the prechamber and instantly ignites. Due to the small amount of oxygen in the prechamber, only part of the fuel burns out, and the rest of the fuel is displaced from the prechamber into the engine cylinder, where it burns out completely.
When fuel is injected through a vortex chamber, the fuel combustion process is carried out 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, the fuel is mixed with air much better, and the combustion process is smoother.
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