Working Principle:
The diesel internal combustion engine differs from the gasoline powered Otto cycle by using highly compressed hot air to ignite the fuel rather than using a spark plug compression ignition rather than spark ignition.In the diesel engine, only air is initially introduced into the combustion chamber. The air is then compressed with a compression ratio typically between 15:1 and 22:1 resulting in 40-bar (4.0 MPa; 580 psi) pressure compared to 8 to 14 bars (0.80 to 1.40 MPa; 120 to 200 psi) in the petrol engine. This high compression heats the air to 550 °C (1,022 °F). At about the top of the compression stroke, fuel is injected directly into the compressed air in the combustion chamber. This may be into a (typically toroidal) void in the top of the piston or a pre-chamber depending upon the design of the engine. The fuel injector ensures that the fuel is broken down into small droplets, and that the fuel is distributed evenly. The heat of the compressed air vaporizes fuel from the surface of the droplets.
The vapour is then ignited by the heat from the compressed air in the combustion chamber, the droplets continue to vaporize from their surfaces and burn, getting smaller, until all the fuel in the droplets has been burnt. The start of vaporization causes a delay period during ignition and the characteristic diesel knocking sound as the vapour reaches ignition temperature and causes an abrupt increase in pressure above the piston. The rapid expansion of combustion gases then drives the piston downward, supplying power to the crankshaft. As well as the high level of compression allowing combustion to take place without a separate ignition system, a high compression ratio greatly increases the engine's efficiency. Increasing the compression ratio in a spark-ignition engine where fuel and air are mixed before entry to the cylinder is limited by the need to prevent damaging pre-ignition. Since only air is compressed in a diesel engine, and fuel is not introduced into the cylinder until shortly before top dead center , premature detonation is not an issue and compression ratios are much higher.
Diesel's original engine injected fuel with the assistance of compressed air, which atomized the fuel and forced it into the engine through a nozzle a similar principle to an aerosol spray. The nozzle opening was closed by a pin valve lifted by the camshaft to initiate the fuel injection before top dead center . This is called an air-blast injection. Driving the three stage compressor used some power but the efficiency and net power output was more than any other combustion engine at that time.
Diesel engines are also produced with two significantly different injection locations. "Direct" and "Indirect". Indirect injected engines place the injector in a per-combustion chamber in the head which due to thermal losses generally require a "glow plug" to start and very high compression ratio. Usually in the range of 21:1 to 23:1 ratio. Direct injected engines use a generally donuts shaped combustion chamber void on the top of the piston. Thermal efficiency losses are significantly lower in Diesel engines which facilitates a much lower compression ratio generally between 14:1 and 20:1 but most DI engines are closer to 17:1.
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