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Basic Working Principle Of Four Stroke Diesel Engine

Nov. 30, 2022

Diesel engine is an indispensable part of diesel generator set, which is a compression ignition internal combustion engine using diesel as fuel. During operation, the air in the cylinder is compressed and the temperature rises. The diesel fuel injected into the cylinder at regular intervals ignites and combusts automatically, generating high-temperature and high-pressure gas expansion to push the piston to do work, transforming heat energy into mechanical work. The working cycle of diesel engine consists of intake, compression, fuel injection, ignition, expansion, work and exhaust. These processes can be realized by four stroke or two-stroke diesel engines.


Basic working principle of four stroke diesel engine (non supercharged):


Four stroke diesel engine: use four strokes and the crankshaft rotates for two cycles to complete a working cycle. When working, the piston moves in a reciprocating straight line and the crankshaft rotates. The instantaneous position where the piston changes the direction of motion is called dead center (dead center), and the instantaneous motion speed of the piston at dead center is zero. The farthest dead center from the crankshaft center is called top dead center, and the nearest dead center is called bottom dead center.


① Intake stroke: the piston moves down from TDC and the intake valve opens. The cylinder is filled with fresh air due to the downward suction of the piston. In order to fill more fresh air, the inlet valve is generally opened in advance before TDC and closed late after BDC. The opening angle of the inlet valve is about 220 °~250 °.


② Compression stroke: the piston moves up from the bottom dead center, and the inlet and exhaust valves are closed. The upward piston compresses the air in the cylinder, increasing its pressure and temperature. The pressure at the end of compression is about 3~6MPa, and the temperature is about 500~700 ℃. Near TDC (compression end point), the atomized fuel is injected into the combustion chamber through the fuel injector, and starts to ignite and burn by itself under the action of high-temperature and high-pressure air.


diesel engine

③ Power stroke: the piston moves downward from TDC, and the inlet and exhaust valves are closed. At the beginning of this stroke, a large amount of mixed gas burns, and the pressure and temperature in the cylinder rise sharply, the maximum values of which can reach 6~9MPa and 1500~2000 ℃ respectively. The high-temperature and high-pressure gas pushes the piston downward to do work. At some time after TDC, the combustion ends and the expansion work is still in progress. At a certain time before the piston reaches BDC, the exhaust valve opens and the work process ends. At this time, the pressure in the cylinder is about 0.2~0.5MPa, and the temperature is about 600~700 ℃. The piston continues to descend to the lower dead center.


④ Exhaust stroke: the crankshaft drives the piston to move upward from BDC, the exhaust valve continues to open, and the exhaust gas in the cylinder is forced out of the cylinder by the upward piston. In order to achieve full exhaust and reduce the work consumed in the exhaust process, the exhaust valve shall not only be opened before the bottom dead center, but also be closed after the top dead center of the exhaust stroke. The continuous opening angle of the exhaust valve is about 230 °~260 °.


A four stroke diesel engine goes through four strokes to complete a working cycle: intake, compression, work and exhaust. The crankshaft rotates twice in one working cycle, that is, the crankshaft angle is 720 °. Only one stroke does work, and the other three strokes consume work. Therefore, in a single cylinder diesel engine, there must be a flywheel that is large enough to supply the energy required for the three strokes, while in a multi cylinder diesel engine, it is supplied by the expansion of other cylinders.


The diesel engine must be driven by external energy to make it start and run, realize the shutdown state and enter the working state, until the fuel injected into the cylinder ignites automatically, so that the diesel engine can run by itself.


Jiangsu Starlight Electricity Equipments Co.,Ltd. is one of the earliest manufacturers of generators and diesel generator sets in China. The company relies on high-quality products and good services. It has been widely recognized by users in many fields. At present, it can provide various specifications of diesel generator sets in the power range of 15KW-2500KW. For more details, please contact us with sales@dieselgeneratortech.com.

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