A two-stroke diesel engine must also complete four processes during operation: intake, compression, power generation, and exhaust. But unlike a four stroke diesel engine, the piston only runs two strokes (i.e. the crankshaft rotates one revolution) to complete these four processes. Compared with a four stroke diesel engine, it simplifies the valve train and has scavenging and exhaust ports around the cylinder, so a scavenging pump needs to be used to meet the needs of the gas exchange process. The following diagram is a schematic diagram of the working process of a single cylinder two-stroke diesel engine, indicating a reflux scavenging two-stroke diesel engine.
The working process is as follows:
(1) First stroke: intake and compression
When the piston is driven by the crankshaft, the scavenging pump is also driven to work by the crankshaft. The piston moves from the bottom dead center to the top dead center. Before the piston covers the scavenging port, the air sent by the scavenging pump enters the cylinder through the scavenging port and forms a reflux route to clean the exhaust gas, allowing it to be discharged from the exhaust port. At this time, fresh air is filled into the cylinder [Figure (a)]. The piston continued to move upwards to cover the exhaust port, compressing the air and beginning the compression process [Figure (b)]. When the piston approaches the top dead center, the air temperature in the cylinder reaches the conditions for diesel to self ignite, and diesel is injected to self ignite.
(2) Second stroke: working and exhaust
At the end of the first stroke, diesel injected into the cylinder rapidly burns, causing a sharp increase in temperature and pressure. The piston is driven by inertia to cross the top dead center, and the gas expands to push the piston to do work [Figure (c)], resulting in a sharp decrease in pressure and temperature inside the cylinder. When the piston is pushed down by gas pressure to open the exhaust port, the exhaust gas is discharged from the exhaust port by the pressure difference inside and outside the cylinder [Figure (d)]. The piston went down again and opened the scavenging port, sending fresh air from the scavenging pump into the cylinder to clean the exhaust gas. In this way, the piston runs two strokes to complete one working cycle.
The advantages of a two-stroke diesel engine are: better power than a four stroke diesel engine, because its compression ratio is higher than that of a four stroke diesel engine and its exhaust emissions are better; A two-stroke engine has a higher efficiency than a four stroke engine because it reduces two strokes compared to a four stroke engine; Two stroke engines often adopt a cross head structure because the piston travel is long, and the explosion pressure required for the same power is lower, which reduces the mechanical requirements for combustion chamber components; Long piston travel, reduced impact, and reduced vibration; The uniformity of torque is better than that of a four stroke diesel engine because it completes one working cycle by turning the crankshaft once; The rotation is relatively uniform; The structure is relatively simple.
The two-stroke principle is less commonly used in the automotive (including engineering machinery) diesel engine industry, and is widely used in large, low-speed marine diesel engines. The main reason is that the rotational speed of marine diesel engines is relatively low, and the two-stroke cycle is conducive to the maximum power and economic performance of diesel engines, without causing too high thermal load and reducing the service life of components, thereby improving the practicality and reliability of marine diesel engines as a whole.
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