Through the two strokes of the piston to complete a cycle of diesel engines called two-stroke diesel engine, oil machine to complete a cycle of crankshaft only one turn, compared with the four-stroke diesel engine, it improves the ability to work, in the specific structure and principle of work there are also major differences.

Two-stroke diesel engine and four-stroke diesel engine basic structure is the same, the main difference in the gas distribution mechanism. Two-stroke

course diesel engines do not have intake valves, some even exhaust valves are not available, but in the lower part of the cylinder to open a sweep port and exhaust port;

or set up a sweep port and exhaust valve mechanism. And specially set up by the moving parts driven by a sweeping pump and storage pressure air

Sweeping box, the use of the piston and the air port with the completion of the gas distribution, thus simplifying the structure of the diesel engine.

The figure is a two-stroke diesel engine working principle diagram. Sweep air pump attached to the side of the diesel engine, its

rotor driven by the diesel engine. Air is drawn in from the pump's suction, compressed and discharged, and stored in a large-volume

sweep tank in which a certain pressure is maintained. The working

principle of a two-stroke diesel engine is illustrated in a diagram.

Combustion Expansion and Exhaust Stroke:

The fuel is ignited and burned in the combustion chamber to produce high temperature and high pressure gas. The piston is pushed by the gas and moves downward from the upper stop point

to do work on the outside. Piston down until the exhaust port open (at this time the crank in the point position, at this time the gas

expansion of the end of the work, a large number of exhaust gas in the cylinder by its own high-pressure free exhaust, from the exhaust port to the exhaust pipe.

When the pressure in the cylinder drops to close to the sweeping pressure (generally the sweeping pressure in the sweeping box is 0

12, the downward piston to open the sweeping port 3 (at this time the crank is in the position of the point 4, the sweeping air into the cylinder,

At the same time, the exhaust gas in the cylinder through the exhaust port to drive out of the cylinder. The piston runs to the lower stop, the end of this stroke, but the sweep air

process continues until the next stroke exhaust port closed (at this time the crank in the point position.).

-4- 342 third marine diesel engine overhaul diagram two-stroke diesel engine working principle schematic

Sweeping and compression stroke:

Piston from the lower stop upward movement, the piston in the cover before the sweeping port, by the sweeping pump to supply the sweeping air stored in the air box

inside, through the sweeping port to enter the cylinder, the cylinder of the residual exhaust gas by the cylinder into the exhaust port

Air through the exhaust port

Air through the exhaust port

Air through the exhaust port

Air from the exhaust port to the cylinder. The residual exhaust gases in the cylinder are swept out of the cylinder by the air entering the cylinder through the exhaust port

. The piston continues to move upward, gradually covering the sweep port, when the sweep port is completely closed (at this time, the crank is in the point

position, the air stops charging, the exhaust is still going on, this stage is called "after the exhaust stage". When the exhaust port is closed

(at this time, the crank is in the point position, the air in the cylinder begins to be compressed. When compressed to the point before the upper stop,

Injectors will inject fuel into the cylinder, mixed with high temperature and high pressure air, and then in the vicinity of the upper stop fires, self-combustion

fire combustion. The end of this stroke, and with the previous stroke to form a complete working cycle.

Two-stroke diesel engine power diagram is shown in the figure, where, for the start of injection, for the piston upper stop, for

combustion end point.

Two-stroke diesel engine and four-stroke diesel engine compared with some obvious advantages, of course, there are also inherent

disadvantages.

2, four-stroke diesel engine working principle

Diesel engine work is done by the intake, compression, combustion expansion and exhaust of these four processes, these four processes constitute a working cycle. The piston to go four processes to complete a working cycle of diesel engines known as four-stroke diesel engines. Now compared to the above animation explains the rationale for its work.

I. Intake stroke

The first stroke - intake, its task is to make the cylinder full of fresh air. When the intake stroke begins, the piston is at the upper stop and some exhaust gas remains in the combustion chamber inside the cylinder.

When the crankshaft rotates the elbow, the connecting rod moves the piston from the upper stop to the lower stop, and at the same time, the intake valve is opened using a transmission mechanism linked to the crankshaft.

With the downward movement of the piston, the volume above the piston in the cylinder gradually increases: resulting in the air pressure in the cylinder is lower than the pressure in the intake pipe, so the outside air is constantly filled into the cylinder.

The gas pressure in the cylinder during the intake process changes with the volume of the cylinder as shown in the animation. The vertical coordinate of the graph indicates the gas pressure P, and the horizontal coordinate indicates the cylinder volume Vh (or the piston's stroke S), and this graph is called the schematic diagram. The pressure curve in the figure indicates that when the diesel engine works, the change rule of gas pressure in the cylinder. From the earth we can see the beginning of the intake, due to the presence of residual exhaust gas, so slightly higher than the atmospheric pressure P0. In the intake process due to the air through the intake pipe and intake valve flow resistance, so the intake stroke of the gas pressure is lower than the atmospheric pressure, the value of 0.085 ~ 0.095MPa, in the entire intake process, the cylinder gas pressure is roughly constant.

When the piston downward movement close to the lower stop, the gas flow into the cylinder still has a very high speed, inertia is very large, in order to use the inertia of the gas flow to improve the inflatable volume, the intake valve in the piston after the lower stop before closing. Although the piston upward at this time, but because of the inertia of the airflow, the gas can still fill the cylinder.

II. Compression Stroke

The second stroke - compression. Compression of the piston from the lower stop between the upper stop movement, the function of this stroke has two, one is to increase the temperature of the air, for the fuel to fire on its own in preparation: the second is for the expansion of the gas to create the conditions for work. When the piston up, after the inlet valve is closed, the air in the cylinder is compressed, as the volume is constantly small, the air pressure and temperature is also rising, the end of the compression of the pressure and humidity and the degree of air compression, that is, with the compression ratio of the end of the compression of the general compression of the pressure and temperature: Pc = 4 ~ 8MPa, Tc = 750 ~ 950K.

Spontaneous ignition of diesel fuel temperature of approximately 543-563K, the temperature of the end point of compression is much higher than the temperature of spontaneous combustion of diesel fuel, enough to ensure that the fuel injected into the cylinder to ignite combustion on its own.

Sprayed into the cylinder of the diesel fuel, not immediately fire, and after physical and chemical changes only after the fire, this period of time is about 0.001 ~ 0.005 seconds, known as the fire delay period. Therefore, to turn the crank to the upper stop before the crank angle of 10 ~ 35 ° crank start atomized fuel sprayed into the cylinder, and make the crank in the upper stop after 5 ~ 10 °, in the combustion chamber to reach the highest combustion pressure, forcing the piston downward movement.

III. Combustion Expansion Stroke

Third Stroke - Combustion Expansion. At the beginning of this stroke, most of the fuel injected into the combustion chamber is burned. Combustion gives off a lot of heat, so the pressure and temperature of the gas will rise sharply, the piston in the high temperature and high pressure gas downward movement, and through the connecting culm to make the crankshaft rotation, external work. So this stroke is also called work or work stroke.

With the downward movement of the piston, the volume of the cylinder increases, the pressure of the gas decreases, and the work stroke ends when the piston travels to the lower stop and the exhaust valve opens.

In the animation, the pressure change of the working stroke of this line rising part of the fuel combustion in the cylinder when the pressure rises sharply, the highest point indicates the highest combustion pressure Pz, the pressure and temperature at this point:

Pz = 6 ~ 15MPa, Tz = 1800 ~ 2200K

The ratio of the highest combustion pressure to the end point of compression pressure (Pz / Pc ), known as the pressure rise during combustion ratio, expressed in λ. Depending on the type of diesel engine, the range of λ value at maximum power cell is as follows: λ = Pz/Pc = 1.2 to 2.5.

IV. Exhaust stroke

The fourth stroke - exhaust. The function of the exhaust stroke is to discharge the expanded exhaust gas in order to fill with fresh air to prepare for the intake of the next cycle. When the working stroke piston movement to the lower stop near, exhaust valve open up, the piston in the crankshaft and connecting rod driven by the lower stop to the upper stop, and the exhaust gas out of the cylinder. Because of the resistance of the exhaust system exists, so at the beginning of the exhaust stroke, the gas pressure in the cylinder plus than the atmospheric pressure is 0.025-0.035MPa high, its temperature Tb = 1000 ~ 1200 K. In order to reduce the resistance of the piston movement in order to reduce the exhaust, the exhaust valve in the lower stop before the opening of the valve. As soon as the exhaust valve is opened, the gas with a certain pressure immediately rushes out of the cylinder, and the pressure inside the cylinder drops rapidly, so that when the piston moves upward, the exhaust gas inside the cylinder is discharged out by the upward movement of the piston. In order to utilize the inertia of the airflow when exhausting to make the exhaust gas discharge clean, the exhaust valve in the upper stop point before closing.

In the animation, the exhaust stroke curve indicates that during the exhaust process, the gas pressure in the cylinder is almost constant, but slightly higher than atmospheric pressure. The pressure Pr at the end of the exhaust stroke is about 0.105~0.115MPa, and the temperature Pr of the residual exhaust gas is about 850~960K.

Because the intake and exhaust valves are opened early and closed late; therefore, at the end of the exhaust stroke and at the beginning of the intake stroke, the piston is at the upper stop near the end, there is a period of time when the intake and exhaust valves are opened at the same time, and the period of time is expressed by the crankshaft angle, which is known as the valve This time is expressed by the crankshaft angle of rotation, known as the valve overlap angle.

After the end of the exhaust stroke, the intake stroke begins, so the entire cycle is repeated in accordance with the above process. Because of this diesel engine work cycle by four piston stroke that the crankshaft rotates two turns to complete, so it is called four-stroke diesel engine.

In the four-stroke diesel engine in the four strokes, only the third stroke that is the work of the strong stroke to produce power to the outside world for work, and the remaining three strokes are consuming power to prepare for the process. For this reason in a single-cylinder diesel engine must be installed on the flywheel, the use of flywheel rotational inertia, so that the crankshaft in the four strokes of continuous and uniform operation.

1- Intake: At this point the intake valve opens, the piston goes down, and the mixture of gasoline and air is sucked into the cylinder

2- Compression: At this point the intake valve and the exhaust valve are closed at the same time, the piston goes up, and the mixture is compressed.

3-combustion: when the mixture is compressed to the smallest possible spark plug ignites the mixture, the pressure generated by combustion pushes the piston down and drives the crankshaft to rotate.

4-exhaust: when the piston goes down to the lowest point, the exhaust valve opens and the exhaust gases are discharged, the piston continues to go up to discharge the excess exhaust gases.