二冲程发动机与四冲程发动机（Two stroke engine and four stroke engine）

二冲程发动机与四冲程发动机（Two stroke engine and four stroke engine） 二冲程发动机与四冲程发动机(Two stroke engine and four stroke engine) What is the difference between two stroke engine and four stroke engine? Working principle is different, regardless of the two stroke engine or four stroke diesel engine, through the intake, compression, combustion, expansion, exhaust four working processes, in order to complete a work cycle. What's different is that: 1, in the four stroke engine, the crankshaft rotates two times, the piston reciprocating movement two times, every four strokes to complete a work cycle. In the two stroke diesel engine, the crankshaft rotates every time, and the piston moves once and again, and completes a working cycle every two strokes. 2, two stroke diesel engine and four stroke diesel engine, each completed a working cycle, its inlet, exhaust valve or inlet, exhaust, scavenging port is only opened and closed once, but its opening and closing time cycle is different. 3. Theoretically, the power generated by a two stroke diesel engine should be equal to two times the power generated by the four stroke diesel engine of the same working volume. 4. Because of the two stroke diesel engine ventilation, a part of the combustible mixture exhaust with exhaust gas, so the fuel and lubricating oil consumption are large. 5, because the two stroke engine work stroke frequency is large, so the work is relatively smooth. The difference of general arrangement The 1 and four stroke engines have a complex valve train valve mechanism, which is controlled by the camshaft to open and close the valve timing to complete the intake and exhaust process. The two stroke diesel engine uses the piston to control the opening of the exhaust port and the scavenging port to complete the scavenging and exhausting process. The scavenging and exhaust of the 2 and two stroke engines are carried out near the lower dead center of the piston. The valve train of the four stroke engine is set on the cylinder head. Most of the 3 and two stroke diesel engines use crankcase scavenging, and the four stroke engine crankcase has a pressure oil duct or tubing. The cylinder head of 4 and two stroke engines has simple structure, no inlet, exhaust port and valve train, and no lubricating oil duct. The cylinder head of a four stroke engine is a very complex part. The piston rings of the 5 and two stroke engines only have gas rings and no oil rings. The piston ring of the four stroke engine has both gas ring and oil ring. The piston skirt of the 6 and two stroke engines should be longer than the piston skirt of the four stroke engine Working principle of two stroke diesel engine By two strokes of the piston to complete a working cycle of the engine is called the two stroke diesel engine, oil engine crankshaft in a work cycle of only one round, compared with the four stroke diesel engine, it improves the work ability, there are great differences in the structure and working principle. The basic structure of the two stroke diesel engine is the same as that of the four stroke diesel engine, and the main difference is the valve train. Two red The diesel engine has no inlet valve, and some of the exhaust valves are not available, but the lower part of the cylinder is provided with a scavenging port and an exhaust port; A scavenging port and an exhaust valve mechanism are also provided. Specially, a scavenging pump driven by moving parts and a storage pressure air are specially set up The gas scavenging box is used to match the piston and the air port, thus simplifying the structure of the diesel engine. Diagram is the working principle diagram of two stroke diesel engine. The scavenging pump is attached to the side of the diesel engine The rotor is driven by a diesel engine. Air is inhaled from the pump, compressed and discharged, stored in a larger volume In the scavenging chamber, a certain pressure is maintained in the scavenging chamber. The work of the two stroke diesel engine is illustrated in this paper Principle. 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 driven by the gas from the TDC Downward movement, external work. The piston goes down until the exhaust port opens (at this point the crank is at the point where the gas is fired) The end of the expansion work, a large number of exhaust gas in the cylinder by their own high pressure free exhaust, exhaust from the exhaust port to the exhaust pipe. When the pressure in the cylinder drops close to the scavenging pressure (the scavenging pressure in the scavenge box is 0 12, the downward piston to open the scavenging port 3 (at this time the crank at the point 4 position, scavenging air into the cylinder, Meanwhile, the exhaust gas from the cylinder is driven out of the cylinder through the exhaust port. The piston runs to the lower dead end, the end of the stroke, but scavenging The process continues until the next stroke, and the exhaust port closes (at this point the crank is at the point. Schematic diagram of working principle of two stroke diesel engine for overhaul of marine diesel engine third 4 342 Scavenging and compression stroke: The piston moves upward from the lower dead point, and the piston is supplied by the scavenging pump before scavenging the air port The air in the cylinder enters the cylinder through the scavenging port, and the residual gas in the cylinder is entered into the air of the cylinder through the exhaust port Swept cylinder. The piston continues upward, gradually covering the scavenging port, when the scavenging port is completely closed (at this time the crank is at the point) Position, air stop filling, exhaust is still in progress, this stage is called "after exhaust stage."". When the exhaust port closes The air in the cylinder begins to compress at the point where the crank is at the point. When the front point of the compression is uppermost, The injector sprays fuel into the cylinder, mixes with the high temperature and high pressure air, and then fires near the TDC Fire combustion. The stroke ends and forms a complete working cycle with the previous stroke. The indicator diagram of the two stroke diesel engine is shown in figure, which is the starting point of the injection and the TDC of the piston Burning end point. Compared with four stroke diesel engine, two stroke diesel engine has some obvious advantages, of course, there are inherent Shortcoming. The working principle of 2 and four stroke diesel engines The work of the diesel engine is accomplished by four processes: intake, compression, combustion, expansion and exhaust. These four processes form a working cycle. The piston takes four processes to complete a working cycle of a diesel engine called a four stroke diesel engine. Now compare the animation above to explain how it works. I. intake stroke The first stroke - air intake, the task is to make the cylinder filled with fresh air. When the intake stroke starts, the piston is at the top dead center, and there are some exhaust gases in the combustion chamber of the cylinder. When the crankshaft rotates the elbow, the connecting rod moves the piston from the TDC to the TDC, and the intake valve opens with the transmission mechanism associated with the crankshaft. With the downward movement of the piston, the volume of the piston in the cylinder increases gradually: 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 varies with the volume of the cylinder as shown in the animation during the intake process. The ordinate in the figure represents the gas pressure P, and the abscissa represents the cylinder volume Vh (or the piston S), which is called the indicator diagram. The pressure curve in the diagram shows the variation law of the gas pressure in the cylinder when the diesel engine is working. From the soil we can see that the initial intake, due to the existence of residual gas, so slightly higher than the atmospheric pressure P0. During the intake process because of the air through the intake pipe and the air inlet valve flow resistance, so the gas pressure below the atmospheric pressure of the intake stroke, its value is 0.085 ~ 0.095MPa, in the whole process of air, gas cylinder pressure remained approximately constant. When the piston moves down close to BDC, air into the cylinder has high speed, great inertia, in order to use the flow inertia toimprove chongqiliang intake valve closes after the piston after bdc. Although the piston up, but because of the inertia of the gas, the gas can still be filled with the cylinder. Two. Compression stroke Second stroke compression. When the piston is compressed, the piston moves from the TDC to the bottom, and the stroke has two functions. One is to increase the temperature of the air, and the other is to prepare for the spontaneous combustion of the fuel. Two is to create conditions for the expansion of the gas. When the piston, after the intake valve closes, the air in the cylinder is compressed, with small volume, air pressure and temperature is rising, the compression end point pressure and humidity and air compression degree, and compression ratio, the general compression end point pressure and temperature is: Pc = 4 8MPa, Tc = 750 ~ 950K. The spontaneous combustion temperature of the diesel fuel is about 543 - 563K, and the temperature at the compression end is much higher than that of the diesel engine. Diesel injected into cylinder, Not immediately angry, and after physical and chemical changes after the fire, this period of about 0.001 to 0.005 seconds, known as the ignition delay period. Therefore, in turn the crank first stopping point before 10 ~ 35 degree crank angle at the start of the fuel atomization spray into the cylinder, and the crank end point after 5 ~ 10 degrees on, the highest combustion pressure in the combustion chamber, the piston downward force. Three. Combustion expansion stroke Third stroke combustion expansion. At the beginning of the stroke, most of the fuel injected into the combustion chamber is burning. During combustion, a large amount of heat is released, so the pressure and temperature of the gas increase rapidly. The piston moves downward under the action of high temperature and high pressure gas, and rotates the crankshaft through the stalk, so that the external work can be done. So this stroke is also called work or work stroke. As the piston goes down, the volume of the cylinder increases, the pressure of the gas drops, and the working stroke ends at the bottom of the piston and the opening of the exhaust valve. In animation, the pressure change of the working stroke indicates the sharp rise of the pressure when the fuel burns in the cylinder. The highest point indicates the maximum combustion pressure Pz, and the pressure and temperature of the point are the same: Pz = 6 ~ 15MPa, Tz = 1800 ~ 2200K The ratio of the maximum combustion pressure to the pressure at the end of compression (Pz / Pc) is called the ratio of pressure rise at combustion, and is expressed by lambda. According to the type of diesel engine, the range of the lambda value is as follows: lambda = Pz / Pc = 1.2 - 2.5. Four. Exhaust stroke Fourth stroke exhaust. The function of the exhaust stroke is to exhaust the exhaust gas so as to fill the fresh air and prepare for the next cycle of air intake. When the piston of the working stroke moves near the lower dead center, the exhaust valve is opened, and the piston is driven by the crankshaft and the connecting rod to move up from the lower dead point to the upper stopping point, and the exhaust gas is discharged out of the cylinder. Because there is resistance exhaust system, so in the beginning of the exhaust stroke, and gas pressure than atmospheric pressure in the cylinder of the 0.025 - 0.035MPa, the temperature of Tb = 1000 ~ 1200K. In order to reduce the resistance of the piston movement during exhaust, the exhaust valve opens before the lower dead center. When the exhaust valve is opened, the gas with a certain pressure immediately goes out of the cylinder, and the pressure in the cylinder drops rapidly. When the piston moves upwards, the exhaust gas in the cylinder is discharged by the piston. In order to make use of the exhaust airflow inertia, exhaust gas is clean, the exhaust valve is closed after the top dead point. In the animation, the exhaust stroke curve indicates that the gas pressure in the cylinder is almost constant, but slightly higher than the atmospheric pressure. The pressure Pr at the end of the exhaust stroke is about 0.105 ~ 0.115MPa, and the temperature of the residual gas is about 850 ~ 960K Pr. Because of the intake and exhaust valves are open as early as late off; so in the end of the exhaust stroke and the beginning of the intake stroke, the piston at TDC, a time of inlet and exhaust valves open at the same time, this time with the crank angle to said called valve overlap angle. After the end of the exhaust stroke, the intake stroke starts again, so the whole working cycle is repeated according to the above process. Because the working cycle of this kind of diesel engine is completed by four piston strokes, that is, the crankshaft rotates two revolutions, so called four stroke diesel engine. In the four stroke of the four stroke diesel engine, only third strokes, that is, the working impulse, will produce power external work, while the remaining three strokes are the preparation process of the work of consumption. For this reason, the flywheel must be installed on a single cylinder diesel engine, and the crankshaft rotates continuously and evenly in four strokes with the inertia of the flywheel.