A working cycle consists of four piston strokes (the so-called piston stroke refers to the process of the piston from the top dead center to the bottom dead center): intake stroke, compression stroke, expansion stroke (power stroke) and exhaust stroke .
Folding intake stroke
During this process, the intake valve of the engine is opened and the exhaust valve is closed. As the piston moves from the top dead center to the bottom dead center, the cylinder volume above the piston increases, so that the pressure in the cylinder will be below atmospheric pressure, that is, vacuum suction is generated in the cylinder, so that the air passes through the intake pipe and into the air. The valve is sucked into the cylinder while the nozzle sprays out the atomized gasoline and mixes well with the air. At the end of the intake, the gas pressure in the cylinder is approximately 0.075-0.09 MPa. At this time, the temperature of the combustible mixture in the cylinder has risen to 370-400K.
Folding compression stroke
In order to make the combustible mixture sucked into the cylinder burn rapidly to generate a large pressure, so that the engine emits a large power, the combustible mixture must be compressed before combustion to reduce the volume, increase the density, and increase the temperature. That is, a compression process is required. In this process, the intake and exhaust valves are all closed, and the crankshaft pushes the piston from the bottom dead center to the top dead center by one stroke, that is, the compression stroke. At this point, the mixture pressure will increase to 0.6-1.2 MPa and the temperature can reach 600-700K.
There is a very important concept in this itinerary, the compression ratio. The so-called compression ratio is the ratio of the maximum volume of gas in the cylinder before compression to the minimum volume after compression. Generally, the higher the compression ratio, the higher the pressure and temperature of the mixture at the end of compression, and the faster the combustion speed, so the greater the power generated by the engine, the better the economy. The compression ratio of a typical car is between 8-10, but the latest Polo has reached a high compression ratio of 10.5, so its torque performance is relatively good. However, when the compression ratio is too large, not only can the combustion situation be further improved, but abnormal combustion such as flash fire and surface ignition may occur.
The flash fire is an abnormal combustion caused by spontaneous combustion of the end combustible mixture farther away from the ignition center in the combustion chamber due to excessive gas pressure and temperature. At the time of the flash, the flame propagates outward at a very high rate. Even in the case where the gas does not have time to expand, the temperature and pressure rise sharply, forming a pressure wave that advances at the speed of sound. When this pressure wave hits the wall of the combustion chamber, a sharp knocking sound is produced. At the same time, it will cause a series of adverse consequences such as overheating of the engine, power reduction, and increased fuel consumption. Severe flash fire is even caused by damage to the valve, such as valve burnout, bearing bush rupture, and spark plug insulator breakdown.
In addition to flash fire, engines with excessive compression ratios may face another problem: surface ignition. This is due to another type of abnormal combustion (also known as hot ignition or pre-ignition) caused by the igniting of the mixture between the hot surface of the cylinder and the hot places (such as the exhaust valve head, the spark plug electrode, and the carbon deposit). When surface ignition occurs, it is also accompanied by a strong knocking sound (more boring), and the high pressure generated will increase the engine load and reduce the life.
Folding expansion stroke
During this process, the intake and exhaust valves are still closed. When the piston approaches top dead center, the spark plug emits an electric spark that ignites the compressed combustible mixture. After the combustible mixture is burned, a large amount of heat is released, and the pressure and temperature of the gas increase rapidly. The maximum pressure that can be reached is 3-5 MPa, and the corresponding temperature is as high as 2200-2800K. The high-temperature and high-pressure gas pushes the piston from the top dead center to the bottom dead center. The crank rotates the crankshaft and outputs the mechanical energy. In addition to maintaining the engine itself to continue to operate, the rest is used for external work. During the movement of the piston, the internal volume of the cylinder increases, and the gas pressure and temperature drop rapidly. At the end of the stroke, the pressure drops to 0.3-0.5 MPa and the temperature is 1300-1600K.
Folding exhaust stroke
When the expansion stroke (work stroke) is nearing the end, the exhaust valve is opened, and the exhaust gas is freely exhausted by the pressure of the exhaust gas. When the piston reaches the bottom dead center and then moves to the upper dead center, the exhaust gas is forcibly discharged to the atmosphere, which is the row. Gas stroke. During this stroke, the pressure in the cylinder is slightly higher than the atmospheric pressure, which is about 0.105-0.115 MPa. When the piston reaches near the top dead center, the exhaust stroke ends, and the exhaust gas temperature at this time is about 900-1200K.
Thus, we have already introduced a working cycle of the engine during which the piston reciprocates four strokes between the upper and lower dead centers, and the crankshaft rotates for two weeks.
The gasoline engine is equipped with an alternator to form a gasoline generator set.
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