A gasoline engine is a power source that uses gasoline as fuel to convert chemical energy into mechanical energy through combustion. The engine is a machine that converts chemical energy into mechanical energy. Its conversion process is actually a process of working cycle. Simply put, it is burned The fuel in the cylinder generates kinetic energy to drive the reciprocating movement of the piston in the engine cylinder, which drives the connecting rod connected to the piston and the crank connected to the connecting rod, it makes a reciprocating circular motion around the center of the crankshaft to output power.

Now, let's analyze this process:

A working cycle includes four piston strokes (the so-called piston stroke refers to the process of the distance between the piston from top dead center to bottom dead center): intake stroke, compression stroke, expansion stroke (power stroke) and exhaust stroke.

Intake stroke

In this process, the intake valve of the engine is opened and the exhaust valve is closed. As the piston moves from top dead center to bottom dead center, the volume of the cylinder above the piston increases, so that the pressure in the cylinder will fall below atmospheric pressure, that is, vacuum suction is created in the cylinder, so that the air passes through the intake pipe and The intake valve is sucked into the cylinder, and the atomized gasoline sprayed from the fuel injector is fully mixed with the air. At the end of the intake, the gas pressure in the cylinder is about 0.075-0.09MPa. At this time, the temperature of the combustible mixture in the cylinder has risen to 370-400K.

Compression stroke

In order to make the combustible mixture sucked into the cylinder burn quickly to generate greater pressure, so that the engine emits greater power, the combustible mixture must be compressed before combustion to reduce its volume, increase its density, and increase its temperature. A compression process is required. In this process, the intake and exhaust valves are all closed, and the crankshaft pushes the piston to move a stroke from bottom dead center to top dead center, that is, the compression stroke. At this time, the pressure of the mixed gas will increase to 0.6-1.2MPa and the temperature can reach 600-700K.

There is a very important concept in this trip, that is, 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 larger the compression ratio, the higher the pressure and temperature of the mixture at the end of compression, and the faster the combustion speed. Therefore, the greater the power emitted by the engine, the better the economy. The compression ratio of a general car is between 8-10, but now 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 not further improve the combustion situation, but there will be abnormal combustion phenomena such as deflagration and surface ignition.

Flame is an abnormal combustion caused by the spontaneous combustion of the combustible mixture at the end far away from the ignition center in the combustion chamber due to the high gas pressure and temperature. When the flame is ignited, the flame propagates outward at a very high rate, and even when the gas is too late to expand, the temperature and pressure rise sharply, forming a pressure wave that moves forward at the speed of sound. When this pressure wave hits the wall of the combustion chamber, it makes a sharp knocking sound. At the same time, it will cause a series of adverse consequences such as engine overheating, power drop, and fuel consumption increase. Severe deflagration can even cause mechanical damage such as valve burning, bearing bush rupture, spark plug insulator breakdown, etc.

In addition to flash, engines with too high a compression ratio may also face another problem: surface ignition. This is due to another abnormal combustion (also called hot ignition or pre-ignition) caused by the ignition of the mixture on the hot surface and hot places in the cylinder (such as exhaust valve head, spark plug electrode, carbon deposit). When surface ignition occurs, it is also accompanied by a strong knocking sound (more dull), and the high pressure generated will increase the engine load and reduce the life.

Expansion stroke (power stroke)

In this process, the intake and exhaust valves are still closed. When the piston approaches the top dead center, the spark plug emits an electric spark to ignite 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 at this time. The maximum pressure it can reach is 3-5MPa, and the corresponding temperature is as high as 2200-2800K. The high-temperature and high-pressure gas pushes the piston to move from the top dead center to the bottom dead center. The connecting rod rotates the crank and outputs mechanical energy. In addition to maintaining the engine itself, 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 this stroke, the pressure drops to 0.3-0.5MPa and the temperature is 1300-1600K

Exhaust stroke

When the expansion stroke (power stroke) is close to the end, the exhaust valve opens, 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 top dead center, the exhaust gas is forced to the atmosphere. Exhaust stroke. During this stroke, the pressure in the cylinder is slightly higher than the atmospheric pressure, about 0.105-0.115MPa. 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.

Therefore, we have introduced a working cycle of the engine, during which the piston reciprocates four strokes between the top and bottom dead centers, and the crankshaft rotates for two weeks accordingly.