In order to make the combustible mixture sucked into the cylinder burn quickly to generate a 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. That is, 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, which 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 journey, which 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 engine's power, the better the economy. The compression ratio of a general 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 the combustion situation cannot be further improved, but abnormal combustion phenomena such as violent combustion and surface ignition will occur.
Flash burning is an abnormal combustion caused by the spontaneous combustion of the combustible mixture at the end of the combustion chamber far from the ignition center due to excessively high gas pressure and temperature. During a violent fire, the flame spreads outward at a very high rate. Even when the gas is too late to expand, the temperature and pressure rise sharply, forming a pressure wave, which advances 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 also cause a series of undesirable consequences such as engine overheating, power drop, and fuel consumption increase. Severe violent combustion can even cause damage to parts such as valve burnt, rupture of the bearing bush, and breakdown of the spark plug insulator.
In addition to flash combustion, engines with too high compression ratios may also have to face another problem: surface ignition. This is due to another kind of abnormal combustion (also called hot ignition or pre-ignition) caused by the ignition of the mixture on hot surfaces and hot places (such as exhaust valve heads, spark plug electrodes, and carbon deposits) in the cylinder. When the surface ignition occurs, there is also a strong knocking sound (more dull). The high pressure generated will increase the engine load and reduce the service life.
