1, the body
in gasoline an abnormal combustion phenomenon. When gasoline normal combustion, a spark plug after the ignition after a brief preparation ignition delay, a flame core in the vicinity of the electrode gap, flame propagation to the unburned mixture region around the flame core at a speed of 30 to 40 m / sec, the combustion indoor sequential combustion gas mixture, until the end (see Figure). Found Gasoline Knock high-speed photography, in a gasoline engine combustion chamber, flame propagation, away from the spark plug of unburned mixture (end gas) is compressed expanded combusted mixture gas, localized temperature here due to thermal radiation action than the fuel self-ignition temperature, spontaneous reaction to produce, form one or more flame core. I.e., in the normal flame propagation to the previous first autoignition combustion, emits a strong fire, the combustion temperature is often above 4000 deg.] C, the flame propagation speed of more than 200 to 1000 m / s, several times higher than the normal combustion flame propagation speed. High speed propagating detonation combustion within the cylinder so that a pressure wave, and repeated reflection and impinges on the cylinder wall surface, resulting in the forced vibration and generate high frequency noise, i.e., knocking phenomenon. Gas pressure shock wave causes the membrane wall thinning, increased heat loss to the cylinder wall, the result of the power decrease, increase in fuel consumption rate, hot gasoline, oil and cooling water temperature increases. Lasting damage to knock the cylinder wall film, increased wear of the cylinder wall, serious cause mechanical damage. Measures to avoid knocking: the use of high-octane gasoline, the fuel gas mixture with too thick, so that the end of the mixed gas per se is not easy ignition; reduce intake air temperature, intensive cooling of the end of the gas mixture, the ignition delay time, the end of the mixed gas to reduce the temperature; the use of turbulence and swirling combustible mixture, the normal increase flame propagation speed, or combustion chamber compact, rational arrangement of the spark plug location, shorten the distance of flame propagation, shortening the time to the end of a normal flame spread mixture. 
2. What knocking
When the mixed gas (air and fuel mixture sufficient) after the intake stroke into the combustion chamber during the compression stroke of the piston compresses put, the spark plug then the high-pressure mixed gas point, which is generated by the combustion pressure is converted into power of the engine operation. Although combustion engine can be used simply to describe in a few words, but just Combustion engine, I do not know how much has created a blog, master’s thesis, even many scholars, engineers are working on a lifetime of knowledge burning, so to really understand the engine, it takes a lot of effort. It is because the combustion engine is very complex, so the need for reasonably accurate design and control, just a little mistake or abnormal control, will result in abnormal combustion, and “knock” is a kind of abnormal combustion. Simply put, knocking is abnormal combustion caused by the combustion chamber pressure disorders. The rightHigh compression ratio set by the case easier to cause knocking, we need to use high-octane fuel to avoid knock.
3 reasons knock
before it comes knocking reason, we need to understand two things. First, the combustion gas mixture in the combustion chamber, the flame is diffused by the ignition point to “wave” approach to the surrounding, so need a short period of time depends on oil and gas from the ignition to complete combustion. Second, although the need to rely on the spark plug to ignite the oil and gas, but too high temperature, high pressure environment also make spontaneous combustion of oil and gas. Is generally because the knocking combustion chamber and gas ignition, flame diffusion wave not yet fully, unburned hydrocarbon remote because high temperatures or pressures, i.e. spontaneous combustion, the flame with the regular wave impinging flame combustion wave generated tremendous pressure such that the engine produced unusual percussion sound. The main cause knock following reasons:
First, the ignition advance angle too:
In order for the compression piston in the dead end of a power stroke can immediately enter to gain momentum, the pre-ignition usually before the piston reaches top dead center (because of incomplete combustion from ignition to take some time). The too early ignition causes the piston during the compression stroke is still the majority of oil and gas has been burned, unburned oil and gas at this time will be under great pressure to spontaneous combustion, causing detonation.
Second, excessive soot Engine:
Engine soot in the combustion chamber excessively, in addition to the compression will also produce high temperature hot spots on surface carbon ratio is increased (to generate high pressure), so that engine knock.
Third, the engine temperature:
in a hot environment such that the engine intake air temperature is too high, or the engine cooling water circulation failure, will cause the high-temperature engine knocking.
Fourth, the air-fuel ratio is not correct:
is too lean fuel-air mixing ratio will be such that the combustion temperature is raised, the temperature increase will cause the combustion engine temperature is raised, of course, easy to knock.
Fifth, the low octane fuel:
octane fuel anti-knock index is, the higher the octane number, the stronger the knock resistance. The engine compression ratio, the higher the pressure of the combustion chamber, ifIt is the use of low fuel anti-knock, knock is likely to occur.
4, know how and knock the influence
knock of English is Knocking, and percussion means, so knocking engine will produce percussion sound. Minor discontinuous knocking sound quite crisp, somewhat similar to the sound of tapping the triangle. The serious and continuous detonation, the engine will “Lili mile” voice. The engine will obviously did not force. Many depot in order to squeeze out the maximum engine performance and reduce fuel consumption, will generally conventional ignition angle rotation number range set in advance the comparison, the engine load is large in some 2000 to 3000 rpm, there is inevitably a slight burst shock, however slight knock have much impact on the engine does not, the owners do not have to worry too much. However, when the engine knock because of problems arising such as poor heat dissipation or serious coke, which is usually very severe detonation, if continuous and severe knocking occurred in the high rotation and high load, in a minute light the piston and the spark plug erosion, serious and even cylinders of the engine body will blow through.
5, Sensor knock (knock sensor)
The most immediate and effective method for suppressing knocking, ignition timing is delayed, combustion pressure is reduced. So Knock Sensor actuation principle, when when it detects engine knock, then delayed ignition timing to the ignition timing will not knock until the engine does not knock, then slowly return the ignition advance . Knock Sensor using an acceleration sensor measuring the acceleration change amount of the engine, i.e. the vibration. Engineers in the adjustment will knock knock Perceptron vibration mode write ECU, once the detonation Perceptron detect the vibration mode, the ECU determines that the engine knock, then delayed ignition timing. More advanced detonation Perceptron even can determine which cylinder is knocking, but for the individual cylinders delayed ignition timing.
On some advanced automobiles, such as Mercedes-Benz engines 600-137 series has been canceled knock sensor, particularly to a spark plugAnd the corresponding abnormality sensing circuit to sense the plasma stream inside the cylinder to determine whether the engine knocks. This method is more sensitive and effective.