Some reasons for the detonation of the engine

Category:Automobile engine repair - Date:2017-02-10

when it comes to knock, what concerns you most is what gasoline is added. In fact, 93, 97 or 98 is the blast resistance of gasoline, that is, its octane number.

So what is the "octane number"?
in the study of the relationship between fuel and detonation, the researchers found that "isooctane" was most resistant to detonation, and that "n-heptane" was quite easy to detonation, so the antioctane was set to 100, while n-heptane was set to be 0. The so-called octane value of 97 gasoline is that its antiknock degree is the same as that of 97% isooctane and 3% heptane mixture.

So this is purely a problem of blast resistance. It is not that the higher the octane number is, the more powerful the engine will be. Of course, if the gasoline with too low octane value causes knock or detonation angle, the engine will be less powerful. In other words, as long as the engine does not detonation, increasing the octane number of the fuel will not make the engine more powerful or more economical.

Find out what is detonation
When the gasoline engine is working, the mixture of air and fuel is inhaled into the cylinder of the engine and then into the compression chamber. The piston will mix the pressure into the combustor, the pressure and temperature of the mixture increase, and the spark plug ignites the high pressure mixture, and the pressure produced by the mixture combustion is converted to the starting point. The power of the operation of the machine.
When the spark plug ignites the high pressure mixture when the spark plug is working normally, the flame spreads from the ignition point to the four sides in the way of "wave" when the engine is working normally. But because of local high temperature, there are two kinds of situations.

One is in the spark plug point fire, forming local;

Another case is the flame propagation process, and the formation of local fire and spread all around;

These two conditions can cause a sharp rise in pressure and temperature, which forms deflagration, and the abnormal flame strikes with the flame of regular combustion to produce great pressure, causing the engine to produce abnormal knocking and vibration called detonation.

The cause of the detonation of the engine

1. The ignition angle is too early:
In order to make the piston at the end of the compression point, an incoming power stroke can immediately gain power and usually ignite before the piston reaches the top stop (since it takes a period of time from ignition to complete combustion). Too early ignition will make the piston still in the compression stroke, most of the oil and gas have been burned, and the unburned oil and gas will withstand a great pressure of self ignition, causing detonation.

Two. Excessive carbon deposition of the engine:
the excessive carbon deposition in the combustion chamber, in addition to increasing the compression ratio (high pressure), will also produce hot hot spots on the carbon surface, which makes the engine detonation.

Three. The temperature of the engine is too high:
when the engine is too hot, the intake temperature is too high, or the cooling water circulation of the engine is not good enough, which will cause the engine to burn at high temperature.

Four. The air-fuel ratio is incorrect:
The excess fuel air mixture ratio makes the combustion temperature rise, and the increase of the combustion temperature will cause the engine temperature to rise, and of course it is easy to detonation.

Five. Low fuel octane number:
octane number is the index of fuel anti knock. The higher the octane number, the stronger the blast resistance. The engine with high compression ratio has higher pressure in the combustor, and detonation is easy to use if the fuel with low blast resistance is used.

The method of restraining detonation
the fastest and effective way to suppress detonation is to postpone the ignition advance angle and reduce the combustion pressure. Therefore, when the engine detonation is detonation, the ignition advance angle is postponed to the ignition time that will not be detonation. When the engine is not detonation, the ignition will be slowly restored. The knock sensor uses accelerometer to measure acceleration of engine, namely vibration. When the engineer is tuned to the detonation sensor, the vibration mode of the knock is written into the ECU. Once the detonation sensor detects the vibration mode, the ECU determines the engine's detonation, and then the ignition advance angle is delayed. At present, the more advanced knock sensor can even determine which cylinder detonation is, and the ignition advance angle is delayed for this cylinder individually.

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