Experts Analyze the Causes of Detonation of Automotive Engines

In fact, engine detonation is not complicated, as long as the cause of the detonation is clearly defined, it can be used to solve the problem. Here is the search for the “culprit” together with car exploration.

What is knocking

In a gasoline engine, when the mixture enters the combustion chamber on the intake stroke, the piston compresses the piston during the compression stroke. After the spark plug ignites the high-pressure mixture, the pressure generated by the combustion is converted into the engine's operating power.

Although the engine combustion can be described in simple words, but the light is the internal combustion engine combustion research, I do not know how many Bo, master's thesis has been created, and even many scholars and engineers are studying the knowledge of combustion in their lifetime, so to truly understand the engine is It takes a lot of work.

It is precisely because the combustion of the engine is very complex, so it needs to have a very precise design and control, a little control mistakes or abnormalities, it will cause abnormal combustion, and "knocking" is an abnormal combustion. Simply put, knocking is the abnormal pressure in the combustion chamber caused by abnormal combustion.

Causes of knock

Before talking about the cause of the explosion, we must first understand two things.

First, the mixture gas is combusted in the combustion chamber. Its flame is diffused from the ignition point in a “wave” manner. Therefore, it takes a short time from the ignition to the complete combustion of oil and gas.

Secondly, although oil and gas need to be ignited by spark plugs, an environment that is too hot and high in temperature can make oil and gas self-ignite.

The general detonation is because after the oil and gas ignition in the combustion chamber, the flame has not completely spread, the remotely unburned oil and gas are spontaneously ignited because of high temperature or high pressure, and the flames and the normal burning flames collide with each other to generate extreme pressure, causing the engine to be abnormally generated. tap.

The main reasons for knocking are the following:

First, the ignition angle is too early:

In order for the piston to gain power immediately after entering the power stroke after the end of compression TDC, it is usually preceded by ignition before the piston reaches top dead center. However, too early ignition will cause most of the oil and gas to burn when the piston is still in the compression stroke. At this time, the unburned oil and gas will withstand extreme pressure auto-ignition and cause knocking.

Second, the engine excessive carbon deposition:

Excessive carbon deposition in the combustion chamber of the engine causes a high-temperature hot spot on the carbon deposition surface in addition to an increase in the compression ratio, thereby knocking the engine.

Third, the engine temperature is too high:

If the engine is in a too hot environment, the intake air temperature is too high, or the engine cooling water circulation is poor, which can cause high engine temperature and knocking.

Fourth, the air-fuel ratio is incorrect:

A too-lean mixture of fuel and air will increase the combustion temperature, while an increase in the combustion temperature will cause the engine temperature to rise. Of course, it is prone to knocking.

V. Fuel octane is too low:

The octane number is an indicator of the anti-knocking of the fuel. The higher the octane number, the stronger the anti-knocking resistance. An engine with a high compression ratio has a high pressure in the combustion chamber. If a fuel with low knock resistance is used, knocking is likely to occur.

How to know the impact of knock and knock

The knocking English is Knocking, which means knocking, so the engine will produce a knocking sound when knocking. The slightly discontinuous knocking sound is quite crisp and somewhat similar to tapping triangle sounds. When the engine is severely and continuously knocked, the engine will have a "click" sound and the engine will obviously lose power.

Many OEMs now usually advance the comparison of the ignition angle settings in the common speed range in order to squeeze out the engine's maximum performance and reduce fuel consumption. Therefore, some engines will inevitably have slight explosions when the load is large between 2000 and 3000 revolutions. Shock, however, slight knocks will not have much impact on the engine, and owners need not worry too much.

However, if there is a knock due to an engine problem, such as serious carbon deposition or poor heat dissipation, the knocking is usually very serious. If the continuous and serious knocking occurs at a high speed and a high load, no one minute, light The spark plug and the piston are melted, and even the cylinder and the engine body are severely blown.

Knock Sensor

The quickest and most effective way to suppress knocking is to retard the ignition advance and reduce the combustion pressure. Therefore, the action mechanism of the knock sensor is that when the engine knock is detected, it retards the ignition advance angle to the ignition timing that will not knock, and when the engine does not knock, then slowly advance the ignition. .

Knock sensor uses an acceleration sensor to measure the acceleration change of the engine, which is vibration. The engineer will write the knock vibration mode into the ECU when the knock sensor is calibrated. Once the knock sensor detects the vibration mode, the ECU determines the engine knock and then retards the ignition advance. The current more advanced knock sensor can even determine which cylinder knock is, and retard the ignition advance separately for that cylinder.

When it comes to knocking, everyone is most concerned about what gasoline is to be added. In fact, 93, 97, or 98 is the knock resistance of gasoline, which is its "octane number." What is "octane"? When studying the relationship between fuel and detonation, the researchers found that "iso-octane" was the best resistance to detonation, and "n-heptane" was quite prone to detonation, so the anti-knocking degree of isooctane was set at 100. Heptane is set to 0. The so-called gasoline with octane rating of 97 means that its antiknock degree is the same as that of a mixture of 97% isooctane and 3% n-heptane.

Therefore, this problem is purely anti-knocking. It does not mean that the higher the octane number is, the more powerful the engine will be. Of course, if you add gasoline with too low an octane number to cause knocking, or if the deflagration occurs when the knocking occurs, the car will indeed be relatively useless. In other words, as long as the engine does not detonate, increasing the octane rating of the fuel will not make the engine more powerful or more fuel-efficient, it will only make your wallet shrink.