Traditional Culture Encyclopedia - Photography major - The antiknock property of gasoline refers to the ability of gasoline to resist detonation combustion when it burns in the cylinder of gasoline engine. What is detonation combustion?

The antiknock property of gasoline refers to the ability of gasoline to resist detonation combustion when it burns in the cylinder of gasoline engine. What is detonation combustion?

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knock

explosion

An abnormal combustion phenomenon of gasoline engine. When the gasoline engine burns normally, after the spark plug is ignited, a flame core is formed near the electrode gap, and the flame propagates from the flame core to the surrounding unburned mixture area at a speed of 30 ~ 40m/s, so that the mixture in the combustion chamber burns sequentially until the end (see Figure [Schematic diagram of gasoline engine combustion]). When studying the knock of gasoline engine by high-speed photography, it is found that the unburned mixture (end mixture) far away from the spark plug is compressed by the expansion of the combustion mixture, and the local temperature here exceeds the self-ignition temperature of the fuel due to thermal radiation, thus generating spontaneous reaction and forming one or more flame cores. In other words, before the normal flame spread, it will spontaneously ignite and emit a strong flame. The combustion temperature is often above 4000℃, and the flame propagation speed is above 200 ~ 1000 m/s, which is several times higher than normal combustion. High-speed detonation combustion produces pressure shock wave in the cylinder, which is reflected on the cylinder wall and repeatedly impacted, resulting in forced vibration and high-frequency noise, that is, cylinder detonation phenomenon. The impact of pressure wave makes the gas film on the wall thinner and the heat transfer loss to the cylinder wall increases. As a result, the power decreases, the fuel consumption rate increases, the gasoline engine overheats, and the temperature of cooling water and engine oil increases. Continuous deflagration damages the oil film on the cylinder wall, intensifies the wear of the cylinder wall and seriously damages the parts. The measures to avoid knocking are: using high octane gasoline and burning too rich mixture, so that the end mixture itself is not easy to ignite; Reduce the intake temperature, strengthen the cooling of the end mixture and delay the ignition time to reduce the temperature of the end mixture; Turbulence and swirl of combustible mixture can be used to improve the normal flame propagation speed, or a compact combustion chamber can be designed, and the spark plug position can be reasonably arranged to shorten the flame propagation distance, thus shortening the time for normal flame to reach the final mixture.

[Edit this paragraph] What is knocking at the door?

When the mixed gas (air and fuel are fully mixed) enters the combustion chamber in the intake stroke, the piston compresses it in the compression stroke, and the spark plug ignites the high-pressure mixed gas, and then the pressure generated by its combustion is converted into the power for engine operation. Although engine combustion can be simply described in a few words, I don't know how many doctoral and master's theses have been published on the combustion research of internal combustion engines, and even many scholars and engineers have spent their whole lives studying the knowledge of combustion, so it takes a lot of time to really understand the engine.

Precise design and control are needed because the combustion of the engine is very complicated. A slight control error or disorder will cause abnormal combustion, and "knocking" is abnormal combustion. Simply put, knocking is the abnormal pressure in the combustion chamber caused by abnormal combustion.

If the compression ratio is set properly, it is easy to cause knocking, so it is necessary to use high octane fuel to avoid knocking.

[Edit this paragraph] The reason for knocking at the door

Before we talk about the reasons for knocking on the cylinder, we need to know two things first. First of all, the mixture burns in the combustion chamber, and its flame spreads around from the ignition point in the form of "wave", so it takes a short time from ignition to complete combustion of oil and gas. Second, although oil and gas need to be ignited by spark plugs, excessive temperature and high pressure environment will also cause oil and gas to spontaneously ignite.

The general detonation is because the flame wave has not completely spread after the oil and gas are ignited in the combustion chamber. Long-distance unburned oil and gas spontaneously combust due to high temperature or high pressure, and its flame wave collides with the flame wave of regular combustion to produce great pressure, which makes the engine produce abnormal knocking sound. The main reasons for cylinder knocking are as follows:

First, the ignition angle is too early:

In order to make the piston get power immediately after compression top dead center and enter the power stroke, it is usually ignited before the piston reaches top dead center (because it takes some time from ignition to complete combustion). But premature ignition will make most of the oil and gas burn while the piston is still in the compression stroke. At this time, unburned oil and gas will spontaneously ignite under great pressure, resulting in knocking.

Second, the engine carbon deposit is too much:

Excessive carbon deposition in the combustion chamber will not only increase the compression ratio (produce high pressure), but also produce high temperature hot spots on the surface of carbon deposition, which will make the engine knock.

Third, the engine temperature is too high:

Excessive intake air temperature or poor circulation of engine cooling water in overheated environment will lead to engine high-temperature knocking.

Fourth, the air-fuel ratio is incorrect:

Too lean air-fuel ratio will increase the combustion temperature, which will increase the engine temperature, and of course it is easy to knock.

Five, the fuel octane number is too low:

Octane number is an index of fuel antiknock, and the higher the octane number, the stronger the antiknock performance. The engine with high compression ratio has high combustion chamber pressure, and it is easy to knock if the fuel with low antiknock property is used.

[Edit this paragraph] How do you know about detonation and its influence?

Knock means knocking in English, so the engine will knock when knocking. The slightly intermittent tapping sound is quite crisp, which is somewhat similar to the sound of tapping a triangle. In the case of severe and continuous detonation, the engine will have a "mile" sound, and the engine will be obviously weak at this time.

At present, in order to squeeze the maximum performance of the engine and reduce fuel consumption, many automobile manufacturers usually set the ignition angle in the range of common speed earlier, so some engines will inevitably have a slight knock when the load is heavy between 2000 and 3000 rpm. However, a slight knock will not have much impact on the engine, so the owner need not worry too much. But if the engine has problems, such as serious carbon deposition or poor heat dissipation, knocking is usually very serious. If there is continuous and severe knocking at high speed and high load, the spark plug and piston will be melted within one minute, and even the cylinder and engine block will be blown through.

[Edit this paragraph] Knock sensor

The most direct and effective way to suppress knocking is to delay the ignition advance angle and reduce the combustion pressure. Therefore, the working principle of the knock sensor is that when the engine knocks, the ignition advance angle will be delayed to the ignition opportunity that will not knock, and then the ignition will be slowly resumed in advance when the engine does not knock. Knock sensor uses an acceleration sensor to measure the acceleration change of the engine, that is, vibration. When the engineer adjusts the knock sensor, he will write the vibration mode of knock into ECU. Once the knock sensor detects the vibration mode, ECU will judge that the engine knocks, and then delay the ignition advance angle. At present, more advanced knock sensors can even determine which cylinder knocks, and delay the ignition advance angle of that cylinder alone.

[Edit this paragraph ]92, 95 or 98

Speaking of knocking, everyone is most concerned about what gasoline to add. In fact, 92, 95 or 98 is the antiknock property of gasoline, which is its "octane number". What is "octane number"? When studying the relationship between fuel and knock, researchers found that "isooctane" is the most resistant to knock, while "n-heptane" is quite easy to knock, so the anti-knock degree of isooctane is set at 100, while that of n-heptane is set at 0. The so-called gasoline with an octane number of 95 means that its antiknock property is the same as that of a mixture of 95% isooctane and 5% n-heptane. So this is purely an anti-explosion problem, not that the higher the octane number of gasoline, the stronger the engine. Of course, if adding gasoline with too low octane number leads to knocking, or if the engine retreats the ignition angle when knocking occurs, the car will really lack power. In other words, as long as the engine does not knock, increasing the octane number of fuel will not make the engine stronger or more fuel-efficient, but will only shrink your purse.