The reasons for the engine knock
With the car in-depth people’s daily lives, more and more car problems exposed. Engine knock is a relatively recent common problem, many of which do not quite understand the word of a friend heard was very confused. Or even give up some of their favorite models, such as a friend like Fox, but asked me whether it is as vulnerable as Internet rumors knock.
In fact, engine knock is not complicated as long as the clear cause detonation can prescribe the right medicine to solve the problem, the following together on the same car to explore to find the “culprit.”
● What is a knock
Gasoline engine, when the mixed gas (mixture of air and fuel sufficient) into the combustion chamber during the intake stroke, the piston in the compression stroke, when their formal compression, spark plugs will be high-pressure gas mixture ignited, its combustion pressures generated by converting running the engine power. Although the engine combustion can be a simple few words to describe, but just the internal combustion engine combustion research, I do not know how much Bo has created a master’s thesis, and even many scholars, engineers struggle with it their whole lives studying the combustion of knowledge, so to truly understand the engine, is takes a lot of effort.
It is precisely because the combustion engine is very complicated, so we need to have quite precise design and control, control failures or a slight disorders, can lead to abnormal combustion, and “knock” is a kind of abnormal combustion. Simply put, it is abnormal combustion knock as a result of abnormal combustion chamber pressure.
● Knock reasons
When it comes to knock reasons, we must understand two things.
First, the mixed gas in the combustion chamber in flames from the ignition point to “wave” approach to the proliferation of four weeks, so oil and gas incomplete combustion from ignition to the need for a short time.
Second, although the need to rely on oil and gas spark plug ignition, but it was too high temperature, high pressure environment also make oil and gas self-ignition.
Knock the general oil and gas because the combustion chamber after ignition, the flame has not yet fully spread, the remote is not burning oil and gas that, because high temperature or high pressure and spontaneous combustion, the flame with the regular burning flame the impact on great pressure, making the engine produce abnormal敲击.
● caused by detonation has the following main reasons:
1, ignition angle is too early:
In order to piston TDC in the compression after the end of one into the power stroke can be immediate access to power, they usually reach the ending point of the piston in advance of the ignition (because of incomplete combustion from ignition to take some time). Be too early still in the ignition would make the piston compression stroke, most of the oil and gas has been burned, this time not burning oil and gas will be under great pressure to spontaneous combustion, which leads to knock.
2, the engine over-coke:
Excessive carbon deposit in combustion chamber engines, in addition to the compression ratio will increase (generation high-voltage), will also heat the surface of hot spots in the coke, so that the engine knock.
3, the engine temperature is too high:
Engine intake air temperature is too hot environment made through the high, the engine cooling water circulation, or bad, will result in the engine knock heat.
4, air-fuel ratio is not correct:
Is too dilute fuel-air mixture ratio, would make the combustion temperature increase, while the combustion temperature rises, the temperature will cause the engine upgrade, of course, easy to knock.
5, fuel octane is too low:
The octane number of fuel anti-knock indicator of the higher octane, anti-knock more intense. Than high compression engines, the combustion chamber pressure is high, if low use of anti-knock fuel, then prone to knock.
● how to know the impact of knock and knock
Detonation English is Knocking, knocking the meaning, so knock knocking sound when the engine will produce. Minor continuous detonation sounds very crisp, somewhat similar to tap triangle’s voice. And serious and continuous detonation, the engine will have a “Lili mile” voice, this time the engine will be a clear no power.
Now a lot of the depot in order to squeeze out the maximum performance of the engine and reduce fuel consumption and speed are usually used to set the regional comparison of the ignition advance angle, so that some engine capacity between 2,000 to 3,000 turn larger, it is inevitable there will be a slight explosion Chen, however, a slight knock on the engine will not have much impact, the owner need not worry too much. However, if because of engine problems arising from the detonation, such as severe coke or heat and poor, this knock is usually very serious, if it is in high-speed high-load continuous and serious detonation occurred in less than a minute, light then the spark plug and piston melt loss, serious and even body will be deep-fried cylinders and engine wear.
● Knock Sensor
The most rapid and effective suppression of detonation method is to delay the ignition advance angle, lower combustion pressure. So knock sensor actuation principle is that when the engine knock is detected when the ignition advance angle will be postponed to not knock the ignition timing until the engine does not knock, the spark advance and then will be slowly Reply . Knock sensor is the use of accelerometers to measure the acceleration of the engine change, which is vibration. Engineers adjusted when the knock sensor will write ECU detonation shock model, once the knock sensor detects the vibration mode, ECU is to determine engine knock, then delayed ignition advance angle. At present, more sophisticated knock sensor can even determine which one cylinder knock, but for that individual to delay the ignition advance angle cylinder.
● 93,97 or 98
『Knock does not occur as long as the engine to raise the octane level of fuel does not make the engine more powerful or more fuel-efficient』
Speaking of knock, we are most concerned about what is or increasing gasoline problem. In fact, 93,97 or 98 is the anti-knock gasoline, which also is its “octane number.” What is “octane number” mean? In examining the relationship between fuel and detonation, the researchers found that “iso-octane” the most resistant to knock, and the “n-heptane was” quite easy to knock, so will the anti-knock degree of iso-octane set at 100, while the n-heptane is set at 0. The so-called 97 octane petrol is its anti-knock degree and 97% iso-octane and 3% n-heptane mixtures of the same degree of resistance to detonation.
So it is a anti-knock issues, not the addition of the higher octane petrol, the engine more powerful. Of course, if added too low an octane number of gasoline which led to detonation, or knock back the time of the engine ignition angle, the car will indeed be relatively less power. In other words, as long as the engine knock does not occur to raise the octane level of fuel does not make the engine more powerful or more fuel-efficient, it will only make your wallet shrink.
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