In fact, there are all kinds of opinions on the Internet about this problem, which makes the drivers confused, but none of them really tells the principle.
Therefore, I think it is necessary to elaborate on it carefully, so as to put an end to those disorderly remarks as soon as possible. To make this clear, we need to popularize the knowledge of air-fuel ratio again: the so-called air-fuel ratio is the weight ratio of air to fuel. For example, pour 1g of standard gasoline into the beaker and ignite it until all the gasoline in the beaker is completely burned. Then, 14.7-14.8g of air is consumed in the whole combustion process.
Any combustion needs oxygen consumption, so this 14.7 ~ 14.8 is the "standard air-fuel ratio". Therefore, if the air-fuel ratio is lower than 14.7, it is called "anoxic combustion", and higher than 14.8 is called "peroxide combustion".
Idling for a long time will not cause carbon deposition at all.
In the "oxygen-deficient combustion" state, gasoline can not be completely burned out because of lack of oxygen, and eventually there will be insufficient combustion; conversely, if it is "peroxide combustion", because of excessive oxygen, nitrogen oxides in the tail gas will increase, which is not good for the environment. Therefore, once the heat engine is finished and the water temperature reaches the normal temperature, the computer will order the engine to adjust the air-fuel ratio to 1.47 ~ 14.8, so as to ensure the cleanest exhaust gas and the best working condition of the three-way catalytic converter.
The above control principle is called "closed-loop control". The engine from the cold state to the "hot engine" completion, generally less than 2 minutes. That is to say, after your car has been warmed up for 2 minutes, the engine is already close to the "optimum air-fuel ratio" state! (usually, the air-fuel ratio of the heat engine is calibrated around 14 under idle condition). Therefore, we know that idle speed will not lead to insufficient gasoline combustion! Therefore, the statement that "long idle time makes combustion insufficient and carbon deposition" is basically nonsense without a scientific basis!
The operation that will really make the engine in the state of "low air-fuel ratio" is actually "floor oil"! When the air-fuel ratio is reduced to about 12.5, the power and torque performance of the engine reaches the peak. So when we try our best to speed up the floor oil, the computer of the engine will "put aside the environmental protection for the time being"! At this time, there will be insufficient combustion in the cylinder (therefore, the frequent "sharp throttle + sharp brake" opening method will really lead to carbon deposition).
Through the discussion of the principle, we finally know that there will be no combustion problem after long idle time. Therefore, long idle time will not lead to carbon deposition in the cylinder! So what's the bad consequence of idling for a long time? ——In fact, the problem caused by long idle time is "intake port coking", not carbon deposition!
In fact, coking is mainly caused by oil. The oil becomes coking at high temperature. So where does the oil come from?
Very simply, it is the oil vapor from the crankcase ventilation (PCV) system. We know that when the engine is running, the vacuum in the intake pipe will produce suction, and the oil vapor in the crankcase will be sucked into the engine to participate in combustion (this is required for environmental protection). The higher the vacuum degree of the engine intake pipe, the greater the suction force!
When the engine is idling, it is also the time when the engine intake pipe vacuum is the highest, so the suction of the engine PCV system is the maximum state under idle condition. The more oil vapor is sucked out from the crankcase, so the speed of oil pollution deposition in the engine intake system is the fastest, and the coking speed is obviously greatly accelerated.
Therefore, the engine idle for a long time does not have any benefits, but also burning oil for nothing. This is one of the reasons why the automatic start-stop system is becoming more and more popular.
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