Everyone knows，Many models now are equipped with gasoline engines with direct injection in the cylinder。Some people will ask:Gasoline engines can now inject gasoline directly into the cylinder，Why can’t it be directly compression ignited like a diesel engine？Why do we still need to use spark plugs to ignite？

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Gasoline is more likely to ignite?

This will first introduce a physical and chemical term - "flash point"。Flashpoint，Under the specified test conditions，The lowest temperature at which a liquid can vaporize and catch fire using an ignition source。

The flash point of gasoline can be less than 28℃，The flash point of light diesel is 45-120℃，Oils with a flash point lower than 61℃ are flammable。

It is difficult to ignite diesel if you put an open flame close to it，Because the temperature of 20℃ is far lower than the flash point of diesel，Diesel is still very stable at this time，No volatilization，So it’s hard to light。

Diesel is not as easy to evaporate into gas as gasoline，Only the surface layer is in contact with seabet iothe air，Gasoline is highly volatile and produces a relatively complete mixture of gasoline vapor and air，It's like the difference between wood and wood shavings。

Diesel can be compression ignited because of its low self-ignition point

The autoignition point refers to the temperature at which combustibles burn spontaneously in the presence of oxygen，The auto-ignition point of diesel is only 220℃，In other words, diesel only needs to be mixed with air，It can burn by itself at 220℃。

The auto-ignition point of gasoline is 427℃，The high temperature generated by the piston compressing the air can reach the auto-ignition point of diesel but cannot reach the auto-ignition point of gasoline，So diesel engines can use compression ignition，Gasoline engines can only be ignited by spark plugs。

Spark plug cannot ignite diesel

The spark plug is a gasoline engine ignition system that introduces high-voltage current into the cylinder to generate electric sparks，Parts used to ignite flammable gas mixtures，Mainly composed of wire nuts、Insulator、Wiring screw、Center electrode、Side electrode and shell composition，The side electrode is welded on the shell。

The spark plug is arc discharge，No open flame，So it can only ignite gaseous combustibles evenly mixed with oxygen，The gasoline we usually see will eventually be fully evaporated and vaporized in the cylinder，Only in this way can a sufficient gaseous flammable mixture be formed。

But diesel itself is denser，And it is much more stable than gasoline，After such fuel is injected into the cylinder, it is difficult to volatilize into a gaseous state，Most of them are small diesel particles，Unable to form sufficient flammable flammable mixture，So the spark plug cannot ignite diesel。

Gasoline engines use "pre-mixed gas" fuel，That is to say, the mixture that needs to be burned during the power stroke has been prepared before ignition，Just wait for the spark plug to ignite it when the ignition time comes。

Because the timing of ignition basically occurs when the piston reaches top dead center，At this time, the piston will have a reversing action，So it can be seen that the combustion space is constant for a short time at this time，The mixture quickly completes combustion and expansion at this moment，This is the Otto cycle of "constant volume heating"。

The diesel engine starts injecting fuel into the cylinder when the piston approaches top dead center and the temperature in the cylinder is higher than its self-ignition point，Atomized diesel enters the cylinder and mixes with air，Because the temperature reaches the auto-ignition point，Diesel starts to burn，Due to slow diesel burning speed，So the diesel fuel required to perform work at one time will not be sprayed out instantly，But it will continue to inject into the cylinder，The result is that fuel is continuously injected while burning in the cylinder，The injected oil continues to mix with the air and is ignited and burned，The expanding air continues to push the piston，This is the Diesel cycle of constant pressure heating，In English, a diesel engine is called a Diesel engine。

Someone asked: Why can’t the compression ratio of gasoline engines be higher，It also injects fuel at top dead center of compression like a diesel engine？This is mainly because gasoline burns too fast，The combustion process is uncontrollable，It is difficult to make the engine smooth、Soft。

The physical properties of oil products determine that they are “treated differently”。(The content seabet appof the article comes from the Internet，If copyright is involved, please contact us to delete)