As we said a few days ago, what we usually call stepping on the accelerator is actually stepping on the throttle, which means that the accelerator is actually controlling the amount of air intake, which is a valve for air intake. By the amount of air entering the engine, the computer determines the amount of fuel injected by itself, and according to the operating conditions at the time, the amount of gasoline that meets the optimal air-fuel ratio is sprayed. And we are used to the saying that we step on the accelerator, so as long as we accelerate, we say that we step on the accelerator or refuel. On the diesel car, it is completely different from the gasoline car, the throttle we step on really controls the amount of fuel injection, and the deeper we step on it, the more fuel is injected. Why is that? Why is there such a big difference between a gasoline engine and a diesel engine?
In fact, the difference lies in the characteristics of gasoline and diesel, we know that everything we want to ignite is gasoline, not diesel, which means that gasoline is easier to ignite than diesel. However, this is not to say that the ignition point of gasoline is lower than that of diesel, on the contrary, the ignition point of gasoline is about 427 degrees, while the ignition point of diesel is only about 220 degrees. So why is gasoline more likely to ignite instead? This is because gasoline is more volatile. What we ignite is not the oil itself, but the mixture of oil and air, and only oil cannot be ignited without air. Diesel is a long-chain hydrocarbon, high viscosity, not easily volatile, and therefore cannot be mixed with air. This results in an ignition type and a compression-ignition type in the engine.
Because gasoline mixes easily with air, it is only by controlling the amount of air intake and then determining the amount of fuel injected that the air-fuel ratio can be optimal, allowing the gasoline to burn completely. If the throttle is also to control the amount of fuel injected, then the air intake must be a fixed value, and it must always be the state of the maximum throttle opening. At low speeds, the amount of fuel injected is very small, and because the mixture concentration is very low, it cannot be ignited, and the engine cannot work. Therefore, it is only how much air is inlet and how much gasoline is mixed to make the engine work properly.
Unlike diesel, the throttle is to control the amount of fuel injected, and the intake air has been kept at the maximum, which is equivalent to the throttle being fully open. Inject less fuel at low rpm, and more and more fuel is injected as you step deeper and deeper on the throttle. So why doesn't diesel have a mixture that is too thin to burn properly? It is due to the difficulty of diesel volatilization. The diesel fuel is sprayed in the form of a fog column, and this fog does not evaporate quickly. Therefore, if you want to burn, you can only burn the outer ring of the fog column first, because only the outer ring can contact the air, and the inner ring cannot contact the air, so it cannot burn.
With the combustion of diesel fuel in the outer ring, the combustion point gradually spreads inward, and with the combustion also increases the heat of diesel, the evaporation is faster, the atomization effect is improved, and then it can also quickly diffuse to the innermost layer, so that all the diesel is burned. It can be seen from this that no matter how much air is waiting to be burned, the diesel simply does not completely mix with the air to form a homogeneous mixture, and can only start burning around. In other words, the concentration of the mixture is always high in or near the fog column, and the concentration of the mixture is not too low to burn.
The combustion of diesel is slow, and after being sprayed, it is burned layer by layer from the outer ring to the inner ring, so it can keep the intake air at the maximum state at all times, and there is no fear that the mixture will be too thin. Why does the diesel engine have to compress the diesel fuel, and can it also be ignited? No, through the above analysis, it can be seen that because there is no uniform mixture, the concentration of mixture around the diesel fog column is very high, and the place far away from the fog column is full of air, and there is no diesel, so it is not lit at all.
It is impossible for the spark plug to jump the fire just at the edge of the fog column, which is very difficult to grasp. And even if it can be lit at the edge of the fog column, there is only one ignition point, and the burning will be slower, and there is even a possibility that it will be extinguished again. Compression ignition is different, there will be many ignition points, and the fog column is surrounded by ignition points, so that the burning speed will be greatly accelerated, and there will be no extinguishment. In addition, the ignition point of diesel is low, and it is easier to compress ignition, so diesel can only be compression ignition, and gasoline can only be ignited.