Differences between the throttle control of a gasoline vehicle and a diesel vehicle

In gasoline vehicles, what we usually call stepping on the accelerator is actually stepping on the throttle, that is, stepping on the accelerator is actually controlling the amount of air intake. The computer will determine the amount of fuel injection based on the amount of air entering the engine, and spray out the amount of gasoline that meets the best air-fuel ratio according to the current working conditions.

In a diesel vehicle, it is completely different from a gasoline vehicle. The accelerator we step on really controls the amount of fuel injected. The deeper we step on the accelerator, the more fuel is injected. Why is this? Why is there such a big difference between gasoline engines and diesel engines?

In fact, the difference lies in the characteristics of gasoline and diesel. We know that if we want to ignite anything, we use gasoline instead of diesel, which means that gasoline is easier to ignite than diesel. However, this does not mean 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 easier to ignite? It is because gasoline is more volatile. What we ignite is not the oil itself, but the mixture of oil and air. Diesel is a long-chain hydrocarbon with high viscosity and is not easy to volatilize, so it cannot be mixed with air. This leads to the ignition method in the engine, one is the ignition type and the other is the compression ignition type.

Because gasoline is very easy to mix with air, only by controlling the intake volume and then determining the injection volume can the air-fuel ratio be optimized and the gasoline can be completely burned. Only when the amount of air entering is matched with the amount of gasoline, can the engine work properly. 

Diesel is different. The throttle is used to control the amount of fuel injection. The intake air is always kept at the maximum value, which is equivalent to the throttle valve being fully opened. So why does diesel not have a situation where the mixture is too lean and cannot burn normally? This is because diesel is difficult to evaporate. Diesel is sprayed out in the form of a mist column. This group of mist will not evaporate and disperse quickly. The diesel on the outer circle of the mist column will burn first because only the outer circle can contact the air.

As the diesel in the outer circle burns, the burning point gradually spreads inwards, and as the combustion increases, the heat of the diesel increases, evaporation becomes faster, the atomization effect is improved, and then it can quickly spread to the innermost layer, so that all the diesel burns. From this, it can be seen that no matter how much air is waiting to be burned, the diesel will never be completely mixed with the air to form a uniform mixture. 

Since there is no uniform mixture, the concentration of the mixture around the diesel mist column is very high, and the area far from the mist column is full of air without diesel, so it cannot be ignited at all. Compression ignition will have many ignition points, so the combustion speed will be greatly accelerated. In addition, the ignition point of diesel is low, so it is easier to ignite by compression. Therefore, diesel can only be compression-ignited.