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Why does a small gasoline engine need to pull the air door when starting, but the car does not? Posted by : admin / Posted on : Aug 14,2020

Many people have the experience of riding a small motorcycle. When starting a motorcycle, it is generally necessary to pull the "hefeng" up, otherwise it will be difficult for the motorcycle to start. Especially when the weather is relatively cold, it is difficult to start the motorcycle if the wind is not applied. However, there is no such device as "Hyfeng" in the car. We don't need any special operation when starting the car. As long as the ignition switch is turned, the general car can start smoothly. So why do these small gasoline engines need to pull the air door when starting, but the car does not need it? Let's analyze this problem below.

In general, small gasoline engines used in motorcycles and lawnmowers have carburetor-type fuel supply systems. When they start, they need the fuel supply system to provide a very rich combustible mixture. In order to achieve this goal, a set of starting device is specially designed on the carburetor. When starting, only a small amount of air is allowed to enter the engine, while supplying more gasoline, so that a very rich combustible mixture can be generated.

In order to block the air from entering the engine, a choke is installed before the carburetor throat to control the air intake of the engine. This is what we commonly call the "combination" or "damper". When the engine is cold-started, the choke valve is closed and the throttle valve is opened at a very small angle, so that little air enters the engine; at the same time, due to the higher vacuum behind the throttle valve, more gasoline can be removed from the carburetor Suck it out into the engine. In this way, a small amount of air and more gasoline are mixed in the intake manifold to form a very rich combustible mixture, and the engine is easier to start. When the engine starts, the choke valve opens automatically, and automatically adjusts the opening angle according to the amount of air intake, so that the engine runs as stably as possible and shortens the warm-up time. But in the end, you still need to manually open the choke valve, otherwise the car will not have enough air intake during normal driving, which will cause the engine to run weak and emit black smoke.

What would happen without this choke? When the engine is started, the throttle valve needs to be opened at a certain angle to meet the demand for air intake; but at this time, the vacuum behind the throttle valve will be greatly reduced, and gasoline needs to be vacuum "inhaled" into the engine, so it is supplied at this time If the amount of air is large and the amount of gasoline is small, the concentration of the generated combustible mixture is relatively low, which cannot meet the needs of the car when starting, and the engine is difficult to start. Therefore, the role of the choke is to block air from entering the engine, while increasing the vacuum in the intake manifold. Without it, it is very difficult to start a cold car.

So why doesn't the car need to pull the choke when it starts? In fact, in the early carburetor engines, there was this choke, which also needed to be closed manually when starting, such as the previous Jiefang CA10B, CA141, Dongfeng EQ140, etc. On the car, more often use automatic choke or semi-automatic choke, which can be automatically closed when starting to enrich the combustible mixture; after the engine is started, it can be automatically opened, and can change with the temperature The change automatically adjusts the opening angle, and fully opens after the engine temperature is normal. The whole process does not need to be manually adjusted, and is completely automatically completed by the choke control device.

Later, the car's engine began to use an electronically controlled fuel injection system. Its fuel injection volume is no longer affected by the vacuum of the intake manifold. It can be fully controlled automatically, almost as much as you want. When the engine is cold-started, the throttle is still opened at a very small angle to allow a small amount of air to enter the engine, and then the engine control unit controls the injector to inject more gasoline, which can generate a very rich combustible mixture, and the engine can It started smoothly. Although there is no choke, it can also generate a very dense combustible mixture, so the choke is completely abandoned. The only gasoline engines that still use choke doors are small motorcycles, lawnmowers, chainsaws, etc., and electronically controlled fuel injection systems have been completely used in automobiles and large motorcycles, and choke doors are no longer needed.