What is the small branch pipe next to the car intake pipe for?

An intake manifold and an exhaust manifold. The function of intake manifold is to distribute the combustible mixture supplied by carburetor to each cylinder. The function of the exhaust manifold is to collect the exhaust gas discharged from each cylinder, send it to the exhaust pipe and muffler, and then discharge it into the atmosphere.

The intake and exhaust manifold is usually made of cast iron, and the intake manifold is also made of aluminum alloy, which can be cast integrally or separately. The intake and exhaust manifolds are bolted to the cylinder block or cylinder head, and asbestos gaskets are installed on the joint surface to prevent air leakage. The intake manifold carries the carburetor with flange, and the exhaust manifold communicates with the exhaust pipe downward.

Extended data:

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Air enters the throttle valve, and after passing through the manifold buffer system, the air flow channel is "divergent" here, corresponding to the number of cylinders of the engine, for example, four for a four-cylinder engine and five for a five-cylinder engine, and air is introduced into each cylinder respectively. For a naturally aspirated engine, because the intake manifold is located behind the throttle, when the throttle opening of the engine is small, enough air cannot be sucked into the cylinder, which will lead to a high vacuum degree of the manifold.

When the engine throttle opening is large, the vacuum in the intake manifold will become smaller. Therefore, a pressure gauge will be installed on the intake manifold of the fuel injection engine and provided to the ECU to determine the engine load and give the appropriate fuel injection quantity.