Fuel Systems: Intake & Exhaust: Intake system components
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Intake manifolds

Summary
The intake manifold carries the air of the air-fuel mixture to each cylinder. In spark-ignition engines, fuel is either mixed with the air at the entrance to the manifold, or injected close to the cylinder head.

The intake manifold is usually a metal part with several tubular branches, though it can also be made of a special plastic. In carbureted engines, the intake manifold carries the air-fuel mixture into the engine. The cross-sectional area of each tube needs to be kept small to maintain the high air speeds that improve vaporization. At the same time, it cannot be too small, since that restricts the airflow to the engine at higher speeds.

Electronic fuel injected engines with throttle body injection also have intake manifolds that carry air-fuel mixture.

With multi-point injection, the intake manifolds carry air only, so heating of the intake manifold is not needed, and the cross-sectional area of the tubes can be larger. More air can flow and the engine can produce more power. Fuel is injected into the intake ports of the cylinder head.

Crossflow head 4-cylinder carbureted engine
This cylinder head has intake and exhaust manifolds for a 4-cylinder carbureted engine. It is a crossflow head. That means the intake manifold is on one side and the exhaust manifold is on the other.

On many cars, the intake manifold has a mounting for the carburetor and a flange that bolts onto the cylinder head. It has a branch for each cylinder to carry air-fuel mixture into the combustion chamber.

Intake manifold with waterjacket
This intake manifold has a waterjacket under the carburetor mounting. Hot coolant from the cooling system flows through the waterjacket and heats the manifold.

Heating is required in a carbureted engine to provide better vaporization of the air-fuel mixture.

Fuel-injected engine manifold with plenum chamber
This fuel-injected engine manifold has a plenum chamber that provides a reservoir of air and helps prevent interference with the flow of air between individual branches. It also acts as a silencer. On this primary manifold, the intake pipes are fixed in length.

Manifold with extra valves
Other kinds of manifolds have extra valves to change the effective pipe length. They’re computer-controlled by the engine management system to open at a specified engine speed and extend the torque output.

A diesel engine intake manifold carries air only, not fuel. And since no fuel is vaporized in the manifold, it isn’t heated.

The diesel engine doesn’t have a carburetor, therefore has no need for the throttle.

Some diesels use a pneumatic or air-operated governor with a butterfly valve at the entrance to the inlet manifold. This butterfly valve is only used to operate the governor. It is not a throttle butterfly valve as seen on gasoline engines.

Materials

Traditionally, intake manifolds were normally made of light alloy metal castings, however in approximately 70% of modern vehicles the intake manifold are now made from special heat resistant polymers and plastics. Manifolds using this type of construction lighten manifold part weight by up to 50% and contribute to higher fuel efficiency.

These intake manifolds are normally molded from a glass fiber reinforced grade of crystalline polymer that consists of a blend a of syndiotactic polystyrene and polyamide.

The polyamide nylon based materials provide a good solution for these components because of their mechanical properties and their ease of processing during their manufacture.

This type of material is ideally suited for replacing metal in under-the-hood applications because of its strength, stiffness, and chemical resistance under high temperature operating conditions.

In addition, intake manifolds made out of polyamide nylon have succeeded in improving the environmental performance of cars because of the materials ability to be recycled.


Application

The intake manifold has several tubular branches and carries air and or air/fuel mixture from the air cleaner to the inlet valves in the cylinder head.

In carbureted engines, the intake manifold carries the air-fuel mixture into the engine.

The cross-sectional area of each tube needs to be kept small to maintain the high air speeds that improve vaporization. At the same time, it cannot be too small, since that restricts the airflow to the engine at higher speeds.

Electronic fuel injected engines with throttle body injection also have intake manifolds that carry air-fuel mixture. With multi-point injection, the intake manifolds carry air only and the cross-sectional area of the tubes can be larger. More air can flow and the engine can produce more power.Fuel is injected into the intake ports of the cylinder head.

Cylinder heads that have separate intake and exhaust manifolds are know as "crossflow heads". That means the intake manifold is on one side and the exhaust manifold is on the other.

On many cars, the intake manifold has a mounting for the carburetor and a flange that bolts onto the cylinder head. It has a branch for each cylinder to carry air-fuel mixture into the combustion chamber.

Many intake manifolds have a waterjacket under the carburetor mounting. Hot coolant from the cooling system flows through the waterjacket and heats the manifold.

Heating is required in a carbureted engine to provide better vaporization of the air-fuel mixture.

Fuel-injected engine manifolds normally have a plenum chamber that provides a reservoir of air and helps prevent interference with the flow of air between individual branches. It also acts as a silencer.

Normally on primary manifolds, the intake pipes are fixed in length. Other kinds of manifolds have extra valves to change the effective pipe length. They may be computer-controlled by the engine management system to open at a specified engine speed and extend the torque output.

A diesel engine intake manifold carries air only, not fuel. And since no fuel is vaporized in the manifold, it isn’t heated.

Some diesels use a pneumatic or air-operated governor with a butterfly valve at the entrance to the inlet manifold. This butterfly valve is only used to operate the governor. It is not a throttle butterfly valve as seen on gasoline engines.