Intake Manifolds

In automotive engineering, an intake manifold or inlet manifold is the part of an engine that supplies the fuel/air mixture to the cylinders. An exhaust manifold or header collects the exhaust gases from multiple cylinders into one pipe. more...

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The word manifold may come from the Old English word manigfeald (from the Anglo-Saxon manig and feald ) and refers to the folding together of multiple inputs and outputs. Alternatively, it may come from the Latin mano (hand) and simply mean "hand-shaped".

Intake Manifold

The primary function of the intake manifold is to evenly distribute the combustion mixture (or just air in a direct injection engine) to each intake port in the cylinder head(s). Even distribution is important to optimize the efficiency and performance of the engine. It may also serve as a mount for the carburetor, throttle body, fuel injectors and other accessories to the engine.

Due to the downward movement of the pistons and the restriction caused by the throttle valve, in a reciprocating spark ignition piston engine, a partial vacuum (lower than atmospheric pressure) exists in the intake manifold. This manifold vacuum can be substantial, and can be used as a source of automobile ancillary power to drive auxiliary systems: (ignition advance, power assisted brakes, cruise control, windshield wipers, power windows, ventilation system valves, etc).

This vacuum can also be used to draw any piston blow-by gases from the engine's crankcase. This is known as a closed crankcase ventilation or positive crankcase ventilation (PCV) system. This way the gases are burned with the fuel/air mixture.

The intake manifold has historically been manufactured from aluminum or cast iron but use of composite plastic materials is gaining popularity (e.g. most Chrysler 4 cylinders, Ford Duratec 2.0 and 2.3, and GM's Ecotec series).

The design and orientation of the intake manifold is a major factor in the volumetric efficiency of an engine. High performing manifolds contain smooth contours and transitions between different segments. Manifolds that are restrictive and contain abrupt changes in contour produce pressure drops at these points. This reduction in manifold pressure results in less air (and fuel) actually entering the combustion chamber.

Modern intake manifolds usually contain intake runners. These are individual tubes extending to each intake port on the cylinder head. The purpose of the intake runner is to take advantage of the rarefaction pulse generated by closing the intake valve. When the valve is open, air is flowing through the valve at considerable speed. When this valve closes the air that has not yet entered the valve still has a lot of momentum and compresses against the valve, creating a pocket of high pressure air. This high pressure air begins to equalize with the lower pressure air in the manifold. This reduction of density at the end of the runner is what generates the rarefaction pulse. This pulse travels at the speed of sound, and in most intakes travels up and down the intake runner many times before the valve opens again.

Read more at Wikipedia.org