Inlet Manifold:
In engineering, a manifold or manifold (in American English) is that a part of an engine that supplies the fuel/air mixture to the cylinders. The word manifold comes from the Anglo-Saxon word manigfeald (from the Anglo-Saxon manig [many]and field [repeatedly]) and refers to the multiplying of 1 (pipe) into many.
In contrast, a manifold collects the exhaust gases from multiple cylinders into a smaller number of pipes – often right down to one pipe.
The primary function of the manifold is to evenly distribute the combustion mixture (or just air in a very direct injection engine) to every intake port within the cylinder head(s). Even distribution is vital to optimizing the efficiency and performance of the engine. It should also function as a mount for the carburetor, throttle body, fuel injectors, and other components of the engine.
Due to the downward movement of the pistons and therefore the restriction caused by the accelerator, during a reciprocating spark ignition piston engine, a partial vacuum (lower than atmospheric pressure) exists within the manifold. This manifold vacuum is often substantial and may be used as a source of automobile ancillary power to drive auxiliary systems: power-assisted brakes, emission control devices, control, ignition advance, windshield wipers, power windows, ventilation valves, etc.
This vacuum can even be wont to draw any piston blow-by gases from the engine’s crankcase. This is often referred to as positive crankcase ventilation, during which the gases are burned with the fuel/air mixture.
The manifold has historically been manufactured from aluminum or forged iron, but the utilization of composite plastic materials is gaining popularity (e.g. most Chrysler 4-cylinders, Ford Zetec 2.0, Duratec 2.0 and 2.3, and GM’s Ecotec series).
Turbulence
The carburetor or the fuel injector spray fuel droplets into the air in the manifold. Due to electrostatic forces and condensation from the boundary layer, some of the fuel will form into pools along the walls of the manifold, and due to surface tension of the fuel, small droplets may combine into larger droplets in the airstream. Both actions are undesirable because they create inconsistencies in the air-fuel ratio. Turbulence in the intake helps to break up fuel droplets, improving the degree of atomization. Better atomization allows for a more complete burn of all the fuel and helps reduce engine knock by enlarging the flame front. To achieve this turbulence it is a common practice to leave the surfaces of the intake and intake ports in the cylinder head rough and unpolished.
Only a certain degree of turbulence is useful in the intake. Once the fuel is sufficiently atomized additional turbulence causes unneeded pressure drops and a drop in engine performance.
Volumetric Efficiency of Inlet Manifold
The design and orientation of the intake manifold is a major factor in the volumetric efficiency of an engine. Abrupt contour changes provoke pressure drops, resulting in less air (and/or fuel) entering the combustion chamber; high-performance manifolds have smooth contours and gradual transitions between adjacent segments.
Modern intake manifolds usually employ runners, individual tubes extending to each intake port on the cylinder head which emanate from a central volume or “plenum” beneath the carburetor. The purpose of the runner is to take advantage of the Helmholtz resonance property of air. Air flows at considerable speed through the open valve. When the 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. This high-pressure air begins to equalize with lower-pressure air in the manifold. Due to the air’s inertia, the equalization will tend to oscillate: At first the air in the runner will be at a lower pressure than the manifold. The air in the manifold then tries to equalize back into the runner, and the oscillation repeats. This process occurs at the speed of sound, and in most manifolds travels up and down the runner many times before the valve opens again.
The smaller the cross-sectional area of the runner, the higher the pressure changes on resonance for a given airflow. This aspect of Helmholtz resonance reproduces one result of the Venturi effect. When the piston accelerates downwards, the pressure at the output of the intake runner is reduced. This low pressure pulse runs to the input end, where it is converted into an over-pressure pulse. This pulse travels back through the runner and rams air through the valve. The valve then closes.
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