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 components of 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 Zetec 2.0, Duratec 2.0 and 2.3, and GM’s Ecotec series).
Turbulence
The carburetor or the fuel injectors spray fuel droplets into the air in the manifold. Due to electrostatic forces some of the fuel will form into pools along the walls of the manifold, or may converge into larger droplets in the air. Both actions are undesirable because they create inconsistencies in the air-fuel ratio. Turbulence in the intake causes forces of uneven proportions in varying vectors to be applied to the fuel, aiding in 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
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. 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 manifold 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 Helmholz 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.
To harness the full power of the Helmholtz resonance effect, the opening of the intake valve must be timed correctly, otherwise the pulse could have a negative effect. This poses a very difficult problem for engines, since valve timing is dynamic and based on engine RPM, whereas the pulse timing is static and dependent on the length of the intake runner and the speed of sound. The traditional solution has been to tune the length of the intake runner for a specific RPM where maximum performance is desired. However, modern technology has given rise to a number of solutions involving electronically-controlled valve timing (for example Valvetronic), and dynamic intake geometry (see below).
Some naturally-aspirated intake systems operate at a volumetric efficiency above 100%: the air pressure in the combustion chamber before the compression stroke is greater than the atmospheric pressure. The additional energy required to compress the air above atmospheric pressure comes from the momentum of the piston. In combination with the exhaust manifold[vague] the valve opening time can be prolonged and friction losses reduced. The exhaust manifolds achieves a vacuum in the cylinder just before the piston reaches top dead center.[citation needed] The opening inlet valve can then—at typical compression ratios—fill 10% of the cylinder before beginning downward travel.[citation needed] Instead of achieving higher pressure in the cylinder, the inlet valve can stay open after the piston reaches bottom dead center while the air still flows in.[citation needed][vague]
In some engines the intake runners are straight for minimal resistance in some other engines the intake runners are have turns. Turns allow for a denser packaging of the whole engine, are needed for some variable length designs, and allow to reduce the size of the plenum. In an engine with at least 6 cylinders the averaged intake flow is nearly constant and the plenum volume can be smaller. To avoid standing waves within the plenum it is made as compact as possible. The intake runner each use a smaller part of the plenum surface than the inlet, which supplies air to the plenum, for aerodynamic reasons. Each runner is placed to have nearly the same distance to the main inlet. Runners, whose cylinders fire close after each other, are not placed as neighbors.
Variable length intake manifold
Variable Length Intake Manifold (VLIM) is an internal combustion engine manifold technology. Four common implementations exist. First, two discrete intake runners with different length are employed, and a butterfly valve can close the short path. Second the intake runners can be bent around a common plenum, and a sliding valve separates them from the plenum with a variable length. Straight high RPM runners can receive plugs, which contain small long runner extensions. The plenum of a 6 or 8 cylinder engine can be parted into halves, with the even firing cylinders in one half and the odd firing cylinders in the other part. Both sub-plenums and the air intake are connected to an Y (sort of main plenum). The air oscillates between both sub-plenums, with a large pressure oscillation there, but a constant pressure at the main plenum. Each runner from a sub plenum to the main plenum can be changed in length. For V engines this can be implemented by parting a single large plenum (at max RPM) by means of sliding valves into it when RPM is reduced.
As the name implies, VLIM can vary the length of the intake tract in order to optimize power and torque, as well as provide better fuel efficiency.
Lower intake manifold on a 1999 Mazda Miata engine, showing components of a variable length intake system.There are two main effects of variable intake geometry:
Venturi effect - At low rpm, the speed of the airflow is increased by directing the air through a path with limited capacity (cross-sectional area). The larger path opens when the load increases so that a greater amount of air can enter the chamber. In dual overhead cam designs, the air paths are often connected to separate intake valves so the shorter path can be excluded by deactivating the intake valve itself.
Pressurization - A tuned intake path can have a light pressurizing effect similar to a low-pressure supercharger due to Helmholtz resonance. However, this effect occurs only over a narrow RPM range which is directly influenced by intake length. A variable intake can create two or more pressurized “hot spots.” When the intake air speed is higher, the dynamic pressure pushing the air (and/or mixture) inside the engine is increased. The dynamic pressure is proportional to the square of the inlet air speed, so by making the passage narrower or longer the speed/dynamic pressure is increased.
Many automobile manufacturers use similar technology with different names. Another common term for this technology is Variable Resonance Induction System (VRIS).