In internal combustion engines, variable length intake manifold (VLIM), or variable intake manifold (VIM) is an automobile internal combustion engine manifold technology. As the name implies, VLIM/VIM can vary the length of the intake tract in order to optimise 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:
Swirl - Variable geometry can create a beneficial air swirl pattern in the combustion chamber. The swirling helps distribute the fuel and form a homogeneous air-fuel mixture which ignites without engine knocking. At low rpms, the speed of the airflow is increased by directing the air through a longer path with limited capacity (i.e., cross-sectional area), but the shorter and larger path opens when the load increases so that a greater amount of air can enter the chamber. In DOHC designs, the air paths are often connected to separate intake valves so the shorter path can be excluded by de-activating the intake valve itself.
Pressurisation - A tuned intake path can have a light pressurising effect similar to a low-pressure supercharger due to Helmholtz resonance. However, this effect occurs only over a narrow engine speed band. A variable intake can create two or more pressurized "hot spots", increasing engine output. 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
The length and the diameter of the intake and exhaust ducts have a great influence on the torque and power curves of an engine. Long pipes of small diameter improve the charging of the cylinders and thus the torque at low revs whereas shorter pipes of a bigger diameter do it at higher revs.
It was not understood until the Fifties that the cylinders charging could be improved by outfitting them with long individual inlet runners. The first production engine to profit of this system was the one of the Mercedes 300 SL in 1954 (below).
The principle rests on the pressure waves of the gas columns inside the intake ducts, waves induced by the cyclic openings and closing of the valves. If the period of these waves enters in phase with the rhythm of opening of the valve, phenomenon which occurs in a rev range determined by the length of the duct, some supercharging effect is obtained without the assistance of a compressor. The exploitation of the intake manifold resonance got widespread especially since the adoption of injection systems instead of carburetors. But it is also possible to obtain the same effect with a carburetor barrel per cylinder or even with a single carburetor upstream of the tuned runners. This system can be also employed in conjunction with a turbocharger to improve the charging at low revs when the boost pressure is low.
With variable length inlet manifolds, it became possible to displace the rev range where this phenomenon occurs. For example all the length of the runner is used below a certain rpm. When the engine revs faster, some valves are open inside a tight case connected to the air filter and the effective length of the duct is reduced.
The latest BMW V8s are equipped with intake runners of continuously variable length. This system, called DIVA, is the first of its kind and is accurately described in the factory text reproduced below.
"The DIVA concept is both effective and compact in design: The four intake manifolds for each row of cylinders are shaped like a p or a q. In each case the straight section merges into the cylinder head intake duct, the circular sections forming a common drum serving as a collector. In this circular section the cross-section of the intake manifold is U-shaped and open to the inside. This opening is sealed by a flat, rotating rotor ring. Through an opening in the rotor ring, intake air is able to flow from inside the drum into the intake manifold. Turning the position of this opening, the intake manifold changes its effective length â€œ and is shortest when the intake funnel merges directly into the intake duct. Then, with the rotor turning around its central pivot point, intake duct length is varied from 231 to 673 mm.
To provide optimum control of this sophisticated process, four rotor rings on each row of cylinders are connected to a joint axle by a fork spoke.
The two axles are adjusted by an electric motor and turn against one another. The adjustment angle depends primarily on engine speed, with the control process starting at 3500 rpm (beneath which the longest intake manifold position is maintained consistently). The total adjustment range is 236Ã‚Â° and is covered in less than one second."
The BMW V8s DIVA intake manifold. (Picture MTZ)
The exhaust pipes are also the place of pressure waves, but we learned how to properly exploit them later, at the beginning of the Sixties. On a multi-cylinder engine, tubular headers i.e. long curved tubes of equal length before their junction in a common pipe, are efficient in helping exhaust flow and cylinders scavenging. For example on a 6 in line whose firing order is 1-5-3-6-2-4, the tubes coming from cylinders 1, 3 and 2 join in a common collector and those of cylinders 5, 6 and 4 in another. The gas flow emerging in each of the two collectors is thus regularly distributed and some depression is created behind the exhaust valve when it opens, which facilitates the gas flow.
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