What is a Variable-Length Intake Manifold?

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Variable-Length Intake Manifold:

A variable-length intake manifold (VLIM) is an enclosed combustion engine manifold technology. First, two discrete intake runners with different lengths are employed, and a valve can close the short path. Second, the intake runners may be bent around a typical plenum, and a sliding valve separates them from the plenum with a variable length. Straight high-speed runners can receive plugs, which contain small long runner extensions.

The plenum of a 6- or 8-cylinder engine is often parted into halves, with the even firing cylinders in one half and also the odd firing cylinders in the other part. Both sub-plenums and therefore the air intake is connected to a Y (sort of the most plenum). The air oscillates between both sub-plenums, with an outsized pressure oscillation there, but a continuing pressure at the most plenum. Each runner from a sub plenum to the most plenum may be changed long. For V engines this could be implemented by parting one large plenum at high engine speed through sliding valves into it when speed is reduced.

As the name implies, VLIM can vary the length of the intake tract to optimize power and torque, moreover as provide better fuel efficiency.


There are two main effects of variable intake geometry:
  1. 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 (DOHC) designs, the air paths are often connected to separate valves therefore the shorter path will be excluded by deactivating the intake valve itself.
  2. Pressurization: A tuned intake path can have a lightweight pressurizing effect kind of like a low-pressure supercharger thanks to Helmholtz resonance. However, this effect occurs only over a narrow engine speed range which is directly influenced by intake length. A variable intake can create two or more pressurized “hot spots.” When the intake airspeed 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 airspeed, 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 that the variable resonance induction system (VRIS).

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