Variable Gemomentry Intake System or Variable Length Intake

Intake Manifold:

                               An intake manifold is a system which connects the engine to the air filter and in modern gasoline engines (Fuel Injection) intake manifold is the position where the fuel injector and rails are mounted.

                                                                                                                                                                                

Part’s coupled with Intake manifold:

                                The following are the parts that are normally found in the intake manifold (Fuel Injected),

                               

·         Throttle body a.k.a Throttle Butterfly attached to one throttle cable

·         Fuel Rail

·         Injectors

·         Throttle position sensor  

·          One Vacuum line which connects to the MAP sensor







A conventional Four cylinder inlet manifold





  Intake Manifold Working:

                                            This is mainly designed to get maximum cylinder flow irrespective of the engine RPM. With Respect to the Cross Sectional Area of the Intake Manifold at lower rpm engine requires longer and narrow manifold and at higher rpm the engine requires a shorter length manifold to pull-in the maximum amount of air at least available time.

                                              

                                             To get maximum volumetric efficiency the inlet manifold's are mainly designed. To achieve this it’s all about getting the positive pulse at the right amount (When the inlet valve opens) so that it creates the effect of supercharging in the engine. Imagine that the pressure wave’s keeps travelling inside the inlet manifold and during the suction stroke and the air flows at higher velocities inside the engine.Suddenly the valve closes it creates some pulse which travels in the manifold and the whole theory of this manifold is to make use this pulse to push in as much as air it can when the valve opens again.

                                              

                                                These waves which travel in the manifold hit the plenum and bounce back and if you open the inlet valve at an exact moment when the pulse regenerates and this create a supercharging effect in the manifold thereby pushing few amount of extra air inside the engine.

                                               For street cars the engineers always prefer a longer inlet manifold to get that extra bit of torque and for race lovers like me we always tune it through out our power band to get the maximum power through out the rpm.





FEA Model of a four cylinder conventional manifold

Variable Geometry Intake Manifold - Need:

                                               As we discussed it’s all about receiving that one positive pulse which aid in better volumetric efficiency that helps in better drivability in various rpm’s.

                                               Imagine if the pulse wants to travel for a longer period of time and if the manifold length is shorter the wave will be travelling to and fro in the manifold which results positive pulse produced in the wrong period of time results in wasting the supercharging effect which will be produced, thus the volumetric efficiency is reduced. The situation can go on the opposite side also. Thus the engineers made a compromise in designing the inlet manifold while designing them for street cars. The length of the manifold also depends on the engine configuration also.

                                               Soon after engineers found of sending some extra amount of air by secondary port opening in the manifold by altering the passage. The below is the example of those system. These ports will open in some specific conditions depending upon the vacuum present in the manifold.





As you can see there is a by pass valve which opens and closes depening upon the engine speed





                                               Later they switched to two varying length one shorter and one longer manifolds which were not electronically controlled again a simpler butterfly was used to operate these systems. As technology improved the manufacturers used electronically controlled engine management systems to control these manifold switching with respect to the engine rpm. Few manufacturers also had electro pneumatic valves which controls the throttle butterfly opening and there by shifting the manifold length.





Volvo's method of secondary butterfly opening with the help of  a diaphragm



                                              For some manufacturers motogp and F1 has always been their proving ground and this resulted in lots of improvement in street cars also. VGIS is comparatively cheaper than adopting VVT (Variable Valve Timing) which again improves the volumetric efficiency of the engine, there by manufacturers are also able to cut cost on their products. Yamaha had it own style of doing it. They attached a stepper motor which controls the linear moment of the manifold. The stepper motor receives the signal from the electronic moment through which the manifold is length varies. The engine can either choose a 65mm shorter manifold or a 140mm longer manifold. As cited earlier longer manifold can be used in the lower and mid range rpm and shorter ones are always used in higher rpm.

Yamaha's Variable length Manifold attached to a stepper motor which receives signal from the ECU for chossing manifold length i.e, shorter or longer

The following videos explain the working of the variable length manifold: