When air is forced in, the pistons create a strong vacuum, which, when the intake valves are open, causes air to move in the intake system. If the intake valves suddenly close, the air «springs» back - oscillation occurs in the suction pipe. Its frequency depends on the length of the suction pipe, the engine speed and the valve timing of the engine. If the oscillations enter into resonance, a certain pumping effect is achieved. By means of dampers it is possible to create conditions of resonance in a wide range of revolutions.
1. At speeds up to 2550 min-1 both flaps are closed, the air volume in the suction system may fluctuate in the area up to the pressure control valve.
Pic. 4.74. E300 Diesel Variable Length Intake Manifold: 1, 2 - shutters; 3 - intake system; 4 - intake manifold
2. From 2550 to 3350 min-1 damper 1 opens (pic. 4.74), oscillations stop at damper 1.
3. At a speed of more than 3350 min-1 flaps 1 and 2 open, vibrations reach the intake manifold.
These switches are used only in full engine load mode. In order to achieve low fuel consumption and clean combustion at part load, other combinations of air volume frequencies and speeds are required. Therefore, by the calculated time, the electronic control activates both the control valve and the exhaust gas return.