BMW engineers (http://www.bmwusa.com/Joy/Drive/Technology/VariableIntake.htm) have further optimized engine performance across the rpm range by continuously varying the air path in the intake system on their V8 engines, the latest application of which is in the 6 Series coupe and roadster.
The idea is to tune the path length so that pressure waves peak at the closed intake valve just before it opens. The suction waves that propagate back down the manifold as the valve opens are reflected back to the valve as pressure waves-ideally arriving just before the valve closes. The greater the engine speed, the less time the valve is open, and the shorter the intake path must be to reflect back the pressure waves so they build at the proper time.
Some previous powerplants have two-stage intake manifolds that offer a long and a short path for the engine inlet air to follow. Two-stage systems usually employ a flap mechanism to switch between paths. The longer path for low and medium speeds improves torque for acceleration and quick engine response. The short path boosts high-end power.
For its continuously variable system, the BMW team uses a Pierburg AG dc stepper motor (http://www.kolbenschmidt.de/index.php?lang=3&fid=315) to rotate twin intertwined helical rotors (one each for the left and right cylinder banks), continuously changing the path length from 23.9 to 8.5 inches (607 to 215 mm). The reduction in path length starts at 3,500 rpm and is complete by 6,200 rpm, boosting torque in the mid-range by 5-10 percent.
The Pierburg motor is mounted on the manifold between the V8 cylinder banks at the back of the engine in a high temperature environment. The motor drives counter rotating shafts linked by spur gears, which turn each rotor. An integral potentiometer on the motor provides position feedback to the engine control module.
EXPAND, CONTRACT: A servo motor rotates intertwined helical elements (one for the left and right cylinder banks), which varies the length (yellow arrow) of the intake airflow path (red arrow) to tune internal pressure waves for maximum airflow through the intake valves as a function of speed.
TOUGH SERVO: The Pierburg servo (circle) that varies intake manifold path length is mounted between the banks of BMW's V8 engine.
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