BMWs V8s Get Servo Under the Hood

Engineers (http://rbi.ims.ca/3852-548) working at BMW have further maximized 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 inlet air, using a flap to switch between paths. The longer path for low and medium speeds improves torque for acceleration. The short path boosts high-end power.

For its continuously variable system, the BMW team uses a Pier-burg AG dc servo motor and worm gear (http://rbi.ims.ca/3852-554). This drive rotates twin intertwined helical rotors (one for each cylinder bank), continuously changing the path length from 23.9 to 8.5 inches. 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 between the V8 cylinder banks and can withstand temperatures from-40 to 140C. The motor drives counter rotating shafts linked by spur gears, which turn each rotor. An integral potentiometer on the motor provides feedback.

Joachim Wekenmann, Pierburg program manager, says the motor was adapted from those used in automobile throttle bodies. But he adds that this motor and gear produce 5-Nm torque, about 10 times that of previous actuators.

Continuous Control: The six-inch long servo motor/work gear actuator (circle) draws just 2A to hold the air inlet position against any vibration- induced movement. The 187-W motor draws a full 12A to move the intake scrolls only when the gas pedal is floored for maximum acceleration.