Breakthrough for Metal Injection Molding in BMW engines

BMW’s patented Valvetronic variable valve lift, which has already enhanced the performance and fuel efficiency of BMW’s V-8 and V-12 engines, has also been introduced into its 6-cylinder engines.

The 6-cylinder versions include a metal injection moulded (MIM) rocker arm produced by Schunk Sintermetalltechnik GmbH at its MIM facility in Thale, Germany, and shipped to INA Schaeffler KG in Germany which supplies the complete variable valve train system to BMW.

The MIM rocker arm is said to be produced in volumes of around 4.5 million pieces annually using a hardenable 50NiCrMo2.2 steel powder alloy. According to Ingolf Langer, development manager for MIM at Schunk, the parts are injection moulded in a four-cavity mould using an ARBURG Allrounder 420C moulding machine having 100 ton clamping force. The parts are then debinderised and sintered in a continuous Cremer sintering furnace followed by heat treatment.

BMW states that its Valvetronic innovation varies valve lift to a far greater degree than other variable-lift systems – so much, in fact, that it replaces the traditional engine throttle. Engine breathing is controlled entirely by the valves themselves.

The Valvetronic mechanism acts on the intake valves, imposing an additional control element between the camshaft lobe and rocker arm for each cylinder, called an ‘intermediate follower’. Upon contact by the lobe, this follower actuates a finger-type rocker arm and, in turn, the valve. The follower’s pivot point is positioned by an eccentric shaft, rotated by a servo motor in response to the driver’s accelerator-pedal movements; this varies the valve lift.

MIM rocker arm for BMW engines. Courtesy Schunk Sintermetalltechnik GmbH, Germany

The system’s advantages include: greater efficiency, more spontaneous engine response, more power, and a ‘fatter’ torque curve. Not only does the engine produce more torque; the torque peak occurs at a lower speed, 2750 rpm vs. the previous 3500. Schunk Sintermetalltechnik follows Nippon Piston Ring which has been producing MIM rocker arms in Japan for a number of years with cost savings said to be 58% compared with wrought rocker arms.