I’ve been trying to work out exactly how the very complex valve system of the BMW motor in my Plus 8 functions.
After a lot of reading of BMW documents and posts on BMW forums I think I understand it all.
The BMW N62 B48 V8 has “Double Vanos” and “Valvetronic”.
But what are they?
Double Vanos is relatively simple to understand: in essence it is a continuous alteration of both inlet and exhaust cam shaft position based on engine speed and throttle opening. In essence a fully automatic “advance and retard” control for the valves, and indirectly, spark timing.
At low engine speeds timing is retarded so the valves are open later to improve idling quality and smooth power development. As engine speed increases the timing is advanced so valves open earlier: this enhances torque, reduces fuel consumption and lowers emissions. At high engine speeds the valves move back to a retarded position for full power delivery. The cam shafts each have a range of movement of +/- 12 degrees, which is quite significant. But it isn’t quite that simple as the timing change of inlet and exhaust cams can be altered independently so altering overlap: this is used to manage gas flow and keep the cats as hot as possible. The timing also works together with Valvetronic, which gives continuous and precise control over variable intake valve lift.
Valvetronic-equipped engines use the amount of valve lift for air flow control, rather than a butterfly valve in the intake tract. In other words, in normal driving, the "accelerator pedal" controls the Valvetronic software rather than the throttle butterfly plate: true "drive by wire". There is a butterfly, it is used for starting and as part of the “limp home” mode if the main system fails…!
The operation of Valvetronic is complex as there are an extra set of rocker arms, called intermediate arms positioned between the valve stem and the camshaft. These intermediate arms are able to pivot on a central point, by means of an extra, electronically actuated third camshaft. This movement alone, without any movement of the intake camshaft, can vary the intake valves' lift from fully open at 9.9mm for maximum power, to almost closed, at 0.18mm, for idle.
Add Double VANOS and Valvetronic together you end up with a very flexible engine that is both powerful and efficient, easy to drive due to good low end torque. I think it is an excellent device, particularly when connected to the ZF 6HP 26 automatic transmission, which is almost as complex as the engine!
But it is all very complex, controlled completly by computer and probably almost impossible for the traditional DIY engineer to work on!
If anyone can add more to this description, please do so!
