In the good old days, engine tuning done by timing adjustments was easy enough. The crankshaft drove the timing chain, which spun gears in the head and turned the cams. It was popular (and still is) to put a different style of cam in to improve performance over a certain spectrum. In those days you had to decide whether or not you wanted low end torque for cruising around town or high end power for performance and racing, both offering significant tradeoffs. The timing and functional performance of the engine was fixed in place.
That is, until technology became more widely available that allowed the timing to change on the fly. Many manufacturers came out with designs to allow Variable Valve Timing (VVT), all with different solutions to the same problem. How can we achieve the flatest power curve possible on a non-turbo engine, while reducing emissions and fuel consumption? The ability to change the timing of the engine on the fly is a wonderful, magical invention that allows an engine to perform well driving around town at low RPM, as well as at higher RPM when your right foot demands some oomf. One of the more famous acronyms is Honda’s ‘VTEC’, which utilizes multiple cam profiles to not only change timing, but duration and lift. I won’t blow your mind today with the complexity of that system. BMW’s Variable Valve Timing system is called VANOS (Variable Nockenwellen Steuerung), and here’s how it works!
This is the single VANOS engine (found in M50 engines). It controls the intake cam gear, which is mechanically linked to the exhaust gear with a chain.
The key of the VANOS design is that the cam gear and the cam itself are independent, and both have splines. The gear/cup in the VANOS system inserts in between these two parts, mechanically linking them. The VANOS gear has two sets of splines, the outer for the cam gear and the inner for the cam itself. The splines have a twist to them, also known as a helical gear, so as the gear inserts itself further between the cam and cam gear, the relative position changes, if by only a few degrees.
At idle, the gear is retracted. As the RPM bumps off idle, the cup inserts further into the gear and advances intake valve timing. This creates intake and exhaust valve overlap, allowing for exhaust gas recirculation (an operation designed to improve emissions while cruising). When accelerating into the higher RPM ranges, the solenoid closes and the cup retracts once again, reducing overlap and going for max power. It should be noted that this solenoid gets a real workout, and is a common failure in these engines as they age.
Double VANOS has a hydraulic pod (found in later cars) extending into both cam gears, and has control over both intake and exhaust cam timing independently. This system is much more advanced than single VANOS, allowing for constantly variable timing. The ECU can change the intake and exhaust cam timing, and however it so pleases. Using different maps for different situations (IE warm up, cruising, thrashing).
In all practical senses, the system is quite simple and ingenious. Many of the best car makers do it. All it takes is putting a few different shaped gears together to pull off something amazing. Now in 2016, you will see that nearly all automakers have some form of variable valve timing, but BMW was one of the few early companies to take full advantage of such a system early on.