Performance,
torque, idle characteristics and exhaust emissions reduction are improved by
Variable Camshaft Timing. The VANOS system is currently used in all BMW engines.BMW’s Variable Valve Timing system is called VANOS
(Variable Nockenwellen Steuerung), and here’s how it works.
The
VANOS units are mounted directly on the front of the camshafts and adjusts the timing
of the Intake and Exhaust camshafts throughout the entire spread range from retarded
to advanced. The ECM controls the operation of the VANOS solenoids which regulates
the oil pressure required to move the VANOS units. Engine rpm, load and temperature
are used to determine VANOS activation.
The
VANOS mechanical operation is dependent on engine oil pressure applied to position
the VANOS units. When oil pressure is applied to the units (via ports in the camshafts
regulated by the solenoids), the camshaft hubs are rotated in the drive sprockets
changing the position which advances/retards the intake/exhaust camshafts timing.
The VANOS system is “fully variable”. When the ECM detects that the camshafts
are in the optimum positions, the solenoids maintain oil pressure on the units to hold
the camshaft timing. The
operation of the VANOS solenoids are monitored in accordance with the OBD II requirements
for emission control. The ECM monitors the final stage output control and the signals from the
Camshaft Position Sensors for VANOS operation.
The VANOS unit lifted off, notice how the cam and the cam
gear are independent of each other without the VANOS gear in place. Image
courtesy of Beisan Systems.
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.
The VANOS solenoid opens, allowing the helical gear cup to
push forward and change the orientation of the cam in relation to the gear a
number of degrees.
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 ( 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 2014, 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.
Here are just some of the benefits of the VANOS.
- It increases torque at lower to mid range engine speeds with no loss of power in the upper range engine speeds.
- It allows for increased fuel economy due to optimized valve timing angles, and reduced emissions.
- You enjoy a smoother idle quality due to optimized valve overlap
No comments:
Post a Comment