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BMW VANOS or Variable Valve Timing VANOS is BMW's answer to variable valve timing.
It is a combined hydraulic and mechanical camshaft control mechanism, managed by the
car's DME engine management system. At the lower end of the engine-speed scale,
the intake valves are opened later, which improves idling quality and smoothness. At
moderate engine speeds, the intake valves open much earlier, which boosts torque and
permits exhaust gas re-circulation inside the combustion chambers, reducing fuel consumption
and exhaust emissions. Finally, at high engine speeds, intake valve opening is once again
delayed, so that full power can be developed. VANOS operates on the intake camshaft in accordance
with engine speed and accelerator pedal position. VANOS significantly enhances emission management,
increases output and torque, and offer better fuel economy. The latest version of VANOS
is double-VANOS, used, among others, in the new M3.
The VANOS system is based on an adjustment mechanism that can modify the position of
the intake camshaft versus the crankshaft. Double-VANOS adds an adjustment of the intake
and exhaust camshafts. VANOS was first introduced in 1993 on the
BMW M45 engine used in the 5 Series. In overhead cam engines, the cams are connected
to the crankshaft by either a belt or chain and gears. In BMW VANOS motors there is a
chain and sprockets. The crankshaft drives a sprocket on the exhaust
cam, and the exhaust cam sprocket is bolted to the exhaust cam. A second set of teeth
moves a second chain that goes across to the intake cam. The big sprocket on the intake
cam has a big hole in the middle. Inside the hole is a helical set of teeth. On the end
of the cam is a gear that is also helical on the outside. There is a little cup of metal
with helical teeth to match the cam on the inside and to match the sprocket on the outside.
The V for Variable in VANOS is due to the helical nature of the teeth. The gear is moved
by a hydraulic mechanism that works on oil pressure controlled by the DME.
At idle, the cam timing is retarded. Just off idle, the DME energizes a solenoid which
allows oil pressure to move the gear to advance the cam 12.8 degrees at midrange, and then
at about 5000 rpm, it allows it to come back to the original position. The greater advance
causes better cylinder fill at midle RPM for better torque. The noise sometimes heard is
the result of tolerances that make the sprocket wiggle a bit as the cup gear is moved in or
out. Double VANOS
Double-VANOS or double-variable camshaft control significantly improves torque since, valve
timing on both the intake and outlet camshafts are adjusted to the power required from the
engine as a function of gas pedal position and engine speed. The exhaust valve control
also serves the function of the EGR valve and re-introduces exhaust as needed.
On most BMW engines that use a single VANOS, the timing of the intake cam is only changed
at two distinct rpm points, while on the double-VANOS system, the timing of the intake and exhaust
cams are continuously variable throughout all of the rpm range.
With double-VANOS, the opening period of the intake valves are extended by 12 degrees with
an increase in valve lift by 0.9 millimeters. Double-VANOS requires very high oil pressure
in order to adjust the camshafts very quickly and accurately, ensuring better torque at
low engine speeds and better power at high speeds. With the amount of un-burnt residual
gases being reduced, engine idle is improved. Special engine management control maps for
the warm-up phase help the catalytic converter reach operating temperature sooner.
Double-VANOS improves low rpm power, flattens the torque curve, and widens the power band
for a given set of camshafts. The advantage of double-VANOS is that the
system controls the flow of hot exhaust gases into the intake manifold individually for
all operating conditions. This is referred to as EGR or exhaust gas re-circulation, allowing
very fine amounts of exhaust gas re-introduced into the engine.
While the engine is warming up, VANOS improves the fuel/air mixture and helps to quickly
warm up the catalytic converter to its normal operating temperature. When the engine is
idling, the system keeps idle speeds smooth and consistent thanks to the reduction of
exhaust gas re-circulation to a minimum. Under part load, exhaust gas re-circulation is increased
to a much higher level, allowing the engine to run on a wider opening angle of the throttle
butterfly in the interest of greater fuel economy. Under full load, the system switches
back to a low re-circulation volume providing the cylinders with as much oxygen as possible.
VANOS and Valvetronic are BMW's high tech answer to keeping emissions in check and delivering
high performance at the same time.