New Generation N62 Engine
Chapter 3 P.37
Course Contents/Background Material
© BMW AG,
Service Training
Physical considerations:
On engines with throttle valve control, the throttle valve is only
slightly open in the idling and part-load ranges. This results in
the formation of a vacuum pressure of up to 500 mbar upstream
of the intake manifold throttle valve, which prevents the engine
from aspirating freely, in turn preventing optimum filling.
The Valvetronic system open throttle valve largely counteracts
this disadvantage. The air-mass flow to the intake valve is
unimpeded. The full ambient pressure (approximately
1,000 mbar) is available directly at the intake valve for cylinder
filling and scavenging.
The Valvetronic system primarily controls the fill by adapting the
valve opening period, and secondly, through the size of the valve
lift (short opening period/small valve lift=lower fill, and vice
versa).
During the valve opening phase, the engine aspirates more
freely via the valves, even with small valve lifts, than via a throttle
valve which is continuously blocked. In the cylinder filling and
scavenging phase with closed valves, on the other hand, the
engine operates almost without loss using a gas spring.
The residual (lower) throttling loss at the valve with partial lift
results in more fill turbulence in the combustion chamber, and
thus faster and better mixture control, and faster and more
thorough fuel conversion.
At lower speeds, this effect is specifically intensified by opening
the intake valve later, after top dead centre (ATDC), using
VANOS. The resulting increased vacuum pressure in the combu-
stion chamber accelerates the filling motion as the valve is
opened, thus inciting the fill turbulence.
On the whole, the additional variability of the Valvetronic system,
made possible by the adjustable valve lift, results in better
adaptation and optimisation of cylinder filling and scavenging
throughout the engine's entire operating range, in terms of
output, torque, consumption and exhaust emissions.
