The Lift-Duration plot depicts
the modes (i.e. the lift-duration combinations) the various known valve systems
and the FVVA can operate with.
To 'sweep' the entire hatched area, two
variables are necessary.
In the Fully Variable Valve Actuation (or FVVA)
case, the 'state' of a first control and the 'state' of a second control are the
two variables.
With one only variable, as is the case for the state of the
art VVAs, the only possibility is to move along a curve, like the O-B-M for the
Lost Motion VVAs, or the D-A-M for the Constant Duration VVAs.
The FVVA
provides not only the modes the Constant Duration VVA and the Lost Motion VVA
can jointly provide, but infinite times more modes: all the modes along the
O-B-M curve, and all the modes along the D-A-M curve and all the modes between
these curves (hatched area). From the one dimension (lines) we move to the
two dimensions (area).
The conventional engine is capable for only one
mode, like the point C, and the VTEC engine is capable for only two modes, like
the pair of points VL and VH.
The
above plots of valve lift versus crank angle (valve lift profile) have all been
taken by the Double Roller FVVA of the following figure:
The result of locking the right control shaft at its
maximum angle, is the degradation of the FVVA down to a Constant Duration VVA
(curve D-A-M). The result of locking the left control shaft at its maximum
angle, is the degradation of the FVVA down to a Lost Motion VVA (curve
O-B-M). By rotating both control shafts, the whole hatched area is
accessible.
To learn more and 'play' with FVVA's
capabilities, download and run the program 'FVVA.exe' (68 KB) by clicking on the
image: The 'FVVA.exe' shows animated the Double Roller Fully Variable Valve
Actuation mechanism, and computes and presents graphically the valve lift
profile according the desirable / selected angles of the two control
shafts.
What does the VVA bring, beyond complication
and cost, compared to the conventional valve system? Nothing more than
additional valve lift profiles to choose from. Picking a more suitable valve
lift profile for the existing operational conditions is the whole idea. And
what the FVVA brings, beyond complication and cost, compared to the VVA? Nothing
more than infinite times more modes to choose from and approach much closer the
ideal (for the existing operational conditions) valve lift profile, without the
restrictions of VVAs.
Restrictions? Rotating the control
shaft of a state of the art VVA (like BMW's valvetronic or like any other VVA)
at an angle A1 the valve lift is L1 (e.g. 4mm) and the valve duration is D1
(e.g. 120 crank deg). Rotating the control shaft of the same VVA at an angle
A2, the resulting valve lift is L2 (e.g. 9mm) and the resulting valve duration
is D2 (e.g. 250 crank degrees). If, for some operational conditions, the
ideal valve motion has 4mm lift and 120 deg duration, the VVA cooperates with
the engine perfectly. If, for some other operational conditions, the ideal valve
motion has 9mm lift and 250 deg duration, the VVA cooperates with the engine
perfectly, again. But imagine the case the ideal, or the desirable, valve
lift profile has 4mm lift with 250 deg duration: all a VVA can do, is to
compromise and approach the 'desirable'/'ideal' valve lift profile with some
'possible'/'accessible' valve lift profile having intermediate lift and duration
(like 6mm lift with 180 deg duration). Is there any reason (or chance, or
possibility) all the ideal valve lift profiles to lie on a line (like O-B-M or
like D-A-M) of accessible valve lift profiles?
No doubt, VVAs is a step
ahead. But they carry their own compromises. Here comes the
FVVA.
After the above analysis, the skilled in the art can think of
ways to upgrade up to a FVVA, his favourite VVA, like:
