How does the VVA work?
To realize the way the VVA works, let's forget
for a while the continuous operation, selecting just four distinct modes:
Mode1: the control shafts are locked at an angle providing
a maximum valve lift of 1 mm, while an injection / ignition map M1 is activated
to provide correct mixture and spark advance (the basic parameters of the M1
are the throttle valve angle and the revs).
Mode2: the control shafts are locked at an angle providing a maximum valve
lift of 3 mm, while a second injection / ignition map M2 is activated.
Mode3: the control shafts are locked at an angle providing a maximum valve
lift of 6 mm, while a third injection / ignition map M3 is activated.
Mode4:
the control shafts are locked at an angle providing a
maximum valve lift of 11 mm, while a fourth injection / ignition map M4 is
activated (so they are needed four different injection/ ignition maps, one for
each mode, while the conventional throttle valve remains in use).
A parenthesis: Honda's VTEC,
Toyota's VVT-Li, Porsche's Variocam plus operate similarly, having not four but
just two modes of operation and two maps of injection / ignition. In these
engines, the transition from one mode of operation to the other is achieved by
providing pressurized oil to the valve actuator assemblies (rocker arms, bucket
lifters etc) to displace some locking pins, etc...
In the VVA case, there is no
need for hydraulic control. An external electromotor is more than adequate.
This electromotor is triggered by the control unit of the engine, according
revs and load, and pushes the control shafts to the next or to the previous
mode. At the same time it is activated the relevant injection / ignition map.
The driver starts the engine
at Mode1.
The starting is easy at all
conditions, the idling is very slow (around 300 rpm), smooth, economic (in city
traffic a significant part of the fuel is consumed at idling) and clean, while
there is usable torque from 500 rpm. The friction in the valve train system is
many times lower than the friction in the valve train of the conventional
engine. The actual overlap is practically zero, even if at mode 4 the actual
overlap is extreme for the sake of the maximum power output. Mode1 is ideal for
idling and for low to very low revs operation (bottlenecked city traffic, very
bad terrains etc). As the revs and the load increase, soon the small valve lift
of Mode1 becomes inadequate to feed the cylinder with the necessary quantity of
mixture.
Here comes the Mode2 to
provide plenty of torque at low to medium revs, with small actual overlap due
to the low valve lift, with economy, with clean exhaust, with excellent
response and easy driving (normal city traffic, lazy driving at open roads,
etc).
As the revs and the load
increase, the Mode3 is activated (fast suburb traffic, legal driving at open
roads, etc).
Finally, when the engine is
pushed to the limit, the mode 4 is selected (racing).
Another parenthesis: Honda's
economy version of VTEC is like the above VVA operating exclusively at modes 2
and 3, while Honda's performance version of VTEC is like the above VVA
operating exclusively at modes 3 and 4.
According the previous, the
VVA engine with the four steps seems to clearly prevail compared to the present
champion engines (Honda's VTEC S2000, Honda's Civic TypeR and the copies)
because it undoubtedly expands at both ends the efficient/usable rev range and
because it achieves better breathing and combustion at all conditions with
fewer compromises than the two step systems. Taking under account the fact that
the quick moving parts of the VVA are simpler, lighter and stronger, the fact
that inside the VVA it is integrated an intelligent VVT system, etc, etc,
reasonable questions arise.
It is now time to realize
that the VVA can operate not only in the four modes described, but in four
hundred modes, or in four thousand modes, or in infinite modes (i.e.
continuously), adding nothing to the mechanism but, quite the opposite,
removing some parts!
Because at continuous operation the electromotor can
be thrown away, as the driver's right foot displaces the control shafts.
Because at continuous operation the throttle valve
can be thrown away too, since the intake valves, which are able to operate at
valve lifts near zero, do the throttling.
And because at continuous operation the maps of
injection and ignition can be reduced to a unique / simple one, with basic
parameters the valve lift and the revs.
Strange it appears, but the
continuous operation (which, without compromises, offers optimized conditions
for the breathing and the combustion) is simpler than the two, three or four
step systems!…
Finally, the conventional
throttle valve can still be used, if desirable, to partly control the idling operation, to create vacuum into
intake manifold for assisting the brakes, and to reduce the necessary
construction accuracy of the system at very low valve lifts.
VVA and throttle valve
cooperation
Lately some minor
modification were applied to the first prototype Renault 19, and now it
operates with either its carburetor's throttle valve permanently opened, or
with its throttle valve in action.
The acceleration pump of the
carburetor is permanently deactivated.
The gas cable, from the gas
pedal to the control shafts, is divided in two parts at its end. The one part
displaces the control shafts, as previously, having a big lash: only after the
first 2 cm of the stroke of the gas pedal the control shafts start to move.
The second part of the gas
cable is dedicated to rotate, through a simple linkage, the throttle valve of
the carburetor, with minimum lash. At maximum valve lift the linkage opens
completely the throttle valve.
In this way, for the first
part of the gas pedal stroke, only the throttle valve is displaced, while the
valve lift remains unchanged (some 0.3 mm valve lift for intake and 0.5 mm for
exhaust), while the quality of the mixture from the carburetor is better
compared to the permanently / completely opened throttle valve case, especially
at low revs and loads.
Above idling the throttle
valve is significantly opened, compared to the intake valve lift, and the flow
of the mixture to the cylinder is restricted mainly from the intake valve lift.
The engine can idle below
325 rpm.
With 390-400 rpm the idling
consumption was metered at 0.35 lit/hour (0.26 Kg/hour), i.e. about three hours
of idling per liter of fuel, vital for the reduction of the consumption and of
the pollution at cities.
The brakes, due to the
throttle valve, are vacuum assistant again, making the driving more
comfortable.
Having an idling at 350 rpm, plenty of torque from
below 1.000 rpm to 7.000 rpm, and good response everywhere, the least you get
is a very pleasant vehicle to drive.
The low idling is
significant help for the driver. With 350 rpm and first gear in gearbox, the
vehicle moves with about 0.65 m/sec (or 40 m/min, or 2.4 Km/hour), which is
slower than a slowly walking man. The driver has the time to check the traffic
at crossroads without the need to touch the clutch pedal, and after the check
he has just to press the gas pedal in order to move across.
The ability of an engine to operate acceptably in a
wide spectrum of revs (the maximum usable revs divided by the minimum usable
revs gives a good meter for this) introduces new capabilities, for instance
very slow driving at really bad terrains, with minimum stress of the tyres, of
the suspension, of the clutch, of the engine and of the driver, and with
minimum consumption.
It is the usable torque and
the response at low revs that makes the difference, and it seems that the
ability of an engine to operate efficiently at very low revs was, for a long
time, underestimated for the sake of the maximum power output.
The VVA comes exactly to
expand at both sides the usable range of revs. At low revs the VVA offers
plenty of useful torque, direct response, pleasant driving, and the most
important: significant reduction of consumption and pollution. At high revs the
same VVA engine achieves a significant increase of power output. All in one.
Thank you for your time.
Waiting for your comments.