Without the conventional throttle valve,
the most difficult job for a VVA (any VVA) is the idling operation.
Regardless of the specific mechanism (lost motion or constant duration
VVA realized either mechanically or hydraulically or electro-magnetically or ...
) what makes the control of the gas flow into the cylinder is only the gap
formed between the intake valves and their valve seats.
More than 1.000 US patents
have already been granted for VVA mechanisms (search query: ccl/123/90.16) at www.uspto.gov
The problem lies with the sensitivity of the breathing system at very
short lifts: a slight imbalance between the valve lifts (for instance due to
uneven thermal expansion or to uneven mechanical wear or to adjustment
differences etc) causes an intolerable uneven distribution of the charge among
the cylinders.
BMW's valvetronic (a lost
motion VVA) uses 0.3 mm intake valve lift for idling. BMW manufactures the
valvetronic parts with a 0.008 mm dimension accuracy. In addition BMW moved to
cross flow cooling to minimize the
temperature differences along engine head.
Pattakon's B16A prototype
(constant duration VVA) with 0.15mm intake valve lift idles at 300 rpm. The gap
that restricts/controls the air flow to the cylinder at idling is like a
rectangle 200mm long (i.e. the periphery of the two intake valves) and only
0.15mm wide (200/0.15=1330!).
The quantity of the charge
entering each cylinder is roughly proportional to the lift of the valves of the
cylinder.
If a 0.02mm tolerance is
attainable for the intake valve lift, the ratio (0.15+0.02)/(0.15-0.02) = 1.3
indicates that a cylinder can suction 30% more charge than its neighbor cylinder!
An idea of what the 0.02mm
is: by changing the temperature of the 102mm long intake valve for 17 degrees
centigrade, its length changes by 0.02 mm.
The intake valve has to
perform quite different tasks: It has to combine high flow capacity, light weight
and robustness for top peak power at high revs, and, on the other hand, as soon
as the gas pedal is released it has to perform a strictly precise 0.15mm stroke
to allow the engine idle at 300 rpm.
Existing solutions:
A way to face the idling
problem is to apply extreme construction accuracy, to use special cooling
system in order to minimize thermal differences along engine, to avoid very
slow idling revs (like 300 and 400 rpm), to avoid very high revs (like 7000,
8000, 9000 and more rpm), and live in the hope that the inevitable (on long
term) wear of the parts involved will be distributed equally to all cylinders.
Another solution is to use
independent (i.e. per cylinder) VVA and use feedback for continuous on-line
adjustment.
Another way is to use a
lambda sensor for each cylinder in order to control independently each
injector 's duration, but in this case the uneven torque pulses will be
noticeable. Etc.
And an alternative way:
Instead of fighting with the
sensitivity of the system at very short lifts, an alternative idea is to keep
the normal intake valves completely closed during idling and to feed the
cylinders with air or mixture through other intake valves (the idle-valves) of
significantly lower flow capacity.
In practice:
Starting with the B16A
Pattakon 's prototype, a hole of 7mm diameter, 25mm deep, is made beside each
pair of intake valves. Inside the hole a "bullet" like, one way ball valve (the
idle-valve) is nailed / bolted.
The ball (from a ball
bearing) inside the idle-valve is 3.2mm in diameter while the orifice is 2.5mm.
With two short side holes (3mm diameter) the input (i.e. the opening near the
ball) of each idle-valve communicates with the space above the heads of the two
intake valves into the intake port.
The minimum intake valve
lift is set to zero. Every time the gas pedal is released, the two intake
valves of each cylinder stay permanently closed and the mixture is exclusively
suctioned through the 2.5mm orifice.
At idling the mixture enters
into the cylinder through the 2.5mm diameter
orifice, while both 33mm
diameter intake valves stay idle. Comparing the
size of the orifice to the
size of the intake valves, the problem is revealed.
The idle-valve schematically.
An idle-valve sliced, the
internals and the actual size (25mm long)
The idling recorded with
Hondata 's Logger
The same prototype engine
recorded at high revs
When the pressure inside the
cylinder is lower than atmospheric, the ball is pushed upwards allowing mixture
from the intake port to enter the cylinder. As the piston moves upwards the
pressure inside the cylinder rises and comes a moment the one way valve closes.
The rest cycle continues as usual.
At idling all intake valves
stay permanently closed, the charge flows into the cylinder exclusively through
the 2.5mm hole and the engine nicely idles at 330 rpm on stoichiometric
mixture.
Note that at 330 rpm the
kinetic energy stored into the rotating masses (flywheel, crankshaft etc) is 5
times less than at 750 rpm of the conventional idling. Any misfiring at 330 rpm
would cause engine stalling.
The transition from no load
(idling) to load operation is excellent.
Under load the B16A
prototype engine behaves as before (i.e. without the idle-valves) providing top
peak power at 12mm intake valve lift and 9000 rpm (where the rev limit is set)
and flat torque curve.
An interesting and useful
result is that any time the gas pedal is released, the intake control shaft
returns to zero lift angle, leaving the intake valves with their rocker arms
and their rollers immovable, even if the engine continues to rev high (for
instance when braking with the engine). Note that the prototype engine involves
no spring other than valve springs.
Fine tuning:
A small adjusting screw is
used as an obstacle to the air flow from the intake port to the input of the
idle-valve. Turning the adjusting screw the resistance of the air path is
changed. This way the charge can equally be distributed to all cylinders. The
adjustment resembles to the air adjustment in old carburetors.
Applicability and variations:
Any type of VVA able to
provide zero intake valve lift can be combined with the idle-valves.
Obviously the idle-valves
are not necessarily one way valves neither self lock valves.
The location of the
idle-valve is not restricted.
The size and weight of the
idle-valves can become so small that they could be formed even onto the heads of
the intake valves.
In more sophisticated
applications the one way ball valve could be replaced by a small and light
popet valve of short stroke. This small popet valve could be controlled by a
cam on the camshaft. A better way to control a small popet idle-valve seems to
be the electromagnetic control: the idle-valve is much lighter than a normal
valve, it operates only at very low revs and performs a much shorter stroke.
Controlling the duration the idle-valve is kept open (a control similar to that
used for the fuel injection), the idling and even the light load operation at
low revs could be fully and accurately controlled.
Side effects:
The complexity added.
The need for some
modification of the electric generator (or its pulley) to provide adequate
voltage from 300 rpm to keep the battery charged.
Idling Consumption:
Procedure:
The power supply to the fuel pump is disconnected.
The pipe from the fuel filter to the injector 's collector is
disconnected.
A 2 liter transparent bottle is used as fuel tank.
A pre-weighted quantity of fuel is poured into the bottle.
Air is pressurized into the bottle at 2.2 bar.
With a pipe from the bottom of the bottle the injector 's collector is
supplied with the pressurized fuel.
The injector table is modified to compensate the lower fuel pressure.
At 330 rpm idling and
stoichiometric mixture the fuel consumption is 1 liter of unleaded regular
gasoline per 3 hours (i.e. 11.5 hours/gallon or 340cc/hour or 250gr/hour, for
this 1600 cc top power engine). Tests with ethanol will follow.
Hybrid technology:
Hybrid technology takes
advantage of the good efficiency of a conventional engine at full load and
medium revs to bypass / avoid the poor efficiency of the same engine at low
revs, high revs, partial loads and idling.
If the efficiency of the
internal combustion engine were about constant at all revs and loads, the
hybrid technology would be useless.
In hybrid cars the internal
combustion engine is not allowed to operate at partial loads and at
idling. Despite the inevitable energy loss during the
transformation, storing and regeneration of the kinetic energy, there is an
overall gain in fuel economy proving the poor
efficiency of the conventional engine at specific operational
conditions.
On the other hand an engine
with VVA and idle-valves is characterised by more than
significant efficiency improvement at partial loads and at idling, leaving
fewer problems for the hybrid technology to deal with.
Conclusion
The ability of a VVA system
to achieve tiny intake valve lifts is quite different than the ability of the
VVA to control precisely and on a long term basis the idling operation.
The problem is the
sensitivity of the breathing system at idling: the slightest difference between
the intake valve lifts results in an intolerable imbalance of the distribution
of the gas among the cylinders.
The sensitivity problem is
critical only for idling.
Under the lightest reasonable
load the intake valve lift is several times higher than the idling
lift and therefore the sensitivity becomes several times lower than idling
sensitivity, making the normal accuracy and cooling adequate.
With Pattakon 's
idle-valves what is achieved is the liberation of the VVA system from the
idling operation. At idling the only duty left to the VVA is to leave the
normal intake valves closed. This, in turn, releases the design from extreme
construction accuracy, from special cooling system, from cost, from often
adjustments etc.
As regards the
quality and the efficiency of the idling operation:
the extreme velocity of the
mixture entering the cylinder through the small orifice of the idle-valve
(improving mixture homogeny, charge turbulence, flame propagation, operation
stability),
the significantly lower
pumping loss than conventional (no vacuum at intake port),
the constant geometry and
dimensions of the idle-valves on a long term,
and above all, the idling
fuel consumption in practice,
is the answer.
Every engine spends a
percentage of its life at idling. In downtown traffic this percentage
increases, some times a lot.
Every drop of fuel
saved during idling is a direct reduction of air pollution and a
direct profit into owner's pocket.
It would help the
idling consumption data of any new car for comparison.
The
idling consumption has an advantage: it can be
measured with simple tools, any time, at any place and
beyond any doubt.
Please feel free to ask
for more details or for a demonstration.
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mailto:vva@pattakon.com