"if you have any info available which you would be
prepared to share on your leaning system Remi, I would very much
appreciate same please. "
Hello Kingsley,
First I want to say that I am not the inventor off the principle of creating a
vacuum in the float bowl using an electric pump to lean the Bing carbs. This
leaning
device was designed about 20 years ago by Michel Colomban, designer of
the Cri-Cri and MC100 on which the well known DynAero MCR series were based. He
designed his own pump to create the vacuum. The vacuum is adjusted through a
potentiometer and the mixture ratio is measured using an O2/ lambda probe.
Attached
are some original Colomban's drawings (in French, sorry)
My system uses an electric fuel pump (Pierburg E1F p/n 7.21440.51.0) to create
the vacuum. The vacuum side of the pump is connected to the two carb bowl vents
and the pressure side is connected to the airbox, through a combination of
polyurethane
and silicone tubing, nylon tees and straight junctions. See attached
pictures. The vacuum is adjusted using a potentiometer and some electronics
to control the pump voltage. The actual voltage at the pump is being measured
during testing.
Initial testing showed that the pump was way too powerful and I had to add a
by-pass
around it which incorporates a calibrated .8 mm restrictor, to reduce the
effective vacuum. In order to calibrate the system on the bench, I measured
the vacuum using a U shaped clear tubing filled with water. (btw I remember
Colomban
saying he was using red wine to better visualize the level). With the by-pass,
the system provides the following vacuum levels: 3 volts at the pump mm
H2O. 4V=30mm, 5.5V=40mm, up to 13V=220mm.
Testing on the aircraft (912ULS + Airmaster prop):
On the ground (altitude 2000ft): at 2700 RPM, progressively increasing the
voltage
at the pump, RPM increases and when reaching 4 volts the RPM peaks at 2900,
then goes down and starts to run rough above 4V.
at 4000 RPM, progressively increasing the voltage, when reaching 4 volts the RPM
decreases and engine starts to run rough.
In flight at 10000ft, full throttle, 5000 RPM, propeller in Manual mode to
freeze
the pitch, when reaching 4 volts the RPM decreases to 4900 and engine starts
to run rough.
Interpretation of the results: the vacuum necessary to lean the mixture on a
Rotax
912ULS at 10000ft is 30mm H2O approx.
Leaning on the ground at high idle increases the power, which means the idle
mixture
was way too rich.
Leaning at 10000ft does not increases the power at all. That means that, without
leaning, the mixture is not rich enough to prevent the engine to deliver full
power as it is with legacy aircraft engines. I suppose leaning would slightly
reduce fuel flow of the Rotax at this altitude but I had no means of measuring
the gain accurately and also no means of being sure I was not damaging the
engine. That is why I stopped the experiment at this point.
Concerns regarding the pump: it is designed to pump fuel not air. In normal use
I suppose the fuel cools and lubricates the pump. Pumping air, even at reduced
voltage, the pump could be damaged. An alternative would be to use a ventilator.
I found some ventilators which are able to provide the required static vacuum
but I think there is a risk of overheating because there would be used at
zero flow, which is not what there are designed for. Some endurance testing
would
be required to find out.
Compared to the Hacman, the electric leaning system has a few advantages: there
is no modification of the carbs. It is fail safe: if the pump stops working your
are back to the normal configuration where Carb bowl pressure equals air box
pressure. In case of a problem, just switch it off.
Now why is the high altitude behaviour of the Rotax so different from the legacy
aircraft engines? On Lycoming and Continental engines carburettors there is
a power enrichment device (improperly called "economizer" by Marvel Schebler)
which provides a lot richer mixture when the throttle nears the full forward
position.
This is necessary to better cool the cylinders heads to prevent detonation
at full sea level power. At 10000ft, full throttle will give only about
55-65% power, depending on the prop, so the power enrichment is not necessary
anymore. Then the power enrichment and the effect of air density combine to
provide
an extremely overrich mixture. At full rich at 10000ft, those engines will
run rough and the spark plugs may foul if you do not lean. When leaning
appropriately,
you will see a very significant increase of power.
On the 912 and 912S, the figures provided by ROTAX in the Owner Manual show that
the specific fuel consumption at full sea level power is the same as at cruise
power. So obviously there is no power enrichment system. (there is one on the
914 but that is another story). The Rotax water cooled cylinder heads temp
are a lot lower than those of the air cooled legacy engines and that makes
detonation
a lot less likely, therefore a richer mixture at full throttle is unnecessary.
So at 10000 ft ,the 912 is running slightly too rich due to the reduced
density but not to the point of limiting the available power. Flying at 16000
ft over the top of the Mont Blanc, the Rotax runs flawlessly. An appropriate
leaning device would for sure provide some fuel saving at high altitude but
more testing would be necessary to find out how much.
The attached photos show the pump installed on top of the starboard foot well,
the connection to the carbs and airbox (blue and black tubing), the by-pass
(blue
tubing connected to two tees), the controller and voltmeter on the passenger
seat.
Remi
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=493867#493867
Attachments:
http://forums.matronics.com//files/p9050002a_2_155.jpg
http://forums.matronics.com//files/p9050004a_2_148.jpg
http://forums.matronics.com//files/p9050005a_2_501.jpg
http://forums.matronics.com//files/p9050006__copie_662.jpg
http://forums.matronics.com//files/correcteur_altimtrique_pour_carbu_bing_2_540.pdf
http://forums.matronics.com//files/correcteur_altimtrique_pour_carbu_bing_1_196.pdf
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