Is anyone flying with a mixture control for 914? How are you doing it, how
does it perform?
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I am about done for a while studying up on 914. Unless someone convinces
me otherwise, I will install a mixture control of sorts.
Instruments I have:
2 UMA EGTs, one for left side and 1 for right
1 CHT but with switch to measure left and right
Split Second O2 monitor, pretty slick readout with a number of LEDs at
center is Lambda of 1. They mapped the O2 sensor they use quite extensive,
so i am pretty certain it will give more information than just being a
switch, like most 3 wire O2 sensors are used on cars.
OK Mixture control. simple, change pressure going to the float bowl! 1
switch, 1 knob, 3 LEDs.
The 914 is turbocharged. When operated with MP under ambient, you could
vent the float bowl to atmosphere. OK until you raise the MP over ambient.
At that point you would begin to spew out the overflow and no fuel would
be flowing into the manifold, it would go the other way. Rotax engineers
most of the time connect the float bowls to airbox pressure. The carbs
are adjusted to work using airbox pressure as a constant.
Side Note, it is absolute critical that you have a good no leak free flow
connection to this connection, as well as the airbox to fuel pressure
regulator OR ELSE!!
Now when you go over approx. 100% power, the ECU looks for combo of
certain RPM and MP to determine that in fact you are making some heat.
Once it determines this, it then switches the plumbing from airbox
pressure to above airbox pressure. The way it does this is there is a 12
volt 3 connection switch/solonoid. Output goes to float bowls, there are 2
inputs, 1 from airbox normal pressure, 1 from a portion of airbox that
gets hit with impact air and is in fact over airbox normal.
First off i will have a 2 position DPDT switch. 1 position will be Auto
Rich where in fact ECU controls Rich, and the second position will be
Rich, where in fact it will be rich all the time. First off Rich Mixture
is much more desirable than lean. So manual rich can be used to determine
if rich of peak EGT, can probably be used to help cool down a hot motor
and can be used in conjunction with choke to get you to a safe place if
any number of things happens that causes a lean mixture. Also if for any
reason my Lean control fails for any reason, by selecting Rich it will
take it out of the circuit. The Lambda display wants things when making
some heat at full scale rich, or 1 LED less.
Will have a Blue LED come on when ECU is asking Solenoid to come on, and
will have another flashing Blue come on when Rich is selected.
Now for lean. Even though the Bing type 64 is somewhat altitude
compensating, and does even a little better if turbocharged, none the less
when you fly in thin air, the thinner the air gets, the richer things run.
OK. Concept is since when you increase pressure to float bowl it gets
richer, then if you decrease, it will get leaner. Now this is a turbo, so
to have a chance at some sort of linear control, you need to have some
means of a lower than airbox pressure that is constant changing with the
airbox pressure. Not just vent or intake from atmosphere. I will plumb
into the balance tubes between the cylinders. This is MP, that from what i
am told (I have yet to test) is about 1 inch below airbox pressure. So
concept is to T the airbox line going to the Airbox normal side of the 3
way solenoid/switch. Connect the extra nipple to a pilot controlled valve
that then goes to the MP Balance tube. In other words leak some pressure
to the Manifold (Which BTW also leans things out, will be limited volume
though). Now if you hit Rich button it bypasses this in event of a
problem. I have all stuff to do a ground test, and after test can
determine what size of restrictor to install on MP leak so just enough can
be allowed to leak. I will have a blinking Red LED situated so the knob
extension will cover the LED when it is in Normal (richest position it can
get). This way if Auto Rich or Rich is asking for rich, then the red LED
will begin to get bright and flash. If not in Normal it will get your
attention. Other times the Red LED will glow dimly.
There you have it.
1 switch, 1 knob, 3 LEDs.
Testing before I install lean control in panel will determine just how
well it works, and if leak serious affects my MP gauge reading, and just
how much the leak leans out due to pushing extra air downwind of carbs.
No matter what will install Rich button. I scratch built a ~ 7,500HP
Carburetor (A 30,000,000 BTU) hot air balloon burner in 1985 that is still
flyin my machine, making this smaller task pay attention superficial seems
to be a PIECE OF CAKE.
(famous last words, overheated cylinders, melted/broken pistons, trashed
gearbox, warped exhaust, seized turbo and bent connecting rods with extra
venting in the crankcase on a new motor that didn't even fly yet.)
Ron Parigoris
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