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Walter,
You understood me correctly.
The test was designed to determine how smoothly the
engine runs at high altitude, lean of peak in two difference
situations:
1) NO RAM AIR - air pressure inside the cowl
exceeds the pressure of the induction system (usual for a normally aspirated
engine)
2) RAM AIR - air pressure inside the cowl is
less than the pressure of the induction system.
It appears that using ram air (with the resultant
reveral of pressure through the screen) makes the engine run rough (at high
altitude, LOP) - as you suggested.
GAMI lean test has already been performed using ram
air (at a lower altitude) with very good results (.2 spread). Of course I
can repeat GAMI lean test, but if we assume that the spread remains low then
improper fuel distribution is not the problem and lack of fuel atomization may
be.
Mike Mahar sells a system to pressurize the
injectors - has anyone tried it and what were the results?
D. Brunner
D. Brunner
----- Original Message -----
Sent: Monday, January 21, 2008 10:05
AM
Subject: [LML] Re: Pressurized
Injectors
Scott:
Nice summary. In addition, if he is using the air pressure under
the cowl to push air into the injector (through the screen on the NA
injector), the addition of RAM air to the induction increases the MP to the
point that he no longer might have enough upper deck cowl pressure
differential to result in anything more than a pencil stream in the FF.
That alone--even with excellent F:A ratios could result in rough running
as the fuel would be more liquid than vapor. That's what I understood
his message to be about. If my interpretation were correct, then he's
right about adding upper deck pressure to the injectors to increase the
differential pressure between MP and cowl pressure into the injector.
Of course, I could have misunderstood his message.
Walter
A while ago there was a thread on the
effect of pressurized injectors on fuel atomization. It was suggested
that pressurizing the injectors improves fuel atomization and leads to a
smoother running engine LOP at high altitudes.
Recently, we ran tests to see what effect the
use of Ram Air had on smoothness of operation at high altitude.
Engine: IO-550N, 10:1 compression, ram
air, magnetos, GAMI injectors, GAMI spread of .2
gallons
Conditions: Altitude = 17,500, OAT = 1
C
Without Ram Air (MAP = 15.2) the engine ran
smoothly down to about 60 deg LOP or 8.8 gal/hr
With Ram Air (MAP = 16.3) the engine only
ran smoothly to Peak or 11.2 gal/hr
The only difference between these two
settings (that I can think of) is that in the first, ambient air
pressure is probably slightly above manifold pressure (accounting
for the filter) - which means the injector is pressurized very
slightly. In the second, ambient pressure is probably about 1.0
less than manifold pressure (accounting for ram effect) so pressurization of
the injector is reversed. This would seem to confirm the fact
that unpressurized injectors reduces fuel atomization and makes
for a rougher running engine LOP at altitude.
I would like to run smoothly LOP at
altitude using ram air. It seems to me that I can go ahead and
pressurize my injectors by running tubes off the ram air tube to the
injectors and using turbo injectors. (Is Mike Mahar still selling
this setup?) Not sure that I want to go to the time, trouble and
expense to do this though.
I am also wondering if using a "hotter" spark
from Electronic Ignition would improve the problem. (I would try to
use an Electronic Ignition system that had the same advance as
mags)
Your thoughts?
Doug,
Well, you asked, so.................
Let's try to get a common understanding of the contribution of
components first. Remember that good combustion is a result of the right
amount of air and fuel (A/F ratio) and something to ignite the mixture.
Work is performed at a reasonable time in the combustion cycle by having
the combustion event reach some appropriate state of completion( good
pressure) at a certain range of motion of the piston after TDC, say around 16
degrees. Each time you change one of the combustion components, it
affects the rest. Higher compression ration, fuel octane, spark effectiveness,
timing, yada, yada, yada.
At some point, after the engine was broken in, the baffling was set and
ram system built, a Gami lean test was performed and used to
get injectors that were sized to deliver A/F ratios so that each cylinder
was receiving the fuel, air and spark that resulted in delivering fairly equal
amounts of power as measured by reaching peak within a narrow fuel delivery
range. Thus, LOP the engine ran smoothly.
Now, a component was changed without recording all the data
necessary to analyze the results. It would be useful to run the GAMI
lean test again, with and without RAM air, since it appears that the
smoothness is gone because the cylinders are producing different power.
One thing that the ram set-up may do is cause an unusual flow thru the
throttle body, thus altering the A/F in some cylinders. Perhaps
the airflow is altered in the cooling plenum that provides air to the
injectors and there could be some effect.
The major benefit of providing controlled air equally to all the
injectors reduces the possible effect of peculiar air flow in the cooling
chamber on atomization. Be careful - the GAMI injectors were tuned to
your engine as it was operating when the GAMI lean test was performed.
Changing to shrouded injector bodies may again change atomization and the
A/F ratio in each cylinder and not result in an improvement. The benefit
of supplying air to the injector at or above the MAP (I don't know what you
mean by ambient pressure) is that there is no detriment to atomization.
Electronic ignitions do not just provide a "hotter" spark. They
also provide a spark every time it is needed (some mags miss although you
wouldn't notice it merely by feel), a longer more powerful spark to insure the
mixture is ignited even if it isn't the best mixture for ignition and,
they can adjust the timing to deliver the same power for less fuel (or
more power for the same fuel) at different power settings as determined by RPM
and MAP. Electronic ignitions may help your engine be
inherently smoother, depending. Without the timing adjustments, you are
assuming the mag timing is right even though you have increased the CR by
quite a bit. High power performance might be improved by using less than
25 DBTDC.
You have started to experiment with ram air to achieve better performance
(for free) from an engine upon which you spent many dollars for
high CR pistons and GAMI injectors to get a high level of
performance. You have to be willing to spend more money and time on
the further experimentation necessary to keep things moving towards
more optimal performance. It seems you had a good configuration
where everything was reasonably optimal (filtered air, GAMI, etc.)
and the next step (free ram air) seemed simple except that the balance in
your engine has been upset.
Welcome to the world of tweaking, resulting in effects similar to those
of the snowball at the top of the avalanche area.
Scott
Krueger AKA Grayhawk
Lancair N92EX IO320 SB 89/96
Aurora, IL
(KARR)
Pilot not TSO'd, Certificated score only >
70%.
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