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%.