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%.