The big AeroCarb/AeroInjector thread

[Onex107]

I, like Jeff, have done some research on the Aerocarb needles, holder, and setup. The main thing I think causes problems is the play in the needle holder. You can't make 1/4 turn adjustments if the play is more than that. It's a 3/8 X 16 setscrew, that's .030 per turn. The play in mine was .025 and that was a problem. Tighten it up anyway you can and eliminate this variable. I was determined to get to the point where the needle worked at WOT and idle without a major mixture change. A friend of mine made needles for me with different tapers until we found one that accomplished this goal. I'm now using needle #7 and it works perfectly. But guess what. After designing and making needle #7 the measurements are exactly the same as the Aeroinjector #3. The taper angle, the location of the WOT sweet spot ( .083 thickness ) from the zero point, and the idle sweet spot. This needle is exactly .125 in diameter and that is important also. If your needle is .001 or .002 under, it will run richer. I have to assume that the orifice diameter is controlled better than the needles.
I mark the zero spot on the bottom of the needle with a sharp file. Move the zero mark to the edge of the slide. Turn the holder screw in (leaner) 2 1/2 turns. It's a good place to start. You will be within a 1/4 turn of the final setting. I start my Aerovee with the mixture pulled out (from full rich) about 3/8 of an inch. It idles great at this setting. I do my run up and takeoff with the same mixture setting. During climb out if the EGT or CHT begin to get too hot, I increase the mixture about 1/8 inch to cool them down during the climb. Once at cruise rpm I lean the mixture about an 1/8 inch to bring the temps back up to a good operating level. I don't touch the mixture again until the next takeoff and I don't make needle adjustments between summer and winter because the mixture is never "all the way in". It has a little room to adjust both leaner and richer. 115 hours and running sweet.
I might add, I shut the fuel off with the main valve because the mixture shutoff leaks. That way I never change the mixture setting from one flight to the next.

Onex 107
Tri Gear
Aerovee

[Onex107]

I agree with everything Jeff said. I am currently running the #3 needle in my AeroInjector equipped Onex. I find it better calibrated to my fuel system. My mixture control is about half way out and the only adjustment I make is during climb out I move it about 1/4 inch richer to keep temps in line and back 1/4 inch leaner during cruise to put the EGT temps in the 12 1300 range. CHT's run in the 280-325 range. No flow meter, no gascolator, eliminated the burps by wrapping the exhaust pipes and insulating the fuel line. L & N air filter helped. WOT in flight 3300, static 2900 with a Sensinich 54 X 44. Climb out rpm 3000 at 90 mph and 6-700 ft/min. No mixture change required during approach and landing and taxi idle at 950 rpm. Don't touch the mixture again until during climb out. Couldn't be simpler. Love my AeroInjector.

[lutorm]

1. The AeroInjector will not work properly with pressurized or ram air. As air pressure builds up the AeroInjector leans out erratically. Is the intake on the bottom of your cowling pressurizing the air to the AeroInjector?

I read this and it didn't seem right to me so I had to sit down and draw a diagram. I was thinking that the dynamic pressure is a small fraction of the atmospheric pressure so it should make very little difference. (The dynamic pressure at sea level and 150mph is about 2.7kPa, so if you have 100% pressure recovery in the intake that would be equivalent to a 2.7% change in air pressure.)

What I didn't realize is that the problem is that the fuel vent isn't pressurized to the same pressure, so while the dynamic pressure is a small fraction of atmospheric pressure, it's a large fraction of the (unregulated) fuel pressure. In fact, a 0.5m gravity pressure is about equal to the dynamic pressure at 180mph and sea level.

However, that's assuming the fuel tank is at static pressure. At least on our Sonex, the fuel vent is pointing into the relative wind, so there should be some pressure recovery there, too. In any case this seems to indicate that it would be good to route the fuel vent down to the intake so both sides of the fuel system see the same pressure. If you did that, the only effect from ram air at 150mph would be equivalent to about a 1000 ft decrease in altitude, which seems unlikely to be noticeable given our .. ahem, less that tight mixture control.

However, it also makes me wonder why there's not a similar mixture difference between full and empty tank. The fluid column height, and thus the fuel pressure, should vary by at least a factor of two between full and empty tank so, since orifice flow goes as sqrt(pressure), that should give a 40% change in fuel flow between full and empty tank. Do people not observe big mixture changes depending on how full the tank is?

[sonex1374]

Do people not observe big mixture changes depending on how full the tank is?

I notice a slight difference in fuel flow from a full tank to an empty tank, but the difference (subjectively) seems minor and not a real problem. Part of this may be due to the nature of your flying with a near-empty tank - the final leg of a trip that usually involves cruise flight reducing to low power for descent and landing. These situations don't require high fuel flows, so the difference an empty tank makes may simply be hidden from view.

One thing that I do on my fuel system is to vent the fuel tank to the lower cowling in the vicinity of the AeroInjector air filter. This ensures the carb and the fuel tank get equal effect of air pressure. This venting scheme might not make any difference, but it eliminates one variable present with a ram-pressure fuel tank vent or a vent line that sticks down below the cowling. Both those vent arrangements may see large variations in air pressure.

Jeff

[mike.smith]

1. The AeroInjector will not work properly with pressurized or ram air. As air pressure builds up the AeroInjector leans out erratically. Is the intake on the bottom of your cowling pressurizing the air to the AeroInjector?

I would also assume that a major issue would be carb heat, which I would also assume would be necessary if air is being fed directly from outside the cowl instead of from within (where the air is warm).

[builderflyer]

One thing that I do on my fuel system is to vent the fuel tank to the lower cowling in the vicinity of the AeroInjector air filter. This ensures the carb and the fuel tank get equal effect of air pressure. This venting scheme might not make any difference, but it eliminates one variable present with a ram-pressure fuel tank vent or a vent line that sticks down below the cowling. Both those vent arrangements may see large variations in air pressure.

Jeff[/quote]


Jeff,

Could you expand your thinking on your tank vent system a bit? It seems counterintuitive that reducing the pressure on the fuel in the tank is a good thing, especially in that the fuel pressure is so low to begin with and is even that much lower with lesser amounts of fuel in the tank. With your setup, It would seem possible to develop a negative fuel pressure in the fuel line with low quantities of fuel. What am I missing?

My system is kinda the opposite of yours in that the vent tube faces directly into the prop blast and the end of the tube is flared to attempt to capture as much air as possible. But, from your earlier descriptions, your setup may very well work better than mine. So, again, what am I missing?

Thanks,

Art,,,,,,,,,,,,,,,,,,Sonex taildragger #95,,,,,,,,,,,,,,,,,,,Jab 3300 #261

P.S. Watched your recent flight to Vicksburg and it seemed so similar to many of my flights that I almost entered it into my logbook.

[lutorm]

It seems counterintuitive that reducing the pressure on the fuel in the tank is a good thing, especially in that the fuel pressure is so low to begin with and is even that much lower with lesser amounts of fuel in the tank. With your setup, It would seem possible to develop a negative fuel pressure in the fuel line with low quantities of fuel. What am I missing?

Not Jeff, but the thinking behind my conclusion that this is the correct way of doing it is that, since the AeroCarb/AeroInjector does not regulated the fuel pressure, you need to minimize changes in fuel pressure to minimize the resulting changes in fuel flow and mixture ratio. By routing the fuel vent to the intake, the two ends of the fuel system will always be at the same pressure regardless of your speed, so there should be no mixture changes as the ram air pressure changes which I would expect you to get otherwise.

[SonexN76ET]

In Tony Bingelis's books he calls for orienting the fuel vent into the direction of flight to help to increase the fuel pressure in a gravity fed fuel tank. Based on the extremely small opening of the fuel vent line I doubt that it causes any major increase in fuel pressure. I think for the operation of the AeroInjector it is insignificant. The negative side to orienting the opening into the direction of flight is that you have an increased chance of incurring a fuel vent blockage by hitting a large bug or the like. My vent was done according to the Sonex plans where it comes out the bottom of the cowling adjacent to the firewall.

The pressurized air that affects the AeroInjector is any ram air pressure. The ram air pressure going into an AeroInjector will cause erratic leaning of the AeroInjector that the AeroInjector does not automatically compensate for. The factory guidance is to avoid any ram air going into the AeroInjector (as in if your are creating a ram air plenum or creating a ram air effect with other custom touches to the lower cowling).

Jake

[Rynoth]

By routing the fuel vent to the intake, the two ends of the fuel system will always be at the same pressure regardless of your speed, so there should be no mixture changes as the ram air pressure changes which I would expect you to get otherwise.

This doesn't seem to be an accurate statement. The fuel vent is ram air into the top of the tank. The fuel line ends at the needle in the aeroinjector, which is effectively a venturi. It seems to me that it would have an inverse effect to what you're describing.... an increase in airspeed would increase the pressure at the top of the tank and reduce it at the venturi.

Furthermore, the surface area of the aeroinjector inlet is far larger than the fuel vent. I think the fuel dynamics of this is much more complicated than meets the eye, and going with the Sonex recommendation is the prudent course (i.e. no ram air to aeroinjector.)

[lutorm]

In Tony Bingelis's books he calls for orienting the fuel vent into the direction of flight to help to increase the fuel pressure in a gravity fed fuel tank.

Yes, but Binggelis was talking about traditionally carbureted engines that have a float bowl. In that situation you do want max fuel pressure to make sure you get fuel to the carb (as long as it's not so high so as to overcome the float valve), because the pressure needed to draw fuel into the airstream only depends on the height of the fuel in the bowl and not on the incoming fuel pressure from the tank.

The AeroCarb/AeroInjector is very different. There is no float bowl and the pressure difference between the fuel tank and the needle orifice will directly determine the fuel flow out of the orifice. (The pressure at the needle orifice is in turn dependent on the pressure at the intake and the airflow, which we assume constant.) The carb has no way of knowing what part of the static pressure at the intake comes from stagnation pressure vs static atmospheric pressure, there is only one variable: the pressure difference between the two ends of the fuel system.

Based on the extremely small opening of the fuel vent line I doubt that it causes any major increase in fuel pressure. I think for the operation of the AeroInjector it is insignificant.

No, pressure doesn't work that way. Because there is no actual airflow through the vent line, the size of the opening is insignificant and the pressure at the vent line opening must be equal to the pressure in the fuel tank. (This is the same reason the diameter of a pitot tube doesn't affect the airspeed reading.)