The big AeroCarb/AeroInjector thread

[lutorm]

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.)

Yes, there is some pressure drop between the static pressure outside the air filter and the pressure at the needle orifice, but that just depends on the velocity of the air flowing in, which should not be affected much by any ram air.

What I'm saying is that, yes, it would be my expectation that with the fuel vent pointed into the wind and the intake at effectively cowl exit pressure, the pressure in the fuel vent should make the engine get richer at higher airspeeds. Once my plane is flying again I will certainly look for just this effect.

If you make a ram air intake, on the other hand, you would probably anticipate that a reasonable ram air intake have higher pressure recovery than the fuel vent line and thus the pressure at the intake would now go up faster than the fuel vent does, making the engine get leaner with speed.

If you route the vent line down to the air filter, on the other hand, it should be neither. A ram air intake would then by design make the pressure change equally on both sides of the fuel system just as flying higher or lower would. There is simply no way the carb can know the difference between airspeed and altitude at that point.

I respect that you want to follow Sonex' recommendations. It's certainly a good starting point. Personally, after seeing the incredibly uneven mixture distribution with the stock intake system on our Aerovee, I'm of the opinion that the virtue of the stock system is that it is very simple. That's the Sonex philosophy and a good maxim to go by, but the tradeoff of simplicity vs function seems to be a bit too far on the side of simplicity for my taste, and if you are willing to experiment there certainly seems to be room for improvement.

[sonex1374]

First off, I'm not going to talk about ram air to the carb, merely fuel tank vent placement. Ram air is another discussion....

There's no question *theoretically* that altering the pressure of the vent line (and thus the fuel tank) will change the fuel flow at the carb. The real question is how much it will vary, and if that's significant. If you accept that the total pressure range the vent is likely to see (say 50 mph to 200 mph) is approx. 1 psi, then how much will that affect things?

http://www.sonex604.com/images/aoa/Ram_ ... rspeed.jpg

Well, 1 psi is equivalent to 27 inches of water (roughly equivalent to fuel for the purpose of this discussion). That is roughly the depth of the tank. So the max effect of varying the vent line pressure should have a similar effect as the tank fuel level varying from empty to full.

A way to test the significance of this for yourself is to simply run a fuel flow test with two runs - one with a full tank, one with an empty tank, then compare the results. I think you'll find the flow rate will indeed vary somewhat between these two extremes. Having said that, however, we fly all the time living with this variance, and it doesn't really affect things all that much. The effect is there, but it's really not that significant.

There's another factor that comes into play beyond simple fuel pressure at the carb, and that's the effect of manifold pressure from the engine. The fuel pressure in the lines (a combination of fuel level "head pressure" and vent line pressure) delivers the fuel to the carb orifice, but then the engine "sucks" on that orifice to draw the fuel in. This is a complicated process to fully define, but it's easy to observe. For those of you that have fuel flow meters installed, run this test. Set the throttle to idle for starting, turn on your fuel valve, move the mixture knob to full rich, and note the fuel flow rate. It will be pretty low, like 0.5 gph or something. Of course, with the engine not running, all this fuel is simply pouring out the carb and into the air cleaner, so we tend to avoid doing this! That being said, then simply start the engine, and watch the fuel flow rate jump way up. We haven't changed anything by starting except the manifold pressure, and the engine "sucking" is flowing more gas. As I said, the flow dynamics are somewhat more complicated then they first seem.

So back to my earlier statement about the vent line placement. Venting the fuel tank to the same location as the air cleaner ensures that the carb intake and the fuel tank intake are exposed to the same air pressure (whatever that may be), and this reduces the variance that occurs with changing airspeeds. It doesn't change the effect of varying fuel level, only airspeed. Is this a good thing? I tend to think so. Is it all that significant? Not really, but we hedge our bets wherever we can.

Jeff

[Msing48]

All of the the technical discussion here is interesting but in practical use I have found no difference in either venting in the lower plenum, out the bottom or above the cowling except that with a taildragger and full fuel the first two locations allow fuel to escape out of the vent tube when the plane is level or in a descending attitude. I started out with the vent at the bottom and barely inside the cowling (Aerovee engine and Aerocarb). Then I extended it to just outside the bottom of the cowling. I didn't like the fuel escaping as soon as I lifted the tail so moved the vent opening to about 4" above the cowling and facing forward. In that position I didn't lose fuel and noticed no change in mixture regardless of speed. When I upgraded to the newer Aeroinjector I could see no change regarding mixture. I also left it there when I replaced the engine with a Jabiru 3300 and the larger Aeroinjector and have the same results regarding mixture. My recommendation would be to put the fuel vent wherever you want it and don't get too obsessed with the tecnicalities.

[builderflyer]

Thanks to all for adding your ideas to this thread. My thoughts, however incoherent.....

It would seem that, ideally, the goal that we are attempting to accomplish is to have the same amount of fuel delivered to the carb by the fuel system as demanded by the engine at any particular point in time, no more and no less, and with the pilot's ability to control the air/fuel ratio as he deems necessary to achieve the appropriate EGTs. In our gravity system, the amount of fuel the fuel system is able to deliver is determined by the size of the fuel line between the tank and the carb, any restrictions in the fuel line such as a gascolator, fuel sender unit or filter, and the amount of "head" available which varies with the fuel level in the tank, the pitch attitude of the aircraft in flight, and any pressure added to or subtracted from the head via the fuel tank vent.

As the pilot advances the throttle and the engine demands for fuel increases, wouldn't it be desirable to increase the head on the system to aid in the delivery of additional fuel as well? Having the fuel vent tube being pressurized by the prop blast and/or slipstream would appear to be helpful here, although the amount of pressurization available is uncontrollable. Placing the fuel vent tube near the air filter would accomplish just the opposite, that is reduce the available head (Bernoulli's equation) and possibly hinder the delivery of the additional needed fuel.

But two factors concern me regarding the placement of the fuel tank vent tube opening in the engine compartment. 1. Unless the vent tube is at a height of several inches above the fuel tank at some point, it will burp raw fuel at times and would that not be dangerous to have that fuel dumped inside of the cowling?, and 2. If one was to experience an engine fire, what could be the ramifications of having an open path (the fuel tank vent line) from the fire to the fuel vapors within the fuel tank?

Just my thoughts,

Art,,,,,,,,,,,,,,,,Sonex taildragger #95,,,,,,,,,,,,,,,,,Jab 3300 #261/ tired old Aerocarb

[sonex1374]

Mike is correct when he says all this vent placement doesn't really matter. In practice, they all seem to work acceptably. To clarify where my vent line terminates, it's at the bottom of the cowl, near the air cleaner (but still a few inches away), right above the fiberglass. Any drips out the vent run out the back side of the cowl in approx 1 inch, essentially like it would if it penetrated the cowl itself. Not sure how this setup may comparatively fair in a fire..... Extending the vent line up a few inches it a great idea. Just take a look at Bob Mika's fuel vent to see a superb example of how to do it!

Jeff

[SvingenB]

Static/dynamic pressure is one thing, but what about differences in G? Pulling two Gs, and the fuel pressure doubles. Has anyone experienced variation in mixture when pulling G?

[SonexN76ET]

In regards to the ArroInjector running rich while pulling G’s, I have noticed while pulling extended G’s (maybe three or four) for more than a few seconds the engine will surge or miss a bit. It does not seem to loose power it just makes a different engine sound than the normal rhythm. I have found that more aggressively leaning my mixture before I start the high G maneuvers reduces this disruption in the engine sound.

Jake

[rizzz]

In regards to the ArroInjector running rich while pulling G’s, I have noticed while pulling extended G’s (maybe three or four) for more than a few seconds the engine will surge or miss a bit. It does not seem to loose power it just makes a different engine sound than the normal rhythm. I have found that more aggressively leaning my mixture before I start the high G maneuvers reduces this disruption in the engine sound.

Jake

Yep, noticed the same thing.

[Arjay]

Regarding the over rich condition on pulling hi g, my mechanic (an experienced airshow pilot) had to make an emergency landing on a road a few weeks ago when the engine quit and he was unable to restart it. He made about a 4 g pull up from straight and level and the engine just quit. Since then he has experimented and found he can mostly keep the engine running by leaning aggressively just before the hi g maneuver as well as continuing to lean throughout the maneuver. He did have the engine quit, even doing this, but was able to restart with the starter by using a hot start technique (mixture to idle cutoff, throttle advanced, push starter button and then bring the mixture to rich).

This was in a sonex tail dragger with aerovee and aerocarb with no air filter. The fuel vent comes out the top of the cowl and faces forward.

Would love to hear anyone else's experiences, inputs, thoughts.

[lutorm]

Over the last couple of days I've been working to get the AeroCarb adjusted for the custom intake plenum I've written about in the Aerovee forum. My conclusion is that there is something very strange about the tuning instructions, which as has been mentioned before here says to make sure you "have a 90F EGT rise between full rich and the point where the engine just starts running rough", which they call "peak lean".

If we interpret "peak lean" as peak EGT, this instruction is wildly inconsistent with any other sources I've found which say that "full rich" usually is 250F rich of peak and that the "red box" where the engine should not be operated extends to 200F ROP for powers over 80%. "Best power" mixture is at 150F ROP, which is too lean to run at full throttle. 90F ROP would put the engine way lean for full throttle, pretty much in the worst possible spot.

The only way to reconcile this contradiction is if what Sonex calls "peak lean" is nowhere near actual peak EGT for most cylinders. That might be pretty true given how bad the mixture distribution is with the stock intake, so the leanest cylinder would start to lean misfire while the richest ones are still many hundreds of F ROP? But if that's the case, which EGT do they want to increase by 90F? If a cylinder is so lean so as to misfire, it will be lean of peak and its EGT dropping.

With my custom intake, however, the mixture distribution is much improved and all the EGTs more or less go up in unison and are much closer together. The highest EGTs I've seen during leaning is 760C (1400F) so that would put a desired "full rich" mixture somewhere around 1150F (620C). The peak EGTs happened at a fuel flow of around 25L/h (6.6gph) and I've found that to get the EGTs down to 620C at full throttle I have to get the fuel flow past 32L/h (8.4gph). This seems like a higher fuel flow than what other people are reporting, but seems confirmed by the fact that I can pick up some RPM by leaning a bit and that CHTs are noticeably lower at fuel flow > 30L/h compared than down around 25L/h. I'm sure the intake system makes the engine make more power than stock, since it picked up about 250 static RPM with the change (3240-3260 now) so that may explain why the fuel flow would be a bit higher, but it seemed people were mentioning fuel flows of around 5gph WOT as being on the rich side. I just don't see how that is possible. 80hp at 5gph would be a BSFC worthy of a Toyota Prius, not a 50 year old engine design at ful power mixture.

I also found that adjusting the needle seems to affect the idle side much more than the WOT flow. I turned the needle by half a turn and there was no effect on the WOT fuel flow, but it wont from pig rich at idle to too lean to run.