The big AeroCarb/AeroInjector thread

[NWade]

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%.

I interpret the tuning instructions as saying that they want you to run the engine with the mixture knob pushed in to the full rich position. Then lean it out until the engine starts to run rough and check to make sure the peak EGT you see during that leaning operation is at least 90 degrees hotter than what you started with (at full rich mixture).

Basically, they want to make sure that your engine is running rich-enough at its richest setting, and 90-degrees of EGT difference is a minimum safe amount of "room" between full-rich and peak-EGT.

NOTE: An engine may not run rough until it gets past peak-EGT, so you want to look for the highest recorded value over that span; not necessarily the EGT you see at the leanest possible setting.

NOTE 2: EGT gauges don't respond totally instantaneously, so I'd do the mixture pull fairly slowly, to ensure that the gauges have a chance to record good values.

My $0.02,

--Noel
Sonex #1339

[lutorm]

I interpret the tuning instructions as saying that they want you to run the engine with the mixture knob pushed in to the full rich position. Then lean it out until the engine starts to run rough and check to make sure the peak EGT you see during that leaning operation is at least 90 degrees hotter than what you started with (at full rich mixture).

Basically, they want to make sure that your engine is running rich-enough at its richest setting, and 90-degrees of EGT difference is a minimum safe amount of "room" between full-rich and peak-EGT.

But that's my point: look at any reference (for example Figure 3 in Mike Bush's article in Sport Aviation) and you'll find universal agreement that 90-degrees rich of peak is exactly where you do NOT want to operate the engine. You need >200F-250F from peak EGT at WOT.

After doing some tuning test runs with the EGT's in that area, I took a look inside the engine with a borescope and the piston tops were full of little shiny marks reminiscent of meteor craters on the Moon. I think these are detonation marks and confirms that running 90F ROP is not a good place to be.

[SonexN76ET]

Those spots on the top of your pistons are probably lead deposits.

I would really caution you about operations contrary to the recommendations of Sonex.

It will be no fun for you if after liftoff you aircraft fails to climb because it is bogged down by an overly rich mixture. Every little modification and change you make will end up with you acting as a test pilot in an unknown area of your plane’s operating envelope.

Mike Bush has experience in running a maintenance management company, not designing and building aircraft engines. His advice often runs counter to the recommendations of major certified aircraft and engine manufacturers. He has no expressed experience with experimental or LSA engines. I would take his advice with a grain of salt. I take Kerry and the Sonex team’s advice as gospel.

Jake

[SonexN76ET]

If you are truly worried about detonation when running your engine, you should check your timing and also that you properly set your compression ratio. Be aware that when you remove your cylinder heads that you need to put on new copper cylinder head gaskets that are of the same thickness as you had previously as that also affects compression ratio.

Jake

[lutorm]

Mike Bush has experience in running a maintenance management company, not designing and building aircraft engines. His advice often runs counter to the recommendations of major certified aircraft and engine manufacturers. He has no expressed experience with experimental or LSA engines. I would take his advice with a grain of salt. I take Kerry and the Sonex team’s advice as gospel.

I tend to take everyone's advice with a grain of salt, but seeing as Mike's, GAMI's, etc agree with my understanding of physics (which does not change based on the certification state of the engine), while the Sonex team has not provided any physical motivation for their procedure and designed the stock intake with its atrociously bad mixture distribution in the first place, I'm not inclined to believe Sonex over the others.

Ultimately I'll believe the data, which we should get in abundance in the air, and then the verdict should become pretty clear. "Education and recreation", that's what it's about, isn't it?

[gammaxy]

On my engine/propeller/airplane, I have about 100F between roughness and full rich when flying WOT at max RPM in level flight. In most other conditions the spread is considerably greater. The mixture tends to be leaner in flight for the same throttle position and mixture setting due the engine being able to spin faster and thus breathe more air for the same amount of fuel. On the ground and in climbout I probably get more than 200F spread at the same throttle position due to the richer mixture.

I'm very doubtful you are experiencing detonation as long as you are running 100LL and have the ignition advance set correctly. I believe the higher RPMs we turn also tend to provide more margin.

When you get a chance, I'm interested in hearing more about the 250 rpm you picked up. If repeatable, that's pretty significant. When I read your blog previously, I interpreted this statement as meaning you got 70rpm:

Static RPM with the plenum was about 3220 while the others were around 3150, so that’s a good sign; whatever else is going on, the engine is making more power than it used to.

50-70rpm is in the ballpark of the improvement I would have expected. 250rpm is really incredible.

[NWade]

I tend to be a Mike Busch devotee, but I will point out that he does specifically state that he doesn't have experience with Experimental engines. Also, IIRC its 50-degrees ROP where he shows the highest stresses occur.

For me, I take the 90 degree rise as a *minimum* in the Sonex manual. They just want to ensure you to have _some_ leaning capability when you pull that mixture cable.

--Noel

[lutorm]

When you get a chance, I'm interested in hearing more about the 250 rpm you picked up. If repeatable, that's pretty significant. When I read your blog previously, I interpreted this statement as meaning you got 70rpm:

Static RPM with the plenum was about 3220 while the others were around 3150, so that’s a good sign; whatever else is going on, the engine is making more power than it used to.

50-70rpm is in the ballpark of the improvement I would have expected. 250rpm is really incredible.

Yeah, I don't have a great amount of data on the unmodified engine simply because the rear cylinders were running so lean it would overheat with 15s or so at full throttle on the ground. I just went back and looked at the two test runs I do have from back in June; they both have a 25.6L/h fuel flow and max static RPMs of 3040 and 3150. Max static now is 3240 - 3260, after tuning the carb but also fixing the stripped and leaky spark plugs... So, to get a real control I would have to put the stock intake back on, which I'm not enough of a scientist to do... ;-) So yeah, I don't know where I got 250, that must be a typo. It's somewhere between 70 - 210 rpm. There are probably differences from the wind and temperature when the tests were made, too.

I'm happy that it picked up some power, but the reason we embarked on this experiment was to try to even out the mixture distribution so we can run it at full power without overheating the back cylinders. In the ground runs now, the back cylinders heat up at about 2/3 the rate they did with the stock intake. Hopefully we''ll also be able to lean much better at cruise power.

[Onex107]

Don't be in a hurry to label "shiny spots on the piston" detonation. After running at a rich setting the piston is coated with a rather thick layer of carbon, and the shiny spot is where it has flaked off and the new piston is visible. The plugs are the clue. If the plugs are sooty, so is the piston. Carburetor engines are not capable of running lean of peak due to the uneven fuel distribution and when you get close to peak, the engine will run rough because one or two cylinders are dropping out. Certified engines that are capable of lean of peak are fuel injected with balanced fuel injectors matched to each cylinder. When one of them goes lean of peak, the engine is still smooth, just using less fuel and making less power. In my case, the hottest cylinder, highest EGT, is the leanest, and that is the one I have adjust to. Until one of you comes up with a new intake manifold and better fuel atomization.

[sonex1374]

Be careful using Lycoming and Continental recommendations on non-Lycoming and Continental engines. Every design is different, and the numbers don't transfer over directly. Think about Rotax 2 strokes and how different their CHT/EGT recommendations are. Using anything from the 4-stroke world on them will kill them quickly. The concepts still apply, such as max stress on the engine being at some point rich of peak mixture, but where that point is, and how to identify it will vary greatly.

Sonex has been flying VW's for a long time, and they have no interest is misleading anyone. When they make a recommendation, I tend to think it's fairly well thought out.

As to adjusting the AeroCarb, I've come to realize that if the mixture is set a bit rich by the needle, the pilot can always lean it a bit with the mixture knob (even on climb out) to smooth things out. However, it's impossible to make it richer, so that's the goal of ensuring at least 100 deg temp rise when leaning. It's possible to adjust the needle so rich that the engine just slobbers and stumbles, then leaning makes it progressively better right up to the point the engine dies because you just cut off all the fuel supply. That's also not a good area to operate, so there is a degree of fine tuning required here.

Kerry talked about the tuning process on Episode 7 of SonexFlight, so that might be worth reviewing again for those just joining the conversation.

http://www.sonexflight.com/7/index.html

Jeff