Now that the Aerovee seems to run OK, I won’t have time to work much more on it for a while. However, as part of trying to get some full-throttle time on it, I did complete a bunch of runs with different mixtures to see what the mixture distribution looks like now.
These tests consist of running the engine at full throttle until the exhaust gas temperatures stabilize (which happens to take about as long as the cylinder head temperatures hitting redline). By doing this for different fuel flows (by changing the mixture lever between each run) and noting at which fuel flow the EGTs on the different cylinders peak, you get an indication of what mixture the cylinders are running at.
In general you do not want to run the engine at full throttle with the mixture anywhere near the peak EGT, since the heat stress and detonation margin is the smallest at this setting, but this is unfortunately the only way to get a real handle on what fuel flow we should tune for at full throttle. Since this is only for a minute at a time, and only a few minutes in total, I doubt it’ll have any adverse effects.
It takes quite a bit of time to collect these data, plot them, and read off the values. (Ideally you would do this automatically, but you need a way of selecting only the points where the temperatures and fuel flows have reached steady state. Since we’re not going to be doing this a lot, manual data collection had to suffice.)
(Apparently WordPress doesn’t insert normal figure captions for SVG’s… Oh well.) The EGT’s are shown as a function of fuel flow at full throttle. The RPM has an arbitrary offset to just show how the max RPM varies with mixture. The dashed black lines indicate the fuel flows at which the richest and leanest cylinder EGT peaks.
While there is a fair amount of noise both in the fuel flow and EGT measurements, it appears #1 is richest, peaking at 22 liters/h. #3 and #4 are quite close and appear to peak around 23-24l/h. #2 is leanest, peaking already at 25.5 l/h. This is a spread of 3.5l/h or, as a fraction, about 15%. This should translate directly into a difference in the operating lambda of the cylinders, so if #1 is running at max power mixture which typically is something like λ=0.8, #2 would run at λ=0.95, perilously lean for full power. On the other hand, if #2 runs at λ=0.8, #1 would be more like λ=0.65, which is pig rich.
Another way of looking at it is that full rich power mixture in airplanes is typically set to be at “200 ROP”, meaning a mixture on the rich side of the peak EGT fuel flow such that the EGTs are 200F below their peak values. 200F is about 110C, so let’s try this.
#1 EGT peaks at 775C, so “200 ROP” would be at an EGT of 665C, which comes out to a fuel flow of about 29l/h. #2, on the other hand, peaks at 715C, so “200 ROP” would be 605C which is off the plot but appears to be about 35l/h.
It’s noticeable in the above plot that the #2 line, which peaks at the highest fuel flow, also seems to have a wider peak than the others. I tried plotting each of the EGTs as a function of the fuel flow normalized to the fuel flow that gives the peak EGT for that cylinder. Since peak EGT basically is stoichiometric mixture, or λ=1, this should be a proportional to the lambda of that cylinder (more precisely proportional to 1/λ.) If you do that, and normalize all the EGTs to their respective peak values, you get this:So the shapes are actually quite similar, with the notable exception that the #2 cylinder stays hot on the lean side for longer than the others. I believe this has to do with the RPM, when the leanest cylinder starts going lean, the RPM does not drop much, if at all, because its lower power is offset by the fact that the other, richer, cylinders are increasing their power. Once the richest cylinder has peaked and all cylinders are operating lean, further leaning will decrease power output quite rapidly and the RPM will drop. Since higher RPM in general means higher EGT’s, this would tend to keep the EGT for #2 on the lean side higher than for #4 which drops very steeply. On the rich side of the peak, the curves have a remarkably similar slope.
In this view, “200 ROP” would correspond to a fuel flow of about 1.32 times that of peak, or λ=1/1.32=0.76. That’s very rich, but full rich is supposed to be very rich to provide detonation margins on climbout. Best power mixture in aircraft is generally assumed to be “140 ROP”, or about -80C on the plot above. This corresponds to a fuel flow of 1.26 or λ=1/1.26=0.79. That’s a bit richer than the 12.5:1 or λ=0.85 that’s often stated when tuning cars, but pretty close.
So where does that leave us? Running the leanest cylinder 200ROP, at 35l/h, is so very rich for the other cylinders that I think we have to compromise on that. 32.5l/h will run the leanest cylinder at at about best power, so we should stay on the rich side of that.
It’s also worth testing if closing the throttle just a little bit, which tends to increase turbulence in the intake, can help even out the mixture. I did some testing at partial throttle and in that situation the mixture seems a lot more uniform. A more turbulent flow in the intake, as well as the lower pressure in the plenum, will help atomize the fuel, so that’s expected. But I’ll leave investigations of that until the plane is actually flying again.