JBrenneise
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Is there such a thing as a kit to add variable ignition timing for GA aircraft?
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By ignition timing, I mean advancing or retarding the spark to compensate for lower octane fuel(s).
I'd like to do some testing with various fuels in the 7, but there was another post mentioning it's almost impossible to find a knock sensor that works in lycomings. Anyone have any info on why knock sensors don't work, or suggestions on how to make them work?
The unleaded fuel available at Reid-Hillview and San Martin is 94 octane and is FAA-certified as safe for use in approximately 68% of the piston-powered aircraft of the type that use the Reid-Hillview and San Martin airports. Larger jet aircraft, such as those operating out of Mineta San José International Airport, use a Kerosene-based fuel that does not contain lead.
Most of you guys are focussing on microphone type knock sensors, and I agree those come with challenges. Not knowing which frequency to listen for can be overcome by using a large range sensor and real-time FFT data analysis. While it varies a little bit based on combustion chamber design, I remember my internal combustion textbook saying its focussed around 3,500Hz for automotive engines. If I run the motor at 2100 rpm, that should give me a mostly noise free zone between 2100 and 4200hz, until the 2/rev vibrations come back into play. As far as mounting the sensor I see 3 options, the primer port which is unused with an SDS system, the CHT port, or one of the 1/4-20 baffle bolts that go through the fins in the head. I would start with a leak-down test to see which cylinder has the best compression, and lean it out a few percent over the other 3 to make it knock first. That would mean I would only have to focus on that 1 cylinder instead of all 4. Using the LOP button with ignition retard instead of advance would allow me to pull massive amounts of timing almost instantly when I do encounter knock.
I think the easiest way to do this would be to remove a spark plug and mount a pressure transducer in its place. We work with this company for a few of our ground test stands, so I might be able to work a deal on a high-temp, 5000psi pressure transducer and amplifier. (https://www.kistler.com/en/solution...n-analysis-and-cylinder-pressure-measurement/) No noise to worry about and I could plot the trace on a battery powered o-scope in the cockpit. The downsides to this approach would be if I did end up losing the other ignition, I would only have 3 cylinders to fall back on. I would be ok with this as I would do all these tests in gliding distance to the runway, so my only massive risk would be losing the other ignition between 1 and 1000 ft. The biggest issues would be 1 vs 2 plug detonation variables.
This would be a ton easier on a dyno stand, but I think I can make it work on the airplane. I'll think about it more after Oshkosk. If the pressure transducers are out of reach price-wise, I can always hook up a good old stethoscope style system and listen to the cylinder through headphones.
We also need to understand that knock sensors in cars are required because they are trying to optimize timing and mixture to a very, very tight standard to meet fleet emissions standards. Our standard is much easier - do not knock. I have shown that the typical Lycoming running typical take off power mixture is very insensitive to timing retard. Put another way, you can back timing off far enough that you will never have to worry about detonation and retain essentially all of your take off performance - and you can do this open loop with a simple lookup table. This capability is available right now, today. Just give SDS a call.
OTOH, if you are really looking for that absolute corner case where you want to ride right on the edge of detonation reliably with a closed loop system, then please let us know what you find out. We will all benefit.
Larry, as far as getting parameters correct, I don't see it being that difficult. Fill one tank with 94 and the other with 100LL. Spend 30 minutes in the pattern to get everything at operating temps and do back to back takeoffs with the 2 fuels. I'm not trying to determine absolute knock margin, which would require having all the temps be super high. I just want to know the relative difference between the 2, which means the parameters only have to be the similar.
My main goal is to find out how 94UL affects detonation margin on an engine....
....If 100LL starts to knock during the takeoff roll at 28 degrees advance, and 94UL starts to knock at 26, then I know I have to pull 2 degrees of timing out whenever I fill up with 94UL....