- Apr 30, 2019
Hi pfarber - thanks again for a good discussion. And thank you for correcting me regarding the number of engines certified for auto fuel!Convince me there is a need for 100LL other than some outdated 'detonation' theory.
Unfortunately, detonation is still a real thing. Even with our relatively low compression engines. Why? Because, all things are not always perfect, especially in the experimental world.
A cylinder is running hotter than you'd like, but you're flying anyway. You inadvertently leave the carb heat on during a go around. You have everything needed for detonation.
You have a clogged air bleed nozzle (fuel injector). You have everything you need for detonation.
A spark plug fails, causing a hot spot. You have everything you need for detonation.
You are running lean at high power settings. You may have everything you need for detonation.
Detonation certainly does happen in aircraft engines. See https://www.lycoming.com/node/17607, or https://www.airmarkoverhaul.com/detonation-pre-ignition/ or https://www.savvyanalysis.com/articles/detonation-and-pre-ignition, or (my favorite)
Which makes several of pfarber's points, and many more. It's a good read, and a very good discussion of detonation (and pre-ignition).
Given that I race my airplane, and regularly run at fairly high power settings, yup, I should be concerned, or at least aware, of detonation. Even with my "low" compression IO-360 and 100LL. I'd argue you should be too.
Which (if you'll allow), get's us back to the original discussion - 100LL and O2 sensors (and from there, a closed loop system). Given the real world things that can (and do) happen to our engines, to really get the kind of performance that's been discussed here, detonation has to be considered and avoided. That's why I argued (and argue) that a knock sensor is part of the solution. The problem is, our engines are so "noisy" that getting a knock sensor to work is a very difficult problem. Given the small market (as has been pointed out), getting people other than experimenters to invest in development of a true closed loop system, with all of the appropriate safeguards, is unlikely.
Thanks again -