Ian

So where we are is that Sensing mechanisms for knock detection are common and low cost Acoustic sensors are probably the simplest approach. No-one is aware of it being done on Lycoming engines. There are many mechanisms for controlling knock including mixture & throttle control but other mechanisms requires tweaking ;-). Of interest is the knock frequency calculator https://phormula.com/knock-frequency-calculator/ The frequency for the an 0360 would be about 4.4 kilohertz.

I wonder if this relates more to the condensate of carbonic acid ie CO2 in liquid water in a long exhaust where the gases cool significantly. This could possibly corrode steel exhausts. Are the exhausts of an airplane likely to get to the temperatures where an the steam would form a condensate and be able to form an acid? Probably only during startup. https://content.ampp.org/materials-performance/magazine-article/2613/Some-Aspects-of-Steam-Condensate-Corrosion

Your understanding of the term differs from mine. From the Oxford dictionary definition improve (a mechanism or system) by making fine adjustments to it. "engineers tweak the car's operating systems during the race" Based upon the above I think that adding telemetry doesn't change the system so it's not a tweak. Happy for you to continue to disagree but I'm not buying. 😉 You can take this to extremes and ask yourself if putting a car on a dyno is a tweak? It's temporary external instrumentation.

As I said, both events can ruin your day. Is AVGAS now a low sulfur fuel? I know that sulfur has been stripped from both diesel and automotive fuels.

Exhaust gas is already full of water, which also goes into the crankcase. Water injection doesn't greatly impact this issue and exits the engine as steam along with combustion steam. If you're using water injection during landing there's other issues at play. Yes water is heavy however water injection is only needed across a small fraction of the flight envelope, so the weight penalty is small in relation to the benefits. Cruise and decent don't as a rule require optimal RON fuel. Corrosion of the fuel system was effectively controlled by mixing a small amount of water soluble oil into the mix. I'm not familiar with severe corrosion being caused by water injection in the exhaust and I haven't seen or heard of this being an issue. I've never seen it in cars and it's not mentioned in the STC for water injection in planes. Water is an existing component of exhaust gases equal in quantity to CO2. Basic organic chem says there's about 2 hydrogen for every Carbon atom in hydrocarbons, water injection just puts a bit more steam into the mix. Wherever there's an air conditioner there's a ready supply of clean distilled water, so availability's not that much of an issue. Water from a rainwater tank also has very low levels of electrolytes and you can test it with a simple electrical device. Water will be far more available than lead in the future.

Knock detection is an out of band event sensor. There's no requirement to direct engine control, it's similar to an EGT sensor. I wouldn't characterise it as tweaking, I'd probably characterise it as instrumentation and telemetry.

For those of you who hold the quality control of AVgas in high regard the following article might temper your beliefs. I'm not sure if standards were changed or adopted as a result of this. Trust but verify is a pragmatic approach & knock sensors might be relevant. https://www.atsb.gov.au/publications/2001/sir200103_001

Knock detection allows a safe way to operate closer to detonation limits especially with lower quality or variable fuels, and also could allow for the detection of a bad batch of fuel on the ground with an engine run up. As has been suggested, knock detection goes hand in hand with digital engine controls which on automotive vehicles add a cost of far less than $100 per engine. Digital systems can and do operate engines far more reliably and intelligently than people and the ongoing reliance on manual engine control is a blight on the industry. There is no reason for FADEC on a plane to cost significantly more than automotive systems or to be anything but standard, however we are where we are through well intentioned but poorly implemented policy controls. It's also worth pointing out the knock and preignition are different beasts. Knock is detonation of fuel air mixture where the chemical reaction is propogated by a shockwave rather than a flame front. Preignition is where something other than the spark is igniting the fuel air mixture, such as an overheated exhaust valve. Both can ruin your day, however it's important to understand the distinction. Knock sensors will detect detonation but not preignition (unless the preignition then also creates a detonation event) There are a number of controls that can be used to stop knock. Making the mixture richer Cooling the intake charge Retarding ignition Octane enhancers such as water or Water/methanol Reducing manifold pressure. Running rich of peak is safer for sacks of meat because humans can't effectively juggle the variables in real time in a consistently manner. In short, digital systems have operated engines for the last 40 years lean of peak through all phases of operation far more safely, reliably, efficiently and at higher power levels than people can. The question is, can we utilise some of the technologies developed in the Automotive space to make flying safer? Knock sensors are a baby step with limited gains however it is an important functional sensor for safe engine operation. For those concerned about the octane ratings of lean vs rich fuel mixes, water injection also can increase the RON by 25 which if implemented would provide significant safety margins for METO operations. It's not new technology, the Germans in WW2 used it effectively to compensate for their poor quality fuels. It's also available, far less toxic and cheaper than Tetra-ethyl Lead. It's also cheaper to retrofit than an engine rebuild. https://pmc.ncbi.nlm.nih.gov/articles/PMC6389520/

Yes however I'd expect direct injection as a retrofit to be at least an order of magnitude more difficult to develop and install on existing aircraft. Is backfiring a common damage mechanism on existing aviation powerplants?

You're absolutely on the money here. Virtually every petrol vehicle that's on the road today detects knock and retards the ignition, however as airplane engines don't have the strict emissions targets of their land based brethren, enriching the mixture or even water/methanol injection can assisting in controling knock. In the experimental world there are a number of providers selling electronic ignition and fuel injection solutions. But none of them appear to have knock sensing logic. Digital systems do a much better job than people in ensuring that engines operate using the best possible parameters be it maximising power, minimising fuel flow and keeping engine temperatures sane. Given the low cost commodity nature of these systems it seems insane that there isn't more development in this area. I can't see a long or even medium term future for leaded fuel, even without a national approach, it's simply too toxic to withstand a concerted effort from a concerned local community worrying that their children are being brain damaged by the folly of a few people who love to fly. Even though the real reason is that they simply don't like the noise. https://pmc.ncbi.nlm.nih.gov/articles/PMC3230438/ I suspect that part of the change will involve a significant push towards mogas as the standard fuel for the GA fleet as High octane lead free fuels will not be cheap. Many flying schools operate today on Mogas due to the economics. To mitigate the risk of fuels with less stringent quality controls electronics appear to be the simplest and safest solution.

Don't mention the war.

Amen

It doesn't matter if you're not convinced. The fact remains underground HVAC is a shitty solution. As stated, the main reason for underground cables is voters and very occasionally safety related issues such as proximity to a runway. Would the subterranean power lines have the (a) same (b) lower (c) higher maintenance cost ? Generally lower maintence costs however things like active soils etc can reduce lifespan. But this is dwarfed by installation costs. I suspect that the service life of underground cables is significantly longer than for aerial. The same line again the pipelines carrying the cables would have a very long service life. The service life of underground power lines is less than above ground. Generally about 40 years for below ground and 60 years above ground but technologies may change this. The service life is lower because it's a harsher environment, water ingress, salts, ground movement etc. The other differences is that you are likely to be able to increase the capacity of above ground powerlines at a later date. This isn't an option with below ground. If you don't believe the service life argument ask a farmer if fenceposts last longer above ground or below ground. Below ground power lines are technically an inferior solution. For example most power lines are heat limited, heat dissipation is worse underground as the ground acts as an insulator. You also get higher capacitance leading to higher power losses in operation. So you lose more power per km underground than above ground. In short underground power-lines cost many times more, don't last as long, can't be uprated, have higher power losses and are heat limited. If you're not convinced, do the research. https://shop.elsevier.com/books/transmission-and-distribution-electrical-engineering/bayliss/978-0-08-096912-1 Note the safety related runway context to make it aviation related 😉

But the question remains, does anyone do it for Lycoming engines? From a safety point of view it would be nice to know when the engine is misbaving. I used to have a Subaru B4 which was designed for 100 octane fuel and detuned for the Australian market. Bad batches of fuel occurred on a regular basis from all the fuel majors and it would ping badly when you accelerated. The "fix" was putting a few litres of ethanol into the fuel which was even easier when E85 became available.

Not all knock sensors are so pricey, acoustic and ion sense are reasonable cheap. Ion sense is build into mazda coil on plug assemblies. https://www.repco.com.au/clearance/fuelmiser-knock-sensor-cks268/p/A5385939?rgfeed=true&cid=google-shopping&utm_source=google&utm_medium=cpc&srsltid=AfmBOoqFnb7sk-BUKrs6KvjrYySLBj1HwytQ-coYDxRczNsUUhGzBPYWcHg&region_id=100001