knock detection Lycoming

[Ian]

On 10/10/2025 at 6:01 AM, turboplanner said:

With the standard Lycoming engine Lycoming's dyno-test, reliability tests save you a lot of experimenting, but it sounds like you are tweaking. 

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.

Edited Saturday at 11:05 PM by Ian

[onetrack]

One of the main problems with water injection, is simply corrosion. Modern materials can be chosen to eliminate corrosion in the water reservoir and injection plumbing, but residual moisture in cylinder bores is a big no-no.

The water injection system must be set up so that water injection ceases long before the engine is shut down, to remove all traces of water from the combustion chambers and intake.

 

In addition, water injection creates severe corrosion of the exhaust system components, so these components must be manufactured from corrosion-resistant materials. A lot of stainless steel is not as corrosion-resistant as it's made out to be.

There's also a need to ensure that the water used is totally free from minerals, additives and chemicals that adversely affect the combustion process, and which may facilitate electrolysis.

This pure water supply must be available on a regular basis, and anywhere you might travel to. Not an easy thing to do.

 

Finally, water is heavy, and water injection uses a fairly substantial volume of water. In a light aircraft, that's a un-needed weight penalty.

[RFguy]

Back on the bad AVGAS storey .

Back when  we used to refine fuels locally. Where does out AVGAS come from these days - Singapore / Malaysia / Indonesia ?

 

of course the killer there is a twin engined avgas  aircraft loses its engine redundancy. you'd be better off in a Cirrus with a parachute over the tiger country

 

I've never ever had what I consider 'bad MOGAS'/ not ever. but I am picky with how long it is left, how subject to evaporation rate etc it is, not hanging onto winter mogas in summer (it has different vapour pressures in some regions) ..

I'm actually looking for a Comanche and intentionally looking for one that has a 8.5:1 MOGAS approved capable engine  (a 250'), rather than the later high compression types (8.7, 8.9)

 

Edited Saturday at 11:27 PM by RFguy

[facthunter]

The Combustion Process itself PRODUCES  water. LOTS of it.  Sulphur in the fuel makes sulphuric acid. Too hot a spark plug's heat range  OR a piece of incandescent Carbon will cause preignition. When detonation OR Preignition occur the CHT's rise rapidly.  to make the situation worse.  Nev

[Ian]

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.  

[Ian]

5 minutes ago, facthunter said:

The Combustion Process itself PRODUCES  water. LOTS of it.  Sulphur in the fuel makes sulphuric acid. Too hot a spark plug's heat range  OR a piece of incandescent Carbon will cause preignition. When detonation OR Preignition occur the CHT's rise rapidly.  to make the situation worse.  Nev

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.

[facthunter]

The RR Dart engines used Water Methanol Mix for every normal take off. Nev

[facthunter]

The Lower (to  10 PPM ) sulphur levels WILL be Implemented HERE by December this year for Auto fuels.   The AVGAS issue was from Mobil's Altona refinery where something went horribly wrong and Caustic soda (NaOH) ended up in the final Product damaging some aircrafts fuel systems so badly some never flew again. Mogas has little security as to Protection from "SOMEONE" adding anything before it gets to You, AVGAS and AVTUR JP4 etc are covered by release notes in either sealed Drums or from a Recognised Fuel Organisation via  a Fuel truck specially equipped .  In MY Own Vehicles I have had 2 instances where  I didn't get to where I wanted to go and I recalled where I got the fuel from Both times and  it turned out Many Locals had had a similar experience it seemed.   This fuel was being doctored adding stuff not subject to Fuel tax. Never buy fuel from a Place that can flood OR where the turnover is not large. You wouldn't like to see what is inside some of those below ground storage tanks. Nev

[onetrack]

No, AVGAS is not a low-sulphur fuel. 

 

Quote

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.

 

The following is a cut and paste from a Quora contribution by one Paul Montgomery, a retired engineer.

 

QUOTE: "I built a steam injection system for a 390 Ford engine back in the 1980s before current technology was employed and got a 3 to 5 mile per gallon improvement on a truck that got 10 miles to the gallon. This is roughly 25% of fuel savings which are substantial for those days. I did a lot of tinkering with this. I heated the water by wrapping an exhaust header with copper tubing. This was a huge improvement over cold water injection as it decreased the amount of water I needed to inject, and caused less damage to the exhaust. Still, I used several gallons for each tank of gas. I also manually cut the injection on after the engine warmed up, and before I cut the engine off. Even with this, I still had trouble rotting out the muffler and exhaust pipes. This problem could be eliminated by installing stainless steel exhaust components.

In the end, even after all this work and enthusiasm, I ended up deciding it was better to just pay for the extra gas. This is the reason why you don’t see many water injection systems."

[turboplanner]

9 hours ago, Ian said:

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.

 If you are modifying a factory engine beyond the factory assembly and settings standards, it's called tweaking; you might use a sensor but that's not necessary.

[Ian]

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.

 

[Ian]

9 hours ago, onetrack said:

Even with this, I still had trouble rotting out the muffler and exhaust pipes.

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

[turboplanner]

5 hours ago, Ian said:

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"

That's the point I was making; I don't use instruments to find the new knocking position.

 

 

😉

 

 

 

[facthunter]

Stainless is more prone to cracking.  A shorter exhaust might get more heat to it. Nev

[turboplanner]

9 hours ago, Ian said:

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.

 

That's the point I was making; I don't use instruments to find the new knocking position.

 

[Sorry about the last post, I was trying to reduce the words to the issue.]

[Ian]

So where we are is that 

1. Sensing mechanisms for knock detection are common and low cost
2. Acoustic sensors are probably the simplest approach.
3. No-one is aware of it being done on Lycoming engines.
4. 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.

[facthunter]

The only incidences  that I'm aware of  is when Avtur has been inadvertently mixed with avgas making the Octane rating so low that the engines have destroyed themselves during the take-off. Preignition and detonation can co exist. Either are a rapid heat generating Phenomenon and impose shockloads on the engine. The only way of limiting damage is to reduce the Power dramatically or shut the engine down. IN Normal operations, adhering to the Proper fuel and running stipulated Power(Boost/RPM settings, correct Plugs and Limit head temps you are OK.  A motor that is burning a lot of OIL is also suss as the normal Lube oils reduce the Octane rating, considerably if they are in the Combustion process. Nev