knock detection Lycoming

[Ian]

Hi All,

I was wondering what the general consensus was with knock detection on lycoming engines was.

Knock detection is standard fare in auto engines, allowing advancing and retarding ignition based upon knock. The cylinders ring like a bell with the sound be loudest at the fundamental or harmonic frequency based around the diameter of the cylinder, knock detection is largely a microphone.

Another approach is to test current using the sparkplug post ignition. This is used by Mazda amongst others and would provide a non-accoustic approach.

 

From what I've read there seems to be a view that aircooled engines are too noisy to detect knock but I haven't actually come any actual source material where someone states this outright.

Lycoming has done laboratory testing where they are detecting knock under various load conditions. If this is acoustic based it might be a reasonable approach.

 

From what I've seen none of the electronic ignition kits for experimental aircraft offer knock detection as a feature, however I could be wrong.

 

Given how destructive knock is to aircraft engines it seem to be quite a desirable feature. Thoughts welcome.

 

[onetrack]

Ian - FYI, there are three separate types of knock sensors fitted to automotive engines.

 

1. Acoustic sensors that use piezoelectric units to detect vibrations in the engine block directly.

2. Ion sensors that measure electrical activity between the spark plug electrodes, post ignition. 

3. Direct combustion pressure sensors that are fitted to the ends of injectors or glowplugs, that measure combustion pressure levels.

 

As far as I understand, acoustic sensors and ion sensors are becoming less common in engine design, and are being superseded by direct combustion pressure sensors.

 

Porsche fitted acoustic sensors to some of their engines, so it's technically feasible for air cooled engines. The argument that air cooled engines are too noisy is not true, because an acoustic sensor on a water-cooled engine must be made more sensitive, as it has to detect vibrations through two thicknesses of metal with water sandwiched between them.

 

https://www.vehicleservicepros.com/service-repair/underhood/article/21174614/making-sense-of-ion-sense-technology

 

There are aftermarket knock sensors available, but they aren't cheap, especially when you consider that you have to install them and wire them up, as well as purchase them.

 

https://bdn-automotive.com/products/pressure-sensors

 

 

Edited September 21 by onetrack

[facthunter]

Good idea to have them. Knock destroys engines. A loose plug can cause it Or a wrong heat range one. Overheated Motor or dud fuel. Too much boost. Incandescant carbon build up etc.  Nev

[sfGnome]

…or when buying a new van, filling it up with ‘standard’ (ok, this was a lot of years ago) because my old van took standard, and then wondering why the engine was rattling so much… 🙄

[facthunter]

Bad way to save Money. The NEW direct Injection PETROL motors don't have that problem.  Nev

[Ian]

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

[Ian]

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.

[facthunter]

As well as the sensor you have to do something about it. Lean mixture can promote it as well as over advanced Ignition timing and LUBE oil. An oil burner can KNOCK prematurely..  Nev

[RFguy]

have done it on Jabs, could do on my O360 I guess. used bosch knock sensor from aliexpress.  you'll need a engine to torture to test

My Kluger SUV advances the timing until it detects knock, and then backs off- how it makes a bit more from ULP98 compared to ULP91. the behaviour is visible in the JBEC connector telemetry.

[Ian]

On 22/09/2025 at 11:12 AM, facthunter said:

As well as the sensor you have to do something about it. Lean mixture can promote it as well as over advanced Ignition timing and LUBE oil. An oil burner can KNOCK prematurely..  Nev

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. 

[facthunter]

Direct INJECTION is another Path. It will also prevent destructive Backfiring. There's NO combustible MIX in the Manifolds to explode. Nev

[Ian]

17 minutes ago, facthunter said:

Direct INJECTION is another Path. It will also prevent destructive Backfiring. There's NO combustible MIX in the Manifolds to explode. Nev

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? 

[facthunter]

More common than generally realised. (poor starting techniques) It's common also with LPG auto engines . Nev

[skippydiesel]

Hi Ian

As you noted above;

Knock detection is only half of the equation,  automatic ignition adjustment is the other half.

Variable fuel injector timing is a further refinement/compliment to the above.

I am sure you realise that you are proposing a system that would, if retrofitted, have only modest benefit for existing powerplants.

To achieve automotive style improvements, would require significant reengineering and a lot of computerisation (fly by wire engine management/control)😈

[facthunter]

Detonation is not a problem generally when engines are operated at the recommended settings and correct fuel. That includes the CORRECT Heat range on the Spark plugs and Making sure they are correctly Tensioned. Observe CHT Limits Boost and Rpm combinations..  Nev

[Thruster88]

Lycoming do have a fully electronic FADEC version of their 540ci 6 cylinder,  the TEO-540-C1A producing 375hp. Two of them are used in the Tecnam p2012 traveller. Not sure why thy don't have this option on the 4 cylinder engines.  

[onetrack]

Quote

Not sure why thy don't have this option on the 4 cylinder engines

Possibly due to cost? All the additional componentry in a FADEC system on a smaller engine, would push that small engine cost up substantially.

[facthunter]

Close Coupled Cowls on those 540s Can cook the engines. Cowl gills are Manually operated generally.  Nev

[Student Pilot]

When everything old is new again? Seems a lot of problems of this subject has solved many times over. Fadec is talked about in newer aircraft, the FW190 from a couple of years ago (80) had a single lever throttle/propellor. The mighty Constellation had a very complicated engineers panel that involved all sorts of trickery to delay detonation on the Wright compounds. Water injection was involved in a lot of WW2 aircraft to delay detonation at higher power settings. Even lean of peak is not a new thing with Lindberg teaching rookie pilots how to get more range from Pratt 2800's in Corsairs during the pacific battles.

[facthunter]

Lean of Peak is not the way to go if you want engine reliability in the long term. AVGAS has 2 Octane ratings  The higher One for  RICH and the Other for LEAN,  The  BIG radials ran on avgas 130-145.There's one as 100-130. I doubt these days you can get the Higher ones at all. All those motors will have to run De-Rated. ( with Less Boost).  The DC-6b Had an onboard Ignition analyser (under a hood) for the P&W R2800's for the Flight engineer to Monitor. The Constellation Often referred to as the Consternation and the Boeing strato Cruiser were Notorious for Engines being shut down in Flight. They were the Big Wrights which were  also fitted to the Neptunes. Nev

[RFguy]

I fly at peak EGT, but always at  <= 70% of max fuel flow . Usually at 65% ~ 8.1 gph.

..chambers and valves look good. (mostly ULP98)

Lycoming have a chart. for lean, you need to stay below the kink , IE <= ~ 72%

many people fly peak EGT at 75%... probably the limit, especially if its hot..

 

 

 

 

[turboplanner]

On 22/09/2025 at 8:17 AM, Ian said:

Hi All,

I was wondering what the general consensus was with knock detection on lycoming engines was.

Knock detection is standard fare in auto engines, allowing advancing and retarding ignition based upon knock. The cylinders ring like a bell with the sound be loudest at the fundamental or harmonic frequency based around the diameter of the cylinder, knock detection is largely a microphone.

Another approach is to test current using the sparkplug post ignition. This is used by Mazda amongst others and would provide a non-accoustic approach.

 

From what I've read there seems to be a view that aircooled engines are too noisy to detect knock but I haven't actually come any actual source material where someone states this outright.

Lycoming has done laboratory testing where they are detecting knock under various load conditions. If this is acoustic based it might be a reasonable approach.

 

From what I've seen none of the electronic ignition kits for experimental aircraft offer knock detection as a feature, however I could be wrong.

 

Given how destructive knock is to aircraft engines it seem to be quite a desirable feature. Thoughts welcome.

 

1920'a cars had variable distributor timing, with a lever on the dash or steering wheel, connected to one side of the distributor. This was to protect your hand from kick-back when you were crank starting. You'd start the engine retarded and then move the lever to advance the distributor. You could feel the power curve from the seat of the pants - increasing as you advanced the ignition to the point where the power started to drop off just before knock. After a while you knew the sweet spot and didn't go further.

 

 When building race engines, where I've shaved the head for  higher compression and converted the fuel system for  Methanol A, the ignition timing will be quite different, and I "feel" for the end of the power curve and the beginning of that slight drop; a dyno solves the need for road testing.

 

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

[facthunter]

It's not only too much advance causing problems. It can be too hot a plug  or an overheated exhaust valve or crook fuel or too much boost. I've never experienced it  personally  in any aircraft  though I've had engine failures for other reasons. Most aero engines run fairly Low compression ratio's and have a limit on how long TO power can be used for. METO is MAX Except Take Off. With certified motors and good  cowling and management I'd suggest it's not a Large issue. A richer Mixture can alleviate it. The damage is typically piston ring land breakage and Piston crown caving in. I've never seen that with an aero engine. Detonation is Hard on Motors. Any diesel added will surely kill a motor during take off It happened when Turbo charged Motors were Introduced. Some fuellers thought TURBO Meant Turbine. Nev

[Ian]

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/

 

 

[Ian]

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