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Adhere to the power settings! Detonation damage in a Rotax 912S3

For those interested in ‘over square’ as an operating guideline, if the 912 were operated at 5200 RPM unthrottled at sea level, this would be equivalent to running a Lycoming at 2425 RPM with circa 29 inches MP. The Rotax would tolerate this better due to small cylinder bore.

With any normally aspirated engine, as altitude rises and available MP decreases the so-called over square condition lessens, it does not worsen, so if the pilot does not pay attention to operating limitations, the chance of detonation is higher at low altitude.

My guess is this piston failure resulted from sporadic long term operation with marginal fuel, with detonation occurring when operating ‘over square’ with a CS prop at high power settings and low altitude.

Last Edited by Silvaire at 10 Nov 15:04

Silvaire wrote:

For those interested in ‘over square’ as an operating guideline,

What do you mean by “oversquare” in this context? It normally means operating an engine with an MP greater than the RPM/100.

ESKC (Uppsala/Sundbro), Sweden

Over square is just a convenient way of describing operation of an engine at low RPM with high manifold pressure, and I used percentage of redline RPM to compare engines on that basis. ‘MP higher than RPM/100’ has no direct technical significance. Other colloquial terms are used elsewhere, e.g. lugging the engine. It doesn’t matter what words you use. The issue for any gasoline engine design is just the time available for fuel to break down and detonate prior to spark ignition, compared with the time required with a given fuel and MP.

Last Edited by Silvaire at 10 Nov 16:09

Silvaire wrote:

‘MP higher than RPM/100’ has no direct technical significance.

Exactly. That’s why I asked.

ESKC (Uppsala/Sundbro), Sweden

CharlieRomeo wrote:

Removal of the cylinder head showed damage to the piston, located at the lower forward quadrant. The cylinder surface showed silvery, non-magnetic deposits in a location matching the piston damage.

Thanks for sharing the story.

You did not comment on the fact, that in the picture you not only see the big piston damage at 4-5 o’clock position, but there is also a small dent with what looks like hard upward bent edges at the opposite side at 10:30 position.
With the additional information that the engine came fresh out of a 400h where the spark plugs have been changed, I would consider and further investigate an alternative explanation to detonations: If a small piece of debris fell into the cylinder during the exchange of the plugs, that moved around for some time and finally got stuck between piston and cylinder wall it could have caused the cylinder to slightly tilt …
A more thorough (microscopic) examination of the dent at 10:30 would show, if this has been there before or only after the cylinder failed.

Germany

Also, would detonation [caused by excess manifold pressure] not normally affect all cylinders? Why would it completely trash one and leave the others without any sign of damage? Or did the other cylinders show similar signs?

Biggin Hill

For what it’s worth, one factor that can cause detonation in a single cylinder is a small oil or coolant leak into the cylinder. There are others, ignition issues etc.

Last Edited by Silvaire at 11 Nov 14:12

Indeed, on the car engines that I’ve worked on for many decades, “knocking” is often heard where valve guides/seals are worn and oil is being ingested.
I’ve often come across engines that have had the ignition timing retarded to combat the knocking without apparent contemplation as to why the knocking exists in the first place.
Do Rotax engines not have knock sensors to prevent detonation?

Forever learning
EGTB

No, the Rotax engines aren’t equipped with knock sensors.

EDLE

Of course they are. One each side of the pilot’s head.🙂

France
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