The original posts are from 2012.
There is no meaning in “over square”. The units of MP and RPM ae totally unrelated. You could measure MP in PSI or RPM in the proper engineering way (radians per second 
). Not using over-square is a totally dud concept.
What you can have is an engine operating limitation to not exceed a particular torque below some particular RPM. For example the IO540-C4 engine has a limit on MP in certain RPM ranges (the shaded area)
It has been speculated this is due to stress on the prop end of the crankshaft.
Another factor allowing modern engines to operate with high compression ratios and specific power outputs is knock sensors and electronic engine management. My helicopter engine, de-rated/de-tuned by 50%, still produces 106 bhp per litre on 91 UL. It would run on premium automobile gasoline, but I prefer not to use pump gas in aircraft. And yes, I “trust” my simple old Lycoming a lot more than the Suzuki.
Silvaire wrote:
Detonation is encouraged by all of the following factors: High compression ratio Large bore cylinders Simplicity – 2V per cylinder Potential full power operation with one dead mag/plug Supercharging Low engine speed Air cooling
To add to this list your could add fixed timing, and the lack on any antiknock sensor device to adjust timing if detonation starts to occur.
Running over square, or a slower RPM retards the timing possibly increasing the risk of detonation, because the charge is more compressed before ignition occurs fully. I.e. it increasing the peak cylinder pressure.
Does the
95RON pump fuel running compressions of 13.5:1.have an antiknock sensor?
Yes I thought the same – there are plenty of oversquare settings in the POH and nothing at all prohibiting or even discouraging them.
Perhaps the mantra is a hangover from the days of big radials?
The point that hasn’t been raised is ignition timing. The earlier the spark the less opportunity for detonation, surely? I have recently been tuning a modified car engine which needs a lot of spark advance to run well, and I have no idea where the timing is for a Lyco/Conti GA engine.
Pilot DAR wrote:
This is why “over square” operation is discouraged or prohibited for most engines with constant speed propellers.
Is it really? Every aircraft with a CS prop that I’ve flown has had “oversquare” settings in the power setting table in the POH, and all of them have flown with less vibration, quieter, and with lower fuel consumption using these “oversquare” settings. Many supercharged engines run a good foot oversquare by design!
There’s also things like this: https://www.experimentalaircraft.info/flight-planning/aircraft-engine-avweb.php
Very interesting thread! May I add a question here? My engine is turbocharged and liquid cooled and the climb settings per POH are 2500 rpm and 37.5 inches.
On hot / high DA days, towards the top of the climb, I need to reduce manifold pressure to keep inlet air temperature at an acceptable value.
I have found out that 2600 RPM and 36.5 inches deliver more or less the same power but with lower inlet air temperature. The engine sounds happy and inlet air temperature drops by a few degrees F – enough for me to complete the climb without reducing climb rate.
Given the fact that the engine is certified for 5 minutes at this level of rpm AND 39.5 inches, can you see any disadvantage to my trick?
Interestingly, while I was detonation testing the 520, I found that having achieved detonation speeding up the engine (fine the prop) and pulling the power back quickly did not immediately end the detonation, it continued out of my control until the engine cooled a little.
Yes, very interesting about the continued effect of overheating on detonation...
why is it that aircraft engines can only control detonation by using 100+ octane fuel, when they have such (relatively speaking) low compression ratios?
Detonation is encouraged by all of the following factors:
All those factors (in addition to CR) require higher octane fuel to prevent detonation but also produce engines which are some combination of lighter, simpler, and/or more efficient. That's why the fuel was developed and why engines were designed around it.
That said, many of the simpler lower powered aviation engines (virtually everything 150 HP and below) were developed for 80/87 octane fuel and accordingly don't need 100 octane or even close.
The highest compression GA engine I know of is 10.5:1, with the most common being 8.5:1, and really low 7:1. I have done instrumented detonation testing on an 8.5:1 Continental O-520. I did cause and measure detonation for brief periods, which was required to validate my measuring equipment.
To create measurable (though happily not damaging) detonation in this engine, I operated it as follows on several separate occasions: Oil and cylinder head temps at maximum (by baffling), RPM 2000 to 2100, MP full throttle, fuel used: 80/87 Avgas. My test conditions to demonstrate compliance were 2 inches of manifold pressure over square relative to the RPM selected, same high temps, and 91 AKI mogas. There was no detonation.
100LL is the default "certified" aviation gasoline, because some aircraft require it for octane, though other aircraft for other characteristics. The Avgas manufacturers just will no longer make two grades of Avgas for a tiny market. Back when I research such things, Avgas was 0.25% of gasoline production, with Mogas being all the rest. Small wonder Avgas is priced and distributed as a specialty product.
100LL is very certainly a compromise gasoline, but compromised high, to keep the big pistons flying. It has other differences which are less in the forefront, with volatility (and no seasonal change being a big one) compared to Mogas.
The 13:1 engines which operate well on 95AKI are probably liquid cooled? Liquid cooled engines operate with more stable temperatures in the combustion chambers, which is a big factor in preventing detonation. Air cooled aircraft engines are terrible for uneven temperatures in local areas, which can reduce detonation margins.
Like car engines, aircraft engines are best operated to prevent detonation by keeping the RPM faster, and power lower. This is why "over square" operation is discouraged or prohibited for most engines with constant speed propellers. Very simply, detonation takes time to happen. The fuel air charge will only self ignite if the conditions are right, and there is time. Time is directly related to RPM - faster RPM, less time between combustion strokes. Less time between combustion strokes, less time for detonation to occur. If you can normally ignite the fuel air charge before it can detonate, you still have normal combustion, ans all is well.
Detonation has two harmful elements: Very high combustion pressures at the wrong time - explosion going down meets piston coming up = lots of force. The other is that to origin of the detonation (original ignition point) could be at a place in the cylinder other than the spark plug, where it is intended. In such a case, the flame front can 'wash" across combustion chamber surfaces in a path other than that intended. When that happens, those surfaces can be subjected to flame much hotter than intended. A flame which moves out evenly, cools at the front as it goes, so it does not subject the surfaces it contacts to the most intense temps. A washing flame can have a very different flame front, and local areas can get really hot. Extra hot, and early extra pressure can blow holes in pistons.
Interestingly, while I was detonation testing the 520, I found that having achieved detonation speeding up the engine (fine the prop) and pulling the power back quickly did not immediately end the detonation, it continued out of my control until the engine cooled a little.
Do not operate aircraft which are not approved so, on Mogas. Detonation is only one reason. Vapour lock due to the wrong volatility is a much greater risk. It too, can be mitigated, but if it goes bad, it'll be at the very worst time, and the consequences will be total.
You'll hear that one of the reasons AVGAS must be used is that it's higher octane rating is essential to stop detonation in aircraft engines. I've never really bought into this argument (that's not to say it's wrong!)
Aircraft engines aren't incredibly high performance engines in terms of specific power output. In terms of bhp/litre, they are compete on a par with garden lawnmowers (approx 30bhp/litre), with the highest compression ratio's that I have seen being approx 8:1.
Many other reciprocating piston engines these days operate perfectly well on 95RON pump fuel running compressions of 13.5:1.
So why is it that aircraft engines can only control detonation by using 100+ octane fuel, when they have such (relatively speaking) low compression ratios?
