Hoping there is an engine guru here that can discuss compression ratios on aircraft engines. I'm curious as to why aircraft engines with 8.5:1 ratio need 100LL while auto engines can get into the 10.5:1 range and still run 91.

I've been doing some reading on the subject, trying to stay away from forums and sticking to articles and books, but I'm just a slow learner I guess. I've gathered that the dynamic compression ratio (DCR) is what really determines the required octane rather than static compression ratio (SCR) which is what is listed in most engine specifications. I have no idea how to figure out the DCR of an aircraft engine but I'm not sure I really need to. Since detonation is the real concern maybe it is the fixed timing mags that cause the issue or perhaps it is the low rpm/high power setting when compared to automotive engines.

Ignition mapping. My old CRF450 has a Compression ratio of 12.5 to 1 and I could run 87 in it all day long, revving to the moon!

The guy who bought it from me installed a 13.6:1 piston in it and he says it likes 91 or better. These are essentially tiny versions of formula 1 motors and not the stone-age motors we fly behind.

Ive always thought our magnetos are the bottleneck for progress/performance.

I am waiting or 172Heavy to reply. He is pretty sharp.

Tim

Most of these engines were originally designed to run 80 octane, high C/R Pistons (9.0:1) can be bought for the o-200/0-300 with impressive results. An o-200 with 9:1 makes something like 140hp and the o-300 makes somewhere around 210hp but this obviously requires the aircraft to be put in the experimental category. I know of guys running the 9:1 in their 0-200's and squirting nitrous on top of that. Probably the biggest reason detonation/pre-ignition is of any risk at all in these low C/R engines is because of the crappy designed intake manifolds/carbaurators delivering uneven mixtures to the cylinders and like you said the fixed timing ignition. Unfortunately since the cost of certification is so high we will have no choice but to live with the old technology, probably forever. It is imprssive seeing the new fuel injection/ignition technology being put to use on aircraft engines, those 120-130 cubic inch engines are making well over 100hp reliably while only burning 3-5gph.

Those 9:1 pistons make it so they have to run 100ll? My reasons for this research are for experimental purposes. My current thinking is that the issue is mostly caused by fixed timing and low rpm, I could be totally wrong. Nothing can be done about the rpm and I'm just not knowledgeable enough on the subject to determine if electronic timing would be enough to make running 91 in a high compression aircraft engine possible.

I thought the issue was more to do with the air cooled nature of aircraft engines, and the higher effective compression ratios of aircraft engines. The higher head temps dictated lower maximum pressure to keep the flame speeds down (and hence the heat generation at the top of the stroke, concentrated on the head). Cylinder head temps are a lot cooler on water cooled engines. Perhaps this is why they can allow significantly higher pressures before preignition. Air cooled Volkswagens also had relatively low compression and retarded timing. The other part of it was that Otto cycle automobiles most commonly run at far lower manifold pressures than planes, which makes the effective compression ratio closer to the mechanical compression ratio for most situations in an airplane engine.

But then again, that's just what I gleaned from some books. These are great read if you are really interested in practical engine design. There are two books in the set. It covers unusual topics like water injection and supercharging as well.
http://www.amazon.com/Internal-Combustion-Engine-Theory-Practice/dp/0262700263

Another book, more recent and brief, also has sections on brayton cycles and addresses some modern fuel systems.
http://www.amazon.com/Internal-Combustion-Engine-Fundamentals-Heywood/dp/1259002071/ref=sr_1_1?s=books&ie=UTF8&qid=1415839212&sr=1-1&keywords=Internal+Combustion+Engine+Fundamentals

Silly me. Said I was a slow learner. Yeah cylinder head temp does affect detonation and I have read that air cooled engines have to run slightly lower compression ratios. The manifold pressure deal makes since too; the throttle on a automobile is usually mostly closed thus a really low manifold pressure vs an airplane that runs at much higher manifold pressures.

Thanks for the links to the books.

Octane requirements get complicated. There are a lot of variables.

Things working against the aircraft engine:

Large diameter open combustion chambers with minimal swirl / tumble. It takes longer for the flame front to get all the way to the walls which provides more time for the pre-flame front reactions to take place in the end gas which can result in abnormal combustion.

Engines optimized for high loads (aircraft) instead of light load fuel economy (automobile).

Fixed ignition timing - you can't retard at speed / load points were you might run into knock.

No EGR - Exhaust Gas Recirculation (either due to an external EGR valve or valve timing) tends to suppress knock.

There are probably a number of other factors, but those are the ones that pop into mind.

Edit:

The books mentioned are good. Heywood goes into a lot more math than Taylor. (I have a much older edition of Taylor and Taylor and a reasonably current edition of Heywood on my desk at work.)

Still, I'm not sure how much MAP itself contributes other than timing vs. MAP.

I swapped the heads and cam on my old LS1 and ended up with 11.2:1 SCR and about 8.4-ish DCR. Ran on 93 octane all day long with a fair number of 1/4mi passes. No detonation.

I also had very good control of spark timing.

Running at WOT within a small RPM range I would think the fixed timing mags would be fine, but with increased altitude and decreased MAP I bet some pretty decent efficiency could be gained from variable spark timing. I don't think it would effect detonation though. As my MAP went down on the TA I would command more advance to keep the power stroke pressure peak in "the sweet spot."

To prevent detonation at lower RPMs I just reduced the advance, which can be handled by fixed timing mags as airplane engines only run in a small RPM band.

I'm no expert, but at first blush I'd put air cooling at the head of the list of culprits causing low compressions to be required.

There are lots of considerations to be observed here, first is what is octane, what is the difference between low octane and high octane fuel? What will or could occur if lower octane fuel is used as a replacement for higher octane fuel?
The two fuels have the same amount of BTU/energy per gallon, however the higher octane is more stable at higher pressures and tempters, in other words higher octane fuel can with stand more pressure and heat without pre ignition. Therefore cylinder head temps and pressure in the combustion chamber are part of the equation; this brings us to cam design and ignition timing. Try to in vision a ball of energy that you can move advance or retard in relation of TDC between compression and power stroke, you can also change the size of it, the smaller it is the more intense it is the larger it is the less intense, the higher the pressure smaller and more intense it becomes, this is what happens when the compression ratio is increased and or timing is advanced, Mike Bush said in one of his articles that an engine runs best when peek cylinder pressure is developed @ 16 Deg. after top dead center, the ignition may start long before that for instance 30 Deg before TDC. Low cylinder pressure requires more ignition lead high cylinder pressure requires less ignition lead, notice I did not say compression ratio, but cylinder pressure, here is where cam design comes into play the closer the cam lobe center lines the more overlap and better the scavenging (the engine sounds like this: rumpity rumpity) but the more overlap the more cylinder pressure is lost and as the cam is retarder peek horse power is developed at a higher RPM, by increasing overlap and retarding the centerline of the cam in relation to TDC we are moving up the point dynamic peek cylinder pressure occurs, this also decreases the dynamic cylinder pressure at lower RPM, this is one reason why we can use or need locked advance on ignition, we need the advance at lower RPM but as the Dynamic compression comes into play it remains the same or less to develop peek horse power at higher RPM. As we increase our compression ratio (with the same cam) we need less timing lead to develop peek torque @ 16 deg ATDC.
Hazards of high compression and too much timing lead, running 50 to 100 ROP or low octane if the peek pressure/heat/energy is developed to early more heat is absorbed by the piston and the cylinder head, the higher the cylinder pressure the more intense and faster this will occur, this is when it most likely to develop pre ignition and detonation this is why any time we fly a high compression engine we should have an engine analyzer, when one or several exhaust tempters drop rapidly and it’s cylinder temps go up on your engine you have pre ignition and thermal runaway and parts start coming out the exhaust pipe, this ain’t good for no body. I hope this helps and is not just the ramblings of an old man.

172heavy wrote:Mike Bush said in one of his articles that an engine runs best when peek cylinder pressure is developed @ 16 Deg. after top dead center

He covers this in his "Leaning Basics" webinar. Lots of questions but I'll have to wait till I have some time to nerd over them. Thanks 172heavy.

whee wrote:

172heavy wrote:Mike Bush said in one of his articles that an engine runs best when peek cylinder pressure is developed @ 16 Deg. after top dead center

He covers this in his "Leaning Basics" webinar. Lots of questions but I'll have to wait till I have some time to nerd over them. Thanks 172heavy.

Way to go Joe.

Tim

Newbizor wrote:........ high C/R Pistons (9.0:1) can be bought for the o-200/0-300 with impressive results. An o-200 with 9:1 makes something like 140hp and the o-300 makes somewhere around 210hp but this obviously requires the aircraft to be put in the experimental category. ........

I think it's a little more involved than just dropping in some high compression pistons. The O-200 in a Formula One racer might (might!) be making 140hp but it's doing so at 3000 or more rpm which requires at the very minimum a short prop-- maybe stronger valve springs as well as probably a big change in timing from a standard O-200, and at least 100 octane fuel if not more.

You all probably know why compression ratio is important in making more power, but here is an easy version:


Going from a stock CR of 7.0 to 9.0 would buy you around 8.6%. A person who has one in a homebuilt told me he has to rebuild his O-200 about every 1100-1200 hours with valve jobs in between with his 9.2 pistons because he likes to go fast all the time.

Thanks Tim

Going from a stock CR of 7.0 to 9.0 would buy you around 8.6%. A person who has one in a homebuilt told me he has to rebuild his O-200 about every 1100-1200 hours with valve jobs in between with his 9.2 pistons because he likes to go fast all the time.

Ethanol to the rescue---it loves very high compression and cools things down.

I thought about that for a half second Marty. 220 Franklin in an experimental set up to run E85, but decided against it.

In a J3 landplane, I've run 9.5 pistons in an O-200 at 28° BTDC on 93 octane mogas with alcohol. Stock MA3-SPA 10-4115. No detonation.

Sea level power at 2750 rpm just under 110 hp.
Most I've turned with it during a 55 mph climb was 114 hp at 2850 rpm running a Mac 1B90CM7535.

At sea level, stock O-200 power is still increasing at 3100 rpm, where it is 109 hp per Continental data.

With 9.5 pistons and no changes to timing or carburetion, at 3100 rpm, power is 119.6 hp (9.76% thermodynamic increase over stock). In a J3 in level flight weight at 1050 pounds and 1100' DA, with a Mac 1B90CM7142, 3100 rpm gives 118 mph (Vne is 122 mph).

For interest and comparison re Franklin engines

335 cu inch, 6 cyl, 7.00:1 comp, 80/87 fuel, 2800rpm, 28 BTC, weight inc mags carb 327lbs. 180hp
335 cu inch, 6 cyl, 7.00:1 comp, 100/130 fuel, 3200rpm, 30 BTC, weight inc mags carb turbo 368lbs, 260hp 5 mins, 240hp cont
350 cu inch, 6 cyl, 7.00:1 comp, 80/87 fuel, 3200rpm, 28 BTC, weight inc mags carb 304lbs, 200hp
350 cu inch, 6 cyl, 10.5:1 comp, 100/130 fuel, 2800rpm, 28 BTC, weight inc mags carb 339lbs, 215hp
350 cu inch, 6 cyl, 10.5:1 comp, 100/130 fuel, 2800rpm, 28BTC, weight inc mags carb 339lbs, 220hp
350 cu inch, 6 cyl, 10.5:1 comp, 100/130 fuel, 3200rpm, 28BTC, weight inc mags carb 304lbs, 230hp
350 cu inch, 6 cyl, 10.9:1 comp, 100/130 fuel, 3200rpm, 28BTC, weight inc mags carb 304lbs, 235hp
350 cu inch, 6 cyl, 7.4:1 comp, 100/130 fuel, 2800rpm, 25BTC, weight inc mags carb turbo 377lbs, 250hp 5 mins, 235hp cont

Another good book to check out

http://www.amazon.com/Four-stroke-Perfo ... g=duc08-21

His Two Stroke Performance tuning book is very good.