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249th_Harrier
08-06-2005, 11:46 AM
I did a short search on octane on the internet. Octane does not have anything to do with energy content, it is only an indication of how easy the fuel is to ignite. Higher octane means the fuel is more difficult to ignite. This is good if you have an engine with a high compression ratio. If you have an engine with high compression ratio, you want high octane so the fuel doesn't ignite prematurely (during the compression stroke of the piston) causing knocking. If an engine is designed to use 100 octane fuel, it won't run right with 80 octane (there will be knocking). But putting 150 octane fuel in it won't make any difference.

According to "To Win the Winter Sky", the problem with '44 - '45 German avgas was:
1) Sometimes octane would dip below the target due to quality problems, causing knocking
2) The percent of aromatic hydrocarbons was relatively high, resulting in incomplete combustion, power loss, overheating because the gas is burning in the exhaust manifold instead of the cylinder, and damage to plastic and rubber parts in the fuel line.

It is possible (I don't have the data) that Allied engines used higher compression ratios than German ones, since they knew that they could supply the higher octane gas. This would result in higher power and efficiency. But to say that such and such an engine would have more hp if it had higher octane gas, as has been stated on this board, that does not make sense, but if someone has a logical explanation I would like to hear it.

LStarosta
08-06-2005, 11:58 AM
You are assuming that the American engines in question didn't have higher compression ratios. Perhaps this higher octane fuel allowed the engines to reach full potential.

Utchoud
08-06-2005, 12:47 PM
Hi,

Some time ago, I posted data charts of British, American and Soviet engines here (http://forums.ubi.com/eve/forums/a/tpc/f/23110283/m/5381095333/p/1). Information on compression ratios and fuel grades are included.

The charts are very old and some mistakes were found in them, but I hope they can help.

Utchoud

han freak solo
08-06-2005, 01:57 PM
Just seat of the pants experience, from my past.

I had a '72 Dodge Charger with a 400 c.i. engine (w/ auto transmission) that normally ran low octane regular gas 87-89 octane. By bumping up the octane to near 100. It allowed me to advance the ignition timing enough to allow the engine to make the horsepower it was capable of.

Since this old car had final drive gearing for high speed it wouldn't spin its rear tires from a dead stop. Until, you upped the octane and advanced the timing. Then it was burn rubber time.

I'm sure the WWII aircraft had the same potential with the proper retuning to take advantage of high octane gas. Meaning, the engine was just detuned for cr@p gas when cr@p gas was all you had.

249th_Harrier
08-06-2005, 02:12 PM
Originally posted by LStarosta:
You are assuming that the American engines in question didn't have higher compression ratios. Perhaps this higher octane fuel allowed the engines to reach full potential.

If a an engine is designed for 100 octane fuel, it will knock if you use 80 octane. This will drastically reduce power and engine life. I can't imagine anyone would intentionally use the wrong octane fuel unless it was a dire emergency. It is my understanding that if you use less than the rated octane, you get knocking. If you use fuel with equal to or greater than the rated octane, then there is no knocking. There is no performance improvement. Performance improvements from better fuel are possible, but this is separate from octane, which is just an "ignitibility" metric. If there is anyone who can explain to the contrary I would like to hear it.

LStarosta
08-06-2005, 02:19 PM
Since this old car had final drive gearing for high speed it wouldn't spin its rear tires from a dead stop. Until, you upped the octane and advanced the timing. Then it was burn rubber time.


Maybe in the time it took you to find 100 Octane, you just wore your tires out.

http://forums.ubi.com/images/smilies/35.gif

NonWonderDog
08-06-2005, 04:24 PM
It's not nearly as simple as all that. This is something I am absolutely horrible at explaining, so stick with me. But in general, higher octane combined with more advanced ignition timings will give slightly more power. This usually isn't much, but you'd logically get a lot more benefit in a big aircraft engine than a small car engine.

At low ignition advance settings, the fuel isn't given much time to burn. The spark doesn't have time to travel all the way through the mixture, and the fuel burns unevenly. Advancing the timing helps... to a point. Low octane gas burns quickly; although it won't all burn at once, it will reach peak pressure quickly. If the ignition is too advanced and peak pressure is reached before the cylinder hits top dead center (pre-ignition), bad things happen.

High octane gas doesn't contain any more energy -- in fact it contains less -- but it burns more slowly. This means you can advance the ignition timings more without worrying about pre-ignition. What this means is that the spark front can travel more completely through the mixture. Also, the fuel burns more evenly because the time it takes the spark to travel through the mixture is less significant compared to burn time. Since you're burning more gas and more of it's reaching combustion at roughly the same time, you make more power.

There's a trade-off, of course. Advanced ignition timings mean more heat in the cylinders. Also, and more significantly, the more you advance the ignition timings, the more power is wasted in slowing the pistons before they reach top dead center. This is generally less than the power gained through more complete combustion, but it means that there are diminishing returns with spark advance. Going from 10deg to 15deg (with the corresponding increase in octane) will gain more power than going from 15deg to 20deg (with the corresponding increase in octane). Stupidly high octane and very advanced spark timings will of course mean a net loss in power.

If all you do is change octane, you'll never see an increase in power. Engines can only be tuned for one specific octane at a time, and most car engines are tuned for normal 87 octane. Putting lower octane gas in will cause pre-ignition, a loss of power, and knocking; putting higher octane gas in can cause detonation, a loss of power, and knocking. Generally a too-low octane is much worse.

p1ngu666
08-06-2005, 04:56 PM
in avaiton higher octane allows u to stuff more air and fuel into the cylinder as it wont ksplode till u ingite it. not many cars alow u to adust boost, but u can adust timing.

interesting on the german pipes and seals, read several things about them failing inflight. saved a few lives ironicaly (ramjadger)

NonWonderDog
08-06-2005, 05:01 PM
Doh!

Yeah, there's that too. That's probably the main thing here; while you can't adjust compression ratios, you can adjust supercharger boost. Higher octane gas means you can run more pressure on the supercharger without pre-ignition.

Monty_Thrud
08-06-2005, 05:15 PM
Yep...what Harrier says is correct, in my Bonnie and Thunderbird(both high comp engines i use 98 octane super unleaded)...you do notice a significant difference between that and 95 octane...why who says this isnt so?

249th_Harrier
08-06-2005, 05:41 PM
Originally posted by NonWonderDog:
It's not nearly as simple as all that. This is something I am absolutely horrible at explaining, so stick with me. But in general, higher octane combined with more advanced ignition timings will give slightly more power. This usually isn't much, but you'd logically get a lot more benefit in a big aircraft engine than a small car engine.

At low ignition advance settings, the fuel isn't given much time to burn. The spark doesn't have time to travel all the way through the mixture, and the fuel burns unevenly. Advancing the timing helps... to a point. Low octane gas burns quickly; although it won't all burn at once, it will reach peak pressure quickly. If the ignition is too advanced and peak pressure is reached before the cylinder hits top dead center (pre-ignition), bad things happen.

High octane gas doesn't contain any more energy -- in fact it contains less -- but it burns more slowly. This means you can advance the ignition timings more without worrying about pre-ignition. What this means is that the spark front can travel more completely through the mixture. Also, the fuel burns more evenly because the time it takes the spark to travel through the mixture is less significant compared to burn time. Since you're burning more gas and more of it's reaching combustion at roughly the same time, you make more power.

There's a trade-off, of course. Advanced ignition timings mean more heat in the cylinders. Also, and more significantly, the more you advance the ignition timings, the more power is wasted in slowing the pistons before they reach top dead center. This is generally less than the power gained through more complete detonation, but it means that there are diminishing returns with spark advance. Going from 10deg to 15deg (with the corresponding increase in octane) will gain more power than going from 15deg to 20deg (with the corresponding increase in octane). Stupidly high octane and very advanced spark timings will of course mean a net loss in power.

If all you do is change octane, you'll never see an increase in power. Engines can only be tuned for one specific octane at a time, and most car engines are tuned for normal 87 octane. Putting lower octane gas in will cause pre-ignition, a loss of power, and knocking; putting higher octane gas in will cause detonation, a loss of power, and knocking. Pre-ignition is much worse, so a too-high octane won't hurt as bad.

Thanks NonWonder! I missed was that octane is not only ignite-ability, but also burn rate, burn uniformity and the ratio of spark propagation time to burn time. The supercharging boost thing makes sense too.

Badsight.
08-06-2005, 05:47 PM
compression ratings are not listed in most superficial stats on most websites

you have to dig deeper , actual books are a good place to start


Originally posted by 249th_Harrier:
I did a short search on octane on the internet. Octane does not have anything to do with energy content, it is only an indication of how easy the fuel is to ignite. Higher octane means the fuel is more difficult to ignite. This is good if you have an engine with a high compression ratio. If you have an engine with high compression ratio, you want high octane so the fuel doesn't ignite prematurely (during the compression stroke of the piston) causing knocking. If an engine is designed to use 100 octane fuel, it won't run right with 80 octane (there will be knocking). But putting 150 octane fuel in it won't make any difference.

According to "To Win the Winter Sky", the problem with '44 - '45 German avgas was:
1) Sometimes octane would dip below the target due to quality problems, causing knocking
2) The percent of aromatic hydrocarbons was relatively high, resulting in incomplete combustion, power loss, overheating because the gas is burning in the exhaust manifold instead of the cylinder, and damage to plastic and rubber parts in the fuel line.

It is possible (I don't have the data) that Allied engines used higher compression ratios than German ones, since they knew that they could supply the higher octane gas. This would result in higher power and efficiency. But to say that such and such an engine would have more hp if it had higher octane gas, as has been stated on this board, that does not make sense, but if someone has a logical explanation I would like to hear it.

Tully__
08-06-2005, 06:39 PM
In most of the aircraft in this game there is an easily adjustable (sometimes from cockpit, sometimes only on the ground) control of supercharger or turbocharger boost. Higher octane allows safe operation at higher boost and better power. The discussion relating to the availability of high octane fuels are presuming that the engine boosts are being correctly adjusted for the octane ratings currently available.

TheCrux
08-06-2005, 07:31 PM
The gearheads have it right ( as usual ) but I'd like to add some more.

Ignition timing can be more aggressive on an engine using higher octane fuel, but it's really the other stuff that is a larger factor, especially on A/C engines.

While you cannot adjust compression ratios higher inflight, you can design/build an engine with a higher compression ratio which, if you've got optimal camshaft settings, will provide higher cylinder pressure at the engines operating peak. Hence this engine, while more powerful, will require a higher octane fuel to 1) deliver this power 2) not blow holes in the tops of the pistons 3) hammer the connecting rod bearings into oblivion

On artificially aspirated engines can run much higher manifold pressures with higher octane as well ( as has been said )

Utchoud
08-07-2005, 01:25 AM
Something to support your statements - quotes from Merle Olmsted's "The 357th over Europe". The 357th FG took part in operation Frantic V in August 1944. Bombers attacked targets in Germany, landed in Russia, were refuelled and flew to US bases in Italy. They were also escorted by 357th FG's Mustangs. (Quotes translated back to English)

Red Conlin:


€œWhen I returned to my G4-C, I realized that the Russian ground personnel filled my fuel tanks with 90-octane instead of 130-octane fuel by mistake.€

Henry Pfeiffer:


€œMy C5-E was on of the planes that was provided with inadequate fuel. From the grass airfield, we were taking off at once in several flights and my flight was one of the last ones. During my take-off, I was adding throttle very slowly and gradually in order to keep my position in the formation. After I reached 40 or 45 in. boost, my engine died away. I throttled back and it started again. I throttled up again and it stopped. I couldn€t break off, so I decided to finish my take-off with 40 in. boost€¦€

Several pilots had these difficulties and one plane was lost during take-off.

Luftwaffe_109
08-07-2005, 03:06 AM
249th_Harrier :
According to "To Win the Winter Sky", the problem with '44 - '45 German avgas was:
1) Sometimes octane would dip below the target due to quality problems, causing knocking


About German fuel octane, my understanding is that the Germans didn't produce fuel of higher than 96 octane for regular service. However there was the German 100 octane C-3 synthetic fuel which had almost as good anti-knocking qualities to the British 100/130 PN fuel due to its high aromatic content. I think the Bf-109 subtypes that could use 100 octane included the DB 601N powered versions (E- /N, F-1/-2,) in 1940-41 and DB 605D, AM, ASM powered versions (late G-14, G-14/AS, G-10, K-4) in 1944-45.

By adding large amounts of tetraethyl lead to cracked crude stocks the fuel octane could be increased up to 96 octane. However, if I am not mistaken to go to higher than 100 octane fuels it would required reprocessing the 87-96 octane avgas by reforming. This would result in the loss of just under a third of the initial fuel stock in the process.

Considering Germany's dire fuel situation, the Allies could afford to do this for regular service fuels while the Germans couldn't.

Regards

alert_1
08-07-2005, 04:10 AM
Interesting discussion..maybe *SOME* LW units were getting 100 octan fuel while other only 87 octan, that would explainfor ex. performance differneces between the same types of AC, also for Fw190 EAST vs. Fw190 WEST (how is modelled in the sim).
Another thougt: most of fuel at disposal of VVS was 90 octan, but later in war about one third of avgas used in VVS was highoctan US fuel. Were VVS engines adjusted or it was only for leand lease AC?

Hurri-Khan
08-07-2005, 09:05 AM
Here is octane effects with blenheim IV's;

"Compared with the Blenheim I Mercury VII of 840hp at +5lb supercharging, the Mercury XVs of the Mark IV could deliver 920hp at +9lb boost (but only with 100 octane petrol and only for an emergency limit of 30 minutes or for just 3 minutes at take-off). The engines would then be consuming 190-plus gallons per hour. The practice was to fuel up with 100 octane in the outer tanks only (maximum 188 gallons), leaving the inners for 280 gallons of 87 octane. To the peril of the crews, this invited re-fuelling (or worse, tank selection) mistakes. The Mercury XVs were liable to suffer damage and potentially serious loss of take-off power if abused with high boost on 87 octane inadvertently filled in the outer tanks."

* Using 100 octane fuel at +9lb boost (take-off limit 3 minutes, emergency limit 30 minutes). * Using 87 octane fuel +5lb boost (take-off limit 3 minutes, emergency limit 5 minutes).

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