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Detonation versus Pre-Ignition Resistance of E85
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Author:  jmarkaudio [ Wed Apr 13, 2011 8:38 pm ]
Post subject:  Detonation versus Pre-Ignition Resistance of E85

It was brought up on another forum that while E85 has good detonation resistance, it is more sensitive to preignition than gas, even more than 93 octane. I did a search, there is an SAE paper on the subject. I have to wonder though, there are a few things that don't figure. Gasoline has an auto ignition temperature of 536˚, ethanol at 689˚. Gasoline is a mix that starts vaporizing at around 100˚ and ends roughly between 220˚ and 270˚ depending on the fuel, Ethanol is fixed at 173˚. I don't know the curve of cylinder pressure and how it affects the auto ignition temps. Also, while the SAE paper varies a lot of things in the tests, it fixes the RPM at 900. Since we almost never run that low and certainly never under load, I wonder how significant it is for our application. My converter stalls at 5900, certainly no significant load until well over 5000 RPM. Any thoughts?

Author:  shrinker [ Wed Apr 13, 2011 10:11 pm ]
Post subject:  Re: Detonation versus Pre-Ignition Resistance of E85

The detonation and pre-ignition resistance of alcohol when mixed with gasoline is worse than straight alcohols. The OH part of alcohols interacts with the petroleum molecules and reactions happen at much lower temps, it even does it in the tank. Distillation temperature when the 2 are mixed are not simply interpolation between the 2 points. Everything changes. Thats why I dont like domestic E85. They use different percentage mixes, different petroleum grades in different seasons etc. Its a mess. if your going to push boundaries then you need consistency. Either pure Petroleum spirits or pure Alcohols.

I always have doubts about testing at the RPM;s used by scientists because the residual component of a burn has to be countered for in analysis. Doing 900 rpm is a lot different to 7000 in cylinder evacuation etc. I have only ever read one test where I thought they got it all right. The difficulty of testing without uncontrolled events is why this whole game is open slather.

Author:  jmarkaudio [ Wed Apr 13, 2011 10:56 pm ]
Post subject:  Re: Detonation versus Pre-Ignition Resistance of E85

What is funny is the SAE paper suggests E100 is more prone to pre-ignition than E85. ????

Author:  shrinker [ Thu Apr 14, 2011 1:38 am ]
Post subject:  Re: Detonation versus Pre-Ignition Resistance of E85

If they didnt chemically clean the engine when testing between E100 and E85 then its all wrong.
All fuels have some preignition tendency. Just what is preignition? Is it caused by hot spots or cool flame reactions or radical residuals? Bit hard to define all that, Did the testing you read address those issues. Not all SAE stuff is faultless. At 900 rpm there is ample time for cool flame reactions to progress to deflagration. Thats not going to occur at 7000. And then what droplet distribution did they use, how much fuel was gasified at various points in the cycle, what was the AFR spread throughout the chamber, what squish? What compression ratio changes were made from E85 to E100. Compression ratio affects heat build up and starts preignition.

Author:  shrinker [ Thu Apr 14, 2011 1:54 am ]
Post subject:  Re: Detonation versus Pre-Ignition Resistance of E85

This whole subject of detonation etc is really difficult to nail down. My experience is this, I made a special carby once(it wasn't commercially viable but i just did it as an exercise in learning way back when i started) I fitted it to a Toyota 3K engine and modified the head to get 17 to 1 compression ratio. The original carby would not even idle without detonating. My test ran smooth even at 70 degrees advance, there was nothing I could do to make that engine knock. It was not overly rich, in fact it was just normal fuel economy mixtures. It was not anything tricky to stop detonation, all it was was a carby to achieve complete vaporization at all operating ranges. It was sized for full power etc no tricks. That test I did back then showed me what detonation really was, and its not in the books.

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