Fuel octane ratings - TCCoA Forums
 
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post #1 of 15 (permalink) Old 07-12-2002, 11:18 AM Thread Starter
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Fuel octane ratings

I posted this one the old TCCoA forum this morning, and I thought that it should have been a new thread. But now I saw this new forum, and had a chance to be the first to post in a section, so why not.

The octane rating of a fuel is actually it's resistance to burn. A higher octane fuel has to reach a higher temperature before it begins to burn.

Gotta include one law: when you compress air, it gets hotter. Don't believe me? Start up an air compressor, let it run a little and then feel how hot the compressor gets.

As the piston travels up the cylinder, it compresses the air/fuel mixture. Close to the top of piston travel (TDC) the computer fires the spark plug, this is called advanced timing. This ignites the air/fuel mixture and forces the piston downward. This is all fine and dandy when the engine is new and clean inside. When the combustion chamber is gets some carbon deposits on the inside, that's when pinging and knocking start to become a problem. If a piece of carbon is deposited on the top of the piston, for example, this chunk can and will be pre-heated by the previous combustion stroke. This chunk can get so hot that as the piston moves up it becomes hot enough to ignite the air/fuel mixture before the spark plug has a chance to fire. This is called pre-ignition, that's bad. A pre-ignition source does not have to be on the piston, it can be an overheated exhaust valve or spark plug, or a deposit somewhere in the combustion chamber.

To fix the pre-ignition, also called pinging, one can either clean out the combustion chamber of all possible pre-ignition sources, or run a fuel that is less affected by those pre-ignition sources. This fuel is a gasoline with a higher octane rating. This fuel will have extra additives to help prevent it from pre-igniting. This should help to eliminate the pinging.

Factors that will increase an engines octane requirement:
1. Ignition timing is advanced
2. Air density rises due to supercharging or a larger throttle opening or higher barometric pressure.
3. Humidity or moisture content of the air decreases
4. Inlet air temperature is increased
5. Coolant temperature is raised
6. Antifreeze (glycol) engine coolant is used
7. Engine load is increased

excerpt from Internal Combustion Engine Fundamentals by John B. Heywood, 1988.

Another factor that would require an increase in octane requirement is increase of compression ratio. The mose you compress air, the hotter it gets. Pre-Ignition can also take place when air/fuel mixture gets too hot just from the compression of the gases. You may notice that all cars that require the higher octane rating are either artificially aspirated (supercharged or turbocharged) or high compression ratio (10.5:1 or so).

Diesel engines don't even have spark plugs. They rely on the heating of the air by compression to ignite the oil mist that is injected into the combustion chamber. Due to the fact that the fuel is not injected until the piston is almost at TDC, Diesel engine designers can utilize higher compression ratios than our gasoline engines because they don't have to worry about pre-ignition and knock.

As for running a fuel that has higher rating that is required for your car: unless your computer is re-programmed to take advantage of the higer octane by advancing the timing and other adjustments, you really shouldn't see any difference. Now there might be a different mixture of additives and detergents between different octane grades that might help to clean the injectors and combustion chamber. This would help,
by cleaning out the engine and make it more effiecient.

I don't want to say that money is wasted by purchasing premium when regular will do, but it might make no difference at all.

Sorry to make this soooooo long, but I tried to explain it as best as I could. This book was my textbook from my ME333 Internal Combustion Engines class that I took as an elective when I was in school getting my Mechanical Engineering degree.

The design of engines is very complex, to say the least.

Scott
1995 T-Bird LX 4.6L
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post #2 of 15 (permalink) Old 07-12-2002, 12:31 PM
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Great Post!

Thanks Scott. If you know anything about HTML, would you be willing to convert that to a tech article for us? If not, is there somebody out there who would volunteer to do that? There's some good info there that should be available all the time.

God Bless and Fly Low!

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post #3 of 15 (permalink) Old 07-12-2002, 01:29 PM Thread Starter
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Sure thing.

just let me know where you'd want it.

There's more info/theory than just that, so let me know how in depth you'd like to see it.

Scott

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post #4 of 15 (permalink) Old 07-12-2002, 01:37 PM
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Hmmm......

Perhaps a two stage article. The basics as above then a second page with more indepth stuff. If you want to do it that is....

Don't worry about page formatting. We'll take care of that. Just HTML-ize the text. E-mail it to Mark and myself when you're done.

Thanks!

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post #5 of 15 (permalink) Old 07-12-2002, 01:48 PM Thread Starter
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I can do that.

1st stage: Simple

2nd stage: Complex

Gotcha

I'm just glad someone read it.

Scott

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post #6 of 15 (permalink) Old 07-12-2002, 06:24 PM
 
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Ya Scott, great stuff, we sould probubly look into putting this up in the artilces section , of course with your permission..


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post #7 of 15 (permalink) Old 07-13-2002, 01:04 AM
 
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yup, all right

also carbod deposits raise compression ratio on an engine that doesnt have the correct timing set for that, and thats not cool. and deisels run about 16:1 compression. the diesel that they use is mostly whats filtered out of gasoline. higher octane is another way to retard timing too. thats all i have to say.
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post #8 of 15 (permalink) Old 07-14-2002, 12:16 PM
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The power strokes run just shy of 21:1 compression

Josh
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post #9 of 15 (permalink) Old 07-14-2002, 11:10 PM
 
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so what is Cetaine booster?

and will it work in gasoline like an octaine booster?
it'd be nice if it did!
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post #10 of 15 (permalink) Old 07-15-2002, 05:00 AM
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Yeah, that's what I wanted to hear. That sounds like correct information and based on data. No more fighting about 114 Octane or something. This information should be documented as Sir William stated. Question for you though, what happens when nitrous is being used. Same as with the Supercharged/turbo equipped cars????

Steve
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post #11 of 15 (permalink) Old 07-15-2002, 07:23 AM
 
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Scott, that is a great piece. Thanks for sharing it.

As for running a fuel that has higher rating that is required for your car: unless your computer is re-programmed to take advantage of the higer octane by advancing the timing and other adjustments, you really shouldn't see any difference. Now there might be a different mixture of additives and detergents between different octane grades that might help to clean the injectors and combustion chamber. This would help,

This has been my experience. It's also been my old man's experience. We found out that running premium kept the carb in his old F-150 cleaner and extended the time periods between rebuilds. He was having some hesitation problems, so his mechanic decided to rebuild it; the mechanic pulled the carb, took one look at it, and said, "this thing is clean and in really good shape; it doesn't need rebuilt."

It also helped him improve his fuel mileage by as much as 2mpg(the efficiency point). We use brand-name gas only - Chevron. Great stuff!!
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post #12 of 15 (permalink) Old 07-15-2002, 07:34 AM Thread Starter
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I don't have factual data to back this up, but, when nitrous is used, it adds extra available oxygen to the fuel/air mixture. Unless more fuel is added by the computer, the engine will run lean.

I'm not 100% certain as to whether or not is actually changes the octane requirement of the engine. It probably does, but I have no written proof of that.

One side benefit of using nitrous is that when it is injected into the intake tract of the engine, it cools the intake charge. That's because when N20 boils (which is the process of turning from liquid to gas) it is an endothermic reaction. This means that it requires heat from the environment. It effectively sucks heat energy out of the air, which makes it cooler, which increases the density. Number 2 on the list in the original post indicates that the octane requirement tends to go up when air density rises due to supercharging or larger throttle opening or higher barometric pressure. But, number 4 indicates that the octane requirement will go up with a rise in inlet air temperature. This would indicate that the cooling effect of N20 would lower octane rating.

Wow, that sure made things confusing.

**Warning, Warning, Opinion Alert**

I would think, since N2O can be turned on and off at will, one should try to run nitrous with 93 octane at first. If that works well, try to see what's the lowest octane fuel that can be run with out pre-ignition. If the engine pings with 91 octane, then run without N2O until that fuel can be replaced with the higher octane fuel.
The ability to turn N2O on and off makes testing like this easy. But if one were to install something permanent, like a supercharger, you can't necessarily do testin like that, because it is running on the engine al the time.

Remember this is an opinion and should be taken with "a grain of salt". I have no personal experience with N2O, and I wish I had supercharger experience. Most of this is textbook information.

**Opinion Section End**

In response to the cetane rating post:

"The ignition quality of a diesel fuel is defined by its cetane number. The method used to determine the ignition quality in terms of cetane number is analogous to that used for determining the antiknock quality of gasoline in terms of octane number, The cetane number scale is defined by blends of two pure hydrocarbon reference fuels." Internal Combustion Engine Fundamentals by John B. Heywood 1988.

Seems like the higher the cetane rating, the better the ignition quality. I would assume that the ignition quality is a rating of how well the fuel burns.

Scott

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post #13 of 15 (permalink) Old 07-15-2002, 12:17 PM
 
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Thumbs down NOS Octaine Requirements

100% of all Nitrious Manufactures state the high octane fuel of at least 92 octane should be used, When Nitrious is introduced it not only adds Oxegen but also adds compression and heat, therefore raiseing the combustion chambers temperature , 87 and 89 would begin to knock considerably if used with No2... If the computer didn't calculate for the added Oxegen with more fuel you'd need to run a wet kit or you would cause serious problems with the pistons and valves..


Jim
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post #14 of 15 (permalink) Old 07-15-2002, 05:23 PM
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Here is something I learned in school and by experience...

And not copied out of a book.

The higher the octane rating of the gasoline, the LESS volitile the gasoline. The lower the RON, the more volitile the gasoline. Meaning, a lower octane rated gasoline will ignite faster. A higher octane rated gasoline will resist the initial burn, but ignites and expands with more force.

Note, a high octane rated gasoline will NEVER add horsepower. Never! If you believe that you need to learn how things work. Using a high octane fuel makes the engine less knock constrained. If you have the ability to add spark advance with the high octane fuel, you will gain power. This is especially true with nitrous or a supercharger.

So if your engine requires 87 octane fuel and you add 110 leaded race gasoline, all you have done is destoyed you catalytic converets, oxygen sensors and wasted the gasoline. Nevermind empty your wallet. So what if you use 100 unleaded fuel on your stock T-Bird? You simply waste your money. The engine will actually make less power on a high octane fuel.

Have fun,

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post #15 of 15 (permalink) Old 07-15-2002, 09:12 PM
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Wink A-TRAIN You make a valid point but

for a stock motor I agree whole heartedly on what has been said on these posts. For your car or mine I agree that Higher octane fuel will not make more power but it will keep the cars from timing detonation due to burning slower as you said. I agree that 99.9 percent of the cars on here don't need 110 octane nor does your or mine but since the burn rate is slower on a higher octane fuel wouldn't it be safer than taking a chance on detonation. Now i,m not talking about a tank full of the stuff just a 2 or 3 to 1 mixture say on race day when the temp is 80 + and you know you are going to be foot through the floorboard all day long. Just a thought!!???

Steve
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"There's a bad, built NA 4.6L COMING Spring-Summer 2018"

Elite member of the Halfshaft snapping, Auburn eating, Tranny crunching, Piston Melting, Tire lifting, 500+ lb ft Torque Club.

Cougar ET-Stock 4.6 shortblock/Stock PI headswap Bullitt NA 12.94 @ 105+
Nitrous 11.75 @ 114 MPH

2008 Ford Powerstroke 6.4 TT F250 4X4 Supercrew: 12.82 @ 105 MPH

"IN MEMORY OF MY FRIENDS JOEL BENDER 1979-2006", JOHNNY LANGTON 1975-2011, and MICHAEL VIVERETTE 1953-2013"
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