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Old 03-07-2009, 05:15 PM   #1
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Does higher octane gas typically burn slower than lower octane gas under the same conditions?

I've read different sites/articles and there's mixed information.

Most sites and some books say higher octane burns slower
Some say, higher octane burns faster than lower octane
Some say, octane doesn't affect burn rate, the additives do.

Ethanol burns faster than gas, right? And has lower peak temperatures than gas?
So a gasohol mix (E10,E85,etc) would burn faster than straight gasoline


The reason why AvGas burns slower than automotive gasoline is because of the Lead added to the fuel, and has nothing to do with the Octane rating?
edit:^I think that's incorrect.

So what does affect the burn rate of gas? O_o



Can running to high of an octane cause an incomplete burn? Worse emissions? Power loss? The effect of retarding ignition timing too much?
Here's a link to Dyno charts comparing lower octane with 100 octane.

http://www.team-integra.net/forum/di...&TopicID=50204


During the compression stroke, the spark plugs ignite the fuel mixture several degrees BTDC. So the gases expand while the piston is moving upwards correct? Wouldn't that cause a power loss?

How does Advancing Ignition Timing increase power output?
By raising the effective compression/increasing cylinder pressure?
So that would mean better mixing of fuel and air?
How does this better "thermal efficiency" work?


Thanks.
Hope to get some good answers.
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Old 03-07-2009, 05:57 PM   #2
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Higher octane burns slower, lower octane faster. Timing advance is designed to allow the combustion process to begin at the point where the pressure created does not reach its peak before the piston reaches TDC or you get detonation or preignition, known as spark knock.

High octane allows you to have higher compression without preignition that would occur if you used lower octane gas. Modern engines with properly designed combustion chambers have allowed compression ratios to increase while still working fine with regular gasoline.

The energy content of regular and premium are generally the same.

A good analogy is gunpowder, pistol powders are very fast burning and do not work well in rifles as a general rule, while rifle powders are slower burning and do not work well in pistols. Think of the difference in barrel lengths as the difference in combustion chambers.

Given the same compression and volume of fuel, in the microseconds when combustion occurs, premium takes microseconds longer to combust, allows more timing advance, and greater compression pressure prior to ignition.

Aluminum cylinder heads also allow higher compression on lower quality fuels because they absorb heat about 4 times as fast as cast iron, which makes the occurance of hot spots in the combustion chamber less likely.

The Atkinson cycle engine in the Toyota Prius has compression of over 13 to 1 while operating fine on regular fuel, something unheard of 40 years ago.

Alcohol in fuel slows the combustion process similar to premium fuel, but alcohol only contains about 2/3rd of the btu energy of gasoline so it takes a greater volume of fuel to accomplish the same combustion pressure.

regards
gary
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Old 03-07-2009, 06:31 PM   #3
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Just a quick technical note, due to the fact it is an Atkinson cycle engine, the actual compression of the Prius is NOT 13:1. As the first bit of the compression stroke is just pushing that air right back out the intake valve. Then the intake valve closes, and actual compression begins. I think it's more like 9:1 or so.
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Old 03-07-2009, 07:09 PM   #4
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Why does higher octane burn slower?
Why does alcohol slow down the combustion process?

This text and many others say that Ethanol burns faster than gasoline.
http://www.fuelandfiber.com/Archive/...E85/age85.html

And this:
http://www.eco-flex.us/pages/myths.htm


3. Ethanol burns faster than gasoline but has a slightly longer ignition delay during the slow burn phase of combustion so the engine does not do as much negative work fighting rising cylinder pressures due to large ignition advances. The total ignition advance for E85 is almost identical to the ideal advance for gasoline so it does not cause the PCM problems when you mix them.
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Old 03-08-2009, 04:35 AM   #5
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The correct way to think of it is this way- "higher octane fuel is harder to ignite".

This means the following:
- It is less likely to "diesel" (i.e. ignite without/before a spark)
- The combustion will propagate more slowly (which is different than burning more slowly, read on...)

So first of all, higher octane fuel combusts slower BECAUSE of its higher octane rating (i.e. an indirect relationship, not a direct relationship).

In order to understand, you have to think about how the "flame" propagates through the combustion process after ignition. Think of it as a bunch of individual fuel droplets spread through the combustion chamber. As one droplet of fuel burns, it gets hotter - this heats up the next drop of fuel, which then initiates the burn and releases its energy to the next droplet, and so on. With a higher octane fuel, since it is harder to ignite it requires that the adjacent droplet gets to a higher temperature before the next droplet will ignite. The result that the "flame" will propagate slower through the combustion chamber.

Therefore, the statement that one fuel "burns" slower or faster is meaningless. Does that mean that each droplet will burn faster? Or that the flame will propagate faster? You can't tell by reading the article, the two phenomena are essentially independent when comparing two fuels that have more differences than just the octane rating.

Now if you just compare high octane pump gas to lower octane pump gas, the rules of thumb are that higher octane fuel will:
- Propagate the flame slower
- Be less prone to self-ignition
... BUT, when you are comparing "octane ratings" between dissimilar fuels, the "higher octane" fuel (i.e. the fuel that is more difficult to ignite) could have a faster OR slower burn rate / propagation rate.

Some other points about higher octane fuel:
- Since it is more difficult to ignite, you'll get a less complete combustion (lower MPG, higher unburned hydrocarbons)
- Since the flame propagation is slower, you are essentially retarding the timing (which means your peak cylinder pressure may occur a bit too late to take full advantage of the mechanics of the engine for optimum efficiency)
... therefore your MPG will DECREASE if you use gasoline with too high of an octane rating.

The conclusion is that you should always use the lowest octane fuel for your engine which does not knock.

-Bob C.

p.s. one important followup point: most OEM ECMs run their timing too far advanced IMO, and rely too heavily on the knock sensor to tweak the timing to the correct point. This system works by retarding the timing after it hears knock. This is fine in the short term, but over 100,000 miles these occasional knocks will have an accumulative affect and you could ruin your head gasket.
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Old 03-08-2009, 05:05 AM   #6
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Quote:
Originally Posted by bobc455 View Post
p.s. one important followup point: most OEM ECMs run their timing too far advanced IMO, and rely too heavily on the knock sensor to tweak the timing to the correct point. This system works by retarding the timing after it hears knock. This is fine in the short term, but over 100,000 miles these occasional knocks will have an accumulative affect and you could ruin your head gasket.
Wouldn't that point support the practice of using higher octane ratings than specified?

Also, your theory sounds ok but in practice we have the longest lasting engines ever. 181,000 miles on my GMC engine, never had the heads off...should I be looking for signs of cumulative occasional knock damage?
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Old 03-09-2009, 09:37 PM   #7
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-"So what does affect the burn rate of gas? "

Air/fuel ratio has a big effect on the burn rate of gas. http://www.gassavers.org/showpost.ph...69&postcount=5

Here's a handy chart from this page: http://www.max-boost.co.uk/max-boost...ion_deeper.htm
-------------AFR; 18.4 16.3 14.7 13.3 12.2 11.3 10.5 9.2
burn rate mm/sec; 250, 270, 330, 350, 360, 320, 250, 220
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Old 03-10-2009, 02:18 AM   #8
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Originally Posted by theholycow View Post
Wouldn't that point support the practice of using higher octane ratings than specified?
Yup - unfortunately, though, it is difficult to tell what's going on with the knock sensor unless you have a scan tool that can give you a KR readout. And even then, most knock is transitional (i.e. during quick throttle changes or shifts).

Quote:
Also, your theory sounds ok but in practice we have the longest lasting engines ever. 181,000 miles on my GMC engine, never had the heads off...should I be looking for signs of cumulative occasional knock damage?
One more benefit of hypermiling! Since we are generally much lighter on the throttle than the average driver, we tend to stay out of the "transitional" areas where average drivers see the most knock.

For my family's cars (my Saturn and the wife's Taurus), we both use 89 octane under most circumstances. We do this because I can hear the occasional knock when using 87 octane fuels. (I am lucky to have a trained ear that can hear that occasional knock, most drivers will never pick up on it). The 89 octane fuel gives that extra safety cushion to stay out of the knock zone.

-BC
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Old 03-10-2009, 05:31 AM   #9
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I don't know. My 98 GMC pickup has 160,000 miles on it. It had 109,000 on it when I bought it. Absolutely no problems with the engine at all. I've always gotten better than average mileage in my vehicles, but i've only been actively hypermiling for about a year. Even at that I still occasionally open up the throttle and let the horses out for a run. I think if you're having head gaskets blow at 100,000 miles its not due to a knock sensor, its more likely due to poor vehicle maintenance or poor engine design.

Even running hard the gaskets hold up a long time. I remember at the rescue squad we had an old 86 Caprice with the police package and a 350. It was a Sheriff's car, and the Sheriff's dept gave us the car when they retired it (after 5 years and 110,000 miles) Then the squad had the car for another 5 or 6 years and the car had well in excess of 170,000 miles on it when the head gasket blew.

No civillian in their normal driving could be as hard on their vehicles as a Sheriff's dept and a rescue squad. The car lasted over 10 years and 170,000 miles. The county was a large county, and the squad cars got a workout.
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Old 03-10-2009, 10:37 AM   #10
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Most people report FE and HP gains by advancing the timing by a few degrees. This tells me the factory uses conservative timing advance to be on the safe side.

At light throttle there is low cylinder pressure, so any knock that occurs at light throttle won't add much more pressure, and the risk for damage, whether cumulative or instantaneous, is low. Likewise the stock knock control in my car's stock ecu is completely inactive at light throttle. I've looked into the ecu code and also found that knock at mid throttle will pull timing, but the ecu gives back the timing almost instantly (1 degree every 0.07 seconds). Knock at WOT pulls timing and keeps it pulled longer (returns 1 degree every 0.75 seconds). If the ecu senses consistant knock it'll subtract a few degrees of timing all the time.

Of course this info is from my specific car and may not apply to other cars, but it's nice to peek at the inner workings of an ecu to see what factory engineers are doing.
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