Landracing Forum
Tech Information => Technical Discussion => Topic started by: DaveB on February 16, 2015, 09:56:37 AM
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There is a common misconception that the higher the octane number the more power the gasoline makes. Most people coming to this site probably understand fuel very well but I thought I'd just throw out some interesting information with regard to octane rating and energy available from gasoline. What I'm saying here are generalities. There are exceptions to everything.
The octane rating is the ability of a fuel (we're talking about gasoline) to resist ignition before you want it to ignite. Ignition can take place by spark (what we want) or by compression and/or heat (we usually don't want this). This resistance to the wrong kind of ignition, higher octane, allows the engine to use higher compression and that's what gives higher horsepower.
Energy available from gasoline is usually measured in BTU and is effected by octane less than by many other factors.
An example may be an engine with a 9.5 to 1 compression ratio and computerized engine management that seems to run OK on 87 octane gas. The computer engine management system has maybe turned back timing advance or increased richness so it did not knock on low octane fuel. When you put 92 octane gas in it it runs better because the engine management is operating it closer to optimum so it makes more power.
Another example may be a lower performance engine that runs fine on 87 octane. No power increase takes place with higher octane fuel because the parameters were tuned to the lower octane fuel. Fuel octane needs matched to the engine set up and high performance engines often need higher octane.
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THANKS that was useful info for myself and probably others :cheers:
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There is a common misconception that the higher the octane number the more power the gasoline makes. Most people coming to this site probably understand fuel very well but I thought I'd just throw out some interesting information with regard to octane rating and energy available from gasoline. What I'm saying here are generalities. There are exceptions to everything.
The octane rating is the ability of a fuel (we're talking about gasoline) to resist ignition before you want it to ignite. Ignition can take place by spark (what we want) or by compression and/or heat (we usually don't want this). This resistance to the wrong kind of ignition, higher octane, allows the engine to use higher compression and that's what gives higher horsepower.
Energy available from gasoline is usually measured in BTU and is effected by octane less than by many other factors.
An example may be an engine with a 9.5 to 1 compression ratio and computerized engine management that seems to run OK on 87 octane gas. The computer engine management system has maybe turned back timing advance or increased richness so it did not knock on low octane fuel. When you put 92 octane gas in it it runs better because the engine management is operating it closer to optimum so it makes more power.
Another example may be a lower performance engine that runs fine on 87 octane. No power increase takes place with higher octane fuel because the parameters were tuned to the lower octane fuel. Fuel octane needs matched to the engine set up and high performance engines often need higher octane.
Exactly.
For more on this idea, read my thread, "Compression Ratios, What do They Mean?"
http://www.landracing.com/forum/index.php/topic,12353.0.html
:cheers:
Fordboy
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fordboy628 said "Exactly"
Coming from you that means a lot.
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Well, this leads to an interesting question.
In the gasoline classes at Bonneville, we're limited in choices to these three ERC products.
R + M / 2 method
A-19A 119
110-K 110.3
MUL/B 101 (unleaded)
http://www.ercracingfuels.com/sxs1.htm
In the "Grenade", we ran the A-19A, despite the fact that we were only at 13.2 CR.
Realistically, seeing what most guys run in road racing events with somewhat higher compression ratios in their BMC engines, the 110-K would likely have been fine at Bonneville.
We tested on a chemically similar fuel to the A-19, C-14, which is made by VP. The octane was similar, as well as the specific gravity. I wanted to keep SG constant between the dyno and Bonneville to remove the variable, because we were running a carburetor, rather than EFI, and were making adjustments on the salt.
So the questions are as follows -
Can a denser fuel compensate for lack of anti-knock properties?
Is it possible to over-octane a combination?
Higher octane usually equates to a slower burning fuel, so is it possible that a less complete burn occurs with a higher octane fuel than a lower octane brethren?
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I'm going to stick my neck out here and assume that your 13.2:1 compression ratio is calc'd CR (swept volume). It will probably be less at Bonneville altitudes with the thinner air.
YMMV
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I'm going to stick my neck out here and assume that your 13.2:1 compression ratio is calc'd CR (swept volume). It will probably be less at Bonneville altitudes with the thinner air.
YMMV
I've been told about 10-12% less.
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The mention of slower burning fuel reminds me of my 283 B gas dragster back in the 60s when I'd go to a different part of the country with unfamiliar local pump gas . It would take several runs of experimenting with spark lead and jetting to get close to normal time slips .
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The ratio is a comparison, and it's constant, and not affected by air density.
What goes down is cranking pressure and combustion pressure.
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The ratio is a comparison, and it's constant, and not affected by air density.
What goes down is cranking pressure and combustion pressure.
True - compression ratio remains constant - the quantity of a/f charge changes with air density, and if left unadjusted, so does the a/f ratio.
Maximizing that charge by varying ignition and cam timing are a given, but . . .
I'm wondering about picking a fuel that burns at a rate that maximizes the combustion pressure during the combustion event - burning as completely as possible, and extracting as much pressure as possible before the exhaust valve opens.
Might a slower burning, higher octane fuel, intended to diminish knock, possibly provide less combustion pressure than a lower octane, faster burning fuel?
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Different in many ways, but doesn't diesel fuel/engine take advantage of the slower burn rate of the fuel - to make a gob of extra torque? The fire is still burning and the charge expanding when gasoline would have long been burned.
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Different in many ways, but doesn't diesel fuel/engine take advantage of the slower burn rate of the fuel - to make a gob of extra torque? The fire is still burning and the charge expanding when gasoline would have long been burned.
Slim, yes to a certain degree. One reason Nitro is used for Fuel, besides the BTU's.
Back some years ago when racing karts or very hi speed motors (14-19k/revs) ran best on minimum octane fuel. I used the oil to also suppress the detonation. If this was done incorrectly, it made quite a mess in a very short amount of time.
J
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I'd run the lowest Octane you dare run. To high on Octane will result in lower top end RPM. There is some room to play with without making to big a difference, but you can get past the "sweat spot" on either side as far as Octane goes.
To high on Octane is cheaper to fix though.
Rouse
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To high on Octane is cheaper to fix though.
Yep - and less to clean up.
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The common knowledge is that octane increase slows the burn that is not true. It "may be true for a specific fuel" but is not due to the octane, but rather the choice of fuel components used in the fuel.
Sunnoco Maximal is their highest octane blend and is also their fastest burning fuel.
http://www.aaoil.co.uk/files/1513/7344/4603/SUNOCO_MAXIMAL_SPEC_050905.pdf
Fuel mixes which are high in toluene tend to burn slower, because of toluenes very low volatility (poor evaporation in the intake tract).
Fuel burn speed also depends strongly on fuel air mixture, so if you significantly alter your fuel air mix you have also unintentionally modified your effective ignition timing. Fastest burn speed typically occurs around max power mixture which is usually assumed to be about 12.5:1 but some fuels have different ideal mixtures for best burn speed.
The slow burn speed of some high octane fuels is not due to the octane rating but is only a coincidental relationship.
On page 9 of this pdf is a chart that shows the effect of changing fuel air mixture on burn speed for both gasoline and fuel ethanol
http://www.epa.gov/otaq/presentations/gni-mjb-051303.pdf
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Yes, there is NO relationship, direct, indirect, inverse or other, between octane rating and burn rate.
As hotrod stated, burn rate is a function of the components of the fuel blend, and the proportion of each component in the mixture.
This also is: "Complicated".
Choosing the correct fuel for your engine depends on MANY factors, and there are probably several choices that will work well enough, unless you are in Nascar or F/1 . . . .
:cheers:
Fordboy
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So it's probably fortunate that we're limited in our choices at Bonneville.
Seeking out and tuning to the "magic pale" could get complicated and expensive.
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Seeking out and tuning to the "magic pale" could get complicated and expensive.
I only, and selfishly, want to "tune" myself to the "magic pale" . . . . . .
And I don't care about the "expense", although something darker, aged in wood, with a higher ABV is always a personal favorite . . . . .
:cheers: :cheers: :cheers:
Fordboy
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In response to Reply #4, switching to fuels with different specific gravity affects the mixture ratio on a carbed motor. It is best to use a fuel with the same specific gravity as the gas that was used to set the mixture and timing on the dyno.
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I would like to throw something out for fodder.
If you consider the modest compression ratio of ~ 13.5:1 both of the A19-A & 110K are able to do the job and then some. The A19-A being able to handle compression ratios more than two points higher AND also deal with engine speeds over 10,000 rpm (burn time).
What many of us fail to look at is the MULA-B 101 unleaded. Unleaded racing fuels tend to have more oxygen molecules. If testing you my find those oxygen molecules will help with the engines power output, especially at the altitude of Bonneville........
Now would be a good time for Harold to stop by.
Carry on.
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I would like to throw something out for fodder.
If you consider the modest compression ratio of ~ 13.5:1 both of the A19-A & 110K are able to do the job and then some. The A19-A being able to handle compression ratios more than two points higher AND also deal with engine speeds over 10,000 rpm (burn time).
What many of us fail to look at is the MULA-B 101 unleaded. Unleaded racing fuels tend to have more oxygen molecules. If testing you my find those oxygen molecules will help with the engines power output, especially at the altitude of Bonneville........
Now would be a good time for Harold to stop by.
Carry on.
In the world I live in, oxygen already in the fuel is always a good thing . . . . . . .
:cheers:
Fordboy
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As an example -- we run the MUL-B (or whatever the latest concoction is) in the '09 Kawasaki ZX14. Rick Gold suggested that we stick with it when we first asked (and were already using it). Enough octane to keep things under control in the 10,000++ stroker motor, a couple of percent oxygenated, and -- well, it sure seemed to work for us.
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Saw the thread title "octane" and rose to the item like an old trout goes for a bug appearing fly...... :-o
Octane is a term that is almost as misunderstood and abused as horsepower and invokes more emotion than logic, methinks. :-D Let us reason together.
What a racer wants to select is the fuel that A) is legal and B) offers the minimum octane to decrease detonation (knock) while not tearing up one's fuel budget. The burn speed is an important issue (relative to bore size and compression ratio) and that kind of info should be available from the fuel mfg. Some fuels such as those like triptane bearing fuels are nice in that they are not prone to detonate even with high CR.....However, they become very expensive and thus hard to come by. :roll:
Oh a guy could rant on and on about fuel but simply put is that A) Bonneville is at a fairly high elevation (thus poor density) and is as a result fairly forgiving of higher CR and B) IF the rules permit doing so, choose a fuel that has oxygen within. 8-)
Talk to a fuel guy that is not into hype but into chemistry because that is what the engine works on in the combustion chamber. :lol: Once the intake valve closes, all the excitement begins and the hype will not illuminate the darkness nearly as well as a happy fuel burn will. :?
Regards to all that like this kind of stuff,
HB2 :-)
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Said it much better than I.......
Good to hear from you Harold!