The "old octane question"(s)...sorry guys
The "old octane question"(s)...sorry guys
(OP)
Does the octane rating effect the temperature within the the combustion chamber (given the same c/r) and how much?
Does higher octane fuel burn "slower" than low octane fuel?
All things being equal as far as tune goes are there ever any measurable power gains to be had by different octane fuels?
I'm really sorry for these redundant and stupid questions and I think I know the answers, but would like people more knowledgeable than I to confirm or prove my thoughts wrong.
Thanks!
Does higher octane fuel burn "slower" than low octane fuel?
All things being equal as far as tune goes are there ever any measurable power gains to be had by different octane fuels?
I'm really sorry for these redundant and stupid questions and I think I know the answers, but would like people more knowledgeable than I to confirm or prove my thoughts wrong.
Thanks!





RE: The "old octane question"(s)...sorry guys
Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: The "old octane question"(s)...sorry guys
RE: The "old octane question"(s)...sorry guys
The original question stated that “All things being equal as far as tune goes are there ever any measurable power gains to be had by different octane fuels?”
Here is a summary:
Example engine: 5.7 GM carbureted 4bbl Q-Jet, 8.5:1 comp ratio, optimized for 87 octane fuel, timing set to manufacturers specifications (8° BTDC), emissions on target to year engine certified (1985 test range) indicating proper fuel mixture.
85 Octane 244 bhp; (note: detonation detected)
87 Octane 247 bhp;
89 Octane 245 bhp;
93 Octane 250 bhp;
Note the 3 bhp horsepower difference is well within dynamometers repeatability discrepancy.
System was run continuously using three fuel supplies and a fuel switching valve. 10 minutes run time allowed between fuel switching for carburetor purging and temperature stabilization.
The second test series run showed slightly different results:
85 Octane 249 bhp; (note: severe detonation detected)
87 Octane 241 bhp;
89 Octane 247 bhp;
93 Octane 245 bhp;
What I did notice is that the power levels occurred at different RPM levels, the higher the octane, the higher the RPM peak power level. Exhaust temperatures were different too. The higher the octane, the hotter the exhaust.
Also, the ignition timing was NOT changed during the test, nor was the carburetor jetting.
Franz
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RE: The "old octane question"(s)...sorry guys
Octane rating is purely a measure of knock resistance, and therefore any influence on other factors is coincidental.
The only way higher octane fuel will increase power is by supporting a higher compression ratio or more timing advance.
It does not necessarily support more advance because it burns slower, but because it resists the detonation or spontaneous combustion across the chamber as the pressure reaches a higher peak before TDC when the fuel has been burning and generating pressure for longer.
Burn rate will be influenced by a number of factors, including how homogeneous the air fuel mixture is, the chemical nature of the components of the fuel, the particle size of the fuel dispersed in the air stream, the air temperature in the manifold, the change in air temperature during the compression stroke, the rate of pressure build up after ignition, the evaporation rate of the fuel, the latent energy of evaporation of the fuel, the A/F ratio, quantity energy released per unit of mass on burning, and other factors I am sure.
Simple tests comparing one octane rated fuel to another, while somewhat useful in practise, can be misleading as to supporting theory if we jump to to many conclusions based on limited tests. If the tests only test one fuel for each octane rating, the differences might not be due to octane rating, but other variables dependant on fuel composition. Fuel cannot have the same composition, but a different octane, as the octane change is dependant on formulation change.
A good example of this is to compare two very different fuel with similar octane rating, like ethanol or methanol with a blend of propane and butane.
Regards
pat pprimmer@acay.com.au
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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: The "old octane question"(s)...sorry guys
Franz
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RE: The "old octane question"(s)...sorry guys
I am suggesting it might be coincidental that the burn rate for the higher octane fuel is because the change in formulation to achieve the higher higher octane in the fuel you used for your tests, also coincidently results in a slower burn rate. It might also be possible to use another formulation for obtaining the same octane, but get a faster burn rate.
I have found it difficult to get reliable data on burn rates of various fuels or fuel components. This is probably because it is difficult to control all variables that effect the rate.
On looking at the data from your results, I see them as inconclusive as they do jump around rather than progressively following a predictable pattern.
I am not trying to be argumentative, but I think this subject is a lot more complex than this thread has indicated so far, and good data has been out of my reach to date.
I have run testing and applications trials laboratories in the past, so I am probably looking at this as more of a laboratory technician than an engine builder or tuner. This might explain why I see things from a slightly different perspective.
Regards
pat pprimmer@acay.com.au
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RE: The "old octane question"(s)...sorry guys
You are exactly correct.
Our client wanted to prove that there would be no power difference when using differnt pump grade gasolines (sorry, I'm in Central Texas). The test results came back that it was inconclusive that simply changing octanes WITHOUT changing any engine parameters (jetting, ignition, compression) would result in any repeatable performance gain.
Our client was in litigation with a major oil company about deceptive advertising, claiming that using a higher octane motorfuel would result in performance gains. In a series of more than 20 dyno runs, there was never more than a 5 bhp difference between the mean readings. Two back to back runs with the same octane fuel would result in horsepower readings up to 10 bhp apart (about 2.5% variance). Personally, I hesitate using a 2.5% variance as a predictable trend, unless it is repeatable, and it wasn't.
Note that this was an engine that used mechanical components, no electronic fuel mixture or ignition support. The goal was to test the fuel, not the engine. The dyno was laboratory grade and certified accurate.
What we find is that since motor fuels are formulated seasonally, there is almost never the same fuel in the tanks. This test used "down the street" fuel, not endolene or lab spec fuel.
Franz
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RE: The "old octane question"(s)...sorry guys
So does the formation of lead oxide from combustion of the TEL. But perhaps to an engineer only Physics is truth, and Chemistry another fairytale?
Permit a scientist to point out that octane number is a purely empirical quantity. This means that any correlation between it and combustion rate, heat of combustion, or even knock resistance in an engine other than the test engine, is only coincidence.
RE: The "old octane question"(s)...sorry guys
It melts at 888 C. I suppose it breaks down somewhere considerably north of that.
And at what temperature does an exhaust valve operate at?
Up to 800 C from estimates I've seen, but I'm sure modern researchers can track it by the millisecond these days.
Has there ever been tests run, on deposition of lead oxides on valves and faces?
You bet- Ethyl and Esso had filing cabinets full of such results.
If it is a "LUBE" thing. Then why does simply changing the materials cure the "WEAR" problems?
Because different materials may have different lube requirements under identical conditions.
I confess I've lost the point among the Socratic method. But dismissing a big chunk of peer-reviewed, published research as "fairytail" (sic) and offering only tenuous analogy and rhetorical questions as backup may seem a tad arrogant to your fellow engineers.
RE: The "old octane question"(s)...sorry guys
Say if a new car manufacturer is recommending RON 93 for a car not equipped with knock sensor. By using RON 97, ignition timing is not changed due to no knock sensor available. The article mentioned that there is no increase in horsepower. There is significant increase in HC emission because of the inability of the combustion to fully burn the mixture.
If I'm not mistaken, Octane number rating is affected by the length of the carbon chain for the molecules that form the fuel. The longer carbon chain is easier to burn thus easier to cause engine knock.
When we look into the knocking phenomena, flame travelling across the combustion chamber will likely to cause knock at the end of the cylinder if the fuel at the end of the combustion can be easily combusted.
So I guess I do think that higher octane number will cause the fuel to burn slower during combustion.
RE: The "old octane question"(s)...sorry guys
You are correct if you simply consider the length of the chain as the indicator of octane rating, but longer chains also evaporate slower which can have an effect on burn rate as the fuel will not burn until after it evaporates.
Also, there are other ways to change octane, like using cyclic chain structures, and having other groups present like alcohol or ketone's, or additives like tetra ethyl lead or even water.
Regards
pat pprimmer@acay.com.au
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RE: The "old octane question"(s)...sorry guys
Yeah you're right, there are many ways to increase the octane rating. So I guess, it is wise to help Rob by using the fuel that is available for him at his closest gasoline station. This means that we need to use the gas station's gasoline that involves the most common processing and blending method.
In this context, his original question about the burn rate has not been answered yet. Let's get him the answer for his question using the conventional and easy to find gasolines.
AO
RE: The "old octane question"(s)...sorry guys
Fair enough, but that answer does only apply to currently available pump fuel in regions with unleaded, but without alcohol.
Also differences in octane at the local pump mainly involve differences in cyclic hydrocarbons rather than chain length. Differences in chain length of the alaphatics is more likely to be dependant on availability/cost and whether or not it's the summer or winter brew.
If the local seasonal brew does have a change in formulation re chain length, the aromatics content will be adjusted to maintain the octane.
Regards
pat pprimmer@acay.com.au
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RE: The "old octane question"(s)...sorry guys
From what I scanned, octane number doesn't seem to affect power statistically, but only if other changes in engine mechanical or operating variables are made. In other words, simply switching from 87 pump Octane to 93 say, won't make a real-world difference.
Unless the vehicle is black (the fastest color, of course).
Will Octane number affect mileage? I have several friends that swear Premium gas provides more miles per gallon than Regular. I doubt it, but I haven't seen any proof one way or the other.
Just wondering ...
. . . Steve
RE: The "old octane question"(s)...sorry guys
Once again it all depends.
Methanol has very high octane (I know it is not used in pump gas) but returns poor mileage as it is low energy per litre or gallon and it requires about 6.5 A:F ratio vs 14.7 for typical petrol.
When talking pump gas, the octane boosting additives might or might not have more energy per litre or might give Stoich at a higher A:F ratio, it all depends on the local brew at the time.
If the fuel has the same energy content and requires the same A:F ratio, which it mostly pretty well does for pump gas in developed countries, then IF, AND ONLY IF, the engine is so high in compression that it would knock on regular fuel, EXCEPT FOR THE FACT THAT THE KNOCK SENSOR CONSTANTLY TRIMS THE TIMING TO AVOID EXCESSIVE KNOCK, THEN AND ONLY THEN WILL YOU GET BETTER MILEAGE from premium or high octane pump gas.
The increase comes from the three way combination of high compression, knock sensor operation and high octane.
Regards
pat pprimmer@acay.com.au
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RE: The "old octane question"(s)...sorry guys
An example consistent with what Pat is saying, but supporting your friends also:
2003 Mini Cooper, naturally aspirated, 10.5:1 CR, manual asks for 91 octane or better. Runs fine on 87 octane (no noticeable degradation in acceleration), but averages 27-28mpg on my typical drive to/from work. With 89 octane, it gets about 29-30mpg on average, and with 93 octane it gets about 31mpg on average. These figures are based on the "reset odometer at each refill" method, with only mental tabulation of the results. I cannot recall ever getting better than 29mpg over a tank-worth of normal driving with 87 octane, and I definitely have hit 32mpg+ with 93 octane. All of these figures are worse than my '97 Honda Civic dx got (34mpg) on 87 octane fuel. The civic didn't benefit noticeably from higher octane.
RE: The "old octane question"(s)...sorry guys
My friends (and I) ride Harley-Davidson motorcycles. They do not compensate for octane, just like most of the vehicles pre-198?.
I would guess that the knocking reduces the power the engine can make, probably as it is destroying it
. . . Steve
RE: The "old octane question"(s)...sorry guys
RE: The "old octane question"(s)...sorry guys
Honda uses knock sensor for cars and motorcycles. The algorithm advances the ignition timing by 0.5 degree in every revolution, the moment slight knock is detected, the ignition timing is retarded by 2 degree. It continues because engine rpm, throttle, etc. are known not to be constant.
Advancing the timing right until you get the maximum brake torque will get you higher combustion pressure, better fuel economy and higher torque.
In Japan, high performance cars require RON 98. However, the knock sensor enables the cars to take up RON rating as low as 92.
RE: The "old octane question"(s)...sorry guys
2 So how come to battle the valve face and seat problems in a HFO engine they didn't use this "lubed" valve face seat theory?
It all boils down to one thing, temperature of operation, and corrosive action.
RE: The "old octane question"(s)...sorry guys
2) Why assume that a technology for a spark-ignition gasoline engine will be applicable to a compression-ignition diesel, especially where an octane increaser like TEL will likely reduce cetane? Might the large amounts of sulfur in HFO have an effect on the valve seats? How should I know? Do you?
temperature of operation, and corrosive action. -Isn't that two things?
RE: The "old octane question"(s)...sorry guys
my current car gets better mileage on 89 than on 87 but doesn't seem to gain anything noticeable by going 91. The car reccomend's 89.
RE: The "old octane question"(s)...sorry guys
I have several friends that swear Premium gas provides more miles per gallon than Regular. I doubt it, but I haven't seen any proof one way or the other.
CC:
A friend recently passed away, so we cannot quiz him on the subject, but he used to measure his MPG on a regular basis and said that the premium gas not only gave better mileage (for some brands - not all), but was more economical in terms of kilometres per euro, despite the price differential.
That is, it was more than worth paying the extra price, for the extra mileage from the premium petrol, at the petrol station he used.
But like many have said, it probably down to blending. This seems to me to be one of those 'do pretty women make better wives' questions. While there is an initial argument and maybe a general tendency to lean towards a particular answer, there seems to be neither a clearcut yes, nor a definite cause and effect; just some plausible reasons why in some cases the answer might be yes, together with counter arguments why in other cases the answer might be no.
RE: The "old octane question"(s)...sorry guys
Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: The "old octane question"(s)...sorry guys
RE: The "old octane question"(s)...sorry guys
Does higher octane fuel burn "slower" than low octane fuel?
Octane rating is how easy the fuel auto-ignites which I assume is dependent on molecules having enough kinetic energy that when they collide they can break apart and start to reform differently, eg a C-C bond breaking and later the loose carbon forming a C-O bond with an oxygen atom.
So naïvely, octane rating would depend enthalpy of formation of the weakest-link in the molecules.
On the other hand, burn rate is likely to depend on the enthalpy of formation of the molecules as a whole.
So I don't see any reason why octane rating and burn rate should be related, although as a very vague generalisation lower octane fuels might give more energy for a given weight of fuel, due to the way this is calculated: energy used to break the old bonds subtracted from energy released when creating the new bonds. So if an old bond was easier to break more energy would result from the reaction.
If burn rate depends on droplet size and that depends on how well the fuel vapourises, etc, I think we may as well regard fuel octane rating and fuel burn rate as independent.
RE: The "old octane question"(s)...sorry guys
RE: The "old octane question"(s)...sorry guys
2) Why assume that a technology for a spark-ignition gasoline engine will be applicable to a compression-ignition diesel, especially where an octane increaser like TEL will likely reduce cetane? Might the large amounts of sulfur in HFO have an effect on the valve seats? How should I know? Do you?
temperature of operation, and corrosive action. -Isn't that two things?
}}}}} This is from Drwebb??? as posted above.
This above does not make sense........
1 TEL is not added as an anti wear, it is an anti knock.
2 Admission of not knowing??? I think the comparison was cause of valve seat wear and what causes it.
And the last part...... Yes its two things.
This makes me think of something. Not to belittle anyone. But my major complaint with a typical degreed program of education is, all the required mind fillers. Like having to know which greek god wore slippers with wings etc.
""""Temperature of operation, and corrosive action. Isn't that two things."""""
I ask what????????
Yes but isn't it easier to oxidize iron / steel at elevated temperatures????? Take for instance, an Oxy acetylene cutting torch. Do we get the picture. At room temps the steel plate won't be harmed as much via the O2, but heat it up and what happens????
RE: The "old octane question"(s)...sorry guys
If you will read a few of the dozens of papers published on the phenomenon of Exhaust Valve Seat Recession you will find that TEL, along with a few other metallic additives are well known to exhibit wear preventing properties in that application.
The full quotation was "It all boils down to one thing, temperature of operation, and corrosive action", which I thought was kind of funny, and hence the smiley.
One of the things any education should teach is to spend a little time in the library reading instead of assuming the ability to speculate makes one much of an authority on anything. Look, if you folks want to barbecue the TEL research (much of it published in engineering journals) have at it- I didn't conduct it. But taking pot shots without even bothering to read the work just makes you look silly.
RE: The "old octane question"(s)...sorry guys
Valve seat recession ? So what do you mean by that?
Do you mean that say, a hard seat that has been inserted into a counter bore, starts to recede?
Or are you talking about the micro welding and valve and seat corosive action of say cast iron and steel valves?
To slow this process we use such materials as stainless steel, and stellite. And the softer stainless, does not require "lubrication". Please present your model of TEL lubricant. And what the coeffient of friction is. Then explain the relative motion of two taper joints that come into contact. And why a stanless valve doesn't seem to need this TEL lubricant?
And here is some reading for you.......
http://www.du.edu/~jcalvert/phys/lead.htm
The qoute is from the above site.
"""Tetraethyl lead was made by treating ethyl bromide or ethyl chloride with an alloy of sodium and lead, NaPb. The sodium grabbed the halogen, the lead latched on to the ethyl radical, and some lead was left over, which could be reused. Some extra ethyl bromide was added to the fuel, so that in the heat of the combustion, it would combine with the lead that had initially been released, and carry it out in the exhaust. Otherwise, the lead would stay around and foul the spark plugs and valves (which it did anyway). Some people thought the lead lubricated the valves and was necessary for the engine, but this is false.""""<-----NOTE
RE: The "old octane question"(s)...sorry guys
http://www.avweb.com/news/columns/182149-1.html
Like I said, he says just another wives tail.
So like he said please present the data. He did .
RE: The "old octane question"(s)...sorry guys
I think the original question asked might be answered by thinking about increased future use of higher octane corn alcohol blend in some of our midwest farm states. This is maybe a politico's idea to further help subsidize our farmers, extend oil resources while also reducing exhaust envirnomental pollutants ? Not sure all the costs per mile driven are being considered or part of the equation at this point. Seems to depend whether your an envirnomentalist, a technical type, or a political type who bends with your voting average joes thoughts on subject.
Regardless, the stored up energy in the fuel source (hydrocarbon gasoline or alcohol fermented from corn) originally came from the sun. Took a bit longer time period though, to make the hydrocarbon....
I have heard that piston engines have been designed and used in auto transporation tests in Brazil that run on 100%alcohol and I believe alcohols have high octane numbers. I believe, in order to make alcohol piston engines have effeciency, they had to be manufactured with very high compression ratios and I suspect also, major changes to normal spark timing.
Even with changes, the specific energy produced, stating as miles per lb of fuel used, should be much lower with the alcohol high octane fuel vs the lower octane gasoline due to differences in specific energy of alcohol vs gasoline. Seem to remember it relates to bond energies contained in the molecules.
RE: The "old octane question"(s)...sorry guys
2000-01-2015 : Valve Seat Recession-An Independent Review of Existing Data 06/19/2000 Paper
2. 962029 : Valve Seat Recession and Protection Due to Lead Phase Out in Thailand 10/01/1996 Paper
3. 872076 : The Effect of Low Lead Gasolines and Some Aftermarket Lead Substitutes on Exhaust Valve Seat Wear 11/01/1987 Paper
4. 871622 : Effects of Using Unleaded and Low-Lead Gasoline and Gasoline Containing Non-Lead Additives on Agricultural Engines Designed for Leaded Gasoline 09/01/1987 Paper
5. 2000-01-2016 : Development of a High-Performance Anti-Wear Additive Providing Protection Against Valve Seat Recession Combined With Octane Enhancement in Treated Fuel 06/19/2000 Paper
7. 860090 : The Effects of Low-Lead and Unleaded Fuels on Gasoline Engines 02/01/1986 Paper
8. 710674 : Influence of Low Lead Fuels on Exhaust Valve Performance 02/01/1971 Paper
14. 710368 : Valve Problems With Lead Free Gasoline 02/01/1971 Paper
21. 861550 : Exhaust Valve Recession With Low-Lead Gasolines
RE: The "old octane question"(s)...sorry guys
Valve seat 'recession' is caused by microwelding of the valve to the seat, which creates small hard lumps of oxidised welded material. These form a grit that speed the wear (known as recession) between the back face of the valve and the insert. Operation at high speeds or temperatures increase the speed of the wear, as does rotation of the valves. TEL, or other products, build up a protective layer on the face of the insert. This layer prevents the microwelds from forming and may reduce the wear rate for a given amount of grit (sorry can't remember if this was discussed).
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: The "old octane question"(s)...sorry guys
Since I have not seen them, I have no idea if they are pro or con.
There is not a mention of it in C Fayette Taylor's books, nor in J B Heywood's book that I have found.
Any flux action or lubrication is all conjecture.
Unless some measured flim is proven in one of those SAE papers. TEL's primary purpose is Octane boost.
And thanks Gregg for the mention of temperature increase.
RE: The "old octane question"(s)...sorry guys
just type the paper # in the search box.
icrman, are you not aware that in the early 70's valve and seat materials where changed in all automobiles for sale in the US due to the phasing out of leaded fuel, because of the need to use materials that resisted recession by the mechanism GregLocock described, in the absence (or dearth) of lead compounds at the valve/seat interface?
You are right lead compounds' (of which TEL is a subset) primary purpose is (was) octane boost. Valve seat "lubrication" was a fortunate windfall benefit. Because of it's relative low cost per octane quotient, it was the favored method for tailoring octane in all gasolines prior to being legislated out. So in the leaded gasoline era (1930's - 1970's), auto manufacturers grew to expect that all gasolines would contain sufficient lead compounds to provide this benefit, when developing and marketing their engines.
RE: The "old octane question"(s)...sorry guys
I am not saying valve seat/face interface problems can not occure with the use of non leaded fuel. I'm saying the reasons for the cure maybe something else other than what is believed. The same type of problems can occure in non gasoline burning engines. And the fix was not leaded fuel.
RE: The "old octane question"(s)...sorry guys
You're also right that the fix for non-gasoline engines is not leaded fuel, but that's not the issue being debated. What has been asserted is:
Problem - valve/seat wear/recession due to microwelding phenomenon
Solution 1 - leaded or other gasoline additives providing protective coating at interface
Solution 2 - revised materials with inherent resistance to this problem
Which of these do you consider a "wives tail"?
RE: The "old octane question"(s)...sorry guys
RE: The "old octane question"(s)...sorry guys
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: The "old octane question"(s)...sorry guys
Now have you ever seen valve and seat temperature charts?
I think the term "micro welding" is kind of a misnomer when talking about the wear that occurs at the valve face-seat interface. The temperatures to cause that aren't present. And if they are then the so called lead film would be boiled off and scavenged. The cure for valve seat recession is to lower the temperature. Which as proven in a 1600 cc VW aircooled engine, TEL does effectively do. And which manufacuters of various large engines accomplish with extensive seat and valve cooling. In those SAE papers do they have reference the various observed temperatures?
RE: The "old octane question"(s)...sorry guys
But yes, temperature is definitely a factor in valve/seat wear, as GregLockock pointed out.
Now you've got me interested... what is the mechanism for TEL to lower valve seat temperatures?
RE: The "old octane question"(s)...sorry guys
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: The "old octane question"(s)...sorry guys