The "old octane question"(s)...sorry guys 4

[RobHughes]

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!

 

[enginebob]

drwebb, so if this pb has a lubing effect on a valve face or seat. I have some questions. 1 Since the whole reason for lubrication is to reduce friction, please explain where there is a friction action? And why this has no effect on a softer "stainless steel" valve?
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.

 

[drwebb]

1) Do you not recognize wear prevention- which may be independent of friction reduction- as a function of a lubricant? And where does it say that all metals respond identically to anti-wear additives like TEL?

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?

 

[carnage1]

I have seen the same results for gasmileage vs octane in cars that asked for higher octane.
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.

 

[crysta1c1ear]

ivymike said:
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.

 

[patprimmer]

The above explanation seems crystal clear to me ;-)

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.

 

[25362]

For all that it's worth, n-octane has a road index octane number of -19 (!)

 

[crysta1c1ear]

Now that this thread has been kickstarted again, I reread some of the early posting.

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.

 

[Lou Scannon]

I for one am dubious about any results being quoted from "mileage tests" between different tanks of gasoline. Why should anyone believe these measurements were performed with any scientifically defensible experimental control and statistical significance?

 

[icrman]

{{{1) Do you not recognize wear prevention- which may be independent of friction reduction- as a function of a lubricant? And where does it say that all metals respond identically to anti-wear additives like TEL?

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????

 

[drwebb]

It does make sense if you read it carefully. My reply was directed to enginebob who preferred to counter informed assertion with hypothetical questions. I tried to lighten the exchange by answering him in kind; I knew a literary device might prove too subtle for some (engineers), but since it rated a couple stars I'm gratified a few more perceptive readers were amused. But since some of the posts have been deleted it's harder to follow the thread now.

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.

 

[icrman]

"Spend some time in the library"

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

 

[icrman]

More

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 .

 

[naturlm]

Thoughts. Chemicals think more in energy per lb terms and not necessarily in energy per gallon terms.

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.

 

[drwebb]

Ah, thanks for misspelling 'wives tail'. I suppose I'll be the one looking silly if I can't smell a troll. All in good fun of course, but now I'll bid adieu with a few tidbits from the hacks at the Society of Automotive Engineers- alas, paltry horsepower to counter that final arbiter of truth The WWW, but I'm kinda sentimental when it comes to good ole' fashioned peer review . . .

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

 

[GregLocock]

2000-01-2015 is a cracker. If I may summarise:

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.

 

[icrman]

Has anyone read these SAE papers? I would like a synopsis.
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.

 

[Lou Scannon]

SAE papers can be purchased online for under $15 at

http://www.sae.org/servlets/techtrack?PROD_TYP=PAPER

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.

 

[icrman]

hemi,

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.

 

[Lou Scannon]

icrman, it's always fair game to question widely accepted "knowledge", but what part of Greg Locock's excellent summary are you calling into question?
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"?

 

[swall]

icrman--I took a look at the AVweb link and have a couple of comments. First of all, aluminum head engines, which would comprise all of the aircraft piston engines for at least the last 60 years, have hard metal valve seat inserts.This alone may account for why the FAA test results performed on two aircraft with flat six engines showed no valve seat wear with the unleaded fuel. Second, it was not clear in the article if the test was done on brand new engines that had never been exposed to leaded fuel. Anecdotal evidence suggests that when a leaded fuel engine is switched to unleaded fuel, there is residual lead film on the valve seats for some time and this delays the onset of valve seat recession that would be encountered with unleaded fuel.So, if the test was done on engines that had previously used leaded fuel, the conclusion would be flawed. Finally, I always chuckle when I hear the term 100 octane "low lead" (avgas), since the stuff has 2 grams of lead per gallon.