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!

 

[patprimmer]

This has been discussed many times before. Try a search and we won't have to do it all again

Regards
pat pprimmer@acay.com.au
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[Rob45]

But the short answer to your questions is, "Yes."

 

[franzh]

Rob45, but that was one dyno study I conducted about 10 years ago with different results than what you presented.

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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[patprimmer]

Octane level in itself does not effect burn rate, nor power level. Some high octane fuels burn fast, some burn slow.

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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[franzh]

What we postulated was that the rate of combustion in relation to the piston position during the expansion stroke contributed to the different combustion temperatures. The higher octanes produce peak combustion temperatures/pressures slightly later during the expansion stroke than the lower octanes, leading credibility to the the statement that octane fuels resist detonation, are harder to ignite, and once ignited have a slightly longer burn duration. Basic fuels have an inherent octane rating without the use of additives, and some additives help increase the relative octane rating.
Franz

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[patprimmer]

Franz

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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[franzh]

Pat:
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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[drwebb]

"Also explains the "lubed" valve face fairytail."

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.

 

[drwebb]

At what temperature does lead oxide breakdown?
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.

 

[Azmio]

Well, I am not a fuel expert. However, there was an article about the use of different octane number for cars equipped with and without knock sensor.

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.

 

[patprimmer]

Azmio

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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[Azmio]

Pat,

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

 

[patprimmer]

Azmio

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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[ICman]

Okay, Pat, I searched and found very little on this subject.

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

 

[patprimmer]

Steve.

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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[ivymike]

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.

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.

 

[ICman]

The Mini probably adjusts timing based on the knock sensor. Perhaps the Honda does not, or compensates for low-octane gas another way?

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 . If so, then mileage may increase with octane number. Yes?

. . . Steve

 

[ivymike]

the honda does not require high octane gas. If it did require high octane gas, the results would likely be different.

 

[Azmio]

Icman,

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.