69427

A few disagreements: Higher octane is there to prevent detonation, not slow the burn rate down. If higher octane fuel burned slower you would lose performance by retarding the piston/crank LPP point, but your 93/103 octane experiment showed that the burn rate and LPP point were unchanged.

Also, advancing the timing because you add additional octane does not increase performance, as you have moved the LPP point earlier, reducing the combustion thermal efficiency.

TJay74

Too bad several of your disagreements are wrong, higher octane fuel literally works by burning slower. I have read many industry articles and research on this topic along with video from inside of a combustion cylinder to show what and how detonation is and how it happens in the combustion chamber.

The higher the octane the slower the fuel burns. High octane fuel literally combats detonation/knock by slowing down the burn speed of the fuel to keep the flame front from pre-igniting and creating 2 individual flame fronts in the combustion chamber that end up colliding and creating detonation/knock (and the audible sound of knock, pinging). When timing is advanced too far it causes hot spots to form inside of the combustion chamber on the leading edges of things like valves, valve reliefs on pistons and spark plugs. Once those edges get hot enough due to the advanced ignition timing they can pre-ignite the fuel air charge before the actual spark plug is able to ignite the mixture. When this happens you end up with a artificially ignited flame front and then another flame front that is created when the spark plug ignites the mixture. These two independent flame fronts travel across the combustion chamber at the speed of sound and end up colliding (creating the knock and the pinging sound) along with hammering the surface of the combustion chamber. Advance the timing far enough and make the vehicle knock hard enough and the detonation will even mar the surface of the combustion chamber.

There was a video done from a scientific laboratory that had a high speed camera in the combustion chamber, it was neat seeing the detonation when filmed with a high speed camera.

Another area where you are wrong, and again this has been proven within the auto industry is with advancing timing, to some degree. Advancing timing can and will increase cylinder temperature and pressure and can increase horsepower, but only so far. Even with a fuel that can control the timing advance and keep detonation from happening, you can only advance the timing so far until reach a point of diminishing gains and where horsepower does not increase relative to the timing increase.

glass slipper

69427 is correct, go to the end of paragraph 6.3 of http://www.faqs.org/faqs/autos/gasoline-faq/part3/ and read the two sub paragraphs but especially the second sub paragraph where it states “flame speed does not correlate with octane”. Octane is the measure of a fuel’s resistance to knock, nothing more and nothing less. Read through all of paragraph 6 to gain more knowledge, especially the portion discussing the effect of higher octane fuels on pre-flame reactions.

I’m a mechanical engineer with a minor in internal combustion engines, I was a teaching assistant in my senior year and ran the engine lab which included a CFR engine (Combustion Fuel Research engine with variable compression ratio to determine the octane of a fuel). If you could provide links to the industry articles and research, I would like to read them in case anything has changed since I graduated 40 years ago.

There are too many variables to say definitely one way or the other about increased timing increasing HP. I will say that for any given set of variables, there is one ignition advance setting that makes best power. I can say that you want peak cylinder pressure to occur approximately 11-14 degrees ATDC to develop maximum HP.

Kingtal0n

https://www.eia.gov/energyexplained/...e-in-depth.php
"causes pressure in the cylinder to spike and causes the knock to occur."

Higher octane is "more stable" we could say that it is "more difficult to burn" this leads to the idea that it "burns more slowly" and under some circumstances yes it will burn more slowly because of its increased stability. The difference between 0-octane fuel and 100-octane fuel is that the 0-octane fuel rapidly reacts in an uncontrolled sudden reaction or can get started from minimal heat and pressure input causing it to rapidly explode in a cylinder and that creates the pressure spike which damages the engine suddenly 'knocking'. But if we have 100 octane fuel and 0-octane fuel side by side in a bucket and light them both on fire it is unlikely that one will 'burn more slowly' than the other under those conditions. I think that is why some ways of measuring the 'burn rate' or reaction rate of gasoline fuels can give us conflicting feeling about whether higher octane fuel actually burns more slowly or not.


To understand octane of gasoline hydrocarbons requires some knowledge of stereochemistry in fuel molecules and how that influences combustion rate.


The term 'burn slower' is dripping with potential misunderstanding. One engineer's "burn slower" is another scientists "poorly accessible carbons" or "highly accessible reaction radical intermediate formation"

I find this best explain by simply looking at the stereochemistry of 0 octane vs 100 octane gasoline fuel

So using our eyes and minds, what difference do we notice between these?
What difference is very obvious from the shape of the gasoline molecules?
It takes a little chemistry background. First, the zero octane fuel stacks very well. You can stack them very close together one on top of the other, very dense and thick stacks are possible. That means they can get very close to each other.
Another thing to notice is how easy it is to access the zero octane molecule. An oxygen radical can't tell any difference from any side of the 0-octane chain, it can basically attack anywhere it wants successfully.

These two things are the main difference between zero and 100 octane gasoline: The 100 octane gasoline stacks poorly, there is more space between them due to their shapes, AND they are more difficult to attack with oxygen radicals seeking to steal carbon covalent bonds. Oxygen radicals form at behest of spark initiation and attempt to break the bonds between carbons of gasoline, but have much more difficulty getting access to the high octane gasoline fuel due to it's shape and wider spreader. This causes many more time between reactions, and more "unsuccessful" attempts on behalf of the oxygen radicals bouncing with no affect at low enough temperature. Temperature is energy in the chemical reaction, higher temperatures speed up reactions because higher temp means more frequent and more powerful collisions between oxygen and fuel molecules. Chemical reactions proceed due to molecular collisions between molecules, collisions require enough energy and the correct orientation to gain access to the covalent bonds between carbon atoms in gasoline, and the high octane fuel because of it's difficult shape and more wider spreading is more resistant to approaching oxygen radicals, and the space between them, which means less frequent energy exchanges, less frequent reactions, a more 'stable' fuel quality.

In terms of energetics this has some implication that the lower the octane fuel, the more easier it is to burn, the less 'stable', which means less energy input is required to break carbon covalent bonds, ... which ultimately means higher fuel efficiency if the fuels contain similar energy content for a comparison. In other words, if the molecule can be broken easier with less energy input to relief it's stored energy, then the fuel efficiency of the engine should increase provided the increased reaction rate under heat and pressure doesn't damage the engine by causing a spike in pressure. Thus lower octane fuels tend to supply increased economy to an engine that can make use of the reduced stability. The pitfall to this is that in performance land, where we run our engine hard and the cylinder tends to increase in temperature, and we often run higher compression (especially in forced induction applications) The higher temp and higher fuel density (remember the spreading of fuel molecules is important factor and squeezing them closer together is increasing their density making it easier for oxygen radicals to encounter and attach the covalent bonds) of high performance engines means there will be an increased reaction rate which causes pressure to rise more quickly potentially damaging the engine. This is ultimately a tuning concept that a professional tuner must be aware of as a difference between the rate of fuel burning and the start of such a reaction, as their respective interplay is NOT mutually dependent . In other words if we time a spark very late, the fuel can still explode so rapidly that it destroys the engine. Imagine trying to run 0-octane fuel in any motor, with any spark timing. There should not be any timing value spark initiation you can try that will be successful to run the engine, no matter when we ignite the fuel it explodes and gas expands so rapidly that the pressure rises dramatically in relation to the moving piston and destroys the engine. Now think about 87 vs 93 octane and how the timing interplay with fuel burn rate. When using 87 octane fuel it might seem obvious that reduced spark timing is required, but it may not be so obvious that even with a very late spark, with enough heat and pressure, the fuel could still expand and explode too rapidly to be useful to the moving piston. This is the point where we should remark upon the moving rate of the piston itself, that is, slower gear ratios like overdrive cause reduced rate of engine acceleration which in turn requires a more stable, slower fuel reaction rate, than say revving in neutral. It gets kind of long winded tuning topics and as this isn't really a tuning discussion I won't keep typing. But I think using the above simple ideology you can see how lower octane fuel could benefit fuel economy and even power if the compression were very low and temperature were very low, under the right circumstances the reduced stability could work out just fine and improve cylinder pressure where it is needed. And how the higher octane fuel increased stability could really help for hot, or slower moving engines, or engines with higher compression, because it gives the piston time to 'move out of the way' move down the bore as the gas is expanding not-as-rapidly as a lower octane fuel would.

E TKIT RIDE

My two cents from something I learned from Kraka on this forum. For 2020 and 21 cars that the manual says requires 93 octane, find a station that sells E85 which is 100+ octane, put two gallons into your tank and the rest 91. You'll then have 93 octane. I do feel like my car runs better this way. I'm told GM fixed this in 2022 with their "fuel injection" updates so the car now makes the same power running 91 octane.

Andybump

In the sections of the Owners Manual about tracking the car, both 2021 and 2022 say 93 Octane is required. In the Driving and Operating section of the 2021 manual, the image show Required 93, but the wording say use 93, but 91 can be used with reduced power. In the 2022 manual the image says Minimum 91 , and the wording says 91 is required. So, while it explicitly says the 2021 will have reduced power with 91 octane, the 2022 wording, which requires 93 for the track, suggests that the 2022 gets some benefit from the 93 octane. So, they both can use 91 octane, and both require 93 octane on the track.

This one is from the Track section of the 2021 Manual.


This next one is from the Track section of the 2022 manual.


This next one is from the 2021 Manual, Driving and Operating Section;




This last one is from the 2022 Manual, Driving and Operating Section