High octane fuel burns slower?

[Enigma]

Flame speed increases as rpm increases, but remains the same in terms of crankshaft degrees.

This. More rpm creates more turbulence in the charge. A more turbulent charge burns quicker, and more efficiently. The burn rate is proportional to rpm, little need for ignition advance.

[MadBill]

...Not really.
You have to control the combustion pressure rise speed or a too fast moving pressure front in the chamber could cause the light end chain gases to pre-ignite,causing the two pressure fronts to collide creating a situation we all know and love so well.
I think I sad that right.

There are a number of commonly-held fallacies in the above statement:
o The worst effect of a 'too rapid' pressure rise due to fast but normal flame front progression is 'rumble', rough noisy combustion, especially at low speed, but not detonation. Otherwise, a faster pressure rise allows for a later spark event, less negative work being done before TDC and less time for the 'delay interval' for detonation to 'time out', i.e., the normal flame front arrives before detonation can occur ('Normal' combustion typically sees a pressure rise of ~ 20-30 psi/degree, rumble usually is noted >35 psi/°)
o Preignition by definition occurs prior to the normal spark event.
o Detonation isn't dependent on some fall out of fuel constituents leading to a concentration of more flammable compounds, it can happens in any combustible mixture. [Many years ago I worked on natural gas engine development and some of our tests resulted in knock/detonation while running on pure methane (RON ~140) at 17:1 CR with a very short duration cam.]
o "Colliding pressure fronts" is a very common fallacy, but detonation is actually an explosion, 'burning' at supersonic velocities. (although collision of the resulting pressure waves can add to the destruction.)

[4sfed]

o The worst effect of a 'too rapid' pressure rise due to fast but normal flame front progression is 'rumble', rough noisy combustion, especially at low speed, but not detonation. Otherwise, a faster pressure rise allows for a later spark event, less negative work being done before TDC and less time for the 'delay interval' for detonation to 'time out', i.e., the normal flame front arrives before detonation can occur ('Normal' combustion typically sees a pressure rise of ~ 20-30 psi/degree, rumble usually is noted >35 psi/°)
o Preignition by definition occurs prior to the normal spark event.
o Detonation isn't dependent on some fall out of fuel constituents leading to a concentration of more flammable compounds, it can happens in any combustible mixture. [Many years ago I worked on natural gas engine development and some of our tests resulted in knock/detonation while running on pure methane (RON ~140) at 17:1 CR with a very short duration cam.]
o "Colliding pressure fronts" is a very common fallacy, but detonation is actually an explosion, 'burning' at supersonic velocities. (although collision of the resulting pressure waves can add to the destruction.)

Good explanation.

[Truckedup]

I've spent some time searching for the answer.I can't find any reliable information to support that premium pump gas burn slower than regular grade. The following statement appears to explain how higher octane suppress detonation and or pre ignition. it seems what burns slower is the undesirable ignition not the desired ignition fired by the spark plug.

The anti-knocking agents reduce the activity of free radicals in the burning mixture, either by eliminating them, or by converting them to radicals that are relatively stable (and thus unreactive).

Burning proceeds as a chain reaction mediated by free radicals. If the radicals are unstable, the reaction is fast, and less heat is needed to initiate it. Branched alkanes (isooctane) and aromatic compounds produce relatively stable radicals which allow the chain reaction to die down. Tetraethyl lead eliminates free radicals by allowing them to combine with its ethyl groups.

[Kevin Johnson]

The idea that high octane fuel burns slower is a trivial consequence of a definition used to measure octane number, where iso-octane is assigned a value of 100 and n-heptane a value of 0. The laminar flame speed of iso-octane is lower than that of n-heptane.

http://www.fisita.com/education/congres ... 06sc40.pdf see figure 5 on page 5.

~~~~~~~~~~~~~~~~~~~

How 'bout them Mets?

[BrazilianZ28Camaro]

Maybe the assumption that high octane fuels burn slower may have arised when trying to burn high octane gas on low CR engines.

The partially burned mixture, (due the lack of initial heating from compression), still burning into the exaust sounds exactly like a engine with retarded ignition and performs like that too. Advancing the timing would supress this problem, but still, the performance was never recovered.

[Kevin Johnson]

I think this is what happens with knowledge.

http://naca.central.cranfield.ac.uk/rep ... tn-452.pdf page 6 of the pdf or page five of the document itself, last paragraph, first and second sentences.*

As time goes by much of the original research is forgotten and bits and pieces of it survive to generate a sort of rumor. The people who did know with first hand knowledge have passed away.



* Some people have complained to me that I need to be more specific with references. I hope that is specific enough. The problem is, though, if you only look at disconnected facts without the context of the document these sorts of "rumors" are perpetuated.

[The Radius Kid]

...Not really.
You have to control the combustion pressure rise speed or a too fast moving pressure front in the chamber could cause the light end chain gases to pre-ignite,causing the two pressure fronts to collide creating a situation we all know and love so well.
I think I sad that right.

There are a number of commonly-held fallacies in the above statement:
o The worst effect of a 'too rapid' pressure rise due to fast but normal flame front progression is 'rumble', rough noisy combustion, especially at low speed, but not detonation. Otherwise, a faster pressure rise allows for a later spark event, less negative work being done before TDC and less time for the 'delay interval' for detonation to 'time out', i.e., the normal flame front arrives before detonation can occur ('Normal' combustion typically sees a pressure rise of ~ 20-30 psi/degree, rumble usually is noted >35 psi/°)
o Preignition by definition occurs prior to the normal spark event.
o Detonation isn't dependent on some fall out of fuel constituents leading to a concentration of more flammable compounds, it can happens in any combustible mixture. [Many years ago I worked on natural gas engine development and some of our tests resulted in knock/detonation while running on pure methane (RON ~140) at 17:1 CR with a very short duration cam.]
o "Colliding pressure fronts" is a very common fallacy, but detonation is actually an explosion, 'burning' at supersonic velocities. (although collision of the resulting pressure waves can add to the destruction.)

While I don't doubt what you're saying,I wonder if all fuels - Alchy,gasoline,etc. will react the same way to the pressure rise stimulus as the pure Methane example you cited?
As I understand it,Alchohol can detonate and you won't even know it until something blows.
I guess what I'm getting at is considering the natural states and properties of the different fuels,would they not react somewhat differently in the chamber?

[MadBill]

I used the methane example to illustrate that because it has only one molecule and yet can detonate, specific constituents in gasoline are clearly not the cause of detonation. My knowledge of methanol fuel is limited, but I believe it is detonation resistant but somewhat prone to preigniton, which is a silent but deadly killer.
Certainly though, as you say, the hundreds (actually thousands) of constituents used as fuel in internal combustion alone or in combination with each other, do react differently in an engine.

[Kevin Johnson]

http://en.wikipedia.org/wiki/Deflagrati ... transition

Under certain conditions, mainly in terms of geometrical conditions such as partial confinement and many obstacles in the flame path that cause turbulent flame eddy currents, a subsonic flame may accelerate to supersonic speed, transitioning from deflagration to detonation. The exact mechanism is not fully understood,[1] and while existing theories are able to explain and model both deflagrations and detonations, there is no theory at present which can predict the transition phenomenon.

Now a paper from 50 years ago. [my emphasis] https://web.anl.gov/PCS/acsfuel/preprin ... 1_0185.pdf

However, there is no way of predicting under what conditions a detonation rather than deflagration will be established.

If one is inspired, a paper on methane oxidation in a shock tube: https://web.anl.gov/PCS/acsfuel/preprin ... 7_0082.pdf

Shock tube studies of iso-octane and n-heptane: http://www.dtic.mil/dtic/tr/fulltext/u2/a422646.pdf

To repeat the answer to the thread subject: the reference fuel chosen for the definition of 100 octane has a slower flame speed than the reference fuel chosen for the definition of 0 octane. This simply represents the thinking at the time the definition of octane rating was established. If you cannot generally predict the transition from deflagration to detonation in a particular fuel then it is empty to try to generally extrapolate flame speed to detonation.

[140Air]

I think this is what happens with knowledge......

As time goes by much of the original research is forgotten and bits and pieces of it survive to generate a sort of rumor. The people who did know with first hand knowledge have passed away.

This is an interesting reflection. Reinventing the wheel is a rather laborious and wasteful exercise.
However to be fair, combustion research has never stopped and the process is better known now than ever before. More recently some of the hotbeds of this research have been nuclear weapons labs which have the plasma expertise and the supercomputers to do the calculations. But, even they regard this problem as extremely difficult and beyond their supercomputers' ability to completely model.

[Kevin Johnson]

I think this is what happens with knowledge......

As time goes by much of the original research is forgotten and bits and pieces of it survive to generate a sort of rumor. The people who did know with first hand knowledge have passed away.

This is an interesting reflection. Reinventing the wheel is a rather laborious and wasteful exercise.
However to be fair, combustion research has never stopped and the process is better known now than ever before. More recently some of the hotbeds of this research have been nuclear weapons labs which have the plasma expertise and the supercomputers to do the calculations. But, even they regard this problem as extremely difficult and beyond their supercomputers' ability to completely model.

Yes, there is that slight difficulty of accurately modeling transfinite processes with a single cardinality.

[Truckedup]

Kevin has present an impressive pile of test facts that gives me the impression the "slower buring high octane" is not exactly true ?

[140Air]

I think this is what happens with knowledge......

As time goes by much of the original research is forgotten and bits and pieces of it survive to generate a sort of rumor. The people who did know with first hand knowledge have passed away.

This is an interesting reflection. Reinventing the wheel is a rather laborious and wasteful exercise.
However to be fair, combustion research has never stopped and the process is better known now than ever before. More recently some of the hotbeds of this research have been nuclear weapons labs which have the plasma expertise and the supercomputers to do the calculations. But, even they regard this problem as extremely difficult and beyond their supercomputers' ability to completely model.

Yes, there is that slight difficulty of accurately modeling transfinite processes

Yes....

with a single cardinality.

...not sure what you mean here, but I'm sure you are right Kevin.

[The Radius Kid]

with a single cardinality.

...not sure what you mean here, but I'm sure you are right Kevin.

You do know what a "Cardinal rule" is?