Octane number

[dovlet]

Dear All,

What do you think about flame speed between octane number 92 & 93? Which one will be expose faster if both will be used in engine with compression of 10:1?

[alberttm]

93 will burn better than 92, and 95 better than 93 mate)) what is that compression for 10:1 ?!!!

[dovlet]

Yes, 10:1.

Why you think so? can you explain?

[alberttm]

Yes, 10:1.

Why you think so? can you explain?
I don't think so, I know due to it's a well known scientific fact that a higher octane burns better, Im not sure I understand what you mean by 10:1 compression, what is it? Compression at a cylinder or fuel to air ratio? Moreover, just a few weeks ago I got missfiring filling 92 instead of 95 (both are ok, but I had bad spark plugs), eventhough they were bad, spark plugs were ok to fire 95 but they couldnt deal with 92, maybe this makes me think 95 burns better than 92, what do you think?

[Mcdiver]

The higher the compression in an engine, the higher octane needs to be used. I don't know why, but I have always been told this by my friend that builds race engines. A normal factory engine rarely benefits from high octane fuel since the compression is lower.

[dovlet]

I don't think so, I know due to it's a well known scientific fact that a higher octane burns better, Im not sure I understand what you mean by 10:1 compression, what is it? Compression at a cylinder or fuel to air ratio? Moreover, just a few weeks ago I got missfiring filling 92 instead of 95 (both are ok, but I had bad spark plugs), eventhough they were bad, spark plugs were ok to fire 95 but they couldnt deal with 92, maybe this makes me think 95 burns better than 92, what do you think?

You could not explain me. I tried to search that scientific facts in google but unfortunately i am fault. 10:1 is compression of your ignition chamber (cylinder). Regarding to Misfire it is probably due to octane number of fuel not for your engine. Do not forget that if your ignition is installed with respect to 93 you most likely with have misefire with 92.

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The higher the compression in an engine, the higher octane needs to be used. I don't know why, but I have always been told this by my friend that builds race engines. A normal factory engine rarely benefits from high octane fuel since the compression is lower.

Yes, that is true. But it is not mean that 93 burn faster than 92.

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I think 92 faster burn than 93 because it has less octane increasing additives.

[alberttm]

The higher the compression in an engine, the higher octane needs to be used. I don't know why, but I have always been told this by my friend that builds race engines. A normal factory engine rarely benefits from high octane fuel since the compression is lower.
Didn't know this, thank you, this will probably mean for a dead engine with low compression the lower octane is better.

[DuckM5]

My understanding is that higher octane burns slower and cooler than lower octane. With lower octane and higher compression, the higher heat may cause pre-detonation which is the knocking. That is why the higher octane helps prevent knocking as the slower and cooler burn help to prevent detonation happening too early.

[alberttm]

Ok, I dont really get what you mean faster, is it something like 92 burns in one minute and 93 in one and half, but we are probably talking different things, yes the lower octane is more explosive, is that supposed to mean faster?

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As far as I know my car is supposed to run fine both with 92 and 95, but 95 is preferred.

[DuckM5]

Maybe this will help explain lower vs higher octane.

https://www.carthrottle.com/post/engineering-explained-high-vs-low-octane-petrol/

[ImTheDevil]

DuckM5 is correct. When a fuel-air mixture is ignited, the flame front expands outward from the point of ignition. A higher octane fuel burns more slowly - the flame front expands more slowly in a more controlled burn. A lower octane fuel burns faster - in a sense it's more volatile.

Higher octane is usually required both on a high compression engine and on a boosted engine because it is less volatile and therefore less likely to detonate, or pre-ignite. In the case of a boosted engine, cylinder temperatures can get very high and a low octane fuel can start to ignite before the plug fires. This pre-ignition (detonation) is bad because the ultrasonic shock waves from the ignition travel downward and hit the piston on its way up. If this happens strongly enough, something has to give, and it's usually a rod or a piston. A high-compression engine has the same risk - remember in physics, compressing a gas heats it up? The higher the CR of an engine, the hotter that mixture gets as the piston travels upward toward TDC. If it gets hot enough to ignite before the plug fires, you can break parts. The higher octane fuel resists this better and allows for high compression and/or boost without as much chance of detonation.

The ultimate example of this is Top Fuel drag racing. Those engines are 1/5 of an a$$ hair away from going thermonuclear at WOT. They run 90:10 nitromethane:methanol fuel, which is so resistant to ignition that you could drop a lit Zippo lighter in a bucket of it and it would just douse the flame. It's the only thing they've developed that can withstand the incredible temperatures and pressures of the cylinders on those engines.

[DuckM5]

I am not an expert so maybe this will help:

https://www.carthrottle.com/post/engineering-explained-high-vs-low-octane-petrol/

[djb2]

The "octane" rating is more specifically called the Anti-Knock Index.

It describes how resistant the fuel is to exploding rather than burning with a smooth flame front.

Two fuels ('gasoline blends') can have the same octane rating, but have different flame front speeds and different energy content.

In general, with very similar blends, the higher octane gas will have a slightly slower flame front and lower energy content. But you can't count on that being true from a pump. The gas will either be identical (the 92 octane actually being identical to the 93 octane, coming from the same refinery and tanks, just branded differently), identical with a different additive package, or a very different blend with different components.

[alberttm]

The "octane" rating is more specifically called the Anti-Knock Index.

It describes how resistant the fuel is to exploding rather than burning with a smooth flame front.

Two fuels ('gasoline blends') can have the same octane rating, but have different flame front speeds and different energy content.

In general, with very similar blends, the higher octane gas will have a slightly slower flame front and lower energy content. But you can't count on that being true from a pump. The gas will either be identical (the 92 octane actually being identical to the 93 octane, coming from the same refinery and tanks, just branded differently), identical with a different additive package, or a very different blend with different components.

Finally, so this means higher octane burns BETTER, and probably 93 burns 0, 00000000000000000001 milliseconds faster than 92, if this was the issue that matters.

[seabass07]

You will not misfire from 93 if your car is tuned for 92. Cars that are tuned for 87 don't misfire on 93 either unless there's a problem. If you're chasing down a problem, I doubt an octane point is your problem.

[ImTheDevil]

Finally, so this means higher octane burns BETTER, and probably 93 burns 0, 00000000000000000001 milliseconds faster than 92, if this was the issue that matters.

The 93 would burn that much slower than the 92, not faster.

[alberttm]

The 93 would burn that much slower than the 92, not faster.

Thanks cap, I finally got it))) Faster is not always better

[dovlet]

You will not misfire from 93 if your car is tuned for 92. Cars that are tuned for 87 don't misfire on 93 either unless there's a problem. If you're chasing down a problem, I doubt an octane point is your problem.

In this case if you use 92 or 93 Octane for engine tuned for 88 Octane you will have less power end due to effective explosion will be in latter angle. Somewhere around 23-30° after your piston reach dead or upper point. Effective angel when explosive will apply more power is 17° after piston reach upper point.

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Thanks cap, I finally got it))) Faster is not always better

Exactly...

[alberttm]

In this case if you use 92 or 93 Octane for engine tuned for 88 Octane you will have less power end due to effective explosion will be in latter angle. Somewhere around 23-30° after your piston reach dead or upper point. Effective angel when explosive will apply more power is 17° after piston reach upper point.

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Exactly...

OK, the next thing I learn would be, "How do gamma rays during sun explosions affect my engine's torque when I get from home to work?".

[seabass07]

In this case if you use 92 or 93 Octane for engine tuned for 88 Octane you will have less power end due to effective explosion will be in latter angle. Somewhere around 23-30° after your piston reach dead or upper point. Effective angel when explosive will apply more power is 17° after piston reach upper point.

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Exactly...

It's not ideal. You'll lose power, but it won't misfire.

You're doing forced induction, right? On a high compression engine? Knock resistance is your friend.

[dovlet]

OK, the next thing I learn would be, "How do gamma rays during sun explosions affect my engine's torque when I get from home to work?".

That is the one of the main point you have to know. To prevent gammary explosion to your engine use sun preventive cream. Should help)))

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It's not ideal. You'll lose power, but it won't misfire.

You're doing forced induction, right? On a high compression engine? Knock resistance is your friend.

Belive me knock sensor will not help if detonation occur. It happens fast & before detonation sensor realize them. If sensors can response immediately for detonation you will never have burnt pistons because when it starts sensor sent signal to ECU to change ignition angels. In my case I have had 2 burnt pistons due to detonation and my ECU did not response immediately and did not send signal to ECU to change angle for ignition.

[seabass07]

I said knock resistance. As in higher octane. But I'm guessing the ecu wasn't able to safely run the boost?

[ImTheDevil]

Dovlet, what was the cause of the detonation? Was it too much boost for your fueling, an injector or pump failure, too much timing advance? I'm always morbidly curious about engine autopsies after failures like that...

[dovlet]

Dovlet, what was the cause of the detonation? Was it too much boost for your fueling, an injector or pump failure, too much timing advance? I'm always morbidly curious about engine autopsies after failures like that...

Cannot give 100% right answers. But I guess it happened due to next:
1. Pump could not supply sufficient fuel.
2. Timing advance normally but lean fuel will create detonation before ignition.
3. Due to lean fuel temperature in cylinder 7&8 increased which means also preignition or detonation.
4. Too much boost not tuned ECU. Pulley o SC was 3.25 which creates around 10 PSI.

Lean Fuel and not right installed timing advance=High Temperature=Detonation and Preignition.

[RToth]

In this case if you use 92 or 93 Octane for engine tuned for 88 Octane you will have less power end due to effective explosion will be in latter angle. Somewhere around 23-30° after your piston reach dead or upper point. Effective angel when explosive will apply more power is 17° after piston reach upper point.

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Exactly...

The ignition actually starts before top dead center though (BTDC), not after. Under cruising / light throttle conditions the ignition advance will be earlier ~30 degrees to get the most efficiency / power from the lean mixture. As throttle input and load increases the ignition timing will actually decrease as the air / fuel ratio gets richer or closer to stoich as that provides the most power with the richer mixture. Of course the actual timing numbers BTDC will vary from engine to engine.

[BimmerBreaker]

Octane is simply a measurement of how much a fuel can be compressed before igniting. The energy in each octane of fuel is the same - give or take (no measurable/noticeable difference for uses such as our engines). Using an octane higher than the engine can use is bad for the engine - using an octane lower than it is designed for would cause major problems on old engines, but modern engines have knock detectors that will retard timing when it detects pinging/knocking and thus for long distance, low load trips, one can use lower octane safely to both lower their gas costs and increase MPG. Lower octane is easier to ignite, thus the engine will not work as hard to ignite it (thus increasing mpg, albeit slightly, since timing will also be retarded - not a real problem during low load driving).

A lot of BMW engines can use higher octane than the "minimum" requirement 91 AKI (Europe and most other places use RON octane measurement system, which will be about 95 RON = 91 AKI). You can damage your engine using an octane much higher than recommended and you will NOT gain any performance if the knock detectors do not adjust for it. Engines like the S62 can make use of up to about 95/96 AKI (~101-105 RON). I'd imagine the M62 could make use of about 92/93 AKI (~96-98 RON), maybe 94 (~100 RON)... but I do not have much (really, any) experience with these engines.

Forced induction changes everything. Do not use lower octane with forced induction - bad things can happen before the knock detectors can say "Back off!!!"

[dovlet]

The ignition actually starts before top dead center though (BTDC), not after. Under cruising / light throttle conditions the ignition advance will be earlier ~30 degrees to get the most efficiency / power from the lean mixture. As throttle input and load increases the ignition timing will actually decrease as the air / fuel ratio gets richer or closer to stoich as that provides the most power with the richer mixture. Of course the actual timing numbers BTDC will vary from engine to engine.

Read carefully again, but perhaps my English is not good because I am not English native. More Russian.
I know that ignition is occurring before upper point -30 degree (BTDC) and exposing proceeding until 17 degree after upper point (BTDC) in ideal case. It is giving you more power to push your piston down. But, if your fuel is high octane than needed your final power to push piston will be at 20 or 23 degree which means you will never have sufficient power as it could be with right octane number.

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Octane is simply a measurement of how much a fuel can be compressed before igniting. The energy in each octane of fuel is the same - give or take (no measurable/noticeable difference for uses such as our engines). Using an octane higher than the engine can use is bad for the engine - using an octane lower than it is designed for would cause major problems on old engines, but modern engines have knock detectors that will retard timing when it detects pinging/knocking and thus for long distance, low load trips, one can use lower octane safely to both lower their gas costs and increase MPG. Lower octane is easier to ignite, thus the engine will not work as hard to ignite it (thus increasing mpg, albeit slightly, since timing will also be retarded - not a real problem during low load driving).

A lot of BMW engines can use higher octane than the "minimum" requirement 91 AKI (Europe and most other places use RON octane measurement system, which will be about 95 RON = 91 AKI). You can damage your engine using an octane much higher than recommended and you will NOT gain any performance if the knock detectors do not adjust for it. Engines like the S62 can make use of up to about 95/96 AKI (~101-105 RON). I'd imagine the M62 could make use of about 92/93 AKI (~96-98 RON), maybe 94 (~100 RON)... but I do not have much (really, any) experience with these engines.

Forced induction changes everything. Do not use lower octane with forced induction - bad things can happen before the knock detectors can say "Back off!!!"

You absolutely right but, energy is the same. But, higher octane has more additives which allow to resist to flame. 91 & 93 octane fuel will burn with different speed on the same engine. Am I right?

[seabass07]

There is less energy in the higher octane.

[alberttm]

Read carefully again, but perhaps my English is not good because I am not English native. More Russian.
I know that ignition is occurring before upper point -30 degree (BTDC) and exposing proceeding until 17 degree after upper point (BTDC) in ideal case. It is giving you more power to push your piston down. But, if your fuel is high octane than needed your final power to push piston will be at 20 or 23 degree which means you will never have sufficient power as it could be with right octane number.

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You absolutely right but, energy is the same. But, higher octane has more additives which allow to resist to flame. 91 & 93 octane fuel will burn with different speed on the same engine. Am I right?

No Dovlet, your English is OK, you just blew it saying fuel burns after piston reaches dead point, not before, he he))

[BimmerBreaker]

You absolutely right but, energy is the same. But, higher octane has more additives which allow to resist to flame. 91 & 93 octane fuel will burn with different speed on the same engine. Am I right?
Octane is only a measurement of how much a fuel can be compressed before igniting and octane rating has no (direct or statistically significant) bearing on flame speed. It is also the chemical structure of the fuel that determines it's octane rating - not the additives. Most fuel additives nowadays are detergents. No TEL here :)

There is less energy in the higher octane.
True - higher octane fuels can contain less energy than lower octane fuels. The reason higher octane fuels can generate more power with less energy content is because they can be compressed further. This is also why running higher octane on an engine calling for low octane is not only not beneficial but can be bad for the engine.

[vinyldude]

It is also the chemical structure of the fuel that determines it's octane rating - not the additives. Most fuel additives nowadays are detergents.

Ever heard about lead, MTBE, (or in these modern days) ethanol? All of these are additives to increase the octane rating of gas.
But you are partially correct because big part of the equation is also what comes out of the refining process and how much heavier hydrocarbons such as octane-molecules are in the mix.

[dovlet]

No Dovlet, your English is OK, you just blew it saying fuel burns after piston reaches dead point, not before, he he))

�� is it? Sorry, fuel starts burn before upper point of piston and continue after reaching upper point. And all power applying in 17 degree. Best case scenario.

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So, 2 different Octane numbers have 2 different burn speed and energy due to power explosion time increased in comparing to lower Octane fuel. Lower Octane will burn in 15 m/sek and give 100 power (example). Higher Octane will burn 20 m/sek and give 95 power due to burn process increased.

[alberttm]

�� is it? Sorry, fuel starts burn before upper point of piston and continue after reaching upper point. And all power applying in 17 degree. Best case scenario.

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So, 2 different Octane numbers have 2 different burn speed and energy due to power explosion time increased in comparing to lower Octane fuel. Lower Octane will burn in 15 m/sek and give 100 power (example). Higher Octane will burn 20 m/sek and give 95 power due to burn process increased.

Dovlet there is NO SUCH THING FASTER OR SLOWER, words fast or slow can not be used is this context, http://www.2strokeheads.com/tech-octane-detonation.htm,

Also read this,

The above posting on Detonation is a good article, the only thing I have to add to it is a quick note on fuel octane. I also for a long time thought that a high-octane fuel was very slow burning; it makes perfect sense. A few months ago I read an article in Hot Rod magazine on fuel octane rating that changed my perception... the article's authors did some tests with various gasolines and additives in one engine on a dyno, ranging from pure pump gas to gas with octane additives to race gas. The results were as one might expect, except for this fact: the higher the octane of the fuel, the LESS ignition advance was required to obtain maximum power! This meant that the high octane fuel was burning more quickly, not slower as according to conventional wisdom. The article prompted me to do some further research.

READ MORE...

Let me try to explain it, OK if you just take low and high octane fuel into a cup and try to burn it, yes, high octane IS MORE RESITANT to ignition, which makes it safer, less detonative, and slower, BUT WHEN COMPRESSED HICH OCTANE IS COMPRESSED MORE THAN LOW, so LESS IGNITION ADVANCE IS NEEDED AND THE SAME POWER IS OBTAINED. When making high octane the idea is not to make it SLOWER the idea is to make it more effective and less dangerous of harmful. So fuel burns the same but under different compression, if you have low compression low burns better, because high will not be compressed enough to burn the same and realease the same power.

[dovlet]

Dovlet there is NO SUCH THING FASTER OR SLOWER, words fast or slow can not be used is this context, http://www.2strokeheads.com/tech-octane-detonation.htm,

Also read this,

The above posting on Detonation is a good article, the only thing I have to add to it is a quick note on fuel octane. I also for a long time thought that a high-octane fuel was very slow burning; it makes perfect sense. A few months ago I read an article in Hot Rod magazine on fuel octane rating that changed my perception... the article's authors did some tests with various gasolines and additives in one engine on a dyno, ranging from pure pump gas to gas with octane additives to race gas. The results were as one might expect, except for this fact: the higher the octane of the fuel, the LESS ignition advance was required to obtain maximum power! This meant that the high octane fuel was burning more quickly, not slower as according to conventional wisdom. The article prompted me to do some further research.

READ MORE...

Let me try to explain it, OK if you just take low and high octane fuel into a cup and try to burn it, yes, high octane IS MORE RESITANT to ignition, which makes it safer, less detonative, and slower, BUT WHEN COMPRESSED HICH OCTANE IS COMPRESSED MORE THAN LOW, so LESS IGNITION ADVANCE IS NEEDED AND THE SAME POWER IS OBTAINED. When making high octane the idea is not to make it SLOWER the idea is to make it more effective and less dangerous of harmful. So fuel burns the same but under different compression, if you have low compression low burns better, because high will not be compressed enough to burn the same and realease the same power.

Yes, but we are talking in this thread about 1 engine and different octane numbers. 2 different octane will have different burning properties in the same engine. Am I right or not?

[alberttm]

Yes, but we are talking in this thread about 1 engine and different octane numbers. 2 different octane will have different burning properties in the same engine. Am I right or not?

Dont think there will that much difference if you talk about one engine and 92 or 93, its not the difference you can see, feel and even hard to prove that one is slower than another, if you tak about 10:1 compression. Its something like take two same volume glasses of water and state that one has probably more H2O moleculas than another.

The best way to state something for sure is by proving it, if can not be proved then yes in theory its like you say, but still in fact we dont know which one is actually faster or slower, it might be that different substances act or react differently under different, severe or extreme conditions, so the only way ti check it out is to check it out. Pour some 92 and 93 into that engine which this thread we are talking about, take a stopwatch and check, why not?

[ViolinARC]

This reminds me of the "Which oil should I use?" threads...:stickoutt

My '00 clearly states "87AKI Minimum" and "91AKI Recommended" on the gas cap while my '03 shows "91AKI Only". Same engine, same software but for some strange reason, for the '03 version, 87AKI is no longer "good enough". Maybe it's the M-bumpers and 18" rims vs. the 17"??? :dunno

I've been running 87AKI (I do use 91AKI every two or three fills) in the '00 for YEARS without any adverse effects and average around 18-19 MPG mixed (mountain, city, highway and I drive hard). I have gotten as high as 22MPG average. So far the '03 (running 91AKI only) has been averaging 15.5 MPG so far (same roads and driving style). Just for kicks, I may switch to 87AKI on the '03 to see what happens to my fuel economy...:confused

[seabass07]

No idea what the reason for that is, but it could be a DME change or could be from failures. Different timing/fueling can easily make it no longer safe to run 87. Interesting that they changed their position on that.


My reading comprehension sucks today. Missed the same software part...