Octane Question

[SrDriver]

In some areas regular unleaded is shipped 84 octane then blended with Ethanol to 87 octane.

Here in the south we still have 87, 89, and 93 octane at the pumps. 89 octane is a blend of 87 and 93 octane mixed by the pump.

Not sure about the 91 octane up north but assume it is 87 octane mixed with Ethanol to reach 91 octane?

 

[SilverSurfer]

Originally Posted By: kschachn
It's a standard ASTM test (D2700?) which at the time used a standardized single-cylinder engine. I did it as part of a fuels & lubricants class I took in college. If you Google around enough you will probably find descriptions of the ASTM test.

As far as flame front propagation speeds between hydrocarbons and alcohols go.... That's going to have to be from someone with a pay grade higher than mine. I'm sure pressure makes a difference but that's about all I can guess.

Octane rating is the relative resistance to detonation, detonation (I assume) meaning the transition to rapid combustion caused by a shockwave.

Originally Posted By: SilverSurfer
I am new around here, what test are you speaking of? Can you send a link?
Any validity to a correlation between octane rating of gasoline and flame speed?
I guess while I am asking, what about comparing flame speed of pure gasoline to ethanol?


Thanks.

I did find this article from Sunoco (http://www.racegas.com/article/10).

Quote:
If you’re not sure which fuel burns faster than others, one indicator is specific gravity. “Lighter” fuels – fuels with a lower specific gravity – tend to burn faster because fast burning hydrocarbons are themselves light. Look for a specific gravity close to 0.70 and you’ll likely find a fast burning fuel.
...
You might be surprised to learn that some of the highest octane fuels may also be some of the fastest burning fuels!

 

[Pontual]

You could test roughness comparing E85 with premium gasoline 95ROM+MON/2 or even better, race gas, since their resistance to ignition (octane rating) are closer than E85 and 87 gasoline, of course. Even with a comparatively advanced ignition spark, the gasoline engine is always smoothier than a alcohol engine feeded.

 

[kschachn]

Also just to be precise, the octane rating of a fuel that is posted at the pump is an average of a motor (M) and research (R) ratings, not just the motor rating I described. I believe the average is called the "anti-knock index" or AKI. You probably already know that but I just wanted to point it out.

And, I'm not sure that knocking is really detonation, it is also called pre-ignition. Once again I can't recall the details but actual detonation (a reaction propagated by a shock wave) may not actually be occurring despite the use of the term. In any case, it is ignition of the fuel/oxygen mixture at a time prior to when it is desired.

 

[SilverSurfer]

Originally Posted By: kschachn
Also just to be precise, the octane rating of a fuel that is posted at the pump is an average of a motor (M) and research (R) ratings, not just the motor rating I described. I believe the average is called the "anti-knock index" or AKI. You probably already know that but I just wanted to point it out.

And, I'm not sure that knocking is really detonation, it is also called pre-ignition. Once again I can't recall the details but actual detonation (a reaction propagated by a shock wave) may not actually be occurring despite the use of the term. In any case, it is ignition of the fuel/oxygen mixture at a time prior to when it is desired.


My understanding is that knocking occurs when two different flame fronts meet. First flame front from the spark plug causing the initial desired combustion, the second as caused by the "spontaneous" combustion due to pressure rise in the cylinder (due to heat from spark/expanding gas, piston moving up compressing things, and any existing latent heat in combustion chamber -- made worse by a hot engine, high ambient air temp, etc).

 

[4wheeldog]

Originally Posted By: SilverSurfer
Originally Posted By: 901Memphis
You can always get race gas fuel additive if you want more octane.

http://race-gas.com/


Yes I have seen that, and it seems legit. One of the vehicles is my daily driver, so I don't want to get some radical tune that requires me to bring race-gas additive with me everywhere I go. If I go on a roadtrip or something I just want to be able to use what is available. I just want to get the most bang for my buck!


Most bang for your buck......Assuming both are priced the same, and also assuming an EFI engine, you will get lower mileage with the E10 version. This is because ethanol only has about 60% of the BTUs in actual gasoline.

Since the ethanol produces a minor amount of oxygen when it burns (Which is why it is added as an "Oxygenator"), and may serve to cool the whole shebang a tiny bit, it is possible that your engine may be able to produce a tiny bit more horsepower witht E10......But I doubt you could find a dyno sensitive enough to measure it, and you would not be able to distinguish the cause from other background differences.....Temperature, humidity, etc.

 

[A_Harman]

Originally Posted By: kschachn
No, not all octane molecules are alike. It depends on how highly branched the 8C molecule is. n-octane has a very low octane number, whereas iso-octane is high.

And the octane rating is an aggregate of the gasoline's composition including lower and higher carbon chains.

Originally Posted By: SilverSurfer
Not exactly, but you have the basic idea. Octane is the name of a molecule. The more of these molecules, the greater the gasoline's ability to resist detonation caused by high compression ratios and too much ignition advance. That is, it raises the temperature at which the fuel begins to combust. Pre-ignition is usually caused by hot spots, like carbon build up on the exhaust valve and dirty spark plugs. Either way, its bad news for engines.


But the octane fuel that is used in the Octane Test is 100% iso-octane, technically called 2,2,4-trimethyl pentane. That is the branched form of octane.

 

[badtlc]

Originally Posted By: bigt61
Remember, that octane is an explosion inhibitor - lower octane is more volatile and has higher energy content (BTU). High octane is needed in high performance engines so they can squeeze the air-fuel tighter(high compression) for a bigger bang which equals more horsepower. Low octane fuel will explode too easily causing pre-ignition (it ignites all by itself from being squeezed - like a diesel) This pre-ignition causes power loss and engine damage in a gasoline engine. Octane additives are expensive - therefor the price difference.


Not completely correct. BTU between 87 and 91 is the same. Deflagration is different between the two. That is it.

 

[kschachn]

Originally Posted By: A_Harman
But the octane fuel that is used in the Octane Test is 100% iso-octane, technically called 2,2,4-trimethyl pentane. That is the branched form of octane.


Well as the standard, right? It receives a rating of 100, everything else is compared to that.

 

[A_Harman]

Originally Posted By: SilverSurfer


But yes, you have identified why I am asking the question! I have E0 91 octane and E10 91 octane available locally to me (no reliable 93 octane sources). So what do I pick? Where is the power? Pure gasoline has 3% more energy (BTU's), but ethanol can withstand more compression and spark advance for more power. If E10 91 starts life as E0 91 and ethanol is added to it, then it is higher than 91 octane, and I can use more spark advance to make more power. On the other hand, if it is E0 87 and ethanol is added to it, then I am still at 91 octane, but with less BTU's, so even worse.



Don't assume you're getting fuel that has higher (R+M)/2 than is posted on the pump.

 

[A_Harman]

Originally Posted By: kschachn
Originally Posted By: A_Harman
But the octane fuel that is used in the Octane Test is 100% iso-octane, technically called 2,2,4-trimethyl pentane. That is the branched form of octane.


Well as the standard, right? It receives a rating of 100, everything else is compared to that.


The CFR Test is done with a variable compression ratio single cylinder engine using fuels that are mixtures of iso-octane and normal-heptane. n-heptane is defined as having an Octane Number of 0, and iso-octane has an Octane Number of 100. The engine is run on a test fuel of unknown octane number, and the compression ratio at which a cylinder pressure transducer records autoignition is recorded. Then the engine is run on various mixtures of iso-octane and n-heptane until it records autoignition at the same compression ratio. If a test fuel exhibits autoignition at the same compression ratio as a mixture of 87% iso-octane and 13% n-heptane, it is said to have an Octane Number of 87.

The Research and Motor octane numbers are the result of testing the CFR engine at different intake charge temperatures and spark advance settings. The Motor Octane test is run at higher charge temperature and spark advance, so it produces lower octane numbers than the Research test. It is the average of the Research and Motor numbers that is posted on fuel pumps. The difference between the Research and Motor numbers is called the Sensitivity, and is generally 6-10 points.

 

[MNgopher]

In Minnesota, 91 octane E10 is created by blending 84 octane E0 with 91 octane E0 to get approximately 88 octane, and then blending in 10% ethanol to bring it back up to 91 Octane E10.

It is legal in MN to sell non-oxygenated premium gas (E0) for boats, small engines, collector cars, etc, but you can use it in your car too as it isn't monitored or policed. That is the 91 octane E0. It is usually a lot more expensive. That is part of why the E10 91 octane is a blend of 87 E0, 91 E0, and Ethanol - its cheaper.

 

[UrS4]

Ok guys. first I have to point out that I'm not trying to offend anyone at all. If someone find this post offending then please excuse me. Also English is my 2nd language so please bear with me.


Why is so many so focussed on the "two flame fronts colliding" theory regarding detonation..?!! I guess it's a theory posted in some old books about SI engines.
By searching the web I can't really find any valid proof to back up this theory (if you know about some decent evidence on this theory I would really like to read it) - what I mostly get is mechanics, backyard mechanics and car enthusiasts arguing back and forth - the thing they typically have in common is that they lack basic understanding of thermodynamics.


Try to look at it this way..
Let's say we have an engine operating at high load (high cylinder filling),
1. As the air fuel mix is being compressed in the cylinder the mixture will then heat up but not so much that it will self ignite

2. The spark plug fires at the time it is supposed to, say 20 deg. before tdc, and the air fuel mix is lit

3. The flame progresses from the spark plug like a growing hemisphere.
From the moment the fire is lit till several deg. after tdc the temperature in the cylinder rises remarkably and therefore the pressure will also rise. Meanwhile the piston will also contribute a little to the pressure and temp gain on its last way to tdc.

4. Air fuel mix is lit but flame front has not reached the outer edges of the combustion chamber and the piston has passed tdc, okay.
4.1 Now.. Imagine that the air-fuel temperature near the piston edge and cylinder wall gets so hot that it can't resist auto ignition any longer and then makes virtually all the unburnt air fuel mix detonate at the same time which will make a super rapid increase in pressure that potentiality may damage the engine. I will kinda claim that it happens so fast that the that pressure develop and collapse so fast that it will end up being a sonic pressure wave bouncing back and forth in the chamber till the pressures has equalized.


Lets say we have some glowing deposits in the combustion chamber that starts the burn before the spark plug fires..(That's what I call pre-ignition) Then the temperature rise rapidly much earlier and create much larger pocket of air and fuel that will detonate and create even higher pressures that may cause catastrophic engine damage


I don't require anyone to agree with my view on pinging/detonation in SI engines. But I hope someone will find it interesting and maybe dig deeper into the processes and phenomenons happening in SI engines.
Maybe someone got a deeper insight of what exactly goes on during detonation in SI engines. If so I will encourage you to share it so that we learn.

 

[SilverSurfer]

Originally Posted By: UrS4
Ok guys. first I have to point out that I'm not trying to offend anyone at all. If someone find this post offending then please excuse me. Also English is my 2nd language so please bear with me.


Why is so many so focussed on the "two flame fronts colliding" theory regarding detonation..?!! I guess it's a theory posted in some old books about SI engines.
By searching the web I can't really find any valid proof to back up this theory (if you know about some decent evidence on this theory I would really like to read it) - what I mostly get is mechanics, backyard mechanics and car enthusiasts arguing back and forth - the thing they typically have in common is that they lack basic understanding of thermodynamics.


Try to look at it this way..
Let's say we have an engine operating at high load (high cylinder filling),
1. As the air fuel mix is being compressed in the cylinder the mixture will then heat up but not so much that it will self ignite

2. The spark plug fires at the time it is supposed to, say 20 deg. before tdc, and the air fuel mix is lit

3. The flame progresses from the spark plug like a growing hemisphere.
From the moment the fire is lit till several deg. after tdc the temperature in the cylinder rises remarkably and therefore the pressure will also rise. Meanwhile the piston will also contribute a little to the pressure and temp gain on its last way to tdc.

4. Air fuel mix is lit but flame front has not reached the outer edges of the combustion chamber and the piston has passed tdc, okay.
4.1 Now.. Imagine that the air-fuel temperature near the piston edge and cylinder wall gets so hot that it can't resist auto ignition any longer and then makes virtually all the unburnt air fuel mix detonate at the same time which will make a super rapid increase in pressure that potentiality may damage the engine. I will kinda claim that it happens so fast that the that pressure develop and collapse so fast that it will end up being a sonic pressure wave bouncing back and forth in the chamber till the pressures has equalized.


Lets say we have some glowing deposits in the combustion chamber that starts the burn before the spark plug fires..(That's what I call pre-ignition) Then the temperature rise rapidly much earlier and create much larger pocket of air and fuel that will detonate and create even higher pressures that may cause catastrophic engine damage


I don't require anyone to agree with my view on pinging/detonation in SI engines. But I hope someone will find it interesting and maybe dig deeper into the processes and phenomenons happening in SI engines.
Maybe someone got a deeper insight of what exactly goes on during detonation in SI engines. If so I will encourage you to share it so that we learn.

Good insight! Good graph! Your English is good! Don't worry, nobody is going to execute you!

I think that graph might need to be adjusted just a little bit. How is detonation going to occur if the cylinder pressure is decreasing as indicated in the graph? I suppose heat could be increasing due to burning gasoline, but pressure is going down due to piston movement? That is the only thing I can think of.

Perhaps the two flame fronts comment is not accurate. Detonation is just a shock due to all the fuel combusting all at once, creating a force so great that it will crack pistons and rings.

 

[y_p_w]

Originally Posted By: MNgopher
In Minnesota, 91 octane E10 is created by blending 84 octane E0 with 91 octane E0 to get approximately 88 octane, and then blending in 10% ethanol to bring it back up to 91 Octane E10.

It is legal in MN to sell non-oxygenated premium gas (E0) for boats, small engines, collector cars, etc, but you can use it in your car too as it isn't monitored or policed. That is the 91 octane E0. It is usually a lot more expensive. That is part of why the E10 91 octane is a blend of 87 E0, 91 E0, and Ethanol - its cheaper.

There were some older threads where there was discussion about the earlier days of ethanol blended fuels where the base fuel wasn't specifically meant for oxygenates in mind. They'd take an 87 octane E0 or a 92 octane E0 and blend in MTBE or ethanol with a really high blending octane rating and call it a day. It might say 87 or 92 on the pump, but in reality it was higher. The industry called that "octane giveaway".

 

[y_p_w]

Originally Posted By: A_Harman
The CFR Test is done with a variable compression ratio single cylinder engine using fuels that are mixtures of iso-octane and normal-heptane. n-heptane is defined as having an Octane Number of 0, and iso-octane has an Octane Number of 100. The engine is run on a test fuel of unknown octane number, and the compression ratio at which a cylinder pressure transducer records autoignition is recorded. Then the engine is run on various mixtures of iso-octane and n-heptane until it records autoignition at the same compression ratio. If a test fuel exhibits autoignition at the same compression ratio as a mixture of 87% iso-octane and 13% n-heptane, it is said to have an Octane Number of 87.

I understand there's some sort of trick to testing for an octane rating higher than 100.

 

[MNgopher]

You are exactly right y_p_w.

Until recently, the large Magellan pipeline that can carry fuel up this way carried 87 octane E0. In Iowa, it meant that E0 87 octane was available, but more expensive than 89 octane which was an E10 blend of the 87 octane and ethanol. About a year ago, that pipeline system no longer carries 87 E0. Its all now 84 and 91 octane E0. Everything else from 87 octane to 93 octane can be made by blending those two and ethanol.

Local refiners had been making 84 octane E0 for years - ethanol use was mandated here in the 1990's.

 

[A_Harman]

Originally Posted By: SilverSurfer

I think that graph might need to be adjusted just a little bit. How is detonation going to occur if the cylinder pressure is decreasing as indicated in the graph? I suppose heat could be increasing due to burning gasoline, but pressure is going down due to piston movement? That is the only thing I can think of.



The detonation occurs ATC because the end gases reached autoignition temperature, then ignited all at once, sending shock waves bouncing around the cylinder. That is what causes the spikes on the negative slope portion of the cylinder pressure curve.

 

[A_Harman]

Originally Posted By: y_p_w
Originally Posted By: A_Harman
The CFR Test is done with a variable compression ratio single cylinder engine using fuels that are mixtures of iso-octane and normal-heptane. n-heptane is defined as having an Octane Number of 0, and iso-octane has an Octane Number of 100. The engine is run on a test fuel of unknown octane number, and the compression ratio at which a cylinder pressure transducer records autoignition is recorded. Then the engine is run on various mixtures of iso-octane and n-heptane until it records autoignition at the same compression ratio. If a test fuel exhibits autoignition at the same compression ratio as a mixture of 87% iso-octane and 13% n-heptane, it is said to have an Octane Number of 87.

I understand there's some sort of trick to testing for an octane rating higher than 100.


I think they use iso-octane with defined amounts of tetraethyl lead added as reference fuels once the Octane Rating goes over 100.

 

[SilverSurfer]

Originally Posted By: A_Harman
Originally Posted By: SilverSurfer

I think that graph might need to be adjusted just a little bit. How is detonation going to occur if the cylinder pressure is decreasing as indicated in the graph? I suppose heat could be increasing due to burning gasoline, but pressure is going down due to piston movement? That is the only thing I can think of.



The detonation occurs ATC because the end gases reached autoignition temperature, then ignited all at once, sending shock waves bouncing around the cylinder. That is what causes the spikes on the negative slope portion of the cylinder pressure curve.


If the remaining charge ignited all at once, shouldn't we see an instantaneous pressure spike?