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Author Topic: How does octane number relate to volatility?  (Read 8687 times)

Offline William McCormick

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How does octane number relate to volatility?
« on: 25/08/2012 01:08:50 »
Someone who lives with me but shall remain nameless put a whole load of unleaded in a diesel van we were using to move house; luckily I spotted what she was up to before she'd put more than a third of a tank in. We filled it up the rest of the way with diesel and it ran fine... lucky escape I think...

The diesel engine will run colder on gasoline.

Gasoline is designed to cool the engine block when it enters the compression chamber of a gasoline engine. If you take an engine with no computer, and manually set the degrees before top dead center, that the spark plugs fire. You will notice that with high octane fuel, that you can actually increase the number of degrees before top dead center, and run the engine, giving much more torque and horse power, with no ping, then you could with lower octane fuel.

The ping is caused by not enough fuel or to much advancement of the timing. The ping is the diesel effect. It is very bad for gasoline cars, there connecting rods, the rods that connect the pistons to the crank shaft are not built for that kind of firing, so many degrees before top dead center. So most car manufacturers do not tweak the amount of timing advancement very much. It is easier to just use a safe setting. Spark plugs can actually melt, if the engine pings to much.

High Octane fuel actually evaporates much faster then lower octane fuel, if something is evaporating, creating heavy dense cool vapor, you can get more fuel into a cylinder. And more air and more oxygen. Cold air contains more oxygen.

If something is boiling, it creates hot, light gases, that do not allow the right amount of fuel into the cylinder. High Octane fuel is designed not to boil, in the carburetor, or intake manifold. In fuel injected systems high octane fuels, remain atomized fuel, to the very last second. Creating maximum cooling, and offering the maximum amount of before top dead center degrees possible.

So if you put gasoline into the diesel, and I do not know what type of diesel, some diesels have spark plugs, some do not, I would guess that your vehicle will have less energy, and run a bit cold, possibly causing more fuel to be used, because sensors may see it as a cold engine.

                      Sincerely,

                            William McCormick



« Last Edit: 28/08/2012 23:48:03 by chris »


 

Offline damocles

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Re: octane rating and evaporation
« Reply #1 on: 25/08/2012 02:59:00 »
William,

once again some of your post (expressed in a very matter-of-fact style) is complete and utter nonsense.
I will pick particularly on


Quote
High Octane fuel actually evaporates much faster then lower octane fuel, if something is evaporating, creating heavy dense cool vapor, you can get more fuel into a cylinder. And more air and more oxygen. Cold air contains more oxygen.

High Octane fuel does not necessarily evaporate much faster. Both rates of evaporation are very similar. Both types of fuel are mixtures that contain some components that evaporate faster and some that evaporate slower, and rates of evaporation, provided they exceed a certain minimum, are quite irrelevant to the performance of a petrol engine.

There is some cooling associated with evaporation of fuel, but the total amount of cooling is proportional to the amount of fuel that evaporates, not to its rate of evaporation.

The most important difference between a low octane and a high octane fuel is that the latter can more effectively limit the chain branching steps in the detailed mechanism of the combustion reaction that lead to explosions 

Cold air does not contain more oxygen than hot air. Obviously 2 gram of cold air contains more oxygen than 1 gram of hot air. You can use that argument to say that a lemon is sweeter than a grape, because it contains more sugar! In terms of mixing ratio (= proportions by number of molecules) or proportions by mass, cold air and hot air are identical in composition.

About the other parts of your post, some is OK, but boiling in the carburettor is equally undesirable for motors whether they run on high octane or low octane fuel, and no less likely to happen for high octane than low octane. In fact there is even a contradiction between your ideas that faster evaporation and less chance of boiling in the carburettor can go together -- boiling is simply a fast evaporation that occurs when the temperature of a liquid generates a vapour pressure that is equal to atmospheric pressure (or, more precisely, the pressure in the local environment).
 

Offline William McCormick

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Re: octane rating and evaporation
« Reply #2 on: 26/08/2012 03:44:04 »
William,

once again some of your post (expressed in a very matter-of-fact style) is complete and utter nonsense.
I will pick particularly on


Quote
High Octane fuel actually evaporates much faster then lower octane fuel, if something is evaporating, creating heavy dense cool vapor, you can get more fuel into a cylinder. And more air and more oxygen. Cold air contains more oxygen.

High Octane fuel does not necessarily evaporate much faster. Both rates of evaporation are very similar. Both types of fuel are mixtures that contain some components that evaporate faster and some that evaporate slower, and rates of evaporation, provided they exceed a certain minimum, are quite irrelevant to the performance of a petrol engine.

There is some cooling associated with evaporation of fuel, but the total amount of cooling is proportional to the amount of fuel that evaporates, not to its rate of evaporation.

The most important difference between a low octane and a high octane fuel is that the latter can more effectively limit the chain branching steps in the detailed mechanism of the combustion reaction that lead to explosions 

Cold air does not contain more oxygen than hot air. Obviously 2 gram of cold air contains more oxygen than 1 gram of hot air. You can use that argument to say that a lemon is sweeter than a grape, because it contains more sugar! In terms of mixing ratio (= proportions by number of molecules) or proportions by mass, cold air and hot air are identical in composition.

About the other parts of your post, some is OK, but boiling in the carburettor is equally undesirable for motors whether they run on high octane or low octane fuel, and no less likely to happen for high octane than low octane. In fact there is even a contradiction between your ideas that faster evaporation and less chance of boiling in the carburettor can go together -- boiling is simply a fast evaporation that occurs when the temperature of a liquid generates a vapour pressure that is equal to atmospheric pressure (or, more precisely, the pressure in the local environment).

You obviously have no experience with high octane fuels. High octane fuels have to be kept tightly capped or they will literally evaporate away on you, in a short period of time. I would put their ability to evaporate with the best lacquer thinners there are.

We use them to take the gum out of carburetors because they are so volatile. There vapor pressure is extremely high compared to normal fuels. This is something anyone would notice if you work with them.

I used to put a little 100 octane fuel in my car, because I had nitrous oxide, and tubular fuel injection under the carburetor. In the worst winters when everyone else could not start their newer model car, my car would kick over, in one firing of a spark plug. High octane works very much like ether works.

I would appreciate an apology because you are just on some kick to make light of what I am saying. If you ever worked with the stuff you would be laughing at yourself right now. And I am saying that as a friend. We would be laughing together if we had some of each fuel and tested them together.

The only reason you can advance the timing, well beyond top dead center with high octane fuel over an equal amount of low octane fuel, and not get ping is because the high octane evaporates, and does not boil. 

You can also open up or drill out the jets in a carburetor, or change the injector output, and burn more low octane fuel, to run a car at an advanced timing. But the outcome will not net you the same performance. Because you will not be able to get enough air in to burn it. 

The problem with low octane fuel is that it boils in the carburetor in race cars. Causing a light vapor not a cool dense vapor to make it to the piston. The hot light vapor also heats air it is traveling with expanding it. Making it impossible to get all the fuel and air into a cylinder. This is very well known amongst race car experts.

                      Sincerely,

                            William McCormick 

 
« Last Edit: 26/08/2012 04:26:01 by William McCormick »
 

Offline William McCormick

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Re: octane rating and evaporation
« Reply #3 on: 26/08/2012 04:04:33 »
The ping is caused by not enough fuel or to much advancement of the timing. The ping is the diesel effect. It is very bad for gasoline cars, there connecting rods, the rods that connect the pistons to the crank shaft are not built for that kind of firing, so many degrees before top dead center. So most car manufacturers do not tweak the amount of timing advancement very much. It is easier to just use a safe setting. Spark plugs can actually melt, if the engine pings to much.
"Dieseling" is when you shut off the key of a gasoline engine, and the engine keeps running due to compression ignition like a Diesel engine.  It is caused by a hot engine and high compression.  I suppose it would only happen with the key off with a carbureted engine, and not an injected engine, and I haven't heard it happen for years. 

The Dieseling could also occur while the engine is running in certain situations causing a "ping".  It is independent of the engine timing as the fuel is ignited without the spark. 

The higher octane rating of the fuel would tend to prevent this compression related pre-ignition, or dieseling which is why it is used in high performance cars.

Ethanol (CH3-CH2-OH) behaves much like high octane fuel, and is often used in race cars.  However, it is in fact much less volatile (evaporates less readily) than most of the gasoline.

Diesel in a gasoline engine would be much less likely to cause compression ignition as the in cylinder pressures aren't high enough.

Intercoolers are sometimes used with turbos because the turbo compresses the air making it hotter (like a heat pump or refrigerator).  The intercooler then cools the air which would effectively increase the air density and increase the amount of air being pushed into the cylinder. 

I don't know if fuel atomization/vaporization would also affect the air capacity per stroke.  Certainly it would only make a difference if the fuel entered the system prior to, or during the intake stroke, but not if it was injected during the compression stroke.

Dieseling is when a fuel self ignites, from compression. Dieseling takes place when an engine is very hot, and causes the engine to run on even after the gasoline electronic ignition system is turned off.

Ping is also the diesel effect, the piston has prematurely fired, without the electronic ignition system. You usually get no power from the detonation. Sometimes it drastically slows down a motor.

A car that is tuned near the limits will start to ping if the cooling system fails in that automobile. We know that high octane fuel cools.

Lower octane fuel has more BTU's then higher octane fuel, and can create more heat per gallon then higher octane fuels. It is all just a tweaking to make the most of the compression engine model as it exists. Just like diesel fuel makes more BTU's then regular gas. Number six industrial building fuel has more BTU's then diesel or home heating fuels.

I don't think we are at odds. We might just place importances at different levels.


                      Sincerely,

                            William McCormick   
 

Offline damocles

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Re: octane rating and evaporation
« Reply #4 on: 26/08/2012 05:08:14 »
William,

once again some of your post (expressed in a very matter-of-fact style) is complete and utter nonsense.
I will pick particularly on


Quote
High Octane fuel actually evaporates much faster then lower octane fuel, if something is evaporating, creating heavy dense cool vapor, you can get more fuel into a cylinder. And more air and more oxygen. Cold air contains more oxygen.

High Octane fuel does not necessarily evaporate much faster. Both rates of evaporation are very similar. Both types of fuel are mixtures that contain some components that evaporate faster and some that evaporate slower, and rates of evaporation, provided they exceed a certain minimum, are quite irrelevant to the performance of a petrol engine.

There is some cooling associated with evaporation of fuel, but the total amount of cooling is proportional to the amount of fuel that evaporates, not to its rate of evaporation.

The most important difference between a low octane and a high octane fuel is that the latter can more effectively limit the chain branching steps in the detailed mechanism of the combustion reaction that lead to explosions 

Cold air does not contain more oxygen than hot air. Obviously 2 gram of cold air contains more oxygen than 1 gram of hot air. You can use that argument to say that a lemon is sweeter than a grape, because it contains more sugar! In terms of mixing ratio (= proportions by number of molecules) or proportions by mass, cold air and hot air are identical in composition.

About the other parts of your post, some is OK, but boiling in the carburettor is equally undesirable for motors whether they run on high octane or low octane fuel, and no less likely to happen for high octane than low octane. In fact there is even a contradiction between your ideas that faster evaporation and less chance of boiling in the carburettor can go together -- boiling is simply a fast evaporation that occurs when the temperature of a liquid generates a vapour pressure that is equal to atmospheric pressure (or, more precisely, the pressure in the local environment).

You obviously have no experience with high octane fuels. High octane fuels have to be kept tightly capped or they will literally evaporate away on you, in a short period of time. I would put their ability to evaporate with the best lacquer thinners there are.

If you are talking about high octane fuels in terms of specialized racing blends, then you are quite right -- i do not have much experience. If you are talking about what you would buy as a premium blend at a petrol station then I have plenty of experience. If you are talking about methanol, I also have plenty of experience of its general properties, though not of its use as an automotive fuel.

When I am talking, on the other hand, about radical chain reactions and radical explosions, which are the scientific explanation of combustion processes, then I would venture to say that you probably have absolutely no experience nor even second hand knowledge.

"Pinging" and the inefficiency or low octane fuels in vehicles not tuned to run on such materials is quite definitely associated with straight chain rather than branched molecular structures, and relates to the much greater tendency of the former to burn explosively. Boiling in the carburettor would certainly cause performance problems, but they do not bear any great connection with pinging, other than reducing fuel supply so that the engine runs lean, which does increase the balance of a very complicated combustion reaction more towards explosion.

There are two main internal factors that control rate of evaporation: boiling point, and heat of vaporisation.

Some important components of automotive fuels:
methanol -- used only in blends for motor sports
ethanol -- a cheap way of raising the octane rating
n-heptane -- the standard 0 point for octane numbers
2,2,4-trimethylpentane or iso-octane -- the standard 100 point for octane numbers
toluene -- a relatively expensive way of raising the octane number

If you check out the boiling points, heats of vaporisation, and octane numbers, and the blend of these and similar materials in standard and premium petrol bowser fuel, you will find (remembering that fast evaporation goes with low boiling point and low heat of vaporisation) that apart from the extremely fast evaporation of methanol, there is no particular pattern. Incidentally, in terms of what this thread is about, there is absolutely no prospect of diesel fuel boiling in the carburettor, though blocking it may well be an issue.

Quote
I would appreciate an apology because you are just on some kick to make light of what I am saying. If you ever worked with the stuff you would be laughing at yourself right now. And I am saying that as a friend. We would be laughing together if we had some of each fuel and tested them together.

The only reason you can advance the timing, well beyond top dead center with high octane fuel over an equal amount of low octane fuel, and not get ping is because the low octane evaporates, and does not boil. 

You are not going to get that apology, William. Among other reasons that is because the last sentence makes little sense, unless you are in the business of trying to mis-tune motors. As far as scientific considerations are involved here evaporation and boiling are the same thing. I suppose the real problem here has to do with a gas bubble blockage in the liquid feed line, because the only real difference between evaporation and boiling is that the latter term is used when gas bubbles are formed within the bulk of the liquid. In any case, this is far from the only reason or even the major reason -- it has more to do with the relationship between explosive limits, pressure, composition, and temperature in a very complicated combustion reaction.

Also I am not on a kick to make light of what you say. I am interested in seeing that answers on the science parts of these forums make reasonable sense in the light of current scientific knowledge, and that discussions are carried out in a tone where we all recognize our limitations.

Quote
The problem with low octane fuel is that it boils in the carburetor in race cars. Causing a light vapor not a cool dense vapor to make it to the piston. The hot light vapor also heats air it is traveling with expanding it. Making it impossible to get all the fuel and air into a cylinder. This is very well known amongst race car experts.

You are probably quite right here. I have no experience with race cars. A failure of evaporative cooling would certainly lead to less fuel and oxygen intake, and a lower amount of vapour if there was a reduced intake of fuel would lead to less evaporative cooling. BUT if we are talking only about ordinary petrol in the absence of "boiling in the carburettor" there is almost no difference between low octane and high octane fuels in this regard.

 

Offline William McCormick

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Re: octane rating and evaporation
« Reply #5 on: 26/08/2012 14:21:01 »
William,

once again some of your post (expressed in a very matter-of-fact style) is complete and utter nonsense.
I will pick particularly on


Quote
High Octane fuel actually evaporates much faster then lower octane fuel, if something is evaporating, creating heavy dense cool vapor, you can get more fuel into a cylinder. And more air and more oxygen. Cold air contains more oxygen.

High Octane fuel does not necessarily evaporate much faster. Both rates of evaporation are very similar. Both types of fuel are mixtures that contain some components that evaporate faster and some that evaporate slower, and rates of evaporation, provided they exceed a certain minimum, are quite irrelevant to the performance of a petrol engine.

There is some cooling associated with evaporation of fuel, but the total amount of cooling is proportional to the amount of fuel that evaporates, not to its rate of evaporation.

The most important difference between a low octane and a high octane fuel is that the latter can more effectively limit the chain branching steps in the detailed mechanism of the combustion reaction that lead to explosions 

Cold air does not contain more oxygen than hot air. Obviously 2 gram of cold air contains more oxygen than 1 gram of hot air. You can use that argument to say that a lemon is sweeter than a grape, because it contains more sugar! In terms of mixing ratio (= proportions by number of molecules) or proportions by mass, cold air and hot air are identical in composition.

About the other parts of your post, some is OK, but boiling in the carburettor is equally undesirable for motors whether they run on high octane or low octane fuel, and no less likely to happen for high octane than low octane. In fact there is even a contradiction between your ideas that faster evaporation and less chance of boiling in the carburettor can go together -- boiling is simply a fast evaporation that occurs when the temperature of a liquid generates a vapour pressure that is equal to atmospheric pressure (or, more precisely, the pressure in the local environment).

You obviously have no experience with high octane fuels. High octane fuels have to be kept tightly capped or they will literally evaporate away on you, in a short period of time. I would put their ability to evaporate with the best lacquer thinners there are.

If you are talking about high octane fuels in terms of specialized racing blends, then you are quite right -- i do not have much experience. If you are talking about what you would buy as a premium blend at a petrol station then I have plenty of experience. If you are talking about methanol, I also have plenty of experience of its general properties, though not of its use as an automotive fuel.

When I am talking, on the other hand, about radical chain reactions and radical explosions, which are the scientific explanation of combustion processes, then I would venture to say that you probably have absolutely no experience nor even second hand knowledge.

"Pinging" and the inefficiency or low octane fuels in vehicles not tuned to run on such materials is quite definitely associated with straight chain rather than branched molecular structures, and relates to the much greater tendency of the former to burn explosively. Boiling in the carburettor would certainly cause performance problems, but they do not bear any great connection with pinging, other than reducing fuel supply so that the engine runs lean, which does increase the balance of a very complicated combustion reaction more towards explosion.

There are two main internal factors that control rate of evaporation: boiling point, and heat of vaporisation.

Some important components of automotive fuels:
methanol -- used only in blends for motor sports
ethanol -- a cheap way of raising the octane rating
n-heptane -- the standard 0 point for octane numbers
2,2,4-trimethylpentane or iso-octane -- the standard 100 point for octane numbers
toluene -- a relatively expensive way of raising the octane number

If you check out the boiling points, heats of vaporisation, and octane numbers, and the blend of these and similar materials in standard and premium petrol bowser fuel, you will find (remembering that fast evaporation goes with low boiling point and low heat of vaporisation) that apart from the extremely fast evaporation of methanol, there is no particular pattern. Incidentally, in terms of what this thread is about, there is absolutely no prospect of diesel fuel boiling in the carburettor, though blocking it may well be an issue.

Quote
I would appreciate an apology because you are just on some kick to make light of what I am saying. If you ever worked with the stuff you would be laughing at yourself right now. And I am saying that as a friend. We would be laughing together if we had some of each fuel and tested them together.

The only reason you can advance the timing, well beyond top dead center with high octane fuel over an equal amount of low octane fuel, and not get ping is because the low octane evaporates, and does not boil. 

You are not going to get that apology, William. Among other reasons that is because the last sentence makes little sense, unless you are in the business of trying to mis-tune motors. As far as scientific considerations are involved here evaporation and boiling are the same thing. I suppose the real problem here has to do with a gas bubble blockage in the liquid feed line, because the only real difference between evaporation and boiling is that the latter term is used when gas bubbles are formed within the bulk of the liquid. In any case, this is far from the only reason or even the major reason -- it has more to do with the relationship between explosive limits, pressure, composition, and temperature in a very complicated combustion reaction.

Also I am not on a kick to make light of what you say. I am interested in seeing that answers on the science parts of these forums make reasonable sense in the light of current scientific knowledge, and that discussions are carried out in a tone where we all recognize our limitations.

Quote
The problem with low octane fuel is that it boils in the carburetor in race cars. Causing a light vapor not a cool dense vapor to make it to the piston. The hot light vapor also heats air it is traveling with expanding it. Making it impossible to get all the fuel and air into a cylinder. This is very well known amongst race car experts.

You are probably quite right here. I have no experience with race cars. A failure of evaporative cooling would certainly lead to less fuel and oxygen intake, and a lower amount of vapour if there was a reduced intake of fuel would lead to less evaporative cooling. BUT if we are talking only about ordinary petrol in the absence of "boiling in the carburettor" there is almost no difference between low octane and high octane fuels in this regard.

I had corrected that low octane fuel evaporates as soon as I posted it. If you had read my previous posts you would have seen I said many times that high octane fuel evaporates more quickly then low octane fuel causing the fuel itself to stay very cool.

So if you want to play confusing word games I cannot stop you, however I hope others see you for the unscientific fellow you are. If anyone was such a great scientist we would be living on other planets by now.

You almost cannot boil high octane fuel. Because it will just evaporate with violence. I don't think you could throw a cup of high octane fuel across a room and have much if any hit the floor.

Ping is caused by a lack of fuel, to cool the cylinder. That is the only thing that can cause it. You can create or simulate a lack of fuel by advancing the timing of the engine many degrees before top dead center. You can heat the engine and intake manifold, from a poorly designed cooling system or a failed cooling system and get ping. Or you can feed the engine with a low octane fuel, that does not evaporate with vigor, but rather boils and does not allow in enough fuel to reach the cylinder in atomized form to keep the cylinder cool.

There are other things that can take a perfectly running engine and lean it out. A fellow that I built a bonnet for to go over his eight pack injector intake manifold, in the early eighties, found that the bonnet caused the air to increase in velocity as it entered the eight pack. He melted spark plugs while racing, and it took him a while to figure out what happened. We put it on a flow bench and found that when you put a flat plate over a round hole a certain distance above the tube, that you actually cause in increase in flow and velocity.

Some people around my area had that problem with chimney caps. When a cold front moved in, the funnel like design caused the air to rush down chimneys so fast that, when the oil burner fired, it fired into the house. There was that much pressure created by the flow setup by the cap over the chimney outlet.  Imagine a college cap kind of looking design, that is about what will do it.



                      Sincerely,

                            William McCormick



 

Offline Bored chemist

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Re: octane rating and evaporation
« Reply #6 on: 27/08/2012 10:20:54 »
According to wiki the room temperature vapour pressures of isooctane (octane number 100) and heptane (octane number 0) are 5.33 Kpa and 5.5 KPa
That's bugger all difference.

On the other hand, both methanol and toluene have octane numbers over 100.
Methanol is a vapour pressure of about 100 mmHg i.e. 13 KPa
Toluene has a vapour pressure of about 11 mmHg or 1.4Kpa

There is simply no relationship between volatility and octane number.
Got that?
So things like
"You almost cannot boil high octane fuel."
are plainly bollocks.
You can. To take an extreme case, hydrogen has a high octane number and is rather easy to boil.
Xylene also has a high octane number but it's sufficiently involatile to be a major constituent of some paint thinners.

"Ping is caused by a lack of fuel, to cool the cylinder.  That is the only thing that can cause it.  "
Nope, it can be due to a perfectly adequate source of the wrong fuel- i.e one with a low octane number.
 

The Naked Scientists Forum

Re: octane rating and evaporation
« Reply #6 on: 27/08/2012 10:20:54 »

 

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