New 906 owner needs gas, fuel suggestions for the U.S.

[DesmoDog]

Now as to whether high octane fuel burns slower than low octane fuels I offer up the following.
Knocking is caused by the rapid explosive and INCOMPLETE burning of vaporized fuel ahead of the normal flame front. High octane fuels have chemicals added to them to make the fuel vaporizing occur at a more uniform rate attempting to resist this explosion.

Ok... I'm with you so far. I'm not entirely sure why there's an emphasis on INCOMPLETE burning after the explosive part... the unburned mix goes BOOM and you've got knock.

In effect they raise the pressure at which this knocking occurs by raising the pressure at which combustion occurs.

Now you're losing me. Replace combustion with autoignition and it would make more sense to me. Say it raises the time before autoignition occurs at this temp and pressure and I'd be right there with you.

High octane fuels at ground level in low compression engines in effect will burn slower and incompletely simple because the pressure is not there in a low compression engine.

And you've lost me. This is the root of the whole disagreement. You're tieing flame speed to resisting explosion. I'm not. It doesn't control knock by burning slower. It controls knock by not exploding as quickly. Which gives the mixture more time to burn rather than explode.

Which reminds me of something else. So far it seems this entire discussion has been tied to compression and pressure. Temperature matters too.

[DesmoDog]

Written by Racetech, Inc. - reproduced with permission

...Fuel Characteristics
Low compression engines usually run well on low octane fuel because they have relatively low charge densities and the burn rate within these confines is usually predictable. A low compression engine switched to 118-octane race fuel will always lose power unless the ignition advance is increased to compensate for the slower burn rates. Even then, a low CR engine may lose power with the timing optimized for high-octane fuel.

A high compression or turbocharged engine operates with much higher charge densities and consequently faster burn rates. The high-octane fuel permits these rapid burn rates because it has far less tendency to auto ignite and detonate under these conditions. As a result, high compression and turbo engines cannot realize their full hp potential without high-octane fuel.

...© COPYRIGHT 2004, VITEK PERFORMANCE, INC.

Forgive the massive snipping (or rejoice, depending on your view) but it seems these were the most applicable statements.

The first paragraph shown seems to go against what Shell has stated (that they want race fuels to burn quickly - quote posted earlier). However... as we agreed ealeir we were leaving race fuels out of this. I think we both agree there's a lot more difference between 89 octane pump gas and 118 octane race fuel (notice they dont even call it gas - I do not think this is by chance) and 89 vs 93 octane pump gas.

The second paragraph mentiones a resistance to auto-ignition. That's been my point all along. I say it resists auto-ignition through mechanisms other than burnign slower. And the conditions that make it autoignite are only made worse by burning slower. I still haven't seen anything that explains why burning the mix faster is a great thing but the speed that the fuel itself burns should be slower. This makes no sense to me. It doesn't explode because the flame front is too fast.

So I think we both agree we don't want it to explode. That's a bad thing.

We don't agree on how a higher octane gas avoids this. And likely never will.

[higgy]

Ok... I'm with you so far. I'm not entirely sure why there's an emphasis on INCOMPLETE burning after the explosive part... the unburned mix goes BOOM and you've got knock.

It is important to note that during a knock only part of the charge is ignited. the vast majority of the charge goes out the tailpipe as HC and soot. in a modern emission controlled engine it means your cat gets destroyed in a matter of minutes

Now you're losing me. Replace combustion with autoignition and it would make more sense to me. Say it raises the time before autoignition occurs at this temp and pressure and I'd be right there with you.

Time has no bearing on compression ignition cycles,it is simply a matter of pressure. pressure causes knock. Temperature only plays a part in that it raises pressure when the volume is constant or decreasing, again Boyles law.


And you've lost me. This is the root of the whole disagreement. You're tieing flame speed to resisting explosion. I'm not. It doesn't control knock by burning slower. It controls knock by not exploding as quickly. Which gives the mixture more time to burn rather than explode.

Which reminds me of something else. So far it seems this entire discussion has been tied to compression and pressure. Temperature matters too.


On that much I agree,it is the one thing in all this causing confusion.
It is the root of our disagreement. Yes temperature matters. but internal combustion engines are compression engines. Air is what they derive their power from, not the fuel used to heat the air. Temperature only matters in that it has an effect on the fuel used.


Read further into my response. That part I get. The part I dont get is why it would be beneficial to burn the mix faster with dual plugs, yet also beneficial to burn the fuel slower with a slower flame speed. Flame fronts don't create shock waves, explosions do.

And THIS has me really confused:
Quote:
Two plugs,two flame fronts propagating burning the complete charge faster not faster flame propagation

Why confused? Two plugs,you ignite the fuel at two points in the charge, each point propagates at the same speed but the entire charge gets burned in half the time.
For example,if you take a jug and poke a hole in it the water will leak out at a certain speed. if you you poke a second hole in it
while each hole will leak at the same speed,because there are now two holes it will be empty in half the time as one hole.

Read further into my response. That part I get. The part I dont get is why it would be beneficial to burn the mix faster with dual plugs, yet also beneficial to burn the fuel slower with a slower flame speed. Flame fronts don't create shock waves, explosions do.

But that's the whole point Flame fronts, the controlled heating and expansion of the air is what engines derive their power from. Not the fuel used to heat the air.
explosive ignition does not heat the air sufficiently to provide the a driving force the mechanical components of an engine require. Not to be critical of you, but from what I see it is the source of your confusion. Internal combustion engines run on air not the fuel used to heat the air.

You seem to understand a lot about gasoline,you seem to have a handle on engines. The problem is engines run on air not gasoline. I think that is the source of your confusion. please feel free to correct me If I am wrong.


Yeah I hear the collective sighs as well, and I have the same time constraints.Am currently working 60+ hours a week and I do tend to ramble on............

don't worry about being a smartass, I have a thick skin
but the point of high octane gas is to burn more slowly( more uniformly) when the pressure is high and normal compression exaggerates the properties of additives designed to have the effect of controlling the evaporation of the fuel in an effort to avoid autoignition or knock. But all of this at its root is to provide a consistant heat source to expand the air.

[DesmoDog]

...Time has no bearing on compression ignition cycles,it is simply a matter of pressure. pressure causes knock. Temperature only plays a part in that it raises pressure when the volume is constant or decreasing, again Boyles law.

Oh, but time does have a bearing on it, as do pressure AND heat. BTW - Boyles law makes no sense in this context. Boyles law assumes constant temp. When this started you posted PVT1 = PVT2. That's obviously a false statement in the case of a running engine. Lets take case 1 being 20 BTDC on the compression stroke. Case 2 is 20 ATDC. V1 = V2, P1 < P2, and T1 < T2. (If not, you've got problems.) Under these conditions, there is no way PT1 = PT2, Hence my earlier comment about the fuel not being accounted for. Perhaps I should have said the energy - heat - released by the combustion of the fuel isn't taken into account in the formula you posted.

Regarding the thought temp doesn't matter outside of raising the pressure. Taken to an extreme this would imply that if I put the mixture in a balloon and kept it at a constant pressure, I could raise the temp indefinitely and it would never explode. Obviously not true. There is a temp it will autoignite at. There is also a pressure it will autoignite at. But in fact it is the combination of those two, temp AND pressure, that control where it autoignites. Lower either one and the tendancy to knock drops. Raise either one, the tendancy to knock will rise.

On that much I agree,it is the one thing in all this causing confusion.
It is the root of our disagreement. Yes temperature matters. but internal combustion engines are compression engines. Air is what they derive their power from, not the fuel used to heat the air. Temperature only matters in that it has an effect on the fuel used.

I think we're talking about two different things. ABSOLUTELY temp matters in that it has an effect on the fuel, I agree. The higher the temp, the faster the mixture autoignites. And by that I mean goes boom. I never meant to imply that the engine was getting any power from this autoignition. I'm not talking about making power in any of this, I'm talking about keeping the unburned mixture from exploding.

But that's the whole point Flame fronts, the controlled heating and expansion of the air is what engines derive their power from. Not the fuel used to heat the air... Not to be critical of you, but from what I see it is the source of your confusion. Internal combustion engines run on air not the fuel used to heat the air.

Yep, I know that. But the fuel mixture is what knocks and ruins the party. I'm afraid I didn't get my question across clearly - when it comes to dual plugs we both agree on what the benefit is. I'll leave it at that.

You seem to understand a lot about gasoline,you seem to have a handle on engines.

Well thanks, I guess those years spent getting an Aerospace Engineering and Mechanics degree with an emphasis on propulsion wasn't a complete waste of time. Seriously though, I in no way shape or form think that makes me an expert here.... it certainly does not.

[higgy]

I guess the basic point is you are taking a fuel designed to operate at one set of pressures and putting in an engine that operates at significantly lower pressures. My understanding is that for complete combustion to take place in an emissions controlled engine the temperature has to be within very confined ranges both to prevent knock and Nox production regardless of octane . The simplest test I can think of if you have access to an gas analyzer is have you engine tested for hc on regular octane and then on high octane. What you will see is a rise in hc on the high octane fuel. All that extra hc eventually will lead to more carbon and more hc dilution of your engines oil or in the vernacular turn your pride and joy into a sludge puppy
if the engine you test has a cat,all you need to do is measure the temp at the back of the cat. You will find the high octane fuel raises the temp of the cat about 150 t0 200 degrees,not a good thing at all. the normal temp differential of your typical working cat is 100 degrees,with the rear of the cat being the higher temp.