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 Posted: Tue Aug 28, 2007 2:54 am Post subject: Non-aviation question about fuel (octane and "knocking") |
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Hi Dave,
I am writing off list. Mostly, this is because I wanted to try to answer some of your questions but I am FAR from an expert in all of this stuff. I just have some old knowledge that might or might not be accurate or useful.
First let me talk about octane and detonation. Please note I am not talking about pre-detonation which is an entirely different subject. When you want a reciprocating gasoline engine to operate properly you want the fuel air mixture to burn in a controlled fashion in response to a spark from the ignition system. This is what the engine is designed for. If you run a mixture that is too lean then the fuel can detonate instead of burning. In this case it all "burns" at once in an explosion rather than a nice sequenced reaction. The impulse in pressure made in this explosion happens not only faster than a burn but it comes at the wrong time too. In the normal detonation case, it happens when the spark plug goes off (usually about 15 degrees rotation of the crank shaft before top-dead-center) so the force is applied against the rotation of the crank instead of pushing it forward. This is a good way to break your engine - right now.
Pre-detonation (pre-ignition) is a variation on this theme. In this case, the fuel-air mixture goes off because it is in the wrong place in the wrong condition at the wrong time. This can be ignited by red-hot carbon deposits in the cylinder or by the simple pressure of the high compression (and super-charged pressure) cylinder. A higher octane rating means the fuel is more resistant to this sort of ignition than lower octane rated fuel. I don't have experience with supercharged engines, but I suspect adding boost makes this kind of ignition more likely. This is also the normal way a diesel engine works, but it is designed to work this way and doesn't even have spark plugs since the fuel burns instantly when injected in the high pressure cylinder.
I don't know if you are a gun person or reloader, but there is a corresponding issue for reloaders. The powder charge in a normal load burns and propels the bullet down the barrel in an orderly manner. As you reduce the charge, the amount of acceleration is reduced -- to a point. If you get the charge too small then it detonates instead of burning. The result is always bad -- often a gun barrel split open and shrapnel flying around the head of the shooter. When reloading, this is the reason you must pay attention to the minimum powder load as well as the maximum load.
The only other comment I want to make is about fuel color codes. In the old days 80/87 was red and 100LL was blue. If you mixed them at all the result was clear. This is no longer an issue since you can't get 80/87 any more.
OK, one last comment . . . I intend to use only avgas in my aviation engine and I suggest you consider doing the same. There are a number of reasons for this. Firstly, this is what they sell at airports and I don't feel like hauling fuel in little cans from questionable autogas sources to put in my plane. Secondly, the higher you fly the more likely your fuel will evaporate right out of the tank. The real difference between avgas and autogas is the additives in avgas that make it less likely to evaporate under low pressure (less volatile). Lastly, if you think it is a pain to haul fuel to your normal airport try hauling fuel to a remote location where you land for fuel on a cross country flight. In other words, using autogas is very inconvenient if you fly your plane outside the local area.
Best regards,
Paul
XL fuselage
At 07:59 AM 8/27/2007, you wrote:
[quote]I suppose this could be aviation related, especially for the 912ULS owners, but my cause for posting is not.
I have a supercharged Miata that I have to feed 91+ octane gas or it will knock pretty severely when I put my foot in it, especially at high boost. I normally run around 5-7 pounds of boost around town and have no problems with knocking when I use 91. I run 13+ on the track and will only light up the knock sensor light under pretty extreme loads or on really hot days when the intercooler can't keep up.
I have been buying 91 octane gas at a local station and lately I have been getting lots of knocking at the low boost levels, and I am not driving very hard. (I live in a town with 2500 people and three of them are full time cops. Not much chance for spirited driving.) My best guess was that I was starting to lose a valve seal or maybe I had some rings going bad and had carbon building up on the piston heads and that was causing detonation. But now I am not so sure. I filled up an almost empty tank out of town over the weekend and I haven't seen the knock light blink even once. I am now wondering about the gas at my local station.
I did notice something interesting when looking at the pumps at my local station last time I was there. The pump only has two Hobbs-meter like counters at the top of the pump instead of three. I always thought those were total counters, one for each underground tank but this station sells three grades of gas. Could they be storing only two grades of gas and mixing 85 and 91 to get the 87 grade? Or worse, could they be selling 87 gas as 91? (I see one of the counters ticking away when I fill up with 91. I will put 87 in the truck next time it needs gas and I will see if both counters run at the same time.)
A petro-chemist friend once tried to explain to me about gasoline molecule chain length and detonation, "cracking" during the refining process, and who knows what else. I am not a chemist (I don't even play one on TV) so I got lost pretty quickly. What I did take away from the conversation was that his opinion was that the lower grade octane gas (the shorter molecule chains?) would pre-detonate under compression even if mixed with the high octane gas causing the whole mix to go up. (It was also his strongly expressed opinion that "octane boosters" were pretty useless as well.) I want to make it clear that I am not sure I understand what is going on in the cylinder when "knocking" occurs, so I am starting at a bit of a disadvantage on this whole issue.
So I guess my questions to this (incredibly) knowledgeable group are these: - Does mixing different octane fuel "average" the effective octane of each grade?
- How can a regular schmoe like me find out of they are selling 89 octane gas as 91? I am pretty sure that asking isn't going to get me very far.
- Could there be additives in the gas that might encourage knocking? Would the 10% Ethanol gas cause more knocking than the undiluted fuel?
- What is really going on when the engine "knocks"? (The sensor replaces one of the engine mount bolts on the engine side. It looks like an epoxy filled hollow bolt with a wire lead coming out the end. That wire runs back to the after market ECU. Is this sensor really just a microphone listening for "knocks"? Can it be that simple?)
- What (if anything) is the difference between "knock" and "pre-detonation"? I hear both terms used and have always wondered if they meant something different.
- What do you 912ULS owners do when you can't find 91 octane gas? Is there an octane booster that you like? Do they even work at all?
- What are the minimum octane requirements for the non-912ULS engine options like the Jabiru 3300? (I am planning on building a 701 one of these fine days. Honest.)
Please forgive the posting of a not totally aviation related question to this board, but the depth of knowledge here is astounding and I figured this was about as good a place to go for help as I was likely to find. Asking technical questions on most of the auto lists usually gets responses like "Buy a real car". Besides, if I put wings on this car I sometimes think it would take to the air! Too bad it is too heavy to qualify as an E-LSA plane. The power to weight ratio is about right...
Thanks in advance, and feel free to contact me directly if you don't want to clutter up the list with this topic.
Dave
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