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Any aircraft which did have problems, probably broke some of the important rules in their fuel system designs which have been discussed at length on various threads here on VAF. |
Thanks
Ok guys. I did include the following statement in my initial post of this thread.
"I realize this can be a controversial subject, and I may be overreacting." That does seem to be the case, and that's...Ok I think. I have learned a a lot from all the responses. The debate over the use of pump gas seems to a point of contention. What I learned from the replies is that it does seem completely feasible to run SOME engines on "pump gas". However, in order for this to take place safely, several steps need to be taken. Everything from building the airplane fuel system to accept this fuel, to transporting, and eventually the proper operation of the engine needs to be carefully planned and executed. I think the problem some of the posters have with stating that "it's ok to do..I've done it for years", is that you get guys like me on here that are not that familiar with piston engines, and I run down to the corner gas station and fill up my 5 gal lawn-mower gas can and head to the airport. That's just an inherent problem of the internet and msg boards specifically. It sounds like the posters that are using some fuel other than 100LL have in fact taken all the necessary precautions and are operating safely. I think that's great and one of the huge benefits of experimental aviation. However, the majority of operators may not be able to safely operate within all the "benefits" of experimental aviation. Regarding this 100LL phaseout issue, most pilots just need a fuel that will be easily accessible at their local airport, and at a fair price. Hopefully RV10inOz speaks the truth and the issue will "soon" be a thing of the past. Fingers crossed! Thanks again for all the information. It's time to head to the shop and work on flaps:) |
Why do we run into knock problems using MOGAS when we have CRs above 8.5?
Modern cars have CR well above 10 and racing bikes even above 12. So what's the problem with our aircraft engines? Too low air density/pressure? Would it be easier with turbo charged engines? Malte |
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Water cooled heads remove heat more efficiently and less prone to hot spots, cylinder head combustion chamber design, smaller piston diameter, ECT.
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The larger the cylinder volume (all other things equal), the longer it takes the flame front to burn all the way to the corners of the combustion chamber. If the combustion event takes too long the heat and pressure inside the cylinder reach the point where the unburnt mixture at the far (from the plugs) corners of the chamber will spontaneously combust (AKA detonate). Add in a low RPM engine like ours, and we play close to the detonation margins. On an engine with smaller combustion chambers, the flame front burns all the way to the extents of the chamber faster, reducing the risk of detonation, since the fuel burns before the conditions inside the combustion chamber cause detonation. This is why we see 10+:1 compression ratios on today's smaller, higher revving engines, but not on yesterday's engines with big combustion chambers and which turn relatively slowly. Notice when you get pinging in your car - generally under load and at relatively low RPM, like starting off going up a hill. Kinda/sorta like our engines see all of the time. |
Kyle, great explanation! Thank you!
Malte |
Indeed, thanks Kyle and Bret. Makes sense.
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This is from the Shell link I listed on post 1. (Avgas Fact and Future).
Is this relative to mjanduda's question? Last paragraph mainly. "Octane rating is a measure of how resistant a fuel is to detonation or "pinking"; the higher the Octane rating, the more the fuel / air mixture can be compressed without detonation happening. To make this clear, octane rating is not a measure of the amount energy in the fuel, but is a measure of its resistance to detonation. The advantage or higher octane fuels is that a higher compression ratio or supercharging ratio can be used, which then leads to a higher engine cycle efficiency, which in turn means more power output for a given fuel burn. However, to confuse things further, there are four principal ways to measure Octane rating, RON, MON, Lean Mixture and Rich Mixture ratings. Road fuels tend to be measured on a RON scale, for which unleaded fuels tend to be 95 - 98 RON but are only 85 - 87 MON. Avgas is measured on Lean Mixture (similar to MON) but also has a Rich Mixture Octane rating. The Lean Mixture rating is 100 octane (15 octane higher than the comparable 85 MON for unleaded Mogas) but Avgas also has a Rich Mixture rating of 130 which allows higher supercharger boost pressures to be used without detonation occurring. This is particularly a problem when using high power settings at low altitude, for example during take off. As you can see TEL in Avgas makes a significant difference to the octane rating and without it Octane ratings would be back down to 80 - 85 Lean Mixture - the level for road fuels - instead of 100 / 130. This is not a problem for most typical modern normally aspirated engines as their compression ratios are quite modest and detonation would not be a problem with 80 - 85 Lean Mixture Octane fuel." |
Modern chambers are designed with a lot of computer simulation of the mixing and combustion events and then validated in real life on a instrumented dyno.
Lycomings had none of this tech or science available when they were designed. Cars are running up to 14 to 1 CRs on 87 octane these days. Even turbo engines are running 10.5 CRs on 87 octane. Computer controlled fuel and spark has lots to do with it too. At least we can add those parts to help out the old Lyc these days. We can retard the spark at high MAP to avoid detonation and re-advance when MAP is lower to gain back the power and efficiency. Can't do that with mags. |
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