Aviation fuel in 4-stroke engine?

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Vietti

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Just a thought. I went to av gas when Coleman fuel got so expensive, couldn't tell any difference. At a show last year a widely respected modeler next to me said he thought av gas was formulated to not be so volatile(?) at altitude and I'd be better off with pump gas so I switched to non ethanol pump gas, again couldn't tell the difference. Even cheaper.

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dieselpilot

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Back to compression. If you are getting 7:1 you need at least 88 Octane petrol. If 10:1 you need over 95 octane petrol. But if only 4:1 you can use paraffin with a glow plug, not a spark. If the compression in the engine is lower than planned, you need a richer mixture to get git to run. That is the likely reason you "pop" but not run.
This isn't a development project, but a proven engine design. It's safe to say that if a well respected designer operated the engine at the design compression ratio, we can completely ignore "theory". There are several running examples on youtube built by others.

Compression ratio and ignition/combustion characteristics are very dependent on engine design. There is no hard rule. At model scale many things go out the window so generalizing really doesn't help. What I can say is that in general octane rating in model scale bores is all but irrelevant.

Leakage (piston and/or valves) is always the most critical factor in model engines, especially very low power low speed designs.
 

74Sprint

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Here is some information that I researched and found interesting and may solve your problem. I was given 2 Pulling Poulan chain saws, one was like new, I also purchased a Pro model, my neighbor also bought a used one. Now as some might know is the reason the call them Pulling-Poulans is because you pull and pull and they fart but won't run, even my new Poulan Pro. So like I said I started researching these engines and why is it mostly Poulan that has the problem. Oh I purchased my Pulling-Poulan because Poulan was purchased by Husqvarna so I just assumed that Husky would have corrected the problem based on their reputation but, nope. So here I am all pissed off because I can't get my new Poulan running. I got tired out and called my son-in-law over to help. Now he was a O-line and D-line football player here in Canada, so he is big very big. Well he grabs this Poulan Pro and is whipping out the pull cord like nothing and it starts and runs, well what the hell.

My research led me to find out that Poulan uses a Walbro preprogrammed CDI ignition. Side note here: Some snowmobile makers have reverse on their sleds. Some gearbox and some just run the 2-stroke engine backwards based on ignition timing. Now Poulan in their infinite wisdom decided to be nice to people who buy one of their chainsaws to program the ignition timing so that it doesn't give a kickback when starting. This is a case of where the fix is worse than the problem it was supposed to fix. It turns out the ignition timing is retarded by 8 degrees after TDC until the engine reaches 200 RPM. So by time the air/fuel mixture detonates the piston is down the bore to where there is not enough cylinder pressure to keep the engine turning. Now I know that I can by adjusting the gap at the magneto portion I can change the timing. Oh I did manage to get a new CDI/coil pack from China but the motor did the same but it had a bit more adjustment, so that engine I got started but it slowly puttered out over and over. At 200 RPM the timing goes to 0 degrees and at 400-500 it goes from 2 degrees to 8 degrees BTDC. Depending on the model the max timing is 8 - 12 degrees BTDC.

So maybe it's not your fuel but lack of ignition timing, please try advancing your ignition timing.

Ray
 

stackerjack

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I have an Enya 4 stroke, which relies on leakage past the piston ring from the combustion chamber to lubricate the bottom end. There is enough that the engine has a drain from the crankcase.
Yes Peter, I believe that is what happens, but I still find it difficult to swallow. The Enya is an excellent engine and I'm sure the designers knew what they were doing.
A few years ago I had an OS 4-stroke, which had the camshaft in front of the engine, unlike the previous model, where it was at the rear. On both engines, the crankcase breather/drain nipple was at the rear of the engine. I could understand why it was at the rear on the early model, because the camshaft was also at the rear. My problem was: why have the drain nipple at the rear on the later model, when the crankshaft was at the front?
I wrote a letter to OS in Japan with my observations. I got a reply which told me not to worry, everything was being taken care of.
On their next engine, they moved the drain nipple to the front of the engine, just under the camshaft.
Jack
 

Steamchick

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This isn't a development project, but a proven engine design. It's safe to say that if a well respected designer operated the engine at the design compression ratio, we can completely ignore "theory". There are several running examples on youtube built by others.

Compression ratio and ignition/combustion characteristics are very dependent on engine design. There is no hard rule. At model scale many things go out the window so generalizing really doesn't help. What I can say is that in general octane rating in model scale bores is all but irrelevant.

Leakage (piston and/or valves) is always the most critical factor in model engines, especially very low power low speed designs.
Fair comment. Unfortunately, in my experience, you can't ignore theory. Engines will run with "poorer" fuels, but may do long term damage from pre-ignition, etc. But also, I have helped numerous motorcyclists with "poor starting" with a few suggestions. I can never predict what works, but careful attempts with different factors being changed can find a solution that works. Many motorcycle engines I have met have had lower compression after not running for 6 months or so. Sometimes oil will restore the compression and wash-out the tiny amount of muck that caused a ring or valve to leak excessively. Sometimes a richer starting mixture fixes it, especially when it fires then doesn't run. After a good run, it often resolves itself as rings free-off or muck is cleared from valves, etc. when good compression returns. However, petrol degrades with time. It is a mix of a base fuel and additives. Some of these aid cold starting, but 'old fuel' - more than 3 months old - has these broken down so on some engines doesn't start so easily. Fresh fuel fixes the issue.
It is a complex issue, that no-one can fix from a remote computer. But we simply offer various possible solutions from our knowledge and experience. So changing "old" (degraded) 95 octane fuel for fresh 85 octane fuel may fix a starting problem, but introduce a detonation problem... Therefore a fuller understanding helps figure a best solution...
I'll shut up now, as I am rambling...
K2
 

Chiptosser

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Aviation fuel, is fuel used in full size aircraft.
A lot of confusion can be saved if you would say Model Aviation fuel in this discussion..

I do believe some people are getting confused, not knowing the differences.

Aviation fuel has to meet preset standards for piston aircraft engines, used in general aviation and commercial aviation, called Avgas.
LL100, low leaded 100 octane gasoline.

Not to be confused with Model airplane engine fuel, or Jet fuel.
I hope this helps.
 

petertha

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A few years ago I had an OS 4-stroke, which had the camshaft in front of the engine, unlike the previous model, where it was at the rear. On both engines, the crankcase breather/drain nipple was at the rear of the engine. I could understand why it was at the rear on the early model, because the camshaft was also at the rear. My problem was: why have the drain nipple at the rear on the later model, when the crankshaft was at the front? I wrote a letter to OS in Japan with my observations. I got a reply which told me not to worry, everything was being taken care of. On their next engine, they moved the drain nipple to the front of the engine, just under the camshaft.
I'm of the general opinion that everything inside a conventional methanol/glow engine gets wetted by fuel mixture (containing oil) via proximity blowby. Usually the issue is how to vent it without having puddles build up & worrying about excess (hydraulic lock) especially inverted cylinders. So kind of suggests there is an abundance of lubrication. Even gasoline burners with smaller oil % in the fuel seem to operate this way. They have no external fuel pump.

But I admit, its kind of a mystery how fuel wetting can migrate into the far reaches & nooks of the engine, even including external mechanisms like rockers & pushrod ends. The rockers must get lubricated by blowby via the valve stem annulus. The engines just seem to be 'sloppy' enough that it works. I never hear about people oiling their rockers. In fact you can see stroke fuel streaks with a bad gasket.

Most of the bigger multi-cylinder methanol RC engines are similar despite radial, opposed, in-line... just a drain plug somewhere on the crankcase despite being proportionately longer flow path. Two exceptions I can think of is Edwards radial with minimal/no oil in the methanol fuel but external circulating oil pump. The Ohrndorf 5-cyl radial I'm building calls for a nose case partial filled with oil for the gears & cams, but its segregated from the crankcase. The crankpin, master rod bottom assembly is wetted by intake charge entering crankcase but on its way to intake tubes.

Back to commercial RC 4S engines, I've seen a few 4S engines with breather tube on rear. I think its more about venting slight positive pressure & some (but probably not all) residue is going along for the ride. So I'm not sure there is a flow path per say.

The current OS FSa-72II has breather tube on rear cover plate, the cams/gears are on front of engine.

The FS-120 III Surpass pump engine is same mechanical layout, but different again. Looks like the rear tap is used for pressure pulsing the carb & interestingly has a tube (C) running from induction manifold forward to cam area.
 

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Steamchick

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Just an idea about blow-by. It is greater than you may think. If you disconnect the hose from your rocker box on your car, and idle the engine, you can feel a good whoosh of blow-by gas coming through. This will be wet with oil from splash and vapour in the crankcase and other chambers. Considering pressures and sizes of pipe - rocker breather versus exhaust pipe - it isn't hard to believe about 1% sort of volume of gas is passing the piston rings instead of doing its job and powering the engine.Now imagine how much gas is passing through on wide open throttle. 1% of a lot of gas? Even if this is made up of 14 parts to 1 air to fuel, and the fuel is 2% oil, then there is adequate oil for lubrication of all the moving parts in the chambers of the engine. But, because I started with the car engine, you may wonder why there is relatively so much oil in the car engine? Answer, because it washes around the engine internals taking heat (from friction of contact surfaces) from the hot spots to where it is cooler, IE. Surfaces on the outside or surfaces with Coolant (water) passing behind them. The oil can easily be 80 degrees C hotter in parts of the engine than the coolant, which may be at 80 to 115deg.C anyway. But models relatively have a much better cooling - by being so small they have relatively huge external surfaces - so manage without a bucket of oil moving the heat away from hot-spots. Many cars even have an oil-water heat exchanger... which seems curious when the coolant can be at 110deg.C. under pressure!
Or at least that's what I learned working in Engine Design ....
So oil does more than lubricate....
K2
 

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