Aviation fuel in 4-stroke engine?

[blockmanjohn]

Hi,

I have completed Jan Ridder's scuderi cycle engine. As of yet it will not run except for a few "pops" here and there. I am fairly sure that the problem is low compression. This engine uses no rings and relies on a very close and smooth fit between the piston and cylinder. He suggests no more than about .0004 inches clearance. Mine is a bit more than that, maybe 5 or 6 tenths.

My plan is to cut a groove in the pistons and install viton o-rings. The problem is that there is no oil used in this engine. High carbon cast iron for the pistons and cylinders is all it uses.

Is it possible to use an aviation type 2 stroke fuel which contains oil and possibly some nitro? Might it be better to just add some 2-stroke oil to the coleman fuel that I am using? Could it run with no oil in the fuel?

I guess I could just experiment with different fuels and see what happens, but I would really appreciate any input I can get from those who have more experience than me in these matters first.

Thanks in advance, John.

 

[SmithDoor]

I have used aviation fuel in trucks , motorcycle, lawn mower and B & S engines.

My parents had a FBO and from time to time you have to drain bottom of tanks looking for water. So it was free fuel that could not be used in aircraft.
It was 80 octane and 100 octane they ran great.

Dave

 

[Nerd1000]

Just add some 2-stroke oil to your fuel methinks.

 

[deverett]

Just a thought... Some small 'steam' engines use some grooves cut in the piston wall to act as a labyrinth seal. (so does the Robinson hot air engine). Would it be worth trying the same in this engine?

Dave
The Emerald Isle

 

[Jasonb]

Why not make a pair of new pistons and lap them into the bore, these are fits that are not really measured you just lap the bore and then make the piston to just about fit by feel and then finally lap it into the bore as Jan suggests. This is the way many glow and model "diesel" engines are made.

 

[lohring]

The main advantage of aviation fuel is high octane. Very few model engines need that. More oil or any method to get a better seal as mentioned above is a better approach.

Lohring Miller

 

[propclock]

I really like the True Fuel 40-1 and 50-1, 2 cycle fuel . Available at Home Depot and others.
Very long shelf life and also holds up well in model engine tanks. High octane also.
Just my 1.414 cents worth.

 

[74Sprint]

It sounds like you are talking about ring-less RC model engines that use a tapered bore and the pistons are lapped to fit. On these engines they same it should almost feel like the engine is seizing when the piston reaches TDC. Yup actual aviation fuel is high octane and what is considered a dry fuel, little to no lubrication from it being burnt. RC fuel that I use is either alcohol with 15% castor oil or the same with 15% nitro. This fuel does need a higher compression and a glow plug to burn. It can be burnt with a spark but sparkplugs tend to foul easy because of the castor oil. Low compression engines need low octane fuel to run, i.e. distillate. Coleman fuel is about 50-60 octane, alcohol depending on type and where you get it from will be between 100-120 octane For small engines like 0.40 - 0.90 CID gas to oil ratio it is usually between 16:1 to 25:1, so lots of oil. Check out RC Groups for help.

Ray

 

[bluejets]

The lower compression of your engine will certainly be helped by the rc aircraft methanol/nitro/oil mix.
The first thing one does when running nitro content is to lower the engine compression ratio from that of standard 4:1 methanol/caster mix.
The "lapping" used to be done on rc glow (and diesel) engines in the old days with cast piston/liners ...these days as Ray says, the approach is rather high tech where the expansion of the different metals allows engines to "sieze" in a slightly tapered bore at startup.
No more "running -in" at rich mixture for 2 or 3 hours and a shed full of deadly smoke.
Bit like balsa dust, no one gave a toss in those days.

 

[Mark-V]

Hi John,
Are you using the “vapour carb” system that Jan uses? That relies on petrol and the vapour of it to run the engine. No oil required as you say. Adding oil to the fuel (petrol) will not help as it will not get into the vapour. Coleman will most likely not work either and aviation fuel certainly not since it is paraffin and will not produce any vapour.
If you are using a standard carb then Coleman or petrol and thick 4 stroke oil (around 20 to 40%) should work with the loose fit you have.
However I would stay with the vapour system and straight petrol and since they are cast iron, try and “grow” the piston(s) for a tighter fit.
Mark

 

[Peter Twissell]

I assume the "aviation fuel" referred to by John is 100LL (petrol), not Jet A (kerosene).
100LL is 100 octane, so it can be used with higher compression than lower octane fuels. The LL refers to low lead. Tetraethyl lead is added as an octane booster, also providing some degree of protection for valves and seats in high performance engines.
Neither of these properties are likely to help a non starting engine.
Pete.

 

[blockmanjohn]

Hi John,
Are you using the “vapour carb” system that Jan uses? That relies on petrol and the vapour of it to run the engine. No oil required as you say. Adding oil to the fuel (petrol) will not help as it will not get into the vapour. Coleman will most likely not work either and aviation fuel certainly not since it is paraffin and will not produce any vapour.
If you are using a standard carb then Coleman or petrol and thick 4 stroke oil (around 20 to 40%) should work with the loose fit you have.
However I would stay with the vapour system and straight petrol and since they are cast iron, try and “grow” the piston(s) for a tighter fit.
Mark

 

[blockmanjohn]

Hi Mark,

How does one "grow" a piston?

 

[Jasonb]

CI pistons can be grown by heating to red heat and holding them at that temperature which draws the carbon to the surface and therefore they become slightly larger.

Myself I don't think you need to go to the high oil contents that are used on model aero engines but a few drops in the tank to give a 1:30-1:40 mix should be enough if you do go for the Viton option. It is not as though you are using the oil to lubricate bearings and the conrod as it's an open crank engine.

I have several engines with Viton rings and most just use a drip feed oiler so I just use neat fuel either pump petrol or Colmans. Those that don't have drip feed or an enclosed crankcase just get the small amount of oil added to the fuel as per ratio above. I use good quality 2-stroke oil for the 4-stroke engines and mostlly mix it with Colmans. If you can get it then the Aspen readymix is also said to be good. In the US "Marvel mystery Oil" is a popular one if you want to mix your own

As someone else said if you are using a vapour carb then it won't do anything adding oil to the fuel as it will just get left in the tank

 

[blockmanjohn]

Jason,
Thanks for the input. As I am using a vapor carb I guess the viton rings and oil in the fuel is not n option. Growing the piston as you explained may be my best option. How long does the piston have to stay at temp? I no longer have access to a lathe, so turning new pistons cannot happen. Since the cast iron is pretty slippery dry, might the engine run awhile with viton rings and no added lubrication?

Thanks, John

 

[Jasonb]

On those sort of size pistons hold at bright red but not orange heat for about 5mins so they can soak through and then quench in oil, it also re arranges the flakes in the CI so they take up more space.

You should be OK just manually adding a drop of oil every so often or a squirt of WD40, One of mine the RMC Type B runs well like that on it's vapour carb and has a vitron ring.

Also don't use the commercial figures for ring compression a sthe engine will be too tight, about 5% at most.

 

[Steamchick]

Hi,

I have completed Jan Ridder's scuderi cycle engine. As of yet it will not run except for a few "pops" here and there. I am fairly sure that the problem is low compression. This engine uses no rings and relies on a very close and smooth fit between the piston and cylinder. He suggests no more than about .0004 inches clearance. Mine is a bit more than that, maybe 5 or 6 tenths.

My plan is to cut a groove in the pistons and install viton o-rings. The problem is that there is no oil used in this engine. High carbon cast iron for the pistons and cylinders is all it uses.

Is it possible to use an aviation type 2 stroke fuel which contains oil and possibly some nitro? Might it be better to just add some 2-stroke oil to the coleman fuel that I am using? Could it run with no oil in the fuel?

I guess I could just experiment with different fuels and see what happens, but I would really appreciate any input I can get from those who have more experience than me in these matters first.

Thanks in advance, John.

Hi John. You pose a few questions here. First, if you can give us a clue about the actual compression you have, it will "sort the wheat from the chaff". I think there is a lot of valuable opinion, along with some mixed messages here, and you are likely to be misled by some comments. Frankly, I am confused by some.
For starters. I have no experience of o-rings so won't offer any ideas there. But also, I see a lot of I formation about various fuels beyond my experience, so will only pose the advice- be sure you are talking of the same thing when you mention something like "Aviation fuel". This generic term has been used referring to very high octane leaded petrol (gasoline) over 120 octane. A mate used to get it for racing cars on over 12:1 compression. He offered me some for a race tuned Yamaha motorcycle I had at 17:1. Pistons lasted a week on 98 octane petrol plus "octane boost additive"! Yet aviation fuel can also refer to the many mixes of fuel you can buy for model aircraft. 2-strokes, 4-strokes, and of various firing methods: compression ignition, glow-plug and spark... All needing different chemistry and mixtures of fuel.
Spark plug engines are not good with oil in the combustion chamber, yer spark plugs were developed 100 years ago to cope with oil, and until the legislature banned most they were very successful small engines. Now banned in most places over 150cc. The oil cools the combustion considerably, but still needs to be hot enough to avoid "wet oil" condensing on spark plugs. So a special heat-rating of plug is needed. The wrong spark plug will foul quickly, or burn away and possibly destroy the engine.
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. So maybe try a richer mixture, delayed spark or other tweaks? Record what you do and tell us in an organised table, and maybe the clever guys can be more precise with their advice?
I had a 3/4 cc marine engine that ran a short while on 30% oil, 30% ethylene, 40% paraffin. But when the compression dropped after running in I changed to 50% alcohol and oil with a glow plug, until the cylinder and piston wore out completely. ( Wrong cast iron for piston).
Have fun,
K2

 

[Peter Twissell]

But if only 4:1 you can use paraffin with a glow plug, not a spark.

Presumably that would be methanol with a glow plug at low compression?
Paraffin will work with a glow plug, but at much higher compression (e.g. diesel engine).

 

[stackerjack]

Speaking of model aircraft engines for a minute: does anybody know exactly, how the oil in the fuel gets into the crankcase to lubricate the big end etc. not to mention the valve gear.
I'm talking about such engines as O.S./Saito/Laser 4-strokes, or similar.
Jack

 

[Peter Twissell]

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.