Does nitro fuel evaporate ?
 

When I have left a small amount of fuel (Duraglo 16) in my nitro race car overnight, what's left is darker and does not run the engine too well. The LHS owner said that this because the nitro evaporates and leaves a potentially damaging residue.

Now, for my flight box I have a modified fuel container cap with two holes in it, one with a metal fitting to join the pump line and the 'clunk' line. The other hole is used as a 'breather' hole. I've yet to use it on the bottle of fuel (Duraglo 5). Is it ok to leave it all in situ and not worry about the fuel going of during the next few days / weeks ? Or do I have to plug the breather hole and / or the fuelling 'nipple' ?

Fuel info is here

[link]http://www.aabt-collectibles.co.uk/cat443-434.htm[/link]

Thanks

RE: Does nitro fuel evaporate ?
 

Yes nitromethane evaporates. So does methonal, which also sucks up water. I don't know about damaging residues though. I would cap the bottle.
-John

RE: Does nitro fuel evaporate ?
 

Always cap your bottle of fuel!!! It's not the nitro evaporating that's the problem, it's methanol's hunger for humidity. Humidity in the air quickly contaminates the fuel and your engine runs like crap.
Nitromethane has a pretty high boiling and flash point, much higher than methanol's boiling and flash points. You can extinguish kitchen matches in a pan of pure nitro without igniting it on a cool day. Yes, I have tried it, it's not something that I simply read.

RE: Does nitro fuel evaporate ?
 

Some fuel constituents are volatile and as other have said, open fuel will suck up moisture from humid air. The amount depends on the amount of air circulation (a few litres of humid air doesn't contain very much moisture). As fuel evaporates, the composition changes. Methanol is the most volatile component and will tend to evaporate the most. Nitromethane is substantially less volatile and much less will evaporate at normal temperatures. For all practical purposes, the oil is considered to be non-volatile and can't evaporate appreciably except at very elevated temperatures - long after the methanol and nitro are gone. Old fuel left with the cap off can run poorly due to adsorbed moisture and/or lower methanol + higher oil content. If you started out with 100 ounces of fuel with 10% nitro, 20% oil and 70% methanol and you allowed 30% loss due to evaporation (mostly methanol) you would end out with fuel having ~ 14% nitro and 29% oil. Not many engines would run well with that much oil. Keep fuel tightly capped and store in a cool area away from direct sunlight.

Ross

RE: Does nitro fuel evaporate ?
 

Actually, fuel won't usually evaporate much of the good stuff or absorb much water overnight. And it also won't do much of that when almost sealed in the tank. Overnight isn't much problem. But leave it more than overnight and you could have problems after awhile. It's not a good idea to get into the habit of leaving fuel overnight just because "overnight" very often winds up being "next weekend" or "next month".

Some nitro mixes need igniters in the fuel. Those are often very volatile. They wick off and the nitro won't pop. So leaving fuel in the tank certainly has no positive value.

BTW, a lot of times, the fuel you see left in your tank is darker simply because muffler pressure lines feed residue into it. The stuff is darker from obvious reasons and is thicker because a lot of the light components have burned away in the combustion chamber.

If you run a brass clunk and brass tubing in the tank, leaving that in raw fuel isn't a great idea. Alcohol and nitro do a number on the brass. And it does more damage when the brass is immersed in the fuel.

It's worthwhile to pump out your tank for another reason. You almost always clear the lines when you're doing it. And fuel that's had a chance to evaporate in the lines does more to screw up the next engine restart than the stuff left in the tank. Of course, overnight won't do much of this but...............

RE: Does nitro fuel evaporate ?
 

To take it a step farther, even fuel left in the carb can make starting a week or so later a little harder. I used to just empty my tank but now I also run the engine with the fuel line off to burn off anything left and have found it starts alot easier next time I fly. It may just be me but it seems that way anyway.

RE: Does nitro fuel evaporate ?
 

Burning the fuel out of the engine is a good idea. As has been mention'd methonal is hydroscopic (sucks up water). Left in the engine it will attract water to where u don't want it (the bearings).
-John

RE: Does nitro fuel evaporate ?
 

On the last flight of the day always run the engine completely dry of any fuel. The usual deal on the last flight is to taxi back to the pits, shut the engine down and empty the tank. But the low speed running before shutting down lets fuel collect in the crankcase even if you've got a good idle mixture. This is what starts bearings rusting. The fuel sucks up water, the water attacks the steel bearings. The ideal way to clear that excess fuel is to run at full throttle and either pinch or remove the fuel line until the engine stops then wait a few seconds for the pressure in the tank to bleed back through the muffler pressure line. Then empty the tank.

RE: Does nitro fuel evaporate ?
 

Thanks for all the comprehensive answers. It's pretty obvious what the situation is.

RE: Does nitro fuel evaporate ?
 

Interesting. I've always heard the opposite(throttle-wise, anyway) practice recommended: shutting off the engine, draining the fuel tank, and cranking it over with the throttle closed and the glow clip attached. The theory is that the engine requires less fuel and has more vacuum with the throttle closed and so will more completely burn what is left in the system. Whether it is correct or not, I know that my engines will restart and run for a few seconds several times at idle setting after pinching the fuel line at full throttle to shut them off. Maybe it's one of those things that depends on whether you're in the Northern or Southern hemisphere?:)

That said, I don't know that it is critical one way or the other--the important thing is to crank it until it doesn't fire after removing the fuel source, and if it makes you feel better, add some of your favorite after run oil.

RE: Does nitro fuel evaporate ?
 

I don't completely agree with some of what is said here. Nevertheless, keep your fuel capped, drain your tank, and run the fuel residue out of your engine. ;)

RE: Does nitro fuel evaporate ?
 

Thanks. I think that we have an agreement on that one !

RE: Does nitro fuel evaporate ?
 

On the burn out at the end of the day, I use a low throttle setting (fast idle), but then again all of my engines are YS 4-strokes. A full throttle dry run would be very dangerous on a YS and probably other 4-strokes, as it could cause the engine to seriously backfire, throw the prop, or even break the engine.

The procedure I use is shut down engine, pump tank dry, restart engine and let it run dry at idle, then attempt to restart again until I get about 2 seconds of cranking with the starter and no fire in the engine. I'm sure there are other rituals that work well for other pilots, but this seems to do the trick for me. No problems with rust, easy starts next flying session, etc.

RE: Does nitro fuel evaporate ?
 

BTW, guys, when nitromethane burns one of it's products is nitric acid. When methanol burnse, a couple of it's products are formic acid and carbonic acid.

One of the reasons for using afterrun fluid is to displace the remaining unburned fuel. It's also good at dealing with the residues from the burned fuel: the nitric, formic, and carbonic acids.

RE: Does nitro fuel evaporate ?
 

You're right there, I wasn't even thinking about 4 strokes because I've never used one and they don't collect fuel inside the crankcase anyway. But with a 2 stroke at idle there's very little turbulence to sweep up the excess fuel and send it on up into the cylinder. That's why they stutter and fumble if they idle mix is a bit too rich when you open the throttle. More air gets let in which sweeps up that excess and it's suddenly too rich until it gets burnt off. If it's done at full throttle then there's a lot of air and turbulence plus when the mixture gets too lean for the engine to fire then it takes a short while for it to run down to a stop, maybe another 100 revs or so, which should clear out the last few drops.

Actually this is the way all CL engines stop at the end of every flight and rusted bearings are almost completely unknown in them. OK, I'll admit it :), when I first heard about rusted bearings I started to wonder why it only seemed to happen with RC so I figured the only real difference was in the way the engines were shut down. Plus the common use of castor in CL...draw your own conclusions :)

RE: Does nitro fuel evaporate ?
 

Many four-stroke engines get little benefit from running them dry of fuel. Raw fuel rarely gets into the lower-end of the engine. Mainly oil residue forced past the piston ring due to combustion pressures. The residue contains all of the products you'd find in the exhaust...'cooked' oil, a tiny amount of water, tinier amounts of acids, and so forth. This residue contains just enough electrolytes to cause corrosion to occur on the steel parts. We've seen four-stroke engines that have been run for a few tanks when new, and then not used for 4-6 weeks. Then the engine was severly-corroded.

You'll need to use after-run oil very generously in the lower-end of your four-stroke engine. It's best applied by using a syringe of oil and forcing it into the crankcase through the breather port.

Some engines use the crankcase for pumping action, and fresh fuel may flow through them. You need to check your engine manufacturer's instructions to see what they recommend for corrosion protection.

RE: Does nitro fuel evaporate ?
 

after each day at the model club pump out eny fuel in the tank then start the engine a few times to get rid of eny fuel left in the fuel lines...this will help the engine to stay dry till the next time u want to start it.......oh and yes nitro and methonol evaperait and if u dont get it out of the fuel system the left over oil will alter your fuel mixture....the oil duznt evaperait!!!:D