Here is a good question - why is castor oil preferred in some engines? Cox, for example, their fuel was 25% castor oil. Granted that 30 years ago they didn't have the synthetic oil that they have today, but I understand that even today for the little Cox engines you still want to use Castor oil. Why? What is special about Castor oil, what will it do that other oils won't?
I understand we can't use automotive engine oil - it's way too thick for the tight clearences the small engines have. What about regular 2 stroke oil? Or lightweight machine oil? Marvel mystery oil? Etc.? What is special about castor?
It sticks to metal well even at high temperature so it provides goo lubrication at temperatures that most of the synthetic lubricants (use in model fuels) quit working.
Its relatively cheap.
The main negative property of Castor is that it turns into a gumm varnish just before it burns. This causes problems with stickin valves in 4-stroke model engines.
There are petroleum based oils that mix with alcohol... I've mixe motor oil in alcohol fuel by adding a little gasoline. (don't do thi at home... strange chemical mixes can be dangerous
It withstands high temperatures better than other oils. I still have some Fox breakin fuel that lists 27% castor oil. Fox recommended 20% to
22% castor with all of their lapped piston engines. I still use a castor blend in my ABC engines. A good after run oil is a 50% mix of Marvel mystery oil and transmission fluid.
Castor will protect your engine from a *lean* run. That hated varnish that castor generates at high temps is actually a lubricant. The castor has a higher flash point than the synthetics we use so when the synthetic oil breaks down because of high temps the castor is still doing it's job.. A lean run (extremely high temp) will destroy an engine running synthetics only. Also using castor eliminates the need for an after run oil. BTW, using a castor blend will not gum up a 4c unless you lean it out to the point that it overheats. If you do that the engine using castor will still survive.
I definitely recommend a fuel that has some castor oil in it. All synthetic may run ok and keep from getting stuff all over the plane..but if you ever run the motor too hot or get a lean neede valve settiing, especially on a 2 cycle engine, the synthetic crap is long gone and you have no protection. The very high flash point of castor is your protection. I have engines that are over thirty years old...still running strong on a castor or castor blend fuel. Can't imagine why any thinking modeler would want to use a total synthetic fuel anyway. Frank Schwartz
Because it smell good. I was at the field Sunday before last flying glow for the first time this year and was having a good time when I caught a whiff of that castor burning.......mmmm almost like my wife's cologne(ok not really) mk
Hi, guys, excellent answers, thanks! So, Castor can withstand the high temps that synthetics can't, and castor will mix with methanol while most petroleum based oils won't.
Hmm...so why don't we use kerosene or some other fuel oil in our engines? I understand that alcohol is safer to store and use - you want to spill a couple of ounces of gasoline and watch it ignite - POOOFFFF. Spill a couple ounces of alcohol and ignite. It just slowly burns with a low blue flame. OK, so maybe I answered my own question :-P
When I was a kid I used to put a match to a bottle of alcohol and watch it gently poof out the top. If I did that with gasoline, it would BLOOOEEYYY out the top and probably blow my head half off. I don't know how I survived my childhood.
No the reason that we use methanol is that there is a catalytic reaction between it and the platinum in the plug that maintains the heat in the element until the next compression stroke.
If you change the methanol or plug metal then the engine won't run without the glow plug heater attached. That's why an old plug which glows when the heater is attached won't run an engine as the platinum is a very thin coating on the wire and has probably been burnt off at some point.
Huh. No kidding. Learn something new every day. I've flown these things for over 30 years, and I didn't know that....but then, I didn't know why we use castor, either.
Model 'diesels' do run on kero, the ether is there to aid ignition and allow solution with the castor, the castor for lubrication. A property of castor not directly mentioned in the foregoing is that it has exceptionally good anti-seizure properties. Probably second only to lanolin.
Methanol has a lower BTU content per oz than gasoline, but you burn muc
more alcohol per cubic ft of air. The end result is you get more powe per combustion stroke with alcohol.
The glow plug feature is an added bonus... that eliminated the need t carry the hevy battery, coil and points ignition system which needed lot of tinkering on early model engines. Much simpler more power an weight savings.. that wins.
Gasoline is more economical because its cheaper and you use less pe "horsepower hour". Thats why the 40% scale IMAC planes are almost al gasoline powered. The added weight of the ignition system and th heavier engine is not as big a factor because the big planes usuall need the nose weight anyway.
Early gasoline ignition systems were an RF nightmare, especially wit the early home-made AM radio systems. Modern gasoline engine ignitio systems put out far less RF interference and the FM radios are bette at resisting the interference. Also a modern electronic ignition i lighter, more powerful and more reliable
Furthermore, you probably already know that any modeler who uses castor in his fuel is never constipated. The body absorbs enough of it through the skin....and you know the rest. Subject closed.... Frank Schwartz
Methanol also runs cooler and has higher horsepower potential, because although it has lower BTU per pound than gasoline, it burns at about 6:1 air:fuel, versus gasoline up around 12-13:1 - therefore per intake stroke you get a much larger MASS of fuel air mixture per stroke since so much more of that mixture is dense liquid versus gas. This more than makes up for the small deficiency in energy/unit mass.
Castor oil is an interesting molecule - it is actually a prepolymer, with functional hydroxyl groups on it, and is often used as a modifer in urethane chemistry. When heated beyond a certain temperature, it cross-links with itself (homopolymerization) and forms higher molecular weight molecules with correspondingly higher vaporization temperatures. While they are not as desireable as lubricants as the unaltered material they provide some semblance of lubrication beyond the temps that synthetics tend to break down.
There are many lubricants in the chemical world, and while many of them look and feel similar (hey, it's oil, right?) there are differences in their properties that make them good for one application and not another, hence the reason there is no universal oil for internal combustion engines.
The area of highest concern in Cox engines is the ball and socket joint between the connecting rod and the piston - it is under a lot of duress, and survives the best with a notable percentage of castor in the fuel mix. You really don't need all castor, but I strongly suggest 50% castor (i.e. 50% of the oil is castor..) and 20-22% total oil for Coxes. Sig Champion 25% is actually a really good 1/2a fuel, with a 50/50 syn/cas blend and 20% total oil content, and 25% nitromethane which is baby formula to .049's.
Really, the major negatives about castor are that it is more prone to varnish buildup, which BTW is preferred to flashing off and metal-metal wear, and the tenacious nature of the goop that comes out of the exhaust. But wipe down soon after flight with some Fantastik or mindshield washer fluid (not the anti-streaking kind) whic is water, detergent and methanol, and you'll live through the experience.
That's OK - just take a dose of castor oil and go to bed. ;-)
There's a bloke in Australia, Brian Winch, who writes a column on engines in several magazines, who swears that castor is no longer the best. In the USA he appears in R/C Report. I suppose if one is extremely careful never to run lean, and never runs a lapped iron engine (some Foxes and all Coxes) castor isn't worth the mess, but I'll just keep on wiping off after spooging oil all over the right side of my glow engine-powered toys.
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