Fuel Choices

An article appeared in the Oct 2000 issue of MODEL WORLD relating to the pro's and con's of different types of fuel. Below is a precise of that report.

The modelling "die hards" should recognize that engines and fuels have advanced over the years. CNC techniques along with modern metallurgy have reduced the need for high content castor fuels, engines these days develop much more power, are more efficient as well as reliable. Even so the debate still rages, Castor or Synthetic. Let's list the pro's & Con's for each in a Straight mix first.

Synthetic

Pro's: Good lubrication - little carbon or varnish deposits - cleaner exhaust residue - range of viscosities - totally soluble in nitromethane.

Con's: Can be corrosive - burns at approx 100 degrees less than castor - much more expensive.

Castor

Pro's: Very good lubrication - less needed pro-rata than synthetic - higher temperature tolerance (will not burn off).

Con's: Carbon and varnish deposits - very messy exhaust deposits - sensitive to cold temperatures - insoluble in nitromethane - available in one viscosity only - gums up the valve gear something rotten on four strokes.

It appears that no impartial body can conclusively prove that one is better than the other, each has it's own different advantages and disadvantages. The answer is probably to use a mix of the two in proportions that would maximize the benefits of both, indeed a lot of modellers have over the past ten years adopted this approach.

Manufacturers so called "magic fuels" are probably, if my nasal passages don't deceive me, a "Synthetic Mix" with a few drops of Castor thrown in, which is just fine for the general engine. However when it comes to specialized engines i.e. Helicopters, ducted fan, pylon etc., then the manufacturers recommendations should be adhered to.

The Facts about nitromethane

The common belief that nitromethane is a "HOT" constituent is totally wrong, it is in fact quite benign, it needs to be heated to 96 deg F before it vaporizes enough to be ignited with a naked light. Whereas Methanol would burst into flames at nominal room temperature.

So why use nitromethane.? Without boring you with too much detail about calorific values etc., it boils down to this. For a given volume of air (one cycle of the engine), 2.25 times more nitromethane than Methanol will be burnt by that air. The more fuel that's burnt the more energy is generated. (ED. Why not use more nitromethane than methanol. Probably because a gallon of nitromethane would cost more than a £100).

The practical facts about the benefits are that for every 5% nitromethane added to the fuel, at full throttle the increase in power only generates about an extra 100 RPM. The biggest gain is a much improved and reliable tick over.

Once again take note of the manufacturers recommendations since the engine will have been designed for the fuel range specified, indeed overdosing with nitromethane in most cases REDUCES the maximum power. Perhaps the only exception to this is the FOUR stroke where using the maximum specified nitromethane mix will greatly enhance the otherwise very fallible tick over.

The MYTHs about nitromethane

Nitromethane does NOT cause acid corrosion, indeed Chemical experts say it's impossible for it to do so. The other rumour about nitromethane mixes allowing less fuel consumption is also NOT true, indeed the reverse is the fact since for the same air volume through the engine requires more nitromethane to complete the burn, otherwise you get a lean burning engine which is not good for lubrication. You would normally expect the needle valve to be opened one more click for good combustion.

Notes From ED.

My quite extensive experience with combustion equipment (hot rod cars, industrial boilers & turbines, small engines etc.) leads me to believe that all of the above is true. In fact the most likely cause of acid corrosion is the fuel itself. No matter what fuel you burn, unless its burnt efficiently and completely carbon/soot is left behind, this with moisture generates the acid everybody gets carried away about. The unfortunate requirement of the glow engine is the over rich mixture necessary for all through lubrication and cooling. Rich mixture means part burnt fuel and part burnt fuel is carbon. Perhaps that's why the engine manufacturer advices a clean-up at the end of the season. A following paragraph from "Model Technics" web site appears to support these points.............

"Never store the model nose down in the corner of your garage, going home in the car, or even while cleaning it at the field, if it is fitted with a tuned pipe or an extra large silencer. The exhaust residue which collects in these, is highly corrosive and would run straight back into the engine."

For more detailed information about FUELS and GLO-PLUGS see the Model Technics Web Site

Another point of note is that glow-fuel does "go off" unless its kept under ideal conditions. The main component being methanol is hygroscopic, meaning it literally drinks water (most of the car screen wash liquids are mixed with methanol). If you store the fuel in a damp atmosphere or with a substantial air space in the container the methanol will absorb all the moisture. You should decant part used large containers into smaller bottles filled to the brim and corked tight. How many of you have trouble starting uncleaned engines with last years fuel at the beginning of the season, then zip into a frenzy of necessary tweaking and fiddling?