RC piston engines needed to be optimized for power:weight ratio, they needed to be mechanically simple just to fit in the requisite form factor, and they needed to overcome the very poor thermal efficiency associated with scaling down heat engines and decreasing the displacement to surface area ratio.
The solution to this has been what the automotive world would call two-stroke diesel ('Glow') engines, but using a very volatile fuel mix of methanol and nitromethane. Methanol is an alcohol (!not safe-to-drink ethanol!) that can be derived from fermented cellulose, and serves as the primary component of the fuel. Unlike pure hydrocarbons, nitromethane is both oxidized so that it can achieve some carbon-hydrogen-oxygen combustion reaction without air intake, and it's got extra nitrogen molecules to hold that oxygen; The freeing of nitrogen atoms to atmospheric N2 is an energetic reaction held in common with most explosives. According to HowStuffWorks, it's "a little like gasoline that has been pre-mixed with nitrous oxide". The fact that it supplies its own oxygen means that a given quantity of nitro can mix with a lot less air in order to reach a mixture that can fully combust, for a given volume container like a combustion cylinder. As engine mass scales with some function of combustion cylinder volume, this allows tiny engines to still produce a reasonable amount of power.
Methanol is hygroscopic: it absorbs water preferentially to oils, even sucking it out of water vapor in the air. Alcohols that go into fuels first go through water-absorbing chemicals to make them annhydrous (99.99% free of water). Once it does absorb water, that water refuses to allow mixing with any oils, and the fuel separates (and goes bad). This means that nitro fuel (just like ethanol-laden gasoline) has a natural shelf life on the order of a year or two once it's been exposed to air; This is shortened the more air it's had an opportunity to suck the water out of, so big containers that are half-empty will kill fuel much faster than a full container, or one with the air squeezed out before sealing.
For added specific power, essentially all RC engines are two-stroke, a lighter weight design meaning the lubrication oil burns right along with the fuel, and indeed, it's mixed in at a low ratio to the other two ingredients. This makes RC engines a very dirty option; the complete replacement of 2-strokes with 4-strokes in automotive applications reduced some visible types of air pollution a hundred-fold, and efforts are underway to replace even smaller motors in mopeds and lawn mowers as well. A 4-stroke engine would be a very difficult thing to put on an RC plane because of the much heavier body per power.
The fact that oil-laden fuel is messy before and after combustion is one of the more powerful reasons that RC enthusiasts have switched en masse to electric.
Glow engines include RC engines and traditional diesels. They do not use spark plugs, but a permanently hot element on one side of the combustion chamber. This element remains a sufficient temperature to ignite the fuel, but only once it has been compressed to near its minimum volume at the very top of the engine cycle.
- Understanding Nitro Fuel - RCHobbies
- Wouldn't more nitro allow an engine to run cooler? - RCGroup