Glow fuel is a generic term used to describe a fuel source used in model engines -- generally the same or similar fuels can be used in model airplanes, helicopters, cars and boats. Glow fuel can be burned by very simple two stroke engines or by more complicated four stroke engines and these engines can provide impressive amounts of power for their very small size. Glow fuel is primarily for two stroke engines with the need for oil mixed in the fuel and limited exhaust and fuel/air between cycles. Top Fuel race cars with 4-cycle engines also use methanol nitromethane mixtures but do not contain appreciable oil.
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Other commonly used names are nitro or just model fuel. Note that the nitro name is generally inaccurate, as nitromethane is usually not the primary ingredient, and in fact many glow fuels, especially the so-called "FAI" type, named for the Fédération Aéronautique Internationale, which requires such fuel in some forms of aeromodeling competition, contain no nitromethane at all.
Glow fuel is generally a mixture of methanol, nitromethane, and oil.
Methanol is usually the primary ingredient, as it provides the bulk of the fuel and is also needed as a solvent for the other ingredients. Its presence is also required for the glow plug generally found in model engines to burn via a catalytic reaction from the platinum metal in the glow plug's wire element glowing in the presence of methanol vapor, to keep the ignition going between strokes.
Nitromethane is generally added to the methanol to increase power and to make the engine easier to tune. Typically glow fuel is about 0-30% nitromethane, with higher percentages of nitromethane generally giving better performance but costing more as nitromethane is considerably more expensive than methanol. A given amount of nitromethane contains less energy than the same amount of methanol, but it requires less oxygen to burn approximately half, so more of it can be put into an engine during each stroke and this leads to a higher power output.
For racing use, the nitromethane content can be increased well above 30% with a corresponding increase in cost, but some methanol is still required as it's needed to allow the glow plug to ignite the fuel in the first place, and as a solvent to allow the lubricants and nitromethane to mix, so the nitromethane content usually doesn't go any higher than 65%.
Nitromethane is often difficult to obtain in many countries since it is known as an explosive, so in these countries glow fuel generally has no nitromethane at all. Without at least a small amount of nitromethane, glow engines are harder to tune (get the correct fuel/air mixture) correctly and generally provide less power for a given displacement.
Most model engines require oil to be included with the fuel as a lubricant, as it's not provided by the engine itself, and so model engine fuel is typically 8-22% oil, with the higher percentages run in older design two-stroke glow engines that use bushings for the crankshaft bearings. The most commonly used lubricants are castor oil and synthetic oils, and many glow fuels include a mixture of the two. The oils included in glow fuel generally are not burned by the engine, and are expelled out the exhaust of the engine. This also helps the engine dissipate heat, as the oil emitted is generally hot.
Four stroke model engines, since they are generally designed to be simple powerplants while still incorporating the usual camshaft, rocker arms and poppet valves of larger sized four stroke engines, are generally meant to use glow ignition and their fuel. Often, the oil percentage for four stroke glow fuel can be lowered from the 18-20% figure used for some two-stroke engines, down to as low as a 12-15% percentage per unit of blended glow fuel, but use of such low-percentage lubricant fuel can also mandate the need for a small percentage of castor oil in the mix to avoid having too little oil in the mix, and also mandates setting the high-speed fuel mixture carefully by using a handheld digital tachometer to check engine speed to avoid over-leaning of the fuel mixture.
Glow engines generally have to be run slightly rich with a higher fuel/air ratio than is ideal to keep the engine cool as the fuel going out the exhaust also takes heat with it, and so vehicles with glow engines generally get coated with lots of oil. Almost all the oil comes out the exhaust, and some nitromethane and methanol as well (as it's not all burned) requiring some cleaning when one is done using the model.
The nitromethane that exists in many glow fuel blends can cause corrosion of metal parts in model engines, especially four-stroke designs, due to the nitric acid residue formed from combustion of nitromethane-content glow fuel, making the use of a so-called "after-run oil" a common practice after a model flying session with a four-stroke glow engine-powered model.
Glow fuel is not difficult to make, and so many modelers mix their own to save money, but some of the ingredients are flammable and/or explosive and so can be dangerous, especially in large quantities. Most modelers buy their glow fuel premixed from such manufacturers such as Byron, Blue Thunder, FHS Supply, Model Technics, Morgan, Powermaster, Tornado, Wildcat, and many others.
Nitromethane is sometimes replaced or supplemented by nitroethane. Propylene oxide is sometimes added in small percentages. Another form of model fuel, for small compression ignition engines, is called "Diesel Fuel", which generally consists of kerosene, ether, oil and some sort of ignition improver, usually amyl nitrate or isopropyl nitrate, it is in no way related to the automotive fuel of the same name.