Lubrication
Most model engines require oil to be included with the fuel as a lubricant since the engine has no independent oiling capability. 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.
Read more about this topic: Glow Fuel