A glow plug engine, or glow engine, is a type of small internal combustion engine typically used in model aircraft, model cars and similar applications. The ignition is accomplished by a combination of heating from compression, heating from a glow plug and the catalytic effect of the platinum within the glow plug on the methanol within the fuel.
American inventor Ray Arden invented the first glow plug for model engines in 1947. Born in New York on February 24, 1890, Arden sold over 300 inventions, mainly toys, in the 1920s. In the late 1940s Arden invented miniature ignition engines while attempting to build the world's smallest model airplane engine. Methanol fuel came onto the market after World War II as a source of fuel for model engines. A couple of Arden's friends were running an ignition model engine and to their amazement, when they turned it off, the engine kept running. They investigated why this was and discovered that a fault in the spark plug had caused a part of it to become red hot. They called their friend Arden and the glow plug was born. From then on, model engines became much simpler, no longer requiring the additional weight, bulk, and complexity of points, ignition coils, condensers, timers and batteries.
Model glow plug design
The glow plugs used in model engines are significantly different from those used in full-size diesel engines. In full-size engines, the glow plug is used only for starting. In model engines, the glow plug is an integral part of the ignition system because of the catalytic effect of the platinum wire. The glow plug is a durable, mostly platinum, helical wire filament recessed into the plug's tip. When an electric current runs through the plug, or when exposed to the heat of the combustion chamber, the filament glows, enabling it to help ignite the special fuel used by these engines. Power can be applied using a special connector attaching to the outside of the engine, and may use a rechargeable battery or DC power source.
Glow fuel generally consists of methanol with varying degrees of nitromethane content as an oxidizer for greater power, generally between 5% and 30% of the total blend. These volatiles are suspended in a base oil of castor oil, synthetic oil or a blend of both for lubrication and heat control, again, in varying degrees of overall content. The lubrication system is a "total loss" type, meaning that the oil is expelled from the exhaust after circulating through the engine. The fuel ignites when it comes in contact with the heating element of the glow plug. Between strokes of the engine, the wire remains hot, continuing to glow partly due to thermal inertia, but largely due to the catalytic combustion reaction of methanol remaining on the platinum filament. This keeps the filament hot, allowing it to ignite the next charge, thus sustaining the power cycle.