A
glow plug is used to ignite the fuel in nitro engines by a heated coil of
wire, it has no moving parts or method of adjustment.
Various types are available with differing housings, wire elements, plating and hole size which determine the temperature of the coil element.
When
a glow starter is attached to the glow plug it causes the plug's coil to 'glow'
igniting the air/fuel mixture in the engine cylinder. Once the engine is running
the heat generated by combustion keeps the glow plug's element hot enough
to continue re-igniting the fuel without the eternal power source (glow starter)
Glow plugs are usually identified by a code indicating the operating temperature of the element. However different manufacturers use different temperature rating codes. The lack of a universal standard can cause confusion. Usually engine manufacturers recommend a glow plug no./brand best suited for the engine. Similarly glow plug manufacturers offer guidelines/recommendations for intended engine type/application.
Signs of a glow plug being 'too hot' are pre-ignition, high running temperature. Extreme cases with excessive compression may cause 'detonation' of the fuel rather than burning. Tell tale signs of detonation (which can damage the engine) are a 'miss' in the exhaust sound at high speed and pitted cylinder head around the glow plug/top of piston. It may also cause the coil to come loose from the glow plug causing engine damage.
Signs of a glow plug being 'too cold' are loss of acceleration/top speed and poor engine idle. Extreme cases may have excessive fuel discharge from the exhaust but this can also be caused by incorrect fuel mixture settings.
Generally small engines need 'hot' glow plugs and larger 'cool' plugs.
Engines using higher nitro content fuel often run better with 'cooler' glow plugs and conversly using lower nitro content often run better with 'hot' plugs.
Where 'optimum' engine performance is critical e.g. racing, then compression ratio of the engine is also a factor in plug selection. High compression engines favour 'hot' glow plugs and lower compression 'cold' plugs.
Several aspects determine a glow plug's temperature range. Main factors are : thickness, length and composition of the wire used to form the coil. Manufacturers tend to be secretive about this information. Another factor is size of hole in which the element is held, type of plating on glow plug housing and the material the housing is made from.
Most glow plugs dont have information recorded on them so keeping track of which glow plug is which can become confusing once out of their original packaging.
Generally glow plugs will last equally long regardless of type assuming the correct fuel mixture is used. However 'hot' plugs are more susceptible to damage through excessively lean fuel settings causing damage to their elements which are generally thinner than those of 'colder' plugs.
Obtaining 'optimum' engine/glow plug performance is generally not critical for most flyers as long as satisfactory engine performance/reliability is achieved However useful gains in performance/reliabilty can be gained by experimenting with different plugs.
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