I think there have been a number of mis-leading answers to this question....

Firstly, the filament in-and of itself does not contribute chemically, nor catalytically to the combustion. It's only function is to produce a heat source. The length of the filament (because it is a coil) is surprising, and has a substantial surface area which is good for heat transfer.

Fuel combustion is a funny thing. Regular "gas" (that you put in your car) is a prime example. The actual fuel is non-flammable. It is the fuel vapour that burns.... and it only burns in a discrete boundary of oxygen/vapour concentrations at a given pressure... Too much vapour, and there is no combustion. Too much oxygen, and there is no combustion. Too little pressure, and there is no combustion. Too much ... etc. All combustion takes place at a temperature point that is specific to the pressure and mix of the fuel.

To proove this, just look at a candle.... Candles are quite safe, until they are lit. Candles are not considered to be explosive, although, in the right environment they can explode.

Your engine works as follows:
fuel/air is mixed in just the right proportions inside the carbeurattor. The fuel is atomised (sprayed very finely into the air) so that the fuel has a large surface area. The larger the surface area, the quicker the fuel vaporizes. The fuel/air mixture gets fed into the cylinder. The glow-plug is glowing inside the cylinder. It does not cause any combustion yet becuase there is not enough pressure. The Up-stroke of the piston compresses the fuel. Because your engine is well tuned, the fuel/air is fully compressed fully before it combusts. The fuel-air that is near the glow element combusts first, because it is the most heated. The heat, combined with the pressure brings the remaining un-combusted fuel above the flash-point temperature, and the explosion that started near the glow plug progresses through all the fuel/air mix.

Basically, combustion only happens in your engine when the fuel/air mixture is just right, and the mixture is significanly pressurised, and a heat source is applied. At that point, the mixture combusts. Combustion is an exothermic reaction (causing heat), and this heat is partially used to heat the glow filament. Enough heat is retained in the filament that on the next compression cycle of the engine there is enough heat left to initiate the next combustion.

To start the engine, you need the heat in the filamnt, otherwise the fuel/air mix will not reach the flash-point.

It is theoretically possible to tune your engine so that the fuel will combust without a heated glow-plug, but I don't believe that a normal engine will be able to obtain enough compression. If you were to use a different fuel though, you may get different results.

If there is enough residual heat in the engine, and glow plug then the fuel may combust without actually charging the glow plug. i.e. If your engine has been running, and you stop it, then it may be possible to re-start it without actually re-appling the current to the glow plug.

Because of all these things, your most efficient engine operation will be affected by air temperature, humidity, pressure, and fuel content. This is why it is important to re-tune your hish-end engine operation every time you go to the field.

So, The glow plug is just a filament that stores heat! It is specailly designed so that it does not cause ignition too early in the engine cycle (or too late). Changing the length, size, and composition of the filament will all affect it's operation. A longer glow-plug (with a longer filament - more surface area) may advance the combustion. possibly causing back-fires (until you re-tune the engine to operate under the new conditions).

So, there is an amateur intro to combustion. (mostly learned from automobile maintenance).

gus