Glow plugs are used to heat the combustion chambers of diesel engines in cold conditions to help ignition at coldstart. In the tip of the glow plug is a coil of a resistive wire or a filament which heats up when electricity is connected.
Glow plugs are required because diesel engines produce the heat needed to ignite their fuel by the compression of air in the cylinder and combustion chamber. Petrol engines use an electric spark plug. In cold weather, and when the engine block, engine oil and cooling water are cold, the heat generated during the first revolutions of the engine is conducted away by the cold surroundings, preventing ignition. The glow plugs are switched on prior to turning over the engine to provide heat to the combustion chamber, and remain on as the engine is turned over to ignite the first charges of fuel. Once the engine is running, the glow plugs are no longer needed.
Indirect-injection diesel engines are less thermally efficient due to the greater surface area of their combustion chambers and so suffer more from cold-start problems. They require longer pre-heating times than direct-injection engines, which often do not need glow plugs at all in temperate or hot climates even for a cold start.
In a typical diesel engine, the glow plugs are switched on for between 10 and 20 seconds prior to starting. Older, less efficient or worn engines may need as much as a minute (60 seconds) of pre-heating.
Large diesel engines as used in heavy construction equipment, ships and locomotives do not need glow plugs. Their cylinders are large enough so that the air in the middle of the cylinder is not in contact with the cold walls of the cylinder, and retains enough heat to allow ignition.
Modern automotive diesel engines with electronic injection systems use various methods of altering the timing and style of the injection process to ensure reliable cold-starting. Glow plugs are fitted, but are rarely used for more than a few seconds.
Glow plug filaments must be made of materials such as platinum and iridium that are resistant both to heat and to oxidation and reduction by the burning mixture. These particular materials also have the advantage of catalytic activity, due to the relative ease with which molecules adsorbed on their surfaces can react with each other. This aids or even replaces electrical heating.
Source: Wikipedia
