<P> Diesel engines, unlike gasoline engines, do not use spark plugs to induce combustion . Instead, they rely solely on compression to raise the temperature of the air to a point where the diesel combusts spontaneously when introduced to the hot high pressure air . The high pressure and spray pattern of the diesel ensures a controlled, complete burn . The piston rises, compressing the air in the cylinder; this causes the air's temperature to rise . By the time the piston reaches the top of its travel path, the temperature in the cylinder is very high . The fuel mist is then sprayed into the cylinder; it instantly combusts, forcing the piston downwards, thus generating power . The pressure required to heat the air to that temperature, however, requires a large and strong engine block . </P> <P> The temperature at the top of the compression stroke depends on many factors, particularly the compression ratio of the cylinder and the starting temperature of the inducted air . When the engine is cold, the temperature of the inducted air is low and it receives little heat from the engine cylinder walls . In addition, as the air is compressed and becomes heated, some of this heat is lost to the cold cylinder walls, further reducing the temperature at the top of the compression stroke . The glow plug solves this . </P> <P> There are two different glow plug types: the in - cylinder variety and the in - manifold ("Thermostart") variety . In the case of in - cylinder, there is a plug in every cylinder direct injected (or in the case of indirect injected, the glow plug is in the prechamber providing a hot spot to encourage ignition . In the case of the in - manifold one, there is only one for all the cylinders . </P> <P> In older generation diesel - engine vehicles, unlike in a gasoline - engine vehicle, for a' cold start' the operator did not simply turn the key to the "start" position and have the engine immediately start . Instead, the operator activated the glow plug (s) for a time first . Early diesels used the' Thermostart' type glowplug in the inlet manifold . These take 20 seconds to achieve working temperature and the vehicle operator had to manually time (or guess) when the 20 seconds had elapsed . With in - cylinder glow plugs, technological improvements included a warning light on the dash to indicate how long the preheating should last . The preheating phase was also made to be automatically activated when the operator turned the key to the "on" position for a long duration; the glowplug relay switches the glowplugs on, and a light (see picture at right) on the instrument cluster illuminates . This process is called "pre-heating" or "glowing". Many modern diesels automatically activate their glow plugs when the operator unlocks the vehicle or opens the door to the car, thus simplifying the process and shortening the time the operator has to wait before the engine will start . According to Bosch: "Older engines with Thermostart manifold plugs used a glow period of up to 20 seconds whereas more modern engines use around a 6 to 8 second heat period and provide after glow at a reduced voltage ." </P>

Glow plugs are used in diesel engines to