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A glowplug (alternatively spelled as glow plug or glow-plug) is a heating device used to aid starting diesel engines. In cold weather, high speed diesel engines can be difficult to start because the mass of the cylinder block and cylinder head absorb the heat of compression, preventing ignition (which relies on that heat). Pre-chambered engines make use of small electric heaters inside the pre-chambers—glowplugs—while the direct-injected engines have these glowplugs in the combustion chamber. The glowplug is a pencil-shaped piece of metal with a heating element at the tip. This heating element, when electrified, heats due to its electrical resistance and begins to emit light in the visible spectrum, hence the term "glow"-plug. The visual effect is very similar to that of a toaster. The fuel injector spray pattern then impinges directly upon the hot tip of the glow plug during the injection of fuel at top dead center. This enables the fuel to ignite even when the engine is insufficiently hot for normal operation. This reduces the cranking time needed to start the diesel engine.
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 will combust 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, necessitates the use of a large and very strong engine block.
The temperature at the top of the compression stroke is dependent upon many factors, the most influential of which are 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 will be given up to the cold cylinder walls, further reducing the temperature at the top of the compression stroke. This problem is solved by the glow plug.
There are two quite different types of glow plug: the in-cylinder variety; and the in-manifold ("Thermostart") varieties. 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.
Method of operation
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 glow plug 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. Also the preheating phase was 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". 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".
Before any attempt is made to start a vehicle engine, the vehicle's parking brake should be firmly applied and gear selection should be in Neutral (or Park). Any engine stop control should be returned to the run position and in cold weather, any 'excess fuel' starting aid should be set or used - such as Ki-Gass.
With in-cylinder glow plugs, when a pre-set time has elapsed, the glowplug relay switches off the "wait-to-start" light. A pre-heating cycle usually lasts for 2 to 5 seconds. The operator then proceeds to turn the key to the "start" position. The relay switches off the glowplugs after the engine is running (or, in older cars, at the same time the "wait to start" light goes out). In some cars, glow plugs continue to operate for up to 180 seconds after engine start to keep the engine within emissions regulations, as combustion efficiency is greatly reduced when the engine is very cold.
With a Thermostart plug in the inlet manifold, within a few seconds diesel vapours start to fill the inlet manifold. As the plug continues to heat up, it opens a valve permitting diesel from a special reservoir mounted directly above the Thermostart into the Thermostart plug. This fresh diesel is also vaporised and adds to that in the inlet manifold. At 20 seconds, provided air is available, the diesel near the plug ignites and as the engine is cranked, the ignited diesel is drawn into the combustion chambers - to which more diesel is added after the compression stroke. This additional diesel immediately ignites, starting the engine with ease. Vehicles fitted with Thermostart glow plugs do not usually activate via the normal ignition switch. A button elsewhere is provided (along with, in some cases, the button to activate the starting motor). Where a Thermostart is activated by the same switch as the ignition, it is usually activated by turning the switch one 'notch' anticlockwise. After the 20 second period has elapsed, cranking is achieved by turning the ignition switch a further notch anticlockwise. Once the engine has fired and is running, the ignition switch is released, allowing it to spring back to the off position. The operator should then turn the ignition switch to the on position - usually one notch to the right.
Warm engine start
If the car had been running very recently, or if the ambient temperature was hot, the "wait to start" light might not come on. In this case, the operator may proceed to turn the key to the "start" position and start the engine without having to wait.
With a Thermostart, it is entirely up to the operator to choose whether to activate this glow plug. In the case of ignition key controlled Thermostart, turning the key two notches to the right will effect unheated cranking.
A glowplug resembles a short metal pencil. The heating filament is fitted into its tip. Glowplug filaments must be made of certain materials, such as platinum and iridium, that are resistant to both oxidation and high temperature.
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.
Model engine glow plugs are also used as re-usable igniters in theatrical pyrotechnics and the special effects industry to remotely ignite pyrotechnic devices using flash and smoke composition powders.