Glowplug

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Glowplug

''For the similar device used in model aircraft engines, see Glow plug (model engine).

A glowplug (alternately spelled as glow plug or glow-plug) is a heating device used to aid starting diesel engines.

Purpose

Diesel engines, unlike petrol engines, do not use spark plugs to induce combustion. Instead, they rely solely on compression. The piston rises, compressing the air in the cylinder; this, by natural effect, causes the air's temperature to rise. By the time the cylinder 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 problem posed is that in cold weather, if the engine has not been running (as is the case when the car is left to sit overnight), that large engine block becomes very cold; when one then attempts to start the engine, the cold engine block acts as a heat sink, quickly dissipating the heat generated by the pistons compressing air. The engine is then unable to start, because it can not generate and maintain enough heat for the fuel to ignite.

One common, and very effective, method for fixing this problem is to fit each cylinder with a glowplug. For that reason, many smaller diesel engines come pre-fitted from the factory with glow-plugs.

Method of operation

In a diesel-engine car, unlike in a gasoline-engine car, the operator does not simply turn the key to the "start" position and have the engine immediately start. Instead, the operator turns the key to the "on" position; 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".

If the car has been running very recently, or if the ambient temperature is hot, the "wait to start" light may 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.

A glowplug is a pencil-shaped piece of metal with a heating element at the tip; that heating element, when electrified, heats due to electrical resistance and begins to emit light in the visible spectrum, hence the term "glow" plug; the effect is very similar to that of a toaster. The heat generated by the glowplugs is directed into the cylinders, and serves to warm the engine block immediately surrounding the cylinders. This aids in reducing the amount of thermal diffusion which will occur when the engine attempts to start.

When internal sensors detect that the core of the engine block has reached a certain designated temperature, or when a certain amount of time elapses, the glowplug relay switches off the "wait-to-start" light. A pre-heating cycle usually lasts for 10 to 20 seconds. The operator then proceeds to turn the key to the "start" position, as in a gasoline engine. The glowplug 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 newer 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.

Construction

A glowplug resembles a short metal pencil. The heating filament is fitted into its tip. Glowplug filaments must be made of materials, such as platinum or iridium, which are resistant to both oxidation and high levels of heat.

Problems

In some cars, including all older cars, it is possible to attempt to start the engine (crank it over) while the glowplugs are switched on (relays in many modern cars prevent this). The heat caused by the engine's combustion, when combined with the heat from being electrified, is destructive to the glowplugs; the overheating can easily cause them to burn out. Thus, it is important to wait for the "wait to start" light to go out before cranking the engine.

Glowplugs have a limited lifespan. Certain factors, such as the aforementioned overheating, can greatly shorten that lifespan. The amount of heat which a glowplug produces diminishes over time; this causes the engine to become progressively harder to start in cold weather.

Glowplugs are inexpensive, and typically easy to replace -- making it worthwhile to replace them annually in areas with a cold winter season.

Modern advancements

Indirect-injection engines are less thermally efficient, due to the greater surface area of their cylinders, and so suffer more from cold-starting issues. 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. Most modern diesel cars use direct-injection engines (see Volkswagen TDI).

Modern automotive diesel engines use various electronic methods of altering the timing and style of the injection process to ensure reliable cold-starting. Glow plugs are still fitted, but are rarely used for more than a few seconds.

Large engines

Large diesel engines, such as those used in heavy construction equipment and locomotives, do not need glow plugs. Their cylinders are large enough that the air in the middle of the cylinder is not in contact with the cold walls of the cylinder, and thus retains enough heat to allow ignition.