A glowplug is a heating device used to aid starting diesel engine

Diesel engine

A diesel engine is an internal combustion engine that uses the heat of compression to initiate ignition to burn the fuel, which is injected into the combustion chamber...

s.

Purpose

Diesel engine

Diesel engine

A diesel engine is an internal combustion engine that uses the heat of compression to initiate ignition to burn the fuel, which is injected into the combustion chamber...

s, unlike petrol engine

Petrol engine

A petrol engine is an internal combustion engine with spark-ignition, designed to run on petrol and similar volatile fuels....

s, do not use spark plug

Spark plug

A spark plug is an electrical device that fits into the cylinder head of some internal combustion engines and ignites compressed fuels such as aerosol, gasoline, ethanol, and liquefied petroleum gas by means of an electric spark.Spark plugs have an insulated central electrode which is connected by...

s to induce combustion

Combustion

Combustion or burning is the sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species. The release of heat can result in the production of light in the form of either glowing or a flame...

. 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

Cylinder (engine)

A cylinder is the central working part of a reciprocating engine or pump, the space in which a piston travels. Multiple cylinders are commonly arranged side by side in a bank, or engine block, which is typically cast from aluminum or cast iron before receiving precision machine work...

; 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 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 the large engine block becomes very cold; when one then attempts to start the engine, the cold engine block acts as a heat sink

Heat sink

A heat sink is a term for a component or assembly that transfers heat generated within a solid material to a fluid medium, such as air or a liquid. Examples of heat sinks are the heat exchangers used in refrigeration and air conditioning systems and the radiator in a car...

, quickly dissipating the heat generated by the pistons compressing air. The engine is then unable to start, because it cannot generate and maintain enough heat for the fuel to ignite

Combustion

Combustion or burning is the sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species. The release of heat can result in the production of light in the form of either glowing or a flame...

.
For that reason indirect injected

Indirect injection

In an internal combustion engine, the term indirect injection refers to a fuel injection where fuel is not directly injected into the combustion chamber...

 diesel engines are manufactured with glow-plugs in each prechamber, and direct injected

Direct injection

Direct injection may refer to:*A music recording technique more commonly referred to as Direct Input*A method of fuel injection for an internal combustion engine, as in a gasoline direct injection petrol engines and most diesel engines...

 diesel engines are manufactured with glow-plugs in each combustion chamber.

Method of operation

In an older generation diesel-engine vehicles, unlike in a gasoline-engine vehicle, the operator did not simply turn the key to the "start" position and have the engine immediately start. Instead, 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".

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.

A glow plug is a pencil-shaped piece of metal with a heating element

Heating element

A heating element converts electricity into heat through the process of Joule heating. Electric current through the element encounters resistance, resulting in heating of the element....

 at the tip; that heating element, when electrified, heats due to its electrical resistance

Electrical resistance

The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...

 and begins to emit light in the visible spectrum, hence the term "glow" plug; the effect is very similar to that of a toaster

Toaster

The toaster is typically a small electric kitchen appliance designed to toast multiple types of bread products. A typical modern two-slice toaster draws anywhere between 600 and 1200 W and makes toast in 1 to 3 minutes...

. 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

Heat transfer

Heat transfer is a discipline of thermal engineering that concerns the exchange of thermal energy from one physical system to another. Heat transfer is classified into various mechanisms, such as heat conduction, convection, thermal radiation, and phase-change transfer...

 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 2 to 5 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

Pencil

A pencil is a writing implement or art medium usually constructed of a narrow, solid pigment core inside a protective casing. The case prevents the core from breaking, and also from marking the user’s hand during use....

. The heating filament is fitted into its tip. Glowplug filaments must be made of certain materials, such as platinum

Platinum

Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...

 and iridium

Iridium

Iridium is the chemical element with atomic number 77, and is represented by the symbol Ir. A very hard, brittle, silvery-white transition metal of the platinum family, iridium is the second-densest element and is the most corrosion-resistant metal, even at temperatures as high as 2000 °C...

, that are resistant to both oxidation and high levels of heat.