Heating element used to aid in starting diesel engines
A typical glowplug
Glowplug in operation
In a diesel engine, a glowplug (also spelled glow plug) is a heating device used to aid starting of the engine in cold weather. This device is a pencil-shaped piece of metal with an electric heating element at the tip.
A glowplug system consists of either a single glowplug in the inlet manifold, or one glowplug per cylinder. In older systems, the driver is required to manually activate the glowplug system and wait approximately 20 seconds before starting the engine. Newer systems automatically activate the glowplug(s) before the engine is started and have a quicker warm-up time.
Design
Glow plugs installed in a small engine (beneath the horizonal bar
Diesel engines, unlike gasoline engines, do not use spark plugs to ignite the air/fuel mixture. 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. When the engine is cold, the intake air receives little heat from the cylinder walls, rather it loses heat (once compressed) to the cold cylinder walls, further reducing its thermal energy. Glow plugs are used to help overcome this issue by heating the engine.
A glowplug resembles a short metal pencil. The heating element is fitted into its tip. Glowplug filaments must be made of certain materials, such as platinum and iridium, that resist oxidation and high temperature. The most common arrangements are in-cylinder glow plugs and in-manifold glow plugs.
However, some diesel engines (especially direct-injection engines) do not use starting aid systems such as glowplugs. The minimum starting temperature without using glowplugs is 40 °C for precombustion chamber injected, 20 °C for swirl chamber injected, and 0 °C for direct injected engines. Engines with a displacement of more than one litre per cylinder usually incorporate a flame-start system rather than glowplugs, if a starting aid system is required.[1]
In-cylinder type
In-cylinder glow plugs are installed either in every cylinder (for direct-injection engines or in every pre-chamber (for indirect injection engines).
For this design, a valve opens to allow diesel fuel into the inlet manifold while the glowplug is heating up; this diesel near the glowplug typically ignites after approximately 20 seconds. The ignited air diesel is then drawn into the combustion chambers and more diesel is injected after the compression stroke. The additional diesel is ignited by the diesel that is already burning in the combustion chamber, and the engine starts.
In-manifold type
In-manifold glow plugs are a single glow plug is shared between all of the cylinders.
Activation method
Glowplug indicator light in the dashboard of a car
For older vehicles using an in-cylinder glowplug, the driver needs to activate the glow plug for approximately 20 seconds before starting the engine. This was achieved by leaving the ignition key in the "on" position, and only moving it to "start" once the glowplug had finished pre-heating the engine. Once the engine is running, some cars continue to use the glowplugs until the engine has reached its operating temperature, in order to reduce the diesel exhaust emissions. Similarly, some engines re-activate the glowplugs if the temperature of an engine under light loads reduces below a certain threshold, in order to improve the efficiency of the engine.
Engines with an in-manifold glowplug typically either use a button (separate to the ignition key) to activate the glowplug system, or by turning the ignition key in the counter-clockwise direction.
Many modern engines automatically activate the glowplugs when the operator unlocks the vehicle or opens the door to the car, thus simplifying the process and shortening the waiting time the operator has to wait before the engine will start. In addition, the time needed to pre-heat the engine is typically 6-8 seconds.[2] Some vehicles include a warning light on the dashboard that extinguished once the engine was pre-heated.
Glow plugs in small model engines are quite different to those discussed above for full-size diesel engines. Full-size engines only use the glow plug for starting. Model engines use a glow plug as an integral part of the ignition system for the methanol-based fuel used by these engines.
