Glowplugs are found inside diesel engines, which use force of compression to ignite the fuel/air mixture instead of the conventional spark plug that appears in petrol engines.
However, compression ignition means diesel can be a bit reluctant to start, especially when it's cold. Hence, the glowplug.
It is a small cylindrical piece of metal with a heating element at one end. When the ignition of the vehicle is turned on, an electrical current is sent to the glowplug, which then start to glow hot.
When the fuel is squirted into the cylinder and the fuel/air mixture is compressed the glowplug helps to ignite everything, even when the engine itself is too cold to perform the task on its own.
This greatly reduces the time taken to crank a diesel engine into life, thereby placing much less strain on the vehicle’s battery.
Modern glowplugs heat up much more quickly than those in older engines. When the driver turns on the ignition in a modern diesel vehicle, the glowplug symbol (a curly wire) is usually illuminated for just two or three seconds; when it goes out the engine is ready to be started.
However, in older vehicles, glowplugs took much longer to heat up, and there was no indicator on the car’s dashboard to inform the driver that the engine was ready to be cranked in to life.
Instead, the driver would have to switch on the ignition, and count in their head for as long as 20 seconds before the engine was ready to be started.
Advances in engine technology are not the only reason that glowplugs take less time to warm up; diesel fuel itself is much more advanced than it used to be, and so suffers far less from the problem of ‘waxing’ (basically becoming more solid) at low temperatures.