Case Studies

Audi, BMW, Mercedes, Sprinter – Simple Guide to the Diesel Glowplug

Simple Guide to the Diesel Glow Plug

In a diesel engine, a glow plug (also spelled glowplug) is a heating device used to aid the starting of the engine in cold weather. This device is a pencil-shaped piece of metal with an electric heating element at the tip. Operation of the glow plug system is usually indicated by a warning light on the dash, the same light can also be used to indicate failures.

A glow plug system consists of either a single glow plug in the inlet manifold or one glow plug 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.

However, compression ignition means diesel can be a bit reluctant to start, especially when it’s cold. Hence, the glow plug.
As its name implies, the glow plug is a simple device, and generally quite reliable, that glows at a high temperature when in operation.
Construction 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 glow plug, which then starts to glow hot.

When the fuel is squirted into the cylinder and the fuel/air mixture is compressed the glow plug 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 into 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.

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