What are glow plugs?

In a gasoline or spark-ignition engine, the air/fuel mixture is ignited by a spark from the spark plug. Diesel engines, on the other hand, are compression-ignition engines, which operate according to the principle of auto-ignition: intake air is compressed in the cylinders, heating it to a temperature between 700 and 900°C. When fuel is injected into the cylinders, it ignites automatically as a result of exposure to the high temperature.

If the outdoor temperature is low, then the engine and the cylinders are cold, as is also the intake air. To counteract heat loss, the glow plugs compensate for unfavorable ignition conditions by “pre-glowing” to heat the combustion chamber. Modern glow plugs continue to glow until the engine has reached the required operating temperature. This also ensures smooth operation of the engine and low emissions during cold starts. After-glow reduces blue smoke and white smoke by almost 50 percent. “Cold start knocking” is a thing of the past.

A glow plug has to be robust: it is mounted in the cylinder head and the glow element projects into the combustion chamber of the engine, where it is exposed to extreme heat and high pressure.

NGK offers glow plugs for a wide range of engines and control units. Each type is precision work. Glow plugs differ in terms of the materials used and the design. There are steel and ceramic glow plugs with different thread diameters and lengths, in addition to glow plugs for different battery voltages and special glow properties.