Requirements on glow plugs

In principle, the glow plug functions like an immersion heater: Electrical energy is introduced through a coil resistor, which heats up to very high temperatures (up to 1000 °C).

Operating conditions in the diesel engine

Unlike the petrol engine, the diesel engine is a self-igniter. The air suctioned into the cylinders is heated up to a temperature of approx. 700-900 °C through compression, which results in self-ignition when fuel is injected. Therefore, a diesel engine requires higher compression (compression ratio 20-24:1) and a correspondingly more stable construction than the petrol engine. In order to guarantee that the necessary temperature is reached even in unfavourable conditions, such as cold-starting or frost, additional heat must be introduced to the combustion chamber.

Method of operation of glow plugs

In principle, the glow plug functions like an immersion heater: Electrical energy is introduced through a coil resistor, which heats up to very high temperatures (up to 1000 °C).

However, in practice this simple principle causes some difficulties in regard to service life, overheating protection and current consumption. As a result, starting processes in the 1960s still lasted up to 30 seconds. In the 1980s this was already reduced to starting times of 3-5 seconds. With the introduction of TDI engines, differences from petrol engine became almost imperceptible with outside temperatures above 0 °C. Only temperatures below 0 °C require the additional pre-glowing.

Glow plugs - the development continues

In no way has this development reached its full potential. Plugs which are capable of after-glowing are needed. The glow plugs should take action both during the starting process and up to three minutes into the warm-up phase, depending on the temperature. Only in this manner can a high level of running performance and low emissions be assured. This places increased demands on the service life of glow plugs.

In addition, the low-compression diesel engines of the future will achieve high performance with simultaneously low emissions as a result of turbo charging. However, such concepts have poor starting performance because of their design. Ceramic high-temperature glow plugs offer some advantages here, because they become significantly hotter than metal glow plugs and provide a long service life.

Thanks to advanced glow plug technology, the difference between the starting performance of a diesel and petrol engine will soon be almost imperceptible.

And those are the requirements on a modern glow plug:

  • Since the glow plug is exposed to the combusting fuel, it must be able to withstand extreme heat, high pressure, vibrations and the attack of corrosive chemicals.
  • The glow plug must reach its operating temperature quickly and have a long service life.
  • Glow plugs also require a control mechanism that regulates the generated voltage to a specified length of time, which takes place either manually or through a control unit. The system is often provided with a corresponding control lamp which indicates that the pre-glowing is underway.
  • For the environment's sake - reduction of white/blue-smoke: Through afterglowing the fuel is completely combusted and development of smoke is reduced by up to approx. 49 %.
  • Remedy of cold-start knocking: The fuel burns evenly and completely; as a result, more energy is released and the combustion chamber temperature increases more rapidly.

Related topics

Glow plug technologies: Glow plugs are basically differentiated between metal rod glow plugs and ceramic glow plugs. The two technologies have special characteristics in regard to thermal behaviour.

Influential factors in the product development: The application of new technologies and legal provisions directly influences the development of new products.