Requirements on glow plugs
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.