Thermographic study of the preheating plugs in diesel engines

The use of direct injection diesel engines has been widely applied during the past ten years. In such engines, the preheating plugs are a key element which has a significant contribution in the pollutant emissions. In this paper, two different plug designs from Renault are analyzed. The new plug reduces substantially the required electrical consumption. Nevertheless, the pollutant emissions are higher (fundamentally CO and HCs) and hereby a thorough analysis is required to understand the possible reasons. Firstly, an infrared thermography analysis of the plugs has been carried out. The new plug tip presents 100–200 °C higher temperatures than with the former design. Secondly, a thermal model has been developed and validated with temperature measurements. The latter model has helped to obtain the energy flow diagram. Finally, a thermography analysis of the head of the cylinders has been carried out. The results show that the higher exhaust emissions are related with an incomplete combustion process due to a thin air gap which surrounds the tip of the plug.