Fuel injection pressure decreases in four-stroke diesel engines after extended operations. Poor combustion caused by the pressure drop is a serious problem that can lead to environmental pollution. Several factors have been considered to be behind mechanisms to decrease fuel injection pressure, but the details of such mechanisms are still unknown. Up until now, only limited numbers of studies have been conducted to define these mechanisms. In this study, we clarified that stress relaxation, buckling and metal fatigue of the pressure regulating spring in the fuel injection valve contribute to lowering injection pressure. We also found that the lowering of injection pressure causes early and prolonged injection in diesel engines.
