Investigation of the hydrogen pre-ignition induced by the auto-ignition of lubricating oil droplets

With the application of hydrogen in marine engines, greenhouse gas emissions can be effectively reduced. However, abnormal combustion restricts the development of hydrogen engines. Lubricating oil is considered the main factor responsible for the onset of abnormal combustion modes. The pre-ignition that occurred in hydrogen engines may have some different characteristics, due to lower ignition energy and higher burning rate of hydrogen. To understand the characteristics of the pre-ignition induced by lubricating oil, experimental research was carried out based on a rapid compression machine (RCM). The pre-ignition is usually accompanied by an engine knock. Both the increase of temperature and pressure intensifies the occurrence tendency of pre-ignition for hydrogen engines. In particular, when the temperature is increased by 70 K, the oil droplet (0.1 mm) ignition delay is shortened by about 70 % and the flame diffusion speed is increased by about 40 %. The reduction of air-to-fuel equivalence ratio (λ) promotes the occurrence of pre-ignition accompanied by varying degrees of engine knock. To avoid knocking associated with pre-ignition, an air-to-fuel equivalence ratio in the range of 2.5 to 3.0 is appropriate while maintaining thermal efficiency. Compared with methane, the effect on the physical ignition delay of oil droplets is significantly greater, whereas the effect on the chemical ignition delay is less pronounced.