On the hydrogen pre-ignition phenomenon induced by engine lubricating oils with different calcium contents in a rapid compression expansion machine

This study is a follow-up to our previous work (Yeganeh et al., Proc. Combust. Inst., 2024) on the hydrogen (H₂) pre-ignition phenomenon induced by engine lubricating oils. While the earlier study compared the H₂ pre-ignition induced by two different engine lubricating oils from different API (American Petroleum Institute) Groups, the current work focuses on two API Group II lubricants with differing calcium (Ca) contents in their additive packages. Since Ca content plays a major role in low-speed pre-ignition (LSPI) in gasoline direct injection (GDI) engines, it is essential to investigate its influence on H₂ pre-ignition as well. Pre-ignition phenomenon is investigated across various equivalence ratios (ϕ = 0.5, 0.4, 0.33) and compression ratios (ɛ = 11–14). The findings reveal that ɛ value has a more pronounced effect on H₂ pre-ignition than ϕ value. Most importantly, unlike gasoline engines, where Ca content significantly affects LSPI, it has only a minor effect on H₂ pre-ignition. Instead, the base oil chemistry seems to have a greater influence on H₂ pre-ignition. Chemical simulations support these findings, showing that the longer chain length of n-hexadecane, used as a surrogate for the tested lubricants, increases reactivity and promotes H₂ pre-ignition at intermediate temperatures. The simulations also indicate that the pre-ignition behavior observed in the experiments can be replicated with specific molar fractions of n-hexadecane: 2 mol- % at ɛ = 13 and ϕ = 0.5, 2.4 mol- % at ɛ = 13 and ϕ = 0.4, and 2.8 mol- % at ɛ = 12 and ϕ = 0.33.