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

• H2 pre-ignition induced by lubricants is influenced more by ɛ than by changes in ϕ. • Ca content has a minor influence on lubricant reactivity for causing H2 pre-ignition. • n-Hexadecane serves as a surrogate for lubricants to replicate H2 pre-ignition behavior. This study is a follow-up to our previous work (Yeganeh et al., Proc. Combust. Inst., 2024) on the hydrogen (H 2 ) pre-ignition phenomenon induced by engine lubricating oils. While the earlier study compared the H 2 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 H 2 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 H 2 pre-ignition than ϕ value. Most importantly, unlike gasoline engines, where Ca content significantly affects LSPI, it has only a minor effect on H 2 pre-ignition. Instead, the base oil chemistry seems to have a greater influence on H 2 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 H 2 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.