Investigation of the impact of base oils on the auto-ignition behavior of hydrogen–air mixtures

Using hydrogen as a fuel in internal combustion engines (ICEs) poses several challenges that have yet to be resolved. One issue is the difficulty of controlling combustion, arising from the interaction of lubricant oil during the combustion process. This study examines the effect of lubricant oil on hydrogen's ignition behavior by measuring the minimum auto-ignition temperature (AIT) using a heated constant-volume autoclave at 20 bar. AIT measurements were performed in air and stoichiometric hydrogen–air mixtures to evaluate the effect of the base oil on ignition characteristics. Experimental results demonstrate that auto-ignition of a small volume of 0.2 ml in a total volume of 200 ml of base oil significantly altered the ignitability of H2, reducing the AIT of a stoichiometric H2-air mixture from 460 °C to 270 °C. Further increases in base oil volume up to 1 ml result in a further decrease in AIT in both air (260 – 240 °C) and H2-air mixtures (270 – 250 °C), indicating the need for further investigation into whether this suggests a higher risk of pre-ignition in H2 ICEs. Four common base oils – two mineral oils (Groups II and III) and two synthetic oils (Groups IV (polyalphaolefin) and V (ester)) – were tested to assess their ignition characteristics. The experimental results revealed that all four base oils exhibited similar AITs in air (260 °C) and in hydrogen–air mixtures (270 °C). However, their reactivity differed at a given temperature and pressure; the synthetic oils were less reactive than the mineral oils. Of the synthetic oils tested, the ester base oil exhibited the lowest reactivity, reflected in its longer ignition delay time. Additionally, a numerical investigation was conducted in an adiabatic, constant-volume reactor to examine the impact of base oil surrogates on H2 ignition across the temperature range. The simulated results showed that the base oil surrogates are more reactive than H2 at low temperatures. Of these tested oils, the ester showed the lowest reactivity at a given temperature and pressure, indicating its beneficial potential as a base lubricant for hydrogen engines.