Open Access

The paper presents the results of the experimental and reaction kinetic investigation of hydrogen ignition at different pressures, highlighting its non-linear behaviour and effects of radical wall termination. The reaction kinetic simulation predicts the three characteristic ignition limits caused by radical and thermal auto-ignition and is in close agreement with the experimental measurements. The introduction of radical wall termination in the reaction mechanism allowed us to investigate the effect of the autoclave wall and vessel size on the hydrogen ignition behaviour. The first ignition limit is determined by the chain initiation reaction H_2+O_2→2 OH and shows a strong sensitivity towards wall termination of O, H and OH radicals. The third ignition limit is dominated by the reaction paths HO_2+HO_2→H_2 O_2+O_2 and H_2 O_2+M→2 OH+M which is why it shows a strong sensitivity towards wall termination of HO₂ and H₂O₂ radicals. The second ignition limit is influenced by the wall termination of O, H, OH, HO2 and H2O2 radicals. Increasing the radical wall termination rate by increasing the adsorption rate of the radicals at the wall leads to an increase of the auto-ignition temperature at the same pressure.

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.