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The presented research work examines the effects of different fuel-air equivalence ratios and initial pressure on ignition temperature.
This study shows a non-monotonic pressure dependence in stoichiometric mixtures at varying pressures from 1 to 10 bar and a low sensitivity to changes in the equivalence ratio (0.5-2). The results form the basis for our future research into the complex interaction between lubricant characteristics and hydrogen-air mixtures to better understand pre-ignition phenomena in internal combustion engines.
The paper presents the results of an experimental and reaction kinetic investigation of hydrogen ignition at different pressures in a closed vessel, highlighting its non-linear behavior and the effects of radical wall termination. The reaction kinetic simulation predicts the three characteristic ignition limits of hydrogen caused by radical and thermal auto-ignition and is in close agreement with the experimental measurements. The first ignition limit is determined by the chain branching reaction H+O_2→O+ OH. This limit shows strong sensitivity towards the wall termination of O, H and OH radicals. The second ignition limit is influenced by the wall termination of O, H, OH, HO2 and H2O2 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 strong sensitivity towards wall termination of HO₂ and H₂O₂ radicals. Increasing the radical wall termination rate by increasing the sticking coefficient of the radicals at the wall or the surface-to-volume-ratio leads to an increase of the auto-ignition temperature at the same pressure. The introduction of radical wall termination reactions improved the prediction of ignition limits and highlighted the profound effect of the autoclave wall and vessel size on the hydrogen ignition behavior.
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.
A successful transition to clean energy depends on ensuring that the world's growing energy needs are met while reducing greenhouse gas emissions. Achieving this goal requires the use of carbon-free energy sources. Many utilities have already begun the shift from conventional carbon-based fuels to cleaner alternatives, such as hydrogen. However, numerous challenges persist regarding the use of hydrogen in internal combustion engines. The use of hydrogen in engines, which have extremely lower ignition energy, has raised concerns about the risk of pre-ignition due to the undesirable presence of lubricating oil during combustion. Due to its low auto-ignition temperature, lubricating oil is widely regarded as the main cause of abnormal combustion.
To verify and understand the potential influence of lubricating oil on hydrogen reactivity, the auto-ignition temperature was measured using a constant volume reactor. Auto-ignition was identified by a sudden increase in pressure combined with a rapid rise in temperature.
The experimental results verified that the addition of 0.2 ml lubricating oil to stoichiometric hydrogen-air mixtures at 20 bar reduced the auto-ignition temperature from 460°C to 270°C, reducing the ignition temperature by 190K. Moreover, the auto-ignition temperature of oil showed a decreasing trend as the initial amount of oil increased, indicating that the ignition process mainly occurs by vaporization.
The formation of hydrogen-oxygen mixtures for example due to cross-over, malfunction or start-up and shut-down processes is a hazard very specific to given electrolysis processes that must be properly addressed. In this work the explosion limits of hydrogen-oxygen-mixtures at conditions up to 30 bar and 300 °C were determined experimentally. It was found that the existing experimental data can be interpolated with good accuracy using empirical approaches. Moreover, explosion limits at atmospheric conditions were also determined with reduced ignition energy, down to 1 mJ. Although in the literature it can be found that the ignition energy of flammable gases increases strongly when the concentration changes from stoichiometric to near the explosion limits, no significant influence on the mixture concentration was found within tested ignition energy range for H2/O2 mixtures. Finally, hot surface ignition for mixtures with 6 mol% hydrogen in oxygen, thus slightly above the explosion limit, were experimentally studied at different pressures up to 30 bara. Similarly, only slight difference from the ignition temperatures determined for stoichiometric mixtures were found. A 0D adiabatic, constant-volume reactor model was used to calculate the ignition temperatures. The model was tested for its prediction of ignition temperatures of hydrogen mixtures at different pressures.
Hydrogen internal combustion engines (H₂ ICEs) present a promising alternative to conventional fuels, but they face challenges such as pre-ignition, where lubricating oils play a critical role. This study investigates the auto-ignition behavior of two base oils — Group II (mineral) and Group V (Ester) — and three formulated oils (Oils A, B, and C) at 20 bar using a heated constant-volume autoclave. Oil A and Oil B share a Group II (mineral) base, with Oil A containing lower levels of calcium-based detergents and higher levels of phosphorus-based antioxidants compared to Oil B. In contrast, Oil C is formulated with a Group V (ester) base oil, incorporating magnesium-calcium detergents. The auto-ignition temperature was measured in both air and stoichiometric hydrogen-air mixtures to assess the influence of oil composition, additives, and hydrogen addition on ignition characteristics. Results show that hydrogen’s AIT at 20 bar is 460°C but drops to 270°C with the addition of 0.2 ml of base oil. Base oils exhibited similar AITs in air (260°C) and hydrogen-air mixtures (270°C), with reactivity differences linked to molecular composition — ester (Group V) displayed lower reactivity compared to mineral oil (Group II). Formulated oils demonstrated slightly higher AITs (up to 290°C), where phosphorus-based additives reduced reactivity, while lower calcium content further slowed ignition. Among the tested oils, the ester-based oil with a mixture of calcium and magnesium detergents exhibited the lowest reactivity, making it a promising candidate for hydrogen engines. Additionally, reduced oxygen availability increased AIT by 10°C and prolonged ignition delay. A chemical analysis was also performed to evaluate the ignition properties of Group II (mineral) and Group V (ester) oils under varying temperatures. These findings highlight the impact of lubricant composition on pre-ignition behavior in H₂ ICEs, offering valuable insights for optimizing lubricant formulations.
Effect of lubricant oil composition on hydrogen auto-ignition in a heated constant-volume autoclave
(2026)
This study examined the impact of the base oil and additives in formulated lubricant oils on the ignition behavior of hydrogen. The minimum auto-ignition temperature (AIT) was measured using a heated, constant-volume autoclave at 20 bar. Four fully formulated lubricant oils (Oils A, B, C, and D) with different additive compositions and base oils were tested to evaluate their influence on hydrogen ignition. The focus was on additives containing calcium (Ca) and magnesium (Mg), as these elements are known to affect pre-ignition. The study also investigates the individual effects of Ca and Mg additives in base oil. Ca-based additives are known to be strong promoters of pre-ignition, while Mg-based additives tend to have no effect. However, the results indicate that the Ca-based additive has no significant effect, while the Mg-based additive has a minor inhibitory effect, on hydrogen ignition behavior in the heated constant-volume-autoclave. Base oil, on the other hand, has a greater influence on H₂ ignition. The results also show that different lubricant formulations have different auto-ignition characteristics. Lubricants with a full package are more resistant to auto-ignition. Among the lubricants tested, the ester-based formulation exhibited the highest AIT, while the lubricant oil had a lower tendency to auto-ignite under hydrogen-rich conditions.
