Characterisation of hydrogen engine combustion and mitigation of knock in dual-fuel operations

Abstract

Over the past decades, significant efforts have been focused on reducing fossil fuel dependency by promoting sustainable energy sources. Recently, major corporations have shifted their attention to H2 as a fuel for internal combustion engines, as the development of H2 fuel cells has not progressed as rapidly as expected. H2, with its higher calorific value and carbon-free molecular composition, offers a promising clean fuel alternative. However, the limited H2 infrastructure necessitates the continued development of dual-fuel combustion. This dissertation focuses on H2 combustion characterisation and improving the performance of existing dual-fuel engines by leveraging the thermodynamic properties of fuels. H2 combustion characterisation was performed utilising in-cylinder pressure measurements obtained through experimental testing. These in-cylinder pressure measurements were processed using LabVIEW software to analyse key combustion parameters such as the rate of heat release and combustion duration. These parameters were subsequently compared to those obtained from conventional fuels. Accurate determination of the air-fuel ratio during lean operation is critical. This was achieved through simultaneous and separate measurements of fuel and airflow rates. To address the pulsating airflow, a critical flow orifice was designed and incorporated into the setup based on choked flow theory, which depends solely on upstream conditions. The major highlight of this combustion characterisation investigation is the high brake thermal efficiency of 23% achieved by H2 under λ3 mixture, compared to the 21% obtained with stoichiometric petrol testing at wide open throttle. A cryogenic setup was developed specifically for liquid natural gas injection to enhance combustion and mitigate engine knock in dual-fuel engines. However, due to safety constraints, experimentation with liquid natural gas was substituted with injections of liquid nitrogen and liquid propane. This approach aims to reduce intake air temperatures, thereby mitigating engine knock. Temperature measurements during liquid nitrogen injection revealed a reduction of approximately 45 °C at a 60% substitution ratio with vapour propane. Liquid propane injection resulted in temperature reductions of 4 °C and 7 °C at 60% and 70% substitution ratios, respectively. Across all substitution ratios and intake conditions tested, the use of liquid dual-fuel injection consistently decreased the maximum amplitude of pressure oscillations, indicating improved knock resistance.