Investigation of Combustion Dilution Strategies on Performance and Emissions of a Single-Cylinder Spark-Ignition Direct-Injection Hydrogen Engine

Hydrogen internal combustion engines research is particularly relevant for propulsion systems requiring long range and fast refuelling while offering advantages in fuel purity requirements and operational efficiency. Leveraging the existing engine industry offers the potential for a fast response in the reduction of greenhouse gas emissions. However, abnormal combustion and high NOx emissions limit the feasibility of stoichiometric operation in hydrogen engines, making the evaluation and optimisation of dilution strategies essential. While lean combustion has been widely studied, the impact of exhaust gas recirculation (EGR), particularly in combination with lean mixtures, has yet to be comprehensively assessed. This study investigates the effectiveness of different dilution strategies, with a specific focus on the combined effects of air dilution and EGR on hydrogen combustion. Their joint influence on thermal efficiency, combustion stability, and emissions is evaluated across a range of engine loads. EGR is shown to effectively reduce in-cylinder temperatures and NOx emissions due to the high heat capacity of its water content, while also lowering boosting requirements. However, increased water concentrations can alter combustion chemistry, and the high water production inherent to hydrogen combustion introduces additional challenges related to condensation management within the EGR system. Experimental results are presented from a boosted 0.5-litre spark-ignition single-cylinder research engine equipped with high-tumble ports and direct hydrogen injection. Emission analysers are used to track gaseous emissions with major focus on NOx and unburned hydrogen concentration. The results analyse combustion, emissions, and efficiency trends across different levels of dilution and help identify load-dependent optimal dilution strategies that balance efficiency, emissions reduction, and boosting requirements in hydrogen-fuelled engines.