Optimizing NOx reduction in small hydrogen-powered IC engines using water injection and EGR

With the growing push for low-emission and sustainable transport technologies, hydrogen-fuelled internal combustion engines (HICEs) have become an attractive transitional solution. These engines offer a zero-carbon alternative to conventional fuels, emitting only water vapor during combustion. However, hydrogen’s high flame temperature and fast burn rate can lead to elevated nitrogen oxide (NOx) emissions, which remain a significant environmental concern.

This research explores a practical, engine-based solution to this challenge by combining two in-cylinder NOx reduction strategies: water injection and exhaust gas recirculation (EGR). A small single-cylinder spark ignition engine was modified to operate on hydrogen fuel, and then equipped with systems for controlled water injection and EGR. Through a series of experiments at steady-state conditions, the individual and combined effects of these techniques on NOx emissions, combustion temperature, and engine performance were evaluated.

Results showed that water injection effectively lowered in-cylinder peak temperatures by absorbing combustion heat, leading to a noticeable reduction in NOx. EGR contributed by reducing the oxygen content and thermal intensity of the combustion process. When both methods were applied together, NOx emissions were reduced by more than 90% compared to baseline hydrogen operation. The engine continued to run stably, and only a modest drop in thermal efficiency (around 3.5%) was observed.

This dual-strategy approach presents a cost-effective and scalable pathway for reducing NOx emissions in hydrogen engines, particularly for compact mobility applications. It aligns well with global clean-air goals and can accelerate the adoption of hydrogen-powered technologies in sectors where battery-electric solutions may not yet be feasible.