Assessment of geometric variations on efficiency and emissions of a Hydrogen–Diesel dual-fuel engine using CFD approach

The global shift towards sustainable energy solutions has increased the demand for cleaner fuels in internal combustion engines, with hydrogen offering significant potential due to its zero-carbon nature and superior combustion properties. This study conducts a comprehensive numerical analysis of how engine geometry—specifically stroke length, bore size, and the associated compression ratio—affects the combustion, performance, and emission characteristics of a hydrogen–diesel dual-fuel (HDDF) compression ignition engine. Validated three-dimensional CFD simulations were performed for hydrogen energy shares (HESs) of 24 %, 57 %, 73 %, and 90 %, under high and low load conditions. Results show that increasing stroke length improves in-cylinder pressure, heat release rate (HRR), and brake thermal efficiency (BTE), with BTE rising by up to 9 % at high load and 73 % HES. However, under low load, longer strokes led to a BTE drop of up to 29 % at 90 % HES. Bore enlargement consistently reduced BTE—by up to 17 % at high load and 27 % at low load—due to reduced compression ratios and weakened combustion intensity. Emission analysis revealed that NOx emissions increased by 38 % at high load and 73 % HES with stroke extension, and by as much as 64 % under low load at 90 % HES. Conversely, increasing bore size reduced NOx by up to 16 % at high load and 13 % at low load, but resulted in sharp rises in CO, UHC, and soot—exceeding 100 % in some cases. These findings underscore the importance of optimising stroke–bore geometry based on hydrogen substitution level and engine load to balance efficiency gains with emission control in HDDF engine applications.