Investigating the effects of stroke-to-bore ratio on the performance of hydrogen-fuelled diesel engines

• Longer strokes raised brake efficiency by up to 20.4% at higher hydrogen shares. • Soot and CO emissions dropped by up to around 80% at longer strokes. • NOx emissions increased with stroke length, reaching a 38.4% rise at 145 mm. • Unburned hydrogen fell by over 99.9% at longer strokes and high HES levels. • Bore–stroke ratio changes strongly influenced hydrogen use and dual-fuel combustion. Hydrogen–diesel dual-fuel (HDDF) combustion offers a promising pathway for reducing carbon emissions from compression ignition (CI) engines while maintaining high efficiency. However, the influence of engine geometry on HDDF combustion behaviour remains insufficiently understood. This study numerically investigates the effect of stroke length variation at a fixed compression ratio on the performance and emissions of a CI engine operating at hydrogen energy shares (HES) of 24%, 57%, and 73%. A validated three-dimensional (3D) computational fluid dynamics (CFD) model incorporating detailed hydrogen–diesel reaction mechanisms was employed. Six geometric configurations were analysed with stroke lengths of 125, 130, 135, 140, 145, and 150 mm, corresponding to stroke-to-bore ratios of 1.087, 1.153, 1.220, 1.288, 1.358, and 1.429, respectively. Results show that stroke length strongly influences combustion characteristics and emission behaviour. Compared with the baseline 125 mm stroke, brake thermal efficiency increased by up to 20.4% at 57% HES for the 145 mm configuration, attributed to enhanced expansion work and improved air–fuel mixing. Longer strokes also reduced soot and carbon monoxide (CO) emissions by up to 78.6% and 84.3%, respectively. At 73% HES, the 130–135 mm configurations exhibited more uniform carbon dioxide (CO 2 ) formation, while the 145 mm case showed lower CO 2 due to reduced diesel participation. Nitrogen oxides (NO x ) increased moderately at low HES (up to 6.07%) but more significantly at high HES (up to 38.39%). These results highlight the critical role of stroke-to-bore ratio in shaping HDDF engine performance and emissions.