Experimental studies on the performance of low-carbon, high-efficiency heavy-duty dual-fuel combustion engines

Abstract

This thesis presents an experimental investigation into dual-fuel combustion strategies aimed at decarbonising heavy-duty engines through the use of low- and zero-carbon gaseous fuels. A single-cylinder research engine and its fuelling system were upgraded for dual-fuel operations with hythane and hydrogen. Systematic experiments were performed at a constant engine speed of 1200 rpm and loads of 0.6, 1.2, and 1.8 MPa IMEP, corresponding to 25%, 50%, and 75% of full engine load. The study explored both conventional and advanced combustion strategies by varying effective compression ratio and diesel injection timing to maximise thermal efficiency and minimise engine-out emissions. The diesel-hythane dual-fuel system demonstrated strong potential for short-term decarbonisation. An advanced combustion strategy using early diesel injection combined with Miller cycle delivered significant improvements in thermal efficiency by up to 4% at low load and reduced CO₂ emissions by up to 40% relative to conventional diesel combustion. Total GHG emissions were lowered by approximately 25%, and NOx and soot emissions were reduced by as much as 89% and 69%, respectively, compared to diesel-only operation. The diesel-hydrogen system, while facing limitations in diesel substitution due to combustion phasing constraints, achieved the highest CO₂ and GHG reductions – by up to 56% – when operated with a lower effective compression ratio. Although NOx levels increased under the baseline configuration, mitigation strategies such as external EGR, water injection, and leaner mixtures were shown to effectively reduce NOx without compromising efficiency. Notably, green hydrogen use allowed the diesel-hydrogen powertrain to exceed the EU’s 2030 CO₂ reduction target. A comparative assessment across diesel-CNG, diesel-hythane, and diesel-hydrogen systems confirmed that while methane-based fuels offer substantial NOx reduction, their GHG benefits are limited by methane slip. Hythane emerged as the best short-term solution due to its balance of efficiency and emissions performance, while green hydrogen showed the greatest promise for long-term decarbonisation, provided that NOx control strategies and injection optimisation are fully implemented. Overall, this research confirms that dual-fuel combustion with hythane and hydrogen – when paired with advanced engine strategies – can significantly lower the carbon and pollutant emissions of heavy-duty diesel engines. The findings provide a solid foundation for the further development of clean, efficient dual-fuel systems aligned with upcoming emissions regulations and climate targets.