Diesel vs. hydrogen-diesel dual-fuel engines for a sustainable future: A review of combustion, fuel properties, emissions, and performance

The transportation sector remains a major contributor to global greenhouse gas emissions, primarily due to its heavy reliance on internal combustion engines. Diesel engines are renowned for their durability, high energy density, and fuel efficiency. They also pose significant environmental challenges, including emissions of nitrogen oxides (NOx), particulate matter (PM), and carbon dioxide (CO2). In pursuit of cleaner and more sustainable alternatives, hydrogen-diesel dual-fuel (HDDF) technology has emerged as a promising transitional solution. This review provides a diesel-centric comparison of conventional diesel and HDDF engines, focusing on fuel properties, combustion behaviour, engine performance, and emission characteristics under heavy-duty operating conditions. Quantitative analysis of published experimental studies shows that hydrogen substitution typically improves brake thermal efficiency by 5 to 13% at medium to high loads for hydrogen energy shares (HES) of 10 to 30%. Corresponding reductions in brake specific fuel/energy consumption of 4 to 30% are widely reported. Near-zero CO2 emissions are achievable at very high HES under low-load conditions. However, NOx emissions generally increase by 10 to 70% at elevated HES levels due to higher in-cylinder temperatures, necessitating effective mitigation strategies. Exhaust gas recirculation (EGR) is shown to reduce NOx by up to 96% with minimal efficiency penalty. The review further identifies abnormal combustion phenomena particularly backfire and knock as key constraints. By synthesizing combustion regimes, performance trends, and emission trade-offs, this review highlights HDDF engines as a technically viable bridge toward low-carbon heavy-duty transport. It outlines critical research needs in injection strategies, sensing, modelling, and real-time combustion control.