Decarbonizing urban public transport: development and final assessment of a hydrogen-fueled hybrid propulsion system for city buses

The H2ICE project focuses on the investigation of the feasibility and the development of a new class of hybrid powertrains incorporating a hydrogen-fueled Internal Combustion Engine (ICE) designed for 12-meter urban buses, with the goal of achieving low emissions and competitive efficiency. A combined experimental and simulation-based approach was adopted. Achieving the full potential of this propulsion system requires significant advancements over the current state of the art, as several technical challenges remain. Chief among these is effectively managing the combustion process. First, a three-dimensional CFD model was calibrated and validated against an extensive optical-engine campaign, achieving predictive accuracy on mixture formation and flame development adequate for supporting design decisions in ultra-lean operation. This model supported the development of a complete one-dimensional engine model, coupled with a dedicated control strategy that improved combustion stability in the ultra-lean regime while enabling reduced NOx formation and high efficiency. Several Waste Heat Recovery (WHR) configurations were assessed through a synergy between experimental analysis and numerical simulations, including an electric turbocompound and an Organic Rankine Cycle (ORC). The combined two-stage WHR system achieved a maximum recovery efficiency of 14% (6.4% on average), delivering up to 16 kW of net electrical power under high exhaust energy conditions. In parallel, an innovative H2-SCR concept was experimentally validated, reaching up to 50% NOx conversion efficiency at exhaust temperatures around 250 ◦C and oxygen concentrations of approximately 12.5%. A causal Rule-Based (RB) Energy Management System was designed as an implementable solution for realtime ECU application. In the final virtual-vehicle assessment over SORT driving cycles, hydrogen consumption reached values as low as 9 kg/100 km (i.e., meeting the project target of ~ 10 kg/100 km under standardized conditions), while real-world variability may lead to higher values, as discussed in the manuscript. Tailpipe NOx emissions ranged from approximately 0.14 g/kWh under standardized type-approval cycles to as low as 0.009 g/ kWh under real-world operating conditions, remaining below the forthcoming Euro 7 limits for heavy-duty applications. These results demonstrate that H2ICE-based hybrid powertrains represent a technically feasible and realistic solution for urban buses, capable of delivering low pollutant emissions and competitive efficiency, and offering a viable decarbonization pathway in contexts where battery-electric solutions face infrastructure or range limitations.