Between 2024 and 2025, fleets across Europe, North America, and Australia have initiated pilot programs to convert diesel heavy-duty trucks to hydrogen setups. This includes Cummins' deployment of its Euro 7–compliant H2-ICE within its HELM modular platform, and the EDAG Group's collaboration with Cummins to develop high-pressure storage and plug-and-play retrofit kits. In the UK, ULEMCo tested its H2ICED dual-fuel system, while Canada’s Hydra Energy is already commercializing Class 8 co-combustion trucks. Researchers at UNSW Sydney have demonstrated up to a 90% hydrogen mix in diesel engines, achieving over 85% CO₂ reduction.Converting a diesel internal combustion engine to run on hydrogen involves overhauling fuel delivery and ignition systems. This typically includes replacing injectors with hydrogen-capable units and switching to spark ignition. Engine management software is re-calibrated for optimal hydrogen flow and air-fuel ratios, often using a lean-burn mix to minimize NOₓ. The ultimate benefit is near-zero CO₂ emissions on pure hydrogen.Dual-fuel systems offer a pragmatic transition, allowing operators to blend hydrogen with diesel, reducing diesel consumption by 30-90%. This maintains operational reliability while significantly cutting the carbon footprint. Fleet managers can adjust the hydrogen proportion based on availability and operational needs, gradually increasing hydrogen adoption without compromising range or power.These retrofits rely on advanced hydrogen storage solutions, primarily Type IV composite tanks designed for 350–700 bar pressure. These lightweight tanks provide sufficient range for regional hauls. However, their viability is intrinsically linked to the parallel development of hydrogen refueling infrastructure, including stations, safety standards, and robust supply chains.The primary drivers for retrofitting existing fleets, rather than purchasing new hydrogen or electric vehicles, are speed and cost-effectiveness. Retrofitting allows companies to rapidly reduce emissions and bypass substantial upfront investments and long lead times for new zero-emission rigs. These conversions often qualify for subsidies in regions with tightening CO₂ and NOₓ regulations. OEMs like Cummins are developing modular H2-ICE platforms that offer fuel flexibility, adapting to diverse market conditions.Leading innovators include Cummins (HELM architecture), EDAG Group (integration and certification), ULEMCo (dual-fuel kits), and Hydra Energy (commercial co-combustion trucks). While full fuel cell electric vehicle (FCEV) retrofit kits exist, hybrid and dual-fuel setups are often preferred where hydrogen supply is inconsistent or costs are high.Challenges persist, including upfront costs (tens to hundreds of thousands per truck) without strong financial incentives. Integrating large hydrogen tanks can reduce cargo space, and regulatory frameworks vary significantly. The most critical hurdle remains limited hydrogen refueling infrastructure outside established corridors, necessitating meticulous planning for fleets.Nevertheless, collaboration between policymakers, tech firms, and fleet operators is growing. Initiatives like Clean Logistics in Europe are standardizing retrofit protocols to reduce costs and streamline approvals. Early deployments in North America suggest potential for lower total cost of ownership. As global efforts towards sustainable energy and industrial decarbonization intensify, hydrogen retrofits are poised to serve as a crucial bridge to a fully hydrogen-powered transport sector, provided infrastructure scales commensurately.