Light duty engines with low pressure PFI burn hydrogen in a lean premixed combustion principle. The engine load can be varied from low to high load by adjusting the equivalence ratio, due to the wide flammability region. Emissions of NOx will therefore depend on the load and operating strategy, but after-treatment will likely be required to meet Euro 6 and upcoming Euro 7. As the engines will likely operate with lean mixtures, SCR and EGR are the most relevant aftertreatment technologies. LNT may be an option as well. Particulate filters should not be required, as hydrogen combustion does not form particulate matter.

Medium and heavy-duty engines, operating with lean burn principles will likely also require combinations of SCR and EGR to comply with the relevant emission standards. If the engines are constructed for the dual fuel principle, they must furthermore be equipped with DPF to remove particulate matter formed by the diesel combustion.

Engines intended for marine use must be either Tier II or Tier III compliant. The emission limits for NOx in ECA zones (Tier III) may necessitate the use of SCR. Particulate filters are relevant for DF hydrogen marine engines when used for inland waterways and in areas where PM emissions are subject to local regulations.