The rising share of electricity from renewable energy sources results in a need for longterm storage options to balance load and volatile supply in the future. A promising option for the long-term storage of surplus electricity is the production of hydrogen from water and its reconversion to generate electricity. An economic way to implement this reconversion in the near future is proposed in this paper. The designed engine process allows for efficient zero-emission combustion of hydrogen with pure oxygen, which is available as a byproduct of hydrolysis. To limit the high temperatures that arise during stoichiometric combustion of hydrogen and oxygen, the engine process has been combined with a steam cycle, resulting in temperatures not exceeding those in common diesel engines. The resulting two-stage process uses part of the exhaust gas energy for evaporation and overheating in a steam cycle. The exhaust gas energy is partly recovered and the required compression work is provided by a pump rather than a compression stroke, leading to higher efficiencies than in existing hydrogen internal combustion engines. The designed process consists of a two-stroke engine cycle and has been modeled as a standard cycle using real gas data and considering charge cycle losses, wall heat losses and mechanical losses. With an assumed maximum cylinder pressure of 150 bar, the process can achieve an effective efficiency of more than 50%. With an achievable power density almost twice as high as in conventional hydrogen engines this concept is quite promising. However, there are some challenges with regard to oil-free lubrication, high combustion temperatures and high-pressure injection of hydrogen and oxygen, which require further research. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.