The role of hydrogen for future internal combustion engines

In order to successfully cope with the trend of mitigating climate change as outlined in the recommendations of Paris (COP21) and Glasgow (COP26) Climate Agreements, propulsion technologies must be able to achieve the highest CO2 reduction, within very short time scales. To achieve this challenging goal, electric powertrains powered by batteries charged using renewable energy represents not only a public mandate but also the focus of research efforts of the relevant academic and industrial communities. However, this technology cannot answer all the various needs concerning personal mobility, sustainability and feasibility. Hence, in parallel an important role will be played by internal combustion engines (ICE) fed with non-fossil hydrocarbons and hydrogen (H2).1 Today, internal combustion engines using fossil fuels generate about 25% of the world's power and they are responsible for about 17% of the world's greenhouse gas (GHG) emissions, while producing other main pollutant emissions such as carbon monoxide (CO), carbon dioxide (CO2), nitrogen oxides (NOx) and particulate matter (PM) with strong negative impact on air quality in urban spaces.In the current energy landscape, hydrogen is perceived as a flexible energy carrier with potential applications across all energy sectors. Hydrogen represents a promising energy carrier to store renewable electric energy when available in excess during peak production, due to the typical intermittent character of renewable wind and photovoltaic energy plants. Hydrogen may be used to feed fuel cells (FCs). The current state, hydrogen FC technology is expensive and requires pure hydrogen and a high specification compressor to supply the compressed air. In addition, large batteries would be needed to store the electricity required to cope with the transient nature of power demands for vehicle applications. As a result, the overall current FC-based powertrain efficiency is much lower than that of the FC alone.10 A recent study showed how the overall efficiency of a FC powertrain system is very similar to that of ICE system for commercial vehicles.