Using hydrogen as a combustion fuel has been around for some time. Liquid hydrogen was used in rockets during the space race. Airbus is developing jet engines that will run on hydrogen. Many will remember when Mercedes-Benz developed hydrogen combustion buses for the 2008 World Cup in Germany.
In the UK, ULEMCo has been providing a diesel engine dual-fuel solution for trucks and vans for several years. Through the Low Emission Freight Trial, ULEMCo vehicles covered over 60,000km in dual-fuel mode, displacing between 10-45% of diesel fuel with hydrogen and with all showing an ability to be as fuel efficient as the standard diesel alternative.
The benefits of hydrogen combustion
A key advantage of hydrogen combustion for road transport is that much of the knowledge, resources and supply chains used for petrol and diesel internal combustion engines can transfer. Fewer rare-earth metals mean a much lower upfront cost compared to battery-electric vehicles and hydrogen fuel cells. The short fuelling times and long range of hydrogen vehicles also add to their attractiveness.
A dual-fuel option is ideal for operators that seek to reduce emissions while providing the flexibility to use a 100% diesel option. However, the continued use of fossil diesel will not help us to achieve net-zero greenhouse gas (GHG) emissions.
As such, other companies are focusing on dedicated ‘mono-fuel’ hydrogen combustion engines. Cummins was recently awarded £14.6m through the APC18 funding round to develop a zero-carbon dioxide hydrogen combustion engine for heavy-duty vehicles. CMB Tech is also working on a mono-hydrogen combustion engine for trucks and gensets.
Thermal efficiency comparison is worth considering
From a GHG perspective, the combustion of hydrogen produces only water vapour. That is not a GHG in the same way as carbon dioxide and methane are.
However, the challenge with combustion at high temperatures is the production of other emissions, such as NOx. It has been the main contributor to poor air quality in the last few years. While there is an option of using SCR systems to scrub out NOx, that process converts it to nitrous oxide, which is a greenhouse gas. That does not support the net-zero ambition.
Another challenge for hydrogen combustion is thermal efficiency. Historically, dedicated hydrogen-only combustion engines have achieved around 25-50% thermal efficiency. That compares to a conversion efficiency of 50-60% for hydrogen fuel cells, which do not produce any of the air quality pollutants that combustion engines do.
That appears to be in line with the thought process of many major bus OEMs in the UK, the EU and around the world, where the focus has very much been on developing hydrogen fuel cell-powered vehicles.
Never rule it out, but…
But it is not all about the tailpipe. How the hydrogen is produced is also a crucial factor in the overall net carbon savings of its use as a transport fuel. That is highlighted in Zemo Partnership’s recently published Hydrogen Vehicle Well-to-Wheel GHG and Energy Study. It demonstrates that using renewable energy throughout the hydrogen production process is crucial. Otherwise, we could be increasing overall emissions in some cases.
I would never rule out the possibility of technically producing a hydrogen combustion engine that can achieve zero tailpipe GHG emissions. However, the focus on fuel cells and the association of combustion engines with fossil fuels means that we are probably unlikely to see them in coaches or buses in the UK.
