Burning hydrogen in internal combustion engines: a smart and affordable option to reduce CO2 emissions
The transport of people and goods represents nearly 20% of global CO2 emissions. All transport-related sectors, such as automotive and aeronautics, are therefore under increasing pressure to innovate and adapt, and find ways to reduce their carbon footprint.
These sectors must take steps towards decarbonisation, not only to meet society’s expectations of a more sustainable future, but also to meet the environmental impact reduction targets set by the European Union.
Overwhelming electrification…for now
The most common market reaction to this challenge is to switch to electrification, replacing internal combustion engines (ICEs) with electric motors, with batteries as the sole power source. Considerably more efficient than their internal combustion counterparts, electric motors generate zero “tailpipe emissions” and can be 100% clean if green energy is used.
The automotive industry in particular has seen a significant shift towards electric vehicles (EVs), backed by considerable financial support from governments. For now, this shift is largely focused on personal vehicles and some buses.
The adoption of electrification in large utility vehicles such as trucks, construction machinery and agricultural vehicles is currently hampered by some battery-specific challenges, such as low gravimetric energy density (meaning that generating enough energy would add too much weight to the battery), and the length of time it takes to recharge. An alternative clean approach has been to explore hybrid electric architecture, using hydrogen fuel cells to generate electric power in what is known as a fuel cell electric vehicle (FCEV).
VE and FCEV in the aerospace industry
There’s no reason why batteries couldn’t be used for light aircraft flying short to medium ranges, and electric vertical take-off and landing (EVTOL) vehicles – in fact, a number of vehicles early-stage electricity already exist in these areas. However, the low gravimetric energy density (MJ/kg) of current battery technology compared to other power sources means that electrification is not yet a suitable solution for use in large aircraft. commercial. It is also likely that some smaller aircraft could be FCEVs. Again, however, the low volumetric energy density of hydrogen, even in the liquid stage, means the technology could not be used for anything larger.
For example, while testing the possibilities and limits of a hybrid-electric aircraft propulsion system, Airbus and Rolls Royce took the joint decision to end their flagship E-Fan X demonstrator project after three years in 2020.
Overview of fuel types
But electrification isn’t the only game in town. Internal combustion engines may still remain relevant, if we use them to burn hydrogen rather than fossil fuels.
High volumetric energy density, due to long carbon chains and reasonable gravimetric energy density, means the fossil fuel has been best in class since the late 19th century. But burning carbon chains generates CO2, making it the worst in its class for the 21st century.
Graph 1: Challenges of volumetric versus gravimetric fuels (sources: Mdpi, Research gate)
Today, advances in technology mean it is possible to convert ICEs, allowing them to burn hydrogen instead of fossil fuels. It’s not without its challenges, however.
The first of these is to address the problem of low volumetric energy density when storing hydrogen in a vehicle. To illustrate this, the formula PV=KT describes the perfect gas mixture and justifies why pressure, volume, and temperature are so closely related.
One answer is hydrogen gas. Stored at 700 bar and at ambient temperature, it is the current solution for road and rail vehicles.
Liquid hydrogen, meanwhile, stored at 10 bar and -253ohC, is the only viable solution for aviation, whether used in fuel cells to generate electricity, or to power turboprops and turbofans.
Burning hydrogen in ICEs
Given the maturity of thermal engine technology, burning hydrogen in an ICE is an interesting option, both from a technical and economic point of view.
Nevertheless, some technical challenges need to be overcome. For instance:
Develop and adapt current technologies to hydrogen, while maintaining a reasonable cost for the engine
Improving pollution control systems, despite improved environmental performance – i.e. no CO2 and limited NOx emissions
To further explore this concept, we should consider the differences between piston ICEs for motor vehicles and turbine ICEs for aircraft.
Car piston engines
ICEs can, in principle, run on hydrogen to produce mechanical energy, releasing only carbonaceous water vapor and NOx. Switching from an ICE to hydrogen does not change the principle – only a few modifications are necessary, even if the control of NOx emissions requires precise management of the combustion process.
Figure 2: Energy efficiency of e-Fuel WTW (source: Concawe Report on Role of e-fuels in the European transport system)
So how is the auto industry adapting?
Toyota is looking to burn hydrogen in ICEs, while Honda will focus on EVs and FCEVs
Renault will unveil a concept car equipped with a “hydrogen engine” 
ORECA Magny-Cours will evaluate hydrogen technology, while developing its own hydrogen ICE 
Aircraft turbine engines
Initiatives are underway – in the air and on the ground – to test hydrogen-powered ICEs and the direct use of hydrogen as fuel in a gas turbine.
What is the aviation industry doing in this regard?
Airbus and CFM International have launched a joint project to test on the ground and in the air a hydrogen-powered direct combustion engine with a view to the entry into service of a zero-emission aircraft by 2035 
Rolls Royce believes that although hydrogen can be used directly as fuel in a gas turbine, it is likely to start in the shorter transport segments. Sustainable Aviation Fuel (SAF) gas turbines will remain the most likely solution for long-haul flights in the future. 
For many players in the transport sector, reducing carbon emissions will first mean adopting EVs or FCEVs. But that doesn’t mean burning fuels will immediately be relegated to the engineering history books, we just need different fuels.
The combustion of hydrogen in a piston or turbine thermal engine does not generate CO2, just water and a little NOx. The maturity of ICE technology means that systems can be adapted to hydrogen and other e-fuels – synthetic fuels derived from the use of an electrochemical process – at a cost far below the development of FCEVs. In addition, the adaptation of ICEs to hydrogen will protect and save a large number of jobs threatened with disappearance following the advent of EV technology.
In aviation, the appropriate solution for decarbonization will depend on the uses. For example, electric batteries and fuel cells are a good solution for short and medium-haul aircraft with a limited number of seats. For short-haul commuter flights, replacing the original turboprops with a fuel cell and electric powertrain is a potential solution, with the option of using hydrogen as fuel. For long-haul aircraft, however, the only viable option is to burn fuel, which could be SAF or hydrogen.
In summary, although EVs and FCEVs are considered by many to be the most sustainable answer to the environmental challenges of the transport industry, the use of hydrogen and e-fuel powered ICEs represents a cost-effective solution, pragmatic and sustainable.
 Inside electric vehicles, Toyota’s hydrogen-burning plans ‘not feasible’, says Honda CEO
 Motor1.com, Renault Concept teased with a hydrogen-powered combustion engine
 ORECA, ORECA Magny-Cours develops its first hydrogen engine test bench
 Airbus, Airbus and CFM International to pioneer hydrogen combustion technology
 Rolls Royce, hydrogen energy, for a cleaner future