The deployment of green hydrogen requires mobility. But this energy vector can be used in several forms: fuel cell or internal combustion engine for hydrogen (100% or mixed with diesel or methane). What engines are emerging for what uses? Elements of response.
The hydrogen internal combustion engine (MCI) is above all a so-called “thermal” engine using hydrogen as fuel. MCI engines are also available with a hydrogen/CNG (compressed natural gas) or hydrogen/diesel mixture.
100% hydrogen or mix?
The efficiency of the MCI will depend on the type of application and the installed power (between 25 and 40%), but it should be noted that hydrogen allows a high flame speed in the combustion chamber and therefore better efficiency. Easy to start with a mixture low in hydrogen, it must however be well regulated and secure to avoid the phenomenon of pre-ignition and flashback. Hydrogen MCI emits nitrogen oxides due to high combustion temperatures, but they are now neutralized by urea (Euro VI standards). Used pure, hydrogen does not emit CO2. This engine operating with liquid hydrogen at very cold temperatures or stored in tanks under very high pressure, however, requires special attention: vigilance on leaks, specific materials for high pressure or intense cold.
Different fuels can be used on this type of engine. The diesel/hydrogen mix requires very few engine modifications, but still emits CO2 despite everything. Note that 100% hydrogen allows the use of less pure hydrogen than for a fuel cell, which can help reduce the cost of fuel per use. Another possibility is the use of Hytane, a hydrogen/methane mixture tested in Dunkirk by the Althytude project in anticipation of a future similar mixture in the national gas network. Also note that this type of engine can use syngas (pyrolysis and partial combustion gases CO and H2).
In terms of engine, the choice also exists between a new engine and the possibility of retrofitting from a thermal truck. This approach makes it possible to reduce purchasing costs while new solutions become industrialized.
In terms of uses, MCI engines seem not very suitable for individual cars due to the complexity of hydrogen management and the lack of current distribution. BMW worked on hydrogen MCIs until 2013 before abandoning, mainly due to cost problems and technical obstacles (lack of infrastructure for recharging, ecological problems posed by hydrogen extraction, quantity of energy required to store liquid hydrogen at − 253 °C, and size of the liquid hydrogen tank) to focus on hybrid and electric, then on fuel cells by partnering with Toyota. The Mazda RX‑8 (2003) and Ford Focus C‑MAX H2 ICE (2004) were also equipped with MCI.
With direct injection, hydrogen under pressure in the combustion chamber, new ignition technologies, the treatment of nitrogen oxides, the development of hydrogen infrastructure, these engines are however regaining a place in the landscape, but it is mainly on high powers: utility vehicles, trucks, but also boats and trains and, statically, generators seem better suited to these engines.
MCI VS PAC
Indeed, the larger a fuel cell, the more complex the system becomes and the higher the losses. For MCI, it is the opposite. We will therefore favor MCI on high power applications where the battery is not yet present and will stand out less in terms of its performance and its cost.
PEM technology heat pumps are currently suitable for small and medium powers, up to 600 kW per unit. Unit powers of the order of megawatts are in development, but still absent from the market. Fuel cells generate lower temperatures than MCIs (which still require cooling). Of course, the advantage of heat pumps compared to MCI remains that they only emit water and no GHGs. Likewise, the efficiency of PACs is higher (around 50%) than that of MCIs, which requires less hydrogen to be carried on board the vehicle. But, on the market, low and medium power electric motors equipped with heat pumps can be more easily competed by battery-powered vehicles which, due to the volumes, will undoubtedly display more competitive prices.
Beyond emissions, PACs require rare metals like platinum while MCIs use very little (only for the catalytic converter which contains a small portion). Another advantage of MCI: they benefit from an existing industry allowing a large volume and a rapid reduction in costs. It is also an industrial reconversion issue for the historic thermal engine sector. We thus observe that industrial players are implementing a dual strategy, such as Toyota which is pushing the fuel cell while adapting its thermal models to hydrogen.
There are therefore real market prospects for MCI, complementary to the PAC on very high powers, but also competitive on medium powers (buses, trucks). These engines can benefit from the implementation of hydrogen infrastructure (a hydrogen station can power a PAC bus like an MCI bus).
There remains, for MCIs as for PACs, the problem of the political choice of all-electric battery which could slow down the implementation of hydrogen mobility.
Article produced in collaboration with Karel Hubert, president of the hydrogen research firm Enerka (karel.hubert@enerka-conseil.com).