The role of hydrogen for future internal combustion engines
Résumé
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,2 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, hydrogen3–9 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.11,12
Furthermore, a PEMFC-based powertrain system rejects nearly all of its heat loss via the coolant, which also needs to be kept at a significantly lower temperature than the coolant of an ICE, and hence mandates significantly larger radiators and cooling systems. This is particularly critical for heavy-duty vehicles operating at low speeds, which is relevant considering that this is the most promising short-term application for FC powertrain systems.
A detailed analysis of benefits and costs could highlight the impact of different hydrogen mobility technologies with respect to alternative solutions in terms of societal and private costs and gains, taking into account a variety of socioeconomic and infrastructural contexts.13 In the transport sector, which has very few near-zero emission energy carriers (i.e. electricity and advanced biofuels), hydrogen has the potential to address some of the key emission reduction challenges when combined with ICE technologies. In particular, the H2 fuelled ICE (H2ICE) is the only alternative keeping the ICE powerplant that does not produce any tank-to-wheel CO2 emissions at the tailpipe (as well as IC engines fed with ammonia,14 however more suitable for ship applications). Contrarily to FC powertrain systems, H2 ICEs can be fuelled with non-purified hydrogen, resulting in significantly lower production cost of hydrogen fuel. H2ICEs can take advantage of the existing advanced combustion and engine control technologies, such as direct injection, Miller cycle, lean/diluted combustion, pre-chamber ignition, etc. Thus, the thermodynamic efficiency of direct injection H2ICEs can be similar to the overall efficiency to the FC powertrain.
In terms of pollutant emissions, a certain amount of NOx is generated during combustion, with traces of particulates due to the combustion of very small portions of lubricating oil, but all these can be reduced to zero-impact by means of a lean mixture and a suitable after-treatment system, together with the choice of a specific lubricating oil.15 In particular, advanced hydrogen SCR catalysts16 and particulate filters can remove these pollutant emissions. The availability of hydrogen on-board as a reducing agent can represent an innovative and convenient approach for NOx reduction, eliminating the need of urea/NH3 storage in tanks and the possible ammonia slip at the tailpipe.
The H2ICE is attractive because it takes advantage of the current advanced state of ICE technologies, such as reliability, durability, existing supply chain, existing manufacturing plus recycling infrastructure and affordability, which makes it a near-term, widespread solution to accelerate the large-scale introduction of H2 into transportation market, for both transitional and long-term usage. In fact, the existing worldwide know-how on ICEs and the widespread large-scale manufacturing and supply-chains can continue to be utilised, without any critical interruption. The H2ICE could provide a reliable, durable and cost-efficient solution based on a well-known existing technology, contributing to a fast transition towards carbon-free mobility. Moreover, it is characterised by low total cost of ownership total cost of ownership (TCO), especially in the field of heavy-duty on-road and off-road applications. It can be argued that H2ICE technology can be less expensive than the current state of technology for EV powertrains, due to its minor dependence on low available and expensive materials as rare earth metals. Last but not least, considering that H2ICEs are manufactured in the same production facilities and following the same manufacturing processes as the conventional fossil-fuel ICEs, they contribute to secure jobs by providing sustainable industrial and employment opportunities in the automotive industry.
Hydrogen can be produced from diverse resources. It is abundant in our environment, stored in water (H2O), hydrocarbons (such as methane, CH4), and other organic matter. One challenge of using hydrogen as a fuel is the efficiency of extracting it from these compounds. More specifically, hydrogen can be produced by several methods: the most widely used is steam methane reforming, for which the relevant efficiency of hydrogen production is very high (65%–75%) and the production cost is relatively low. Nevertheless, it results in high levels of CO2 emission. Another widely used method of hydrogen production is coal gasification. In this case, however, the efficiency of hydrogen production is low (45%) while the CO2 emissions are still high, if the CO2 is not sequestered at the production site.
Electrolysis of water is another method of producing hydrogen, but it requires the use of large amounts of electricity and therefore this becomes expensive. In this case, the level of CO2 emissions depends on the electricity source. Less common methods can also be used to produce hydrogen, such as biomass gasification, biomass-derived liquid reforming, or microbial biomass conversion.17 However, only the solar–hydrogen and wind-hydrogen system allows emission-free—but expensive—hydrogen production. The universal applicability of hydrogen for modern energy needs has boosted significant investment and development of renewable hydrogen production in many countries.
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