Hydrogen is recognized as a flexible carrier of renewable energy: it can be produced from any energy source, and also converted into various forms. The main challenges with hydrogen are related to its production and storage. Global annual hydrogen demand is below 2% of the world energy production. Furthermore, most of the hydrogen today is produced from fossil sources, 50% from natural gas, 30% as a by-product from the petroleum refining, 18% from coal, and only 4% by water electrolysis.

Hydrogen is colorless, odorless, and non-toxic substance; the lightest and smallest of the elements. Due to its small molecule size, hydrogen embrittles some metals and generally, hydrogen needs to be handled properly. Hydrogen has high energy content by weight, but low energy content by volume, and thus its storage is challenging.  Hydrogen can be stored and transported as compressed (CH2), liquefied (LH2) or reversivebly binded into solid materials or liquid organic hydrogen carriers (LOHC). Stored hydrogen can be used later in turbines, in internal combustion engines (ICEs), in high-efficiency fuel cells (FCs) or for chemicals. Hydrogen is also important in refinery processes to upgrade raw fossil and bio-based fuels to final products.  Hydrogen can also be used as blending component in methane (hytane).

Hydrogen can also be converted into synthetic liquid electro-fuels (e-fuels), such as e-methanol, e-methane, liquid e-hydrocarbons, e-formic acid, e-ammonia and its derivatives (Table 1). With electro-fuel concept, using renewable hydrogen and atmospheric CO2 or nitrogen, the total cycle can be carbon-neutral. E-fuels resembling conventional fossil fuels are readily compatible with internal combustion engines of today.

Advanced Motor Fuels is one of the International Energy Agency’s (IEA) transportation related Technology Collaboration Programmes (TCP). AMF TCP has not been working directly on hydrogen as motor fuel, however, hydrogen based elecro-fuels are handled in Annexes of chemically similar fossil fuels (diesel, methane, methanol). (AMF, www.iea-amf.org).

Hydrogen TCP focuses on the management of coordinated hydrogen research, development and demonstration activities on a global basis (http://ieahydrogen.org/). Recent work of Hydrogen TCP covers:

• Task 37 - Hydrogen Safety
• Task 38 - Power-To-Hydrogen and Hydrogen-To-X
• Task 39 - Hydrogen in Marine Applications
• Task 40 - Energy Storage and conversion based on hydrogen
• Task 41 - Data and Modelling (Sub-Task C – Cooperation with ETSAP)

The following links will provide information of hydrogen properties, its purity and storage opportunities

• Link to basic information: https://www.ieahydrogen.org/why-hydrogen/
• Link to publication repository: https://www.ieahydrogen.org/publications/

Advanced Fuel Cells TCP works on standards and regulations as well as application areas in the transport sector. The information is provided by the https://www.ieafuelcell.com/index.php?id=33.

Engine principles

Experimental research and demonstration level use of hydrogen has mostly been performed in spark ignition (SI) engines operating with premixed combustion. Due to the very wide flammability limit of hydrogen, this combustion principle enables a high combustion efficiency even in part load. The air-fuel ratio can be very lean, allowing the engine load to be varied without throttle control, just by varying the fueling rate.

There are however many drawbacks and potential risks with using hydrogen in the SI principle, many of which are associated with the risk of pre-ignition and excessive combustion rates of premixed hydrogen when operating near the stoichiometric condition. This often limits the obtainable power with hydrogen in this engine principle.  

Recent research and development projects have focused on high pressure direct injection principles, in which hydrogen is used as supplementary fuel in dual fuel engines.  

Port fuel injection – PFI

Low pressure systems for hydrogen metering can be implemented as manifold or port fuel injection with solenoid injectors.

Single point manifold injection is a low-cost solution which can provide a high degree of air-fuel mixture homogeneity by injecting the fuel upstream of the cylinder inlet ports. This strategy was used with the first generations of fuel injection for light duty gasoline and natural gas-powered SI engines. With hydrogen, however, there is a risk associated with ignition of the fuel in the manifold, which can be destructive to the inlet system. As such, this solution can make sense with small engines, if the implementation is done with careful consideration to this risk, such as limiting the manifold volume and making sure the gas can expand back through the inlet. It should however not be used for two-stroke engines in which the crankcase is filled with air-fuel mixture, where an explosion can damage the engine. 

With port fuel injection, the fuel is injected only during the intake stroke through individual injectors placed at the inlet valves for each cylinder. This reduces the risk and consequences with backfiring, as there is no premixed charge in the inlet manifold.

Low pressure PFI injection retrofit kits, which are in principle identical to those made for natural gas, are available for light and medium duty engines from companies specialized in hydrogen retrofitting [1]. Such an installation can enable the engine to run partly on hydrogen, while diesel is used as the primary fuel. 

Light duty combustion engines

The most recently available production cars with hydrogen combustion engines were the BMW 7-series, which was produced in a limited series of 100 cars between 2005 and 2007. Other car companies have made similar prototype/demonstration and production cars in limited series. Mazda has presented several vehicles equipped with Wankel engines capable of running on hydrogen.

There are currently no light duty vehicles or engines in production, which are capable of using hydrogen as fuel.

High pressure direct injection – HPDI

Attention has increased on the use of hydrogen for heavy duty applications, which require power and torque at low engine speeds. The solution is to use direct injection of hydrogen, which not only eliminates the risk of back-firing, but also increases specific power. Direct injection can either be used for premixed combustion or diffusion-controlled combustion, but the later requires a high pressure (above 200 bar) injection system, which is technically more complicated.

Low pressure direct injection can improve the specific power with the premixed combustion principle, compared to PFI, as the gas does not displace the air during the intake stroke. To match the power delivered by turbo charged heavy duty diesel engines, it is however necessary to use the diesel combustion principle. This allows for higher compression ratios and higher charge air pressures, as the hydrogen burns in a diffusion-controlled combustion. This combustion principle is already in use with natural gas (as LNG or CNG) and has been adopted by several large truck manufacturers in the US and EU.

Heavy duty engines

Cummins has presented a fuel agnostic engine platform developed for several alternative fuels, including hydrogen. The engine block is the same for all fuels, with variations mainly to the cylinder head configuration. With hydrogen, the engine is equipped with direct injection that creates a premixed lean charge which is spark ignited. NOx emissions are controlled with SCR to meet the relevant emissions standards.    

Liebherr has developed hydrogen injection systems for PFI and DI dual fuel operation[2] for their own engine range. The two injection systems share the same architecture, but operate at different pressures, with 20 bar for PFI and 30 bar for DI. The combustion principle is premixed with spark ignition. The efficiency is higher with the DI solution, as it does not displace intake air. Aftertreatment technology is not announced. 

The company ULEMCo Ltd has developed and sells retrofit solutions for hydrogen to road and non-road applications. These are based on PFI systems developed for medium and heavy-duty diesel engines, which enables these engines to use hydrogen in a dual fuel combustion principle.

Westport Fuel Systems Inc. have developed hydrogen DF injectors, fuel tanks and pumps, in complete solutions ready for OEM integration into new vehicles and engines[3]. The concept and component design for hydrogen is largely based on that developed for methane, which has been on the market for more than a decade. The Westport H2 HPDI system is currently the only commercially available concept which operates with high pressure (up to 300 bar) hydrogen injection, which enables diffusion-controlled combustion of hydrogen with diesel pilot injection.

Marine engines

The companies Anglo Belgian Corperation (ABC) and CMB.tech has developed marine 4-stroke hydrogen engine, in a joint venture named BEHYDRO[4]. The engines are available from 1 to 2.67 MW with engines from 6 to 16 cylinders, and as either dual fuel (with 15 % marine fuel) or hydrogen monofuel. Both engine types are equipped with PFI.

BEHYDRO engines have been installed in a tug (Hydrotug 1, IMO 9940875) which is operating in the port of Antwerp. The tug is powered by 2 12 DZD engines, each 2 MW.  Exhaust emissions are EU Stage V compliant with SCR and DPF.

Wärtsilä has demonstrated operation with a blend of 75 % natural gas and 25 % hydrogen, on a large 4-stroke engine of the 50 SG type (50 cm bore, SI gas engine)[5]. Wärtsilä is furthermore researching pure hydrogen combustion for 4-stroke engine applications.

Emissions and aftertreatment options

Light duty engines with low pressure PFI burn hydrogen in a lean premixed combustion principle. The engine load can be varied from low to high load by adjusting the equivalence ratio, due to the wide flammability region. Emissions of NOx will therefore depend on the load and operating strategy, but after-treatment will likely be required to meet Euro 6 and upcoming Euro 7. As the engines will likely operate with lean mixtures, SCR and EGR are the most relevant aftertreatment technologies. LNT may be an option as well. Particulate filters should not be required, as hydrogen combustion does not form particulate matter.

Medium and heavy-duty engines, operating with lean burn principles will likely also require combinations of SCR and EGR to comply with the relevant emission standards. If the engines are constructed for the dual fuel principle, they must furthermore be equipped with DPF to remove particulate matter formed by the diesel combustion.

Engines intended for marine use must be either Tier II or Tier III compliant. The emission limits for NOx in ECA zones (Tier III) may necessitate the use of SCR. Particulate filters are relevant for DF hydrogen marine engines when used for inland waterways and in areas where PM emissions are subject to local regulations.