Several organizations, encompassing companies, research labs, and academia, have formed the Hydrogen Opposed Piston Engine Working Group. The Working Group consists of members undertaking research and development in the field of hydrogen combustion in an opposed-piston engine.

An opposed piston engine with hydrogen combustion could well provide the best-known thermal efficiency from a reciprocating engine, with the potential to match the in-vehicle efficiency of a hydrogen fuel cell. If so, it is a valuable potential option for long haul transit in our quest for sustainable transportation.

—James Turner, Professor of Mechanical Engineering, Clean Combustion Research Center, King Abdullah University of Science and Technology (KAUST)

The newly formed organization will host a series of meetings of its members to exchange research results, insights, and ideas.

The direct injection two-stroke engine could be a very promising and interesting option for hydrogen combustion to achieve zero NO_x because of its advantages of high-power density and inherent much lower NO_x emissions. These two-stroke advantages are even more significant with an opposed-piston engine thanks to its higher power density and efficiency.

—Pierre Duret of DI2S Consulting & Training and former director Powertrain and Sustainable Mobility, IFP School (France)

In a recently published whitepaper on hydrogen opposed-piston engines (H₂-OPE), opposed-piston engine maker Achates Power explained that since there are no carbon molecules in hydrogen fuel, neither unburned hydrocarbons nor particulate matter is a concern for emission control from a hydrogen combustion engine. The only criteria emission of concern is NO_x.

If hydrogen combustion is sufficiently lean—i.e., with an air/fuel equivalence ratio (lambda) λ ≥ 2.23—very low levels of NO_x are created. A reciprocating engine, operating sufficiently lean, will produce very little NO_x and may be able to meet ultralow NO_x tailpipe requirements without an aftertreatment system. Papers from BMW and Hydrogen Energy Research Lab show that sufficiently lean hydrogen combustion in properly designed combustion chamber results in close to zero NO_x. Compared with conventional hydrogen ICE, the opposed-piston engine has a significant advantage for low NO_x hydrogen combustion.

Hydrogen fuel cells must operate at higher than ambient air pressure to operate efficiently; however, the air compressors used to pressurize the fuel cell system consume significant parasitic energy, particularly at high load. A study of a 2017 Toyota Mirai fuel cell by Argonne National Laboratory found fuel cell stack efficiency below 50% and fuel cell system efficiency dropping below 40% at high loads. The sharp drop in system efficiency is due to parasitic losses to the air compressor and other system accessories.

According to Achates, a heavy-duty opposed-piston diesel engine (configured for ultralow NO_x) has peak thermal efficiency of just below 50%, centered around high load areas of the operating map, with broad areas of high efficiency (>45% brake thermal efficiency). Achates Power expects close to the same level of thermal efficiency with hydrogen as a fuel.

According to Achates Power, the two main technical challenges for H₂-OPE are fuel injection and ignition source.

• Fuel Injection. OP engines utilize direct injection after ports closure to avoid the risk of fuel short circuiting out the exhaust. High pressure injectors for hydrogen are challenging because the small molecule size of the gas lends itself to leakage, but direct injection enables high compression ratios for improved efficiency and increased power and torque. Direct injection also avoids the risk of backfire. Several companies are developing hydrogen high pressure injectors, including Bosch and Westport Fuel Systems.

• Ignition Source. Hydrogen has high resistance to autoignition, making it unsuitable for compression ignition. A spark plug, or other ignition source, is generally required. Achates Power prefers compression ignition in its OP engines because it enables combustion in the middle of the combustion chamber for clean, efficient combustion and minimal heat loss. The addition of a spark plug is a new design element. Fortunately, the OP engine has room for multiple spark plugs around the cylinder circumference. Coupled with the fast flame speed of hydrogen combustion, stable and robust combustion can be expected from spark ignition in an OP engine.

The founding members of the Hydrogen Opposed Piston Engine Working Group include:

• Achates Power, San Diego, CA. Created with the mission to build cleaner, more efficient engines, Achates Power has an experienced staff of engineers and scientists working with leading engine manufacturers to bring the opposed piston engine to market. Achates Power is backed by the Oil and Gas Climate Investments and other investors.

• Aramco Americas. Aramco Services Company (d/b/a Aramco Americas) is the US-based subsidiary of Aramco, a world leader in integrated energy and chemicals, which has had a presence in the US for more than 60 years. Aramco Americas is a contributor to the US energy sector through research and development, venture fund activities, asset ownership, as well as technology and digital transformation.

• Bourns College of Engineering – Center for Environmental Research and Technology, University of California, Riverside, Riverside, CA.  UC Riverside Bourns College of Engineering, Center for Environmental Research & Technology strives to be a recognized leader in environmental education as it collaborates with industry and government to improve the technical basis for regulation and policy, and to be a creative source of new technology.

• Combustion and Propulsion Systems, Chalmers University of Technology, Gothenburg, Sweden.  Chalmers University of Technology Combustion and Propulsion Systems uses it expertise in combustion and emissions formation to contribute to a sustainable future by minimizing emissions from combustion engines.

• Department of Automotive Engineering, Clemson University, Clemson, SC. Clemson is a leading public research institution in South Carolina.  The University’s International Center for Automotive Research brings together world class faculty, state-of-the art facilities, and graduate students to lead basic and translational research with an emphasis on industry relevance. Vehicle Propulsion is one of the three strength areas, pursuing a range of topics focused on advanced IC Engine concepts and powertrain electrification.

• DI2S Consulting & Training, Paris, France. DI2S Consulting and Training is run by Pierre Duret, the former Director of Powertrains and Sustainable Mobility at IFP School in Paris.  The firm provides consulting services in the field of two-stroke direct injection engines.

• Engine Research Center, University of Wisconsin-Madison, Madison, WI. The University’s Engine Research Center is dedicated to investigating the fundamental thermo-physical process that control combustion performance and pollutant emissions formed during combustion in internal combustion engines.

• ID-Technologies, Fribourg, Switzerland. ID-Technologies is a Swiss engineering company specialized in energy technologies. The core business is the production of renewable fuels and clean, efficient energy conversion technologies.

• Indian Institute of Science, Bangalore, India.  IISc Bangalore is a public institution of higher learning, research and teaching, consistently ranked every year as the best university in India. The Combustion & Spray Research Laboratory and the Engines & Energy Systems Laboratory in the Department of Mechanical Engineering undertake cutting edge research in the fields of combustion, atomization, plasma ignition using laser-based diagnostics and develop alternate fuel-based technologies, particularly for small engines.

• King Abdullah University of Science and Technology, Thuwal, Saudi Arabia. King Abdullah University of Science and Technology advances science and technology through distinctive and collaborative research integrated with graduate education.

• Mahle Powertrain, Plymouth, MI. MAHLE Powertrain is a leading global engineering and consultancy services provider, wholly owned by the MAHLE Group, a global Tier 1 supplier to the automotive industry. MAHLE Powertrain specializes in research, development, and application of future powertrain systems.

• Marquette University, Milwaukee, WI. The mechanical engineering department at Marquette University has an up-and-coming research program focused on alternative fuels for internal combustion engines with support from the DOE, ARPA-e, and NSF.

• Powertrain Control Laboratory, University of Michigan, Ann Arbor, MI.  The Powertrain Control Laboratory undertakes leading research in the fields of automation of hydrogen fuel cells, fuel reforming for hydrogen on-demand, and hydrogen storage for hybrid and electrified powertrains with support from US Army’s Automotive Research Center, DOE and NSF.

• Shell plc, London, England. Shell, a global energy company with operations in more than 70 countries, uses advanced technologies and takes an innovative approach to help build a sustainable energy future.

• SuperTurbo Technologies, Inc. SuperTurbo specializes in the design, development, and commercialization of the SuperTurbo mechanically driven turbocharger for on-highway and off-highway commercial vehicles.