Although the auto industry is rushing to electrify, batteries, in their current state of development, aren’t the answer for many modes of transportation, or even many use cases in the auto and commercial-vehicle sectors. This leaves the clean-up demanded by tightening global emissions regulations to better internal-combustion engines – or better fuels to burn in those engines.

As efficiency gains for IC engines and their exhaust-aftertreatment systems reach the point of diminishing returns, “What you can do is change the fuel,” said Stefan Mayer, manager, diesel and turbo at MAN Energy Solutions, at the “Current and Future State of Transportation Fuels” expert panel discussion at SAE International’s annual WCX conference in Detroit. With increasing attention on climate change and “decarbonization” measures in every industry, Mayer and other speakers outlined a variety of efforts to expand the application of alternative fuels to provide potential big reductions in greenhouse gas emissions.

Some alternative fuels already are gaining mainstream attention – and increasing expectations for their adoption. Renewable electricity and hydrogen are under pressure to scale rapidly, while hydrogen capacity needs to dramatically increase to address quickly expanding strategies for its use in many transportation applications.

Scrutiny on shipping
MAN’s Mayer indicated that the marine-propulsion sector is intent on decarbonization. Some of the company’s most widely-known marine engines – many are 2-stroke designs of low speeds and massive size (stroke dimensions of upwards of 4 m [13.1 feet]) – can burn up to 10 tons of fuel per hour. Mayer said that 80-90% of all global freight is at some point transported by sea and that the shipping industry is responsible for around 3% of all global CO2 emissions.

Since 50% of the industry is served by MAN engines, around 1.5% of global CO2 is generated by its powerplants. With current thermal efficiencies running around 50%, MAN’s marine engines are effectively optimized for the unique duty cycle of large-vessel shipping and have reached a certain state of the art. To make serious GHG reductions, he said, attention must shift to fuels.

“It is not engine technology that is the barrier for decarbonization of shipping,” Mayer said, “it is and will be the production capacity of renewable fuels.” One such effort, he said, is the company’s Project AEngine, a full-scale testing and demonstration of a “zero-carbon marine engine” fueled by ammonia. The burning of ammonia, he said, generates no CO2 emissions because ammonia contains no carbon.

The AEngine project is studying the unique demands of engine process, onboard fuel handling and safety and regulation. The company’s Copenhagen research center is conducting the program with a target of a “commercial design” by 2024, he added. The company in 2021 initiated the “AmmoniaMot” (Ammonia Engine in German) collaboration with partners from industry and research institutes to develop a “dual-fuel, medium-speed engine capable of running on diesel-fuel and ammonia,” Man Energy Solutions said in a release.

MAN already has alternative-fueled marine engines in service, Mayer asserted, which operate on liquified natural gas (LNG), methanol, ethane and liquified propane gas (LPG). And, he added, there is potential for biofuels such as fatty acid methyl esters (FAME) and bio-crudes that can deliver up to a 40% reduction in GHG. There also is ongoing research in advanced “blue” biofuels based on carbon capture-and-storage (CCS) processes, as well as carbon-free synthetic e-fuels – all “stuff you can’t buy yet” but with realistic promise. He added that MAN recently developed the world’s first methanol-fueled engine for ultra-large shipping vessels.

Ammonia the ‘wild card’
Frederic Meyer, strategy and projects director at Total Energies, also indicated that he expects more attention on development of ammonia fuel and onboard CCS to improve the complex Energy Efficiency Design Index (EEDI) for ships, which mandates a 30% CO2 reduction (compared to the 2000-2010 average) for newly built vessels starting in 2025. Meyer said the future may hold a new dollars/CO2 content price structure for all fuels.

Safety and handling considerations also must be factored into the decision-making process, said Brian Kaul, a research staff member in the Fuel Science and Engine Technologies Research Group at Oak Ridge National Laboratory. For marine applications, biofuels, methanol and ammonia show promise, he said, stressing that “there is no one low-carbon fuel that is going to solve all the problems.”

Kaul referred to ammonia as the “wild card”: “It’s one of the most widely produced and handled industrial chemicals in the world, but no one is really using it as a fuel yet,” he said. Ammonia reacts chemically with water when spilled, producing ammonium hydroxide that is toxic and caustic, but it will rapidly dissipate. Its flammability/explosion risk is low.

From an engineering perspective, double-walled fuel lines with an interstitial purge gas typically are required for safety considerations. “Because of its toxicity, because you need respirators for safe handling, I think marine and maybe agriculture are the only areas you’re going to see this as a fuel. This is not going to be an on-road transportation fuel for cars and trucks,” Kaul said.

Here and now
Although the WCX 2022 panel’s emphasis was on alternative fuels, possible improvements for existing “conventional” fuels weren’t omitted. Shailesh Lopes, senior fuels engineer at General Motors, presented compelling figures for the potential hydrocarbon and oxides of nitrogen (NOx) reduction of improving the quality of diesel fuel. Lopes said diesel cetane numbers of less than 40 result in high emissions, while cetane numbers of 50 or better can deliver a significant emissions reduction.

Lopes advocated for “desired” diesel-fuel properties of 15% or less aromatics content and a cetane rating of 55 or better. He said such properties also can reduce the constituency of harmful metals in the fuel that can accelerate degradation of aftertreatment systems. “Vehicles and fuels are a system,” he asserted.

–reporting by Ryan Gehm