EXCLUSIVE: Ammonia-hydrogen engines challenge conventional combustion models

Engine manufacturers developing ammonia-hydrogen combustion engines may need to rethink how they analyse in-cylinder pressure data, after research by Kistler and the University of Nottingham identified potentially significant errors in conventional heat-release calculations.

The study examined whether models developed around hydrocarbon fuels remain suitable for zero-carbon fuel blends. Using a single-cylinder research engine, researchers tested pure ammonia and ammonia-hydrogen mixtures containing up to 60% hydrogen. Their findings suggest that relatively small changes in fuel composition can have a substantial effect on calculated combustion characteristics.

“Our findings suggest that engine manufacturers will need to use heat release models developed specifically for ammonia-hydrogen combustion,” Dr David Rogers, head of ICE Systems at Kistler told The Motorship. “Greater emphasis will also be required on experimental validation to ensure that combustion behaviour is accurately captured.”

The research focused on the polytropic index, a thermodynamic parameter used in first-law heat-release models to describe pressure and volume changes during compression and expansion. Conventional combustion analysis often applies values established through years of research with gasoline and other hydrocarbon fuels.

Kistler compared those standard assumptions with fuel-specific, cycle-resolved calculations based on measured pressure data. Under pure ammonia operation, the polytropic index during compression differed by about 4% from the gasoline default value. The researchers found that discrepancies were particularly relevant during the later stages of combustion, while increasing the hydrogen share generally accelerated combustion and reduced modelling differences.

Rogers said continuing to use conventional models could have practical consequences for engine development.

“Based on our experience, most equipment and algorithms used for online in-cylinder pressure analysis was developed with hydrocarbon fuels in mind,” he said. “We found that applying heat release models developed for conventional fuels can lead to inaccurate predictions of combustion phasing when used with ammonia-hydrogen mixtures, increasing the risk of poor calibration decisions and longer development cycles.”

For sectors such as shipping, however, the research represents only one part of the challenge. Ammonia-hydrogen engines must also demonstrate reliable operation under demanding real-world conditions, while developers need to understand emissions, fuel-path effects and long-term durability.

“Further work is needed to better understand ammonia-hydrogen combustion when these fuels are used together, particularly their behaviour in real engine conditions,” Rogers said. “A deeper understanding is also required of fuel-path effects on emissions formation and long-term engine durability.”

He added that large-scale testing under representative marine operating conditions would be essential, noting that research results within the wider industry are not always widely shared.

The researchers were surprised by the scale of some of the discrepancies, although they expected ammonia-hydrogen chemistry to produce differences from conventional fuels.

“We anticipated some differences because of the unique chemistry and properties of ammonia-hydrogen fuels,” Rogers said. “But quantifying them in the context of practical applications showed that this was an area worth exploring further.”

Kistler now intends to incorporate the findings into its measurement technology, including its KiBox2 combustion analysis system, while continuing research across different engine types and zero-carbon fuel mixtures.

The broader objective, Rogers said, is to give engineers more reliable data earlier in the development process. “Kistler is taking a holistic approach to these challenges to provide customers and users with the highest possible data quality in the most efficient way,” he said.