Zero-carbon shipping: Ammonia engine combustion mode cuts greenhouse gases by over 70%

As global economies race to fulfil net-zero carbon pledges, green ammonia has emerged as one of the most promising carbon-free liquid fuels to decarbonise the transportation sector, particularly the heavy-duty marine shipping industry

However, ammonia’s unfavourable combustion properties, such as its low flame velocity and high resistance to ignition, have long hindered its practical application in internal combustion engines (ICEs).

Traditional combustion approaches often suffer from severe fuel slip, poor efficiency, or elevated emissions of nitrous oxide (N₂O), a greenhouse gas nearly 300 times more potent than carbon dioxide.

In a study published in ENGINEERING Energy, a collaborative team of scientists from the State Key Laboratory of Engines at Tianjin University (China) and the Department of Energy Sciences at Lund University (Sweden) has unveiled a breakthrough solution: the Ammonia Thermal Atmosphere Compression Ignition (TACI) combustion mode.

This innovative approach successfully bypasses ammonia’s inherent kinetic limitations, achieving highly efficient, stable, and clean ammonia diffusion combustion under mild operating conditions.

Key research highlights and innovations

• Pioneering TACI combustion mode:

  • The study introduces the novel Thermal Atmosphere Compression Ignition (TACI) mode to achieve stable and efficient ammonia diffusion combustion in internal combustion engines. By utilising n-heptane combustion to create a highly active thermal atmosphere, the system successfully triggers the auto-ignition of directly injected ammonia spray.

• Decoding the thermodynamic boundary:

  • The researchers discovered that achieving stable ammonia diffusion combustion requires a high in-cylinder temperature of approximately 1500 K. This critical thermal boundary effectively overcomes the severe cooling effect caused by ammonia’s high latent heat of vaporisation during spray evaporation.

• Exceptional emission control:

  • Under stable operating conditions, the TACI mode enables highly efficient combustion characterised by high indicated thermal efficiency (ITE). Importantly, it simultaneously achieves low nitrogen oxide (NO_x) emissions, ultra-low nitrous oxide (N₂O) emissions, and negligible unburned ammonia (NH₃) slip.

• Advanced intake control strategies:

  • The team systematically explored the effects of modulating intake pressure and applying intake air heating to maximise the ammonia energy fraction and minimise overall greenhouse gas (GHG) emissions.

• Meeting IMO 2040 targets:

  • The study reveals that while intake heating significantly improves the ammonia substitution ratio (ASR), it must be coupled with high intake pressure to ensure a sufficient oxygen supply and prevent the formation of N₂O. This optimised combined strategy increased the ASR by 17% and the GHG reduction ratio by 9%.
  • Ultimately, the system achieved an ASR of over 80% and a GHG reduction exceeding 70% under medium-load conditions, successfully meeting the International Maritime Organisation (IMO) 2040 decarbonisation targets.

Breaking down the multi-stage strategy

Unlike conventional premixed systems, the TACI mode leverages a unique multi-stage combustion strategy. First, a highly reactive and volatile fuel, n-heptane, is introduced into the intake pipe and undergoes Homogeneous Charge Compression Ignition (HCCI) within the engine cylinder.

his primary combustion creates a highly active “thermal atmosphere” characterised by elevated temperatures, increased pressures, and an abundance of reactive chemical radicals. Liquid ammonia is then precisely injected at high pressure into this pre-activated environment, triggering instantaneous auto-ignition and establishing a stable diffusion flame.

“By dominating the process through fuel-air mixing rather than relying solely on flame propagation, diffusion combustion effectively overcomes ammonia’s sluggish burning rate,” explain researchers. “Furthermore, the local rich-burn conditions inherent to this diffusion mode naturally facilitate the selective non-catalytic reduction (SNCR) of nitrogen oxides (NO_x), which intrinsically minimises harmful pollutant formation.”

Crucially, the TACI system maintains exceptionally high thermal efficiency while ensuring negligible unburned ammonia slip and ultra-low N₂O emissions under stable operating states. This performance directly aligns with the rigorous 2040 greenhouse gas reduction targets established by the International Maritime Organisation (IMO), marking a critical leap forward in bringing zero-carbon internal combustion engines from theoretical research into practical, commercial reality.