The majority of experiments on ammonia use in the literature concerned spark-ignited (SI) engines, although some have also achieved satisfactory combustion using compression ignition (CI) engines. High compression ratios, low speeds and high loads have been found preferable for ammonia fueled engines, which is primarily related to ammonia’s low flame speed. Engine tests though showed good results for an ammonia fuelled SI-engine with a small amount of hydrogen added.

Ammonia is considered interesting for marine engines, which are mostly large diesel engines today. Marine engines fueled by ammonia with pilot diesel fuel injection could be an option. Large displacement volume and operation at a constant low speed with high loads are favorable for ammonia combustion.Ammonia carriers already have experience with handling and storage of ammonia, and could thereby benefit in terms of lower CO₂ emissions and economic savings from using already on-board fuel by implementation of ammonia fueled engines.

Engines for ammonia

Ammonia engines can be constructed for both premixed (Otto) and diffusion controlled (Diesel) combustion principles. The Otto principle is the most simple and applicable for small and medium sized 4-stroke engines, while the Diesel principle has more advantages in large dual fuel engines, including large 2-stroke marine engines.

Light duty engines

Experimental ammonia-fueled Otto engines have been demonstrated to operate most efficiently with near-stoichiometric combustion. There is a trade-off between high NOx formation at lean combustion, and increased emissions of ammonia and hydrogen at fuel rich combustion. Lean burn does not appear to be feasible with ammonia engines operating with the Otto principle, so load control must be based on the control strategies normally used in SI engines.

Ammonia has not received much attention or consideration as fuel for light duty engines. There have, so far, not been any factors motivating the development of small ammonia engines. However, in June 2023, the Chinese owned engine manufacturer GAC and Toyota presented a prototype 2.0 L, 120 kW SI engine, operating on pure ammonia [16]. The engine is built for passenger car use, as a proposed near zero carbon alternative to battery electric vehicles.

Marine engines

Due to the increasing interest in the potential for reduction in CO2 emissions in the marine sector, marine engine designers are now developing large marine engines for ammonia.

2-stroke dual fuel ammonia engines are currently being developed, building on the experience and technology developed for dual fuel solutions with direct gas and liquid fuel injection. 2-stroke propulsion ammonia DF engines are expected to be ready for delivery and installation in new ships from 2024, while retrofit solutions are expected to be available for existing ships from 2026 [17, 18].     

Wärtsilä has recently released a 4-stroke ammonia DF engine and fuel system intended for marine power generation and propulsion [19]. The engine is equipped with low pressure port fuel injection of ammonia, with diesel pilot injection. The engine is capable of both DF and fuel oil only operation.