Proving the feasibility of a large clean ammonia marine engine (ZEWT Partnership) Deep sea shipping (maritime intercontinental transport) and ship operations with very high power demand require the storage of large amounts of energy carriers aboard in order to ensure the required autonomy and the ability to navigate safely in adverse and extreme conditions. The use of low energy density green fuels will in these operational environments lead to a significant loss of cargo volumes or useable space (e.g. passenger cabins) which directly impacts the economic parameters of the vessels and consequently leads to reluctance by ship owners to invest in clean ships.Ammonia is a zero carbon fuel, with a (slightly) higher volumetric energy density than liquid hydrogen and with comparatively high liquefaction temperatures and pressures. Due to this, and its combustion characteristics, green ammonia has been widely advocated as a potential sustainable alternative marine fuel. Yet its possible use within a large low-speed marine engine has yet to be proven. A practical and easy use of ammonia may be a game changer in making shipping climate neutral, in particular if it can be used in the existing fleet through retrofitting.The aim is to develop, demonstrate and validate a multi-cylinder internal combustion engine of at least 10 MW power output running on ammonia as its main fuel, with IMO-Tier III or lower NOx emissions and negligible emissions of SOx, particulates and other harmful substances or odours. As an indication total tank-to-wake GHG emission reduction versus an MGO baseline should be at least 80% (taking into account that the climate-neutral upstream supply of ammonia is not part of this topic). Risks should be assessed and the engine should also comply with all relevant safety rules and regulations as stipulated by classification societies and flag states.Projects should demonstrate the engine operating at its rated power in a laboratory or on board of an actual vessel, thus going beyond the state of the art which is currently demonstrating ammonia combustion in smaller (road vehicle-based) test engines, or rapid compression machines. Assuming feasibility, pathways toward deployment will be proposed.This topic implements the co-programmed European Partnership on ‘Zero Emission Waterborne Transport’ (ZEWT).