An Experimental Study on Aqueous Ammonia and Diesel Dual-Fuel Combustion with Ignition Improving Additives in a Compression Ignition Engine

Ammonia is a potential vector of renewably produced hydrogen for combustion systems and decarbonisation of transport. However, anhydrous ammonia remains dangerous and difficult to handle due to its volatility and toxicity. Therefore, a water-based solution of ammonium hydroxide (NH4OH) was proposed to investigate the potential use as a fuel in a compression-ignition engine. Ammonium hydroxide, also referred to as aqueous ammonia, is liquid phase under atmospheric conditions; therefore, the storage of such a fuel does not require high pressure. Previous work has established that ammonium hydroxide solution could contribute to energy release during co-combustion with fossil diesel. However, the presence of water reduced combustion stability and limited the extent to which aqueous ammonia could displace diesel. In addition, the characteristics of pollutant emissions of burning such a fuel remained less understood. This study therefore explores the potential of ignition improving additives for ammonium hydroxide and diesel dual-fuel combustion in a diesel engine. Two chemical additives, hydrogen peroxide and ammonium nitrate, were selected to blend with ammonium hydroxide at varied concentrations. The solution was aspirated to the engine via port injection into the preheated air intake, while diesel was supplied via direct injection at 550 bar. Tests were undertaken at constant engine work but with varying levels of displacement of diesel fuel by aspirated aqueous ammonia, with measurements of combustion characteristics and both particulate and gaseous emissions in the exhaust. The addition of hydrogen peroxide reduced the duration of ignition delay relative to aqueous ammonia and diesel only co-combustion, especially at higher additive concentrations of 2% and greater. Furthermore, the presence of both ignition improvers saw an equivalent energy release from aqueous ammonia achieved with reduced injection duration and a higher proportional contribution to overall engine load. The aqueous ammonia and diesel dual-fuel combustion resulted in a general increase in both particulate mass and number. This trend was especially noticeable with 5000ppm ammonium nitrate added to the fuel, where the number of particles was 164% greater than diesel only combustion and particle diameter mainly ranged between 10-50 nm. Meanwhile, despite an increase in fuel-bound nitrogen with the use of ammonia, exhaust emissions of nitrogen oxides did not linearly increase, and both ignition improvers reduced nitrogen oxides relative to ammonia and diesel co-combustion without additives.