Experimental Activity on a 4-Stroke SI Engine Fuelled with Ammonia and Hydrogen

Storing hydrogen is one of the major problems concerning its utilization on board vehicles. A promising solution is storing hydrogen in the form of ammonia that is liquid at roughly 9 bar at environmental temperature and therefore involves relatively small volumes and requires light and low-cost tanks. Moreover, liquid ammonia contains 1.7 times by volume as much hydrogen as liquid hydrogen itself. It is well known that ammonia can be burned directly in I.C. engines, however a combustion promoter is necessary to support combustion especially in the case of high-speed S.I. engines. As a matter of fact, the best (and carbon-free!) promoter is hydrogen, which has very high combustion velocity and wide flammability range, whereas ammonia combustion is characterised by low flame speed, low flame temperature, narrow flammability range (combustion is impossible if mixture is just slightly lean), high ignition energy and high self-ignition temperature. The experimental activity shown in the paper was aimed at determining proper air-ammonia-hydrogen mixture compositions for the actual operating conditions of a twin-cylinder 505 cm3 S.I. engine. Hydrogen and ammonia are separately injected in the gaseous phase. The experimental results confirm that it is necessary to add hydrogen to air-ammonia mixture to improve ignition and to speed up combustion, with ratios that depend mainly on load and less on engine speed. This activity is correlated with a larger-scale project, founded by Tuscany Region, in which a partnership of research and industry entities has developed a fully-working plug-in hybrid electric vehicle equipped with a range-extending 15 kW IC engine fuelled with hydrogen and ammonia. Hydrogen is obtained from ammonia by means of on-board catalytic reforming.