Neat ammonia use in a heavy-duty diesel engine converted to spark ignition focused on lean operation

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Ammonia is a prime fuel alternative in the global effort of decarbonizing the transportation and power generation sectors. However, ammonia suffers from poor combustion characteristics including a lower burning velocity, a higher ignition energy, a lower calorific value, and narrower flammability limits when compared to conventional hydrocarbons. This study investigated the potential for fueling neat ammonia to a single-cylinder heavy-duty four-stroke research diesel engine converted to natural gas spark ignition operation. Stable neat ammonia combustion, for the first time focused on lean operation, was performed without any hardware modifications or engine control strategies other than the adjustment of the spark timing. Experiments were performed at various low to medium engine loads when compared to the maximum load of the non-converted original diesel engine, for engine speeds ranging from 800 rpm to 1400 rpm and equivalence ratios from 0.8 to 1.05. Results indicated that pure ammonia operation did not produce any penalty on engine performance or combustion stability when compared to methane operation at similar conditions and same fuel energy per cycle. Moreover, neat ammonia engine performance was better at the lean equivalence ratio, mostly driven by the associated increase in intake pressure when fuel energy per cycle was kept constant. In addition, as the data showed the capability of stable ammonia combustion for a wide range of speeds and loads (at least for the engine platform used in these experiments), this study confirmed the potential of converting current heavy-duty diesel engines to neat ammonia spark operation with minimum changes in hardware and/or control.