Stable combustion of ammonia in an internal combustion engine: A single fuel approach enabled by multi-pulse transient plasma ignition

It was demonstrated that the use of nanosecond pulse transient plasma enables stable combustion of ammonia in an internal combustion engine (i.e., a modified natural gas engine) over a wide range of equivalence ratios from phi = 0.78 to phi = 1.23 Additionally, the ammonia combustion was investigated in an oxygen-rich environment to demonstrate the effectiveness of nanosecond pulse transient plasma for combustion initiation and increased rate of pressure rise (in a constant volume environment) compared to a conventional spark ignition system. Previous attempts to burn ammonia in internal combustion engines have used a dual fuel approach (i.e., diesel:ammonia, gasoline:ammonia, natural gas:ammonia) and have been limited to less than 30 %vol ammonia content. Herein, stable combustion was achieved with pure ammonia mixed with air in a single-fuel approach. The stable combustion of ammonia is enabled by two key mechanisms: 1.) ignition timing that needs to be substantially advanced relative to the top dead center of the compression stroke because of the relatively low flame speeds associated with ammonia, and 2.) multi-pulse high voltage discharge gives rise to ionic winds, multiscale turbulence and mixing, and increased flame surface area of the flame kernel. The latter mechanism is crucial, as it was not possible to achieve stable combustion using conventional spark ignition (i.e., magneto-type ignition) regardless of ignition timing advancement and equivalence ratio. The engine stability was quantified by measuring the coefficient of variation of indicated mean effective pressure (COVIMEP). COVIMEP = 6.8 % was achieved for pure ammonia combustion with air at an equivalence ratio of phi = 1.23, which is very close to the industry standard of 5 %, indicating stable engine operation. The value of COVIMEP increases at lower equivalence ratios, as expected, reaching COVIMEP = 16.5 % at phi = 0.78 but still supporting stable engine operation. This general approach enables pure ammonia to be burned in internal combustion engines using a transient plasma ignition system that could serve as a drop-in replacement of conventional magneto-type ignition systems without requiring any further engine modifications.