Transient plasma enhanced combustion of ultra-lean H2 in an internal combustion engine for reduced NOx emission

•Plasma ignition enables a 2X improvement in engine stability (a 52% drop in COVIMEP).•Plasma ignition enables a 4X reduction in NOx emissions under ultra-lean conditions.•Plasma ignition increases the maximum power by 175% under ultra-lean conditions.•Augmented flame surface area and reduced ignition delays are achieved with plasma. Hydrogen is considered one of the leading candidates among all carbon-free (i.e., “green”) fuels. While the combustion of hydrogen does not produce any CO2 emissions, it results in high NOx emissions (i.e., NO and NO2) due to its high flame temperatures. Here, we report substantial improvements in the combustion of H2 under ultra-lean conditions using transient plasma ignition (TPI), which supports stable engine operation with minimal NOx emissions. The transient plasma-based ignition system was tested on a retrofitted natural gas engine using zero-carbon fuel (i.e., H2). The flame characterization was conducted in a canonical static combustion chamber together with a high-speed camera and a pressure sensor. Compared to conventional spark ignition (magneto type), the TPI enables a 2-fold improvement in engine stability (i.e., 52 % drop in coefficient of variation of indicated mean effective pressure (COVIMEP) at an engine speed of 1500 RPM) and a 4-fold reduction in NOx emissions at constant power at an engine speed of 2400 RPM. In this ultra-lean range, we also observe a 175 % increase in the maximum mechanical power produced by the engine using TPI (0.2 kW at an engine speed of 1100 RPM) by achieving more complete combustion. We compare the results of engine performance (stability, power, and NOx emissions) with those using a conventional (magneto-type) spark ignition (CSI) over a wide range of equivalence ratios (ϕ) from 0.2 to 0.7. The transient plasma-based enhancement arises from hydrodynamic effects, (i.e., ionic winds), which gives rise to turbulence and multi-scale mixing. This is supported by Schlieren imaging, which shows an augmented flame surface area and reduced ignition delays with TPI compared to that with CSI. This general approach enables hydrogen fuel to be burned in internal combustion engines, while maintaining low NOx emissions 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.