Transient Plasma Enhanced Engine Combustion of Ultra-lean H2 for Reduced NOx Emission

Hydrogen is considered one of the leading candidates among all carbon-free (i.e., “green”) fuels. While the oxidation of hydrogen does not produce any CO₂ emissions, its combustion in air results in high NO_x emissions (i.e., NO and NO₂) due to its high flame temperatures. In the work presented here, we evaluate the effect of transient plasma ignition (TPI) on the combustion of hydrogen in an internal combustion piston engine. The transient plasma is produced by high-voltage nanosecond pulses, which generate highly reactive radical species including atomic oxygen, atomic hydrogen, and ozone that accelerate the oxidation process providing more stable combustion with this “green” fuel. These transient high-voltage nanosecond pulses also produce turbulence and multiscale mixing, which are advantageous for combustion. In the present study, we compared the results of engine performance (stability, power, and NO_x emissions) with those using a conventional (magneto-type) spark ignition over a wide range of equivalence ratios (f) from 0.2 to 0.7. We observed a substantial enhancement in the engine stability, as quantified by the coefficient of variation (COV_IMEP), over the entire f range. Of particular interest is the engine performance (stability and power) under lean conditions f < 0.4, where minimal NO_x emissions can be detected. Under ultra-lean conditions (i.e., 0.2 ≤ f  ≤ 0.4), we observed up to 52% reduction in COV_IMEP and up to 175% increase in the maximum mechanical power produced by the engine using TPI. This approach enables hydrogen to be used in internal combustion engines, while maintaining low NO_x emissions requirements 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.