Transient Plasma Ignition for Clean, Fuel-efficient Transportation Vehicle Engines

Flame ignition experiments by the principal investigators (PIs) have shown that non-equilibrium transient plasma discharges (TPDs) provide much shorter ignition delays and burn durations (by a factor of about 3) compared to conventional sparks under otherwise identical conditions (including the ignition energy). Compared to conventional sparks, TPDs are characterized by very high voltages (typically 30 kV), high currents (typically 100 A), extremely short durations (typically 50 nanoseconds), very high wall-plug efficiencies (ratio of energy deposited in the gas to the electrical energy consumed in producing the discharge) (typically 50%) and less electrode heating and erosion. This type of discharge process is entirely different from that used in any prior internal combustion engine (ICE) study and cannot be produced by conventional ICE ignition systems. This work assesses the value of TPD ignition for premixed-charge engines for transportation vehicles under applications typically assigned to diesel engines whose high NOx and particulate emissions are becoming increasingly unacceptable. The inherently shorter burn duration attainable with TPDs can be exploited in at least two different ways that will be tested in this work, possibly enabling diesel-like fuel economy with premixed-charge-like NOx and particulate emissions. Use leaner mixtures that would otherwise burn too slowly for use in engines. This approach will be emphasized since it does not require additional equipment beyond the ignition system. We estimate that a 12-fold reduction in thermal NOx emissions with no loss of thermal efficiency is feasible with this approach. Redesign the intake port and piston shape to minimize turbulence levels in the engine. By employing TPD ignition, the turbulence level necessary to accelerate combustion to an acceptably rapid rate can be lower and in turn, the heat loss would be reduced. We estimate that a 7% - 10% improvement in thermal efficiency with no increase in NOx emissions is feasible in this approach. The combinations of electrode design, equivalence ratio, compression ratio, ignition timing and turbulence level that optimize fuel efficiency and emissions performance for a given application will be determined. Experiments will be performed in a 4-cylinder, 2.5 liter automotive engine at USC then, if warranted, extended to large-bore transportation vehicle engines. This study will focus on the use of TPDs for ignition in engines employing alternative fuels including natural gas, methanol, ethanol, hydrogen, with gasoline also tested solely for comparison purposes. An additional benefit of TPD ignition that no other ignition source can offer is the possibility of post-combustion reduction in NO levels. Prior work by Prof. Gundersen has shown that TPDs in engine exhaust leads to substantial reduction in NO emissions that is competitive on an energy consumption per unit of NO removed with other methods. This raises the possibility of having two or more TPD events per engine cycle - at the end of the compression process to initiate combustion, and later at the end of the expansion process and/or during the exhaust process in order to obtain further reductions NO emissions. Since it requires no additional hardware and little additional testing, this possibility will be pursued at no additional project cost.