Nanosecond Pulsed Ignition for Automotive Applications: Performance and Emissions Characteristics of Gasoline Combustion in an Optical Engine 2021-01-0475

Performance and emissions characteristics were measured for a part- load operating point using an optically-accessible single-cylinder gasoline research engine equipped with three different exploratory nanosecond repetitively pulse discharge (NRPD) igniters. The three igniters investigated are as follows: 1) a four-prong advanced corona ignition system (ACIS) that produces large ignition volumes from streamer discharges, 2) a barrier discharge igniter (BDI) that generates strong surface plasma along the insulator that completely encases the power electrode, and 3) a J-hook non-resistive nanosecond spark (NRNS) igniter. For select conditions, high-speed imaging (20 kHz) of excited state hydroxyl (OH*) chemiluminescence was performed to measure flame development in-cylinder. An available NRPD pulse generator was used to supply positive direct current (DC) pulses (~ 10 ns pulse width) to each igniter at a fixed 100 kHz frequency. The minimum pulse number (1 - 200) and primary voltage (900 - 1300 V) required to achieve stable ignition was used for each operating point. The pulse generator featured a sense and control system that would interrupt pulse trains if arc transition was detected. For all conditions, engine speed and load were fixed at 1300 revolutions per minute (rpm), and 3.5 bar indicated mean effective pressure (IMEP), respectively. Sweeps of equivalence ratio and stoichiometric charge dilution were separately investigated, with lean and dilute combustion limits identified for each igniter. Results were benchmarked against the engine equipped with a double fine wire spark igniter driven by a high- energy (93 mJ) inductive coil.

For ACIS, the NRPD generated high-energy streamers led to faster early flame development due to the larger initial discharge volume. However, ignition timing advance for lean and dilute mixtures was limited by arcing propensity to the injector tip from the closest prong due to the lower in-cylinder density and the time of discharge. Conversely, ignition for the BDI occurred at the interface between the insulator surface and ground where the electric field strengths are strongest. Given the separation between the nearest ground surface and the power electrode by the insulator, arcing into the combustion chamber was never observed. Relative to the inductive coil spark igniter, both BDI and ACIS featured much more repetitive positioning of the early flame kernel. Finally, for NRNS, primary energy utilization was lowest at all equivalence or dilution ratios, while both the lean and dilute limit extension was greatest with this igniter.