Pulsed nanosecond discharges (PND) can achieve ignition in internal combustion engines through enhanced reaction kinetics as a result of elevated electron energies without the associated increases in translational gas temperature that cause electrode erosion. Atomic oxygen (O), including its electronically excited states, is thought to be a key species in promoting low-temperature ignition. In this paper, high-voltage (17-24 kV peak) PND are examined in oxygen/nitrogen/carbon dioxide/water mixtures at engine-relevant densities (up to 9.1 kg/m3) through pressure-rise calorimetry and direct imaging of excited-state O-atom and molecular nitrogen (N2) in an optically accessible spark calorimeter, with the anode/cathode gap distance set to 5 mm or with an anode-only configuration (DC corona). The conversion efficiency of pulse electrical energy into thermal energy was measured for PND with secondary streamer breakdown (SSB) and similar low-temperature plasmas (LTP) without. The calorimetry measurements confirm that, similar to inductive spark discharges, SSB discharges promote ignition by increasing the local gas temperature. LTP discharges, on the other hand, had very little local gas heating, with electrical-to-thermal energy conversion efficiencies of ~1% at 9 bar. Instead, LTP discharges were found to generate substantial electronically-excited O-atom populations at lower pressures, but the observed image intensity decreased rapidly as the initial pressure was increased. The observed O-atom emission peaked ~20 ns after the start of the pulse and was concentrated near the anode and cathode tips, indicating that the presence of the cathode was beneficial for increasing radical production (although the likelihood of SSB increased). Decreasing oxygen and increasing carbon dioxide concentrations were found to reduce the observed image intensity, but had minimal impact on SSB probability and electrical-to-thermal conversion efficiency. The impact of changes in collisional quenching and the electron energy distribution on image intensity were evaluated.
