Flame initiation by nanosecond plasma discharges:
Putting some new spark into ignition
The transient plasma that occurs during the formative phase (typically 50 nanoseconds) of conventional spark discharges is comprised primarily of spatially-distributed streamers prior to the formation of a single intense arc with equilibrated electron energy distribution. The use of non-thermal transient plasma ignition (TPI) was investigated as an alternative to spark ignition (SI) for applications including internal combustion engines and pulse detonation engines (PDEs). Bench tests in quiescent and turbulent constant-volume combustion chambers showed typically 3x shorter ignition delays and pressure rise times with TPI than with SI. These benefits were shown to be a result of both the geometrical advantages of TPI, namely the multiple ignition sites, as well as inherent chemical effects. Testing on a 2.5 liter 4-cylinder engine showed typically 15 - 20% increases in indicated mean effective pressure and much shorter burn durations at identical operating conditions. Moreover, the tradeoff between thermal efficiency and brake specific NOx emissions was found to improve with TPI. It is proposed that these advantages of TPI may be exploited by either (1) the use of leaner fuel-air ratios or (2) by designing engines with lower turbulence levels, thereby reducing heat loss to cylinder walls and increasing thermal efficiency, and employing TPI to obtain sufficiently rapid burning. Reduction in the time for deflagration to detonation transition by a factor of 4 in laboratory-scale PDEs was observed, thus TPI may prove to be an enabling technology for this application.