Flame initiation by
nanosecond plasma discharges:
Putting some new spark
into ignition
Abstract
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.