Left: transient plasma with custom plug and 12ns generator. Right: Arc with spark plug and automobile ignition coil. Photo credit: Dan Singleton
Comparison of pressure traces between transient plasma ignition and traditional spark ignition in quiescent CH4-air mixture.
Left: transient plasma ignition flame front. Right: spark ignition flame front. Photo credit: Dan Singleton
Ignition delay comparison between two different transient plasma ignition systems and traditional spark ignition omn quiescent C2H4-air mixture.
Left: photograph showing ten 12 ns, 50 kV pulses in air. Right: ignition occurring at multiple locations after a single pulse (phi=1.1). Image from Singleton et al 2011.
Our work is an experimental study of non-equilibrium plasma in the transient,
formative phase of an arc, applied to ignition and combustion. Transient plasma generated by nanosecond
pulsed power as applied to ignition (henceforth called transient plasma ignition or TPI) has several
advantages over traditional spark ignition, consistently demonstrating reductions in ignition delay,
lean-burn capability, and the ability to ignite higher mass flow rates, resulting in improved efficiency
and reduced emissions from a variety of airborne engines, as well as internal combustion engines.
The approach to transient plasma ignition is different from traditional approaches
in that a short (typically < 100 ns) high-voltage pulse is used to initiate ignition,
rather than a many microsecond to several milisecond high-voltage pulse for spark ignition.
An arc from a traditional spark plug commonly used in both automobile and aircraft engines
is an equilibrated (thermal) or nearly equilibrated plasma. Ignition is achieved by local
heating of the gas, which increases the dissociation rate, and reactions of chain prolongation
and development. In a non-equilibrated (transient) plasma, the ionization process is dominated
by field-driven, energetic electrons impacting with "cold", non-excited atoms and molecules.
Significant energy goes into creating highly energetic electrons instead of heating of the gas.
An array of streamers (thin plasma channels) propagate across a gap by ionizing the gas in front
of their charged heads. The streamers branch throughout the volume, rather than being concentrated
at a single point like an arc, and have high energy electrons (up to 15 eV) in their heads.
These electrons produce excited species through impact dissociation, excitation, and ionization
of background gas molecules in the system, initiating combustion via electron impact, which is
a more efficient and faster process than via traditional thermal methods.
To realize this technology, it is critical to both reduce the size, weight,
and reliability of nanosecond pulsed power systems, and to understand the science of highly non-equilibrium
energy states that occur between matter and energy. The focus of our research has been both engineering more
compact and reliable TPI systems and applying them in experiments to improve the understanding of the physics
of the process.
Contact: Dan Singleton, Yung-Hsu Lin