Plasma Jet Ignition

[Robert Harris]

More Heywood Factoids - chapter 9.5

All spark ignition is divided into three stages:  Breakdown, arc and glow.
Breakdown is when the spark is first established, arc is after the plasma
explosion and glow is the termination of spark.

The breakdown stage is where the plasma column is generated. Its characterizes
by high (> 10 KV) voltage, high peak current (~200 A) and short duration (~10
ns ) and is about 40 micro m in diameter.  All energy is transferred without
loss to this column.  The temperature and pressure rise extremely rapidly in
this column to about 60,000 k and a few hundred atmospheres.  A strong shock
wave or blast wave propagates outward, the channel expands and in consequence,
the plasma temperature falls.  Typically, 30% of the plasma energy is carried
away by the shock wave: however most of this is regained since spherical blast
waves transfer most of their energy to the gas within a small sphere (~ 2mm)
into which the breakdown plasma soon expands.

The breakdown phase ends when a hot spot develops on the cathode and the
column degrades to an arc.

The arc phase voltage is low (< 100v), though the current can be as high as
the external circuit permits.  In contrast to the breakdown phase where the
gas in the channel is fully disassociated and ionized, in the arc phase the
degrees of disassociation may still be high at the center of the discharge,
but the degree of ionization is much lower (~ 1%).  Voltage drops at the
cathode and anode electrodes  are a significant fraction of the arc voltage
and the energy deposited in these electrode sheath regions, which is conducted
away by the metal electrodes, is a substantial fraction of the total energy.

About 94% of the spark energy  is converted to plasma in the breakdown phase
vs only 50% in the arc stage - plus the arc stage is only ~6000K.

About .2mj of energy is required to ignite a quiescent stoichiometric fuel-air
mixture at normal engine conditions.  Swirl, turbulence, etc raise this number
by an order of magnitude in a working normal engine.

Conventional coil type ignitions deliver 30 to 50 MJ, but a plasma jet
typically requires 1 joule or more stored energy and this is released in  < 20
microseconds.   

There is a race condition - to pump as much energy as fast as possible into
the plasma column after it is established and before the plasma blast so as to
maximize the blast effect.  Before the blast and cathode hot spotting occurs,
the plasma column has almost infinite conductance.  Since greater than 90% of
the pumped energy will convert to plasma at sun like temperatures in the
breakdown stage, its critical to pump it early.

The blast wave from the plasma chamber into the main chamber is comparable to
trying to light a pool of gasoline with a match ( conventional ) or a blow
torch ( Plasma Jet )  

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