Results

It is well understood that combusting under a fuel-lean condition can result in higher thermal efficiency, better fuel economy, and less NOx production. In an ideal internal combustion engine (ICE), constant volume combustion occurs at the minimum cylinder volume under a high compression ratio. With spark ignition, lean combustion does not occur rapidly enough to approach this ideal constant volume cycle, as flame temperature is lower, and reactions speed is slower. Both the lean burn capability and the volumetric effect of the transient plasma are of interest for increasing fuel economy and potential for reduction of NOx production and emission.
The experimental setup is comprised of a single cylinder gasoline test engine (Bore: 93mm, Stroke: 81.4mm, Compression ratio: 12), a control and analysis station, a pulse generator and a HV supply. Pulsed excitation was generated with a pseudospark switched lumped Blumlein that created 85 ns, 44 kV, 60 mJ pulses, and a solid state opening switch (SOS) pulse generator that created 20 ns, 52 kV, 57.2 mJ pulses. Both generated pulses that are considerably shorter than traditional spark ignition (typ. 2 ms).

In this experiment it was found that by using transient plasma ignition the ignition delay period, which is an index of performance in a conventional gasoline engine, can be improved relative to a spark plug. Figure 8 illustrates images taken in 200 µs intervals with a high speed camera showed the transient plasma generated flame speed is 200- 400 µs faster than that of a spark. In frames 1 of the transient plasma ignited case, ignition is seen to occur along the center electrode. It is interesting to note that transient plasma induced ignition does not occur over the entire discharge volume, but instead is initiated solely along the anode, which is consistent with the WPAFB experiment results.

Additionally, the potential utility of the SOS type pulse generator was shown as an ignition source for automobile engines. By reducing the pulse length from 85 ns to 20 ns, we were able to increase the operating pulse voltage from 44 kV to 52 kV. The 20 ns pulse allowed a substantial increase in the applied electric field while holding off spark breakdown. The mean electron energy scales with reduced electric field. The higher applied voltage of the 20 ns pulse increased the reduced electric field and thus the number of high energy electrons capable of dissociation and ionization, which then increased number of free radicals that initially seed the discharge volume. Peak pressure achieved with the 20 ns pulse, was 20% greater than that from spark gap, indicating a larger net heat release ratio. This result correlates well with previous work done, and is indicative of reduced heat loss to the walls. A possible cause is the large number of ignition kernels generated by the transient plasma.

In addition to the shorter pulse, the inherent architecture of the SOS pulse generator makes it attractive for ignition applications. The use of a solid state switch eliminates the needed overhead associated with pseudospark or thyratron, such as pre-heating to determine the gas pressure in the pseudospark, and heating of cathode in a thyratron. While the SOS pulse generator was successful used as an ignition source, further pulse generator development is needed to make it a practical ignition system. A potential solution for multi-cylinder operation is to have compression stages for each cylinder and position them as close to the load as possible, thus minimizing any voltage drop across the transmission line. A distributing mechanism could then be used to switch the relatively slow low voltage pulse to the appropriate cylinder and its associated compression stages. This methodology attempts to bypass the problems of rapidly switching high voltages, and the transmission line losses associated with fast pulses.
The results indicate that transient plasma ignition is a viable ignition methodology with current engine design, and suggest that by going to a shorter pulse and overvolting the gap to a greater extent will further extend stable lean combustion while reducing the total amount of heat lost to the walls.

Conclusions

Transient plasma has consistently shown reductions in ignition delay and increased lean burn capability relative to traditional spark ignition. However, there has been relatively little work attempting to explain the physical mechanism behind these effects. An OH PLIF experiment was performed and calibrated such that OH streamer-induced production was confirmed over the streamer volume at peak OH number densities of ~4x1014 molecules-cm-3. Additionally, it was determined that OH produced by the transient plasma decays to negligible values within 100 µs of the applied pulse.
Transient plasma ignition is on the cutting edge of ignition methodologies for combustion engines. The PDE community in particular is interested in this technology for several reasons: 1) the capability to ignite in a manner that achieves detonation in hydrocarbon-air mixtures (without a previously required oxygen supplement); 2) the extension of the lower flammability limit of mixture and 3) the capability to reduce ignition delay times by factors of 2 to 9 offers comparable potential for increase in repetition rates. For these reasons transient plasma has the potential to overcome the traditional capacitive and inductive spark discharge, and laser discharge ignition techniques.
C. Brophy and J. Sinibaldi are with the Naval Postgraduate School, and C. Carter and M. Ryan are with the WPAFRL at Dayton OH.

Acknowledgment
We thank H. Wang, R. Hanson, J. Jefferies, T. Urushihara, and T. Shiraishi for valuable discussions.