Charles Cathey

Nanosecond Plasma Ignition for Improved Performance of an Internal Combustion Engine

Transient plasma, or plasma during a formative true nonequilibrated phase, is studied as an ignition methodology in comparison with traditional spark ignition (2.5-3 ms and 80 mJ) in a single-cylinder gasoline internal combustion engine. Transient plasmas were generated by applying high-voltage pulses that had comparable energy but that were applied for times that were three to four orders of magnitude shorter-85-ns 60-mJ and 20-ns 57-mJ pulses. These created volume-distributed arrays of streamers, which produced electronically excited species during nanosecond time scales. Reductions in ignition delay, higher peak pressure, and increased net heat release ratio relative to conventional spark ignition were observed in these studies. Transient plasma ignition is demonstrated to initiate combustion rapidly, approaching an ideal constant volume cycle; has potential for improving lean combustion operation; is energy efficient; and is potentially useful for gasoline engine emissions reduction.

Effects of Corona, Spark and Surface Discharges on Ignition Delay and Deflagration-to-Detonation Times in Pulsed Detonation Engines (Postprint)

Abstract : The purpose of the research described herein is to compare the ignition delays in an experimental pulsed detonation engine produced by thermal and non-thermal ignitions. The commercial thermal ignition has a pulse duration of about 1 microsec, whereas the non-thermal ignitions have pulse durations of 100 nanosec. Ignition delay is an important factor, along with fill and purge times, that limit the maximum repetition rate and thrust of pulsed detonation engines. For stoichiometric fuel-air mixtures with aviation gasoline at 1 atmosphere and 360 - 480 K, an ignition delay of 6 millisec was observed with a non-thermal ignition, whereas the ignition delay was 11 millisec with an aftermarket automotive ignition. By replacing the resistive cable and resistor of the aftermarket ignition with a non-resistive cable and surface discharge igniter, its ignition delay was reduced to 7 millisec, which is comparable to that produced by the non-thermal ignitions.

Investigation of Flow Field Properties on Detonation Initiation

The ignition characteristics for ethylene/air mixtures have been explored for varying refresh flow conditions in a simple pulse detonation combustor configuration. A 7.62 diameter combustor with four inlet arms 90 degrees to the combustor access was used to evaluate the effects of flow uniformity, turbulence, temperature, and wall spiral size on the ignition and detonation of ethylene/air mixtures. The effect of creating a more uniform flow field through the use of turbulence-generating screens demonstrated mixed results. The baseline configuration, without a turbulence screen installed, produced large recirculation zones which supported the initiation of high aggregate mass flow rates, but had longer ignition delay times. The 3.175 mm-hole turbulence screen generated the smallest turbulence scales and also produced the shortest ignition delay times, but could not support the reliable ignition of mass flow rates above 0.30 kg/s. The use of two turbulence-producing spirals resulted in detonation initiation for most conditions, but the 6.35 mm spiral produced detonations in shorter distances than the 3.175 mm spiral and was determine to be the preferred spiral. The total pressure loss associated with both spirals was determined and the total pressure loss for the 3.175 mm spiral was found to be approximately 30% of the value for the 6.35 mm spiral. The 6.35 mm diameter spiral had the best detonation initiation performance, but with the associated higher flow loss. A factor of two reduction in ignition delay and deflagration-to-detonation timescales was observed over an initial reactant temperature range of 300K to 480 K. This result directly benefits the development of PDC systems with elevated inlet temperature conditions. The substantial reduction will allow for a substantial increase in operating cycle frequency for these systems and should be applicable for supersonic and hybrid PDC applications.

Applications of Power Modulator Technology to Ignition and Combustion

We report recent studies of applications of power modulator technology to ignition and combustion at USC, in collaboration with the Naval Postgraduate School, the Air Force Research Laboratory at Wright Patterson Air Force Base, and the Nissan Research Center. Transient plasma is under investigation as a technology for ignition of pulse detonation engines and other applications. It is attractive as an ignition source for most engines because of its ability to reduce ignition delay over broad ranges of temperature and pressure and combust leaner mixtures. Transient plasma is generated by applying high voltage nanosecond pulses that create volume-distributed arrays of streamers, which produce electronically excited species during nanosecond time scales. It has been demonstrated to be an energy efficient means of NOx reduction for engine effluent, and has been applied effectively to ignition of pulsed detonation engines (PDE) and more recently extended to internal combustion engines. Several power modulator architectures for ignition applications are discussed, including a magnetic compression based solid state opening switch (SOS) pulse generator. The use of a solid state switch is compared with pseudospark or thyratron based architectures previously used. The pulse amplitude required depends on the exact geometry of the ignition chamber, combustor and corona electrode, but typical voltages used in our experiments are 50 to 70 kV with pulse energies on the order of 100 mJ to 1 J and pulse lengths from 17 to 150 ns.

Transient Plasma Induced Production of OH and its Effects on Ignition in Atmospheric CH4-AIR Quiescent Mixtures (Postprint)

Abstract : Transient plasma from a 60 kV, 70 ns pulse induced OH production in air and CH4/air quiescent mixtures inside a cylindrical chamber is analyzed. The resulting OH from the plasma discharge, ignition, and subsequent combustion is analyzed using planar laser induced fluorescence. A high-framing-rate camera was also used to image ignition and flame propagation in the chamber, providing spatial and temporal resolution over the entire combustion event. Results indicate OH structures produced during the discharge in humid, ambient air are less branched, thicker, and last longer when compared to structures in CH4/dry-air. Transient plasma successfully ignited the CH4/air mixture, populating the discharge volume with radicals. Mean OH number densities produced by the discharge were found to decay within 100 s of the plasma. Ignition under these conditions was found to occur approximately 1 ms after the discharge along the anode, creating multiple ignition kernels whose proximity to the anode is consistent with the region of highest field and, thus, maximum radical density.

Enhanced OH Chemiluminescent Emission from Transient Plasma Ignited Methane-Air Mixtures Relative to Spark Ignition

Summary form only given. Ignition phenomena in gaseous CH4-air mixtures from transient plasma and spark discharges were studied using optical techniques. It was found that transient plasmas, composed of pulsed discharges persisting for times shorter than the plasma formative phase in gas mixtures, produced shorter ignition delay times - a critical factor in applications such as pulsed detonation engines. 1. The transient plasma is generated via 66 kV, 70 ns FWHM pulses with pulse energies of the order 800 mJ/pulse. The free radical OH is present in many chain propagation and branching reactions, therefore is indicative of the intensities of chemical reaction processes before and after ignition. CH4-air combustion tests were performed at 1 atm, at equivalence ratios of phi =1 and phi =1.2, and chemiluminescent emission of OH at 308 nm was recorded. Emission intensity after ignition, induced by transient plasma, was typically a factor of 5-8 greater than by capacitive spark discharge. In addition, before ignition a large OH signature was present immediately after the application of the transient plasma, whereas there was none detected when using a spark. Associated pressure curves were also compared: the transient plasma case had larger peak pressures, with shorter ignition delays consistent with previous data. 2. Additionally a PicoStar ICCD was used to obtain images of streamer propagation, channel formation, and spark formation. The imaging of the streamers and the strong OH signal indicates that transient plasma in the tested geometry yielded a volumetric ignition process populating the discharge volume with reactive free radicals. Physics and chemistry issues resulting from the combination of these effects, volumetric ignition and direct production of free radicals, will be discussed.

Pseudospark-Based Power Modulator Technology for Transient Plasma Ignition

Studies of power modulator technology for transient plasma generation for pulse detonation engine ignition are reported. Transient plasma ignition can significantly reduce delay times in static and flowing systems. The igniter operates reliably in repetitive modes with pulse length down to 50 ns, and in some situations, with less electromagnetic interference than conventional spark ignition. Pseudospark-based line type pulse generation is described in this report. Research directed toward more compact versions of the pulse generation technology, including investigations of miniaturized back-lighted thyratron switches, is briefly discussed. During the previous year, experiments have been conducted in collaboration with researchers at the Naval Postgraduate School, and Wright Patterson Air Force Research Laboratory, testing ignition of pulse detonation engines (PDE). These experiments demonstrated considerable (factors of 2 to 5) reduction in delay to detonation, and improvement in repetition rate, while retaining low energy cost. The results suggest a potential solution to one of the most serious limitations to the development of PDEs. Desired pulse amplitude depends on the exact geometry of the combustor, ignition chamber, as well as corona electrode. Typical voltages employed are 50 kV - 70 kV for these studies, with pulse energies of the order 100 mJ to 1 J. The pulse generator employed a pseudospark switch. The principles of the pulse generator reported here are described in the paper submitted for this meeting last year. It is designed to deliver pulses of Vp=90 kV peak amplitude and taup=50 ns duration