Energy-efficient transient plasma ignition
(with Prof. Martin Gundersen, USC Dept. of Electrical Engineering - Electrophysics)
Supported by the U. S. Air Force Office of Scientific Research and U.S. Department of Energy
The electric arc has been the ignition source of choice for most types of propulsion and automotive combustion engines for over 100 years. It has many advantages including simplicity, low cost, size and weight of the electronics, and it produces sufficiently high temperatures to dissociate and partially ionize most fuel and oxidant molecules. Nevertheless, there are also numerous disadvantages of arc discharges, including the limited size of the discharge, the necessity for supporting electrodes that may interfere with the flow or combustion process, and the low "wall-plug" efficiency (i.e. ratio of energy deposited in the gas to the electrical energy consumed in producing the discharge.) For these reasons, many investigations of ignition of deflagrations and detonations by alternate energy sources such as lasers have been conducted in recent years. Still, laser ignition sources present many practical difficulties, especially the need for reliable optical access, extremely low wall-plug efficiency, and extremely high optical intensities needed to induced breakdown in the gas which in turn makes it difficult to control the location and intensity of the discharge.
The subject of this investigation is the use of corona discharges (the portion of an electric discharge before the onset of the low-voltage, high current arc discharge) for the initiation of combustion in propulsion systems and internal combustion engines. The corona discharge is basically a plasma that is in a transient, formative phase. Corona discharges have the potential to overcome many of these limitations of conventional electric discharges and laser discharge for reasons that include: (1) there is better coupling into gas because the cross-section for dissociation and ionization more nearly matches the electron energy distribution function; (2) there are lower losses through lower radiation, lower anode and cathode losses, and lower gasdynamic disturbance formation; (3) there are many streamers, each of which has a similar energy content, as opposed to a single, unnecessarily large and intense arc, which in turn can initiate combustion in a larger volume and (4) the size and shape of the ignition volume can be tailored using the geometry of the anode and cathode. With recent advances in pulsed power electronics, such discharges can be produced with very high wall-plug efficiencies in a system of reasonable cost, size and weight. Prof. Martin Gundersen of the USC Department of Electrical Engineering - Electrophysics has developed energy-efficient corona discharge systems that will be used for these investigations.
Recent highlights include the first testing of corona ignition in an internal combustion engine. Results were very promising; indicated efficiencies were consistently 15 – 20% higher than spark ignition under identical operating conditions and burn rates were typically twice as fast with corona as spark ignition. See powerpoint presentation below for more details.
View powerpoint presentation about corona ignition in general (more detailed than the description below)
Wang, F., Liu, J. B., Sinibaldi, J., Brophy, C., Kuthi, A., Jiang, C., Ronney, P. D., Gundersen, M. A., "Transient Plasma Ignition of Quiescent and Flowing Fuel Mixtures, " IEEE Transactions on Plasma Science, Vol. 33, pp. 844 – 849 (2005). Download .pdf version from IEEE website
Liu, J. B., Wang, F., Li, G., Kuthi, A., Gutmark, E. J., Ronney, P. D., Gundersen, M. A., "Transient plasma ignition," IEEE Transactions on Plasma Science, Vol. 33, pp. 326-327 (2005). Download .pdf version from IEEE website
Despite the potential advantages, there have been no systematic studies of the initiation of deflagrations or detonations using corona discharges. Preliminary experiments on the ignition of quiescent CH4-air mixtures at 1 atm total pressure have recently been obtained in our laboratory. To our knowledge these are the first data on flame ignition by corona discharge sources. Figure 1 shows a block diagram of the experimental apparatus for quiescent tests. It consists of the corona discharge generator system and a test cylinder for introducing fuel, and studying initiation of combustion. The 5.1 cm diameter test cylinder has gas inlets, outlet and vacuum pump inlet in one end plate, high-accuracy pressure gauge for measuring the partial pressures of the reactants, and a fast-response pressure transducer for use during the combustion experiments.
Figure 1. Schematic diagram of corona discharge ignition system for combustion experiments
Figure 2 shows sequential images of a flame in a very lean mixture ignited using the corona discharge. It can be see that the corona discharge ignites a cylindrical volume whose diameter is more than half of the combustion chamber diameter in a very short period of time. The ignition region surrounds the central cathode, where the streamers are more closely spaced. The effect of the chamber diameter will assessed to determine whether this initial flame kernel diameter is determined mostly by the discharge or the physical size of the chamber.
Figure 2. Sequential photos (33 ms between images) of axial view of corona discharge ignition of a 6.5% CH4-air mixture at 1 atm. Diameter of chamber is 5.1 cm.
Figure 3 shows the energy deposited in the gas as a function of the corona power supply voltage. For sufficiently low voltages (< 8 kV), no discharge can be initiated. Above this voltage, the energy deposited increases rapidly with increasing voltage. Thus, it is possible to specify a particular amount energy deposition depending on the application. An order of magnitude range of energy deposition is shown in Fig. 3. Figure 4 shows the ignition delay time (time lapse between the discharge and the pressure reaching 10% of the peak pressure in a constant-volume chamber) as a function of the discharge energy. It can be seen that there is an "optimal" energy of about 200 mJ for this case, below which the delay time increases rapidly, and above which the delay is nearly constant. Thus, there is little motivation to increase the energy above this optimal value. This behavior was seen for all mixtures tested in our preliminary experiments. As expected, the optimal energy was found to be higher for leaner mixtures.
Figure 3. Energy deposition vs supply voltage for corona discharge sources
Figure 4. Combustion rise time as a function of pulse energy for CH4-air mixtures at 1 atm showing presence of "optimal" energy (~ 200 mJ in this case).
Figure 5 shows the ignition delay time and Figure 6 shows the combustion rise time (time lapse between the pressure reaching 10% and 90% of the peak pressure), both as a function of equivalence ratio. For all corona ignition cases shown, the "optimal" energy was used. Also shown are corresponding results for an arc discharge (~ 70 mJ) at different locations within the chamber. These figures show the most significant finding of the preliminary experiments - the corona discharge leads to much more rapid combustion (by about a factor of 3) than an arc discharge for all mixtures tested, even for the most advantageous arc discharge location.
Figure 5. Ignition delay time as a function of equivalence ratio for CH4-air mixtures at 1 atm for arc and corona ignition sources having "optimal" energy (see text).
Figure 6. Combustion rise time as a function of equivalence ratio for CH4-air mixtures at 1 atm for arc and corona ignition sources having "optimal" energy (see text).
There are many factors that may contribute to the advantageous results of the corona discharge. Clearly the geometrical advantage of the corona is present (many optimal streamers vs. one unnecessarily large and intense arc.) Still, it is expected that the performance of the corona ignition system would be superior to an arc discharge system having several independent discharges because the corona streamers are distributed throughout the annular region between the coaxial electrodes whereas the arcs would be restricted to the regions between their respective electrode pairs. Moreover, these multiple electrode pairs would certainly yield greater heat and radical losses and would be a less energetically efficient means of ignition.
This more rapid combustion with corona discharges can be exploited in a number of ways. In the case of pulse detonation engines, it could lead to smaller, lighter engines with higher specific impulse. For premixed lean-burn gas turbines, it could provide the needed acceleration and stabilization of combustion to make such devices practical in a wider range of applications. Moreover, even for mixtures that are not especially lean, the corona discharge decreases the time required for combustion to occur in a given volume, which means that the residence time of reactants in the high-temperature region of the combustor can be decreased, which in turn leads to lower thermal NOx formation.