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A microwave-enhanced plasma generation technique was combined with laser-induced ignition to improve ignition characteristics. A locally intensified microwave field was formed near the laser-induced breakdown plasma. As the plasma absorbed the microwaves, the plasma emission intensity increased. The plasma lifetime could be controlled by changing the microwave oscillation duration. Furthermore, the microwave-enhanced laser-induced breakdown plasma improved the minimum ignition energy of the methane/air pre-mixture with just a small amount of absorbed microwave energy.
© 2013 Optical Society of America
1. Introduction
The requirement to achieve higher thermal efficiencies for internal combustion engines has been increasing. Dilution of the mixture with air and use of exhaust gas recirculation (EGR) are among the solutions that have been used to improve the fuel economy of spark-ignition engines. However, under high-dilution conditions, the ignition of the mixture becomes unstable, and the cyclic fluctuations of the combustion increase. Thus, several innovative ignition technologies have been developed to improve ignition stability under these conditions [1–12]. Laser-induced ignition is one of these technologies [4–7]. It is free from heat loss to electrodes or abrasion of electrodes owing to its non-intrusive characteristics. Furthermore, laser-induced ignition enables ignition of leaner premixed gas than does spark-plug ignition due to its higher temporal energy density. The use of laser-induced plasma provides a means of controlling energy density by changing the laser power, but the plasma lifetime cannot be controlled.
Microwave-enhanced plasma ignition has been developed to improve the ignition stability of spark-ignition engines [8–12]. The plasma source is a standard spark discharge generated by a spark plug, but a locally intensified microwave field is formed near the spark gap. When the microwaves radiate into the plasma, the electrons are accelerated by the electric-field component of the waves. The electron temperature is increased, and non-thermal plasma is generated. Microwave-enhanced plasma has a high potential to instigate chemical reactions that cannot progress under normal environmental conditions because it contains high energy electrons and active radicals. For example, OH radicals, which have a very high oxidization potential, are generated by the collision of high-energy electrons and H2O molecules. High levels of OH radicals measured spectroscopically during the microwave discharge event suggest that electron-impact reactions with H2O molecules in the microwave plasma increase the pool of oxidizing radicals, allowing enhancement of the early stages of combustion through chemical effects [8].
Several studies have combined laser-induced plasma and microwave for igniting the methane/air mixture and applying to laser-induced breakdown spectroscopy [13–15]. Michael et al. investigated the ignition in methane/air mixtures using low energy seed laser pulses and an overlapping subcritical microwave pulse [13]. The rectangular microwave resonator was used as a chamber. The heating effect of microwave was investigated experimentally and numerically and the potential was showed to improve performance of high speed combustors.
In this study, a microwave-enhanced plasma generation technique was combined with laser-induced ignition to investigate the ignition process in detail. Laser-induced breakdown was used as the source of microwave-enhanced plasma instead of a spark discharge. Microwaves were oscillated and radiated in the cylindrical combustion chamber with antenna to create microwave-enhanced laser-induced breakdown plasma and to expand the plasma temporally and spatially. This method can be used to control the plasma size and extend the plasma lifetime [16]. The effects of the microwaves on the plasma emission intensity and ignition limit of a methane/air premixed mixture were investigated.
2. Experimental setup
Figure 1 shows a schematic diagram of the experimental apparatus used for laser-induced ignition. The cylindrical combustion chamber was 60 mm in diameter and 30 mm in width. It contained three quartz windows; one was used for optical measurements, and the other two were intended for laser incidence and transmission measurement. All of the windows had an antireflection coating. The occurrence of ignition and history of combustion were recorded by a pressure sensor at the bottom of the combustion chamber. A second harmonic Nd:YAG laser (Spectra Physics, Inc., PIV-200, λYAG = 532 nm) was used as the laser source. Oscillations were controlled using a pulse-delay generator. The pulse duration of the Nd:YAG laser was 5 ns, and the initial beam diameter was 8 mm. The laser pulse energy was controlled by a combination of a half-wavelength plate and a polarization beam splitter to avoid spatial or temporal changes. The oscillating Nd:YAG laser produced a linear polarized laser pulse. The half-wavelength plate rotated through the polarization plane of the laser; the horizontal polarization beam component was transmitted through the polarization beam splitter while the vertical polarization beam component was reflected. Therefore, this system could control the power of the laser pulse without changing its spatial or temporal profile. Two pyroelectric power meters were used to detect the laser pulse energy: the incident laser energy was measured by one power meter, and the transmitted laser energy was measured simultaneously by the other. The focus was set to the center of the combustion chamber using a flat-convex lens with a focal length of 100 mm.
Figure 2 shows a schematic diagram of the microwave system and antenna. Microwaves were generated from a semiconductor device (720 W) with frequency of 2.45 GHz and transmitted by a co-axial cable. A three-stub tuner was used for impedance matching. A directional coupler and diode detectors were installed in the microwave transmission line to measure the input and reflected microwave power. The antenna inside the combustion chamber was 3 mm in diameter, with a tapered tip configuration to form a locally intensified microwave field. The plasma source was generated by focusing the laser pulse at a local point and creating a highly intensified electric field by concentrating the microwave radiation. The position of the antenna tip was 0.5 mm away from the laser breakdown point.
The plasma emission intensity was measured through the optical window. The plasma emission was collected with an optical fiber, and the spectrum was measured by a spectrometer (Imagineering, Inc., SPB-2000) with a dichroic mirror, band pass filter, and photomultiplier. Images of the microwave-enhanced plasma were obtained from a high-speed camera (Shimadzu Corporation, HPV-1).
3. Experimental results
3.1 Microwave-enhanced laser-induced plasma
Figure 3 shows the time history of the plasma emission at 308 nm. Microwave-enhanced plasma generates OH radical and remarkable spectrum of OH radical was observed around 308 nm [16]. OH radical has very high oxidization potential and important role to combustion process [17]. Therefore, the emission intensity at 308 nm was measured with high temporal resolution. The ambient conditions were atmospheric pressure and room temperature. The plasma emissions obtained with and without microwaves were compared. Without microwaves, the plasma emission intensity increased suddenly at the point of laser breakdown and then decreased immediately. Normally, laser-induced breakdown plasma has high energy density at breakdown, but the plasma cools after laser termination. With microwaves, the microwaves oscillated after breakdown with oscillation duration of 500 μs. The plasma emission was observed throughout the microwave oscillations as shown in Fig. 3(a). Therefore, the plasma lifetime could be controlled by altering the duration of the microwave oscillations. The plasma emission intensity decreased just after breakdown but gradually increased after 20 μs as shown in Fig. 3(b). As laser-induced plasma absorbed the microwaves, the plasma emission intensity increased.
Fig. 3 Plasma emission intensity at 308 nm with and without microwave enhancement. (a) Whole duration of microwave oscillation. (b) Just after breakdown (t = 0~50 μs).
Figure 4 shows plasma images taken by the high-speed camera at a frame speed of 500 kFPS. At the point of laser breakdown (t = 0 μs), strong plasma emission caused by the laser-induced breakdown was observed. The plasma size diminished at 10 μs, but increased again after 20 μs due to the growth of microwave-enhanced plasma.
Figure 5 shows the input and reflection microwave power together with the plasma emission intensity. In Fig. 5(a), the input microwave power was almost constant at 720 W, but the reflected microwave power decreased and reached zero at 400 μs. The decrease in the reflected microwave power was due to the increase in the absorption microwave power from the plasma. Some loss of microwave transmission was indicated in Fig. 5(a). The microwave energy absorbed by the plasma, indicated by the blue region in Fig. 5(a), was calculated by subtracting the microwave transmission and reflection losses from the input microwave power. The microwave power absorbed by the plasma is indicated in Fig. 5(b). The plasma emission intensity increased with the absorbed microwave energy.
Fig. 5 Plasma emission intensity and microwave power. (a) Input and reflection microwave power. (b) Absorbed microwave power by plasma.
3.2 Minimum ignition energy improvement by microwaves
The effect of microwaves on the minimum ignition energy was investigated. The mixture was a methane/air pre-mixture with a 1.0 equivalence ratio. The initial pressure before ignition was 0.1 MPa. First, the breakdown threshold level and minimum ignition energy without microwaves were investigated. Figure 6 shows the relationship between the incident and transmitted laser energy. When the incident laser energy was less than 5 mJ, the transmitted laser energy was equal to the incident energy, and breakdown did not occur. However, when the incident laser energy was more than 5 mJ, breakdown occurred, and the transmitted laser energy was less than the incident energy due to the laser energy absorbed by the plasma. The minimum ignition energy was around 8 mJ. Here, the ignition success was determined by the existence of pressure rise in the combustion chamber. When the input laser energy was between 5 and 8 mJ, laser-induced breakdown occurred, but the plasma could not ignite the mixture.
The incident laser energy was fixed at 6.6 mJ to investigate the effect of microwaves on the minimum ignition energy. This incident laser energy was less than the minimum ignition energy without microwaves. The ignition success rate was examined by changing the microwave oscillation duration. Figure 7(a) shows the effect of the microwave oscillation duration on the ignition success rate. Without microwaves (microwave oscillation duration = 0 μs), the ignition success rate was 20%. As the microwave oscillation duration was increased, the ignition success rate also increased and reached 100% at 20-μs microwave oscillation duration. The microwave oscillation duration was converted to absorbed microwave energy, and the results are plotted in Fig. 7(b). The ignition success rate reached 100% with 0.5-mJ absorbed microwave energy. Figure 8 shows the effect of microwave-enhanced laser-induced breakdown plasma on the minimum ignition energy. This figure was made by integrating the results of Fig. 7(b) in Fig. 6. The minimum ignition energy was reduced by 0.5 mJ by the absorbed microwave energy.
Fig. 7 Relationship between microwave oscillation and ignition success rate. (a) Microwave oscillation duration. (b) Microwave absorbed energy.
Summary
Microwaves were applied to laser-induced breakdown plasma to enhance the plasma and improve the minimum ignition energy. Laser-induced breakdown plasma was generated by a pulsed Nd:YAG laser. Locally intensified microwaves formed near the laser-induced breakdown plasma. As the plasma absorbed the microwaves and increased in size, the plasma emission intensity also increased. The laser-induced plasma decayed rapidly after termination of the laser pulse, regardless of the microwaves used. However, when the decaying plasma absorbed microwave energy, it started to grow again. The plasma lifetime was controllable by changing the microwave oscillations. The effect of the microwave-enhanced laser-induced plasma on the minimum ignition energy of methane/air pre-mixture was investigated. The equivalence ratio of the mixture was 1.0, and the initial pressure before combustion was 1.0 MPa. The minimum ignition energy was improved by microwave oscillations with only 0.5 mJ of absorbed microwave energy.
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