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The behaviors of laser-induced plasma and fuel spray were investigated by visualizing images with an ultra-high-speed camera. Time-series images of laser-induced plasma in a transient spray were visualized using a high-speed color camera. The effects of a shockwave generated from the laser-induced plasma on the evaporated spray behavior were investigated. The interaction between a single droplet and the laser-induced plasma was investigated using a single droplet levitated by an ultrasonic levitator. Two main conclusions were drawn from these experiments: (1) the fuel droplets in the spray were dispersed by the shockwave generated from the laser-induced plasma; and (2) the plasma position may have shifted due to breakdown of the droplet surface and the lens effect of droplets.
© 2013 Optical Society of America
1. Introduction
Ignition characteristics fundamentally influence the combustion process in spark-ignition (SI) engines, especially direct-injection spark-ignition (DISI) engines [1, 2]. DISI engines are operated unthrottled at ultra-lean conditions by stratifying the charge and preparing a fuel-rich mixture around the spark plug. Wall-guided DISI (WG-DISI) engines, in which the interaction of the fuel spray with a piston crown leads to the desired fuel–air mixture near the spark plug, have been used as the first generation of commercial DISI engines. Attention has focused on spray-guided DISI (SG-DISI) engines to improve the thermal efficiency by using a fuel that is less wall wetting on the piston crown compared with wall-guided DISI engines [3–7]. The spray-guided system generates a stratified fuel concentration near the spark plugs because the fuel is aimed directly toward the plugs. Fuel spray can affect spark discharge, resulting in poor ignitability and cycle-to-cycle fluctuations in combustion for SG-DISI engines [6]. The position of the spark plug is sensitive to the combustion characteristics and mixture formation in SG-DISI engines [8]. Additionally, heat loss occurs from the spark plug on the cylinder head of an SI engine [9]. To overcome these drawbacks, research has focused on laser-induced plasma ignition. Laser-ignition devices do not affect the fuel spray, and the variable ignition position can be selected by adjusting the optical setting. Laser ignition device is not interfered with the fuel spray and it can select variable ignition position by adjusting the optical setting. These advantages make laser ignition well suited for ignition systems using a fuel spray.
In laser ignition, a flame kernel forms laser-induced plasma, which is generated by focusing a high-energy-density pulse laser on a combustible mixture. Laser ignition has several advantages over spark-plug ignition, including ignition point selection via optical settings, ignition energy control, decreased heat loss, and an interaction between the ground electrode of the spark plug and the spray via an ignition device [10–12]. The laser-induced breakdown process generated using a nanosecond pulse from a Q-switched Nd:YAG laser is described as follows.
- A multi-photon ionization or cascade breakdown generates an electron cascade process at sufficient levels of laser irradiance.
- High-temperature and high-pressure plasma is then generated.
- A shockwave expands around the plasma.
- The shockwave velocity decreases, quickly becoming subsonic.
- Flame ignition can occur. Radicals are generated as the plasma cools, followed by flame kernel formation, chemical branching phenomena, and a self-sustaining flame.
Previous reports have shown images and emission spectra of sparks produced by laser-induced breakdown in air [13]. Different ionization levels in the plasma kernels, which were observed using a high-spatial-resolution multi-fiber Cassegrain optics system, occurred during plasma formation and cooling and were observed at different locations within the plasma. This was due to the thickness of the plasma relative to the laser wavelength, which created different ionization levels and energy absorption rates throughout the plasma. The analysis demonstrated the validity of a laser-supported wave model during the first stages of laser-induced breakdown and illustrated the weak dependence of the plasma temperature on the input energy. Beduneau et al. showed images and emission spectra of sparks produced by laser-induced breakdown of methane and propane air mixtures [14]. The results provided information about the different stages of laser-induced breakdown, with a specific focus on the transition from a flame kernel to a self-sustaining flame. They also showed shockwave detachment and defined the timing of this phenomenon, which occurred before the point when the Taylor–Sedov formulation was appropriate for describing shockwave evolution (adiabatic expansion).
Considerable research has been conducted on laser ignition in gaseous mixtures. However, few studies have examined laser-induced plasma generation containing both gas and liquid phases, such as a fuel spray. Groß et al. investigated laser ignition in engines with a spray-guided combustion mode with strongly inhomogeneous fuel/air mixtures and concluded that reliable combustion initiation by a laser-pulse is more difficult to achieve due to the strong mixture fraction fluctuations at the location of laser-spark [11]. Pickett et al. showed laser ignition in diesel spray and discussed the fuel jet lift-off stabilization in the presence of laser-induced plasma ignition [15]. To apply laser ignition to transient fuel spray with high ignitability, further investigation is required to understand the dependence of the plasma position on the laser, the optimal laser focus point, the effect of the shockwave generated by plasma on spray formation, attenuation and scattering of laser energy due to droplets, and the minimum ignition energy under simplified condition, such as constant volume experiments.
In this study, the behaviors of laser-induced plasma and fuel spray were investigated by visualizing images with an ultra-high-speed camera. Iso-octane and ethanol were used as fuels. Time-series images of laser-induced plasma in a transient spray were visualized using a high-speed color camera. The effects of the shockwave generated from laser-induced plasma on evaporated spray behaviors are discussed. The interaction between a single droplet and laser-induced plasma was investigated using a single droplet levitated by an ultrasonic levitator [16].
2. Experimental apparatus and conditions
2.1 Experimental apparatus
Figure 1 shows the experimental apparatus. A two-hole fuel injector [17] was set on a constant-volume vessel, which had four optical windows. One of the two fuel sprays was injected vertically downward into the vessel. Iso-octane and ethanol were used as the fuels. Laser-induced plasma was generated in the injected fuel spray by focusing a nanosecond pulse of the second harmonic (wavelength λ = 532 nm) of a Q-switched Nd:YAG laser. Laser incident energy was controlled by a half-wavelength plate and a polarizing beam splitter, and the laser beam was focused by a convex lens with 100-mm focal length. Laser beam diameter at the lens focal point is ϕ8.5μm, and length is 260μm theoretically. The incident energy and transmitted energy were measured by energy meters. The transmitted energy was used to estimate the energy loss caused by attenuation and scattering. The state of the plasma was visualized by an ultra-high-speed color camera (nac Image Technology, Inc., ULTRA cam HS-106E), which can record images up to 1.25 Mfps (minimum exposure time of 100 ns). The injection pressure, Pi, was changed to observe the effects of flow velocity and droplet diameter on the plasma and fuel formation. Ambient gas pressure, Pa, was 0.1 MPa in all experiments, and ambient temperature was changed using an electric heater around the vessel.
2.2 Laser-ignition images
Figure 2 shows images of the laser-induced plasma in dry air and in a fuel spray. The images were used to investigate the effects of the interaction between the laser-induced plasma and the fuel spray. Iso-octane was used as the fuel. The energy value on the image describes the breakdown threshold (the energy required for breakdown) for each condition. In Fig. 2, the laser beam was applied from the right side. The dashed line indicates the center of the fuelspray, and the focal point was on the line. From this figure, it is clear that the plasma was generated at the laser side in the fuel spray. Because the plasma was not displaced when the laser was focused on dry air, it is likely that the plasma shift was caused by the fuel spray and droplets. This behavior has been discussed from the point of view of the lens effect of droplets [18]; however, many phenomena are still not fully understood.
Figure 3 shows the time-series images of a laser-ignited flame under various ambient temperature conditions. These images were used to investigate the combustion behavior of laser-ignited fuel spray at ambient temperatures, Ta, of 290 K, 400 K, and 430 K. Incident energy was constant at Ein = 90 mJ. The value Q, shown above the image, is the heat production estimated by the pressure history inside the vessel. At Ta = 290 K, the flame was small and luminous. At Ta = 400 K, the flame size and heat release were larger than those at Ta = 290 K, but the flame was also luminous. At Ta = 430 K, the heat production was larger than that at Ta = 400 K, and the flame became blue. These results showed that the ambient temperature is an important parameter that affects combustion. Temperature influences the air–fuel mixture formation, and it is useful to determine whether there is a combustible mixture at each focal position.
3. Experimental results and discussions
Plasma in a fuel spray containing liquid and gas phases is different from plasma containing only the gas phase. Thus, it is necessary to investigate the laser-induced plasma formation process in a fuel spray. Displacement of the plasma position is a problem because the ignition position is important for spray combustion. Additionally, it was observed that the shockwave generated from laser-induced plasma dispersed the fuel droplets. Thus, it is necessary to consider the presence of fuel droplets when selecting the ignition point. To understand the interaction between the spray and plasma, we visualized plasma generation using an ultra-high-speed camera.
The laser-induced plasma generation in fuel spray was recorded by an ultra-high-speed camera. The frame speed was 1 Mfps, and the exposure time was 100 or 200 ns. Ethanol, which has a relatively low boiling point, was used as the fuel because the droplet diameter is known to decrease at higher ambient temperatures due to evaporation. The images show an area around the plasma generation (10.0 × 8.7 mm). The injection pressure, Pi, was set at Pi = 1 and 7 MPa to investigate the effects of flow velocity and droplet diameter on the formation of laser-induced plasma. Figure 4 shows plasma images at Pi = 1 MPa and an ambient temperature of Ta = 296 K. The experimental conditions were the same in images #1 and #2 (Fig. 4). The laser beam was applied from the right side of the image, and the focal point of the convex lens was along the dashed line. Figure 4 (#1) shows plasma generated around the lens focal point. This is similar to laser-induced plasma generation in the gas phase. The shockwave was detected in the image at 4 μs after the laser energy was applied. The droplets were dispersed and broken by the shockwave. On the other hand, the images in #2 (Fig. 4) captured discrete regions of plasma, even though #1 and #2 were under the same conditions. Each region of the plasma emitted a shockwave and caused droplet dispersion that differed from the images shown in #1. Compared with #1, the plasma decay was faster when the plasma was generated at the locations shown in #2. If the same laser energy was applied, the incident energy was partitioned when the plasma generation was discrete, as shown in #2. As a result, the energy consumption per area of plasma was lower than that in #1, causing faster decay of laser-induced plasma.
Figure 5 shows plasma images at Pi = 7 MPa and Ta = 296 K and 390 K. Comparing the images in Fig. 5 with those in Fig. 4 at Ta = 296 K reveals that the droplet size decreased with increasing injection pressure and ambient temperature. In Fig. 5, the point of plasma generation shifted to the laser side compared with Fig. 4. The plasma position shifted because the spray angle expanded with increasing injection pressure, and the contact points of the laser and the droplets moved toward the laser side. In Fig. 5, the plasma was generated at points similar to those in Fig. 4, but the dispersion was greater than that in Fig. 4. When the ambient temperature was increased to 390 K, the dispersion of the fuel spray became significant. Because the ambient temperature (Ta = 390 K) was higher than the boiling point of ethanol (351 K), the fuel evaporated after injection, and the diameter and weight of the droplets decreased. As a result, the droplets became sensitive to the shockwave produced by the laser-induced plasma.
When the injection pressure and the ambient temperature were low, the fuel spray produced many larger droplets due to poor atomization and evaporation. In this case, the laser-induced plasma formed at several positions due to breakdown at the surface of the larger droplets. As the injection pressure and ambient temperature increased, the injected fuel spray produced smaller droplets due to improved atomization and evaporation of ethanol. In this case, smaller droplets were blown away by the shockwave generated from the laser-induced plasma. The interaction between the laser-induced plasma (and the shockwave) and the evaporating fuel spray was captured using an ultra-high-speed video camera. Next, the lens effect of larger droplets on the laser focusing position is discussed.
It is possible that the droplets acted as a convex lens, causing the plasma position to shift. To investigate the lens effect of droplets, the laser was applied to a single droplet of water, which was levitated by an ultrasonic levitator [16]. A droplet having a diameter of 2.5–3 mm was levitated using this device. The position of the droplet was moved backward and forward around the focal point of the convex lens. At each droplet position (DP), the laser was applied to the droplet, and the process was observed by the ultra-high-speed video camera, as shown in Fig. 6. The DP shows the distance from the focal point of the convex lens. The DP has a positive (negative) value when it is longer (shorter) than the focal length. The cross symbol on the image shows the lens focal point. At DP = 1 mm, the laser entered the droplet after passing through the focal point. At this location, the plasma generated in the air contacted the droplet, causing the droplet to scatter from the laser side due to the shockwave from laser-induced plasma. The water droplet has higher viscosity, therefore the droplet was swelled from the laser side due to the shockwave. At DP = −4 mm, the lens focal point was behind the droplet. However, at this location, the droplet scattered from the inside. Thus, the laser beam was refracted by the droplet, and the focal length became shorter than the original focal length of the convex lens. As a result, the combined focal point was inside the droplet, causing the droplet to explode from the inside due to the shockwave. Additionally, the droplet exploded from behind at DP = −18 mm. This result also supports the lens effect of the droplet. When the droplet diameter was smaller, the focal length also became shorter. The plasma was likely generated at the laser side (Figs. 2 and 4) because the focal length became shorter due to the lens effect of the droplets. One explanation for the plasma generation at discrete positions may be that each droplet in the spray acted as a lens.
Here, we discuss the lens effect of levitated droplets on the laser focusing position. The curvature of the droplet was determined by the captured image of a levitated droplet. Levitated droplets are nearly elliptical. Based on high-quality images of the levitated droplet, the image analyzing software provided the position and shape of the outer edge and the curvature of the horizontal and vertical semiaxes. By considering the refractive index of water, we estimated the laser focusing position according to Snell’s law. Figure 7 shows the original laser focusing position and the simulated laser focusing position due to the lens effect of the levitated droplet at (a) DP = –4 mm and (b) DP = –18 mm. At DP = –4 mm, the laser was focused outside the levitated droplet; however, the simulated laser focusing position was inside the droplet due to the lens effect of the levitated droplet. Laser-induced breakdown occurred inside the droplet in this case; the droplet scattered due to the shockwave from the laser-induced plasma inside the droplet. Even at DP = –18 mm, the simulated laser-focusing position was on the left side of the surface of the levitated droplet. Therefore, the droplet scattered from the left side because the shockwave was generated on the left side of the surface of the levitated droplet. These results confirmed that the laser-induced plasma position was shifted due to the breakdown at the droplet surface and the lens effect of the droplet.
4. Conclusions
The behaviors of laser-induced plasma and fuel spray were investigated by visualizing images with an ultra-high-speed camera. The effects of the shockwave from the laser-induced plasma on evaporated spray behaviors were investigated. The interaction between a single droplet and the laser-induced plasma was investigated using a single droplet levitated by an ultrasonic levitator. The conclusions are as follows:
- Ultra-high-speed visualization showed the interaction between the laser-induced plasma (and the shockwave) and the fuel spray. Laser-induced plasma was generated at the surface of the larger droplet under poor atomization conditions. The fuel droplets in the spray were dispersed by the shockwave generated from the laser-induced plasma under improved atomization conditions.
- The interaction between the laser-induced plasma (and the shockwave) and a single levitated droplet can be visualized at various laser-focusing positions. The laser-induced plasma position may have been shifted by breakdown at the droplet surface and the lens effect of the droplet.
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