Effects of pulse rise time and repetition frequency on nanosecond pulsed plasma ignition for combustion

In this study each combustion event was initiated with a burst of four pulses. The voltage and current of each pulse of two different t_r at a PRF = 2 kHz are shown in figure 2. The current is displayed upside down for clarity. For the first pulse, same voltage amplitude was applied to the electrode for t_r = 5 ns and 10 ns pulses, respectively, as shown in figure 2(A). The current is relatively in phase with the voltage, implying a highly resistive impedance of the electrode system. Similar in-phase voltage and current pair was observed for the 2nd pulse (figure 2(B)) but the peak voltage dropped by approximately 20%–25% for both rise times. For the 3rd pulse, 2 times increase in peak current and a drop to half of the initial peak voltage were observed for the case of t_r = 5 ns, as shown in figure 2(C). A phase shift indicating a more reactive impedance of the load was also observed along with the changes in voltage and current. This impedance change in the electrode system implies a mode change in the transient plasma; a less conductive, weakly ionized plasma is transitioning to a more conductive ionization channel. Similar mode change was reported by others [10]. This mode change in plasma electrode system is associated with the combustion occurrence; the combustion is initiated when the mode transition is observed in the voltage and current waveforms. In comparison, the mode change was observed for t_r = 10 ns on the 3rd pulse, as shown in figure 2(C). In addition, PRF could also play an important role in impacting combustion ignition. Figure 3 shows that at a higher PRF, e.g. 6 kHz, the mode change occurred earlier, on the 2nd pulse, for the faster rise time pulses, i.e. t_r = 5 ns. However, ignition consistently occurred on the 3rd pulse without difference for t_r = 10 ns pulses. The results showed that faster t_r can reduce the number of pulses necessary for combustion ignition. Additionally, increased PRF combined with faster t_r can further reduce the number of pulses necessary for ignition. Impact of PRF on ignition energy for combustion is detailed later with figures 4 and 5.

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A significant takeaway from the mode change phenomena is that, within our experimental conditions, combustion closely follows the large increase in peak current. Increasing PRF enabled combustion with less required number of pulses, which will allow less ignition energy as well as ultimately increase the lifetime of the electrodes.

The energy delivered by each pulse, E_p, was calculated as a function of the integral over the pulse duration, of the product of voltage V(t) and current i(t).

The total energy of a four-pulse burst, E_t, was calculated as the sum of the energy of each pulse.

TPI for combustion ignition driven by pulses with pulse repetition frequencies of 1–6 kHz of the fast t_r pulse and PRF of 1–10 kHz of the slower t_r pulse were examined. The difference in the range of PRF's is due to equipment limitations. Figures 4 and 5 show the PRF dependence of the energy deposition in TPI for t_r = 10 ns and 5 ns, respectively. The energy per pulse of each pulse in a four-pulse burst varies with respect to PRF, as shown in figures 4(A) and 5(A). Energy of the first pulse was recorded to be 3.95 mJ ± 0.02 mJ, which was two times energy of the 3rd or 4th pulse regardless of PRF. Since the energy data corresponds to a single pulsing event, up to 14% fluctuation in energy was observed particularly for the 2nd pulse over the range of PRFs tested and is attributed to the ionization state of the transient plasma forming across the gap between the electrodes. The fluctuation does not appear in the 1st pulse because the initial condition always starts off the same, i.e. the same mixture, pressure, and electrically appears to be an open circuit (with relatively high impedance). The fluctuation does not appear in the 3rd and 4th pulse because the conductivity of the plasma remains relatively consistent once an ionized channel is formed across the gap. The differences in the energies of 3rd and 4th pulses are small. Importantly, the energy per pulse of the 3rd/4th pulses dropped to half or less of the energy of the 1st pulse, indicating that the deposited energy in the system decreases significantly when the combustion was initiated. With the shorter rise time, i.e. t_r = 5 ns, the energy of the first pulse, was 5.44 mJ ± 0.21 mJ, higher than that with t_r = 10 ns, as shown in figure 5(A). This indicates that more energy can be delivered to the electrode system with faster pulses. Interestingly, the energy at the 3rd and 4th pulse was about 2 mJ, similar as those measured for t_r = 10 ns and when combustion was initiated. The differences between the applied total energy and the actual energy needed for combustion ignition were compared in figures 4(B) and 5(B). It is evident that the less pulse numbers needed to initiate the combustion, the greater the difference between the applied energy and ignition energy. When the combustion was initiated at the 2nd pulse for t_r = 10 ns at 6 kHz, for instance, the required ignition energy dropped to 70% of the total applied energy comparing to only 8% for others at 1–5 kHz, even thorough the total energy was already the lowest at 6 kHz. These pulsed energy measurements are important as they provide a simple indicator showing the ignition status and agree well with the mode transitions observed in voltage and current waveforms. In addition, they agree well the findings reported by others where similar nanosecond discharges at different voltages and pulse numbers in bursts were used [2, 12].

In addition to applications of repetitive 2–4 pulses in a burst to drive the plasma and achieve combustion ignition, the fuel-air mixture can also be ignited by one single pulse. Experiments were conducted to compare combustion initiated with single-pulsed discharges driven by pulses with two different rise times. A minimum of 23 and 21 kV of the peak voltage are needed to achieve combustion ignition using the single-pulsed TPI for t_r = 10 ns and 5 ns, respectively. The corresponding energy of the single-shot ignition was 7.9 mJ and 6.2 mJ for the slower and faster rise times, respectively.

Note that single shot ignition uses similar or slightly lower energy compared with that required when using 2 or 4 pulse bursts but requires almost twice of the peak voltage in order to achieve ignition. The higher voltage requirement incurred in the single-shot TPI may pose challenges in the pulsed power source design and electrode insulation of the combustion system, which makes the high-PRF pulse burst TPI more appealing to the application. In addition, electrode or spark-plug erosion is one of the most important factors that impact the lifetime of the internal combustion engines [10]. It is hence important for the application to know the minimum number of pulses needed to initiate combustion, so that the electrode lifetime can be extended with reduced erosion [10]. Moreover, applying the minimum number of pulses also reduces energy consumed per event, and improves the energy efficiency of the system.

Peak pressure and ignition delay were measured for different voltage t_r and PRF at three different equivalence ratios: lean, stoichiometric, and rich. OH emission from OH(A-X) transition indicating the relative production of OH(A) during the combustion ignition was also recorded and compared for these different TPI conditions. As an example, figure 6 shows the temporal development of pressure and OH emission during the combustion initiation for different air-fuel mixtures that were ignited by four-pulse bursts consisting of 10-ns high voltage pulses with t_r = 5 ns at 6 kHz. The maximal peak pressure and shortest ignition delay were obtained for the stoichiometric air-fuel mixture comparing with the other mixtures. Under this pulse condition, the use of lean fuel mixture resulted in shorter ignition delay comparing with the rich mixture. The similar maximal value and delay time were also observed in the PMT signal for three mixture ratios, indicating that the OH emission aligns well with the combustion performance in pressure, as shown in figure 6.

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The impact of the voltage rise time t_r on the combustion performance was evaluated by comparing the peak pressure and delay time for three different mixture ratios ignited with 4-pulse bursts with t_r = 5 ns and 10 ns at a PRF of 6 kHz, as shown in figure 7. Each combustion measurement under the same pulse and mixture condition was repeated 10 times to obtain the average peak pressure, ignition delay and the corresponding standard deviations. For both the voltage rise times, the shortest ignition delay time and maximal peak pressure were observed for the equivalence ratio λ = 1. With shorter t_r, less or similar ignition delay time and higher peak pressure were obtained for all three mixture ratios, as shown in figure 7. The advantage of the application of pulses with shorter rise times was also observed in our previous studies, where transient plasma ignition of stoichiometric mixtures was driven by single pulses. Comparison of TPIs driven by 10 ns-duration pulses with two different rise times showed that TPI powered by faster pulses (pulses with a shorter rise time) consistently produced combustion with greater peak pressure and reduced ignition delay time [17]. The advantage of shorter risetime concurs with the studies by Zhang, Xie et al for plasma actuators where a higher reduced electric field E/N was obtained at a shorter rise time, enabling higher energy gain in the starting vortex by the surrounding air flow [18, 19].

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The effects of PRF on the combustion performance were evaluated similarly. Figure 8 compares the ignition delay and peak pressure of TPI driven by four-pulse bursts with a voltage rise time of 10 ns at two different PRF's: 2 kHz and 6 kHz. Each measurement was repeated 10 times to obtain the average value and standard deviation. Although the difference is small considering the error of ∼5%, shorter ignition delay and higher peak pressure were obtained for TPI driven by pulses at 6 kHz comparing with 2 kHz for all the equivalence ratios. Using the stoichiometric mixture, TPI driven by voltage pulses with the same rise time, e.g. t_r = 10 ns, at different PRF's ranging from 2 kHz to 10 kHz was tested to further evaluate the effects of PRF. Figure 9 shows the ignition delay time decreases and the peak pressure increases with increasing PRF from 2 kHz to 6 kHz. Further increasing the PRF from 6 kHz to 10 kHz, the changes in both ignition delay time and peak pressure become less; although the ignition delay decreases slightly with PRF, the difference in peak pressure is insignificant.

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Although the residual plasma species and accumulated heat due to increased PRFs [18] could benefit the combustion ignition, research has shown that there is an optimal PRF for given an interelectrode gap distance under a constant pulse waveform with different PRF's [2, 7]. In this study, the reduced dependence of the ignition delay and peak pressure on the PRF that is increasing from 6 kHz to 10 kHz suggests that a similar optimal PRF exists for this TPI system, where a 3 mm spark-plug gap is used. Applications of pulses with higher PRF's are needed to determine the optimal PRF for this system but have not been carried out due to the limitation in the pulse generators used in this study.

Gas temperature of the transient plasma is important to understand the chemical kinetics during the combustion initiation. Gas temperature during combustion was well investigated by others with similar pulse characteristics [5, 7, 9, 10, 20]. Gas temperature, of 10 pulses of 5 kV amplitude at 30 kHz PRF, atmospheric pressure, and a 1.5 mm discharge gap, of the first three pulses were not measured, the temperature of the fourth pulse, the one before break down measured approximately 300 K [10]. The fifth pulse, where break down occurred, measured approximately 1000 K [10]. All the pulses following breakdown pulse measured 3000 K [10]. Similarly, the temperature of the first pulse of this work was not measured. In this study, the gas temperature of the plasma driven by the four-pulse burst was determined by measurements of the rotational temperature of the second positive system of nitrogen [21]. The emission spectra of N₂ (C³Π_u − B³Π_g) transition was collected and compared with the simulation using Specair (Specair.com, version 2.2). The gas temperature of each plasma driven by the 2nd–4th pulse during a four-pulse burst was measured, as shown in figure 10. The rotational temperature was measured only for the t_r = 10 ns pulse characteristic, it was measured over a range of PRF's from 2–10 kHz PRF, with an interval of 2 kHz. Each emission image was obtained from an accumulation of ten repeated images taken at the same pulse condition (including a constant pulse sequence number and a constant PRF) for a stoichiometric fuel-air mixture at atmospheric pressure. An average emission spectrum was obtained based on the spatial integration of the image over the entire interelectrode gap where a full length of the plasma was developed. The full-length plasma was not detectable at the first pulse and hence only the 2nd, 3rd and 4th pulsed plasma was recorded for temperature measurements. Figure 10 shows that the gas temperature of the plasma increases with the pulse number. At the 2nd pulse, the gas temperature was measured in the range of 900–1300 K. The temperature increased to 1500–1800 K at the 3rd pulse and remained almost constant or increased slightly up to 2000 K at the 4th pulse. The deviation in temperature is estimated to be ±200 K, which is relatively big due to the fluctuation in plasma emission as well as the uncertainty in the electrode location where the plasma occurs.

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The difference in temperature for the same pulse number with different PRF's is hence insignificant. The gradual increase in gas temperature with the pulse number increasing for the tested PRF range, i.e. >2 kHz, indicates that the relaxation time for heavy particles such as N₂(C) is longer than 500 μs and the heating from the pulsed energy accumulates with pulses.