Research Article  Open Access
Vi H. Rapp, Anthony DeFilippo, Samveg Saxena, JyhYuan Chen, Robert W. Dibble, Atsushi Nishiyama, Ahsa Moon, Yuji Ikeda, "Extending Lean Operating Limit and Reducing Emissions of Methane SparkIgnited Engines Using a MicrowaveAssisted Spark Plug", Journal of Combustion, vol. 2012, Article ID 927081, 8 pages, 2012. https://doi.org/10.1155/2012/927081
Extending Lean Operating Limit and Reducing Emissions of Methane SparkIgnited Engines Using a MicrowaveAssisted Spark Plug
Abstract
A microwaveassisted spark plug was used to extend the lean operating limit (lean limit) and reduce emissions of an engine burning methaneair. Incylinder pressure data were collected at normalized airfuel ratios of , , , , and . For each , microwave energy (power supplied to the magnetron per engine cycle) was varied from 0โmJ (spark discharge alone) to 1600โmJ. At lean conditions, the results showed adding microwave energy to a standard spark plug discharge increased the number of complete combustion cycles, improving engine stability as compared to sparkonly operation. Addition of microwave energy also increased the indicated thermal efficiency by 4% at . At , the spark discharge alone was unable to consistently ignite the airfuel mixture, resulting in frequent misfires. Although microwave energy produced more consistent ignition than spark discharge alone at , 59% of the cycles only partially burned. Overall, the microwaveassisted spark plug increased engine performance under lean operating conditions but did not affect operation at conditions closer to stoichiometric.
1. Introduction
Concerns with greenhouse gases, air quality, and shortage of fossil fuels have encouraged the development of new technology and alternative fuels that reduce emissions of combustion engines. Natural gas (containing mostly methane) offers lower greenhouse gas emissions than other hydrocarbon fuels because of its high hydrogentocarbon ratio. Natural gas can also be combusted at high compression ratios without the risk of producing engine knock. Burning natural gas (containing mostly methane) with airfuel ratios larger than stoichiometric (lean conditions) in sparkignited engines has the potential to produce lower emissions and higher thermal efficiencies than petroleumburning engines [1, 2]. However, if the airfuel mixture is too lean, the high amount of air dilution destabilizes combustion, decreasing flame speeds and making the airfuel mixture more difficult to ignite [3, 4]. When combustion becomes unstable due to increased air dilution, engine performance and efficiency deteriorate, limiting the full potential of lean combustion. This unstable combustion point is well known as the โlean limit.โ
One method of increasing burn rates of lean natural gas mixtures and extending the lean limit is to increase turbulent mixing inside the combustion chamber. Das and Watson [1] modified an engine to increase turbulent mixing by swirling the charge during induction and using a squish motion during the compression stroke, which broke up the intakegenerated turbulence into smallscale turbulence inside the combustion chamber. Using this technique and changing the compression ratio, Das and Watson were able to operate the engine at normalized airfuel ratio () of 1.88. However, the peak thermal efficiency occurred at . The engine also produced less carbon dioxide (), hydrocarbon, and nitrogen oxide () emissions than equivalent petroleum engines. Evans [5] achieved similar results using a โsquishjetโ combustion chamber but also investigated the effects of a partially stratifiedcharge mixture near the spark plug to enhance ignition of lean mixtures. Evansโ results showed that the partially stratifiedcharge combustion system extended the lean limit of operation and reduced emissions. Cho and He [6] found that delaying fuel injection reduced CO and HC emissions at 25% throttle and 100% throttle.
Another method for stabilizing lean operation and reducing emissions is addition of hydrogen to natural gasfueled engines. Several researchers have shown that addition of hydrogen increases flame propagation of the airfuel mixture, extending the lean operating limit and reducing emissions [7โ14]. Smutzer [8] developed a hydrogenassisted lean operation (HALO) engine that achieved operation at ultralean conditions (), eliminating production and reduced spark ignition energy by 22%. However, nonuniformities in fueling lead to high cylindertocylinder variation in the engine. Wang et al. [10] investigated the cycletocycle variations in natural gashydrogen blends and found that addition of hydrogen is strongly correlated with peak incylinder pressure, pressure rise rate, and crank angle degree of peak incylinder pressure. They also found that addition of hydrogen decreased cycletocycle variation under stoichiometric conditions and lean conditions. Kornbluth et al. [11] found that adding hydrogen (30%โ50% by volume) to landfill gas (containing mostly methane and ) while operating under lean conditions increased engine stability and power, while decreasing emissions, especially . Their results also show that increasing the concentration of hydrogen extended the lean limit [11].
Highenergy ignition devices have also been used to extend lean limits beyond the capabilities of a traditional spark discharge ignition systems [15, 16]. One such highenergy ignition technology of current research interest is the microwaveassisted spark plug, under development by Imagineering Inc., which delivers microwaves into the combustion chamber in addition to the standard spark discharge [17, 18]. A capacitive discharge spark initiates plasma in the combustion chamber, and microwaves expand the plasma. As compared to arc discharges, which traditionally ignite internal combustion engines, microwave plasma discharges are characterized by higher spatial uniformity, lower plasma potential, higher electron energy, and increased excitation of vibrationally and electronically excited states of molecules [19]. Highenergy electrons, having received energy from microwaves, can enhance mixture reactivity through electron impact reactions which generate radicals and metastable electronically excited chemical species [19]. Past studies have shown that a microwaveassisted spark plug can extend the lean limit of gasolinefueled engines [17, 20, 21] and that microwaves can enhance flame speed in wallstagnated methane flames [22].
Although the microwaveassisted spark plug has proven effective for gasolinefueled engines operating under lean conditions [17, 20, 21], its effectiveness has not previously been demonstrated for use on engines burning methanebased fuels (such as natural gas). For this study, a singlecylinder engine burning methaneair was modified to operate using a microwaveassisted spark plug system. The performance of the microwaveassisted spark discharge was compared to the sparkonly discharge over a range of airfuel ratios and microwave energy levels.
Following this section, the instrumentation, experimental design, and variables used to measure engine stability and performance are described. Next, results and discussions are presented. Last, conclusions are made, and future work is suggested.
2. Material and Methods
2.1. Engine Specifications
Experiments were conducted using a single cylinder Waukesha ASTMCooperative Fuel Research (CFR) engine. Engine specifications can be found in Table 1. A schematic of the CFR engine with the microwaveassisted spark plug system is shown in Figure 1. A compressed gas cylinder supplied methane to the CFR gaseous fuel injector. The CFR engine was fitted with the Imagineering, Inc. microwaveassisted spark plug system, which transmits 2.45โGHz microwaves into the combustion chamber through the spark plug insulator. The microwaves interact with plasma initiated by a 30โmJ capacitive spark discharge [17].

2.2. Measurement Instrumentation
Incylinder pressure was measured using a 6052B Kistler piezoelectric pressure transducer in conjunction with a 5044A Kistler charge amplifier and was recorded every 0.1 crank angle (CA) degree. The cylinder pressure transducer was mounted in the cylinder head. Intake pressure was measured using a 4045A5 Kistler piezoresistive pressure transducer in conjunction with a 4643 Kistler amplifier module. Intake temperature and exhaust temperature were measured using Ktype thermocouples. Crank angle position was determined using an optical encoder, while an electric motor, controlled by an ABB variable speed frequency drive, controlled the engine speed. A Motec M4 ECU (Engine Control Unit) controlled injection timing, injection pulse width, and injection duty cycle. A Horiba analyzer measured emissions and equivalence ratio. A wideband lambda sensor was used to establish stoichiometric conditions and compared with emissions results. Figure 1 shows a schematic of the engine with the location of each sensor.
2.3. Experimental Design
Microwaveassisted spark plug performance was explored over a range of microwave energy inputs, normalized airfuel ratios (), and spark timings. Compression ratio and engine speed were held constant, and the engine was operated naturally aspirated at wideopen throttle. Engine operating conditions are outlined in Table 2. At each operating condition, spark timing was adjusted to achieve Maximum Brake Torque (MBT), at which point 300 thermodynamic cycles of incylinder pressure data were recorded (each cycle consisting of 720โCAD). The parameter space of normalized airfuel ratio included , and . The parameter space of total energy supplied to the magnetron per engine cycle included 0โmJ (spark only), 130โmJ, 900โmJ, and 1600โmJ. The 130โmJ energy input condition was achieved by supplying the magnetron with peak power of 2.6โkW at a 12.5% duty cycle over a 0.36โms duration. The 900โmJ and 1600โmJ conditions also operated with peak input power of 2.6โkW, but with 25% duty cycle and durations of 1.32โms and 2.65โms, respectively. The power supplied to the magnetron falls off with increasing duration, explaining why total energy input does not scale linearly with energy input duration. The total microwave energy delivered to the combustion chamber is estimated to be approximately 17% of the energy supplied to the magnetron after accounting for reflection and transmission losses.

2.4. Data Analysis
The performance of the microwaveassisted spark plug is characterized by using the indicated mean effective pressure (IMEP) and the indicated thermal efficiency [23]. Engine stability is characterized by two parameters. The first parameter is the coefficient of variation of the IMEP (COV_{IMEP}): where is the standard deviation in IMEP and is the mean IMEP [23]. COV_{IMEP} increases when engine operation becomes unstable, leading to partial burn cycles and misfires [23].
The second parameter is the modified pressure ratio (MPR): where is the maximum pressure with ignition and is the maximum pressure while motoring (no ignition) [24]. The MPR determines if a cycle has completely combusted, partially burned, or misfired (no combustion) [24]. For methane, at , the limits of complete combustion, partial burn, and misfire were identified asโcomplete ,โpartial burn ,โmisfire .
Under lean conditions, the normalized airfuel ratio, , was calculated using where is the stoichiometric mass of the fuel injected per cycle and is the actual mass of the fuel injected per cycle [23, 25].
The change in emissions is determined by where ppm_{MW} is the measured emission in ppm when microwave energy is added and ppm_{Spk} is the measured emissions in ppm when no microwave energy is added (spark only).
Uncertainty in measured data is reported as mean ยฑ uncertainty with a confidence level of 95%. Uncertainty in calculated parameters is reported from an uncertainty analysis, and the details can be found in the Appendix.
3. Results and Discussion
The performance of the microwaveassisted spark plug is gauged by the following: engine stability, engine power, and emissions output. Engine stability is expressed by COV_{IMEP} and the percentage of complete combustion cycles. Engine power and performance are given by IMEP and the indicated thermal efficiency. Emissions output for each experiment is provided and compared with stability and performance variables.
3.1. Engine Stability
Engine operation was considered to be stable when COV_{IMEP} was no more than 10%, and the percentage of complete combustion cycles was at least 95%. Shown in Figure 2 is the dependence of COV_{IMEP} on for all operating conditions. At the leanest operating condition (), stable combustion could not be achieved, and multiple misfires occurred using a standard spark discharge. Supplying 1600โmJ per engine cycle to the magnetron consistently ignites the mixture, shown in Figure 2; however, the COV_{IMEP} is 41%, and the percentage of complete combustion cycles is 37% (59% partial burn and 4% misfire), indicating unstable operation.
Decreasing to 1.68 continued to result in unstable operation when using a standard spark discharge. Figure 2 shows the COV_{IMEP} is 35% and the percentage of complete combustion cycles is 77%, while 17% of the cycles partially burn and 6% of the cycles misfire as seen in Figure 3. Adding 130โmJ of energy to the microwave system decreases COV_{IMEP} to 25% and increases the number of complete combustion cycles to 89% (7% partial burn cycles and 4% misfire). Further addition of microwave energy (900โmJ) decreases COV_{IMEP} to 8% and increases the complete combustion cycles to 98% (2% partial burn cycles), resulting in stable operation. Figure 2 shows that addition of 1600โmJ of microwave energy decreases engine stability; however, the difference in partial burn cycles and thermal efficiency from the 900โmJ case was less than 2% for both. The observed small difference implies that adding 1600โmJ of energy neither decreases nor further improves engine stability. These results also suggest that the microwave input may be most effective during the earliest stage of ignition since the 1600โmJ and the 900โmJ energy inputs have the same power, but the 1600โmJ setting is operated for a longer duration, as stated in the methods. Addition of microwave energy did not affect COV_{IMEP} at operating conditions closer to stoichiometric () because almost all the cycles (>99%) completely burned.
3.2. Engine Power and Performance
Engine power and performance are given by IMEP and the indicated thermal efficiency. Figure 4 shows IMEP decreases with increasing for all operating conditions. For , the microwaveassisted spark plug increases engine performance by enhancing burning, leading to better engine stability. At , addition of microwave energy slightly increases IMEP. For , the microwave appears to have almost no effect on IMEP.
For the standard spark discharge, Figure 5 shows an increase in indicated thermal efficiency as airfuel ratio increases from to . Beyond , the indicated thermal efficiency decreases due to incomplete combustion. Addition of microwave energy at increases indicated thermal efficiency by up to 4% but shows minimal improvement between and . The peak indicated thermal efficiency occurred at , which are slightly leaner conditions than previous studies burning only natural gas [1, 6].
3.3. Engine Emissions
The effects of the microwave sparkplug system on emissions were explored by normalizing the microwave operating condition emissions with the standard spark discharge emissions and finding the percent difference (see (4)). Figure 6 shows production increases when the engine is run with 1600โmJ of microwave energy. At , when stable combustion is unaffected by microwaves, microwaveenhanced ignition increases production by 13% (about 100โppm), indicating a small direct contribution of the added microwave energy towards formation. As operating conditions become leaner ( and ,) the difference in production between sparkonly and microwaveenhanced operation widens. This increase is due to an increase in complete combustion cycles when the engine is operating with microwave energy in addition to the direct contribution of when adding microwave energy. The minimum measurement for all operating conditions was 3.75โg/kWh.
Figure 7 shows that addition of 1600โmJ of microwave energy decreases carbon monoxide (CO) and total unburned hydrocarbons (THC) as operating conditions become leaner. This trend agrees with previous results [11] and is due to an increase in complete combustion cycles with the addition of microwave energy under leaner operating conditions ().
4. Conclusions and Recommendations
In this study, we found that a microwaveassisted spark plug extends the lean stable operating limit of an engine burning methaneair. The addition of microwave energy improved engine stability and as a result also increased engine performance. At the leanest operating condition (), a standard spark discharge did not consistently ignite the airfuel mixture and resulted in multiple misfires. However, with the addition of microwave energy, the mixture ignited, resulting in mostly partial burn cycles (59%) and few misfire cycles (4%). Decreasing the normalized airfuel ratio from to allowed for engine performance comparisons between the spark discharge alone and the spark discharge with microwave energy assistance. Addition of microwave energy at increased the number of complete combustion cycles, the indicated mean effective pressure, and the indicated thermal efficiency.
Increasing the total microwave energy input to the magnetron per cycle from 130โmJ to 900โmJ improved engine stability. Further increasing input energy to the microwave system from 900โmJ to 1600โmJ by increasing the duration of energy input showed no additional improvements. These results suggest that microwave input may be effective only during the earliest stage of ignition, when a small flame kernel is still present near the spark plug electrode. For richer mixtures (), microwave energy did not improve engine stability because almost all the cycles completely burned.
Addition of microwave energy decreased CO and total unburned hydrocarbon emissions but increased emissions. The increase in is due to an increase in complete combustion cycles when microwave energy is added to the ignition. Also, under leaner operating conditions (), combustion advances slightly, allowing more time for formation.
The present microwaveassisted spark plug fully eliminated misfires under lean conditions () and could likely allow for steady, lean operation when burning other alternative fuels. However, performance of the microwaveassisted spark plug should be further tested using alternative fuels. Using a higherturbulence engine with the microwave sparkplug system may also increase power output and further extend the lean limit. Additionally, investigating flame occurrence and propagation using optical techniques may also provide further insight on the effectiveness of a microwaveassisted spark plug.
Appendix
A. Uncertainty Analysis
An uncertainty analysis was performed on all engine measurements and calculations, except emissions. For the measured values, the random uncertainty was calculated using 95% confidence intervals (approximately 2 times the standard error, for normally distributed errors) [26]: where is the measured value, is the number of cycles, is the standard deviation, defined as and is the mean of the measured values.
The general formula for error propagation in an equation with multiple variables was calculated using [26, 27] where uncertainty is being measured in and are independent variables with measured uncertainties. This error propagation formula was used to determine the uncertainty in calculated variables.
A.1. InCylinder Pressure
As mentioned previously, the incylinder pressure was measured using a 6052B Kistler piezoelectric pressure transducer in conjunction with a 5044A Kistler charge amplifier. The systematic error for the transducer, , is listed as 0.2 bar. Random uncertainty in the pressure data was calculated using (A.1) and a minimum of 300 cycles.
Using the random uncertainty and the systematic error in the pressure, the total uncertainty in pressure can be calculated using [26, 27]
A.2. Volume
The volume at each crank angle degree can be calculated by [27] where is the clearance volume, is the bore, is the connecting rod length, and is crank radius [23]. Top Dead Center (TDC) of the motoring traces was calculated using the method outlined by Tunestal [28]. The uncertainty in crank angle degree, using Tunestalโs method to find TDC, is 0.05ยฐ. The uncertainty in volume can be determined using where
A.3. Indicated Work
The indicated work is calculated using the trapezoidal method of integration: where is the number of divisions and is the pressure [23]. Using (A.3), the uncertainty in the work can be determined by
A.4. Indicated Power, IMEP, and Indicated Thermal Efficiency
Knowing the uncertainty in the work, the uncertainty in the indicated power indicated mean effective pressure (IMEP), and the indicated thermal efficiency () can be determined using the equation for uncertainty in products and quotients: where uncertainty is being measured in and are independent variables with measured uncertainties [27].
Using (A.10), the uncertainty in indicated power (), IMEP, and can be determined by the following:
Acknowledgments
This research was partially supported by the University of Michigan, Award no. 3001397038, through a cooperative agreement with the US Department of Energy entitled โA University Consortium on High Pressure Lean Combustion (HPLC) for Efficient and Clean ICE.โ The authors wish to acknowledge the assistance of T. Dillstrom, N. Killingsworth, and M. Wissink in conducting experimental measurements.
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Copyright
Copyright © 2012 Vi H. Rapp et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.