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Article An Analysis of Exhaust Emission of the Internal Treated by the Non-Thermal Plasma

Ming-Hsien Hsueh 1,*, Chia-Nan Wang 1 , Meng-Chang Hsieh 2, Chao-Jung Lai 3, Shi-Hao Wang 1, Chia-Hsin Hsieh 1, Tsung-Liang Wu 1 and Jo-Hung Yu 1

1 Department of Industrial Engineering and Management, National Kaohsiung University of Science and Technology, Kaohsiung 807, Taiwan; [email protected] (C.-N.W.); [email protected] (S.-H.W.); [email protected] (C.-H.H.); [email protected] (T.-L.W.); [email protected] (J.-H.Y.) 2 Institute of Undersea Technology, National Sun Yat-Sen University, Kaohsiung 804, Taiwan; [email protected] 3 Department of Fashion Design and Management, Tainan University of Technology, Tainan 71002, Taiwan; [email protected] * Correspondence: [email protected]

Academic Editors: Chiara Bisio and Monica Pica   Received: 18 November 2020; Accepted: 18 December 2020; Published: 21 December 2020

Abstract: Industries’ air causes serious challenges to modern society, among them exhaust from internal combustion , which are currently one of the main sources. This study proposes a non-thermal plasma (NTP) system for placement in the of internal combustion engines to reduce the toxic contaminants (HC, CO, and NOx) of exhaust gases. This NTP system generates a high-voltage discharge that not only responds to the ion chemical reaction to eliminate NOx and CO, but that also generates a combustion reaction at the local high temperature of plasma to reduce HC. The NTP system was designed on both the front and rear of the exhaust pipe to analyze the difference of different exhaust flow rates under the specified frequency. The results indicate that the NTP system can greatly reduce toxic contaminants. The NTP reactor placed in the front of exhaust pipe gave HC and CO removal efficiency of about 34.5% and 16.0%, respectively, while the NTP reactor placed in the rear of exhaust pipe gave NOx removal efficiency of about 41.3%. In addition, the voltage and material directly affect the exhaust gases obviously. In conclusion, the proposed NTP system installed in the exhaust system can significantly reduce air . These results suggest that applying NTP to the combustion engine should be a useful tool to simultaneously reduce both emissions of NOx and CO.

Keywords: non-thermal plasma (NTP); exhaust emission; internal combustion engine; ion chemical reaction

1. Introduction The last few years have witnessed rapid development in industrial technology, with an increasing demand for -based energy. The combustion of fuel is the primary source of energy for vehicle engines, aircraft engines, and industrial machinery. Fuel combustion produces a combination of chemical energy, thermal energy, and exhaust gases. Portions of combustion gases harmful to the environment such as unburned , oxides, , and particulate matter are released into the environment. The exhaust gases emitted into the atmosphere cause due to the incomplete combustion of carbonaceous fuel. Inhaling gases are harmful to humans and cause health issues such as , , cardiovascular diseases [1–3]. Inhaling the

Molecules 2020, 25, 6041; doi:10.3390/molecules25246041 www.mdpi.com/journal/molecules Molecules 2020, 25, 6041 2 of 20 exhaust gases damages the airway and may impair their function, especially the lungs [4]. Therefore, with mounting environmental concerns, the purification of exhaust gases from the internal combustion engine is becoming increasingly urgent to address. Balki et al. (2014) investigated the exhaust emission of an SI engine using , , and methanol at different engine speeds and observed that the use of alcohol instead of gasoline as the fuel in a low power engine caused a decline mainly in NOx, (HC) and CO emissions. In addition, when methanol and ethanol were used in the combustion engine, the emissions of NOx, CO, and HC were reduced by 49% and 47.6%, 22.6% and 21.25%, and 21.6% and 19.13%, respectively [5]. Hsueh et al. (2016) studied a fuel temperature control device using a thermoelectric module (TEC) chip to measure engine performance and exhaust emission at various fuel temperatures and air/fuel (A/F) ratios. The emission ratios of HC and CO decreased but that of NOx increased as the fuel temperature increased [6]. Do˘ganet al. (2017) investigated ethanol-gasoline blends as fuel for a four- and four- spark-ignition engine and found that ethanol added to gasoline in the combustion engine fuel caused a reduction in CO and NOx emission ratios [7]. In recent years, the development of non-thermal plasma (NTP) has attracted high interest and is considered as the green strategy for exhaust remediation. It can be used to remove various air pollutants such as SO2, NOx, HC, CO, and VOC [8,9]. In NTP, the electron temperature is higher than the temperature, which is close to room temperature. High energy electrons collide with surrounding gas molecules to produce reactive species such as dissociated molecules, ions, free radicals, and secondary electrons [10]. The reactive species actively combine with the environmental pollutants to form a decomposition product [11]. Various NTP systems such as the spark, dielectric barrier discharges, gliding arc, corona, microwave, and glow have been investigated for the direct conversion of exhaust gases [12–16]. The use of a plasma reactor with a catalyst to treat exhaust gases from gasoline engines was investigated by Dan et al. (2005), who observed that particulate matter removal efficiency ranged approximately from 25 to 57% [17]. Kim et al. (2017) investigated the effects of NTP on a lean premixed model combustor of NOx and CO by changing the mixing nozzle exit velocity and the equivalence ratio. The result showed a reduction in NOx and CO emissions [18]. Adnan et al. (2017) tried to increase the rate of NTP decomposition by increasing the flow rates of the exhaust gases and could reduce the concentrations of CO, CO2, HC, and NOx by more than 95% [19]. In this study, the variation of toxic contaminants in the exhaust system of the internal gasoline engine was investigated by using the NTP system. The NTP reactor was placed in the front or rear of the exhaust pipe to treat and the variation of emission was analyzed. The exhaust gas variation of the engine was observed by controlling the position, voltage, and types of NTP reactors with different engine speeds.

2. Experimental Apparatus and Techniques

2.1. Experimental Setup The schematic of the experimental setup is shown in Figure1, which mainly includes three sections, the production of exhaust gas by the engine, the device for exhaust emissions reduction by the NTP system, and the exhaust gas analyzer of the detection system. The engine used for this experimental test was the model GP-125 manufactured by KYMCO Co. Ltd. (Taiwan), and there was no catalyst inside the exhaust pipe. The specifications of the engine in the power and torque measurement test are mentioned in Table1. The experimental control conditions of the engine are mentioned in Table2. The engine speeds were set in the range of 4000–6000 rpm, a frequently used range for practical motorcycle engines. The detailed experimental setup of the NTP system is shown in Figure2, which is consisted mainly of three parts, the NTP reactor, the voltage converter, and the NTP control device. The NTP reactors used for this experiment test were model CR8EGP and CR8EIX manufactured by NGK Co., Ltd. (Aichi, Japan). The specifications of NTP reactors are mentioned in Table3. NTP reactor was placed in the model A, in the front of the exhaust pipe or model Molecules 2020, 25, x FOR PEER REVIEW 4 of 21

Fixed output current (A) 3 Resolution (mV/mA) 100/10 Precision ±(1% reading + 2 digits) Molecules 2020, 25, 6041 3 of 20 Power supply (V, Hz) AC110/220 ± 10% selectable, 50/60 Dimensions (mm) 250W × 150H × 310D B, in the rear of the exhaust pipe, as described in Figures2 and3. The voltage converter used for this experiment test was model 3051A-LGL3–900Table 5. The range (KYMCO of each Co.exhaust Ltd., gas. Taiwan). The NTP control device used for this experiment test was model DP-30032 (HILA International Inc, Taiwan). The specifications Value of the NTP control device areParameter mentioned in Table4. Pulsed voltages with a frequency of 10 Hz and adjustable amplitudes from 2 to 5 V were appliedMeasuring to the Range electrodes Tolerance to produce a spark. The exhaust gas analyzer used for this experimentHC (ppm) test was model 0–2000 EF-306EN (Exford, ±12 Taiwan), which measures the range of each exhaust gas, as mentionedCO (%) in Table 0–105. The exhaust gas ±0.06 was measured at the end of the exhaust pipe and the data were recorded for 5 min under normal conditions to calculate an average. CO2 (%) 0–20 ±0.5 The A/F ratio was maintained at 13.7 to fix the stable fuel supply condition and observe the change in engine power performance andNOx emission. (ppm) 0–5000 ±25

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Fixed output current (A) 3 Resolution (mV/mA) 100/10 Precision ±(1% reading + 2 digits) Power supply (V, Hz) AC110/220 ± 10% selectable, 50/60 FigureDimensions 1. Schematic (mm) of this experimentexperiment 250W and × setup150H location.× 310D Table 1. The engine specification of motorcycle. Table 5. The range of each exhaust gas. Parameter Value Value ParameterModel GP125 StokeMeasuring Range four-stoke Tolerance Engine type Single cylinder HC (ppm) 0–2000 ±12 Displacement (c.c.) 124.6 BoreCO (%)Stoke (mm) 0–10 52.4 57.8 ±0.06 × × 9.9:1 CO2 (%) 0–20 ±0.5 Fuel Unleaded gasoline FigureNOMax 2. Thex power(ppm) setup (kw of /non-thermalrpm) 0–5000 plasma 7.8/7500 (NTP) ±25 system. Max torque (N-m/rpm) 10.3/5500 Wheelbase (mm) 1220 Air/fuel ratio (A/F) 13.7

Table 2. The control conditions of engine.

Parameter Value Engine speed (rpm) 4000, 5000, 6000 Fuel 95 Engine temperature (◦C) 100–110 air temperature ( C) 22–26 Figure 1. Schematic of this◦ experiment and setup location. Figure 3. Photographic view of the device set up on (a) model A and (b) model B of the experimental motorcycle.

2.2. Experimental Procedure The measurement equipment was adjusted and corrected before the experiment began. The preparation and measurement procedures were as follows:

FigureFigure 2. 2.The The setup setup of of non-thermal non-thermal plasma plasma (NTP) (NTP) system. system.

Figure 3. Photographic view of the device set up on (a) model A and (b) model B of the experimental motorcycle.

2.2. Experimental Procedure The measurement equipment was adjusted and corrected before the experiment began. The preparation and measurement procedures were as follows:

Molecules 2020, 25, x FOR PEER REVIEW 4 of 21

Fixed output current (A) 3 Resolution (mV/mA) 100/10 Precision ±(1% reading + 2 digits) Power supply (V, Hz) AC110/220 ± 10% selectable, 50/60 Dimensions (mm) 250W × 150H × 310D

Table 5. The range of each exhaust gas.

Value Parameter Measuring Range Tolerance HC (ppm) 0–2000 ±12 CO (%) 0–10 ±0.06

CO2 (%) 0–20 ±0.5

NOx (ppm) 0–5000 ±25

Molecules 2020, 25, 6041 4 of 20 Figure 1. Schematic of this experiment and setup location.

Table 3. The types of NTP reactor.

Value Parameter Type One Type Two Model NGK-CR8EGP NGK-CR8EIX Material Platinum Iridium alloy Discharging Gap (before used) (mm) 0.7 0.8 Discharging Gap (after used) (mm) 0.7 0.7 Center electrode diameter (mm) 0.8 0.8 Outer electrode diameter (mm) 2.20 1.85 Figure 2. The setup of non-thermal plasma (NTP) system.

Figure 3. 3. PhotographicPhotographic view view of the of device the device set up seton ( upa) model on (a A) modeland (b) Amodel and B (b of) the model experimental B of the experimentalmotorcycle. motorcycle.

2.2. Experimental Procedure Table 4. The specification of NTP control device. The measurement equipmentParameter was adjusted and corrected Valuebefore the experiment began. The preparation and measurementModel procedures were as follows: DP-30032 Main output voltage (V) 0–30 Main output current (A) 0–3 Fixed output voltage (V) 2.5/3.3/5 Fixed output current (A) 3 Resolution (mV/mA) 100/10 Precision (1% reading + 2 digits) ± Power supply (V, Hz) AC110/220 10% selectable, 50/60 ± Dimensions (mm) 250W 150H 310D × ×

Table 5. The range of each exhaust gas.

Value Parameter Measuring Range Tolerance HC (ppm) 0–2000 12 ± CO (%) 0–10 0.06 ± CO (%) 0–20 0.5 2 ± NOx (ppm) 0–5000 25 ±

2.2. Experimental Procedure The measurement equipment was adjusted and corrected before the experiment began. The preparation and measurement procedures were as follows: 1. The engine condition of the motorcycle, such as engine oil, air filter, and fuel was checked. 2. The exhaust gas analyzer was calibrated and warmed up.

3. The engine was initiated for 30 min for the temperature to reach 100 ◦C. 4. The NTP system was checked and the type of NTP reactor was selected. 5. Before each experiment, a new NTP reactor was replaced. Molecules 2020, 25, x FOR PEER REVIEW 5 of 21

1. The engine condition of the motorcycle, such as engine oil, , and fuel was checked. 2. The exhaust gas analyzer was calibrated and warmed up. 3. The engine was initiated for 30 min for the temperature to reach 100 °C. 4. The NTP system was checked and the type of NTP reactor was selected. Molecules5. Before2020, 25each, 6041 experiment, a new NTP reactor was replaced. 5 of 20 In this study, the engine speed was 4000, 5000, and 6000 rpm, and the engine combustion producedIn this study,exhaust the gas engine when speed the engine was 4000, started. 5000, The and 6000exhaust rpm, gas and was the discharged engine combustion to the exhaust produced gas exhaustanalyzer gas after when passing the engine through started. the TheNTP exhaust reactor. gas The was data discharged were directly to the exhaustrecorded gas with analyzer the help after of passingMicrosoft through Excel. the The NTP sparking reactor. frequency The data of were the directlyNTP control recorded device with was the 10 help Hz. of The Microsoft experimental Excel. Theprocedure sparking is frequency presented of inthe Figure NTP control 4. In devicethese experiments, was 10 Hz. The the experimental different rpm procedure of the isengine presented were inmeasured Figure4. and In these compared experiments, to determine the di thefferent optimal rpm type of the and engine location were of the measured NTP system. and compared Based on the to determineresults of thethis optimalexperiment, type the and optimal location condition of the NTP of system.the NTP Based system on was the obtained results of and this later experiment, applied theto an optimal actual condition vehicle. of the NTP system was obtained and later applied to an actual vehicle.

Figure 4. The procedure of experimental. Figure 4. The procedure of experimental. The dissociation process of exhaust gas was defined as: The dissociation process of exhaust gas was defined as: The removal efficiency of pollutants (RE) for HC, CO, and NOx: The removal efficiency of pollutants (RE) for HC, CO, and NOx: [Gas]off [Gas]on RE (%) = [Gas]−− [Gas] 100% (1) RE (%) = [Gas] ×× 100% (1) [Gas]off

TheThe conversion conversion e efficiencyfficiency of of pollutants pollutants (CE) (CE) for for CO CO2:2: [Gas] − [Gas] [Gas] [Gas] CE( (%))== on − off × 100% (2) CE % [Gas] 100% (2) [Gas]off × where [Gas]off is the exhaust emission value without the NTP system; [Gas]on is the exhaust emission wherevalue [Gas]with otheff is NTP the exhaustsystem. emission value without the NTP system; [Gas]on is the exhaust emission value with the NTP system. 2.3. NTP Reactor 2.3. NTP Reactor The spark is generated by the electrode gap of the NTP reactor. (Tables 3 and 4). The combustible The spark is generated by the electrode gap of the NTP reactor. (Tables3 and4). The combustible gas is ignited in the exhaust gas. The exhaust gas composition of engine combustion includes HC, gas is ignited in the exhaust gas. The exhaust gas composition of engine combustion includes HC, CO, CO, CO2, NOx, SO2, and PM2.5, etc. But the instrument used in this article can only measure HC, CO, CO2, NOx, SO2, and PM2.5, etc. But the instrument used in this article can only measure HC, CO, CO2, and NOx. Therefore, the major reactions in the exhaust gas reduction by the NTP reactor are listed below [20–23]. Molecules 2020, 25, 6041 6 of 20

The short high current spark with excited atomic radicals, excited molecules, and ions are generated, and the temperature during the spark phase can be as high as 3000 K. Therefore, we considered the Zeldovich thermal mechanism (see the equations below) of NOx generation.

N + O NO + N (3) 2 → N + O NO + O (4) 2 → molecules are broken down into oxygen radicals by the free electrons. The reactions of these oxygen molecules are listed below [20–23].

O + e− O + O (5) 2 → O + O O (6) → 2 O + e− O + O (7) 3 → 2 O + O O (8) 2 → 3 O + O O + O (9) 3 → 2 2 Under a higher energy electron collision, N2 transforms into N radicals and the water molecules partially dissociate to form active species, such as OH and H radicals. The results of H2O and N2 molecules at higher energy electron collision reactions are listed below [21,23].

N + e− N + N (10) 2 → N + N N (11) → 2 H O + e− OH + H (12) 2 → H O + O OH + OH (13) 2 → OH + O O + H (14) → 2 OH + O HO + O (15) 3 → 2 2 OH + HO O + H O (16) 2 → 2 2 The mechanism for the destruction of CO2 in a non-thermal plasma involves the electron-induced dissociation of CO2 to give CO and O followed by recombination of the oxygen atoms to give the final products, CO and O2. CO can also recombine with atomic oxygen to reform CO2. The reactions are listed below [19,20]. CO + e− CO + O (17) 2 → CO + e− C + O (18) 2 → 2 CO + O CO (19) → 2 CO + e− C + O (20) → CO + H O CO + H (21) 2 → 2 2 C + O CO + O (22) 3 → 2 The process of NOx removal mainly occurs through the oxidation pathway. Once N and O radicals are formed, the most important reactions of the oxidation of NO to NO2 are listed below [21–23].

N + O NO (23) → Molecules 2020, 25, 6041 7 of 20

Molecules 2020, 25, x FOR PEER REVIEW N + O NO + O 7 of(24) 21 3 → 2 NO + e− N + O (25) NO + O→ → NO (27) NO2 + e− N + O + O (26) NO + O→→NO +O (28) NO + O NO2 (27) NO + N →→ N +O (29) NO + O NO + O (28) 3 → 2 2 NO + HO→NO +2H (30) NO + N N + O (29) → 2 NO +O →NO +O (31) NO + H O NO + 2H (30) 2 → 2 NO +HO →HNO +O (32) NO + O NO + O (31) 2 3 → 3 2 NO NO+ HO +OH→HNOHNO + O(33) (32) 2 2 → 3 NO2 + OH HNO3 (33) 3. Results and Discussion → 3. Results and Discussion 3.1. HC 3.1. HC Figures 5–8 present the HC emission value and the removal efficiency using different materials (TableFigures 3) and5 –at8 presentvarious thelocations HC emission (Figures value 2 and and 3) of the the removal NTP reactor. e fficiency The HC using emissions different are materials known (Tableto be a3 )result and at of various the poor locations combustion (Figures of 2the and fuel3) of[24]. the We NTP found reactor. that The the HCcontrol emissions voltage are of known 2 to 5 V to in bethe a NTP result system of thepoor could combustion reduce the of HC the fuelemissions [24]. We at found different that theengine control speeds. voltage The of minimum 2 to 5 V in HC the NTPremoval system efficiency could reducewas 1.2% the at HC the emissions control voltage at diff erent2 V for engine platinum speeds. (type The one) minimum at 6000 HCrpm removal and for ereactorfficiency in wasthe front 1.2% of at the controlexhaust voltagesystem 2(model V for platinumA). The control (type one) voltage at 6000 1 V rpmexhibited and for little reactor change in thein the front HC of removal the exhaust efficiency, system possibly (model because A). The controlthe voltage voltage and 1the V exhibitedspark were little too change small. Therefore, in the HC removalit does not effi showciency, in possiblythe figure. because The control the voltage voltage and 0 V the indicates spark were the original too small. exhaust Therefore, gas without it does notthe showNTP system. in the figure. Our results The control show voltagethat HC 0 removal V indicates effic theiency original increased exhaust with gas increasing without thevoltage NTP because system. Ourof the results increase show in that shape HC and removal density effi ciencyof the increasedspark with with increasing increasing voltage. voltage With because the increase of the increase in the incontact shape area and of density the large of the spark spark and with high increasing density wi voltage.th the HC With exhaust the increase gas, the in re-burning the contact probability area of the largeof the spark HC exhaust and high gas density increased. with theParticularly, HC exhaust the gas, shape the and re-burning density probability of the spark of approached the HC exhaust the gascritical increased. value at Particularly,the control voltage the shape 4 V, and and density the spark of variation the spark was approached small from the the critical control value voltage at the of control5 to 4 V. voltage 4 V, and the spark variation was small from the control voltage of 5 to 4 V.

FigureFigure 5.5. TheThe HCHC ((aa)) emissionemission values values and and (b ()b removal) removal effi efficiencyciency (RE) (RE) using usin platinumg platinum in thein the front front of the of exhaustthe exhaust pipe. pipe.

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Figure 6. The HC (a) emission values and (b) RE using iridium alloy in the front of the exhaust pipe. Figure 6. The HC (a) emission values and (b) RE using iridium alloy in the front of the exhaust pipe. FigureFigure 6.6. TheThe HCHC ((aa)) emissionemission valuesvalues andand ((bb)) RERE usingusing iridiumiridium alloyalloy inin thethe frontfront ofof thethe exhaustexhaust pipe.pipe.

Figure 7. The HC (a) emission values and (b) RE using platinum in the rear of the exhaust pipe. FigureFigure 7.7. TheThe HCHC ((aa)) emissionemission valuesvalues andand ((bb)) RERE usingusing platinumplatinum inin thethe rearrear ofof thethe exhaustexhaust pipe.pipe. Figure 7. The HC (a) emission values and (b) RE using platinum in the rear of the exhaust pipe.

Figure 8. The HC ( a) emission values and ( b) RE using iridium alloy in the rear of the exhaustexhaust pipe.pipe. Figure 8. The HC (a) emission values and (b) RE using iridium alloy in the rear of the exhaust pipe. Figure 8. The HC (a) emission values and (b) RE using iridium alloy in the rear of the exhaust pipe. Figures9–11 show the HC emission value and removal e fficiency at control voltage 5 V using FiguresFigures 9–119–11 showshow thethe HCHC emissionemission valuevalue andand removalremoval efficiencyefficiency atat controlcontrol voltagevoltage 55 VV usingusing differentFigures materials 9–11 show and atthe various HC emission locations value of theand NTP removal reactor. efficiency Table6 atshows control the voltage maximum 5 V using and differentdifferent materialsmaterials andand atat variousvarious locationslocations ofof thethe NTPNTP reactor.reactor. TableTable 66 showsshows thethe maximummaximum andand minimumdifferent materials values of and HC at removal various e ffilocationsciency atof controlthe NTP voltage reactor. 5 VTable using 6 dishowsfferent the materials maximum and and at minimumminimum valuesvalues ofof HCHC removalremoval efefficiencyficiency atat controlcontrol voltagevoltage 55 VV usingusing differentdifferent materialsmaterials andand atat variousminimum locations values of of the HC NTP removal reactor. ef Theficiency maximum at control HC removalvoltage e5ffi Vciency using was different 34.5% formaterials iridium and alloy at variousvarious locationslocations ofof thethe NTPNTP reactor.reactor. TheThe maximummaximum HCHC removalremoval efficiencyefficiency waswas 34.5%34.5% forfor iridiumiridium (typevarious two) locations at 6000 rpmof the and NTP for reactor. reactor inThe the maximum rear of the HC exhaust removal system efficiency (model was B). The34.5% minimum for iridium HC alloyalloy (type(type two)two) at at 6000 6000 rpmrpm andand forfor reactorreactor inin thethe rearrear ofof thethe exhaustexhaust systemsystem (model(model B). B). TheThe minimumminimum removalalloy (type effi two)ciency at was6000 11.9% rpm and for platinumfor reactor (type in the one) rear at of 6000 the rpmexhaust and system for the (model reactor inB). the The front minimum of the HCHC removalremoval efficiencyefficiency waswas 11.9%11.9% forfor platinumplatinum (type(type one)one) atat 60006000 rpmrpm andand forfor thethe reactorreactor inin thethe frontfront exhaustHC removal system efficiency (model A).was These 11.9% results for platinum indicate (type that HCone) exhaust at 6000 gas rpm concentration and for the reactor increases in withthe front the ofof thethe exhaustexhaust systemsystem (model(model A).A). TheseThese resultsresults indicateindicate thatthat HCHC exexhausthaust gasgas concentrationconcentration increasesincreases increaseof the exhaust in engine system speed. (model This isA). because, These results with the indicate increase that in HC the ex enginehaust speed, gas concentration the gasoline increases fuel gets withwith thethe increaseincrease inin engineengine speed.speed. ThisThis isis because,because, withwith thethe increaseincrease inin thethe engineengine speed,speed, thethe gasolinegasoline nowith time the to increase burn in in the engine engine. speed. The This NTP is reactorbecause, materials with the ofincrease type one in the and engine type two speed, reduce the gasoline the HC fuelfuel getsgets nono timetime toto burnburn inin thethe engine.engine. TheThe NTPNTP reactorreactor materialsmaterials ofof typetype oneone andand typetype twotwo reducereduce exhaustfuel gets gas noconcentration time to burn andin the type engine. two exhibitedThe NTP areactor better ematerialsffect than of the type type one one. and This type is becausetwo reduce the thethe HCHC exhaustexhaust gasgas concentrationconcentration andand typetype twotwo exhibitedexhibited aa betterbetter effecteffect thanthan thethe typetype one.one. ThisThis isis lowthe resistanceHC exhaust and gas small concentration electrode area and of type type two two exhibited material cause a better the effect formation than ofthe a largertype one. spark This with is becausebecause thethe lowlow resistanceresistance andand smallsmall electrodeelectrode areaarea ofof typetype twotwo materialmaterial causecause thethe formationformation ofof aa abecause higher internalthe low density.resistance With and the small increase electrode in the contactarea of areatype of two large material spark andcause high the density formation with of the a largerlarger sparkspark withwith aa higherhigher internalinternal density.density. WithWith thethe increaseincrease inin thethe contactcontact areaarea ofof largelarge sparkspark andand HClarger exhaust spark gas, with the a higher re-burning internal probability density. of With HC exhaustthe increase gas increased. in the contact The NTParea of reactor largelocation spark and in highhigh densitydensity withwith thethe HCHC exhaustexhaust gas,gas, thethe re-burnire-burningng probabilityprobability ofof HCHC exhaustexhaust gasgas increased.increased. TheThe modelshigh density A and with B reduced the HC the exhaust HC exhaust gas, the gas re-burni concentration.ng probability In model of HC A, exhaust the HC removalgas increased. efficiency The NTPNTP reactorreactor locationlocation inin modelsmodels AA andand BB reducedreduced thethe HCHC exhaustexhaust gasgas concentration.concentration. InIn modelmodel A,A, thethe increasedNTP reactor with location decreasing in models engine A speed and B because reduced of the the HC slow exhaust flow rate gas ofconcentration. exhaust gas withIn model decreasing A, the HCHC removalremoval efficiencyefficiency increasedincreased withwith decreasingdecreasing enenginegine speedspeed becausebecause ofof ththee slowslow flowflow raterate ofof HC removal efficiency increased with decreasing engine speed because of the slow flow rate of exhaustexhaust gasgas withwith decreasingdecreasing engineengine speed.speed. TheThe re-bre-burningurning probabilityprobability ofof HCHC exhaustexhaust gasgas increasedincreased exhaust gas with decreasing engine speed. The re-burning probability of HC exhaust gas increased

Molecules 2020, 25, 6041 9 of 20 Molecules 2020, 25, x FOR PEER REVIEW 9 of 21 Molecules 2020, 25, x FOR PEER REVIEW 9 of 21 Molecules 2020, 25, x FOR PEER REVIEW 9 of 21 engine speed. The re-burning probability of HC exhaust gas increased with the increase in contact with the increase in contact time between the spark and HC exhaust gas. In model B, the HC removal timewith betweenthe increase the sparkin contact and HCtime exhaust between gas. the In spar modelk and B, HC the exhaust HC removal gas. In effi modelciency B, increased the HC removal with an efficiencywith the increase increased in withcontact an increasetime between in engine the spar speed.k and This HC is exhaustbecause gas.the flow In model rate of B, exhaust the HC gasremoval was increaseefficiency in increased engine speed. with Thisan increase is because in engine the flow speed. rate ofThis exhaust is because gas was the flow almost rate the of same exhaust in model gas was B. almostefficiency the increasedsame in model with anB. Theincrease re-burning in engine probability speed. This of HC is because exhaust the gas flow increased rate of with exhaust an increase gas was Thealmost re-burning the same probability in model B. of The HC re-burning exhaust gas probability increased withof HC an exhaust increase gas in HCincreased gas flow with per an unit increase area. inalmost HC gas the flowsame per in model unit area. B. The The re-burning reduction probability in HC removal of HC efficiency exhaust gas of modelincreased B was with higher an increase than Thein HC reduction gas flow in per HC unit removal area. e ffiTheciency reduction of model in HC B was removal higher efficiency than that of of model model B A. was This higher is because than thatin HC of gasmodel flow A. per This unit is becausearea. The of reduction the larger in cross-sectional HC removal efficiency area in model of model B, which B was leads higher to thanthe ofthat the of larger model cross-sectional A. This is because area in of model the larger B, which cross-sectional leads to the area complete in model release B, which of spark leads and to slow the completethat of model release A. of This spark is andbecause slow ofgas the flow larger rate, cross-sectionalincreasing the re-burningarea in model probability B, which of HCleads exhaust to the gascomplete flow rate, release increasing of spark theand re-burning slow gas flow probability rate, increasing of HC exhaustthe re-burning gas. Therefore, probability HC of exhaust HC exhaust gas gas.complete Therefore, release HC of exhaust spark and gas slow emissions gas flow get rate, reduced. increasing the re-burning probability of HC exhaust emissionsgas. Therefore, get reduced. HC exhaust gas emissions get reduced.

Figure 9. The HC (a) emission values and (b) RE of control voltage 5 V using platinum. Figure 9. The HC (a) emission values and (b) RE of control voltage 5 V using platinum. FigureFigure 9.9. TheThe HCHC ((aa)) emissionemission valuesvalues andand ((bb)) RERE ofof controlcontrol voltagevoltage 55 VV usingusing platinum.platinum.

Figure 10. The HC (a) emission values and (b) RE of control voltage 5 V using iridium alloy. FigureFigure 10.10. The HC (a)) emissionemission valuesvalues andand ((bb)) RERE ofof controlcontrol voltagevoltage 55 VV usingusing iridiumiridium alloy.alloy. Figure 10. The HC (a) emission values and (b) RE of control voltage 5 V using iridium alloy.

Figure 11. TheThe HC HC RE RE at at control control voltage 5 V. Figure 11. The HC RE at control voltage 5 V. Figure 11. The HC RE at control voltage 5 V. Table 6. The maximum and minimum values of HC removal efficiency at control voltage 5 V.

HC RE Value Parameter Maximum Minimum Platinum and in the front of the exhaust pipe (Type one and model A) 15.3% 11.9% Iridium alloy and in the front of the exhaust pipe (Type two and model A) 18.5% 14.3% Platinum and in the rear of the exhaust pipe (Type one and model B) 29.8% 20% Iridium alloy and in the rear of the exhaust pipe (Type two and model B) 34.5% 24.6%

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Table 6. The maximum and minimum values of HC removal efficiency at control voltage 5 V. Table 6. The maximum and minimum values of HC removal efficiency at control voltage 5 V. HC RE Value Parameter HC RE Value Parameter Maximum Minimum Maximum Minimum Platinum and in the front of the exhaust pipe (Type one and model A) 15.3% 11.9% Platinum and in the front of the exhaust pipe (Type one and model A) 15.3% 11.9% Iridium alloy and in the front of the exhaust pipe (Type two and model A) 18.5% 14.3% Iridium alloy and in the front of the exhaust pipe (Type two and model A) 18.5% 14.3% Platinum and in the rear of the exhaust pipe (Type one and model B) 29.8% 20% MoleculesPlatinum2020 and, 25, in 6041 the rear of the exhaust pipe (Type one and model B) 29.8% 20%10 of 20 Iridium alloy and in the rear of the exhaust pipe (Type two and model B) 34.5% 24.6% Iridium alloy and in the rear of the exhaust pipe (Type two and model B) 34.5% 24.6%

3.2.3.2. CO FiguresFigures 1212–15–15 presentpresent thethe COCO emissionemission valuevalue andand removalremoval eefficienfficiencycy ofof thethe didifferentfferent typestypes andand locationslocations ofof NTPNTP reactor.reactor. The control voltage of 2 to 5 VV inin thethe NTPNTP systemsystem couldcould reducereduce thethe COCO emissionsemissions atat didifferentfferent engineengine speeds.speeds. The minimumminimum CO removal eefficiencyfficiency waswas 1.2%1.2% atat thethe controlcontrol voltagevoltage 22 VV forfor PlatinumPlatinum (type(type one)one) andand forfor in the frontfront of the exhaust system (model A) at 6000 rpm. TheThe resultsresults showshow thatthat thethe removalremoval eefficiencyfficiency ofof COCO increasesincreases withwith increasingincreasing voltage.voltage. ThisThis isis becausebecause withwith increasingincreasing voltage,voltage, thethe shapeshape ofof thethe sparkspark getsgets larger,larger, leadingleadingleading tototo ananan increaseincreaseincrease ininin thethethe productionproductionproduction ofof freefree radicals.radicals. AbundantAbundant reactivereactive radicalsradicals andand higherhigher combustioncombustion reactionreaction temperaturetemperature causedcaused thethe increaseincrease inin thethe dissociationdissociation probabilityprobability ofof COCO exhaustexhaust gas.gas.

Figure 12. The CO (a) emission values and (b) RE using platinum in the front of the exhaust pipe. FigureFigure 12.12. The CO (a)) emissionemission valuesvalues andand ((b)) RERE usingusing platinumplatinum inin thethe frontfront ofof thethe exhaustexhaust pipe.pipe.

Molecules 2020, 25, x FOR PEER REVIEW 11 of 21 FigureFigure 13. 13.The The CO CO (a ()a emission) emission values values and and ((bb)) RERE usingusing iridium alloy alloy in in the the front front of of the the exhaust exhaust pipe. pipe. Figure 13. The CO (a) emission values and (b) RE using iridium alloy in the front of the exhaust pipe.

FigureFigure 14.14. TheThe COCO ((aa)) emissionemission valuesvalues andand ((bb)) RERE usingusing platinumplatinum inin thethe rearrear ofof thethe exhaustexhaust pipe.pipe.

Figure 15. The CO (a) emission values and (b) RE using iridium alloy in the rear of the exhaust pipe.

Figures 16–18 present the CO emission value and removal efficiency of control voltage 5 V using different materials and at various locations of the NTP reactor. Table 7 shows the maximum and minimum values of CO removal efficiency at control voltage 5 V using different materials and at various locations of the NTP reactor. The maximum CO removal efficiency was 16.0% for iridium alloy (type two) at 5000 rpm and for reactor in the rear of the exhaust system (model B). The minimum CO removal efficiency was 3.3% for platinum (type one) at 6000 rpm and for reactor in the front of the exhaust system (model A). The NTP reactor material of types one and type two reduced the CO exhaust gas concentration. The effect of type two NTP reactor material was found to be better than that of type one because, low resistance forms larger spark and high internal density. The production of free radicals increases by increasing the spark size. The larger amount of radicals and higher combustion reaction temperature caused an increase in the dissociation probability of CO exhaust gas. The dissociation of CO is broadly divided into two parts. In the first part, the dissociation of CO gives carbon and oxygen atoms. Carbon atoms get deposited on the surface of the NTP reactor, as shown in Figure 19. The second part involves the recombination of CO and oxygen atoms to form the CO2. The location of the NTP reactor in models A and B reduce the CO exhaust gas concentration. In model A, the CO removal efficiency increased with a decrease in the engine speed, which caused the flow rate of exhaust gas to be slow. The dissociation probability of CO exhaust gas increased with the increase in contact time between the spark and CO exhaust gas. The CO removal efficiency was the highest in model B at 5000 rpm because the flow rate of exhaust gas was almost the same in model B. The dissociation probability of CO exhaust gas increased with an increase in CO gas flow per unit area. The reduction in CO removal efficiency of model B is higher than that of model A. This is because in model B, the dissociation of free radical and CO exhaust gas has a larger space, and the flow rate of exhaust gas was slow. This reduced the CO exhaust gas emissions. However, the amount of change in the CO exhaust gas measurement is small, there may be random errors in this experiment which may be caused by ambient temperature, humidity and . The random errors of CO exhaust values is about 0.1.

Molecules 2020, 25, x FOR PEER REVIEW 11 of 21

Molecules 2020, 25, 6041 11 of 20 Figure 14. The CO (a) emission values and (b) RE using platinum in the rear of the exhaust pipe.

FigureFigure 15.15. TheThe COCO ((aa)) emissionemission valuesvalues andand ((bb)) RERE usingusing iridiumiridium alloyalloy inin thethe rearrear of of the the exhaust exhaust pipe. pipe.

FiguresFigures 16 16–18–18 present present thethe COCO emissionemission valuevalue andand removalremoval eefficiencyfficiency of of controlcontrol voltagevoltage 5 5 VV usingusing didifferentfferent materialsmaterials andand atat variousvarious locationslocations ofof thethe NTPNTP reactor.reactor. TableTable7 7shows shows the the maximum maximum and and minimumminimum valuesvalues ofof COCO removalremoval eefficiencyfficiency atat controlcontrol voltagevoltage 55 VV usingusing didifferentfferent materialsmaterials andand atat variousvarious locations locations of of the the NTP NTP reactor. reactor. The The maximum maximu COm CO removal removal efficiency efficiency was 16.0%was 16.0% for iridium for iridium alloy (typealloy two)(type attwo) 5000 at 5000 rpm rpm and forand reactor for reactor in the in the rear rear of theof the exhaust exhaust system system (model (model B). B). The The minimum minimum COCO removalremoval eefficiencyfficiency waswas 3.3%3.3% forfor platinumplatinum (type(type one)one) atat 60006000 rpmrpm andand forfor reactorreactor inin thethe frontfront ofof thethe exhaustexhaust systemsystem (model(model A).A). TheThe NTPNTP reactorreactor materialmaterial ofof typestypes oneone andand typetype twotwo reducedreduced thethe COCO exhaustexhaust gasgas concentration.concentration. TheThe eeffectffect ofof typetype twotwo NTPNTP reactorreactor materialmaterial waswas foundfound toto bebe betterbetter thanthan thatthat ofof typetype oneone because,because, lowlow resistanceresistance formsforms largerlarger sparkspark andand highhigh internalinternal density.density. TheThe productionproduction ofof freefree radicalsradicals increasesincreases byby increasingincreasing thethe sparkspark size.size. TheThe largerlarger amountamount ofof radicalsradicals andand higherhigher combustioncombustion reactionreaction temperaturetemperature causedcaused anan increaseincrease inin thethe dissociationdissociation probabilityprobability ofof COCO exhaustexhaust gas.gas. The dissociation of of CO CO is is broadly broadly divided divided into into two two parts. parts. In Inthe the first first part, part, the the dissociation dissociation of CO of COgives gives carbon carbon and and oxygen oxygen atoms. atoms. Carbon Carbon atoms atoms get getdeposited deposited on onthe thesurface surface of the of the NTP NTP reactor, reactor, as asshown shown in inFigure Figure 19. 19 The. The second second part part involves involves the the recombination recombination of CO of CO and and oxygen oxygen atoms atoms to form to form the the CO . The location of the NTP reactor in models A and B reduce the CO exhaust gas concentration. CO2. The2 location of the NTP reactor in models A and B reduce the CO exhaust gas concentration. In Inmodel model A, A, the the CO CO removal removal efficiency efficiency increased increased with with a a decrease decrease in in the engine speed, whichwhich causedcaused thethe flowflowrate rateof of exhaustexhaust gasgas toto bebe slow.slow. TheThe dissociationdissociation probabilityprobability ofof COCO exhaustexhaust gasgas increasedincreased withwith thethe increaseincrease inin contactcontact timetime betweenbetween thethe sparkspark andand COCO exhaustexhaust gas.gas. TheThe COCO removalremoval eefficiencyfficiency waswas thethe highesthighest inin modelmodel BB atat 50005000 rpmrpm becausebecause thethe flowflow raterate ofof exhaustexhaust gasgas waswas almostalmost thethe samesame inin modelmodel B.B. TheThe dissociation dissociation probability probability of of CO CO exhaust exhaust gas gas increased increased with with an increasean increase in CO in gasCO flowgas flow per unit per area.unit Thearea. reduction The reduction in CO removalin CO removal efficiency efficiency of model of B model is higher B is than higher that ofthan model that A.of Thismodel is because A. Thisin is modelbecause B, in the model dissociation B, the dissociatio of free radicaln of andfree COradical exhaust and CO gas hasexhaust a larger gas space,has a larger and the space, flow and rate the of exhaustflow rate gas of wasexhaust slow. gas This was reduced slow. This the COreduced exhaust the gasCO emissions. exhaust gas However, emissions. the However, amount of the change amount in theof change CO exhaust in the gas CO measurement exhaust gas measurement is small, there is maysmal bel, there random may errors be random in this errors experiment in this whichexperiment may bewhich caused may by be ambient caused by temperature, ambient temperature, humidity and humidi dust.ty The and random dust. The errors random of CO errors exhaust of CO values exhaust is aboutMoleculesvalues 0.1. is 2020 about, 25, x0.1. FOR PEER REVIEW 12 of 21

FigureFigure 16.16. TheThe COCO ((aa)) emissionemission valuesvalues andand ((bb)) RERE ofof controlcontrol voltagevoltage 55 VV usingusing platinum.platinum.

Figure 17. The CO (a) emission values and (b) RE of control voltage 5 V using iridium alloy.

Figure 18. The CO RE at control voltage 5 V.

Table 7. The maximum and minimum values of CO removal efficiency at control voltage 5 V.

CO RE Value Parameter Maximum Minimum Platinum and in the front of the exhaust pipe (Type one and model A) 7% 3.3% Iridium alloy and in the front of the exhaust pipe (Type two and model A) 10.8% 5.9% Platinum and in the rear of the exhaust pipe (Type one and model B) 13% 10.1% Iridium alloy and in the rear of the exhaust pipe (Type two and model B) 16% 13.2%

MoleculesMolecules 20202020,, 2525,, xx FORFOR PEERPEER REVIEWREVIEW 1212 ofof 2121

Molecules 2020, 25, 6041 12 of 20 FigureFigure 16.16. TheThe COCO ((aa)) emissionemission valuesvalues andand ((bb)) RERE ofof controlcontrol voltagevoltage 55 VV usingusing platinum.platinum.

FigureFigure 17.17. TheThe COCO ((aa)) emissionemission valuesvalues andand ((bb)) RERE ofof controlcontrol voltagevoltagevoltage 555 VVV usingusingusing iridiumiridiumiridium alloy.alloy.alloy.

FigureFigure 18.18.18. TheThe COCO RERE atat controlcontrol voltagevoltage 55 V.V.

Table 7. The maximum and minimum values of CO removal efficiency at control voltage 5 V. TableTable 7.7. TheThe maximummaximum andand minimumminimum valuesvalues ofof COCO reremovalmoval efficiencyefficiency atat controlcontrol voltagevoltage 55 V.V. COCO RE RE Value Value Parameter CO RE Value ParameterParameter MaximumMaximumMaximum MinimumMinimumMinimum PlatinumPlatinum andand inin thethe frontfront ofof thethe exhausexhaus exhausttt pipe pipe (Type (Type one one and and model model A)A) 7% 7% 7% 3.3% 3.3% 3.3% Iridium alloy and in the front of the exhaust pipe (Type two and model A) 10.8% 5.9% Iridium alloy and in the front of the exhaust pipe (Type two and model A) 10.8% 5.9% IridiumPlatinum alloy and and in the in the rear front of the of exhaust the exhaus pipet pipe (Type (Type one andtwo modeland model B) A) 13% 10.8% 10.1% 5.9% PlatinumPlatinumIridium alloy andand andinin thethe in rearrear the rearofof thethe of exhausexhaus the exhausttt pipepipe pipe (Type(Type (Type oneone two andand and modelmodel model B)B) B) 16% 13% 13% 13.2% 10.1% 10.1% Molecules 2020, 25, x FOR PEER REVIEW 13 of 21 IridiumIridium alloyalloy andand inin thethe rearrear ofof thethe exhaexhaustust pipepipe (Type(Type twotwo andand modelmodel B)B) 16% 16% 13.2% 13.2%

Figure 19.19. AA photographic photographic view view of NTPof NTP reactor reactor (a) platinum (a) platinum and (c) iridiumand (c) alloyiridium before alloy this experimentbefore this experimentand (b) platinum and (b and) platinum (d) iridium and alloy(d) iridium after this alloy experiment. after this experiment.

3.3. CO22

Figures 20–2320–23 present the CO2 emission value and conversion efficiency efficiency using the different different materials and at various locations of the NTP reactor. The The control control voltage voltage of of 2 2 to 5 V in the NTP system could increase the CO2 emissions at different engine speeds. The maximum CO2 conversion efficiency was 0.9% at the control voltage 2 V for platinum (type one) at 4000 rpm and for reactor in the front of the exhaust system (model A). The results show that CO2 conversion efficiency increases with decreasing voltage. This is because decreasing voltage leads to a smaller shape of spark and this reduction causes a decline in the production of free radicals. Ultimately, this reduces the dissociation probability of CO2 exhaust gas.

Figure 20. The CO2 (a) emission values and (b) CE at platinum in the front of the exhaust pipe.

Figure 21. The CO2 (a) emission values and (b) CE using iridium alloy in the front of the exhaust pipe.

Molecules 2020, 25, x FOR PEER REVIEW 13 of 21 Molecules 2020, 25, x FOR PEER REVIEW 13 of 21 Molecules 2020, 25, 6041 13 of 20

system could increase the CO2 emissions at different engine speeds. The maximum CO2 conversion efficiency was 0.9% at the control voltage 2 V for platinum (type one) at 4000 rpm and for reactor in the front of the exhaust system (model A). The results show that CO2 conversion efficiency increases with decreasing voltage. This is because decreasing voltage leads to a smaller shape of spark and this reduction causes a decline in the production of free radicals. Ultimately, this reduces the dissociation probability of CO2 exhaust gas. Figures 24–26 show the CO2 emission value and conversion efficiency at control voltage 5 V using different materials and at various locations of the NTP reactor. Table8 shows the maximum and minimum values of CO2 conversion efficiency at control voltage 5 V using different materials and at various locations of the NTP reactor. The maximum CO2 conversion efficiency was 0.6% for platinum

(type one) at 4000 rpm and for reactor in the front of the exhaust system (model A). The minimum CO2 Figure 19. A photographic view of NTP reactor (a) platinum and (c) iridium alloy before this conversionFigure e19.fficiency A photographic was 0.3% view for iridiumof NTP alloyreactor (type (a) two)platinum at 4000 and rpm (c) iridium and for reactoralloy before in the this rear of experiment and (b) platinum− and (d) iridium alloy after this experiment. the exhaustexperiment system and (model (b) platinum B). Most and of (d the) iridium NTP reactoralloy after materials this experiment. of type one and type two increase CO2 exhaust gas concentration, while the type one exhibits a better effect than the type two. The reduced 3.3. CO2 3.3. CO2 production of free radicals causes the dissociation probability of CO2 exhaust gas to decrease. CO2 Figures 20–23 present the CO2 emission value and conversion efficiency using the different is alsoFigures obtained 20–23 from present the combination the CO2 emission of CO and value oxygen and atoms.conversion The dissociationefficiency using of CO the2 is different divided materials and at various locations of the NTP reactor. The control voltage of 2 to 5 V in the NTP intomaterials two parts. and at The various first part locations involves of the dissociationNTP reactor. of The CO control2 to give voltage CO and of oxygen 2 to 5 V atoms. in the In NTP the system could increase the CO2 emissions at different engine speeds. The maximum CO2 conversion secondsystem part,could the increase dissociation the CO of2 COemissions2 gives at carbon different and oxygenengine speeds. atoms. CarbonThe maximum atoms get CO deposited2 conversion on efficiency was 0.9% at the control voltage 2 V for platinum (type one) at 4000 rpm and for reactor in theefficiency surface was of the 0.9% NTP at reactorthe control (Figure voltage 19). Most2 V for of platinum the NTP reactor(type one) locations at 4000 of rpm model and A for and reactor model in B the front of the exhaust system (model A). The results show that CO2 conversion efficiency increases increasethe front the of COthe 2exhaustexhaust system gas concentration. (model A). The This results is because show the that amount CO2 conversion of CO2 dissociation efficiency is increases less than with decreasing voltage. This is because decreasing voltage leads to a smaller shape of spark and this thewith amount decreasing of recombination voltage. This is of because CO and decreasing oxygen atoms. voltage This leads result to a is smaller similar shape to that of of spark CO exhaustand this reduction causes a decline in the production of free radicals. Ultimately, this reduces the dissociation gasreduction emissions. causes However, a decline the in amountthe production of change of infree the radicals. CO2 exhaust Ultimately, gas measurement this reduces wasthe dissociation small, thus, probability of CO2 exhaust gas. thereprobability may be of errors CO2 exhaust in measurement. gas.

Figure 20. The CO2 (a) emission values and (b) CE at platinum in the front of the exhaust pipe. FigureFigure 20.20. The CO22 (a) emission values and (b) CE at platinum in the frontfront ofof thethe exhaustexhaust pipe.pipe.

FigureFigure 21.21. TheThe COCO22 (a) emission values andand ((bb)) CECE usingusing iridiumiridium alloyalloy inin thethe frontfront ofof thethe exhaustexhaust pipe.pipe. Figure 21. The CO2 (a) emission values and (b) CE using iridium alloy in the front of the exhaust pipe.

Molecules 2020, 25, 6041 14 of 20 Molecules 2020, 25, x FOR PEER REVIEW 14 of 21

Figure 22. The CO2 (a) emission values and (b) CE using platinum in the rear of the exhaust pipe. FigureFigure 22.22. The CO22 (a) emission values and (b) CE using platinum in thethe rearrear ofof thethe exhaustexhaust pipe.pipe.

Molecules 2020,, 25,, xx FORFOR PEERPEER REVIEWREVIEW 15 of 21 Figure 23. The CO2 (2a) emission values and (b) CE using iridium alloy in the rear of the exhaust pipe. Figure 23. The CO2 (a) emission values and (b) CE using iridium alloy in the rear of the exhaust pipe.

Figures 24–26 show the CO2 emission value and conversion efficiency at control voltage 5 V using different materials and at various locations of the NTP reactor. Table 8 shows the maximum and minimum values of CO2 conversion efficiency at control voltage 5 V using different materials and at various locations of the NTP reactor. The maximum CO2 conversion efficiency was 0.6% for platinum (type one) at 4000 rpm and for reactor in the front of the exhaust system (model A). The minimum CO2 conversion efficiency was −0.3% for iridium alloy (type two) at 4000 rpm and for reactor in the rear of the exhaust system (model B). Most of the NTP reactor materials of type one and type two increase CO2 exhaust gas concentration, while the type one exhibits a better effect than the type two. The reduced production of free radicals causes the dissociation probability of CO2 exhaust type two. The reduced production of free radicals causes the dissociation probability of CO2 exhaust gas to decrease. CO2 is also obtained from the combination of CO and oxygen atoms. The dissociation gas to decrease.Figure CO 24. 2The is also CO 2obtained (a) emission from values the combinationand (b) CE of controlof CO andvoltage oxygen 5 V using atoms. platinum. The dissociation FigureFigure 24.24. TheThe COCO22 (a) emission valuesvalues andand ((b)) CECE ofof controlcontrol voltagevoltage 55 VV usingusing platinum.platinum. of CO2 is divided into two parts. The first part involves the dissociation of CO2 to give CO and oxygen atoms. In the second part, the dissociation of CO2 gives carbon and oxygen atoms. Carbon atoms get deposited on the surface of the NTP reactor (Figure 19). Most of the NTP reactor locations of model A and model B increase the CO2 exhaust gas concentration. This is because the amount of CO2 dissociation is less than the amount of recombination of CO and oxygen atoms. This result is similar to that of CO exhaust gas emissions. However, the amount of change in the CO2 exhaust gas measurement was small, thus, there may be errors in measurement.

FigureFigure 25.25. The CO2 (a) emission values and ( b) CE of control voltage 5 V using iridium alloy. Figure 25. The CO22 (a) emission values and (b) CE of control voltage 5 V using iridium alloy.

Figure 26. The CO2 CE at control voltage 5 V. Figure 26. The CO2 CE at control voltage 5 V.

Table 8. The maximum and minimum values of CO2 conversion efficiency at control voltage 5 V. Table 8. The maximum and minimum values of CO2 conversion efficiency at control voltage 5 V.

CO2 CE Value CO2 CE Value Parameter Maximum Minimum Platinum and in the front of the exhaust pipe (Type one and model A) 0.6% 0.3% Iridium alloy and in the front of the exhaust pipe (Type two and model A) 0.5% 0.3% Platinum and in the rear of the exhaust pipe (Type one and model B) 0.1% −0.1% Iridium alloy and in the rear of the exhaust pipe (Type two and model B) 0% −0.3%

x 3.4. NOx Figure 27 presents the oil temperature of the engine at different engine speeds. Figures 28–31 x present the NOx emissionemission valuevalue andand thethe removalremoval efficiencyefficiency atat differentdifferent typestypes andand locationslocations ofof thethe NTPNTP x reactor. The principal source of NOx emissionsemissions isis thethe oxidationoxidation ofof atmosphericatmospheric nitrogen.nitrogen. WeWe foundfound x that a control voltage between 2 to 5 V in the NTP system, could reduce NOx emissionsemissions atat differentdifferent x engine speeds. The maximum NOx removalremoval efficiencyefficiency waswas 41.3%41.3% atat thethe controlcontrol voltagevoltage 44 VV forfor iridiumiridium alloy (type two) at 3000 rpm and for reactor in the front of the exhaust system (model A). The x minimum NOx removalremoval efficiencyefficiency waswas 4.7%4.7% atat thethe controlcontrol voltagevoltage 22 VV forfor platinumplatinum (type(type one)one) atat 60006000 x rpm and for reactor in the rear of the exhaust system (model B). The results show that NOx removalremoval

Molecules 2020, 25, x FOR PEER REVIEW 15 of 21

Figure 24. The CO2 (a) emission values and (b) CE of control voltage 5 V using platinum.

Molecules 2020, 25, 6041 15 of 20 Figure 25. The CO2 (a) emission values and (b) CE of control voltage 5 V using iridium alloy.

Figure 26. The CO2 CE at control voltage 5 V. Figure 26. The CO2 CE at control voltage 5 V.

Table 8. The maximum and minimum values of CO2 conversion efficiency at control voltage 5 V. Table 8. The maximum and minimum values of CO2 conversion efficiency at control voltage 5 V. CO2 CE Value Parameter CO2 CE Value Parameter Maximum Minimum Maximum Minimum Platinum and in the front of the exhaust pipe (Type one and model A) 0.6% 0.3% PlatinumIridium alloy and andin the in front the front of the of exhaust the exhaust pipe pipe (Type (Type one two and and model model A) A) 0.5% 0.6% 0.3% 0.3% IridiumPlatinum alloy and and in the in the rear front of the ofexhaust the exhaus pipet pipe (Type (Type one andtwo modeland model B) A) 0.1% 0.5% 0.1% 0.3% − Iridium alloy and in the rear of the exhaust pipe (Type two and model B) 0% 0.3% Platinum and in the rear of the exhaust pipe (Type one and model B) 0.1% −−0.1% Iridium alloy and in the rear of the exhaust pipe (Type two and model B) 0% −0.3% 3.4. NOx

3.4. NOFigurex 27 presents the oil temperature of the engine at different engine speeds. Figures 28–31 present the NOx emission value and the removal efficiency at different types and locations of the NTP Figure 27 presents the oil temperature of the engine at different engine speeds. Figures 28–31 reactor. The principal source of NOx emissions is the oxidation of atmospheric nitrogen. We found that present the NOx emission value and the removal efficiency at different types and locations of the NTP a control voltage between 2 to 5 V in the NTP system, could reduce NOx emissions at different engine reactor. The principal source of NOx emissions is the oxidation of atmospheric nitrogen. We found speeds.Molecules The2020, maximum 25, x FOR PEER NO REVIEWx removal efficiency was 41.3% at the control voltage 4 V for iridium16 alloy of 21 that a control voltage between 2 to 5 V in the NTP system, could reduce NOx emissions at different (type two) at 3000 rpm and for reactor in the front of the exhaust system (model A). The minimum engine speeds. The maximum NOx removal efficiency was 41.3% at the control voltage 4 V for iridium NOefficiencyx removal increased efficiency with was increasing 4.7% at thevoltage, control however, voltage NOx 2 V forremoval platinum efficiency (type began one) at decreasing 6000 rpm andafter alloy (type two) at 3000 rpm and for reactor in the front of the exhaust system (model A). The forreaching reactor control in the rear voltage of the 4 exhaustV. This systemis because (model the change B). The resultsin the spark show of that control NOx removal voltage e5ffi Vciency to the minimum NOx removal efficiency was 4.7% at the control voltage 2 V for platinum (type one) at 6000 increasedcontrol voltage with increasing 4 V was small. voltage, The however, free radicals NOx andremoval exhaust effi gasciency degradation began decreasing appeared after to be reaching similar, rpm and for reactor in the rear of the exhaust system (model B). The results show that NOx removal controlbut the voltage combustion 4 V. This reaction is because temperature the change inincrease the sparkd. With of control the increase voltage 5in V tocombustion the control voltagereaction 4 temperature, V was small. Thethe amount free radicals of diatomic and exhaust nitrogen gas degradation dissociated appeared from the to monatomic be similar, butnitrogen the combustion increased reactionand finally temperature resulted in increased. high NO Withx. the increase in combustion reaction temperature, the amount of diatomic nitrogen dissociated from the monatomic nitrogen increased and finally resulted in high NOx.

Figure 27. The oil temperature of the engine at different engine speeds. Figure 27. The oil temperature of the engine at different engine speeds.

Figure 28. The NOx (a) emission values and (b) RE using platinum in the front of the exhaust pipe.

Figure 29. The NOx (a) emission values and (b) RE using iridium alloy in the front of the exhaust pipe.

Molecules 2020, 25, x FOR PEER REVIEW 16 of 21 Molecules 2020, 25, x FOR PEER REVIEW 16 of 21 efficiency increased with increasing voltage, however, NOx removal efficiency began decreasing after efficiency increased with increasing voltage, however, NOx removal efficiency began decreasing after reaching control voltage 4 V. This is because the change in the spark of control voltage 5 V to the reaching control voltage 4 V. This is because the change in the spark of control voltage 5 V to the control voltage 4 V was small. The free radicals and exhaust gas degradation appeared to be similar, control voltage 4 V was small. The free radicals and exhaust gas degradation appeared to be similar, but the combustion reaction temperature increased. With the increase in combustion reaction but the combustion reaction temperature increased. With the increase in combustion reaction temperature, the amount of diatomic nitrogen dissociated from the monatomic nitrogen increased temperature, the amount of diatomic nitrogen dissociated from the monatomic nitrogen increased and finally resulted in high NOx. and finally resulted in high NOx.

Molecules 2020, 25, 6041 16 of 20 Figure 27. The oil temperature of the engine at different engine speeds. Figure 27. The oil temperature of the engine at different engine speeds.

Figure 28. The NOx (a) emission values and (b) RE using platinum in the front of the exhaust pipe. FigureFigure 28.28.The The NONOxx ((aa)) emissionemission valuesvalues andand ((bb)) RERE usingusing platinumplatinum inin thethe frontfront ofof thethe exhaustexhaust pipe.pipe.

Molecules 2020, 25, x FOR PEER REVIEW 17 of 21 Molecules 2020, 25, x FOR PEER REVIEW 17 of 21 FigureFigure 29.29. The NOxx (a) emission emission values and (b) RE using iridium alloyalloy inin thethe frontfront ofof thethe exhaustexhaust pipe.pipe. Figure 29. The NOx (a) emission values and (b) RE using iridium alloy in the front of the exhaust pipe.

Figure 30. The NOx (a) emission values and (b) RE using platinum and in the rear of the exhaust pipe. FigureFigure 30.30. The NOx ((aa)) emission emission values values and ( b) RE using platinum and in the rear of the exhaust pipe.pipe.

FigureFigure 31. 31.The The NO NOx x( a(a)) emission emission valuesvalues andand (b) RE using using iridium iridium alloy alloy in in the the rear rear of of the the exhaust exhaust pipe. pipe. Figure 31. The NOx (a) emission values and (b) RE using iridium alloy in the rear of the exhaust pipe.

Figures 32–34 present NOx emission value and removal efficiency at control voltage 5 V using Figures 32–34 present NOx emission value and removal efficiency at control voltage 5 V using Figures 32–34 present NOx emission value and removal efficiency at control voltage 5 V using didifferentfferent materialsmaterials and atat variousvarious locationslocations of thethe NTPNTP reactor.reactor. Table 9 9 shows shows thethe maximum maximum andand different materials and at various locations of the NTP reactor. Table 9 shows the maximum and minimum values of NONOxx removal eefficiencyfficiency at control voltage 5 V using didifferentfferent materialsmaterials and at minimum values of NOx removal efficiency at control voltage 5 V using different materials and at various locations of the NTP reactor. The maximum NOx removal efficiency was 37.6% for iridium various locations of the NTP reactor. The maximum NOx removal efficiency was 37.6% for iridium various locations of the NTP reactor. The maximum NOx removal efficiency was 37.6% for iridium alloy (type(type two) two) at at 4000 4000 rpm rpm and and for reactorfor reactor in the in front the offront the exhaustof the exhaust system (model system A). (model The minimum A). The alloy (type two) at 4000 rpm and for reactor in the front of the exhaust system (model A). The minimum NOx removal efficiency was 7.5% for platinum (type one) at 6000 rpm and for reactor in minimum NOx removal efficiency was 7.5% for platinum (type one) at 6000 rpm and for reactor in the rear of the exhaust system (model B). The results show an increase in NOx exhaust gas the rear of the exhaust system (model B). The results show an increase in NOx exhaust gas concentration with an increase in engine speed, which then causes the temperature of the combustion concentration with an increase in engine speed, which then causes the temperature of the to increase. At high temperatures, nitrogen and oxygen atoms easily recombine to form chamber to increase. At high temperatures, nitrogen and oxygen atoms easily recombine to form NOx. The NTP reactor materials of type one and type two reduce the NOx exhaust gas concentration. NOx. The NTP reactor materials of type one and type two reduce the NOx exhaust gas concentration. The effect of type two material was found to be better than type one. The production of free radicals The effect of type two material was found to be better than type one. The production of free radicals increased by increasing the spark size and the area of contact between the free radicals and NOx increased by increasing the spark size and the area of contact between the free radicals and NOx exhaust gas also increased. This enhanced the probability of dissociation of NOx exhaust gas to give exhaust gas also increased. This enhanced the probability of dissociation of NOx exhaust gas to give nitrogen and oxygen atoms. The NTP reactor location in models A and B reduced the NOx exhaust nitrogen and oxygen atoms. The NTP reactor location in models A and B reduced the NOx exhaust gas concentration. Moreover, the NOx removal efficiency increased with decreasing engine speed gas concentration. Moreover, the NOx removal efficiency increased with decreasing engine speed because of the slower the flow rate of exhaust gas and the lower temperature of exhaust gas. The NOx because of the slower the flow rate of exhaust gas and the lower temperature of exhaust gas. The NOx removal efficiency of model A was better among the two models because the products of NOx removal efficiency of model A was better among the two models because the products of NOx dissociation can easily recombine to form the NOx at high temperatures. The combination possibility dissociation can easily recombine to form the NOx at high temperatures. The combination possibility of nitrogen and oxygen atoms is lower at the faster flow rate of the exhaust gas. Therefore, the NOx of nitrogen and oxygen atoms is lower at the faster flow rate of the exhaust gas. Therefore, the NOx exhaust gas emissions are reduced. exhaust gas emissions are reduced.

Molecules 2020, 25, 6041 17 of 20

NOx removal efficiency was 7.5% for platinum (type one) at 6000 rpm and for reactor in the rear of the exhaust system (model B). The results show an increase in NOx exhaust gas concentration with an increase in engine speed, which then causes the temperature of the combustion chamber to increase. At high temperatures, nitrogen and oxygen atoms easily recombine to form NOx. The NTP reactor materials of type one and type two reduce the NOx exhaust gas concentration. The effect of type two material was found to be better than type one. The production of free radicals increased by increasing the spark size and the area of contact between the free radicals and NOx exhaust gas also increased. This enhanced the probability of dissociation of NOx exhaust gas to give nitrogen and oxygen atoms. The NTP reactor location in models A and B reduced the NOx exhaust gas concentration. Moreover, the NOx removal efficiency increased with decreasing engine speed because of the slower the flow rate of exhaust gas and the lower temperature of exhaust gas. The NOx removal efficiency of model A was better among the two models because the products of NOx dissociation can easily recombine to form the NOx at high temperatures. The combination possibility of nitrogen and oxygen atoms is lower at the faster flow rate of the exhaust gas. Therefore, the NOx exhaust gas emissions are reduced.

Table 9. The maximum and minimum values of NOx removal efficiency at control voltage 5 V.

NOx RE Value Parameter Maximum Minimum Platinum and in the front of the exhaust pipe (Type one and model A) 24.8% 9.8% Iridium alloy and in the front of the exhaust pipe (Type two and model A) 37.6% 18% Platinum and in the rear of the exhaust pipe (Type one and model B) 19.3% 7.5% MoleculesIridium 2020,, 25 alloy,, xx FORFOR and PEERPEER in the REVIEWREVIEW rear of the exhaust pipe (Type two and model B) 30.1% 17% 18 of 21

FigureFigure 32.32. TheThe NONOxxx ((a)) emissionemission valuesvalues andand ((bb)) RERE ofof controlcontrol voltagevoltagevoltage 555 VVV usingusingusing platinum.platinum.platinum.

FigureFigure 33.33. TheThe NONOxxx ((a)) emission emission values values and and ( (b)) RE RE ofof controlcontrol voltagevoltage 55 VV usingusing iridiumiridium alloy.alloy.alloy.

Figure 34. The NOxx RERE atat controlcontrol voltagevoltage 55 V.V.

Table 9. The maximum and minimum values of NOx removal efficiency at control voltage 5 V.

NOx RE Value Parameter Maximum Minimum Platinum and in the front of the exhaust pipe (Type one and model A) 24.8% 9.8% IridiumIridium alloyalloy andand inin thethe frontfront ofof thethe exhaexhaust pipe (Type two and model A) 37.6% 18% Platinum and in the rear of the exhaust pipe (Type one and model B) 19.3% 7.5% IridiumIridium alloyalloy andand inin thethe rearrear ofof thethe exhaexhaust pipe (Type two and model B) 30.1% 17%

4. Conclusions InIn thisthis study,study, aa motorcyclemotorcycle engineengine waswas examinexamined using the NTP system to measure exhaust gas emission using different materials, voltage, and locations of the NTP reactor. Considering the HC exhaust gas emissions, the NTP system re-burns HC exhaust gas to reduce thethe HCHC exhaustexhaust gasgas concentration.concentration. TheThe HCHC removalremoval efefficiencyficiency increasesincreases withwith increasingincreasing voltage.voltage. TheThe shapes and density of the spark approached critical values at the control voltage 4 V. The HC exhaust gas emissions of model B were lower than model A. The effect of type two NTP reactor material was better than that of type one. The maximum reductionion ofof HCHC removalremoval efficiencyefficiency obtainedobtained withwith thethe NTP system reached 34.5%, while the minimum HC removal efficiency was 1.2%.

Molecules 2020, 25, x FOR PEER REVIEW 18 of 21

Figure 32. The NOx (a) emission values and (b) RE of control voltage 5 V using platinum.

Molecules 2020, 25, 6041 18 of 20 Figure 33. The NOx (a) emission values and (b) RE of control voltage 5 V using iridium alloy.

FigureFigure 34. 34.The The NONOxx RERE atat controlcontrol voltagevoltage 55 V.V. 4. Conclusions Table 9. The maximum and minimum values of NOx removal efficiency at control voltage 5 V. In this study, a motorcycle engine was examined using the NTP system to measure exhaust gas NOx RE Value emission using different materials,Parameter voltage, and locations of the NTP reactor. Maximum Minimum Considering the HC exhaust gas emissions, the NTP system re-burns HC exhaust gas to reduce the HCPlatinum exhaust and gas in the concentration. front of the exhaust The HCpipe removal(Type one eandfficiency model increasesA) with 24.8% increasing 9.8% voltage. The shapesIridium and alloy density and in the of front the spark of the approachedexhaust pipe (Type critical two values and model at the A) control 37.6% voltage 4 V.18% The HC exhaustPlatinum gas emissions and in the of rear model of the B wereexhaust lower pipe than (Type model one and A. Themodel eff ectB) of type two 19.3% NTP reactor 7.5% material was better than that of type one. The maximum reduction of HC removal efficiency obtained with the Iridium alloy and in the rear of the exhaust pipe (Type two and model B) 30.1% 17% NTP system reached 34.5%, while the minimum HC removal efficiency was 1.2%. Considering the CO exhaust gas emissions, the reaction between free radicals and CO exhaust gas 4. Conclusions gates mainly dissociated to obtain carbon and oxygen atoms. Carbon gets deposited on the surface of theIn NTP this reactorstudy, a and motorcycle affects the engine life of was the examin NTP reactor.ed using Furthermore, the NTP system CO andto measure oxygen exhaust atoms also gas recombineemission using to form different CO2. Thematerials, CO removal voltage, effi andciency locations increases of the with NTP the reactor. increase in voltage. The CO removalConsidering efficiency the of model HC exhaust B was foundgas emissions, to be higher the NT thanP thatsystem of modelre-burns A. TheHC exhaust effect of gas the typeto reduce two NTPthe HC reactor exhaust material gas concentration. was better than The type HC one.removal The ef maximumficiency increases reduction with of COincreasing removal voltage. efficiency The obtainedshapes and with density the NTP of the system spark reached approached 16.0%, critical while values the minimum at the control CO removal voltage e ffi4 V.ciency The HC was exhaust 1.2%. gas emissionsIn terms of of themodel CO 2B exhaustwere lower gas than emissions, model theA. The CO effect2 exhaust of type gas two dissociates NTP reactor to form material CO andwas oxygenbetter than atoms, that and of thetype dissociation one. The maximum of CO2 obtains reduct carbonion of atoms HC removal and oxygen efficiency atoms. obtained Like CO, with carbon the getsNTP deposited system reached on the 34.5%, surface while of the the NTP minimum reactor. HC However, removal most efficiency of the amountwas 1.2%. of dissociation was found to be less than the amount of HC re-burning and CO reduction. Therefore, the total amount of CO2 increased and the CO2 conversion efficiency increased with decreasing voltage. The CO2 conversion efficiency of model A was higher than that of model B. The effect of type one NTP reactor material was better than that of type two. The maximum CO2 conversion efficiency obtained with the NTP system reached 0.9%, while the minimum CO conversion efficiency was 0.3%. 2 − Considering the NOx exhaust gas emissions, the NOx gets dissociated by the NTP system to reduce the NOx exhaust gas concentration. The removal efficiency NOx increased with increasing voltage, but the NOx exhaust gas removal efficiency began to decrease after reaching control voltage 4 V. As the combustion reaction temperature increased, the amount of diatomic nitrogen dissociated from the monatomic nitrogen increased and finally resulted in high NOx. The NOx exhaust gas emissions of model A were lower than that of model B. The effect of type two NTP reactor material was better than that of type one. The maximum reduction of NOx exhaust gas emissions removal efficiency obtained with the NTP system reached 41.3%, while the minimum NOx removal efficiency was 4.7%. Finally, according to the above discussion, the effect of iridium alloy is better than platinum for HC, CO, CO2, and NOx emission removal. The effect of the NTP reactor in the rear of the exhaust system is better than in the front of the exhaust system for HC, CO, and CO2 emission removal. However, the NOx emission removal for the reactor in front of the exhaust system is better than that in the rear of the exhaust system. Molecules 2020, 25, 6041 19 of 20

5. Patents Taiwan Patent M526623.

Author Contributions: M.-H.H. guided the research direction, found the solutions and designed the approach to set up the testing facility; C.-N.W. wrote the initial draft of the paper; M.-C.H. analyzed the experimental results in this study; S.-H.W., C.-H.H., C.-J.L., T.-L.W., and J.-H.Y. conducted the experiments and collected the results. All authors have read and agreed to the published version of the manuscript. Funding: This paper is supported by the Ministry of Science and Technology (MOST) and the National Kaohsiung University of Science and Technology under MOST-106-2622-E-151-007-CC3. Acknowledgments: The authors would like their thanks to the Ministry of Science and Technology (MOST) and the National Kaohsiung University of Science and Technology for their financial support during this study. Conflicts of Interest: The authors declare no conflict of interest.

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