Exhaust Gas Characteristics According to the Injection Conditions in Diesel and DME Engines

applied sciences

Article Exhaust Gas Characteristics According to the Injection Conditions in Diesel and DME Engines

Seamoon Yang 1 and Changhee Lee 2,*

1 Industry-University Cooperation (LINC), Chosun University, 309 Pilmun-daero, Dong-gu, Gwangju 61452, Korea; [email protected] 2 Department of Mechanical and Shipbuilding Convergence Engineering, Pukyung National University, Busan 48547, Korea * Correspondence: [email protected]; Tel.: +82-51-629-7816

Received: 18 December 2018; Accepted: 12 February 2019; Published: 14 February 2019

Abstract: In this paper, the effect of high-pressure injection pressure on particulate matter (PM) and nitrogen oxide (NOx) emissions is discussed. Many studies have been conducted by active researchers on high-pressure engines; however, the problem of reducing PM and NOx emissions is still not solved. Therefore, in the existing diesel (compression ignition) engines, the common rail high-pressure injection system has limitations in reducing PM and NOx emissions. Accordingly, to solve the exhaust gas emission problem of a compression ignition engine, a compression ignition engine using an alternative fuel is discussed. This study was conducted to optimize the dimethyl ether (DME) engine system, which can satisfy the emission gas exhaust requirements that cannot be satisﬁed by the current common rail diesel compression ignition engine in terms of efﬁciency and exhaust gas using DME common rail compression ignition engine. Based on the results of this study on diesel and DME engines under common rail conditions, the changes in engine performance and emission characteristics of exhaust gases with respect to the injection pressure and injection rate were examined. The emission characteristics of NOx, hydrocarbons, and carbon monoxide (CO) emissions were affected by the injection pressure of pilot injection. Under these conditions, the exhaust gas characteristics were optimized when the pilot injection period and needle lift were varied.

Keywords: DME; common rail; direct injection engine; pilot injection; main injection; needle lift

1. Introduction In order to prevent air pollution caused by exhaust gas from automobiles, exhaust emission regulations such as Tier-III and EURO-VI have been announced in various countries such as USA, Europe and Japan. Many researchers are working to reduce harmful emissions by gradually increasing the allowable regulation of hazardous emissions from automobiles [1]. Diesel engines have been widely used as power sources for automobiles and ships because of their high thermal efficiency and fuel efficiency. It has the advantage of low carbon monoxide (CO) and unburned hydrocarbons (HC) because it burns under lean conditions where the fuel consumption rate is lower than that of the gasoline engine and the combustion state is excess oxygen. However, since it has the disadvantage of high PM (particulate matter) and NOx (nitrogen oxides) emissions, it cannot satisfy the increasingly stringent emission regulations [2,3]. Using diesel particulate filters (DPF) together with diesel oxidation catalysts (DOC) is a technology that can reduce the emissions of diesel cars to the level of the latest gasoline vehicles. There have been many studies on nitrogen oxides in various countries depending on the social environment and economic conditions. However, in Europe, selective catalytic reduction (SCR) and US prefer the NOx storage catalyst (LNT, Lean NOx trap) [4]. The exhaust emission reduction technique

Appl. Sci. 2019, 9, 647; doi:10.3390/app9040647 www.mdpi.com/journal/applsci Appl. Sci. 2019, 9, 647 2 of 19 using the above-described exhaust after-treatment apparatus reduces emissions discharged through the engine combustion process to a separate device in the exhaust system. The soot purification apparatus and the NOx storage catalyst are excellent in filtering soot and NOx components, respectively Performance. However, in order to apply the actual engine, it is necessary to remove the accumulated soot or SOx through a process called regeneration while monitoring the state before and after the filter [5–7]. A number of studies have been conducted to reduce exhaust gases such as nitrogen oxides and soot using various fuel properties and additives [6,7], multiple injection [8,9], EGR [10–12] and PCCI/SCCI/CAI [13–16]. DME (dimethyl ether, CH3-O-CH3) among alternative fuels for diesel engines has low auto ignition temperature. It also has good evaporative performance when sprayed into the combustion chamber and has a higher cetane number (>55) than the diesel fuel. In particular, DME is an oxygen-containing fuel containing about 34.8 wt% of oxygen, and because there is no direct coupling between carbon and carbon in the fuel characteristics, almost no PM is being emitted in the diesel engine [17–23]. Due to these advantages, it is possible to apply a large amount of exhaust gas recirculation (EGR) without much difficulty to the engine, and it can greatly reduce NOx, and it has many excellent characteristics as an alternative fuels for diesel engines [24–28]. This is because it is easy to change and improve parameters affecting injection characteristics such as injection timing, injection duration, and injection pressure and injection quantity independently of engine rotation through electronic control. It is a great advantage to have superior advantages compared to the existing diesel engines such as output, fuel consumption, noise and vibration, and reduction of NOx in exhaust gas through electronic control according to each 4 conditions [29–31]. Experiments and numerical studies on spray combustion and exhaust characteristics for compression ignition engines using DME fuel have been conducted by many researchers. Numerical studies on compression ignition engines show that the application of diesel alternative fuels is largely dependent on the physical and chemical properties of the fuel and the application of a chemical reaction mechanism for combustion simulation Teng et al. [32] used the results of the study on the thermodynamic properties of DME fuel and the fuel property data of the AVL FIRE program [28] as input to the fuel material subproject of KIVA-3V code. Numerical analysis of the compression ignition engine using DME fuel was performed by applying the detailed reaction mechanism of DME fuel proposed by Fisher and Curran et al. [33,34]. Comparisons with experimental results confirm that the results of the numerical analysis simulate the spray combustion and exhaust characteristics well. In this study, the effects of engine performance, injection pressure and pilot injection period using injection pressure and injection rate on diesel and DME engines, pilot injection rate as a function of injection timing, and needle lift and injection timing on exhaust gas characteristics were investigated to optimize the DME combustion engines.

2. Experimental Apparatus and Methodology

2.1. Experimental Setup To prevent the occurrence of vapor lock because of the DME characteristics, an experimental setup was designed and fabricated, as shown in Figure1, using a vapor lock prevention device (liquid holding device), a booster, and a special plunger pump (gas booster and liquid pump from HASKEL) for the adhesion possibility of the sliding parts. Appl. Sci. 2019, 9, 647 3 of 19 Appl. Sci. 2019, 9, x FOR PEER REVIEW 3 of 21

Figure 1. Schematic diagram of the liquid fuel supply system. Figure 1. Schematic diagram of the liquid fuel supply system. The experimental engine shown in Table1 is an engine from Daedong, which is a water-cooled single-cylinderThe experimental four-cycle engine direct-injection shown in Table diesel 1 is engine. an engine Its specificationsfrom Daedong, are which total is displacement a water-cooled of 673single-cylinder cc, maximum four-cycle output of direct-injection 9.11 kW with 2200 diesel rpm, engine. and maximum Its specifications torque of are 4.22 total kg m displacement with 1800 rpm. of 673 cc, maximum output of 9.11 kW with 2200 rpm, and maximum torque of 4.22 kg m with 1800 rpm. Table 1. Engine specifications in this study. The injection period pulse (using AM503B current probe amplifiers from Tektronix, Beaverton, OR, USA) was performed for the commonItems rail injector injection Specifications characteristics and compared with the actual injection amount. The line pressureCycle according to the two-way 4 pulse was measured (using 4067A2000 amplifiers from KISTLER,Combustion Winterthur, Type Switzerland) to CI confirm the pilot of the injection lift operating period and the pressureCompression fluctuation Ratio of the main injection. 18 The injection needle lift was measured (using signal condition amplifiers from WOLF, Boston, MA, USA) to compare it with the Displacement (`) 0.673 line pressure under the loading condition of the engine. In addition, the injection rate, which is the Bore × Stroke (mm × mm) 95 × 95 core of the common rail high-pressure injection characteristics, was measured (Japan KYOWA Electric Corporation (Tokyo, Japan),Max. Power signal (kW/rpm) condition: CDV700A, 9.11/2200 bridge box: DB120, strain gauge: KFG-03). Figure 2 shows the fuelMax. injection Torque control (kg m) system and test 4.22 engine configuration. The engine control system in this study applied the ECD-U2 common rail system manufactured by NIPPON DENSOThe (Tokyo, injection Japan). period To pulse input (using the engine AM503B revoluti currentons probe and crank amplifiers positions from into Tektronix, the engine Beaverton, control OR,unit USA)(ECU), was Ne performed pulse (crank for position the common pulse) rail for injector injection injection characteristics characteristics experiments and compared and Ge pulse with the(cylinder actual detect injection pulse) amount. embedded The line in pressurethe supply according pump were to the used. two-way For the pulse measurement was measured of exhaust (using 4067A2000gases, non-dispersive amplifiers frominfrared KISTLER, spectrometry Winterthur, (using Switzerland) MEXA 324JK to from confirm HORIBA) the pilot was of performed the injection for liftCO operating and THC, period the chemiluminescence and the pressure fluctuationmethod (using of the the main 10 AR injection. model of The the injection thermo needleenvironmental lift was measuredinstrument) (using for NO signalx, and condition the reflection amplifiers photometer from WOLF, method Boston, (AFT-2000 MA, from USA) World to compare Environment) it with the for linesmoke. pressure The data under obtained the loading from conditionthese instruments of the engine. were used In addition, as a representative the injection value rate, using which the is theaverage core ofvalue the for common the three rail measurement high-pressure results. injection characteristics, was measured (Japan KYOWA Electric Corporation (Tokyo, Japan), signal condition: CDV700A, bridge box: DB120, strain gauge: KFG-03). Figure2 shows the fuelTable injection 1. Engine control specifications system and in this test study. engine configuration. The engine control system in this study applied theItems ECD-U2 commonSpecifications rail system manufactured by NIPPON DENSO (Tokyo, Japan). To input the engineCycle revolutions and crank4 positions into the engine control unit (ECU), Ne pulse (crank positionCombustion pulse) forType injection characteristics CI experiments and Ge pulse Compression Ratio 18 Appl. Sci. 2019, 9, 647 4 of 19

Appl. Sci. 2019, 9, x FOR PEER REVIEW 4 of 21 (cylinder detect pulse) embedded in the supply pump were used. For the measurement of exhaust gases, non-dispersive infrared spectrometry (using MEXA 324JK from HORIBA) was performed for Displacement (ℓ) 0.673 CO and THC, the chemiluminescence method (using the 10 AR model of the thermo environmental Bore × Stroke (mm × mm) 95 × 95 instrument) for NOx, and the reﬂection photometer method (AFT-2000 from World Environment) Max. Power (kW/rpm) 9.11/2200 for smoke. The data obtained from these instruments were used as a representative value using the Max. Torque (kg m) 4.22 average value for the three measurement results.

FigureFigure 2.2. Schematic diagramdiagram ofof thethe fuelfuel injectioninjection controlcontrol system.system. 2.2. Experimental Methodology 2.2. Experimental Methodology Fuel injection conditions were changed by controlling the injection timing, injection duration, Fuel injection conditions were changed by controlling the injection timing, injection duration, injection pressure, pilot injection rate, and injector needle lift using the ECU program. injection pressure, pilot injection rate, and injector needle lift using the ECU program. The injection timing (10◦, 14◦, 18◦, and 22◦ before top dead center (BTDC)) was adjusted using The injection timing (10°, 14°, 18°, and 22° before top dead center (BTDC)) was adjusted using the the common rail injection device manual controller based on the injection timing (22◦ BTDC) of the common rail injection device manual controller based on the injection timing (22° BTDC) of the mechanical mechanical injection pump in accordance with the specifications of the single-cylinder horizontal diesel injection pump in accordance with the specifications of the single-cylinder horizontal diesel engine that engine that was used as the test engine. In this instance, the injection lift (0.14, 0.21, 0.24, and 0.36 mm) was used as the test engine. In this instance, the injection lift (0.14, 0.21, 0.24, and 0.36 mm) was changed was changed based on the injection nozzle (ψ = 0.22 mm × 5 ea), and the injection characteristics, based on the injection nozzle (ψ = 0.22 mm × 5 ea), and the injection characteristics, engine performance, engine performance, and exhaust gas characteristics were investigated according to the pilot and main and exhaust gas characteristics were investigated according to the pilot and main injections. The engine injections. The engine revolutions were 1800 rpm, load ratio was 75%, and common rail injection revolutions were 1800 rpm, load ratio was 75%, and common rail injection pressure was 50 MPa. The pressure was 50 MPa. The braking thermal efficiency was focused on maintaining the current common braking thermal efficiency was focused on maintaining the current common rail engine and reducing NOx rail engine and reducing NOx among the exhaust gases [10–12]. among the exhaust gases [10–12]. 3. Results and Investigations 3. Results and Investigations 3.1. Effects of Atomization on Exhaust Gas 3.1. Effects of Atomization on Exhaust Gas Figure3 shows the effect of high-pressure injection pressure on PM and NO x emissions. Many studiesFigure have 3 been shows conducted the effect by of active high-pressure researchers injection on the high-pressurepressure on PM engine; and NO however,x emissions. the problem Many ofstudies reducing have PM been and conducted NOx emissions by active is still researchers not solved. on In thethe existinghigh-pressure diesel (compressionengine; however, ignition) the engines,problem theof reducing common PM rail and high-pressure NOx emissions injection is still system not solved. has limitations In the existing in reducing diesel PM(compression and NOx emissions.ignition) engines, Therefore, the common to solvethe rail exhaust high-pressure gas emission injection problem system of has the limitations compression in reducing ignition PM engine, and aNO compressionx emissions. ignition Therefore, engine to usingsolve anthe alternative exhaust ga fuels emission was studied problem [21–24 of]. the In thiscompression study, the exhaustignition gasengine, problem, a compression which cannot ignition be solved engine by using the existingan altern commonative fuel rail was diesel studied compression [21–24]. In ignition this study, engine, the wasexhaust solved gas with problem, the optimization which cannot of be a DME solved engine by the system existing that common can satisfy rail bothdiesel efficiency compression and exhaustignition gasengine, emission. was solved with the optimization of a DME engine system that can satisfy both efficiency and exhaust gas emission. Appl.Appl. Sci. Sci. 20192019, ,99, ,x 647 FOR PEER REVIEW 5 5of of 21 19

Figure 3. Effects of injection pressure on the exhaust gas. Figure 3. Effects of injection pressure on the exhaust gas.

Table2 presents the causes of PM and NO x generation and the mitigation measures. To solve the problemTable 2 presents of exhaust the gascauses emission of PM fromand NO conventionalx generation fossil and fuelsthe mitigation of compression measures. ignition To solve engines, the problemDME, which of exhaust is a natural gas gasemission reforming from fuel, conventional was used asfossil an alternativefuels of compression fuel. The thermal ignition efﬁciency engines, of DME,the compression which is a natural ignition gas engines reforming using fuel, the DME was fuelused was as an studied alternative to determine fuel. The the thermal optimal efficiency injection ofconditions the compression to maintain ignition the current engines level using of common the DME rail fuel diesel was engines studied and to simultaneously determine the reduce optimal the injection conditions to maintain the current level of common rail diesel engines and simultaneously PM and NOx emissions. reduce the PM and NOx emissions.

Table 2. Causes of PM and NOx generation and mitigation measures. Table 2. Causes of PM and NOx generation and mitigation measures. Emission Methods Effects of Reduction Emission Injection Methods timing delay Lowering of Effects combustion of Reduction temperature [35] Lowering of combustion temperature [36] Exhaust gas recirculation Injection timing delay LoweringLowering of ofcombustion oxygen thickness temperature [37] [35]

NOx Water fulmination Lowering of combustion temperature [36] Exhaust gas recirculation Lowering of combustion temperature [38] Fuel–water injection Lowering of oxygen thickness [37] WaterPilot fulmination injection Lowering of combustion rate in the ﬁrst period [39] Improving cetane number LoweringLowering of combustion of combusti rateon in thetemperature ﬁrst period [38] [40] Fuel–water injection NOx After treatment NOx recharge and resolve [41–45] Lowering of combustion rate in the first Pilot injection Air inhalation increase in spray [46] Spray improvement period [39] Mixing increase [47] Lowering of combustion rate in the first Improving cetane number Optimization in Mixingperiod increase [40] [48 ] combustion chamber Increase in air usage rate [49] After treatment NOx recharge and resolve [41–45] Mixing increase [50] Confusion combustion PM Air inhalation increase in spray [46] Spray improvement Increase of smoke oxidation [51] LeanMixing burn combustion increase [47] [52] Turbo charging Optimization in combustion Air inhalationMixing increase increase in spray [48] [52] PM Reduction inchamber oil consumption Increase Reduction in air of usage SOF [53 rate] [49] Low sulfur fuel Reduction of sulfur combination matter [53–55] Mixing increase [50] ConfusionAfter treatment combustion Smoke catching and SOF oxidation [56,57] Increase of smoke oxidation [51] Appl. Sci. 2019, 9, x FOR PEER REVIEW 6 of 21

Lean burn combustion [52] Turbo charging Air inhalation increase in spray [52]

Reduction in oil consumption Reduction of SOF [53] Reduction of sulfur combination matter Low sulfur fuel [53–55] After treatment Smoke catching and SOF oxidation [56,57]

3.2. Effects of Injection Pressure on Exhaust Gas Emission Characteristics of Diesel and DME Engines

1. NOx emission characteristics of diesel and DME engines Figure 4 shows a comparison of the exhaust gas emission characteristics of DME and diesel engines depending on the injection speed and pressure of the engines at an injection timing of 14° BTDC and 25% load factor. NOx emissions in DME engines are not significantly different from those in diesel engines; however, the emission characteristics of the fuel injection pressure are different. Appl. Sci. 2019, 9, 647 6 of 19 In particular, NOx emissions are lowest at 2100 rpm in DME engines, unlike those in diesel engines. This suggests that the DME engine used in this study has an unstable combustion state because3.2. Effects of ofthe Injection non-optimized Pressure on fuel Exhaust injection Gas Emission conditions, Characteristics such as ofinjection Dieseland timing DME and Engines amount according to the injection pressure of the fuel. Thus, the combustion state of the DME fuel, which is 1.NO x emission characteristics of diesel and DME engines an oxygen fuel, is generally improved, thereby increasing the combustion temperature and NOx emissionFigure concentration4 shows a comparison[58,59]. This ofis consistent the exhaust with gas the emission emission characteristics characteristics of of DME other and internal diesel combustionengines depending engines. onTherefore, the injection it suggests speed and that pressure it is necessary of the engines to investigate at an injection the possibility timing of of 14 ◦ reducingBTDC and NO 25%x emissions load factor. in DME engines using the fuel injection pressure characteristics. Load 25%/BTDC14deg./NZ0.22×6 140 1200RPM 120 1500RPM 100 1800RPM 2100RPM 80

40 NOx emission[ppm] 20

0 25 30 40 45 25 30 40 45 DIESEL DME Injection pressure [MPa]

Figure 4. NOx emission characteristics of diesel and DME engines without pilot injection as a function of injection pressure and engine speed. Figure 4. NOx emission characteristics of diesel and DME engines without pilot injection as a function of injection pressure and engine speed. NOx emissions in DME engines are not signiﬁcantly different from those in diesel engines; however, the emission characteristics of the fuel injection pressure are different. 2. HC emission characteristics of diesel and DME engines In particular, NOx emissions are lowest at 2100 rpm in DME engines, unlike those in diesel engines. This suggests that the DME engine used in this study has an unstable combustion state because of the non-optimized fuel injection conditions, such as injection timing and amount according to the injection pressure of the fuel. Thus, the combustion state of the DME fuel, which is an oxygen fuel, is generally improved, thereby increasing the combustion temperature and NOx emission concentration [58,59]. This is consistent with the emission characteristics of other internal combustion engines. Therefore, it suggests that it is necessary to investigate the possibility of reducing NOx emissions in DME engines using the fuel injection pressure characteristics.

2. HC emission characteristics of diesel and DME engines

Figure 5 shows the emission characteristics of the unburned HC in DME and diesel engines depending on the engine operating speed and injection pressure with light load and injection timing of 14° BTDC. At the engine operating speed of 1200–1800 rpm, the HC emissions were slightly lower than the diesel engine emissions. However, at the operating speed of 2100 rpm, the HC emissions of the DME engines were about 2.5 times higher than those of the diesel engines. This indicates that the combustion characteristics of oxygenated fuel appeared in the low-speed range but in the case of 2100 Appl. Sci. 2019, 9, 647 7 of 19 rpm the emission of HC was increased compared with 1200–1800 rpm by incomplete combustion. The excessive injection amount because of the high-pressure injection and the inconsistency of the injectioninjection timing timing was was shown shown in in the the high-speed high-speed operatio operationn condition. condition. In In DME DME engines, engines, the the effect effect of of fuel fuel injectioninjection pressure pressure on on the the HC HC emission emission characteristics characteristics were were not not significant signiﬁcant at low at low speeds; speeds; however, however, at highat high speeds, speeds, the the higher higher was was the the fuel fuel injection injection pres pressure,sure, the the lower lower was was the the HC HC emission. emission. To To optimize optimize thethe DME DME engines, engines, it waswas necessarynecessary toto optimize optimize the the fuel fuel injection injection pressure pressure and and injection injection amount amount for for the theengine engine operating operating conditions. conditions.

Load25%/BTDC14deg./NZ0.22×6 1200 1200RPM 1000 1500RPM 1800RPM 800 2100RPM

600

400 HC emission[ppm] 200

0 25 30 40 45 25 30 40 45 DIESEL DME Injection pressure[MPa]

Figure 5. HC emission characteristics of diesel and DME engines without pilot injection as a function Figure 5. HC emission characteristics of diesel and DME engines without pilot injection as a function of of injection pressure and engine speed. injection pressure and engine speed. 3. CO emission characteristics of diesel and DME engines 3. CO emission characteristics of diesel and DME engines Figure6 shows the CO emission characteristics of DME and diesel engines depending on the Figure 6 shows the CO emission characteristics of DME and diesel engines depending on the engine operating speed and injection pressure with light load and injection timing of 14◦ BTDC. engine operating speed and injection pressure with light load and injection timing of 14° BTDC. The The emission characteristics of CO were similar to those of HC. The CO emission concentration of emission characteristics of CO were similar to those of HC. The CO emission concentration of the the DME engine was slightly lower than that of the diesel engine at all operating conditions and not DME engine was slightly lower than that of the diesel engine at all operating conditions and not signiﬁcantly affected by the fuel injection pressure. This was because of the combustion characteristics significantly affected by the fuel injection pressure. This was because of the combustion of the oxygenated fuel of the DME, as described in HC. characteristics of the oxygenated fuel of the DME, as described in HC. 4. Smoke emission characteristics of diesel and DME engines depending on the injection pressure and number of revolutions

Figure7 shows a comparison of the soot concentration with the injection pressure for operating speed of DME and diesel engines at the engine operating speed of 1800 rpm, light load condition, and injection timing of 14◦ BTDC. The exhaust gas emission characteristics of DME engines were not discharged as compared to those of the diesel engine [33,34,60]. As a result of injecting high-pressure liquid fuel DME, which is a compressible oxygen fuel, the mixing ratio with air in the injection characteristic improved as compared to that with diesel, thereby enhancing the diffusive combustion state due to inject the high-pressure fuel. Appl. Sci. 2019, 9, x FOR PEER REVIEW 8 of 21 Appl. Sci. 2019, 9, 647 8 of 19

Load25%/BTDC14deg./NZ0.22×6 1.0 1200RPM 1500RPM 0.8 1800RPM 2100RPM 0.6

0.4 CO emission[%] 0.2

0.0 25 30 40 45 25 30 40 45 DIESEL DME Injection pressure[MPa]

Appl. Sci. 2019, 9, x FOR PEER REVIEW 9 of 21 Figure 6. CO emission characteristics of diesel and DME engines without pilot injection as a function Figure 6. CO emission characteristics of diesel and DME engines without pilot injection as a function of of injection pressure and engine speed. injection pressure and engine speed. Load25%/BTDC14˚/NZ0.22×6 4. Smoke emission15 characteristics of diesel and DME engines depending on the injection pressure and number of revolutions 1200RPM Figure 7 12shows a comparison of the soot concentration1500RPM with the injection pressure for operating speed of DME and diesel engines at the engine operating1800RPM speed of 1800 rpm, light load condition, and injection timing of 14° BTDC. The exhaust gas emission characteristics of DME engines were not 9 2100RPM discharged as compared to those of the diesel engine [33,34,60]. As a result of injecting high-pressure liquid fuel DME, which is a compressible oxygen fuel, the mixing ratio with air in the injection characteristic improved6 as compared to that with diesel, thereby enhancing the diffusive combustion state due to inject the high-pressure fuel. Smoke[%] 3

0 25 30 40 45 25 30 40 45 -3 DIESEL DME Injection Pressure[MPa]

Figure 7. Smoke characteristics of diesel and DME engines without pilot injection as a function of Figureinjection 7. Smoke pressure characteristics and engine of speed. diesel and DME engines without pilot injection as a function of injection pressure and engine speed. 3.3. Effects of Injection Pressure and Pilot Injection Period on Exhaust Gas Emission Characteristics of 3.3.DME Effects Engines of Injection Pressure and Pilot Injection Period on Exhaust Gas Emission Characteristics of DME

Engines1. NO x emission characteristics of diesel and DME engines depending on the injection pressure and pilot injection period 1. NOx emission characteristics of diesel and DME engines depending on the injection pressure and pilot injection period Figure8 shows that the NO x emissions of DME as a diesel alternative fuel was more than those of dieselFigure near 8 the shows top deadthat the center NOx (TDC) emissions at the of same DME diesel as a diesel injection alternative timing offuel the was experimental more than engine.those of diesel near the top dead center (TDC) at the same diesel injection timing of the experimental engine. As the injection pressure increased, the amount of NOx increased, and pilot injection showed that the diesel could be reduced to 20% (100 ppm) and DME to 10% (50 ppm). In this experiment, based on improving the diesel injection timing as much as possible and the pilot injection about 15% of the total injection amount, it was confirmed that NOx could be reduced to about 10–20% of the total emission rate by high-pressure fuel injection and pilot injection. For pilot injection at 6° before the 14° BTDC, where combustion occurred, the thermal efficiency decreased with increasing fuel consumption rate; however, it was expected that the NOx generated in a high-temperature region would be reduced because the temperature of the combustion chamber decreases. Appl. Sci. 2019, 9, 647 9 of 19

As the injection pressure increased, the amount of NOx increased, and pilot injection showed that the diesel could be reduced to 20% (100 ppm) and DME to 10% (50 ppm). In this experiment, based on improving the diesel injection timing as much as possible and the pilot injection about 15% of the total injection amount, it was conﬁrmed that NOx could be reduced to about 10–20% of the total emission rate by high-pressure fuel injection and pilot injection. For pilot injection at 6◦ before the 14◦ BTDC, where combustion occurred, the thermal efﬁciency decreased with increasing fuel consumption rate; however, it was expected that the NOx generated in a high-temperature region would be reduced Appl. Sci. 2019, 9, x FOR PEER REVIEW 10 of 21 because the temperature of the combustion chamber decreases.

1800RPM/Φ0.22×6/BTDC14degree/Load75%/Pilot 10% 600

500 Main inj.[Diesel]

400 Main inj.[DME]

300 Pilot+Main inj.[DME] 200

NOx emissiom [ppm] Pilot+Main inj.[Diesel] 100

0 30 40 50 60 80 Injection pressure[MPa]

Figure 8. NOx emission characteristics of diesel and DME engines with and without pilot injection as a Figure 8. NOx emission characteristics of diesel and DME engines with and without pilot injection as function of injection pressure. a function of injection pressure. 2. HC emission concentration in diesel and DME engines depending on the injection pressure and 2. HCpilot emission injection concentration period in diesel and DME engines depending on the injection pressure and pilot injection period Figure9 shows the HC emission with pressure during high-pressure pilot and main injections. HCFigure was reduced 9 shows by the more HC emission than 50% with in this pressure study during as compared high-pressure to those pilot in other and studiesmain injections. (injection HCpressure was reduced 20–35 MPa) by more [24, 26than] for 50% high-pressure in this study pilot as injection.compared HC to wasthose generated in other studies in the combustion (injection pressurechamber 20–35 wall orMPa) in a [24,26] state where for high-pressure the air–fuel ratio pilot was injection. rich, and HC it was was generated generated in in the a state combustion where the chamberfuel was wall vaporized or in a andstate not where oxidized. the air–fuel Moreover, ratio DME, was rich, as an and oxygenated it was generated fuel, generated in a state almost where no the HC, fueland was generated vaporized higher and fuel not densityoxidized. per Moreover, unit volume. DM Furthermore,E, as an oxygenated unlike thefuel, emission generated characteristics almost no HC, and generated higher fuel density per unit volume. Furthermore, unlike the emission of NOx, the emission rate of HC was reduced by the increase of pressure. It could be expected that, characteristicsas the injection of pressure NOx, the increases, emission the rate injection of HC was amount reduced in the by diesel the increase and DME of engines pressure. for It the could injection be expectedperiod is that, optimized as the injection in the laboratory, pressure increases, and incomplete the injection combustion amount is lessin the likely diesel to and occur. DME engines for the injection period is optimized in the laboratory, and incomplete combustion is less likely to occur.3. CO emission concentration in diesel and DME engines depending on the injection pressure and pilot injection period Figure 10 shows the CO emission rate as a function of pressure when performing high-pressure pilot and main injections. CO is generally generated in an incomplete combustion environment because of lack of oxygen in an excess fuel state, which was not a problem because DME is an oxygenated fuel, and compression ignition engines are not a big problem because they operate in a lean air–fuel ratio region. In this study, the CO emission rate was lower than that of the diesel engine. Appl. Sci. 2019, 9, x FOR PEER REVIEW 11 of 21

1800RPM/Φ0.22×6/BTDC14degree/Load 75%/Pilot 10% 70 Pilot+Main inj. [Diesel] 60 Main inj. [Diesel] 50

Appl. Sci. 2019, 9, 647 10 of 19 Appl. Sci. 2019, 9, 40x FOR PEER REVIEW 11 of 21 Pilot+Main inj. [DME] 30 1800RPM/Φ0.22×6/BTDC14degree/Load 75%/Pilot 10% Main inj. [DME] 70 Pilot+Main inj. [Diesel] HC emission [ppm] 20 60 10 Main inj. [Diesel] 50 0 40 30 40 50 60 80 Pilot+Main inj. [DME] Injection pressure[MPa] 30 Main inj. [DME] Figure 9. HC emission characteristics of diesel and DME engines with and without pilot injection as a

HC emission [ppm] 20 function of injection pressure.

3. CO emission10 concentration in diesel and DME engines depending on the injection pressure and pilot injection period 0 Figure 10 shows the CO emission rate as a function of pressure when performing high-pressure 30 40 50 60 80 pilot and main injections. CO is generally generated in an incomplete combustion environment because of lack of oxygen in an excess fuelInjection state, whichpressure[MPa] was not a problem because DME is an oxygenated fuel, and compression ignition engines are not a big problem because they operate in a Figure 9. HC emission characteristics of diesel and DME engines with and without pilot injection as a lean air–fuelFigure 9. ratioHC emission region. characteristicsIn this study, of the diesel CO andemis DMEsion enginesrate was with lower and thanwithout that pilot of the injection diesel as engine. a function of injection pressure. function of injection pressure. 1800RPM/Pme 0.7MPa/BTDC14degree/Load 75%/Pilot 10% 3. CO emission0.6 concentration in diesel and DME engines depending on the injection pressure and pilot injection period Pilot+Main Inj. [Diesel] Figure 10 shows0.5 the CO emission rate as a function of pressure when performing high-pressure pilot and main injections. CO is generally generated in an incomplete combustion environment Main Inj. [Diesel] because of lack0.4 of oxygen in an excess fuel state, which was not a problem because DME is an oxygenated fuel, and compression ignition engines are not a big problem because they operate in a lean air–fuel ratio region. In this study, the CO emission rate was lower than that of the diesel engine. 0.3 Pilot+Main Inj. [DME] Main Inj.1800RPM/Pme [DME] 0.7MPa/BTDC14degree/Load 75%/Pilot 10% 0.20.6 CO emission [%] Pilot+Main Inj. [Diesel] 0.10.5

Main Inj. [Diesel] 0.40 30 40 50 60 80 Pilot+Main Inj. [DME] 0.3 Injection pressure [MPa]

Figure 10. CO emissionMain Inj. characteristics [DME] of diesel and DME engines with and without pilot injection as a 0.2

function ofCO emission [%] injection pressure.

3.4. Effects of Pilot Injection Rate on Exhaust Gas According to Injection Time in DME Engines 0.1 In this study, the emission characteristics of HC, NOx, CO, and carbon dioxide (CO2) were analyzed for the engine performance0 tests. DME showed unleaded combustion and no special combustion method was required for exhaust gas reduction. However, NOx and HC vary depending on the operating conditions and30 fuel characteristics; 40 thus, the 50 reduction rate 60 was investigated 80 for each pilot Injection pressure [MPa] injection rate, injection timing, needle lift, and injection timing in this experiment. Appl. Sci. 2019, 9, x FOR PEER REVIEW 12 of 21

Figure 10. CO emission characteristics of diesel and DME engines with and without pilot injection as a function of injection pressure.

3.4. Effects of Pilot Injection Rate on Exhaust Gas According to Injection Time in DME Engines

In this study, the emission characteristics of HC, NOx, CO, and carbon dioxide (CO2) were analyzed for the engine performance tests. DME showed unleaded combustion and no special combustion method was required for exhaust gas reduction. However, NOx and HC vary depending onAppl. the Sci. operating2019, 9, 647 conditions and fuel characteristics; thus, the reduction rate was investigated for 11each of 19 pilot injection rate, injection timing, needle lift, and injection timing in this experiment.

1. 1. NONOx xemissionemission characteristics characteristics of of DME DME engines engines depending depending on on the the injection injection pressure pressure and and pilot pilot injectioninjection period period

FigureFigure 11 11 shows shows the the effect effect of of pilot pilot injection injection period period on on NO NOx xemissionsemissions with with injection injection timing; timing; other other studiesstudies [35,39,46] [35,39,46 ]have have shown shown that that the the NO NOx xemissionemission rate rate of of DME DME as as a adiesel diesel substitute substitute fuel fuel decreases decreases asas it it approaches approaches the the TDC. TDC. Therefore, Therefore, in this experiment,experiment, thethepilot pilot injection injection rate rate was was examined examined from from 0% 0%to to 30% 30% of theof the total total injection injection amount amount with with the the injection injection pressure pressure of 50 of MPa 50 MPa and and the injectionthe injection timing timing at the ◦ ◦ atlowest the lowest was 10wasBTDC 10° BTDC and at and the at highest the highest was 22 wasBTDC. 22° BTDC. As a result, As a NOresult,x decreased NOx decreased as the pilot as the injection pilot injectionrate increased. rate increased. This was This because was because the pilot the injection pilot injection before the before main the injections main injections slowed theslowed diffusion the diffusioncombustion combustion because because of the injection of the injection period delay. period Furthermore, delay. Furthermore, it reduced it reduced the combustion the combustion chamber chambertemperature temperature and average and effectiveaverage pressureeffective becausepressure the because injection the delay injection decreased, delay therebydecreased, reducing thereby the reducingemission the of emission NOx generated of NOx atgenerated high temperatures. at high temperatures. NOx wasmostly NOx was composed mostly composed of nitrogen of monoxidenitrogen monoxide(NO) and (NO) nitrogen and dioxide nitrogen (NO dioxide2), which (NO were2), which produced were byproduced the combination by the combination of nitrogen moleculesof nitrogen in moleculesthe air and in oxygenthe air and at high oxygen temperatures. at high temperatures. NOx in the compressionNOx in the compression ignition engine ignition were engine divided were into dividedthermal into NO thermal and fuel NO NO, and oxidized fuel NO, by oxidized the nitrogen by the contained nitrogen in contained the fuel, mostin the of fuel, which most was of thermalwhich wasNO. thermal The generation NO. The mechanism generation was mechanism based on thewas Zeldovich based on mechanism the Zeldovich andhad mechanism a mechanism and (Nhad2 + a O mechanism→ NO + N) (N in2 + which O → NO NO + molecules N) in which dissociated NO molecules during dissociated combustion during in an combustion excess air atmosphere in an excess to airproduce atmosphere NO. Theto produce generation NO. ofThe NO generation by this dissociation of NO by this was dissociation a strong endothermic was a strong reaction, endothermic which reaction,exponentially which increased exponentially at high increased temperatures at andhigh was temperatures discharged duringand was the discharged expansion stroke. during That the is, expansionthe concentration stroke. ofThat NO is, was the greatly concentration dominated of by NO the was combustion greatly temperature,dominated by and the the combustion reaction time temperature,and oxygen and concentration the reaction were time also and important oxygen concentration factors. were also important factors.

1800RPM/Load75%/Lift 0.21/Pinj.50MPa 500

400 Pilot 0% Pilot 10% Pilot 20% 300 Pilot 30%

200 NOx emission [ppm] 100

0 10 14 18 22 Injection time degree[BTDC]

Figure 11. NOx emission characteristics of DME engines with the pilot injection duration rate as a function of injection timing.

2. HC emission characteristics of DME engines depending on the injection timing and pilot injection rate

Figure 12 shows the effect of pilot injection rate on HC generation with injection timing. The comparison in Figure 12 shows that NOx and HC were opposite to each other. Furthermore, the pilot injection quantity increased and, as the TDC was approached, the HC emission amount increased [47,49]. This was because, when injecting more than 10% of the pilot injection, the injection period was delayed, and the amount of HC emission increased because of unstable combustion. Appl. Sci. 2019, 9, x FOR PEER REVIEW 13 of 21

Figure 11. NOx emission characteristics of DME engines with the pilot injection duration rate as a function of injection timing.

2. HC emission characteristics of DME engines depending on the injection timing and pilot injection rate Figure 12 shows the effect of pilot injection rate on HC generation with injection timing. The comparison in Figure 12 shows that NOx and HC were opposite to each other. Furthermore, the pilot injectionAppl. Sci. 2019quantity, 9, 647 increased and, as the TDC was approached, the HC emission amount increased12 of 19 [47,49]. This was because, when injecting more than 10% of the pilot injection, the injection period was delayed, and the amount of HC emission increased because of unstable combustion. Moreover, HCMoreover, were generated HC were by generated unstable by combustion unstable combustion of fuel, generated of fuel, generated because of because lack of of oxygen lack of in oxygen a state in wherea state the where combustion the combustion chamber chamber wall surface wall surfaceor air–fuel or air–fuel ratio was ratio rich, was and rich, discharged and discharged without without being oxidized.being oxidized. It also occurred It also occurred when the when fuel thewas fuel evapor wasated evaporated during fueling during or fueling when the or when fuel was the burning fuel was unsafely,burning unsafely,similar to similar CO. The to CO. ignition The ignition of the offuel the sprayed fuel sprayed from from the thefuel fuel injection injection nozzle nozzle in in the the combustioncombustion process process of of the the compressi compressionon ignition ignition engine engine occurred occurred at at a a point point slightly slightly leaner leaner than than the the stoichiometricstoichiometric air–fuel air–fuel ratio ratio (λ (λ == 1), 1), and and spontaneous spontaneous ignition ignition coul couldd not not occur occur outside outside the the lean lean burn burn limitlimit (A/F (A/F = =40) 40) [49]. [49 ].In In such such ultra-lean ultra-lean region, region, unreacted unreacted fuel fuel did did not not end end well, well, producing producing unburned unburned fuelfuel (HC). (HC). The The concentration concentration ofof HCHC generatedgenerated in in the the ultra-lean ultra-lean region region was was signiﬁcantly significantly affected affected by by the theamount amount of fuelof fuel injected injected during during the the ignition ignition delay dela periody period and and the mixingthe mixing ratio ratio of the of injectedthe injected fuel fuel with withair. Thatair. That is, a largeis, a large amount amount of fuel of injected fuel injected during during the ignition the ignition delay perioddelay period or a delay or a in delay the ignition in the ignitiondelay period delay mightperiod cause might an cause increase an increase in the amount in the amount of generated of generated HC. Moreover, HC. Moreover, when the when injected the injectedfuel became fuel became rich, there rich, could there be could twoscenarios: be two scenar (1) theios: fuel (1) remainingthe fuel remaining in the sack in volumethe sack orvolume sack hole or sackwas hole injected was atinjected a low speedat a low in speed the latter in the stage latter of thestage combustion of the combustion process; process; and (2) whenand (2) the when fuel wasthe fuelexcessively was excessively supplied supplied to the combustion to the combustion chamber ch fromamber the from beginning. the beginning. In particular, In particular, the sack the volume sack volumeand the and volume the volume of HC tended of HC tend to beed almost to be proportional.almost proportional. 1800RPM/Load 75%/Lift 0.21/Pinj.50MPa 45

40 Pilot 30% Pilot 20% 35 Pilot 10%

30 Pilot 0%

25 HC emission [ppm]

15 10 14 18 22 Injection timing degree[BTDC]

Figure 12. HC emission characteristics of DME engines with the pilot injection duration rate as a Figurefunction 12. ofHC injection emission timing. characteristics of DME engines with the pilot injection duration rate as a function of injection timing. 3. CO emission characteristics of DME engines depending on the injection timing and pilot 3. COinjection emission rate characteristics of DME engines depending on the injection timing and pilot injection rate Figure 13 shows the CO emission from the injection timing and the pilot injection amount. It also shows that the amount of CO decreased with the increasing injection delay and pilot injection amount. CO is a colorless, odorless, toxic gas that was generated in the engine when operating at rich air–fuel ratios. When there was not enough oxygen to convert all the carbon in the HC-based fuel to CO2, some of the fuel would not burn and remained as CO. Maximum emission of CO occurred when the engine’s fuel was operating in a rich state, that is, at start-up or under accelerated load. Even when the intake fuel–air mixture was at the theoretical mixture ratio or the lean mixture ratio, CO was also generated in the engine. The fuel–air mixture was poor or there was a local rich mixture, resulting in a knock in incomplete combustion. Appl. Sci. 2019, 9, x FOR PEER REVIEW 14 of 21

Figure 13 shows the CO emission from the injection timing and the pilot injection amount. It also shows that the amount of CO decreased with the increasing injection delay and pilot injection amount. CO is a colorless, odorless, toxic gas that was generated in the engine when operating at rich air–fuel ratios. When there was not enough oxygen to convert all the carbon in the HC-based fuel to CO2, some of the fuel would not burn and remained as CO. Maximum emission of CO occurred when the engine’s fuel was operating in a rich state, that is, at start-up or under accelerated load. Even when the intake fuel–air mixture was at the theoretical mixture ratio or the lean mixture ratio, CO was also generated in the engine. The fuel–air mixture was poor or there was a local rich mixture, Appl. Sci. 2019, 9, 647 13 of 19 resulting in a knock in incomplete combustion.

1800RPM/Load 75%/Lift 0.21/Pinj.50MPa 0.12

Pilot 0% 0.1 Pilot 10% Pilot 20% 0.08 Pilot 30%

0.06

0.04 CO emission [%]

0.02

0 10 14 18 22 Injection time degree [BTDC]

Figure 13. CO emission characteristics of DME engines with the pilot injection duration rate as a Figurefunction 13. CO of injection emission timing. characteristics of DME engines with the pilot injection duration rate as a function of injection timing. 4. CO2 emission characteristics of DME engines depending on the injection time and pilot 4. COinjection2 emission rate characteristics of DME engines depending on the injection time and pilot injection rate Figure 14 shows CO2 emission characteristics of DME engines according to the injection timing andFigure pilot injection 14 shows rate. CO2 CO emission2 was checked characteristics as it is of identiﬁed DME engines as an emissionaccording causing to the injection global warming, timing andand, pilot similar injection to the rate. CO generationCO2 was checked shown inas Figure it is identified 13, from theseas an results,emission the causing reduction global of CO warming,through Appl. Sci. 2019, 9, x FOR PEER REVIEW 2 15 of 21 and,DME similar engine to also the showed CO generation a tendency shown to decrease in Figu asre the 13, pilot from injection these results, amount the increased reduction [39 ].of CO2 through DME engine also showed a tendency to decrease as the pilot injection amount increased [39]. 1800RPM/Pinj.50MPa/Load 75% 7 Pilot 0 % 6

5 Pilot 10 % 4 Pilot 20 % 3 Pilot 30 %

CO2 emission [%] 2

0 10 14 18 22 Injection timing degree [BTDC]

Figure 14. CO2 emission characteristics of DME engines with the pilot injection duration rate as a Figurefunction 14. CO of injection2 emission timing. characteristics of DME engines with the pilot injection duration rate as a function of injection timing.

3.5. Effects of Needle Lift on Exhaust Gas According to Injection Timing in DME Engines

Figures 15–17 show the exhaust gas rates (HC, NOx, and CO) for the engine performance tests. In this experiment, the exhaust gas characteristics were examined by applying the DLL-AP type needle lift of 0.14, 0.21, 0.24, and 0.36 mm nozzle (DL-AP type), which was one of the injector improvements, different from the examination result by the condition change of the ECU program.

1. NOx emission characteristics of DME engines depending on the injection timing and needle lift

Figure 15 shows the effect of needle lift on NOx emissions with injection timing at a loading rate of 75% at an injection pressure of 50 MPa. In this case, the effect of the needle lift on the NOx emission was found to increase as the injection timing was delayed and decreased as the injection angle was advanced. Further, the needle lifts showed low values at 0.36 and 0.14 mm, while at 0.21 and 0.24 mm showed an increasing trend. First, the increase in the injection timing was caused by the temperature rise of the combustion chamber because of diffusion combustion from the delay of the injection period. The decrease of 0.14 and 0.36 mm of needle lift at 22° BTDC caused incomplete combustion at 0.14 mm due to the long supply period because of the common rail characteristics for the required fuel supply when injected with the required injection period. Appl. Sci. 2019, 9, 647 14 of 19

3.5. Effects of Needle Lift on Exhaust Gas According to Injection Timing in DME Engines

1.NO x emission characteristics of DME engines depending on the injection timing and needle lift

Figure 15 shows the effect of needle lift on NOx emissions with injection timing at a loading rate of 75% at an injection pressure of 50 MPa. In this case, the effect of the needle lift on the NOx emission was found to increase as the injection timing was delayed and decreased as the injection angle was advanced. Further, the needle lifts showed low values at 0.36 and 0.14 mm, while at 0.21 and 0.24 mm showed an increasing trend. First, the increase in the injection timing was caused by the temperature rise of the combustion chamber because of diffusion combustion from the delay of the injection period. The decrease of 0.14 and 0.36 mm of needle lift at 22◦ BTDC caused incomplete combustion at 0.14 mm due to the long supply period because of the common rail characteristics for the required fuel supply Appl.when Sci. injected 2019, 9, x withFOR PEER the requiredREVIEW injection period. 16 of 21

1800RPM/Pinj.50MPa/Load 75%/Pilot 10% 1000 0.14 0.21 800 0.24 0.36 600

400

NOx emission [ppm] 200

0 10deg. 14deg. 18deg. 22deg. Injection timing degree [BTDC]

Figure 15. NOx emission characteristics of DME engines with nozzle needle lift as a function of injection Figure 15. NOx emission characteristics of DME engines with nozzle needle lift as a function of injection timing degree. timing degree. 2. HC emission characteristics of DME engines depending on the injection timing and needle lift 2. HC emission characteristics of DME engines depending on the injection timing and needle lift FigureFigure 1616 showsshows the the effect of needle liftlift onon HCHC emissionsemissions withwith injection injection timing timing at at a a loading loading rate rate of of75% 75% at at an an injection injection pressure pressure of of 50 50 MPa. MPa. InIn thisthis case,case, the HC emission fluctuation ﬂuctuation was was not not large large with with differentdifferent injection injection timing timing delays. delays. The The 0.36 0.36 mm mm need needlele lift lift showed showed a a HC HC emission emission concentration concentration lower lower thanthan 0.21 0.21 mm, mm, suggesting suggesting that that the the DME DME supply supply per per unit unit time time was was oversupplied oversupplied to to the the engine. engine. Thus, Thus, thethe needle needle lift lift of of 0.21 mmmm waswas usefuluseful forfor engine engine gas; gas; however, however, it it was was not not good good in termsin terms of exhaustof exhaust gas. gas.

1800RPM/Pinj.50MPa/Load 75%/Pilot 10% 50

20 0.14 0.21 HC emission [ppm] 10 0.24 0.36 0 10deg. 14deg. 18deg. 22deg. Injection timing degree [BTDC]

Appl. Sci. 2019, 9, x FOR PEER REVIEW 16 of 21

1800RPM/Pinj.50MPa/Load 75%/Pilot 10% 1000 0.14 0.21 800 0.24 0.36 600

400

NOx emission [ppm] 200

0 10deg. 14deg. 18deg. 22deg. Injection timing degree [BTDC]

Figure 15. NOx emission characteristics of DME engines with nozzle needle lift as a function of injection timing degree.

2. HC emission characteristics of DME engines depending on the injection timing and needle lift Figure 16 shows the effect of needle lift on HC emissions with injection timing at a loading rate of 75% at an injection pressure of 50 MPa. In this case, the HC emission fluctuation was not large with different injection timing delays. The 0.36 mm needle lift showed a HC emission concentration lower than 0.21 mm, suggesting that the DME supply per unit time was oversupplied to the engine. Thus, theAppl. needle Sci. 2019 lift, 9, of 647 0.21 mm was useful for engine gas; however, it was not good in terms of exhaust15 of 19 gas.

1800RPM/Pinj.50MPa/Load 75%/Pilot 10% 50

20 0.14 0.21 HC emission [ppm] 10 0.24 0.36 0 10deg. 14deg. 18deg. 22deg. Appl. Sci. 2019, 9, x FOR PEER REVIEW Injection timing degree [BTDC] 17 of 21

FigureFigure 16. 16. HCHC emission emission characteristic characteristic of of DME DME engines engines with with nozzle nozzle needle needle lift lift as as a afunction function of of injection injection timingtiming degree. degree.

3. 3. COCO emission emission characteristics characteristics of of DME DME engines engines depending depending on on the the injection injection timing timing and and needle needle lift lift

FigureFigure 17 17 shows shows the the effect effect of of needle needle lift lift on CO emissions withwith injectioninjection timingtiming at at a a loading loading rate rate of of75% 75% at at an an injection injection pressure pressure of of 50 50 MPa. MPa. The The CO CO emission emission slightly slightly ﬂuctuated fluctuated at 0.36at 0.36 mm mm because because of of the theinjection injection timing timing delay; delay; however, however, there there was no was large no ﬂuctuation. large fluctuation. This was This expected was expected to be incomplete to be incompletecombustion combustion because of because the longer of injectionthe longer period injection caused period by the caused injection by the timing injection delay, timing as in thedelay, case asof in HC. the case of HC. 1800RPM/Pinj.50MPa/Load 75%/Pilot 10% 0.08 0.14 0.21 0.24 0.06 0.36

0.04

CO emission [ppm] 0.02

0.00 10deg. 14deg. 18deg. 22deg. Injection timing degree [BTDC]

Figure 17. CO emission characteristics of DME engines with nozzle needle lift as a function of injection Figuretiming 17. degree. CO emission characteristics of DME engines with nozzle needle lift as a function of injection timing degree.

4. Conclusions Based on the results of this study on diesel and DME engines with common rail conditions applied, we examined the engine performance changes and emission characteristics of exhaust gases with varying injection pressure and injection rate. The emission characteristics of NOx, HC, and CO were investigated for different injection pressures of the pilot injection. The exhaust gas emission characteristics for the pilot injection period and needle lift were investigated as follows: 1. For DME engines, the effect of fuel injection pressure on HC emissions was not significant at low speeds; however, at high speeds, higher fuel injection pressures caused lower HC emissions. To optimize the DME engine, it is necessary to optimize the fuel injection pressure and injection amount depending on the engine operating conditions. 2. The emission characteristics of CO were similar to those of HC. The CO emission concentration of DME engines was slightly lower than that of diesel engines. The fuel injection pressure did not have a significant effect at all operating conditions. This was because the combustion characteristics of the oxygenate fuel of DME described in HC. As the liquid fuel DME, which is a compressible oxygen fuel, was injected at high pressure, the mixing ratio with air was improved in the injection characteristic as compared to that with diesel. Further, it was expected to enhance into a diffusive combustion state. Appl. Sci. 2019, 9, 647 16 of 19

1. For DME engines, the effect of fuel injection pressure on HC emissions was not significant at low speeds; however, at high speeds, higher fuel injection pressures caused lower HC emissions. To optimize the DME engine, it is necessary to optimize the fuel injection pressure and injection amount depending on the engine operating conditions. 2. The emission characteristics of CO were similar to those of HC. The CO emission concentration of DME engines was slightly lower than that of diesel engines. The fuel injection pressure did not have a significant effect at all operating conditions. This was because the combustion characteristics of the oxygenate fuel of DME described in HC. As the liquid fuel DME, which is a compressible oxygen fuel, was injected at high pressure, the mixing ratio with air was improved in the injection characteristic as compared to that with diesel. Further, it was expected to enhance into a diffusive combustion state. 3. The HC emission can be extremely low, as it was generated by oxygen deficiency in a state where the combustion chamber wall surface or the air–fuel ratio was rich, and because DME was generated in a state where the fuel was not vaporized and the oxidized fuel was an oxygen fuel. Moreover, as the injection pressure increased, the emission rate of NOx reduced. This was because the injection rate in diesel and DME engines was optimized according to the injection period, and incomplete combustion is not expected.

4. The NOx emission was reduced at the injection pressure of 50 MPa, with pilot injection rate increasing from 0% to 30% of the total injected amount, and the minimum and maximum injection ◦ ◦ timing being 10 and 22 BTDC, respectively. The NOx emission decreased with the increasing pilot injection rate because the pilot injection before the main injection delays the rapid diffusion combustion. This was because the injection period delay lowered the combustion chamber temperature; and it was considered that NOx emissions generated at a high temperature were reduced because the average effective pressure caused by the injection delay decreased.

5. It can be seen from this experiment that NOx and HC were opposite to each other. Further, as the pilot injection amount increased, or as it got closer to TDC, the HC emission amount increased. When pilot injection was applied in this study, it was expected that the injection period would be delayed if more than 10% was applied, so that the HC emissions would increase because of unstable combustion. 6. The increase in injection timing was caused by the delay in injection period, occurring with the temperature rise in the combustion chamber because of diffusive combustion. The decrease of 0.14 mm and 0.36 mm of needle lift at 22◦ BTDC was caused by incomplete combustion because of the longer supply period of the characteristics of common rail for supplying the required fuel when injected with the required injection period at 0.14 mm.

Author Contributions: S.Y. and C.L. conceived and designed the experiments; S.Y. performed the experiments; C.L. analyzed the data; S.Y. contributed reagents/materials/analysis tools; C.L. wrote the paper. Funding: This study was supported by the development of key fusion technology of the industry–academia research cooperation cluster support project. Conﬂicts of Interest: The authors declare no conﬂict of interest. Appl. Sci. 2019, 9, 647 17 of 19

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