Study on Effects of Common Rail Injector Drive Circuitry with Different Freewheeling Circuits on Control Performance and Cycle-By-Cycle Variations

energies

Article Study on Effects of Common Rail Injector Drive Circuitry with Different Freewheeling Circuits on Control Performance and Cycle-by-Cycle Variations

Erxi Liu 1,2,* and Wanhua Su 1,*

1 State Key Laboratory of Engines, Tianjin University, Tianjin 300072, China 2 Postdoctoral Workstation, CATARC (Tianjin) Automotive Engineering Research Institute Co., Ltd, Tianjin 300300, China * Correspondence: [email protected] (E.L.); [email protected] (W.S.)

Received: 19 January 2019; Accepted: 9 February 2019; Published: 12 February 2019

Abstract: This paper provides a new common rail injector drive circuitry for practical use. The new drive circuitry with variable freewheeling circuit was developed based on the requirements for the rate of current drop in the peak-and-hold solenoid model. The variable freewheeling circuit exhibited superior performance in the control accuracy compared to the conventional circuit with a resistor in series with diode (RD) freewheeling circuit. Furthermore, the current cutting process was 30 µs shorter, and the control accuracy of the cycle fuel injection mass was improved by at least 0.36% or exactly 2.86% when a small fuel injection mass was used. In addition, the variable freewheeling circuit consumed less power because the drive power charging was done through the feedback from electromagnetic energy to electrical energy. When the fuel injection mass was large, the fall range of the driving power voltage became 1 V smaller, its recovery time was 1ms shorter, and the highest temperature of the drive circuitry was only 37 ◦C, which was 127 ◦C lower than that of the RD freewheeling due to the decrease in energy consumption. Finally, experimental tests with a multi-cylinder engine showed that the variable freewheeling circuit reduced the cycle-by-cycle combustion variations by 0.5%, and lessened the NOx and soot emissions signiﬁcantly by 3.5% and 4%, respectively, in comparison to the RD freewheeling circuit.

Keywords: injector drive circuitry; freewheeling, performance; combustion; cycle-by-cycle variations

1. Introduction Various advanced engine combustion modes have been developed in order to achieve high efficiency and low engine emissions [1–4]. The engine control unit (ECU) must provide more flexible and accurate control for the start of injection (SOI), end of injection (EOI) and various injection patterns [5–7]. Therefore, the common rail injector must not only have a highly responsive solenoid but also have a suitable injector drive circuitry. The circuitry performance affects the control accuracy of the injection that eventually will influence the combustion performance. The injector solenoid is usually driven through the peak-and-hold model [8]. Figure1 illustrates the typical current shape of the peak-and-hold injector. In order to start a fuel injection, the solenoid current accelerates rapidly from 0 A to Ipeak and the solenoid plunger is lifted quickly. Then, when the plunger is already lifted, the current drops from Ipeak to Ihold, as shown in Stage (a) in Figure1. During the Ihold phase, the current is maintained near the Ihold, and the plunger is still lifted (Stage (b)). Next, Stage (c) is the process in which the current drops from Ihold to 0A causing the plunger to be lowered. The requirements for the rate of current drop are different in the three stages [9]. The rate in Stages (a) and (b) should be slow to ensure a smooth transition and reduce the current vibrating

Energies 2019, 12, 564; doi:10.3390/en12030564 www.mdpi.com/journal/energies Energies 2019, 12, 564 2 of 18 Energies 2018, 11, x FOR PEER REVIEW 2 of 19

44 driverange power and the consumption, drive power consumption,respectively. Meanwhile, respectively. the Meanwhile, rate in Stage the ( ratec) should in Stage be fast (c) should to cut off be fastthe 45 currentto cut off quickly; the current the quicker quickly; the the cutting quicker process, the cutting the process, higher the higheraccuracy the of accuracy EOI and of fuel EOI injection and fuel Energies 2018, 11, x FOR PEER REVIEW 2 of 19 46 massinjection would mass be wouldobtained be obtained[10,11]. [10,11]. 44 drive power consumption, respectively. Meanwhile, the rate in Stage (c) should be fast to cut off the 45 current quickly; the quicker the cuttingIpeak process, the higher the accuracy of EOI and fuel injection 46 mass would be obtained [10,11]. Current shape of Peak&Hold

Ipeak Stage b Current shape of Peak&Hold

Drive Ihold-phase current/A Stage a Stage Stageb c

Drive 0 Ihold-phase 0 current/A Stage a 47 Time /s Stage c 48 FigFigureure 1 1.. TypicalTypical current shape of the peak peak-and-hold-and-hold injector injector.. 0 0 4749 The conventional conventional RD RD freewheeling freewheeling circuit circuit oftenTime often employed /s employed in the in solenoid the solenoid drive circuitry drive circuitry design 50 [12],design as [shown12], as in shown Figure in 2, Figure where2, wherethe circle the line circle outlines line outlines the current the current freewheeling freewheeling path, and path, the and arrow the 48 Figure 1. Typical current shape of the peak-and-hold injector. 51 representsarrow represents the current the currentdirection. direction. The rule Theof current rule of drop current in the drop RD infreewheeling the RD freewheeling circuit is expressed circuit is expressed by Equation (1), which applies to all three stages mentioned above. However, a single 4952 by EquationThe conventional (1), which RD applies freewheeling to all three circuit stages often mentioned employed above. in the However, solenoid drivea single circuitry freewheeling design freewheeling path cannot simultaneously satisfy the different demands of the three stages [13]. 5053 [12],path as cannot shown simultaneously in Figure 2, where satisfy the the circle different line outlines demands the of current the three freewheeling stages [13]. path, and the arrow 51 represents the current direction. The rule of current drop in the RD freewheeling circuit is expressed di (VCCR + r)i + U = − D (1) 52 by Equation (1), which applies to all threedt stages mentionedL above. However, a single freewheeling 53 path cannot simultaneously satisfy the different demands of the three stages [13].

VCC D L

R D L Q R

54 Q 55 Figure 2. Drive circuitry with resistor in series with diode (RD) freewheeling.

54 di ()R r i  U   D (1) 55 FigFigureure 2 2.. DriveDrive circuitry circuitry dtwith with resistor resistor in in L series series with with diode diode ( (RD)RD) freewheeling freewheeling.. 56 Several methods were proposed for designing an injector drive circuitry for the peak-and-holdpeak-and-hold di ()R r i  U 57 model. John John D. D. Mooney Mooney [14] [14 ] suggested suggested  a a simulation simulationD code code for for evaluating evaluating circuitry circuitry designs, designs,(1) 58 introduced the the design design method method for for a adt single or dual L power supply, and analyzed the influenceinﬂuence of different battery voltage on the drive current and DC/DC circuitry. The variable freewheeling circuit 5659 differentSeveral battery methods voltage were on proposed the drive currentfor designing and DC/DC an injector circuitry drive. The circuitry variable for freewheeling the peak-and circuit-hold 5760 model.was employed John D. in his Mooney circuitry [14] design, suggested but without a simulation considering code the for feedback evaluating from the circuitry solenoid designs, to the 5861 introducedpower for energy the design recovery method and forelectric a single energy or dualsaving. power Jin Jin Li supply, [15] [15] analyzed and analyzed the drive the voltage influence on the of 5962 differentsolenoid batteryresponses voltage based on on the a finite ﬁnitedrive element current modand model DC/DCel of solenoid circuitry valves, . The showing variable freewheelingthat the voltage circuit was 6063 wasone employed of of the the key key factorsin factorshis circuitry for controlling for controllingdesign, the but injector. without the injector. Cai considering [16 ] Cai studied [16] the the studied feedback soft-switching the from soft the- circuitswitching solenoid parameters circuitto the 6164 powerparametersand their for characteristicsenergy and their recovery characteristics via and simulation, electric via energysimulation, and the saving. results and Jin showedthe Li results [15] that analyzed showed using soft-switching thatthe driveusing voltage soft technology-switching on the 6265 solenoidtechnology responses in a high based-speed on solenoida finite element drive circuitry model of could solenoid reduce valves, the showingswitching that devices the voltage loss, noise, was 6366 oneand ofenergy the keyconsumption. factors for In controllingaddition, there the exist injector. some Cai patents [16] studied[17–19] about the soft drive-switching circuitries, circuit but 64 parameters and their characteristics via simulation, and the results showed that using soft-switching 65 technology in a high-speed solenoid drive circuitry could reduce the switching devices loss, noise, 66 and energy consumption. In addition, there exist some patents [17–19] about drive circuitries, but

Energies 2018, 11, x FOR PEER REVIEW 3 of 19

67 they failed to provide the structure design principle and performance report. Also, some designs 68 were only explained through simulation, and these designs might be impractical for use. Therefore, Energies 2019, 12, 564 3 of 18 69 in this study, a new drive circuitry with a single power supply combined with a variable 70 freewheeling circuit was proposed and its structure design principle and performance were in a high-speed solenoid drive circuitry could reduce the switching devices loss, noise, and energy 71 discussed. consumption. In addition, there exist some patents [17–19] about drive circuitries, but they failed to provide the structure design principle and performance report. Also, some designs were only 72 2. Variableexplained Freewheeling through simulation, Drive Circuitry and these designs might be impractical for use. Therefore, in this study, 73 Thea new variable drive circuitry freewheeling with a single drive power circuitry supply employs combined withdifferent a variable freewheeling freewheeling paths circuit was in different proposed and its structure design principle and performance were discussed. 74 stages. By changing the switches’ working conditions, a single appropriate freewheeling path will be 75 effective2. enough Variable to Freewheeling control the Drive current Circuitry drop process according to the different requirements in the 76 three stages.The variable freewheeling drive circuitry employs different freewheeling paths in different stages. 77 A predesignBy changing and the switches’example, working which conditions, completely a single satisfied appropriate the drop freewheeling rate re pathquirements, will be effective is illustrated 78 in Figureenough 3a. The to control control the current time dropsequence process of according QH (the to the high different-side requirements MOSFET) inand the threeQL stages. (the low-side A predesign and example, which completely satisﬁed the drop rate requirements, is illustrated in 79 MOSFET) are shown in Figure 3b. The effective freewheeling path for Stages (a) and (b) is illustrated Figure3a. The control time sequence of QH (the high-side MOSFET) and QL (the low-side MOSFET) 80 in Figureare 4 shown(a), in in which Figure3 b.QH The was effective on and freewheeling QL was path off. for Its Stages rule (a) of and the (b) iscurrent illustrated drop in Figure is expressed4a, by 81 Equationin 2, which where QH the was resistance on and QL was of off.QH Its was rule neglected of the current under drop isthe expressed on state, by Equationresulting (2), in where the lowest the rate. 82 In Stageresistance (c), QH of and QH was QL neglected were both under off, the which on state, means resulting that in the the lowest current rate. In immediately Stage (c), QH and became 0A 83 throughQL a so were-called both off,avalanche which means current that thedrop. current Meanwhile, immediately the became effective 0A through freewheeling a so-called path avalanche for Stage (c) current drop. Meanwhile, the effective freewheeling path for Stage (c) is illustrated in Figure4b and 84 is illustrated in Figure 4b and expressed by Equation 3. However, since the solenoid generated a expressed by Equation (3). However, since the solenoid generated a large self-induced electromotive 85 large selfforce,-induced the QH electromotivesource would generate force, a high the negative QH source transient would voltage, gene whichrate would a high then negativedamage the transient 86 voltage, circuitrywhich would [20,21]. then damage the circuitry [20,21].

VCC

D L

87 88 (a) Circuitry structure.

89 90 (b) Control time sequence.

Figure 3. Predesign of the variable freewheeling drive circuit. 91 Figure 3. Predesign of the variable freewheeling drive circuit.

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VCC

D L

92 93 (a) Stages (a) and (b).

VCC

D L

94 95 (b) Stage (c).

96 FigureFigure 4. Effective4. Effective freewheeling freewheeling path in in the the three three different different stages. stages.

didi ri+ri U+ U =−= − D D (2) (2) dtdt L L di di = −∞ (3) dt=−∞ (3) dt 3. Improved Design 97 Considering3. Improved Design the practical application, the improved design derived from Figure3a is shown 98in Figure5Considering, where L isthe the practical injector, application, QH and the QL improved are the high-sidedesign derived MOSFET from Figure and low-side 3a is shown MOSFET, in 99QH-logic Figure and 5, where QL-logic L is are the the injector, corresponding QH and QL control are the time high-side sequence MOSFET inputs and of QHlow-side and QL,MOSFET, IR2101S is 100the high-sideQH-logic and QL-logic low-side are driver the corresponding chips, Ho and control Lo are time the sequence ampliﬁed inputs signal of QH outputs, and QL, R2 IR2101S and R3 are 101the respectiveis the high-side gate resistorsand low-side of QH driver and chips, QL, C3Ho and Lo D3 are are the the amplified corresponding signal outputs,R2 bootstrap and capacitor R3 are and 102 the respective gate resistors of QH and QL, C3 and D3 are the corresponding bootstrap capacitor and diode, C1 is the absorption capacitor, D1 and D2 are the freewheeling diodes, R1 is the pull-down 103 diode, C1 is the absorption capacitor, D1 and D2 are the freewheeling diodes, R1 is the pull-down resistor, and all D1, C1, and R1 connect the QH source to the ground,. The control time sequence was 104 resistor, and all D1, C1, and R1 connect the QH source to the ground,. The control time sequence was 105the samethe same as the as onethe one in Figurein Figure3b. 3b. The The descriptions descriptions ofof symbolssymbols used used in in Figure Figure 5 are5 are detailed detailed in Table in Table 1. 1.

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VCC

12V D3 QH

C3 Energies 2018, 11, x FOR PEER REVIEW1 8 5 of 19 Vcc Vb QHlogic 2 7 R2 Hin Ho QLlogic 3 6 Lin Vs VCC 4 5 L D2 GND Lo C1 R3 IR2101S 12V D3 QH

C3 1 8 Vcc Vb QHlogic 2 7 R2 D1 Hin Ho R1 QL QLlogic 3 6 Lin Vs 4 5 L D2 GND Lo C1 R3 IR2101S 106 R1 D1 QL 107 FigureFigure 5.5. ImprovedImproved designdesign ofof thethe variablevariable freewheelingfreewheeling drivedrive circuit.circuit.

Table 1. The descriptions of symbols used in Figure5. 108 106 Table 1. The descriptions of symbols used in Figure 5. 107 DeﬁnitionsFigure 5. Description Improved design of the variable freewheeling Deﬁnitions drive Descriptioncircuit. Definitions Description Definitions Description 108 C1Table Absorption 1. The description Capacitors of symbols used L in Figure 5. Injector C3C1 Absorption Bootstrap Capacitor Capacitor L QH Injector High Side MOSFET D1C3 Definitions FreewheelingBootstrap Description C Diodeapacitor DefinitionsQH QL High Description Side Low MOSFET Side MOSFET D2D1 C1 FreewheelingAbsorption Diode Capacitor Diode LQL R1 LowInjector Side Pull-down MOSFET Resistor D3C3 BootstrapBootstrap Diode Capacitor QH R2High Side MOSFET Gate Resistor D2 Freewheeling Diode R1 Pull-down Resistor IR2101SD1 GateFreewheeling Drive Chip Diode QL R3Low Side MOSFET Gate Resistor D3 D2 BFreewheelingootstrap Diode Diode R1R2 Pull-downGate Resistor Resistor 3.1. Safety and ReliabilityIR2101SD3 GateBootstrap Drive DiodeChip R2R3 GateGate Resistor Resistor IR2101S Gate Drive Chip R3 Gate Resistor D1, C1, and R1 were set to ensure the safety and reliability of the drive circuitry. Their functions 109 are3.1.109 describedSafety 3.1. and Safety asReliability follows: and Reliability 110 110D1, C1, andD1, C1, R1 and were R1 wereset to set ensure to ensure the the safety safety andand reliability reliability of the of drive the drivecircuitry. circuitry. Their functions Their functions 3.1.1. Function of Freewheeling Diode D1 Set 111 are111 described are described as follows: as follows:

112The QH3.1.1. sourcesFunction of and Freewheeling Vs pin ofDiode IR2101S D1 Set in Figure5. were connected, which is the technical 112 requirement3.1.1. Function of bootstrapof Freewheeling [22]. As Diode shown D1 in S Figureet 6a, a large negative transient voltage was generated 113 The QH sources and Vs pin of IR2101S in Figure 5. were connected, which is the technical − 113 at the114The QH requir QH source sourcesement without of andbootstrap D1. Vs The pin [22]. voltage ofAs IR2101S shown reached in inFigure Figure20 6 Va, and a5. large were would negative connected, damage transient the which voltageIR2101S is was the after technical several 114 workingrequir115 ement cycles.generated of The bootstrap at the D1 QH setting source [22]. voltage withoutAs shown wasD1. The limited in voltage Figure to reached− 16 Va, (i.e.,−20 a largeV theand forwardwould negative damage voltage transient the IR2101S of D1), voltage as shown was in Figure116 6afterb. Setting several working D1 ensured cycles. thatThe D1 the seing negative voltage transient was limited voltage to −1 V (i would.e., the forward not corrupt voltage theof IR2101S. 115 generated117 at the QH source without D1. The voltage reached −20 V and would damage the IR2101S Therefore,D1), the as feasibility shown in Figure of circuitry 6b. Setting was D1 ensured realized. that the negative transient voltage would not corrupt 116 after118 several the IR2101S working. Therefore, cycles. the The feasibility D1 seing of circuitry voltage was realizedwas limited. to −1 V (i.e., the forward voltage of 117 D1), as shown in Figure 6b. Setting D1 ensured that the negative transient voltage would not corrupt 60 118 the IR2101S. Therefore, the feasibility of circuitry was realized. 50

60 40

50 30

20 40 6 10 0 30 -6 on QH Source /V 0 -12 20 -18 Negative transientvoltage -10 6 0.60 0.63 10 -20 0 0.0 0.2 0.4 0.6 0.8 119 -6 Time（ms）

on QH Source /V 0 -12 120 (a) Without D1. -18 Negative transientvoltage -10 Figure 6. Effects of D1 on controlling the0.60 negative transient0.63 voltage in the variable freewheeling drive circuit. -20 0.0 0.2 0.4 0.6 0.8 119 Time（ms） 120 (a) Without D1.

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40 Energies 2019, 12, 564 6 of 18 30

Energies 2018, 11, x FOR PEER REVIEW 6 of 19 4 20

60 0 10 50 -4 on QH Source /V 0 40 30 -8 0.60 0.63 Negative transient voltage -10 4 20

-20 0 0.010 0.2 0.4 0.6 0.8 -4 121 on QH Source /V 0 Time /ms -8 0.60 0.63 122 Negative transient voltage -10 (b) With D1.

-20 123 Figure 6. Effects of D1 on controlling0.0 the 0.2 negative transient0.4 voltage0.6 in the0.8 variable freewheeling 121 Time /ms 124 drive circuit. 122 (b) With D1.

125 3.1.2.123 Function Figure of Absorption 6. Effects of D1 Capacitor on controlling C1 the SetFigure negative 6. Cont transient. voltage in the variable freewheeling 124 drive circuit. 126 At the beginning of Stage (c), a voltage spike of −10 V was generated because D1 was not fully 3.1.2. Function of Absorption Capacitor C1 Set 127 switched125 3.1.2.on, asFunction shown of Absorption in Figure Capacitor 7a. The C1 IR2101SSet (or other high-side driver chips) only had a At the beginning of Stage (c), a voltage spike of −10 V was generated because D1 was not fully 128 negative126 transientAt the voltagebeginning toleranceof Stage (c) , of−5a voltage V. Thespike spikeof −10 V would was generated not damage because D1 the was circuitry not fully but could 129 switchedunexpectedly127 on,switched as cause shown on, the as in IR2101S shown Figure in 7toFigurea. be The latched 7a IR2101S. The [23,24]. IR2101S (or other (orIf latched, other high-side high the-side drivercircuitry driver chips) chips) would only only go had out a aof negative control, 130 transientand128 QH would voltagenegative turn tolerance transient on again. voltage of− The5 tolerance V. Theeffective spike of−5 freewheeling V. would The spike not would damage path not then thedamage turned circuitry the circuitryinto but the could but one could unexpectedly in Figure 4a, 129 131 causeand the the rate IR2101Sunexpectedly of current to be cause latcheddrop the IR2101Sbecame [23,24 to ].slower be If latched latched, (Fig [23,24].ure the If circuitry7 clatched,). The tslower wouldhe circuitry godrop would out rate of go control, delayedout of control, and the QH lowering would 130 turn on again.and QH The would effective turn on freewheelingagain. The effective path freewheeling then turned path intothen turned the one into in the Figure one in4 Figa,ure and 4a the, rate of 132 process131 ofand the the plunge rate of r,current which drop would became subsequently slower (Figure 7prolongc). The slower the dropinjection rate delayed process the loweringand significantly 133 currentincrease132 drop theprocess fuel became ofinjection the slowerplunge mass.r, (Figurewhich would7c). subsequently The slower prolong drop ratethe injection delayed process thelowering and significantly process of the 134 plunger,133Therefore, whichincrease would tothe eliminatefuelsubsequently injection themass. voltage prolong spike, the injection C1 was process set to and connect signiﬁcantly the Vs increaseto the ground. the fuel 134 135 injectionAccordingly, mass. theTherefore, spike towas eliminate eliminated the voltage by capacitor spike, C1 absorpti was set on, to connectas illustrated the Vs into Fig the ure ground. 7b. 135 Accordingly, the spike was eliminated by capacitor absorption, as illustrated in Figure 7b.

60 60

50 50 40 40 30 30 4 20

4 0 20 10

0 -4 10 onQH Source /V 0 -8 0.60 0.63 Negativetransient voltage -10 -4 onQH Source /V 0 -20 -8 0.0 0.600.2 0.40.63 0.6 0.8 Negativetransient voltage -10 136 Time /ms 137 -20 (a) Without C1. 0.0 0.2 0.4 0.6 0.8 136 Time /ms 137 (a) Without C1.

Figure 7. Voltage spike in the variable freewheeling drive circuit.

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2 20 0 10 on QH Source /V -2 0 Negative transient voltage 0.60 0.63

-10 0.0 0.2 0.4 0.6 0.8 138 Time /ms 139 (b) With C1.

30 Gate of QH Source of QH 0 Voltage on Voltage electrode/V 18 12 6 QH Control signal/V 0 16

8 QL Control signal/V 0 28 Abnormal current 14 Normal current

Dtive 0 current/A 0.0 0.2 0.4 0.6 0.8 1.0 140 Time /ms 141 (c) Latched IR2101S.

142 Figure 7. Voltage spike inFigure the variable 7. Cont freewheeling. drive circuit.

Therefore, to eliminate the voltage spike, C1 was set to connect the Vs to the ground. 143 3.1.3. Function of Pull-Down Resistor R1 Set Accordingly, the spike was eliminated by capacitor absorption, as illustrated in Figure7b. 144 If R1 was not set, the Vs was suspended. As shownin Figure 8, when the circuit was ready for 145 3.1.3.operation, Function but the of Pull-DownVs voltage Resistorreached R16 V Set and the voltage difference of C3 was not big enough, then 146 QH couldIf R1 wasnot be not fully set, executed the Vs was when suspended. turned on. As As shownin a result, Figure the current8, when became the circuit atypical. was ready for 147 operation,To eliminate but the Vsthe voltagesuspended reached state, 6 VR1 and was the set voltage to connect difference the Vs of to C3 the was ground. not big After enough, injection, then QHthe 148 couldVs returned not be fully to 0 V executed due to when the pull turned-down on. effect As a result,of the theresistor. current Consequently, became atypical. the aberration was 149 eliminated.

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30 Gate of QH 0 Source of QH Voltage onVoltage electrode/V 18 12 QH 6 Control signal/V 0 16 QL

8 Control signal/V 0 28 Abnormal current Normal current 14 Dtive

current/A 0 0.0 0.2 0.4 0.6 150 Time /ms 151 FigureFigure 8 8.. SuspensionSuspension of of Vs Vs in in the the variable variable freewheeling freewheeling drive drive circuit circuit..

152 3.2. CurrentTo eliminate Drop Rate the suspended state, R1 was set to connect the Vs to the ground. After injection, the Vs returned to 0 V due to the pull-down effect of the resistor. Consequently, the aberration 153 In Stages (a) and (b), QH was turned on when QL was off. The effective freewheeling path is was eliminated. 154 illustrated in Figure 9a, and the rule of current drop rate is expressed by Equation (2). Therefore, 155 with3.2. Current this design, Drop Rate the current drop rate would be the slowest and would fully satisfy the 156 requirements of Stages (a) and (b). In Stages (a) and (b), QH was turned on when QL was off. The effective freewheeling 157 EnergiesIn Stage 2018 ,( 11c),, xboth FOR PEERQH and REVIEW QL were off. The effective freewheeling path of Stage (c) is illustrated8 of 18 path is illustrated in Figure9a, and the rule of current drop rate is expressed by Equation (2). 158 in Figure 9b, and it followed Equation (4). When compared to Figure 4b, VCC, D1, D2, and L form a Therefore,charging withthe drive this design,power from the currentthe solenoid drop becaus rate woulde of the be conversion the slowest from and electromagnetic would fully satisfy energy 159 new freewheeling path. The current did not drop to 0 A immediately; instead, the rate of current theto requirements electrical energy. of Stages The charging (a) and (b). capacity is consequently expressed by Equation (5). 160 drop became slower. However, the feasibility of the circuitry was implemented by setting D1 and

161 charging the drive power from the solenoid becauseVCC of the conversion from electromagnetic energy 162 to electrical energy. The charging capacity is consequently expressed by Equation (5).

VCC QH VCC

QH D2 VCC VCC D1

D1 D2

(a) Stages (a) and (b). 163 Figure 9. Freewheeling pathsVCC in the three different stages. 164 (a) Stages a and b.

QH VCC

D1 D2

L Branch 1

Branch 2

QL Branch 3

(b) Stage (c).

Figure 9. Freewheeling paths in the three different stages.

di ri+++ VCC U U =− DD12 (4) dt L

Ihold E =⋅⋅VCC i dt vcc  (5) 0 Based on the design in Figure 5, the following two methods were commonly employed to improve the rate of current drop in Stage (c).

3.2.1. Adding Impedance Elements Adding impedance elements into the effective freewheeling path in Figure 9b could increase the rate of current drop in Stage (c). The effective freewheeling path of Figure 9b could be broken down into three branches. Branch 2 and Branch 3 were the common parts of Stages (a) and (b). The impedance added into these branches could increase the rate of current drop not only in Stage (c) but

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charging the drive power from the solenoid because of the conversion from electromagnetic energy to electrical energy. The charging capacity is consequently expressed by Equation (5).

VCC

QH VCC

D1 D2

Energies 2019, 12, 564 9 of 18 (a) Stages (a) and (b).

VCC

QH VCC

D1 D2

L Branch 1

Branch 2

QL Branch 3

(b) Stage (c).

Figure 9. Cont. Figure 9. Freewheeling paths in the three different stages. In Stage (c), both QH and QL were off. The effective freewheeling path of Stage (c) is illustrated in Figure9b, and it followed Equationdi (4).ri When+++ VCC compared U U to Figure4b, VCC, D1, D2, and L form =− DD12 (4) a new freewheeling path. The currentdt did not drop L to 0 A immediately; instead, the rate of current drop became slower. However, the feasibility of the circuitry was implemented by setting D1 and charging the drive power from the solenoid becauseIhold of the conversion from electromagnetic energy to E =⋅⋅VCC i dt electrical energy. The charging capacity isvcc consequently expressed by Equation (5). (5) 0

Based on the design in Figuredi 5, theri + followingVCC + U Dtwo1 + UmethodsD2 were commonly employed to = − (4) improve the rate of current drop indt Stage (c). L

Ihold 3.2.1. Adding Impedance Elements Z Evcc = VCC · i · dt (5) Adding impedance elements into the effective0 freewheeling path in Figure 9b could increase the rateBased of current on the drop design in Stage in Figure (c).5 The, the effective following fr twoeewheeling methods path were of commonly Figure 9b employedcould be broken to improve down theinto rate three of current branches. drop Branch in Stage 2 (c). and Branch 3 were the common parts of Stages (a) and (b). The impedance added into these branches could increase the rate of current drop not only in Stage (c) but 3.2.1. Adding Impedance Elements

Adding impedance elements into the effective freewheeling path in Figure9b could increase the rate of current drop in Stage (c). The effective freewheeling path of Figure9b could be broken down into three branches. Branch 2 and Branch 3 were the common parts of Stages (a) and (b). The impedance added into these branches could increase the rate of current drop not only in Stage (c) but also in Stages (a) and (b). Meanwhile, the impedance added intoBranch1 could only increase the rate of current drop in Stage (c), but the negative transient voltage generated by Vs in Stage (c) would be increased from –UD1 to –(UD1+Uimpedance). By doing so, the safety of the circuitry safety would be threatened as described above. Therefore, it is not recommended to add any impedance elements to improve the rate of the current drop in Stage (c).

3.2.2. Increasing the VCC Voltage While considering the injector voltage and current limits, the effects of different VCCs on the drive current were simulated using the PSPICE software as shown in Figure 10a, where QH-logic and QL-logic are the same as Figure3b, r = 0.23 Ω and L = 150 uH are as a circuit model for the injector, Main Pulse = 100 µs, Injection Pulse = 400 µs, VCCs = 24V, 36V, 48V, 60V, 70V, and Hold Pulses are adjusted to guarantee Ihold = 12 A. As can be seen in Figure 10b, increasing the VCC not only could Energies 2019, 12, 564 10 of 18

increase the current growth rate from 0 A to Ipeak but also could increase the current drop rate in Stage (c), as also shown by Equation (4). The drive power voltage is a standardized parameter of injectors, and its value must be within an acceptable range. A higher drive power voltage would reduce the conversion efﬁciency of the boost circuitry. However, an appropriate increase should Energies 2018, 11, x FOR PEER REVIEW 10 of 19 Energiesbe acceptable. 2018, 11, x FOR PEER REVIEW 10 of 19

VCC VCC D3 InjectorQH Signal Synthesis 12V D3 C3 InjectorQH D1 Signal Synthesis 12V C3 D1 4.7uF Injection Pulse U1 4.7uF Injection Pulse 1 U1 8 S1 VCC Vb S1 1 8 R2 0.23 C1 0.1uF QH-logic 2 VCC Vb 7 R2 0.23r 2 C1 0.1uF Hin Ho Main Pulse 1 QH-logic 2 7 100 r 2 Main Pulse2 1 3 QL-logic 3 Hin Ho 6 100 S1 Lin Vs S1 2 12 3 QL-logic 3 6 R3 150uH 3 12 4 Lin Vs 5 R3 150uHL S1 GND Lo S1 3 AND 4 5 L D2 Hold Pulse OR AND IR2101GND Lo 100 R1 D2 Hold Pulse OR IR2101 100 R1100k 1 100k 1 QL QL 191 191 192 (a) PSPICE diagram. 192 (a) PSPICE diagram.

40 40

12 30 12 30 VCC 8 VCC 72V 8 72V 60V 4 60V 4 20 48V

48V 0 20 36V 0.00040 0.00044 0 36V 24V 0.00040 0.00044

Drive 24V Drive current /A

current /A 10 10

0 0 0.0000 0.0002 0.0004 0.0006 0.0000 0.0002 0.0004 0.0006 193 Time /ms 193 Time /ms 194 (b) Effects of VCC on the drive current. 194 (b) Effects of VCC on the drive current.

195 Figure 10. Effects of VCC on the drive current in the variable freewheeling circuit through PSPICE. 195 FigureFigure 10 10.. EffectsEffects of of VCCVCC onon the the drive drive current current in in the the v variableariable freewheeling freewheeling circuit circuit through through PSPICE PSPICE.. 196 4. Experiments 196 4. Experiments 197 The boost circuitry was used as the power source in the drive circuit to evaluate the electric energy 197 The boost circuitry was used as the power source in the drive circuit to evaluate the electric energy 198 consumption; its its schematic schematic diagram diagram is isillustrated illustrated in Fig in Figureure 11. If11 the. If VCC the VCC was lowerwas lower than the than target the targetvalue, 198 consumption; its schematic diagram is illustrated in Figure 11. If the VCC was lower than the target value, 199 value,the boost the circuitry boost circuitry would start would to work, startto and work, vice andversa; vice if higher, versa; ifthe higher, circuitry the would circuitry stop. would stop. 199 the boost circuitry would start to work, and vice versa; if higher, the circuitry would stop.

BAT D VCC BAT D VCC L L Q C Q C Closed-loop Closed-loopControl Control

200 200 201 Figure 11 11.. Boost circuitry circuitry.. 201 Figure 11. Boost circuitry. 202 The effect of different R on the drive current with the RD freewheeling circuit is shown in 202 The effect of different R on the drive current with the RD freewheeling circuit is shown in 203 Figure 1212.. As As the the RR increased,increased, the the rate rate of of current current drop drop in in Stage (c) would be faster, as shown in 203 Figure 12. As the R increased, the rate of current drop in Stage (c) would be faster, as shown in 204 EquationEquation 1. However, However, the the rate rate of of current current drop drop was was basically basically the the same same when when R R= 10=10 Ω Ωandand R =R 15= 15Ω. ΩIn. 204 Equation 1. However, the rate of current drop was basically the same when R = 10 Ω and R = 15 Ω. In 205 InStage Stage (c), (c), the the positiv positivee transient transient voltage voltage of of the the drain drain electrode electrode Q Q (V (V) ) in in Fig Figureure 2 isis expressedexpressed byby 205 Stage (c), the positive transient voltage of the drain electrode Q (V) in Figure 2 is expressed by 206 Equation (6); consequently, as the R increased, the V would grow significantly larger and damage 206 Equation (6); consequently, as the R increased, the V would grow significantly larger and damage 207 the MOSFET. Therefore, a value of 10 Ω was selected as the optimal value of R. 207 the MOSFET. Therefore, a value of 10 Ω was selected as the optimal value of R.

V VCC  iR  U D (6) V VCC  iR  U D (6)

Energies 2019, 12, 564 11 of 18

Equation (6); consequently, as the R increased, the V would grow signiﬁcantly larger and damage the MOSFET. Therefore, a value of 10 Ω was selected as the optimal value of R.

V = VCC + iR + UD (6) Energies 2018, 11, x FOR PEER REVIEW 11 of 19

25 R 6 0Ω Energies 2018, 11, x FOR PEER REVIEW 11 of 19 1Ω 4 2030 5Ω 10Ω 2 15Ω 25 R 6 0 15 0Ω 1Ω 4 0.0006 0.0008 20 5Ω 2 Drive 10Ω 10 15Ω current /A 0 15 0.0006 0.0008

Drive 5 10 current /A

05 0.0000 0.0004 0.0008 0.0012 0.0016 0.0020 208 0 Time /ms 0.0000 0.0004 0.0008 0.0012 0.0016 0.0020 Figure 12. Effects of R on the drive current with RD freewheeling circuit. 209 208 Figure 12. Effects of R on the driveTime current /ms with RD freewheeling circuit. 209 Figure 12. Effects of R on the drive current with RD freewheeling circuit. 210 In order order to to study study the the effect effect of the of two the types two of types the freewheeling of the freewheeling circuit on thecircuit combustion on the performance, combustion 211 210performance,an engineIn test order wasan toenginecarried study thetest out effect usingwas of carried a the self-developed two out types using of the ECU afreewheeling self under-developed the circuit condition ECU on the underthat combustion only the the condition injector drivethat 211circuit performance, was changed. an engine The test test engine,was carried as shownout using in a Figure self-developed 13, was ECU based under on athe heavy-duty condition that six-cylinder diesel 212 212only the injector drive circuit was changed. The test engine, as shown in Figure 13, was based on a engine.only The the detailedinjector drive specifications circuit was changed of the test. The engine test engine, are shownas shown in in Table Figure2 .13, was based on a 213 213heavy heavy-duty-duty six -sixcylinder-cylinder diesel diesel en engine.gine. The The detailed detailed specifications specifications of the test of engine the testare shown engine in Tableare shown 2. in Table 2.

214 215 FigureFigure 13 13.. SchematicSchematic of experiment of experiment setup in setupthe engine in thetest. engine test.

216

214 215 Figure 13. Schematic of experiment setup in the engine test.

216

Energies 2018, 11, x FOR PEER REVIEW 12 of 19

217 Energies 2019, 12, 564 Table 2. Engine characteristics. 12 of 18

Bore × Stroke 126 × 155 mm SwirlTable Ratio 2. Engine characteristics. 1.2 Compression Ratio 17:1 Bore × Stroke 126 × 155 mm Injection system Common rail Swirl Ratio 1.2 CompressionInjection Pressure Ratio 16017:1 MPa Number Injectionof Injector system Nozzle Holes Common8 holes rail InjectorInjection Nozzle Pressure Hole Diameter 0.217160 MPamm InjectorNumber ofSpray Injector Angle Nozzle (included) Holes 1438 holes deg Injector Nozzle Hole Diameter 0.217 mm InjectorPeak Spray Cylinder Angle Pressure (included) 16.5143 Mpa deg PeakRated Cylinder Rotating Pressure Speed 210016.5 r/min Mpa RatedRated Rotating Torque Speed 19702100 N r/min·m RatedRated Power Torque Rating 3531970 kW N·m Rated Power Rating 353 kW Ignition Order 1-5-3-6-2-4 Ignition Order 1-5-3-6-2-4

218 As described above, the RD and variable freewheeling injector drive circuits were established, 219 as shownAs indescribed Figure 2 above, and Figure the RD 5, and respectively, variable freewheeling with R = 10 injectorΩ. The driveBOSCH circuits-CRIN2 were (the established, second 220 generationas shown of in Bosch Figures common2 and5, respectively,rail injector for with truckR =and 10 Ωheavy. The d BOSCH-CRIN2uty vehicles) injector (the second was chosen generation in 221 theof test, Bosch with common standardized rail injector VCC of for48 V, truck Ipeak andof 25 heavy A, and duty recommended vehicles) injector Ihold of 12 was A. The chosen modulation in the test, 222 frequencieswith standardized of both circuitriesVCC of in 48 the V, IholdIpeak phaseof 25 were A, andthe same. recommended The experimentalIhold of 12results A. The are discussed modulation 223 as frequenciesfollows. of both circuitries in the Ihold phase were the same. The experimental results are discussed as follows. 224 4.1. Drive Current 4.1. Drive Current 225 A comparison of the drive current with an injection pulse of 600 µs is illustrated in Figure 14 A comparison of the drive current with an injection pulse of 600 µs is illustrated in Figure 14 below. 226 below.

20 10

inflection point 5

15 0 0.6 0.7 0.8

10 Drive current/A

5 RD freewheeling Variable freewheeling

0 0.0 0.2 0.4 0.6 0.8 227 Time /ms 228 FigureFigure 14. 14.ComparativeComparative effects effects of freewheeling of freewheeling injection injection circuits circuits on onthe the drive drive current current..

229 UsingUsing the the variable variable freewheeling freewheeling circuit, circuit, the rates rates of of current current drop drop in inStages Stages (a) and(a) and (b) were(b) were slower 230 slowerthan thosethan those of the of RD the freewheeling RD freewheeling circuit, circuit, as described as described in Equation in Equation (1) and Equation (1) and (2),Equation respectively. (2), 231 respectively.Therefore, theTherefore, process the duration process using duration the variable using the freewheeling variable freewheeling circuit in Stage circuit (a) in was Stage longer. (a) was In the 232 longer.Ihold phase, In the the Ihold duty phase cycle, the to duty maintain cycle the to current maintain value the near current the I hold valueof the near RD the freewheeling Ihold of the circuitRD 233 freewheelingwas bigger, circuit and its was current bigger, ﬂuctuation and its current rangewas fluct alsouation larger. range was also larger. 234 In InStage Stage (c) (c),, R wasR was 10 10Ω,Ω while, while UDU1 Dwas1 was 1 V 1 from V from Equation Equation (1) (1).. As Asthe the current current became became smaller, smaller, 235 thethe rate rate of ofcurrent current drop drop in inthe the RD RD freewheeling freewheeling circuit circuit would would become become lower. lower. Usi Usingng Equation Equation (4), (4), 236 whenwhen r wasr was nearly nearly 0 Ω, 0 Ω the, the VCCVCC waswas 48 48V; V;hence, hence, when when I wasI was small, small, the the current current drop drop rate rate hardly hardly 237 changed.changed. As Asis illustratedin is illustratedin Figure Figure 14, 14 the, the current current drop drop rate rate in inthe the variable variable freewheeling freewheeling circuit circuit was was faster than that of the RD freewheeling circuit. The former’s solenoid valve seating duration was 30 µs

Energies 2018, 11, x FOR PEER REVIEW 13 of 19

238 faster than that of the RD freewheeling circuit. The former’s solenoid valve seating duration was 30 239 µs shorter than the inflection point of the drive current, which was nearly equivalent to the landing Energies 2019, 12, 564 13 of 18 240 time of the solenoid valve.

241 4.2shorter. Control than Accuracy the inﬂection point of the drive current, which was nearly equivalent to the landing time of the solenoid valve. 242 The cycle variations corresponding to different injection fuel masses at a fuel pressure of 160 243 MPa4.2. Control are shown Accuracy in Table 3. When the injection fuel mass was the same, the pulse width of the 244 variable freewheeling circuit was slightly longer, but the cycle variation was smaller, indicating 245 betterThe control cycle accuracy variations in corresponding the injection of to fuel different mass. injectionThe control fuel accuracy masses at increased a fuel pressure by at least of 160 0.36 MPa% 246 orare exactly shown 2.86% in Table at 35.-mg When fuel the mass injection injection fuel as mass shown was in the Fi same,gure 15 the. This pulse was width a result of the of variable a faster 247 currentfreewheeling drop rate circuit and was a faster slightly end longer, of injection but the in cycleStage variation(c). was smaller, indicating better control accuracy in the injection of fuel mass. The control accuracy increased by at least 0.36% or exactly 2.86% 248 at 5-mgTable fuel 3. massComparati injectionve effect as showns of freewheeling in Figure injection15. This circuits was a resulton the ofcontrol a faster accuracy current of dropfuel mass rate and 249 a fasterinjection. end of injection in Stage (c).

Table 3. Comparative effects ofRD freewheeling freewheeling injection (1000 circuitscycles) on the control accuracy of fuel mass injection.Fuel Mass (mg) 5 10 40 80 120 160 200 Pulse Width (µs) 265 334 585 777 1090 1467 1847 RD freewheeling (1000 cycles) Cycle VariationFuel Mass (mg)(%) 5.945 4.28 10 402.32 801.82 1200.87 1600.82 200 0.71 Pulse Width (µs) 265 334 585 777 1090 1467 1847 Cycle Variation (%)Variable5.94 freewheeling 4.28 2.32 (1000 1.82 cycles) 0.87 0.82 0.71 Fuel Mass (mg) 5 Variable10 freewheeling 40 (1000 cycles)80 120 160 200 Fuel Mass (mg) 5 10 40 80 120 160 200 Pulse PulseWidth Width (µs) (µs) 299299 379 614614 808808 11321132 1502 1502 1876 1876 Cycle Variation (%) 3.08 2.18 1.51 1.1 0.45 0.38 0.35 Cycle Variation (%) 3.08 2.18 1.51 1.1 0.45 0.38 0.35

5.6

5.2 mg 4.8 mass/

Fuel injected 4.4 RD freewheeling COV=5.94% 4.0 Varible freewheeling COV=3.08% 0 200 400 600 800 1000 250 Injection cycle number 251 Figure 15.15. Comparative effectseffects of of freewheeling freewheeling injection injection circuits circuits on on the the cycle-by-cycle cycle-by-cycle variations variations of fuel of 252 fuelinjected injected mass mass at 5 mg.at 5 mg.

253 4.34.3.. Electric Energy Consumption 254 The change in VCC voltage is illustrated in Figure 1166. When the injection fuel mass was 200 mg 255 and done at 160 MPa, the fall range of the drive power voltage became became 1 1 V V smaller, smaller, and its recovery 256 time was 1 ms shorter than those of the RD freewheeling circuit.

Energies 2019, 12, 564 14 of 18 Energies 2018, 11, x FOREnergies PEER 2018 REVIEW, 11, x FOR PEER REVIEW 14 of 19 14 of 19

50 50 50 50

48 48 40 40

46 46 30 30

RD freewheeling RD freewheeling

44 20 Drive

44 20 Drive Drive Drive Varible freewheeling Varible freewheeling current /A current /A voltage /V voltage /V

42 42 10 10

40 40 0 0 0.000 0.001 0.0000.002 0.0010.003 0.0020.004 0.0030.005 0.004 0.005 Time /ms 257 257 Time /ms 258 Figure258 1 6. ComparativeFigureFigure effects 1 16.6. ComparativeComparative of freewheeling effects effects injection of of freewheeling freewheeling circuits on injection the electric circuits energy on the consumption electric energy. consumption.consumption.

259 In Figure259 16, theIn levels Figure of 161 electricity6,, the the levels levels consumption of of electricity electricity consumptionwere consumption the same were whenwere the thethe same samecurrent when when theincreased currentthe current increased increased from 260 from 0 A260 to I peak. fromHowever,0 A to 0 IApeak to there. However,Ipeak. However,was no there energy there was consumption nowas energy no energy consumption when consumption the current when when thedecreased current the current from decreased Idecreasedpeak to from fromIpeak to IpeakIhold to. 261 Ihold. In the261 Ihold IInphase,hold the. InI holdtthehephase, electricIhold phase, the energy electric the electric consumption energy consumption energy of consumption the of variable the variable of freewheeling the freewheeling variable circuit freewheeling circuit was was smaller circuit than was 262 smaller 262than thatsmallerthat of the of the RDthan RD due that due to of a to thesmaller a smallerRD dueduty duty to for a smaller formaintaining maintaining duty forthe the maintainingcurrent current value value the near current near the the I holdvalueI.hold The. near The drivethe Ihold power. The 263 drive power263 waswasdrive charged charged power in was inStage Stage charged (c) (c)when when in usingStage using (thec) the whenvariable variable using freewheeling freewheeling the variable circui circuit,freewheelingt, but but not not whencircui when t, but using not the when RD 264 using the264 RD freewheelingusingfreewheeling the RD circuit. freewheeling Therefore, Therefore, circuit. the the electric electric Therefore, energy energy the consumption electric consumption energy of the of consumption variable the variable freewheeling of the variable circuit 265 freewheeling265 circuitwasfreewheeling smaller.was smaller. circuit was smaller.

4.4. Highest Attainable Temperature 266 4.4. Highest266 Attainable 4.4. Highest Temperature Attainable Temperature 267 267 The highest temperature captured by a thermal infrared imager (FLIR) at 1900 rpm was below The highest temperature captured by a thermal ◦infrared imager (FLIR) at 1900 rpm was below 268 268 the environment temperature of of 20 °CC during a single injection and at a fuel mass of 200 mg, as the environment temperature of 20 °C during a single injection and at a fuel mass of 200 mg, as ◦ 269 269 shown in Figure 1717. The The highest highest temperature temperature of of the the variable variable freewheeling freewheeling circuit circuit was was only only 37 37 °C,C, shown in Figure 17. The highest◦ temperature of the variable freewheeling circuit was only 37 °C, 270 which was270 127 which °C lower was was than127 127 C that °C lower lower of thanthe than RD that that freewheeling of the of RD the freewheeling RD circuit. freewheeling This circuit. was circuit. This due was to This thedue was smaller to the due smaller to the electricity smaller 271 electricity271 consumption electricityconsumption in consumptionthe informer the former drive in the circuit. drive former circuit. The drive element The circuit. element that The had thatelement the hadhighest that the highesthadtemperature the temperaturehighest in temperature in the RD in 272 thefreewheeling RD freewheeling circuit was circuit R, wherewas R, as where it was as D2 it was in the D2 variable in the variable freewheeling freewheeling circuit. circuit. 272 the RD freewheeling circuit was EnergiesR, where 2018 as, 11 it, xwas FOR D2 PEER in REVIEWthe variable freewheeling circuit. 15 of 19

(a) RD freewheeling(a) circuitRD freewheeling. (b) V ariablecircuit. freewheeling circuit.

Figure 17. Comparative effects of freewheeling injection circuits on the highest attainable temperature. 273 Figure 17. Comparative effects of freewheeling injection circuits on the highest attainable 274 temperature.

275 4.5. Cycle-by-Cycle Variations 276 At a load of 1600 rpm, IMEP = 1 MPa, and a circulating fuel injection mass of 100 mg, the 277 cycle-by-cycle variations of the peak cylinder pressure for the two types of the freewheeling circuit 278 are shown in Figure 18. It can be seen that due to the improvement on the control accuracy of the fuel 279 injection mass, the cycle variations (COV) of the peak cylinder pressure using the variable 280 freewheeling circuit were reduced from 1.5% to 1%, indicating a significant performance 281 improvement.

11.0

MPa 10.5

10.0

9.5 Cylinder 1 COV=1.498% Cylinder 3 COV=1.478% Peak cylinder pressure/ Cylinder 6 COV=1.417% 9.0 0 20 40 60 80 100 282 Combustion cycle number 283 (a) RD freewheeling circuit.

Energies 2018, 11, x FOR PEER REVIEW 15 of 19

(b) Variable freewheeling circuit.

273 Figure 17. Comparative effects of freewheeling injection circuits on the highest attainable 274 Energiestemperature2019, 12, 564. 15 of 18

275 4.5. Cycle-by-Cycle Variations 4.5. Cycle-by-Cycle Variations 276 At a load of 1600 rpm, IMEP = 1 MPa, and a circulating fuel injection mass of 100 mg, the 277 cycleAt-by- acycle load variations of 1600 rpm,of the IMEPpeak cylinder = 1 MPa, pressure and a for circulating the two types fuel injectionof the freewheeling mass of 100 circuit mg, 278 arethe shown cycle-by-cycle in Figure variations 18. It can be of seen the peakthat due cylinder to the pressureimprovement for the on twothe con typestrol of accuracy the freewheeling of the fuel 279 injectioncircuit are mass, shown the in Figure cycle 18 variations. It can be (COV) seen that of due the to peak the improvement cylinder pressure on the using control the accuracy variable of 280 freewheelingthe fuel injection circuit mass, were the cycle reduced variations from (COV) 1.5% of to the 1%, peak indicating cylinder apressure significant using performance the variable 281 improvement.freewheeling circuit were reduced from 1.5% to 1%, indicating a signiﬁcant performance improvement.

11.0

MPa 10.5

10.0

9.5 Cylinder 1 COV=1.498% Cylinder 3 COV=1.478% Peak cylinder pressure/ Cylinder 6 COV=1.417% 9.0 0 20 40 60 80 100 282 Combustion cycle number 283 Energies 2018, 11, x FOR PEER REVIEW (a) RD freewheeling circuit. 16 of 19

11.0

10.5

10.0

9.5 Cylinder 1 COV=0.977% Cylinder 3 COV=1.073%

Peak cylinder pressure/MPa Cylinder 6 COV=0.997% 9.0 0 20 40 60 80 100 Combustion cycle number 284 285 (b) Variable freewheeling circuit.

286 Figure 18.18. Comparative effects of freewheeling injection circuits circuits on on the cycle cycle-by-cycle-by-cycle variations of 287 peak cylinder pressure pressure.. 4.6. Combustion Performance 288 4.6. Combustion Performance At three load levels (1300 rpm, 1600 rpm, and 1900 rpm) and IMEP = 1 MPa with the same 289 At three load levels (1300 rpm, 1600 rpm, and 1900 rpm) and IMEP = 1 MPa with the same optimal control strategy, the NOx and soot emissions and the thermal efficiency for the two types of the 290 optimal control strategy, the NOx and soot emissions and the thermal efficiency for the two types of freewheeling circuit are compared in Figure 19. The engine emissions and combustion efficiency were 291 the freewheeling circuit are compared in Figure 19. The engine emissions and combustion efficiency improved due to the decrease in the combustion cycle variations, which could effectively reduce the 292 were improved due to the decrease in the combustion cycle variations, which could effectively reduce influence of deviation of combustion process from the optimal control target. Especially at 1900 rpm, 293 the influence of deviation of combustion process from the optimal control target. Especially at 1900 rpm, where the NOx and soot emissions were reduced by 3.5% and 4%, respectively, and the thermal 294 where the NOx and soot emissions were reduced by 3.5% and 4%, respectively, and the thermal efficiency was slightly increased by 0.3%, while the HC and CO emissions were little improved. 295 efficiency was slightly increased by 0.3%, while the HC and CO emissions were little improved.

45 RD freewheeling 42 40.3565 40.4566 Variable freewheeling 39 37.6622 37.8562 36.2244 36.5015 36

Efficiency /% 0 0.020

） 0.015 0.01266

-1 0.01216 0.010 0.00795 0.00775 0.00805 0.00776 0.005 kW·h) soot / soot

（ 0.000

(g· 5 4 ） 3.26881 3.18493 3.00258 2.89832 -1 3 2.44851 2.3656 2

kW·h) 1 NOx /

（ 0

(g· 1300rpm 1600rpm 1900rpm 296 297 (a) Effects on Efficiency, soot and NOx emissions

Energies 2018, 11, x FOR PEER REVIEW 16 of 19

11.0

10.5

10.0

9.5 Cylinder 1 COV=0.977% Cylinder 3 COV=1.073%

Peak cylinder pressure/MPa Cylinder 6 COV=0.997% 9.0 0 20 40 60 80 100 284 Combustion cycle number 285 (b) Variable freewheeling circuit.

286 Figure 18. Comparative effects of freewheeling injection circuits on the cycle-by-cycle variations of 287 peak cylinder pressure.

288 4.6. Combustion Performance 289 At three load levels (1300 rpm, 1600 rpm, and 1900 rpm) and IMEP = 1 MPa with the same 290 optimal control strategy, the NOx and soot emissions and the thermal efficiency for the two types of 291 the freewheeling circuit are compared in Figure 19. The engine emissions and combustion efficiency 292 were improved due to the decrease in the combustion cycle variations, which could effectively reduce 293 the influence of deviation of combustion process from the optimal control target. Especially at 1900 rpm, 294 Energieswhere 2019 the , 12 NO, 564x and soot emissions were reduced by 3.5% and 4%, respectively, and the the16rmal of 18 295 efficiency was slightly increased by 0.3%, while the HC and CO emissions were little improved.

45 RD freewheeling 42 40.3565 40.4566 Variable freewheeling 39 37.6622 37.8562 36.2244 36.5015 36

Efficiency /% 0 0.020

） 0.015 0.01266

-1 0.01216 0.010 0.00795 0.00775 0.00805 0.00776 0.005 kW·h) soot / soot

（ 0.000

(g· 5 4 ） 3.26881 3.18493 3.00258 2.89832 -1 3 2.44851 2.3656 2

kW·h) 1 NOx /

（ 0

(g· 1300rpm 1600rpm 1900rpm 296

297 Energies 2018, 11, x FOR PEER REVIEW(a) Effects on Efficiency, soot and NOx emissions 17 of 19

RD freewheeling 0.9 ） Variable freewheeling 0.73281 0.72725 -1 0.6 0.45488 0.45375 0.48398 0.4815 kW·h) HC / 0.3 （ 0.0 (g·

0.09 ）

-1 0.06531 0.06515 0.05666 0.05637 0.06256 0.06209 0.06 kW·h) CO / 0.03 （ 0.00 (g· 298 1300rpm 1600rpm 1900rpm 299 (b) Effects on HC and CO emissions

300 FigureFigure 119.9. ComparativeComparative effects effects of of freewheeling freewheeling injection injection circuits circuits on on the the engine engine emissions emissions and and 301 combustioncombustion efficiency efﬁciency..

302 5.5. Conclusions Conclusions 303 (1)(1) Compared Compared to to the the RD RD freewheeling circuit, circuit, the the variable variable freewheeling freewheeling injector injector drive drive circuit circuit 304 metmet the the requirements requirements better better for for the the rate rate of of current current drop drop in in the the three three different stages stages of of the 305 peakpeak-and-hold-and-hold model. 306 (2)(2) Through Through analysis analysis and and structural structural improvements improvements to to improve improve the the safety safety and and reliability reliability of of the the 307 circuitry,circuitry, any anomalies interfering with with the the practical practical use use of of the variable freewheeling freewheeling circuit circuit were were 308 eliminatedeliminated simultaneously. simultaneously. 309 (3)(3) R Reducingeducing the the current current drop drop rate rate in the in theIhold phaseIhold phasecould couldeffectively effectively reduce reduce the energy the 310 consumption.energy consumption. 311 (4)(4) By By charging charging the the drive drive power power supply supply using using the electromagnetic the electromagnetic energy energy released released by the injector by the 312 injectorduring theduring current the current closing closing stage, the stage, electric the electric energy energy consumption consumption could becould reduced be reduced and the and power the 313 powerrecovery recovery time could time becould accelerated. be accelerated. 314 (5)(5) Compared Compared to to the the RD RD fre freewheelingewheeling circuit, circuit, the the variable variable freewheeling freewheeling circuit circuit exhibited exhibited higher higher 315 accuracyaccuracy in in controlling controlling the the injection injection mass, mass, consumed consumed lower lower electric electric energy energy and and lowered lowered the the circuit’s circuit’s 316 highesthighest attainable attainable temperature. temperature. 317 (6) Compared to the RD freewheeling circuit, the variable freewheeling circuit could effectively 318 improve the cycle-by-cycle variations, lower the engine emissions and increase the combustion 319 efficiency. 320 The highlights of this article are as follows: 321 1. The injector drive circuitry with variable freewheeling circuit had better control accuracy of 322 fuel injection mass and lower electrical consumption than with a RD freewheeling circuit. 323 2. The improved design of the variable freewheeling circuit was studied to eliminate anomalies 324 interfering. 325 3. The variable freewheeling circuit could effectively improve the cycle-by-cycle variations, 326 lower the engine emissions and increase the combustion efficiency of diesel engine than RD 327 freewheeling circuit.

328 Nomenclature

Definitions in Circuitry Schematics VCC Driving Power L Injector Solenoid D Diode R Resistor Q MOSFET C Capacitor QH High-side MOSFET

Energies 2019, 12, 564 17 of 18

(6) Compared to the RD freewheeling circuit, the variable freewheeling circuit could effectively improve the cycle-by-cycle variations, lower the engine emissions and increase the combustion efﬁciency. The highlights of this article are as follows: 1. The injector drive circuitry with variable freewheeling circuit had better control accuracy of fuel injection mass and lower electrical consumption than with a RD freewheeling circuit. 2. The improved design of the variable freewheeling circuit was studied to eliminate anomalies interfering. 3. The variable freewheeling circuit could effectively improve the cycle-by-cycle variations, lower the engine emissions and increase the combustion efﬁciency of diesel engine than RD freewheeling circuit.

Author Contributions: Conceptualization, E.L. and S.W.; Methodology, E.L.; Software, E.L.; Validation, E.L.; Formal Analysis, E.L.; Investigation, E.L.; Resources, E.L.; Data Curation, E.L.; Writing-Original Draft Preparation, E.L.; Writing-Review & Editing, E.L.; Visualization, E.L.; Supervision, S.W.; Project Administration, S.W.; Funding Acquisition, S.W. Funding: This research was funded by [the National Natural Science Foundation of China] grant number [CNFS NO.51236005] and by [the Ministry of Science and Technology] grant number [973 National Key Project 2013CB228401]. Conﬂicts of Interest: The authors declare no conﬂict of interest.

Nomenclature

Deﬁnitions in Circuitry Schematics VCC Driving Power L Injector Solenoid D Diode R Resistor Q MOSFET C Capacitor QH High-side MOSFET QL Low-side MOSFET Deﬁnitions in Formulas VCC Voltage of Driving Power i Solenoid Current r Resistance of Solenoid L Inductance of Solenoid UD Forward Voltage of D R Resistance of R

References

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