a coordinated boost control in a twincharged spark ...annastef/papers_tc_diesel/... · a...

A COORDINATED BOOST CONTROL IN A TWINCHARGED SPARK IGNITIONENGINE WITH HIGH EXTERNAL DILUTION

Shima NazariDepartment of Mechanical Engineering

University of Michigan

Ann Arbor, Michigan, 48109

Email: [email protected]

Anna StefanopoulouRani Kiwan

Department of Mechanical Engineering

University of Michigan

Ann Arbor, Michigan, 48109

Vasilios TsourapasEaton

Global Research and Technology

Southfield, Michigan, 48076

ABSTRACT

This paper proposes a novel master-slave control strategyfor coordination of throttle, wastegate and supercharger actu-ators in an electrically twincharged engine in order to guar-antee efficient boost control during transients, while at steadystate a throttle-wastegate coordination provides minimum enginebackpressure hence engine efficiency elevation. The benefits andchallenges associated with Low Pressure Exhaust Gas Recir-culation (LP-EGR) in a baseline turbocharged engine, includ-ing improved engine efficiency, mainly due to better combustionphasing, and sluggish engine response to a torque demand dueto slowed down air path dynamics were studied and quantifiedin [1]. Hence in this paper an electrical Eaton TVS roots type su-percharger at high pressure side of the turbocharger compressor(TC compressor) is added to the baseline turbocharged engineand the performance of the proposed controller in the presence ofLP-EGR, which is a more demanding condition, is evaluated andcompared to the turbocharged engine. One dimensional (1D)crankangle resolved engine simulations show that the proposedmaster-slave control strategy can effectively improve the tran-sient response of the twincharged engine, making it comparableto naturally aspirated engines, while the consumed electrical en-ergy during transients can be recovered from the decreased fuelconsumption due to LP-EGR conditions at steady state in ap-proximately 1 second. Finally, a simple controller is developedto bypass the TC compressor and maximize the engine feedingcharge during the transients in order to avoid TC compressorchoking and achieve faster response.

INTRODUCTIONNowadays turbocharging is the most prevalent mean of

boosting the engine feeding charge. Unfortunately turbochargedengines can suffer from drivability issues such as insufficientboost pressure at low engine speeds and slower torque responseknown as ”turbo-lag”. Supercharging is the next popular methodof boosting the engines, which does not possess the drivabil-

Comp. bypass valve

Throttle

TC Compressor Turbine

Wastegate

EGR coolerEGR valve

Exhaust manifoldCAC

Intake Manifold

180 180 180

Supercharger

CAC

FIGURE 1. Schematic of twincharged SIDI engine

Proceedings of the ASME 2016 Dynamic Systems and Control Conference DSCC2016

October 12-14, 2016, Minneapolis, Minnesota, USA

DSCC2016-9691

1 Copyright © 2016 by ASME

Downloaded From: http://proceedings.asmedigitalcollection.asme.org/ on 11/07/2017 Terms of Use: http://www.asme.org/about-asme/terms-of-use

ity challenges associated with turbocharging but at the expenseof lower engine fuel economy. Including a supercharger in aturbocharged powertrain or twincharging can combine the driv-ability pros of supercharging with fuel economy benefits of tur-bocharging [2–4].

Variable speed supercharging provides the possibilities ofmore flexible boost control, using a smaller supercharger and re-duced supercharger bypass losses, resulting in both engine fueleconomy and peak torque improvement [5]. Variable speed su-percharging can be achieved easily through electrical supercharg-ing. Although several attempts have been reported to use electri-cal roots type superchargers to retrieve otherwise wasted throt-tling losses [6, 7], recovering the fuel penalty associate with su-percharger consumed electricity is not easy. Considering thisfact, electrical supercharging seems to be a good option for tran-sient performance improvement and for enabling high fuel econ-omy concepts such as highly diluted combustion.

During recent years cooled external Exhaust Gas Recircu-lation (eEGR) has gained a lot of attraction as one potentialpractice to decrease vehicles fuel consumption. Boosted gaso-line engines fuel economy improves significantly by includingcooled eEGR, [8–11]. Presented at [1], despite considerable im-provement in engine fuel consumption by introducing LP-EGR,mainly due to closer to Maximum Brake Torque (MBT) sparktiming and less heat transfer and pumping losses, LP-EGR slowsdown the air path dynamics of the turbocharged systems signif-icantly because of increased air path volumes and decreased ex-haust gas specific enthalpy. As a direct result of this effect the en-gine response time to a torque demand deteriorates, which meanshigher drivability challenges for turbocharged engines equippedwith eEGR.

Considering this fact and the ability of variable speed super-charging in turbo-lag reduction, this paper proposes incorporat-ing an electrical supercharger in a turbocharged engine with lowpressure loop eEGR. The application of a supercharger in ad-dition to throttle and wastegate actuators provides an additionaldegree of freedom in air path control of the engine, hence the co-ordination of these three actuators would be very important forengine fuel consumption reduction in addition to faster chargecontrol. The focus of current study would be the design of a con-troller for coordination of throttle, wastegate and superchargerspeed, which minimizes the engine backpressure at steady statefor fuel economy maximization and uses the supercharger onlyduring the transient through a novel master-slave control strat-egy, where the supercharger speed is slaved to both throttle andwastegate.

This paper is organized as follows. First the twinchargedengine structure and its model are described. The controllerstructure and different parameters actuation method is explainednext. The subsequent section compares the transient results in-cluding the response time and the engine fuel economy of thetwincharged engine to the baseline turbocharged engine with two

different control strategies. One without eEGR and fast torqueresponse throttle-wastegate actuation and the second case witheEGR and throttle-wastegate coordination for high fuel econ-omy. Different actuators movement and the supercharger per-formance for the studied transients are illustrated next. In orderto avoid choking in the turbocharger compressor during severtip-ins a simple controller is designed to bypass the compressorusing a bypass valve and help speeding up the engine torque re-sponse even more. The engine transient performance with andwithout this controller is compared at the final section of this pa-per.

SYSTEM AND MODEL DESCRIPTIONThe studied baseline engine is a 1.6 liter 4 cylinder four-

stroke turbocharged spark ignited gasoline direct injection en-gine. In the proposed twincharged configuration, an electricalEaton TVS roots type supercharger at high pressure side of theturbocharger compressor (TC compressor) along with a chargeair cooler (CAC) and a bypass line are included. Figure 1 showsthe schematic of the twincharged engine and its air path. Sparktiming (uSA), intake and exhaust cam timing (uICT and uECT ),throttle (uq ), wastegate (uwg), EGR valve (uegr), TC compres-sor bypass valve (ubp) and supercharger speed (uSC) are variousactuators and their input signals, represented on the figure.

The employed GT-power model, [12], captures 1-D gas dy-namics in the air path, turbocharger performance, heat transfer,valve lift and port flow behavior, fuel injection and vaporization,combustion and other details necessary to predict engine perfor-mance. The model has been calibrated and validated against thebaseline turbocharged engine data.

CONTROL SYSTEMFigure 2 shows the schematic of the proposed controller for

twincharged engine. The aim of this controller is to use the elec-trical supercharger only during the transient part of the responseand minimize the engine backpressure at steady state while hav-ing a desirable response time and overshoot. Desired intakemanifold pressure (p⇤im) is determined for the desired BMEP(BMEP⇤), desired eEGR level (eEGR⇤) and engine speed (N)and the minimum necessary turbocharger boost pressure, p⇤TCb,that ensures least engine backpressure is determined as follow-ing:

p⇤TCb =

⇢p⇤im i f p⇤im � pambient ,pambient i f p⇤im < pambient

(1)

where pambient is the ambient pressure. In the designed controllerconfiguration, the throttle, wastegate and supercharger speed arecoordinated such that the supercharger speed is controlled based



on the error in tracking the desired intake manifold and desiredboost pressure hence slaved to both throttle and wastegate. Thiswill provide fast and effective boost response during the tran-sients and idling at steady state. Assuming a perfect SC mo-tor control, the supercharger speed is commanded through twoproportional controllers and a feedforward part. One of the con-trollers output is proportional to the error in intake manifold pres-sure, pim, and the second one is proportional to the error in tur-bocharger boost pressure, pTCb:

uSC = min(uSC,umaxSC ) (2)

uSC = k1eim + k2eTCb +uidleSC (3)

eim = p⇤im � pim (4)

eTCb = p⇤TCb � pTCb (5)

where uSC is the supercharger speed, k1 and k2 are proportionalcontroller gains. The variable uidle

SC is the idle speed of super-charger in which the pressure ratio across the supercharger isequal to unity and is in form of a look up table calibrated fordifferent engine speed and loads.

When a step is applied in BMEP during a tip-in transient,desired intake manifold pressure, p⇤im and desired turbochargerboost, p⇤TCb change accordingly. The supercharger speed risesand its pressure ratio and pumping flow increase consequently,responding fast to the errors eim and eTCb that gets generated tillthe throttle and wastegate controllers function to reduce these er-rors. Then the intake manifold pressure reaches its desired value(eim ! 0) and the supercharger speed decreases proportionallybut it takes more time for the turbocharger to produce the re-quired boost. Eventually when the turbocharger boost reachesits target steady state value (eTCb ! 0) the supercharger speedapproaches its new idle value (uSC ! uidle

SC ), at which it does notproduce any boost. This controller is gain scheduled for differentoperating points.

The throttle valve (uq ) controls intake manifold pressure andconsists of a model based feedforward part and a PI feedbackpart,

uq = kp,q (p⇤im � pim)+ ki,q

Z t

t0(p⇤im � pim)dt +u f f

q (6)

where in above equation u f fq is the feedforward portion and is

calculated based on desired intake manifold pressure and param-eters such as engine speed, engine size, throttle size and intake

Twincharged ENGINE

N

Throttle Controller

Wastegate Controller

BMEP Spark tableeEGR

Look up tablesN

k1

k2

idle SC RPM

FIGURE 2. Control system schematic

manifold volume. kp,q and ki,q are the proportional and integralfeedback gains respectively. The wastegate (uwg) controls theturbocharger boost pressure and is a PI controller as following:

uwg = kp,wg(p⇤TCb � pTCb)+ ki,wg

Z t

t0(p⇤TCb � pTCb)dt (7)

where kp,wg is the proportional gain and ki,wg is the integral gainand both are gain scheduled for different operating points. Themaster-slave structure of the controller is more clear consider-ing that the throttle and wastegate controllers both possess inte-gral action, hence they will meet the steady state set points ofintake manifold and TC boost pressure, while the superchargercontroller includes only proportional parts for the errors in thesetwo parameters. Besides, targeting minimum required TC boostpressure, wastegate controller guarantees minimizing the engineback pressure and helps improving the fuel economy.

Cam timings are calibrated for different engine speeds andloads. EGR valve position is specified based on engine speedand desired values of load and eEGR level. The spark timingis determined using a look up table which accounts for engineload, speed and instantaneous residuals in intake manifold. Theresidual fraction in intake manifold is measured using a fast O2sensor. The knock model used to generate this look up table ispresented in [13].

In modern turbocharged engines smaller turbochargers areused in order to decrease the engine turbo-lag, however this prac-tice imposes some limitations such as higher choking probability.In the twincharged engine during tip-ins the supercharger aims tofill the intake manifold as fast as possible. While, the TC com-pressor is located at upstream of the supercharger and in extremetransient conditions it chokes and acts as a restriction, limitingthe pumping ability of the supercharger. To avoid this problem,a simple controller is designed to bypass TC compressor whenvacuum is sensed in its down stream using a bypass line and a



valve. It can be stated that for a portion of time the bypass valveand the turbocharger work in parallel to maximize the air flowinto the engine. The bypass valve control law is as following:

ubp =

⇢1 i f prTCC < 0.980 otherwise (8)

where prTCC is the cycle averaged pressure ratio across the TCcompressor, 1 means fully open and 0 means fully closed valve.The bypass valve dynamic behavior is here approximated by afirst order system with 40 ms time constant.

TRANSIENT RESULTSIn this paper three different engine configurations were sim-

ulated and compared against each other:

– Twincharged engine with LP-EGR, denoted by “TwC 15%eEGR” on result plots. The control system configuration forthis case was explained in the prior paragraphs and throughEqn. (1) to (8).

– Baseline turbocharged engine with LP-EGR system, markedwith “TC 15% eEGR” on result plots. In this case the desiredboost pressure is determined using Eqn. (1) and throttle andwastegate controllers actuate based on Eqn. (6) and Eqn. (7)respectively, so in this case minimum engine backpressure isachieved too. Spark timing and cam timings are controlledsimilar to twincharged case.

– Baseline turbocharged engine without LP-EGR, representedby “TC 0% eEGR” on plots. In this system throttle actuatorcontrols intake manifold pressure, with a controller similarto prior two configurations but wastegate is kept closed dur-ing the simulations (uwg = 0) to provide higher boost pres-sure hence minimizing the engine response time to a torquedemand. In this case spark timing and cam timings are alsocontrolled similar to previous configurations.

Engine ParametersFigure 3 compares the twincharged engine parameters

(black lines) for 10% to 90% of full load tip-in at 2000 rpmengine speed to the baseline turbocharged engine parametersin two cases: high fuel economy case, with 15% eEGR andminimum backpressure wastegate control strategy (dash-dot redlines) and fast torque response case with closed wastegate andwithout eEGR (dashed blue lines). The load responses (the firstplot on left) show that the new proposed powertrain is signifi-cantly faster in terms of torque response. The response time (10-90%) for the twincharged system is 0.28 sec, which is as fast as anaturally aspirated engine. Comparing this number to that of tur-bocharged engine without eEGR 1.28 sec (dashed blue line), andwith eEGR 2.30 sec (dash-dot red line), it is evident that the new

powertrain configuration can completely eliminate the turbo-lagin the boosted engine.

The corresponding plot in Fig.4 shows similar results for10% to 60% of full load tip-in at 2000 rpm constant enginespeed. The response time (10-90% ) is 0.2 sec for twinchargedengine, 0.50 sec for turbocharged engine and 0.70 sec for tur-bocharged engine with 15% eEGR, confirming the torque re-sponse improvement of twincharging even for medium loads.

The first plots on right of Fig.3-4 represent the intake man-ifold pressure versus time. As expected the intake manifoldpressure increases much faster for the twincharged cases and itssteady state value is higher for the cases with eEGR. The secondplots on the left demonstrate the variation in Brake Specific FuelConsumption (DBSFC) of three cases. The numbers are changesrelative to BSFC at 10% load for twincharged engine. The tur-bocharged engine with 15% eEGR and the twincharged enginehave lower fuel consumption at steady state.

The supercharger energy consumption is also included in thepresented plots and this is why the specific fuel consumptionof twincharged cases are slightly higher than the turbochargedcases during the transient part of the response. Although someof the electric energy that powers the SC can come from regen-erative braking, it is assumed here that all the electricity requiredto power the supercharger comes from the engine crank train.The calculated supercharger equivalent fuel consumption and thetwincharged engine corrected BSFC depend on the superchargerconsumed power (PSC) and an assumed gross efficiency (he) of70% for electrical energy production, storage and conversion,

PSC,eq = PSC/he (9)

where PSC,eq is the supercharger equivalent crank train powerand PSC includes the required power to compress the flow andthe friction losses. Equivalent fuel flow rate of the supercharger(m f ,SC) can be computed assuming the same brake efficiency forpower generation as the twincharged engine brake efficiency, hbwithout considering an alternate source such as energy stored inthe battery during regenerative braking,

m f ,SC = PSC,eq/hb (10)

hb =Pb

m f QLHV(11)

where Pb is the engine brake power, m f is the fuel flow and QLHVis the lower heating value of the fuel. And finally the twinchargedengine corrected specific fuel consumption is computed as theratio of total fuel flow rate to the engine brake power, Pb



0 1 2 3 40

20

40

60

80

100

Load

(%)

Tw C 15% eEGRTC 15% eEGRTC 0% eEGR

0 1 2 3 4

0.5

1

1.5

2

Inta

ke M

an. P

(bar

)

0 1 2 3 4

-100

-50

0

50

100

"BSFC

(g/k

W-h

)

0 1 2 3 4-100

0

100

200

300

400"

Turb

ine

InletT(K

)

0 1 2 3 40

0.2

0.4

0.6

0.8

time(sec)

Nor

m. T

C R

PM

0 1 2 3 40

0.05

0.1

0.15

time(sec)

EGR

frac

tion

FIGURE 3. Engine parameters for 10% to 90% of full load tip-in

BSFCcor = (m f + m f ,SC)/Pb (12)

The main assumption in these calculations is the assumedbrake efficiency for supercharger equivalent crank train powergeneration. This assumption is somewhat conservative since theengine brake efficiency is very low during the transient response,while in a drive cycle there is a flexibility in electrical energy pro-duction and it can be generated when the engine brake efficiencyis around its optimum value.

Although the twincharged engine consumes more energyduring the transient response compared to the baseline enginewithout eEGR, it has a better fuel economy at steady state mainlydue to included low pressure EGR [1]. So it would be interest-ing to know how much operation time at steady state is requiredto compensate for the electrical energy consumed by the super-charger during the transient. The computations show that for10% to 90% of full load tip-in case only 1.1 sec operation at highload is sufficient to make up for the fuel consumed by the su-percharger during the studied transient. This number for 10% to60% of full load case is 0.8 sec, meaning that if the twinchargedengine operates only 0.8 sec at 60% of full load, it recovers theextra consumed energy for speeding up the response. In addi-tion, in case of free available electrical energy e.g. from brake

0 1 2 3 40

10

20

30

40

50

60

70

Load

(%)

Tw C 15% eEGRTC 15% eEGRTC 0% eEGR

0 1 2 3 4

0.5

1

1.5

Inta

ke M

an. P

(bar

)

0 1 2 3 4

-100

-50

0

50

100

"BSFC

(g/kW

-h)

0 1 2 3 4

0

100

200

300

"Turb

ine

InletT

(K)

0 1 2 3 4

0

0.1

0.2

0.3

0.4

0.5

0.6

time(sec)

Nor

m. T

C R

PM

0 1 2 3 40

0.05

0.1

0.15

time(sec)

EGR

frac

tion

FIGURE 4. Engine parameters for 10% to 60% of full load tip-in

recovery in mild hybrid vehicles, no extra fuel is burnt for thesupercharger consumed electricity and in this condition depend-ing on the batteries state of charge (SOC) it could be efficient touse the supercharger at steady state too.

The second plot on the right compares the turbine inlet tem-perature variation during the tip-in for different models. Thenumbers are changes relative to turbine inlet temperature at 10%load in twincharged engine. For 90% of full load, the exhausttemperature decreases by 140 �C and for 60% of full load case itdrops by 110 �C through adding 15% eEGR and minimum back-pressure wastegate control. The third plots on the left illustratethe normalized turbocharger speed (as the ratio of turbochargerspeed to it maximum allowable speed) versus time for threecases. For the baseline turbocharged engine with 0% eEGR, theturbocharger speed is higher before and after the transient sincethe wastegate is closed in this case and more flow passes throughthe turbine. The turbocharger speeds up faster in twinchargedcase compared to turbocharged case with 15% eEGR. The rea-son is that the target load is achieved faster in twincharged en-gine and the exhaust gas enthalpy follows the engine power, sothe available energy of turbocharger increases much faster for thetwincharged engine.

Finally the third plots on the right represent the residuals inthe intake manifold. For 10% of load (before the step) althoughEGR valve is fully closed at this point (see Fig.5 and Fig.6), the



0 1 2 3 40

20

40

60

80

100

Thro

ttle

(%)

0 1 2 3 4 50

0.2

0.4

0.6

0.8

1

WG

nor

m. d

iam

TwC 15% eEGRTC 15% eEGRTC 0% eEGR

0 1 2 3 4−10

0

10

20

30

40

time(sec)

∆Sparktiming(deg)

0 1 2 3 40

0.2

0.4

0.6

time(sec)

EGR

nor

m. d

iam

.

FIGURE 5. Actuators movement for 10% to 90% of full load tip-in

residual fraction is around 0.05 for all cases. This part of resid-uals enters the intake manifold during the valves overlap fromthe cylinders and exhaust manifold. After the tip-in cam timingschange; this part of residuals is eliminated and for the cases witheEGR it takes some time for the burnt residuals to pass the airpath and fill the intake manifold. For the studied tip-ins the tran-sients start without LP-EGR because the studied engine main-tains high internal residuals at low load and it is not possible tohave high LP-EGR at this condition due to both combustion sta-bility problems and LP-EGR limitations at low loads.

In twincharged engine case this intake manifold residualsundershoot causes an undesirable overshoot in load response. Al-though the intake manifold pressure is regulated with less than0.5% overshoot, the load response of the two studied cases show3.1% and 10.6% overshoot respectively. This overshoot couldbe mitigated by decreasing the supercharger controller gains butthis will slow down the response as well and could make the ben-efits of the electrical supercharging disappear. Nonlinear controldesign will be investigated in the future to attenuate this effect.

Actuator MovementFigure 5 illustrates the actuators movement for 10% to 90%

of full load transient and Fig.6 represents the same parameters for10% to 60% of full load case. In cases with 15% eEGR, whereminimum backpressure wastegate control strategy is employed,the throttle is fully open after the step is applied. This control ap-proach minimizes the boost pressure and pressure drop across thethrottle consequently, and the minimum of this parameter occursat wide open position for both 90% and 60% of full load.

In the cases with minimum backpressure wastegate controlstrategy, the wastegate is open at 10% load, since no boost is re-

0 1 2 3 40

20

40

60

80

100

Thro

ttle

(%)

0 1 2 3 40

0.2

0.4

0.6

0.8

1

WG

nor

m. d

iam

TwC 15% eEGRTC 15% eEGRTC 0% eEGR

0 1 2 3 4−10

0

10

20

30

time(sec)

∆Sparktiming(deg)

0 1 2 3 40

0.2

0.4

0.6

time(sec)

EGR

nor

m. d

iam

.

FIGURE 6. Actuators movement for 10% to 60% of full load tip-in

quired at this load; it closes after the step is applied to help speed-ing up the turbocharger and opens again to its maximum possiblevalue later to avoid producing unnecessary boost pressure. Thisevent happens sooner for the twincharged case because as pre-sented before, the turbocharger speeds up more rapidly in twin-charged engine. Normalized wastegate diameter is calculated asthe ratio of effective wastegate diameter to its maximum value.

The second plot on left shows the variation in spark tim-ing compared to low load value. The spark timing of cases witheEGR is advanced at high load compared to the baseline en-gine without eEGR. During the transient part the spark timingis more retarded for the twincharged case compared to the tur-bocharged case with eEGR. The reason is that as explained be-fore the spark timing is controlled based on the engine speed,BMEP and eEGR level in intake manifold. In twincharged en-gine the BMEP reaches its target value faster than the other case,while the residual fraction is still low in intake manifold. So thespark is retarded to avoid knocking as the result of this specificspark control strategy. The last plots show the EGR valve nor-malized diameter during the studied transients governed by itsactuator dynamics and a look up table value. Specifically thevalve is closed at low load and opens to its desired value after thetip-in is applied for the cases with eEGR.

Supercharger PerformanceFigure 7 shows the supercharger performance parameters for

the two presented transient results (10% to 90% of full load and10% to 60% of full load transients). The first plot shows thesupercharger speed. The supercharger rpm increases during thetransient period and approaches its idle value during the steadystate. The second set of plots represents the operating line of the



0 1 2 3 40

2

4

6

8

time(sec)

Supe

rcha

rger

spe

ed (K

rpm

)

10% to 90% load tip−in10% to 60% load tip−in

r ed u ced mas s fl ow (k g /s .√

K/kpa)

Pres

sure

Rat

io

0 0.005 0.01 0.015 0.02 0.025 0.030.8

1

1.2

1.4

1.6

1.8

2

2.2

0 1 2 3 40.8

1

1.2

1.4

1.6

1.8

time(sec)

Supe

rcha

rger

pre

ssur

e ra

tio

0 1 2 3 4−1

0

1

2

3

4

time(sec)

Supe

rcha

rger

pow

er (k

w)

FIGURE 7. Supercharger parameters

supercharger on its characteristic map and the third set of plotsdisplays the pressure ratio across the supercharger. The super-charger is used just in transient portion of the results and its pres-sure ratio is unity before and after the transient part. The pressureratio across the supercharger drops to less than unity at the be-ginning of the transient. The reason for this effect is that afterapplying the step in desired load the throttle valve which is a fastactuator fully opens but due to its inertia, supercharger responseto the command is slower and it cannot speed up and provide therequired feed charge immediately, as the result its downstreamvolume pressure drops to sub-atmospheric. This effect is similarto pressure ratio drop across the TC compressor at the start oftransient explained before.

The last plot represent the supercharger power consumption.For 10% to 90% of full load tip-in, the supercharger consumesup to 3.7 kW and for 10% to 60% of full load tip-in its maximumpower is 1.1 kW. The supercharger consumed power before andafter the transient part is very small (100 to 200 W) and includesfriction losses.

TC Compressor BypassingIt was mentioned before that a simple controller is employed

in order to bypass the TC compressor when it is acting as a re-striction for the supercharger in severe transients. Figure 8 com-pares the load response of the twincharged engine with and with-out this mechanism. The results clearly show that the case with-out using the bypass valve (dash-dot red line -bpc) has a slowerload response. The response time (10-90%) of this case is 0.14sec larger than the case which uses the bypass valve, equal to0.42 sec. The top right plot represents the pressure in the pipebetween the TC compressor and the supercharger. In the case

−0.5 0 0.5 1 1.5 20

20

40

60

80

100

Load

(%)

bpobpc

−0.5 0 0.5 1 1.5 20.5

1

1.5

2

P af

ter T

C c

omp.

(bar

)

bpobpc

−0.5 0 0.5 1 1.5 20

20

40

60

80

time(sec)

mas

s flo

w (g

/s)

TC comp.TC bypasstotal

−0.5 0 0.5 1 1.5 20

20

40

60

80

time(sec)

com

p. b

ypas

s va

lve(

%)

FIGURE 8. Comparing cases with and without using TC compressorbypass valve

that the bypass valve is not used and stays always closed, thispressure drops to 0.69 bar while in the other case it declines onlyto 0.93 bar. The bottom left plot shows the charge flow throughthe turbocharger compressor (black line), the flow through thebypass valve (dashed red line) and the total feeding flow (dash-dot blue line). The last plot represents the bypass valve openingprofile and shows that the valve opens only for the first half sec-ond of the transient response.

CONCLUSIONThis paper proposed a novel control configuration for coor-

dination of throttle, wastegate and supercharger speed in an elec-trically twincharged engine in order to achieve high fuel econ-omy as well as efficient boost control by using the superchargeronly during the transient part of the response and enabling higheEGR and minimum backpressure wastegate control at steadystate. The performance of the twincharged engine with the intro-duced controller at 15% low pressure eEGR was compared to thebaseline engine for two different transient cases. For the largeand the medium tip-in cases the twincharged engine responsetime decreases to 0.28 sec and 0.20 sec, compared to 2.30 secand 0.70 sec for turbocharged engine with same eEGR level andwastegate controller respectively. Although the twincharged en-gine has lower fuel economy during the transient, 1-D simula-tions show that for 90% of full load case 1.1 sec and for 60% offull load case only 0.8 sec operation at steady state is adequateto compensate for the consumed electricity because of decreasedfuel consumption mainly due to included eEGR and more ad-vanced spark timing shown in [1]. In addition, the simulationsshow choking in the TC compressor during the large tip-in can



increase the response time of the twincharged engine up to 140ms, which was avoided in current study through bypassing TCcompressor when the pressure ratio across it dropped to less thana marginal number.

REFERENCES[1] Nazari, S., Stefanopoulou, A., and Martz, J. “Minimum

backpressure wastegate control for a boosted gasoline en-gine with low pressure external egr”. In ASME 2016 Dy-namic Systems and Control Conference.

[2] George, S., Morris, G., Dixon, J., Pearce, D., and Heslop,G., 2004. Optimal boost control for an electrical super-charging application. Tech. rep., SAE Technical Paper.

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