2012 IEEE International Conference on Control Applications (CCA) Part of 2012 IEEE Multi-Conference on Systems and Control October 3-5, 2012. Dubrovnik, Croatia A Series-Parallel Hybrid Electric Vehicle Control Strategy Including Instantaneous Optimization of Equivalent Fuel Consumption Branimir Škugor, Danijel Pavković, Member, IEEE and Joško Deur, Senior Member, IEEE vicinity of its optimal fuel efficiency (which is closely related to the operation near the maximum engine torque), while at the same time keeping the battery SoC within the predefined bounds. The RB controller is then extended with the ECMS control approach, which determines the fuel consumption-wise optimal engine operating point (i.e. the engine speed). The resulting engine speed operating point is then smoothly adjusted with respect to the battery SoC request, so that the battery SoC drift is effectively avoided. The proposed RB and ECMS-based control strategies are verified by means of computer simulations for the NEDC, UDDS and HWFET certification driving cycles. Abstract — Control strategy for a series-parallel hybrid electric vehicle powertain aimed at operating the engine in its optimal fuel efficiency operating region is proposed in the paper. An instantaneous optimization algorithm based on the equivalent consumption minimization strategy (ECMS) is used in order to improve the hybrid vehicle fuel efficiency, and it is combined with a battery state-of-charge (SoC) controller to honor predefined SoC bounds. The effectiveness of the proposed control strategy is verified by means of computer simulations for three characteristic certification driving cycles. I. INTRODUCTION Hybrid electric vehicle (HEV) powertrain is traditionally controlled by a heuristic (rule-based) control strategy which aims to keep the internal combustion engine (ICE) within an optimal fuel efficiency operating region [1]. In order to improve the fuel efficiency of parallel HEVs, an instantaneous fuel consumption optimization strategy, based on the so-called equivalent consumption minimization strategy (ECMS), can be used instead [2, 3], or in combination with the rule-based controller [4, 5]. In the ECMS approach, the battery power flow is reflected to “additional” fuel consumption rate, and the fuel consumption optimization is carried out over the whole engine operating range. However, the results in [6] have shown that the locally-optimal ECMS approaches cannot easily account for the battery state-of-charge (SoC) sustainability (i.e. SoC can be prone to drift). Therefore, the approach in [6] proposes to penalize the battery-related equivalent consumption in the so-called adaptive ECMS optimization cost function, significantly increases the complexity of the control strategy and it may still be sensitive to the quality/accuracy of the penalty factor adaptation. In order to avoid the above SoC sustainability issues, this paper proposes to integrate a rule-based (RB) controller with the ECMS approach in a novel manner incorporating explicit SoC control, and to apply the resulting concept to a more recent and more complex series-parallel HEV powertrain. In the series-parallel hybrid powertrain, the engine operating point can be chosen with a relatively high degree of freedom, due to the implementation of the so-called electrical continuous variable transmission (eCVT) concept [7]. In this approach the core RB controller is aimed at operating the engine in the II. PROCESS MODEL This section outlines the series-parallel hybrid electric powertrain including the kinematic model and the controloriented battery dynamics model. A. Series-parallel transmission The principal schematic of the common, one-mode seriesparallel hybrid electric vehicle powertrain [8, 9] is shown in Fig. 1a. The hybrid vehicle powertrain comprises the internal combustion engine as the primary power source and two electric machines. The M/G1 electrical machine is typically operated in the generator mode (thus being able to keep the engine in the desired optimal operating point), while the M/G2 operates as a traction motor (during normal driving), or a generator (during regenerative braking intervals) [9]. The electrical power can also be supplied from the battery (e.g. when the driver demand is increased), or it can be stored within the battery during regenerative braking or low-power demand intervals. Fig. 1b shows the HEV transmission bond graph model, which is convenient for mathematical model derivation and power flow analysis [9]. The mechanical power flows are illustrated in Fig. 1b by the bonds which determine the amount of power () and its direction. The junction points 0 and 1 represent the speed and torque summation points, respectively, the transformer elements (TF) denote the speed/torque transformation (through a gearbox), and the modulated gyrator (MGY) elements represent the mechanical-electrical power transformation. The internal combustion engine and the battery are modeled in Fig. 1b by the source effort bond element (SE). The kinematic relationships between the engine, M/G1 and M/G2 torques and speeds are given by the following sets of equations (Fig. 1b, [9]): This work has been supported by the Croatian Science Foundation through the, grant No. 09/128, and logistically by the AVL Company through the AVL-Cruise Academic Software License agreement. B. Škugor (corresponding author), D. Pavković, and J. Deur are with the University of Zagreb, Faculty of Mechanical Engineering and Naval Architecture, Zagreb, Croatia. Corresponding author phone: +385-(0)16168325; fax: +385-(0)1-6168351. E-mails: branimir.skugor@fsb.hr; danijel.pavkovic@fsb.hr; josko.deur@fsb.hr. 978-1-4673-4504-0/12/$31.00 ©2012 IEEE e (h 1) mg1 , 310 (1) mg1 (h 1)e hmg 2 , (2) cd mg 2 / io , (3) cd io ( mg 2 h(h 1) 1 e ) , (4) v where the torque (.) and speed (.) variables are defined in Fig. 1a, h is the (fixed) planetary gear ratio, and io is the final drive ratio. Fig. 2 shows the simplified block diagram representation of power flows and corresponding efficiencies mg1, mg2, batt of the M/G1 and M/G2 machines and the battery, respectively. The reference power flow directions denote the case when the M/G1 machine operates as a generator, the M/G2 machine operates as a traction motor, and the battery is being discharged (Fig. 1b). Fig. 3 shows the efficiency maps of the engine and electrical machines, along with the engine and electrical machines maximum torque curves (denoted by bold lines). The data are adopted from the AVL Cruise simulation software and adapted for the particular implementation by using the data and experimental results given in [7] and [10]. e, e ICE SE r io cd, io cd t1 -1 Diff. c mg1, mg1 M/G1 (b) t1 mg2, mg2 M/G2 B. Battery model Fig. 4a shows an equivalent battery circuit used to build the control-oriented battery model. It includes a nonlinear opencircuit voltage vs. SoC dependence Uoc(SoC) taken from the AVL Cruise data library (Fig. 4b). In this particular application, the battery internal resistance R(SoC, i) [11] is only made dependent on the battery operating mode (i.e. R = Ric for charging, and R = Ridc for discharging). The battery SoC rate depends on the battery current i and the battery charge capacity Qmax: dSoC i (t ) , dt Qmax Battery / Ultracapacitor (a) t2 t2 s kgy1 ubat M/G1 .. MGY i 0 i SE Rmg1 bat mg1 .. r1 1 R img2 M/G2 MGY: kgy2 s= mg1 s= mg1 e e h+1 h.. TF TF: h PG 1 mg2 0 r r 1 r2 mg2 R Rmg2 i..o-1 TF cd cd Fig. 1. Principal schematic of considered series-parallel hybrid powertrain (a), and corresponding kinematic bond graph model (b). Fig. 2. Block diagram illustration of vehicle power flow (battery discharging). (5) while the battery power is given as: Pbatt U oc ( SoC )i(t ) R ( SoC , i)i 2 (t ) . (6) By combining (5) and (6), the final control-oriented battery model can be obtained [3]: U oc2 ( SoC ) 4 R ( SoC , i ) Pbatt U oc dSoC . dt 2Qmax R ( SoC , i ) (7) The battery power Pbatt in (7) is calculated according to the following electrical power balance equation (cf. Fig.3): k k Pbatt mg 2mg 2 mg 2 mg 1mg 1 mg1 , 2 1 (8) where the coefficients k1 and k2 are equal to -1 in the case of electrical machine operating as a motor, while they equal +1 in the case of generator operation. The battery model in Fig. 4a can be used for calculation of the battery charging/discharging efficiency needed in the ECMS approach in Section III. For the case of battery charging, the so-called “local” battery power efficiency (see Fig. 3. Efficiency maps and maximum torque curves of ICE and electrical machines M/G1 and M/G2. 311 e.g. [1]) can be used, because it effectively takes into account the battery roundtrip (charging/discharging) power losses: P (t ) U oc (t ) Ridc i(t ) c d , (9) Pc (t ) U oc (t ) Ric i(t ) where Pc(t) and Pd(t) are battery charging and discharging power requirement, respectively. On the other hand, when battery discharging is considered, a straightforward efficiency relationship may be used: d (1 i 2 (t ) Ridc / Pbatt ) 1 , Fig. 4. Quasi steady-state battery model (a), and open-circuit battery voltage vs. state-of-charge dependence (b). (10) because in that case the round-trip losses do not exist. C. Driver model A driver model corresponding to a ”virtual” proportionalintegral (PI) vehicle speed controller [12] is shown in Fig. 5. It is implemented herein for the generation of the driveline torque command cdR based on the predefined (desired) vehicle speed vdc profile over a certification driving cycle and for the assumed zero road grade case. The driver torque limit, which corresponds to the transmission torque limit, is used within the driver model saturation algorithm. The aerodynamic and rolling resistance effects may be treated as slowly-varying disturbances when designing the driver “controller”. III. CONTROL SYSTEM Fig. 5. Driver as a part of vehicle model. This section presents the HEV powertrain control system structure, which includes (i) the core rule-based engine controller that provides the basic reference (target) values for M/G1 electrical machine and engine control, and (ii) a superimposed controller based on the equivalent consumption minimization strategy (ECMS). A. Basic structure The series-parallel hybrid powertrain combines two torque development paths, first through the engine and second through the M/G2 machine, (Fig. 1b, Eqs. (1) and (4)), while at the same time allowing for the independent control of the engine speed via the M/G1 machine. Thus, it is possible to keep the engine in the vicinity of the optimal operating region (characterized by optimal fuel consumption), which is located around the engine maximum output torque curve (see Fig. 3a, [4, 7, 13]). This type of "enginecentric" control is typically realized through utilization of the so-called rule-based (RB) controller, wherein the engine operating point is determined based on the driver demand and the battery power demand [7]. The block diagram of a RB controller, inspired by the Toyota Prius power flow analysis from [7] is shown in Fig. 6. The controller utilizes the driver power demand Pd = τcdR ωmg2/io and the battery power demand (-PbattR) commanded by the SoC controller in order to determine the engine power demand Pe*. It is further used to obtain the engine torque demand eR0 and the engine speed reference ωeR0 (needed for the low level engine control strategy) based on the requirement that the engine operates on the nearly optimal maximum engine torque output curve Pe* =eemax(e), where the emax(e) curve is approximated by a third-order polynomial. The RB control strategy in Fig. 6 also comprises a typical engine start/stop logic (cf. [7]) which turns the engine off at low power demands (Pe* < Poff), thus avoiding low power Fig. 6. Rule-based (RB) control algorithm. 312 engine operation which corresponds to lower engine efficiencies (especially at low engine speed values, see Fig. 3a). The engine is again restarted only when the engine power demand exceeds the predefined threshold Pon (either due to increased driver demand Pd or due to SoC controller requesting battery charging). Due to the torque limits of the M/G1 and M/G2 electrical machines (especially at high speeds, see Figs. 3b and 3c), the engine torque command eR0 needs to satisfy the following expressions derived from (1) and (4): e (h 1) mg1max ( mg1 ) (11) e (h 1)(hio ) 1 cd (h 1)h 1 mg 2 max ( mg 2 ) (12) ICE constraints, case 240 160 eRl 40 0 0 1000 2000 e mg1R KR 5000 rate ( m eq ) to be optimized: m eq m fuel m batt , (13) Instead of using the constant (average) battery equivalent fuel rate over the whole battery operating range (as proposed in [3]), the battery equivalent fuel rate is made dependent on the engine operating point in the following manner: Aek Pe Aek batt Pbatt , for Pbatt 0 m eq 1 Aek Pe Aek batt Pbatt , for Pbatt 0 (14) where Aek [g/J] is the instantaneous engine specific fuel consumption (see Fig. 9 and cf. Fig. 3a), and Aek is the average engine fuel consumption over the engine operating range on the maximum output torque curve. The significance of the above equation is that during battery charging, for which the engine power is consumed, a negative equivalent fuel consumption of the battery is obtained directly from the instantaneous engine specific fuel consumption. On the other hand, when the battery is being discharged, the discharged energy cannot be directly related to the current engine specific fuel consumption, but rather to a moving average of the past consumption. This fact is reflected in (14) in an approximated manner by using the average engine specific fuel consumption Aek . The goal of the ECMS is to find the optimum engine operating point in terms of minimizing the cost function (13), which relates to finding the optimum feasible engine speed e mg1max emax Km T s +1 mg1 1 + Iemg1s Torque lag e eR mg1L - 4000 defined ( m batt ) and added to the actual engine fuel consumption rate ( m fuel ) to obtain the overall equivalent fuel mg + 3000 [rpm] Fig. 7. Illustration of engine constraints. mg1max - = 4500 rpm 80 C. Equivalent consumption minimization strategy In order to further minimize the HEV fuel consumption, the RB control system is extended by a strictly realized equivalent consumption minimization strategy (ECMS). In the ECMS approach, the so-called battery equivalent fuel rate [2, 3] is + mg2 eRu 120 B. Low-level control The M/G1 machine needs to be speed-controlled, because it is intended to keep the engine in a desired (optimal) operating point. The structure of the M/G1 speed control loop is shown in Fig. 8a, wherein a proportional-integral (PI) speed controller is used. The PI speed controller is tuned according to the symmetrical optimum tuning procedure [12], thus facilitating a fast and well-damped response of the M/G1 speed control loop. The engine low-level control system structure is shown in Fig. 8b. The engine torque is primarily controlled by means of the feedforward torque reference eR (corresponding to gas pedal command in conventional vehicles), which is supplied by the superimposed RB controller (see previous subsection). The auxiliary, relatively slow (dynamically non-dominant) proportional (P) feedback controller is included in order to avoid drifting the engine speed away its target value under the “boundary” conditions when the M/G1 machine torque is saturated and cannot balance the engine torque (e.g. due to dynamic/inertia effects or inaccuracies of torque limit curves). If the engine speed drift tends to occur, the P controller simply corrects (reduces) the engine torque reference eR. KR TR s = 400 Nm, Maximum ICE torque Upper limit Lower limit 200 In the case when the engine torque feedforward reference eR0 violates the upper limit (11) or lower limit (12), the engine speed command ωeR0 is reset to the nearest boundary value (ωeRl or ωeRu, Fig. 7) at which the engine torque demand satisfies the conditions (11) and (12). mg1R cd e + mg1 eR + Equivalent inertia a b Fig. 8. MG1 speed control loop (a), ICE speed control loop (b). 313 Kc - eR + 1 e Te s + 1 + Torque lag eL - 1 IeICE s Equivalent inertia e the RB controller augments the driver power demand Pd without consideration for the engine fuel efficiency, the ECMS should be used to indirectly moderate the SoC controller power request PbattR (see Section III.A). Due to the fact that the engine power request P*e is effectively determined by the requirement of engine operation at or near the maximum torque curve, the overall control strategy should moderate the RB controller output (i.e. the engine speed reference ωeR), while taking into account the value of the battery SoC control error eSoC (Fig. 6). For that purpose, a simple SoC control error weighting approach is proposed herein: eRmod ( eR eopt )W (eSoC ) eopt , (17) where eRmod is the modified engine speed reference, and W(eSoC) is the SoC error weighting function defined as follows: W (eSoC ) tanhbeSoC tanh(eSoC ) Fig. 9. Engine specific fuel consumption plot Aek. = 200 Nm eopt [x 1000 rev/min] 5 4 3 2 1 0 0 1000 rev/min] cd eR [x = 100 Nm eopt - cd cd = 300 Nm cd = 400 Nm Fig. 11 shows the comparative plots of the SoC error weighting functions for different values of the arbitrary shaping factor b. In the vicinity of the target SoC value (eSoC ≈ 0) the weighting function is close to zero, thus giving emphasis on the fuel-optimal ECMS (see (17)). On the other hand, for excessive SoC errors the weighting function tends to unit value (W(eSoC) = 1) one, thus favoring the SoC controller action. The larger the shaping factor b, the more emphasized is the SoC error weighting. 2 1 0 -1 -2 0 30 60 90 120 150 30 60 90 120 150 180 180 v [km/h] vv [km/h] v a b Fig. 10. ECMS optimal speeds e with given torque demands vs. vehicle velocity (a), difference between RB and ECMS optimal speeds with given torque demands vs. vehicle velocity (b). IV. SIMULATION RESULTS This section presents the results of the proposed RB+ECMS control strategy simulation studies for characteristic certification driving cycles. For the purpose of examination of various types of HEV control strategies, where each strategy will generally end up with different final SoC value, a simple equivalent consumption-based criterion is proposed in order to account for the discrepancy in the final SoC value. (cf. Fig. 7) on the target maximum engine torque curve, i.e.: eopt min(m eq (e , e )) , (15) subject to eRl e eRu , and e emax (e ) . (18) (16) A. Compensation of final SoC value variations Effectiveness of the above concept is illustrated by the results of off-line searching for the optimal engine speed eopt according (14)-(16) for a wide range of vehicle speeds. The optimization results, shown in Fig. 10a, point out that the engine speed may be kept at approximately constant low values at low vehicle velocities (and low torque demands), while it grows with the vehicle velocity, otherwise. More importantly, Fig 10b shows the difference of engine speed references provided by the ECMS and the RB controller for the case when the SoC controller is inactive (the battery SoC is assumed to be within the SoC controller deadzone). The presented results indicate that the fuel-optimal ECMS approach results in notably different engine speed references compared to those obtained by the RB controller, thus pointing to a good potential for further RB controller improvement via ECMS. The final SoC discrepancy (SoC) from the target value (SoCR = 50%) is transformed to the corrected fuel consumption mf using a static map obtained by global control-variable optimization method [15] for the same vehicle model and driving cycle and various target values of the final SoC. The corrected fuel consumption mf is then added to the actual consumption mf in order to calculate the equivalent fuel consumption mc,eq, thus facilitating unified comparison of different variants of the control strategy. The fuel consumption correction map obtained by the aforementioned optimization approach for the NEDC cycle is shown in Fig. 12. B. Results The proposed, basic RB control strategy and the combined RB-ECMS strategy have been verified by means of simulations for the case of New European Driving Cycle (NEDC), Urban Dynamometer Driving Schedule (UDDS), and Highway Fuel Economy Test (HWFET) certification cycles. In all simulations D. Incorporation of ECMS into RB strategy The above ECMS intervention is integrated within the RB controller in Fig. 6, in order to reduce the vehicle fuel consumption. Since the low-level battery SoC controller within 314 place the engine operating points onto the engine maximum torque curve. The responses in Fig. 14b show that both strategies gradually discharge the battery over the initial portion of the driving cycle (t < 500 s) corresponding to urban driving. As the driving progresses (t > 500 s), the RB strategy puts the emphasis on recharging the battery by means of engine power (the engine is sporadically turned on), thus resulting in higher RB fuel consumption compared to RB-ECMS that tries to reach a good trade-off between consumption minimization and SoC control. Finally, during highway driving conditions (t > 800 s), the RB and RB-ECMS strategies turn the engine on/off in a similar manner, but the speed/torque operating points may differ (see Fig. 14 and cf. Fig. 10b) since the ECMS component tends to find the locally-optimal engine operating point. Thus obtained fuel consumption and SoC data are then compared based on the equivalent consumption assessment criterion explained in the previous subsection. The main simulation results for the different driving cycles are summarized in Tables I – III. The results show that for the case of NEDC driving cycle the equivalent fuel consumption of the RB-ECMS approach is about 3% lower compared to the RB strategy. The comparative summary results for other driving cycles show that the similar benefit of the ECMS utilization is obtained in the case of HWFET cycle (2.9% fuel consumption reduction), while it is somewhat lower for the UDDS cycle (1.8% fuel consumption reduction). mf [%] Fig. 11. SoC control error weighting function. Fig. 12. Equivalent fuel consumption vs. final SoC discrepancy calculation. TABLE 1. Main simulation results for New European Driving Cycle (NEDC). NEDC RB RB-ECMS mf [g] 268.6 281.4 SoC [%] 47.6 51.9 mc,eq [g] 268.6 260.5 TABLE 2. Main simulation results for Highway Fuel Economy Test (HWFET). HWFET RB RB-ECMS Fig. 13. Vehicle speed profiles for different certification driving cycles. mf [g] 474.1 543.0 SoC [%] 45.4 64.5 mc,eq [g] 474.1 460.4 TABLE 3. Main simulation results for Urban Dynamometer Driving Schedule (UDDS). UDDS RB RB-ECMS the battery SoC reference (target) value is kept at 50%, while the SoC controller deadzone width SoC is set to 10%. Fig. 13 shows the vehicle speed reference profiles for the NEDC, UDDS and HWFET driving cycles, based on which the driveline torque command is calculated by the driver model (see Subsection II.C). The NEDC cycle comprises a relatively long interval of urban-like driving (frequent vehicle starting and stopping), which is followed by a highway-like driving (vehicle speed vv reaching 120 km/h) and final rapid deceleration. The UDDS cycle is characterized by even more frequent starting and stopping intervals with less emphasis on high-speed driving compared to NEDC. The HWFET cycle, on the other hand, is characterized by driving at relatively high and approximately constant speed (i.e. vehicle speed is mostly kept between 60 and 90 km/h). Simulation results for the case of NEDC driving cycle are shown in Fig. 14. The engine operating point plot in Fig. 14a shows that both the RB controller and the RB-ECMS tend to mf [g] 267.1 290.6 SoC [%] 44.5 51.0 mc,eq [g] 267.1 262.2 V. CONCLUSIONS The paper has proposed a rule-based (RB) energy management control strategy for the common series-parallel HEV, which is aimed at operating the internal combustion engine close to optimal efficiency region. In order to further improve the fuel efficiency an exact equivalent consumption minimization strategy (ECMS) has been incorporated into the RB control system, where arbitration between the ECMS instantaneous optimization and SoC feedback control is realized by means of a smooth weighting function. The proposed RB and RB-ECMS control strategies have been validated by computer simulation for three characteristic certification driving cycles (NEDC, UDDS and HWFET). The 315 [4] e [Nm] [5] [6] [7] e [rad/s] [8] [9] [Nm] [10] e [11] SoC [%] [12] [13] mf [g] [14] [15] [16] Fig. 14. Simulation results for NEDC driving cycle: engine operating points (a), and simulation traces (b). presented simulation results have shown that RB-ECMS can reduce the fuel consumption by up to 3% when compared to the simpler RB strategy. The ECMS effectiveness has been particularly emphasized in the case of NEDC driving cycle characterized by moderate urban driving and short-duration high-speed driving, whereas it has been somewhat less effective for the more aggressive UDDS driving cycle. It should be noted that the presented one-dimensional instantaneous optimization (over the maximum engine torque curve) can be readily modified for the two-dimensional optimization case (simultaneous optimization of engine speed and torque operating points over a wider engine map region), in order to achieve further gains in fuel economy [16]. REFERENCES [1] [2] [3] L. Guzzella, A. 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