IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 62, NO. 3, MARCH

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1 IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 62, NO. 3, MARCH Design and Analysis of a Novel Multimode Transmission for a HEV Using a Single Electric Machine Futang Zhu, Li Chen, and Chengliang Yin Abstract This paper presents the mathematical modeling and analysis of a novel multimode transmission (MMT) for a hybrid electric vehicle (HEV) using a single electric machine (EM), which implies compactness and low cost. The single-em solution avoids losses from another EM and its power electronics, which are employed in many existing HEVs. The topology of the MMT planetary gearset is the same as that of conventional four-speed automatic transmissions (ATs). The MMT realizes five power flow modes, which are developed into 16 operation modes, including one Motor_only mode, four Engine_only modes, four Compound driving modes, six Braking modes, and one Charging while parking mode. The properly arranged clutches transmit power flow more flexibly, allow direct mechanical power transmission from the engine to the drive shaft, and avoid spin loss for the engine and energy conversion loss for the electric components. Simulation under the New European Driving Cycle (NEDC) shows that the fuel consumption of the proposed HEV is comparable to a benchmark THS II-like vehicle, which uses a planetary gearset, two EMs, and no clutch, which indicates the fuel economy potential of this concept. Index Terms Hybrid electric vehicle (HEV), multimode transmission (MMT), planetary gearset, single electric machine (EM). I. INTRODUCTION THE increasingly stringent standards on fuel economy and emission levels has dramatically boosted research and applications of new-generation vehicles, in which hybrid electric vehicles (HEVs) are considered to offer the best promise in the short term to midterm [1]. For most currently developed HEVs, two electric machines (EMs) are used [2], e.g., Toyota Hybrid System (THS) [3], [4], Ford FHS [5], GM-Allison AHS [6], Timken EVT [7], and Renault IVT [8]. One of the two EMs works mostly as a driving motor and the other as a generator. However, an EM can be operated in any one of the four quadrants of the torque versus speed coordinate system. The EM works as a motor in the quadrants 1 and 3 and as a generator in quadrants 2 and 4. Manuscript received September 9, 2011; accepted September 7, Date of publication October 3, 2012; date of current version March 13, This work was supported in part by the Clean Vehicle Consortium of the U.S. China Clean Energy Research Center and in part by the Science and Technology Commission of the Shanghai Municipality of the People s Republic of China and Shanghai Automobile Gear Works under Grant 07dz The review of this paper was coordinated by Mr. D. Diallo. The authors are with National Engineering Laboratory for Automotive Electronic Control Technology, Shanghai Jiao Tong University, Shanghai , China ( li.h.chen@sjtu.edu.cn). Color versions of one or more of the figures in this paper are available online at Digital Object Identifier /TVT Some researchers are enlightened to investigate the potential that a single EM helps the HEV to realize necessary operation modes and gain comparable performance to those HEVs with two EMs. The single-em HEV uses one less EM and its associated power electronics system, which imply advantages of compactness, low power loss, and low cost. On the other hand, the single EM cannot work as a motor and a generator at the same time, which makes the power flow management more challenging and might introduce limitation to the HEV performance. One critical component for the HEV operations is the transmission. Several transmissions for single-em HEVs [9] [14] were investigated, and four typical architectures are shown in Fig. 1. Two planetary gearsets and four clutches are employed in the architecture in Fig. 1(a), five shaft gear pairs and one planetary gearset are in (b), and a conventional automatic transmission (AT) and one extra clutch are combined in (c) and (d), respectively. Using the transmission in Fig. 1(b), the efficiency of an HEV improves by 10% compared with that of Toyota Prius [12]. Using the transmission in Fig. 1(d), an Audi A6 HEV gains average fuel consumption of 6.2 L/100 km [14], whereas a similar level Lexus HEV using the third-generation THS achieves fuel consumption of 8.1 L/100 km [15]. This paper presents a novel multimode transmission (MMT) for the single-em HEV. It contains two planetary gearsets and four clutches. The topology of the MMT is the same as that of the popular four-speed ATs, such as GM (4T65E, 4L60E), CHRYSLER (42LE), ZF (4HP-18), etc. [16]. The four clutches are derived from the conventional AT with their axial shafts modified to connect the EM. The proposed MMT makes use of many components of the conventional ATs, which have been refined and perfected for decades. Thus, the reliability and cost effectiveness are likely to be superior to other transmissions that are newly designed. We believe that the proposed architecture is more viable for practical applications because of its evolutionary nature. In this paper, a simulation model is developed for the proposed MMT, which is used to analyze and evaluate its performance. Section II explains the configuration, design, and operation modes of the proposed MMT. The mathematical model is developed in Section III, following the energy management strategy, including feasible mode definition, mode selection, and torque distribution in Section IV. Finally, simulation results and discussions on the factors influencing fuel consumption are given, with the comparison of those of a second-generation THS II-like HEV /$ IEEE

2 1098 IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 62, NO. 3, MARCH 2013 Fig. 2. MMT. Schematic of MMT. (a) Production four-speed AT [16]. (b) Proposed Fig. 3. Schematic of the benchmark THS II-like vehicle [3]. Fig. 1. Schematics of existing single-em HEVs. (a) Proposed by Tsai and Schultz [11]. (b) Proposed by Zhang and Lin [12]. (c) Infiniti M35 hybrid [13]. (d) Audi A6 hybrid [14]. II. SYSTEM ARCHITECTURE AND OPERATION MODES A. Configuration The topology of the proposed MMT is identical to that used in several conventional four-speed ATs, as shown in Fig. 2(a). The MMT consists of two planetary gearsets: input and output. The carrier gear C1 of the input planetary gearset is connected to the ring gear R2 of the output planetary gearset, and the ring gear R1 of the input planetary gearset is connected to the carrier gear C2 of the output planetary gearset. Four clutches are employed: Two of them are rotating clutches (denoted as CR1 and CR2, respectively), and the other two are braking clutches (denoted as CB1 and CB2, respectively). The MMT has two power sources, i.e., the engine and the EM, as shown in Fig. 2(b). The engine is connected to the two planetary gearsets through CR1 and CR2. The EM is connected to the sun gear S1 directly, and the motion of the EM or S1 is manipulated by CB1 and CR2. Compared with the THS II-like vehicle in Fig. 3, which is a typical dual-em HEV and has no clutch, the proposed MMT has four clutches, which have more mechanical components. However, one single-em solution means simpler power electronics. This paper will investigate the advantages of using a single EM, which achieves comparable performance to the dual-em HEV. B. Operation Modes The proposed MMT can realize five basic power flow modes, i.e., Motor_only driving, Engine_only driving, Compound driving, Braking, and Charging while parking. The power flow modes could be realized by different transmission ratios, which are changed through clutch engagement and disengagement. Considering various combinations of clutch operations, the power flow modes are further developed into 16 operation modes, as shown in Fig. 4. The operations of the clutches and the EM are shown in Table I, where denotes the locked phase of a clutch.

3 ZHU et al.: DESIGN AND ANALYSIS OF NOVEL MMT FOR HEV USING SINGLE EM 1099 Fig. 4. Power flow diagrams for operation modes of MMT. (a) Motor_only. (b) Engine_only_1. (c) Engine_only_2. (d) Engine_only_3. (e) Engine_only_4. (f) Compound_1. (g) Compound_2. (h) Compound_3. (i) Compound_4_EVT. (j) Mech_Braking. (k) Reg_Braking. (l) Compound_Braking_0. (m) Compound_Braking_1. (n) Compound_Braking_2. (o) Compound_Braking_3. (p) Charging while parking. TABLE I OPERATION MODES OF THE MMT-BASED SINGLE-EM HEV The HEV in Engine_only driving modes works as a conventional engine-driven vehicle and selects one of the four gear ratios to match the motions of the engine and the vehicle. The gear ratio is realized by locking two clutches. Generally, a large gear ratio matches low vehicle velocity, and a small ratio matches high velocity.

4 1100 IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 62, NO. 3, MARCH 2013 Fig. 6. Torque analysis for the MMT. Fig. 5. Schematic of the MMT. The EM spins in the first three Engine_only modes. When the EM produces torque to aid the engine for a better driving performance, it comes up with three compound driving modes. The EM cannot aid driving in the fourth Engine_only mode because it is fixed by CB1. The fourth compound driving mode, i.e., Compound_4_EVT, locks only one clutch and makes the MMT an electric variable transmission with two degrees of freedom (DOFs). The extra DOF allows much flexibility for the HEV to narrow engine operating points within an efficient region, regardless of vehicle velocity and load [2]. By releasing all the clutches, the Mech_braking mode levers all braking torque on conventional mechanical braking systems. In Reg_braking mode, the EM works as a generator and provides regenerative braking torque by locking CB2. The mechanical braking and regenerative braking are combined in the Compound_braking_0 mode. The other three braking modes, i.e., Compound_braking_1 Compound_braking_3, operate the clutches in the same way as the three Engine_ only driving modes, i.e., Engine_only_1 Engine_only_3, the same way as the three compound driving modes, i.e., Compound_1 Compound_3. These three braking modes introduce engine braking and allow three choices of the gear ratio. III. MODELING A control-oriented MMT model capturing the driveline dynamics is developed here. The vehicle driveline is a multibody system. To simplify the analysis, the damping and compliance are ignored, and all parts of the MMT are assumed to be lumped inertias. The MMT schematic is given in Fig. 5. The derivations for the models of the vehicle, the engine, the EM, the clutch, and the battery are similar to those in literature [17] [20]. In the following, we focus on the MMT model. For the input planetary gearset, I R1, I C1, and I S1 denote the inertia about the shafts of R1, C1, and S1, respectively. In addition, ω R1, ω C1, and ω S1 denote the angular velocities of R1, C1, and S1, respectively. Similar definitions are applied to the output planetary gearset. The EM contributes to I S1 because it has a direct connection with S1. The EM angular velocity is denoted by ω em, which is equal to ω S1. The engine inertia is denoted by I eng, and the engine angular velocity is ω eng. T C2 is the interactional torque between R1 and C2, and T R2 is the interactional torque between C1 and R2. The clutch torque generated by CR1, CR2, CB1, and CB2 is denoted by T CR1,T CR2,T CB1, and T CB2, respectively. T out is the vehicle load torque with respect to the C2 shaft, T em is the output torque of the EM, and T eng is the output torque of the engine. Let R a denote the ratio of the ring-gear tooth number over the sun-gear tooth number of the input planetary gearset, and R b denote the ratio of ring-gear tooth number over sun-gear tooth number of the output planetary gearset. The torque analysis for the engine input shaft is shown in Fig. 6(a), and those for the two planetary gearsets are shown in Fig. 6(b) and (c) by applying the lever analogy method [18]. For the engine input shaft, the equilibrium equation is written as T eng T CR1 T CR2 = I eng ω eng. (1) For the lever that represents the input planetary gearset, the equilibrium equations are written as T em + T CR1 + T CR2 + T CB1 T C2 T R2 = I S1 ω S1 + I C1 ω C1 + I R1 ω R1 (2) T CR1 +(1 + R a )(T CR2 + T CB1 + T em ) T R2 =(1 + R a )I S1 ω S1 + I C1 ω C1. (3) For the lever that represents the output planetary gearset, the equilibrium equations are written as T CB2 + T C2 + T R2 T out = I S2 ω S2 + I C2 ω C2 + I R2 ω R2 (4) T R2 R b T CB2 + R b I S2 ω S2 I R2 ω R2 = 0. (5) ω R1, ω C1, ω S1, ω R2, ω C2, ω S2, and ω eng are the seven state variables in (1) (5). They are subject to four kinematic constraints as follows. Therefore, there are only three independent state variables, i.e., ω S1 + R a ω R1 =(1 + R a ) ω C1 (6) ω S2 + R b ω R2 =(1 + R b ) ω C2 (7) ω C1 = ω R2,ω R1 = ω C2. (8)

5 ZHU et al.: DESIGN AND ANALYSIS OF NOVEL MMT FOR HEV USING SINGLE EM 1101 After eliminating internal torque T C2 and T R2, substituting ω S1 and ω C2 by ω em and ω out, and choosing ω em, ω eng, and ω out as the independence variables, the following matrix equations can be derived, which computes the effect of the input torque on the speeds of the engine, the EM, and the vehicle as follows: or T 1 T 2 = I 11 I 12 I 13 I 21 I 22 I 23 T 3 I 31 I 32 I 33 ω eng ω em ω out (9) ω eng ω em = 1 J 11 J 12 J 13 J 21 J 22 J 23 T 1 T 2 (10) J ω det out J 31 J 32 J 33 T 3 where T 1 = T eng T CR1 T CR2 T 2 = T em T out + T CR1 + T CR2 + T CB1 + T CB2 T 3 =(1 + R a )T em + T CR1 +(1 + R a ) (T CR2 + T CB1 ) R b T CB2 (11) I 11 = I eng I 12 = I 13 = I 21 = I 31 = 0 I 22 = I S1 + I C1 1/(1 + R a ) I S2 R b /(1 + R a )+I R2 1/(1 + R a ) I 23 = I C1 R a /(1 + R a )+I R1 + I S2 (1 + R a + R b )/(1 + R a ) + I C2 + I R2 R a /(1 + R a ) I 32 = I S1 (1 + R a )+I C1 1/(1 + R a )+I S2 R a R b /(1 + R a )+I R2 1/(1 + R a ) I 33 = I C1 R a /(1 + R a ) I S2 (1 + R a + R b ) R b /(1 + R a )+I R2 R a /(1 + R a ) (12) J 11 = I 22 I 33 I 23 I 32 J 12 = (I 21 I 33 I 23 I 31 ) J 13 = I 21 I 32 I 22 I 31 J 21 = (I 12 I 33 I 13 I 32 ) J 22 = I 11 I 33 I 13 I 31 J 23 = (I 11 I 32 I 12 I 31 ) J 31 = I 12 I 23 I 13 I 22 J 32 = (I 11 I 23 I 13 I 21 ) J 33 = I 11 I 22 I 12 I 21 J det = I 11 J 11 + I 12 J 12 + I 13 J 13. (13) IV. ENERGY MANAGEMENT STRATEGY The proposed energy management system consists of two modules: mode selection and torque distribution, as shown in Fig. 7. The mode selection module selects a proper mode from the feasible modes, which are defined by the number of clutch operations. The torque distribution module distributes the optimal torque commands to the engine and the EM. Fig. 7. Structure of the energy management system. A. Feasible Mode Definition Each of the 16 operation modes in Table I has 15 possible transitions to other modes; therefore, there are 240 altogether. Some are realized by the EM operations, such as the transition from Engine_only_1 mode to Compound_1 mode; some are realized by engagement or disengagement of the clutches, such as from Engine_only_1 mode to Engine_only_2 mode; and some need the cooperation of the EM and the clutches. The EM takes the advantage of fast torque response; however, the clutch operation introduces friction-involved nonlinear dynamics and normally takes hundreds of milliseconds or even more. Therefore, the clutch operation is more challenging for mode transitions. Since the conventional AT commonly uses two clutches for gear shifting and never uses three or more clutches due to the complexity, this paper considers that the mode transition by two or less clutches is feasible and that the mode transition by three or more clutches is infeasible. The numbers of the clutch operations during mode transition are given in a matrix, as shown in Table II. The first column lists the operation modes and their sequence numbers, and the first row cites the sequence numbers. For the mode transition from the column to the row mode, the cell at this column and row counts the clutch operation number. The matrix is symmetrical about the diagonal; therefore, the blank cells are the same as their counterpart cells. It is found that 22 cells below the diagonal, which are highlighted by bracket, are greater than 2. They indicate that the corresponding mode transitions are infeasible. In addition, the other 98 transitions are feasible. Similarly, for the transitions above the diagonal, 22 are infeasible, and 98 are feasible. B. Mode Selection The inputs of the mode selection module are the required torque, vehicle states, and the battery state of charge (SOC). Extensive algorithms have been applied in estimation and control development, such as fuzzy logic [19], optimization methods [20], and neural networks [21]. Still, rule-based control strategies, which are developed based on engineering intuition and simple analysis of component-efficiency tables, are widely used in many HEVs due to its ease in handing changes in the operating mode. In this paper, a rule-based control strategy is developed for mode selection. The engine is the primary power source, and the EM is the supplemental power source. The control strategy aims to operate the engine working in the high-efficiency or subhigh-efficiency range through the following eight rules. The

6 1102 IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 62, NO. 3, MARCH 2013 TABLE II CLUTCH OPERATION MATRIX Fig. 8. State flowchart of the energy management controller. rules are implemented in MATLAB state flow, and an example for the propulsion case is given in Fig. 8. The detailed control rules are as follows. 1) To avoid inefficient engine operations, the engine shuts off, and the electric motor supplies all driving torque (i.e., use Motor_only mode) below a certain vehicle velocity denoted by v EM_ max. The required driving torque is within the EM torque capacity, which is calculated from T EM_ max and the gear ratio. 2) When the required torque exceeds the EM capacity, the engine is started up using Compound_1 mode when the desired velocity is below v EM_ max, orusing Compound_2 mode when the velocity is above v EM_ max or the SOC is below the lower limit SOC min. Then, the EM provides assistive torque to make the engine operate in high- or subhigh-efficiency area. The selection among the four compound driving modes depends on the vehicle velocity and the required driving torque. Compound_1 mode is selected when the vehicle velocity is below v 1_ max, which is calculated from ω up and the first gear ratio, and the required driving torque is above T 2_ max, which is calculated from T EM_ max, T e_up, and the second gear ratio. Similar rules are designed for the selection of Compound_2, Compound_3, and Compound_4_EVT modes. 3) When the battery SOC is below the maximum limit SOC max and the required torque does not exceed the maximum torque limit T e_up of the high-efficiency region, the engine provides additional torque, which passes through the EM to charge the battery.

7 ZHU et al.: DESIGN AND ANALYSIS OF NOVEL MMT FOR HEV USING SINGLE EM ) When the battery SOC is within SOC min and SOC max and the engine can work in high or subhigh-efficiency region [T eng_only_low, T eng_only_up], the EM does not work, and the HEV is operated in one of the four Engine_only driving modes. The selection of the four modes depends on the vehicle velocity and the required torque, which is the same as that for a traditional-engine-driven vehicle. 5) When the required torque is negative and the SOC is below SOC max, Reg_braking mode is activated. Since the regenerative braking power is limited by the rated capacity of the battery, the regenerative braking torque denoted by T reg_ lim is also limited and calculated from T EM_ max and the gear ratio. 6) When the required braking torque is more negative than T reg_ lim and SOC is below SOC max, the mechanical braking system provides the additional braking torque in Compound_braking_0 mode. 7) When the required braking torque is more negative than the summation of T reg_ lim and the mechanical braking system, the engine enhances braking by its friction and air resistance, in Compound_braking_1, Compound_braking_2 or Compound_braking_3 mode. Since Compound_braking_1 mode has a bigger amplifier ratio, the braking torque generated by the engine resistance is larger than the other two modes. When SOC reaches SOC max, regenerative braking is not used for battery durability consideration, i.e., T reg_ lim is zero. Thus, the braking levers on the mechanical system and the engine. 8) To maintain battery capacity, the engine is allowed to charge the battery when the vehicle is parking. This way, the battery can be always ready whenever the vehicle begins to run (Charging-while-parking mode). Based on the given rules, some of the feasible transitions in Table II are frequently used, whereas some are seldom used. For example, the transition from Motor_only mode to Compound_2 mode is always operated to start up the vehicle. The transitions from Compound_2 mode to other compound driving modes often occur when the vehicle velocity falls into the corresponding range. However, the transition from Motor_only to Compound_4_EVT is hardly used because of the big gap between v EM_ max and v 3_ max, unless Compound_2 and Compound_3 do not work for failure reasons. Therefore, the proposed MMT allows more flexible mode choices for the HEV. C. Torque Distribution The relationship between the MMT input torque and output torque is specified in the dynamical equation (9). The torque distribution module solves this equation to get the input torque based on measured data and predefined parameters. The left side of (9) involves seven torque values, i.e., T eng, T em, T out, and four clutch torque values. T out is the actual torque with respect to the C2 shaft and is supposed to be equal to T out_req, which comes from predefined required torque. T eng and T em are to be decided by this torque distribution module. The modes with the same clutch status have the same expression for T eng and T em calculated from (10). Therefore, the solutions for the 16 operation modes are classified in terms of the clutch status. The clutches are either locked or open when the MMT is in one mode. The transmitted torque is zero when the clutch is open, and is an internal torque value when locked. Therefore, the expressions of T eng and T em do not use the entries of clutch torque values. They are derived, respectively, as follows. 1) Motor_only, Reg_Braking, and Compound_Braking_0 modes In these modes, clutch CB2 is locked, whereas all the other clutches are open. The state equation is rewritten as T em T out + T CB2 = I 22 ω em + I 23 ω out (14) (1 + R a )T em R b T CB2 = I 32 ω em + I 33 ω out. (15) T em can be calculated from T em = T out_reqr b /(1 + R a + R b )+ ω out (I 23 R b + I 33 )/ (1+R a +R b )+ ω em (I 22 R b +I 32 )/(1+R a + R b ). (16) 2) Engine_only_1, Compound_1, and Compound_ Braking_1 modes In these modes, clutches CB2 and CR2 are locked. The state equation is rewritten as T eng T CR2 = I eng ω eng (17) T em T out + T CR2 + T CB2 =I 22 ω em +I 23 ω out (18) (1 + R a ) T em +(1 + R a ) T CR2 R b T CB2 = I 32 ω em + I 33 ω out. (19) The summation of T eng and T em is T em + T eng = T out_req R b /(1 + R a + R b )+ ω eng I eng +[ ω out (I 23 R b +I 33 )+ ω em (I 22 R b +I 32 )]/(1+R a +R b ). (20) In Engine_only_1 mode, T em is zero; therefore, T eng is equal to the right side of (20) with ω em = ω eng.in the Compound_1 mode, T eng is decided by the engine high-efficiency area, and T em supplements the rest. In the Compound_braking_1 mode, T eng is the engine resistance torque, and T em supplements the rest. 3) Engine_only_2, Compound_2 and Compound_ Braking_2 modes In these modes, clutches CB2 and CR1 are locked/ The state equation is rewritten as T eng T CR1 = I eng ω eng (21) T em T out + T CR1 + T CB2 = I 22 ω em + I 23 ω out (22) (1 + R a )T em + T CR1 R b T CB2 = I 32 ω em + I 33 ω out. (23)

8 1104 IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 62, NO. 3, MARCH 2013 A combination of T eng and T em is (1 + R a + R b )T em +(1 + R b )T eng = T out_req R b + ω out (I 23 R b +I 33 )+ ω eng I eng (1+R b )+ ω em (I 22 R b +I 32 ). (24) TABLE III SPECIFICATIONS FOR THE MMT VEHICLE AND THE THS II-LIKE VEHICLE T eng and T em are decided in the same way as those in Engine_only_1, Compound_1, and Compound_ Braking_1 modes, respectively. 4) Engine_only_3, Compound_3, and Compound_ Braking_3 modes In these modes, clutches CR1 and CR2 are locked. The state equation is rewritten as T eng T CR1 T CR2 = I eng ω eng (25) T em T out + T CR1 + T CR2 = I 22 ω em + I 23 ω out (26) (1 + R a )T em + T CR1 +(1 + R a ) T CR2 = I 32 ω em + I 33 ω out. (27) ThesummationofT eng and T em is T em + T eng = T out_req + ω out I 33 + ω eng I eng + ω em I 32. (28) In addition, T eng and T em are decided in the same way as those in the Engine_only_1, Compound_1, and Compound_braking_1 modes, respectively. 5) Engine_only_4 mode In this mode, clutches CR1 and CB1 are locked. The state equation is rewritten as T eng T CR1 = I eng ω eng. (29) T em T out + T CR1 + T CB1 = I 22 ω em + I 23 ω out (30) (1 + R a )T em + T CR1 +(1 + R a ) T CB1 = I 32 ω em + I 33 ω out. (31) Since T em is zero, T eng is calculated as T eng =T out_req (1+R a )/R a + ω out ((1+R a )I 23 I 33 ) /R a + ω eng I eng + ω em ((1 + R a )I 22 I 32 ) /R a. (32) 6) Compound_4_EVT mode In this mode, clutch CR1 is locked, whereas all the other clutches are open. The state equation is rewritten as T eng T CR1 = I eng ω eng (33) T em T out + T CR1 = I 22 ω em + I 23 ω out (34) (1 + R a )T em + T CR1 = I 32 ω em + I 33 ω out. (35) T eng and T em are calculated from T eng = T out_req (1+R a )/R a + ω out ((1+R a )I 23 I 33 ) /R a + ω eng I eng + ω mg ((1+R a )I 22 I 32 ) /R a (36) T em = T out_req 1/R a + ω out (I 33 I 23 )+ ω em (I 32 I 22 ). (37) 7) Charging while parking mode In this mode, clutch CR2 is locked, whereas other clutches are open. The state equation is rewritten as T eng T CR2 = I eng ω eng (38) T em +T CR2 T out T brake = I 22 ω em +I 23 ω out (39) (1+R a )T em +(1+R a ) T CR2 = I 32 ω em +I 33 ω out. (40) T eng is decided by the engine high-efficiency area. With ω out = 0 when parking, T em is calculated by T em = T eng + ω em I 32 /(1 + R a )+ ω eng I eng. (41) V. S IMULATION AND ANALYSIS The performance of the proposed MMT vehicle is demonstrated by comparing it against a THS II-like vehicle, which uses two EMs without a clutch but all the other vehicle parameters are the same as the target vehicle studied in this paper. The diagram of the THS II electric variable transmission is shown in Fig. 3. The second planetary gearset is considered as a reduction gear because its carrier is fixed. The way to connect the engine EM1 of the THS II-like vehicle is the same as that of the proposed HEV. The comprehensive New European Driving Cycle that consists of four identical urban cycles and one extra urban cycle with frequent start/stop is used to assess vehicle performance [22]. A. Specifications and Control Parameters To reduce fuel consumption, the engine used in the MMT vehicle is downsized from 80 to 57 kw. The engine is augmented by the EM, which has a maximum power of 30 kw. A lithium

9 ZHU et al.: DESIGN AND ANALYSIS OF NOVEL MMT FOR HEV USING SINGLE EM 1105 TABLE IV SPECIFICATIONS FOR THE MMT TABLE V PARAMETERS OF THE RULE-BASED ENERGY MANAGEMENT CONTROLLER iron phosphate battery is used as the energy storage system. The maximum power of EM1 and EM2 in the THS II-like vehicle is 15 and 30 kw, respectively. Specifications of the vehicle and MMT are listed in Tables III and IV, respectively. The control parameters in Fig. 8 are selected, as shown in Table V, i.e., P eng_ max. B. Simulation Results The engine, battery, and vehicle parameters of the proposed MMT vehicle and the THS II-like vehicle are the same. The two vehicles use the same rule-based control strategy. They differ in calculations of torque distribution due to their different mechanisms. The torque distribution for the proposed MMT vehicle is given in Section IV-C, and the torque distribution for the THS II-like vehicle is calculated based on the fundamental torque and speed formulas of the planetary gearsets, Fig. 9. Simulation results of the NEDC. (a) Vehicle velocity. (b) MMT operation mode. (c) Engine power. (d) EM power. (e) Battery power. (f) Battery SOC. which are applied to several EVTs with one or more planetary gearsets [18]. In Fig. 9, vehicle velocity, operation mode, engine power, EM (EM, EM1, and EM2) power, battery power, and SOC are shown. The black solid line represents the results of the proposed MMT vehicle, and the blue or red dashed line represents the results of the THS II-like vehicle.

10 1106 IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 62, NO. 3, MARCH 2013 It is shown in Fig. 9(a) that both the MMT vehicle and the THS II-like vehicle track the velocity designated by the NEDC very well. This indicates that both vehicles gain identical kinetic energy through the NEDC process. Fig. 9(b) shows that 10 of the 16 operation modes are used. These operation modes cover four of the five power flow modes. At the very beginning of the NEDC, the vehicle is kept still in Mech_braking mode and then started in Motor_only mode. When the vehicle speed exceeds v EM_ max, Compound_2 and Compound_3 modes are activated subsequently to combine the engine torque and the EM torque. One notices that Compound_1 mode is not applied. The reason is that the simulation sets the same value for v EM_ max and v 1_ max, below which Motor_only mode is used and above which Compound_2 mode is used for improving engine efficiency. Other than that, Engine_only_1 is not applied because the required torque does not fall into the engine high- or subhigh-efficiency region when the vehicle velocity is below v 1_ max and when the battery SOC is within the lower and upper boundaries during the NEDC simulation. Since the regenerative braking combined with the mechanical braking can provide the required braking torque, only Reg_braking and Compound_braking_0 modes are applied. It is unnecessary to introduce engine resistance to assist braking; therefore, Compound_braking_1 3 modes are not used. The Charging-while-parking mode is not activated because the battery SOC can be balanced during vehicle running. The engine operating points of the MMT vehicle are shown in Fig. 10(a). Aside from the points with negative torque, which occurs when the engine is starting up or going to be resting without fuel supply, most operating points spread in the highefficiency region defined by the engine speed from 1500 to 4000 r/min and the engine torque from 70 to 100 Nm; at the same time, some points scatter in the subhigh-efficiency region with engine torque from 40 to 70 Nm. As a comparison, the engine operating points of the THS II-like vehicle concentrate on the high-efficiency region, as shown in Fig. 10(b). The specific fuel consumption of these operating points is below 240 g/(kwh), whereas those of the MMT vehicle are up to 260 g/(kwh). The vehicle fuel economy is affected by the engine specific fuel consumption and other factors. The following gives the explication. C. Factors Influencing Fuel Consumption Over the whole NEDC cycle, the proposed MMT vehicle achieves fuel consumption of 3.81 L/100 km, and the fuel consumption of the THS II-like vehicle is 3.9 L/100 km. The results show that the fuel economy of the two vehicles is comparable, and the MMT vehicle gains a minor advantage. Fig. 10(c) shows the accumulative fuel-produced energy from the engine. For the MMT vehicle, the energy produced by fuel combustion all along the NEDC is totally accumulated to 4788 kj, in which 3285 kj comes from the high-efficiency operating points, and 1503 kj comes from subhigh and other points. However, for the THS II-like vehicle, the accumulative energy is totally up to 5322 kj (534 kj more than in Fig. 10. Engine operating points and accumulative energy. (a) MMT vehicle. (b) THS II-like vehicle. (c) Fuel-produced energy. the MMT vehicle), in which 5222 kj from the high-efficiency points and only 10 kj from other points. It is interesting to reveal a phenomenon that high-efficiency usage of the engine does not definitely lead to low fuel consumption of the vehicle. There should be factors other than the engine efficiency affecting the fuel consumption. Generally, the energy flow of an HEV vehicle can be depicted as in Fig. 11, without including plug-in electricity input and auxiliary component consumption for simplification consideration. The energy balance is described as the input energy subtracted by the losses equal to the output energy. Compared with the THS II-like vehicle, the engine of the MMT vehicle inputs less energy because it consumes less fuel, and the fuel is combusted in the lower efficiency region. Since the driving cycles of the two vehicles are identical, their external losses and vehicle kinetic energy are the same. In addition, because the initial and ending SOC values during the NEDC are almost identical for the two vehicles, as shown in Fig. 9(f), their energy for battery storage can be regarded as the same. Therefore, a possible explanation for the MMT vehicle that

11 ZHU et al.: DESIGN AND ANALYSIS OF NOVEL MMT FOR HEV USING SINGLE EM 1107 TABLE VI ACCUMULATIVE OPERATING TIME OF THE EMS Fig. 11. Energy flow for a HEV. accomplishes the NEDC by using less energy from the engine is more braking regeneration as supplement energy source and/or less internal losses. The internal losses come from the EMs, the battery, and their power electronics. The statistic data and detail explanation for the influence of these factors are given as follows. 1) Influence of Regenerative Braking: The accumulative regenerative energy of the MMT vehicle is kj, whereas that of the THS II-like vehicle is kj. Different operations of engine braking are found to be a major reason for the regenerative energy gap of 101 kj. In the MMT vehicle, engine braking is not applied by opening the clutches, i.e., the braking torque is provided by the combination of regenerative braking and mechanical braking. Moreover, regenerative braking is preferred, and mechanical braking is supplemented to supply enough braking torque according to the control rules. Thus, engine braking does not consume any regenerative energy from the vehicle body, and the regenerative energy can flow to the battery as much as possible. However, in the THS II-like vehicle, engine braking is inevitable due to the no clutch structure, even if regenerative braking and mechanical braking are applied simultaneously. Negative engine power is recognized in the braking process, as shown in Fig. 9(c). By integral calculation, engine braking energy in the THS II-like vehicle is accumulated to kj, whereas the engine braking energy in the MMT vehicle is only kj (which comes from the kinetic energy of the engine itself). In other words, engine braking in the THS II-like vehicle dissipates by kj more than that in the MMT vehicle. The other reason for the 101-kJ regenerative energy gap is related to power electronics efficiency, which is discussed in the following. 2) Influence of the EMs and their Power Electronics: It is shown in Fig. 9(d) that the intervals with zero power of the EM in the MMT vehicle are much more than the intervals in the THS II-like vehicle. In the MMT vehicle, open or lock operations of the clutches allow efficient usage of the EM. Table VI shows that the EM accumulatively rests for s, about one-third of the total NEDC time, which can extend its life cycle effectively. However, in the THS II-like vehicle, EM1 and EM2 always rotate when the vehicle is running. EM1 must provide a balance torque, and EM2 has to rotate simultaneously with the output shaft. Therefore, they can rest for only s when the vehicle is at still. As a consequence, the more frequent usage of EM1 and EM2 means more power conversions, which lead to more losses. Since an EM and its corresponding power electronics, i.e., an inverter, are working simultaneously, they are considered as one unit in this discussion. The MMT vehicle uses one unit (EM unit) including one EM and one inverter, whereas the THS IIlike vehicle uses two units (EM1 unit and EM2 unit), including two EMs and two inverters. To give quantitative statistics, the operating points of the three units are shown in Fig. 12(a) (c), respectively, and the accumulative output energy for the motor and generator working status are given in Fig. 12(d), respectively; furthermore, the losses are calculated and illustrated. The value of the efficiency contour is a combination of the two components in a unit. The operating points of the EM unit spread in the first, third, and fourth quadrants are shown in Fig. 12(a). The vertically and horizontally scattered points represent, respectively, the operating points during constant speed driving and accelerating (or braking). Points in the first and fourth quadrants represent the motor and generator status of the EM, respectively. The horizontally scattered points in the third quadrant occur in Compound_4_EVT mode, in which EM modulates the engine speed to its high-efficiency region. In Fig. 12(b), most of the operating points scatter in the second and fourth quadrants, representing that EM1 works mostly as a generator. Aside from the points in the fourth quadrant, most of them concentrate on low efficiency area, particularly the operating points near the zero torque line, which indicate EM1 spinning. The points along the torque limit line occur in the moments when engine torque changes suddenly from negative to positive during starting. In Fig. 12(c), the operating points of the EM2 unit spread evenly in the whole speed and torque range. In Fig. 12(d), when the EMs are operated as motors, the accumulative output energy of the EM unit is 950 kj less than the summation of the EM1 and EM2 units; when they are operated as generators, the accumulative energy of the EM unit is 1463 kj less than the summation. In other words, the EM unit of the MMT vehicle is used less than the EM1 and EM2 units of the THS II-like vehicle. The energy loss through the EM unit is 1242 kj, and the energy loss through the EM1 and EM2 units is 1522 kj, as

12 1108 IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 62, NO. 3, MARCH 2013 TABLE VII ACCUMULATIVE OPERATING TIME OF THE EMS Fig. 13. Accumulative energy of the batteries. Fig. 12. Operating points and accumulative energy of the EM units. (a) EM unit. (b) EM1 unit. (c) EM2 unit. (d) Accumulative energy. shown in Fig. 12(d). Therefore, the EM unit reduces the energy loss by 280 kj for the MMT vehicle. 3) Influence of the Battery: Fig. 9(e) shows that the battery charging and discharging operations of the THS II-like vehicle are more frequent than those of the MMT vehicle. Table VII shows that the battery in the MMT vehicle rests for s, whereas that in the THS II-like vehicle only rests for s. It is shown in Fig. 13 that 1076-kJ less energy is charged into the battery of the MMT vehicle, and 844-kJ less energy is discharged from the battery of the MMT vehicle. In addition, the battery charging depth of the THS II-like vehicle is deeper than that of the MMT vehicle, as shown in Fig. 9(f). Frequent and deep usage of the battery leads to more energy losses (thermal dissipation in Joule) on the battery. Fig. 13 shows that the energy loss through the battery in the MMT vehicle is 268 kj, and the energy loss through the THS II-like vehicle is 493 kj. In other words, the battery of the MMT vehicle reduces the energy loss by 225 kj. 4) Synthesis Analysis: From the given analysis, three main aspects are found to compensate the 534-kJ less energy output from the engine of the MMT vehicle: 1) kJ more braking regenerative energy, 2) 280-kJ less loss in the EM units, and 3) 225-kJ less loss in the battery. Thus, due to the limitation of the single-em structure, the MMT vehicle operates the engine in a relative lower efficiency region but achieves less loss from the electric power devices; on the contrary, the THS II-like vehicle operates the engine in an ideally high-efficiency region but produces more loss from the electric power devices. Therefore, the fuel consumption of the MMT vehicle with proper control strategy can be comparable with that of the THS II-like vehicle. An extreme case of the MMT vehicle is the Engine_only driving power flow mode, which transmits the engine power to the vehicle without passing any electric power devices and never occurs in the THS II-like vehicle. In Fig. 9(b), the Engine_only driving power flow mode (applied by Engine_only_2 4 modes) accumulatively takes s; in other words, it occupies % of the total NEDC duration. In this power flow mode, the engine works within a high or subhigh-efficiency region. Owing to the four less intermediate

13 ZHU et al.: DESIGN AND ANALYSIS OF NOVEL MMT FOR HEV USING SINGLE EM 1109 elements (generator, charging, discharging, and motor), the synthetic efficiency from the engine to the MMT vehicle is reasonably higher than that from the engine to the THS II-like vehicle. VI. CONCLUSION An MMT combining two planetary gearsets, four clutches, and a single EM has been proposed in this paper for a HEV. The single-em solution avoids losses from another EM and its power electronics, which are employed in many existing HEVs. The MMT proposed here is very similar to the planetary gearsets used in several existing four-speed ATs and can have 16 operation modes, which are classified into five power flow modes: Motor_only driving, Engine_only driving, Compound driving, Braking, and Charging while parking. A model of the MMT-based HEV is developed in the MATLAB/SIMULINK environment. A rule-based energy management control strategy, using the efficiency maps of the components for estimating the power performance of the HEV, is presented. Simulation under the NEDC with the same initial and close SOCs shows comparable fuel economy performance to the THS II-like vehicle, which indicates the fuel economy potential of this concept. Two factors are considered for the efficiency advantage of the single-em HEV over the dual-em HEV. First, the energy conversion loss in electric components can be reduced due to the existence of Engine_only modes. Second, clutches are used to avoid spin loss of the engine and EMs. Notice that, due to the no-clutch structure, the THS II-like vehicle does not allow direct mechanical power transmission from the engine to the drive shaft. Although the engine can be modulated to its high-efficiency region, the overall system efficiency is still lower than the efficiency of the engine-only driving under some driving conditions. The fundamental reason is that the fuel efficiency improvement is counterbalanced by the energy loss in the power conversions among mechanical and electrical components. The EM works as a motor for 34.82% of the total NEDC running time, as a generator for 27.15%, and rests for 30.89%. Because the open or lock operations of the clutches allow efficient usages in the proposed HEV, the life cycle of the EM is expected to be extended effectively. [7] A. Villeneuve, Dual mode electric infinitely variable transmission, in Proc. Int. CVT Hybrid Transm. Congr., Davis, CA, [8] X. Ai, T. Mohr, and S. Anderson, An electro-mechanical infinitely variable speed transmission, presented at the Soc. Automotive Eng. World Congr., Detroit, MI, 2004, SAE Paper [9] S. I. Jeon, Y. I. Park, and J. M. Lee, Multi-mode driving control of a parallel hybrid electric vehicle using driving pattern recognition, ASME J. Dyn. Syst., Meas. Control, vol. 124, no. 1, pp , Apr [10] K. Aoki, S. Kuroda, S. Kajiwara, H. 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Rockwood, Automatic Transmission & Transaxles. Englewood Cliffs, NJ: Prentice-Hall, [17] K. B. Sheu, Simulation for the analysis of a hybrid electric scooter powertrain, Appl. Energy, vol. 85, no. 7, pp , Jul [18] L. Chen, F. Zhu, M. Zhang, Y. Huo, C. Yin, and H. Peng, Design and analysis of an electrical variable transmission for a series-parallel hybrid electric vehicle, IEEE Trans. Veh. Technol., vol. 60, no. 5, pp , Jun [19] W. Xiong, Y. Zhang, and C. Yin, Optimal energy management for a series parallel hybrid electric bus, Energy Convers. Manag., vol. 50, no. 7, pp , Jul [20] J. Liu and H. Peng, Modeling and control of a power-split hybrid vehicle, IEEE Trans. Control Syst. Technol., vol. 16, no. 6, pp , Nov [21] J. Moreno, M. E. Ortúzar, and J. W. Dixon, Energy-management system for a hybrid electric vehicle, using ultracapacitors and neural networks, IEEE Trans. Ind. Electron., vol. 53, no. 2, pp , Apr [22] O. Bitsche and G. Gutmann, Systems for hybrid cars, J. Power Sources, vol. 127, no. 1/2, pp. 8 15, Mar [23] J. Meisel, An analytic foundation for the two-mode hybrid-electric powertrain with a comparison to the single-mode toyota prius THS-II powertrain, presented at the Soc. Automotive Eng. World Congr. Exhib., Detroit, MI, Apr., 2009, SAE Paper REFERENCES [1] C. C. Chan, The state of the art of electric, hybrid, and fuel cell vehicles, Proc. IEEE, vol. 95, no. 4, pp , Apr [2] J. D. Wishart, Y. Zhou, and Z. Dong, Review, modeling and simulation of two-mode hybrid vehicle architecture, in Proc. IDETC/CIE, LasVegas, NV, [3] R. H. Staunton, C. W. Ayers, L. D. Marlino, J. N. Chiasson, and T. A. Burress, Evaluation of 2004 Toyota Prius hybrid electric drive system, U.S. Dept. Energy Freedom CAR Veh. Technol., Washington, DC, ORNL/TM-2006/423, May [4] A. Takasaki, Development of new hybrid transmission for 2009 prius, in Proc. EVS24 Int. Battery, Hybrid Fuel Cell Elect. Veh. Symp., Stavanger, Norway, [5] J. M. Miller, Hybrid electric vehicle propulsion system architectures of the e-cvt type, IEEE Trans. Power Electron., vol. 21, no. 3, pp , May [6] M. R. Schmidt, Two-mode, input-split, parallel, hybrid transmission, U.S. Patent , Sep. 24, Futang Zhu was born in Shandong, China, in He received the B.S. degree in vehicle engineering from Jilin University, Changchun, China, in He is currently working toward the Ph.D. degree with the School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai, China. From 2011 to 2012, he was a Visiting Scholar with ETAS GmbH, BOSCH Group, Stuttgart, Germany. His research interests include modeling and control of electric vehicles, particularly for power coupling mechanisms of hybrid electric vehicles.

14 1110 IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 62, NO. 3, MARCH 2013 Li Chen was born in Hunan, China, in She received the B.S. and M.S. degrees in mechanical engineering from Hunan University, Changsha, China, in 1994 and 1997, respectively, and the Ph.D. degree in vehicle engineering from Shanghai Jiao Tong University, Shanghai, China, in She is currently an Associate Professor with the School of Mechanical Engineering, Shanghai Jiao Tong University. Her research interests include dynamics, control, and thermal performance for advanced powertrain systems. Chengliang Yin was born in Inner Mongolia, China, in He received the Master s and Ph.D. degrees in vehicle engineering from Jilin Industrial University, Changchun, China, in 1996 and 2000, respectively. He is currently a Full Professor with Shanghai Jiao Tong University, Shanghai, China, where he is also the Vice Dean with the Institute of Automotive Engineering and the Vice Director with the National Engineering Laboratory for Automotive Electronic Control Technology. He is also currently an Advanced Technical Adviser with the Shanghai Automobile and Dongfeng Automotive Group. His research interests include the control of automotive electronics and electric vehicles, particularly research and development of hybrid electric vehicles. Dr. Yin received the General Motors Innovative Talent in Automotive Industry Award in 2009.