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Performance and energy management of a novel full hybrid electric powertrain system
Energy 89 (2015) 626e636
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Energy journal homepage: www.elsevier.com/locate/energy
Performance and energy management of a novel full hybrid electric powertrain system Cheng-Ta Chung a, Yi-Hsuan Hung b, * a b
Department of Vehicle Engineering, National Formosa University, Yunlin, 63201, Taiwan Department of Industrial Education, National Taiwan Normal University, Taipei, 106, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 December 2014 Received in revised form 27 May 2015 Accepted 30 May 2015 Available online 2 July 2015
This study compared the performance and energy management between a novel full hybrid electric powertrain and a traditional power-split hybrid system. The developed planetary gearset and dual clutch configuration provides five operation modes. Equations for the torque and speed of power sources for the planetary gearset and dual clutch system and the Toyota Hybrid System are firstly derived. By giving vehicle performance of gradability, maximal speeds in hybrid and pure electric modes, the power sources of the 210 kg target vehicle are: a 125 cc engine and two 1.8 kW motor and generator. The optimal tankto-wheel efficiencies, ratios of circulating power, and operation points at specific vehicle speeds and out loads are calculated. Simulation results show that the dual-motor electric vehicle mode offers superior performance regarding electric drive; the low capacity of the battery is conducive to reducing manufacturing and maintenance costs; the tank-to-wheel efficiency is mainly operated above 20% while the power split electronic-continuously-variable-transmission mode is the major operation mode, and a maximum of 17% fuel economy improvement is achieved compared with the Toyota Hybrid System in most of the vehicle speed ranges. The outstanding performance warrants further real-system development, especially regarding the implementation in plug-in and sport hybrid powertrain designs. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Powertrains Energy management Power-split hybrid Simulation Optimization
1. Introduction Recently, fulfilling the demand on high energy density of batteries for extending the cruising mileage of electric vehicles remains a challenge. Developing engine/motor HEVs (hybrid electric vehicles) with advanced management of energy efficiency is still one of the most favorable solutions with regard to environmental issues. For example, in Ref. [1], the comparative study of the use of energy carrier based on renewable energy sources indicates that energy-efficiency improvements with the use of renewable energy would have significant influence on energy environmental cost. For HEV system design and control, in Ref. [2], an overview of power management of HEVs was studied; in Ref. [3], HEV configurations, energy management strategies and electronic control units were compared. In Ref. [4], a hydraulic/electric synergy system for heavy hybrid vehicles was designed with the multi-energysource power distribution. In Ref. [5], a dynamic analysis on energy
* Corresponding author. E-mail addresses: [email protected] (C.-T. Chung), [email protected] (Y.-H. Hung). http://dx.doi.org/10.1016/j.energy.2015.05.151 0360-5442/© 2015 Elsevier Ltd. All rights reserved.
management of hybrid systems was conducted. Furthermore, various optimization techniques for energy management of HEVs or EVs (electric vehicles) have been exploited. For instance, in Ref. [6], an optimization method based on dynamic programming was utilized for EV fleet; in Ref. [7], a control optimization of a power-split HEV was carried out. PHEVs (plug-in HEVs) may receive much attention in the near future because of their high EV-mode range and chargeable characteristics. Amjad et al. [8] evaluated battery energy and power requirements for a plug-in HEV for different EV-mode range. Khayyam et al. [9] proposed an adaptive intelligent energy management system for PHEV to further reduce fuel consumption and emissions. Arslan et al. [10] introduced a concept of virtual power plant to investigate the impacts on mass penetration of PHEVs into the electricity grid. Bradley et al. [11] reviewed the design of PHEVs and assessed their sustainability. By recognizing the benefits of PHEVs, one aim of this research is to develop a new type of power-split HEV that not only enhances performance and energy efficiency but also provides excellent EV drive suitable for plug-in applications. Compared with serial and parallel powertrain configurations, the power-split configuration is characterized by qualities such as high overall energy efficiency, low levels of pollutants and fuel
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consumption, and a flexible power distribution. The THS (Toyota Hybrid System) are the most well-known powertrains that have been used to equip commercial vehicles for a long time [12]. Sasaki [13] optimized engine fuel consumption to minimize exhaust emissions and to apply regenerative braking for the THS-I. To enhance the driving performance, the THS-II was developed. It improved the driving performance by enhancing the power density of the electric drives and boosting the voltage of the energy source [14]. The key technology, namely power-split electronic continuously variable transmission (power-split e-CVT), flexibly facilitates energy management by one engine and two motor/generators [15]. Because the THS configuration is the most widely used hybrid configuration employed in commercial applications, it is used for performance comparison. Other configurations of advanced powertrains for other international automakers have been researched in previous studies. In Ref. [16], a power-split HEV was compared to the THS and control rules were optimized. In Ref. [17], the four operation modes and two-speed transmission of the GM multimode electric transaxle was proposed, and a so-called single mode full hybrid system was proposed in Ref. [18]. In Ref. [19], comparison of technologies of e-CVT transmission for Toyota, Ford, and General Motors companies were conducted. Considering the future commercialization of the developed powertrain in Taiwan, light-duty HEVs are selected in this research. In Ref. [20], two planetary gear sets and a chain set combined with an engine and a motor/generator were equipped for four operation modes to maximize the vehicle performance and to reduce emissions. In Ref. [21], a novel parallel-hybrid drive train was developed for a light-duty scooter. In Ref. [22], a control unit for an engine/motor hybrid scooter that effectively reduced pollutants was implemented. In a similar previous study, the PDOC (planetary gearset and dual one-way clutch) hybrid electric system with power-split eCVT have been analyzed [23]. The results demonstrated a superior driving performance and fuel economy in cruise driving, especially compared with those achieved using the conventional rubber-belt CVT system (a maximum of 32% fuel economy improvement). In this paper, a new hybrid powertrain with a modified PDOC, called the PDC (planetary gearset and dual clutch) hybrid electric system, is proposed. The PDC system features the engine-driven and power-split e-CVT operating modes, similar to those of a PDOC system. However, one major advantage of the PDC system is the capability to operate the pure electric drive using two motors since its motor and generator are arranged as a torque coupler; the PDOC system does not have this capability because its motor and
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generator are connected in a speed-differential manner in the pure electric drive. Thus, the PDC system is suitable for plug-in applications. The performance and operation characteristics of the PDC system were evaluated through simulation and compared with those of the THS to determine the potential and feasibility for various applications. The remainder of this paper is organized as follows: Section 2 presents a comparison of the system design and operation modes between our powertrain and the THS. Section 3 describes performance indices and the search for optimal operation points (energy management) of the PDC system and THS. Section 4 discusses the vehicle performance and optimal energy management according to the concepts and indices described in Sections 2 and 3. Section 5 presents the conclusion and lists the contributions of this study.
2. Powertrain architecture and operation modes 2.1. Comparison of the architecture of the PDC system and that of the THS The powertrain architecture and a power system diagram of the PDC system are shown in Fig. 1. A PDC system consists of an engine (E), a generator (G), a driving motor (M), a set of planetary gears (i.e., power splitter) (P), a CC (controllable clutch), a OC (one-way clutch), and several components arranged to form a complete drive train. The planetary carrier of the planetary gear (c) is connected to the G by a chain (Ch). The E is connected to the G by a reduction gear (Gr). The CC is coupled to the output shaft of the G and connects or disconnects the G to or from the Gr. The sun gear of the planetary gear (s) is connected to the M through the OC. The ring gear of the planetary gear (r) is connected to the wheel (W) through a final drive (F). A power control unit consists of power drivers for the M and G and a power electric device for transmitting and modulating the electric power among the ESS (energy storage system), M, and G. The ESS may be a battery, ultracapacitor, or both. The OC is placed between the s and c of the P to prevent relative rotation when the s operates faster than the c does. A schematic diagram of a THS motorcycle is shown in Fig. 2. As in the THS, the E, M, and G are mechanically connected to the c, r, and s, respectively. However, in adapting the system to a 125 cc motorcycle, a slightly different arrangement was used. The E was connected using a Gr. Additionally, the M was connected using a Ch and coupled to the real W through a final drive (F).
Fig. 1. (a) Powertrain architecture and (b) schematic diagram of the PDC system.
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(a) Dual-motor electric vehicle mode When started and driven at low speed, PDC is operated at a dual-motor EV (electric vehicle) mode that both the G and M can offer driving torque for enhanced electric drive. This is achieved by locking the sun gear of the P connected to the M and the planetary carrier of the P linked to the G against relative rotation using the one-way clutch OC, as shown in the speed diagram of the P in Fig. 3(a) and the power flow diagram of the PDC in Fig. 3(b), where Zr and Zs represents the number of teeth of the ring gear and the sun gear respectively. In addition, the CC is disengaged so that the power flow between the E and G is disconnected and no further driving effort is consumed to drive the unfired E. The driving torque of the W, Tw, transferred from that of M, Tm, and G, Tg, in the dualmotor EV mode can be expressed as:
Tw ¼ rf $hf $hp $Tm þ rf $rch $hf $hch $hp $Tg
(1)
where r and h indicate reduction ratio and transmission efficiency respectively. Moreover, the subscripts f, p, ch represent F, P and Ch respectively. (b) Switching CVT mode
Fig. 2. Powertrain architecture of the THS motorcycle.
2.2. Operation modes of the PDC system and the THS This section presents a comparison of the operation modes of a PDC system and those of the THS. Because the configurations differ, the operation modes vary. The PDC system contains five modes, whereas the THS operates with four modes.
2.2.1. Operation modes of the PDC system The PDC system is operated through five modes, namely the dual-motor electric vehicle (dual-motor EV), switching CVT (sCVT), engine-driven, power-split e-CVT, and boost modes. To determine which of these modes should be in operation, the road conditions and performance requirements regarding the driving of the vehicle are required.
The switching CVT (s-CVT) mode can be regarded as a transient switching from the dual-motor EV mode to the hybrid modes, (i.e. the engine-driven and the power-split e-CVT modes). As the vehicle speed is increased to a switching point, the E is started and the M acts as a generator to release the OC from locking, freeing the P from direct drive. Subsequently, a function of CVT can be performed because the rotation speed of the c, uc, and that of the r, ur, can be continually varied by modulating the rotation speed of the s, us, as shown in Fig. 4. The speed relationship among them is stated as follows:
uc ¼
1 k us þ ur ; kþ1 kþ1
(2)
where k is the basic ratio of the planetary gear, defined as Zr/Zs. The corresponding speed relationship for M, G, E, and W is described as follows:
ug ¼
ue 1 k um þ r $uw ¼ kþ1 f re=g k þ 1
where the subscript e/g represents Gr.
Fig. 3. (a) Speed diagram for the power splitter and (b) power flow diagram of PDC at dual-motor EV mode.
(3)
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Fig. 4. (a) Speed diagram for the power splitter and (b) power flow diagram of PDC at power split e-CVT mode.
Fig. 5. (a) Speed diagram for the power splitter and (b) power flow diagram of PDC at engine-driven mode.
In principle, the ratio of the generative load of M, Tm, to the driving torque of E, Te, is fixed. The parameter Tm is expressed as follows:
where the subscripts ch and e/g represent the Ch and Gr, respectively. (d) Power-split e-CVT mode
Tm
re=g $rch $he=g $hch $hp Te ¼ kþ1
(4)
(c) Engine-driven mode As the sun gear of the P and M are held stationary, as shown in Fig. 5, the vehicle operates in the engine-driven mode and is driven by the E alone. The G may act as a generator for charging the ESS when required. As in the PDOC system, the engine-driven mode rather than the power-split e-CVT mode is preferably applied under low-speed and high-load conditions in hybrid driving to improve fuel economy. The driving torque of the W, Tw, transmitted from that of the E, Te, in this mode can be expressed as follows:
k $r $r $h $h $h $h $Te ; Tw ¼ rf $ k þ 1 ch e=g f p ch e=g
(5)
The power output of the E, Pe, splits into two paths. One path leads toward the P via the Ch; the other leads toward the G to generate electric power, as expressed in the following:
Pe ¼
Pg Pp þ he=g hch
(6)
Generating electricity through the G has two functions. A small fraction of Pg is used to charge the ESS and, thus, supplement the energy in the ESS consumed primarily in the EV mode. Most of Pg directly flows to the M; therefore, modulating the rotation speed of the M, um, provides additional degree-of-freedom that can be used to optimize the operation of the entire powertrain. The power relationship among the G, M, and ESS is described as follows:
Pg ¼
Pm Pess þ hg $hm hg $hchg
(7)
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where the subscripts ess and chg represent the ESS and charging, respectively. The power-split e-CVT mode rather than the engine-driven mode functions as the primary operating strategy in hybrid driving, especially at medium and high speeds, because it enables high flexibility and the capability to modulate the engine speed as well as engine load according to the maximal efficiency of the entire system. This is achieved through electric circulation; specifically, a portion of the engine power is circulated in the form of electrical power through the G and M without excessively charging or discharging the ESS. Here, Pg flows to the M; thus, Eq. (7) becomes
Pg ¼
Pm hg $hm
(8)
The operating speed of the E is lowered by increasing the rotation speed of the M in reverse, as shown in Fig. 6. Performing this procedure and widening the throttle opening of the E by raising the generating load of the G may greatly reduce the brakespecific fuel consumption (bsfc in grams per kilowatt hours) of the E, providing compensation for the loss of circulating driving power and yielding a net gain in the efficiency of the entire system. For a given driving torque and rotation speed of the W, Tw and uw, under a certain driving condition, a value of the rotation speed of the M, um, may be initially set for the e-CVT control, and the corresponding rotation speed of the E and G can be determined using the following equation:
ug ¼
ue 1 k um þ r $uw ¼ kþ1 f re=g k þ 1
1 Tw k$rf $hp $hf
1 um Tm hg $hm ug
(12)
(e) Boost mode If the required driving torque exceeds the maximal available torque when a hybrid mode is engaged (i.e., s-CVT, engine-driven, or power-split e-CVT mode), then additional power supplied by the ESS provides a short-term boost effect, depending on the amount of energy remaining in the ESS. In this boost mode, the G provides this supplemental torque. However, the maximal torque capacity of the G may not be fully used, because there is a constraint regarding the torque condition among all components of the P, as expressed in the following:
Ts : Tc : Tr ¼ 1 : k þ 1 : k
(13)
Consequently, the torque generated by the G is limited by the following condition:
Tg
kþ1 $Tm;max re=g $he=g $Te;max rch $hch
(14)
where Tm,max and Te,max are the maximal torque of the M and E, respectively.
2.2.2. Modified THS operation modes
(10)
According to Eq. (8) and P ¼ 2p$u$T, the generative torque of G is calculated using
Tg ¼
1 kþ1 Tg þ Tw re=g $he=g re=g $rch $rf $he=g $hch $hp $hf
(9)
The required torque of the M is calculated as follows:
Tm ¼
Te ¼
(11)
Finally, the required torque output of the E for the power-split eCVT control is expressed as follows:
The operating characteristics of the THS have been extensively described in numerous documents. The operation in pure electric drive by using the THS is shown in the speed diagram for the power splitter of Fig. 7(a). The vehicle is solely driven by the M, while the G is rotated in reverse to hold the E without delivering traction power. This operating mode is called the single-motor EV mode compared to the dual-motor EV mode for the PDC system. Moreover, the inadequate traction effort provide by merely one motor can be supplemented by turning the E on using the G as a starter. This operation is called the engine-assisted EV mode and is also shown in Fig. 7(a). Under normal driving conditions at medium and high vehicle speeds, the THS motorcycle is mostly operated in either the enginedriven mode or the power-split e-CVT mode, as illustrated in
Fig. 6. (a) Speed diagram for the power splitter and (b) power flow diagram of PDC at power split e-CVT mode.
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Fig. 7. Speed diagram for the power splitter of THS at various modes.
Fig. 7(b). When operated in the engine-driven mode, the vehicle is driven by the engine alone with the G held stationary. In the powersplit e-CVT mode of the THS, the vehicle is operated through circulation of the engine power in the form of electric power through the G and M. Therefore, extra degree-of-freedom in varying the engine speed and load by changing the rotation speed of the G (ug) as well as the corresponding power of circulation provides an opportunity to determine the optimal operating point of energy efficiency for a particular driving condition. In addition, as in the PDC system, a boost mode can be engaged to provide an instantaneous and enhanced synergy drive by using the extra electric power extracted from the ESS. The related equations for the operation modes of the modified THS can be referred to Eqs. (1)e(14). 3. Power circulation ratio, parameter evaluation, and optimal operation points 3.1. Power circulation ratio and required traction torque To quantize the degree of power split in the simulation, a parameter called the ratio of circulating power (z) is used and defined as the generative power circulated through the G and M, Pg, in CVT operation divided by the engine power Pe:
z¼
Pg Pe
(15)
This parameter represents the proportion of engine power that is converted to electric power circulated from the G to the M in CVT operation. However, increasing this proportion leads to a greater loss of energy conversion. Therefore, a tradeoff in the efficiency of the entire system was analyzed. The power-split e-CVT mode was operated under a condition of z > 0; the engine-driven mode was regarded as a specific case of power-split e-CVT with z ¼ 0. To evaluate the road load of the vehicle, the traction torque of the W, Tw, in the simulation was modified from Ref. [23] and is expressed in the following equation:
Tw ¼
1 rACd v2 þ mMt g cos q þ Mt g sin q þ Mt a $R 2
(16)
where r is the density of air, A is the frontal area of the test vehicle, Cd is the drag coefficient, n is the vehicle speed in meters per
second, m is the coefficient of rolling resistance, Mt is the test mass of the vehicle, g is the acceleration caused by gravity, q is the slope angle of climbing in degrees, a is the acceleration of the vehicle in meters per second squared, and R is the radius of the W.
3.2. Parameter evaluation The values of the coefficients and design variables in Eq. (16) used for the simulation are listed in Table 1. A standard commercial 125-cc motorcycles from Taiwanese motorcycle manufacturers is used for comparison, where the parameter values are referred to [23]. A 125-cc single-cylinder gasoline engine with a maximal torque of 9.0 N m at 6500 rpm and a maximal power of 6.8 kW at 7500 rpm was used as the E. A performance map of its torque and brakespecific fuel consumption (bsfc in grams per kilowatt hour) versus rotation speed is illustrated in Fig. 8(a). The efficiencies of the transmission components were set to be constant and are listed in Table 2. The maximal torque, rated power, and maximal rotation speed of the M and G were determined according to their efficiency maps scaled from that of a permanent magnet synchronous motor/ generator used for HEVs and modified from the map in Ref. [23]. In addition, the design parameters of the PDC powertrain and the baseline system were determined, such as k, re/g, rch, and rf. The performance requirements for determining the aforementioned values are described as follows: 1. The vehicle is required to maintain a cruising speed of 10 km/h when driving uphill at q ¼ 12 (or grade ¼ 21.3%). 2. The switching point of the vehicle speed between the EV and a hybrid mode is at approximately 20 km/h. 3. The maximal vehicle speed at which the vehicle can operate purely in the EV mode is approximately 40 km/h.
Table 1 Values of various coefficients and design variables in Tw. Parameter
Value
1 rAC d 2
0.279 N/(m/s)2 0.013 210 kg 9.81 m/s2 0.215 m
m
Mt g R
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Fig. 8. Performance maps of (a) the engine [23] and (b) the motor and the generator.
3.3. Optimal operation points
Table 2 Efficiency of various components. Component
Efficiency
Final drive Power splitter Clutch Chain Reduction gear
hf ¼ 0.95 hp ¼ 0.95 hc ¼ 0.95 hch ¼ 0.95 he/g ¼ 0.95 he/p ¼ 0.95
4. The maximal vehicle speed at which the vehicle can operate in a hybrid mode is approximately 90 km/h. 5. The total reduction ratio of the powertrain in a hybrid mode must be carefully adjusted to provide a sufficient driving force covering the entire range between 20 and 90 km/h. 6. The power capacity of the M and G must be sufficient, at least to afford the circulating load in Eq. (8) required for power-split eCVT operation, especially at a high speed. 7. Adopting the same type of motor/generator for both the M and G is favorable because of the simplicity of operation and control. The proposed PDC hybrid system was treated as the primary system in selecting the suitable values of the design parameters and the desired specifications of both the M and G. The rated maximal torque, rated power, and maximal rotation speed of the M and G were 8.5 N m, 1.8 kW, and 6000 rpm, respectively. The corresponding performance map of torque and efficiency versus rotation speed is shown in Fig. 8(b). The design values of the related parameters for the PDC system are listed in Table 3. For the same specifications of the E, M, and G, the related design parameters of the THS powertrain were evaluated and are listed in Table 4.
Table 3 Values of various design parameters for PDC.
According to a classification proposed by Miller [19], the PDC system involves a type of output split, whereas the THS involves a type of input split. As for the other definition documented in the work of M. Ehsani etc. [24], both are a type of integrated speedcoupling and torque-coupling hybrid electric drive train. However, PDC consists of a speed coupler connected to output, whereas THS possesses a torque coupler linked to output. Therefore, a comparative assessment of the operating characteristics of these two distinct systems is valuable. The energy efficiencies over the entire operating region of the power-split e-CVT and engine-driven modes were investigated. Because these two modes are operated with most of the generative power that is circulated between the G and M. The energy efficiency of the entire system can be evaluated according to the so-called tank-to-wheel efficiency, htw, neglecting the charging and discharging power from the ESS. This parameter is defined as the ratio of the available driving power of the W, Pw, to the power stored in the fuel, Pf, and is expressed in the following equation:
htw ¼
Pw Pf
(17)
where the charging and discharging power of the ESS is neglected. A global search method [25] of the optimal operating points with a maximal tank-to-wheel efficiency was performed, covering the entire operating range of the power-split e-CVT and engine-driven modes; the PDC system and THS were comparatively assessed to understand the operating characteristics and potential of the PDC system.
Table 4 Values of various design parameters for THS.
Parameter
Value
Parameter
Value
k rf rch re=g
2.0 6.8 1.0 2.2
k rf rch re=p
1.9 5.2 1.0 2.2
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The simulation algorithms for the PDC system are expressed as follows: 1. For a specific v, one specific value of the W driving force is initially selected within an affordable range. Tw is calculated according to Eq. (16) and uw is obtained as shown in the following:
uw ¼
v 2p$R
(18)
2. Tm can thus be computed according to Eq. (10). 3. By varying um in favor of optimal e-CVT control, a value of um in an allowable range is chosen, and the values of ug and ue are then calculated using Eq. (3). 4. Because Tm and um are known, hm can be determined by referring to the performance map of the M. However, an iteration process must be performed to identify the related operating point of the G, where the corresponding value of hg must satisfy the condition of Eq. (8). 5. After hg are solved through an iteration, Tg and Te can be determined using Eqs. (11) and (12). 6. As Te and ue are calculated, the corresponding value of bsfc in grams per kilowatt hours can be determined by locating the operating point on the performance map. 7. The tank-to-wheel efficiency htw is obtained using the following equation:
htw ¼ a
Pw bsfc$Pe $Qf
(19)
where a is a conversion factor (¼3.6 109) and Qf is the heating value of the fuel (¼4.3 107 J/kg). By repeating Steps 3e7 with various values of um for a given v, a maximal outcome from a set of calculated htw values can be obtained. Accordingly, the corresponding value of um as well as those of ug and ue are treated as the optimal operating points at this specific v and Fw. The simulation algorithm for the THS is similar to that for the PDC system except that ug, rather than um, is varied to determine the optimal operating point with a maximal htw, and the corresponding operating conditions of the M, G, and E can be regarded as the optimal operating points at the given v and Fw.
Fig. 9. Driving performance with respect to maximum driving force for PDC system.
engine-driven mode rather than the power-split e-CVT mode enables the maximal driving force to be achieved in a speed range between 25 and 72 km/h. The corresponding driving performance for the THS is shown in Fig. 10. The single-motor EV mode for the THS did not satisfy the required gradeability at low speed and reached a low maximal speed of only 58 km/h. The reason for this insufficiency in pure EV drive is that one single-stage reduction ratio of the final drive F must be determined according to a tradeoff between sufficient driving force for hill climbing at low speeds and the functionality of power-split e-CVT over a wide range of medium to high speeds. 4.2. Optimal efficiency and operation point The contour map of the maximal htw versus v and Fw calculated in Section 3.3 is shown in Fig. 11(a). It shows that the operating region with an htw greater than 20% is considerably broad, covering most of the available area except for a small area of cruising and light acceleration for v at speeds below 55 and above 83 km/h. The related contour map of z versus v and Fw is shown in Fig. 11(b). It reveals that most of the operating area is in the power-split e-CVT mode (i.e., z > 0), except for one small region with cruise operation and light
4. Simulation results and discussion In this study, to determine the application potential of the proposed system, a simulation of the PDC based on a conventional 125-cc motorcycle was conducted and the THS architecture was for comparison. 4.1. Driving performance comparison The driving performance of the PDC system is illustrated in Fig. 9. Because both the M and G provide propulsive power in the dual-motor EV mode, hills can be climbed at a low speed and the maximal speed in pure EV drive can reach 70 km/h. This superior performance in pure EV drive makes the PDC system especially suitable for a plug-in hybrid design. Moreover, operation in the
Fig. 10. Driving performance with respect to maximum driving force for THS.
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Fig. 11. Contour maps of (a) maximal htw and (b) z vs. v and Fw for PDC.
acceleration at v < 35 km/h and another region with a high load at low and medium speeds. This also indicates that the operating points of the E are highly concentrated in the high-efficiency zone. Furthermore, the highest z occurring along the line for cruise driving indicates that maximal benefits of using circulating power are gained when the vehicle is driven at a constant medium or high speed (i.e., v > 40 km/h) or with light acceleration. The contour map of the maximal htw versus v and Fw for the THS is shown in Fig. 12(a). It exhibit a large area of operation with htw > 0.2. The related contour map of z versus v and Fw for the THS is shown in Fig. 12(b). Moreover, for the THS, the engine-driven mode rather than power-split e-CVT mode dominates over a wide range of medium and high speeds (v > 32 km/h) and with a light to medium driving load (50 N < Fw < 250 N). In other words, most of the operations in hybrid mode cannot be benifitted by using electric circulation, causing the corresponding operating points of the E remain in the lower-efficiency zone.
4.3. Fuel economy It has been widely accepted that the fuel economy of a full HEV with a power-split e-CVT (e.g., Toyota Prius) is higher in urban driving than in highway driving. When driving on highways, the vehicle mostly runs at a constant speed, and the charging and discharging load of the ESS is low. The E acts as the sole supplier of driving power, while the G and M circulate only the power extracted from the E and adjust its operating points. Therefore, the vehicle cannot benefit from the frequent use of the M as the main high-efficiency driving device as it does in urban driving. Therefore, driving at a constant speed and in the power-split e-CVT and engine-driven modes without substantially charging or discharging the ESS is crucial for fuel economy in both the PDC system and the THS. The fuel economy FE for cruising can be obtained from the simulated results of htw as expressed in the following:
Fig. 12. Contour maps of (a) maximum htw and (b) z vs. v and Fw for THS motorcycle.
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Fig. 13. (a) Fuel economy vs. vehicle speed and (b) percentage of improvement for fuel economy for PDC with respect to THS at cruise driving.
rf $Qf FE ¼ htw Fw
(20)
where FE is in kilometers per liter and rf is the density of fuel (760 kg/m3). Fig. 13(a) and (b) show the simulated results for fuel economy and the corresponding improvements of the PDC system with respect to the THS system in cruising driving. Improvements of more than 10% and a maximum of 17% are attained over a wide speed range of 50 km/h to 70 km/h. This may be because the optimal operating points for the PDC system are located considerably farther away from those of the THS and lie in a region with considerably higher E efficiency. By contrast, the performance of the PDC system at low cruising speeds (i.e., v < 45 km/h) and at high cruising speeds (i.e., v > 82 km/h) is lower than that of the THS. In this zone, the engine-driven mode is favored because the gain in the bsfc of the E attained by shifting the operating point upward by using circulating power is insufficient to compensate for the loss consumed along this electric power path. Furthermore, although the PDC system exhibits poor fuel economy at 40 km/h, its dualmotor EV mode, which supports pure electric drive at a speed of up to 40 km/h, provides a favorable alternative enabling frequent operation of a hybrid mode at 40 km/h to be avoided and the prevailing plug-in function to be added. 5. Conclusion The novel full hybrid PDC system proposed in this paper is based on a revision of the PDOC system presented in Ref. [23] and features a similar capability to adopt a power-split e-CVT strategy. A performance simulation was conducted and the results were compared with those of the THS to evaluate the feasibility and potential of the proposed system. The major characteristics of the PDC system, calculated results, and contributions are summarized as follows: 1. Compared with the single-motor EV mode for the PDOC system and the THS, the dual-motor EV mode accomplished by the automatic lock of the OC in the PDC system provides superior electric drive performance. This facilitates enhancing flexibility in an energy management strategy to optimize the switch
2.
3.
4.
5.
between the EV and hybrid mode, and thus, the proposed system may be suitable for plug-in application. According to the simulation results, when identical E, M, and G specifications are applied, the PDC system can achieve greater gradeability as well as synergy drive in the boost mode than the THS can. According to a global search for the optimal operation over the entire driving range in both the engine-driven and power-split e-CVT modes, the PDC system can be efficiently operated by taking full advantage of the electric circulation for power-split eCVT, ranging from cruise driving to maximal traction, especially at medium and high vehicle speeds, whereas the THS operates mostly in the engine-driven mode. The optimized results showed that the well-to-wheel efficiency was mainly operated above 20%, while the power split e-CVT mode was the major operation mode. The optimal fuel economy in cruise driving achieved by the PDC system is superior to that of the THS (e.g., a maximum of 17% fuel economy improvement, at most medium and high speeds except for the low and high ends). Based on a power-split e-CVT architecture, the PDC system requires a low battery capacity, reducing manufacturing and maintenance costs. The outstanding driving performance and fuel economy of the PDC system compared with that of the THS system suggest that motorcycles based on the PDC system warrant further development, especially regarding plug-in and sport hybrid designs.
Acknowledgments The authors would like to thank the Ministry of Science and Technology of the Republic of China, Taiwan for its financial support for this research under Contract No.: 103-2221-E-003-022-. References [1] Johansson B, Ahman M. A comparison of technologies for carbon-neutral passenger transport. Transp Res Part D 2002;7:175e96. [2] Chau KT, Wong YS. Overview of power management in hybrid electric vehicles. Energy Conv Manag 2002;43:1953e68. [3] Bayindir KC, Gozukucuk MA, Teke A. A comprehensive overview of hybrid electric vehicle: powertrain configurations, powertrain control techniques and electronic control units. Energy Conv Manag 2011;52:2393e404.
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[4] Sun H, Yang L, Jing J. Hydraulic/electric synergy system (HESS) design for heavy hybrid vehicles. Energy 2010;35:5328e35. [5] Garcia HE, Mohanty A, Lin WC, Cherry RS. Dynamic analysis of hybrid energy systems under flexible operation and variable renewable generationdpart I: dynamic performance analysis. Energy 2013;52:1e16. [6] Skugor B, Deur J. Dynamic programming-based optimization of charging an electric vehicle fleet system represented by an aggregate battery model. Energy 2015;XXX:1e10. [7] Liu J, Peng H. Control optimization for a power-split hybrid vehicle. In: American control conference, Minneapolis; June 2006. p. 14e6. [8] Amjad S, Rudramoorthy R, Neelakrishnan S, Sri Raja Varman K, Arjunan TV. Evaluation of energy requirements for all-electric range of plug-in hybrid electric two-wheeler. Energy 2011;36:1623e9. [9] Khayyam H, Bab-Hadiashar A. Adaptive intelligent energy management system of plug-in hybrid electric vehicle. Energy 2014;69:319e35. [10] Arslan O, Karasan OE. Cost and emission impacts of virtual power plant formation in plug-in hybrid electric vehicle penetrated networks. Energy 2013;60:116e24. [11] Bradley TH, Frank AA. Design, demonstrations and sustainability impact assessments for plug-in hybrid electric vehicles. Renew Sustain Energy Rev 2009;13:115e28. [12] Koichiro K, Yamazaki M, Tokieda J. Development of new-generation hybrid system THS II e drastic improvement of power performance and fuel economy. Paper#2004-01-0064, SAE 2004 World Congress & Exhibition. 2004. [13] Sasaki S. Toyota's newly developed hybrid powertrain. In: Proceedings of the 1998 IEEE 10th international symposium on power semiconductor devices & IC's, Kyoto, Japan; 3e6 June, 1998. [14] Kimura A, Ando I, Itagaki A. Development of hybrid system for SUV. In: 2005 SAE world congress, SAE 2005-01-0273, Detroit, Michigan; April 11e14, 2005.
[15] Gelb GH, Richardson NA, Wang TC, Berman B. An electromechanical transmission for hybrid vehicle power trains e design and dynamometer testing. In: Society of automotive engineers, paper No. 710235, SAE Congress, Detroit, MI; January 11e15, 1971. [16] Liu J, Peng H. Modeling and control of a power-split hybrid vehicle. IEEE Trans. Control Syst Tech 2008;16:1242e51. [17] Miller MA, Holmes AG, Conlon BM, Savagian PJ. The GM “Voltec” 4ET50 multimode electric transaxle. SAE Int J Eng 2011;4:1102e14. [18] Bing H, Tong Z, Yixi G. Development of a single-mode full hybrid system. In: 2011 int. conf. on IEEE electr. inf. and control eng (ICEICE); 2011. April 15e17, 2108-11. [19] Miller JM. Hybrid electric vehicle propulsion system architectures of the eCVT type. IEEE Trans Power Electron 2006;21:756e67. [20] Sheu KB, Hsu TH. Design and implementation of a novel hybrid-electricmotorcycle transmission. Appl Energy 2006;83:959e74. [21] Ceraolo M, Caleo A, Capozzella P, Marcacci M, Carmignani L, Pallottini A. A parallel-hybrid drive-train for propulsion of a small scooter. IEEE Trans Power Electron 2006;21:768e78. [22] Hsu YY, Lu SY. Design and implementation of a hybrid electric motorcycle management system. Appl Energy 2010;87:3546e51. [23] Chung CT, Hung YH. Energy improvement and performance evaluation of a novel full hybrid electric motorcycle with power split e-CVT. Energy Conv Manag 2014;86:216e25. [24] Ehsani M, Gao Y, Williamson SE, Lukic SM. Hybrid electric, and fuel cell vehicles: fundamentals, theory, and design. Boca Raton, FL: CRC; Dec. 2004. [25] Hung YH, Wu CH. An integrated optimization approach for a hybrid energy system in electric vehicles. Appl Energy 2012;98:479e90.
Contents lists available at ScienceDirect
Energy journal homepage: www.elsevier.com/locate/energy
Performance and energy management of a novel full hybrid electric powertrain system Cheng-Ta Chung a, Yi-Hsuan Hung b, * a b
Department of Vehicle Engineering, National Formosa University, Yunlin, 63201, Taiwan Department of Industrial Education, National Taiwan Normal University, Taipei, 106, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 December 2014 Received in revised form 27 May 2015 Accepted 30 May 2015 Available online 2 July 2015
This study compared the performance and energy management between a novel full hybrid electric powertrain and a traditional power-split hybrid system. The developed planetary gearset and dual clutch configuration provides five operation modes. Equations for the torque and speed of power sources for the planetary gearset and dual clutch system and the Toyota Hybrid System are firstly derived. By giving vehicle performance of gradability, maximal speeds in hybrid and pure electric modes, the power sources of the 210 kg target vehicle are: a 125 cc engine and two 1.8 kW motor and generator. The optimal tankto-wheel efficiencies, ratios of circulating power, and operation points at specific vehicle speeds and out loads are calculated. Simulation results show that the dual-motor electric vehicle mode offers superior performance regarding electric drive; the low capacity of the battery is conducive to reducing manufacturing and maintenance costs; the tank-to-wheel efficiency is mainly operated above 20% while the power split electronic-continuously-variable-transmission mode is the major operation mode, and a maximum of 17% fuel economy improvement is achieved compared with the Toyota Hybrid System in most of the vehicle speed ranges. The outstanding performance warrants further real-system development, especially regarding the implementation in plug-in and sport hybrid powertrain designs. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Powertrains Energy management Power-split hybrid Simulation Optimization
1. Introduction Recently, fulfilling the demand on high energy density of batteries for extending the cruising mileage of electric vehicles remains a challenge. Developing engine/motor HEVs (hybrid electric vehicles) with advanced management of energy efficiency is still one of the most favorable solutions with regard to environmental issues. For example, in Ref. [1], the comparative study of the use of energy carrier based on renewable energy sources indicates that energy-efficiency improvements with the use of renewable energy would have significant influence on energy environmental cost. For HEV system design and control, in Ref. [2], an overview of power management of HEVs was studied; in Ref. [3], HEV configurations, energy management strategies and electronic control units were compared. In Ref. [4], a hydraulic/electric synergy system for heavy hybrid vehicles was designed with the multi-energysource power distribution. In Ref. [5], a dynamic analysis on energy
* Corresponding author. E-mail addresses: [email protected] (C.-T. Chung), [email protected] (Y.-H. Hung). http://dx.doi.org/10.1016/j.energy.2015.05.151 0360-5442/© 2015 Elsevier Ltd. All rights reserved.
management of hybrid systems was conducted. Furthermore, various optimization techniques for energy management of HEVs or EVs (electric vehicles) have been exploited. For instance, in Ref. [6], an optimization method based on dynamic programming was utilized for EV fleet; in Ref. [7], a control optimization of a power-split HEV was carried out. PHEVs (plug-in HEVs) may receive much attention in the near future because of their high EV-mode range and chargeable characteristics. Amjad et al. [8] evaluated battery energy and power requirements for a plug-in HEV for different EV-mode range. Khayyam et al. [9] proposed an adaptive intelligent energy management system for PHEV to further reduce fuel consumption and emissions. Arslan et al. [10] introduced a concept of virtual power plant to investigate the impacts on mass penetration of PHEVs into the electricity grid. Bradley et al. [11] reviewed the design of PHEVs and assessed their sustainability. By recognizing the benefits of PHEVs, one aim of this research is to develop a new type of power-split HEV that not only enhances performance and energy efficiency but also provides excellent EV drive suitable for plug-in applications. Compared with serial and parallel powertrain configurations, the power-split configuration is characterized by qualities such as high overall energy efficiency, low levels of pollutants and fuel
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consumption, and a flexible power distribution. The THS (Toyota Hybrid System) are the most well-known powertrains that have been used to equip commercial vehicles for a long time [12]. Sasaki [13] optimized engine fuel consumption to minimize exhaust emissions and to apply regenerative braking for the THS-I. To enhance the driving performance, the THS-II was developed. It improved the driving performance by enhancing the power density of the electric drives and boosting the voltage of the energy source [14]. The key technology, namely power-split electronic continuously variable transmission (power-split e-CVT), flexibly facilitates energy management by one engine and two motor/generators [15]. Because the THS configuration is the most widely used hybrid configuration employed in commercial applications, it is used for performance comparison. Other configurations of advanced powertrains for other international automakers have been researched in previous studies. In Ref. [16], a power-split HEV was compared to the THS and control rules were optimized. In Ref. [17], the four operation modes and two-speed transmission of the GM multimode electric transaxle was proposed, and a so-called single mode full hybrid system was proposed in Ref. [18]. In Ref. [19], comparison of technologies of e-CVT transmission for Toyota, Ford, and General Motors companies were conducted. Considering the future commercialization of the developed powertrain in Taiwan, light-duty HEVs are selected in this research. In Ref. [20], two planetary gear sets and a chain set combined with an engine and a motor/generator were equipped for four operation modes to maximize the vehicle performance and to reduce emissions. In Ref. [21], a novel parallel-hybrid drive train was developed for a light-duty scooter. In Ref. [22], a control unit for an engine/motor hybrid scooter that effectively reduced pollutants was implemented. In a similar previous study, the PDOC (planetary gearset and dual one-way clutch) hybrid electric system with power-split eCVT have been analyzed [23]. The results demonstrated a superior driving performance and fuel economy in cruise driving, especially compared with those achieved using the conventional rubber-belt CVT system (a maximum of 32% fuel economy improvement). In this paper, a new hybrid powertrain with a modified PDOC, called the PDC (planetary gearset and dual clutch) hybrid electric system, is proposed. The PDC system features the engine-driven and power-split e-CVT operating modes, similar to those of a PDOC system. However, one major advantage of the PDC system is the capability to operate the pure electric drive using two motors since its motor and generator are arranged as a torque coupler; the PDOC system does not have this capability because its motor and
627
generator are connected in a speed-differential manner in the pure electric drive. Thus, the PDC system is suitable for plug-in applications. The performance and operation characteristics of the PDC system were evaluated through simulation and compared with those of the THS to determine the potential and feasibility for various applications. The remainder of this paper is organized as follows: Section 2 presents a comparison of the system design and operation modes between our powertrain and the THS. Section 3 describes performance indices and the search for optimal operation points (energy management) of the PDC system and THS. Section 4 discusses the vehicle performance and optimal energy management according to the concepts and indices described in Sections 2 and 3. Section 5 presents the conclusion and lists the contributions of this study.
2. Powertrain architecture and operation modes 2.1. Comparison of the architecture of the PDC system and that of the THS The powertrain architecture and a power system diagram of the PDC system are shown in Fig. 1. A PDC system consists of an engine (E), a generator (G), a driving motor (M), a set of planetary gears (i.e., power splitter) (P), a CC (controllable clutch), a OC (one-way clutch), and several components arranged to form a complete drive train. The planetary carrier of the planetary gear (c) is connected to the G by a chain (Ch). The E is connected to the G by a reduction gear (Gr). The CC is coupled to the output shaft of the G and connects or disconnects the G to or from the Gr. The sun gear of the planetary gear (s) is connected to the M through the OC. The ring gear of the planetary gear (r) is connected to the wheel (W) through a final drive (F). A power control unit consists of power drivers for the M and G and a power electric device for transmitting and modulating the electric power among the ESS (energy storage system), M, and G. The ESS may be a battery, ultracapacitor, or both. The OC is placed between the s and c of the P to prevent relative rotation when the s operates faster than the c does. A schematic diagram of a THS motorcycle is shown in Fig. 2. As in the THS, the E, M, and G are mechanically connected to the c, r, and s, respectively. However, in adapting the system to a 125 cc motorcycle, a slightly different arrangement was used. The E was connected using a Gr. Additionally, the M was connected using a Ch and coupled to the real W through a final drive (F).
Fig. 1. (a) Powertrain architecture and (b) schematic diagram of the PDC system.
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(a) Dual-motor electric vehicle mode When started and driven at low speed, PDC is operated at a dual-motor EV (electric vehicle) mode that both the G and M can offer driving torque for enhanced electric drive. This is achieved by locking the sun gear of the P connected to the M and the planetary carrier of the P linked to the G against relative rotation using the one-way clutch OC, as shown in the speed diagram of the P in Fig. 3(a) and the power flow diagram of the PDC in Fig. 3(b), where Zr and Zs represents the number of teeth of the ring gear and the sun gear respectively. In addition, the CC is disengaged so that the power flow between the E and G is disconnected and no further driving effort is consumed to drive the unfired E. The driving torque of the W, Tw, transferred from that of M, Tm, and G, Tg, in the dualmotor EV mode can be expressed as:
Tw ¼ rf $hf $hp $Tm þ rf $rch $hf $hch $hp $Tg
(1)
where r and h indicate reduction ratio and transmission efficiency respectively. Moreover, the subscripts f, p, ch represent F, P and Ch respectively. (b) Switching CVT mode
Fig. 2. Powertrain architecture of the THS motorcycle.
2.2. Operation modes of the PDC system and the THS This section presents a comparison of the operation modes of a PDC system and those of the THS. Because the configurations differ, the operation modes vary. The PDC system contains five modes, whereas the THS operates with four modes.
2.2.1. Operation modes of the PDC system The PDC system is operated through five modes, namely the dual-motor electric vehicle (dual-motor EV), switching CVT (sCVT), engine-driven, power-split e-CVT, and boost modes. To determine which of these modes should be in operation, the road conditions and performance requirements regarding the driving of the vehicle are required.
The switching CVT (s-CVT) mode can be regarded as a transient switching from the dual-motor EV mode to the hybrid modes, (i.e. the engine-driven and the power-split e-CVT modes). As the vehicle speed is increased to a switching point, the E is started and the M acts as a generator to release the OC from locking, freeing the P from direct drive. Subsequently, a function of CVT can be performed because the rotation speed of the c, uc, and that of the r, ur, can be continually varied by modulating the rotation speed of the s, us, as shown in Fig. 4. The speed relationship among them is stated as follows:
uc ¼
1 k us þ ur ; kþ1 kþ1
(2)
where k is the basic ratio of the planetary gear, defined as Zr/Zs. The corresponding speed relationship for M, G, E, and W is described as follows:
ug ¼
ue 1 k um þ r $uw ¼ kþ1 f re=g k þ 1
where the subscript e/g represents Gr.
Fig. 3. (a) Speed diagram for the power splitter and (b) power flow diagram of PDC at dual-motor EV mode.
(3)
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Fig. 4. (a) Speed diagram for the power splitter and (b) power flow diagram of PDC at power split e-CVT mode.
Fig. 5. (a) Speed diagram for the power splitter and (b) power flow diagram of PDC at engine-driven mode.
In principle, the ratio of the generative load of M, Tm, to the driving torque of E, Te, is fixed. The parameter Tm is expressed as follows:
where the subscripts ch and e/g represent the Ch and Gr, respectively. (d) Power-split e-CVT mode
Tm
re=g $rch $he=g $hch $hp Te ¼ kþ1
(4)
(c) Engine-driven mode As the sun gear of the P and M are held stationary, as shown in Fig. 5, the vehicle operates in the engine-driven mode and is driven by the E alone. The G may act as a generator for charging the ESS when required. As in the PDOC system, the engine-driven mode rather than the power-split e-CVT mode is preferably applied under low-speed and high-load conditions in hybrid driving to improve fuel economy. The driving torque of the W, Tw, transmitted from that of the E, Te, in this mode can be expressed as follows:
k $r $r $h $h $h $h $Te ; Tw ¼ rf $ k þ 1 ch e=g f p ch e=g
(5)
The power output of the E, Pe, splits into two paths. One path leads toward the P via the Ch; the other leads toward the G to generate electric power, as expressed in the following:
Pe ¼
Pg Pp þ he=g hch
(6)
Generating electricity through the G has two functions. A small fraction of Pg is used to charge the ESS and, thus, supplement the energy in the ESS consumed primarily in the EV mode. Most of Pg directly flows to the M; therefore, modulating the rotation speed of the M, um, provides additional degree-of-freedom that can be used to optimize the operation of the entire powertrain. The power relationship among the G, M, and ESS is described as follows:
Pg ¼
Pm Pess þ hg $hm hg $hchg
(7)
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where the subscripts ess and chg represent the ESS and charging, respectively. The power-split e-CVT mode rather than the engine-driven mode functions as the primary operating strategy in hybrid driving, especially at medium and high speeds, because it enables high flexibility and the capability to modulate the engine speed as well as engine load according to the maximal efficiency of the entire system. This is achieved through electric circulation; specifically, a portion of the engine power is circulated in the form of electrical power through the G and M without excessively charging or discharging the ESS. Here, Pg flows to the M; thus, Eq. (7) becomes
Pg ¼
Pm hg $hm
(8)
The operating speed of the E is lowered by increasing the rotation speed of the M in reverse, as shown in Fig. 6. Performing this procedure and widening the throttle opening of the E by raising the generating load of the G may greatly reduce the brakespecific fuel consumption (bsfc in grams per kilowatt hours) of the E, providing compensation for the loss of circulating driving power and yielding a net gain in the efficiency of the entire system. For a given driving torque and rotation speed of the W, Tw and uw, under a certain driving condition, a value of the rotation speed of the M, um, may be initially set for the e-CVT control, and the corresponding rotation speed of the E and G can be determined using the following equation:
ug ¼
ue 1 k um þ r $uw ¼ kþ1 f re=g k þ 1
1 Tw k$rf $hp $hf
1 um Tm hg $hm ug
(12)
(e) Boost mode If the required driving torque exceeds the maximal available torque when a hybrid mode is engaged (i.e., s-CVT, engine-driven, or power-split e-CVT mode), then additional power supplied by the ESS provides a short-term boost effect, depending on the amount of energy remaining in the ESS. In this boost mode, the G provides this supplemental torque. However, the maximal torque capacity of the G may not be fully used, because there is a constraint regarding the torque condition among all components of the P, as expressed in the following:
Ts : Tc : Tr ¼ 1 : k þ 1 : k
(13)
Consequently, the torque generated by the G is limited by the following condition:
Tg
kþ1 $Tm;max re=g $he=g $Te;max rch $hch
(14)
where Tm,max and Te,max are the maximal torque of the M and E, respectively.
2.2.2. Modified THS operation modes
(10)
According to Eq. (8) and P ¼ 2p$u$T, the generative torque of G is calculated using
Tg ¼
1 kþ1 Tg þ Tw re=g $he=g re=g $rch $rf $he=g $hch $hp $hf
(9)
The required torque of the M is calculated as follows:
Tm ¼
Te ¼
(11)
Finally, the required torque output of the E for the power-split eCVT control is expressed as follows:
The operating characteristics of the THS have been extensively described in numerous documents. The operation in pure electric drive by using the THS is shown in the speed diagram for the power splitter of Fig. 7(a). The vehicle is solely driven by the M, while the G is rotated in reverse to hold the E without delivering traction power. This operating mode is called the single-motor EV mode compared to the dual-motor EV mode for the PDC system. Moreover, the inadequate traction effort provide by merely one motor can be supplemented by turning the E on using the G as a starter. This operation is called the engine-assisted EV mode and is also shown in Fig. 7(a). Under normal driving conditions at medium and high vehicle speeds, the THS motorcycle is mostly operated in either the enginedriven mode or the power-split e-CVT mode, as illustrated in
Fig. 6. (a) Speed diagram for the power splitter and (b) power flow diagram of PDC at power split e-CVT mode.
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Fig. 7. Speed diagram for the power splitter of THS at various modes.
Fig. 7(b). When operated in the engine-driven mode, the vehicle is driven by the engine alone with the G held stationary. In the powersplit e-CVT mode of the THS, the vehicle is operated through circulation of the engine power in the form of electric power through the G and M. Therefore, extra degree-of-freedom in varying the engine speed and load by changing the rotation speed of the G (ug) as well as the corresponding power of circulation provides an opportunity to determine the optimal operating point of energy efficiency for a particular driving condition. In addition, as in the PDC system, a boost mode can be engaged to provide an instantaneous and enhanced synergy drive by using the extra electric power extracted from the ESS. The related equations for the operation modes of the modified THS can be referred to Eqs. (1)e(14). 3. Power circulation ratio, parameter evaluation, and optimal operation points 3.1. Power circulation ratio and required traction torque To quantize the degree of power split in the simulation, a parameter called the ratio of circulating power (z) is used and defined as the generative power circulated through the G and M, Pg, in CVT operation divided by the engine power Pe:
z¼
Pg Pe
(15)
This parameter represents the proportion of engine power that is converted to electric power circulated from the G to the M in CVT operation. However, increasing this proportion leads to a greater loss of energy conversion. Therefore, a tradeoff in the efficiency of the entire system was analyzed. The power-split e-CVT mode was operated under a condition of z > 0; the engine-driven mode was regarded as a specific case of power-split e-CVT with z ¼ 0. To evaluate the road load of the vehicle, the traction torque of the W, Tw, in the simulation was modified from Ref. [23] and is expressed in the following equation:
Tw ¼
1 rACd v2 þ mMt g cos q þ Mt g sin q þ Mt a $R 2
(16)
where r is the density of air, A is the frontal area of the test vehicle, Cd is the drag coefficient, n is the vehicle speed in meters per
second, m is the coefficient of rolling resistance, Mt is the test mass of the vehicle, g is the acceleration caused by gravity, q is the slope angle of climbing in degrees, a is the acceleration of the vehicle in meters per second squared, and R is the radius of the W.
3.2. Parameter evaluation The values of the coefficients and design variables in Eq. (16) used for the simulation are listed in Table 1. A standard commercial 125-cc motorcycles from Taiwanese motorcycle manufacturers is used for comparison, where the parameter values are referred to [23]. A 125-cc single-cylinder gasoline engine with a maximal torque of 9.0 N m at 6500 rpm and a maximal power of 6.8 kW at 7500 rpm was used as the E. A performance map of its torque and brakespecific fuel consumption (bsfc in grams per kilowatt hour) versus rotation speed is illustrated in Fig. 8(a). The efficiencies of the transmission components were set to be constant and are listed in Table 2. The maximal torque, rated power, and maximal rotation speed of the M and G were determined according to their efficiency maps scaled from that of a permanent magnet synchronous motor/ generator used for HEVs and modified from the map in Ref. [23]. In addition, the design parameters of the PDC powertrain and the baseline system were determined, such as k, re/g, rch, and rf. The performance requirements for determining the aforementioned values are described as follows: 1. The vehicle is required to maintain a cruising speed of 10 km/h when driving uphill at q ¼ 12 (or grade ¼ 21.3%). 2. The switching point of the vehicle speed between the EV and a hybrid mode is at approximately 20 km/h. 3. The maximal vehicle speed at which the vehicle can operate purely in the EV mode is approximately 40 km/h.
Table 1 Values of various coefficients and design variables in Tw. Parameter
Value
1 rAC d 2
0.279 N/(m/s)2 0.013 210 kg 9.81 m/s2 0.215 m
m
Mt g R
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Fig. 8. Performance maps of (a) the engine [23] and (b) the motor and the generator.
3.3. Optimal operation points
Table 2 Efficiency of various components. Component
Efficiency
Final drive Power splitter Clutch Chain Reduction gear
hf ¼ 0.95 hp ¼ 0.95 hc ¼ 0.95 hch ¼ 0.95 he/g ¼ 0.95 he/p ¼ 0.95
4. The maximal vehicle speed at which the vehicle can operate in a hybrid mode is approximately 90 km/h. 5. The total reduction ratio of the powertrain in a hybrid mode must be carefully adjusted to provide a sufficient driving force covering the entire range between 20 and 90 km/h. 6. The power capacity of the M and G must be sufficient, at least to afford the circulating load in Eq. (8) required for power-split eCVT operation, especially at a high speed. 7. Adopting the same type of motor/generator for both the M and G is favorable because of the simplicity of operation and control. The proposed PDC hybrid system was treated as the primary system in selecting the suitable values of the design parameters and the desired specifications of both the M and G. The rated maximal torque, rated power, and maximal rotation speed of the M and G were 8.5 N m, 1.8 kW, and 6000 rpm, respectively. The corresponding performance map of torque and efficiency versus rotation speed is shown in Fig. 8(b). The design values of the related parameters for the PDC system are listed in Table 3. For the same specifications of the E, M, and G, the related design parameters of the THS powertrain were evaluated and are listed in Table 4.
Table 3 Values of various design parameters for PDC.
According to a classification proposed by Miller [19], the PDC system involves a type of output split, whereas the THS involves a type of input split. As for the other definition documented in the work of M. Ehsani etc. [24], both are a type of integrated speedcoupling and torque-coupling hybrid electric drive train. However, PDC consists of a speed coupler connected to output, whereas THS possesses a torque coupler linked to output. Therefore, a comparative assessment of the operating characteristics of these two distinct systems is valuable. The energy efficiencies over the entire operating region of the power-split e-CVT and engine-driven modes were investigated. Because these two modes are operated with most of the generative power that is circulated between the G and M. The energy efficiency of the entire system can be evaluated according to the so-called tank-to-wheel efficiency, htw, neglecting the charging and discharging power from the ESS. This parameter is defined as the ratio of the available driving power of the W, Pw, to the power stored in the fuel, Pf, and is expressed in the following equation:
htw ¼
Pw Pf
(17)
where the charging and discharging power of the ESS is neglected. A global search method [25] of the optimal operating points with a maximal tank-to-wheel efficiency was performed, covering the entire operating range of the power-split e-CVT and engine-driven modes; the PDC system and THS were comparatively assessed to understand the operating characteristics and potential of the PDC system.
Table 4 Values of various design parameters for THS.
Parameter
Value
Parameter
Value
k rf rch re=g
2.0 6.8 1.0 2.2
k rf rch re=p
1.9 5.2 1.0 2.2
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The simulation algorithms for the PDC system are expressed as follows: 1. For a specific v, one specific value of the W driving force is initially selected within an affordable range. Tw is calculated according to Eq. (16) and uw is obtained as shown in the following:
uw ¼
v 2p$R
(18)
2. Tm can thus be computed according to Eq. (10). 3. By varying um in favor of optimal e-CVT control, a value of um in an allowable range is chosen, and the values of ug and ue are then calculated using Eq. (3). 4. Because Tm and um are known, hm can be determined by referring to the performance map of the M. However, an iteration process must be performed to identify the related operating point of the G, where the corresponding value of hg must satisfy the condition of Eq. (8). 5. After hg are solved through an iteration, Tg and Te can be determined using Eqs. (11) and (12). 6. As Te and ue are calculated, the corresponding value of bsfc in grams per kilowatt hours can be determined by locating the operating point on the performance map. 7. The tank-to-wheel efficiency htw is obtained using the following equation:
htw ¼ a
Pw bsfc$Pe $Qf
(19)
where a is a conversion factor (¼3.6 109) and Qf is the heating value of the fuel (¼4.3 107 J/kg). By repeating Steps 3e7 with various values of um for a given v, a maximal outcome from a set of calculated htw values can be obtained. Accordingly, the corresponding value of um as well as those of ug and ue are treated as the optimal operating points at this specific v and Fw. The simulation algorithm for the THS is similar to that for the PDC system except that ug, rather than um, is varied to determine the optimal operating point with a maximal htw, and the corresponding operating conditions of the M, G, and E can be regarded as the optimal operating points at the given v and Fw.
Fig. 9. Driving performance with respect to maximum driving force for PDC system.
engine-driven mode rather than the power-split e-CVT mode enables the maximal driving force to be achieved in a speed range between 25 and 72 km/h. The corresponding driving performance for the THS is shown in Fig. 10. The single-motor EV mode for the THS did not satisfy the required gradeability at low speed and reached a low maximal speed of only 58 km/h. The reason for this insufficiency in pure EV drive is that one single-stage reduction ratio of the final drive F must be determined according to a tradeoff between sufficient driving force for hill climbing at low speeds and the functionality of power-split e-CVT over a wide range of medium to high speeds. 4.2. Optimal efficiency and operation point The contour map of the maximal htw versus v and Fw calculated in Section 3.3 is shown in Fig. 11(a). It shows that the operating region with an htw greater than 20% is considerably broad, covering most of the available area except for a small area of cruising and light acceleration for v at speeds below 55 and above 83 km/h. The related contour map of z versus v and Fw is shown in Fig. 11(b). It reveals that most of the operating area is in the power-split e-CVT mode (i.e., z > 0), except for one small region with cruise operation and light
4. Simulation results and discussion In this study, to determine the application potential of the proposed system, a simulation of the PDC based on a conventional 125-cc motorcycle was conducted and the THS architecture was for comparison. 4.1. Driving performance comparison The driving performance of the PDC system is illustrated in Fig. 9. Because both the M and G provide propulsive power in the dual-motor EV mode, hills can be climbed at a low speed and the maximal speed in pure EV drive can reach 70 km/h. This superior performance in pure EV drive makes the PDC system especially suitable for a plug-in hybrid design. Moreover, operation in the
Fig. 10. Driving performance with respect to maximum driving force for THS.
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Fig. 11. Contour maps of (a) maximal htw and (b) z vs. v and Fw for PDC.
acceleration at v < 35 km/h and another region with a high load at low and medium speeds. This also indicates that the operating points of the E are highly concentrated in the high-efficiency zone. Furthermore, the highest z occurring along the line for cruise driving indicates that maximal benefits of using circulating power are gained when the vehicle is driven at a constant medium or high speed (i.e., v > 40 km/h) or with light acceleration. The contour map of the maximal htw versus v and Fw for the THS is shown in Fig. 12(a). It exhibit a large area of operation with htw > 0.2. The related contour map of z versus v and Fw for the THS is shown in Fig. 12(b). Moreover, for the THS, the engine-driven mode rather than power-split e-CVT mode dominates over a wide range of medium and high speeds (v > 32 km/h) and with a light to medium driving load (50 N < Fw < 250 N). In other words, most of the operations in hybrid mode cannot be benifitted by using electric circulation, causing the corresponding operating points of the E remain in the lower-efficiency zone.
4.3. Fuel economy It has been widely accepted that the fuel economy of a full HEV with a power-split e-CVT (e.g., Toyota Prius) is higher in urban driving than in highway driving. When driving on highways, the vehicle mostly runs at a constant speed, and the charging and discharging load of the ESS is low. The E acts as the sole supplier of driving power, while the G and M circulate only the power extracted from the E and adjust its operating points. Therefore, the vehicle cannot benefit from the frequent use of the M as the main high-efficiency driving device as it does in urban driving. Therefore, driving at a constant speed and in the power-split e-CVT and engine-driven modes without substantially charging or discharging the ESS is crucial for fuel economy in both the PDC system and the THS. The fuel economy FE for cruising can be obtained from the simulated results of htw as expressed in the following:
Fig. 12. Contour maps of (a) maximum htw and (b) z vs. v and Fw for THS motorcycle.
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Fig. 13. (a) Fuel economy vs. vehicle speed and (b) percentage of improvement for fuel economy for PDC with respect to THS at cruise driving.
rf $Qf FE ¼ htw Fw
(20)
where FE is in kilometers per liter and rf is the density of fuel (760 kg/m3). Fig. 13(a) and (b) show the simulated results for fuel economy and the corresponding improvements of the PDC system with respect to the THS system in cruising driving. Improvements of more than 10% and a maximum of 17% are attained over a wide speed range of 50 km/h to 70 km/h. This may be because the optimal operating points for the PDC system are located considerably farther away from those of the THS and lie in a region with considerably higher E efficiency. By contrast, the performance of the PDC system at low cruising speeds (i.e., v < 45 km/h) and at high cruising speeds (i.e., v > 82 km/h) is lower than that of the THS. In this zone, the engine-driven mode is favored because the gain in the bsfc of the E attained by shifting the operating point upward by using circulating power is insufficient to compensate for the loss consumed along this electric power path. Furthermore, although the PDC system exhibits poor fuel economy at 40 km/h, its dualmotor EV mode, which supports pure electric drive at a speed of up to 40 km/h, provides a favorable alternative enabling frequent operation of a hybrid mode at 40 km/h to be avoided and the prevailing plug-in function to be added. 5. Conclusion The novel full hybrid PDC system proposed in this paper is based on a revision of the PDOC system presented in Ref. [23] and features a similar capability to adopt a power-split e-CVT strategy. A performance simulation was conducted and the results were compared with those of the THS to evaluate the feasibility and potential of the proposed system. The major characteristics of the PDC system, calculated results, and contributions are summarized as follows: 1. Compared with the single-motor EV mode for the PDOC system and the THS, the dual-motor EV mode accomplished by the automatic lock of the OC in the PDC system provides superior electric drive performance. This facilitates enhancing flexibility in an energy management strategy to optimize the switch
2.
3.
4.
5.
between the EV and hybrid mode, and thus, the proposed system may be suitable for plug-in application. According to the simulation results, when identical E, M, and G specifications are applied, the PDC system can achieve greater gradeability as well as synergy drive in the boost mode than the THS can. According to a global search for the optimal operation over the entire driving range in both the engine-driven and power-split e-CVT modes, the PDC system can be efficiently operated by taking full advantage of the electric circulation for power-split eCVT, ranging from cruise driving to maximal traction, especially at medium and high vehicle speeds, whereas the THS operates mostly in the engine-driven mode. The optimized results showed that the well-to-wheel efficiency was mainly operated above 20%, while the power split e-CVT mode was the major operation mode. The optimal fuel economy in cruise driving achieved by the PDC system is superior to that of the THS (e.g., a maximum of 17% fuel economy improvement, at most medium and high speeds except for the low and high ends). Based on a power-split e-CVT architecture, the PDC system requires a low battery capacity, reducing manufacturing and maintenance costs. The outstanding driving performance and fuel economy of the PDC system compared with that of the THS system suggest that motorcycles based on the PDC system warrant further development, especially regarding plug-in and sport hybrid designs.
Acknowledgments The authors would like to thank the Ministry of Science and Technology of the Republic of China, Taiwan for its financial support for this research under Contract No.: 103-2221-E-003-022-. References [1] Johansson B, Ahman M. A comparison of technologies for carbon-neutral passenger transport. Transp Res Part D 2002;7:175e96. [2] Chau KT, Wong YS. Overview of power management in hybrid electric vehicles. Energy Conv Manag 2002;43:1953e68. [3] Bayindir KC, Gozukucuk MA, Teke A. A comprehensive overview of hybrid electric vehicle: powertrain configurations, powertrain control techniques and electronic control units. Energy Conv Manag 2011;52:2393e404.
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