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Development of New Plug-In Hybrid System for Compact-Class Vehicle.

1. INTRODUCTION

Automakers are expanding the availability of plug-in hybrid vehicles (PHVs) to help satisfy increasingly stringent automotive environmental standards around the world.

As part of this trend, the development of the new Prius Prime aimed to achieve an appealing vehicle capable of meeting the expectations of customers by combining world-class environmental performance (i.e., power consumption efficiency and fuel economy) with greatly improved dynamic performance in electric vehicle (EV) mode. This paper describes the development of this new plug-in hybrid system, as well as its components and systems.

2. DEVELOPMENT OBJECTIVE

2.1 Dynamic Performance in EV Mode and Environmental Friendliness

The first objective of this development was to create a highly efficient system with greatly improved dynamic performance in EV mode, while maximizing the performance of the hybrid system components in the new fourth generation Prius hybrid vehicle (HV) to maintain high-performance in HV mode [1]. At the same time, this development also aimed to improve the driving feeling of the PHV in EV mode. This high EV mode performance was achieved by increasing the output current and battery voltage of the power control unit (PCU), adding a new Dual Motor Drive System with a one-way clutch installed on the transaxle, and adopting a high-capacity battery pack system. In addition, a new battery warm-up system was developed to help improve EV mode dynamic performance in cold temperatures.

2.2 Charging System

Charging is an essential function for PHVs. This development increased the efficiency of the charging system and boosted the output of the charger with the aim of minimizing increases in charging time caused by the expanded EV mode range.

The charging system also features a new charging function based on weekly scheduling. The new function includes a timer and coordinated control between the pre-air conditioning and battery warm-up system to help improve user friendliness.

3. CONFIGURATION OF PLUG-IN HYBRID SYSTEM

Figure 1 shows the configuration of the plug-in hybrid system.

The engine in the new Prius Prime is the same 2ZR-FXE engine used in the Prius HV. This engine has a peak thermal efficiency of 40%[2]. The configuration of the plug-in hybrid system is essentially the same as in the Prius HV [1]. However, to improve performance in EV mode, the output of the boost converter in the PCU was increased. A new Dual Motor Drive System was also adopted, which is capable of using the generator in addition to the motor to drive the vehicle in EV mode. These improvements helped to greatly enhance EV mode performance.

The traction battery system features a larger capacity battery than the Prius HV, which helps to substantially extend range in EV mode. A new battery warm-up system was also developed to improve EV mode performance in winter. At the same time, a higher efficiency charger was developed to minimize increases in charging time caused by the larger battery capacity. The following sections describe the details of these components and systems.

4. TRACTION BATTERY SYSTEM

4.1 Structure of Battery System

Figure 2 shows the layout of the parts in the battery system. Control devices such as the battery ECU are consolidated at the top of the system and the battery stack is installed at the bottom. This layout reduces the length of the battery and allows a storage space to be provided in the vehicle behind the battery. The air-cooling system is applied to this system. The electric heater elements of the battery warm-up system are installed below the battery stack to save space while providing efficient battery warm-up.

Although the total energy of the new battery system is approximately twice as high as the previous system, the new component layout and an optimized frame structure kept the increase in weight to approximately 1.5 times and the increase in volume to approximately 1.6 times (Table 1). The developed structure also enables a flat luggage compartment surface with enough space to fit two golf bags.

4.2 Battery Warm-Up System

Battery output generally becomes restricted in low temperatures. When a battery is cold, output is reduced even when the state of charge (SOC) is high. For hybrid vehicles, this means that the engine may have to be started to satisfy the power demand from the driver (Figure 3).

Therefore, an electric heater is installed in the new Prius Prime below the battery stack to warm up the battery in cold climates (Figure 2). During charging, the electric heater is controlled to maintain the battery at the appropriate temperature. This increases battery output as shown in Figure 3 and allows a more higher battery output power in EV mode.

Figure 4 shows the electric heater pattern. The heater pattern has a variable areal density that optimizes the amount of heat generation over 35 areas and ensures that the cells in the battery pack are warmed up uniformly.

The pattern applies higher heater output to the corners to counteract the increased cooling that occurs in areas of the battery case with larger surface area exposure. Lower heater output is applied to the center of the battery, where heat is more likely to accumulate.

Figure 5 shows an example of the battery warm-up system operation. Under normal charging conditions, the system automatically maintains the target battery temperature. When a scheduled departure is set using the timer function, the warm-up system operates automatically by calculating the necessary heater start time to achieve the target battery temperature at the scheduled departure time. This ensures that the battery is warmed up without wasting energy.

5. PHV TRANSAXLE

5.1 Transaxle Structure

Figure 6 shows a cross-section of the PHV transaxle.

The development objectives of the new PHV transaxle were as follows.

1. Help achieve powerful EV driving force by placing a one-way clutch between the engine and power split device to realize the Dual Motor Drive System, which enables the vehicle to be driven using two motors.

2. Help achieve powerful EV mode driving force and greatly extend the EV mode driving range by enhancing the cooling performance of the planetary gear and motor during EV mode through the use of an electric oil pump (EOP).

3. Utilize the existing components and capabilities of the HV transaxle in the Prius HV when possible to maintain high performance in HV mode while also greatly improving performance in EV mode.

The PHV transaxle has a four-axis structure and consists of a torsional damper with a torque limiter, one-way clutch, input shaft, planetary gear, generator, motor, reduction device, and differential device. The planetary gear functions as the power split device that determines whether engine power is supplied to the generator or used as vehicle driving force. The motor and motor reduction gear are laid out on a different axis to the input shaft, and the motor speed reduction device uses a parallel shaft layout. Power is applied from both the engine and motor to the counter driven gear, which transmits this power to the differential gear. The one-way clutch receives the reaction torque of the generator and transmits it to the tires when driving force is required in EV mode.

5.2 Dual Motor Drive System

Figure 7 shows a monographic chart of the power split device (planetary gear) when the Dual Motor Drive System is operating. The one-way clutch is placed to permit rotation in the forward direction of the engine on the carrier axis of the planetary gear, while preventing engine rotation in the reverse direction. If reverse direction torque is outputted from the generator on the sun gear axis, the one-way clutch on the carrier axis receives this reaction torque, and transmits the generator torque to the output connected to the ring gear axis. In addition to this generator torque, output of torque from the traction motor allows the vehicle to be driven using two motors.

5.3 One-Way Clutch

Figure 8 shows a cross-section of the one-way clutch installation configuration.

The one-way clutch is connected to the flywheel on the crankshaft and is located on the engine side from the flywheel. This simple and innovative design enables the adoption of the Dual Motor Drive System while carrying over as many parts from the HV engine and transaxle as possible.

Figure 9 shows the internal structure of the one-way clutch. To achieve a thin structure with a high torque capacity, a mechanical one-way clutch with pawls that does not rely on the coefficient of friction was adopted.

The one-way clutch consists of a housing, outer race, pawls, springs, inner race, plate, and rivets. Of these parts, the housing, outer race, pawls, springs, and plate are integrated and rotate together. The inner race engages with the plate between the engine and transaxle through spline engagement, and is fixed in the rotation direction.

In the forward rotation direction of the engine, the one-way clutch can rotate because the pawls and inner race are not engaged. As the engine speed increases, the centrifugal force on the pawls also increases and exceeds the spring force that was pushing the pawls into the inner race. This enables rotation without contact between the pawls and inner race. In addition, in the reverse rotation direction of the engine, the one-way clutch is fixed in the rotation direction because the pawls and inner race are engaged.

One potential issue of a one-way clutch with pawls is the ratchet noise generated by contact between the pawls and inner race during rotation. However, in this structure, the pawls and inner race begin to separate at an engine speed of approximately 400 rpm. Since this is less than the designed engine idling speed (approximately 1,000 rpm), ratchet noise is not generated in the normal operating regions of the engine. This design also minimizes the drag torque of the one-way clutch because the pawls and inner race are not in contact while in HV mode.

5.4 Planetary Gear/Motor Cooling Structure

The Dual Motor Drive System allows generator output torque to be applied to the planetary gear when the engine is stopped. In addition, since the engine is stopped for longer period of time in a PHV compared to a normal HV, heat generation by the traction motor is a challenging issue. Therefore, an EOP was adopted to enhance the planetary gear lubrication and cooling performance when the engine-driven mechanical oil pump (MOP) cannot supply the necessary lubrication.

In addition, rotation of the planetary carrier also stops when the engine is not operating. To ensure that enough lubricant is supplied to the planetary gear even in this state, a closed-circuit oil passage is provided from the EOP to the input shaft and planetary gear. In this configuration, the pump supplies pressurized lubrication instead of the conventional splash lubrication method using centrifugal force.

Figure 10 shows the lubrication and cooling structure. The EOP is laid out in parallel to the MOP, and check valves are provided in between. This structure ensures that lubricant and coolant is always supplied from one of the oil pumps. Both pumps also share the same discharge passage, which allows reuse of parts from the HV transaxle, and minimizes the increase in transaxle size due to the additional EOP.

These technologies help to greatly enhance driving force and range in EV mode.

6. POWER CONTROL UNIT

6.1 Objectives of Series Development

Figure 11 shows the PCU adopted in the new Prius Prime.

The PCU was developed with the aim of minimizing the number of parts that need to be changed when applied to a series of vehicles by unifying the basic cooling and electrical structure concept of the vehicle series. The new Prius Prime uses a newly adopted reactor due to the wider range of current flow compared to the Prius HV. This section describes the reactor, which was developed with fewer components than the previous Prius PHV to facilitate compatibility with a wide range of specifications.

6.2 Reactor Configuration

To achieve the desired magnetic characteristics, the reactor consists of I- and U-cores, a coil, and gap plates. These components are integrated into an assembly and installed into the case (Figure 12).

Starting with the new Prius HV, the PCU is now installed directly on the transaxle. Therefore, the PCU is affected by engine vibration, which results in greater vibration acceleration (G) and a wider vibration frequency range. With the reactor, the component parts have to be held together, and the assembly must be held to the case. Therefore, the positions and shapes of the installation portions were optimized and made stiffer, and an integrated molding structure was developed to hold the components (Figure 13).

This configuration enables a cooling structure that compresses the cooling sheets that contain highly thermally conductive heat dissipating filler within. This lowers heat resistance and improves cooling performance. The previous reactor also included potting that required time to harden. The new reactor is assembled using completely fastened structure that helps to save work hours.

6.3 Electrical and Magnetic Structure

The required electrical characteristics of the reactor are determined from the standpoint of the system specifications and protecting the power devices. The new PCU includes a current sensor that monitors the reactor current directly and improves the accuracy of the boost voltage control. As a result, the inductance characteristics permit magnetic saturation, which allows the number of gap plates to be reduced (Figure 14). The newly developed reactor achieves the direct current (DC) superimposition characteristics that are required over a wide current range by optimizing the core sectional area and number of coil turns while reducing the number of gap plates from eight in the previous Prius PHV to four. Due to these measures, the new reactor is lower cost than the previous reactor with 27% fewer parts.

7. CHARGING SYSTEM

7.1 Onboard Charger

The charging system installed in the new Prius Prime is capable of fully charging the battery in approximately two hours, despite the substantial increase in EV mode range. This was achieved by improving the efficiency and increasing the output of the onboard charger. Table 2 shows the charging methods and associated charging times. The user can select the method in accordance with the usage environment.

The onboard charger is designed to be compatible with input voltages from AC 100 V to AC 240 V, to match the normal charging infrastructure available in Japan, the U.S., and Europe (Table 3). This charger is air-cooled to improve installation flexibility, which means that it can be installed under the rear seats, thereby preventing the use of valuable space in the occupant, luggage, or engine compartments.

This charger uses a higher operating frequency for the DC/DC converter and power factor corrector, which reduces the size of the coil and transformer. This also enables the adoption of a generalpurpose power device module, thereby reducing the size of the power devices. Figure 15 shows the developed charger and Table 4 shows the results of the size-reduction measures. Overall, the size of the charger was reduced by more than 50% compared to the previous charger.

This charger also features an optimized transformer turn ratio in the DC/DC converter, lower steady-state loss due to low-loss power devices, and lower device switching loss due to the adoption of zero-volt switching (ZVS). As a result, the power conversion efficiency of the charger was greatly improved from 89% to 94%. In addition, the adoption of a low-capacity sub DC/DC converter to supply power to auxiliary devices during charging helps to ensure high charging efficiency for the whole charging system (Figure 16).

Furthermore, the charger and control system comply with the Mode 2 and Mode 3 charging standards of IEC 61851-1, and the Level 1 and Level 2 charging standards of SAE J1772.

7.2 Charging Inlet

The connector and lid of the charger have a lock function that communicates with the smart keyless entry system to release the lock simply at the press of a switch.

To improve visibility when attaching or detaching the charging cable at night, the lid of the charger is provided with a light near the charging inlet (Figure 17). A charging indicator is also provided close to the inlet so that the user can confirm charging has started correctly, or that charging that was scheduled using the timer is in progress.

8. PLUG-IN HYBRID SYSTEM CONTROL

8.1 Control Development Objectives

Based on the Prius HV, the new Prius Prime uses a large-capacity traction battery and the newly developed Dual Motor Drive System to differentiate performance in EV mode and achieve an unsurpassed acceleration performance.

8.2 Driving Force Characteristics in EV Mode

The new Dual Motor Drive System enables the generator to be used as a motor and increases the rotation speed of the transaxle. As a result, the new Prius Prime can be used continuously in EV mode over a wider range of driving than the previous Prius PHV. Specifically, the new Prius Prime does not start the engine even if the accelerator pedal is depressed firmly at low vehicle speeds. EV mode can also be maintained up to a top speed of 135 km/h, which is 35 km/h higher than the previous Prius PHV. In addition, smooth acceleration characteristics particular to EV mode driving were achieved considering the vehicle speed control characteristics of the accelerator pedal (Figures 18 and 19).

EV mode has two available driving modes depending on the driving force demanded by the driver: one that uses the motor (MG2) alone, and the Dual Motor Drive System mode that supplements the driving force of the motor with the driving force of the generator (MG1). Figure 20 shows the usage ranges of each mode.

In addition, an EV-auto mode is provided that starts the engine under high load driving conditions or to reduce energy consumption. As a result, the new Prius Prime provides various functions that satisfy a wide range of user requirements.

8.3 Battery Charge Mode

The new Prius Prime also features a mode that increases the SOC of the traction battery even without AC charging, to maximize use of EV mode. This function is activated when the system enters battery charge mode and enables the driver to re-enter EV mode by charging the traction battery without the use of external charging infrastructure.

9. CONCLUSIONS

Toyota Motor Corporation performed verification tests of a prototype PHV from 2009 to 2013 [3]. Figure 21 shows the actual miles traveled in EV mode (e-VMT) extracted from the results of these tests. It is assumed that the annual total mileage is 15000 miles. The blue line indicates the EV mode output power level equivalent to the previous Prius PHV and the red line indicates the EV mode output power level equivalent to the new Prius Prime. The green line indicates the battery costs required to realize that EV mode range. In addition, Table 5 shows the main specifications of the new Prius Prime achieved by the components and controls described in this paper.

As indicated in Table 5, the EV mode range of the new Prius Prime is 25 miles, more than twice as far as the previous Prius PHV [4]. The graph in Fig. 21 confirms that the greater EV mode range and power of the new Prius Prime can improve the actual e-VMT by approximately three-fold compared to the previous Prius PHV.

Figure 22 shows the ratio of battery cost and e-VMT extracted from Figure 21. A higher ratio means greater cost performance (i.e., higher usage efficiency). This figure demonstrates that the EV range of the new Prius Prime was developed aiming for the highest point of system usage efficiency.

A PHV combines the merits of a fuel-efficient HV with the driving enjoyment of an EV. As a result, it achieves a high degree of environmental friendliness without the range concern of an EV. As shown in Fig. 21, due to its longer e-VMT than the previous Prius PHV, widespread adoption of the new Prius Prime has the potential to help reduce fossil fuel consumption.

REFERENCES

[1.] Fushiki, S., Taniguchi, M., Takizawa K., Kikuchi T., Hara, K., Kumagai, T., Muta, K., "Hybrid Technologies for the New Prius," TOYOTA Technical Review. Vol.62, P.60-P.69, 2016

[2.] Taniguchi, M., Yashiro, T., Takizawa, K., Baba, S. , "Development of New Hybrid Transaxle for Compact-Class Vehicles," SAE Technical Paper 2016-01-1163, 2016, doi:10.4271/2016-01-1163.

[3.] Hashimoto, K., Takeuchi, H., Itagaki, K., "Analysis of the Environmental Performance of the Plug-in Hybrid Vehicle Based on the Validation Test Data", JSAE Annual Congress (Spring), Vol.46 No.6, P.1079-P.1085, 2015

[4.] Matsumoto, S., Takeuchi, H., Itagaki, K., "Development of Plug-in Hybrid System for Midsize Car," Proceedings of the FISITA 2012 World Automotive Congress 387-399, 2012

Shinji Ichikawa, Hiroaki Takeuchi, Shigeru Fukuda, Shigeki Kinomura, Yoshiki Tomita, Yosuke Suzuki, and Takahiko Hirasawa

Toyota Motor Corporation

doi:10.4271/2017-01-1163
Table 1. Main specifications of battery system.

                     New          Previous      Difference
                     system       system

Total electric        8.8          4.4         Approximate
energy [kwh]                                   2x increase
Weight [kg]         120           80           Approximate
                                               1.5x increase
Volume [L]          145           87           Approximate
                                               1.6x increase
Voltage [V]         351.5        207.2         --
Cells                95           56           --

Table 2. Charging times.

Charging Method         Charging Time

240V/16A                Approx. 2 hours
120V/12A                Approx. 5 hours 10 minutes

Table 3. Onboard charger specifications.

Input Voltage                86 - 264 Vrms
Output Voltage              149 - 396 V
Maximum Input Current        16.9 Arms
Maximum Output Power          3.3 kW

Table 4. Sizes of each component.

Component            Size of new component

Coil                 0.15 L (42% smaller)
Transformer          0.15 L (28% smaller)
Power device         0.054 L (18% smaller)
Overall charger      3.8 L (52% smaller)

Table 5. Main specifications of new Prius Prime.

                              Prius Prime        Prius PHV

EV range                       25mile             llmile
Top speed in EV driving       135km/h            100km/h
Fuel consumption                54MPG              50MPG
Electricity consumption       133MPGe             95MPGe
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Article Details
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Author:Ichikawa, Shinji; Takeuchi, Hiroaki; Fukuda, Shigeru; Kinomura, Shigeki; Tomita, Yoshiki; Suzuki, Yo
Publication:SAE International Journal of Alternative Powertrains
Article Type:Report
Date:May 1, 2017
Words:3636
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