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Development of Electric Powertrain for CLARITY PLUG-IN HYBRID.

Introduction

Today, the development and popularization of environmentally conscious vehicles is essential as a response to global warming, air pollution, and energy issues. Honda has now developed three vehicles in its CLARITY series employing three different power-plants in a shared body platform: the CLARITY FUEL CELL, a fuel cell vehicle; the CLARITY ELECTRIC, an electric vehicle (EV); and the CLARITY PLUG-IN HYBRID, a plug-in hybrid electric vehicle (PHEV).

Hondas CLARITY PLUG-IN HYBRID features a higher-capacity battery, which extends the vehicle's all-electric range (AER) to 47 miles. Increased battery and Voltage Control Unit (VCU) power have made it possible to extend engine-free EV operation across the entire vehicle speed range, realizing a maximum speed of 100 mph. As a result, the majority of everyday driving can be conducted using the vehicle as an EV. The expansion of the range of EV operation offers users the smooth acceleration and quietness that are characteristic of EV. During hybrid operation, the vehicle realizes a high 42 mpg level of fuel economy and is capable of long-distance driving. This means that the majority of everyday driving (commuting to work or school, etc.) can be conducted as an EV, while longer distances (weekend trips, etc.) can be covered using the vehicle as a hybrid, with the engine generating power. As an EV able to offer a greater number of users the unique appeal of EV driving together with a sufficient range, the CLARITY PLUG-IN HYBRID can be expected to contribute to the popularization of environmentally conscious vehicles in the future.

This article will discuss the features of the powertrain system and the newly developed Power Control Unit (PCU) and Intelligent Power Unit (IPU) employed in the CLARITY PLUG-IN HYBRID.

Overview of Powertrain System

Powertrain Layout and Specifications

Figure 1 shows the powertrain layout in the vehicle. The drive motor, generator, engine, and PCU are positioned in the engine room at the front of the vehicle, while the IPU is positioned under the floor. An onboard charger for charging the battery is fitted to the rear sub-frame.

Figure 2 shows a side view of the powertrain layout. The under-floor positioning of the IPU has made it possible to realize a spacious cabin seating five passengers, while also allowing the fitting of the high-capacity battery. In addition, because there is no need to fit the battery in the trunk, the layout also helps to realize a spacious trunk.

Table 1 shows the specifications of the vehicle and individual components.

Overview of Vehicle and Powertrain Operation

Figure 3 shows an image of PHEV operation and the battery state of charge (SOC). The vehicle operates as an EV when there is a high SOC following charging of the battery. (When power is demanded that exceeds the range for EV operation, the engine is also used.) When the SOC declines as a result of charge-depleting (CD) operation, the system makes the transition to charge-sustaining (CS) operation, in which the engine is also used. During CS operation, the powertrain operates to maintain the SOC by means of power generation by the engine.

Figure 4 shows the configuration of the powertrain and its driving modes. The power-plant is made up of a motor, for vehicle operation and power regeneration; a generator, for generating power and starting the engine; an engine; a battery; and a PCU. The PCU integrates an inverter that drives the motor, an inverter that drives the generator, and a DC-DC converter that boosts battery voltage (VCU). The engine is directly connected to the generator and is connected to an axle shaft via the clutch.

The system s driving modes are EV Driving Mode, in which the motor is driven by battery power; HV (hybrid electric vehicle) Driving Mode, in which both engine-generated power and battery power are used; and Engine Driving Mode, in which both engine power (via the connection to the clutch) and motor power are used.

Maximum Speed and AER as EV

During EV Driving Mode, as shown in Figure 4, the motor is driven exclusively by power from the battery. When the vehicle is driving at high speeds or is operating under a high load, such as when climbing a hill, the VCU boosts the battery voltage in order to drive the motor. The motor drive voltage is controlled by the VCU in order to realize the demanded power while maintaining overall efficiency at the optimum level.

Figure 5 shows the AER at each vehicle speed and maximum speed during EV operation with a fully charged battery. The capacity of the battery in the developed vehicle has been increased to 17 kWh, increasing the vehicle's AER approximately 3.6-fold. Increased power in the battery and VCU has made it possible to realize a maximum speed of 100 mph during EV operation.

Figure 6 shows the proportion of weekday US user driving distance. The capacity of the CLARITY's battery was set at 17 kWh to realize an AER of 47 miles, representing approximately 80% of weekday US user driving distance.

These AER and speed range during EV operation make it possible for the vehicle to be used as an EV for weekday driving by the majority of users.

Power-Plant Operation and Engine Downsizing

Figure 7 shows the system operation at each vehicle speed and motor torque. When power is demanded that exceeds the range for EV operation, the system operates in HV Driving Mode or Engine Driving Mode. When the demand for power declines, the system returns to EV operation.

During CS operation, the range for EV operation is reduced in response to the decline in the battery SOC, and the system operates to maintain the battery SOC by means of series generation by the engine. H V Driving Mode or Engine Driving Mode is selected in order to help ensure optimal total powertrain efficiency.

During both CD and CS operations, battery power is the main source of the demanded drive power in the high-load range, with the shortfall being supplemented through generation by the engine.

The increased battery and VCU power realized in this system has expanded the range for EV power against the previous PHEV. Engine power has been reduced while increasing the motor power, making it possible to downsize the engine from 2.0 to 1.5 liters.

Figure 8 shows the relationship between power generated by the engine and battery power in H V Driving Mode. In H V Driving Mode, the battery is used as a buffer for power charge and discharge. The engine operating point (speed and torque) is determined to follow the minimum brake-specific fuel consumption (BSFC) for every possible power demand. Moreover, the engine load is increased or decreased by charging or discharging the high-voltage battery in order to move engine operating point toward the higher efficiency point on the minimum BSFC line [1]. Because the system is fitted with a high-capacity battery, it is possible for the battery to supply power in response to changes in demand for power, thus controlling changes in the amount of power generated by the engine. The engine generates power while maintaining, to the extent possible, operation at the operating point for optimum BSFC. The engine is able to maintain high-efficiency generation while controlling sudden changes in power output to the greatest extent possible.

During Engine Driving Mode, as shown in Figure 4, both engine power, via the direct connection to the clutch, and motor power are used. When the vehicle is cruising at high speeds, high-efficiency operation is made possible by the direct connection between the engine and the output shaft via the lock-up clutch.

Figure 9 shows an overview of drive power output in Engine Driving Mode. In order to realize the power necessary for acceleration in Engine Driving Mode in Hondas previous plug-in hybrid, it was necessary to disengage the lock-up clutch, boost engine power, and increase the amount of power generated by the generator. Because power can be supplied by the high-power battery and high-power VCU in the developed powertrain, it is possible to accelerate using the motor to supplement drive power while maintaining the operating point for optimum engine BSFC, with the lock-up clutch engaged.

Figure 10 shows the frequency of engine speed at each vehicle speed during city and highway driving in CS operation. The frequency of high-efficiency engine drive in HV Driving Mode is increased in the developed powertrain, controlling fluctuations in engine speed. This makes it possible to realize quiet operation. In Engine Driving Mode, the range for maintenance of lock-up is increased, making it possible to control increases in engine speed in the high vehicle speed range. The realization of high engine efficiency and increased quietness with reduced engine speed has made it possible to downsize the engine from 2.0 to 1.5 liters.

Increased power in the battery and VCU has made it possible to realize operation in the high-efficiency range for the engine, helping to enable the vehicle to achieve a high 42 mpg level of fuel economy.

Overview of IPU and PCU

Overview of IPU

IPU Configuration Figure 11 shows the layout of the IPU. The IPU contains a 17 kWh lithium-ion battery, a 12 V DC-DC converter, and a battery ECU. And it is positioned under the floor of the vehicle Table 2 shows the specifications of the IPU.

Figure 12 shows a comparison of the battery power and energy for Honda's previous PHEV and the CLARITY PLUG-IN HYBRID.

The energy of the battery employed in the CLARITY PLUG-IN HYBRID has been increased 2.5-fold and its power 1.4-fold against the battery employed in the previous PHEV, making it possible to realize a speed of 100 mph during EV operation. In addition, by improving the electrode material, SOC usage range has been expanded, and the energy of CD operation has been increased fourfold.

IPU Technologies Figure 13 shows the parts layout in the IPU. The battery module is divided into two, situated front and rear, and the rear module is fitted in two levels. The 12 V DC-DC converter is positioned in the center of the IPU, between the battery modules.

Figure 14 shows the position of the IPU in the vehicle. In Honda's previous PHEV, the IPU was positioned in the trunk. CLARITY PLUG-IN HYBRID has increased the battery capacity of IPU by 2.5-fold but realized a cabin and luggage space equivalent to gasoline cars due to the under-floor installation. Because it is necessary for the floor in a sedan package to offer adequate leg room in the front and rear seats and allow for rear seat trunk access, the battery modules were divided and positioned under the front and rear seats.

Figure 15 shows the layout of the high-voltage wiring and components in the IPU. The high-voltage wiring and communications wiring are concentrated in the center of the IPU between the battery modules, allowing for an integrated configuration.

Battery Table 3 and Figure 16 show the specifications and size of the battery cells.

Figure 17 shows the evolution in the performance of the battery cells. The goal for the battery employed in the CLARITY PLUG-IN HYBRID was to extend AER against the previous PHEV, and as a result the battery's specific power was increased 1.4-fold and its specific energy 2.1-fold.

IPU Heat Management

Cooling System. Table 4 and Figure 18 show battery cooling performance and IPU size of the FIT EV and the CLARITY PLUG-IN HYBRID. The configuration of the under-floor IPU in the CLARITY PLUG-IN HYBRID was developed based on the unit employed in the FIT EV. The cooling system was selected taking into consideration the amount of heat generated by the battery and the volume of the cooling system itself. The FIT EV employed a configuration in which the bottoms of the cells were air-cooled. In the CLARITY PLUG-IN HYBRID, the amount of heat generated by each cell is higher. When using the air cooling system, it was found that the temperature of the battery rises, and when a high-temperature environment is realized, or high-load running is performed, power performance decreases. A system was therefore adopted in which the bottoms of the cells are coolant-cooled, realizing 3.5 times higher cooling performance than the air cooling.

Figure 19 shows changes in battery temperature in a high-temperature environment. The adoption of the coolant-cooling system has helped to ensure that it is unnecessary to limit the vehicle's driving performance due to high battery temperatures, even during high-load operation like the US06 mode in a high-temperature environment.

Cooling System Configuration. Figure 20 shows the IPU cooling circuit. The components cooled by the system are the battery, the 12 V DC-DC converter, and the onboard charger. The system uses both an electric water pump (EWP) and a radiator (RAD) dedicated exclusively to the IPU, positioned at the front of the vehicle. After the battery (which has a low management temperature) is cooled by the circulation of long life coolant (LLC) by the EWP, the 12 V DC-DC converter and the onboard charger are cooled. Because each component has a different management water temperature and the operating states in which each component produces heat differ, electric water valve (EWV) and a bypass circuit have been employed to isolate the battery from the cooling circuit.

Table 5 shows vehicle operating states and the heat generation of each component. The battery is the main source of heat generation during vehicle operation, but when the vehicle is being charged, onboard charger is the main source of heat generation. If all the components were cooled using the same coolant channel, the heat generated by the charger during charging would cause the battery temperature to increase, resulting in the degradation of the battery. To help prevent this, the developed system employs a bypass water channel and a EWV that switches coolant circuits when the temperature of the coolant is higher than the temperature of the battery.

Figure 21 shows the IPU cooling channel during vehicle operation (the main circuit). Because the battery and the 12V DC-DC converter generate heat during vehicle operation, coolant is circulated through the battery (BATT), the 12 V DC-DC converter (DCDC), and charger (CHG).

Figure 22 shows the IPU cooling channel (the bypass circuit) when the battery is being charged. During charging, a high level of heat is generated by the onboard charger, increasing the temperature of the coolant. Increases in battery temperature are therefore avoided by sending the coolant through the bypass circuit when the coolant temperature is higher than the battery temperature.

Figure 23 shows the battery temperature transition when driving and charging twice a day. During charging, the water temperature rises due to the heat of the charger, but when we switch to the bypass channel, the battery temperature rise suppressed.

Figure 24 shows the frequency of battery temperature. It was found that the bypass channel reduced the high-temperature frequency of the battery. Also, by using the bypass channel, the durability of the battery was improved by 15%.

Overview of PCU

The PCU integrates an inverter that drives the traction motor, an inverter that drives the generator, and a VCU, which is a battery voltage boost converter.

Figure 25 shows the configuration of the power-plant. The PCU is positioned above the transmission and is directly connected to it. This configuration has realized a level of compactness in the power-plant, including the PCU, that makes it possible to fit the power-plant in the front engine room.

Figure 26 shows the configuration of the PCU. The power module is cooled by means of an integrated water jacket. The water jacket makes contact with the middle case containing the inductor. Coolant cooling of the inductor has made it possible to increase continuous power output.

Table 6 shows the specifications of the PCU [2].

In order to expand the EV Driving Mode shown in Figure 7. it was necessary to increase the power of the VCU that boosts battery voltage. An interleaved circuit configuration was employed to increase the power of the VCU, making it possible to increase the maximum power of the VCU 1.3-fold and its continuous power output 3.3-fold against the VCU employed in the previous PHEV. As a result, the vehicle realizes a speed of 100 mph during EV operation.

An increase in the volume of the PCU as a result of the increased power of the VCU would have made it challenging to fit the PCU in the engine room. The employment of a coupled inductor in the VCU made it possible to reduce the size of the inductor, allowing the unit to be positioned in the engine room.

Figures 27, 28. and 29 show the PCU circuit diagram, the VCU switching waveform, and the configuration of the coupled inductor, respectively. The two-phase coupled inductor has Phase 1 paired with Phase 2. For each phase, two coils are wound on a single core and current is run through them so that the magnetic fluxes generated in the core by the direct current are in reverse directions, thereby reducing the direct current magnetic flux (Figure 29). In addition, the timing for switching the phases coupled with each other is offset by 180[degrees] so that the alternating current component is also reduced. The effect of the former is to make the inductor more compact, and that of the latter is for the ripple currents in the inductor to cancel each other out, enabling the smoothing capacitor to be made more compact [3].

Figure 30 compares the power density of the developed and previous VCU. Figure 31 shows the layout of the power-plant in the engine room. As shown in Figure 30. The power density of the developed VCU is 2.8-fold that of the previous VCU. As shown in Figure 31. The increased power density of the VCU has made it possible to control any increase in the front-rear length of the PCU, allowing the positioning of the unit in the engine room.

Conclusions

A powertrain has been developed for the 2018 model CLARITY PLUG-IN HYBRID, as follows:

* The capacity of the IPU battery has been increased 2.5-fold against Honda's previous PHEV, increasing power by 1.4-fold and making it possible to realize a speed of 100 mph during EV operation. The range of use of the SOC has been increased, increasing energy approximately fourfold during CD operation. As a result, the vehicle realizes an AER of 47 miles. The positioning of the IPU under the floor of the vehicle has made it possible to realize cabin space for five passengers and a spacious trunk.

* The use of an interleaved circuit configuration in the VCU has increased the maximum power of the unit 1.3-fold and its continuous power output 3.3-fold against the VCU employed in the previous PHEV, making it possible to realize a speed of 100 mph during EV operation. The employment of a coupled inductor has increased the power density of the VCU 2.8-fold, making it possible to reduce the size of the unit and position it in the engine room.

* Increased power in the battery and VCU has made it possible to operate the engine at optimum BSFC, helping to enable the vehicle to achieve a high 42 mpg level of fuel economy. It has been possible to downsize the engine from 2.0 to 1.5 liters while realizing the necessary power and quiet operation.

References

[1.] Higuchi, N., Sunaga, Y., Tanaka, M., and Shimada, H., "Development of a New Two-Motor Plug-in Hybrid System," SAE Int. J. Alt. Power 2(1):135-145, 2013, doi:10.4271/2013-01-1476.

[2.] Saito, K. and Otsuka, H., "Development of PCU for a New HEV Drive," JSAE Technical Paper No. 46-13, 13-16, 2013,20135063.

[3.] Kitamoto, R., Sato, S., Nakamura, H., and Amano, A., "Development of Fuel Cell Boost Converter Using Coupled-Inductor for New FCV," SAE Technical Paper 2017-01-1224, 2017, doi:10.4271/2017-01-1224.

Tomoya Yamagishi and Takashi Ishikura, Honda R&D Co., Ltd., Japan

History

Received: 20 Jul 2018

Revised: 16 Nov 2018

Accepted: 10 Jan 2019

e-Available: 11 Feb 2019

doi:10.4271/2018-01-0415
TABLE 1 Vehicle and powertrain specifications.

                              CLARITY PLUG-IN    Previous
Model                         HYBRID             PHEV

Vehicle   Length/width/       4895 mm/1875       4910 mm/1850
          height              mm/1475 mm         mm/1465 mm
          Wheel base          2750 mm            2775 mm
          Vehicle weight      1843 kg            1730 kg
          Number of           5                  5
          passengers
          All-electric range  47 miles           13 miles
          (EPA)
          Fuel economy        CD 110 MPGe        CD 115 MPGe
          (EPA) (combined)    CS 42 MPG          CS 46 MPG
Engine    Type                L4 1.5 L           L4 2.0 L
                              Port injection     Port injection
                              Atkinson cycle     Atkinson cycle
          Power               76.5 kW            105 kW
          Torque              134 Nm             165 Nm
Traction  Type                DC brushless       DC brushless
motor                         motor              motor
          Power               135 kW             124 kW
          Torque              315 Nm             307 Nm
Battery   Type                Li-ion             Li-ion
          Capacity            17.0 kWh           6.7 kWh
PCU       Maximum output      360 kVA            400 kVA
          (motor inverter)
          Maximum voltage     650 V              700 V
          VCU maximum         (1.3-fold) kW (in  (Undisclosed)
          output              comparison with
                              previous PHEV)
          VCU continuous      (3.3-fold) kW (in  (Undisclosed)
          power               comparison with
                              previous PHEV)
Charger   Output (at AC       6.6 kW             6.6 kW
          240V)

TABLE 2 IPU specifications.

                      CLARITY PLUG-IN
Item            Unit  HYBRID           Previous PHEV

Capacity        kWh    17.0              6.7
Rated voltage   V     310.8            320
Cell (module)   -     168(14)          100
Cooling system  -     Coolant-cooled   Air-cooled
Position        -     Under-floor      Cabin

TABLE 3 Cell specifications.

                     CLARITY PLUG-IN
Item           Unit  HYBRID           Previous PHEV

Type           -     Li-ion           Li-ion
Capacity       Ah    27.3             20.8
Rated voltage  V      3.7              3.2

TABLE 4 Battery cooling performance.

                        CLARITY PLUG-IN
Item             Unit   HYBRID           FIT EV

Dimensions       mm     1787/1076/315    2112/1146/376
Battery energy   kWh      17.0             20.0
Number of cells  -       168              432
Cooling system   -      Coolant-cooled   Air-cooled
Cooling          w (*)   665              190
performance

(*) When the temperature difference between the battery and the outside
air is 12 K.

TABLE 5 Vehicle operating state and heat generation.

Operation                   12V DC-DC      Onboard
state          Battery (W)  converter (W)  charger (W)

High-load CS   400          150              0
operation
6.6 kW charge  30            80            750

TABLE 6 PCU specifications.

                         CLARITY PLUG-IN
Model                    HYBRID               Previous PHEV

PCU    Maximum output    360 kVA              400 kVA
       (motor inverter)
       Maximum voltage   650 V                700 V
       Weight             17.7 kg              17.4 kg
       Volume             11.7 L               11.5 L
       VCU maximum        (1.3-fold) kW (in     -
       output            comparison with
                         previous PHEV)
       VCU continuous    (3.3-fold) kW (in      -
       power             comparison with
                         previous PHEV)
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Author:Yamagishi, Tomoya; Ishikura, Takashi
Publication:SAE International Journal of Alternative Powertrains
Date:Dec 1, 2018
Words:3732
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