Plug-In Hybrid System In The Toyota Prius PHV

Technology May 2018 Plug-In Hybrid System Toyota Prius PHV
Plug-In Hybrid System In The Toyota Prius PHV

The new Prius PHV uses a plug-in hybrid system based on the Toyota Hybrid System (THS). Electric driving performance has been greatly enhanced from the original THS for the current Prius without plug-in function. Environmental performances of the vehicle, the electricity consumption and fuel consumption, are top level in midsize cars.

DEVELOPMENT TARGETS

After gaining experience with the previous generation of Prius PHV, the target in developing a new Prius Plug-in Hybrid Vehicle (PHV) was to create a highly efficient system with greatly improved dynamic performance in EV mode. Also, the performance of the hybrid system components in the current Prius Hybrid Vehicle (HV) had to be maximised to maintain high-performance in HV mode [2].

At the same time, the drive feeling of the vehicle in EV mode was a target for improvement including cold temperature performance. Additionally, the efficiency of the charging system and the output of the charger had to be improved to minimise the increase of charging time due to the expanded EV mode range. The charging system needed to include new features – for example, based on weekly scheduling, timer and coordinated control between the pre-air conditioning and battery warm-up system to help improve user friendliness. These development steps of the new units and systems were presented at the Aachen Colloquium 2017 [1].

(1) Configuration of the new plug-in hybrid system (© Toyota)

SYSTEM ARCHITECTURE

The hybrid system architecture in the new Prius PHV, (1), is based on the current Prius hybrid system [2]. A new Dual Motor Drive System was adopted, allowing the use of a generator (MG1) additionally to the motor (MG2) to drive the vehicle with greatly enhanced EV mode performance. For enhanced EV mode range, the traction battery system features a different battery technology and a larger capacity battery than the one for Prius HV. The high capacity lithium-ion battery was developed for long electric driving range and high electric driving power. A battery warming system was developed to keep good battery temperature under low temperatures. Cabin comfort is ensured by a heat pump climate system, allowing to realise a good electric driving range also in cold condition.

The Power Control Unit (PCU) is strongly enhanced including output of the boost converter in order to realise high electric output power from the lithium-ion battery, which then translates through appropriate driving force control to a good electric drive feeling. The power of the new charger is 1.6 times higher than the one in the previous Prius PHV to decrease the charging time. The pre-climate system and the battery warming system can be activated by a timer during the charging process. Thus, drivers can enjoy comfortable and powerful driving from the very beginning of their ride on the vehicle. The engine is the same 2ZR-FXE gasoline engine used in the Prius HV with a maximal thermal efficiency of 40 % [3].

(2) Cross-section of the PHV transaxle (© Toyota)

PHV TRANSAXLE

A new PHV transaxle aims for the EV driving force. To achieve this, a one-way clutch is placed between the engine and power split device realising the Dual Motor Drive System, which enables the vehicle to be driven using two motors. An Electric Oil Pump (EOP) enhances the cooling of motor and planetary gear during EV mode. Using the existing components and capabilities of the HV transaxle in the Prius HV allows maintaining high performance in HV mode.

The PHV transaxle has a four-axis layout including a torsional damper with a torque limiter, (2). The planetary gear works as power split device, supplying engine power to the generator or to the vehicle as driving force. The traction motor and motor reduction gear as well as motor speed reduction device are set in a parallel layout to the input shaft. Engine and motor apply power to the counter driven gear, which transmits this power to the differential gear. The one-way clutch allows rotation in the forward direction of the engine on the carrier axis of the planetary gear. When driving force is required in EV mode, the one-way clutch is activated based on torque of the generator, transmitting the power to the differential.

The Dual Motor Drive System is operating, when reverse direction torque from the generator is put on the sun gear axis. The one-way clutch is connected to the flywheel on the crankshaft and located on the engine side from the flywheel on the carrier axis. It transmits the generator torque to the output connected to the ring gear axis. In addition, output of torque from the traction motor allows the vehicle to be driven using two motors. A thin structure of the one-way clutch with a high torque capacity was achieved by a mechanical one-way clutch with pawls that does not increase friction, (3).

(3) One-way clutch structure (© Toyota)

To avoid ratchet noise, the engine speed of separation of the pawls and inner race was designed around 400 rpm, which is significantly lower than the engine idling speed of approximately 1,000 rpm. As pawls and inner race are not in contact while in HV mode, this design also minimises the drag torque of the one-way clutch [4].

The Dual Motor Drive System allows the engine to be stopped for longer period of time compared to a normal HV. Thus, heat generation by the traction motor has to be controlled by an EOP, enhancing the planetary gear lubrication and cooling when the engine-driven Mechanical Oil Pump (MOP) cannot supply the lubricating fluid. A closed-circuit oil passage from the EOP to the input shaft and planetary gear allows pressurised lubrication, when the engine is stopped. Parallel layout of EOP and MOP, plus the use of check valves, ensure lubrication and cooling in all conditions. Both pumps use the same discharge line in order to use common parts with the standard HV transaxle and to minimise the size impact of the transaxle by the EOP.

POWER CONTROL UNIT (PCU)

The Prius PHV uses a newly adopted reactor with wider range of current flow compared to the Prius HV. The PCU is installed directly on the transaxle. Engine vibrations for both acceleration (g) and frequency range affect the PCU more than in a vehicle body mounted position. Therefore, the positions and shapes of the installation portions were optimised and stiffened, using an integrated moulding structure, (4). Sufficient cooling is ensured by cooling sheets with highly thermally conductive heat dissipating filler. The reactor’s structure also allows efficient assembly in the production process.

The PCU includes a current sensor monitoring the reactor current directly and improving the accuracy of the boost voltage control. Thus, the inductance characteristics permit magnetic saturation, allowing a reduction of the number of gap plates. The newly developed reactor achieves excellent direct current (DC) superimposition characteristics over a wide current range by optimising the core sectional area and number of coil turns [5].

(4) Reactor molding structure (© Toyota)

TRACTION BATTERY SYSTEM

The traction battery system includes control devices such as the battery ECU at the top of the system. The battery system is air-cooled. While the total energy of the new system was nearly doubled to 8.8 kW/h compared to the previous PHV system, weight increased only about 1.5 times to 120 kg and volume about 1.6 times to 145 l. 95 cells provide a voltage of 351.5 V. The structure of the battery pack enables a flat luggage compartment surface.

To avoid strong restrictions of battery output in low temperatures and engine start in order to be able to satisfy the power demand from the driver, an electric heater below the battery stack warms up the battery in cold climate, (5).

During charging, the electric heater maintains the appropriate battery temperature. The heater pattern has a varying density to optimise the heat generation in all areas, ensuring uniform warm-up of the cells in the battery pack. Higher heater output in the corners considers the increased heat loss in areas with larger external surface of the battery case, while in the centre of the battery lower heater output is applied.

(5) Internal structure of the battery system (© Toyota)

CHARGING SYSTEM

The Prius PHV can be fully recharged in approximately 2 h using European standard voltage (~230 V) using its maximum power of 3.3 kW. The on-board charger is compatible with input voltages from AC 100 V to 240 V considering the charging infrastructure in Europe, Japan, and the USA. The air-cooled charger is installed under the rear seats, not to take space of occupants or luggage.

The charger uses a high operating frequency for the DC/DC converter and a power factor corrector to limit the size of the coil and transformer. A general-purpose power device module is applied. To achieve a power conversion efficiency of 95 %, the charger features an optimised transformer turn ratio in the DC/DC converter, low-loss power devices, and low device switching losses based on the adoption of Zero-volt Switching (ZVS). A low-capacity sub DC/DC converter supplies power to auxiliary devices during charging. This supports to achieve high charging efficiency for the whole charging system.

The charger and control system comply with the mode 2 and mode 3 charging standards of IEC 61851-1, and level 1 and level 2 charging standards of SAE J1772 [5].

SYSTEM CONTROL

The Prius PHV is designed not to start the engine even in case of deep accelerator pedal stroke at low vehicle speeds. EV mode can be kept up to a top speed of 135 km/h. Accelerator pedal characteristics are calibrated carefully to achieve smooth acceleration in EV mode driving, (6). The EV mode uses either the traction motor alone or the Dual Motor Drive System depending on the driving force demanded by the driver.

(6) Schematic illustration of the driving force characteristics in EV mode (accelerator stroke: 0 to 90 %) (© Toyota)

CONCLUSION

The Toyota Prius PHV achieves outstanding environmental performance as confirmed by homologation values of 22 g/km CO2 in the NEDC as well as the five-star rating in the ADAC EcoTest [6]. Based on customer feedback since 2009 during PHV trials and sales of the previous Prius PHV, EV range and maximum EV driving speed are optimised through development of every component in the PHV system, balancing customer’s requirements and battery costs.

REFERENCES

[1] Uehara, T.; Ichikawa, S.; Takeuchi, H.; Fukuda, S.; Kinomura, S.; Tomita, Y.; Suzuki, Y.; Hirasawa, T: Development of New Plug-in Hybrid System for C-Segment Vehicles. Aachen Colloquium, 2017

[2] Fushiki, S., Taniguchi, M.; Takizawa, K.; Kikuchi, T.; Hara, K.; Kumagai, T.; Muta, K.: Hybrid Technologies for the New Prius. In: Toyota Technical Review 62 (2016), pp. 60-69

[3] Taniguchi, M.; Yashiro, T.; Takisawa, K.; Baba, S.; Tsuchida, M.; Mizutani, T.; Endo, H.; Kimura, H.: Development of New Hybrid Transaxle for Compact-Class Vehicles. SAE Technical Paper 2016-01-1163, 2016, doi:10.4271/2016-01-1163

[4] Kato, K.; Suzuki, Y.; Nishimine, A.; Sunenaga, S.; Shimbayu, K.; Tsuchida, M.; Baba, S.; Miyazaki, T.: Development of New Plug-in Hybrid Transaxle for Compact Class Vehicles. 17. International VDI congress “Getriebe in Fahrzeugen”, Bonn, Germany, 2017

[5] Ichikawa, S.; Takeuchi, H.; Fukuda, S.; Kinomura, S. et al.: Development of New Plug-in Hybrid System for Compact-Class Vehicles. SAE Int. J. Alt. Power. 6(1):95-102, 2017, doi:10.4271/2017-01-1163

[6] Plug-in-Hybride im ADAC EcoTest. Online: www.adac.de/infotestrat/adac-im-einsatz/motorwelt/pluginhybride_im_ecotest.aspx/, access: July 2017

AUTHORS

TAKASHI UEHARA is Chief Engineer in Powertrain Product Planning Division of the Powertrain Company in Toyota Motor Corporation in Toyota (Japan).

SHINJI ICHIKAWA is Project Manager in Powertrain Product Planning Division of the Powertrain Company in Toyota Motor Corporation in Toyota (Japan).


THANKS

The authors thank all colleagues in Toyota Motor Corporation and development partners, who made this project a success. They would also like to thank Hiroaki Takeuchi, Shigeru Fukuda, Shigeki Kinomura, Yoshiki Tomita, Yosuke Suzuki, Takahiko Hirasawa, Tamaki Ozawa of Toyota Motor Corporation, Japan for their contribution to this article.