Automotive World 2019: Innovations in xEV drive systems
Evolution of components for the proliferation of electric vehicles
This paper introduces the topics presented at the AUTO-4 technical session entitled "Technological Innovations of Drive Systems for xEVs" on January 17th at the Automotive World Conference 2019. This session consisted of lectures by two automakers and one electronics supplier on the features and component technologies of electric vehicles.
The lectures by both automakers focused on the series hybrid type of electrified vehicles, which have different characteristics from that of pure electric vehicles such as large and small battery capacities, the presence or absence of a direct drive mechanism connecting the engine, and the system’s configuration elements. The concepts and advantages of series hybrids are also different from that of EVs.
Nissan's e-POWER shares components common with EVs, with the 100% pure electric motor drives of EVs demonstrating improved driving performance and excellent fuel economy with the characteristics of responsiveness and features such as single pedal operation. In contrast, the Honda Clarity PHEV has a relatively large 17 kWh battery (with a range of 114 km in pure EV mode) and a direct drive mechanism connecting the engine which were adopted to alleviate driver concerns regarding highway driving and range anxiety.
The lecture of Hitachi explained its efforts to solve the issues associated with the miniaturization of inverters (cooling, low loss, high reliability) that is indispensable for the proliferation of electric vehicles and its efforts to develop quiet motors.
Nissan LEAF Teardown: By-wire technologies for driving, braking, and stopping (Jan. 2019)
Nissan LEAF Teardown: Powertrain with electromechanical structure, and drive system (Nov. 2018)
Evolving EV Technologies; Nissan and Honda Approaches to Electrification (Mar. 2018)
Approaches to Combustion Engine Improvements and Electrification (Feb. 2018)
The growth of xEVs and improvements to ICE vehicles (Part 1) (Jul. 2017)
Research on revolutionary combustion technology(2): Contemplating the limits of innovation (Jan. 2017)
Current Status of Nissan e-POWER and Further Improvements in the Future (Nissan）
|"Current Status of Nissan e-POWER and Further Improvements in the Future" lecture scene
(Automotive World 2019)
Lecturer: Mr. Akihiro Shibuya, Alliance PED, e-POWER Project Promotion Group, Powertrain and EV Engineering Division, Nissan Motor Co., Ltd.
Levering the EV technologies used in the Nissan Leaf, the lecturer explained Nissan’s e-POWER series hybrid system and its component technologies.
The components of the e-POWER electrified powertrain and technologies such as drive motor control were equipped on the Nissan Note released in November 2016 and the Serena released in March 2018, and are common with those on the Nissan Leaf released in November 2017. The small battery has a capacity ranging from 1.5kWh to 1.8kWh, and although the range in the pure EV drive mode is low, the system features excellent fuel consumption and smooth driving performance due to the extensive use of the engine operating at its point of maximum thermal efficiency.
In addition, Nissan has hybrid variants of the Fuga, Skyline, and X-Trail that are equipped with a single motor, dual clutch system.
Expanding EVs and e-POWER systems as part of its powertrain strategy
Amidst strengthening emissions and fuel efficiency regulations around the world, Nissan Motor is continuously improving the efficiency of its internal combustion engines, with plans to expand its line-up of EVs which is aimed at reducing CO2 emissions by 90% from 2000 to 2050. By 2022, Nissan is targeting to electrify 50% of its vehicles, with plans to achieve sales of 1 million units per year for EVs and vehicles equipped with e-POWER systems.
As of 2018, 60% to 70% of all Note customers have selected the e-POWER option.
e-POWER uses electric components and drive motor control technologies common with those of EVs
The diagram below shows the configurations for EV, e-POWER, and conventional hybrid vehicle systems. At the system level, the inverter, motor, and axle are the same as that of EVs, while the e-POWER system differs with respect to the electrified powertrain and technologies such as drive motor control. The e-POWER system is equipped with a smaller capacity battery pack and includes an engine that connects to a generator motor that either powers the electric propulsion motor or recharges the battery. Common to all systems that power the drivetrain in 100% pure electric drive mode, is an electric motor that realizes seamless acceleration without the shifting that is common in mechanical transmissions.
Since the power generation system and the drive system are separated, electricity can be generated at the optimal efficiency operating point of the engine to maximize fuel consumption performance.
|System configurations||Photo of the e-POWER unit configuration (Cut model)|
(Source: Nissan newsroom)
"e-POWER Drive" capable of one-pedal operation
Along with the convenience of being able to accelerate or decelerate by using only the accelerator pedal (e-Pedal), fuel efficiency is improved through regenerative braking energy that continues to be recovered until the vehicle comes to a stop.
Reference report: e-Pedal = Accelerator pedal and brake-by-wire
4WD electric motor systems
Although e-POWER vehicles equipped with standard 2WD drive systems are advantageous from the perspective of driving on slippery road surfaces, e-POWER vehicles are also available with 4WD systems that exhibit superior driving performance on all surfaces such as climbing steep inclines, icy roads, and driving in deep snow.
The 4WD system has small motors located at the rear of the 4WD layout, and although 4WD e-POWER vehicles do not always operate in 4WD mode, the 4WD version mainly assists when accelerating the vehicle from a full stop.
Although an e-POWER system vehicle is equipped with an internal combustion engine, the purpose of the engine is to supply electric power to the high voltage battery instead of the alternator, which makes it possible to improve control with good responsiveness and to improve driving performance on surfaces such as the snow covered roads.
Increased power by expanding the application of e-POWER and calibration
Starting with the e-POWER system used for the Nissan Note, the system can be calibrated to expand the adoption of the same system to other applications. The Note’s specifications are 254 Nm of torque and 80 kW of power output for the motor, and 58 kW of power output for the engine, while the Serena’s specifications are 320 Nm of torque and 100 kW of power output for the motor, and 62 kW of power output for the engine. However, the specifications of the motors used in both the Note and the Serena are identical, but the power output is improved only by increasing the inverter specifications and calibrating the engine accordingly.
Mr. Shibuya mentioned that he has heard people question the feasibility of powering a 2-ton vehicle such as the Serena with a 1.2L engine, but the engine only needs to generate electricity at the most efficient point, with the idea being that the engine only needs to generate enough energy to charge the battery.
The e-Power NISMO S version of the Note also utilizes the Serena system, and Nissan is expanding its lineup of model variants using a combination of engines calibrated accordingly using the e-POWER system based on that used for the Nissan Note.
Nissan believes that it is necessary to improve driving performance, emissions, and fuel efficiency to expand the models (Body type, Segment) on which these systems are equipped and the regions in which they are sold. According to Mr. Shibuya, Nissan also recognizes the challenges associated with improving the efficiency of the electronic components common with EVs, and will continue to improve the motor's magnetic circuits and the semiconductors used in the inverter to expand the high efficiency operating range of permanent magnet type motors, the performance of which is not acceptable in high rotation ranges. In addition, Nissan will endeavor to realize both high performance and high output in its engines dedicated to generating power by limiting the optimal operating range of the engines. In addition, Nissan will limit the number of engine variations to achieve operational efficiencies. And, Mr. Shibuya explained that by doing so Nissan will be able to manage the expansion of model variations, with plans to adopt the electronic components common with EVs to models in the B, C, and D-Segments as well as other various body types.
Technology Innovation of the new i-MMD Plug-in for the Clarity PHEV (Honda）
|"Technology Innovation of the New i-MMD Plug-in for the Clarity PHEV" lecture scene
(Automotive World 2019)
Lecturer: Mr. Teruo Wakashiro, Chief Engineer, Dept. 2, Technical Development Division 1, 4-Wheeler R&D Center, Honda R&D Co., Ltd.
Honda's hybrid vehicle lineup includes the Sport Hybrid i-DCD model and the 4WD Sport Hybrid SH-AWD model, both of which are equipped with a single motor hybrid system. However, the lecture by Mr. Wakashiro focused on the new i-MMD two motor hybrid system adopted for the Clarity PHEV that was released in July 2018. The Clarity PHEV, which is the first vehicle where Honda has integrated the i-MMD two motor hybrid system with a 1.5L Atkinson-cycle engine, was explained by Mr. Wakashiro by comparing it to the previous generation Accord PHEV.
Furthermore, other Honda vehicles believed to be equipped with the common i-MMD two motor hybrid system in recent years include the Odyssey which was released in February 2016 and the Accord which was released in May 2016.
In December 2018, Honda announced that for the Insight’s i-MMD hybrid system, the two motors would be using magnets that are not made of rare earth elements, but the lecture by Mr. Wakashiro gave no mention of this development.
Overview of the development of the new i-MMD Plug-in
Air pollution, CO2 and energy problems are topics to be addressed, and even the IEA (International Energy Agency) is anticipating the proliferation of ZEVs (zero emission vehicles). Environmental regulations are becoming stricter, even in emerging countries, which cannot be addressed solely by improving the efficiency of internal combustion engines.
Honda has accumulated a vast amount of knowledge since it introduced the EV Plus in 1997 to today’s technology that is equipped on the Accord PHEV, and the company aims for electrified vehicles to make up two-thirds of its global automobile sales by 2030 (HV and PHEV: 50%, FCV and EV: 15%).
In reference materials released around 2008, it was believed that EVs and ULCVs (Ultra-Low Carbon Vehicles) would be required to reduce the average of CO2 emissions of EVs and ULCVs from the 157 g/km in 2007 to 70 g/km by 2025, and that the realization of ULCV technology would be necessary for PHEVs to meet the CO2 emission regulation values.
Development of the i-MMD Plug-in hybrid system
Two major issues for EVs are range and the charging infrastructure. Data derived from the field testing of EVs showed that most people leave 40% of the remaining battery power in their EVs due to concerns related to range anxiety. This brings us to PHEV’s where the purpose is to provide “a vehicle in which you can enjoy how far you can travel in an EV” rather than focusing on “range anxiety”. PHEVs were developed with the aim of satisfying 75% of the average traveling distances in the U.S. and deliver performance at levels that would allow EV mode driving even on highways.
In the case of HEVs, conventional i-MMD hybrid systems used engine power for acceleration, whereas Honda’s new i-MMD (Intelligent Multi-Mode Drive) two-motor hybrid system enables an HEV to accelerate by drawing from the power stored in the battery, resulting in acceleration that is quieter and highly efficient.
The first prototype car was equipped with a 660cc engine, an SHEV (series hybrid electric vehicle) with no direct drive mechanism connecting the engine, which confirmed that it would be possible to use a small engine. Subsequently, the new i-MMD hybrid system was used to develop PHEVs with the same layout as an HEV by adopting a PCU (Power Control Unit) which integrates a high output VCU (Voltage Control Unit) to improve fuel efficiency and quietness at high speeds with a direct drive mechanism connecting the engine. The main components of the new i-MMD Plug-in system are shown in the diagram below on the left.
Technology of the i-MMD Plug-in system
・ Compared with the battery cells adopted for the Accord PHEV, the new system has battery cells that have 2.1 times the energy density, 1.4 times the power density, and 2.5 times the power with an output of 17 kWh, which results in 4 times the EV drive mode energy and 1.4 times the maximum power output.
・ High power output is achieved by improving the cooling performance of the battery. By integrating the battery with a DC-DC converter, battery charger and water cooling system, the system has been downsized and a bypass circuit prevents heat from being generated during charging.
・ By improving the VCU (Voltage Control Unit), Honda was able to increase the power output by 3.3 times to achieve a PCU (Power Control Unit) with high output. As a result, the maximum driving speed in EV mode is now 160 km/h.
・ To avoid having to use a larger PCU to deliver high power output, a new magnetic two-phase coupled inductor was adopted to limit the size of the PCU to the same level as that of a conventional PCU. The core of the inductor is T-shaped (lower right diagram), inverting the two coils by 180 degrees to cancel any magnetic flux leakage, and to arrange the sensors and harnesses near the inductor.
・ Compared to the Accord PHEV, the size of the PCU remains the same, but the output density of the VCU was increased by 2.8 times, while increasing the continuous power output by 3.3 times while driving in EV mode.
・ The two motor system is similar to that developed for the Odyssey, but the system for the conventional Accord PHEV is 23% smaller, lighter, and has 8 Nm greater torque and 11 kW greater output.
・ A 2.0L engine was used for the Accord PHEV, but later a downsized 1.5L inline 4-cylinder Atkinson-cycle engine was adopted. A maximum thermal efficiency of 40.5% was realized by improving combustion and reducing friction.
・ The EV Drive, Hybrid Drive, and Engine Drive mode are the same as in conventional HEV systems, but the Clarity PHEV when used in the pure electric EV drive mode can meet most consumer driving needs for daily driving situations.
・ When switching from EV drive to engine drive, there is a pedal clip function that applies resistance to the accelerator pedal for a short period of time, which contributes to the maintenance of the driving speed in the EV mode.
・ By expanding the region of the electric drive mode, Honda has been able to increase the EV drive range to 114.6 km using the JC08 test mode and 101 km using the WLTC test mode.
(Source: Honda news release）
According to Mr. Wakashiro regarding the future deployment of electrified vehicles, the progress of electrification over the past several years has been truly remarkable. Therefore, going forward, Honda needs to further improve the efficiency of the hybrid vehicles and accelerate the deployment of PHEV and ZEV vehicles.
Technical Trends for Electrification of Powertrains & Control Systems (Hitachi)
|"Technical Trends for Electrification of Automotive Powertrain & Control Systems" lecture scene
(Automotive World 2019)
Lecturer: Mr. Kinya Nakatsu, Senior Chief Researcher, Center for Technology Innovation-Controls, and Chief Researcher, Electrification System Lab., Research and Development Group, Hitachi, Ltd.
Since the 1970s, Hitachi has been developing motors and inverters for electric vehicles, with the power density of its inverters almost tripling. This lecture introduced technologies being developed by Hitachi such as inverters adopting a double-sided cooling method with direct water cooling and fully immersed cooling fins that are adopted for the inverter in the PHEVs of Mercedes-Benz (see photo below).
Background and outline of Hitachi’s efforts
Toward the future low-carbon society, the IEA has established a CO2 reduction goal that requires the transportation sector to reduce CO2 emissions by 12.4 billion tons in 2050, with automobiles accounting for 86% of that target. To achieve this requires the electrification of vehicles such as EVs and PHEVs, which requires the development of high performance components so as to not interfere with the achievement of the CO2 targets.
To extend the range of EVs, in addition to improving battery performance, it is necessary to further miniaturize inverters and motors, and it is expected that the output density of inverters will double in 2030.
Key challenges for inverters
Hitachi introduced the initiatives and technological measures that it is taking to address the key challenges for inverters: (1) cooling, (2) low loss, (3) high reliability.
The size of the inverter is determined by the dimensions of the power module located below the cooling water passage of the inverter. Therefore, instead of mounting the power module vertically or using thermal grease to improve heat dissipation, Hitachi believed that it was preferable to adopt a cooling mechanism that uses direct cooling of the power modules with double-sided direct cooling by immersing the fins on the base of the power module in cooling fluid. And, by adopting a direct cooling structure, Hitachi was able to modify the shape of the power module so that it could be inserted it into the hole of the water channel lid, and by optimizing the shape of the radiating fins it was able to reduce thermal resistance by 50%, while improving power output density and miniaturizing the inverter.
The photo below on the right shows a power semiconductor made of SiC (silicon carbide) material, and is an example of a power module with a shape similar to that of a double-sided direct cooling mechanism.
(2) Loss reduction
Semiconductor technology is evolving at a rapid pace, such as higher processing speeds for reducing energy losses during On/Off switching and decreasing voltages when power semiconductors are in operation, but currently the evolution of peripheral technologies is currently unable to keep pace with the rate at which semiconductor technology is evolving.
For inverters that use power modules with double-sided direct cooling, Hitachi uses low inductance mounting to prevent increases in surge voltages, resonance, and the deterioration of EMC (electromagnetic compatibility) caused by wiring inductance.
To reduce the wiring inductance of the power module, the wiring and the base of the heat dissipation conductor are located closer to each other, so that the magnetic flux is canceled by the induced current, resulting in a reduction of surge voltages from the 629 V of conventional inverters to 485 V.
Although SiC power semiconductors have low ON voltages compared to conventional silicon semiconductors, SiC semiconductors are effective for reducing On/Off switching energy losses, but since the wafer size of fabricated semiconductors is small, the size of the chips is also small, requiring a layout connecting a large number of chips in parallel.
The current balance when a large number of small chips are arranged in parallel becomes an issue, because such a configuration results in uneven wiring lengths, a biased temperature distribution, and a deterioration in the life and performance of the inverter. In a double-sided direct cooling structure, the current path length is uniform and current fluctuation is reduced from the 10% of conventional structures to 2%.
(3) High reliability
In an effort to improve the reliability of the soldered joints of power semiconductors, Hitachi is engaged in soldering joints using sintered copper and silver powder instead of ordinary lead-free tin solder, which results in a temperature cycle life that is equivalent to silicon semiconductors and even that of semiconductors made of durable SiC material.
Making motors quieter
Audible motor driving noise is becoming quieter in automotive applications as well as in trains and elevators. But, because motors involve complex controls, configurations and mechanisms, it is important to first understand the principles associated with motors to control noise. Because there are times when vibration and noise must be sacrificed to give priority to efficiency, there are ways to make motors quieter in the future by reducing noise using semiconductors made of SiC material to improve noise and vibration performance. Mr. Nakatsu also mentioned the possibility of making electric motors quieter by changing the ratio of the number of poles to slots in the design and construction of electric motors, by optimizing the selection of magnets with weak magnetic flux, and by adjusting the flux barrier of the magnetic circuit.
According to Mr. Nakatsu, Hitachi needs to improve the power density and manufacturability of its electronic components in the future. He also went on to explain that the company will likely face challenges arising from the need to develop components that are compatible with changes in the charging infrastructure, such as charging methods, that are inevitable when the EV society becomes a reality.
Automotive World, Nissan, Honda, Hitachi, Hitachi Automotive Systems, Leaf, e-POWER, Clarity, Electric, EV, HV, PHEV, FCV, Hybrid system, Traction motor, Inverter, Battery, PCU, Inductor, SiC, All wheel drive, Pedal
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