“With the increasing attention to environmental conditions (especially air quality) around the world, the CO2 emissions of automobiles need to be reduced by reducing the average fuel consumption of automobiles. One way to achieve this goal is to use hybrid engines instead of purely internal combustion engine (ICE) powered cars.
With the increasing attention to environmental conditions (especially air quality) around the world, the CO of automobiles2Emissions need to be reduced by reducing the average fuel consumption of cars. One way to achieve this goal is to use hybrid engines instead of purely internal combustion engine (ICE) powered cars.
In a typical automobile, the traction power system must be able to operate within a very wide range of power and speed conditions, which is usually expressed in the “torque-speed” range. The adoption of hybrid power systems allows system designers to freely arrange multiple power sources in a way that optimizes the performance of each system within different torque-speed ranges. The electric power source can provide very high torque and is very useful when accelerating the car, but it is only available for a limited period of time. The specific time involved depends on the size of the battery and the torque output of the motor. With this high torque that can generate a power source, the size of the ICE can be significantly reduced. This greatly improves its fuel efficiency. However, adding an electric power source is certainly not a simple engineering problem. It requires a method for many automotive system design considerations.
Traditionally, electrification has been achieved by adding high-voltage (~350V) batteries and high-performance motors directly coupled to the ICE power system. The defining category of these full-hybrid vehicles has always been energy-efficient vehicles, and they are also quite attractive from the perspective of improving energy efficiency. However, they also greatly increase the cost and weight of the car.
Recently, the 48V automotive system architecture has received considerable attention. These systems can be said to be a step towards a full hybrid vehicle. Under normal circumstances, they are called “Mild Hybrids”, but when designed at lower power levels, they can also be classified as “Micro Hybrids.” They use relatively compact 48V batteries, high-performance motors, and at least one additional 48V electrified subsystem. The lower cost of 48V systems makes them attractive in the eyes of many car OEMs, and they will soon become part of most car manufacturers’ product portfolios.
The choice of 48V architecture is wide and increasing. The most basic system includes a battery, a starter generator, a 48V to 12V converter, and usually has at least one 48V load. As 48V cars still retain 12V batteries and multiple 12V loads, these systems may currently exist in the form of dual-voltage systems.
Figure 1. Electrical topology of a typical 48V mild hybrid system
With these dual-voltage systems, a large number of new configurations are possible. Since the 48V system is basically able to provide higher power levels, it will support new higher-power peripherals, such as 48V E-Turbo and 48V E-Roll stable systems. In addition, higher power availability will promote the migration of power-consuming 12V loads to the 48V bus to take full advantage of higher energy efficiency.
Initially, the 12V system side of the dual voltage system will remain as it is, minus the 12V alternator. Since there is no 12V power source, a converter is needed to transfer the power generated by 48V to the 12V side. Although the converter requires high energy efficiency, it still imposes a loss penalty on all 12V loads because it needs to obtain power through the converter. There is a strong incentive between increased losses and power limitations to be able to move 12V peripherals to 48V operation.
These converters are bidirectional in design and can use both batteries at the same time during periods of high demand. The bidirectional converter can convert the power from any battery to another battery, and may exist for a certain period of time in the future.
Except for redundancy, there is no technical reason to keep the 12V starter, and it may become a future trend to remove it. If it is no longer needed, the size of the 12V battery can be significantly reduced, or even completely removed. But this will be a bold move, and the converter needs to be designed very carefully.
For the 48V system side, the starter generator is the main component. It is responsible for all electric power generation of the car and car starting. It can also perform regenerative energy recovery during car braking. In this mode, the machine acts as a generator to provide negative torque to the power system, slow down the vehicle speed and restore battery power. Starter generators come in multiple configurations and power levels, each with very specific implementation goals.
The hybrid field uses a shorthand code to determine the position of the motor in the chassis subsystem. The system uses a set of Px indicators to generate tags that indicate each position where the motor is coupled to the power system. The value of the indicator (P0 to P4) will increase when the power insertion point passes through the rear of the vehicle.
Figure 2. Px hybrid system terminology and example power levels
Since the insertion point of the BSG is located at the front end accessory drive (FEAD) of the engine, the power level of the BSG is the smallest, and the torque transmission must be connected by a steel belt. Because it is coupled by steel gears, the remaining insertion points (P2-P4) of the chassis have a higher power level. In addition, higher-power machines have additional features that can provide traction assistance to the ICE. This means that in addition to the power provided by the ICE, the electric power source can also increase the acceleration of the car. In some configurations, it is conceivable that the car can be started only by the electric power source when the ICE is turned off. This depends on the amount of traction assistance available to the motor and its location in the chassis subsystem.
The dual-voltage system requires the addition of a 48V to 12V power converter. In the absence of a 12V alternator, this component is needed to supply power to the 12V system. Due to the need for two-way behavior and high energy efficiency, special design methods are required. The typical power range of these converters is in the range of 1kW to 3kW. In order to maintain high energy efficiency in this high power range, multi-stage buck-boost converters are currently the most popular topology. The buck topology allows power to flow from the higher voltage side to the lower voltage side. Similarly, the boost topology allows power flow in the opposite direction. The multi-level design allows sharing many individual converter sub-circuits to be combined into a high-power design. When the converter output load is heavy, all the sub-circuits can work. When the converter outputs a light load, many sub-circuits will be turned off, thereby providing lower losses and higher energy efficiency.
There are many kinds of 48V loads that can be imagined, and many higher power loads cannot be realized by a 12V system. The highest among them is the electronically controlled supercharger. Since the supercharger needs to accelerate to an extremely high speed within a fraction of a second, it requires a relatively high transient power. A typical booster drive consists of a low-inertia three-phase motor driven by a three-phase inverter. Although the average power is relatively low, the peak power can reach more than 8kW. Such a wide power range configuration can perfectly match the 48V system. Many other automotive subsystems are also well-suited to 48V architectures, whether in single-phase or three-phase configurations. The possible 48V loads are listed in Figure 3.
Figure 3. Summary of mild hybrid subsystem components
Other 48V systems
The 48V battery system consists of lithium-ion batteries, which require more attention and handling than lead-acid batteries. In view of this, 48V cars need a battery management system (BMS). The system is responsible for monitoring the battery voltage and battery temperature so that the battery can be charged safely. As the 48V system has regeneration capabilities, this situation has become more complicated. When the remaining power of the car battery is low enough, the regeneration command can be issued, but the control of the BMS needs to be very careful, which is essential to prevent overcharging or overheating.
The 48V circuit also has more complicated requirements for fusing and contact. It is currently uncertain whether the 12V blade fuse can provide adequate arc protection if used in a 48V system. Moreover, since the relay contact distance required by the 48V system will be greater than that required by the 12V system, the fuses and relays need to be redesigned. Since the requirements of these components can be easily met by using semiconductor devices, these problems are likely to be solved by Electronic solutions.
Adding a 48V system to a 12V car will give designers the opportunity to achieve the fuel efficiency improvements required by today’s cars. It will also greatly increase the demand for new and innovative power electronic circuits. Although many variants of the 48V architecture will appear, the final judgment will be made after automotive customers weigh the advantages and costs of the features.
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