The current vigorous promotion of vehicle electrification has made the development of vehicle internal combustion engines deteriorating. In response, AVL has developed a combined power system of high-efficiency internal combustion engines and 48 V plug-in power equipment. This approach has achieved a promising future. The alternative solution not only reduces the CO2 emissions of the OEM manufacturer’s fleet, but also enables users to achieve emission-free electric driving in the city.
1 Strong political pressure
With the continuous implementation of automobile electrification, the development of internal combustion engines for vehicles is stretched, especially in Europe. In addition to the strong political driving force generated by the CO2 emission regulations based on transportation, it also requires air quality. Make significant improvements. Such requirements also reflect the stringency of the future Euro 7 exhaust emission regulations, especially based on the real driving emissions (RDE) boundary conditions. The requirements of this regulation will exceed the limits of the US-20 regulations for ultra-low emission vehicles in the United States.
The micro-vehicles used for driving in the city are limited in terms of attachment quality, structural space requirements, and especially the cost of fully electrified. Therefore, hybrid power, especially the 48 V system, is of great significance.
2 The current 48V drive system
The integration of the 48 V belt drive starter-generator (BSG) into the powertrain system has shown its unique application prospects, which can improve the adaptability of internal combustion engine exhaust emissions, driving maneuverability and driving dynamics. Nevertheless, this type of system is based on the layout in the belt drive, and it cannot provide sufficient user experience in terms of electric driving, flexible driving, and braking driving. In addition to the above limitations, the limited torque through the belt drive will also bring additional technical challenges. If the belt drive needs to transmit more torque, it needs to increase the belt ribs and/or increase the pre-tightening force of the V-ribbed belt, which will cause more severe friction and increase fuel consumption. compensate.
The fuel consumption advantage of the 48 V system compared to the 12 V system is based on the greater energy recovery potential, and the generator still has good efficiency even considering the efficiency loss of the DC/DC converter. These restrictions and the fact that the internal combustion engine does not disconnect from the powertrain system when recovering operating energy have gradually limited the effectiveness of the system in reducing CO2 emissions.
3 Next-generation 48 V system
Compared with 48 V-BSG, the system where the motor is directly arranged in the powertrain system and can separate the internal combustion engine can reduce CO2 emissions more effectively, thus not only can recover more energy, but also improve the realization of electric vehicles. possibility.
The P2, P3, and P4 layout patterns shown in Figure 1 can realize the above-mentioned electric travel mode, and can achieve the same effect of reducing CO2 emissions as a high-voltage full-hybrid system with the same structure.
Figure 1 P0～P4 48 V powertrain system architecture and the energy recovery curve according to the size of the motor in the WLTC test cycle
The technological improvement potential of hybrid power to reduce CO2 emissions mainly depends on the energy recovery system, so the research focus of this article is mainly on improving the efficiency of energy recovery. The boundary conditions considered here are determined by vehicle size, mass, driving resistance and driving cycle regulations. System analysis (based on the simulation tool AVL Cruise) shows that the P2 hybrid solution for the front axle transverse drive structure can provide the highest energy recovery potential.
The peak power of about 20 kW is a good compromise between energy utilization and system power when the generator is running. In view of structural space requirements, system cost, and modularity/scalability, side-mounted (offset) systems offer good potential. Due to the additional transmission ratio of the bias structure, a high-speed motor with an integrated frequency converter and a speed of up to 18 000 r/min can be used. This solution of the motor can be designed as a module, because it can also be used as an electric drive axle or as the main drive device of a small electric vehicle (such as A0 class three-wheel motorcycle).
In development, the existing solution can provide approximately 25 kW or 20 kW of generator power during drive operation. The power difference is based on the system voltage. Due to the internal impedance of the battery, obtaining 20 kW of power from the battery will cause a voltage drop, and passing current to the battery will increase the voltage accordingly, thereby obtaining a higher rated power.
4 48 V plug-in hybrid electric vehicle
In order to achieve the required CO2 emission reduction target by 2025 or 2030, it is necessary to continuously optimize the friction loss of vehicles, the gear shift strategy of the transmission, and the thermal management plan of the internal combustion engine itself. The effect of CO2 emissions still needs to be improved.
From a logical point of view, it is necessary to increase the market share of electric vehicles. The target effect can be experienced by means of a system simulation of a demonstration vehicle based on the VW Golf Ⅶ sedan. To this end, the efficiency-optimized internal combustion engine developed by AVL is combined with a 20 kW electric rear axle and a 48 V battery with a capacity of 5.3 kW·h developed by AVL and tested in real traffic. This optimized powertrain system can drive a mileage of more than 20 km in an electric state at a maximum speed of 50 km/h (Figure 2).
Figure 2 CO2 emissions and architecture of the AVL demonstration vehicle (PHEV = plug-in hybrid)
5 Daily partial no exhaust gas emissions
Research to reduce exhaust emissions (such as CO2) is not entirely about users. Free access to the city’s zero-emission areas and electric driving experience are very attractive to it. For users, in addition to cost price, their main concern is the characteristics of the luggage compartment, net load quality, driving power and other aspects of the vehicle. In order to meet the relevant requirements in the system, the design needs to analyze the real-life usage conditions, such as the need to correctly drive out of the garage, cross the edge of the sidewalk, pass through the ramp and longer distances, and also need to consider not only from -30 to The temperature influence of 60 ℃, but also includes the use of small cars with a net load mass of 480 kg and a dead weight of 1 250 kg.
Extensive analysis of various important usage conditions can accurately determine the requirements. For example, the system integration of the usage conditions in China and Europe results in an average electric driving range of 22 km required, which can fully meet the requirements for driving in the city. .
For C-class cars and vehicles with a mass of 1 500 kg, the maximum wheel power demand is about 25-30 kW, and the average continuous power of the vehicle for driving is about 5 kW, such as heating/cooling devices, entertainment Auxiliary facilities such as information equipment, automobile headlights, and wiper devices need to increase the power by 1 to 3 kW on average, so the total continuous power of the battery needs to reach about 6 to 8 kW. Figure 3 The upper graph shows the comparison of power requirements for the urban part of the WLTC test cycle and for real urban driving.
Figure 3 Comparison of WLTC city part and real city driving and power required by WLTC
When considering the transmission path from the wheel to the battery, the required electrical system power is about 30 to 35 kW. If the minimum voltage to maintain normal operation is 34 V, the peak current at 30 kW is 880 A.
The main factor that affects the cost of the electrical system is the current rather than the voltage, which involves the correctness of choosing this type of 48 V system, as specified in the ISO 6469-3 standard (road transmission safety standard), which will be even higher. The high voltage of 75 V as the system voltage can only reduce the current by about 20%.
Figure 3 The following figure shows through the comprehensive analysis of the system that high power only takes a short time (mostly 1 to 5 s), so the demand for these power peaks is relatively small. If the maximum power of 8 kW needs to be continuously provided, a wire with a greatly reduced diameter can meet its requirements.
The requirements of real driving and RDE regulations bring additional considerations such as low temperature performance and service life. If considering the relevant requirements for the system standard architecture, necessary adjustments should be made for this.
Critical challenges such as high current and the loss caused by it, voltage breakthrough during high current pulses, and low temperature performance of batteries are usually difficult to solve under the current system architecture. Therefore, in order to model these requirements, the application of system engineering methods will face three interesting situations:
(1) High power-limited duration-high current;
(2) Low power—long duration—small current;
(3) Low temperature-power does not decrease.
The 48 V battery used in this type of system architecture has an energy of 400 W·h. The resulting limitation is higher energy loss and reduced efficiency (Figure 4). As shown in the system architecture, this type of battery and energy transmission path will show corresponding drawbacks when meeting the above-mentioned 1 and 3 requirements.
Figure 4 Power output and internal loss of 12s2p and 400 W·h－48 V batteries
The accumulator that can fully meet such requirements is a double-layer capacitor (DSK). The disadvantage of this technology is that the voltage rise is relatively large. The combination of storage batteries and double-layer capacitors is a promising solution to the above-mentioned tasks. In order to minimize the loss, unlike most other solutions, it is more feasible to integrate the capacitor in the power module. At this time, if it needs to be integrated into a module, there are two possibilities: The first possibility is to use a simple semiconductor switch to connect the capacitor to the battery voltage, thereby reducing costs. In this case, a small DC/DC converter can charge and discharge in an inactive state. The second possibility is to apply a high-current DC/DC converter in advance, and the design of this type of converter can be optimized as a frequency converter.
In both cases, the number of double-layer capacitors needs to be determined based on power and energy requirements, rather than on the voltage level, so that a design that is conducive to cost reduction can be achieved.
The battery applied based on the separate system architecture has lower requirements for low-temperature performance and peak power, allowing the use of battery designs with energy units, which can achieve higher energy density and lower cost in terms of installed capacity .
Finally, the higher current requirements of motors and inverters should be considered. If the design is changed from 3 phases to 6 phases without star connection points, then the current will be reduced by half, the loss for every 3 phases will be reduced to a quarter, and the loss for every 6 phases will be reduced to half. Similar to the equation P=I·R.
6 New high-power system architecture
The new system architecture (Figure 5, top) can meet the ever-increasing requirements. In the WLTC test cycle, the CO2 emissions can be lower than 65 g/km according to the installed accumulators.
The core component of this system architecture is the AVL-48 V electric bridge. In addition to the electric motor, it also includes a reduction transmission mechanism and a clutch, which can disconnect the electric motor from the power system at high vehicle speeds. The frequency converter integrated in the motor (marked in blue in the bottom left figure of Figure 5) is installed in a common housing (the bottom figure of Figure 5) and separates the water cooling circuit from the motor.
Figure 5 System architecture based on system synthesis and AVL-48V high-power electric bridge, as well as design of energy unit and 6-phase motor