48 volt starter generator
In 2017, the entire automotive industry achieved significant growth. - From automotive OEMs to small device manufacturers, automotive electrification has become a hot topic, especially the 48 volt architecture. In fact, searching for the term "48 volts" in any search engine, you will search for a large number of results, which indicates This engineering solution for automotive systems has taken a place in the market.
In the previous "The emergence of the 48 volt mild hybrid system," I briefly introduced the new 48 volt automotive architecture and its differences from the existing 12 volt system. In this topic we will touch on the reasons behind determining the new voltage level and delve into one of its main applications - the 48 volt starter generator.
Why is 48 volts?
The obvious first and foremost question is: “Why is it 48 volts?” This is a very important issue. Remember, in the 1990s, a 42 volt electrical power standard was proposed to replace the 12 volt standard. Although this standard has not been developed. But its intent is to solve some of the same problems we face today, such as more powerful electric drive accessories and lighter wiring harnesses. There are two main reasons for choosing 48 volts as a standard - safety and efficiency.
A major problem involved in increasing voltage is that it can endanger personal safety. While some people are still arguing whether 48 volts is safe enough, this voltage level provides the higher power we need without entering the "high voltage" range. Figure 1 shows how a 48 volt battery operates at different voltage levels. As stated in ZVEI's document "Voltage Levels for Electric Travel", 60 volts (DC) is the maximum safety limit, and its battery voltage is considered too dangerous. In the "normal operation" range, the battery can achieve the best performance, but the electronics inside the car should be able to withstand the worst case high voltage conditions.
Figure 1: Safety voltage margins
As I’ve previously stated, the current 12-volt system is unable to cope with the growing demand for electrical power within conventional vehicles. However the 42-volt proposal was a complete replacement of the 12-volt electrical architecture while 48-volt complements it. The 48-volt battery simply adds an additional power source for new applications which also contribute to a smoother driving experience. Furthermore, the size and cost of wiring and components is significantly reduced due to the higher voltage of the 48-volt battery.
A closer look at the 48-volt starter generator options
Having a similar appearance as the car alternator (Figure 2a) but slightly bigger in size, the 48-volt starter generator’s initial topology position is at the engine’s belt. The belt-driven starter generator (BSG), also known as P0 architecture (Figure 2b) is a cost-effective solution which can provide up to a 15% reduction in CO2. Looking at some boost recuperation systems (such as Bosch’s), the maximum power ratings are around 10 kW for mechanical output in boost mode and 12 kW for electrical output during recuperation – both at 48-volts. While these numbers are rated for short periods of time, the BSG’s continuous power can reach up to 5 kW with maximum efficiency of 85%.
Figure 2a and 2b: Car alternator and P0 starter generator topology
However with tightening emission regulations, automotive Tier 1 suppliers have developed different starter generator topologies to further reduce the CO2 footprint of 48-volt mild hybrid vehicles. In ascending order, these configurations offer better emission reduction but become increasingly complex and costly.
Figure 3: 48-volt mild hybrid starter generator topologies
Crankshaft mounted starter generator (P1)
As the name suggests this solution has the starter generator mounted directly on the crankshaft (which converts the linear motion of pistons into rotary motion). This provides higher torque than the P0 architecture due to the absence of a belt drive, and with no belt losses there is greater efficiency. The maximum power required is 10 kW but the efficiency goes up to 94%. However one significant limitation of this solution is that torque requirements can be demanding, due to no torque/speed ratio between the crankshaft and the starter generator. An example of this topology is the 2010 Mercedes-Benz S400 BlueHybrid.
Shaft mounted machine (P2/P3)
Both P0 and P1 architectures are mounted on the engine, but there are other mounting options such as having the 48-volt electrical machine on the gearbox’s input/output shaft (P2/P3 respectively). By providing a mechanical disconnect, this translates to improved energy flow efficiency and allows for the provision of hybrid functions (e.g. e-drive).
The P2 architecture is ether integrated into the transmission on the input shaft or attached on the side, and results in increased energy recuperation and electrical drive capabilities. Mounting the solution on the output shaft (P3), provides the highest level of the above-mentioned benefits. The obvious disadvantage of the shaft mounted electrical machine is the cost of integration.
Rear axle mounted electrical machine (P4)
The ultimate architecture at this time involves mounting at the rear axle drive (P4). This provides the vehicle with 4-wheel drive capabilities, with the combustion engine at the front and the electrical machine at the back. Maximum power requirement of the P2-P4 architectures can reach up to 21 kW with an efficiency of 95%. Moving the starter generator closer to the rear axle also provides more hybrid functionality to the vehicle. The new 48-volt machine is able to reduce the CO2 emissions by up to 21% in urban driving environment depending on its architecture.
What is more, this high-power application requires a significant portion of electronics to drive it. Naturally power MOSFETs play a key role in these electronic modules, but they need to be capable of withstanding worst case scenarios such as excessive currents and thermal leakages. Hence, the topic of my next blog will be to delve into the semiconductor content inside 48-volt applications such as the starter generator. Stay tuned!
