The Nature and Promise of 42 V Automotive Power: An Update

Transcription

1 The Nature and Promise of 42 V Automotive Power: An Update Power Area and CEME Seminar, December 2002 P. T. Krein Grainger Center for Electric Machinery and Electromechanics Department of Electrical and Computer Engineering University of Illinois at Urbana-Champaign

2 Outline Why 42 V? Safety and other reasons. Target power levels. Architectures. Points about engineering research needs. Major applications: power steering, starter-alternators, etc. Mild hybrid designs based on 42 V. Research opportunities. Conclusion. 2

3 Why 42 V? The electrification of the automobile is a major step in its evolution. Electrical applications are beneficial for the same reasons as for systems in aircraft: Better efficiency More flexible control Ease of energy conversion Low-cost control and conversion of energy is a key point. Electric power is rising because of electric auxiliaries as well as more features. 3

4 Why 42 V? When electricity is used to power various components (steering, brakes, suspension, air conditioning), the results are better efficiency and more flexible performance. Performance is decoupled from the engine. Many estimates have been made, such as 10% fuel economy improvements by simple electrification of existing functions. 4

5 Why 42 V? Possible new features: Combined starter-alternator to reduce costs and enhance performance. Regenerative braking. Start on demand arrangements to avoid idle engines. Improved, more efficient power steering and other subsystems. Active suspensions. Electrical valves and engine elements -- ultimately the self-starting engine. 5

6 Why 42 V? The conventional car is rapidly becoming more electric. The total electric load is about 1500 W today, and is increasing toward 5000 W. Conventional alternators cannot deliver more than about 2000 W, and are not efficient. A higher voltage system supports lower current and loss. 6

7 Three alternatives: Why 42 V? Stick with 12 V. This limits effective power levels. Get the voltage as high as possible (>100 V). This requires a major overhaul of safety systems and basic designs. Push the voltage as high as possible before significant safety issues come into play. 42 V tries to do the last: get the voltage as high as possible while avoiding severe safety issues. 7

8 Safety Issues A car s electrical system is typically open. Complicated wiring harnesses with close contact and hundreds of connections. Regulatory agencies have set a level of about 60 V dc as the maximum reasonable level in an open system. Headroom is required to stay below this level under all allowed conditions. 8

9 Safety Issues Industry premise: stay with an open electrical system for the foreseeable future. This philosophy supports the option for evolutionary change of automotive electric power. 9

10 Safety Issues There are also fully regulated and battery regulated systems. Battery-regulated system ultimately defer to the battery to set the voltage level. A battery-regulated system must allow for Polarity reversal Disconnection: momentary or continuous Wide voltage swings Inductive spikes from corrosion or deliberate disconnect are significant. 10

11 Safety Issues 12 V battery systems require undamaged operation at 12 V or from short-term spikes up to 75 V. At higher battery voltages, surge suppressors and other add-ons will be needed to limit these extremes to present levels. In a battery regulated system, 36 V is about the highest possible level (but these are charged at 42 V) without excessive possibility of damage and spikes much beyond 60 V. 11

12 Safety Issues In a fully regulated system, there is some buffering between the battery and the rest of the system. With full regulation, the wide swings of a battery system are not necessarily encountered by the user. 48 V batteries are possible within the 60 V limit, with such regulation. The higher voltages also support extra efforts, such as anti-reversing diodes. 12

13 Safety Issues The term 42 V refers to a range of choices with nominal battery levels in the range of 36 V to 48 V. While there is incomplete consensus, the evolutionary approach would favor 36 V batteries (charging at 42 V). For comparison, we should take 42 V to mean a tripling of present voltage, to give at least triple the power. With better generation, power up to 5x is available. 13

14 Safety Issues We can also consider a closed system, in which electrical contact is more protected. Closed systems are used in today s hybrid and electric cars. The voltage levels there can exceed 300 V dc. 14

15 Power Levels Voltage Typical power level Maximum power level 12 V 1200 W 2000 W 42 V 5000 W 10 kw 300 V 30 kw 100 kw At 42 V, a car s electrical system rivals that of a house. But, 10 kw is not enough for traction power. 15

16 Architectures Each automotive voltage level has advantages for some loads. 12 V or less for lamps, sensors, electronics, controls. 42 V for motors, pumps, and fans. High voltage for electric traction power. Incandescent lamps, for example, are more rugged and more reliable at low voltages (but they are disappearing). 16

17 Architectures Many possible architectures are possible. Most retain some 12 V capacity. They are typically divided into single-battery and dual-battery systems. There is no consensus on which to select, and we are likely to see several. 17

18 Architectures Single battery at 42 V: ENGINE 42V ALTERNATOR 42V BATTERY 42V LOADS Problem: jump starts? Problem: charge balance. DC DC 12V LOADS 18

19 Dual battery: ENGINE 42V ALTERNATOR Architectures 42V BATTERY 42V LOADS The dc-dc converter must be bidirectional to support starting and reliability. 12V BATTERY BIDIRECTIONAL DC DC 12V LOADS 19

20 Architectures 12 V battery REGULATOR ENGINE 42V STARTER/ ALTERNATOR 42V LOADS Here a starter-alternator is shown as well. BIDIRECTIONAL DC DC Source: Mechanical Engineering Magazine online, April V BATTERY 12V LOADS 20

21 Architectures Distributed converters with 42 V battery. ENGINE 42V STARTER/ ALTERNATOR 42V BATTERY 42V LOADS Here there are many dc-dc converters at the various loads. LOCAL DC/DC LOADS 21

22 Architectures The ultimate is a true multiplexed system: Deliver a single 42 V power bus throughout the vehicle, with a network protocol overlaid on it. Local dc-dc converters provide complete local operation and protection. A ring bus or redundant bus structure could be used to enhance reliability. What about fuses? No central point is available. 22

23 Architectures Costs would seem to dictate a single-battery arrangement. However, this involves either a high-power 42V to 12V converter (bidirectional) or a troublesome 42 V battery. Some researchers talk about a small dc-dc converter just for jump starts. Most systems are partially multiplexed (power and network distribution rather than individual loads). 23

24 Issues Key off loads: sensors, alarms, clocks, remote systems. All draw down power. Flat loads draw roughly fixed power, although the alternator output can vary. Connectors. Fusing. Arcs: much above 12 V, it becomes possible to sustain an arc in close quarters. 24

25 Connectors 150 A connector for 42 V (AMP, Inc. prototype). 25

26 Points About Research Needs Many of the new challenges of 42 V have been addressed in other contexts: 48 V systems throughout the telephone network (with battery regulation) Higher dc voltages in several aerospace applications (with bigger arcing problems in lowpressure ambients) Methods need to be adapted to the low-cost high-vibration automotive case. 26

27 Points About Research Needs Motors are of keen interest. Dc motors are cheap to build because of the convenient wound-rotor structure. The small machine design methods for cars do not translate well to 42 V. At 42 V, ac motors make sense. But small ac motors have been expensive in most contexts. How to build cheap, small ac motors (with electronic controls)? 27

28 Points About Research Needs Fusing is critical. Power semiconductor circuits capable of acting as self fuses active devices used as circuit breakers based on local sensing. Actual fuses and circuit breakers with costeffective arc management suitable for automotive environments. Fusing issues (among others) have slowed down the development of 42 V systems. 28

29 Major Applications Electric power steering. Two forms: assist pump and direct electric. The assist pump uses an electric motor to drive a conventional hydraulic unit. The direct system uses electric motors with the steering rack. In both cases, action can be controlled independent of the engine. Source: Delphi Corp., Saginaw Steering Systems Div. 29

30 Major Applications Electric air conditioning. Remove the air conditioning system from engine belt drive. Provides much better control and flexibility. Easier cycling,possible heat pump application. 30

31 Major Applications Integrated starter-alternator (ISA). Build an electric machine into or around the flywheel. Both permanent magnet and induction types are being studied. Source: Mechanical Engineering Magazine online, April

32 Major Applications Provides on-demand starts. Supports regenerative braking. The very fast dynamics of an ac machine allows even active torque ripple cancellation. If ripple can be cancelled, there is promise for much quieter engines and much lower vibration levels. 32

33 Major Applications Electromechanical engine controls. Valves. Fuel. Source: FEV Engine Technology, Inc. 33

34 Major Applications Active suspensions. Use electromechanical actuators in conjunction with mechanical suspension members. With enough actuator power, road bumps (large and small) can be cancelled with an active suspension. 34

35 Major Applications Catalyst management systems and exhaust treatment. Today, most automotive emissions occur in the first few minutes of operation, when the catalyst is too cold to be effective. Catalyst heaters or short-term exhaust management systems can drastically reduce tailpipe emissions in modern cars and trucks. Electrostatic precipitator methods can be of value with diesel particulate exhaust. 35

36 Additional Applications 36

37 Mild Hybrids The key limitation of 42 V is that it really does not support electric traction power levels. As the promise of electric and hybrid vehicles becomes clearer, engineers push for higher power levels beyond the reach of 42 V. A compromise is possible: the mild hybrid vehicle. 37

38 Mild Hybrids A light hybrid or mild hybrid uses a small motor to manage performance. The engine can be shut down at stops. Braking energy can be recovered. The car does not operate in an all-electric regime. The Honda Insight is a good example. Source: 38

39 Mild Hybrids For a mild hybrid approach, about 5 kw or so can provide a useful level of traction power. The technique is accessible in a 42 V system, although higher voltage (144 V in the Insight) is beneficial. A 42 V ISA has substantial promise for fuel economy improvements, and straddles the boundary between a conventional car with an ISA and a mild hybrid. 39

40 Other Hybrids Higher-power hybrids require high voltage (240 V and up) for traction power. Electrical accessories are essential. Such cars can benefit from 42 V systems. 40

41 Other Hybrids Source: The Toyota hybrid system operates at 288 V, and reaches 30 kw. All key accessories are electric. 41

42 Research Opportunities Low-cost small ac motor systems: 42 V dc bus Cheap inverters Small ac motors that can be manufactured easily Engine electromechanical devices and controls. Protection and semiconductor fusing. System-level analysis. 42

43 Conclusion The continuing increase in electric power levels in automobiles will require higher voltages. 42 V systems (batteries at 36 V or 48 V) are the highest possible in an open electrical system. There are fuel economy improvements just at this level, but the extension to mild hybrids offers much more. While the industry is now is a go slow mode for 42 V, no one doubts its eventual use. 43

44 The End! 44

45 Why Not Just Big Batteries? Lead-acid battery energy density is only about 1% of that in gasoline. Our test car: 600 lb battery pack equivalent to one gallon of gas! 45

46 Electric and Hybrid Gallery General Motors EV lb battery pack at 312 V, 102 kw motor mph in less than 9 s. Volvo turbine-based hybrid prototype. 46

47 Electric and Hybrid Car Gallery This Ford Escort was the first true practical prototype hybrid a complete station wagon. Second-gen diesel hybrid. 47

48 Electric and Hybrid Car Gallery 48

49 Toyota Hybrid Specs Small NiMH battery set, 288 V. 40 HP motor, ac permanent magnet type. Continuously-variable transmission with sunplanet gear set for energy control mph in about 17 s cc engine can hold 75 mph indefinitely. Atkinson cycle engine ( 5-stroke ) gets better thermal efficiency but lower output torque. Rated 54 mpg city, 48 highway. 49

50 Electric and Hybrid Car Gallery Toyota architecture Honda architecture: 50