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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
Outline
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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.
Grainger Center for Electric Machines and Electromechanics
University of Illinois at Urbana-Champaign
2
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.
Grainger Center for Electric Machines and Electromechanics
University of Illinois at Urbana-Champaign
3
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.
Grainger Center for Electric Machines and Electromechanics
University of Illinois at Urbana-Champaign
4
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.
Grainger Center for Electric Machines and Electromechanics
University of Illinois at Urbana-Champaign
5
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.
Grainger Center for Electric Machines and Electromechanics
University of Illinois at Urbana-Champaign
6
Why 42 V?
• Three alternatives:
– 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.
Grainger Center for Electric Machines and Electromechanics
University of Illinois at Urbana-Champaign
7
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.
Grainger Center for Electric Machines and Electromechanics
University of Illinois at Urbana-Champaign
8
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.
Grainger Center for Electric Machines and Electromechanics
University of Illinois at Urbana-Champaign
9
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.
Grainger Center for Electric Machines and Electromechanics
University of Illinois at Urbana-Champaign
10
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.
Grainger Center for Electric Machines and Electromechanics
University of Illinois at Urbana-Champaign
11
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.
Grainger Center for Electric Machines and Electromechanics
University of Illinois at Urbana-Champaign
12
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.
Grainger Center for Electric Machines and Electromechanics
University of Illinois at Urbana-Champaign
13
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.
Grainger Center for Electric Machines and Electromechanics
University of Illinois at Urbana-Champaign
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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.
Grainger Center for Electric Machines and Electromechanics
University of Illinois at Urbana-Champaign
15
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).
Grainger Center for Electric Machines and Electromechanics
University of Illinois at Urbana-Champaign
16
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.
Grainger Center for Electric Machines and Electromechanics
University of Illinois at Urbana-Champaign
17
Architectures
• Single battery at 42 V:
ENGINE
42V
BATTERY
42V
LOADS
42V
ALTERNATOR
• Problem: jump starts?
• Problem: charge balance.
DC–DC
12V LOADS
www.hoppecke.com
Grainger Center for Electric Machines and Electromechanics
University of Illinois at Urbana-Champaign
18
Architectures
• Dual battery:
ENGINE
42V
BATTERY
42V
LOADS
42V
ALTERNATOR
• The dc-dc converter must
be bidirectional to support
starting and reliability.
BIDIRECTIONAL
DC–DC
12V
BATTERY
12V LOADS
Grainger Center for Electric Machines and Electromechanics
University of Illinois at Urbana-Champaign
19
Architectures
• 12 V battery
ENGINE
REGULATOR
42V
STARTER/
ALTERNATOR
• Here a starter-alternator
is shown as well.
42V
LOADS
BIDIRECTIONAL
DC–DC
12V
BATTERY
12V LOADS
Source: Mechanical
Engineering Magazine
online, April 2002.
Grainger Center for Electric Machines and Electromechanics
University of Illinois at Urbana-Champaign
20
Architectures
• Distributed converters with 42 V battery.
ENGINE
42V
BATTERY
42V
STARTER/
ALTERNATOR
42V
LOADS
• Here there are many dc-dc
converters at the various
loads.
LOCAL
DC/DC
LOADS
Grainger Center for Electric Machines and Electromechanics
University of Illinois at Urbana-Champaign
21
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.
Grainger Center for Electric Machines and Electromechanics
University of Illinois at Urbana-Champaign
22
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).
Grainger Center for Electric Machines and Electromechanics
University of Illinois at Urbana-Champaign
23
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.
Grainger Center for Electric Machines and Electromechanics
University of Illinois at Urbana-Champaign
24
Connectors
• 150 A connector for 42 V (AMP, Inc.
prototype).
Grainger Center for Electric Machines and Electromechanics
University of Illinois at Urbana-Champaign
25
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.
Grainger Center for Electric Machines and Electromechanics
University of Illinois at Urbana-Champaign
26
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)?
Grainger Center for Electric Machines and Electromechanics
University of Illinois at Urbana-Champaign
27
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.
Grainger Center for Electric Machines and Electromechanics
University of Illinois at Urbana-Champaign
28
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.
Grainger Center for Electric Machines and Electromechanics
University of Illinois at Urbana-Champaign
29
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.
Grainger Center for Electric Machines and Electromechanics
University of Illinois at Urbana-Champaign
30
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 2002.
Grainger Center for Electric Machines and Electromechanics
University of Illinois at Urbana-Champaign
31
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.
Grainger Center for Electric Machines and Electromechanics
University of Illinois at Urbana-Champaign
32
Major Applications
• Electromechanical engine controls.
• Valves.
Source: FEV Engine Technology, Inc.
• Fuel.
Grainger Center for Electric Machines and Electromechanics
University of Illinois at Urbana-Champaign
33
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.
Grainger Center for Electric Machines and Electromechanics
University of Illinois at Urbana-Champaign
34
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.
Grainger Center for Electric Machines and Electromechanics
University of Illinois at Urbana-Champaign
35
Additional Applications
Grainger Center for Electric Machines and Electromechanics
University of Illinois at Urbana-Champaign
36
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.
Grainger Center for Electric Machines and Electromechanics
University of Illinois at Urbana-Champaign
37
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.
Source: www.familycar.com
• The car does not operate in an
“all-electric” regime.
• The Honda Insight is a good example.
Grainger Center for Electric Machines and Electromechanics
University of Illinois at Urbana-Champaign
38
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.
Grainger Center for Electric Machines and Electromechanics
University of Illinois at Urbana-Champaign
39
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.
Grainger Center for Electric Machines and Electromechanics
University of Illinois at Urbana-Champaign
40
Other Hybrids
Source: www.familycar.com
• The Toyota hybrid
system operates at
288 V, and reaches
30 kW.
• All key
accessories are
electric.
Grainger Center for Electric Machines and Electromechanics
University of Illinois at Urbana-Champaign
41
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.
Grainger Center for Electric Machines and Electromechanics
University of Illinois at Urbana-Champaign
42
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.
Grainger Center for Electric Machines and Electromechanics
University of Illinois at Urbana-Champaign
43
The End!
Grainger Center for Electric Machines and Electromechanics
University of Illinois at Urbana-Champaign
44
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!
Grainger Center for Electric Machines and Electromechanics
University of Illinois at Urbana-Champaign
45
Electric and Hybrid Gallery
• General Motors EV1.
• 1300 lb battery pack at
312 V, 102 kW motor.
• 0-60 mph in less than 9 s.
• Volvo turbine-based
hybrid prototype.
Grainger Center for Electric Machines and Electromechanics
University of Illinois at Urbana-Champaign
46
Electric and Hybrid Car Gallery
• This Ford Escort was the first “true practical”
prototype hybrid – a complete station wagon.
• Second-gen
diesel hybrid.
Grainger Center for Electric Machines and Electromechanics
University of Illinois at Urbana-Champaign
47
Electric and Hybrid Car Gallery
Grainger Center for Electric Machines and Electromechanics
University of Illinois at Urbana-Champaign
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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.
• 0-60 mph in about 17 s.
• 1500 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.
Grainger Center for Electric Machines and Electromechanics
University of Illinois at Urbana-Champaign
49
Electric and Hybrid Car Gallery
• Toyota architecture 
• Honda architecture:
Grainger Center for Electric Machines and Electromechanics
University of Illinois at Urbana-Champaign
50