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ABSTRACT Designing and making cars differently ± emphasizing ultra light weight,
ultralow drag, and integrated design ± can reduce required propulsive power by
about two-thirds. This can make direct-hydrogen fuel cells and commercially
available compressed-hydrogen-gas tanks practical and affordable even at
relatively high early prices. Coordinating such vehicles with deployment of fuel
cells in buildings permits a rapid transition to a climate-safe hydrogen economy
that is profitable at each step starting now.
New manufacturing and design methods can also make these radically more
efficient vehicles cost-competitive and uncompromised, as illustrated by a 2.38-
litre-equivalent-per-100-km midsize sport-utility concept car designed in 2000 by
Hyper car, Inc. Major reductions in the required capital, assembly, space, and
product cycle time can offer key competitive advantages to early adopters. These
changes are increasingly recognized as portents of unprecedented technological
and market transitions that can make cars climate-safe and the car and oil
industries more benign and profitable.
INTRODUCTION
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The hypercar design concept combines an ultralight, ultra-aerodynamic
autobody with a hybrid-electric drive system. This combination would allow
dramatic improvements in fuel efficiency and emissions. Computer models predict
that near-term hypercars of the same size and performance of today’s typical 4–5
passenger family cars would get three times better fuel economy. In the long run,
this factor could surpass five, even approaching ten. Emissions, depending on the
power plant, or APU, would drop between one and three orders of magnitude,
enough to qualify as an “equivalent” zero emission vehicles (EZEV).
In all, hypercars’ fuel efficiency, low emissions, recyclability, and durability
should make them very friendly to the environment. However, environmental
friendliness is currently not a feature that consumers particularly look for when
purchasing a car. Consumers value affordability, safety, durability, performance,
and convenience much more. If a vehicle can not meet these consumer desires as
well as be profitable for its manufacturer, it will not succeed in the marketplace.
Simply put, market acceptance is paramount. As a result, hypercars principally
strive to be more attractive than conventional cars to consumers, on consumers’
own terms, and just as profitable to make.
HISTORY
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Since 1991, Rocky Mountain Institute, a 15-year-old, 43-person
independent non profit resource policy centre, has applied to cars its experience
from advanced electric end-use efficiency. In many technical systems, buildings,
motors, lights, computers, etc., big electrical savings can often be made cheaper
than small savings by achieving multiple benefits from single expenditures. The
marginal cost of savings at first rises more and more steeply (“diminishing
returns”), but then often “tunnels through the cost barrier” and drops down again,
yielding even larger savings at lower cost. RMI hypothesized that the same might
be possible in cars. By 1993, this concept had been established and published and
by 1995 it refined into papers advised by hundreds of informants; and by 1996,
expanded into a major proprietary study emphasizing manufacturing techniques
for high volume and low cost.
Most big changes in modern cars were driven either by government
mandate, subsidy, or taxation motivated by externalities, or by random
fluctuations in oil price. However, the more fundamental shift to hypercars can
instead be driven by customers’ desire for superior cars and manufacturers’ quest
for competitive advantage. Customers will buy hypercars because they’re better
cars, not because they save fuel—just as people buy compact discs instead of vinyl
records. Manufacturers, too, will gain advantage from hypercars’ potentially lower
product cycle time, tooling and equipment investment, assembly space and effort,
and body parts count. Since these features offer decisive competitive advantage to
early adopters, RMI chose in 1993 not to patent and auction its intellectual
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property, but rather, like the open- software development model, to put most of it
prominently into the public domain and maximize competition in exploiting it. In
late 1993, the concept won the Nissan Prize at ISATA (the main European car-
technology conference); in 1994, it was the subject of an ISATA Dedicated
Conference, and began attracting considerable attention. By late 1995, RMI was
providing compartmentalized and nonexclusive support, strategic and technical, to
about a dozen automakers and a dozen intending automakers from other sectors
(such as car parts, electronics, aerospace, polymers, and start-ups, including a
number of alliances and virtual companies).
PRINCIPLES OF HYPERCAR DESIGN
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After a century’s devoted effort by excellent engineers, only ~15–20%
of a modern car’s fuel energy reaches the wheels, and 95% of that moves the car,
not the driver, so only 1% of fuel energy moves the driver. This is not very
gratifying. Its biggest cause is that cars are conventionally made of steel—a
splendid material if mass is either unimportant or advantageous, but heavy enough
to require for brisk acceleration an engine so big that it uses only 4% of its power
in the city, 16% on the highway. This mismatch halves an Otto engine’s
efficiency. Rather than emphasizing incremental improvements to the driveline,
the hypercar designer starts with platform physics, because each unit of saved road
load can save in turn ~5–7 units of fuel that need no longer be burned in order to
deliver that energy to the wheels.
Thus the compounding losses in the driveline, when turned around
backwards, become compounding savings. In typical flat-city driving (Fig. 2),
road loads split fairly evenly between air resistance, rolling resistance, and
braking. Hypercars could have lower curb mass, lower aerodynamic drag, ´ lower
rolling resistance, and lower accessory loads than conventional production
platforms. In a near-term hypercar, irrecoverable losses to air and road drag
plummet. Wheel power is otherwise lost only to braking, which is reduced in
proportion to gross mass and largely regenerated by the wheel motors (70%
recovery wheel-to-wheel has been demonstrated at modest speeds). The hybrid
decouples engine from wheels, eliminating the part-load penalty of the
Otto/mechanical drive train system, so the savings multiplier is no longer 5–7%
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but only ~2–3.5%. Nonetheless, even counting potentially worse conditions in
high-speed driving (because aero drag rises as the cube of speed and there’s less
recoverable braking than in the city), the straightforward parameters illustrated
yield average economy ~41 km/l.7
ULTRALOW DRAG
Hypercars would combine very low drag coefficient CD with compact
packaging for low frontal area A. Several concept cars and GM’s productionized
EV-1 have achieved on-road CD 0.19 (vs. today’s production average ~0.33 and
best production sedan 0.255, or Rumpler’s 0.28 in 1921). With a longer platform’s
lesser rear-end discontinuity; Ford’s 1980s Probe concept cars got wind-tunnel CD
0.152 with passive and 0.137 with active rear-end treatment. Some noted
aerodynamicists believe £0.1, perhaps ~0.08, could be achieved with passive
boundary-layer control analogous to the dimples on a golf-ball. Between that
idealized but perhaps ultimately feasible goal and the 1996 reality of 0.19 lie many
linked opportunities for further improvement without low clearance or excessively
pointy profile.3, 7 Thin-profile recumbent solar race cars illustrate how well side
wind response can be controlled, as in the Spirit of Biel III’s on-track CD of 0.10
at 0° yaw angle but just over 0.08 at 20°.
Production cars have A £2.3 (US av.) to 1.8 m2 (4-seat Honda DX);
well-packaged 4-seat concept cars, 1.71 (GM Ultralite) to 1.64 (Renault Vesta II).
For full comfort, we assume 1.9 for 4–5 or 2.0 for 6 (3+3) occupants. Rolling
resistance is reduced proportionally to both gross mass and coefficient of rolling
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resistance r0. Steel drum test values of r0 are 0.0062 for the best mass produced
radial tires, 0.0048 for the lowest made by 1990 (Goodyear), and the low 0.004s
for the state of the art. On pavement, with toe-in but not wheel-bearing friction, we
assume the EV-1’s empirical 0.0062 (Michelin), which might be further reduced
without sacrificing safety or handling. Such tires are typically hard and relatively
narrow, increasing pressure over the contact patch to help compensate for the car’s
light mass. The wheel motors, being precise and ultra strong digitally controlled
servos, could also be designed to provide all-wheel anti-slip traction and antilock
braking superior to those now available.
ULTRALIGHT MASS
Today’s production platforms have curb mass mc ~1.47 t (RMI’s
simulations add 136 kg for USEPA test mass). Some 1980s concept cars made of
light metal achieved mc <650 kg (Toyota 5-seat AXV diesel 649 kg, Renault 4-
seat Vesta II 475, Peugeot 4-seat ECO 2000 449). But advanced composites can
do better, with carbon-fibre composites acknowledged by Ford and GM experts to
be capable of up to a 67% body in- white (BIW) mc reduction from the 273-kg
steel norm without/372 kg with closures—to ~90/123 kg, vs. the 5- seat Ultra
Light Steel Auto Body’s 205/– kg or the 5–6- seat Ford Aluminium-Intensive
Vehicle’s 148/198 kg. RMI assumes near-term advanced-composite 4–5-seat
BIWs not of 90/123 kg but ~130/150.3,4 In contrast, the 4-seat Esoro H301’s BIW
weighed only 72/150 (using lighter-than-original bumper and door designs for
comparability) far below the carbon GM Ultralite’s 140/191, even though 75% of
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the Esoro’s fiber was glass, far heavier than carbon fibre.2 Of carbon-and-aramid
BIWs, Viking 23’s (1994) weighed 93 kg with closures, while Esoro composites
expert Peter Kägi’s 1989 2-seat OMEKRON’s weighed only 34 kg without
closures. Though these examples differ in spaciousness and safety, they confirm
carbon fibre’s impressive potential for BIW mass reduction. A 115-line-item mass
budget benchmarked to empirical component values indicates that a 130/150-kg
BIW corresponds to mc ~521 kg. Near-term values for a full-sized 3+3 sedan
range upwards to ~700 kg but can be reduced at least to ~600 kg with further
refinement.
Advanced composites are used in Hypercars they offer the greatest
potential for mass reduction. Reducing a vehicle's mass makes it peppier and/or
more fuel-efficient to drive, nimbler to handle, and easier to stop. Experts from
various U.S. and European car companies have estimated that advanced composite
auto bodies could be up to 67 percent lighter than today's steel versions. In
comparison, aluminium is estimated to be able to achieve a 55-percent mass
reduction, and optimized steel around 25-30 percent. So for mass reduction and
fuel economy, advanced composites look especially promising. Their superior
mechanical properties allow them largely to decouple size from mass enabling cars
to be roomy, safe, and ultralight.
HYBRID-ELECTRIC DRIVE
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Hypercars build on the foundation of recent major progress in electric
propulsion, offering its advantages without the disadvantages of big batteries.
Batteries’ deliverable specific energy is so low (~1% that of gasoline) that, as P.D.
van der Koogh notes, “Battery cars are cars for carrying mainly batteries ,but not
very far and not very fast, or else they’d have to carry even more batteries.” This
nicely captures the mass compounding snowballing of weight that limits battery
cars, good though they’re becoming, to niches rather than to the general-purpose
family-vehicle role that dominates at least North American markets. It is
unimportant to this discussion whether Hypercars use series or parallel hybrids.
Both approaches, and others, may offer advantages in particular market segments.
Either way, an onboard auxiliary power unit (APU) converts fuel into electricity as
needed; the APU can be an internal- or external-combustion engine, fuel cell,
miniature gas turbine, or other device. The electricity drives special wheel motors
(conceivably hub motors, but at least in early models probably mounted inboard to
manage sprung/unsprung mass ratios).
The motors may be direct drive or use a single gear, though some designs
might benefit from two gear ratios. A load-levelling device (LLD) buffers the
APU, temporarily stores recovered braking energy, and augments the APU’s
power for hill climbing and acceleration. The LLD can be a high specific- power
battery, ultracapacitor, superflywheel, or combination, typically rated at ~30–50
peak kW. High braking-energy recovery efficiency and reducing the APU map
nearly to a point require high kW/kg plus excellent design and controls.
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Fuel cells are also used as APU because they're very efficient,
produce zero or near-zero emissions (depending on the type and origin of the fuel
used), could be extremely reliable and durable (since they have almost no moving
parts), and could offer a high degree of packaging flexibility. Currently, however,
they're very expensive because they're not produced in volume, and a widespread
refuelling infrastructure doesn't yet exist for some of the fuels considered for their
use. Fuel cells generate electricity directly by chemically combining stored
hydrogen with oxygen from the air to produce electricity and water. The hydrogen
can be either stored onboard or derived by "reforming" gasoline, methanol, or
natural gas (methane). Reforming carbon-containing fuels generates more
emissions than using hydrogen created directly with renewable energy, but these
fuels are much more readily available and may be used as a transitional step until a
hydrogen infrastructure develops. Fuel-cell technology has advanced significantly
in the past few years, and a handful of automakers have shown prototype fuel-cell-
powered vehicles. However, these prototypes have been quite heavy, requiring
large (and therefore expensive) fuel-cell power plants, which have led some
observers to predict that it may take 15 to 20 years for fuel cells to become
economical. Yet Hypercar® vehicles could accelerate the adoption of fuel cells,
because the Hypercar® vehicle's much lower power requirements would require
far less fuel-cell capacity than a heavy, high-drag conventional car.
REVOLUTION CONCEPT CAR DESIGN
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OVERVIEW
The Revolution fuel-cell concept vehicle was developed by Hypercar, Inc. in
2000 to demonstrate the technical feasibility and societal, consumer, and
competitive benefits of holistic vehicle design focused on efficiency and
lightweighting. It was designed to have breakthrough fuel economy and emissions,
meet US and European Motor Vehicle Safety Standards, and meet a rigorous and
complete set of product requirements for a sporty five-passenger SUV crossover
vehicle market segment with technologies that could be in volume production
within five years (Figure 1).
Fig. 1 Revolution concept car photo and layout
The Revolution combines lightweight, aerodynamic, and electrically and
thermally efficient design with a hybridized fuel-cell propulsion system to deliver
the following combination of features with 857 kg kerb mass.2.
Seats five adults in comfort, with a package similar to the Lexus RX-300
(6% shorter overall and 10% lowers than a 2000 Ford Explorer but with
Slightly greater passenger space)
1.95-m3 cargo space with the rear seats folded flat
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2.38 L/100km (99 miles per US gallon) equivalent, using a direct-hydrogen
fuel cell, and simulated for realistic US driving behaviour
530-km range on 3.4 kg of hydrogen stored in commercially available 345-
bar tanks
Zero tailpipe emissions
Accelerates 0±100 km/h in 8.3 seconds
No body damage in impacts up to 10 km/h (crash simulations are described
below)
All-wheel drive with digital traction and vehicle stability control
Ground clearance adjustable from 13 to 20 cm through a semi-active
suspension that adapts to load, speed, location of the vehicle's centre of
gravity, and terrain
Body stiffness and torsional rigidity 50% or more higher than in premium
sports sedans
Designed for a 300 000á-km service life; composite body not susceptible to
rust or fatigue
Modular electronics and software architecture and customizable user
interface
Potential for the sticker price to be competitive with the Lexus RX-300,
Mercedes M320, and BMW X5 3.0, with significantly lower lifecycle cost.
LIGHTWEIGHT DESIGN
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Every system within the Revolution is significantly lighter than
conventional systems to achieve an overall mass saving of 52%. Techniques used
to minimize mass, discussed below, include integration, parts consolidation, and
appropriate application of new technology and lightweight materials. No single
system or materials substitution could have achieved such overall mass savings
without strong whole-car design integration. Many new engineering issues arise
with such a lightweight yet large vehicle. While none are showstoppers, many
required new solutions that were not obvious and demanded a return to
engineering fundamentals.
For example, conventional wheel and tyre systems are engineered
with the assumption that large means heavy. The low mass, large size and high
payload range relative to vehicle mass put unprecedented demands on the
wheel/tyre system. Hypercar, Inc. collaborated with Michelin to design a solution
that would meet these novel targets for traction and handling, design appeal, mass,
and rolling resistance. Another challenge in this unusual design space is vehicle
dynamics with a gross mass to kerb mass ratio around 1.5 (1300 kg gross
mass/857 kg kerb mass). To maintain consistent and predictable car-like driving
behaviour required an adaptive suspension. Most commercially available versions
are heavy, energy-hungry, and costly. Hypercar, Inc. collaborated with Advanced
Motion Technology, Inc. (Ashton, MD) to design a lightweight semi-active
suspension system that could provide variable ride height, load levelling, spring
rate, and damping without consuming excessive amounts of energy. Other unique
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challenges addressed included crosswind stability, crashworthiness, sprung-to-
unsprung mass ratio, and acoustics.
EXTERIOR STYLE AND AERODYNAMICS
The Revolution concept vehicle is designed as a mid-sized, entry-level luxury
sport- utility crossover vehicle (i.e., combining sport-utility with passenger car
characteristics). Its design is contemporary and attractive but aerodynamic.
Fig. 2 An Example Of Aerodynamic Analysis
Some of the aerodynamic features include:
a smooth underbody that tapers up toward the rear to maintain neutral lift
underbody features that limit flow out of the wheel wells
tapered roofline and rear `waistline'
clean trailing edge
rounded front corners and A-pillar
gutter along roofline to trip crosswind airflow
radiator intake at high-pressure zone on vehicle nose
wheel arches designed to minimize wheel-induced turbulence
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aerodynamic door handles.
In addition to the `fixed' design features, other systems also contribute to
the Revolution's aerodynamic performance. For example, the suspension system
lowers ride height during highway driving to minimize frontal area. Also, the
suspension and driveline components do not protrude significantly below the floor
level; this maintains smooth underbody airflow and minimizes frontal area.
Having the rear electric motors in the wheel hubs also eliminates the need for a
driveshaft and differential under the vehicle.
POWERTRAIN
The Revolution powertrain design integrates a 35kW ambient pressure
fuel cell developed by UT Fuel Cells, 35kW nickel metal hydride (NiMH) buffer
batteries, and four electric motors connected to the wheels with single-stage
reduction gears. Three 34.5MPa internally regulated Type IV carbon-fibre tanks
store up to 3.4 kg of hydrogen in an internal volume of 137 L (Fig 3).
The fuel cell system's near- ambient inlet pressure replaces a costly and
energy-intensive air compressor with a simpler and less energy-intensive blower,
raising average fuel efficiency and lowering cost. The commercially available foil-
wound NiMH batteries provide extra power when needed and store energy
captured by the electric motors during regenerative braking. The _3kWh of stored
energy is sufficient for several highway-speed passing manoeuvres at gross
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vehicle mass at grade, and can then gradually taper off available power until the
batteries are depleted, leaving only fuel-cell power available for propulsion until
the driving cycle permits recharging.
Fig3: Revolution Component Packaging
The front two electric motors and brakes are mounted inboard,
connected to the wheels via carbon-fibre half shafts. This minimizes the unsprung
mass of the front wheels and saves mass via shared housing and hardpoint
attachments for the motors and brakes. The front motors are permanent magnet
machines, each peak-rated at 21kW. The rear witched reluctance motors are each
10 peak kW, so they're light enough to mount within the wheel hubs without an
unacceptable sprung/unsprung mass ratio. Hubmotors also allow a low floor in the
rear, and improve underbody aerodynamics by eliminating driveshaft, differential,
and axles. The switched reluctance motors also have low inertia rotors and no
electromagnetic loss when freewheeling, improving overall fuel economy
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especially at high speed. More efficient four-wheel regenerative braking is also
possible with this system, further increasing fuel economy.
Proprietary innovations within the Revolution manage and distribute
power among the drive system components. Powertrain electronics are currently
expensive, and typical fuel cell systems require extensive power conditioning
(using a DC -- DC converter) to maintain a consistent voltage, since at full power,
the stack voltage drops to approximately 50% of its open circuit voltage.
Hypercar, Inc. developed a power electronics control methodology that simplifies
power conditioning while optimally allocating power flows under all conditions.
This cuts the size of the fuel cell DC -- DC converter by about 84%, reducing
system cost, and improves power distribution efficiency, increasing fuel economy.
The normal doubling of radiator size for a fuel cell vehicle doesn't handicap the
Revolution because its tractive load, hence stack size, are reduced more than that
by superior platform physics. The Revolution's cooling system efficiently
regulates the temperature of each powertrain component without resorting to
multiple cooling circuits, which would add weight and cost.
The common-rail cooling has a branch for each main powertrain
component and a small secondary loop for passenger compartment heating. This
loop also includes a small hydrogen-burning heater to supply extra start-up heat
for the passengers when required (though this need is minimized by other aspects
of thermal design). The variable-speed coolant pump, larger-diameter common rail
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circuit, and electrically actuated thermostatic valves ensure sufficient cooling for
all components without excessive pumping energy.
The Revolution's fuel economy was modelled using a second-by-
second vehicle physics model developed by Forschungsgesellschaft
Kraftfahrwesen mbH Aachen (`FKA'), Aachen, Germany. All fuel-economy
analyses were based on the US EPA highway and urban driving cycles, but with
all speeds increased by 30% to emulate real-world driving conditions. Each
driving cycle was run three times in succession to minimize any effect of the
initial LLD state of charge on the fuel economy estimate. In addition to fuel
economy, Hypercar, Inc. simulated how well the Powertrain would meet such load
conditions as start-off at grade at gross vehicle mass, acceleration at both test and
gross vehicle mass, and other variations to ensure that the vehicle would perform
well in diverse driving conditions. Illustrating the team's close integration to
achieve the whole-vehicle design targets, the powertrain team worked closely with
the chassis team to exploit the braking and steering capabilities allowed by all-
wheel electric drive to create redundancy in these safety-critical applications. The
powertrain, packaging, and chassis teams also worked closely together to
distribute the mass of the Powertrain components throughout the vehicle in order
to balance the vehicle and keep its centre of gravity low.
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STRUCTURE
ALUMINIUM AND COMPOSITE FRONT END
The front end of the Revolution body combines aluminium with
advanced composites using each to do what it does best (Fig 4). The front bumper
beam and upper energy-absorbing rail are made from advanced composite. The
rest of the front-end structure is aluminium, with two main roles: to attach all the
front-end Powertrain and chassis components, and as the primary energy-
absorbing member for frontal collisions greater than 24 km/h. Aluminium could
do both tasks with low mass, low fabrication cost (simple extrusions and panels
joined by welding and bonding), and avoidance of the more complex provision of
numerous hardpoints in the composite structure.
Fig4: Aluminium And Composite Front End
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COMPOSITE SAFETY CELL
The overarching challenge to using lightweight materials is cost-
effectiveness. Since polymers and carbon fibre cost more per kilogram and per
unit stiffness than steel, their structural design and manufacturing methods must
provide offsetting cost reductions. Hypercar, Inc.'s design strategy minimized the
total amount of material by optimal selection and efficient use; simplified and
minimized assembly, tooling, parts handling, inventory, scrap, and processing
costs; integrated multipurpose functionality into the structure wherever practical;
and employed a novel manufacturing system for fabricating the individual parts.
OCCUPANT ENVIRONMENT
The occupant environment typically accounts for 30% of the mass and cost of a
new vehicle. Since it is also what users most intimately experience, automakers
pay close attention to design for aesthetic appeal, ergonomics, and comfort. The
Revolution development team was challenged to provide a lightweight interior that
would still meet aggressive safety, comfort, acoustic, thermal, and aesthetic
requirements. The result: much of the inner surface of the carbon-fibre safety cell
is exposed to the interior, and energy-absorbing trim is applied only where needed
to meet FMVSS requirements (Fig 5). The carbon fibre `look' is becoming
increasingly popular in several automotive and non-automotive markets, so this
feature should meet all requirements (light weight, aesthetically appealing, low
cost, and safe) though it may not fit the tastes of all market segments. Other
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interior safety features integrated into the Revolution include front and side
airbags, pretension seatbelts, and sidestick control of steering, braking, and
acceleration. While using a sidestick to control automobiles may take some time to
gain wide consumer acceptance, its safety benefits are compelling. It gets rid of
the steering column and pedals -- the leading sources of injury in collisions
because they are the first things that the driver hits. Without these obstacles, the
seat belt and airbag system have more room to decelerate the driver more gently.
This is especially important for short drivers who typically have to pull their seat
far forward in order to reach the pedals, putting them dangerously close to the
airbag in conventional vehicles.
Fig5 : Revolution Interior
In the Revolution, the seat does not adjust forward and back, only
vertically, so drivers of all sizes will be the same distance from the airbags,
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improving its deployment-speed calibration and increasing overall safety.
Sidesticks also improve accident avoidance. Studies have shown that after a short
familiarization period, sidestick drivers are much better at performing emergency
evasive manoeuvres than are stick-and-pedal drivers, due to finer motor control in
hands than in feet, and greater speed and ease of eye-hand than of eye-hand-foot
coordination. Clearly, more work would be required in this area for sidesticks to
be feasible, but for the purposes of this concept vehicle, the team could
demonstrate sufficient safety benefits to keep them in the final design.
DaimlerChrysler, BMW, and CitroÈ en appear to share this view.
Another user interface safety feature is the LCD screen that replaces
numerous traditional gauges and displays. Placing the screen at the base of the
windshield, centred on the driver's line of sight, allows the driver to change any
vehicle settings via a common interface without greatly shifting the driver's
viewline or focal distance. The multi-function display and the software-rich design
of the vehicle also add such non-safety benefits as the ability to customize the
interface and add new software-based services without adding new hardware.
To adjust settings, the driver or passenger would use voice commands or a small
pod with four buttons and a jogwheel located in the centre console between the
front seat occupants..
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CLIMATE CONTROL
The climate control strategy illustrated in the Revolution design is
intended to deliver superior passenger comfort using one-fourth or less of the
power used in conventional vehicles. This required a systematic approach to
insulation, low thermal mass materials, airflow management, and an efficient air
conditioning compressor system. The foamcore body, the lower-than-metal
thermal mass of the composites, ambient venting, and spectrally selective glazings
greatly reduce unwanted infrared gain, helping cooling requirements drop by a
factor of roughly 4.5. Power required for cooling is then further reduced by heat-
driven desiccant dehumidification and other improvements to the cooling-system
and air-handling design.
Similarly, the Revolution was designed to ensure quick warm up,
controllability, and comfort in very cold climates. The heating system is similar to
that of conventional vehicles, but augmented by radiant heaters, a small hydrogen
burner for quick initial warm up if needed, and a nearly invisible heater/defroster
element embedded in the windshield.
CHASSIS
The chassis system combines semi-active independent suspension at
each corner of the vehicle, electrically actuated carbon-based disc brakes, modular
rear corner drive train hardware and suspension, electrically actuated steering, and
a high- efficiency run-flat wheel and tyre system. This combination can provide
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excellent braking, steering, cornering, and manoeuvrability throughout the
vehicle's payload range and in diverse driving conditions.
SUSPENSION
The Revolution's suspension system combines lightweight aluminium
and advanced-composite members with four pneumatic/electromagnetic linear-
ram suspension struts developed by Advanced Motion Technology, a
pneumatically variable transverse link at each axle, and a digital control system
linked to other vehicle subsystems (Figure 7). The linear rams comprise a variable
air spring and variable electromagnetic damper. The pressure in the air spring can
be increased or decreased to change the static strut length under load and to adjust
the spring rate. The resistance in the damper can be varied in less than one
millisecond, or up to 1000 times per vertical cycle of the strut piston. The overall
suspension system takes advantage of the widely and, in the case of damping,
rapidly tunable characteristics of these components. Thus the same vehicle can
pass terrain that requires high ground clearance, but also ride lower at highway
speeds to improve aerodynamics and drop the centre of mass.
Fig6: Revolution Chassis System
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Each strut is linked transversely (across the vehicle) to counter body roll.
The link itself is isolated so that a failure that might compromise anti-roll stiffness
would not compromise the pneumatic springs. Hydraulic elements connect the
variable pneumatic element at the center of the transverse link to the left and right
struts. The stiffness of the transverse link is adjusted by varying the pressure in the
isolated pneumatic segment. Oversized diaphragms reduce the pressure required in
the variable pneumatic portion of the roll-control link (normally at about 414 - 828
kPa), minimizing the energy required to tune the anti-roll characteristics. The anti-
roll system works in close coordination with the individual electromagnetic struts
to control fast transients in body roll and pitch during acceleration, braking,
cornering, and aerodynamic inputs. Many technologies can provide semi-active
suspension, but the linear rams best fit the Revolution's energy efficiency needs by
regenerating modest amounts of power when damping.
BRAKES
The Revolution's brakes combine electrical actuation with
carbon/carbon brake pads and rotors to achieve high durability and braking
performance at low mass. The front brakes are mounted inboard to reduce
unsprung mass. Carbon/carbon brakes' non-linear friction properties depending on
moisture and temperature are compensated by the electronic braking control,
because the caliper pressure is not physically connected to the driver's brake pedal,
so any nonlinearities between caliper pressure and stopping force are
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automatically corrected. Electrical actuation also eliminates several hydraulic
components, which saves weight, potentially improves reliability, and allows very
fast actuation of anti-lock braking and stability control. The brake calipers and
rotors should last as long as the car.
STEERING
The Revolution's steer-by-wire system has no mechanical link between
the driver and the steered wheels. Instead, dual electric motors apply steering force
to the wheels through low-cost, lightweight bell cranks and tubular composite
mechanical links.
This design permits continuously adjustable steering dynamics and maintains
Ackerman angle over a range of vehicle ride heights, in a modular, energy-
efficient, and relatively low cost package.
Fig7 : Steer by Wire System
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WHEEL AND TYRE SYSTEM
Hypercar collaborated closely with Michelin on the design of the wheel
and tyre system for the Revolution. The PAX1 run-flat tyre system reduces rolling
resistance by 15%, improves safety and security (all four tyres can go flat, yet the
vehicle will still be driveable at highway speeds), and improves packaging (no
need for a spare). The PAX technology is slightly heavier per corner than
conventional wheel/tyre systems, but eliminating the spare tyre reduces total net
mass.
POWER DISTRIBUTION, ELECTRONICS, AND CONTROL SYSTEMS
The Revolution's electrical and electronic systems are network- and
bus-based, reducing mass, cost, complexity, failure modes, and diagnostic
problems compared with traditional dedicated point-to-point signal and power
wiring and specialized connectors. The new architecture also permits almost
infinite flexibility for customer and aftermarket provider upgrades by adding or
changing software. In effect, the Revolution is designed not as a car with chips but
as a computer with wheels.
POWER DISTRIBUTION
All non-traction power is delivered via a 42-volt ring-architecture
power bus, providing fault-tolerant power throughout the vehicle. Components are
connected to the ring main via junction boxes distributed throughout the vehicle,
via either a sub- ring (to maintain fault-tolerance to the device) or a simple branch
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line for non-fault- tolerant devices. The junction boxes are fused so that power can
be supplied to the branches from either leg of the ring main. The benefits of this
system include low mass, high energy efficiency, fault-tolerance, simplicity, and
cost-effectiveness.
COST ANALYSIS
Given the many new technologies in the Revolution, one might
wonder how much such a vehicle might cost to produce.. The engineering team
estimated the vehicle's production cost at a nominal volume of 50 000 units per
year, using extensive anonymous supplier price quotations (for 82% of the
components), plus some in-house and independent consultants' bottom-up cost
modelling for technologies not yet in production. As designed, the vehicle could
be sold profitably at standard mark-ups for US$ 40 -- 45 000 retail. With further
development, Hypercar, Inc. estimates that this price could be reduced to
approximately US$35000 competitive with existing vehicles of similar
performance, features, size, and amenity but lacking Revolution's exciting features
and quintupled fuel economy. This cost estimate is directly related to the starting
point the product requirements. Part of Hypercar, Inc.'s reason for designing a
vehicle for the entry-level luxury sport utility segment was that its price point
would make many of the advanced features affordable. If the product requirements
were instead for a small economy car, it would be designed differently to meet
those requirements, it may not include all the features of the Revolution, and cost
reduction requirements could become more stringent.
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CONCLUSION
The automobile industry is on the threshold of potentially dramatic
change in its materials use and platform design. Ultralight-hybrid hypercars, using
advanced composites for the auto body, may be more attractive to the consumer,
just as profitable to the producer, and much friendlier to the environment than
conventional cars. With careful design and the industrialization of recycling
technologies, hypercars may even increase the recyclability of cars in the future.
Hypercars’ reduced power requirements could make the drive system smaller and
simpler, enabling components to be modular for easy removal and upgrading.
Hypercars using fuel cells are very heavy and very expensive
nowadays. Therefore using composite materials for body parts may not reduce the
mass of the body to the desirable extent. Yet Hypercar vehicles could adopt the
fuel cells, because their much lower power requirements would require far less
fuel-cell capacity than a heavy, high-drag conventional car.
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REFERENCES
1) Lovins, A.B. (1995) `Hypercars: the next industrial revolution', 1993
Asilomar Summer Study on Strategies for Sustainable Transportation 75-95,
American Council for an Energy-Efficient Economy, Washington DC, Publ.
#T95-30.
2) Lovins, A.B. and Lovins, L.H. (1995) `Reinventing the wheels', Atlantic
275(1): 75-86 (Jan.), and `Letters', id. 275(4): 16-18 (Apr.), submitted in
1993, both available as Publ. #T94-29, www.rmi.org/images/other/HC-
ReinventWheels.pdf
3) Cramer, D.R. and Brylawski, M.M. (1996) `Ultra light-hybrid vehicles
design: implications for the recycling industry', Society of Plastics
Engineers Recycling Division,Proceedings 3rd Annual Recycling
Conference, Chicago, IL, 7-8 Nov., Publ. #T96-
4) Website: www.rmi.org, www.hypercar.com.
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