Hypercar - Seminar Report

INTRODUCTION

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.

PRINCIPLES OF HYPERCAR DESIGN

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% 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 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 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

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.

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

Lightweight design

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 challenges addressed included crosswind stability, crashworthiness, sprung-to-unsprung mass ratio, and acoustics.

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 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.

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 especially at high speed. More efficient four-wheel regenerative braking is also possible with this system, further increasing fuel economy.

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 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.

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. 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.

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.

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 drivetrain hardware and suspension, electrically actuated steering, and a high- efficiency run-flat wheel and tyre system. This combination can provide excellent braking, steering, cornering, and maneuverability 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 center of mass.

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 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

(Figure 8). 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.

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.

Control system architecture and software

The vehicle control system architecture relies on distributed integrated control. Intelligent devices (nodes) perform real-time control of local hardware and communicate via multiplexed communications data links. Nodes are functionally grouped to communicate with a specific host controller and other devices using well- developed controller-area-network (CAN) or time-triggered network protocols. (The latter includes redundant hardware and deterministic signal latencies to ensure accurate and timely control of such safety-critical functions as steering, braking, and airbag deployment.) Each host controller manages the objectives of the devices linked to it. Host controllers of different functional groups are mounted together in a modular racking system and communicate via a high-speed data backplane. This modular, three-level architecture provides local autonomous real-time control, data aggregation, centralized control of component objectives, centralized diagnostics, and high reliability and resilience. The central controller runs additional services and applications related to the operation of the vehicle entertainment systems and data communications. It also provides a seamless graphical user interface to all systems on the vehicle for operation and diagnostics.

This system, developed in collaboration with Sun Microsystems and STMicro-electronics has many advantages. First, networking allows data to be shared between components and aggregated to create knowledge about the car's behaviour and its local environment and to create new functions in the vehicle. Networking also reduces the weight, cost, failure modes, and complexity of wiring harnesses: for example, a typical vehicle has approximately 25 wires routed to the driver's-side door, while the Revolution uses four.

The central controller and user interface and the user communications are all handled by a Java embedded server developed by Sun Microsystems and conforming to the Open Services Gateway Initiative (OSGI) standard. This network-centric approach provides high security, resilience, and reliability. Adding approved hardware devices or certified applets is simple and robust, with automatic installation and upgrading during continuous operation. The Revolution's specific software design contains many useful, innovative, and valuable features.

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 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.

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 drivesystem 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.