Ultralight Hybrid Vehicles: Principles and Design

The technical feasibility of superefficient family cars has been demonstrated. Yet it has typically compromised vehicle performance, safety, cost, manufacturability, or marketability. Industry experimentation has tended to focus on improving performance, or on implementing hybrid-electric drivesystems in essentially conventional vehicles, or on reducing mass and drag, or on improving safety—but has rarely attempted to optimize all of these as a system. Maximizing benefits through synergies between platform, chassis-component, and drivesystem design parameters seems poorly understood. Whole-system engineering-design is essential to move toward commercial viability. Second-by-second simulations and performance modeling provide evidence for automobiles 3–4x more fuel-efficient than today’s, with emissions approximating the California Air Resource Board’s proposed Equivalent Zero Emission Vehicle requirement for hybrids (~0.1 x ULEV), and with safety, performance, and marketability surpassing that of many current automobiles. The commercial success of such designs depends on the concurrent optimization of numerous parameters, with emphasis on tractiveand accessory-load reduction and on component and control optimization. Platform optimization, subject to appropriate design criteria, must precede or accompany new drivesystem technologies, because only tractive-load reduction makes hybrid drivesystems commercially viable. Thus the artful combination of hybridelectric drive with lightweight, low-drag platform design appears requisite to the cost-effective optimization of efficiency, emissions, performance, and safety for production worthy and marketable automobiles. 1. DESIGN CRITERIA Industry design criteria for efficient vehicles have tended to focus on limiting compromises in performance rather than on improving it. The one criterion of marketability not typically spelled out is that such vehicles must not only be equivalent to those they displace, but be in some way more attractive to customers. RMI’s analysis suggests that efficient designs could yield generally improved acceleration, handling, braking, safety, and durability. Since fuel economy and emissions are low on the list of criteria for most consumers today, and may be lower in the future, efficient vehicles must be better in other respects if they are to gain the large market share required to provide significant societal benefits. The following criteria (based in part on similar criteria developed by the US Partnership for a New Generation of Vehicles) appear essential for the U.S. market and were assumed for this analysis (all improvements are relative to current touring-class production sedans): • Acceleration from 0–100 km/h in 8.5 s at test mass (half-full fuel tank; two 68-kg occupants), and 12 s at gross mass (five 68-kg occupants; 91 kg luggage). • Gradability sufficient to maintain 105 km/h on a 6.5% grade at test mass, and 90 km/h at gross mass for 20 min. Acceleration should also be reasonable on grades to facilitate safe merging on steep highway entrance ramps, suggesting 0–100 km/h acceleration in ~15 s at gross mass on a 5% grade. • Improved handling, maneuverability, tire adhesion, antilock braking, and traction control. • Improved crashworthiness, interior safety features, and ease, speed, and safety of post-crash extrication. • Combined urban/highway range of 640 km. • At least equivalent ride, handling, and control of noise, vibration, and harshness. • Carrying capacity for the gross-mass load and occupants with equivalent comfort and cargo space. • Useful life of 320,000 km, maintenance of original performance and emissions specifications for at least 160,000 km, improved service intervals, and comparable reliability and refueling time. • Equivalent or improved customer features, such as climate control and entertainment systems, and total real cost of ownership. 2. PROPULSION SYSTEMS Efforts to meet California’s Zero-Emission-Vehicle and Ultra-Low-Emission-Vehicle or similar mandates have spawned major advances in propulsion systems: e.g., motors and controllers with high specific power and system efficiencies well over 90% for much of their usable range –6 and load-leveling devices (LLDs) capable of meeting real-world hybrid vehicle requirements with careful systems integration. –10 These advances enable auxiliary power unit (APU) technologies that aren’t well suited for conventional cars, but work well in hybrids when accompanied by efficient electric drives. New APU options include gas turbines, Stirling engines, thermophotovoltaic burners, and fuel cells. Among them, Stirling engines capable of maintaining η ≥0.38 over a wide range of speeds and loads while far surpassing ULEV standards , and hydrogen fuel cells with peak η ~0.60 at part load, stand out as strong contenders for nearand mid-term introduction, respectively. Most of these technologies have been around for decades, but until recently were not sufficiently developed or were not enabled by other key technologies for automobiles. Many would still be overly complex, bulky, and probably cost-prohibitive if applied to conventional cars or to heavy battery-electric cars.