Significant energy efficiency technologies that less than ten years ago were thought too long-term to be considered in an assessment of fuel economy potential through 2005 (NRC, 1992), are now available for purchase in at least some OECD countries. The US Partnership for a New Generation of Vehicles (PNGV), the European Car of Tomorrow and Japanese Advanced Clean Energy Vehicle programmes have helped achieve these striking successes. In December 1997, a commercial hybrid electric vehicle was introduced in Japan, demonstrating a near doubling of fuel economy over the Japanese driving cycle for measuring fuel economy and emissions. In 1998, a practical, near zero-emission (considering urban air pollutants) gasoline-powered passenger car was developed, and demonstrated. This achievement established the possibility that modern emissions control technology, combined with scientific fuel reformulation, might be able to achieve virtually any desired level of tailpipe emissions at reasonable cost using conventional fossil fuel resources. Emissions problems now limit the application of lean-burn fuel economy technologies such as the automotive diesel engine. Advanced technologies and cleaner fuels may achieve similar results for lean-burn gasoline and diesel engines in the near future. Such advances in urban air pollutant emissions controls for fossil fuel burning engines reduce the environmental incentives for curbing fossil fuel use by road vehicles. Automotive fuel cells also realized order of magnitude reductions in size and cost, and dramatic improvements in power density. The status of these key technologies is reviewed below.
A hybrid electric vehicle combines an internal combustion engine or other fuelled power source with an electric drivetrain and battery (or other electrical storage device, e.g., an ultracapacitor). Potential efficiency gains involve: (1) recapture of braking energy (with the motor used as generator and captured electricity stored in the battery); (2) potential to downsize the engine, using the motor/battery as power booster; (3) potential to avoid idling losses by turning off the engine or storing unused power in the battery; and (4) increasing average engine efficiency by using the storage and power capacity of the electric drivetrain to keep engine operation away from low efficiency modes. Toyota recently introduced a sophisticated hybrid subcompact auto, the Prius, in Japan and has since introduced a version into the US market. Honda also began selling in model year 2000 its Insight hybrid, a two seater. Ford, GM, Daimler/Chrysler and several others have hybrids in advanced development. The most fuel-efficient hybrid designs can boost fuel economy by as much as 50% at near-constant performance under average driving conditions. The added complexity of the dual powertrain adds significantly to the cost of hybrids, and this could hinder their initial market penetration in countries with low fuel prices, unless policies are adopted to promote them.
Hybrids attain their greatest efficiency advantagepotentially greater than 100%over conventional vehicles in slow stop-and-go traffic, so that their first applications might be urban taxicabs, transit buses, and service vehicles such as garbage trucks. An assessment of the potential for hybridization to reduce energy consumption by medium-sized trucks in urban operations concluded that reductions in l/100km of 23% to 63% could be attained, depending on truck configuration and duty cycle (An et al., 2000).
Testing the Toyota Prius under a variety of driving conditions in Japan, Ishitani et al.,(2000) found that the hybrid electric design gave 40%50% better fuel economy at average speeds above 40 km/h, 70%90% better in city driving at average speeds between 15 and 30 km/h and 100%140% better fuel economy under highly congested conditions with average speeds below 10 km/h. Actual efficiency improvements achieved by hybrids will depend on both design of the vehicle and driving conditions. Much of the efficiency benefit of hybrids is lost in long-distance, constant high-speed driving.
Mass reduction via materials substitution is a potentially important strategy for improving light-duty vehicle fuel economy, because it permits synergistic reductions in engine size without loss of performance. The use of alternative materials to reduce weight has been historically restrained by cost considerations, manufacturing process technology barriers, and difficulty in meeting automotive requirements for surface finish quality, predictable behaviour during crash tests, or repairability. The past few years have seen significant developments in space frame structures, advanced new manufacturing technology for plastics and aluminium, and improved modelling techniques for evaluating deformability and crash properties. Ford has displayed an advanced lightweight prototype that is a mid-size car with a weight of only 900 kg, as compared to vehicles weighing 1450 kg today. Even if some of the more exotic weight-saving materials from Fords prototype were discarded, a weight reduction of 30% or more appears possible. With engine downsizing to maintain a constant ratio of kW/kg, this should produce a 20% fuel economy improvement. Some aluminium-intensive luxury cars have already been introduced (for example, the Audi A8 and the new Volkswagen Lupo with 3l/100km consumption), and Ford is known to be considering the introduction of such a vehicle in the mass market.
According to Bouwman and Moll (1999), 85% of life cycle vehicle energy use occurs in the vehicle use phase, with about 15% accounted for in vehicle production and about 3% recovered in recycling. Mass reductions of 30% to 40% via extensive substitution of aluminium for steel have been incorporated in the designs of advanced, high fuel economy prototypes, improving fuel economy by 20% to 25%. Because the production of aluminium requires more energy than production of steel, and the recycling of aluminium auto bodies is more difficult given current recycling technology, the benefits of substituting aluminium for steel must be assessed by a life cycle analysis of greenhouse gas emissions (efforts are being made to improve aluminium recycling technology, however). Analyses have shown that accounting for life cycle impacts diminishes, but does not eliminate GHG emission reductions caused by the use of aluminium for mass reduction in motor vehicles (Figure 3.9). The amount of reduction, however, is sensitive to several key assumptions. Considering the total life cycle emissions for a typical passenger car in the USA, Das (2000) concluded that higher net emissions in the production plus recycling stages would reduce the potential GHG benefits of aluminium in the vehicle use stage by 6.5% versus conventional steel auto bodies, but by 15.8% versus advanced, ultra-light steel body (ULSAB) designs.
Because the increased emissions come first in the production stage, there is a recovery period before net emissions reductions are realized. Das (2000) found a recovery period of four years versus steel but 10 years versus ultra-light steel auto-bodies (ULSAB) for an aluminium-intensive vehicle. An analysis by Clark (1999) of aluminium versus conventional steel, assuming fewer lifetime kilometres, found a cross-over point at approximately eight years for a single vehicle, but at 15 years for an expanding fleet of aluminium-intensive vehicles. In comparison to ULSAB, the car fleet crossover point was found to be at 33 years. In other OECD countries where lifetime vehicle kilometres may be one-half, or less, the levels of the USA, the cross-over points would be even farther in the future. Sensitivity analyses have shown that the results depend strongly on key assumptions, especially the sources of energy for aluminium production and lifetime vehicle miles.
Bouwman and Moll (1999) obtained similar results in scenarios based on the growing Dutch passenger car fleet. A scenario in which aluminium vehicles were introduced in 2000achieved lower energy use than a steel scenario after 2010. By 2050, the aluminium scenario energy use was 17% below that of the all steel scenario.
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