We examine the life cycle implications of a wide range of fuels and propulsion systems that could power cars and light trucks in the US and Canada over the next two to three decades ((1) reformulated gasoline and diesel, (2) compressed natural gas, (3) methanol and ethanol, (4) liquid petroleum gas, (5) liquefied natural gas, (6) Fischer–Tropsch liquids from natural gas, (7) hydrogen, and (8) electricity; (a) spark ignition port injection engines, (b) spark ignition direct injection engines, (c) compression ignition engines, (d) electric motors with battery power, (e) hybrid electric propulsion options, and (f) fuel cells). We review recent studies to evaluate the environmental, performance, and cost characteristics of fuel/propulsion technology combinations that are currently available or will be available in the next few decades. Only options that could power a significant proportion of the personal transportation fleet are investigated.Contradictions among the goals of customers, manufacturers, and society have led society to assert control through extensive regulation of fuel composition, vehicle emissions, and fuel economy. Changes in social goals, fuel-engine-emissions technologies, fuel availability, and customer desires require a rethinking of current regulations as well as the design of vehicles and fuels that will appeal to consumers over the next decades.The almost 250 million light-duty vehicles (LDV; cars and light trucks) in the US and Canada are responsible for about 14% of the economic activity in these countries for the year 2002. These vehicles are among our most important personal assets and liabilities, since they are generally the second most expensive asset we own, costing almost $100 000 over the lifetime of a vehicle. While an essential part of our lifestyles and economies, in the US, for example, the light-duty fleet is also responsible for 42 000 highways deaths, and four million injuries each year, consumes almost half of the petroleum used, and causes large amounts of illness and premature death due to the emissions of air pollutants (e.g. nitrogen oxides, carbon monoxide, hydrocarbons and particles).The search for new technologies and fuels has been driven by regulators, not the marketplace. Absent regulation, most consumers would demand larger, more powerful vehicles, ignoring fuel economy and emissions of pollutants and greenhouse gases; the vehicles that get more than 35 mpg make up less than 1% of new car sales. Federal regulators require increased vehicle safety, decreased pollution emissions, and better fuel economy. In addition, California and Canadian regulators are concerned about lowering greenhouse gas emissions. Many people worry about the US dependence on imported petroleum, and people in both countries desire a switch from petroleum to a more sustainable fuel.The fuel-technology combinations and vehicle attributes of concern to drivers and regulators are examined along with our final evaluation of the alternatives compared to a conventional gasoline-fueled spark ignition port injection automobile.When the US Congress passed laws intended to increase safety, decrease emissions, and increase fuel economy, they did not realize that these goals were contradictory. For example, increasing safety requires increasing weight, which lowers fuel economy; decreasing emissions generally decreases engine efficiency. By spending more money or by reducing the performance of the vehicle, most of the attributes can be improved without harming others. For example, spending more money can lighten the vehicle (as with an aluminum frame with greater energy absorbing capacity), improving performance and safety; a smaller engine can increase fuel economy without diminishing safety or increasing pollution emissions, but performance suffers; modern electronics have improved performance, fuel economy, and lowered emissions, but have increased the price of the vehicle. However, low price and performance are important attributes of a vehicle. To resolve these contradictions, regulators in the US and Canada need to specify the desired tradeoffs among safety, emissions, fuel economy, and cost, and a single agency needs to be designated in each country to oversee the tradeoffs among the regulators’ attributes and those desired by consumers.We discuss methods needed to evaluate the attractiveness of vehicles employing alternative fuels and propulsion systems including:Predicting the vehicle attributes and tradeoffs among these attributes that consumers will find appealing;assessing current and near term technologies to predict the primary attributes of each fuel and propulsion system as well as its externalities and secondary effects;applying a life cycle assessment approach;completing a benefit–cost analysis to quantify the net social benefit of each alternative system;assessing the comparative advantages of centralized command and control regulation versus the use of market incentives;characterizing and quantifying uncertainty.An especially important feature of the analysis is ensuring that vehicles to be compared are similar on the basis of size, safety, acceleration, range, fuel economy, emissions and other vehicle attributes. Since it is nearly impossible to find two vehicles that are identical, we use the criterion of asking whether consumers (and regulators) consider them to be comparable. Comparability has proven to be a difficult task for analysts. No one has managed a fully satisfactory method for adjustment, although some have made progress. Absurd comparisons, such as comparing the fuel economy of a Metro to that of an Expedition, have not been made because of the good sense of analysts. However, steps should be taken to achieve further progress in developing methods to address this issue.Comparing fuels and propulsion systems require a comprehensive, quantitative, life cycle approach to the analysis. It must be more encompassing than ‘well-to-wheels’ analysis. Well-to-wheels is comprised of two components, the ‘well-to-tank’ (all activities involved in producing the fuel) and ‘tank-to-wheel’ (the operation/driving of the vehicle). The analyses must include the extraction of all raw materials, fuel production, infrastructure requirements, component manufacture, vehicle manufacture, use, and end-of-life phases of the vehicle. Focusing on a portion of the system can be misleading. The analysis must be quantitative and include the array of environmental discharges, as well as life cycle cost information, since each fuel and propulsion system has its comparative advantages. Comparing systems requires knowing how much better each alternative is with respect to some dimensions and how much worse it is with respect to others. Since focusing on a single stage or attribute of a system can be misleading, e.g. only tailpipe emissions, we explore the life cycle implications of each fuel and propulsion technology. For example, the California Air Resources Board focused on tailpipe emissions in requiring zero emissions vehicles, neglecting the other attributes of battery-powered cars, such as other environmental discharges, cost, consumer acceptance and performance. The necessity of examining the whole life cycle and all the attributes is demonstrated by the fact that CARB had to rescind its requirement that 2% of new vehicles sold in 1998 and 10% sold in 2003 be zero emissions vehicles.No one fuel/propulsion system dominates the others on all the dimensions in Table 8Table 8Evaluation of attributes for fuel/propulsion technologies relative to a conventional automobile (see scale at bottom of table)RFG+SIPIRFG+SIDIDiesel+CIDICNG+SIPIEthanol+SIPIBattery EVGasoline+HEVGasoline+fuel cellH2 Ren.+fuel cellEnvironmentalNear term—local air pollution (vehicle)A-1: ozone++−−+++−++++A-2: PM+−−−−+=++++++A-3: air toxics+++−−++=++++++A-4: fuel cycle emissions−++++−−+??Long termA-5: global warming=++++++=+=+A-6: fossil fuel depletion=+++++=+=++Vehicle attributesB-1: range=+++−−−−−++??B-2: performance===−−++−??CostsC-1: vehicle cost==−−=−−−−−−−−C-2: fuel cost−++++−−−−+=?C-3: infrastructure cost===−−−−−=−−−−Other social issuesD-1: energy independence=+++++++=++D-2: safety=====++=??RFG+SIPI: reformulated gasoline fuelled spark ignition port fuel injection (SIPI) vehicle; RFG+SIDI: reformulated gasoline fuelled spark ignition direct injection; Diesel+CIDI: conventional diesel fuelled compression ignition direct injection; CNG+SIPI: compressed natural gas fuelled SIPI; Ethanol+SIPI: ethanol from biomass feedstocks with no fossil fuel inputs fuelled SIPI; Battery EV: battery powered electric vehicle; Gasoline+HEV: gasoline fuelled hybrid electric vehicle with SIPI; Gasoline+fuel cell: gasoline fuel cell vehicle (with reformer); H2 Ren.+fuel cell: hydrogen from renewables fuelled fuel cell vehicle.Relative to SIPI federal standard gasoline combination. =: fuel/propulsion system combination is about the same; +: fuel/propulsion system combination is slightly better; −: fuel/propulsion system combination is slightly worse; ++: fuel/propulsion system is better; −−: fuel/propulsion system is worse; ?: unknown.. This means that society must decide which attributes are more important, as well as the tradeoffs among attributes. For example, higher manufacturing cost could be offset by lower fuel costs over the life of the vehicle. Changes in social goals, technology, fuel options, customer desires, and public policy since 1970 have changed vehicle design, fuel production, manufacturing plants, and infrastructure. In particular, gasoline or diesel in an internal combustion engine (ICE) is currently the cheapest system and is likely to continue to be the cheapest system through 2020. These vehicles will continue to evolve with improvements in performance, safety, fuel economy, and lower pollution emissions. However, if society desires a more sustainable system or one that emits significantly less greenhouse gases, consumers will have to pay more for an alternative fuel or propulsion system.We review a dozen life cycle studies that have examined LDV, comparing different fuels and/or propulsion systems. The studies are summarized in . The studies vary in the fuel/propulsion options they consider, the environmental burdens they report, and the assumptions they employ, making it difficult to compare results. However, all of the studies include the ‘well-to-tank’ and ‘tank-to-wheel’ activities and the majority of the studies include a measure of efficiency and greenhouse gas emissions associated with these activities. We limit our comparison to these activities and measures. The life cycle studies match most closely for the well-to-tank portion and for conventional fossil fuels. See for a summary of the ranges of efficiency and greenhouse gas emissions reported in the studies for the well-to-tank portion for the various options. For the well-to-tank portion for the production of electricity, renewable fuels, and hydrogen, differing fuel production pathways are most important. Due to the range of different production options for these fuels (as well as other issues such as study assumptions), results are much more variable. In addition, there is less experience with producing these fuels, resulting in more uncertainty. It is important to distinguish between total and fossil energy required for production when comparing efficiencies among the fuels. Petroleum-based fuels have the highest efficiency for the well-to-tank portion when total energy is considered. However, if only fossil energy is considered, biomass-based fuels such as ethanol become more attractive.The tank-to-wheel portions are more difficult to compare. Each study uses its selected vehicle (e.g. conventional sedans, light-weight sedans, pickup trucks); many present assumptions regarding the vehicle efficiencies. However, the studies do not generally report the range of assumptions or test conditions.The well-to-wheel results (the sum of the well-to-tank and tank-to-wheel activities) of the studies are still more difficult to compare. The baseline vehicle (with a few exceptions) is a current gasoline fueled ICE port fuel injection vehicle; it combines an efficient well-to-tank portion with a relatively inefficient tank-to-wheel portion. A direct injection diesel vehicle is considerably more efficient and therefore results in lower emissions of carbon dioxide even though the carbon content in the diesel is higher than that in gasoline. Fuel cell vehicles have a high theoretical efficiency but generally a low efficiency well-to-tank portion, which offsets some of the vehicle efficiency benefits. shows the ranges of values reported in the life cycle studies for the well-to-wheel greenhouse gas emissions. All of the fossil fuel options result in emissions of large amounts of greenhouse gases. Ethanol and hydrogen have the potential to reduce greenhouse gas emissions significantly. However, this is highly dependent on the pathways for ethanol and hydrogen production, especially the amount of fossil fuel inputs during production. Some of the hydrogen options result in higher greenhouse gas emissions than those of a gasoline ICE vehicle. Results for hybrid electric vehicles (HEVs) are dependent on the efficiency improvements over conventional vehicles that are assumed.As noted above, Table 8 summarizes our best judgment as to how each fuel/propulsion system combination would be evaluated on each attribute desired by consumers or society. No one system beats the alternatives on all dimensions. The most desirable system is defined by the properties that the evaluator thinks are most important.Despite the many difficulties and complexities, there are some broad conclusions regarding LDV for the next two to three decades. The vehicle options likely to be competitive during the next two decades are those using improved ICEs, including HEVs burning ‘clean’ gasoline or diesel. An extensive infrastructure has been developed to locate, extract, transport, refine, and retail gasoline and diesel. Any alternative to petroleum would require a new infrastructure with attendant disruption and costs running to trillions of dollars. The current infrastructure is a major reason for continuing to use gasoline and diesel fuels.Absent a breakthrough in electrochemistry, battery-powered vehicles will remain expensive and have an unattractive range. The failure to produce a breakthrough despite considerable research does not give much hope that vastly superior, inexpensive batteries will be produced within our time frame.Fuel cell propulsion systems are unlikely to be competitive before 2020, if they are ever competitive. Although, fuel cells have high theoretical efficiencies, and do not need a tailpipe and therefore have vehicle emissions benefits over conventional vehicles, generating the hydrogen and getting it to the vehicle requires large amounts of energy. The current well-to-wheel analyses show that using a liquid fuel and onboard reforming produces a system inferior to gasoline powered ICEs on the basis of efficiency and environmental discharges. Storage of the hydrogen onboard the vehicle is another challenge.Fischer–Tropsch liquids from natural gas and ethanol from biomass may become widespread. The Fischer–Tropsch liquids will penetrate if there are large amounts of stranded natural gas selling for very low prices at the same time that petroleum is expensive or extremely low sulfur is required in diesel fuel. Ethanol could become the dominant fuel if energy independence, sustainability, or very low carbon dioxide emissions become important—or if petroleum prices double.Absent major technology breakthroughs, a doubling of petroleum prices, or stringent regulation of fuel economy or greenhouse gas emissions, the 2030 LDV will be powered by a gasoline ICE. The continuing progress in increasing engine efficiency, lowering emissions, and supplying inexpensive gasoline makes it extremely difficult for any of the alternative fuels or propulsion technologies to displace the gasoline (diesel) fueled ICE.This conclusion should not be interpreted as one of despair or pessimism. Rather, the progress in improving the ICE and providing gasoline/diesel at low price has obviated the need for alternative technologies. Many of the technologies that we examine, such as cellulosic ethanol or Fishcher–Tropsch fuels from natural gas or HEVs are attractive. If there were no further progress in improving the gasoline/diesel fuel ICE or the fuel became more expensive, one or more of these options would take over the market. Thus, the fact that the current fuel and technology is so hard to displace means that society is getting what it wants at low cost.Extensive progress has been made by analysts in examining the life cycles of a range of fuels and propulsion systems for personal transportation vehicles. The most important contribution of these methods and studies is getting decision-makers to focus on the important attributes and to avoid looking only at one aspect of the fuel cycle or propulsion system or at only one media for environmental burdens. The current state of knowledge should avoid the recurrence of the fiasco of requiring battery-powered cars on the grounds that they are good for the environment and will appeal to consumers. [Copyright &y& Elsevier]