Transportation Research

Energy Efficient Transportation

Energy efficient automobiles depend on the right automotive systems, energy efficient engines and power trains, appropriate fuels, and policies that encourage energy efficiency. Public policy to promote diversity in transportation modes can also reduce fuel consumption.

Automobile Energy Efficiency Regulatory Standards

U.S. policy to promote energy efficiency in automobiles has relied on the Corporate Average Fuel Economy (CAFE) Standards, first passed into law in 1975. Although CAFE standards led to rapid increases in average fuel economy from the time of their passage until 1985, since that year, the standards have increased only insignificantly. In addition, the averaging provision, separately applied to passenger cars and light duty trucks, has led to many distortions. The National Research Council (NRC) has studied CAFE standards and has proposed many changes in the structure of these standards and in their levels. Professor James Sweeney was a member of that NRC panel and chaired its Policy subcommittee.

Although some of these changes have been applied to light duty trucks, most of the recommendations have not been implemented. However, while gasoline prices were at particularly high levels after hurricanes Rita and Katrina, members of Congress proposed reconsidering increases in the CAFE standards. PEEC continues to study the CAFE standards, their possible improvement, and alternatives to CAFE.

But CAFE standards are not the only regulatory rules that can influence the energy efficiency of vehicles. Other possibilities include feebates, gas-guzzler vehicle scrappage programs, provision of privileges to low-fuel-consuming vehicles (e.g., access to high occupancy "diamond" lanes), application of cap-and-trade carbon limitations to the "carbon commitment" of new vehicles, incentives to create telecommuting infrastructure, and the creation of innovative car/ride share systems. Coupled with technological innovations, these transportation policy initiatives can create a change in the demand for driving. In addition, regulatory policies can decrease use of petroleum by changing the use of vehicles or by mandating changes in the mix of fuels. For example, restrictions against long-duration diesel truck idling, coupled with the provision of electrical air-conditioning at truck stops, could reduce diesel use. Pay-as-you-drive insurance can help consumers to understand the variable costs of driving and can motivate consumers to drive less. Improvements in land-use planning can reduce the need for long-distance commuting to work. PEEC studies a broad range of such policies.

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Electrification of Vehicles

Vehicle-to-grid (V2G) systems using electric-drive vehicles (battery, fuel cell, or plug-in hybrid) with 2-way energy/information meters to allow buying and selling of power offer an intriguing combination of distributed generation, load management, and energy storage. The advantages of developing V2G might include an additional revenue stream for cleaner vehicles, increased stability and reliability of the grid, and, eventually, inexpensive storage and backup for intermittent renewable electricity.

The need to reduce our dependence on oil for our personal vehicles is obvious and extremely important. While national attention has been focused on hydrogen and fuel-cell vehicles, this is, at best, a long-term solution that will not have much impact for at least another decade or two. Meanwhile, there is a groundswell of interest in adding extra batteries to hybrid-electric vehicles (HEVs) so that, for a large fraction of the time, those cars would run on electricity rather than gasoline.

These "plug-in hybrids" (PHEVs) might have on the order of 10 kWh of extra battery storage, charged at night from the grid, which would give these cars the potential to drive about 30 miles each day on electricity alone. Depending on driving patterns, studies suggest that these cars would be capable of in excess of 100 miles of driving per gallon of gasoline used, substituting electricity for gasoline as the prime energy source in the vehicle. At $3.50/gallon and 45 mpg, to the extent PHEVs run on gasoline, the fuel cost would be 7.8 cents/mile while miles driven on off-peak electricity at, say 8 cents/kWh, would cost about 2 cents/mile. If the U.S. Advanced Battery Consortium goals for lithium-ion batteries can be achieved, a 10-kWh battery pack would add about 50 kg (110 lbs) of weight, take up about 1.2 cubic feet of space, cost about $1,000 and last about 3 years with deep discharge every day and considerably longer with more modest discharges. Until such time, however, PHEVs may be less marketable than hybrid vehicles. PHEVs enjoy considerable political support by providing a bridge between those who worry most about imported oil and national security with environmentalists who focus on reducing urban smog and global carbon emissions.

The additional electric power available on board a PHEV would facilitate converting traditional mechanical steering systems to steer-by-wire systems, analogous to the fly-by-wire designs of modern aircraft. Such steering capability could, in turn, lead to vehicles with unprecedented levels of safety and adaptability. Previous work at Stanford has demonstrated how steer-by-wire can be used to deploy a lane keeping assistant that keeps the vehicle from drifting out of the lane without hindering driver commands. Given that 40% of the 40,000 annual traffic fatalities in the U.S. are caused by a collision with a fixed obstacle in the environment, a lane keeping assistant could have an immediate impact on traffic safety.

A steer-by-wire car can also modify its handling characteristics in software, enabling the car to perform differently for different members of the family or respond in a predictable manner regardless of the weight distribution. Such capability could, for instance, allow drivers of more economically or environmentally designed cars to experience the same responsiveness and precision found in sports cars. Best of all, the energy consumption of these advanced steering systems is a fraction of the consumption of conventional hydraulic power steering. Taken as a whole, these advantages provide powerful incentives for increased electrification of vehicles and thus could be expected to increase the market penetration of such vehicles.

Following just behind PHEVs would be battery-powered, all-electric vehicles (EVs). Such EVs would use no gasoline at all, although indirectly they would use additional natural gas in the generation of electricity. Although the General Motors EV1 had very little consumer acceptance and was withdrawn from the market, the long-life of advanced batteries and the steer-by-wire capabilities could create an EV vehicle that would have large competitive advantages over gasoline-fueled vehicles. Such a vehicle could have strong market acceptance.

It is worth exploring the possibilities of a fleet of EVs with a predictable fraction connected to the grid at any given time. The ability of the fleet to both provide power and absorb power suggests using them for grid ancillary services. One ancillary service market, now worth about $9 billion per year, is helping the grid to adjust to frequent (say 400 times/day), short-duration (a few minutes at a time), imbalances between supply and demand. This fine-tuning service is referred to as 'regulation' or 'automatic generation control' (AGC). One study suggests a fleet of 100 EVs participating in this market could produce revenues of almost $5000 per vehicle per year (Kempton and Tomic, 2005). After subtracting the added electronics and decreased battery life, net revenues were estimated at about $2500/vehicle-yr. It is dubious whether such net revenues would be available if significantly more vehicles were connected to the grid, since increasing numbers would reduce the price paid for such AGC. But such a decrease in price would reflect reductions in cost to the electric generation/transmission system. And whether such vehicles could be adequately controlled by the Independent System Operator is not obvious. But exploration of the viability and economic desirability of such a system may lead to new options that would increase the market acceptance of EVs.

With current technology, an all-electric, grid-connected EV displaying the acceleration and handling characteristics of a sports car together with unparalleled safety is indeed possible. The 300 mile range generally desired in passenger vehicles, however, is prohibitively expensive with current battery technology and requires significant compromises to the vehicle design, limiting applicability to light trucks, for instance. While advances in battery technology will continue to improve the feasibility of EVs, it is also worth considering concepts of power modularity in which the power train could be optimized around usual usage patterns. By designing a modular vehicle that could add additional battery or energy capacity as needed, the same car could adapt to its current role on an energy basis. This could result in earlier adoption of electric and fuel cell vehicles, further reductions in energy consumption as a result of lower mass and design flexibility that could provide increased trunk and interior space.

Current driving patterns indicate that a vehicle with a range of 50-100 miles and the ability to quickly recharge could handle almost all trips for almost all families. With advances in lithium-ion batteries resulting in charging times of minutes, ubiquitous grid connectivity could result in continuous recharging at home, work or even while running errands, reducing the range requirements on a vehicle. Thus by carrying a light battery load, it would be possible to have a very light, sporty and extremely efficient vehicle appropriate for everyday use. By reducing the number of batteries, the vehicle could be designed to have more space for cargo or passengers than a conventional vehicle of similar size and performance.

There will of course be times when longer trips are necessary. Given the customization enabled by electrification, these longer trips could be enabled by simply dropping in additional energy capacity. While the simplest extension would be to add additional batteries, it may also be possible to add another power plant, such as a fuel cell, for longer trips. With the electric infrastructure on the EV, this plant would only be required to provide the 30 kW or so necessary to overcome road loads at highway speeds. The existing electric drive train and steer-by-wire would reconfigure to provide the acceleration and handling expected by the driver. In comparison to the plug-in hybrid, such a design would offer increased efficiency by avoiding the need to carry an unused engine during urban driving. The vehicle would be optimized for the vast majority of trips instead of compromising this performance for the occasional longer distance drive. Furthermore, additional energy capacity such as a fuel cell could be rented for short periods instead of owned, enabling adoption before costs and reliability of fuel cells come into line with the demands of the automotive application.

One major question, of course, is whether the additional benefits to the customer will outweigh the inconvenience of switching modules on a vehicle. To gain acceptance, reconfiguring a car for short in-town trips, moderate distances or extended travel should be as simple as changing seats in a minivan. This suggests the value of a prototype vehicle that can convincingly demonstrate advantages of modularity and the benefits arising from electrification.

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Stanford's Mobile Laboratory: The X1 Modular by-Wire Car

Researchers at Stanford have previously built an all-electric drive-by-wire car to demonstrate fault detection and safe by-wire system design. Although it resembles a dune buggy in physical appearance, the P1 testbed vehicle displays handling comparable to a sport sedan and steering more responsive than any production vehicle. With independent left and right front steering and independent rear wheel electric drive, the vehicle has the actuation capability to compensate for failures in one system by using other actuators (driving one wheel while regeneratively braking the other to turn, for instance).

Since it was intended to be a prototype (and was built by four students in a period of only 15 months), P1 is neither highly optimized nor modular. While P1 continues serves as a research test bed, a group of students is currently designing its successor, X1, as a flexible, reconfigurable platform on which different suspensions, interiors or drive train elements can be attached. This modularity, and the addition of a body for a more car-like appearance, will make X1 an ideal technology demonstrator for concepts of electric vehicle safety, performance and grid connectivity.

By connecting to the green dorm, the full system of a grid-connected infrastructure can be demonstrated. Through test drives and performance goals, the desirability and practicality of electric vehicles can be demonstrated to the broader public. Finally, the modular structure will enable a demonstration of the advantages of power modularity in electric vehicles. Professor Chris Gerdes will lead the development of the X1 vehicle and the investigation of power modularity for vehicles. The goal is to add a physical dimension to the research in vehicle electrification and have a tangible prototype of advanced concepts that stakeholders can drive and experience.

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