FEATURE ARTICLE
Plug-in Hybrid Vehicles for a Sustainable Future
Appropriately designed hybrid cars will help wean society off petroleum. The necessary technology is available now
Andrew Frank
Going the Last Mile
The more-typical hybrids one finds on American roads today, those being sold by Toyota, Honda and Ford, for example, follow a design philosophy that puts most of the burden of powering the vehicle on the engine. Were these cars to offer an all-electric mode (which they don't), the batteries would hold only enough energy to drive a very short distance.
My colleagues and I like to characterize hybrids according to the ratio of electric power to total power, a number we call the degree of hybridization. This statistic would be 0 for a conventional car and 1 for a purely electric vehicle (although one wouldn't, of course, calculate the degree of hybridization in either of these two extreme cases). Another useful way to gauge the degree of hybridization is simply by noting the all-electric range. A hybrid electric vehicle that can travel, say, 60 miles on battery power alone would be termed an HEV 60.
The hybrid cars being sold in the United States today have degrees of hybridization close to 0.1. And they all operate so as never to let their batteries become appreciably depleted, thus making them all in essence of the HEV 0 class. What is needed are hybrid cars built with larger electric motors, smaller engines, greater amounts of battery storage and energy-management schemes that allow the batteries to become drawn down to a small fraction of their capacity. Such vehicles could go for several tens of miles without using gasoline at all.
The advantage of that approach becomes obvious when one considers how most passenger cars and trucks are used by private citizens. The typical driver travels less than 40 miles a day. Thus having a car with at least a 40-mile range on battery power alone would allow most people to use no gasoline at all on a daily basis if they could recharge their car's batteries at night by plugging them into an electric outlet. This practice would not only save consumers money at the pump, it would at the same time reduce their tailpipe emissions to zero. Those who drive farther than 40 miles a day would, of course, have to use some gasoline in their cars, but much less than they now do.
What are the impediments to building such plug-in hybrid vehicles? The first is cost. The "power-split" propulsion system Toyota currently uses for its immensely popular Prius (an approach that General Motors, DaimlerChrysler and BMW have also chosen) contains two electric motor-generators and a complex arrangement of gears in the transmission. The problem is that these power trains must compete with conventional engines and transmissions, which cost comparatively little, meaning that such hybrid cars would be expensive even if they required no batteries at all.
A simpler propulsion system is needed to be competitive with conventional cars. One possible strategy is to use what is called a series configuration, whereby the engine is employed only to generate electricity, which is then used to charge the battery and to power one or more electric motors coupled to the wheels. That arrangement has the advantage of being mechanically simple, perhaps requiring no transmission at all. It also allows the engine to run always at maximum efficiency. But a series hybrid also has some important drawbacks. For one, it needs to include a separate electric generator distinct from the motor (or motors) driving the wheels. More important, it suffers from the inherent inefficiency of having to convert the mechanical power produced by the engine into electrical power and then back to mechanical power.
The best strategy, in my view, is to use a single electric motor coupled with a simple transmission that links both it and the car's internal combustion engine directly to the wheels, which is termed a parallel hybrid configuration. In particular, I believe that the key is using a continuously variable transmission, which does away with the usual fixed gears and instead allows the ratio between rotation of the engine and the rotation of the wheels to take on whatever value will allow most efficient operation.
Although engineers have been doing research on continuously variable transmissions since the advent of the automobile, the first truly practical and simple design emerged only in the 1980s with work done in the Netherlands by Van Doorne's Transmissie (which is now owned by Bosch). An alternative design, which came from work done at another Dutch firm, Gear Chain Industries, in collaboration with two other companies, DAF and Volvo, soon showed improvements over the Bosch unit. Since that time, further efforts have yielded continuously variable transmissions that can now boast overall efficiencies above 95 percent and that are every bit as durable as gears while producing much less noise. What is more, because these transmissions require only about 25 parts (rather than the several hundred to more than 1,000 pieces found in conventional transmissions), their cost should be considerably lower once manufacturers begin producing them in large numbers. (I should note that the Prius transmission is also continuously variable, but the means used to attain this feature require two motor-generators, making the system more expensive than it needs to be.)
In addition to appropriate transmissions, plug-in hybrids also need high-power electric motors and controllers. Advances in the fabrication of the permanent magnets used in motors has allowed them to shrink considerably over the past three decades. These motors can be used with today's microcomputer controllers and with special semiconductor switches that are capable of handling many amps of current at high voltage levels, both to transfer power to the wheels and to recapture it when the car slows. The total package—including a down-sized internal combustion engine, a continuously variable transmission, modern batteries, an electric motor and control electronics—need weigh no more than a conventional engine and transmission yet can provide up to 60 miles of all-electric range while driving at up to 60 miles per hour using the electric motor alone (and much faster in hybrid mode with the engine running too).
The technology for plug-in hybrids is now advanced enough to allow all classes of vehicles to be manufactured, from the smallest to the largest. In an effort to help demonstrate that fact, my students and I have put together nine plug-in hybrids in the past 15 years, everything from two-seat sports cars to full-size sport-utility vehicles—all of which have 60 miles of all-electric range using ordinary metal-hydride batteries.
The hybrids we have fabricated weigh roughly the same as standard vehicles because the internal combustion engines employed are less than one-third the size of those found in typical cars and because their continuously variable transmissionsaremuch lighter and simpler than conventional multi-speed geared transmissions. The pounds we were able to save in this way could then be put into good-sized electric motors and batteries.
For example, a hybrid car we constructed in 1997 named "Coulomb" (a converted Mercury Sable sedan) has an engine with a displacement of only 660 cubic centimeters, something one finds more typically powering a modest-size motorcycle. Yet that diminutive engine can produce 36 kilowatts, which is more than sufficient for sustained hill climbing. Coulomb also contains an electric motor capable of putting out 75 kilowatts peak power, which allows the car to accelerate from a standstill to 60 miles per hour in only 9 seconds when used in conjunction with its gasoline engine. With the car's 18-kilowatt-hour pack of metal-hydride batteries, the motor can carry the car for 60 miles in all-electric mode.
Advanced lithium-ion batteries now becoming available for automotive use are smaller and lighter than the metal-hydride cells we have so far employed, which will allow for lighter vehicles with the same electric range or ones that can go even farther before they begin to use gasoline. At the moment, the main roadblocks to lithium-ion cells are higher cost, reduced longevity and concerns about safety, but some battery makers claim to have solved these issues with their newest designs. I look forward to testing some of the latest lithium-ion batteries in one of the plug-in hybrids that I am now building with my students. I fully expect that lithium-ion cells of one variety or another will eventually replace metal-hydride batteries in hybrid cars, offering a two- to threefold increase in energy storage for a pack of a given weight, along with a greater ability to absorb energy quickly during regenerative braking and, perhaps, with adequate durability to last for 15 years and 150,000 miles.
Charging time for the batteries in a plug-in hybrid is not nearly as much of an issue as it is for a purely electric car, because the engine can always provide propulsion. Thus the batteries can be charged relatively slowly, which can be done quite efficiently from nothing more elaborate than an ordinary household outlet. What is more, because the power requirements for slow charging are quite modest, the electricity doesn't necessarily have to come from the electric grid—it can also be derived from rooftop photovoltaic panels or from a small wind turbine.
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