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
Space-Age Quest
The inherent inefficiency of the engines used to power cars and trucks has long been recognized. So when the price and availability of gasoline became a concern three decades ago, automotive engineers began to search for better alternatives—and the hybrid-car concept looked quite interesting.
In the 1970s, I was a new professor at the University of Wisconsin, having come from more than a decade in industry where I worked on helicopters, missiles and spacecraft sent to the Moon. I was thus armed with knowledge of the latest aerospace technology, so it was natural for me to want to tackle the problem of greatly improving the fuel efficiency of the automobile. I set the personal goal of designing a car that could get 100 miles per gallon while providing performance equal to or better than the conventional car up to 65 miles per hour (somewhat faster than the 55-mile-per-hour national speed limit set in 1974). But I didn't announce my aims to the agencies funding me at the time for fear that I would be scoffed at.
I studied the possibility of reaching my targets using a hybrid that exploited the technology of the day. But I quickly discovered that the kinds of batteries then available (lead-acid, nickel-cadmium or iron-nickel Edison cells) were too heavy and could not produce the needed power or store the energy required to provide the desired power-leveling effect. In addition, although the concept of an automobile with two energy supplies on board remained conceptually sound, it was difficult to imagine the average American grandmother driving a vehicle with more controls than a steering wheel, accelerator and brake pedal. How then was the driver to manage the two separate power sources? Another thorny issue was the need to have enough power available to perform any maneuver desired by the driver—whether that be passing a truck on a grade or bringing the vehicle swiftly to highway speed on an uphill on-ramp.
One way to satisfy this requirement is to keep the engine at about the same size as it is in a conventional car. Then a hybrid vehicle can always revert to its engine to provide the power needed. Another way is to have a large battery pack with enough energy for the longest anticipated maneuver—an approach that allows the designer to reduce the size of the engine and thus save weight. Beginning in 1972, my students and I began exploring these strategies by designing and constructing various prototype vehicles for contests run by the U.S. Department of Transportation and later by the Department of Energy (DOE). With our early work, the challenges of engineering hybrid vehicles became abundantly clear.
One of the missing pieces at the time was an efficient and lightweight energy-storage device that could instantaneously produce high levels of power. Another gap in technology was a computer that could manage multiple sources of energy efficiently and automatically. It would have to deliver high power to the wheels when required and also, when the car needed to be slowed, to transform the kinetic energy of motion into electricity for charging the battery so this energy could later be reused. Another element that was lacking was a transmission system that could efficiently handle more than one power plant.
My research, and that of many of my contemporaries in automotive engineering, immediately moved to finding technical solutions for these challenges. Our first efforts were directed at developing a way to store energy other than in an electrochemical battery. About this time the DOE learned of research on energy-storage flywheels and started a program to develop mobile flywheels that could store enough energy to drive 100 or more miles. Rough calculations published in an article in Scientific American showed that it was possible to design a flywheel to do this. There were, however, a few technical issues not mentioned in the article. In particular, it glossed over the difficulty of making a device for vehicular use that could safely carry an enormous amount of kinetic energy in a spinning mass.
The idea that had sold everyone on this approach was that a flywheel made of composite material would be inherently safe, because if it failed it would suddenly become a group of randomly arranged fibers, and all the energy would be dissipated as heat. The problem is that the law of conservation of momentum makes other, more dangerous failure modes more likely, and safety precautions were needed to protect from them. The best researchers in industry and government labs missed this simple fact. After a few disastrous accidents and a lot of money spent, the DOE program was canceled at the end of the ‘70s. In the meantime, I had constructed two flywheel-powered cars and demonstrated that such a vehicle could achieve 50 percent better fuel economy than was then typical. The flywheel hybrid could get about 35 miles per gallon, but that would be about the limit—a long way from my original 100-mile-per-gallon goal.
One reason that these flywheels didn't work out better was that they stored very little energy for their weight and bulk. To give a modern perspective on the problem, the flywheel systems my colleagues and I built into these hybrid cars weighed about 500 pounds, but the same amount of mass today in advanced electrochemical batteries can hold 20 to 30 times more energy. Because the flywheels could not store enough energy for long maneuvers, the engines in these hybrid cars could not be made any smaller than normal. Thus the considerable added weight of the flywheel could not be balanced by adopting a lighter engine.
In the 1990s, the California Air Resources Board, the authority charged with cleaning up the state's smoggy skies, realized that it would be possible to build zero-emission cars powered only with electricity from the power grid. Use of such vehicles would shift emissions from tailpipes to central power plants, where pollutants could be more easily controlled. The key was a good electrochemical battery. Research by Ovonic Battery Company and a consortium of other battery makers showed that the metal-hydride battery could store enough energy to make an electric vehicle practical, and many companies around the world began research in this area.
Further thrust came from a government-industry effort (spearheaded by Al Gore, then the vice president) called the Partnership for a New Generation of Vehicles. The managers of that program decided that California's electric-car initiatives were too ambitious, so they set the goal of constructing a gasoline-burning vehicle that could get three times the fuel economy of a conventional automobile. The only way to achieve that kind of mileage was with a hybrid, which would require electrochemical cells of one sort or another. The prospect that auto makers might build such cars in larger numbers further spurred research on electrochemical cells.
By about 1995, battery technology had advanced to a point where it was sufficient for hybrid cars. The next task facing designers was to bring the overall cost down to a level that would be competitive. In the late 1990s, Toyota and Honda introduced hybrid vehicles into the marketplace, demonstrating that considerably improved fuel economy can be achieved at a reasonable price. Today every other car company is suddenly realizing that many people are willing to pay more for better gas mileage, which should create enough sales volume to drive the costs down even further. Thus the hybrid power train is being established as a modern automotive standard.
The hybrid cars being sold today are showing improvements of 20 to 30 percent in fuel economy, but they still fall far short of the goalposts I set for myself in 1970: 100 miles per gallon, along with better performance. I and many others are convinced that the solution is to design hybrid cars in a way that many car makers are resisting—so that they can be recharged by plugging them into an ordinary electrical outlet and can travel a considerable distance on electric power alone. Only two plug-in hybrids are now on the market, and both are being sold only in Europe: a version of the Mercedes Sprinter delivery van and the "elect'road" variant of the Renault Kangoo, which is really an electric car with a gasoline-powered "extender" option. General Motors has just recently announced plans to offer a plug-in hybrid in the United States, a version of its Saturn Vue sport-utility vehicle, which is expected to be able to travel something like 10 miles on battery power alone. And in January GM unveiled a concept car dubbed the Volt, a plug-in hybrid with 40 miles of all-electric range. However, it remains unclear when these new GM vehicles might go into production.
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