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HOME > PAST ISSUE > March-April 2007 > Article Detail

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

The idea of a hybrid vehicle—one that propels itself using both a conventional engine and an electric motor—is not new. Indeed, some hybrid automobiles were produced more than a century ago, when the internal combustion engine was still in its infancy. These cars were designed to address the limited range of existing electric vehicles and the difficulty of starting the early engines, which had to be cranked by hand (a procedure that resulted in not a few broken arms). The early hybrid vehicles, and the purely electric cars that dominated the fledgling automobile industry early in the 20th century, eventually gave way to a proliferation of cars based on the now-ubiquitous internal combustion engine, a development made possible by the relatively low cost and widespread availability of gasoline.

Figure 1. “Yosemite,” a Ford Explorer that was transformed into a plug-in hybridClick to Enlarge Image

As a result, the hybrid concept remained dormant until the 1970s, a decade during which the price of a gallon of gas tripled. The higher cost brought to light an inherent problem of using internal combustion engines in cars: inefficiency. For example, until recently it's been universal practice to keep the engine running at all times, even when the car comes to a stop, because having to restart the engine was viewed as problematical. So the engines in our cars are simply idled, which uses fuel to do essentially nothing. Even when the car is moving, much energy is wasted because the engine is usually throttled—the amount of air it is allowed to take in is restricted, which lowers power to an appropriate level but also acts to diminish efficiency. What's more, fuel is even consumed in slowing the car, because this is normally done by throttling back the engine after running at speed.

Such inefficiencies are not so worrisome for aircraft or ships, which seldom idle and infrequently run at anything other than an optimal speed. But automobiles typically operate at very low power levels. In the city, the average speed of a car is just 19 miles per hour, which requires around 9 kilowatts (equivalent to 12 horsepower). Yet the driving public demands rapid acceleration from its cars, which requires something like 150 kilowatts or 200 horsepower. Thus our cars usually operate with their engines throttled way back so as to use only a small fraction of their available power. With highway driving, which uses 12 kilowatts or about 16 horsepower (for a medium-sized sedan cruising at 65 miles per hour), somewhat better fuel economy can be realized. But even at highway speeds, the efficiency is far from what could be achieved were automobile engines not throttled so much.




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