When engineers at the University of Washington set out to create a vehicle that is cleaner and safer to operate than gas or electric cars, they jokingly named it the smogmobile after a L'il Abner cartoon depicting a car fueled by air pollution. But the vehicle developed by the UW team almost lives up to its name. Running on liquid nitrogen, the smogmobile generates no harmful emissions and actually creates an opportunity for pollutants to be removed from the air as its fuel is produced.
"If you're going to talk about a truly non-polluting car, you have to do something different than gas or electric," explains Abe Hertzberg, professor emeritus of aeronautics and astronautics at the UW and head of the smogmobile project. "We believe a liquid nitrogen vehicle can match the performance and range of an electric car, while still being affordable and easy to maintain and operate. And ecologically, it's a dream come true."
Carl Knowlen, a UW research scientist working on the project, will present a paper on the Smogmobile at the Society of Automotive Engineers Future Transportation & Technology conference and exposition Aug. 7 in San Diego.
The smogmobile is powered by energy from pressure built up when super-cooled liquid nitrogen is heated by ambient air and converts to a gas. The nitrogen gas turns an air motor, which propels the car, then exits the tailpipe. Since the atmosphere already is 78 percent nitrogen, the environmental effect of driving smogmobiles -- even millions of them -- would be virtually undetectable, Hertzberg says.
What really excites this veteran researcher, however, is the potential of liquid nitrogen production to actually reduce air pollution. To make liquid nitrogen, Hertzberg explains, a plant would simply run air through a large refrigeration system and collect the liquid nitrogen as it condenses. In the process, pollutants such as carbon dioxide also are removed from the air and could be disposed of in a benign manner.
Hertzberg pursued the smogmobile project in part to pull the plug on the electric car bandwagon being propelled by laws in California and elsewhere that require 10 percent of all cars sold to be zero-emission vehicles. Heralded as the most environmentally friendly alternative to smog-belching, gas-powered vehicles, electric cars offer chronically poor performance and present pollution and safety problems of their own, UW researchers contend.
Lead-acid batteries, which are used in General Motors' EV1 electric car, have a limited 70- to 90-mile range and threaten to increase heavy-metal pollution. Newer nickel- metal hydride and lithium-ion batteries offer better range and performance, but they are prohibitively expensive and potentially dangerous.
"The safe batteries are an ecological mess and can't go anywhere," Hertzberg explains "The ones that can go anywhere are incredibly expensive and quite dangerous. As the amount of stored energy increases in these batteries, they become like bombs and could do a lot of damage in a traffic accident. You'd have to work really hard to hurt yourself with a liquid nitrogen vehicle. Liquid nitrogen isn't combustible, corrosive or toxic. It's just cold."
Ironically, Hertzberg says, the cold temperature is precisely what stalled previous efforts to develop a liquid nitrogen vehicle. Researchers have known for decades that pressure built up when super-cooled liquid nitrogen is converted to a gas could power a car. But the UW group was the first to develop a heat exchanger system that prevents frost build-up from impeding the conversion process.
The smogmobile's heat exchanger pulls liquid nitrogen from an insulated fuel tank through a series of aluminum tubing coils and specially designed pipes. Engine exhaust and outside air are circulated around the coils and pipes to gradually warm up the nitrogen from a minus-320 F liquid to an ambient-temperature gas. The conversion from liquid to gas expands the volume of the nitrogen 700 times, building sufficient pressure to turn an air motor much like pressure from burning gasoline drives an internal combustion engine.
One major difference, however, is that gasoline is a non-renewable resource so internal combustion engines have become more efficient over the years. With the abundance of air, there hasn't been the same motivation to make air motors use compressed air more efficiently. The motor employed in the smogmobile prototype was built to power a winch for raising ship anchors. It has the high torque needed to propel a vehicle but consumes the converted nitrogen gas fuel as if it, too, was as abundant as air.
"This motor operates at about 20 percent of the efficiency we think is possible, so it greatly reduces the mileage we can achieve on a tank of fuel," explains John Williams, a master's student in the department of aeronautics and astronautics who has worked on the smogmobile project for two years.
Under a $360,000 U.S. Department of Energy grant, Knowlen, Williams and fellow students Peter Vitt and Helene DeParis have built a smogmobile prototype from a converted mail truck. The prototype musters only a fifth of a mile per gallon using the inefficient winch motor. But Hertzberg is seeking a follow-on grant to design a more efficient motor that could achieve two to three miles per gallon in an optimally designed vehicle. This would enable the smogmobile, using a 100-gallon tank, to match the average range for gas-powered vehicles of 250 miles between fill-ups. As large as the 100-gallon tank sounds, Williams said it would still weigh less than the batteries used in electric cars.
The liquid nitrogen vehicle also has the potential to be more economical to operate than either electric- or gas-powered vehicles, according to the UW researchers. Assuming an 8-cent-per-gallon price for mass-produced liquid nitrogen, they predict the smogmobile would cost 3.8 cents per mile to drive. This compares favorably with the 6.2-cent-per-mile cost of operating gas-powered vehicles and electric cars (including the cost of battery replacement every two to three years).
Despite the environmental and economic benefits of liquid nitrogen vehicles, Hertzberg realizes it will be difficult to attract the interest of an automotive industry and public already committed to gas- and electric-powered cars. But he hasn't let that stop him.
"I don't expect any trouble in proving that a liquid nitrogen vehicle can work, I expect trouble in selling the idea," he said. "But this has become a bit of a holy grail for me because it's the right thing to do. This is a car that will work with the environment instead of against it."