Alternative Energy: Cryogenic Heat Engine

Another version of an air-powered car is being developed by researchers at the University of Washington using the concept of a steam engine, except there is no combustion. The Washington researchers use liquid nitrogen as the propellant for their LN2000 prototype air car. The researchers decided to use nitrogen because of its abundance in the atmosphere -- nitrogen makes up about 78 percent of the Earth's atmosphere -- and the availability of liquid nitrogen. There are five components to the LN2000 engine:

• A 24-gallon stainless steel tank.
• A pump that moves the liquid nitrogen to the economizer.
• An economizer that heats the liquid nitrogen with leftover exhaust heat.
• A heat exchanger that boils the liquid nitrogen, creating a high pressure gas.
• An expander, which converts nitrogen's energy into usable power.

The liquid nitrogen, stored at -320 degrees Fahrenheit (-196 degrees Celsius), is vaporized by the heat exchanger. The heat exchanger is the heart of the LN2000's cryogenic engine, which gets its name from the extremely cold temperature at which the liquid nitrogen is stored. Air moving around the vehicle is used to heat the liquid nitrogen to a boil. Once the liquid nitrogen boils, it turns to gas in the same way that heated water forms steam in a steam engine.

Nitrogen gas formed in the heat exchanger expands to about 700 times the volume of its liquid form. This highly pressurized gas is then fed to the expander, where the force of the nitrogen gas is converted into mechanical power by pushing on the engine's pistons. The only exhaust is nitrogen, and since nitrogen is a major part of the atmosphere, the car gives off little pollution. However, the cars may not reduce pollution as much as you think. While no pollution exits the car, the pollution may be shifted to another location. As with the e.Volution car, the LN2000 requires electricity to compress the air. That use of electricity means there is some amount of pollution produced somewhere else.

Some of the leftover heat in the engine's exhaust is cycled back through the engine to the economizer, which preheats the nitrogen before it enters the heat exchanger, increasing efficiency. Two fans at the rear of the vehicle draw in air through the heat exchanger to enhance the transfer of heat to the liquid nitrogen.

The Washington researchers have developed a crude prototype of their car, using a converted 1984 Grumman-Olson Kubvan mail truck. The truck has a radial five-cylinder that produces 15 horsepower with the liquid nitrogen fuel. It also features a five-speed manual transmission. Currently, the vehicle is able to go only about two miles (3.2 km) on a full tank of liquid nitrogen, and its top speed is only 22 mph (35.4 kph). However, because a liquid nitrogen-propelled car will be lighter, the researchers think that a 60-gallon (227 liters) tank will give the LN2000 a potential range of about 200 miles (321.8 km).

With gas prices soaring, as they have over the past two years, it might not be long before many motorists turn to vehicles powered by alternative fuels. Although air-powered vehicles are still behind their gasoline counterparts when it comes to power and performance, they cost less to operate and are arguably more environmentally friendly, which makes them attractive as the future of highway transportation.