FUEL CELLS—ENERGY FOR A POWER-HUNGRY WORLD - Chemistry: Notable Research and Discoveries - page 148

Yet the advantages of fuel cells over heat engines are not just
economical. In addition to heat, gasoline engines emit a lot of
pollutants.
AClEAnSourCEoFEnErgy
Gasoline consists of a mixture of hydrocarbon molecules, which are
long chains of hydrogen and carbon, along with a few other added
chemicals. Octane, for example, contains eight carbon atoms and 18
hydrogen atoms and is an important component of gasoline. Burning
a substance such as a hydrocarbon is the result of a rapid reaction with
oxygen (or some other oxidizing agent), which produces a lot of heat. In
a cylinder of an automobile gasoline engine, a spark from a spark plug
ignites a mixture of gasoline and air, resulting in an explosive reaction
that creates hot, expanding gases. These gases push against a piston,
turning the car's crankshaft. The gases are then released into the exhaust
to prepare for the next combustion cycle. (Exhaust gases are still rela-
tively hot—the second law of thermodynamics requires this heat loss.)
If all of the hydrocarbons in the fuel were burned, the gaseous products
would be water, H 2 O, and carbon dioxide, CO 2 .
The release of carbon dioxide into the environment has come un-
der scrutiny lately because of its possible connection to global climate
change. A single car's emission is not a big problem, but the exhaust of
millions of cars, along with other combustion engines, adds up.
The emission problem is made worse by the incomplete combus-
tion that occurs in most heat engines. Automobile engines produce a
lot of power, which requires a lot of activity in each cylinder (up to 15
or 20 combustions per second, and even more for high-performance
vehicles). The cycle happens so quickly that some of the fuel does not
burn completely. As a result, the gases emitted in the car's exhaust not
only consist of water and carbon dioxide, they also contain unburned
hydrocarbons as well as noxious chemicals such as carbon monoxide,
which is a poisonous gas. In addition, nitrogen oxides such as nitrogen
dioxide (NO 2 ) are created by the high temperature of the engine and
the presence of air, which is about 78 percent nitrogen and 21 percent
oxygen by volume. Nitrogen oxides contribute to the stifling smog and
haze that is often experienced in large cities.
Polluted air smells bad and makes city dwellers long for fresh air.
But air pollution has even more dire effects for both animals and hu-