The Energy Evolution - Hydrogen and Fuel Cells: Advances in Transportation and Power - page 15

and more conventional fuels is its form. Natural gas is gaseous rather than
liquid in its natural state.
The United States has been a major producer and user of natural gas,
but only a few percent of annual production is used for vehicles, construc-
tion and other equipment including power generation. Compressed natu-
ral gas has been used in about 30,000 vehicles in the United States, which
includes school buses, delivery trucks, and fleet vehicles. Worldwide, about
a million vehicles in thirty-five countries operate on natural gas. Some of
the countries where natural gas is widely used include New Zealand, Italy
and countries of the former Soviet Union. Vehicles that operate on liquid
natural gas have also been used in taxis in Korea and Japan.
Most of the 300 NG filling locations in the United States are used by
private fleets, but about one-third are open to the public. This fuel is more
appropriate for fleet vehicles that operate in limited geographical regions
and that return to a central location every night for refueling.
In 1991 the California Air Resources Board certified a compressed
natural gas (CNG) powered engine as the first alternative fueled engine
certified for use in California. The board also sponsored a program to fuel
school buses with CNG. While CNG has been used for fleet and delivery
vehicles, most tanks hold enough fuel for a little over 100 miles.
While natural gas has been plentiful, supplies are limited and in-
creased demand has caused the cost to increase. Besides the range lim-
itation, natural gas vehicles can cost more due to the need to keep the
fuel under pressure. The weight and size of the pressure tank also reduces
storage space and affects fuel economy.
NATURAL GAS VEHICLES
Most gasoline-powered engines can be converted to dual-fuel en-
gines with natural gas. The conversion does not require the removal of
any of the original equipment. A natural gas pressure tank is added along
with a fuel line to the engine through special mixing equipment. A switch
selects either gasoline or natural gas/propane operation. Diesel vehicles
can also be converted to dual-fuel operation.
Natural gas engines can use lean-burn or stoichiometric combustion.
Lean-burn combustion is similar to that which occurs in diesel engines,
while stoichiometric combustion is more similar to the combustion in a
gasoline engine.
Compressed natural gas has a high octane rating of 120 and produc-