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Natural gas is a fossil fuel that generates less
air pollutants and greenhouse gases.
COMPRESSED NATURAL GAS (CNG) AS A TRANSPORTATION FUEL
What is CNG
Compressed natural gas, or CNG, is natural gas under
pressure which remains clear, odourless, and non-corrosive. Although
vehicles can use natural gas as either a liquid or a gas, most vehicles
use the gaseous form compressed to pressures above 3,100 pounds per
square inch.
How is natural gas
produced
Most natural gas comes from three types of wells:
natural gas-and-condensate wells, oil wells, and coal bed methane wells.
In 2003, California had over 1,200 natural gas-and-condensate wells
operating. Well-extracted natural gas requires a cleanup process before
it can be used in vehicles or residences.
What are the benefits of
using natural gas in transportation
Natural gas is produced both worldwide and domestically
at relatively low cost and is cleaner burning than gasoline or diesel
fuel. Natural gas vehicles show an average reduction in ozone-forming
emissions of 80 percent compared to gasoline vehicles.
Natural Gas Vehicles
Compressed natural gas (CNG) and liquefied natural gas
(LNG) are considered alternative fuels under the Energy Policy Act of
1992. Natural gas vehicles (NGVs) are either fuelled exclusively with CNG
or LNG (dedicated NGVs) or are capable of natural gas and gasoline
fuelling (bi-fuel NGVs).
What
is a natural gas vehicle
Dedicated natural gas vehicles (NGVs) are designed to run
only on natural gas; bi-fuel NGVs have two separate fuelling systems that
enable the vehicle to use either natural gas or a conventional fuel
(gasoline or diesel). In general, dedicated NGVs demonstrate better
performance and have lower emissions than bi-fuel vehicles because their
engines are optimized to run on natural gas. In addition, the vehicle does
not have to carry two types of fuel, thereby increasing cargo capacity and
reducing weight.
Natural gas vehicles are fuelled with compressed natural
gas (CNG) or liquefied natural gas (LNG). These fuels are considered
alternative fuels under the Energy Policy Act of 1992 and qualify for
alternative fuel vehicle tax credits. As a new twist, tests are being
conducted using natural gas vehicles fuelled with
HCNG, a blend of CNG and hydrogen.
Compared with vehicles fuelled with conventional diesel
and gasoline, NGVs can produce significantly lower amounts of harmful
emissions. In addition, some natural gas vehicle owners report service
lives two to three years longer than gasoline or diesel vehicles and
extended time between required maintenance.
The driving range of NGVs generally is less than that of
comparable gasoline and diesel-fuelled vehicles because of the lower
energy content of natural gas. Extra storage tanks can increase range, but
the additional weight may displace payload capacity. NGV horsepower,
acceleration, and cruise speed are comparable with those of an equivalent
conventionally fuelled vehicle.
Other benefits of NGVs include increasing U.S. energy
security and paving the way for fuel cell vehicles.
How Natural
Gas Vehicles Work
Light-duty natural gas vehicles work much like
gasoline-powered vehicles with spark-ignited engines. This schematic shows
basic CNG fuel system components.
CNG enters the vehicle through the natural gas fill valve
(A) and flows into high-pressure cylinders (B). When the engine requires
natural gas, the gas leaves the cylinders and passes through the master
manual shut-off valve (C). The gas travels through the high-pressure fuel
line (D) and enters the engine compartment. Gas enters the regulator (E),
which reduces the gas pressure used for storage (up to 3,600 psi) to the
required vehicle fuel injection system pressure. The natural gas solenoid
valve (F) allows natural gas to pass from the regulator into the gas mixer
or fuel injectors. The solenoid valve shuts off the natural gas when the
engine is not running. Natural gas mixed with air flows down through the
carburettor or fuel-injection system (G) and enters the engine combustion
chambers where it is burned to produce power, just like gasoline.
Schematic of a natural gas vehicle fuelling system.
Adapted from Compressed Natural Gas.
Some heavy-duty vehicles use spark-ignited natural gas
systems, but other systems exist as well. High-pressure direct injection
engines burn natural gas in a compression-ignition (diesel) cycle. See
Development of the High-Pressure Direct-Injection ISX G Natural Gas
Engine. Heavy-duty engines can also burn diesel and natural gas in a
dual-fuel system.
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