United States
Environmental Protection
Agency
Transportation and Air Quality
Transportation and Regional
Programs Division
EPA420-F-00-033
March 2002
www.epa.gov
SUCCESS STOfly
In 1999, the U.S. Depart-
ment of Energy (DOE) con-
ducted a study that provided
direct, side-by-side cost and
emissions comparisons of
CNG and gasoline-fueled
taxi-cab operations. The
report, Alternative Fuels Case
Study: Barwood Cab Fleet
Study Summary, compared 10
CNG-fueled cars from 1996
with 10 gasoline-fueled cars of
the same make and model to
determine operating costs.
The study found that the
fuel economy of the CNG
and gasoline cabs were identi-
cal and that operating costs for
CNG cabs were 25 percent
less than the gasoline-powered
cabs. Average fuel costs for the
CNG cabs were 32 percent
less, and reportedly cost about
15 percent less to maintain
than the gasoline vehicles.
Tailpipe emissions tests on 14
of the 20 vehicles indicated
that CNG exhaust emissions
were significantly lower than
their gasoline counterparts for
non-methane hydrocarbons
and carbon monoxide.
A copy of this report is
available at .
Fuels:
Compressed Natural Gas
One in a series of fact sheets
Natural gas is one of the most widely used forms of energy today.
It is commonly used to heat and cool homes and businesses
nationwide. In addition, more than 85,000 compressed natural
gas (CNG) vehicles, including one out of every five transit buses, are operating
sucessfully today. CNG's popularity stems, in part, from its clean-burning prop-
erties. In many cases, CNG vehicles generate fewer exhaust and greenhouse
gas emissions than their gasoline- or diesel-powered counterparts.
CNG is odorless, colorless, and tasteless.
It consists mostly of methane and is
drawn from gas wells or in conjunction
with crude oil production. CNG vehicles
store natural gas in high-pressure fuel
cylinders at 3,000 to 3,600 pounds per
square inch. An odorant is normally
added to CNG for safety reasons.
AVAILABILITY
Two types of CNG fuel systems are on the
market: dedicated vehicles, which operate
exclusively on natural gas, and dual-fuel
vehicles, which can use both natural gas
and gasoline. Auto manufacturers offer a
variety of both dedicated and dual-fuel
CNG vehicles, including compacts,
trucks, vans, and buses.
The United States has vast natural gas
reserves distributed across the country
through extensive pipeline systems extend-
ing from the wellhead to the end-user. As
a result, CNG is currently available at
approximately 1,300 refueling stations in
46 states, and this number continues to
grow. In addition, CNG vehicle owners
can refuel their cars at home by installing
small compressors connected directly to
the home's natural gas supply.
EMISSIONS
CHARACTERISTICS*
Actual emissions will vary with
engine design; these numbers reflect
the potential reductions offered by
compressed natural gas, relative to
conventional gasoline.
• Reductions in carbon monoxide
emissions of 90 to 97 percent, and
reductions in carbon dioxide emis-
sions of 25 percent.
• Reductions in nitrogen oxide emis-
sions of 35 to 60 percent.
• Potential reductions in nonmethane
hydrocarbon emissions of 50 to 75
percent.
• Fewer toxic and carcinogenic pollu-
tants, and little to no paniculate
matter produced.
• No evaporative emissions in dedi-
cated engines (such as those associ-
ated with gasoline or diesel).
* Estimates based on CNG's inherently
"cleaner" chemical properties with an engine
that takes full advantage of these properties.
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AFFORDABILITY
The CNG market is more stable than
the gasoline market. CNG generally
costs 15 to 40 percent less than gaso-
line or diesel. CNG requires more
frequent refueling, however, because
it contains only about a quarter of
the energy by volume of gasoline. In
addition, CNG vehicles cost between
$3,500 to $6,000 more than their
gasoline-powered counterparts. This
is primarily due to the higher cost of
the fuel cylinders. As the popularity
and production of CNG vehicles
increases, vehicle costs are expected
to decrease.
PERFORMANCE
The octane rating for CNG is higher
than that for gasoline; in a dedicated
engine, a CNG vehicle's power, accel-
eration, and cruise speed can be
greater than that of a gasoline-pow-
ered vehicle. In addition, due to the
cleaner burning characteristics of nat-
ural gas, CNG vehicle engines can
run more efficiently than a gasoline-
powered vehicle, thereby extending
the life of the vehicle. In heavy-duty
vehicles, CNG engines are also gen-
erally less noisy than diesel engines.
SAFETY
Although CNG is a flammable gas, it
has a narrow flammability range,
making it an inherently safe fuel.
Strict safety standards make CNG
vehicles as safe as gasoline-powered
vehicles. In the event of a spill or
accidental release, CNG poses no
threat to land or water; it is non-
toxic. CNG also disperses rapidly,
minimizing ignition risk relative to
gasoline. Natural gas is lighter than
air and will not pool as a liquid or
vapor on the ground. Nevertheless,
leaks indoors may form a flammable
mixture in the vicinity of an ignition
source. CNG is primarily methane,
however, which is a greenhouse gas
that could contribute to global cli-
mate change if leaked. Methane is
slightly soluble in water and under
certain environmental conditions
(anaerobic) does not biodegrade; if
excess amounts accumulate, the gas
can bubble from the water, possibly
creating a risk of fire or explosion.
Reported incidences of bus fires
are related to engine failures, not the
use of natural gas. Natural gas buses
have onboard gas detectors and other
safety devices, such as tank safety
valves that allow fuel flow only when
the engine is keyed on. Also, the
tanks must be inspected and
approved by the U.S. Department of
Transportation after certain periods
of use.
There are some different safety
concerns with CNG buses than
diesel fuel buses, such as greater
breaking distance due to increased
fuel storage system weight. This is a
relatively small concern, however,
because the fuel system is a small
fraction of a bus' total weight. CNG
buses also might accelerate slower
than their diesel counterparts.
MAINTENANCE
Proper training is required for all
maintenance personnel working on
CNG vehicles. The oil in a CNG
vehicle does not need to be changed
as frequently because CNG burns
more cleanly than gasoline, produc-
ing less deposits in the oil.
For More Information
EPA Alternative Fuels Web Site
www.epa.gov/otaq/consumer/
fuels/altfuels/altfuels.htm
Natural Gas Vehicle Coalition
1100 Wilson Boulevard
Suite 850
Arlington, VA 22209
Phone: 703 527-3022
Fax: 703 527-3025
Web site: www.ngvc.org
Alternative Fuel Refueling
Station Locator
Web site: afdcmap.nrel.gov/nrel
Alternative Fuels Data Center
Web site: www.afdc.nrel.gov
National Alternative Fuels
Hotline
Phone: 800 423-1 DOE
Printed on paper that contains at least 30 percent postconsumer fiber.
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