Natural Gas Vehicles, General
Information
The vehicle can be equipped with a compressed
natural gas (CNG) fuel system, or a liquefied natural
gas (LNG) fuel system. CNG is made by compress-
ing natural gas to less than 1% of its volume at stan-
dard atmospheric pressure. LNG is created by con-
densing natural gas into a liquid by cooling it to
approximately -259°F (-162°C). CNG often contains
an odor-producing chemical, while LNG does not
have any odor.
When vaporized at ambient temperatures, natural
gas is less dense than air, and it rises and disperses.
However, LNG is intially heavier than air due to its
cryogenic temperature. When released in large
amounts, it may drop or pool on the ground before
its temperature rises enough to vaporize. Cold at-
mospheric conditions may prevent natural gas from
dispersing quickly when released in large amounts.
Natural gas is nontoxic, but it can cause asphyxiation
if present in very large concentrations.
For natural gas to burn, it must first vaporize, then
mix with air in the proper proportions (flammable
range is 5 to 15% by volume in air), and then be ig-
nited.
A typical natural gas fuel system consists of:
•
Fuel supply cylinders that store compressed
gas at high pressure (CNG), or a tank that
stores liquefied gas at an extremely low tem-
perature (LNG).
•
A vaporizor or heat exchanging device that
changes LNG to gaseous form (LNG fuel sys-
tems only).
•
Pressure relief and manual fuel shutoff valves.
•
A filling connection with a check valve that pre-
vents the gas from flowing back out of the fuel
filling line.
•
High-pressure and low-pressure fuel filters.
•
A pressure control regulator that reduces the
high fuel tank or cylinder pressure to the lower
pressure needed for the engine.
•
A fuel contents gauge that indicates the fuel
supply in the tank or cylinders.
Natural Gas Vehicles, Safety
Precautions
DANGER
Natural gas is highly flammable. Failure to ob-
serve the following safety precautions could lead
to the ignition of the natural gas, which could
cause serious bodily injury or death.
Follow these safety precautions when operating or
repairing a natural gas vehicle.
•
Do not start the engine if a natural gas leak is
detected.
•
Do not transfer CNG from one vehicle to an-
other, as a buildup of static electricity could
cause a spark and ignite the fuel.
•
Do not attempt to cut any fuel supply piping
components.
•
Avoid open flames or sparks near a natural
gas vehicle.
•
Cover eyes and exposed skin when working on
a natural gas fuel system or fueling a natural
gas vehicle.
•
Permit no smoking or other ignition sources
within thirty feet of a natural gas vehicle.
•
Have CO
2
fire extinguishers (ABC minimum)
located in a highly visible and easily accessible
location.
•
Use Purple-K powder (PKP) or a similar dry
powder to extinguish an LNG fire. Do not at-
tempt use water around an LNG fire.
•
Always purge the fuel lines before performing
maintenance or repairs on a natural gas fuel
system. To purge the lines, close the manual
fuel shutoff valve, and start the engine. Let the
vehicle idle until the fuel lines are empty and
the engine stops.
•
Always use a natural gas detector to test the
system for leaks, whether an odor is present or
not. Routinely inspect the fuel filtering and
regulating mechanisms, and the fuel lines. A
bubble solution can be used to pinpoint the
exact location of leaks.
Natural Gas Vehicle
19.1
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