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Compressed Natural Gas Fuel System
CNG General Information
Natural gas is a mixture of hydrocarbon gasses with
a high concentration of methane gas, which burns
completely into carbon dioxide and water, resulting in
lower emissions than with diesel or gasoline.
Natural gas is less energy dense than diesel, there-
fore, a larger volume of natural gas is required to
produce the same power. This is one reason for the
slightly lower maximum power in natural-gas-
powered vehicles. The power difference is a reduc-
tion in maximum power only and does not affect nor-
mal driving. The power reduction is evidenced only in
wide-open throttle, high-torque conditions.
IMPORTANT: The natural gas fuel system
should be routinely inspected for gas leakage.
Use a natural gas detector to check the fuel
tanks, fuel filtering and regulating mechanisms,
and the fuel lines. If necessary, replace leaking
fuel tanks; repair or replace any lines, devices,
or connections that are leaking.
Natural gas is nontoxic, but it can cause asphyxiation
in certain circumstances. Commercial compressed
natural gas normally contains an odor-producing
chemical, which enables users to detect gas leaks,
but a natural gas detector is recommended for
checking for leaks.
A typical natural gas fuel system consists of:
•
Fuel supply containers or tanks storing the gas
at high pressure.
•
A pressure relief valve and manual shutoff
valves.
•
A filling connection with a check valve to pre-
vent the gas from flowing back out of the con-
nection.
•
A valve that automatically closes if the engine
stops.
•
A high-pressure fuel filter and a low-pressure
fuel filter.
•
A pressure regulator to reduce the fuel tank
pressure to the lower pressure needed for the
engine.
•
A gas-air mixer to produce a flammable mix-
ture for the engine.
8.1
•
A pressure gauge to indicate the fuel supply in
the tanks.
CNG Fuel Tank Filling
Procedure
CNG is moved from the fuel station storage tanks to
the vehicle cylinders through a metered dispenser.
During fueling, as pressure inside of the vehicle cyl-
inders increases, so does the temperature. The dis-
penser shuts off when it determines the CNG cylin-
ders are full.
CNG fuel cylinders cannot be filled to their full capac-
ity of 3600 psi (24 821 kPa) in cooler weather. Since
temperature change causes gas to expand and con-
tract, less pressure in the cylinders on a cold day is
equal to the same amount of gas at a higher pres-
sure on a warm day. The fuel station dispenser has a
temperature-pressure compensating device, which
accounts for ambient temperature during fueling. See
Table 8.1
for approximate full cylinder pressures at
various temperatures.
CNG Temperature/Pressure Compensation Values
Fill Station Pressure Set Point
Temperature:
3000 psi (20 684
°F (°C)
kPa) Set Point
100 (37.8)
3415 (23 546)
90 (32.2)
3276 (22 587)
80 (26.7)
3138 (21 636)
70 (21.1)
3000 (20 684)
60 (15.6)
2861 (19 726)
50 (10)
2723 (18 774)
40 (4.4)
2584 (17 816)
30 (-1.1)
2446 (16 865)
20 (-6.7)
2307 (15 906)
10 (-12.2)
2169 (14 955)
0 (-17.8)
2031 (14 003)
-10 (-23.3)
1893 (13 052)
-20 (-28.9)
1755 (12 100)
-30 (34.4)
1616 (11 142)
-40 (-40)
1477 (10 184)
Table 8.1, CNG Temperature/Pressure Compensation
Use the following steps to fuel a CNG vehicle.
1.
Shut down the engine and apply the parking
brake.
3600 psi (24 821
kPa) Set Point
4086 (28 172)
3909 (26 952)
3754 (25 883)
3600 (24 821)
3445 (23 752)
3288 (22 670)
3131 (21 587)
2973 (20 498)
2814 (19 402)
2655 (18 306)
2495 (17 202)
2336 (16 106)
2180 (15 031)
2024 (13 955)
1868 (12 879)
Values
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