General Information
Caution: Service and repair of the compressed natural gas (CNG) fuel system should only be performed by trained CNG service technicians. To prevent personal injury or damage to the fuel system, do not attempt repairs yourself.
Note: Information provided in this chapter is specific to the CNG engine, however, other chapters in this manual pertain to the CNG operated vehicle. These include, but are not limited to, the controls and pretrip inspection and daily maintenance chapters.
CNG is made by compressing natural gas to less than 1% of its volume at standard atmospheric pressure. CNG often contains an odor-producing chemical.
When vaporized at ambient temperatures, natural gas is less dense than air, and it rises and disperses. Cold atmospheric 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 ignited.
A typical natural gas fuel system consists of:
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Fuel supply cylinders that store compressed gas at high pressure.
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Pressure relief and manual fuel shutoff valves.
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A filling connection with a check valve that prevents the gas from flowing back out of the fuel filling line.
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High-pressure and low-pressure fuel filters.
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A pressure control regulator that reduces the high fuel tank or cylinder pressure to the lower pressure needed for the engine.
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A fuel contents gauge that indicates the fuel supply in the tank or cylinders.