CNG Fuel System Fundamentals, Safety, and Regulations
What a CNG system is really doing
A compressed natural gas vehicle stores methane at high pressure (commonly 3,000 or 3,600 psi service pressure) and delivers it to the engine as a low-pressure gas. Every component between the cylinder valve and the injector exists to step that pressure down safely, isolate fuel when the engine is off, and vent or contain gas during abnormal events. Candidates must know the major subsystems: storage, high-pressure plumbing, receptacle/fill path, manual and electrically operated shutoff valves, pressure regulator(s), low-pressure delivery, and the engine control interface.
Safety concepts to name
- Methane is lighter than air, so it disperses upward in open areas but accumulates in ceilings, pits, and enclosed spaces.
- CNG has a narrow flammability range (roughly 5-15% by volume in air) and a high autoignition temperature, so it is hard to ignite but burns vigorously when it does.
- Natural gas is odorized, but odorant fade and the fact that you may not smell it at low concentration mean you never rely on smell alone.
- High-pressure gas stores enormous energy; a pinhole leak can cut or inject.
Regulations and standards
Know the roles, not the paragraph numbers: FMVSS 304 covers compressed natural gas fuel system integrity; NFPA 52 governs vehicular fueling and repair facilities; CSA/ANSI NGV standards cover cylinders, valves, and components; SAE J1616 addresses fueling station practice. Inspectors should know that cylinder requalification/inspection intervals and the vehicle fuel-system inspection are separate requirements.
Traps
Questions love to test "vent to atmosphere vs. capture," the difference between service pressure and test pressure, and whether a component is a shutoff or a relief device. Do not confuse working pressure ratings with burst ratings.
Rule of thumb
If a component can trap gas between two closed devices, it needs a way to relieve or bleed that trapped volume - always ask "where does this gas go?"