ASE Practice Test (F1: Compressed Natural Gas (CNG) Vehicles) 2 Flashcards

What is the standard storage pressure for compressed natural gas (CNG) in vehicle fuel systems?

Answer: 3,600 PSI (248 bar)

CNG vehicle fuel systems typically store natural gas at 3,600 PSI (248 bar) at standard conditions. This high pressure allows sufficient fuel volume to be stored in a reasonable tank size for adequate driving range. The pressure regulator reduces this to a lower working pressure for the engine. Some older systems used 3,000 PSI, but 3,600 PSI is the current standard for most light and medium-duty CNG vehicles in the US.

Before performing any maintenance on a CNG vehicle fuel system, what safety step is REQUIRED?

Answer: Close the manual fuel shutoff valve(s) at the cylinder(s), bleed down the system pressure using proper procedures, and verify with a pressure gauge that pressure has been safely relieved

CNG systems store gas at extremely high pressure (3,600 PSI). Before any fuel system work, the manual shutoff valves on each cylinder must be closed to isolate the high-pressure supply. Residual pressure in the lines must be safely relieved following OEM procedures. Pressure must be verified at zero using an appropriate gauge before opening any fitting. Failure to properly depressurize can result in explosive fuel release and fire or asphyxiation.

What material are CNG vehicle fuel cylinders commonly made of, and how are they rated for service life?

Answer: Type 1 (steel), Type 2 (steel with composite wrap), Type 3 (aluminum with composite wrap), or Type 4 (full composite) — each with a service life expiration date stamped on the cylinder

CNG cylinders are classified by type based on construction. All types have a stamped or labeled expiration date (typically 15–20 years from the manufacture date) after which the cylinder must be removed from service regardless of apparent condition. Using an expired cylinder is illegal and dangerous because the composite overwrap and/or metal liner degrades over time. Inspecting and retiring expired cylinders is mandatory in CNG vehicle service.

A CNG vehicle has a check engine light with a lean fuel condition code. The pressure regulator output pressure is within specification. What should the technician inspect next?

Answer: CNG injectors for leaking or restricted flow, and the fuel temperature/pressure sensor for accurate readings

If regulator output pressure is correct, a lean condition in a CNG engine points to either inadequate fuel delivery at the injectors (restricted or failing injectors) or inaccurate sensor feedback causing the ECM to miscalculate fuel requirements. CNG injectors can become restricted with oil contamination or deposits. The fuel temperature/pressure sensor affects fuel density calculations — an incorrect reading causes incorrect injector pulse width.

What is the purpose of the excess flow valve (EFV) in a CNG cylinder valve assembly?

Answer: To automatically shut off gas flow if the flow rate exceeds a preset limit, such as during a fuel line rupture

The excess flow valve is a safety device inside the cylinder valve that senses abnormally high gas flow rates. If a fuel line ruptures, a hose fails, or a fitting breaks loose, the escaping gas flow rate exceeds the EFV's threshold and the valve automatically closes, stopping gas flow from the cylinder. This significantly reduces the risk of large fuel release and fire in the event of a serious fuel system failure. It does not affect normal engine operation.

When a CNG vehicle is stored indoors overnight, which precaution is most important?

Answer: Ensure the facility has adequate ventilation since natural gas is lighter than air and can accumulate near the ceiling creating a flammability hazard

Natural gas (methane) is lighter than air (specific gravity approximately 0.55 relative to air). Any gas leaks from a CNG vehicle indoors will rise and accumulate near the ceiling. In sufficient concentration (5–15% in air is flammable), a spark can ignite the accumulated gas. Facilities must have overhead ventilation systems and gas detection at ceiling height. Natural gas vehicles should only be stored in properly equipped, ventilated facilities.