ASE Practice Test (F1: Compressed Natural Gas (CNG) Vehicles) 2 — Questions and Answers
Question 1: What is the standard storage pressure for compressed natural gas (CNG) in vehicle fuel systems?
- 3,000 PSI (207 bar)
- 3,600 PSI (248 bar) (Correct answer)
- 500 PSI (34 bar)
- 10,000 PSI (689 bar)
Correct 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.
Question 2: Before performing any maintenance on a CNG vehicle fuel system, what safety step is REQUIRED?
- Only close the manual shutoff valve on the high-pressure regulator
- 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 (Correct answer)
- Disconnect the negative battery cable only
- Simply turn the ignition off and begin work
Correct 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.
Question 3: What material are CNG vehicle fuel cylinders commonly made of, and how are they rated for service life?
- Aluminum only, rated for unlimited service life
- 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 (Correct answer)
- Stainless steel only, with no expiration date
- Copper alloy rated by operating cycles only
Correct 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.
Question 4: 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?
- The CNG cylinder pressure gauge calibration
- CNG injectors for leaking or restricted flow, and the fuel temperature/pressure sensor for accurate readings (Correct answer)
- The thermostat and cooling system
- The oxygen sensor heater circuit only
Correct 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.
Question 5: What is the purpose of the excess flow valve (EFV) in a CNG cylinder valve assembly?
- To regulate pressure from 3,600 PSI down to engine working pressure
- To automatically shut off gas flow if the flow rate exceeds a preset limit, such as during a fuel line rupture (Correct answer)
- To allow refueling only at certified CNG stations
- To measure and display remaining fuel quantity
Correct 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.
Question 6: When a CNG vehicle is stored indoors overnight, which precaution is most important?
- Keep the vehicle fully fueled to 3,600 PSI
- Ensure the facility has adequate ventilation since natural gas is lighter than air and can accumulate near the ceiling creating a flammability hazard (Correct answer)
- Store it near a heat source to prevent fuel freezing
- Park with windows open to allow fuel vapors to escape downward
Correct 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.
ASE Practice Test (F1: Compressed Natural Gas (CNG) Vehicles) 2
What is the standard storage pressure for compressed natural gas (CNG) in vehicle fuel systems?