CNG Vehicle Conversion and Installation 2 — Questions and Answers
Question 1: Which federal standard governs the design and installation of CNG fuel systems on motor vehicles in the United States?
- FMVSS 303 (Correct answer)
- NFPA 52
- SAE J2406
- EPA 40 CFR Part 86
Correct answer: FMVSS 303
FMVSS 303 (Federal Motor Vehicle Safety Standard 303) sets requirements for CNG fuel system integrity on motor vehicles.
Question 2: What is the maximum allowable leakage rate for a CNG fuel system during a post-installation leak test per NFPA 52?
- No detectable leakage at operating pressure (Correct answer)
- 0.5% volume per hour
- 10 cc per minute
- 1 psi pressure drop over 10 minutes
Correct answer: No detectable leakage at operating pressure
NFPA 52 requires that no detectable leakage be present when the system is tested at operating pressure.
Question 3: When converting a gasoline vehicle to CNG, which component must be upgraded to handle the higher fuel pressure?
- Fuel pressure regulator (Correct answer)
- Catalytic converter
- Oxygen sensor
- Throttle body
Correct answer: Fuel pressure regulator
The fuel pressure regulator must reduce high-pressure CNG (up to 3,600 psi) to the lower pressure required by the engine's fuel injection system.
Question 4: A Type 4 CNG cylinder is constructed of:
- Full carbon fiber overwrap on a polymer liner (Correct answer)
- All-aluminum construction
- Steel with fiberglass overwrap
- Hoop-wrapped carbon fiber on a steel liner
Correct answer: Full carbon fiber overwrap on a polymer liner
Type 4 cylinders use a thermoplastic polymer liner fully wrapped with carbon fiber composite, making them the lightest cylinder type.
Question 5: During a CNG vehicle conversion, the installer must verify that the CNG cylinder mounting brackets can withstand a static load of at least how many times the cylinder's weight?
- 8 times in all directions (Correct answer)
- 2 times forward, 1 time laterally
- 4 times forward only
- 3 times in all directions
Correct answer: 8 times in all directions
NFPA 52 requires that cylinder mounting must withstand 8 times the cylinder weight in all directions to ensure crash safety.
Question 6: Which type of sealant is prohibited when assembling CNG fuel line fittings?
- Petroleum-based thread sealant (pipe dope) (Correct answer)
- PTFE tape rated for gas service
- Anaerobic thread sealant rated for CNG
- Teflon-coated compression fittings
Correct answer: Petroleum-based thread sealant (pipe dope)
Petroleum-based thread sealants can swell or deteriorate in CNG service and are not compatible; only CNG-rated sealants or PTFE tape should be used.
Question 7: What is the primary purpose of the Manual Shutoff Valve (MSV) required on every CNG cylinder?
- To allow the fuel supply to be isolated manually during maintenance or emergencies (Correct answer)
- To regulate delivery pressure to the engine
- To vent excess pressure to the atmosphere
- To prevent overfilling of the cylinder
Correct answer: To allow the fuel supply to be isolated manually during maintenance or emergencies
The MSV provides a means to manually stop gas flow from the cylinder, which is essential for safe maintenance and emergency shutoff.
CNG Vehicle Conversion and Installation 2
Which federal standard governs the design and installation of CNG fuel systems on motor vehicles in the United States?