CNG Vehicle Conversion and Installation 3 — Questions and Answers
Question 1: What minimum bend radius is generally required for rigid stainless steel CNG fuel tubing to prevent cracking?
- 3 times the outside diameter of the tubing (Correct answer)
- 1 times the outside diameter
- 10 times the wall thickness
- Equal to the tubing length
Correct answer: 3 times the outside diameter of the tubing
A minimum bend radius of 3x the outside diameter prevents work-hardening and cracking in stainless steel CNG tubing.
Question 2: After completing a CNG conversion, which test should be performed FIRST before any operational test?
- Static leak test at full operating pressure using approved leak detection solution (Correct answer)
- Road test to check for vibration-induced leaks
- Cylinder proof pressure test at 1.5x working pressure
- Emission compliance test
Correct answer: Static leak test at full operating pressure using approved leak detection solution
A static leak test confirms all connections are sealed before the system is operated, preventing dangerous fuel releases.
Question 3: Which document must a CNG conversion installer provide to the vehicle owner upon completion of the installation?
- Installer certification label and all applicable cylinder certification documents (Correct answer)
- Only the receipt for parts and labor
- EPA emissions test report only
- State DMV form for alternative fuel registration
Correct answer: Installer certification label and all applicable cylinder certification documents
The installer must affix a certification label and provide cylinder documentation including manufacturing date, service life, and pressure rating.
Question 4: CNG cylinders installed in a vehicle's trunk or enclosed cargo area require what safety provision?
- A pressure relief device vented to the outside of the vehicle (Correct answer)
- A fireproof enclosure rated for 30 minutes
- An automatic pressure gauge visible from outside
- A CO2 suppression system
Correct answer: A pressure relief device vented to the outside of the vehicle
PRDs in enclosed spaces must vent to the exterior to prevent accumulation of natural gas inside the vehicle if they activate.
Question 5: Which vehicle system component is most likely to require recalibration after a dedicated CNG conversion to maintain proper air-fuel ratio?
- Engine Control Module (ECM) (Correct answer)
- Alternator voltage regulator
- ABS control module
- Power steering pump
Correct answer: Engine Control Module (ECM)
The ECM must be recalibrated with CNG-specific fuel maps because CNG has different stoichiometry and energy content compared to gasoline.
Question 6: When routing a CNG fuel line through the vehicle's firewall, the penetration must be:
- Sealed with a grommet or fitting rated for the pressure and the fuel (Correct answer)
- Left open to allow heat dissipation
- Sealed with silicone caulk only
- Covered with heat-shrink tubing rated for 200°F
Correct answer: Sealed with a grommet or fitting rated for the pressure and the fuel
All firewall penetrations must be sealed with a properly rated grommet or bulkhead fitting to prevent gas migration and chafing of the tubing.
Question 7: What CNG component is specifically designed to prevent gas flow if the fuel line is severed in a collision?
- Excess Flow Valve (EFV) (Correct answer)
- Check valve
- Thermal PRD
- High-pressure regulator
Correct answer: Excess Flow Valve (EFV)
An Excess Flow Valve automatically closes when gas flow exceeds a preset rate, such as what occurs when a fuel line ruptures.
CNG Vehicle Conversion and Installation 3
What minimum bend radius is generally required for rigid stainless steel CNG fuel tubing to prevent cracking?