CNG Fuel System Components 3 — Questions and Answers
Question 1: A Type 4 CNG cylinder is constructed from:
- All-steel with no liner
- Aluminum liner wrapped with carbon fiber composite
- All-aluminum construction
- Plastic liner fully wrapped with carbon fiber composite (Correct answer)
Correct answer: Plastic liner fully wrapped with carbon fiber composite
Type 4 cylinders use a high-density polyethylene (HDPE) or nylon plastic liner that is completely overwrapped with carbon fiber composite for maximum strength at minimum weight.
Question 2: What is the standard service life of most Type 3 and Type 4 CNG cylinders in the US?
- 5 years
- 10 years
- 15 years
- 20 years (Correct answer)
Correct answer: 20 years
FMVSS 304 and NGV2 standards typically allow CNG cylinders a 20-year or 15-year service life depending on cylinder type and manufacturer specifications.
Question 3: The CNG excess flow valve closes automatically when:
- The tank reaches full pressure
- Gas flow exceeds a preset rate due to a downstream leak or rupture (Correct answer)
- The engine is turned off
- Ambient temperature drops below freezing
Correct answer: Gas flow exceeds a preset rate due to a downstream leak or rupture
The excess flow valve senses abnormally high flow caused by a broken line or fitting and shuts off to prevent rapid uncontrolled gas release.
Question 4: Why are CNG fuel system components required to be electrically bonded?
- To power the electronic fuel injectors
- To prevent static electricity buildup that could ignite gas (Correct answer)
- To provide a ground path for the vehicle's battery
- To allow remote monitoring of fuel levels
Correct answer: To prevent static electricity buildup that could ignite gas
Electrical bonding provides a continuous conductive path so that static charges dissipate safely rather than sparking in the presence of natural gas.
Question 5: Which valve on a CNG cylinder prevents gas from flowing backward from the system into the filling nozzle during fueling?
- Pressure relief device
- Excess flow valve
- Check valve (Correct answer)
- Manual shutoff valve
Correct answer: Check valve
The check valve allows gas to flow only one direction—into the cylinder—and prevents backflow toward the dispensing equipment when the nozzle is disconnected.
Question 6: At what pressure are most light-duty CNG vehicle cylinders typically rated for service in the US?
- 1,800 psi
- 2,400 psi
- 3,000 psi
- 3,600 psi (Correct answer)
Correct answer: 3,600 psi
US light-duty CNG vehicles use cylinders rated at 3,600 psi (248 bar) service pressure, which is the standard for the NGV2 and equivalent standards.
Question 7: What is the purpose of the odorant injection system in a CNG vehicle fuel system?
- To improve fuel combustion efficiency
- To add a detectable scent so leaks can be identified by smell (Correct answer)
- To lubricate injector tips
- To neutralize sulfur compounds in the gas
Correct answer: To add a detectable scent so leaks can be identified by smell
Natural gas is naturally odorless, so mercaptan odorant is added at the distribution level to give it a distinctive smell that makes leaks detectable.
CNG Fuel System Components 3
A Type 4 CNG cylinder is constructed from: