ASE Compressed Natural Gas (CNG) Vehicles 2 — Questions and Answers
Question 1: What is the typical service pressure of a CNG fuel storage cylinder in a light-duty vehicle?
- 3,600 psi (Correct answer)
- 150 psi
- 600 psi
- 9,000 psi
Correct answer: 3,600 psi
Light-duty CNG cylinders are commonly rated for a service pressure of 3,600 psi (about 248 bar).
Question 2: Which component reduces high cylinder pressure to a usable pressure before the gas reaches the engine?
- Fuel pressure regulator (Correct answer)
- Catalytic converter
- Throttle body
- Fuel rail damper
Correct answer: Fuel pressure regulator
A CNG pressure regulator (sometimes multi-stage) steps the high storage pressure down to injection pressure.
Question 3: A CNG cylinder has an expiration date stamped on it. Why must it be removed from service when expired?
- The composite or steel structure can fatigue and fail under pressure over time (Correct answer)
- The fuel quality degrades inside
- It loses its color coating
- The mounting bolts rust
Correct answer: The composite or steel structure can fatigue and fail under pressure over time
CNG cylinders have a finite service life because pressure cycling and material aging can compromise structural integrity.
Question 4: What odorant is added to CNG so leaks can be detected by smell?
- Mercaptan (Correct answer)
- Chlorine
- Ammonia
- Ethanol
Correct answer: Mercaptan
Mercaptan gives natural gas its characteristic 'rotten egg' odor for leak detection.
Question 5: Where does CNG tend to accumulate if it leaks inside an enclosed space?
- Near the ceiling, because it is lighter than air (Correct answer)
- On the floor, because it is heavier than air
- It disperses evenly
- It sinks into drains
Correct answer: Near the ceiling, because it is lighter than air
CNG (methane) is lighter than air, so it rises and collects at the highest points.
Question 6: Which device on a CNG cylinder vents gas to prevent rupture if the tank is exposed to fire?
- Pressure relief device (PRD) (Correct answer)
- Check valve
- Solenoid shutoff
- Fill receptacle
Correct answer: Pressure relief device (PRD)
A thermally activated pressure relief device releases gas safely when exposed to excessive heat.
Question 7: Before servicing the CNG fuel system, what is the correct first step?
- Isolate the cylinder valve(s) and depressurize/purge the lines (Correct answer)
- Disconnect the battery only
- Drain the engine oil
- Open the radiator cap
Correct answer: Isolate the cylinder valve(s) and depressurize/purge the lines
The fuel system must be isolated and depressurized to safely work on high-pressure CNG components.
ASE Compressed Natural Gas (CNG) Vehicles 2
What is the typical service pressure of a CNG fuel storage cylinder in a light-duty vehicle?