CNG Systems 2 — Questions and Answers
Question 1: What is the primary function of the high-pressure regulator in a CNG fuel system?
- Ignite the compressed gas
- Reduce storage pressure to a usable level for the engine (Correct answer)
- Filter contaminants from the gas
- Monitor tank pressure electronically
Correct answer: Reduce storage pressure to a usable level for the engine
The high-pressure regulator steps down CNG from storage pressure (up to 3,600 psi) to an intermediate or low pressure the engine can use.
Question 2: Which component prevents CNG from flowing back from the fuel rail into the storage cylinders?
- Manual shutoff valve
- Check valve (Correct answer)
- Pressure relief device
- Fuel filter
Correct answer: Check valve
A check valve allows fuel flow in only one direction, preventing backflow from the fuel rail toward the cylinders.
Question 3: On a bi-fuel CNG vehicle, what determines which fuel the engine uses when both fuels are available?
- Ambient temperature sensor
- The engine control module (ECM) or driver selection switch (Correct answer)
- The pressure relief device
- The fuel filter condition
Correct answer: The engine control module (ECM) or driver selection switch
On bi-fuel vehicles, the ECM or a manual driver switch controls which fuel system (CNG or gasoline) is active.
Question 4: What does a pressure relief device (PRD) on a CNG cylinder do in a fire scenario?
- Seals the cylinder permanently
- Vents gas to prevent cylinder rupture from excessive heat (Correct answer)
- Switches fuel supply to a backup tank
- Activates the vehicle's fire suppression system
Correct answer: Vents gas to prevent cylinder rupture from excessive heat
A PRD is thermally activated and vents CNG safely to atmosphere to prevent catastrophic cylinder failure when exposed to fire.
Question 5: Which material is commonly used for CNG storage cylinders (Type 3) due to its strength-to-weight ratio?
- Pure aluminum
- Aluminum liner with carbon fiber overwrap (Correct answer)
- Steel with rubber lining
- High-density polyethylene
Correct answer: Aluminum liner with carbon fiber overwrap
Type 3 cylinders use an aluminum liner fully wrapped with carbon fiber or glass fiber composite for high strength at reduced weight.
Question 6: What is the standard maximum operating pressure for most CNG vehicle storage systems in the United States?
- 1,800 psi
- 2,400 psi
- 3,600 psi (Correct answer)
- 5,000 psi
Correct answer: 3,600 psi
U.S. CNG vehicle systems are designed for a maximum operating pressure of 3,600 psi (248 bar) at 70°F.
Question 7: What happens to CNG system pressure when the ambient temperature drops significantly in cold weather?
- Pressure increases
- Pressure decreases (Correct answer)
- Pressure stays exactly the same
- The fuel converts to liquid
Correct answer: Pressure decreases
Gas pressure drops with temperature according to the ideal gas law, so cold weather reduces the pressure reading in CNG tanks.
CNG Systems 2
What is the primary function of the high-pressure regulator in a CNG fuel system?