CNG Fuel System Components 2 — Questions and Answers
Question 1: What is the primary function of a CNG pressure regulator in the fuel system?
- Filter contaminants from the gas stream
- Reduce high-pressure gas to a usable engine inlet pressure (Correct answer)
- Measure the volume of gas consumed
- Prevent backflow of gas toward the cylinder
Correct answer: Reduce high-pressure gas to a usable engine inlet pressure
The pressure regulator steps down the high storage pressure (up to 3,600 psi) to the low pressure required by the engine's fuel injection system.
Question 2: Which material is most commonly used for CNG high-pressure fuel lines between the cylinder and the regulator?
- Reinforced rubber hose
- Aluminum tubing
- Stainless steel tubing (Correct answer)
- Copper tubing
Correct answer: Stainless steel tubing
Stainless steel tubing is used for high-pressure CNG lines because it resists corrosion and can safely handle pressures up to 3,600 psi.
Question 3: A CNG manual shutoff valve is typically located:
- Between the regulator and the engine
- Downstream of the fuel injectors
- At the cylinder valve or close to the storage tank (Correct answer)
- Inside the pressure relief device
Correct answer: At the cylinder valve or close to the storage tank
The manual shutoff valve is placed at or near the cylinder to allow technicians to isolate the fuel supply during service or in an emergency.
Question 4: What does a CNG fuel filter primarily remove from the gas stream?
- Excess oxygen
- Particulates, oil mist, and moisture (Correct answer)
- Carbon monoxide byproducts
- Nitrogen compounds
Correct answer: Particulates, oil mist, and moisture
CNG filters remove solid particles, compressor oil mist, and water vapor that could damage regulators, injectors, and other precision components.
Question 5: In a bi-fuel CNG vehicle, the gasoline and CNG fuel systems are connected through:
- A shared fuel pump
- A fuel selector switch and dedicated solenoid valves (Correct answer)
- A single common rail injector
- The same fuel filter housing
Correct answer: A fuel selector switch and dedicated solenoid valves
A bi-fuel system uses a selector switch to open and close solenoid valves, directing fuel flow from either the CNG or gasoline system to the engine.
Question 6: The pressure relief device (PRD) on a CNG cylinder is designed to:
- Vent gas slowly to maintain constant pressure
- Open automatically when cylinder temperature exceeds a threshold (Correct answer)
- Refill the cylinder from an external source
- Signal a low-fuel warning to the driver
Correct answer: Open automatically when cylinder temperature exceeds a threshold
The PRD is thermally activated to vent gas safely if the cylinder is exposed to excessive heat, such as during a vehicle fire, preventing a catastrophic rupture.
Question 7: Which CNG system component is responsible for delivering precise amounts of fuel into the engine intake?
- High-pressure regulator
- CNG fuel injector (Correct answer)
- Manual shutoff valve
- Cylinder valve
Correct answer: CNG fuel injector
CNG injectors are electronically controlled to meter exact fuel quantities into the intake manifold based on signals from the engine control module.
CNG Fuel System Components 2
What is the primary function of a CNG pressure regulator in the fuel system?