CNG - Compressed Natural Gas Fuel System Components Questions and Answers 1 — Questions and Answers
Question 1: Which of the following is the primary function of a thermally-activated Pressure Relief Device (PRD) on a CNG cylinder?
- To regulate the flow of fuel to the engine.
- To prevent over-pressurization during normal fueling by venting excess pressure.
- To safely vent the cylinder's contents in the event of a fire or high-temperature exposure. (Correct answer)
- To act as a manual shut-off valve in an emergency.
Correct answer: To safely vent the cylinder's contents in the event of a fire or high-temperature exposure.
A thermally-activated PRD is a critical safety component designed to open and release the gas from the cylinder if the temperature reaches a specific point (e.g., ~230°F), such as in a vehicle fire. This controlled venting prevents the cylinder pressure from rising to a level that could cause a rupture.
Question 2: A technician is servicing a CNG vehicle and notices that after turning the ignition off, the high-pressure fuel supply to the regulator is immediately cut off. Which component is responsible for this action?
- The manual shut-off valve
- The fuel pressure regulator
- The check valve at the fill receptacle
- The main fuel solenoid valve (Correct answer)
Correct answer: The main fuel solenoid valve
The main fuel solenoid valve is an electronically controlled valve that opens to allow fuel flow when the ignition is on and the engine is running, and closes to stop the flow of high-pressure gas when the ignition is turned off.
Question 3: What is the primary purpose of the check valve located within the CNG fueling receptacle?
- To filter contaminants from the incoming gas.
- To regulate the speed at which the cylinder can be filled.
- To prevent the backflow of gas from the vehicle's fuel system. (Correct answer)
- To measure the pressure of the gas during fueling.
Correct answer: To prevent the backflow of gas from the vehicle's fuel system.
The check valve in the fueling receptacle is a one-way valve that allows high-pressure gas to flow into the vehicle's cylinders during fueling but prevents it from escaping back out through the receptacle once the fueling nozzle is removed.
Question 4: A technician needs to replace a damaged section of high-pressure fuel line between the CNG cylinder and the primary pressure regulator. Which material is most appropriate for this application?
- Copper tubing
- Reinforced rubber hose
- Seamless stainless steel tubing (Correct answer)
- Braided aluminum line
Correct answer: Seamless stainless steel tubing
High-pressure fuel lines in a CNG system must be made from seamless stainless steel to safely withstand the extremely high pressures (up to 3,600 psi) and resist corrosion and vibration.
Question 5: The main pressure regulator in a CNG fuel system performs which critical function?
- It increases the gas pressure to ensure proper injection.
- It reduces the high pressure from the storage cylinders to a much lower, usable pressure for the engine. (Correct answer)
- It stores a small amount of fuel for cold starts.
- It filters oil and other contaminants from the natural gas.
Correct answer: It reduces the high pressure from the storage cylinders to a much lower, usable pressure for the engine.
The primary function of the main pressure regulator is to reduce the storage cylinder pressure (e.g., 3,600 psi) down to a consistent and significantly lower operating pressure required by the engine's fuel injection system (e.g., around 125 psi).
Question 6: Before performing service on the high-pressure regulator, a technician must isolate the CNG cylinders from the rest of the fuel system. Which component should be used for this purpose?
- The fuel filter
- The manual shut-off valve (Correct answer)
- The pressure relief device (PRD)
- The fueling receptacle check valve
Correct answer: The manual shut-off valve
The manual shut-off valve is a quarter-turn valve located on the cylinder or in the high-pressure line that allows a technician to manually stop the flow of gas from the storage cylinders to the downstream components for service or in an emergency.
CNG - Compressed Natural Gas Fuel System Components Questions and Answers 1
Which of the following is the primary function of a thermally-activated Pressure Relief Device (PRD) on a CNG cylinder?