CNG Systems 3 — Questions and Answers
Question 1: A CNG technician notices the fuel gauge reads empty but the pressure gauge shows 200 psi. What is the most likely explanation?
- The cylinders are full
- The gauge is malfunctioning
- Gas remains but at too low a pressure for practical use (Correct answer)
- The vehicle is ready to refuel with gasoline
Correct answer: Gas remains but at too low a pressure for practical use
At 200 psi there is still some CNG in the cylinders, but it is below the pressure needed for reliable engine operation and represents near-empty.
Question 2: Which CNG system component is responsible for metering the correct amount of gas into the engine's intake?
- Pressure relief device
- Gas injectors or mixer (Correct answer)
- Manual shutoff valve
- High-pressure fill coupling
Correct answer: Gas injectors or mixer
Gas injectors (in sequential systems) or a mixer (in single-point systems) meter CNG into the intake airstream for combustion.
Question 3: What is the purpose of the manual shutoff valve on a CNG cylinder?
- To automatically shut off gas during a collision
- To allow manual isolation of gas flow for service or emergency (Correct answer)
- To regulate operating pressure
- To indicate remaining fuel level
Correct answer: To allow manual isolation of gas flow for service or emergency
The manual shutoff valve lets technicians or emergency responders manually stop gas flow from the cylinder for safety or maintenance.
Question 4: What type of sensor is used to detect CNG leaks inside a vehicle enclosure?
- O2 sensor
- Methane or combustible gas sensor (Correct answer)
- CO2 sensor
- NOx sensor
Correct answer: Methane or combustible gas sensor
Methane sensors or combustible gas detectors are used to detect CNG leaks because methane (CH4) is the primary component of natural gas.
Question 5: During a fast-fill at a public CNG station, why may the displayed fuel gauge reading drop shortly after fueling ends?
- Fuel leaks out immediately after filling
- Gas cools and contracts after the fast-fill heat of compression dissipates (Correct answer)
- The check valve has failed
- The ECM resets the gauge automatically
Correct answer: Gas cools and contracts after the fast-fill heat of compression dissipates
Fast-fill compresses gas rapidly, raising its temperature and pressure; as the gas cools to ambient temperature, pressure and the gauge reading drop slightly.
Question 6: Which regulation governs the design and testing of CNG cylinders used on vehicles in the United States?
- NFPA 52
- ANSI/CSA NGV2 (Correct answer)
- SAE J1488
- DOT FMVSS 303
Correct answer: ANSI/CSA NGV2
ANSI/CSA NGV2 (Natural Gas Vehicle Cylinder Standard) sets the design, testing, and performance requirements for CNG vehicle cylinders in the U.S.
Question 7: What is the function of the CNG fuel filter in the low-pressure side of the fuel system?
- Removes water and particulates that could damage injectors or the mixer (Correct answer)
- Increases fuel pressure before the engine
- Measures fuel flow rate
- Converts methane to propane
Correct answer: Removes water and particulates that could damage injectors or the mixer
The low-side filter removes moisture and fine particulates from CNG to protect sensitive components like injectors and the regulator.
CNG Systems 3
A CNG technician notices the fuel gauge reads empty but the pressure gauge shows 200 psi.
What is the most likely explanation?