Fuel System Components Flashcards
7 cards from real CNG practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Fuel System Components flashcards as text
What is the role of the CNG electronic fuel control unit (FCU) or dedicated ECU?
Answer: It manages injector timing and duration based on sensor inputs for optimal CNG combustion
The FCU or CNG-specific ECU processes sensor data (pressure, temperature, O2, MAP) and controls injector pulse width and timing for correct air-fuel ratio with natural gas.
A technician detects ice forming on the CNG pressure regulator during vehicle operation. This most likely indicates:
Answer: Normal Joule-Thomson cooling effect during pressure drop
When high-pressure gas rapidly expands through the regulator, it cools dramatically (Joule-Thomson effect), causing ambient moisture to freeze on the regulator body.
Which document or standard governs the installation of CNG fuel systems on vehicles in the United States?
Answer: NFPA 52 (Vehicular Natural Gas Fuel Systems Code)
NFPA 52 is the primary US standard covering the design, installation, operation, and maintenance of vehicular compressed and liquefied natural gas fuel systems.
When a CNG cylinder fails its visual inspection due to impact damage, the correct action is to:
Answer: Remove the cylinder from service and follow OEM/NGV2 disposition guidance
Cylinders with impact damage must be removed from service because structural integrity cannot be confirmed visually, and repair of composite cylinders in the field is not permitted.
The CNG fuel temperature sensor is used by the ECM primarily to:
Answer: Compensate fuel delivery calculations since gas density varies with temperature
Natural gas density changes with temperature, so the ECM uses fuel temperature data to adjust injector pulse width and deliver the correct fuel mass regardless of gas temperature.
Which of the following best describes the difference between a CNG solenoid valve and a manual shutoff valve?
Answer: The solenoid valve is electrically actuated and can close automatically; the manual valve requires physical operation
A solenoid valve is electronically controlled and closes automatically when the ignition is off or a leak is detected, while a manual shutoff requires a technician to physically turn it.
After replacing a CNG high-pressure fitting, the technician should verify the connection using:
Answer: An electronic combustible gas detector or approved leak detection solution
Open flame detectors are never used near CNG; technicians must use an electronic combustible gas detector or a listed leak detection solution (soap-based) that is safe for use with natural gas.