CNG Cheat Sheet 2026

The 30 highest-yield CNG facts, distilled from real exam questions. Print it, save it as a PDF, or study it here — free, no sign-up.

1. What does the shut-off valve at the compressed natural gas cylinder serve as? → To cut the gas flow from tank to lines.
2. What pressure does NFPA 52 define as the maximum allowable working pressure (MAWP) for a standard CNG vehicle fuel system? → 3,600 psi
3. In a CNG cascaded storage system, which bank is drawn from FIRST to fill a vehicle? → The low-pressure bank
4. Compared to diesel, CNG vehicles typically reduce particulate matter (PM) emissions by approximately how much? → Up to 90%
5. What is the standard maximum service life of a DOT-specification CNG vehicle cylinder from its manufacture date? → 20 years
6. What is the main environmental advantage of CNG over diesel fuel? → CNG combustion produces significantly lower NOx, particulate matter, and CO2 than diesel
7. What does the Wobbe Index measure in the context of CNG fuel quality? → The interchangeability and energy delivery rate of the gas relative to a standard
8. What material are piston rings and seals in oil-free CNG compressors commonly made from? → PTFE (polytetrafluoroethylene) and carbon composites
9. In the event of a leak, how will compressed natural gas (CNG) behave in an open environment? → It will dissipate rapidly upwards into the atmosphere.
10. Which gas does CNG contain? → Methane
11. When performing a CNG injector balance test, an injector shows significantly lower flow than the others. The BEST next step is to: → Inspect and clean or replace the suspect injector
12. A CNG fuel rail is different from a gasoline fuel rail primarily because it: → Must withstand significantly higher working pressures
13. What is the purpose of the thermal PRD (pressure relief device) vent tube on a vehicle-mounted CNG cylinder? → To direct vented gas away from the vehicle body and toward a safe location
14. Which federal agency regulates tailpipe emission standards that CNG vehicles must meet in the United States? → EPA
15. What does 'CNG equivalent' mean when comparing fuel economy ratings? → Fuel economy expressed in miles per gasoline gallon equivalent (mpgge)
16. Which of the following materials is NOT acceptable for the construction of a CNG compressor room wall that separates it from an occupied area? → Unprotected wood-frame construction
17. Which of the following CNG system components is MOST likely to cause a rich-running condition if it fails in the open position? → A CNG injector stuck open
18. What type of ventilation is required for enclosed spaces where CNG vehicles are parked or serviced? → High-level or ceiling ventilation to prevent methane accumulation
19. What is the typical pressure configuration for a three-bank CNG cascade storage system? → 1,200 PSI / 2,400 PSI / 3,600 PSI
20. Which standard governs the nozzle-to-receptacle interface for CNG vehicle fueling in the United States? → ANSI/CSA NGV1
21. What must be done BEFORE reconnecting and pressurizing a CNG compressor after maintenance? → Leak-test all disturbed connections with an approved method before applying pressure
22. When should a CNG cylinder be immediately removed from service? → If it has been involved in a fire or shows signs of heat damage
23. In a bi-fuel CNG/gasoline vehicle, which fuel is typically used during cold engine starts? → Gasoline
24. On a bi-fuel CNG vehicle, what determines which fuel the engine uses when both fuels are available? → The engine control module (ECM) or driver selection switch
25. What does CNG stand for? → Compressed Natural Gas
26. CNG has an autoignition temperature of approximately 1,004°F (540°C), much higher than gasoline's. This property means CNG engines: → Are more resistant to engine knock and pre-ignition
27. According to US standards, what is the typical maximum working pressure for a CNG vehicle fuel system? → 3,600 psi (248 bar)
28. When converting a gasoline vehicle to CNG, which component must be upgraded to handle the higher fuel pressure? → Fuel pressure regulator
29. Which training requirement is mandated for employees who fuel CNG vehicles at a fleet's private fueling station under OSHA standards? → Hazard Communication (HazCom) training on compressed gas hazards and emergency procedures
30. What type of sensor is used to detect CNG leaks inside a vehicle enclosure? → Methane or combustible gas sensor