Mixed Deck — All CNG Topics Flashcards

What type of portable fire extinguisher is most appropriate for use at a CNG fueling station?

Answer: Dry chemical ABC-rated extinguisher

ABC dry chemical extinguishers are appropriate at CNG stations because they handle ordinary combustibles, flammable gases/liquids, and energized electrical equipment.

What material are piston rings and seals in oil-free CNG compressors commonly made from?

Answer: PTFE (polytetrafluoroethylene) and carbon composites

PTFE and carbon-filled composite seals are used in oil-free CNG compressors because they are chemically resistant, maintain integrity at high pressures, and require no lubrication.

During a post-maintenance pressure test, a technician pressurizes the system and leaves it for 10 minutes. Pressure drops 20 psi. What does this indicate?

Answer: A leak exists in the system that must be located and repaired before returning to service

Any pressure drop during a static pressure hold test indicates a leak; the system must not be returned to service until the leak is located and corrected.

When using a manometer to check CNG low-pressure regulator output, the reading fluctuates widely with engine speed changes. This indicates:

Answer: The regulator diaphragm is damaged or the spring is fatigued, causing inconsistent pressure regulation

A healthy regulator maintains steady output pressure across engine speed ranges; wide fluctuations indicate a diaphragm or spring failure inside the regulator.

A Type 4 CNG cylinder is constructed from:

Answer: Plastic liner fully wrapped with carbon fiber composite

Type 4 cylinders use a high-density polyethylene (HDPE) or nylon plastic liner that is completely overwrapped with carbon fiber composite for maximum strength at minimum weight.

What is the primary chemical component of compressed natural gas?

Answer: Methane (CH4)

Natural gas is composed primarily of methane (CH4), typically 90% or more by volume, with small amounts of ethane, propane, and other gases.

Which scenario would most likely cause a CNG cylinder to fail the visual inspection portion of a requalification test?

Answer: Deep corrosion pits, bulging, or impact damage exceeding manufacturer limits

Deep corrosion pits, bulging, dents, or impact damage exceeding the manufacturer's specified tolerances are grounds for cylinder rejection during visual requalification inspection.

Because CNG is composed almost entirely of methane, it produces lower vehicle emissions of which specific pollutant class compared to gasoline?

Answer: Non-methane volatile organic compounds (NMVOCs)

Since methane is essentially the only hydrocarbon in CNG, combustion produces very few non-methane VOCs, unlike gasoline which contains dozens of heavier hydrocarbon compounds.

How must penetrations through a fire-rated wall in a CNG compressor building be treated?

Answer: Sealed with listed fire-stop materials to maintain the wall rating

All penetrations through fire-rated assemblies must be sealed with listed firestop systems to preserve the intended fire resistance of the separation.

Which of the following CNG system components is MOST likely to cause a rich-running condition if it fails in the open position?

Answer: A CNG injector stuck open

An injector stuck open delivers continuous fuel to that cylinder, creating an excessively rich air-fuel mixture, misfires, and potential catalyst damage.

Which federal standard governs the design and installation of CNG fuel systems on motor vehicles in the United States?

Answer: FMVSS 303

FMVSS 303 (Federal Motor Vehicle Safety Standard 303) sets requirements for CNG fuel system integrity on motor vehicles.

Which DOT specification cylinder type is most commonly used for CNG vehicle storage because it offers the best weight-to-strength ratio?

Answer: Type 4 (non-metallic liner with full-wrapped composite)

Type 4 cylinders use a non-metallic (polymer) liner fully wrapped in carbon fiber composite, giving the best strength-to-weight ratio for vehicle use.

Which of the following facility configurations requires the highest level of ventilation per NFPA 52?

Answer: Fully enclosed CNG vehicle storage and maintenance building

Fully enclosed buildings trap gas most readily and therefore require the most stringent mechanical ventilation to maintain safe atmospheric conditions.

A CNG vehicle with a dedicated natural gas engine has excessive oil consumption. The technician should inspect:

Answer: Valve stem seals and piston rings, as oil contamination in the combustion chamber is accelerated by dry CNG combustion

CNG burns very dry and removes lubrication from valve stems; worn valve stem seals and rings are a common cause of high oil consumption in dedicated CNG engines.

Under NFPA 52, when must a CNG cylinder be hydrostatically retested?

Answer: Cylinders certified to NGV-2 standard are not retested; they are retired at end of service life

NGV-2 certified cylinders used in vehicles have a defined service life and are removed from service rather than retested, unlike industrial cylinders.

An operator discovers a CNG dispenser hose with visible outer jacket damage. Under NFPA 52, the correct action is to:

Answer: Tag it out of service and replace it immediately

NFPA 52 requires that damaged fueling hoses be immediately removed from service to prevent a hazardous failure.

A driver reports that the CNG dispenser nozzle would not lock onto the vehicle's fill receptacle. What is the safest correct action?

Answer: Do not force the connection; report the incompatible equipment to maintenance and use an alternative dispenser

Forcing a mismatched CNG nozzle-receptacle connection risks high-pressure gas release; the driver should report the issue and use a compatible dispenser.

Compared to gasoline vehicles, CNG vehicles typically produce approximately what percentage less carbon monoxide (CO)?

Answer: 70–90% less

CNG combustion is cleaner and more complete than gasoline, resulting in 70–90% lower carbon monoxide emissions.

What is the compress natural gas engine mixer used for?

Answer: To blend air and gas before entering the intake.

Explanation: The mixer in a compressed natural gas (CNG) engine blends air and gas before entering the intake manifold of the engine. This mixture is essential for proper combustion within the engine.

Which component connects the vehicle's CNG fuel system to the refueling nozzle at the station?

Answer: NGV1 or NGV2 receptacle (fill valve)

The NGV1 receptacle (light-duty) or NGV2 (heavy-duty) is the standardized fill valve on the vehicle that accepts the refueling nozzle.