Compressed Natural Gas (CNG) Vehicles Flashcards

A customer reports their CNG vehicle won't switch from gasoline to CNG. What should be checked first?

Answer: CNG tank pressure and the fuel selector/control logic

Low tank pressure or a fault in the switching logic commonly prevents a bi-fuel system from going to CNG mode.

What can cause a CNG regulator to freeze or ice up during operation?

Answer: The expansion of gas across the regulator cools it (Joule-Thomson effect)

Rapid pressure drop across the regulator cools the gas, which can freeze moisture; many regulators are coolant-heated to prevent this.

Which leak-detection method is approved for finding CNG leaks at fittings?

Answer: Electronic combustible gas detector or approved leak solution

A combustible gas detector or non-corrosive leak-detection solution safely locates leaks without ignition risk.

A CNG vehicle gets fewer miles per fill than expected. The least likely mechanical cause is:

Answer: A worn windshield wiper

Wiper wear has nothing to do with fuel range; leaks, incomplete fills, and sensor faults all reduce usable fuel.

Why must only approved, rated fittings and lines be used on the high-pressure side of a CNG system?

Answer: Standard fuel-line parts cannot withstand 3,000+ psi and may rupture

High-pressure CNG components must be specifically rated; ordinary fuel parts can fail dangerously under cylinder pressure.

After replacing a CNG component, what verification step is essential before returning the vehicle?

Answer: Pressurize and perform a leak test on all disturbed fittings

Every disturbed high-pressure joint must be leak-tested after reassembly to confirm a safe seal.

Which condition would trigger a CNG cylinder to be condemned rather than re-inspected?

Answer: Visible cuts or impact damage exceeding allowable limits

Structural damage beyond allowable limits requires the cylinder to be permanently removed from service.