CNG Properties and Safety 5

Question 1

A CNG vehicle technician notices frost forming on a fuel line fitting during fueling. What does this indicate?

Accepted Answer

Normal operation due to gas expansion and cooling

Answer

Frost forming on CNG fittings during fueling is normal and results from the Joule-Thomson effect — rapid gas expansion causes significant cooling of the fittings.

Question 2

What is the main environmental advantage of CNG over diesel fuel?

Accepted Answer

CNG combustion produces significantly lower NOx, particulate matter, and CO2 than diesel

Answer

CNG combustion results in significantly lower NOx, particulate matter (PM), and CO2 emissions compared to diesel, making it cleaner for air quality.

Question 3

What type of regulator reduces CNG pressure from cylinder pressure to engine operating pressure?

Accepted Answer

A multi-stage pressure regulator

Answer

CNG fuel systems use a multi-stage pressure regulator to step down cylinder pressure (3,600 psi) progressively to the low pressure required by the engine (typically under 10 psi).

Question 4

Under DOT regulations, what information must be permanently marked on a CNG cylinder?

Accepted Answer

DOT specification, service pressure, serial number, and requalification date

Answer

DOT regulations require CNG cylinders to be permanently marked with the DOT specification number, service pressure, serial number, manufacturer, and requalification/expiration date.

Question 5

Why are Type 4 CNG cylinders (full composite with plastic liner) potentially problematic for fire situations compared to Type 1 metal cylinders?

Accepted Answer

Composite materials can delaminate or fail without visible warning when exposed to heat

Answer

Composite cylinders can suffer internal delamination or degradation from heat exposure that is not visible externally, making post-fire inspection critical before returning to service.

(CNG) Compressed Natural Gas Practice Test

(CNG) Compressed Natural Gas Practice Test

CNG Properties and Safety 5