CNG - Compressed Natural Gas Vehicle Conversion and Installation Questions and Answers

Question 1

A technician is installing a CNG fuel system on a vehicle. According to NFPA 52, what is the minimum required clearance between a CNG fuel line and the engine exhaust system?

Accepted Answer

6 inches (150 mm)

Answer

NFPA 52, the Vehicular Gaseous Fuel Systems Code, mandates specific clearances to prevent heat from the exhaust system from damaging CNG fuel system components. A minimum clearance of 6 inches (150 mm) is required between any CNG fuel line and the exhaust system. If this clearance cannot be maintained, a suitable heat shield must be installed.

Question 2

When installing CNG cylinder mounting brackets to a vehicle's frame, which of the following is a critical requirement to prevent premature cylinder failure?

Accepted Answer

Ensuring a rubber or other non-metallic isolating material is placed between the cylinder and the metal brackets.

Answer

To prevent galvanic corrosion and fretting (wear caused by repeated small movements) between the cylinder and its metal mounting brackets, it is essential to use a durable, non-metallic isolating material, such as a rubber gasket. This prevents direct metal-to-metal contact, which can lead to damage and weaken the cylinder over time. Welding brackets is prohibited, and while proper bolt grade and torque are important, the isolation material is key to preventing specific types of long-term damage.

Question 3

A vehicle is being converted to a bi-fuel CNG system. After the hardware installation, the Engine Control Unit (ECU) must be calibrated. What is the primary goal of adjusting fuel trims during the CNG calibration process?

Accepted Answer

To match the long-term and short-term fuel trims on CNG as closely as possible to the values when running on gasoline.

Answer

The primary goal of CNG calibration is to make the engine run as efficiently and cleanly on CNG as it does on gasoline. The ECU uses short-term (STFT) and long-term (LTFT) fuel trims to adjust the air-fuel ratio. A proper calibration ensures that the fuel trims while operating on CNG are very close to the baseline gasoline trims, indicating the engine is operating within its designed parameters and not running too rich or too lean.

Question 4

Which of the following labels is required by the EPA to be affixed in the engine compartment of a vehicle that has been converted to operate on CNG?

Accepted Answer

A label with the name and address of the conversion system manufacturer and a statement that the vehicle complies with EPA regulations.

Answer

The U.S. Environmental Protection Agency (EPA) requires that vehicles with certified or compliant conversion systems have a specific label installed under the hood. This label must identify the conversion system manufacturer's name, address, and phone number, and state that the system complies with the applicable EPA regulations for the vehicle's model year.

Question 5

During a CNG conversion, a technician must route the high-pressure fuel line from the cylinders in the trunk to the engine bay. According to NFPA 52 guidelines, where should the fuel line be routed?

Accepted Answer

Attached securely to the exterior of the vehicle's frame or body.

Answer

NFPA 52 explicitly prohibits routing high-pressure CNG fuel lines through the passenger or driver compartments. Lines must be protected from physical and heat damage. The standard practice is to securely mount the fuel line to the exterior of the vehicle's frame or along the underbody, away from the exhaust and moving parts, ensuring it is protected from road debris.

(CNG) Compressed Natural Gas Practice Test

(CNG) Compressed Natural Gas Practice Test

CNG - Compressed Natural Gas Vehicle Conversion and Installation Questions and Answers