CNG - Compressed Natural Gas Vehicle Conversion and Installation Questions and Answers 1 — 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?
- 1 inch (25 mm)
- 3 inches (75 mm)
- 6 inches (150 mm) (Correct answer)
- 9 inches (225 mm)
Correct answer: 6 inches (150 mm)
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?
- Welding the brackets directly to the frame for maximum strength.
- Using grade 5 bolts torqued to the highest possible specification.
- Ensuring a rubber or other non-metallic isolating material is placed between the cylinder and the metal brackets. (Correct answer)
- Applying a thick layer of paint to the brackets and cylinder contact points to prevent corrosion.
Correct answer: Ensuring a rubber or other non-metallic isolating material is placed between the cylinder and the metal brackets.
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?
- To maximize engine horsepower on CNG.
- To ensure the vehicle's diagnostic trouble codes are suppressed.
- To match the long-term and short-term fuel trims on CNG as closely as possible to the values when running on gasoline. (Correct answer)
- To decrease the fuel rail pressure to its lowest possible setting for efficiency.
Correct answer: To match the long-term and short-term fuel trims on CNG as closely as possible to the values when running on gasoline.
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?
- A label indicating the cylinder's last hydrostatic test date.
- A label showing the vehicle's new, lower weight capacity.
- A label with the name and address of the conversion system manufacturer and a statement that the vehicle complies with EPA regulations. (Correct answer)
- A diagram of the CNG fuel flow and a list of all installed components.
Correct answer: A label with the name and address of the conversion system manufacturer and a statement that the vehicle complies with EPA regulations.
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?
- Through the passenger compartment for easy inspection access.
- Inside the main frame rail for maximum protection.
- Directly adjacent to the vehicle's battery cables for efficient routing.
- Attached securely to the exterior of the vehicle's frame or body. (Correct answer)
Correct answer: Attached securely to the exterior of the vehicle's frame or body.
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.
Question 6: A customer wants to convert their 2-year-old light-duty truck to CNG. According to EPA regulations for converting 'new and relatively new vehicles,' what is required for the conversion system?
- A letter from the original vehicle manufacturer approving the conversion.
- An EPA-issued Certificate of Conformity for that specific vehicle/engine group. (Correct answer)
- A simple declaration from the installer that the system meets emissions standards.
- Proof that the vehicle has been driven at least 50,000 miles before conversion.
Correct answer: An EPA-issued Certificate of Conformity for that specific vehicle/engine group.
For vehicles that are in their model year or one year past it, the EPA considers them 'new or relatively new.' Conversion systems for these vehicles must undergo a rigorous testing and certification process similar to that of an original equipment manufacturer. The EPA then issues a Certificate of Conformity for the specific engine family, which verifies the system meets all applicable emissions standards.
CNG - Compressed Natural Gas Vehicle Conversion and Installation Questions and Answers 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?