ASE Practice Test (F1: Compressed Natural Gas (CNG) Vehicles) 3 — Questions and Answers
Question 1: What is the primary difference between a bi-fuel CNG vehicle and a dedicated CNG vehicle?
- A bi-fuel vehicle uses both CNG and electric power; a dedicated vehicle uses CNG only
- A bi-fuel vehicle can operate on either CNG or gasoline using separate fuel systems; a dedicated vehicle operates on CNG only (Correct answer)
- A bi-fuel vehicle has two CNG tanks; a dedicated vehicle has one tank
- A bi-fuel vehicle is factory-built; a dedicated vehicle is always converted
Correct answer: A bi-fuel vehicle can operate on either CNG or gasoline using separate fuel systems; a dedicated vehicle operates on CNG only
Bi-fuel CNG vehicles retain a conventional gasoline fuel system and add a separate CNG system, allowing the driver to switch between fuels. Dedicated CNG vehicles are designed to run only on natural gas and do not have a gasoline system. Bi-fuel vehicles offer flexibility when CNG stations are unavailable but carry the weight of two complete fuel systems. Dedicated vehicles are optimized for CNG operation.
Question 2: When leak-testing a CNG fuel system after repair, which method is approved?
- Pressurize with compressed air and apply soap solution to joints
- Pressurize with natural gas or approved inert gas (nitrogen) to working pressure and use an approved electronic CNG leak detector or approved soap solution on joints (Correct answer)
- Use open flame to detect escaping gas
- Visual inspection only
Correct answer: Pressurize with natural gas or approved inert gas (nitrogen) to working pressure and use an approved electronic CNG leak detector or approved soap solution on joints
CNG leak testing requires using natural gas or nitrogen (to simulate system conditions) combined with an electronic gas detector calibrated for methane, or an approved non-corrosive soap solution applied to all joints, fittings, and connections. An electronic leak detector can find very small leaks. Open-flame testing is absolutely prohibited — the slightest leak near an open flame would result in immediate ignition and explosion.
Question 3: A CNG-powered bus has a fuel gauge reading that does not correspond to the actual pressure in the cylinders. What is the MOST likely cause?
- Empty CNG cylinders
- A faulty pressure transducer or sensor in the fuel system, or a faulty gauge/instrument cluster reading (Correct answer)
- A failed manual shutoff valve
- An excess flow valve that is stuck open
Correct answer: A faulty pressure transducer or sensor in the fuel system, or a faulty gauge/instrument cluster reading
CNG fuel gauges do not measure liquid fuel level like gasoline vehicles. Instead, they read cylinder pressure from a pressure transducer. The displayed fuel quantity is calculated from pressure data. If the pressure transducer fails or reads inaccurately, the gauge will not reflect actual tank pressure. The transducer, its wiring, and the gauge/instrument cluster are the components to diagnose when gauge readings are inaccurate.
Question 4: Why must CNG cylinders be inspected before each fill cycle at a high-pressure refueling station?
- To check for proper color coding of the cylinder
- To identify damage, dents, heat exposure marks, or expired service date that would make refueling the cylinder unsafe or illegal (Correct answer)
- To verify the cylinder label matches the vehicle VIN
- To measure the cylinder's remaining wall thickness with ultrasound
Correct answer: To identify damage, dents, heat exposure marks, or expired service date that would make refueling the cylinder unsafe or illegal
CNG cylinders are pressure vessels that can fail catastrophically if damaged, heat-exposed, or expired. Visual inspection before filling identifies obvious concerns: dents or impact damage to composite layers, discoloration from heat or fire (which destroys composite fiber strength), cuts in the fiber wrap, or an expired service date stamp. Refueling a damaged or expired cylinder is a serious safety hazard. NFPA 52 and CNG station protocols require visual inspection before filling.
Question 5: What is the maximum allowable concentration of methane (natural gas) in an enclosed facility before mandatory evacuation?
- 5% of the Lower Flammable Limit (LFL)
- 25% of the Lower Flammable Limit (LFL) — approximately 1.25% methane in air (Correct answer)
- 50% of the LFL
- 100% of the LFL before concern
Correct answer: 25% of the Lower Flammable Limit (LFL) — approximately 1.25% methane in air
Methane's Lower Flammable Limit (LFL) is approximately 5% concentration in air. NFPA and facility safety standards typically require action — increased ventilation, evacuation, and elimination of ignition sources — when methane concentration reaches 25% of the LFL (approximately 1.25% methane by volume in air). Gas detection systems in CNG facilities are set to alarm at this level to provide safe warning well below the actual flammability threshold.
Question 6: After a CNG vehicle is involved in a collision, what is the FIRST action emergency responders or technicians should take regarding the fuel system?
- Immediately cut the CNG lines to release pressure quickly
- Turn off the vehicle ignition, close the manual shutoff valve(s) on the cylinders (if safely accessible), and move away from the vehicle — do not approach if there is active fuel release or fire (Correct answer)
- Open all windows to ventilate the vehicle interior
- Remove the CNG cylinders from the vehicle immediately
Correct answer: Turn off the vehicle ignition, close the manual shutoff valve(s) on the cylinders (if safely accessible), and move away from the vehicle — do not approach if there is active fuel release or fire
After a collision involving a CNG vehicle, the priority is to stop ignition sources (key off) and isolate the fuel supply at the cylinder manual shutoff valves. If gas is actively leaking, the area must be evacuated and ignition sources eliminated. CNG cylinders should never be removed from a damaged vehicle without specialized training and equipment. Emergency response procedures for CNG vehicles are published by NFPA and vehicle manufacturers.
ASE Practice Test (F1: Compressed Natural Gas (CNG) Vehicles) 3
What is the primary difference between a bi-fuel CNG vehicle and a dedicated CNG vehicle?