ASE Compressed Natural Gas (CNG) Vehicles 3 — Questions and Answers
Question 1: Compared to gasoline, CNG has a higher octane rating of approximately what value?
- 120-130 (Correct answer)
- 87
- 60
- 100
Correct answer: 120-130
CNG's effective octane rating of about 120-130 allows higher compression ratios and resists knock.
Question 2: Which exhaust emission is significantly reduced when an engine runs on CNG instead of gasoline?
- Carbon monoxide and particulates (Correct answer)
- Oxygen
- Nitrogen
- Water vapor
Correct answer: Carbon monoxide and particulates
CNG combustion produces lower CO and particulate emissions due to its clean-burning, simple molecular structure.
Question 3: A bi-fuel CNG vehicle is one that:
- Can run on either CNG or gasoline from separate tanks (Correct answer)
- Mixes CNG and diesel together
- Uses only CNG
- Runs on CNG and electricity
Correct answer: Can run on either CNG or gasoline from separate tanks
A bi-fuel vehicle has both CNG and gasoline storage and can switch between the two fuels.
Question 4: Why might a CNG engine experience slightly reduced power compared to the same engine on gasoline?
- Gaseous fuel displaces intake air, lowering volumetric efficiency (Correct answer)
- CNG has no energy content
- The spark plugs are removed
- The throttle stays closed
Correct answer: Gaseous fuel displaces intake air, lowering volumetric efficiency
Because CNG enters as a gas it occupies cylinder volume that would otherwise hold air, reducing charge density and power.
Question 5: What type of fuel injector is typically used in a dedicated CNG engine?
- Gaseous/low-pressure port injector (Correct answer)
- High-pressure diesel injector
- Carburetor jet
- Direct liquid injector at 30,000 psi
Correct answer: Gaseous/low-pressure port injector
CNG systems use gaseous fuel injectors designed to meter low-pressure natural gas into the intake ports.
Question 6: When refueling a CNG vehicle, the temperature compensation matters because:
- Gas heats up during fast fill, so the tank may read full before reaching true capacity (Correct answer)
- Water condenses in the tank
- The fuel freezes
- Pressure does not change with temperature
Correct answer: Gas heats up during fast fill, so the tank may read full before reaching true capacity
Fast-fill heating raises pressure and can give a 'full' indication that drops as the gas cools (temperature/pressure relationship).
Question 7: Which inspection is required on CNG cylinders at regular intervals per safety standards?
- Visual inspection for damage, abrasion, and corrosion (Correct answer)
- Replacing the windshield
- Rotating the tires
- Flushing the coolant
Correct answer: Visual inspection for damage, abrasion, and corrosion
Periodic visual inspection of CNG cylinders checks for cuts, gouges, corrosion, and impact damage that affect safety.
ASE Compressed Natural Gas (CNG) Vehicles 3
Compared to gasoline, CNG has a higher octane rating of approximately what value?