CNG Engines and Fuels 3 — Questions and Answers
Question 1: What is the primary chemical component of compressed natural gas?
- Propane (C3H8)
- Methane (CH4) (Correct answer)
- Butane (C4H10)
- Ethane (C2H6)
Correct answer: Methane (CH4)
Natural gas is composed primarily of methane (CH4), typically 90% or more by volume, with small amounts of ethane, propane, and other gases.
Question 2: CNG has an autoignition temperature of approximately 1,004°F (540°C), much higher than gasoline's. This property means CNG engines:
- Must use glow plugs instead of spark plugs
- Are more resistant to engine knock and pre-ignition (Correct answer)
- Require lower compression ratios than gasoline engines
- Cannot use catalytic converters
Correct answer: Are more resistant to engine knock and pre-ignition
CNG's very high autoignition temperature means it resists premature detonation far better than gasoline, which is why it achieves an octane rating of 120-130.
Question 3: The stoichiometric air-to-fuel ratio for CNG is approximately:
- 14.7:1 (same as gasoline)
- 17.2:1 (Correct answer)
- 9.0:1
- 25.0:1
Correct answer: 17.2:1
CNG requires approximately 17.2 parts of air per 1 part of fuel by mass for complete combustion, compared to 14.7:1 for gasoline, due to methane's higher hydrogen-to-carbon ratio.
Question 4: CNG has a lower volumetric energy density than gasoline, which means CNG vehicles:
- Have lower fuel cost per equivalent mile driven
- Require larger or multiple storage tanks to achieve comparable driving range (Correct answer)
- Always produce more power than gasoline engines
- Can be refueled significantly faster than gasoline vehicles
Correct answer: Require larger or multiple storage tanks to achieve comparable driving range
Because CNG stores less energy per unit volume than liquid gasoline, CNG vehicles need larger or additional cylinders to achieve driving range comparable to a gasoline vehicle.
Question 5: The flammability range of natural gas in air is approximately:
- 1–4% concentration by volume
- 5–15% concentration by volume (Correct answer)
- 20–30% concentration by volume
- 35–50% concentration by volume
Correct answer: 5–15% concentration by volume
Natural gas is flammable only when its concentration in air is between approximately 5% and 15%; below 5% the mixture is too lean, and above 15% it is too rich to ignite.
Question 6: Because CNG is composed almost entirely of methane, it produces lower vehicle emissions of which specific pollutant class compared to gasoline?
- Nitrogen oxides (NOx)
- Carbon dioxide (CO2)
- Non-methane volatile organic compounds (NMVOCs) (Correct answer)
- Water vapor
Correct answer: Non-methane volatile organic compounds (NMVOCs)
Since methane is essentially the only hydrocarbon in CNG, combustion produces very few non-methane VOCs, unlike gasoline which contains dozens of heavier hydrocarbon compounds.
Question 7: Why do CNG engines generally use spark plugs with a tighter electrode gap than standard gasoline engines?
- CNG burns faster than gasoline mixtures
- Methane requires more ignition energy than gasoline vapor, needing a more concentrated spark (Correct answer)
- CNG produces more carbon fouling that widens the gap over time
- CNG operates at lower cylinder pressures requiring a smaller spark
Correct answer: Methane requires more ignition energy than gasoline vapor, needing a more concentrated spark
Methane has a higher minimum ignition energy than gasoline vapors, so CNG engines use tighter spark plug gaps to generate a more concentrated, reliable arc for consistent ignition.
CNG Engines and Fuels 3
What is the primary chemical component of compressed natural gas?