Practice Test Geeks(CNG) Compressed Natural Gas Practice Test
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Engines and Fuels Flashcards

7 cards from real CNG practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.

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  1. What is the primary chemical component of compressed natural gas?

    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.

  2. CNG has an autoignition temperature of approximately 1,004°F (540°C), much higher than gasoline's. This property means CNG engines:

    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.

  3. The stoichiometric air-to-fuel ratio for CNG is approximately:

    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.

  4. CNG has a lower volumetric energy density than gasoline, which means CNG vehicles:

    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.

  5. The flammability range of natural gas in air is approximately:

    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.

  6. Because CNG is composed almost entirely of methane, it produces lower vehicle emissions of which specific pollutant class compared to gasoline?

    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.

  7. Why do CNG engines generally use spark plugs with a tighter electrode gap than standard gasoline engines?

    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.