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.

Read the first 7 Engines and Fuels flashcards as text

  1. What is the approximate octane rating of compressed natural gas (CNG)?

    Answer: 120-130

    CNG has an octane rating of approximately 120-130, significantly higher than any common gasoline grade, enabling higher compression ratios in dedicated engines.

  2. Dedicated CNG engines often use a higher compression ratio than gasoline engines primarily because:

    Answer: CNG's high octane rating resists engine knock at higher compression

    CNG's octane rating of 120-130 means it resists autoignition, allowing engineers to design higher compression ratios that improve thermal efficiency without knock.

  3. In a bi-fuel CNG/gasoline vehicle, which fuel is typically used during cold engine starts?

    Answer: Gasoline

    Bi-fuel vehicles typically default to gasoline at cold start because CNG vaporizes less reliably in very cold temperatures and gasoline provides more consistent cold-start performance.

  4. What is the primary function of the pressure regulator in a CNG fuel system?

    Answer: Reduce high-pressure tank CNG to a lower delivery pressure for the engine

    The pressure regulator steps down CNG from tank pressure (up to 3,600 psi) to the low working pressure required by the engine fuel delivery system.

  5. Compared to equivalent gasoline engines, dedicated CNG engines typically emit significantly lower levels of:

    Answer: Carbon monoxide (CO) and non-methane hydrocarbons

    CNG combustion produces substantially lower carbon monoxide and non-methane hydrocarbons than gasoline due to methane's simpler molecular structure and cleaner combustion.

  6. Converting a gasoline vehicle to CNG using a simple mixer system typically results in:

    Answer: Reduced engine power output of roughly 10-15%

    Mixer-based CNG conversions reduce power by 10-15% because gaseous CNG displaces intake air, reducing volumetric efficiency compared to liquid-injected gasoline.

  7. A venturi-type CNG mixer introduces fuel into the engine by:

    Answer: Blending CNG with intake air upstream of the throttle body

    A venturi mixer uses the pressure differential created by airflow to draw CNG into the intake air stream before the throttle body, similar in principle to a carburetor.