CNG Engines and Fuels 2 — Questions and Answers
Question 1: What is the approximate octane rating of compressed natural gas (CNG)?
- 87
- 91
- 100
- 120-130 (Correct answer)
Correct 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.
Question 2: Dedicated CNG engines often use a higher compression ratio than gasoline engines primarily because:
- CNG has a higher volumetric energy density
- CNG's high octane rating resists engine knock at higher compression (Correct answer)
- CNG requires less oxygen for combustion
- CNG produces less heat during combustion
Correct 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.
Question 3: In a bi-fuel CNG/gasoline vehicle, which fuel is typically used during cold engine starts?
- CNG only
- Both fuels simultaneously
- Gasoline (Correct answer)
- The driver must select manually each time
Correct 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.
Question 4: What is the primary function of the pressure regulator in a CNG fuel system?
- Filter contaminants from the CNG stream
- Reduce high-pressure tank CNG to a lower delivery pressure for the engine (Correct answer)
- Increase fuel pressure for direct injection
- Measure the amount of fuel remaining in the tank
Correct 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.
Question 5: Compared to equivalent gasoline engines, dedicated CNG engines typically emit significantly lower levels of:
- Carbon dioxide (CO2)
- Water vapor (H2O)
- Carbon monoxide (CO) and non-methane hydrocarbons (Correct answer)
- Oxygen
Correct 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.
Question 6: Converting a gasoline vehicle to CNG using a simple mixer system typically results in:
- Increased horsepower over the original gasoline engine
- Reduced engine power output of roughly 10-15% (Correct answer)
- Identical power output to the gasoline version
- Doubled fuel efficiency
Correct 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.
Question 7: A venturi-type CNG mixer introduces fuel into the engine by:
- Injecting high-pressure CNG directly into the combustion chamber
- Blending CNG with intake air upstream of the throttle body (Correct answer)
- Vaporizing liquid CNG into the intake manifold
- Mixing CNG with engine oil before combustion
Correct 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.
CNG Engines and Fuels 2
What is the approximate octane rating of compressed natural gas (CNG)?