CNG Engines and Fuels 2

Question 1

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

Accepted Answer

120-130

Answer

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:

Accepted Answer

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

Answer

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?

Accepted Answer

Gasoline

Answer

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?

Accepted Answer

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

Answer

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:

Accepted Answer

Carbon monoxide (CO) and non-methane hydrocarbons

Answer

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

(CNG) Compressed Natural Gas Practice Test

(CNG) Compressed Natural Gas Practice Test

CNG Engines and Fuels 2