CNG Engines and Fuels 3

Question 1

What is the primary chemical component of compressed natural gas?

Accepted Answer

Methane (CH4)

Answer

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:

Accepted Answer

Are more resistant to engine knock and pre-ignition

Answer

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:

Accepted Answer

17.2:1

Answer

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:

Accepted Answer

Require larger or multiple storage tanks to achieve comparable driving range

Answer

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:

Accepted Answer

5–15% concentration by volume

Answer

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.

(CNG) Compressed Natural Gas Practice Test

(CNG) Compressed Natural Gas Practice Test

CNG Engines and Fuels 3