Alternative fuels — typical octane number of CNG for spark-ignition use Compressed natural gas used in SI engines typically has an octane number nearest to which of the following values?

Introduction / Context:
CNG (mostly methane) is an excellent spark-ignition fuel due to its high resistance to knock. Knowing its octane number helps set compression ratio and ignition timing strategies.

Given Data / Assumptions:

• CNG composition dominated by methane with minor ethane, propane, and inert gases.
• Octane number asked as a representative single value.
• Automotive-grade CNG considered.

Concept / Approach:
Methane has very high knock resistance; research octane numbers for CNG are commonly around 120 or higher. This allows higher compression ratios than typical gasoline without detonation, improving thermal efficiency in dedicated CNG engines.

Step-by-Step Solution:
Identify fuel: CNG for SI engines.Recall typical RON values: near or above 120.Choose the closest option: 120.

Verification / Alternative check:
Engine calibration sources cite effective methane number and RON for CNG in the 120–130 band depending on composition.

Why Other Options Are Wrong:

• 90, 100, 110: appropriate for some gasoline grades or alcohol blends, but below CNG's common rating.

Common Pitfalls:
Confusing octane (knock resistance in SI engines) with cetane (ignition quality in CI engines); CNG is not suitable for compression ignition without pilot injection.

Final Answer:
120