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By higher octane rating of SI fuels, it is meant that the fuel has


1. Higher heating value
2. Higher flash point
3. Lower volatility
4. Longer ignition delay

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Correct Answer - Option 4 : Longer ignition delay

Explanation:

The S.I. engine fuel has been given an anti-knocking rating which is known as Octane Rating. It basically shows the resistance of the fuel to create knocking by auto-ignition.

According to a standard practice, the antiknock value of an SI engine fuel is determined by comparing its antiknock property with a mixture of two reference fuels, normal heptane (C7H18) and iso-octane (C8H18).

Iso-octane chemically being a very good antiknock fuel, is arbitrarily assigned a rating of 100 octane number. Normal heptane (C7H18) has very poor antiknock qualities and is given a rating of 0 octane number. So, if normal heptane (C7H18) is present in the fuel then it will create knocking which will result in the auto-ignition. Thus, Normal heptane content in fuel for the S.I. engine accelerates auto-ignition.

Higher octane is related only to longer ignition delay which prevents knocking.

Cetane rating, also known as cetane number is a measurement of the quality or performance of diesel fuel. The higher the number, the better the fuel burns within the engine of a vehicle. The cetane number is similar to the Octane Rating in that it is a rating assigned to fuel to rate the quality of its combustion.

The main difference between cetane and octane ratings is that the octane rating represents how well a fuel can resist pre-ignition due to compression—ensuring the fuel ignites only from a spark from the spark plug. However, the cetane number measures the delay in the ignition time of the fuel. 

A higher cetane number resulting in quicker ignition of the fuel leads to less non-ignited fuels building up inside the combustion chamber, as well as more complete fuel combustion. Better fuel combustion and quick ignition lead to quicker starting for vehicles, as well as an engine that operates more quietly as only misted fuel is ignited and build-up inside the engine is minimized. Moreover, fuel efficiency improves with more complete combustion and harmful emissions are reduced.

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