ANS:B - lower octane number.

Straight run petrol, when compared to methyl or ethyl alcohol, has different properties. Let's examine each of the provided options:

1. Lower Calorific Value:
   • Methyl and ethyl alcohols have lower calorific values compared to petrol. Petrol has a higher energy content per unit volume.
2. Lower Octane Number:
   • Methyl and ethyl alcohols typically have higher octane numbers compared to straight run petrol. Higher octane numbers indicate better resistance to knocking in engines.
3. Higher Specific Gravity:
   • Petrol generally has a lower specific gravity than alcohols. Alcohols are denser than petrol.
4. Higher Ignition Temperature:
   • Alcohols have higher ignition temperatures compared to petrol. This means they require more heat to ignite.

Given these comparisons, the correct statement is that straight run petrol has a lower octane number than methyl or ethyl alcohol. So, the correct answer is: lower octane number. The octane number (or octane rating) of a fuel is a measure of its ability to resist knocking or pinging during combustion, caused by the air-fuel mixture detonating prematurely in the engine. A higher octane number indicates better resistance to knocking, which is crucial for the efficient and smooth operation of internal combustion engines.

Explanation of Lower Octane Number:

1. Definition and Measurement:
   • The octane number is determined by comparing the fuel's performance to a mixture of iso-octane (which resists knocking) and heptane (which knocks easily). For example, a fuel with an octane number of 87 performs the same as a mixture of 87% iso-octane and 13% heptane.
2. Straight Run Petrol:
   • Straight run petrol, also known as straight-run gasoline, is the product directly distilled from crude oil without further refining processes to enhance its properties.
   • This type of petrol typically has a lower octane number, often ranging between 50 to 70, depending on the crude oil source and distillation process.
3. Alcohols (Methyl and Ethyl Alcohol):
   • Methyl alcohol (methanol) and ethyl alcohol (ethanol) have higher octane numbers, generally above 100. This makes them excellent additives for increasing the octane rating of gasoline.
4. Why Octane Number Matters:
   • Engine Performance: Higher octane fuels prevent knocking, allowing for more efficient engine performance and preventing damage to engine components.
   • Compression Ratio: High-performance and high-compression engines require fuels with higher octane ratings to avoid knocking.
   • Fuel Economy: Using the correct octane-rated fuel ensures optimal fuel economy and engine power.

Comparison:

• Straight Run Petrol: Lower octane number, prone to knocking in high-compression engines.
• Methyl/Ethyl Alcohol: Higher octane number, better resistance to knocking, used as additives to boost gasoline octane ratings.

Conclusion:

Straight run petrol has a lower octane number compared to methyl and ethyl alcohol. This means it is more likely to cause knocking in engines, especially those with higher compression ratios, making alcohols with their higher octane numbers preferable for improving fuel performance and engine efficiency.