Question

Given below are two statements, one is labelled as Assertion (A) and the other one labelled as Reason (R).

Assertion (A): A good CI engine fuel, like diesel oil, is a bad SI engine fuel, and a good SI engine fuel, like petrol, is a bad CI engine fuel.

Reason (R): A good CI engine fuel requires high self-ignition temperature and good SI engine fuel requires low self-ignition temperature.

In light of the above statements, choose the correct answer from the options given below.

• A. Both (A) and (R) are true and (R) is the correct explanation of (A).
• B. Both (A) and (R) are true but (R) is NOT the correct explanation of (A).
• C. (A) is true but (R) is false.
• D. (A) is false but (R) is true.

Hint:

Compare what self-ignition temperature means for compression ignition versus spark ignition.

Answer 1

The Correct Option is C

We can check this through the octane and cetane rating system used for these two fuels.

Petrol, used in SI engines, is rated by octane number, and a HIGHER octane number means the fuel resists knocking, which in practical terms means it has a higher self-ignition temperature and does not fire on its own under compression alone.

Diesel, used in CI engines, is rated by cetane number, and a HIGHER cetane number means the fuel ignites more readily under compression, which in practical terms means it has a LOWER self-ignition temperature.

So the actual fuel requirements are opposite to what Reason (R) claims, CI engines need a low self-ignition temperature fuel and SI engines need a high self-ignition temperature fuel.

Assertion (A), about diesel performing poorly in SI engines and petrol performing poorly in CI engines, remains a correct and independent fact, confirmed by how poorly petrol resists compression ignition and how poorly diesel responds to spark ignition.

Therefore Assertion (A) is true while Reason (R), as worded, is false.

\[\boxed{\text{(A) is true but (R) is false}}\]