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

Step 1: Assertion (A) says a good CI engine fuel like diesel is a poor SI engine fuel, and a good SI engine fuel like petrol is a poor CI engine fuel. This is a well established fact in engine technology, so Assertion (A) is true.

Step 2: Reason (R) says a good CI engine fuel needs a high self-ignition temperature and a good SI engine fuel needs a low self-ignition temperature.

Step 3: This is actually the reverse of the truth. A CI engine relies only on the heat of compression to ignite the fuel, so it needs a fuel with a LOW self-ignition temperature so that it ignites promptly once injected into the hot compressed air.

Step 4: An SI engine uses a spark to start combustion at a controlled instant, so its fuel must resist auto-ignition from compression heat alone. That means SI fuel needs a HIGH self-ignition temperature, or a high octane rating, so it does not pre-ignite or knock before the spark fires.

Since Reason (R) reverses the correct requirement, it is false, even though Assertion (A) is true. So the correct choice is option 3, (A) is true but (R) is false.