Question

Which of the following is true for self-ignition temperature of CI engine and SI engine fuel?
(A). A good CI engine fuel requires high self-ignition temperature
(B). A good SI engine requires low self-ignition temperature
(C). A good CI engine fuel requires low self-ignition temperature
(D). A good SI engine requires high self-ignition temperature

Choose the correct answer from the options given below:

• (A) and (B) only.
• (B) and (C) only.
• (A) and (D) only.
• (C) and (D) only.

Hint:

Remember the opposite ignition requirements:
- SI Engine Fuel (Gasoline): High Octane number \(\implies\) High SIT \(\implies\) High resistance to auto-ignition.
- CI Engine Fuel (Diesel): High Cetane number \(\implies\) Low SIT \(\implies\) Ignites easily under compression.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Self-Ignition Temperature (SIT) is the lowest temperature at which a fuel ignites spontaneously in normal atmosphere without an external spark or flame.

Step 3: Detailed Explanation:
1.

Compression Ignition (CI) Engines: CI engines rely on the high temperature of compressed air to ignite the injected diesel fuel.
To ensure rapid, reliable auto-ignition and minimize ignition delay (preventing diesel knock), a good CI engine fuel must have a

low self-ignition temperature. Thus, statement (C) is correct.
2.

Spark Ignition (SI) Engines: SI engines ignite an air-fuel mixture using a spark plug.
If the fuel's self-ignition temperature is too low, the unburned end-gas will auto-ignite before the spark flame front arrives, causing destructive engine knock.
To prevent knock, a good SI engine fuel must have a

high self-ignition temperature. Thus, statement (D) is correct.

Step 4: Final Answer:
The correct option is 4, which corresponds to (C) and (D) only.