Knocking in a fuel is

Options

A. Tendency to spill
B. Explosion of unburnt fuel-air mixture
C. Late burning of fuel
D. Difficult ignition of fuel in combustion Chamber.

Answer

**Step-by-Step Solution:**

1. **Understanding Knocking**: Knocking in a fuel refers to a phenomenon that occurs in internal combustion engines. It is characterized by a sharp sound that results from the premature combustion of the air-fuel mixture in the engine's cylinder.

2. **Cause of Knocking**: The main cause of knocking is the uneven burning of the fuel within the engine cylinder. Ideally, the fuel should burn in small, controlled packets. However, when the combustion is uneven, it leads to knocking.

3. **Mechanism of Knocking**: When the air-fuel mixture is compressed in the cylinder, it should ignite in a controlled manner. If it ignites all at once or in an uncontrolled manner, it creates shock waves. These shock waves can cause the knocking sound and can also lead to potential damage to engine components.

4. **Identifying the Correct Option**: Among the given options:
- Option 1: Tendency to spill - Incorrect
- Option 2: Explosion of unburned fuel-air mixture - Correct
- Option 3: Late burning of fuel - Incorrect
- Option 4: Difficult ignition of fuel in combustion chamber - Incorrect

5. **Conclusion**: The correct answer to the question "Knocking in a fuel is" is **Option 2: Explosion of unburned fuel-air mixture**.

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