_________ is an important anti knock compound added to petrol used in internal combustion engines .

Answer

**Step-by-Step Solution:**

1. **Understanding the Question:**
- The question asks for an important anti-knock compound that is added to petrol used in internal combustion engines.

2. **Identifying Anti-Knock Compounds:**
- Anti-knock compounds are substances that help to prevent knocking in engines. Knocking occurs when fuel ignites prematurely in the engine cylinder, leading to inefficient combustion and potential engine damage.

3. **Common Anti-Knock Compounds:**
- The most well-known anti-knock compound historically used in petrol is tetraethyl lead (TEL).

4. **Chemical Composition:**
- Tetraethyl lead has the chemical formula Pb(C2H5)4. It consists of a lead atom bonded to four ethyl groups.

5. **Function of Tetraethyl Lead:**
- Tetraethyl lead increases the octane rating of petrol, which helps to improve engine performance by allowing for higher compression ratios without knocking.

6. **Conclusion:**
- Therefore, the appropriate word to fill in the blank is "tetraethyl lead".

**Final Answer:**
- Tetraethyl lead is an important anti-knock compound added to petrol used in internal combustion engines.

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