Compression Ratio & Knocking Tendency in SI Engines – SSC JE / RRB JE Concept

In this post, we’ll solve an important conceptual question from Spark Ignition (SI) Engines that often appears in exams like SSC JE, RRB JE, and State Engineering Services.

Question:

If the compression ratio is increased in an S.I. engine, the knocking tendency will:

(a) Increase
(b) Decrease
(c) Not be affected
(d) Cannot be predicted

Video Solution

I’ve explained the concept with diagrams and real-life examples in my video.

Watch Full Video Solution Here

Correct Answer:

(a) Increase

Explanation:

In an S.I. (Spark Ignition) engine, the fuel-air mixture is compressed before ignition.
When compression ratio increases:

1. End-gas temperature and pressure rise sharply.

2. Higher temperature and pressure reduce the self-ignition delay period.

3. As a result, the unburnt end-gas may auto-ignite before the flame front arrives, causing knocking.

Key Formula Reminder:

Knocking in SI engines is primarily influenced by:

$$\text{Knocking tendency} \propto \text{Pressure} \quad \& \quad \text{Temperature of End Gas}$$

$$\text{Knocking tendency}\propto \text{Pressure}\&\text{Temperature of End Gas}$$

Higher compression ratio → Higher end-gas temperature → Increased knocking tendency.

Effects of Knocking:

• Loss of power

• Engine overheating

• Piston and valve damage

• Reduced engine life

Exam Tip:

• In SI engines → Higher compression ratio → Knocking increases.

• In CI (Diesel) engines → Higher compression ratio → Knocking decreases.