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<h1>Compression Ratio & Knocking Tendency in SI Engines – SSC JE / RRB JE Concept</h1>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.<hr data-start="323" data-end="326"><h3 data-start="328" data-end="345">Question:</h3><h5>If the compression ratio is increased in an S.I. engine, the knocking tendency will:</h5><h5>(a) Increase 
(b) Decrease 
(c) Not be affected 
(d) Cannot be predicted</h5><hr data-start="517" data-end="520"><h3 data-start="522" data-end="547">Video Solution</h3>I’ve explained the concept with diagrams and real-life examples in my video. <h4><a data-start="631" data-end="666" class="" rel="noopener" href="https://www.youtube.com/watch?v=6BAnm-ubENw&list=PLjQMEOXj4i1_I4Lg19iJ7L43an_XWUWg7&index=3" target="_blank">Watch Full Video Solution Here</a></h4><hr data-start="669" data-end="672"><h3 data-start="674" data-end="697">Correct Answer:</h3>(a) Increase<hr data-start="718" data-end="721"><h3 data-start="723" data-end="743">Explanation:</h3>In an S.I. (Spark Ignition) engine, the fuel-air mixture is compressed before ignition. 
When compression ratio increases:<ol data-start="879" data-end="1126">
<li data-start="879" data-end="934">
End-gas temperature and pressure rise sharply.
</li>
<li data-start="935" data-end="1014">
Higher temperature and pressure reduce the self-ignition delay period.
</li>
<li data-start="1015" data-end="1126">
As a result, the unburnt end-gas may auto-ignite before the flame front arrives, causing knocking.
</li>
</ol><hr data-start="1128" data-end="1131"><h3 data-start="1133" data-end="1162">Key Formula Reminder:</h3>Knocking in SI engines is primarily influenced by:

<math xmlns="http://www.w3.org/1998/Math/MathML" display="block"><semantics><mrow><mtext>Knocking tendency</mtext><mo>∝</mo><mtext>Pressure</mtext><mspace width="1em"></mspace><mi mathvariant="normal">&</mi><mspace width="1em"></mspace><mtext>Temperature of End Gas</mtext></mrow><annotation encoding="application/x-tex">\text{Knocking tendency} \propto \text{Pressure} \quad \& \quad \text{Temperature of End Gas}</annotation></semantics></math> Knocking tendency∝Pressure&Temperature of End Gas
Higher compression ratio → Higher end-gas temperature → Increased knocking tendency.<hr data-start="1408" data-end="1411"><h3 data-start="1413" data-end="1441">Effects of Knocking:</h3><ul data-start="1442" data-end="1534">
<li data-start="1442" data-end="1459">
Loss of power
</li>
<li data-start="1460" data-end="1482">
Engine overheating
</li>
<li data-start="1483" data-end="1510">
Piston and valve damage
</li>
<li data-start="1511" data-end="1534">
Reduced engine life
</li>
</ul><hr data-start="1536" data-end="1539">Exam Tip:

<ul data-start="1560" data-end="1710">
<li data-start="1560" data-end="1630">
In SI engines → Higher compression ratio → Knocking increases.
</li>
<li data-start="1631" data-end="1710">
In CI (Diesel) engines → Higher compression ratio → Knocking decreases.</li></ul>

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