Question

Generally, in spark ignition engines, the compression ratio cannot be enhanced beyond certain limit because to avoid

• A. Auto ignition
• B. Over heating
• C. Knocking
• D. Preheating

Hint:

While auto-ignition is the chemical trigger, Knocking is the actual structural phenomenon and operational hazard that engineers must limit the compression ratio to avoid. - Higher compression ratio in SI engines $\rightarrow$ premature auto-ignition of end-gas $\rightarrow$ Severe Knocking.

Answer 1

The Correct Option is C

Concept: In Spark Ignition (SI) engines, a homogeneous mixture of fuel and air is drawn into the cylinder during the intake stroke and compressed during the compression stroke. Combustion is designed to be initiated precisely by an electrical spark from the spark plug, producing a smooth flame front that travels across the combustion chamber. The compression ratio ($r$) is defined as: \[ r = \frac{V_{\text{total}}}{V_{\text{clearance}}} \] As the compression ratio increases, the temperature ($T_2$) and pressure ($P_2$) of the fuel-air mixture at the end of the compression stroke rise following the ideal gas relationship: \[ T_2 = T_1 \cdot (r)^{\gamma-1} \]

Step 1: Explore the mechanism of Knocking.
If the compression ratio is set too high, the temperature and pressure of the unburned end-gas mixture exceed the self-ignition threshold of the fuel. Instead of waiting for the spark-initiated flame front to reach them, these localized pockets of end-gas undergo rapid spontaneous auto-ignition. This sudden explosion creates high-amplitude, high-frequency shock waves that bounce off the cylinder walls, producing a sharp metallic ringing sound known as Knocking or detonation.

Step 2: Identify the primary operational limitation.
Severe knocking can cause engine overheating, erode piston crowns, damage cylinder head gaskets, and cause mechanical failure. To prevent this destructive knocking phenomenon, the compression ratio in commercial petrol (SI) engines is strictly limited, typically ranging between 6 and 11. This constraint aligns with Option (C).