Indicate any 4 locations within SI engine cylinder where unburnt HC form.

 Here are four key locations within an SI (Spark Ignition) engine cylinder where unburnt hydrocarbons (HCs) are commonly formed:

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🔧 1. Crevice Volumes (e.g., Piston Ring Lands and Cylinder Head Gasket Area)

• What happens: Air-fuel mixture enters narrow gaps (crevices) during compression.

• Result: Flame front cannot penetrate these tight spaces due to quenching, so fuel remains unburnt.

• Example areas:

  • Between piston and cylinder wall

  • Around the spark plug threads

  • Gasket regions

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🔧 2. Near the Cylinder Wall (Wall Quenching Zone)

• What happens: The flame loses heat rapidly to the cool cylinder wall, which quenches the flame.

• Result: Incomplete combustion near walls leaves behind unburnt HCs.

• Occurs more in: Cold-start conditions or engines with low wall temperature

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🔧 3. In the Piston Head Recesses or Bowl (Dead Zones)

• What happens: Poor air motion or turbulence in recessed areas prevents full combustion.

• Result: Local fuel-rich pockets or low flame speed lead to HC formation.

• Affected by: Poor combustion chamber design or carbon buildup

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🔧 4. Late Burning or Incomplete Combustion Zones

• What happens: Some areas may burn too slowly, especially near the end of the expansion stroke.

• Result: The burning is cut off by the opening of the exhaust valve, leaving partially burned or unburnt fuel.

• Common in: Poor ignition timing or lean mixtures

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✅ Summary Table:

Location	Reason for HC Formation
Crevice volumes	Flame cannot enter; fuel trapped unburnt
Near cylinder walls	Flame quenched by cold surfaces
Piston bowl/recesses (dead zones)	Poor mixing or turbulence; local rich mixtures
Late burning zones	Combustion interrupted before completion

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Unburnt hydrocarbons are a major pollutant and are minimized using design improvements, fuel injection control, and catalytic converters.