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:


🔧 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


🔧 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


🔧 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


🔧 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


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

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

0 Comments:

Post a Comment

Thank you for the comment