SPARK IGNITION ENGINES

Air-fuel ratio requirements, Design of carburetor –fuel jet size and venture size, Stages of combustion-normal and abnormal combustion, Factors affecting knock, Combustion chambers, Introduction to thermodynamic analysis of SI Engine combustion process.

Air-fuel ratio requirements

Air-fuel ratio requirements

Variation of Power output , BSFC with A/F ratio.

Automotive A/F ratio requirements

Carburetor

Stages of combustion

Stages of combustion

Stages of combustion

Normal Combustion

Under ideal conditions the common internal combustion engine burns the fuel/air mixture in the cylinder in an orderly and controlled fashion.

Abnormal Combustion

When unburned fuel/air mixture beyond the boundary of the flame front is subjected to a combination of heat and pressure for a certain duration (beyond the delay period of the fuel used), detonation may occur. Detonation is characterized by an instantaneous, explosive ignition of at least one pocket of fuel/air mixture outside of the flame front. A local shockwave is created around each pocket and the cylinder pressure may rise sharply beyond its design limits.

Knocking

Knocking (also called knock, detonation, spark knock, pinging or pinking) in spark-ignition IC engine occurs when combustion of the air/fuel mixture in the cylinder starts off correctly in response to ignition by the spark plug, but one or more pockets of air/fuel mixture explode outside the envelope of the normal combustion front.

The fuel-air charge is meant to be ignited by the spark plug only, and at a precise point in the piston's stroke. Knock occurs when the peak of the combustion process no longer occurs at the optimum moment for the 4 stroke cycle. The shock wave creates the characteristic metallic "pinging" sound, and cylinder pressure increases dramatically. Effects of engine knocking range from inconsequential to completely destructive.

Factors affecting knock

1. Density Factors

1. Density Factors

2. Time Factors

2. Time Factors

3. Composition Factors

Variables affecting Knocking in Si engine.

Combustion Chambers

Combustion Chambers

1. Smooth Engine Operation

Combustion Chambers

2. High Power Output and thermal Efficiency

Combustion Chambers

Typical Combustion Chamber

Combustion Chambers

Typical Combustion Chamber

Combustion Chambers

Typical Combustion Chamber