Flame Propagation and Flame Speed Across the Combustion Chamber and Factors Affecting Flame Propagation
Flame propagation and flame speed
Flame propagation is defined as the progress (spread) of the flame inside the engine cylinder (combustible environment) outward from the point at which the combustion started. Flame generally propagates spherically. The rate of radial propagation of flame is known as flame speed. It is typically measured in m/s. It gives an idea about how rapidly fame propagates from a reference point. The flame speed determines the ability of a fuel to undergo controlled combustion without detonation.
Flame propagation is a result of both the reaction rate and the transposition rate. The reaction rate is a purely chemical reaction in which flame penetrates its way into the unburned fuel/ charge. Transposition rate is due to the physical movement of the flame front (i.e. mass transfer), and it is the result of the pressure difference between the burning charge and unburned charge in the combustion chamber. The rate of flame front propagation within the engine cylinder is critical to efficient combustion. Concept of flame propagation is more crucial to Spark-ignition engine than compression ignition engine because it dictates the behaviour of detonation. In contrast, the ignition delay is vital to CI engine.
Factors affecting flame propagation
Compression ratio: Flame propagation increases with the increase of compression ratio. But the high compression ratio results in high temperature, which increases the chance of detonation.
Engine speed: Flame propagation speed increases almost linearly with engine speed.
Fuel-air ratio: Velocity of flame diminishes if fuel is too lean or too rich.
Turbulence: Turbulent motion of mixture increases the heat-mass transfer, and hence the flame propagation.
Intake temperature and pressure: Increase in intake temperature and pressure increases the flame propagation.
Engine load: When engine load increases the cycle pressure also increase and hence the flame speed.
Some other factors are -type of combustion chamber of engine, spark timing in SI engine.
Flame front propagation across the combustion chamber
Above figure shows the rate of flame propagation inside the combustion chamber of SI engine. Area-I (A→B) Shows the initial stage of flame propagation. Since the spark plug is placed at the quiescent layer (inactive and still) that close the cylinder wall, the beginning of flame propagation lacks the turbulence. Due to low transposition rate and low turbulence, the flame progress will be relatively slow at this area.
The Area-II (B→C) is more turbulent, where the flame consumes more mass of the fuel-air mixture. Flame progress in this region is more rapid and at a constant rate.
The Area-III (C→D) shows towards the end of flame propagation, where the quantity of the unburned mixture is low. So, the transposition rate is low. Since the turbulence in this area is low, the reaction rate is also low. For these reasons, the rate of flame propagation is also get reduced.
👉Detonation velocity/ explosive velocity is the velocity at which shock wave front travel through detonated explosive.