1. Generation and Control of Turbulent Flames in SI Engine
P M V Subbarao
Professor
Mechanical Engineering Department
Safe, Reliable and Fast Combustion needs the Help of Turbulence….

2. A Three Step Approach to Control Flame Speed in SI Engines
In a Homogeneous mixture of fuel and air
Study & Control of Planar (constant Area) Laminar Flames.
Control of Non-planar (variable area) Laminar Flames.
Control of Turbulent Flames

3. Description of Turbulent Flow in An Engine Cylinder

4. Phases of the Flow inside an Engine Cylinder
The flow in the cylinder can be divided into several distinct phases:
The flow of air-fuel mixture into the cylinder through the intake valve or valves as a turbulent jet.
Decaying turbulent in-cylinder flow during (the later part of) the intake stroke.
Growing turbulent flow during the compression stroke.
Amplification of turbulence level due to combustion.
Flow with a strong attenuation of the turbulence during expansion stroke.

5. Instantaneous Local Velocity in an I.C. Engine Cylinder : A Sketch

6. Evidence of Organized Vortical Structure in an Engine 1200 rpm motored engine
Instantaneous Velocity Distribution
In Mid Plane During Cycle 1
Instantaneous Velocity Distribution
In Mid Plane During Cycle 20
90 Cycle Mean Velocity Distribution
In Mid- Plane

7. Turbulent Energy Spectrum
The turbulent incylinder flow consist in many eddies of varying sizes and orbital speeds, d and uO.
Each of these may not be a of a single value but vary within a certain range.

8. Vortical Description of In-Cylinder Flow
The size of most energetic eddy is found to be roughly B/6 , where B is the bore.
A general engine flow, at middle third of the intake stroke has an tangential velocity of roughly 10 ×Sp , where Sp ,is the average piston speed.
This means that this vortex has cycle time of (a time scale) of roughly, B/ 60 ×Sp .
What is the use of this knowledge?

9. Energetic Characterization of Turbulent Flow
The largest eddy at TDC will be roughly the clearance height, while at BDC it will be roughly the cylinder bore
The largest eddies are relatively weak.
The kinetic energy of the eddies varies with size or scale.
As the size drops, the energy initially rises rapidly to a peak, and then falls continually down to the smallest eddies.
The most energetic eddy, at the peak, which is responsible for most of the transport, is about 1/6 the size of the largest eddy ‐ thus, 1/6 of the bore at BDC, or 1/6 of the clearance height at BDC.

10. Fast Vortex Flow Faster than the Average Piston Velocity
The most energetic vortex completes a 60 times faster than intake stroke of piston.
How to use of this knowledge?
It tells us that this vortex can mix the gas 60 times during an intake stroke, if it is alive throughout the stroke.

11. Size of the Largest Eddy
The span of length scales in a turbulent flow is related to its Reynolds number.
Indeed, in terms of the largest velocity scale, which is the orbital velocity of the largest eddies, UO= uO(dmax).
The energy supply/dissipation rate equation is:
L is a general symbol for dmax.

12. Clues to Develop a Healthy Turbulent Flow in An Engine Cylinder
Eddy turnover time:
Characteristic Chemical Reaction Time:
The ratio of the characteristic eddy time to the laminar burning
time is called the Damkohler Number Da.

13. Span of Multi-scale Turbulent Flames
Multi-scale turbulent flames are essential for operation of high speed engines.
Turbulent flames are characterized by rms velocity fluctuation, the turbulence intensity, and the length scales of turbulent flow ahead of flame.
The Largest scale, L is also called as an integral length scale.
The smallest length scale is known as Kolmogrov scale .
The span of eddy scales:

14. Identification of A Control(Dimensionless) Parameter
Turbulent Reynolds Number:
Important controlling dimensionless parameter: