Air Motion Inside a Cylinder

1. INTERNAL COMBUSTION ENGINES-II Air Motion & Combustion
University of Petroleum & Energy Studies
Dehradun

2. I C ENGINES - II Air Motion & Combustion
•Air Motion :
- Air or gas motion within the engine cylinder is one of the major factors that controls the fuel- air mixing and combustion.
- It also has significant impact on heat transfer.
- Both the bulk gas motion and the turbulence characteristics of the flows are important.
- The initial in-cylinder flow pattern is set- up by the intake process and subsequently modified during compression.

3. I C ENGINES - II Air Motion & Combustion
 Swirl :
engine concepts to promote rapid mixing of air and injected fuel.
- In prechamber engine swirl is created during compression to promote fuel-air mixing in the prechamber.

4. Characterizing Turbulence
•Swirl
–Axis of rotation is parallel to cylinder
–Generate swirl about valve axis (inside port)

5. Swirl
•Impulse Swirl Meter
•Honeycomb flow straightener measures total torque exerted by swirling flow.
•A swirling ratio is defined:
Rs=s/2N
•This ratio is the angular velocity, s, of a solid- body rotating flow (equal to angular momentum of actual flow) divided by the crankshaft angular rotational speed

6. Swirl

7. Characterizing Turbulence
•Tumble
–Axis of rotation is perpendicular to cylinder axis
–Associated with swirl

8. Characterizing Turbulence
•Rt is the tumble ratio,
Rt=t/2N
•This ratio compares the angular velocity,
•t, of the solid-body rotation with same angular momentum as actual velocity distribution in tumble to angular velocity of the crankshaft (N)

9. I C ENGINES - II Air Motion & Combustion
 Squish :
- The radially inward or transverse gas motion that occurs towards the end of the compression stroke when a portion of the piston face and cylinder head approach each other closely is called squish. It manifests in getting the gas
displaced into the combustion chamber (space).

10. I C ENGINES - II Air Motion & Combustion
 Squish :
- The amount of squish is defined by the percentage of squish area, that is , the piston area which closely approaches the cylinder head as compared to the total piston area.
- Squish- generated gas motion results from using compact combustion chamber geometry.

11. Squish
•Radially inward gas motion that occurs toward end of compression stroke
•Or
•The Squish Band or "Quench" is defined as that area between the flat of the piston and the flat of the cylinder head at top dead center (TDC). On the compression stroke, as the piston approaches TDC, the compressed mixture of fuel and air is "squished" to the remaining space of the combustion chamber where the spark plug and valves reside.

12. I C ENGINES - II Air Motion & Combustion
 Crevice Flows & Blowby :
- The engine combustion chamber is connected to several small volumes usually called crevices because of their narrow entrances.
- Gas flows into and out of these volumes during engine operating cycle as cylinder pressure changes.
- The largest crevices are the volumes between the piston, piston rings and cylinder walls.
- Some gas flow out of these regions into the crankcase. These are called blowby gases.

13. I C ENGINES - II Air Motion & Combustion
 Crevice Flows & Blowby :
- Other crevices volumes are ; threads around spark plug, space around central electrode of spark plug, gap around injectors, intake & exhaust valves, and gaskets.

14. •

17. I C ENGINES - II Air Motion & Combustion
•Fuel - Air Mixing and Initiation of Combustion :
- In C.I.Engines, only air is compressed to raise its temperature and pressure to a high value. Highly pressurized fuel is injected through one or more jets into this highly compressed air in the combustion chamber.
- The fuel jet disintegrates within the core of fuel surrounded by a spray envelope of air and fuel particles created both by the atomization and vaporization of fuel.

18. I C ENGINES - II Air Motion & Combustion
•Fuel - Air Mixing and Combustion :