Air-Motion in Engine Cylinder

INTERNAL COMBUSTION ENGINES-II Air Motion & Combustion

University of Petroleum & Energy Studies

Dehradun

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.

I C ENGINES - II


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.

Characterizing Turbulence

• Swirl

– Axis of rotation is

parallel to cylinder

– Generate swirl about

valve axis (inside port)

Swirl

• Impulse Swirl Meter

• Honeycomb flow straightener measures total torque exerted by swirling flow.

• A swirling ratio is defined:

Rs=s/2N

• 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


• Tumble

– Axis of rotation is

perpendicular to

cylinder axis

– Associated with swirl


• Rt is the tumble ratio,

Rt=t/2N

• 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)

I C ENGINES - II


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).

I C ENGINES - II


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.

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.

I C ENGINES - II


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.

I C ENGINES - II


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.

I C ENGINES - II


• 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.

I C ENGINES - II


• Fuel - Air Mixing and Combustion :

Air-Motion in Engine Cylinder

Engineering

Transcript of Air-Motion in Engine Cylinder