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INTERNAL COMBUSTION ENGINE
(SKMV 3413)
Dr. Mohd Farid bin Muhamad Said
Room : Block P21, Level 1, Automotive
Development Centre (ADC)
Tel : 07-5535449
Email: mfarid@fkm.utm.my
Topic 7: Fluid Motion within
Combustion Chamber
Combustion chamber is a definite closed space formed by three separate engine parts:
cylinder head
cylinder wall
piston top face
Such closed space when subjected to pressure rise and change in volume, forces the gases within it to move in
various kind of motions, such as mentioned above.
COMBUSTION CHAMBER AND AIR
MOVEMENT
Efficient operation of an engine depends on the level of mixing process between air and fuel , and so to have a
good combustible air/fuel mixture ready to ignite at the
proper timing.
Such mixing process is enhanced by various motions of both fluids during intake and compression strokes, and so
enhance proper operation of the engine.
These motions can be summarized as follows :
(1) Turbulence, (2) Swirl, (3) Squish, (4) Tumble, (5) Crevice
TYPES OF MOTIONS
This type of motion is created by piston action, specifically during compression stroke.
However, due to high velocities involved, all flows into, out of, and within engine cylinders are turbulence flow.
During turbulence flow, all flowing particles experience random motion fluctuation in all direction.
The advantages of turbulence motion :
Distribute fuel
Mix fuel with air
Assist combustion
Reduce after burn
TURBULENCE MOTION
It is a rotational (circular) motion of gases imparted during suction stroke.
It can be generated by designing intake system components to allow tangential entry of gases.
The advantages of swirl motion may be summarized as :
Enhance mixing of air and fuel .
Giving homogeneous mixture .
Speed up the spreading of flame front.
SWIRL MOTION
SWIRL MOTION
a) Eccentric induction
process
b) Air entering engine
cylinder from tangential
direction
c) Air contoured intake
runner
d) Air contoured intake
valve
SWIRL MOTION
Swirl ratio can be defined as :
it is a dimensionless parameter used to quantify rotational motion within the cylinder.
Mathematically expressed in two different expressions :
SWIRL MOTION
Swirl Ratio
One simple way of modelling cylinder swirl is using the paddle wheel model.
The volume within the cylinder is idealized to contain an imaginary paddle wheel that has no mass.
As the paddle wheel turns, the gas between the blades turns with it, resulting in a cylinder of gas all rotating at one angular velocity.
The mass moment of inertia of this cylinder is:
The angular momentum is:
SWIRL MOTION
=2
8
= mass of gas mixture in the cylinder = bore = diameter of rotating mass
= = solid body angular velocity
Example 1
A four-cylinder, 3.2 liter engine running at 4500 RPM has a swirl ratio
(SR1) of 6.0. The stroke and bore are related as S = 1.06B. Calculate:
1. Angular velocity of gas mixture in the cylinder.
2. Swirl ratio (SR2)
Other types of motion include:
1) Squish
It is a radial motion towards the centerline of the
cylinder
2) Tumble
It is rotational motion around a circumferential axis.
3) Crevice motion
It is a flow into the very small crevices of the
combustion chamber
OTHER MOTION
Squish and reverse squish is a radial motion towards the center line of the cylinder generated by a recess
located in either the piston or the cylinder head.
Squish motion occurs during compression stroke.
Reverse squish motion occurs during expansion stroke.
Due to very small clearance volume, when the piston approaches TDC, radial flow of air from the periphery
to the recess is produced, this is squish.
During expansion, volume increases, and the burning gases are forced outward , this is reverse
squish.
OTHER MOTION
Squish motion
It is actually a rotational motion, but it is in fact generated by squish motion around
circumferential axis.
Therefore, tumble (caused by squish as piston approaches TDC), is a motion about a
circumferential axis near the edge of the
clearance volume in the piston bowl or the
cylinder head.
It is become one of the important parameters in enhancing air-fuel mixture for modern engine.
Tumble ratio is the dimensionless parameter used to characterize the magnitude of tumble:
OTHER MOTION
Tumble motion
TR = (angular speed of tumble)/(engine speed) = /
OTHER MOTION
Tumble motion
Crevices are very small volumes such as :
clearance between piston and cylinder wall.
gaps in the gaskets between head and block.
unrounded corners and edges.
OTHER MOTION
Crevice motion
Example 2
An engine with pistons as shown in Figure below operates at 3500
RPM, with each cylinder containing 0.0014 kg of air-fuel. When a piston
approaches TDC, the gas inward squish velocity equals 7.66 m/s. At
TDC half of the cylinder gases then create a tumble rotation of 2.2 cm
diameter. Calculate:
1. Angular momentum of gases in tumble
rotation.
2. Tumble ratio, assuming a paddle wheel
model for the rotation.
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