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.