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In
-
Cylinder Pressure and Flame Measurement

Dr. Manoochehr Rashidi

Engine Research Center

Shiraz University

http://succ.shirazu.ac.ir/~motor/

motor@shirazu.ac.ir

Part 2

Flame Observation

and Measurement

Schematic arrangement of the transparent piston engine


Quartz piston assembly used for obtaining
high speed motion picture of flame


HYCAM rotating prism camera, 10 000 frames/sec on 16

mm film

1.
Object lens

2.
Image size limiter

3.
Segment shutter

4.
1st field lens

5.
1st prism

6.
plane compensation
prism

7

2nd field lens

8

2nd prism

9

Intermediate lens

10

U prism

11

Image

12

Prism

13

Ocular

14

Synchronized gear
drive

Color

photographs

from

high

speed

movie

of

spark

ignition

engine

combustion

process,

taken

through

glass

piston
.

Ignition

timing

30

before

TC,

1430

rpm
.


لاعتشا هقرج روتوم رد هلعش تکرح

Non uniform flame propagation is one of
the causes of cyclic variation in engine


Microshadowgraphs of flame at
various engine speeds showing effect
of turbulence


Schematic arrangement of the transparent piston engine

Synchronization arrangement, for simultaneous

flame front and pressure measurement.

Typical profile of flame front at various crank angle

Inner circle is the visible part of the combustion chamber

Consecutive measured pressure

Variation of flame radius with crank angle or time

Measured flame velocity from photographs

Calculated gas velocity just ahead of flame

Entrained (or burning) velocity

Full line is least square, and doted line is from model

Variation

of

flame

geometry

and

velocity

parameters

during

four

individual

combustion

cycles
.

Variables

shown

in

the

figure

are,

flame

radius

r
f
,

burned

gas

radius

r
b
,

normalized

entrained

volume

y
f
,

burned

volume

y
b
,
.

normalized

flame

front

area

a
f
,

laminar

area

a
L
,

flame

front

speed

u
b
,

burning

speed

S
b
,

and

laminar

flame

speed

S
L
.

Figure

in

previous

page

shows

results

from

an

analysis

of

cylinder

pressure

data

and

the

corresponding

flame

front

location

information

(determined

from

high
-
speed

movies

through

a

window

in

the

piston)

of

several

individual

engine

operating

cycles
.

The

combustion

chamber

was

a

typical

wedge

design

with

a

bore

of

102

mm

and

a

compression

ratio

of

7
.
9
.

The

flame

radius

initially

grows

at

a

rate

that

increases

with

time

and

exhibits

substantial

cycle
-
by
-
cycle

variation

in

its

early

development

(Fig
.

a)
.

Later

(r
f

>

30

mm)

the

growth

rate,

which

approximates

the

expansion

speed

u
b

,

reaches

an

essentially

constant

value
.

The

flame

radius

r
f

is

initially

equal

to

the

burned

gas

radius

r
b

it

increases

above

r
b

as

the

flame

grows

and

becomes

increasingly

distorted

by

the

turbulent

flow

field

(Fig
.

b)
.

Eventually

r
f

-

r
b

goes

to

an

essentially

constant

value

of

about

6

mm
.

This

difference,

is

approximately

half

the

thickness

of

the

turbulent

flame

brush
.


Normalized

enflamed

and

burned

volumes,

and

flame

front

area

and

laminar

burning

area,

are

shown

in

Fig
.

c
.

Volumes

are

normalized

by

the

cylinder

volume,

and

areas

by


Rh,

where

h

is

the

average

clearance

height

and

R

the

cylinder

radius
.

Discontinuities

occur

in

the

flame

area

a
f

at

the

points

where

the

flame

front

contacts

first

the

piston

face

and

then

the

near

cylinder

wall
.

The

laminar

area

A
L

is

initially

close

to

the

flame

area

A
f

and

then

increases

rapidly

as

the

flame

grows

beyond

10

mm

in

radius
.

During

the

rapid

burning

combustion

phase

(y
f

>

0
.
2
)

the

value

of

y
f

is

significantly

greater

than

y
b

.

During

this

phase,

the

laminar

area

exceeds

the

flame

area

by

almost

an

order

of

magnitude
.

These

observations

indicate

the

existence

of

substantial

pockets

of

unburned

mixture

behind

the

leading

edge

of

the

flame
.

The

ratio

of

the

volume

of

the

unburned

mixture

within

the

turbulent

flame

zone,

to

the

reaction
-
sheet

area

within

the

flame

zone,

defines

a

characteristic

length

l
r

,

which

can

be

thought

of

as

the

scale

of

the

pockets

of

unburned

mixture

within

the

flame
.

l
r

is

approximately

constant

and

of

order

I

mm
.

These

flame

geometry

results

would

be

expected

from

the

photographic

observations

of

how

the

flame

grows

from

a

small

approximately

spherical

smooth
-
surfaced

kernel

shortly

after

ignition

to

a

highly

wrinkled

reaction
-
sheet

turbulent

flame

of

substantial

overall

thickness
.

Initially,

the

amount

of

unburned

gas

within

the

enflamed

volume

is

small
.

During

the

rapid

burning

phase

of

the

combustion

process,

however,

a

significant

fraction

(about

25

percent)

of

the

gas

entrained

into

the

flame

zone

is

unburned
.

The

front

expansion

speed

u
f
,

burning

speed

S
b
,

and

laminar

flame

speed

S
L

are

shown

in

Fig
.

d
.

The

expansion

speed

increases

as

the

flame

develops

to

a

maximum

value

that

is

several

times

the

mean

piston

speed

of

3
.
1

m/s

and

is

comparable

to

the

mean

flow

velocity

through

the

inlet

valve

of

18

m/s
.

The

burning

speed

increases

steadily

from

a

value

close

to

the

laminar

flame

speed

at

early

times

to

almost

an

order

of

magnitude

greater

than

S
L

during

the

rapid

burning

phase
.

During

this

rapid

burning

phase,

since

(r
f



r
b
)

is

approximately

constant,

the

flame

front

expansion

speed

and

the

mean

burned

gas

expansion

speed

are

essentially

equal
.

The

difference

between

u
b

and

S
b

is

the

unburned

gas

speed

u
g

just

ahead

of

the

flame

front
.

Note

that

the

ratio

u
f
/S
b

decreases

monotonically

from

a

value

equal

to

the

expansion

ratio

e
u
/e
b

at

spark

to

unity

as

the

flame

approaches

the

far

wall
.


Superimposed

tracings

of

flame

fronts
.


Illustration

showing

best

fit

circle

to

the

18
th

flame

front
.

Schematic

diagram

of

combustion

chamber

geometry

and

spherical

flame

front
.


Angle

versus

distance,

showing

qualitative

trajectories

for

flame

center,

flame

fronts,

and

gas

particle
.

CONCLUSIONS

Simultaneous

pressure

measurements

and

highs

peed

motion

pictures

of

the

visible

flame

in

a

spark

ignition

engine

show

that

the

initial

flame

front

propagation

speed

is

very

close

to

that

of

a

laminar

flame

for

the

same

charge
.

As

the

flame

grows,

its

speed

increases

rapidly

to

a

quasi
-
steady

value

of

order

10

times

the

laminar

value
.

During

the

rapid

quasi
-
steady

propagation

phase,

a

significant

fraction

of

the

gas

entrained

behind

the

visible

flame

front

is

unburned
.

The

measurements

also

suggest

that

the

final

combustion

phase

can

be

approximated

by

an

exponentially

decreasing

burning

rate

with

a

time

constant

of

order

1

ms
.

Detailed

analysis

of

the

data

has

led

to

the

development

of

a

set

of

empirical

differential

equations

that

correlate

well

the

experimental

observations
.

The

burning

equations

contain

three

parameters
:

the

laminar

burning

speed

of

the

charge

S
L
,

a

characteristic

speed

u
T
,

and

a

characteristic

length

l
T
.

Measurements

of

S
L

under

engine

like

conditions

can

be

made

in

constant

volume

combustion

bombs,

and

values

for

a

number

of

common

fuels

are

available
.

Values

for

u
T

and

l
T

can

be

obtained

from

engine

experiments,

and

preliminary

correlation

for

relating

these

parameters

to

engine

geometry

and

operating

variables

have

been

given
.

The

data

suggest

that

u
T

increases

and

l
T

decreases

during

compression

of

the

unburned

gas
.

For

a

given

engine

cycle,

the

parameters

in

the

burning

equations

can

be

adjusted

to

fit

the

observed

pressure

curve
.

Cycle
-
to
-
cycle

fluctuations

in

pressure

can

be

caused

by

variations

in

any

of

the

parameters

S
L
,

u
T
,

and

l
T
.

Variations

in

S
L

can

be

caused

by

incomplete

mixing

of

the

fresh

charge

with

burned

residual

gas

in

the

cylinder

and

by

variations

in

the

stoichiometry

of

the

fresh

charge
.

Variations

in

u
T
,

and

l
T

are

presumably

associated

with

the

statistical

character

of

turbulence
.

An

additional

parameter

required

to

close

the

burning

equations

with

a

geometrical

description

of

the

enflamed

region

is

the

vector

r
e

giving

the

position

of

the

apparent

flame

center
.

The

nominal

value

of

r
e

is

determined

by

spark

plug

position,

but

convection

of

the

flame

kernel

at

early

times

during

propagation

can

produce

significant

displacement
.

It

is

observed

that

substantial

cycle
-
to
-
cycle

fluctuations

can

be

caused

by

variations

in

the

parameter

r
e
.

Variations

in

r
e

are

presumably

caused

by

convection

of

the

initial

flame

kernel

in

the

flow

field

near

the

spark

plug
.

In

this

connection,

it

may

be

noted

that

a

correlation

between

the

pressure

and

the

flow

velocity

.
r
e

near

the

spark

has

been

observed

in

laser

doppler

measurements
.

Although

the

proposed

empirical

burning

equations

provide

a

relatively

simple

and

accurate

method

of

predicting

the

observed

burning

rates

in

spark

ignition

engines,

the

range

of

engine

geometry

and

operating

variables

investigated

in

this

experiment

is

relatively

small

and

needs

to

be

considerably

extended
.

In

particular,

systematic

investigations

over

a

wide

range

of

engine

speeds,

spark

angles,

valve

lifts,

and

compression

ratios

are

needed

to

establish

the

proper

correlation

for

u
T
,

and

l
T
.

the

origin

of

the

cyclic

variations

in

the

values

of

S
L
,

u
T

,

l
T

and

r
e

needs

to

be

more

closely

examined,

and

correlation

for

relating

their

magnitudes

to

engine

geometry

and

operating

conditions

need

to

be

developed
.

The

range

of

validity

of

the

burning

equations

and

their

applicability,

for

example,

to

engines

with

significant

swirl

and

squish

also

needs

to

be

established
.

Finally,

the

experimental

evidence

presented

and

the

proposed

empirical

equations

need

to

be

better

understood

in

terms

of

the

underlying

physical

mechanisms
.

References

1.
C
.

R
.

Ferguson,

“Internal

combustion

engines

applied

thermodynamics”

Wiley,

1985
.

2.
R
.

Stone,

“Introduction

to

internal

combustion

engines”

Macmillan

Press,

1999
.

3.
J
.

B
.

Heywood,

“Internal

combustion

engine

fundamentals”

McGraw

Hill

1988
.

4.
G
.

P
.

Beretta,

M
.

Rashidi,

J
.

C
.

Keck,

"Turbulent

flame

propagation

and

combustion

in

spark

ignition

engines"
.

Combustion

and

flame,

V
52
,

N
3
,

P
217
,

1983
.


5.
M
.

Rashidi,

"The

nature

of

cycle
-
by
-
cycle

variation

in

the

SI

engine

from

high

speed

photographs"
.

Combustion

and

flame
.

V
42
,

P
111
,

1981
.


6.
M
.

Rashidi,

"Calculation

of

equilibrium

composition

in

combustion

products
.
"

Journal

of

applied

thermal

engineering,

V
18
,

No

3
-
4
,

pp
.

103
-
109
,

1998

7.
M
.

Rashidi,

"Measurement

of

flame

velocity

and

entrained

velocity

from

high

speed

photographs

in

the

SI

engine"
.

The

institution

of

mechanical

engineers
.

Proceedings
.

V
194
,

N
21
,

P
231
,

1980
.


8.
M
.

Rashidi,

"The

sensitivity

of

elementary

reactions

for

hydrogen

oxygen

in

a

well

stirred

rector"
.

International

journal

of

hydrogen

energy
.

V
5
,

P
515
,

1980

9.
M
.

Rashidi,

M
.

S
.

Massoudi,

"A

study

of

the

relationship

of

street

level

carbon

monoxide

concentrations

to

traffic

parameters"
.

Journal

of

atmospheric

environment
.

V
14
,

P
27
,

1980

10.
G
.

P
.

Beretta,

M
.

Rashidi,

J
.

C
.

Keck,

"Thermodynamic

analysis

of

turbulent

combustion

in

a

spark

ignition

engine
;

experimental

evidence"
.

Paper

WSS/CI

80
/
20

presented

at

the

spring

meeting

of

the

western

states

section

of

the

combustion

institute
.

April

1980

11.
M
.

Rashidi,

M
.

S
.

Massoudi,

"The

use

of

gas

engines

for

motor

vehicles

in

oil

exporting

countries"
.

SAE

paper

770145
,

1977
.