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(1)

AN INVESrriGATION OF THE: fCFFJl~C~r~3 OF MULTIPLE ARK IGNI'l1ION ON PERF'ORli'LANCI!:: Of:t., A HIGH ~3PJI:IW PE'I'ROL

IGNITION ENGINE.

A Thesis

:Presented to

the Faculty of the School of Englneerinr; 'J.1he Uni versl ty of NeW Zealand

In Partial Fulfillment

of the Requirements for the Degree

Batchelor of If,'ngineering vll th Honours (lVIechc:mlcal)

(2)

0

I.

'r

OF'

TIGATION

'l'l:1e problHm

fJtatement. of the problem

Importance of the inves t:l on

Object of the investigation

Scope of investi tion •

sation of the remainder of the

thesis

.

.

II. OF LITEHNWHJl:

theory of :tgni ti.on .

Statement of the theory •

ssions for a simple quanti

treatment •

Discus on of

Ionize,tion theory of 1 tion .

General statement of t11.e theory

Inve ons into the i tion

propert:i.es ve,rious types of

Chain reaction theory or ignition

Summary of the es of

i on

• L

. 2

. 2

. 3

. 4

• 4·

• 5

. 7

. 7

• 7

. 7

8

(3)

ii:t

Gen chare,ct.ertstics of the

dJschex·r;e as indtcated by t;he theory 17 Propagat1on of the explo on .

Ob ons vli t.h bomb explosions 19 surernent of flame velocity 20

Combustion in engine

Flame movement and velocity

ssure development

~:emperature ,rarie.tions in

23 23

products

of combustion 21+

Factors affec flexne velooi ty 25

The delay od • 26

1J:lhe precess of tion

Character.1.stics of the spe,rlr dtscharge 29 Factors affecting sparldng potential 30 Effect of heat energy of the spark •

tion properties of the components

of the discharge

35

~)wnme.ry of tJhe fac affecting ignition in the internal combustion engine

Dual i gnj. tl on • 37

Experimental results vri th dual ignition 37

scussion of the ts obtained with

(4)

GHAFI'ER III.

IV.

DESCRIPTION OF THE APPARATUS

The engine test bed • 41

General arrangement of the equipment

41

Measurement of speed • 42

Measurement of power . 42

Measurement of fuel consuJllptlon 45

Ivreasurement of air consumption 45

Measurement of sparlc advance 46

Measurement of temperature . lJ.7

IVfeasu.rement of manifold pressure • 47

]£nglne cooling . 47

Oarburett1on 49

Special Equipment 50

Multiple sparking plug 50

Distributor .

51

TJWHNIC1UE AND Rl;::SUL'J~G OF TEtVl'.U \rVITH S'J:'ANDARD ENGINE

Full throttle test

'l'echnique

Results

Fuel consumption tests •

~L'echnique Results 58 58 58 60 60 60 60

V. 'J:'IWHNIQUE AND RICEWUl'S 0'81

INDIVIDUAL I~X.PEii.IlVIEWJ.'fl WI'l1

(5)

CHAP'.rE~IZ

Tests with

a.u.

carburettor

Technique

Hesults

Tests with Zenith carburettor

~:echnique

Hesults

Tests with primary ignition coil

circuits in parallel.

Technique

Hesults

H;xperiments with the primary coils

1n series

Technique

Hesults

Experiments to determ1ne the effects

v PAGE 64. 64 65 68 68 69 77 77 78

of changes in coil circuit parameters 78

Technique 80

Hesults 80

General observations . 80

VI. tHJl\{fVIARY OF RICSUVI'S, RECOIV1lVJf!.:NDArl'IONS AND CONCLUSIONS

Burnmary of results

Power output

Fuel consumption

Mixture rarige

84

84·

84

84·

(6)

B

Effect of circuit parameters

Combustion time

Discussion of results with

reference to the theories of

i tion necommen<'lationB

tl)R

cope of lnvesti n:et.1on

Cone

ch

ons

cal d.eveloproent of t1-le r·eacti on trteor'y

'l'heory of j i t,j_on coll ld u.p of primary curr<:mt Vo across e

sudden interruption

Voltage induced :1 n trw sonondary after s lnterx•uption of the

lYf'irnaJ:>y nurrent

Circuit equations for the 1 t:ton coil under sparking c tlons

CaJ.cuJ.atlons for coil circuit

cheJ:"'Etc 86 E39 91

93

95 97

. 100

. 106

. 108

• 108

. 110

5

(7)

CI-1'\PTER

c

D

E

PAGJ~

Two coils in series 116

~['hree coils in series~ standard cond.e:nsel' 117 Two coils ln parallel, 0.5 mf. condenser 117 Air flow metel' design calm1lations 0 119

Air flow calculattons for' o:rif':Loe gauge Ccn'r'ection factor for temperatU.r'l-'l and

pi'eSSUl"e 0 G 0 12:5

Determination of' cornpression ratio & 126

Measul"ement of clearance voluJlle 126 lVleasurement of the swept volume 126

11esults

..

..

& G • 127

Specifications of the test engine 128 F ~.1her•mod;ynarnj_c co.lculat:Lons

:for trw

.ideal cycle

CalcnJat:i.oruJ foi' L'<I,C; COCIJJiticHI o''' ', o\:;

1.31

o1)·(;a1n(;(J fron1 i.ndivLclual expe::rhnerJt:~ 137

co:nf:J i.ant C':eeqttcnc~y TGxpcri.wcn:l~:o; wlth coils

couotanL fre~1ency

i.n pars.ll l anCi

l,)[)

151

(8)
(9)

1. IJ'emperatnrE':l 1,oJ'eve Forms . for Four Dl ffercmt Methods of HeatinB in Air

2. Influence of F'requency and ~'faximum Current on Igni tabill ty of Carbon Monox:tcle - Oxyr;>;.en Mixtures

Minimwn Ignition Currents (in primary) for Hydrocarbon - Air Mi.xtures

4. ·

Pressure Rise During Combustion in a Closed

Combustion Bomb .

5.

Variation of Combustion T:Lme and DRle,y .Period

6.

wj th ·f.i'uel - A:Lr. Ratio e,nd 'rur.bulence • Minimum sparlt energ,y requ:l.red to ignl te a

petrol-air mixture •

7. Val:>iation of minimum sparlt energy wi tJ1.

mixture strength

8. Relative posjtions of valves and sparking plup;s for Rlcardo 's experiments vvi.th. dual ignition

9. Cross-section of multiple sparking plug boss

11

15

18

22

27

33

38

52

(10)

1.3. 14. 15. 1.6.

17.

19 21

1£ffect of A:ir-J1'ual o on Performance

( G tandard ll}.J.gi ne, i3. lJ. Oarbure t tor) Effect of A:ir-Fu Ratio on Performance

(~:J.U. Garb., Coils i.n rleriE~s)

Fuel Consumptlon Loops

(f::>.U. Carb., Coils in es)

JCffect of Air- Ratlo on Performance (Zenith Oarb., Coils in Sel"ies)

:B:ffeot of Fuel Ratio on l)erformance (Zenith Oe.rb. , Ooi in Geri es)

Po'i'ler Output at Constant Air-Fuel 0

(Zenith Oarb. , Ooi ls ln es)

TCffect of tlon Advance on Performance (Zenith Carb., Coils in Series)

Effect of Air- I1\.:tel tio on Performe.nce (Zenith Carb., Coils j_n lel)

o.

r:; mf condenser)

Effect of Fuel Ratio on :Performc:mce (Zenith Oarb. , Cotl.s tn Parallel

0.5

mf condenser)

t of Air- Rat:io on Performance (Zenith Carb. , Co:i ls et:~

0.1. mf. Condenser)

23 Gffect of Circuit Parameters on Performance

25

26

( e Coil)

t of Circuit Parameters on Performance (Coils in Parallel)

Varia'U.on on Leanest Practicable xture with Pressure at the Point of Ignition

tionship of Temperature a.nd JL'nere;y throue;h a Cross- tion of a Combu on

(11)

c an cluct Co

:it ion >:';ill 107

and Volt Rolattonship an I~n1t1on. Co:tl

r Deta.i and Atten1.1.at:i.mt Cha:eacter:L .Lc

1~3

rmance foP Ideal Oyelt:nJ

(~.t.·J·_ Oa·J~l:l ~ • ·- . • , •• ~~--M··') fc) :U1 SfH' .l • • es, c )

139-J.lt5~

3£3-ltiD. ct of Ai Ratio on l'formanco

(Zenith •;; Coils c

onorny.

(Coil in serie , st 150

Air/Puel Hatio on

15

(Co sol~L s 0.1 rn:C. condonsex')

ct of A.Le/Fuel Ratj.o on l?erf'ol'Inance (Coils in paPall 0.5 • c

5 i io:n on B.M.Ji:.P. w:i.th s f:31IJ:e

(Co:Ll 1n )

feet of A:lr/::ruel H.at;:i.o on Pe~rf'orn1ance

(12)

'J.1ABLJTI PACEtTI

Ir>.:f'lue:nce of Plug Po tt:ion

on tonation (Ricardo)

il'0(1 Ie:nitio:n Advance :for lmuut

Power w:tth

Determination of

Temperature

t Nurnbers o:f

Operation

et Manlf'olci

1!. 'l~arjulatecl Calculation for a Constant

6.

Volume Cycle foP a SpaPl(-I tion Rne;ine

Appr•oxir;mte ssure at Different of Conrpresston Stroke

:lnt.s

Vollunetl"ic tciency of the Standard

Engine at Full Throt

90

136

(13)

PI1Nlll~

I~ 0

OJil

IJayout Test, Equipment (Near S:Lcle) Layout of' st Jl:quiprnent, . ( OJ"'f S ) III. Cylinder Head wit b. Spm"ldng Plues

s:i.ti.on

IV.

ide OyliBder Head Showing the

PAGE!

1!)!

Posi.tlon anc1 Arrange:men:l; of the Electroctes

5h

V. Engine n.:nder Motor:tng Teats c'J:ur:l:ng Init:i.al

(14)

CHAPTER 1

THE PROBLEM AND Tl-IE OBJECT OF THE INVESTIGATION

At the present time ignition by high tension

electric spark is almost universal on high speed

reciprocating internal combustion engines other than

the compression ignition type. The growing clemands

for greater efficiency, higher specific power output

and increasing speed range -vd th the associated high compression pressures has made greater demands on the

ignition system. The modern magneto and coil1 having

progressed from the early hot wire and hot tube ignition,

have been developed. to a high stage of efficiency, although

othrsystems have been suggested.l

Improvements in performance have been noted \vi th

dual ignition2 and this system is standard on aircraft

engines, although in this case improved performance is

only a secondary consideration to safety.3

1 Vf. Beye Smits and P.F.I-I.M. Pont, "The Smitsvonk Low Tension Ce,pacity Ignition System," S.A.E. Journal, Volume 59, No.4,(April 1951) p. 61-64·.

2 H.R. Ricardo and. H.S. Glyde, "The High Speed Internal Combustion Engine." (London: Blackie and Son Ltd., 1950) p. 52.

3 A.P. Young and. L. Griffiths, · II Automotive

(15)

2

Wi tr.t dual 1 tion, one s of plugs i connected

to first magneto and the other set to the second

magneto, the sparks occurring simultaneously e,t the two

plugs. a result explosion is propagated from two

points in the cylinder.

The sparking plugs are usually placed as apart

as possible and 1 t was Swaine4 \'lho suggested that better results might be obtained with sparking plugs placed

very close together. As as the author is aware, no

experimental '\lrork has been carried out along lines.

1. THE

It was the purpose of

the investigation to examine the effects on

(a) power output and (b) fuel consumption of a modern

petrol spark-ignition engine using a dual coil ignition

system with the sparking plugs very close together

in combustion chamber. It was also proposed to

examine any associated effects on ignition time,

pressures a.nd range of burning, and where applicable,

to compe,re results with those obtained i'li th the standard

tion system.

4 J. Swaine, 11Some Design Aspects of Poppet

Valve Cylinder for Spark-Ignition, quid Cooled s, 11 Proceedings of the Institute of Meche,ni

(16)

II.!!J2ortance of the investigation. Although the

days of the high power reciprocating aircraft engine

appear to be numbered, the demand for a light, highly

efficlent and reliable powerplant in light aircraft and

automobiles will probably be 1met, for many years yet, by the spark ignition petrol engine. Intensive

development of the engine with increased compressi.on

ratios and refinements in design have resulted in

higher power outputs per unit weight and improvements

in fuel economy. Further improvements may have to be met

by improvements in ~he ignition system especially in the use of low octane fuels and weak mixtures. There are

some indications5 that present day coil ignition systems

will be inadequate in the near future if the trend

towards greater speed range and higher compression

ratios continues.

As it has been stated before, no experimental

work has been performed with thi.s type of ignition

system, the results may throw some light on one of the

least understood phenomena in internal combustion engine

design.

5 L.H. Middleton and M.F. Peters, "Optimum Rate of Voltage Rise for Minimum .Energy Loss in Ignition .Systems," S.A.I~ . • Journal, Volume 5, No.3, (July 1951)

(17)

4

OBJfi:CJT OF THE INVES'l'IG.l~'riON

The experimental re ts wert~ of e q11 :11· VA

on] • a comparison heing made wtth performance

of the engl ne fl t ted l'li th the B tandard i t:l on sys tern.

This was done because the ne was not des1. for

experimental v10rk and for this reason a number of eomplex

and variable fe.ctors could not be controlled to a high

of accuracy.

~rhe effects on pm<Ter output and fuel consumption were of first consideration~

poe bilities of using a common circuit breaker and

stributor suitable for a multi-cylinder engine without

undue complication ltle<s also investigated. '11he effect of

increasing number of plugs per cylinder up to three

was determined.

attempt was made to get some correlation

between the igniting power of the sparl{ and the condition

of the charge at the point of ignition. The effect of

altering the characteristics of the primary coil circuit

was also investigated vli th a theoretical consideraM.on

of secondary current and voltage.

JVIuch attention hEJ,S been paid to combustion 1n

egines at large throttle openings, but in automobile

(18)

installations, low load factors are encountered. 'fuls

means that the engine may be called to operate at full

throttle for only a very small percentage of i total

operating Investigations into effec on pO\'fer

e.nd fuel consumption were therefore made a -;,vide rango

of throttle openings.

The

following order has ass~ooed for the presentation of the remaind.er of the thesis:

(1) A review of the literature in three parts.

first section is a summary of the theories of ignlti.on

that have been advanced, with a discussion of experimental

worl~ leading up to the modern ignition theory. 'fhe second section is a discussion of the combustion process in

internal combustion engine with reference to the modern

theory of spark-ignition. The third section is a summary

of results obtained with dual igniti.on.

(2) A description of the apparatus and methods of

measurement used the experiments. technique used

the preliminary tests is given together with that used.

in speci tests devi to sheck the conclusions reached

in the preliminary tests.

(3) A discussion of the results obtained. The effects on power, fuel consumption, lgni.tion time are·

(19)

6

(4)

A summary chapter of findings, conclusions

and recommendations. Here an attempt is made to relate

the results vlith those forecast by the ignition theory.

(5) criticism of the test apparatus v-rith

recommendations for future testing.

All design calculations for special st appe,ratus

is given in the appendix together with theoretical eye

calculations. A mathematical treatment for the discharge

of a,n ignition coil under v/Orking oondi tions is also

(20)

A tremendous amount of research has been carried.

out into ignl tion and combustion problems. From early

experimental work two theories arose, namely the thermal

and the ionizatlon theory, to explain the phenomena of

sps,rk ignition. The latest explans,tion that has been

advanced, the chain reaction theory, has been evolved from

further evidence but retains some of the concepts of the

ealier theories.

I. rnmRMAL THEORY OF IG·NITION

For an exploslon to take

place in a conbustible gas mixture, a certain minimum

volume of the gas must be heated to a definite ignition

temperature. The minimum ignition volume is determined

by the composition and nature of the gas mixture. If the

explosion is to be self propagating, the hea,t liberated

by the progressively explosive reaction must be greater

ths,n the heat ssipated by conduction to the surrounding

cold layers of the gas.

Fr·om a purely thermal consideration, disregarding the

(21)

8

and reaction, if T is temperature at a distance r

from the originat time t after heating begins, if k is the terme,l conduc ti vi ty, then the aqua ti on for heat

conduction is:

kr2

b

2 T

-~

+

2lc

o

;)r T

1l1he solution of this equat:ton viill depend on the quantity

of heat suppli Q, the temperature and type of

source.

F'rom this equation, assuming ah ignition temperature,

i t can be easily indicated what amounts of energy should

added by point sources, or sources lasting over a

certain od of time.

J!:xperiments

by Coward and lVIei with methane-air mixtures showed

that 1 t was possible to brine; an appreciable volwne of the xture to combustion by a spark without causing

general ignition. U~der certain condi ons it was possib

to send Uwusand.s of sparicE:i throu,~h a l1if~h1y oornbustl

m.i without an explo on occurring. atione from

(22)

energy for ignition, and i t was shovm that the

volum<~ of the gas that could be consumed before ign:l t:\.on rises rapidly. From these results and other·s 2 , wi t.h

methane-air mixtures, Coward and Meitar calculated

minimum ignition volumes, assuming an ignition temperature, vlhich satisfactory results. Coward and Meiter

concluded,

Nothing in the results of (their) experiments

sts the intervention of any electrical effect of the spark .... other than the normal effect of the degradation of its electrical energy,3

4

Thornton shm"''ed th1:tt the thermal energy of a

spark just capa.ble of igniting a mixture varies considerably

vli th the electr:ke.l conditions e.nd pu.t forw·ard the stion that some sort of ionization preceded combustion. This

can be anticipated on thermal considerations alone, hov1ever, as vras pointed out by Taylor- Jones, Morgan and

£:::: Vfheeler. J

2 J.D. r4organ, "Ignition of Combustible Gases by li:lectt•ic Sparks, 11 IJhllosophical JVJagazine, Volume

.!J.5,

p.

968

(1923)

3 Coward and. Meiter, op. cit. p.

399

.lJ. W .M. Thornton, 11li:lectr:tcal Ignition of :rr;xplosi ve Mixtures, 11

Proceed:tngs Hoye.l Society (London A) Volurne 90,

p. 274, (1914·)

5 J£. Taylor-Jones, J·.D. Morgan, R. V. 1rfueeler, 11 0n the B'orm of 'remperature Wave Spreading by Conduction, from Point and Spherical Sources; vli th a Suggested Application to the Problem of Spark Ignition, 11

(23)

10

Considering electric sparks of two types:

(1) .Exceedingly short (capacity sparks) emd (2) Relatively

long (inductance sparks) and considering four cases of

heating as follows:

(A) Instantaneous at the origin

(B) the origin at a uniform rate during the

time L

(C) Instantaneous over a spherical surface of

radius a

(D) Instantaneous through a spherical volume of

radius a

If Q is the quantity of heat suppli and c is therms,l capacity, the equation for thermal conduction (p.8)

as a solution for Case (A)

T

.Similarly, given the boundary concH tions, the equation

can be solved for the other three cases and the results

are shown graphically in Figure 1. It can be seen from the

figure that

(1) There is no advantage in rai the source,

from a point to a uniformly supplied spherical volume

1 mm. diameter.

(24)

400

5000

~

~

0

t

1000

a..

Of HEAT I NGr AIR. [Taylor-

Jone10J

Mo~Cl~

&.

\-Jneeier]

A. lnstonto"eous

f6,...,t

Source.

t

=

0·00275

B

Contmued

Potnt Soure

t

111 0 ·

OOb

sect

L

=

0 ·

005 sec.

C.

lnstontotu~ous

Sphencol SurfQCe

Q,r:t.

0·1

em.

t

1:1

O·oooo

D.

(nstanioneous

Sphencol Volume.,

a=-

0·05 em. t-::= 0·002

f 7 0 0 - - - . ;

~

OS'

o·IO

0·15

(25)

(3) An increase in the size of the source, however small, will give better heat conduction.

12

These results will be considered again later in

the general discussion on ignition theories.

II. IONIZATION ~rHEORY OF IGNITION

General St~ment of the !Q~~ry. Ignition in

combustible gas mixtures by means of a spark discharge is

in some way associated with the ionization process. 'I'he

t type of electrical discharge is therefore of fundamental

importance in the initiating of the explosion.

Early investigations showed

that the electrical conditions affected the igniting

properties of the spark discharge. Thornton6 found that

it v.ras possible to have brilliant sparl{S v•rhich clid not

cause ignition of the most imflammable mixtures. 'J.lhornton

suggested, 11That a gas has a particular temperature of

inflc.unmation may mean that ionization begins at this

temperature."

6

·vv.M.

Thornton, "Ignition of Gases by Impulsive Electrical Discharge," Proceedings Royal Society

(26)

A series of experiments by Finch and Cowen7

shovred the. t a high tension arc dissipating energy at a

surprisingly high ra,te could be maintained in explosive

gaseous mixtures vli thout causing ignition. In the case

of hydrogen-oxygen mixtures it was found

(1) Ignition occurs without lag immediately on

the attainment of a certain limiting current,

(2)

=

conste,nt over a considerable range. Where pz is the pressure and iz is th.e minimum

ignition current.

It was concluded that as the concentration or ions,

or of molecules or atoms excited to any particular state,

is also approximately a hyperbolic function of the ge,s

pressure in vrhich the potential drop across the discharge

is constant, it followed that the ignition vias determined

by the attainment of a certain definite concentration of

suitably excited molecules or atoms. (Note that this

conclusion does not specify ionisation but 1suitably

excited atoms or molecules1

• As it is seen later, the

modern theory of spark-ignition uses e, similar concept).

7 G. J. Finch and L. G. Cowen, "Reactions in Electrical Discharges, 11 Proceedings Royal Society,

(27)

14

A further series of experimentsB were made with

carbon monoxide-air mixtures ignited by condenser discharp;es of lmo\vn oBolJJ.ati on frequrmcy.

determin minimum ip;n:lt:ton pressure. 'I'l1e effect, of frequency ciould be studied independently of

(a)

(b)

rn1.e total amount of energy d:l.ss:tpated

'.rhe :rate of d.issipation The r'esul of the expe:r:l.ments shmred

(1) ~rhe :l.gni ting pOlver of the spa,rk was independent of the value of the peak current.

(2) The igniting power was determined by the natural

frequency of the circuit to such e,n extent that a sui table decrease in frequency could out\-Jelgh the effect on any possible red.uction in ie;ni tine; power due to a decreased

amount of rate of energy dlssipation, or both, either by the first half oscillation ofthe sparlc or the t'lhole

eli scharge.

:t:hese results, shown graphically in Figure 2, conflict with the thermal theory but consistent wi.th an

exaltation theory as a high frequency spark is a rich source of ionization.

8 G.J. Finch and ,J. 'rhompson, 11 ~('he I~ffect of

Frequency on the Condenser Discharge Ignition of Carbonic-Oxide-Air Detona.ting Gas. 11

(28)

2·0

1·8

1·6

15

INF'~U~NCE

OF

FREQUENC\

AND

~1AXIMUM CURRENT

OF A CONDENSER

DISCHARGE"

ON

_IGNITABILITY

(.

Braclfor~

&

r:1nc.h

J

Carbon Monox1de -

Oxy9en

M1l(ture

0 •041 /~!= (Cl)XaC:ItCH.,Ct:.

1400

_,

tL

l200t +-c

~

'-:J

IOOOU

1·0

" - - - J

0

600

soo

\000

(29)

16

III. CHAIN Rli:AdTION 'J1HEORY OF IGNITION.

In a gas mixture

ignited by an electric spark, a small spherical volume of the

gas is heated instantaneously, and at the same time a small

quantity of active particles is created. It is of no importance

whether these particles are ions, atoms or molecules; nor is

it important to state how such particles are created. It is

merely sufficient to state that a heat generating reaction

takes place at a rate proportional to the concentration of

the e.cti ve particles, a.nd that this reaction varies in intensity

as the active particles diffuse through the and increase

at a rate proportional to their local concentration. Now

the temperature at the centre of the sphere tends to fe.ll

as heat ·is conducted av1ay and rise as heat is generated.

'l'hus, for> r>eaction to be self sustaining, we must assume

a reaction rate such the.t this temperatu.re never decree.ses

du.ring the combustion process.

A

simile.ri ty between the chain reaction theory and the thermal

theory is noticed, but it is obvious that ignitlon cannot

be understood exclusively as a thermal process, simply

raising the temperature of a combustible mixture vrill

not make it burn -- a certain type of chemical activity

(30)

Jost9 is emphatic in his statement that activation

of ions and molecules occurs before the spark energy

depreciates to heat and is, therefore, of prime importance

in the ignition process. Jost suggests, hov1ever,

...• that with a theory not purely thermal in cha:r'acter, diffusion, perhaps especially that of free atoms and radicals, comes into play. Since diffusion follows the same le)tlS as those for heat conduction, i t is completely

conceivable that

1bhe concepts of a pure thermal theory can be retained.

It is possible to conclude that the igniting power

of the spark is dependent mainly on its molecular stimulation

and eventual dissociation, partly through its thermal effect

and only very slightly through its ionizing effect. 'l'he

tonization is probably a supplementary rather than a

fundamental mechanism of ignition.

It is possible to reach some general

conclusions from the evidence.

( 1) For a given spark energy, the explosion vfi 11

occur within certain limits of composition. JYiore energy

is required to initiate the explosion near these composition

limits, the exnount of energy being required rising rapidly

9

vv.

Jost, "Explosion and Combustion Processes in Gases." (New York: McGraw-Hill Book Co. Inc.) 1946, p. 56

(31)

Mm1mum

\qnttlOn

Curren1~

(m

pnman.)j

for

Hydrocarbon -

Atr

Mtxtul"e.-S

[!o\l)

\·0

Hexane

c 2:.

0·.2

0 ~---~

(32)

as the ltmits are approached.

(2) li'or any gas mixture, there is a limiting ignit1on pressure emd energy, the igni tabili ty decreases as the

pressure is lowered.

(3) Cooling effects of boundary walls and electrodes may have considerable influence detennining whether or

not the flame "Ti 11 propagate.

IV. PROPAGATION OF EXPLOSION

In eng:lne combustion, we are dealing with a

"progressive explosive reaction," that is, the react:l.on

velocity keeps increasing and the only limiting factor is

the complete consumption of th.e charge. Although a number

of factors prevent the results being applied directly, results

wlth this type of reaction with hydrocarbon fuels do give

a good ind.ication of the funde,ments of the process.

:a;xplosions in

clo vessels show that there j.s always a noticeable

interval of time e~fter the passage of the spark to \V'hen there

a perceptible rise of pressure.1 1 rrhi s interval or

induction period, after which the reaction begins to accelerate

11 G. 1v1 throvv and J. Boyd, "Combustion Processes 1n Closed Vessels, 11 Industrial e,nd Engineering Oherni stry, ·

(33)

v1as originally thought to be "ignition lag, 11

that is, the time required for a propagating nucleus of flame be

bUi up. Flame photographs12 , however, clearly show that a derable volume of combusti.on takEB place during thls pe od and igni. tion lag accounts fur only a minute fraction of 1. t.

IJ.'he effect of turbulence wa.s investigated by Da.vid13

who t tecl a. fa.n insi. the combustion chamber whlch could be run at various It vms found that turbulence bad very l:lttle effect on tb.e delay per:i but a marl\~ed effect

01:1 subsequent

uently shown111 ssl.Jre r•:ise is

on of

perlod,

slmv but nevertheless

a. speo:t f:l c case, pressure se over the fjrst 20% of ssure rise over the last

O.OlC?%).

Attempts to measu-re velocity of propagation in a combustton bomb are

complicated by the t, the pressure seo and the progress of the explo on is affected by the compression

of the fresh gas by combustion gases. •ro overcom(:1 this

0, d. de d. s, 11Ii'lame tographs of Combustion

ceases in Gases, 11 Jot.U"'na.l Ohemi stry .Society, ( F'ebruary, 192'7)

p.

l3 1/1,

rr:.

David., "Process of Oornbust.ion in dpark Engines," O~nc~ineer1 Volume 169, No. LflW4, (J'une,l940) p.

lif· R. \'1. Fennine; A. 0. Whi ff:l.n "Pressure se during Combustion in Closed. Vessels, 11 1. 'l'rans. Hoyal

Volume 238, p. 149 (1939)

on 509.

(34)

difficulty, Fiock and Hoeder15 initiated the explosion at the centre of a soap bubble contatning a combustible gas

mixture. 'J:lhe bubble was photor;raphed through a narrow sll t

vlhich left only tts horizontal dlameter visible. 'I'he film

~>ras carried on a drum rote,ting a.t a l{nown constant speed about an axis parallel to tb.e slit so that as the diameter

of the flame increased, the lengthening image moved alone;

the screen and produced a V - shaped trace which constituted a time - displacement record of the flame front.

F'rom this trace the velocl ty of combustion could be

calculated dlrectly. Results showed that the flame speeds

are a maxtmum at approximately chemtcally correct mixture

strength but decreases as the mlxture becomes too rich or

too lean.

V. COJVl'BUG'I'ION IN THE :U:NCHN.II:

Combustion in the internal combustion engine is a

highly compllcated process and is still not fully understood.

Considering the normal combustion process and disregarding

effects such as detonation, the problem is still far from easy as in the combustion chamber the mixture is turbulent, the combustion vollune is continually changing, and the

(35)

F'IrJURE 2J.. 22

PRESSURE

RISE

J)URING

COMBU~TION IN A CLOSED

125 .

-

---"-~-COMBUSTION BOMB WITH TUB_BUL~NCE

[:DmndJ

FCJr'l RI.JI''IYII ~

ISOO R.P.Il')

t

-0 ~--~--L---~---~---~---~---~

0 0·3

0·4

O·S

(36)

incoming charge is heated by conduction from the cylinder

walls. Using hydrocarbon fuels, this heating of the charge

ce,n ce,use clissociatton and chain branching, the charge me,y

also d.iluted with quanti ties of hot resldual exhaust

It was previously

thought that after the passage of the spark, turbulence

broke up the nucleus flame into small parts which v1ere

carried into various parts of the combustion chamber thus

virtually setting up large numbers of ignition centres.

Flame Photor;raphs16 shmv, however, that after the passage

of the spark there is a short interval before the flame

appears (ignition lag)t the flame then spreads outwards,

s.

slowly at first, then more rapidly. The entire flame front

is extremely turbulent, turbulence in the remainder of the

combustion chamber is of a similar nature but on a smaller

scale.

From pressure - time diagrams

taken from engine cylinders, it is possible to make a strict

distinction between two phases of pressure development; a

delay period without an apprecie,ble pressure rise and a

combustion time proper. The pres~ure rise is caused by

l6 L.C. Rass'.'/eiler and G.l\1. Vfithrow, 11Slow Motion

Shows Knocking and Non.,..Knocl\:ing Explosions, 11 S. A.ll~

(37)

heat released by combustion causes the unburned portion

of the charge to expand and so compress the burned and

unburned portions isentropically, the increase in pressure

being directly proportional to the percentage of the

charge that has been consumed. 1

7

Following the pressure varlation in the eye , the first

part of the charge burns at constant pressure with a large

temperature rise, then undergoes an adlabatic compression.

The last portion of the charge is compressed adiabatically

and then burns at constant pressure. temperature rise

during the adiabatic compression in greater for the first

part of the charge and the resulting temperatures are

different, even although the amount of heat added during

combustion at constant pressure is essentially the same

for both.

The temperature of the charge decreases progressively

from the first to the last part of the charge to be burned.,

the temperature of the gas in the vicinity of the spark

plug is therefore higher than the,t of any other portion

17

C.D. ller, 11 Roles of Detonation Waves and

Autoigni tion in Spark Ignition l:!"":tlgine Knock as Shown by Photographs Taken at 40.000 and 200,000 Frames per Second,"

(38)

of the charge.

By far the most

combust:1 on reactjon can he expressed in the follmvlng

manner:

JV!ass rate of burning = ( tnstante:meous area of the flame front) x ( Veloc'L ty of advance) x ( Densl ty of the mlxttirE'i)

The Affect of turbulence l s to mak8 the flame front. rac;ged

and so increase its area. T'he flame is propagated in muoh

the same vmy as in staP~nant mixtures, the flame tending to follm'l the shape of the retaining walls. Ivtovements occur ln

the unburned chare;e due to general swirl. and local turbulence.

It should be noted at this stage the above dlscussion

applies to combustion in the second stage of pressure

development. Ricardo 18 regards the tv1o ste.ges as quite

distinct, " ••. one the growth and development of a self

propagating nucleus of flame, the other as the spread of

that flame throughout the combustion chamber." Taldng this

concept as a working definition for the present, the second

stage is purely thermal in character, the flame speed

being relatively unaffected by changed in fuel-air ratio,

tem1:3erature, exhaust gas dilution e,nd other factors.

(39)

26 Increased. turbulence is limited by the incr<?ase ln direct heat losses to the cylinder v.ralls, for smooth

running and optimum efficiency a pressure rise of 30-35

lb./sq.in. per degree of crank ant~le rotation is indicated. 19 the degree of turbulence is proportional to the engine

speed, the time required for the second stage of combustion wlll occupy approximately t,he same amount of cranksha:ft rotation at a.ll crankshaft speects.

The delay period. is oherrd ca.l in origin and an analogy can be drawn here with experimenta.l results obtained with combustion bombs (Induction peri.od).

Brooze20, showed that the delay periocl, for a given condition of the engine, is dependent on the nature of the fuel, the mixture strength, temperature a.nd density of the charge.

sul ts (Figure 6) shovl tha.t the combustion time

is only affected slie~htly by changes in mixture strength, but is dependent on the type and degree of turbulence.

The delay period, however, is seen to be a minimum

when the mixture is about 20% rich and increases with d.eviations from this value. 1.Vhe delay period 1 s e.lso

19 Ricardo, op.clt. p.l3

(40)

Var

1a.t1on

of_~~~~~~s-hon -~T1

l'!le ___

g_~i__]_e:

la__LJ _

_P~~£~

Fue\- A1r Ra11o ond Turbule~

50

40

c Q -iiJ

-Po 3o

Ck:

45

( Dav,cJ)

- -l

I ...

. I

T.O,C.

_j

Combu.f,o,.,

·1 /

1ime

J)&lo~ 1>enod

I. Un5hr'ouded Inlet Va\Ye 2. Shl"'ouclecl " "

3.

Cas"t

e \\a

ted "

··

(low turbuleV'Iu) (5w•rl)

( ht~h Thrbu\wce)

3

_c.

-<(t,<'o<l-/

3

\1)

.:;,t.

'

:w

e

'1)1/h\0.~ .

v

b

\{) 10 Q)

~

Z"'

A

0

0·6 0·9 1·0 l·l

1·4

1·6

Ffac.to..,

ot

Correc:.t

Fue\ ..

A1r

Rot1o

27

a

(41)

28

influenced by the type and of turbulence, ~:n'llrl

being more effective than turbulence in reducing ignition

delay.

The exple,nation may be that the decreases in the

delay period due to S\'firl is due to the decreased boundary

layer thickness. Broeze suggests,

the boundary layer along the combustion chamber vlall the combustion progress velocity is at first slow until it is caught up by the main body of the charge,

in which combustion the combustion chamber wall

combustion progress velocity is increased by its state of movement.21

This vTould explain the efficiency of long reach

plugs in igniting lean Jhixtures as observed by Taub. 22

These results can also be called upon to account

for the cyclic variations observed in indicator diagrams

taken from spark-ignition engines. rrhe variations ta1ce place

mainly in the initial delay period at low compression ratios

or reduced loads. 23 This may be o.ue to differences in the

fuel-air ratio, especially in the case of carburetted

mixtures, or residual dilution, as it is no ced that

the variations are greater when sparking plug is not

well scavenged.

21 Ibid.'

P•

465.

A.

Taub, 11\1/hat about JI:ngine,

"

S.A.E:. Journejl

Volume

44,

p. 201

(1939)

(42)

A cleteJ.led study of th.e theory of coil ignition is

glven Appendix B. A summary of the general terlstics

of spark d.ische,rge is first , together wl th the definitions of terms used. The discussion is

confined. here to the main factors of the sparlr ells

that d.etermine ignition.

A swmnary of the properties of the spark discharge is as follows:

(1)

The spark consists of two components, the

first being the capacity component which ionizes the

e,nd renders it conductive. 'J.lhe capacity component 1 s

di of the stored in the capacitance of

system.

of

The inductive component which follows consists

energy stored in the magnetic of the circuit.

( 2) The capa.ci ty component is of very short dure,tion

and carries a high current, the inductive component

lasts considerably longer carrying a much smaller current,

as the energy can only be released slowly.

(3) JI:very spark gap has a certain time lag,

being a, small interve,l ionization appear, then for

process to develop until the space between the electodes

is completely ionized to a stage that produces a spark

d.l scharge.

(43)

30

is constant, and the voltage will rise to a value

e ra of vol

ratio :l.s

t.lJo 11tmpulse r'a tio. 11

~'h.e factors

affecting the sparking vol are numerous and complex,

from the electrical charac sties of the circuit,

any factors fluenei the ionization of the s bot.ween

the trades will alter the vol tap;e requfred for the

Among the fstctors affectlng sparld.ng potent.tal are

{a) dimensions (b) 13hape and. disposition of the

e trades (c) material of the electrodes (d) nature of

the gas mixture (e) sure and temperature of the

(f) electrode temperature (g) vrave form of the appli

e.m.f.

F~xperiments

by Patterson and Oampbell24 shovved that the effect of substituting a m:lxture of petro air vapour

for pure air was to reduce the sparking voltage slightly

24

a.o.

Paterson and N. Campbell, ";3ome Characteristics of the Spark Discharge and its

tion Explosive Mixtures,11 Proc. Physical

(44)

clecr•ee,se the spa.,rldng potential, 1 t 1 s due to thl s effect that the val tage of the ordinax'y ignl tion system is kept within bounds, a

50%

reduction has been observed with a

rise in electrode temperature of 600°0.25 1'his j.s probably du.e to the fact that hot electrode is surrounded by

a layer of heated gas at a density below that of the remaining charge. The effect of turbulence vrould i.ncrease the sparking val tage by remov:l.ng this he a ted

layer. 'rhi s is observed. vrhen sparking val tages are

26

d.etermined in an air blast.

For a given pair of electrodes, the effect of

increasing the gap vTidth is to increase the sparking potential in almost direct proportion. A similar relationship 1 s observed v-ri th increase of pressure. applying these results to the calculation of sps,rldng

to

voltage to an engine running under operating conditions,

however, the cumulative effect of all the variables is very difficult to determine, for example, an increase in

pressure should increase the spe,rldng potenti , but the temperature of the plug may be raised at the same time.

25 IC.A. vvatson, " :uaectrical Characteristics of Sparlc Gaps and Sparldng-Plugs, u Proc. Inst. Automobi

Englneers, Volume 22, p. 440 (January, 1928)

(45)

32

ConsidE1r:tng

the process of ignition from a purely thermal viewpoint,

from the earlier discussion it is seen that the heat must

be supplied as quickly as possible (Figure 1) to minimise

the heat loss during the critical period of initiating the

combustion. r.rhe energy requ:lrernents for ignition, shown

graphically in Figure 2, indicate that a large increase

in spark energy is required to extend the mixture range at

any given pressure. It should be noted that in this case it

is the capacity component that will cause ignition, any

e,ugmentl=ltion of the spar1c energy cannot be obte~ined by alter-ing the magnet:l.c characteristics of the circuit, tha.t is

by making the spark 11

fatter. 11

Experimental results h~:.we produced evidence to support this view, showing that the capacity sparlc is the

normal form of spark di schar·ge and that the energy required

for ignition decreases as the spark potential increases.27

The electostatic energy in the capacity component

1 s

·ilr

cv

2 ' where 0

=

capacitance of the clrcui t and

v :::

sparking voltage. 'l'he only way to 1ncrease this energy is

to increase the breakdown voltage of the gap by increasing

the gap length, or by increasing the ce.paci ty of the

circuit. The former method is to be preferred, as

(46)

;;;--Mmtmum

Sp9rl<.

Ene(q1e~ Requ1red

io

lgn,te.

a Petro\-

Alr

Mnc.ture

('Poter6on

&

Campbell)

V/t.

of etro \

1n

ecied

1nto Combu~ton

Chamber

Wt o Q\r t.oV\ICM wou

\d

occup~ "fue d~m ber

at i'ne

same tem~r"Qi'ure Qnd pres~ure.

I

.4

SJ

)(

1/')

...!M

J

PS

~

::.7" ~

Q)

c

L.LJ

:::.£

l..

0

a_

(/)

3

2

0

~---~---~---~---~---·

::5000

5000

S~cuK1~

Vo\ta9e.

(47)

.;--'2

"'

.,

...!:!

,

~

' - '

:r

r

c

w

~

~

Q

0..

V\

E

::3 E

G

'-8

6

4

2

34

Vorroiton

of

M1n1murn

SparK

E'nerq~

wt1'h

Mn<.1ure

stre~th

oi' 'J),fterent

SparKtl"'~

1-hten"ttc:tls

(R::,Ier.so~'~

6 CClmpbell)

W+ ot eiro\

1rt'

ectec\ 1nio

The

Cot-nbus\tof"'

Chan,ber

P

::r \;Jt. a1r wl-'uc.h

~ld O<..<.up~

c.hum ber

wt

ihe

· Same tempe.rc,iure u~ pre.ss~r'c:..

0

~----~---~---~---._---~--~

·07

·0~ •10 • I I

(48)

increasi.ng th.e gap length e,lso decreases the eooling effect

of the electrodes. The li.miting factor is the maximum

voltage that the ignition system can supply.

Although a purely thermal theory can give

useful results, as has been stated previously, ignition

cannot be fully understood from a simple consideration of

this kind. IA3,ter experiments with coil ignition showed

that the capacity component was not necessarily responsible

for bringing about ignition. It was found that the

igniting power of the spark decreases with suppres on of 28

the inductive component.

A further series of experiments, carried out with a,

viey,r to determining the igniting power of the components of

the coil discharge for the operating conditions of an

internal combustion engine, shovTed that the performance of

the engine was unaffected by changes in the duration of the

discharge. It was shown, by means of cathode ray

oscillograph analysis, that the only portion of the

discharge required for ignition, is the short initial

portion of the inductive component. ~he duration of the

28 H.Vf. Bradford, G.I. Finch, and .J.VI. Prior, "Coil Ignition of fJome :a::xplosive Gaseous ]\.IJixtures, 11 Jounral Chern.

(49)

)6

dJ.s

value without affecting 1 t,s lp;ni tlng properties. 29

i t can sean from

above dtscu.ssion, the problem of 1 t:ton j s stl obscure

and not fully understood.

Probe.,bly the rate of clis pation of the spark

is the most important factor in the process of ie;nition~ the sparkinP;", voltage, spark energy in the capacity component

and the rate of voltage rise he,ving lesser degrees of lnfluence.

problem is further complice,ted by the wide range of

operating concH tions the ip~ni tion system is called upon to handle. The inductance component me,y bE'l required to ignite mixtures under extreme condttions of imperfect carburettio:n

and more especially, u:ncler cold starting condi ttons requlring high heat energy output of long duration as the mixture may

be in the form of droplets which must be vapourised before normal ignition wi take place.

Cons:tclering the problem in the light of the chain reaction theory, during the early stages of ignition and propagation, a surface to volume ratio betvveen the burned

(50)

and unburned mlxture favourab to reduced. heat trans fer vrill be desirable. Appendix).

VII. DUAL IGNITION

In this system, used in aircraft engines, the ignition systems are completely lndepend.ent. Tv1o sparks occur in

the combustion chamber and the explo on is propagated from two sources instead of one.

ce.rried out a series of carefully conducted tes for the purpose of showing the influence of the sparking plug position on detonation. The engine us for the sts was a single cylinder

"E.35

11 variable compression engine

provided vri th four radi plugs, any two of \·.fhich could be

fired simultaneously. The plug positions and thelr

relation to that of the valves is shown in Figur•e 8. The results of the tes are given in 'J:lable 1 and show that

the tion advance required and t,he highest useful compression ratio (H.U.C.R.) are both dependent very greatly upon the length of flame path from the sparldng

plug to the f~lrtbest point reached by the flame. 1 fi1.e

relat:lve positlons of the active sparking plugs and that of the var:lous va.lves are shown to exert a secondary influence

(51)
[image:51.605.158.457.189.540.2]

FIGURE 8.

R1~LATIVF. QJ\1'8 OF VALVEB AND SP ARJGNG PUJGn FOR J11XPJDRI.MENTS WITH DUAL IGNITION

(52)

I.

INFLUENCE OF SPARKING PLUG POSI'riON ON DETONATION

( from Hi cardo )

Test conditions:

Speed 1500 r.p.m.

Iviixtux•e strength 157~ rich

(maximum power setting) Jacket temperature 50° C.

Heat lnput to the induction system

1350 watts

Fuel "Texas oline11

11

E.3511

variab compression engine

Ignition Timing

Plug Position giving max. H.U.C.R. B.M .• l~.P.

lb./sq.in. torqUe

1 and 3

.

.

• 30° early 5.30 138.0

2 and 4

.

30° II

s.

136.6

3 and 4-

.

.

34° II 4.98 134.5 1 and 2 • •

.

32° II 4.90 133.8

1 and 4

.

32° II 4.80 132.1

2 and 3 32° II 4.84 1 .8

1 only

. .

39° early 4.95 133.5

2 only 39° II

4.90 133.1

.

3 only 40° II

4. 8L~ 132.8

• •

.

4 only 42° fl

4.85 132.8

(53)

40

a.s well.

Still more recently, experim.ents to determine

effect of combustion time on knock have been carried out.

In this case a single cylinder engine vlas fl tted v'li th 17

spark pluga v;hich fired simultaneously. By selecting

the number of plugs firing the combustion time could be

varied. In these experiments, a ficant decrease

in the octane requirements was.observed. 31

ignition. The results obtained by cardo appear to

rather disappointlng. This is perhaps due to the t

that last po of the charge to burn is jected

to intensive radiation from the t\•To advancing fronts.

i t has been shown b ~ a consideration of the 1 tion process in the on a thermal 'Qe,sl s s very reasonab

results. 'I'vw ignition sources very close together, on

thermal constderations, ltfould theoretically give bet

performance as thls would increase the size of the source.

( c • f. p. 12) • r.I'he st.igation llows this line of

thought.

31 D. R. Diggs, 11Hovr Combustion

Automotive Industries, Volume 108, No.

6.

Affects Knock, 11

(54)

An elabo:t~ate test apparat·u_~:~ was out of' th8 q_ueBtLo:o. so that whel~c post>ible, ex.ist:ing eq_u:Lpmont :l.n tt~.e laboratory

The problem of getting two or more sparking

points :l.nstde the cowbust:ton cluJ.mber was :form:i.Cial)le.? aH it

was nn1 Lkely that an.y New Zca1and 1nanul'aeturex• wm11c1 l!.ave

I'ail1U

s~va:i_1alllt::) with Ute f'oFI.IlE:JI' typn. cuul:i."llt,'

V'.·a::: rna(l e avallable an(J wao tall thmtgh o. r:.lt nglc

c:

given in the Appendix.

:Ll:o:n bedplate iJJ.r•oue;h s1mple m.onnt:LrJ£:;13 macl.e ·up out o:C I

bearnfe~., '11he mornd;j.ngs were bolted clovvrt th:eoue;I:t holes

(55)

li2 i3imilarly mm:mt.od cttrectl;y bellind the engine.

TJ:.1.e ive :from the ertg:Ln.e to

through two univer j 0 t n·{· ~--~--Jl-JP q +l·'e" u,!.- and brake Bha:fts

·were reg_ui to be par·a11el lmt c1:ld not need to

exactly in 1 advance indicator was :Cit

to the fPont of the ln•al\:e .. instruments foP 1:x:corc1ing teruperature wer•e housed a test panel d.ir•ect;ly

the The instruments for reco r• and :f'rtel consuJnpt.io:o ·wel''e part of the :L:pment of the

R:Lcax•clo TIJ6/S enghle and wel"e mounteCJ_ some distance along

the wall at the baclr of the test bed ..

Plates I and show the layout. the equtpment ..

A revolut.ion counter f:t ttecl to the bralce gave an appPoxirnation to the tr•ue speed

which vvas check.ec1 finally by a st:eoboscope. The could be deteromtned accui'ately tn steps

'rhe powex• was absorbed i;hrongh

a Fx•oude \'Vater brake .. The capacj.ty of the brake vvas 100

H.P., l outs the capacity of the For this

reason the 1;alance ~ which recorded. torque,

wan not as tive as it could been1 :tally

vV:hen the was l:Lght load.

tive pressure could d.etex•m:Lned to an accu.racy +

(56)
(57)

44

PLA'R II .

(58)

was rneaau.recl on a volurnetric bas :iJ3, the weight o:r :fuel

Lng d.ecJnc after tak':Lnc; r::;pec th

rl1

WO bulbs, upper 50 COo :1

t;y-the lower of 100 cc. capacity, suppli f'uel by

gr•avi ty to the carbtU'Ed.;·cor. The time f'oi' the :fn el level to pass gracluation a measure of the ra of' fuel f'low. A select:ion thPee volurneB of 'Naf3

ava able to su:it the engi:ne load.

It was found that the original :fuel-·ail .. rneter f:Lt.ted vvas inaccnlrate shox•tly

ing began~ The ternative was to f'it an meter and to obtain the 1" :r.•at io by measuP the air f'low and fu flow separately~

rnethods were lJossiblltties: (l) o:d.fice rneter with pnlBatLng tank, (2) v:Lscorts flow meter (3)

hot wire instrwnents.

or

these, the last two were scarded as both re~1ire calibrat s would be di:ff:lcul t as no su:t table apparatuB vvas availalJle.

The orifice meter would be relatively easi the s.i.z of the pulsattng tan}{ requ.i

'

however, would be of the order of ~0

" TestJ11g of

)~onclon: Ohapll18.1!.

• c .J. '·{·, l ·Ye Tl . .. 1J.S

(59)

was too bulky. 'I'he prolJlern was f:wlved 1Jy ut iLL sine; r:wme

of tb.e results obtained f:t'orn silencei' rt.:sear•ch (see il.ppendi"x).

An ori:f:lce, cal :tln•ated prev:i.onsly from steam. :flow,

was :C:l tted to a s :Llencer on the intake side of the caPbt.n•etto:e., The silencer was desitsned to dmnp out all oscillations :in

tho manifold within the speed range of the engine. A lead to an alcohol filled inanometer measu.P<:3cl the head acrosB the orif:lce, tlle manometer ha vlng three Blopes for low,

rnedil:tltl and hie;lt c:dr :flow ra:t;e:'-1.

This apparatus proved satisfactory, little oscillation on the manornet.er- l.)(:;Jrlt:; observed even when r•mmJ:ng at full throttltJ at low E:lpet:Cl .• W1th coi'I'ectJo:rt foP amb:Lent a:i.:e temperatu1•e an.cl presr:~ur>e, it was est1matec!

that the a:Lr flow could be rnearc;1.1X'ed accl:ti•ately to ± 1?~.

M0a~:ru .. rexnent .of e£ark ~vance. 1.'he ::.{mrk~ad:varwe i:ndicato:r was a n~ort tube connected to tb.e spar1cLn£; p1ug ter-1rdnal throngll a 100,000 ohm resistance. A slot was cut

in the j:ndlcatoi' o.isc vvldcb. was f:l.tted to the brake conplint:;.

On the outer fixed part of the indicator, a scale was set

showine the position of top dead centre (T.D1D.) The

position of the neon tube could be altered aeainst the scale so that whor1 the neon tu1Je firec1 opposite the slot, the ane;le of advance coulcl be read o:C.f di.t•ectly aga1.n::d:,

(60)

flash would fluctuate anything up to 2°, pass

irr·E;;gnlarit s :!.n the cam pX'of'

clue to

Thtn•womete el Sp

working on the 1Nheatsto.r1e bPi_clge prtnc :i.ple ~ we:t:?e at (a) cooling wate:r:• jnl.et (b) cool water outlet and (c) ilrair1. plug the to recurcl oil temperature.

thermometers were mermu·y in glass thei'rnometers.

pPeviou.sly EJ.gaLnst Before each test each was A<.lditional th.En•mocouple elements coulc1

or plue; temperature :Lf requtred.

An ajrc:t1aft

mm1:LC'old gauge was J'i ·t.t to a tappine; in the Jnanit'old just below the 11hot spot, u reacl.J.ne; to 0~ l in

xneJ:>cnry.,

Tb.e cooling stem used was that :r: anothe:r:' test i:n the lalJoi'atol"y COltS t.

t10 galL water tanh: connect.oct to a small electrically driven cj.r•culat pump by ~ • p:lp to the eng the cooling circrdt be completect \IV:l.ti1 a p to

the ·to:p ad.<.li tjonal 1 fPorn the town

supply allowed. to tlw

(61)

1!8

overflow pipe was fitted to top of tanl';: Control

of incoming supylyand flow rate of tho circulat water was provided by two valves on the test panel.

With this ar'Pangernent, it was found that wrllle the water· tnnJ<: was colcl, the f'low rate throu.gh the engine

was Af3 the tempe1•at.l.n~e of the water :r•ose 1 however, the flow rate had to be unt1l it x>eache<1 a stage wher•e it was r;eyond the capacity of the ch•culatin.g pmnp ..

sh water then b.ad to be ac1ndttecl. to the positioning

of the inletpipe, incoming cold water had a tendency to flow dix>ectly to the engine, the flow having to be

clrastically eel to lceep the outlet tempt:n•ature constant ..

the meantime, the original hot circulating water was

1)e pushed out into the overflow p:i.pe.

As t t was clesirable to have a ::n:nal1 ten.tpe

e across the , th.e rnethod u.seo_ in later• tests was to keep the inlet valve open :i.ghtly. This gave the

st compx•ornise, the small quantity water entering kept

the t ratu.re of the water the tanlc at a roasonal)le

level and only a srnall amount er.tt the was with some of the hot ln the tanlz. tb.is the temperat>ure of the t wator could 11elcl at

J. I l-)0('

l• Lj ,. •

The uncertainty of control :i.nl wate:r

(62)

Carburett:lon. It vvao der3i:rable to hmre a lnean:c:; of altering the mixture strength while the engine was running.

had no mumlfJ of allj"Ltstment aD the jetD ·were :fixed. It was su~gested that a needle valve be fitted to the main jet

but there was so little metal housing the jet that i.t was thought that the die car:>t alloy would b:rea1\ away i:f an

attempt wal:\ macJe to clr>ill and tap :Co:e the shan}\. o:f a needle

valve. An 0. U. caPburet tor bavine; a rneans of rrdxtu:re

'r.llc ortly· type ava:LlalJle suitalJle for the engine eapo.c i ty; wo.s a r::: hie draught type an(l an elbow had to l)e made up to adapt i t to the manifold. It wo.s i'ounc1 w~i.th

thL> UI'I'angcrnent tlw.t the:; vo.lue of thu '"··c)-'G J .. \. ' 1=>l "" C''J()''t-" J 1t·I!Cl v~ (...-\.U c• lost

a:n.d that ti1e cha:rge was p:r.•ol)ab1y t11Jt::v r-.mly d1utP:Llrut.ed to

the cyl in.clers. It waB oJ.Ho founcl that the range of nee(i.lc sizeB was not wide enough to allow sufficient alteration to

An o.ttc:rnpt war; then mo.dc to f:i.t a tJ£-;eclle valve to This was successful, elvine a

s.Lze of tbe mutn jet vdth :ei ch ttJ:i.xtnres.

rrwo sets of results, wJth different type caMn;rettors,

Figure

FIGURE 8. QJ\1'8 OF VALVEB AND SP ARJGNG PUJGn
FIGURE 28.
FIGURE 30 ..
FIGURE 38.
+3

References

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