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)
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
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
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
CHAP'.rE~IZ
Tests with
a.u.
carburettorTechnique
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·
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
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 • 127Specifications 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
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 ClosedCombustion Bomb .
5.
Variation of Combustion T:Lme and DRle,y .Period6.
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
1.3. 14. 15. 1.6.
17.
19 211£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
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
'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
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 thePAGE!
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
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
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
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)
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
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·
6
(4)
A summary chapter of findings, conclusionsand 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
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
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-~
+
2lco
;)r T1l1he 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
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
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.
400
5000
~
~
0t
1000a..
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~ousSphencol SurfQCe
Q,r:t.0·1
em.
t
1:1O·oooo
D.
(nstanioneousSphencol Volume.,
a=-
0·05 em. t-::= 0·002f 7 0 0 - - - . ;
~
O·
OS'
o·IO
0·15(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 SocietyA 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 minimumignition 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,
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
2·0
1·8
1·6
15
INF'~U~NCE
OF
FREQUENC\
AND
~1AXIMUM CURRENTOF A CONDENSER
DISCHARGE"
ON_IGNITABILITY
(.
Braclfor~
&
r:1nc.h
J
Carbon Monox1de -
Oxy9en
M1l(ture0 •041 /~!= (Cl)XaC:ItCH.,Ct:.
1400
_,
tL
l200t +-c
~
'-:J
IOOOU
1·0
" - - - J
0
600soo
\00016
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
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. 56Mm1mum
\qnttlOn
Curren1~
(m
pnman.)j
for
Hydrocarbon -
Atr
Mtxtul"e.-S
[!o\l)
\·0
Hexane
c 2:.
0·.2
0 ~---~
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, ·
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.
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
F'IrJURE 2J.. 22
PRESSURE
RISEJ)URING
COMBU~TION IN A CLOSED125 .
-
---"-~-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
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~
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 andAutoigni tion in Spark Ignition l:!"":tlgine Knock as Shown by Photographs Taken at 40.000 and 200,000 Frames per Second,"
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.
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
Var
1a.t1onof_~~~~~~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 /
1imeJ)&lo~ 1>enod
I. Un5hr'ouded Inlet Va\Ye 2. Shl"'ouclecl " "
3.
Cas"t
e \\ated "
··
(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
1·6Ffac.to..,
ot
Correc:.tFue\ ..
A1r
Rot1o
27
a
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:. JournejlVolume
44,
p. 201(1939)
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, thefirst 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.
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 vapourfor pure air was to reduce the sparking voltage slightly
24
a.o.
Paterson and N. Campbell, ";3ome Characteristics of the Spark Discharge and itstion Explosive Mixtures,11 Proc. Physical
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 arise 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)
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 andv :::
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
;;;--Mmtmum
Sp9rl<.
Ene(q1e~ Requ1redio
lgn,te.
a Petro\-
Alr
Mnc.ture
('Poter6on
&Campbell)
V/t.
of etro \
1necied
1nto Combu~tonChamber
Wt o Q\r t.oV\ICM wou
\d
occup~ "fue d~m berat 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.
.;--'2
"'
.,
...!:!
,
~
' - '
:r
r
c
w
~
~
Q
0..
V\
E
::3 EG
'-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 berwt
ihe· Same tempe.rc,iure u~ pre.ss~r'c:..
0
~----~---~---~---._---~--~·07
·0~ •10 • I Iincreasi.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.
)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
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 engineprovided 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
FIGURE 8.
R1~LATIVF. QJ\1'8 OF VALVEB AND SP ARJGNG PUJGn FOR J11XPJDRI.MENTS WITH DUAL IGNITION
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 5.30 138.02 and 4
.
30° IIs.
136.63 and 4-
.
•.
34° II 4.98 134.5 1 and 2 • •.
32° II 4.90 133.81 and 4
.
32° II 4.80 132.12 and 3 • 32° II 4.84 1 .8
1 only
. .
• 39° early 4.95 133.52 only 39° II
4.90 133.1
•
.
•3 only 40° II
4. 8L~ 132.8
• •
.
4 only 42° fl
4.85 132.8
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
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
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 +
44
PLA'R II .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
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:,
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
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
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,