THEORYOF THE IGNITION COIL

MATI LAL D"TT

". S "niv ersity o f Illino is , 1 9 1 1

THESIS

Submitted in Partial Fulfillment ofthe Requirements fo r the

Degree of

MASTER OF SCIENCE

IN ELECTRICAL ENGINEERING

IN

THE GRAD"ATE SCHOOL

OF THE

"NIVERSITYOF ILLINOIS b e

1 9 1 2

"N IVE R S I TYO F I L L I NO I S

THE GRAD"ATE SCHOOL

I HE RE"YRECOMMENDTHATTHE THES I S PREPARED"NDE R MYS"PE RVI S ION"Y

MATI LAL D"TT

ENTITLED

"E ACCEPTED AS F"LF ILLING THI S PART OF THE REQ"I REMENTS FOR THE

DEGREE OF MASTER OF

of Major W ork

ead ofDepartment

Recommendation concurred in

CONTENTS

Intro duct i on

Phys i cal The ory and Princi ples

Of Operat i on

Fundamental Equat i ons in

Trans i ent s

Numeri cal Appl i cat i ons

Conclus i ons

THEORYOF THE MODERN IGNITIONCOIL

I Introduct i on

The modern igni t i on co il,o therwi s e known as

Induc ti on c o il,Induct orium

,or Rhumkorff c o il was regarded

a few years ago principally as a us eful lab orat ory apparatus

and was chi efly us ed t o i llus trate the principl e s o f ele c tro

magne t i c indu ct i on . T o - day i t has be c ome an art i cle of ne

ces s i ty t o thousands o f pe opl e,and the indus try o f manu

factur i ng induct i on co ils has be c ome a very important one .

Among the us es t o whi ch induct i on co i ls are put,may be

ment i one d the wi rele s s te legraph,the exc itat i on of X- ray

tubes,igni t ing the eXpl os i ve mixture of gas and gas ol ine

in the internal c ombus t i on engines of var i ous types,ele ct ri c

gas li ght ing,e t c .

In Sp i t e o f the ei tens i ve us e of the induct i on

co i l and the troubl e ari s ing fr om ignorance in the handling

of su ch c o i ls,very l i t tle has been wr i tt en regarding i ts

princ ipl e s and Ope rat i on fr om an engineering and prac t i cal

po int of Vi ew . Of c ourse,all text b o oks on phys i c s give an

el ementary explanat i on of i t s act i on,and a few highly mathe

me t i cal art i cle s,wr i tten more from the s tand p o int of the

mathemat i c ian than from that of the engineer,have be en

publ i she d . The practi cal enginee r having ordinary mathemat i c ~

al knowledge,finds very l i t tle informat i on on the sub je ct

and hi s knowledge i s l imi t ed t o the usual text bo oks . Even

thi s meager informat i on i s s o s cattered that i t i s qui te b e

yond the reach o f pract i cal engineers . The att empt of thi s

the s i s wi ll be t o pre s ent the who l e subje ct in a l ogi cal

manner and devel op s ome e quat i ons from fundamental princ iple s

t o be of us e t o the every day enginee r .

I I Phys i cal The ory and Princ iple s of Ope rat i on

When we examine the devel opment o f the igni t i on

c o i l,we find that the evolut i on foll owed the general law

" that improvement s in exper imental ins truments advance al ong

de fini te l ines by a pro ce s s o f evolut i on in whi ch rudimentary

forms are succ ess ively replaced by more and more c omple t ely

devel ope d machine s ,

" wes t pe ople who are interes te d in the

induct i on co il think that Faraday made the di s c ove ry o f

ele ctr o - magne t i c induc t i on wi th hi s rude ring trans former,

but the fact is that Jos eph Henry,a young teache r in the

Albany Academy,was an independent di s c overe r o f el e ctro

magne t i c induc t i on . This i s pr qven by an ac count of the

manne r in whi ch he had independently performed a s imi lar

expe riment in the previ ous autumn before re ce iving an account

o f Faraday 's work? "ut Wi lli am S turge on,an eminent Engli sh

phys i c i s t,was the firs t pe rs on t o c ons truc t a co il whi ch

was pract i cally the same in the gene ral form as the induc t i on

c o il o f t o ~ day . The next great improvement was that of i n

s erting a condens er acro s s the "break" o f the primary c i r

cui t of the c oi l . Thi s was rendere d by Fi z eau,

a French

phys i c i s t?’

Thi s appl i cati on of the c ondens e r in the primary

circui t made pos s ible the universal us e of the induct i on

c oil . The mo de rn i gni t i on c o il i s e s s ent i ally the s ame as

the Rhumkorff c o il or inducti on c o il . It c ons i s ts o f two

c oi ls , primary and s e c ondary,and a central i ron c ore . The

latt er i s usually c omp os ed of a bundl e of s oft i r on wires

(insulate d from each othe r by shel lac ). The primary c o il

whi ch i s wound ar ound thi s core i s made o f a few turns of

thi ck insulated c oppe r wi re ; the se c ondary co il whi ch

surrounds the pr imary c o il i s made o f very fine,well i n

sul ated wire of cons i de rable length having a large number

of turns .P

Fig . I repres ents

the pi cture o f such a c o il

showing the de tai ls . The cur

rent fl owing from the bat te ry

" through the b inding s crew A,

the c ontact s crew b and the

hammer h,ente rs the primary co i l ,

where i t acts induct ively on thePhy

1

s e c ondary c oil . Having trave rs ed the primary c o i l i t c ome s

out at f,and re turns t o the battery through the b inding

s crew C . When the current fl ows through the primary co il i t

magnet i s e s the i ron c ore whi ch attracts the i r on hammer or

vibrat or h,thus breaking the primary c i rcui t at c . Thi s

int errupt i on of the primary c i rcui t caus e s the magne t i c flux

t o d i e away . The hammer i s then releas ed thus c omple t ing

the e le c tri c ci rcui t,and the current fl ows again . Thi s i s

repeate d at a great fre quency Whi ch depends on the inert i a

of the hamme r and the c ons tants o f the c oi l . The repeate d

appearance and di s appearance of the current in the primary

co il induce s an al te rnat ing ele ctro - mot ive force of h igh voltage

in the s e c ondary co il . The s trength o f this el e ctr o - mot ive

force dep ends on the rati o o f the number of turns in the

se c ondary c o il t o that of the primary co i l . When the el e ctro

mo t ive force i s large enough t o overcome the res i s t ance be

tween the ne edle po ints at p,a S park pas s e s .

T o make the break more sudden and t o s et up an

el e ctr i cal os c i llat i on in the c ircu i t it i s usual t o put a

condens er a cros s the "break p oints " of the primary ci r cuit .

This c ondens er i s thus s o c onne c te d that i t doe s not a ffe ct

the current rush in making but only in breaking . Owing t o

the magne t i c induct i on in the co il,the current in the primary

co i l cannot ri s e suddenly t o i ts maximum s t rength . It foll ows

the law E77 0— 6

where i primary current in ampere s

E impre s s e d e le ctro—mot i ve force in vo lts

res i s tance o f primary c oil in ohmsr

L a s el f induct i on in henr ie s

6

t t ime in s e conds

The current there fore re quire s an appre ci able fract i on of a

s e c ond t o fl ow through the primary c oi l containing inductance

and re s i s tance only . C onse quently at the make o f the pr imary

ci rcui t the rat e of change o f flux i s relat ively sl ow . In the

cas e of the break,howe ver

,the pr imary ci rcui t c ontains i n

ductanc e, res i s tance and capaci ty

,as a c ons e quence the rate

of change o f flux in the case o f bre ak is greater than in the

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at the moment o f "break" as i f i t we re a shunt ci rcui t of

negat i ve s el f- induct i on,only wi th thi s di fference - that

ins t ead of di s s ipating energy l ike a c ircui t of res i s tance

and induc t i on i t re turns t o the primary ci rcui t in the form

of a revers e d current,and increas e s the t o tal change o f i n

du ct i on through the s e c ondary

F "NDAMENTAL EQ"ATIONS IN TRANS IENTS .fio ’ fl CD ' b - ‘w w

Th e induct ion c o il

str ipp ed of al l me chani cal

d etail may‘

b e r epre sent ed

d iagrammat i cally by F i g . 2 .

E i s the con stant e l ec tr omo

t ive forc e impr e s s ed,L and

Ffiaar are the s el f- induct i on

an d re s i stanc e of the pr imary co il . C s a cap ac ity conne c ted

acro s s the switch or int errup ter S e i s th e el e c tromo tive fo r c e

of the c ondenser . Al s o,Lii s the s elf- indu c t i on of the se c ondary

co il, and q i s the sum of the re s i stanc e s

, r; o f the c o il, and

n'

o f the sp ark g ap ,and M i s e qual t o the mutual induction of the

two c o il s,p r imary and s econdary .

F rom the fundamental l aw that in a clo s ed c i r cuit the

algebra i c sum of the ele c tromo t ive for c e s i s e qual to z ero,we

find at the instant aft er op ening the swi t ch 8,that the fol

l owing e quat i ons ar e t rue .

— E LI

g o

._o

eya a z/l bfld

'

3 ¢n d 4 a n d, fir cu e

79 77

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N"MERICAL EXAMPLES

To i llus trate the us e of the equati oa rev i ous l y de

duc ed the cons tants of an induct i on co il were de termine d for

numeri cal appli cat i on . The re s i s tance s of b oth the primary

and the s e c ondary c o i l s were de termine d by means o f a r e

s i s tence bri dge whi ch i s fami li ar t o every ele ctri cal engine er .

The me thod us e d in measuring the inductance was the well known

impedance method . Each o f the primary and the s e condary co i ls

was c onne ct ed as shown in Fig . 4. An al te rnating e le ctro - mot i ve

force was impre s se d on the ci rcui t .

The t o tal current in the c o i l and the

vol tme ter was measure d in each cas e

and this current was later corre c te d

by deduc ting the current taken by the

vol tme ter . The current fl owing through

the vol tmete r was de te rmined by the

e quat i on ]; -§L where c i s the voltme te r re s i s tance,and E i s

V

the impres s ed el e c tro - mo tive force . The fre quency o f the ci rcui t

was als o de termine d by a fre quency me te r . The impedance of the

c o il Z 5 there fore the reactance,x “2

2R2 where R

the res i s tance of the c oil,the s elf- induct i on L where

f a the fre quency of the ele c tro - mo t i ve force .

The unknown capaci ty was de termine d by c ompar ing the

de fle ct i on caus ed by i t s full charge in a galvanome te r wi th the

defl e ct i on caus ed by a known capac i ty charge d wi th same vol tage

in the same galvanome te r . The conne ct i ons are shown in F i g . 5 ,

G i s a galvanome ter . The unknown capac i ty was charge d by the

bat te ry and then di s charged int o the galvanome ter c ircui t and the

defl e c t i on no te d . Then the s tandard capac i ty of a known value

was conne cte d in place of CX

and the de

fle cti on was no ted again . Then

08

ds

where d i s the de fle ct i on o f the

galvanome ter and Cthe capac i ty .

The foll owing cons tants were de te rmined

the primary res i s tance r . 1 696 ohms

the primary reactance x . 61 2 ohms

th s e condary re s i s tance r1

3 0 60 ohms

the s e c ondary reactance x1

6580 ohms

capa c i ty C . 1 22 mi cro farads

the primary inductance L . 0 0 1 7 henry

the s e condary induc tance LI

henri es

d the mutual inductance M AJTTSEETE

. l 495 (as sumed)

General Case .

Subs t i tut ing the s e c ons tants o f the c oi l under i h

ves i tgati on ,previ ously determined

,in the s olut i on o f the

general di fferent i al e quat i on 8 and the auxi li ary e quat i on 9 .

’7 5 X o /6 75'

0 0 0 / 7 x zz o g g' 0 0 74 5

K / O—C

x 0 0 7 9 5

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I f we refer t o the s impl er cas e,when r

1r c c

,

we find fr cme quat i on 1 7,the pe ri od o f os ci llati on equal t o

2. In a part i cular c oil the terms

L,L1 ,

and M remain cons tant . The deduct i on i s that the

peri od of o s ci llat i on for the c oi l i s di re c tly pr op ort i onal

t o the s quare - ro ot of the capac i ty . The refore the fre quency

i s invers ely pr op ort i onal to the s quare - ro ot o f the capaci ty .

Hence smaller the capac i ty,the larger the fre quency .

The peri od of the me chani cal os ci llat i on o f the

vibrat or of the inducti on c o il i s much large r than the peri od

of the e le c tri cal os ci llat i on . As s o on as the hammer i s attract

ed,the primary circu i t i s broken and the e le ctri cal os ci lla

t i on,previ ously de s cr ibe d; s tarts and goe s on for s ome t ime

be fore the hammer c ompl ete s the c i r cui t again . I t is a phys

i cal imp os s ib i l i ty for a me chan i cal vibrat or t o have the s ame

pe ri od o f os c i ll ati on as that of an el e c tri cal c i rcuit o f thi s

nature,there fore a s erie s of revers al s o f magne t i c l ine s takes

place in the c oi l and s everal S parks pas s be twe en the te rminals

o f the s e condary coi l during one break and make of the primary

c ircui t

I t mi ght als o happen that when the current di es down

in the a ct o f charging the condens er,for the fi rs t t ime

,the

hammer i s releas ed . "ut before i t goe s a reas onable di s tance

in the S pace be tween the at tracte d p os i t i on of the hamme r and

the contact s crew,the revers ed current from the condense r

magne t i s es the i r on core i n the c o i l . Thi s magnet i s e d i r on

iron core,though of reverse d p olari ty

,reattrac ts the hammer

t o a ce rtain extent and dampens or hinders i t s o s c illat i on

and ke eps i t l onger in the S pace . The reverse d current dying

down charge s the c ondens e r and the hamme r i s releas ed again

and s o on .

Thi s pro ce ss c ont inually repeate d keeps the hammer

in space whi le S parks pas s be tween the te rminals o f the s e c

ondary c oi l,unt il the s trength of the current be c ome s s o

small that i t cannot hol d the hammer any l onger in space .

Then the hammer rec ompl etes the primary c i rcui t and s ome more

current fl ows through i t,at the s ame time short - c i rcu i t ing

the c ondens er . Thi s phenomenon i s p os s ibl e be cause the

fre quency o f thi s o s ci llat i on i s s o great . Thus what happens

i s almos t e quival ent t o holding the hamme r in Space . Thi s

i s not no t i ceable when the frequency i s great ."ut by i n

creas ing the capaci ty,the frequency be c ome s smalle r and i t

i s no ti ce d that the hammer be c ome s s luggi sh . The act i on,

explaine d above,caus e s i t as i s evi dent from the di s cussi on .

The mos t inte res ting feature o f thi s phenomenon i s

that the pr imary c oil take s a fresh charge of energy only when

the os c illat ion cannot c ont inue whil e working in uni s on wi th

the o ther me chani cal fact ors involve d in the Op erat i on .

The te rm . 1 752 x 3- 5 1 5 t C o s (1 3 80 0 0 t )

of equat i on 3 1 repre s ents the amount o f the c ondens er charge

whi ch caus e s the os ci llat i on and . 0 0 0 046 s e c ond

i s i ts pe ri od . It i s al s o intere s t ing t o note that the peri od

- 5 /5 t ~ 6 s t

be ing . 0 0 0 046 of a s e cond,the expo nent i al terms 6 E

de c rease ve ry Sl owly and the refore the os c i llat i ons prac t i cal

l y re tain thei r in i t ial s t rength for a large number of re

versals o f magne ti c l ine s .

All thi s i s true when i deal c ondi t i ons are ob tain

able,but such c ondi t i ons do not exi s t in pract i ce . The break

of the primary current i s always ac c ompanie d by a S park even

when a c ondens er i s c onne cted acros s the "break po ints " .

Thus a part of the energy i s l os t . The os c illat i on

there fo re s tarts wi th a small er value o f q and i and di es

down fas ter than the theore t i cal curve s .

Almos t all igni t i on c oi ls,whi ch are s een in the

marke t t o day,are di fferent modi fi cati ons o f the induct i on

c oi l . In the i gni t i on of the expl os ive mixtures of gas engine s,

the batte ry ci rcui t i s c onne cte d t o a me chani cal devi c e mount

ed on the engine support . Fi g . 8 Shows the S imple s t arrange

ment su ch a devi ce . In the c ommerci al type s a large vari e ty

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t o permi t S parks t o pas s be tween the terminal s of the s e c ond

co il .

Re ferences Quo ted .

Sull ivan 's Ameri can Jornal of S ci ence July 1 83 2

C omptes Rendus Vel ume XXXVI p . 41 8

Alternating Current Trans former Fleming .

"ibl i ography

Internal C ombus ti on Engines H . E . Wimper i s

Electri c Igni t i on for Gas Eng ine J . A . Newman

Induct i on C0 1 1 ac t i on of condens er in Rhunkorff‘s

C o il Ele c tri ci an,L ondon

,November 1 1

,1 91 0

S e c ondary current o f the induc t i on c o i 1 ,~ os c i ll ograph

by Clyde Snock Journal Frankl in Ins t i tut e ,Oc t . 1 90 7

Induc ti on C oi ls ". F . "ailey Ele ctr i cal World

April 14 and 21,1 91 0

The Des i gn of Induc ti dn C oi ls W. M; O . Eddy

Ele ctri cal World Dec . 26 ,1 90 6 ; Jan . 5

,Feb . 2 ,

1 90 7

Induct i on C oi l in Pract i cal Work Lewi s Wright

The Des ign and Operat i on o f Spark C o i ls F . W . Springer

Ele ctri cal World,De c embe r 14

,1 90 7

Heat ing Effe ct of the Ele ctr i c Spark H . A . Perkins

Ele c tri cal World,March 24

, 1 90 6

The S c i ence of Jump Spark C oi l J . A . Will i ams Gas

Engine,July 1 90 9

Ele c tri c Igni t i on Jones