Investigation by:

G .ii1.Newell. R.G.H.Slaughter.

Written by:

G.F.NevJoll.

IGNITION INTERFERENCE AT FRE0UEl'ifCIES - Bg7l!Ll-oQ iEZ§: - l4

CONlnDENTIAL

Research Department October 1953

Fig.Nos. 1 - 7 '

IGNITION INTERFERENCE AT FRES.F~}.'J"OIES BELOW 100 Mc/a . . , '

The mechanism producing the characteristic train of impulses in the sparldng plug circuit of a motor car ignition sys tern is discussecl, and the theoretical current waveform is derived for a simplified equivalent circuit. Measurements of the frequency spectrum are described and found to be in reasonable agreement wi th the theoretical analysis. The field strength of the interference is calculated, making simplif,ying assumptions; it is found to rise linear~ ydth frequency up to a peak, and then to remain substantially constant, at a slightly lower level, at higher frequencies. The peak occurs at a frequency of between 20 MC/s and 60 MC/s depending on the arrangement of' the ignition system. Means of reducing the level of interference are suggested.'

The results obtained by other workers are not inconsistent vdth the theory advanced in this report.

1. Ji.llr.9Slf!.~~

lolany workers l -6 have contributed to the present knowledge of electrical interference produced by motor ~ar ignition systems. Measurements are um;tally made by tuning a specially ca}.ibrated receiver and aerial system qver the frequency range to be investigated, and measuring the amplitude of the intorference at the output of a receiver of constant bandvr.i.dth. These measurements show that, at a fixed distance from a given vehicle, the field strength of the interferenco is approximate~ constant in terms of peak volts ,Per kC/s of receiver bandvvidth, in thc frequencJ range from 40 Mc/s to 600 MC/s. The interference appears to decrease at lovrer frequencies and is relativeJs' unimpor~'1Jltin' the medium and long wave bands. Li ttle work covering the band below 40 MO/S

'c.ppoars to havo been published. The purpose of this investigation was to. attGmpt to fill in this gap, and to explain the mechanism

-2 ..

which accounts for the variation of the interference with frequency below 100 MC/s.

The interference produced on a television screen, each time a motor oar cylinder firos, takos thoform of a train of bri::;ht spots (Fig. 1 ), the duration dopending on the condi tion of' the plugs and the pressure inside the cylinder. The time interval 'bet;;yoen successive spots is fair~ Uluform, but depends to some exte~t upon the plug gap ro1d the cylinder pressure. The first spot in the train is alwa.ys brighter (or larger) than the succeeding spots, which are thc.mselves reasonablY unifon~. The presence of a calTier introduces vario.tions of brilliance from one spot to another in the train; this is due to the unrelated phase of the individual impulses and the carrier .If the amplitude of the interference relative to the c[J.tTier is'smo.ll, tpere will be equal numbers of spots lightor and darker than the nonnal screen brilliance.

2. .~r:~e! outline of prev19}lsly publi.sqod work

In March 1939, Daniel1 pUblished measurements of the field strength of interference rncliated from a largo nurnber of motor cars. The field strength at ·45 HC/s V/nS found to bE approxiInately 15 db hir;her than at 13 MC/s.

In Septc1)lber 1911-0, George2 published measurements of the field strength of interference and stated that this ~-{as substantially constant between 40 l'Ie/s amI h'50 MO/s.

7. .

In October 19l1-6, Eaglesficlcr published a theory in 'which he considered the spark current as resulting from a constant voltage applied to a swi teh in series VIi th AA inductance; the fie Id . strength V>Tas calCulD.ted assurning this current flowed in the loop fOIned by the ignition system. In other,words, the radiated field is a short pulse, which has ,a frequency spectrum of uniform ompli tilde. Eaglesfield concluded that thissirople picture did not explain the characteristic train of pulses obtained with a Car ignition system. These were ascribed to "osci~lations in the sparking ooil or magneto ll. Ina later article Eaglesfield considered a more complicate,]. equivalent circuit for the ignition system; ho concluded that t.he field strength fell off rapidly as the frequency was reduced but Was SUbstantially consto.nt between 40 and 650 HC/s. The production of' ntrain of pUlses.remained unexplained.

4 ' In January 1949, Pressey and Ashwell published measurements of

the field strength of interference between 40 and 650 MC/s; this was

.... 3-

found, to be sensibly tmiform over' the frequency band.

In August 1949, Nethercot5 published a theory of ignition interference from which he deduced that the spark current consists of a damped oscillation, of frequency between 30 and 50 MC/s, modulated by a savrtooth waveform having a repetition frequency of some .300 Me/s, associated with sucoessive reflections in the iGnition system.

In the next section a now approach to the problem is described in an attempt to explain the way in which the characli:;eristic train of pulses is produced, and to calCUlate the resulting spectrum.

An ignition systerrl can be approximately represented by the circui t of Fig.2( a). The condenser Cl represents the fixod oapaci ty across the contact breaker and includos tho self capacity of the pr~ary vvinding. of the igni tion coi~. Rl . an~ Ll e.re the series res~stance 8nd ~nductance of the prUi1D.I'y w~nJ.~ng. R2, L2, and O2 are the secon~~y resistance, inductance and self capacity respectively. Land R represent the series indlwtance and resistance of the.3sP8I'kirlg plug lea(1s, "here R~ includes the series resist8l1ce of C2} this resist8l1ce is 01" importance when O2 is discmrginG through the sparking plug circuit.

Initially the contact breaker is closed, pennitting a direct current to flow from the battery through the primary circuit. Lct us assume that this current has existed for'a suffiCiently long time to have become substantially constant at a value determined by the battery voltago and tho primary circuit resistance. The contact breaker now opens and the resulting change in the pr:i.ma.ry current produces a voltase across 02~ ,

If the sparking plug were disconnected, the voltage across O2 woul(::' consist of two superimposed damped osoillations with frequencies and decrements determined by the constants cf the primary and secondary circuits Vi"i. th their ooupling parameters. An oscillosoope photograph of' this waveform is shovm in Fig • .3( a). If, however, the sparking plui3 circuit is connected, the voltage across -0 2 increases until it is sufficien'~ to break down the

spark gap, with. the distributor gap in series; 02 is then rapidly discharged. It is this dischargo current in whioh we are vi tally interested as it ignites the cylinder gas.Qs and is the cause of the interferenoe.

The condenser 02 centinuestodisoharge until the spark is quenched; this occur's when the discharge ·current has fallen to about six milliamps. The discharge 01 02 occurr::: in a very shor·~ time, but the secondary current continues to flow, thus. recharging 02 after thu spark is quenched. The gap breaks down El. second time at a lower voltage due to residual ionisation of the gap, and theoycle continues until the current floWing in the secondary oircuit is insuffioient to charge 02 up to the breakdovmvoltage. It is also possible that the build up of pressure in the cylinder when the gas ignites may be sufficiently rapid to terminate the train of sparks, The cJ.uration of the train is ce:tainly vexiablc even with the so.mO plug and circuit in any onc engine.. Fig.l. shows an extreme instance of this variation, the interference patterns resultinD from tvvo isolated sparks from the smue car isni t:i..on sys l{,'l1l.

Fig.4 is a photograph of the sparkinG pluS current vravefonn due to a single spark rocorcled on aTl. oscilloscope having a bandwidth of 5 MC/s. It consists of a series of dDmped oscillations; the first impulso is larger .than the following ones, which are fairly uniform in Dmpli tude. The resulting effect vfOuld be o.s shovm in the lower photograph of Fig.l. Those impulses are . lengthened due to the lL~ited b,illdvddth of the oscilloscopo, bu~ not to a greater extent .than in a teleVision. receiver. This report is ooncerned mainly 'wi th the effeot of interf'erence on a television receiver; in this Case the important pr.1.I'ameter is the peak amplitude of the first impulse.

3.2 The spctE,k current

. The circuit of Fig.2(b) represents the sparking plug and distributor gaps, each wi th its shunt capacity, connected to the secondary of the ignition circuit. AS the ourrent flovving in the ignit~an co~l se?ondary ch~rges.up 02' the voltage across the ~yo gaps J.ll serJ.es WJ.ll be substantJ.ally, equal to that across 02; the total vcl tage will be dis tributcd unequ~lly because the capacity across the distributor is only about 2 ~~F whilst the oapacity across the sparld.:ag plug gap, including ,that due to the ignition lead, is approximately 9 ~IiF. The leakage resistances across " the t\vo gaps are usually extrE-'mely high under rtonnal oonditions, but again the resistance across "th9sparking plug gap is usually lower than that across tho distributor gap.

As a result of those inequalities the greater part of the vol tage v,rill be developed across the distributor gap. ltIeasurements shovf that the latt~r ,-rill break dovm when the voltage is about 2.5 kV. When the vel tago across the "bifO gaps in sorios roaches about 3.0 kV the distributor !3ap breaks dO'hi1l and a surge of current flows uXltil the voltage across the sparking plug gn.p is sufficient to spark over; 02 then discharges through tho tyvo gaps in series with L"Z and R'S" 'This dischLtI'!3o current consis'Gs of alovr frequency cOInpom?nt chEt1'e;ing up the capacities across the tvvo gaps and in additi:,)ll two short duration surges. One of those, occurs when the distributor g[tP breaks dowYl, and the other a few microseconds later ~fhon the sparking plug gap breaks dovm. The time interval betwoen these tvYu surges 'will depcmd on the voltage necessary to break dovVTl the sparking plug gap, which in turn depend.s on the gap length and the cylinder pressure, The wavefonn of the two Burges will be of a similar form and Can be calculated by considoring a unit step voltage applied to' 0. series R Land ° circuit. The current in such conditions is given by

E ...a.t i ' = - e sin gt. Lg

whore E is the voltage at the instant of the gap break-dovvil,

2 1 Wo = Le' (i.e. Wo is the reson[',nt frequenoy of the circuit)

~'he spoct!'lundensity in amperes per radian of angular velocity is given by :

f(w) __________ ~--~E~~_

If the spectrum is measurocl by a receiver having a bandvvidth

(1)

(2)

B cyclos pGr second and a linear phase/frequeney oharacteristic, the pepl<: amplitude of the current would be measured as:

I (ponk)= ~---- ~.,_,._~.2--mIi, ___ _ ;---2----2" 2 -- 2 2

L.; (w ... Wo) + 4W CL

amps (3)

-6-

A quasi-peak indicator, such as the picture tube of a television receiver, or a meter having a sufficiently rapid response, 'would give an indication proportional to tbis peak value. A slower moter would .give a response determinocl both by the ini ti8.l peak and by the subsequent peaks, vvhich are smaller.

At frequencies far from the rosonant frequerJ,cj W , the damping factor a. can be neglected, 0

For the case 'whcre w« W oquation (3) reduces to : o

I (ped(~) = 2EBC amps

Por w» w equation (3) becomes o

( 2EB I ponk) = ~

(4-)

(5)

The dotted curves in Figs. 5(£1.) - 5(0.) shoW' the theor6tical' spectra of the current surge for various cases. Fig,S(a) applios 'when only the oistributor gap breaks dmvn. E is 2.5 kV and the capaei ty 7 IJ.i·I.F (C . ancl 02 of l!'ig. 2 Cb) in sories). R WaS taken as 90 (2 for this illid all subsequent calculations in order to give the best fit "Yi th the experimental results, which vvill be described lntor. i?ig.5(b) applies when only the sparking plug (at atmospheric pressure) breaks dmID, E is 2.5 kV and the oD,pacity 40 IJ.IJ.F (°2 of Fig. 2(b). The timo interval between the ourrent surge produced by tho distributer gap breaking dmm and that produced by the sparking plug, is determined by the brea.;~downvoltages of the tvlO gaps; these depend upon gas pressure, temperature and residual ionisation, This results in a random vo.riation of the time interval, and consequently, a ranclom v£:.riation in the relative phD.se of simile.r frequency components of the two surges, If' vro use a sufficiently narrovV'-band receiver to investigate the spectrum produced by the ~1fO surges, and if the receiver is tlmed to a frequency 3reater than the reciprocal of the randomly vo.riable portion of the time interval, the pe.n.k current observed vdll be that resulting from the 2.!'i thrnetic addition of the currents <luo to each surge. This is not a serious lirni tation in our case bcc::tUiJC below this frequency the two COl:l.ponents have ve'rlJ different ruuvlitudes so that the resultant is not matcrio.lly

-7-

changed by the smo.ller component. peo.1c is therefore given by

The amplitude of the lnrgcst

( 2B I peak)-L3

E .lL._.

-- 2 2 . - 2 2 - w ) + 4-w CL op

'Where E d = din tributor gap breakdO'viJU vol to.g0

Ep = sparking plug breakdovm vol tago

c = p

sories oapaoity in circuit vn1cn the distributor gap breaks dovV11

series capacity in circuit when the sparking plug broo.ks dO\V11

L3 = inductanco of ignition circuit

2 ,1 wop = L*3·C·

p is the resonant frequency of the circuit when

the sparkin.S!: plu.g gap breaks do,\;vn

2 1 00 - - ; w is the resonant frenuency of the circuit when od - L

3C

d od ":I.

the distrioutor gap breaks do,m

If 00 «00 d and w «00 o op

(7)

If 00»00 d o.nd 00 »00 equo.tion ,(6) becor.1es: o op 2B(E + Ed )

I (peak) = - --~ 00 3 (8)

Tho dotted curve in Fig.5( c) shO\m ,!;he resulting theoretical spectrum whon the cylinder pressure is atmospherio; Fig.5(d) shows a Similar spectrum when the sparldng pluZ is operating in a cylinder at a prossure of 75 lb~/sq.in. (gauge); the valuo of E Was taken as 9.0 kV and Ed as 2.5 kV. P

This is the case ai' most practical interest, and shoW's that below 20 - 30 Mols the dominating component is the surge due to tho sparking plug breakclo'livn. A t higher f'requencies both components a.re important.

The full linos in Fi~?;s. 5(a) ... 5(d) show the measured spectra, I/hich will be cliscussed "later ,"

It is not possible precisely to calculate the field strength of the interference radiated from the ignition system due to the complicated effective circuit. Moreover, the results will depend on the screening of the car bonnet and radi~tor, which will be different n.t different frequencies. The simplest approximation is to regard tho ignition system as a loop aerial in free space carrying a unifonn current, and to remember the results will be approximate in the extreme.

The field strength due to a single spectral component, in the direction of' maximum radiation, at a distance large compared with the Wavelength, is :

l2C~2AI " E =,' ~~

Where E ::: fielcl strength in plane of "loop in volts/metre

A = area of loop in square metres

;... ::: vv[).velength in metres

D = distance from loop in metres

I = runpli tuc1e of the spectral component in amperes

The peak field strength for ~receiver bandwi(lth of B cycles per sec)nd centred on a frequency 21c vdll be:

(9)

+ V/m (10)

... 9-

At frequencies higher tl1EUl the resonant frequencies of the spark curren t circui t (w > > w 0. ond w > > W , 1.. e. fro quencio swell above 50 MC/s in our ca~e) we have: op

V/m (11)

This shovvs that above the resonant frequancies of~ho secondary circuit of the iGnition coil the radiated field isindepondent of frequency. For these high frequenoies Eaglesfieldfs simplified treatraont 3 should give the same result. In faot it does, with tv/a qualifioations. First, Eaglesfiold negleots the surge due to the breakdown of the distributor. Seoond, Eaglesfield takes into aCC01.mt refleotionfrom a perfectly oonducting ground plane, and hence defines A differontly: as a rosult the factor 2 in equation (11) does not apfear.

Fig.7(d) shows the theoretical field strength oalculated from equation (10) taking A = 0.037 sq.m..; this is the locp areo. used by Eaglesfield3 and is typioo.l of practical c[,ses. .

In order to obtain experimental results for compD..rison with the foregoing theory, a dummy cylinder blook was constructed and fitted with a standard ignition system; a photograph of the arrangement is shown in Fig.6. It consistB of a metal cylinder vd th six sparking plW!:s inserted, onu CL separate shoet metal box to approximate to AA

engine block. Perspex windows permit inspection of the plug contacts vvhon sparking occurs; provision is made for cormecting the cylinder to 2n air punp so that the effect of increasing the air pressure can bo studied. It was not possible to use an explosive mixture in the cylinder but useful, if limited, information was obtained using air alone. n1e cylinder was connected to the sheet steel 'block' by a 9 (2 resistor across which a volta~~e was produced by the spark current. This vol to.ge was fod via an attel1u["tor to the input of a receiver. The 9 (2 rosistor was found to have a series incluctnnco, when in the ignition Circuit, of' 0.014- IlH, and a shunt capacitance of 30 IlI-lF. Thc moclulus of tho impedance therefore increasecl from 9 (1 at 1.0 MC/s to approximately 15 n at 100 11c/s. This caused on error in the :measurod spoctrum of approxili1o.tely + 4- db at 100 MC/s and proportionally less at lovvor fre que no ies ; the neo.suromento were corrected accordin£:ly. The spectral Dlflpli tudes have beon expressed in torms of

... 10-

peak EUJlperes per kilocycle for compariBon with the results of othor workers.

The spectra were measurod by feeding the voltage wavefonn to the input of a receiver of :known bandwidth anrl adjusting an attenuator before the receiver input until the quasi-peak (i.e. the peak value neglecting occasional larger values) of the vvaveform at the recoi ver input Vias the same amplitude as that of a.l'.1oclulat;ed signal generutor con..l1octocl in parallel. This procedure was ropeatedat each test frequency betl'veen 1.0 Mcls and 100 Mc/s.

The full lino in ]!'ig. 5 ( a) shows the measurocl spectrum of the ourrent cirou.bting in the sparking pJ,u..g circuit when the air pressure in the cylinder was too high for the spark gap to break dm;m. This ctu~ront Was therefore clue to the distributor gap breaking down and oausing a trans~_ent ourrent to flow rounl.'l the circuit through the shunt capacity of the sparkins plug. Tho calculated spectrum for this condition is also shown clotted in Pig.5(a).

Pig.5(b) shows the oaloulated !.lnll measured speotr!.l of the current vd th one spl.1rldng plue connected directly to the ignition coil wi thout the clistributor gap i.n series. The air in the cylinder was at atmosphoric pressure for this test. The pO[Jlc at 85 Mcls appears in othormeasl~od spectra with the sparldng plUG in cirouit and the cause is not knovTn.

Figs.5(c) an(l 5(cl) show the speotre.. fol:' the complete ignition sys tem, 1. e. s parldng plug and dis tri bu tor gap - Yli th cylincler at .atl';].osphorie l?rosst~e arid 75 Ib·,/sq.in.(ga,uge) respeotively.

Fig.7(a) shows spectra measured with the cylinder at !.ltnospheric pressure al1d. also n.t 75 lbs/sq.in. (S8.U:3e). These spectra were measured wi ththe sp!.lrldng plugs ana dj.stributor gaps in series and "nth c. 3000 resistor insertecl in cn.oh plug lead at the oonnection to the plug. The vnlue of 300 (1 WIlS chosen because, vd tb the estimated 90 n existing as serios resistanoe intho oircuit, the total scriesresistance should bo sufficient to damp out the peak which occurs near 20 11C/8 in Fi2s.5(b), 5(c) and 5(d). From the previous theory it follows that, if the series rosistnnco in the pIu:::; circuit rr;-is ffi!.lcle equal to R :: 2 j 5;, a spectrum ydll be obtainerl haVing

a uniform runplitude up to near the frequency w = -1-j L302

... 11-

It vdll be seen from Fie.7(a) that the additional 3000 does in faot remove the peak at arotUlcl. 20 Ee/s.

Fig~7(b) shows the effect of conneoting 8.11. 8,500 n resistance in the lead fl.'om the ignition coil secondary to the distributor. For this case the equivalent circuit of Fig.2(b) is no longer valid. We can, however, regpxd. the rosistanco 3..3 effectively isolating the seeono.ary of the ignition coil; the cirGu~ t capacity ,viII therefore be the distributed capacity of the sparking plug load. It follows tha-l; the resonant frequency is increased, and, from equation (7) that the amplitude of components belOV'f the resonant frequency is reduced. Al though these trends may be soen by comparing Figs .5( c) ancl 5( d) vd th 7(b) they are by no means dofini te. In particular the (lii'forence betvveon the suppression ratios at the two differont cylinder pressures is·diffic'U.lt to explain.

Fig. 7( c) shovls tho effect of' inserting 15,000 0 resistors in each plug 10ac1 in addition to the 8,500 0 suppressor mentioned above. Horo vfe sce a considerable reduction in the cOLlponents over most of the frequency raneo considered; tho measured suppression ratio is 14. db at 1 Uc/s and .30 db at 10 MC/s. The corresponcling theoretical figures for the 15,000 n resistors alone are 12 db and 32 db respectively and the effect of the extra 8,500 0 should increase these ratios by 4. db. The self capacity of the resistance (approximately 1 flflF) will reduce the impedance to about 1,500 0 at 100 HC/s and therefore consid,orably reduce the suppression ratio at tho higher frequencies.

Figs.7('h) and 7(c) illustrate that the suppression obtained wi th D. single re~iis tnnce in series 'with the dis tri butor lead is unso,tisi'o.ctory; acldi tional resif?tanoes in series with the sparking plugs are necessary.

By inspection of the waveform of the voltagedevelopec1 across the ignition coil secondary winding, 'it Yl1'as possible to estimate the voltage at which the sparking plug gap broke clo\m a't various cylinder pressuros. The results with no distributor :i.n series aro given in Table 1.

/

Oylinder prolJsuro in lb./sq.in. gauGe

o

25

50

75

100

-12-

Broakdol'm ,vol tc,gc in kV for a sparJ.":l.ng plUG g::>.}! of' 0.023 in.

2.3

h·.5

6.8

11.2

Thq brcn.kclo\Tll vo1 tages at different air pressures were uoalJurod clireetly, llitb a sparking plug gn.p of 0.020 in •. amI n.ro 8ho'l'111 in Table 2.

--;~~:-dor-~~oss::·~:~·--l--~~~~~o\~~~a~e·i~ kV·~ lb./sq.in. gauge for a spn.rking pIu:::; gn.p

of 0.020 in.

o 2.5

10

20

-13-

The results in Table 2 are probably more relic.ble thnn those of Table 1, due to the method of' n08.suroracnt. ,The breakcloVJl1 vol tage of the distributor gap Vir,S sll:liln.rly measurod and found to

'be 2.5 kV. .

l'hc voltage acrQssa spnrking plug gap and the current flovring were measured irrrr:lediately before the spark quenched. The results are shavvn in Table 3.

TjillLE 3

r---;::-------- y.'Jitage - . -_ .. Cylinder pressure in across gf'.p Current in gap Ib .. /sq.in. s.GI:\uge at instant of at insti'.nt of

quench quench r------.------..

0 360 volts 5.5 nA

10 400 volts 5.8 mA

20 h.15 volts 6.1 mA

30 430 volts 6.5 nA

. -These mea;JtU'oT:lonts wore then repeated Y/ith the distributor

gap in series with the plugs;ap. The total vol ta.r;o across the ge.p . and the current flowing at the instc.nt of quench are shown in Table 4.

---.----- _ ..... --""1"""---------.------------. Cylinder pressure VoltaGe aoross the Current in gaps in Ib. ... /sq. in. gau;~e tvvo gaps at instant . at instDnt of

of quench quenoh

o 680 volts 6.1 m.A

10 710 volts 6.1 m.A

._----_._--------_..:...._------l!. further seric3f:) of moasurements of Gap voltage was made

whon varj.ous currents were flovling. The nparking plUZ~ gap 'Vv'[-tS

0.002 in. aLu was at amlospherio pressuro. The results are shovll1 in Table 5.

(.--..... - .... -.-.----'------II-.------.. ~--Currcn t in gap Voltage aoross Gap

------.-----.-.-.---------4--------------------------+ 70 ml!.

60 mA

50 mA.

30 rn1~

25 mL

20 m.A

12 mA

5 mA

260 volts

2(x\ vol ts

26,5 volts

26,5 volts

26,5 volts

26,5 volts

275 volts

275 volts

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

-15-

These measuremonts suggest that the gap, when struck, behaves as a oonstant voltage device and that the spark quenches at betv{cen 5 and 6 milliamps for a wide range of' gap lengths.

Tho photographs shown in l!'ig.l were obtained by means of a oircuit vrhich produood one ormoro televisi:Jn f:cames, as desired, cornmenoing from the frame synohronising pulse following the pressing of a push-button. This increased the brightness of the tube faoe fcr the exposure, while the oamera was left with the

,shutter open. The repetition frequenoy of the interferenoe oorreflponded to an engine speed suffioient to produoe one ignition stroke for eE'.ch frame of the pioture.

The osoilloscope images 'r{ero obtained using a time base oircui t and brightness ~ulse triggored by the voltage waveform under investigation. This was fed to the osoilloscope plates through u delQy network so that tho vT8.veform at the (lcflcetor plates coinoided wi th tho time base end brightness pulses. The time base ciroui t could only be triggered by one impulse oocurring subsequently to each operation of thc push-1utton svvi8ch. R55 record.ing film was used, developed for ten minutes at 65 F in developer D196. The oscilloscope tube was a type 005 which hus a green fluoresoenoe. The first [mode voltage wus 2.5 IcV and the finul anode voltage was 5.0 kV. The brillianoe was adjusted so that a five oentimetre trace repeated 100 times per seoond gave a reading of 25 foot­l~unberts at its centre using $1 S.E.I.photometer vdth a green filter in the eyepiece. The camera Was fitted yd th an f' /1.9 lens.

The general shape of the measured speotra agree reasonably well with the theoretical curves in Figs. 5(a) - 5(0.). The measuroment of the peak waveform on .':1n oscilloscope tube is open to possible error when the suocessi ve E'JUpli tudes are random. The estllfu~tion of quasi-peak values visually, despite the difficulty of maintaining the same criterion throughout tho measurements, wns thought to be suffioiently aoourate for an investigation of this nature. The minor Clisorepanoies P.t the higher frequenoies Hk'"IY be assooiated with the over-simplifioation of the equivalent cirouit of the ignition ooil.

-16-

6. .92r~!Ear:hso~2v:i.. ~_ J?}'ev~ousl'y.J.?~:£].isho~

Of the previously published analyses, only that of Nethercot5

deals quant it a ti voly '1Ivi th resonance effects in the secondary circuit.

Compn.red with the present treatment Nethercot negleots the effcct of seoondary cirouit clarnping, but i.noludes the effect of the distributed induot~Ulce and co.paci ty of the igni t:.OYl leads. Nethercot shows that 0. distributed circuit gi vcs rise to an additiorU1.1 component of approximately 300 MC/8 which modulates the spark current waveform; there is 0. corresponding peak in the ampli tudo of the speetral components near this frequency.

Hmvever, when attonuation consistent vd. th the value of' R used in Section 3.2 of thisroport is takon into I'.ccQunt, th:i.s additional poak in the spc:Jtrutl substantially (lisappeors; it appoF!.rs, therefore, tha"~ a 'lumped circuit' analysis is valid. This theoreti'2a! conclusion is, noroovor, in agroement vvith the measured results ' + which show no pronounced peaks in the spectruEl botv-,roen 100 and 500 HC/s.

ConSidering next the (wD.i1ablo measured results, those of Gcorgc 2 were r.l8,de using a recei vor bandvddth of 10 kC/s at a distance of 100 ft. . Table 6 shows both the horizontally and vertically polarised peak field strongc~hs for 50% and 90}& of the

. total of cars tested. The difference between the results for the two polarisations is nogligible in vicvv of the approximate nature of the measureMents. For comparison purposes the calculated field strengths are also shown for 10 kC/s bandwidth; those 'Vvera derivocl frora ]'ig.7(cl) for cylino.er pressures of atr!lospheric and 75 lb./sq.in. gauge. The calculated figur,esare within the measured values for the 50% and 90% figures. This suggests that

. "bhe screeninG effect of' the bonnet of the cor is not great; hOYiOvor, the information on this point is too moagre to c1:cavrany firn conclusion.

,. ...... ----,._ . Measurod field strength Oalculated field strength

in IlV /lnotl'e in IlV /metre -- --]lrequency Horizontal Vortical Atmospheric 75 Ibe/sq. in MO/s polarisation polarisation ·prossure ins. pressure

. _."'"

50% 90% 50% 90% cars cnrs cars cers ~----

JJ.() !:O 250 70 250 h.Q 90

60 25 100 50 140 50 90 ,

100 JJ.() 100 50 170 30 70

I - .. . --1-_ . . . .- - -Pressey and Ashwel14 give results for only one car, end these

are therefore of only limited use. Measu:.t;'oments were made with a reoeiver ~'lvin!3 8. bandwidth of 2.5 Mc/s, and at 8: distance of 30 ft.; both horizontally and verticD .. lly polarised components vvere measured. If the rosults are averaged and smoothed, the· field strength, corrected to correspond vii. th a bandv;idth of 10 kC/s, and to a distance of 100 ft. assuming an inverse distance relationship, increases from approxir:k'l.t·ely 10 IlV/m at 50 MC/s to 30 ~LV/rn at 100 MC/s; this trend vvi th frequency is not reflected in tho measurements by George.

It is in fact surprising t~'l.t the measured and theoretical results are in such relatively 13000. agreement, remembering thD .. t the field strencth of the interference varies considerably from Cnr to car, that soreoninz of the ignition system has not beon taken into account, and that the calCUlation of tho .radiated field is extremely approximate.

..J.8-

7. Methods of reclucing the interference -- .... _ ... ""_.--... _-_._._----~'Yrom l~igs. 7(0.) and 7(b) it will be seon that with the experimental

ignition system layout, the use of the 8.5 k (1 suppressor resistor in the cJ.is'trlbutor 100.0. only does not appreciably reduce the interference level. In fact the 300· (1 resistors in each plug l;ad were at least ['.8 effective cvermuch of the frequency rDnge. Fig. 7( c) shows' that the use of 15 k(1 suppressors in oo.ch plug load, in addition to the suppressor in the distributor lead, CnUses a Qonsidernble reduction in the r:unplitude of the spectr1.lt1 over most of the frequency range.

At all frequencies tho int8rferonco radiated is proportional to the nrea enelosed by the ignition leads and the engine block surface, so that reduction of this area should reduco the interferenee level. The radiated interference is proportional to the veltage required to break dovm the spr1.rking plug ['.Bel clistributor gaps; a reduction of this voltaGe, iJy reclueing the 8C'..ps as far as is possible without cleterioratir.g the engine perforL1D.nce, should therofore reduce the interferenc3. At frequencies below' 20 Mc/s the interferenee is proportionnl to the cnpacity across the seeondary winding of the ic;nition coil so that the reduction of this capacity vvould result in some inprovernent. At fr0quoncies r,bove about 30 Mcls thc interference is inVersely propol'ti;)nC1l to tho inductance of the igni tion le[:)'c!B so thr.t increasod distributed inductance in thesc leads shoulcl help so long 2.S the loop area L.>. not increased.

Tho investigation did not include the interference rndiated from the primc.ry circu:i t of the igni ti',n coil and its associatecl leads. If apprecir:'"ble reduction of the re.c1iatic)n from the sooondnry circuit vmre eff'o ctec1, the radia ti ~m from the primnry circuit would be relatively more imporhu1t. It apper.rs from inspection that the re.te of increase of the contact breaker gap is not usuP.lly sufficicntly rapid to prevcnt the increasingprimtlry voltr.ge, corisequent upon the contact bref:~ker oponing, brco.ldng c.cross the gap. Fig.3(b) shows thc initial portion of thc voltage ViTavefonn devolopecl aoross thc priTIk~y coil, after the oontact brec.kor has opened; for this measureT;lent the soooncl8.ry circuit Was not connected to the sparking plugs. It "\'1ill be so en that the primc.ry voltage is 'bivice reduced to a sr;lall value as a re suI t of arcing o.cross the contact breaker as the l[tttor opens.

-19-

Tho s:i.In.ple theory developed in this report appoCtrs to be in reasonable agreement vd th measurements both of the spectra and with the interferpnce levels published by other worker8. The predicted falling, level of radiatccl interference below 20 }r.~/s h'ls not been ()hecked by measurement, but is consistent: with the lack of trouble due to interference with medium wave rcccpt::'Jn.

Thero does not appear to be any simple means of reducing interference other than the cDnventi:mal resistance suppressors. The nuppression ratio obtained .. vi th a sinGle resistance in series .. vith the clistributor lead is not in general considered satisfactory; an additional resistance in seritJs with the sparking plug lead gave better results in the case described in this report. It is thought that resistances in series 'ivi th each plUS lead will also be Dore satisfactory in practical CtlSes, but insufficient work has been (lone to say whether this is necessarily the best arrangement.

Of the other remedies, reduction of the spark gap length is . prob~:tbly incompatible with good. engine performru1.cej reduction of

the shunt capacity of the iglli tion coil secondary is likely to be expensive 8nd to give only a limited improvement. Increasing the distributed inductlUlce of the ignition leads 'lid thout increasing the loop area enclosed by these leads is a possibility, but no attempt has been made to check this experimentally,

DANJEL L.H.

2. GEORGE R.W.

... 20-

"The Rac:linted Short-'wave Interf.erence from Automobile IGnition Systems." 'E.R.A. Report

, No. M/T 63, 1939.

tlFielc: strength of Motor Ignition Interference between 4-0 o.n(l If.50 HC/s". Proe. I.H.E~, September 194-0.

3. EAGLESFJELD C.C."j:"1otor Car IGnition Interference", Wireloss Engineer, October, 1946.

4. . PRESSEY B.G. and

ASHWEI.L G.B.

5. NET}mRCOT W.

"Radintion from Car I5l1i tion Systems", Wireless Engineer, January, 191+9.. .

"Car Isni tion Interference ll Wireless Engineer, August, 194-9.

6. EAGLESI!'JELD C.C."Car IGnition &t1iation", Wireless EnGineer, January, 1951.

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