Journal of the Institute of Petroleum, Vol. 26, No. 205

P. 1 0 1 /lfQ \ ^

J O U R N A L O F

THE INSTITUTE OF PETROLEUM

F O U N D E D 1 9 1 3 I N C O R P O R A T E D 19 1 4

Vol. 26 N O V E M B E R 1940 No. 205

C O N T E N T S

" . P A G E

T h e Clem ent-Pigneguy Knock Indicator. By D . M.

C lem ent and P. G. Pigneguy . . . .

Synthesis and Properties of M o n o -N o rm al-A lk y l-

benzenes. Part II. By T . Y. Ju, G. Shen, and C. E. W o o d ...

O bituary . . . . . . . .

Abstracts . . . . . . . .

Institute N otes . . . . . . . i-ii

P u b lish e d by T h e In s titu te of P e t r o l e u m . A d d re ss : c /o T h e U n iv e rs ity o f B irm in g h a m , E d g b a sto n ,

B irm in g h am . 15.

P r in te d in G re a t B rita in by R ic h ard C lay and C o m p an y , L td ., B ungay, S uffolk.

A l l rig h ts o f P u b lic a tio n o r T ra n sla tio n a re R e serv ed . P r i c e 7s. 6d.

T H E I N S T I T U T E O F P E T R O L E U M

C O U N C IL , 1 9 3 9 -4 0

PRESID EN T:

Prof. A . W . N ash, M .Sc.

A lfred C. Adam s Lf.-Col. S. J. M. Auld,

O.B.E., M .C., D.Sc.

Prof. J. S. S. Brame, C.B.E., F.I.C.

The Rt. Hon. Lord Cadm an, C .C .M .C ., D.Sc., F.R.S.

PAST-PRESID EN TS :

T. Dew hursf, A.R .C.S.

A . E. Dunstan, D.Sc., F.I.C.

Sir Thomas H. Holland, K .C .S.I., K.C.I.E., D.Sc., F.R.S.

J. K e w le y , M .A ., F.I.C.

V IC E -P R ES ID E N T S:

A sh le y C arter, A.M .I.M ech.E.

C. D ailey, M.I.E.E.

J. M cConnell Sanders, F.I.C.

F. B. Thole, D.Sc., F.I.C.

F. H. G arn e r, Ph.D., M .Sc., F.I.C.

MEMBERS O F C O U N C IL : G . H. Coxon

A . Frank Dabell, M.I.M ech.E.

E. A . Evans, M .I.A.E.

E. B. Evans, Ph.D., M .Sc., F.I.C.

W . E. G o o d a y , A .R.S.M ., D.I.C.

A . C. H artley, O.B.E., F.C.G .I.

Prof. V . C. Illing, M .A.

J. S. Jackson, B.Sc., F.I.C.

J. A . O rie l, M .C., M .A.

E. R. Redgrove, Ph.D., B.Sc.

C. A . P. Southwell, M .C., B.Sc.

H. C . Tett, B.Sc., D.I.C.

A . Beeby Thompson, O.B.E.

A . W a d e , D.Sc., A .R .C .S.

W . J. W ilson , F.I.C., A .C .C .I.

C. W . W o o d , F.I.C.

Arthur W . Eastlake, A.M .I.M ech.E., H onorary Secretary

H O N O R A R Y E D IT O R : Dr. A . E. Dunstan H O N O R A R Y A S S O C IA T E E D IT O R : Dr. F. H. G arner H O N O R A R Y T R E A S U R E R : The Rt. Hon. Lord Plender, C.B.E.

S E C R E T A R Y : S. J. A slb u ry , M .A.

________________________________________________________________________

Vo l. 26. N o . 205. No v e m b e r 1940.

T H E C L E M E N T - P I G N E G U Y K N O C K I N D I C A T O R * B y D . M.

an d P . G.

Sy n o p s is.

A lo n g -felt n eed in eng in e-fu el te s tin g lias boon a n in d ic a to r of k n o c k in te n s ity w h ich is u n a ffe c te d b y c o m b u s tio n p re ssu re a n d ono also in w hich th e a c c u ra te fu n c tio n in g does n o t re ly o n t h e in te rd e p e n d e n t s e ttin g o f its co m p o n e n t p a r ts .

T h e in d ic a to r d escrib e d in th is p a p e r is sh o w n to po ssess t h e d esired q u a litie s, w hich en ab les te s tin g to bo carrie d o u t o v er th o e n tire o c ta n e scale w ith o u t a lte ra tio n of th o original se ttin g . W hile th e c o n s tru c tio n m a y a p p e a r to be on sim ila r lines to th o A .S .T .M ./C .F .R . b o u n cin g -p in in d ic a to r, th e ch an g e in p rin cip le in v o lv ed is fu n d a m e n ta l.

T ho now in d ic a to r h a s so fa r o n ly b e en u se d in th o s ta n d a r d C .F .R . engine a n d also a h ig h -sp eed C .F .R . engine, b u t i t is fo lt t h a t th o p rin cip le c a n bo successfully a d a p te d fo r full-scale c ar a n d aero-engino te s tin g .

I

th e anti-knock ra tin g of fuels it is well known th a t th e C .F.R . engine is in considerable use and has shown itself reasonably reliable. I f criticism can be m ade ag ain st a n y p artic u la r p a r t of it, th a t p a r t is th e bouncing- pin, which is a source of direct error in th o ra tin g of fuels. T his ap p a ra tu s is th e m edium b y w hich th e knock in te n sity in th e engine is registered q u a n tita tiv e ly as a knock-m eter reading. As is known, th e m eth o d of fuel ra tin g is b y com paring tw o know n fuels ag ain st th e unknow n sam ple, t h e . com parison being effected b y th e use of th e knock-m eter readings.

Should these knock-m eter readings n o t be tru ly rep resen tativ e o f th e knock in te n sity occurring in th e engine on th e various fuels, th e n a n im m ediate error in ra tin g is m ade.

In v estig atio n has been carried o u t for some tim e p a s t on th e actu al physical phenom ena w hich ta k e place in th e S ta n d a rd A .S.T.M ./C .F.R . bouncing-pin. T he knock in d icato r described in th is article has been designed to overcome certain fau lts in h ere n t in th e operation of th e sta n d a rd a p p aratu s.

A b rief outline of th e sta n d a rd C .F.R . bouncing-pin a p p a ra tu s is as follows :—

A steel pin ab o u t 7 in. in length a n d £ in. d iam eter rests in a vertical position on a diaphragm , th e d iaphragm an d p in being su p p o rted in th e m ain body, w hich is screwed in to th e engine cylinder. On to p of th e m ain body is a n assem bly of tw o leaf-spring contacts a n d a bum per spring m ounted one above tho other. The lower leaf-spring co n tact rests on th e p in-top an d exerts sufficient pressure on it, to hold th e p in ag ain st th e d iaphragm during norm al com bustion of th e engine. Above th is lower leaf-spring co n tact sep ara ted b y a b o u t a 0-003-in. gap is th e u p p er leaf- spring c o n tac t arranged in a buffer system w ith a plunger an d spring sliding in a guide. This u pper leaf-spring and plunger m ay be ad ju sted to v a ry th e gap betw een th e two contacts, and th e y a rre st th e m otion of th e lower co n tact w ith o u t shock, w hen it is driven up by d etonation, so t h a t an im pulse of cu rren t flows in th e circuit th ro u g h th e contacts. T he

* P a p e r received 24t.h S e p te m b e r, 1940.

t “ Shell ” R efin in g & M a rk etin g Co., C e n tra l L a b o ra to rie s, L o n d o n . L L

490 CLEMENT AND PIGNEGUY :

tim e of co n ta ct is registered on th e knock-m eter as a m ean value of cu rren t flowing.

U nder d eto n atin g conditions th e steel bouncing-pin is driven off th e diaphragm b y th e im p act of d eto n atio n an d is b rought to rest by the re tard in g forces of th e lower leaf-spring and th e action o f closing th e contacts. T he p in h as th u s p a rtly sto red its energy o f upw ard m ovem ent in th e resilience of th e lower leaf-spring, an d th is m em ber retu rn s th e pin to th e diaphragm w ith such a velocity th a t it rebounds an d th e process is repeated. T hus for one knock im p act in th e engine cylinder, th e p in is se t in to a sta te of oscillation. T he frequency of th is oscillation is dependent on th e pressure on th e p in dow nw ard an d th e m ass of th e pin itself. W ith th e norm al setting o f th e sta n d a rd p in th e frequency is of approxim ately 800 cycles p er second. W ith a very lig h t lower leaf-spring pressure this v ib ratio n m ay continue th ro u g h o u t th e engine cycle u n til th e n e x t deto­

natio n occurs. U nder norm al leaf-spring pressure th e v ib ratio n dies aw ay well before th e com m encem ent of th e n e x t engine cycle.

I t is im p o rta n t to note now w h a t is occurring to th e diaphragm while th is v ib ratio n o f th e pin upon i t is tak in g place. I ts vertical deflection is altering according to th e pressure in th e cylinder. T hus, as th e knock im p act is being recorded b y th e vertical trav el of th e lower co n tact under th e action o f th e v ib ratin g pin, i t m ay well be th a t th e tim e of closure of th e contacts is due m ore to th e com bustion pressure in th e cylinder th a n to th e ra p id v ib ratio n o f th e p in set u p b y detonation. T he effect o f com­

bustion pressure being recorded as d e to n atio n is m ore troublesom e a t th e higher com pression ratio s w here th e deflection o f th e diaphragm due to higher com bustion pressures is m uch greater. F o r exam ple :—

I n recent experim ents i t was determ ined th a t w hen an engine was n o t d eto n atin g , a considerable deflection of th e diaphragm took place, due to com bustion pressure : th is w as found to be 0-0015 in. a t 4 : 1 compression ratio , 0-0025 in. a t 8 : 1 compression ratio , a n d th e diaphragm deflection p lo tte d ag ain st com pression ratio w as alm ost ex actly a stra ig h t line.

W ith this evidence i t is clear th a t, w hen th e engine is being ru n on an y n o n-detonating fuel, if th is diaphragm deflection curve rises above th e leaf-spring co n tact clearance, a reading on th e knock-m eter is recorded duo to com bustion pressure. (A d etailed account of th is experim ental w ork an d fund am en tal th e o ry underlying i t is given la ter in this paper (A ppendix A).)

I t was also determ ined th a t w ith a norm al pressure dow nw ards on th e p in by th e lower leaf-spring, th e pin rem ained in co n tact w ith th e diaphragm w hen no d eto n atio n was present.

F ig. 1 shows th e displacem ent of th e diaphragm , on which is super­

im posed th e m ovem ent o f th e lower leaf-spring contact, u nder th e action of th e v ib ratin g pin. I t w as evident th a t w hen a sta n d a rd bouncing-pin was set w ith a co n tact gap o f 0-003 in., p a rt of th e closure of th e gap was due to com bustion pressure, an d th e rest due to th e oscillation of th e pin and lower leaf-spring caused b y detonation. As expected, it was determ ined t h a t th e bouncing-pin contacts closed th ree to four tim es per engine cycle (see Fig. 2).

Owing to th e sm aller deflection of th e diaphragm shortly afte r com­

b ustion w hen th e gases begin to expand, the recording o f th e oscillation

P.

THE CLEMENT—I'IGNEGUY KNOCK INDICATOR, 4 9 1

on th e sta n d a rd bouncing-pin is m asked after th e first th ree or four vibrations.

T h u s th e sta n d a rd bouncing-pin records only a p ortion of th e v ibration or oscillation due to d etonation, a n d in the case of a heavier d eto n atio n

■ 0 0 4

co/^srAcr po/swr CL EA AArs/C .E*

/& £ S i/0 £ > Q £ C O N T A C T O F T H E

boc/l^c/ajg e/lw £>o//^srs- -H^Avy kmock. C O N T A C T P Q / f s / r

C t £ A / Z A £ S C £ .

T. 0 . 0 .

O E G K E E S C M /Si A S N A F T

Fig. 1.

th e p in relies on a wave of larger am plitude to increase th e tim e o f co n tact an d th u s increase th e knock-m eter readings.

I f th e sta n d a rd p in could be relied upon to record th e sam e num ber of vib ratio n s on all fuels— e.g., th e first th ree —w ith o u t a n y influence due to com bustion pressure, th e n th e knock-m eter readings would be com parative for th e degree o f deto n atio n in th e engine.

7TD.C.

D E O A E E S C A A f ^ / K S H A E r .

Fig. 2.

4 9 2 CLEMENT AND FIGNEGUY t

T h e foregoing criticism of th e sta n d a rd C .F.R . bouncing-pin m ay be sum m arized as follows :—

T h e knock-m etcr readings being proportional to th e d u ratio n o f co n tact o f th e points, ac tiv a te d by th e p in m ovem ent, are alw ays influenced by th e m ovem ent of th e diaphragm u n d er com bustion pressure. T he pin-gap selected is usually such th a t th e co n tact points close th ree to four tim es per engine cycle, due to th e v ib ratio n of th e pin, an d fu rth e r v ib ratio n s of th e pin are n o t recorded because of th e m ovem ent of th e diaphragm in a direction w hich increases th e pin-gap. T h u s th e knock-m eter records th e m ovem ent o f th e p in during th e first few oscillations of its to ta l v ib ratin g period.

I t has been observed t h a t w hen com paring tw o fuels th e ra te s o f pressure rise of which are different— for instance, a stra ig h t-ru n sam ple an d a benzene blend—while th e knock-m eter reading m ay be th e sam e, th e audible knock in te n sity is very different. T his m ay be explained as follows : when th e control gap is n o t large enough to p rev en t com bustion-pressure interference, th e tim e of closure of th e co n tact points is influenced b y th e shape o f th e com bustion diagram , a n d m ay be affected unequally on th e tw o fuels b y com bustion, depending on th e broadness o f th e peak of th e diagram , q u ite a p a rt from th e d eto n atio n wave accom panying it.

F rom th e foregoing analysis, advantageous changes in th e design o f the pin and recording circuit were m ade, and arc as follows :—

1. R ecording all pin m ovem ent relative to th e diaphragm .

2. M easuring by a n extrem ely sm all cu rren t th e tim e of bounce of th e p in o ff th e diaphragm , b y including th e p in an d diaphragm in th e recording circuit in place of th e leaf-spring assem bly.

A design for a n a p p a ra tu s em bodying th e desirable features given above was proposed, an d th e description o f th is a p p a ra tu s is as follows.

T he bouncing-pin, now referred to as th e pick-up, has been re-designed (see F ig. 14), a n d th e general description is as follows :—

A tu b u la r steel body (a) houses a sta n d a rd C .F.R . diaphragm (

) w hich is held by a sleeve (c) an d n u t (d) from th e inside of th e pick-up u n it. I n th is body a central core is su p p o rted on bakelite bushes (e) so th a t a sta te of high insulation exists betw een th e tw o p a rts. T he cen tral core consists of a brass guide (/) in w hich slides a steel p in (¡

), loaded by a coil-spring (h), th e pressure on which is controlled b y a thum b-screw (j) in th e to p end of th e brass guide. W hen fully assem bled, co n tact is m ade betw een p in and diaphragm .

T h e p in an d diaphragm com plete a circuit for a negative bias voltage on to th e grid of a therm ionic valve, which is in effect a n electronic relay.

T he cu rren t o u tp u t or anode side of th e valve is controlled b y th e negative

electric field inside th e valve. On th e rem oval of th is negative field, by

the breaking of th e circuit b y th e p in as i t bounces on th e diaphragm , a full

anode cu rren t will flow in th e o u tp u t circuit w hich includes th e sta n d a rd

C .F.R . knock-m eter. T he length of tim e during which th is o u tp u t cu rren t

flows is th e d u ra tio n o f th e absence of th e negative field, w hich in tu rn is

equal to th e tim e of break of th e circuit by th e oscillating pin.

K A / O C / < / O O P / ? . E .

CLEMENT—PION EG U Y KNOCK INDICATOR. CIRCUIT DIAGRAM— SUITABLE FOR U SE W ITH COIL OR MAGNETO IG N ITIO N C .F .R . EN G IN E.

THECLEMENT—PIGNEG0YKNOCKINDICATOR.

4 9 4 CLEMENT AND PIGNEGUY :

El e c t r i c a l Ci r c u i t De s i g n.

A d iagram of th e electrical circuit is shown in Fig. 3.

I t will be seen th a t tw o power o u tp u t te tro d e valves O sram K .T . 32 are connected in parallel a n d bo th function in exactly th e sam e m anner.

Tw o valves are used because of th e large im m ediate flow required u nder the conditions of sm all operating tim e, so th a t th e average c u rren t flowing is

sufficient to w ork th e knock-m eter. T he supply of cu rre n t for th e valvc- lieaters is ob tain ed from th e sta n d a rd C .F.R . generator, th e voltage being reduced from 110 V. to 26 V. b y tw o resistances in series w ith th e valve- heaters. One resistance is o f 100 ohm s an d th e o ther o f 45 ohms. The 45-ohm resistance is placed in th e outgoing or negative side o f th e valvc- heaters. Besides its function of reducing th e 110-V. supply, it serves as a source of negative bias for application to th e grid via th e p in an d diaphragm . T he m an n er in which th is is effected is as follows :—

T he valve-heater cu rren t flowing in th is 45-ohm resistance is 0-6 am pere,

/ O

G R / O y O L T S - Fig. 4.

THE CLEMENT—PIGNEGUY KNOCK INDICATOR. 4 9 5

th u s tho voltage drop along it is 27 volts. (From th e laiv E — R I — 45

0-6 = 27 volts.) R eferring to Fig. 3, position A of th e resistance is a t th e sam e p o ten tial as th e cathodes of th e valves, an d th u s, as th ere is a voltage d rop of 27 volts along th e resistance to B , th e position B is o f lower p o ten tial th a n A , or, in o th er words, it is negative w ith respect to th e cathodes by 27 volts. F rom th e grid v o lt/an o d e cu rre n t characteristic (Fig. 4) it can be seen t h a t th is 27 volts w hen applied to th e grid will reduce th e anode c u rre n t flowing to zero.

T his action is due to th e intense negative field inside th e valve, w hich stops th e passage of electrons across tho space betw een cathode an d anode.

T hus no cu rren t can flow in th e o u tp u t circuit u n til th is bias is com pletely rem oved, an d this occurs only w hen th e circuit is broken betw een p in an d d iaphragm . T o ensure a stable o u tp u t w hen th e above circuit is broken, th e cathode an d grid of th e valve are p erm an en tly connected b y a 1-megohm (one m illion ohms) re sistan ce; th is fixes tho w orking position of tho grid a t zero volts relativ e to th e cathode.

T h u s in th e sta te of re st o f th e a p p a ra tu s th e bias o f 27 volts is acting across th e 1-mcgohm resistance, causing a sm all c u rren t to flow in th e pin a n d d iaphragm circuit. T he a c tu al value of th is c u rren t is 0-027 milli- am pere.

w here

R — resistance I — current.

I — J qqq qqq am peres = 0-027 milliampere.

27

T his cu rren t is for all practical purposes zero. W hen th e circuit betw een p in and diaphragm is broken, th e bias on th e grid rises to zero, and this allows th e anode c u rren t to rise in stan tan eo u sly to its m axim um value.

T h u s th e v ib ratio n of th e p in previously referred to allows anode or o u tp u t cu rre n t to flow for a tim e w hich is precisely th e sam e in length an d n a tu re as th e vibration.

Th e Kn o c k-m e t e r Ci r c u i t.

I t now rem ains to m easure th e average o u tp u t b y applying i t to a suitable m eter. I t was required th a t th e sta n d a rd knock-m eter (w ithout h ea ter elem ent) be used, an d as th is in stru m e n t is a moving-coil m eter w ith no dam ping of an y sort, th e o u tp u t from th e valves has to be electrically dam ped to deliver a stead y D.C. cu rren t to th e m eter.

W ith reference to th e circuit Fig. 3c, i t will bo seen th a t th is dam ping consists o f a netw ork o f capacity an d inductance.

Tho effect o f inductance is to oppose an y change in tho value o f c u rren t flow ing; cap acity has th e effect o f storing current. T hus in th e arrange­

m e n t shown, assum ing th e c u rren t to be rising, in d uctance L I offers a large resistance to th e c u r r e n t; C l, which is a capacity of 2000 or 4000 u F , offers an alte rn a tiv e p a th , a n d th e cu rre n t flows in to i t an d is stored.

T he inductance L I, however, will n o t resist all th e rap id ly changing

496 CLEMENT AND PIGNEGUY :

current, and, -whatever cu rren t passes, i t m eets a sim ilar inductance an d cap acity L2 a n d C2 w ith th e m eter in series. C apacity C2 is charged, a n d th e m e ter registers a reading. Now, if th e c u rren t is c u t off a t the source, C l a n d C2, being charged w ith a definite am o u n t of current, now discharge th ro u g h th e m eter— i.e., in th e sam e direction as th e original current— an d th u s th e constancy of th e reading is m aintained.

In th e case of th e o u tp u t from th e valve, which is a rap id ly pulsating supply (when d eto n atio n is being m easured), th e condensers a t each in te rv al betw een pulsations discharge th e correct am ount to m a in tain a co n stan t reading on th e m eter.

The surges o f cu rren t in th e m eter circuit are illu stra te d graphically, w ith an accom panying explanation, in A ppendix B.

The cu rren t passing th ro u g h th e m eter to th e anodes of th e valves is also passed th ro u g h a variable resistance D of 2000 ohm s an d a relay E.

R esistance D allows of a d ju stm e n t to th e anode supply voltage so as to v ary th e m eter reading as required.

R elay E is a m agnetic relay w ith one b reak an d one m ake contacts.

T he reason for its inclusion in th e circuit m a y be explained as follows :—

The o u tp u t of th e set th ro u g h th e m eter an d relay is controlled by th e tim e of absence of th e negative grid bias. Should th e wires connecting th e set to th e p in on th e engine be accidentally parte d , th e m eter would be dam aged b y th e high cu rren t w hich would flow. W ith th e relay in circuit, w hen th e cu rre n t flowing rises to a value slightly in excess of th a t required for full-scale m eter reading— i.e., 2 m illiam peres— th e relay operates an d opens th e circuit th ro u g h th e m eter an d by-passes th e cu rren t s tra ig h t th ro u g h to th e valves. T he relay locks itself in th is position, th u s safeguarding th e m eter, u n til i t is released by th e action o f again com pleting th e negative bias circuit.

T he set has been found to give sufficient o u tp u t a t a supply voltage of 110 volts from th e D.C. generator, w hich is p a rt o f C .F .R . equipm ent.

The to ta l pow er required is 0-6 am pere a t 110 volts — 66 w atts. T h e set operates satisfactorily on either coil or m agneto ignition en g in e s; th ere are several m inor checks to th e wiring, however, w hen first fittin g th e a p p a ra tu s to th e engine. These are as follows :—

Engine with magneto ignition.

Check th a t there is no e a rth on th e positive pole of th e generator.

Engine with coil ignition.

1. T he end o f th e 500-ohm resistance in th e coil ignition circuit m u st be tran sferred from th e negative pole of th e gen erato r to th e positive.

2. T he e a rth on th e positive pole of th e g enerator m u st be rem oved a n d th e e a rth placed on th e negative pole.

T he essential p o in t in b o th cases is to ensure th a t no e a rth exists on th e positive pole o f th e generator, because th e negative pole is e arth ed w hen th e se t is operating correctly, and th u s a direct sh o rt circuit on th e generator m u st be avoided.

T he following perform ance is claim ed for th is a p p a ra tu s :—

(a) The ap p a ra tu s m easures th e tru e knock in te n sity over th e

THE CLEMENT—PIGHEGUY KNOCK INDICATOR. ^{4 9 7}

entire testin g range o f octane num bers w ith o u t an y change of original setting.

(6) Com bustion pressure effects are n o t recorded. I f th e engine is ru n on 100-octane fuel a t th e compression ra tio giving sta n d a rd knock intensity, w ith th e knock-m eter reading a t m id-scale, an d th e fuel is th e n changed over to pu re benzene, th e knock-m eter reading will fall aw ay to zero even though th e fuel is “ bum ping ” q u ite heavily.

(c) T he operation o f th e ap p a ra tu s is m ore sim ple th a n t h a t of th e sta n d a rd pin.

(d) The sen sitiv ity p er octane n u m b er is controllable.

10 knock- m eter divisions p er octane num ber is practicable.

(e) T he current, 0-027 m illiam pere, is so sm all th a t th e re can be no trouble due to arcing a t th e p o in t of co n tact betw een p in an d

diaphragm .

(/) T he a p p a ra tu s can be fitted in a q u a rte r o f an h o u r to operate satisfactorily from th e D.C. generator on eith er coil or m agneto ignition C .F.R . engines.

(;g) M aintenance o f th e set should be negligible. N ew valves m a y be required a t in terv als o f two years. Cleaning of th e pick-up u n it should be carried o u t a t th e sam e tim e as decarbonizing th e engine—

i.e., approxim ately every 100 hours ru n n in g tim e.

T he au th o rs wish to th a n k th e A siatic P etroleum Co., L td ., a n d Shell Refining an d M arketing Co., L td ., for perm ission to publish th e results o f th e experim ental work described, which was carried o u t in th e C om pany’s research lab o rato ry in London.

A P P E N D IX A.

De t a i l e d Ex p e r i m e n t a l In v e s t i g a t i o n o n Mo t i o n o f St a n d a r d Bo u n c i n g- Pi n.

A n a p p a ra tu s called th e W eidenhoff distribuscope which w as designed to m easure th e tim e of m ake an d b reak o f th e co n tact points of autom obile ignition distrib u to rs was used to stu d y th e m ovem ent of th e co n tact points of th e sta n d a rd bouncing-pin. T he d u ra tio n o f co n tact of th e points is indicated on a circular scale o f degrees b y th e flashing of a neon lam p which is incorporated in th e ro ta tin g arm of th e distribuscope. B y ro ta tin g th e arm a t a speed synchronous w ith th e engine speed, th e d u ratio n and num ber of contacts of th e bouncing-pin points during one engine cycle can be read ily estim ated.

T he shaded diagram s on th e draw ing Fig. 5

a n d

represent th e periods o f co ntact, on cran k sh aft degree scale, for various bouncing-pin co n tact p o in t clearances a t 5-3 : I and 8-0 : I com pression ra tio respectively.

D iagram X represents c o n tac t tim e on th e d eto n atin g fuel, a n d diagram Y

n o n -detonating fuel, th e la tte r therefore showing interference due to

com bustion pressure. F o r sta n d a rd knock in ten sity a t 5-3 to 1 com pression

ra tio (C.R.) on a 65-oetane-num ber fuel, th e co n tact p o in ts usually close

th re e tim es. On reducing th e gap from 0-004 in. to 0-0025 in. th e closure

of th e co n tac t p o in ts is due to th e sum o f th e deflection o f th e diaphragm

CONTACT GAP.

/NCEES- X

Y

‘ m 1m yA / l

X

A T COM PR ESS/O N fZAT/O

■ 004. <

•0 0 3 |

■003. </

C 0 2 i |

■004.

• 0 0 3 i . ^

■003. |

OF ¿r-3 : <

r^/O C 0x^73 US TXOX^y EFEEC 7". 65 OCTA N E A/o f/JEL A T YS/TY.

STANDARD AS/OCA //ore/

X

y

m y / x Y m fi V/A \ y /=/ A TBL—A/f) OETONAT/OPJ.

B

X

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t b y y m \ \

Vx y y y / i

X

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W / y //x ■■///.

y / x

X

y ■A/a

V/X/ \y. %

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B . A T COM P RES.7/OP/ R.4 7 7 0 CO/vft r /o A / EFFECT

X

O F < S -o : /

3 2 E OCTANE N® AT r^s/ry.

X Y y / x y / x <1 I m % a STANDARD KNOCK /NTT

7 X " / -/ TEL - /No DETONA T/OPJ.

b y V A X

y

m y y yv / y V // XX/ m W X x x y . 'A y/A y y . y /A ’/ / / y / \ y /A y / x^{y y}

£ o .2o

3

DC/ R A T / O r Y O E G O P / T A C T O F N / T Y p o/p v t s. D E G R E E S G R A P / N S N A F T - --- >---

F i o . 5 .

498 CLEMENTAND PIGNEGUY

TIIE CLEMENT—PIGNEGUY KNOCK INDICATOR. 4 9 9

caused b y com bustion pressure an d th e induced v ibrations o f th e pin caused b y detonation. W hen th e gap is less th a n th e deflection of th e diaphragm caused b y com bustion pressure, th e knock-m eter does n o t record tru e d e to n atio n effect.

A t a C.R. o f 8 : 1 an d pin-gap of 0-003 in. (see Fig. 5

) th e d u ra tio n of co n tact of th e points is g reater th a n th a t a t th e C.R. o f 5-3 : 1 w hen th e engine is running on th e sam e fuel. T his observation confirms th a t th e m ovem ent of th e lower c o n tac t caused b y th e deflection o f the diaphragm increases w ith a n increase in com pression ratio.

I t was found w hen an engine was n o t d eto n atin g th a t a deflection of th e diaphragm took place am ounting to 0-0015 in. a t a compression ra tio of 4 : 1 an d 0-0025 in. a t 8 : 1 C.R. due to com bustion pressure. I t was determ ined th a t u n d er these non-detonating conditions th e pin rem ained on th e diaphragm th ro u g h o u t th e cycle a t all compression ratios, provided th a t th ere w as a norm al pressuro dow nw ards b y th e lower leaf-spring.

T his was carried o u t b y insulating th e pin in its guides a n d com pleting th e circuit th ro u g h th e p in an d diaphragm and th ro u g h th e neon tu b e referred to above. I f th e spring pressuro w as insufficient th ere was a bounce of th e pin due to com bustion w hen toluene, benzene, an d arom atic fuels were used, although th ere w as no detonation.

T he to ta l period of m ake o f th e bouncing-pin contacts caused b y d eto n a­

tio n has a d u ratio n o f ab o u t 15° cran k sh aft w hen th e gap is set to 0-004 in.

F rictio n al losses an d o ther dam ping cause th e p in eventually to come to re st before th e end of th e engine cycle.

D iagram s of p in m ovem ent were also observed w ith th e aid o f th e S tan d a rd -S u n b u ry cathode-ray oscillograph. These diagram s are re ­ produced in Fig. 0. T he vertical and horizontal axes rep resen t displace­

m e n t a n d tim e respectively. A lthough th e y carry no scale, th e y are all relative in size to one an other. T hese were obtain ed b y placing a m agnetic pick-up above a steel low er-leaf spring resting on th e pin-top. T hus m ovem ent of th e spring was recorded.

Fig. 6

shows th e leaf-spring co n tact m ovem ent a t sta n d a rd knock in te n sity on 65 O.N.-5-3 : 1 C.R.

Fig. 6

—leaf-spring c o n tac t m ovem ent a t 5-3 : 1 C.R. w hen running on non-detonating fuel— i.e., no v ib ratio n of pin.

Fig. 6

sta n d a rd knock in ten sity a t approxim ately 8 : 1 C.R. on 92-5 octane fuel.

Fig. 6

leaf-spring c o n tac t m ovem ent a t 8

1 C.R. w hen running on no n-detonating fuel. N ote th e relative position o f diaphragm or leaf- spring deflection a t 65 O.N. settin g Fig. 6

(non-detonating).

T hus, while i t can be seen th a t th e v ib ratio n of th e pin w hen d eto n atio n is occurring is ap p roxim ately of th e sam e n a tu re an d am plitude a t both 5-3 : 1 and 8 : 1 C. R a tio s ; in th e case o f Fig. 6

—th a t is, w hen a t sta n d a rd knock in te n sity on 92-5 octane fuel— th e v ib ratio n is displaced vertically u n d er th e action o f th e diaphragm deflection.

B y increasing th e thickness of th e diaphragm from th e sta n d a rd 0-015 in.

to 0-025 in. th e deflection b y com bustion pressure is g reatly reduced. A t th e sam e tim e th e v ib ratio n s of th e p in due to d eto n atio n have ap p ro x i­

m ately th e sam e am plitude. D iagram s 6 n , c,

an d

are th e corresponding

diagram s w ith th e thicker diaphragm .

5 0 0 CLEMENT AND PIGNEGUY *.

D / A P H / C A G M ' Q 2 S "

C O A / P R P S S / O M R A t / O — S 3 •• /

PUEC /=/ / TE L*

T O C-

D/AP HPAG M ' G / S n C .Q A / P R E R S / Q P / P A r / o

3 0

P U E C P / + T E L . -

r o c -

O/APHRAGM --- ' 0 2 C O M P R E S S / O M P A T / O - S 3 •' f

P i / E C— 6 5 O C r/1/vg PUQ .

D / A P f t P A G A i --- ‘Q / S O O M A R E S S / Q / ^ / . P A r / o — p ‘ Q : / P O P C --- E 2 S O C T A P / E RSQ .

D / A P P R A G M 0 2 5 "

C O M P R E S S / O P / R A T / O - 3 0 / P U P L —--- P / / T E L .

D / A P f t R A G M '0 2 S * C O M P R E S S / Q P J R A T / O — p - Q , /

P U E C --- B 2 5 O O T A M E M O

THE CLEMENT—PIGNEGUY KNOCK INDICATOR. 5 0 1

T hus, although th e d iaphragm is m uch thicker, th e energy tra n sm itte d to th e m ass of th e p in has n o t been m aterially reduced. Reference is m ade to th is im p o rta n t and interesting fa c t in th e following discussion.

Di s c u s s i o n o n t h e Re s p o n s e o f t h e Cl e m e n t- Pi g n e g u y Eo u n c i n g- Pi n t o Kn o c k In t e n s i t y.

I t is obvious th a t th e m echanics of th e sta n d a rd bouncing-pin are identical w ith those of th e C.P. pin. T he m ass of th e p in an d th e spring tension applied to i t are closely sim ilar.

T hus any reasoning based on experim ental d a ta ob tain ed w ith th e s ta n ­ d ard a p p a ra tu s can, w ith in th e lim its o f ex perim ental error, be applied to th e C.P. pick-up u n it. T h a t th is is so is borne o u t b y th e several identical characteristics possessed and obtain ed experim entally by them .

N on-linearity o f Knock-meter Headings with Change in K nock Intensity.

I t has been determ ined th a t w ith eith er th e sta n d a rd p in or th e C.P. pin th e knock-m eter readings recorded do n o t change in proportion w ith regular changes o f octane num ber when th e engine is running a t a fixed com ­ pression ratio. W hile th e non-linearity is n o t serious w hen th e spread of octane num ber during testin g is considered, i.e., approxim ately 4 octane num ber, i t is interesting to investigate why th e non-linearity should exist a t all. In th e following work an d proofs it is shown th a t th e non-linearity is n o t due to th e knock-m eter indications being non-linear w ith th e knock occurring in th e engine, b u t i t is due to th e fa c t th a t th e actual knock in

T he increase in engine knock is n o t linear w ith decrease in octane num ber, a n d ten d s to increase very rap id ly (probably som ething n ear square law) as th e knock becomes heavy.

I n th e work of exam ining th e m ovem ents o f th e sta n d a rd C .F.R . pin, m uch assistance w as derived from th e W eidenhoff D istribuscope, details of which hav e already been given. T he bouncing-pin was insulated in its guides and an electrical circuit com pleted th rough th e pin and diaphragm a n d th ro u g h th e distribuscope. U n d er a norm al spring pressure on th e pin, w hen d eto n atio n occurred, th e distribuscope showed a v ib ratio n or series of regular breaks in th e circuit betw een p in an d diaphragm . This v ib ra tio n was of ap p ro x im ately 800 cycles per second, an d th e pin com pleted ab o u t 9 or 10 cycles for each d eto n atio n o f th e engine.

These 9 or 10 cycles s ta r t wide (showing th e pin off th e diaphragm a long tim e), an d ta p e r aw ay to zero w ith a slight increase in frequency. I t is im p o rta n t to note th a t these oscillations o f th e p in fall to zero before the n e x t knocking com bustion o f th e engine. I t is th e to ta l tim e th a t th e p in is v ib ratin g off th e diaphragm w hich we wish to m easure. T he reasons for th is can be shown in an app ro x im ate m ath em atical p roof as follows :—

F rom th e law K inetic E nergy = W ork Done.

Some o f th e kinetic energy o f th e d eto n atio n does work in driving th e m ass o f th e bouncing-pin ag ain st th e force of th e lower leaf-spring pressure an d its own w e ig h t:—

w v *

th u s

where IF — w eight o f pin, V = velocity o f pin, (j — g rav itatio n al acceleration 32-2 feet per second, per second, F = force retarding-pin,

T he various figures m easured on th e sta n d a rd pin were as follows :—

F — lower leaf-spring pressure (assum ed to rem ain c o n stan t although it is

deflected slightly) plus p in w eight

o m ,

210 + 3 3 ., 2 4 3 ,,

= :2 1 0 + S 3 g m . - - s r - l b . - j ^ l b .

>F = w eight of pin = 33 gm. =

lb.

S =

th e actu al distance m oved b y th e p in u nder d eto n atio n m easured w ith th e aid of V ernier gap ad ju stm en t.

A A A A r • 0 ' 0 0 0 5 f t

= 0-0005 in. = —— - ft,

IF F 2

therefore from — — — F S 2?

T, 2 . 243 0-0005 454

F > =

« X f f i j X — X

T his is th e velocity of th e pin im m ediately a fte r th e im p act of d etonation on th e diaphragm .

Now from th e law

... (2)

¡7

where F = force retarding-pin, t = tim e for p in to come to rest, TF = w eight o f pin,

= g ra v ity ,

I f F _ 33 0 0 5 454

’ ’ -

= 0-00064 second.

This is th e tim e for th e pin to come to re st off th e diaphragm , th u s to ta l tim e for one com plete oscillation

= 2t = 2 X 0-00064

= 0-00128 second.

T he pin having th u s retu rn e d to th e diaphragm , i t has still considerable energy which is stored in th e resilience o f th e d ia p h ra g m ; i t is th u s throw n off again, an d oscillates a t th e n a tu ra l frequency o f th e pin system . This frequency can be calculated from our results above :—

frequency = y, = ~ = 780 cycles p er second.

T his calculated frequency coincides very closely w ith th e frequency of S00 cycles p er second m easured on the W eidcnhoff D istribuscopc, proving

502 CLEMENT AND PIGNEGUY !

THE CLEMENT—ITGNEGUY KNOCK INDICATOR. 5 0 3

th a t our app ro x im ate calculations are correct in principle. T h e following conclusions can bo reached from this proof :—

W V -

K in etie energy o f d eto n atio n K — — given to p m . (1) T his can bo re-w ritten in th e form :—

K X | = V2 or V = ^ ¡ K X |

and from equation (2)

inserting in (2) th e value for V obtain ed from (1) / = w

g F 'V W

4

1

1

_

F X V g X F V g X ^

b u t for an y p a rtic u la r pin settin g F , W, a n d g are c o n stan ts and can be expressed as C an d therefore t can be w ritte n ;—

t = c V k

or

I n words, th e tim e th e p in is off th e diap h rag m is proportional to the square ro o t of th e kinetic energy o f th e im p a ct of d eto n atio n given to th e pin.

A curve to th is eq u atio n has been draw n (see Fig. 7).

T he n e x t im p o rta n t point— does th e radio set o u tp u t cu rren t v a ry in proportion to th e tim e of break o f th e pin and diaphragm circuit. This p o in t has been accu rately checked in th e following m an n er

A to o th ed wheel was obtained an d a brush-gear was m o unted so as to m ake good co n tact w ith each to o th as th e wheel was m ade to ro ta te . T he wheel was th e n m arked o u t in to 9 p a rts, a n d plates were m ade so th a t w hen fixed in position on th e wheel th e y each blanked off a n u m b er of th e te e th o f th e wheel. T hus as th e wheel ro ta te d a definite num ber of te e th could be left unblanked, an d therefore a definite tim e of break a t th e brush could be generated, providing only th a t th e wheel speed was co n stan t. T he wheel used has 96 te e th , an d 8 o f th e p lates b lan k ed off 10 te e th each. O ther plates were provided w hich m ade i t possible to have open 5 teeth or a n y m ultiple o f 5 te e th . T hus th e co n tact tim e could be varied in steps o f 5 te e th from 5 u p to 90, and finally 96 teeth . ;

T he speed o f th e wheel was selected a t 500 r.p.m . so as to generate a m ake and b reak of th e circuit sim ilar to th e bouncing-pin vibration.

T hus 96 breaks p e r revolution a t 500 r.p.m . 500 X 96 , ,

— xx breaks p er second.

60

= S00 per second.

KR/QCKrKETERĘĘ-ĄO/ RIO. ER//TSOF T/ME

5 0 4 CLEMENT AND PIGNEGTJY :

f e/a t/o r j s f h y e e r j t/m e o f p/n

O F F D / A P / i R A Q P f A N D K I N E T I C E N E R G Y

U R / / T S O F K IN E T I C . E N E R G Y O F D E T O N A T /O N . F ig . 7.

T E E T H O P E / N - F io . 8.

THE CLEMENT—PIGNEGUY KNOCK INDICATOR. 5 0 5

T h e circuit th ro u g h th e wheel was ta k e n to th e radio set in place of th e pick­

up u n it, an d th e knock-m eter readings noted as th e p lates referred to above were rem oved. Fig. 8 shows th e straight-line response o f th e m eter readings w ith regular change in circuit break tim e, a t th ree set o u tp u ts—

m inim um , m edium , m axim um .

As a m a tte r o f in terest th e curves in Fig. 8 have been redraw n in Fig. 0 and instead of th e abscissa being “ te e th open,” th e c o n ta c t tim es for th e

o 0 0 5 o / o O / S 0 2 0 0 2 S ■030 0 3 5

T O T A L T / M B O F B R E A AC 0 5 C / H C U / T P E R K N O C K — S E C O O T Q S .

Fi g. 9.

te e th h av e been accurately m easured an d th e calculations involved arc as follows :—

To find to ta l tim e o f break of circuit p er m inute W heel running a t 500 r.p.m . T o tal angle ru n th ro u g h p er m inute

= 500 X 360 degrees

= 3000° p e r second

1 tooth-space w as accurately m easured as giving a break of I f 0.

Now 1° is m oved th rough in second.

. ■. to ta l tim e of b reak of circuit p e r revolution

= N

. •. T o ta l tim e of b reak o f circuit p er m inute

■— A" X

1

-nV

500 seconds

= N

V-, seconds per m inute w here N is th e num ber o f te e th left open on th e wheel.

M M

5 0 6 CLEMENT AND PIGNEGUY :

I t is convenient to express th is tim e, however, in some term relativ e to th e C .F.R . e n g in e ; th u s as th e speed is 900 r.p.m . of th e engine, th ere are 450 theoretical knock im pacts p er m inute. T hus i t is perh ap s b e tte r to express th e break of th e circuit as :—

T im e of b reak o f circuit p er knock o f engine

N -j'j- x T gx> — 0 0006468 N seconds.

U sing this equation, th e values in Fig. 8 have been converted to Fig. 9 from th e following tab le :—

N . T im e (seconds). N . T im e (seconds).

0 . 0 25 0-016170

5 . 0-003234 30 0-019404

10 . 0-000468 40 0-025872

15 . 0-000702 50 0 0 3 2 3 4 0

20 . 0-012936

T hus we have p roof th a t th e knock-m eter readings are p roportional to th e tim e of break o f th e bouncing-pin circuit.

O C T A A / E

Flo. 10.

As we have alread y shown, th e tim e th e p in is off th e diaphragm is a function o f th e K \ , where K is th e kinetic energy of d eto n atio n given to th e p in ; th u s Fig. 7 also represents th e change in knock-m eter reading w ith change of kinetic energy given to th e pin.

Now, th e im p o rta n t p o in t is, if th e change in kinetic energy o f d etonation in the engine were linear w ith change in th e octane num ber o f th e fuel (at

a fixed com pression ratio), th e n Fig. 7 would also rep resen t th e curve of knock-m eter reading versus octane num ber a t th e fixed compression ratio.

W ith th e above sta te m e n t in m ind, Fig. 10 was obtained in th e following m an n er :—

T he engine was ru n u nder sta n d a rd 65-octane-num ber conditions and

th e knock-m eter reading se t to m id-scale. T hen, w ith no alteratio n in

com pression ratio , th e engine v-as ru n on fuels of various octane num ber and

th e knock-m eter readings were taken.

/OO 90 80 70 60 SO

vw/rs o f

ea/emgy o f deeoma rroM

.

C/M/TS OF K/NSr/C £/VEA6Y OF PEfOMA T/OM.

Fig. 11.

THECLEMENT—PIGNEGUYKNOCKINDICATOR. 507

508 CLEMENT AND PIGNEGUY :

Fig. 10 represents th e resu lt o f tw o variables :—

1. I f th e knock in th e engine were linear w ith change in octane num ber, th e n th e knock-m eter readings versus octane-num ber curve should be represented b y Fig. 7.

2. As th e actual knock-m eter reading versus octane-num ber curve o btained (Fig. 10) does n o t coincide w ith th e •previously proven theoretical curve (Fig. 7), th e n th e only reason for th e ir n o t agreeing is th e fa c t th a t th e engine knock is n o t linear w ith change in octane num ber.

B y cross-projection from th e com m on side (tim e = A knock-m eter divisions) o f curves Figs. 7 an d 10, a fu rth e r curve (Fig. 11) can be draw n connecting k in etic energy of d etonation and octane num ber a t th e above- sta te d fixed com pression ratio.

(Note.—T h e “ U n its of K in etic E n erg y ” do n o t, a t th e m om ent, represent

a n y practical value.)

T h u s i t can be seen th a t th e knock im p ac t or in te n sity of d eto n atio n in th e engine m u st follow th e curve of Fig. 11 for th e knock to be recorded in th e m anner o f curve, Fig. 10, having reg ard to th e fa c t th a t th e response o f th e kinetic energy versus tim e curve of th e pin itself is th a t of Fig. 7.

T he th ree curves are shown to g eth er in Fig. 11, showing how th e cross- projection was obtained.

As w as sta te d a t th e beginning o f th is explanation of non-linearity, th e error in ra tin g fuels is sm all, am ounting to ap p ro x im ately a m axim um of 0-2 octane n u m b er w hen th e bracketing reference fuel difference is 3-8 octane num bers. T his error is com m on to b o th th e s ta n d a rd C .F.R . pin an d to th e C lem ent-Pigneguy pin.

I t is suggested th a t th e error could be reduced to a negligible facto r in th e C lem ent-P igneguy p in by reducing th e bracketing reference fuels to 2 octane num bers. A ny suggestion th a t th is w ould involve difficulty in th e selection of bracketing fuels can be satisfactorily answ ered as follows :—

T he C.P. pin gives sta n d a rd knock in te n sity a t all compression ratio s w ith o u t com bustion-pressure interference. T hus, if a compression ra tio versus octane-num ber curve be obtain ed w ith th e a p p a ra tu s for each te s t engine concerned, th e re p e ata b ility a n d stab ility of th e C.P. ap p a ra tu s w ould allow th e accu rate selection o f suitable bracketing reference fuels for all subsequent octane-num ber determ inations. A n a d v an tag e w ith th e C.P. pin over th e sta n d a rd pin u n d er th e reduced octane-num ber b rac k et is th e fa c t t h a t th e sen sitiv ity can be m aintained a t ap p roxim ately te n k nock-m eter divisions p e r octane num ber.

Di s c u s s i o n o n Kn o c k Me a s u r e m e n t a n d Ob s e r v e d Da t a.

T he following sta te m e n ts are based on d a ta w hich have been obtained

on one engine only, an d m ay be biased b y th e personal opinions of th e

au th o rs. The sta te m e n ts are included, however, to prom ote discussion,

and i t is hoped in th e n e a r fu tu re to carry o u t fu rth e r experim ents in an

effort to su b sta n tia te them . R eferring to D iagram s Fig.

and

i t has been show n th a t although th e diaphragm thickness was increased b y

0-010 in., th e energy o f d eto n a tio n tra n sm itte d th ro u g h th e diaphragm to

V E N T I L A T I O N G R I L L E

M A I N S t Y / T C H

F R O N T P A N E L .

P L A N .

V I E W S O F “ R A D I O ” S E T .

[To face p. 508.

C O N D E N S E R S

G E N E R A L I N T E R N A L V I E W O F “ R A D I O ” S E T .

THE CLEMENT—PIGNEQUY KNOCK INDICATOK. 5 0 9

th e p in was n o t reduced. T his gave rise to th e suggestion t h a t th e thickness o f th e diaphragm could be increased w ith o u t lim it— i.e., up to th e thickness o f th e cylinder head. I n o th er words, if a sm all m ass is m o u n ted ex tern ally on a solid p a r t of th e cylinder head, v ib ratio n s are se t up in th e m ass due to th e im p act of detonation.

Accordingly a sm all a p p a ra tu s was m ade up as follows :—

A -&-in. d iam eter steel ball loaded w ith a coil spring was m o unted in an insulated fram e so t h a t co n tact was only m ade b y th e ball on th e cylinder head, w hich w as polished to ensure a sm ooth co n tact surface. I t was determ ined th a t th is “ pick-up u n it ” responded in a n identical m anner

■with th e screwed-in pick-up u nit.

A n interesting use was m ade of this “ bouncing-ball pick-up u n it.” As, o f course, it is used externally, th e bouncing-pin hole in th e engine was available for use a t th e sam e tim e. T hus, using th e sta n d a rd C .F.R . p in a n d th e bouncing-ball u n it, to g eth er w ith tw o knock-m eters, sim ultaneous readings on b o th system s were ob tain ed a n d com pared, and showed close agreem ent over th e range where th e sta n d a rd p in w as n o t recording com bustion pressure—i.e., a t low com pression ratios. T his ex tern al pick-up shows th e possibilities of th e use of such an arran g em en t on engines n o t equipped for th e fittin g of screw ed-in indicators, such as airc ra ft engines, etc.

T he C.P. p in and th e bouncing ball h av e b o th been trie d o u t on a C .F.R . engine running u nder special high-speed conditions. D uring developm ent of a m ethod o f te s t for ra tin g high-octane fuels th e u n its have shown th a t th e y each record d eto n a tio n satisfactorily an d respond to knock in ten sity described as “ freq u en t slig h t.” The engine speed du rin g th e te sts was 1800 r.p .m ., and ja c k e t te m p eratu re using ethylene glycol was ap p ro x i­

m ate ly 180° C., an d th e o u tp u t was 120 B .M .E .P. One or tw o m inor modifications m ay be necessary to th e pick-up u n its— e.g., g reater rigidity an d resistance to high te m p era tu res—b u t, generally speaking, i t can be sta te d th a t th e system of m easuring d eto n atio n is satisfacto ry a t th e high speed, a n d could be used for th e ra tin g o f fuels.

A t th is p o in t in th e discussion i t is considered ap p ro p riate to m ake reference to some experim ental results w hich are of fu n d am en tal im portance.

U p to th e m om ent we hav e considered th e effects of d eto n atio n on a m ass situ a te d on a d iaphragm or h i a n y case ex tern al to th e engine, an d hav e expressed its effects th eo retically in “ u n its of kinetic energy.”

T he following w ork was carried o u t on one engine, once only, a n d th e re ­ fore th e following th e o ry p u t forw ard should be accepted w ith somo reserve.

In an a tte m p t to see w h a t relatio n existed betw een knock-m eter readings

a n d th e gas w ave inside th e cylinder, a “ ball u n it ” referred to above was

used sim ultaneously w ith a norm al cathode-ray oscillograph, th e pick-up

o f which was screwed in to th e bouncing-pin hole. T hen th e engine was

ru n a t sta n d a rd knock in te n sity on 65 O.N. fuel. A t th is engine setting

b o th a p p a ra tu s were set to record th is knock in ten sity . I n deference to

accepted th e o ry th a t d eto n a tio n is high ra te of change o f pressure during

th e d eto n atio n wave, th e oscillograph w as set to record th e ra te of change

of pressure diagram . T he ball u n it w as ad ju sted to record a m id-scale

reading on th e knock-m eter.

510 CLEMENT AND PIGNEGT7Y :

W ith th e a p p a ra tu s th u s set, th e engine was ru n on fuels of various increasing octane num bers an d th e com pression ra tio correspondingly increased to give a m id-scale reading on th e knock-m eter recorded b y th e ball u n it. A t each individual reading so obtain ed i t was n o ted th a t th e heig h t of th e ra te of change o f pressure o f d eto n atio n above th a t o f norm al com bustion (i.e., Y l — Y in diagram , Fig.

was constant. T hus it would a p p ear th a t to m easure th e height o f th e rate-of-change diagram a t th e p o in t o f d eto n atio n would give a tru e indication o f knock in ten sity .

I n criticism of th e C lem ent-P igneguy principle, i t has been sta te d th a t

N O R M A L . C O M B U S T / C U N .

Fi g. 12.

th e m ass o f th e pin is a c tiv a te d in its m ovem ent b y th e r a te of change of pressure, a n d th u s if th e diagram o f ra te of change of pressure becomes sim ilar to Fig.

by v irtu e o f th e n a tu re o f th e fuel or th e m ix tu re stren g th used for th e m ethod of te st, th e n th e C.P. pin can no longer record d e to n a ­ tion, b u t will respond to th e m axim um ra te o f change o f pressure F 2, and n o t to th e d eto n atio n ra te o f change F 3 — Y^.

T he following te s t results are inten d ed to disprove a n y sta te m e n t th a t th e C.P. pin or th e ball u n it is ac tiv a te d b y th e ra te o f change of pressure, and an a tte m p t is m ade to show th a t th e principle of th e a p p a ra tu s is such th a t i t is only activ a te d b y th e second ra te of change o f pressure— i.e., th e ra te of change of th e ra te o f change of pressure. F u rth erm o re, i t is suggested th a t m easuring th e height o f th e rate-of-change diagram is only ap p ro x i­

m a tely w h a t is required. T h a t th e m ethod is reasonably satisfacto ry is

because in general th e height of th e rate-of-change diagram is p roportional

to its slope.