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MOTORING NEWS AND VIEWS

THE FAT SPARK FALLACY

By Donald H. Smith

Probably the majority of people take their cars and all parts and functions thereof for granted. Even so there are quite a few enthusiastic amatour engineers who like to know why and how things work even if the modern car has killed their tinkering spirit and driven it nostalgi- cally backwards into vintage and "special" activities.

Of course, the man who takes, his car for granted will say, "So what? My engine starts easily and runs well-why worry?" Or the mechanic says, "Give me a good fat spark and I don't mind."

Agreet that it is chiefly the technician who is greatly concern ed; it should not be forgotten that it is really be who has evolved the car that can be taken for granted and who, incidentally, has created the Job that

provides the mechanic with his daily bread,

We might leave the matter at that were it not for the fact that Mr. Take-it-for-Granted, having run into a spot of bad starting or misfiring, reeks the advice of his garnze, which, after demonstra- ting that the ailing car hasn't got ite

good fat spark," institutes an impressive and possibly expensive routine to provide one.

Mr. Take-it-etc. pays out his cash and in proportion thereto Is impressed with the need for this "good fat apark," and ever after may be counted among the true believers.

kind of generator and characteristics employed. Such a spark, or more accurately such an arc, is determined chiefly by the amount and disposition of the Iron and copper in the generator or coll.

Here

pately

Some

to the

of the

To all these groups and even to some highly practical professional mechanics it may be rather surprising to state that the process of ignition is not necessarily finally understood even by the experts.

phase is the result of the slower lower voltage after the capacity ignition of the gas in the vicinity discharge of the energy stored component has

reduced the xel of the plug, volatilises magnetically in the coil and is sistance across the gap.

of the adjacent mixture. Into therefore a function of coil design. Investigation has shown that it this more ignitabio zone tho From the forgoing it will be to the capacity component which longer flaming component feeds clear that there is no direct or determines the initiation of igni- heat during a suficiently Jong proportional relationship between tion and that the secod phase period to

combustion the capacity and inductive com- makes no

further contribution that is not

quenched ponents of the spark. It is pos- provided that perfect mixture and by surrounding wet mixture alble to have a spark entirely of plug conditions are present. This

may be explained that the "capacity" typo-one that is provides the basis for the elce- lonisation of the plug gap la a "all volts and no amps is one trostatic theory of ignition and it phenomenon that occurs when a may be permitted to use

upplied such would be well founded and com- high voltago

The gas in the an expression; this type of spark pletely satisfying if Ideal condi- plug, electrode.

positively and can be produced by an electrostations always obtained. But the pls split into the machine; eg, the Wimshurst. perfect mixture and the ideal plug fons, thereby reducing its rests- negatively charged, elements or However, a spark can also be aro a ruro combination, with reproduced of the purely "in- tion and that the second phase which the current can pass.

tance and providing a path across ductive" type which is "all amps of the spark comes into its own. and no volts," according to the Let us consider. The first phase Firing economy mixtures

Increase of Intensity cireult of the spark is exceedingly brie

and has little of the total spark first phase of the spark is most energy, whereas the second part readily effected by increasing the contains the bulk of the energy sap width and this is the usual and is of appreciable duration. It procedure when it is desired to is true that the first phase fires fire weak or economy mix- tures in an engine which in other. the mixture within its path across

is adjusted for such also the plug gap.

L'especis But if that mixture is "wet" combustion condition; the ilmit-

ing factor

factor here in the high the little bit that is touched-off voltage performance of the igni It may be interpolated here loses its initial heat to adjacent that the phrase "all volts and no droplets and to the plug points

tion

equipment. amps," like most attempts to pro- more rapidly than further heat are

It must be realized that there. duce an aphorism, in epen to a la released, and so combustion Eigh voltages, such as liability objections to excessively pedantle quibble. All yolts and dies out, or at least: would die out, to: corona discharge no joules" would be better, be it were not supported by the cessive burning of plug points. cause of the importance

can be or dame phase of the No very precise data gecond. time element involved.

given relating to the relation Indeed, to quote Dr. Watson spark-the inductive component: again, the first phase of the spark the ionisation of the gap itself since so many variables occur.

At one time it, was thought that ship of

plug

gap and voltago (one millionth of a second), cap initiated ignition,, thin being.a carry instantaneous current of the purely electrostatic phenomenon.

For example: a rise of

of plug electrodo

from temperature. order of 50 amps, whereas the But eventually it was shown that to 750 deg C may result in a drop As applied to spark-ignited (flaming phase, although, of a thou the actual spark of the Arst phase from 4,260 to 3,800 volts. Other petrol engines this is the purely sand times longer duration, is did the work and to that extent factors Influencing....

voltago thermal theory of ignition. But produced by

of only 50 a current

pressure,

chargo the thermal theory gained ground. examination. of the spark at the milliamps.?

depression,

position of plug, shape plug shows that there are two When these current values are which is as it should be, since all of electrodes and electrode con phasos, the arst being a thin blue-related to time, the energy re be looked upon as a thermal pro diflag clean, polished, rough white line or "lighting flash" oc- lensed in each phase is 0.0025 and

pro- curring, the Instant the contact 0.0175 joules respectively, which era. ` breaker polita separate. The means that the second phase dis- second is a flaming portion, not poses of seven times more energy blue-white but reddish-yellow and than the first. of longer duration.

Amps and volts Why should there be any doubt about the process of ignition, and why should two schools of thought exisi, "Burely," says the

plain man from the standpoint of his

sense,

•Bound common

"If you want to start a fire you must first strike a light." And the bigger the light you strike the more ikely that the resulting fire will be a good one.

..

Condition of mixture

and plug..

” ་་

of the

one those rather picturesque, descrip].48 "a fat-spark” adding more heat

and

000

are

σπ

It was shown by Dr. J. D. The first build-up of voltage at Morgan of Birmingham Univer the plug electrode tonices the gap sity during World War I that, and establishes that conductive provided sparkc energy is suffi When this difference is con- condition which is essential to en- cient to give consistent, ignition, Dr. E. A. Watson has quoted sidered in association with a drop able any kind of spark to pass, there is no point, in Increasing It in en- the relative time of these two from approximately 6,000 volts to The capacity component initiates because no improvement phases as of the order of one the few hundred necessary to ignition, followed by the induc-gine performance will result.

"The Principles of micro-second "or less" and one maintain the

the tive phase which passes over what

His book. spark once milli-second "or more"-in other capacity phase has established it, is virtually a thort circuit bridge Ignition, deals very fully with

the subject and the words one millionth and

the particular mentioned than the first Bro is losing to its thousandth of a second respections become excusaple,

"Was recently by Dr. E. A tively.

In the normal ignition system durroundings and lasting long

1949-50

Chair The bright part of the spark is the high tension, circuit (call enough to ensure the spreading Watson in his technically referred to

man's Address to the Automobile distributor, cable and of: Ignition throughout the com- Division of the Institution of the secondary, capacity component, while the plug electrode) is in effect one quation space.

According to the standard taxt- Mechanical Engineers with the flarning phase is the inductive plate of a condenser (or cnpack book of Young and Grimths on alicant added comment

tator) component. As ito name implies,

which becomes highly

withstanding all the propaganda "Automobile Electrical Equip- Instantaneous charged at the moment of separa-men!" It is held that "a "fat" or the former is the

Fut forward in support of discharge of energy stored in the tion of the contact breaker points, flaming spark is simply one giving statement is still true today."

'racing' and other colls, this capacitance of the high tension the voltage building up uratil it ctreult and is therefore greatly can jump the weakest spot in the influenced by the dimensions and circuit-the plug gap.. the dielectric strengths of the Widening the gap obviously ro- components thereof, c.g, the plug, sults in the build-up of a higher h.t. cable, external winding and voltage before the spark occurs. insulation of the coll secondary The Inductive component of the and the nature of the coll casing. spark, on the other hand, is the On the other hard, the second result of current flow of much

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a large energy output....and this attribute is not the determining factor in the process of imition

Adverse conditions (will) de-

mand certain amount of energy over and above that required for ignition."

Again, in his "Electrical Equip ment" textbook prepared for the Royal Technical College, Glas- row, and first published in 1927, Professor. 8. Parker Smith stated that "Theoretically.. the small pilot spark produced by the capacitance component is all that is required providing the pluk is properly eltunted and clean.... dovices producing little energy but sudden high voltages are satisfactory. As far as firing correct mixture in concerned the Inductance component would thus seem to be practically useless. In other words the idea that a 'fat' spark, 16, spark with great pear to be ETTOUCwould ap- energy, is casenfint...

A

At high speeds, when there is less time for heat loss, when the parts are hot and when' gas con- ditions are nearer the Ideal, the fret phase does the essential Job of ignition, and the "fat"

and

Maming phase ceases to crank-

In a cold engine at low ing speed with mixture loaded with wet droplets and possibly with the proseries of condensed water vapour on the plug points. "the flaming" "spark does indood bring fire to the fuel: it needs the capacity component, however," to lay the fame path, as it wore, for without it the inductive com ponent in Belpless.

* Questionable claims ("Só a clearer picture of the pro- cess of Ignition emergos, the validity of the claims for the at spark" being questioned by

the

authorities At high japoode when the engine is warm

the capacity component alone mificient for Ignitión -- and, the subsequent “fat 2 inductive com= ponent has no

no infuence whatever on engine, performance. In this condition ignition -Muy be-FECRET dedan dectrostatio

Paterson and Campbell, at the National Physical Laboratory In 1919, entablaned that the spark chers required for ignition de

starting

lises comb

"not-

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