But don’t for­get, it also plugs a hole

Cycle World - - Fundamentals - By STEVE AN­DER­SON / Pho­tog­ra­phy by JEFF ALLEN

“Be it known that I, Nikola Tesla, a cit­i­zen of the United States…have in­vented cer­tain new and use­ful Im­prove­ments in Elec­tri­cal Ig­niters for Gas-en­gines….” So be­gins the 1898 patent granted to Tesla for an elec­tri­cal ig­ni­tion sys­tem that in­cluded spark plugs. Peer into the early his­tory of spark plugs and you find more fa­mous names, names of men rather than of their later name­sake com­pa­nies: Robert Bosch, Al­bert Cham­pion, the Lodge broth­ers. Back when mak­ing in­ter­nal-com­bus­tion en­gines work at all re­li­ably was the cut­ting edge of tech­nol­ogy, engi­neers were mak­ing their rep­u­ta­tions and for­tunes de­vel­op­ing ig­ni­tion systems and spark plugs.

Spark ig­ni­tion in con­cept is sim­ple. In­duce a high-ten­sion cur­rent at ex­actly the pre­cise mo­ment, and you can make a spark jump a small gap in­side an en­gine’s com­bus­tion cham­ber. Spark gaps from early on were about 0.8mm, or 0.031 of an inch. With such a gap, 15,000 volts or more is re­quired to over­come the in­her­ent re­sis­tance of the air and mix­ture in the gap and start a cur­rent flow­ing, but when it jumps across, light­ning strikes. Ini­tial tem­per­a­tures in the plasma formed by the spark can be more than 100,000 de­grees, as all mol­e­cules where the cur­rent be­gins are dis­as­so­ci­ated into their highly re­ac­tive in­di­vid­ual atoms. As the cur­rent flow con­tin­ues, the av­er­age tem­per­a­ture in the sus­tained arc drops to just over 10,000 de­grees. This is more than suf­fi­cient to be­gin to burn the oxy­gen and com­po­nents of fuel mix­ture in the gap, form­ing an ini­tial flame ker­nel that will spread out to burn es­sen­tially all the mix­ture in the com­bus­tion cham­ber.

The spark plug provides the gap at a pre­cise lo­ca­tion and in re­place­able form; as you might sus­pect, the high cur­rent and hos­tile en­vi­ron­ment in­side an en­gine even­tu­ally leads to a non­ser­vice­able spark gap. The plug it­self con­sists of three main parts:

an outer steel shell that screws into a hole tapped into the cylin­der head and acts as the ground for the cur­rent, a cen­ter con­duct­ing rod (the cen­tral elec­trode) to trans­mit the cur­rent (and heat as well), and a re­sis­tant elec­tri­cal in­su­la­tor be­tween them. In the early days, the in­su­la­tor was porce­lain (frag­ile) or stacked mica (dif­fi­cult to seal), and the cen­tral elec­trode might be highly con­duc­tive cop­per. Later, the in­su­la­tor would be sin­tered alu­mina, a strong, elec­tri­cally non­con­duc­tive ce­ramic that has the added ben­e­fit of con­duct­ing heat rel­a­tively well— more on that later.

Be­cause pressure in­side a com­bus­tion cham­ber can be well over 1,000 psi at its peak, seal­ing on a spark plug is crit­i­cal. Typ­i­cally, the joint be­tween the outer shell and cylin­der head is sealed with a crush washer, but it’s also nec­es­sary to seal be­tween shell and in­su­la­tor, as well as the in­su­la­tor and cen­tral elec­trode. This is done with com­pressed glass/powdered metal joints dur­ing man­u­fac­ture on cur­rent plugs, but was han­dled by braz­ing these joints on early plugs.

Dur­ing op­er­a­tion, the tip of the elec­trode and sur­round­ing in­su­la­tor in­side the en­gine is ex­posed to raw fuel, com­bus­tion byprod­ucts, and even en­gine oil that slips past pis­ton rings. It is heated by com­bus­tion it­self. If the elec­trode tip gets too hot, it can serve as a hot spot caus­ing preig­ni­tion and det­o­na­tion, or even melt. Not sur­pris­ingly, tem­per­a­ture con­trol in a spark plug is a ma­jor de­sign issue, and spark plugs are de­signed in var­i­ous heat ranges, the choice of which de­pends on the char­ac­ter­is­tics of a par­tic­u­lar en­gine or even duty cy­cle. You can rec­og­nize these de­sign choices by look­ing at how the in­su­la­tor at the elec­trode end is ta­pered; a long ta­pered elec­trode that reaches far down into the shell be­fore mak­ing con­tact is a “hot” plug be­cause the heat path from tip through in­su­la­tor to shell to cylin­der head is longer. A “cold” plug might have the ce­ramic come almost to the end of the outer shell. The idea is to have the in­su­la­tor run at a high enough tem­per­a­ture that it is self-clean­ing for car­bon and other de­posits, and still have enough heat flow away from the elec­trode tip that it doesn’t over­heat. This balanc­ing act has been pre­de­ter­mined

by fac­tory engi­neers, so gen­er­ally the rec­om­mended plug is best un­less you dra­mat­i­cally change en­gine tune or run some­place with an un­usual duty cy­cle, like Day­tona In­ter­na­tional Speed­way with its 50-per­cent-wide-open-throttle laps.

Ad­di­tion­ally, the elec­trode ma­te­rial on more-ex­pen­sive plugs may be made of a high-tem­per­a­ture, “noble” cor­ro­sion-re­sis­tant metal such as plat­inum or irid­ium. With their tol­er­ance of higher tem­per­a­ture, these can be re­duced to fine wires, which both makes form­ing the ini­tial spark eas­ier elec­tri­cally and re­duces the heat flow back into the plug.

An­other trend in re­cent years is a re­duc­tion in di­am­e­ter, though it is plain to see from the tiny NGK plug used in the 1964 Honda RC165 250cc six-cylin­der that size has been im­por­tant in high-per­for­mance en­gines for a long time. A smaller plug al­lows room for larger valves as well as mak­ing a smaller hole in the cylin­der head, thus increasing cylin­der-head strength. In high­per­for­mance-mo­tor­cy­cle en­gines, plugs with 10mm threads have be­come com­mon, while For­mula 1 has taken this trend even fur­ther, with tiny, 7.6mm-di­am­e­ter spark plugs that weigh less than an ounce each. The Du­cati Pani­gale V-4 plugs fea­ture a hex-size re­duc­tion from 14mm to 12, with a nar­rower, longer thread sec­tion to al­low more coolant flow around the com­bus­tion cham­ber.

But per­haps the most amaz­ing thing about a spark plug is this: Av­er­age plugs en­com­pass all the tech­nol­ogy of more than a cen­tury of in­ter­nal-com­bus­tion-en­gine de­vel­op­ment, and many can be bought for about $2 each. Even Nikola Tesla would mar­vel at that.

ABOVE: Clas­sic mo­tor­cy­cle plug, an NGK B7ES, this one headed for a Nor­ton Com­mando. OP­PO­SITE, TOP: Spark­ing tip of tiny new old stock 1960s Honda RC165 250cc six-cylin­der GP racer plug. OP­PO­SITE, TOP RIGHT: Dou­ble ground elec­trode and ul­tra­nar­row...

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