Spark Plugs Technical Paper A spark plug by plug by definition is a device that carries (or leads) a high volta voltage ge into into a comb combus usti tion on cham chambe berr and and conv convert erts s it into into a spar spark k (by jumping a gap) to ignite any available air-fuel mixture. The ideal air-fuel mixture for an internal combustion engine is considered to be 1.!"1. The reason that 1.!"1 is considered the ideal air-fuel mixture mixture is that tests have proven that at this ratio #$%s& '#%s and $x are at their loest levels. At the point of explosion& under average operating conditions& the maximum combustion chamber of the internal combustion engine reaches *&+,, to &,, degrees ahrenheit and pressures of !,, pounds per s/uare inch. #ond #ondit itio ions ns hic hich h creat create e the the vari variabl ables es incl includ ude e (but (but are are not not limi limite ted d to) to) sudden sudden tempera temperature ture changes changes or by sudden sudden pressur pressure e change changes s that are brought to bear by high temperature of burned gases or lo temperature of the incoming incoming air-fuel air-fuel mixture. mixture. The spark plugs plugs are subjected to typical firing conditions that heat the 0nose% of the spark plug in plug in the firing chamber to beteen 1&,, and 1&!,, degrees ahrenheit and suddenly cooled by intake air-fuel mixture. 2ndustry standards re/uire testing of a spark plug to plug to 1, pounds per s/uare inch& hereas 34 5park 6lug 6lugs s pass tests to 71, poun pounds ds per per s/ua s/uare re inch inch in inte intern rnal al comb combus usti tion on engi engine nes. s. 2n high higher er comp compre ress ssio ion n engi engine nes s the the sp spar ark k ga gap p tradi traditi tion onal ally ly is loer loered ed as the the comp compre ress ssio ion n rati ratio o incr increa ease ses. s. The The seco second ndar ary y re/u re/uir irem emen entt (gro (gross ssly ly overloo overlooked) ked) is that that the spark plug is plug is the major path by hich heat being generated by the ignition of air-fuel leaves the firing chamber. The center electro electrode de is commonly commonly made up of a nickel nickel alloy88 alloy88 ith chrome& manganese and silicone added. 2t is press formed ith a nickel tip elded onto a pure copper core for thermal conductivity. 9ore recent 34 5park 6lugs 6lugs center electrodes have platinum laser-elded onto a nickel alloy alloy center center electro electrode. de. 34%s 34%s neest exotic center electrode has iridium laser-elded onto a nickel alloy center electrode to offer longevity and loer loered ed resi resista stanc nce e to carry carryin ing g the the volt voltag age e nece necessa ssary ry to jump jump the the gap gap beteen the center electrode and some form of ground (hether it be the shell itself (for surface-gapped plugs) or standard ground straps elded onto onto the shell). shell). or enduranc endurance& e& 34 34 offers offers racing spark plugs that have iridium-tipped center electrodes ith platinum alloy or iridium alloy ground electrodes. The insulator is is made of high purity alumina (Al7$*) that is fired at approximately 7&:,, degrees ahrenheit. This is necessary to ithstand the extreme conditions the 0nose% of the insulator is exposed ithin the firing chamber plus contain the high secondary voltages necessary to jump the gap thereby firing the plug under high cylinder pressures. The metal shell can shell can be formed by either cold forming (rolling) and mach machin ined ed or by cutti cutting ng and and mach machin inin ing g from from a soli solid d hexa hexago gon n bar. bar. The The
ground electrode is elded in position and the entire finished product is surface plated. 5ome 5ome spar spark k plug plugs s may may be desi design gned ed to have have to& to& three three or four four ground electrodes. 2t must be emphasi;ed as this point that according to the la la of phys physic ics& s& the spar spark k occu occurs rs at the poin pointt of least least resist resistan ance ce.. 9ultip 9ultiple le ground electrodes electrodes do $T indicate that there is more than one spark at a time to ignite the air-fuel mixture. 9ultiple ground electrodes are electrodes are designed designed to reduce the possibility possibility of the ground electro electrode de heating heating to the point of potential pre-ignition. 9ultiple ground electrode designs electrode designs also have a tendency of eroding (opening the preset gap) at a higher rate than normal (due to the firing taking place from the very tip of the ground to the center elect electro rode de). ). There Therefo fore& re& in a stand standard ard igni ignitio tion n syste system& m& mult multip iple le ground electrode designs are created to allo the spark to occur unobstructed electrode dire directl ctly y in the the air-f air-fue uell mixtu mixture re&& but but in no ay should should be cons constru trued ed as alloing multiple sparks to occur. 'eat range by range by definition is the range in hich the spark plug orks ell thermally. thermally. 2f the firing end temperature temperature of a spark plug drops belo its 0self-c 0self-clea leanin ning g temper temperatur ature%& e%& carbon carbon ill ill accumu accumulat late e on the firing firing end& end& causing the voltage supplied by the ignition system to 0leak aay% and $ spark ill occur at the gap. 34 5park 6lugs offers 6lugs offers a heat range <7= for the hottest heat range up to a <17= for the coldest heat range. That is& the loer the number& the hotter the 34 34 plug. plug. The higher the number& the colder the plug is considered in overall heat range. range. 2n a simple demonstration& the distance the heat must travel from the 0firing end% to the shell (for heat dissipation through the engine head and air or ater jacket) is shorter in a 0colder% plug thereby alloing the tip to cool at a faster rate. This design conse/uently is considered a
selected for a particular application should be in the +, degree ahrenheit to 1&+,, degree ahrenheit operating range ith the goal to maintain the self-cleaning range of 1&*, degrees ahrenheit. >here extreme conditions exist (upards to 1&,,,? @69 for some to-cy to-cycl cle e AT%s& T%s& moto motorcy rcycl cles es or ater ater craft craft and and raci racing ng engi engine nes s that that generat generate e in excess excess of ,, to 1&,,, '6) heat range issues range issues are particularly importa important nt to prevent prevent perman permanent ent damage damage to the engine. engine. $f course& course& the average engine of today can sustain a certain amount of abuse& but over time may result in permanent damage as ell. The spark plug structural factors that determine heat range are" •
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3as volume being determined by space formed beteen the metal shell and the insulator nose at the firing end. 5urface area andBor length of insulator nose at the firing end. Therm Thermal al cond conduc ucti tivi vity ty of the the mater materia ials ls for for the the insu insula lato tor& r& cent center er electrode and more importantly the overall design. The total structure of the center electrode. The relative position of the insulator tip to the shell end. umber of ground electrodes present. The spark plug operational factors affecting temperature are"
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The internal combustion engine%s air-fuel ratio. The overall compression ratio of the engine. The internal combustion engine ignition timing. The octane of the fuel. The engine @69 and overall load. The spark plug operational factors affecting affecting the re/uired voltage of the internal combustion engine are"
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The re/uired re/uired voltag voltage e (to (to jump jump the the gap gap ther thereb eby y firi firing ng the the plug plug)) decreases ith the increase in richness of the airBfuel ratio. The re/uire re/uired d voltag voltage e (to (to jump jump the the gap gap ther thereb eby y firi firing ng the the plug plug)) increases proportionally to the increase of (cylinder) pressure. 2gniti 2gnition on timing timing definitely definitely affects affects the re/uired voltage voltage (hether the timing is advanced or retarded). As the compression pressure at top dead dead center center (T.C.# (T.C.#.) .) reaches reaches the maximum maximum&& the re/uir re/uired ed voltage voltage reac reache hes s the the maxi maximu mum m as ell ell.. The The re/u re/uir ired ed volta oltage ge loe loers rs in rel relati ationsh onship ip to the the adv advance ance of the the igni gnition tion timi timing ng sin since the the compression pressure loers and the spark plug temperature rises. uel types and mixtures ill affect re/uired voltage. voltage .
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The spark plug gap (and resulting temperature) affects the re/uired voltage.. voltage The The pola polari rity ty of the the avai availa labl ble e seco second ndar ary y volt voltag age e il ill affe affect ct the the re/uired voltage. voltage. The re/uired voltage is loer ith minus polarity than ith positive polarity.
>hen >hen makin making g a visua visuall analy analysis sis of the spark plug the folloing may be considered in the cause and effect issue" #auses of overheating" • • • • • • • •
$ver advanced ignition timing. 2nsufficient cooling and lubricating ithin the engine. Too lean air-fuel mixture. Too hot of a spark plug heat range. 2nsufficient tor/ue of the spark plug gasket. Dxcessive deposits of unburned fuel in combustion chamber. Too lo octane fuel. #ontinuous driving under too heavy load for engine application. #auses of oil and carbon fouling" 888
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2mproper use or abuse of choke. #ontinuous lo-speed driving. 8888 Too rich air-fuel mixture. Too cold spark plug heat range. Eoered compression compression pressure and engine engine oil entering combustion combustion chamber. $ver retarded ignition timing. $verall deterioration of system. 6re-delivery fouling (consistent ith continuous lo-speed driving.
Footnotes:
88 ickel alloy has a melting point of 7&7,, to 7&,, degrees ahrenheit continuous. 888 #omm #ommon on depo deposi sits ts incl includ ude e carbo carbon& n& lead lead&& brom bromin ine& e& calc calciu ium& m& sulf sulfur& ur& barium& ;inc& iron& silicon or aluminum. 8888 2f engine is operated at constant lo speeds& a larger surface area at the ceramic tip improves the self-cleaning characteristics of the spark plug.
