JP4270784B2

JP4270784B2 - Spark plug

Info

Publication number: JP4270784B2
Application number: JP2001398549A
Authority: JP
Inventors: 友聡加藤; 守無笹
Original assignee: NGK Spark Plug Co Ltd
Current assignee: NGK Spark Plug Co Ltd
Priority date: 2000-12-27
Filing date: 2001-12-27
Publication date: 2009-06-03
Anticipated expiration: 2021-12-27
Also published as: JP2002260817A

Description

【０００１】
【発明の属する技術分野】
この発明はスパークプラグに関する。
【０００２】
【従来の技術】
自動車用ガソリンエンジンなどの内燃機関の着火用に使用されるスパークプラグにおいては、近年、エンジンの高性能化に伴いエンジンヘッドの構造も複雑化し、スパークプラグの取付けスペースも減少しているため、小型化への要求が喧しくなっている。スパークプラグの小型化は、すなわちエンジンヘッドへの取付部が形成された主体金具の小径化を意味するが、その内側に挿通される絶縁体は、耐電圧確保のため、むやみに小径化することができない。
【０００３】
ここで、スパークプラグの絶縁体は、先端側が段部により縮径されるとともに、主体金具の内周面に形成された凸条部に該段部を係合させる形にて組み付けられている。従って、このような構造において、主体金具の小径化を図る場合、絶縁体の外径縮小には限界があるので、主体金具側の上記凸状部の内周面と、これに対向する絶縁体外周面との隙間を縮小させる方法が採用される。
【０００４】
【発明が解決しようとする課題】
ところが、上記の隙間が小さくなると、スパークプラグの耐汚損性が悪化する問題がある。例えば、スパークプラグは、プレデリバリ時のように電極温度が４５０℃以下の低温環境で使用されると未燃ガスが多く発生する。こうした未燃ガス発生状況が長時間続くと、絶縁体がいわゆる「燻り」や「かぶり」の状態となり、表面がカーボンなどの導電性物質で汚損して作動不良が生じやすくなる。特に、未燃ガスの侵入により上記隙間内にて絶縁体表面が汚損すると、該隙間で火花放電が生じ、正常な着火が不能となる。
【０００５】
本発明の課題は、耐汚損性を損ねることなく小型化を図るのに好適な構造を有したスパークプラグを提供することにある。
【０００６】
【課題を解決するための手段及び作用・効果】
筒状の主体金具と該主体金具の内周側に係止された軸孔を有する絶縁体と該絶縁体の軸孔に保持された中心電極と、該中心電極の先端と対向することで火花放電ギャップを形成する接地電極とを備え、
絶縁体の軸線方向において火花放電ギャップの位置する側を前方側、これと反対側を後方側として、絶縁体は、前端部が周方向の段部により縮径されて該段部が絶縁体側係合部とされ、主体金具に対し後方側開口部から挿入されるとともに、絶縁体側係合部が主体金具の内周面から突出する金具側係合部と係合し、かつ、絶縁体の絶縁体側係合部よりも前方側に位置する部分の基端部の円筒面状外周面（隙間形成外周面）が、金具側係合部の円筒面状内周面（隙間形成内周面）と、所定量の係合位置隙間を形成する形にて対向するとともに、隙間形成外周面の外径をｄ１、隙間形成内周面の内径をＤ１として、
β＝（Ｄ１−ｄ１）／２ ‥‥▲１▼
にて表される係合位置隙間量βが０．０５ｍｍ以上０．４ｍｍ以下に調整されており、該係合位置隙間量βを確保する長さβＬが、０．５ｍｍ以上２．５ｍｍ以下であり、また絶縁体の隙間形成外周面よりも先端側は外周面が縮径していることを特徴とするスパークプラグ。
【０００７】
なお、隙間形成外周面の外径ｄ１と隙間形成内周面の内径Ｄ１との径差Ｄ１−ｄ１が軸線方向の位置によって異なる場合は、係合位置隙間量βは、該径差が最小となる位置での値により代表させるものとする。また、金具側係合部は例えば環状の凸条部とすることができるが、係合部としての機能を果たすことができれば形態はこれに限定されない。
【０００８】
前述の通り、スパークプラグの耐電圧特性を損ねることなく主体金具の外径を縮小しようとすると、絶縁体の肉厚をそれほど減少させることができないから、上記の係合位置隙間量βを小さくせざるを得ない。しかしながら、従来は、汚損時にこの隙間にて飛火することを極力避けるため、該βの値は可及的に大きく設定することが一つの技術常識として定着していた。従って、スパークプラグ小型化の要請により係合位置隙間量βを縮小することは、汚損時の飛火防止の観点からは一つのジレンマであると考えられていたのである。
【０００９】
しかしながら、係合位置隙間量βを中途半端に大きく設定するのではなく、むしろ積極的に縮小すると、意外にも耐汚損性が顕著に向上し、汚損時に該係合位置隙間にて飛火する不具合を効果的に防止できることを見出し、本発明を完成させるに至った。具体的には、上記係合位置隙間量βを０．４ｍｍ以下に調整することで、係合位置隙間への未燃ガスの侵入を確実にブロックすることができ、該係合位置隙間内において絶縁体表面が汚損することを防止できる。その結果、耐汚損性を損ねることなく、スパークプラグの小型化を有効に図ることができる。
【００１０】
係合位置隙間量βが０．４ｍｍを超えると未燃ガスの侵入を食い止めることが困難となり、係合位置隙間内における絶縁体表面の汚損を防止できなくなる。なお、係合位置隙間量βが極端に小さくなると、係合位置隙間内への汚損物質の侵入は生じないが、係合位置隙間から前方側に延びる絶縁体表面に汚損物質が付着した場合に、その汚損物質の堆積層が、係合位置隙間を挟んで反対側に位置する主体金具側係合部と接触して短絡を生じやすくなり、着火性が却って損なわれる場合がある。この点を考慮して、係合位置隙間量βは例えば０．０５ｍｍ以上確保すること、より望ましくは０．２ｍｍ以上確保することが望ましい。また、この係合位置隙間量βを確保する長さβＬは、０．５ｍｍ以上であることが望ましい。長さβＬが０．５ｍｍ未満であると、未燃ガスの侵入を食い止める効果が低下しやすくなる。一方、この長さβＬが長くなると係合位置隙間が前方側に延長してくることになる。このとき、汚損物質が係合位置隙間近傍に付着すると、この係合位置隙間近傍において飛火しやすくなる。従って、この長さβＬは２．５ｍｍ以下であることが望ましい。
【００１１】
【発明の実施の形態】
以下、本発明の実施の形態を図面を参照して説明する。
図１及び図２は、本発明の一実施形態としてのスパークプラグ１００を示す。図１は全体の縦断面図であり、図２はその先端側要部を拡大して示す図である。該スパークプラグ１００は、筒状の主体金具１、先端部２１が突出するようにその主体金具１の内側に嵌め込まれた絶縁体２、先端部３ｅを突出させた状態で絶縁体２の内側に設けられた中心電極３、及び主体金具１に一端が溶接等により結合されるとともに他端側が側方に曲げ返されて、その側面が中心電極３の先端部と対向するように配置された接地電極４等を備えている。また、図２に示すように、接地電極４と中心電極３との間には、間隔αの火花放電ギャップｇが形成されている。接地電極４及び中心電極３の本体部３ａはＮｉ合金等で構成されている。また、中心電極３の本体部３ａの内部には、放熱促進のためにＣｕあるいはＣｕ合金等で構成された芯材３ｂが埋設されている。
【００１２】
主体金具１は、低炭素鋼等の金属により筒状に形成されており、スパークプラグ１００のハウジングを構成するとともに、その外周面には、スパークプラグ１００を図示しないエンジンブロックに取り付けるための取付ねじ部７が形成されている。なお、１ｅは、主体金具１を取り付ける際に、スパナやレンチ等の工具を係合させる工具係合部であり、六角状の軸断面形状を有している。また、絶縁体２は、全体がアルミナ系セラミック焼結体として構成され、軸線方向Ｏに沿って貫通孔６が形成されており、その一方の端部側に端子金具１３が固定され、同じく他方の端部側に中心電極３が固定されている。また、該貫通孔６内において端子金具１３と中心電極３との間に抵抗体１５が配置されている。この抵抗体１５の両端部は、導電性ガラスシール層１６，１７を介して中心電極３と端子金具１３とにそれぞれ電気的に接続されている。これら抵抗体１５と導電性ガラスシール層１６，１７とが焼結導電材料部を構成している。なお、抵抗体１５は、ガラス粉末と導電材料粉末（及び必要に応じてガラス以外のセラミック粉末）との混合粉末を原料とする抵抗体組成物で構成される。
【００１３】
絶縁体２の軸方向中間には、周方向外向きに突出する突出部２ｅが例えばフランジ状に形成されている。そして、絶縁体２には、軸線方向Ｏにおいて、中心電極３の火花放電ギャップｇに向かう側を前方側として、該突出部２ｅよりも後方側がこれよりも細径に形成された本体部２ｂとされている
。一方、突出部２ｅの前方側にはこれよりも細径の第一軸部２ｇと、その第一軸部２ｇよりもさらに細径の第二軸部２ｉがこの順序で形成されている。なお、本体部２ｂの外周面後端部にコルゲーション部を形成してもよい。
【００１４】
他方、中心電極３の軸断面径は抵抗体１５の軸断面径よりも小さく設定されている。そして、絶縁体２の貫通孔６は、中心電極３を挿通させる略円筒状の第一部分６ａと、その第一部分６ａの後方側（図面上方側）においてこれよりも大径に形成される略円筒状の第二部分６ｂとを有する。端子金具１３と抵抗体１５とは第二部分６ｂ内に収容され、中心電極３は第一部分６ａ内に挿通される。中心電極３の後端部には、その外周面から外向きに突出して電極固定用凸部３ｃが形成されている。そして、上記貫通孔６の第一部分６ａと第二部分６ｂとは、図２の第一軸部２ｇ内において互いに接続しており、その接続位置には、中心電極３の電極固定用凸部３ｃを受けるための凸部受け面６ｃがテーパ面あるいはアール面状に形成されている。
【００１５】
絶縁体２には、主体金具１に対し後方側開口部から挿入されるとともに、第一軸部２ｇと第二軸部２ｉとの接続部が周方向の段部が形成されている。該段部は絶縁体側係合部２ｈとして、主体金具１の内面に形成された金具側係合部としての周方向の環状の凸条部１ｃと、リング状の板パッキン６３を介して係合することにより、軸方向の抜止めがなされている。他方、主体金具１の後方側開口部内面と、絶縁体２の外面との間には、フランジ状の突出部２ｅの後方側周縁と係合するリング状の線パッキン６２が配置され、そのさらに後方側にはタルク等の充填層６１を介してリング状の線パッキン６０が配置されている。そして、絶縁体２を主体金具１に向けて前方側に押し込み、その状態で主体金具１の開口縁をパッキン６０に向けて内側に加締めることにより加締め部１ｄが形成され、主体金具１が絶縁体２に対して固定されている。
【００１６】
図２に示すように、絶縁体の絶縁体側係合部２ｈよりも前方側に位置する部分である第二軸部２ｉの基端部外周面（隙間形成外周面）２ｋが、金具側係合部たる凸条部１ｃの内周面（隙間形成内周面）５２と、所定量の係合位置隙間Ｑを形成する形にて対向している。そして、隙間形成外周面２ｋの外径をｄ１、隙間形成内周面５２の内径をＤ１として、
β＝（Ｄ１−ｄ１）／２（前記▲１▼式）
にて表される係合位置隙間量βが０．４ｍｍ以下（望ましくは０．０５ｍｍ以上）に調整されている。
【００１７】
上記係合位置隙間量βを０．４ｍｍ以下に調整することで、例えばプレデリバリ時等の汚損の生じやすい使用環境下においても、係合位置隙間Ｑへの未燃ガスの侵入を確実にブロックすることができ、該係合位置隙間Ｑ内において絶縁体２の表面（隙間形成外周面２ｋ）が汚損することを防止できる。その結果、耐汚損性を損ねることなく、スパークプラグ１００の小型化を有効に図ることができる。例えば、主体金具１の前端側外周面に形成される取付ねじ部７の呼びをＭ１２以下に縮小しても耐汚損性を良好に保持できる。具体的には、取付ねじ部７は、具体的にはＭ１２あるいはＭ１０等の値を採用できる（本明細書において取付ねじ部の呼びは、ＩＳＯ２７０５（Ｍ１２）及びＩＳＯ２７０４（Ｍ１０）に規定された値を意味し、当然に、該規格に定められた寸法公差の範囲内での変動を許容する）。本発明によると、該係合位置隙間Ｑが０．４ｍｍ以下と、従来のスパークプラグよりも小さい値に設定されるから、取付ねじ部７の寸法を縮小した場合も、主体金具側係合部との係合位置における絶縁体２の肉厚をそれほど小さくしなくて済む。従って、係合位置隙間Ｑの縮小により耐汚損性が向上するとともに、絶縁体２の耐電圧特性も良好に維持できる。
【００１８】
なお、本実施形態では、第一軸部２ｇの外周面は略円筒状とされ、他方、隙間形成外周面２ｋをなす第二軸部２ｉの基端部の外周面は、軸線方向Ｏにおいて、係合位置隙間Ｑが略一定（かつ最小値）となるように、隙間形成内周面５２と略同軸的な円筒面状とされている。また、第二軸部２ｉの隙間形成外周面２ｋよりも先端側の外周面は、先端に向かうほど縮径する円錐面状とされている。
【００１９】
また、上記のように取付ねじ部７の呼びを小さくしようとした場合、係合位置隙間Ｑよりも前方側に延出する第二軸部２ｉの外周面と、主体金具１の内周面との間に形成される隙間、すなわちガスボリューム部ＧＶの幅Ｊも小さくなりやすい。該幅Ｊが小さくなりすぎると、係合位置隙間Ｑ内が清浄であっても、それよりも前方側にて第二軸部２ｉが汚損すれば、主体金具１の内周面と第二軸部２ｉの外周面との間で飛火する、いわゆる横飛火の問題が生じやすくなる。そこで、これを防止するために、主体金具１の前端面側開口部の内径をＤ２、当該前端面位置における絶縁体２（第二軸部２ｉ）の外径をｄ２として、
Ｅ＝（Ｄ２−ｄ２）／２ ‥‥▲２▼
にて表されるガスボリューム部端面幅Ｅを、火花放電ギャップｇの間隔をαとして、
１．１α≦Ｅ ‥‥▲３▼
を満足するように調整することが有効である。
【００２０】
絶縁体２は火花放電ギャップｇに近い先端部付近が電界集中しやすく、また、主体金具１の端面内周縁には電界集中しやすいエッジが形成されることから、ガスボリューム部ＧＶにおける横飛火の問題は、主体金具１の先端面位置にて生じやすい傾向にある。しかしながら、この位置でのガスボリューム部ＧＶの幅、つまりガスボリューム部端面幅Ｅを、正規の飛火位置である火花放電ギャップｇの間隔αよりも大きくしておくことで、絶縁体２（第二軸部２ｉ）の表面が汚損した場合でも横飛火の問題を効果的に抑制できる。なお、本明細書においては、▲２▼式に示すように、ガスボリューム部端面幅Ｅを主体金具１と絶縁体２との径差を用いて定義しているが、例えば絶縁体２を主体金具１に組み付ける際に若干の偏心が生じたりすると、主体金具１の内周面と絶縁体２（第二軸部２ｉ）の外周面との実際の距離が局所的に縮小し、該位置での横飛火発生が問題となることも考えられる。そこで、その影響を吸収できるよう、Ｅの値は▲３▼式のように、若干の余裕を見込んだ値に設定している。しかし、組み付け時の偏心等を確実に防止できる場合は、α＜Ｅとしても差し支えない。
【００２１】
また、絶縁体２（第二軸部２ｉ）の先端部の汚損に伴う横飛火は、常に主体金具１の端面位置で生ずるとは限らず、ガスボリューム部ＧＶの幅によっては主体金具の多少奥まった位置で発生することもありえる。そこで、このような横飛火を防止するには、絶縁体側係合部２ｈよりも前方側において、軸線Ｏと直交する仮想平面による絶縁体２の断面外形線の直径をｄ３、これに対応する位置における主体金具１の内径をＤ３としたときに、主体金具１の前端面位置から少なくとも７ｍｍまでの区間の任意位置において、
α＜（Ｄ３−ｄ３）／２ ‥‥▲４▼
を満足していること、つまり、主体金具１の前端面位置から７ｍｍ以上確保された区間Ｌにおいて、α＜（Ｄ３−ｄ３）／２を満足していることが有効である。
【００２２】
軸線方向Ｏのある位置におけるガスボリューム部ＧＶの幅Ｊ（＝（Ｄ３−ｄ３）／２）が、火花放電ギャップｇの間隔αよりも大であれば、その位置での横飛火は本質的に生じにくくなる。他方、横飛火発生に影響する絶縁体表面の電界強度は、火花放電ギャップｇに近い先端部付近にて高くなるが、軸線方向Ｏにおいて後方側に離間するにつれ次第に減少する。しかしながら、有限要素法による電界強度分布シミュレーションを用いて本発明者らが検討したところによると、主体金具の前端面位置から軸線方向において７ｍｍ程度までの区間では、絶縁体表面の電界強度がある程度高くなると予想され、横飛火発生が懸念された。そこで、少なくともこの区間において、上記のガスボリューム部幅Ｊを、正規の放電場所である火花放電ギャップｇの間隔αよりも大きくなるように調整すると、主体金具１の奥まった位置での横飛火が実際に効果的に抑制できるようになる。
【００２３】
次に、金具側係合部たる凸条部１ｃにおいて隙間形成内周面５２には、絶縁体２の軸線Ｏ（ここでは、主体金具１の軸線とも一致している）を含む仮想平面による断面外形線が、隙間形成外周面２ｋと対向する平坦部５２ａと、該平坦部５２ａの前方側端部から主体金具１の内周面に向けて下る傾斜部５２ｂとを有する。そして、それら平坦部５２ａと傾斜部５２ｂとのなす角度θが、
１４０゜≦θ≦１６０゜ ‥‥▲５▼
を満足するものとなっている。該平坦部５２ａと傾斜部５２ｂとの交差位置にはエッジ部が形成されるが、それらのなす角度θを▲５▼のようにやや大きめに設定しておけば、形成されるエッジ部への過度の電界集中が回避でき、耐電圧性能をより向上させることができる。ただし、θが１４０゜未満では効果が小さく、θが１６０゜を超えると、傾斜部５２ｂが主体金具１の内周面に向けてだらだらと長く裾を引く形となり、ガスボリューム部ＧＶにおいて電界強度の高い領域が、肉厚の小さい絶縁体２（第二軸部２ｉ）の先端部にまで広がって、耐電圧性能が却って損なわれる場合がある。また、ガスボリューム部ＧＶの幅Ｊの小さくなる区間が長くなるので、横飛び防止の観点においても不利に作用する場合がある。本実施形態では、平坦部５２ａが第二軸部２ｉの基端部外周面２ｋと同軸的な円筒面をなしており、他方、傾斜部５２ｂは円錐面状に形成されている。
【００２４】
以下、スパークプラグ１００に付加可能な種々の変形について説明する（なお、図１及び図２と共通の部分には同一の符号を付与して詳細な説明を省略する）。まず、図３においては、ガスボリューム部ＧＶの幅Ｊが、火花放電ギャップｇの間隔αよりも大となる区間Ｌの長さをなるべく大きくできるように、第二軸部２ｉの円筒状の基端部２ｒに対し、縮径部２ｊを介して先端本体部分２ｓを接続した形態としている。本実施形態では電界集中しやすい急角度のエッジを生じにくくするため、縮径部２ｊを円錐面状（テーパ状）としている。
【００２５】
また、図４（ａ）の実施形態においても、金具側係合部たる凸状部１ｃの隙間形成内周面５２は、軸線Ｏを含む仮想平面による断面外形線が、隙間形成外周面２ｋと対向する平坦部５２ａと、該平坦部５２ａの前方側端部から主体金具１の内周面に向けて下る傾斜部５２ｂとを有する。そして、それら平坦部５２ａと傾斜部５２ｂとの交差位置に面取り部５２ｃが形成されている（図４（ｂ）に拡大図を示す）。この構成により、平坦部５２ａと傾斜部５２ｂとの交差位置への電界集中が起こり難くなり、平坦部５２ａと傾斜部５２ｂとの角度θを大きくするのと同様の効果を達成できる。図４の実施形態においても絶縁体１の第二軸部２ｉは、図３と同様に、円筒状の基端部２ｒに対し、縮径部２ｊを介して先端本体部分２ｓを接続した形態を有する。なお、図３では先端本体部分２ｓの外周面が円錐面状となっていたが、図４では、主体金具１のより奥まった位置までガスボリューム部ＧＶの幅Ｊがなるべく広くなるように、先端本体部分２ｓの外周面を円筒面状としている。なお、図４（ｃ）に示すように、面取り部５２ｃに代えてアール部５２ｒを設けてもよい。
【００２６】
また、中心電極３の前端面に、Ｉｒ又はＰｔを主成分とする直径１ｍｍ以下の貴金属発火部を固着してもよい。電極先端部を直径１ｍｍ以下に縮径すれば、火花放電ギャップｇに面する電極先端部に電界を集中できるため、放電電圧を下げることができる。また、該電極先端部を貴金属発火部となすことで、火花消耗が抑制されスパークプラグの長寿命化を図ることができる。そして、主体金具１の取付ねじ部７の小径化により絶縁体２が多少薄肉化しても、放電電圧が下がるので、その分、耐電圧性能に余裕を持たせることができる。ただし、過度の電界集中による火花消耗の進行を抑制する観点において、貴金属発火部の直径は０．２ｍｍ以上とすることが望ましい。
【００２７】
さらに、中心電極３の先端部にＩｒ合金（合金成分は例えばＲｈ、ＰｔあるいはＮｉ等）からなる貴金属発火部がレーザー溶接により固着され、接地電極４には上記発火部に対向する形で、ＰｔあるいはＰｔ合金（合金成分は例えばＮｉ等）からなる発火部が抵抗溶接により固着されるとともに、それら発火部と、対向する発火部との間の隙間を火花放電ギャップｇとしてもよい。
【００２８】
【実施例】
本発明の効果を確認するために、以下の実験を行なった。（実施例１）
図１及び図２に示すスパークプラグにおいて、取付ねじ部７の呼びをＭ１２、火花放電ギャップｇの間隔αを１．１ｍｍ、ガスボリューム部端面幅Ｅとαとの比Ｅ／αを１．４、ガスボリューム部幅ＪについてＪ＞αとなる区間Ｌの長さを７ｍｍ、凸状部１ｃの平坦部５２ａと傾斜部５２ｂとの角度θを１５０゜とし、係合位置隙間量βの値を０．１〜０．６ｍｍの種々の値に設定したものを試験品として用意した。そして、各スパークプラグの耐汚損性を調べるために、下記の条件でプレデリバリ耐久試験を行った。すなわち、スパークプラグを、接地電極４側を正、中心電極２側を負とする電圧印加極性で試験用自動車（排気量：１５００ｃｃ、直列４気筒）に取り付け、ＪＩＳ：Ｄ１６０６に例示されている走行パターン（テスト室温：−１０℃）を１サイクルとして、スパークプラグの絶縁抵抗が１０ＭΩ以下に低下するまでこれを繰り返し、そのサイクル数により１０サイクル以上を「○」、８〜９サイクルを「△」、６サイクル以下を「×」（○と△は可、×は不可）として判定した。以上の結果を表１に示す。
【００２９】
【表１】

【００３０】
これによると、係合位置隙間量βの値を０．４ｍｍ以下とすることにより、スパークプラグの耐汚損性が顕著に向上していることがわかる。
【００３１】
（実施例２）
図１及び図２に示すスパークプラグにおいて、取付ねじ部７の呼びをＭ１２、火花放電ギャップｇの間隔αを１．１ｍｍ、ガスボリューム部端面幅Ｅとαとの比Ｅ／αを１．４、凸状部１ｃの平坦部５２ａと傾斜部５２ｂとの角度θを１５０゜、係合位置隙間量βの値を０．４ｍｍとし、ガスボリューム部幅ＪについてＪ＞αとなる区間Ｌの長さを５〜８．３ｍｍの種々の値に設定したものを試験品として用意した。そして、各スパークプラグの低温始動性を調べるために、下記の条件で試験を行った。すなわち、スパークプラグを、接地電極４側を正、中心電極３側を負とする電圧印加極性で試験用自動車（排気量：１５００ｃｃ、直列４気筒）に取り付け、アイドリング３０秒＋停止３０分のサイクルを繰り返し、始動不能となるまでのサイクル数を求める試験を、室温−３０℃と−１０℃との二条件にて行なった。いずれも、そのサイクル数により５サイクル以上を「○」、４サイクル以下を「×」（○は可、×は不可）として判定した。以上の結果を表２に示す。
【００３２】
【表２】

【００３３】
この結果によると、−１０℃の試験ではどの試験品も問題は生じなかったが、より低温で過酷な条件となる−３０℃の試験では、Ｌを７ｍｍ以上確保した試験品において良好な結果が得られた。なお、Ｌが７ｍｍ未満の試験品にて始動不能サイクルが小さくなった理由は、絶縁体汚損の進行により横飛火が発生しやすくなったことが原因と考えられる。
【００３４】
（実施例３）
図１及び図２に示すスパークプラグにおいて、取付ねじ部７の呼びをＭ１２、火花放電ギャップｇの間隔αを１．１ｍｍ、凸状部１ｃの平坦部５２ａと傾斜部５２ｂとの角度θを１５０゜、係合位置隙間量βの値を０．４ｍｍ、第二軸部２ｉの外周面傾斜角度を種々に変更することによりガスボリューム部端面幅Ｅとαとの比Ｅ／αを０．９〜１．７の種々の値に調整した試験品として用意した。これらのスパークプラグは、発火部を予め燻らせた後、可視チャンバーに取り付け、チャンバー内の空気圧を０．４ＭＰａに設定して火花放電させるとともに、放電１０００回中、金具への横飛火が何回発生したかを目視確認することにより、横飛火発生頻度を調べた。結果を図５に示す。これによると、Ｅ／αを１．１以上とすることにより、横飛火発生頻度が顕著に減少していることがわかる。
【００３５】
（実施例４）
図１及び図２に示すスパークプラグにおいて、取付ねじ部７の呼びをＭ１２、火花放電ギャップｇの間隔αを１．１ｍｍ、ガスボリューム部端面幅Ｅとαとの比Ｅ／αを１．４、ガスボリューム部幅ＪについてＪ＞αとなる区間Ｌの長さを７ｍｍとし、凸状部１ｃの平坦部５２ａと傾斜部５２ｂとの角度θを１３５〜１７０゜としたものを試験品として用意した。また、角度θを１２０゜とする代わりに、図４のような面取部５２ｃ（面取り幅０．５ｍｍ）を設けた試験品も用意した。
【００３６】
これら試験品の寸法・形状を初期条件として用い、中心電極３への印加電圧レベルを１０ｋＶとしたときのガスボリューム部ＧＶ内の電界強度分布を、市販のソフトウェアにより有限要素法によりシミュレーションするとともに、平坦部５２ａと傾斜部５２ｂとの交差部直近位置での電界強度を読み取った。結果を表３に示す。
【００３７】
【表３】

【００３８】
これによると、角度θを１４０゜以上としたもの、あるいは面取りを施したものは、電界強度が顕著に小さくなっていることがわかる。図６（ａ）はθ＝１３５゜、同図（ｂ）はθ＝１５０゜のもののシミュレーション結果を示すものであり、図中明るく表れている領域ほど電界強度が高いことを示す。これによると、角度θの小さい前者においては、交差部直近位置に電界集中部が顕著に現われているのに対し、θの大きい後者においては電界集中の度合いが和らげられている様子がよくわかる。
【００３９】
次に、上記各試験品から接地電極を取り除き、その状態で主体金具の開口側をシリコンオイル等の液状絶縁媒体中に浸漬することにより、絶縁体の外面と主体金具の内面との間を該液状絶縁媒体で満たして絶縁した。この状態で、主体金具と中心電極３との間に、高圧電源により交流高電圧又はパルス状高電圧を印加するとともに、その電圧波形をオシロスコープ等により記録し、その電圧波形から、絶縁体に貫通破壊が生じたときの電圧値を、貫通破壊耐電圧として読み取った。なお、各試験品ともｎ＝４０にて試験を行い、耐電圧の平均値と最小値とを求めた。以上の結果を表４に示す。
【００４０】
【表４】

【００４１】
これによると、角度θを１４０゜〜１６０゜としたもの、あるいは面取りを施したものは、耐電圧の平均値及び最小値がいずれも高く、安定した耐電圧性能を有していることがわかる。他方、角度θが１４０゜未満では、耐電圧の平均値及び最小値がともに低下し、耐電圧性能が相対的に低下する傾向が認められる。また、角度θが１６０゜を超えると、耐電圧の平均値は比較的良好であるが、最小値が低下し、耐電圧性能にばらつきが生じやすくなる傾向を示している。
【図面の簡単な説明】
【図１】本発明の一実施形態たるスパークプラグの全体構成を示す縦断面図。
【図２】図１の先端側要部を拡大して示す縦断面図。
【図３】図１のスパークプラグの第一変形例を示す要部縦断面図。
【図４】同じく第二変形例を示す要部縦断面図。
【図５】実施例３の実験結果を示すグラフ。
【図６】実施例４のいくつかのシミュレーション結果を示す図。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a spark plug.
[0002]
[Prior art]
In recent years, spark plugs used to ignite internal combustion engines such as gasoline engines for automobiles have become more compact as the engine head structure has become more complex and the space for installing the spark plugs has decreased as the performance of the engine has increased. The demand for conversion is becoming quarreled. Miniaturization of the spark plug means that the diameter of the metal shell on which the mounting portion to the engine head is formed, but the insulator inserted inside it must be reduced in size to ensure withstand voltage. I can't.
[0003]
Here, the insulator of the spark plug is assembled in such a manner that the tip side is reduced in diameter by the step portion and the step portion is engaged with the protruding portion formed on the inner peripheral surface of the metal shell. Therefore, in such a structure, when reducing the diameter of the metal shell, there is a limit to the reduction of the outer diameter of the insulator, so the inner peripheral surface of the convex portion on the metal shell side and the insulator outer surface facing this A method of reducing the gap with the peripheral surface is adopted.
[0004]
[Problems to be solved by the invention]
However, when the gap is reduced, there is a problem that the antifouling property of the spark plug is deteriorated. For example, when a spark plug is used in a low temperature environment where the electrode temperature is 450 ° C. or lower as in pre-delivery, a large amount of unburned gas is generated. If such an unburned gas generation state continues for a long time, the insulator is in a so-called “blow” or “fogging” state, and the surface is easily fouled by a conductive material such as carbon, and malfunction tends to occur. In particular, when the surface of the insulator is soiled in the gap due to the intrusion of unburned gas, a spark discharge occurs in the gap and normal ignition is impossible.
[0005]
An object of the present invention is to provide a spark plug having a structure suitable for downsizing without impairing stain resistance.
[0006]
[Means for solving the problems and actions / effects]
An insulator having a cylindrical metal shell, a shaft hole locked to the inner peripheral side of the metal shell, a center electrode held in the shaft hole of the insulator, and a spark by being opposed to the tip of the center electrode A ground electrode that forms a discharge gap;
With the spark discharge gap side in the axial direction of the insulator as the front side and the opposite side as the rear side, the insulator has a front end that is reduced in diameter by a step in the circumferential direction. And is inserted into the metal shell from the rear opening, and the insulator-side engagement portion engages with the metal-side engagement portion protruding from the inner peripheral surface of the metal shell, and the insulator is insulated. Of the part located in front of the body side engaging part Cylindrical surface of the base end The outer peripheral surface (gap-forming outer peripheral surface) Cylindrical surface The inner circumferential surface (gap forming inner circumferential surface) is opposed to form a predetermined amount of engagement position gap, the outer diameter of the gap forming outer circumferential surface is d1, and the inner diameter of the gap forming inner circumferential surface is D1,
β = (D1-d1) / 2 (1)
The engagement position gap amount β represented by is adjusted to 0.05 mm to 0.4 mm, and the length βL for securing the engagement position gap amount β is 0.5 mm to 2.5 mm. Ah In addition, the outer peripheral surface is smaller in diameter than the outer peripheral surface forming the gap of the insulator. A spark plug characterized by that.
[0007]
When the diameter difference D1-d1 between the outer diameter d1 of the gap forming outer peripheral surface and the inner diameter D1 of the gap forming inner peripheral surface varies depending on the position in the axial direction, the engagement position gap amount β is the smallest in the diameter difference. It shall be represented by the value at a certain position. Moreover, although a metal fitting side engaging part can be made into a cyclic | annular protruding item | line part, for example, if the function as an engaging part can be fulfill | performed, a form will not be limited to this.
[0008]
As described above, if it is attempted to reduce the outer diameter of the metal shell without impairing the withstand voltage characteristics of the spark plug, the thickness of the insulator cannot be reduced so much. I must. Conventionally, however, it has been established as one technical common sense that the value of β is set as large as possible in order to avoid flying in this gap as much as possible when fouling. Therefore, reducing the engagement position gap amount β in response to a request for downsizing of the spark plug was considered to be a dilemma from the viewpoint of preventing flying at the time of fouling.
[0009]
However, if the engagement position gap amount β is not set to a large value rather than halfway, but rather actively reduced, the antifouling property is unexpectedly improved, and the failure occurs in the engagement position gap at the time of contamination. Has been found to be effectively prevented, and the present invention has been completed. Specifically, by adjusting the engagement position gap amount β to 0.4 mm or less, it is possible to reliably block the intrusion of unburned gas into the engagement position gap. It is possible to prevent the insulator surface from being soiled. As a result, it is possible to effectively reduce the size of the spark plug without impairing the stain resistance.
[0010]
When the engagement position gap amount β exceeds 0.4 mm, it becomes difficult to prevent the intrusion of unburned gas, and it becomes impossible to prevent the insulator surface from being contaminated in the engagement position gap. Note that when the engagement position gap amount β is extremely small, the entry of the pollutant into the engagement position gap does not occur, but when the fouling substance adheres to the insulator surface extending forward from the engagement position gap. The deposited layer of the fouling substance comes into contact with the metal shell side engaging portion located on the opposite side with the engagement position gap interposed therebetween, so that a short circuit is likely to occur, and the ignitability may be impaired instead. Considering this point, it is desirable to secure the engagement position gap amount β, for example, 0.05 mm or more, and more desirably 0.2 mm or more. Also this engagement position The length βL for securing the gap amount β is preferably 0.5 mm or more. If the length βL is less than 0.5 mm, the effect of stopping the intrusion of unburned gas tends to be reduced. On the other hand, when the length βL becomes longer, the engagement position gap extends forward. At this time, if the fouling substance adheres in the vicinity of the engagement position gap, it becomes easy to fly near the engagement position gap. Therefore, this length βL is desirably 2.5 mm or less.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
1 and 2 show a spark plug 100 as an embodiment of the present invention. FIG. 1 is an overall vertical cross-sectional view, and FIG. 2 is an enlarged view of a main portion on the tip side. The spark plug 100 includes a cylindrical metal shell 1, an insulator 2 fitted inside the metal shell 1 so that the tip 21 protrudes, and an insulator 2 inside the insulator 2 with the tip 3 e protruding. One end of the center electrode 3 and the metal shell 1 are connected to each other by welding or the like, and the other end side is bent back to the side so that the side surface faces the tip of the center electrode 3. The electrode 4 etc. are provided. As shown in FIG. 2, a spark discharge gap g having an interval α is formed between the ground electrode 4 and the center electrode 3. The main body 3a of the ground electrode 4 and the center electrode 3 is made of Ni alloy or the like. A core material 3b made of Cu or Cu alloy is embedded in the main body 3a of the center electrode 3 to promote heat dissipation.
[0012]
The metal shell 1 is formed in a cylindrical shape from a metal such as low carbon steel, and constitutes a housing of the spark plug 100. On the outer peripheral surface thereof, a mounting screw for attaching the spark plug 100 to an engine block (not shown) Part 7 is formed. In addition, 1e is a tool engaging part which engages tools, such as a spanner and a wrench, when attaching the metal shell 1, and has a hexagonal axial cross-sectional shape. The insulator 2 is entirely configured as an alumina ceramic sintered body, and has a through hole 6 formed along the axial direction O. A terminal fitting 13 is fixed to one end side of the insulator 2, and the other The center electrode 3 is fixed to the end portion side. A resistor 15 is disposed between the terminal fitting 13 and the center electrode 3 in the through hole 6. Both ends of the resistor 15 are electrically connected to the center electrode 3 and the terminal fitting 13 through the conductive glass seal layers 16 and 17, respectively. The resistor 15 and the conductive glass seal layers 16 and 17 constitute a sintered conductive material portion. In addition, the resistor 15 is comprised by the resistor composition which uses the mixed powder of glass powder and electrically-conductive material powder (and ceramic powder other than glass as needed) as a raw material.
[0013]
In the middle of the insulator 2 in the axial direction, a protruding portion 2e protruding outward in the circumferential direction is formed in a flange shape, for example. The insulator 2 has a main body 2b in which the side toward the spark discharge gap g of the center electrode 3 in the axial direction O is defined as the front side, and the rear side of the protrusion 2e is formed with a smaller diameter than the main body 2b. Has been
. On the other hand, on the front side of the protruding portion 2e, a first shaft portion 2g having a smaller diameter and a second shaft portion 2i having a smaller diameter than the first shaft portion 2g are formed in this order. In addition, you may form a corrugation part in the outer peripheral surface rear end part of the main-body part 2b.
[0014]
On the other hand, the axial sectional diameter of the center electrode 3 is set smaller than the axial sectional diameter of the resistor 15. The through-hole 6 of the insulator 2 has a substantially cylindrical first portion 6a through which the center electrode 3 is inserted, and a substantially cylindrical shape having a larger diameter on the rear side (upper side in the drawing) of the first portion 6a. Second portion 6b. The terminal fitting 13 and the resistor 15 are accommodated in the second portion 6b, and the center electrode 3 is inserted into the first portion 6a. At the rear end portion of the center electrode 3, an electrode fixing convex portion 3c is formed so as to protrude outward from the outer peripheral surface thereof. The first portion 6a and the second portion 6b of the through-hole 6 are connected to each other within the first shaft portion 2g in FIG. 2, and the electrode fixing convex portion 3c of the center electrode 3 is located at the connection position. The convex receiving surface 6c for receiving the tape is formed in a tapered surface or a rounded surface.
[0015]
The insulator 2 is inserted into the metal shell 1 from the rear side opening, and a connecting portion between the first shaft portion 2g and the second shaft portion 2i is formed with a step portion in the circumferential direction. The stepped portion is engaged as an insulator-side engaging portion 2 h via a ring-shaped plate packing 63 and a ring-shaped plate packing 63 in the circumferential direction as a fitting-side engaging portion formed on the inner surface of the metal shell 1. By doing so, the axial direction is prevented. On the other hand, a ring-shaped wire packing 62 that engages with the rear peripheral edge of the flange-shaped protrusion 2e is disposed between the inner surface of the rear opening of the metal shell 1 and the outer surface of the insulator 2, and further On the rear side, a ring-shaped wire packing 60 is disposed via a filling layer 61 such as talc. Then, the insulator 2 is pushed forward toward the metal shell 1, and in this state, the crimping portion 1d is formed by crimping the opening edge of the metal shell 1 toward the packing 60 inward. It is fixed with respect to the insulator 2.
[0016]
As shown in FIG. 2, the base end outer peripheral surface (gap-forming outer peripheral surface) 2k of the second shaft portion 2i, which is a portion located on the front side of the insulator-side engaging portion 2h of the insulator, is engaged with the bracket side It faces the inner peripheral surface (gap forming inner peripheral surface) 52 of the protruding ridge portion 1c as a part in a form that forms a predetermined amount of engagement position gap Q. The outer diameter of the gap forming outer peripheral surface 2k is d1, and the inner diameter of the gap forming inner peripheral surface 52 is D1,
β = (D1-d1) / 2 (formula (1))
Is adjusted to 0.4 mm or less (preferably 0.05 mm or more).
[0017]
By adjusting the engagement position gap amount β to 0.4 mm or less, it is possible to reliably block the intrusion of unburned gas into the engagement position gap Q even in a use environment where contamination easily occurs, for example, during pre-delivery. It is possible to prevent the surface of the insulator 2 (gap-forming outer peripheral surface 2k) from being contaminated in the engagement position gap Q. As a result, it is possible to effectively reduce the size of the spark plug 100 without impairing the fouling resistance. For example, even if the nominal size of the mounting screw portion 7 formed on the outer peripheral surface of the front end side of the metal shell 1 is reduced to M12 or less, the stain resistance can be maintained well. Specifically, the mounting screw portion 7 can specifically adopt a value such as M12 or M10 (in this specification, the designation of the mounting screw portion is a value defined in ISO 2705 (M12) and ISO 2704 (M10)). Naturally, variation within the dimensional tolerances defined in the standard is allowed). According to the present invention, the engagement position gap Q is set to 0.4 mm or less, which is smaller than the conventional spark plug. Therefore, even when the size of the mounting screw portion 7 is reduced, the metal shell side engagement portion It is not necessary to make the thickness of the insulator 2 at the engagement position so small. Therefore, the anti-fouling property is improved by reducing the engagement position gap Q, and the withstand voltage characteristic of the insulator 2 can be maintained well.
[0018]
In the present embodiment, the outer peripheral surface of the first shaft portion 2g is substantially cylindrical, while the outer peripheral surface of the base end portion of the second shaft portion 2i forming the gap forming outer peripheral surface 2k is in the axial direction O. The engagement position gap Q is a substantially cylindrical surface that is substantially coaxial with the gap forming inner circumferential surface 52 so that the engagement position gap Q is substantially constant (and the minimum value). Further, the outer peripheral surface on the tip side of the gap forming outer peripheral surface 2k of the second shaft portion 2i has a conical surface shape whose diameter decreases toward the front end.
[0019]
Moreover, when it is going to make the size of the attachment screw part 7 small as mentioned above, the outer peripheral surface of the 2nd axial part 2i extended ahead from the engagement position clearance gap Q, and the inner peripheral surface of the metal shell 1 The gap formed between the gas volumes, that is, the width J of the gas volume GV is also likely to be small. If the width J becomes too small, even if the inside of the engagement position gap Q is clean, if the second shaft portion 2i is contaminated on the front side, the inner peripheral surface of the metal shell 1 and the second shaft The problem of so-called side-fire, which causes a fire with the outer peripheral surface of the portion 2i, is likely to occur. Therefore, in order to prevent this, the inner diameter of the front end surface side opening of the metal shell 1 is D2, and the outer diameter of the insulator 2 (second shaft portion 2i) at the front end surface position is d2.
E = (D2-d2) / 2 (2)
The end face width E of the gas volume represented by
1.1α ≦ E (3)
It is effective to adjust so as to satisfy the above.
[0020]
The insulator 2 is likely to concentrate an electric field in the vicinity of the tip near the spark discharge gap g, and an edge that tends to concentrate an electric field is formed on the inner peripheral edge of the metal shell 1. The problem tends to occur at the position of the front end surface of the metal shell 1. However, by setting the width of the gas volume GV at this position, that is, the end face width E of the gas volume, to be larger than the interval α of the spark discharge gap g which is the normal spark position, the insulator 2 (second Even when the surface of the shaft portion 2i is soiled, the problem of side fire can be effectively suppressed. In the present specification, as shown in the formula (2), the gas volume portion end face width E is defined using the difference in diameter between the metal shell 1 and the insulator 2. If a slight eccentricity occurs when the metal fitting 1 is assembled, the actual distance between the inner peripheral surface of the metal shell 1 and the outer peripheral surface of the insulator 2 (second shaft portion 2i) is locally reduced. It is also possible that the occurrence of a side fire is a problem. Therefore, in order to absorb the influence, the value of E is set to a value that allows for a slight margin as shown in equation (3). However, if it is possible to reliably prevent eccentricity during assembly, α <E may be satisfied.
[0021]
Further, the side fire due to the contamination of the tip of the insulator 2 (second shaft portion 2i) does not always occur at the position of the end surface of the metal shell 1, and depending on the width of the gas volume portion GV, the metal shell is slightly deepened. It can also occur at different locations. Therefore, in order to prevent such a side fire, the diameter of the cross-sectional outline of the insulator 2 by the virtual plane orthogonal to the axis O is d3 and the position corresponding to this in front of the insulator side engaging portion 2h. When the inner diameter of the metal shell 1 is D3, at an arbitrary position in the section from the front end surface position of the metal shell 1 to at least 7 mm,
α <(D3-d3) / 2 (4)
In other words, it is effective to satisfy α <(D3-d3) / 2 in the section L in which 7 mm or more is secured from the position of the front end face of the metal shell 1.
[0022]
If the width J (= (D3−d3) / 2) of the gas volume GV at a position in the axial direction O is larger than the interval α of the spark discharge gap g, the side fire at that position is essentially It becomes difficult to occur. On the other hand, the electric field strength on the surface of the insulator that affects the occurrence of side-fire is high near the tip near the spark discharge gap g, but gradually decreases as it is separated rearward in the axial direction O. However, according to a study by the present inventors using electric field strength distribution simulation by the finite element method, the electric field strength on the insulator surface is somewhat high in the section from the front end face position of the metal shell to about 7 mm in the axial direction. It was expected that this would occur, and there was concern about the occurrence of side fire. Therefore, at least in this section, if the gas volume portion width J is adjusted to be larger than the interval α of the spark discharge gap g, which is a normal discharge location, a side fire at the position where the metal shell 1 is recessed is caused. Actually, it can be effectively suppressed.
[0023]
Next, a cross section by a virtual plane including the axis O of the insulator 2 (here, also coincides with the axis of the metal shell 1) on the gap forming inner peripheral surface 52 in the protruding strip portion 1 c which is the metal fitting side engaging portion. The outline has a flat portion 52a facing the gap forming outer peripheral surface 2k, and an inclined portion 52b descending from the front end of the flat portion 52a toward the inner peripheral surface of the metal shell 1. And the angle θ formed by the flat portion 52a and the inclined portion 52b is
140 ° ≦ θ ≦ 160 ° ………… 5
Is satisfied. An edge portion is formed at the intersection between the flat portion 52a and the inclined portion 52b. If the angle θ between them is set to be slightly larger as shown in (5), the edge portion is formed. Excessive electric field concentration can be avoided, and the withstand voltage performance can be further improved. However, when θ is less than 140 °, the effect is small, and when θ exceeds 160 °, the inclined portion 52b has a long and long skirt toward the inner peripheral surface of the metal shell 1, and the electric field strength in the gas volume portion GV. In some cases, the high-voltage region extends to the tip of the insulator 2 (second shaft portion 2i) having a small thickness, and the withstand voltage performance may be impaired. In addition, since the section where the width J of the gas volume portion GV becomes smaller becomes longer, it may be disadvantageous in terms of preventing side jumping. In the present embodiment, the flat portion 52a forms a cylindrical surface coaxial with the outer peripheral surface 2k of the base end portion of the second shaft portion 2i, while the inclined portion 52b is formed in a conical shape.
[0024]
Hereinafter, various modifications that can be added to the spark plug 100 will be described (in addition, the same reference numerals are given to the same parts as those in FIGS. 1 and 2 and detailed description thereof will be omitted). First, in FIG. 3, the cylindrical base of the second shaft portion 2i is set so that the length of the section L where the width J of the gas volume portion GV is larger than the interval α of the spark discharge gap g can be made as large as possible. The tip body portion 2s is connected to the end portion 2r via the reduced diameter portion 2j. In the present embodiment, the diameter-reduced portion 2j is conical (tapered) in order to make it difficult to produce sharp edges that easily concentrate electric fields.
[0025]
Also in the embodiment of FIG. 4A, the gap forming inner peripheral surface 52 of the convex portion 1c that is the metal fitting side engaging portion has a cross-sectional outer shape line with a virtual plane including the axis O, and the gap forming outer peripheral surface 2k. It has the flat part 52a which opposes, and the inclination part 52b which goes down toward the inner peripheral surface of the metal shell 1 from the front side edge part of this flat part 52a. And the chamfering part 52c is formed in the crossing position of these flat parts 52a and the inclination part 52b (FIG.4 (b) shows an enlarged view). With this configuration, it is difficult for electric field concentration to occur at the intersection between the flat portion 52a and the inclined portion 52b, and the same effect as increasing the angle θ between the flat portion 52a and the inclined portion 52b can be achieved. Also in the embodiment of FIG. 4, the second shaft portion 2i of the insulator 1 has a configuration in which the distal end main body portion 2s is connected to the cylindrical base end portion 2r via the reduced diameter portion 2j, as in FIG. Have. In FIG. 3, the outer peripheral surface of the tip main body portion 2 s has a conical shape, but in FIG. 4, the tip of the gas volume GV is widened as much as possible to the deeper position of the metal shell 1. The outer peripheral surface of the main body portion 2s is a cylindrical surface. In addition, as shown in FIG.4 (c), it may replace with the chamfering part 52c and may provide the round part 52r.
[0026]
In addition, a noble metal ignition portion having a diameter of 1 mm or less, whose main component is Ir or Pt, may be fixed to the front end face of the center electrode 3. If the diameter of the electrode tip is reduced to 1 mm or less, the electric field can be concentrated on the electrode tip facing the spark discharge gap g, so that the discharge voltage can be lowered. Moreover, by using the electrode tip as a noble metal ignition part, spark consumption is suppressed and the life of the spark plug can be extended. And even if the insulator 2 is somewhat thinned by reducing the diameter of the mounting screw portion 7 of the metal shell 1, the discharge voltage is lowered, so that the withstand voltage performance can be afforded accordingly. However, from the viewpoint of suppressing the progress of spark consumption due to excessive electric field concentration, it is desirable that the diameter of the noble metal firing portion is 0.2 mm or more.
[0027]
Further, a noble metal ignition part made of an Ir alloy (alloy component such as Rh, Pt or Ni) is fixed to the tip of the center electrode 3 by laser welding, and the ground electrode 4 is opposed to the ignition part in the form of Pt. Alternatively, an ignition part made of a Pt alloy (alloy component such as Ni) is fixed by resistance welding, and a gap between the ignition part and the opposing ignition part may be used as a spark discharge gap g.
[0028]
【Example】
In order to confirm the effect of the present invention, the following experiment was conducted. (Example 1)
In the spark plug shown in FIGS. 1 and 2, the mounting screw portion 7 is called M12, the interval α of the spark discharge gap g is 1.1 mm, and the ratio E / α of the gas volume end face width E to α is 1.4. For the gas volume portion width J, the length of the section L where J> α is 7 mm, the angle θ between the flat portion 52a and the inclined portion 52b of the convex portion 1c is 150 °, and the value of the engagement position gap amount β is Those set to various values of 0.1 to 0.6 mm were prepared as test products. And in order to investigate the fouling resistance of each spark plug, a pre-delivery durability test was performed under the following conditions. That is, the spark plug is attached to a test vehicle (displacement: 1500 cc, in-line 4-cylinder) with a voltage application polarity in which the ground electrode 4 side is positive and the center electrode 2 side is negative, and the travel exemplified in JIS: D1606 The pattern (test room temperature: −10 ° C.) is taken as one cycle, and this is repeated until the insulation resistance of the spark plug drops to 10 MΩ or less. Depending on the number of cycles, 10 cycles or more are “◯” and 8-9 cycles are “△”. , 6 cycles or less were determined as “×” (◯ and Δ are acceptable, × is not possible). The results are shown in Table 1.
[0029]
[Table 1]

[0030]
According to this, it can be seen that the antifouling property of the spark plug is remarkably improved by setting the value of the engagement position gap amount β to 0.4 mm or less.
[0031]
(Example 2)
In the spark plug shown in FIGS. 1 and 2, the mounting screw portion 7 is called M12, the interval α of the spark discharge gap g is 1.1 mm, and the ratio E / α of the gas volume end face width E to α is 1.4. The angle θ between the flat portion 52a and the inclined portion 52b of the convex portion 1c is 150 °, the value of the engagement position gap amount β is 0.4 mm, and the length of the section L where J> α with respect to the gas volume portion width J What set this to various values of 5-8.3 mm was prepared as a test article. And in order to investigate the low temperature startability of each spark plug, it tested on the following conditions. That is, a spark plug is attached to a test vehicle (displacement: 1500 cc, inline 4 cylinder) with a voltage application polarity with the ground electrode 4 side being positive and the center electrode 3 side being negative, and a cycle of idling 30 seconds + stop 30 minutes The test which calculates | requires the number of cycles until it becomes impossible to start was performed on two conditions of room temperature -30 degreeC and -10 degreeC. In any case, 5 cycles or more were determined as “◯” and 4 cycles or less as “×” (◯ is acceptable, × is not possible) depending on the number of cycles. The results are shown in Table 2.
[0032]
[Table 2]

[0033]
According to this result, no problem occurred in any of the test products at −10 ° C., but in the test at −30 ° C., which is a severe condition at a lower temperature, good results were obtained in a test product in which L was secured at 7 mm or more. Obtained. In addition, it is thought that the reason why the non-startable cycle is small in the test product with L of less than 7 mm is that side fire is likely to occur due to the progress of the contamination of the insulator.
[0034]
(Example 3)
In the spark plug shown in FIGS. 1 and 2, the mounting screw portion 7 is called M12, the interval α of the spark discharge gap g is 1.1 mm, and the angle θ between the flat portion 52a and the inclined portion 52b of the convex portion 1c is 150. The ratio E / α of the gas volume portion end face width E to α is 0.9 by changing the engagement position gap amount β to 0.4 mm and changing the inclination angle of the outer peripheral surface of the second shaft portion 2i in various ways. Prepared as test products adjusted to various values of ~ 1.7. These spark plugs are ignited in advance and attached to a visible chamber. The spark pressure is set by setting the air pressure in the chamber to 0.4 MPa, and how many side fires occur on the bracket during 1000 discharges. The occurrence frequency of side fire was examined by visually checking whether it occurred. The results are shown in FIG. According to this, it can be seen that by setting E / α to 1.1 or more, the side fire occurrence frequency is remarkably reduced.
[0035]
(Example 4)
In the spark plug shown in FIGS. 1 and 2, the mounting screw portion 7 is called M12, the interval α of the spark discharge gap g is 1.1 mm, and the ratio E / α of the gas volume end face width E to α is 1.4. As for the gas volume portion width J, the length of the section L where J> α is 7 mm, and the angle θ between the flat portion 52a and the inclined portion 52b of the convex portion 1c is prepared as a test product. did. Further, instead of setting the angle θ to 120 °, a test product provided with a chamfered portion 52c (chamfer width 0.5 mm) as shown in FIG. 4 was also prepared.
[0036]
Using the dimensions and shapes of these test products as initial conditions, the electric field strength distribution in the gas volume GV when the applied voltage level to the center electrode 3 is 10 kV is simulated by a finite element method using commercially available software, The electric field strength at the position closest to the intersection between the flat portion 52a and the inclined portion 52b was read. The results are shown in Table 3.
[0037]
[Table 3]

[0038]
According to this, it can be seen that the electric field strength is remarkably reduced when the angle θ is 140 ° or more or when chamfered. FIG. 6A shows the simulation results for θ = 135 ° and FIG. 6B shows the simulation results for θ = 150 °. The brighter the region in the figure, the higher the electric field strength. According to this, it can be clearly seen that the electric field concentration portion appears remarkably near the intersection in the former with a small angle θ, whereas the degree of electric field concentration is reduced in the latter with a large θ.
[0039]
Next, the ground electrode is removed from each test product, and the opening side of the metal shell is immersed in a liquid insulating medium such as silicon oil in this state, so that the gap between the outer surface of the insulator and the inner surface of the metal shell is Filled and insulated with liquid insulating medium. In this state, an AC high voltage or a pulsed high voltage is applied between the metal shell and the center electrode 3 by a high voltage power source, and the voltage waveform is recorded with an oscilloscope or the like, and the voltage waveform is penetrated to the insulator. The voltage value at the time when the breakdown occurred was read as the penetration breakdown voltage. Each test product was tested at n = 40, and an average value and a minimum value of withstand voltage were obtained. The results are shown in Table 4.
[0040]
[Table 4]

[0041]
According to this, it can be seen that those with an angle θ of 140 ° to 160 ° or chamfered have high withstand voltage average values and stable withstand voltage performance. . On the other hand, when the angle θ is less than 140 °, both the average value and the minimum value of the withstand voltage are lowered, and the withstand voltage performance tends to be relatively lowered. Further, when the angle θ exceeds 160 °, the average value of the withstand voltage is relatively good, but the minimum value is lowered, and the withstand voltage performance tends to vary.
[Brief description of the drawings]
FIG. 1 is a longitudinal sectional view showing an overall configuration of a spark plug according to an embodiment of the present invention.
2 is an enlarged longitudinal sectional view showing a main part on the front end side of FIG. 1;
3 is a longitudinal sectional view of an essential part showing a first modified example of the spark plug of FIG. 1. FIG.
FIG. 4 is a longitudinal sectional view of an essential part showing a second modified example.
5 is a graph showing the experimental results of Example 3. FIG.
6 is a diagram showing some simulation results of Example 4. FIG.

Claims

A cylindrical metal shell (1), an insulator (2) having a shaft hole locked to the inner peripheral side of the metal shell (1), and a center electrode held in the shaft hole of the insulator (2) (3) and a ground electrode (4) that forms a spark discharge gap (g) by facing the tip of the center electrode (3),
In the axial direction (O) of the insulator (2), the side where the spark discharge gap (g) is located is the front side, and the opposite side is the rear side, and the insulator (2) is the front end (2i). Is reduced in diameter by a step portion in the circumferential direction so that the step portion serves as an insulator-side engagement portion (2h) and is inserted into the metal shell (1) from a rear-side opening, and the insulator-side engagement portion (2h) engages with the metal fitting side engaging portion (1c) protruding from the inner peripheral surface of the metal shell (1), and more than the insulator side engaging portion (2h) of the insulator (2). cylindrical surface outer circumferential surface of the proximal end portion of the portion (2i) located on the front side (hereinafter, referred to as gap forming the outer peripheral surface) (2k) is, the fitting-side engaging part cylindrical surface shape in the peripheral surface of (1c) ( (Hereinafter referred to as a gap forming inner circumferential surface) (52) and a shape that forms a predetermined amount of engagement position gap (Q), Serial gap forming the outer peripheral surface of the outer diameter of the (2k) d1, the inside diameter of the gap forming the peripheral surface (52) as D1,
β = (D1-d1) / 2
The engagement position gap amount β represented by is adjusted to 0.05 mm to 0.4 mm, and the length βL for securing the engagement position gap amount β is 0.5 mm to 2.5 mm. Ah is, also the spark plug tip side outer peripheral surface than the gap formed outer circumferential surface (2k) of said insulator (2) is characterized that you have a reduced diameter.

The inner diameter of the front end face side opening of the metal shell (1) is D2, and the outer diameter of the insulator (2) at the front end face position is d2.
E = (D2-d2) / 2
The end face width E of the gas volume part represented by the following formula is defined, where α is the interval of the spark discharge gap (g):
1.1α ≦ E
The spark plug according to claim 1, wherein:

On the front side of the insulator-side engagement portion (2h), the diameter of the cross-sectional outline of the insulator (2) by a virtual plane orthogonal to the axis (O) is d3, and the metal shell at a position corresponding thereto When the inner diameter of (1) is D3,
In an arbitrary position in a section from the front end face position of the metal shell (1) to at least 7 mm,
The spark plug according to claim 1 or 2, wherein α <(D3-d3) / 2 is satisfied.

The gap forming inner peripheral surface (52) of the metal fitting side engaging portion (1c) has a flat portion (a cross-sectional outline line formed by a virtual plane including the axis (O) is opposed to the gap forming outer peripheral surface (2k)). 52a), and an inclined portion (52b) that descends from the front end of the flat portion (52a) toward the inner peripheral surface of the metal shell (1). The flat portion (52a) and the inclined portion ( 52b) makes an angle θ
4. The spark plug according to claim 1, wherein 140 ° ≦ θ ≦ 160 ° is satisfied. 5.

The gap forming inner peripheral surface (52) of the metal fitting side engaging portion (1c) has a flat portion (a cross-sectional outline line formed by an imaginary plane including the axis (O) facing the gap forming outer peripheral surface (2k)). 52a), and an inclined portion (52b) that descends from the front end of the flat portion (52a) toward the inner peripheral surface of the metal shell (1), and the flat portion (52a) and the inclined portion The spark plug according to any one of claims 1 to 4, wherein a chamfered portion (52c) or a rounded portion (52r) is formed at a position intersecting with the portion (52b).

The spark plug according to any one of claims 1 to 5, wherein a noble metal ignition part (31) having a diameter of 1 mm or less, whose main component is Ir or Pt, is fixed to the front end face of the center electrode (3).

The spark plug according to any one of claims 1 to 6, wherein a mounting screw portion (7) having a nominal size of M12 or less is formed on a front end side outer peripheral surface of the metal shell (1).