CVTでも無くならないATのトルクコンバータとは一体何なのか?

08

トルクコンバーターの疑問を解決

一世風靡のCVTに呈される問題

AT(オートマチックトランスミッション)車では、ながらくトルコン式といわれる機構が主流でした。CVTが登場してそのよいところに注目が集まりAT車がCVTで動くようになって大きく支持を集め、そうこうするうちに、またトルコン式のAT車が見直されつつあります。そんななかでデュアルクラッチのAT車もスポーツドライブ志向の人からは注目を集めています。

一般的なプーリーを組み合わせるCVTは小さな車であまり速度を出さない場面では優秀なのですが、場合によってはなかなか解決できない問題を抱えています。

ロックアップ機構が洗練されてきて従来型のトルコン式ATが見直されています。ビークルナビではCVTそのものやロックアップ機構についての詳細の記事も準備していこうと思っています。

▼関連記事 デュアルクラッチのトランスミッションについてはこちらを

デュアルクラッチ(DCT)は何がすごいのか?もうMTは要らない?

ところでCVTのクラッチはどうなっているの?

CVTのクラッチはトルコンが主流!

CVTの詳細については少々お待ち頂くとしてクラッチの部分はどうなっているのでしょうか。

▼関連記事 クラッチが何か分からない、なぜ必要なのかも分からないならばこちらをどうぞ

MT車(マニュアル車)のクラッチとは何なのか?  大事な役目を理解しよう

実はCVTの場合、主流となっているクラッチはトルクコンバーター(トルコン)です。えっ!! CVTというのはトルコン式のATとは違うのではないの? と当然お思いになりますね。MT車のクラッチの記事に書いてありますが、クラッチでエンジンからの出力を切ったり繋いだりするのは発進や変速のために必要なことです。

CVTというのは変速に関わる機構のほうのこと

CVTという略語になっている言葉を訳するならば無段階変速機ということになります。歯車がありませんから違う大きさの歯車の噛み合わせを切り替えるための従来のクラッチとは構造が別にできました。

ただしこの場合、トルコン式のクラッチの便利な作用のクリープ現象が使えないという問題があったのです。クリープ現象のおかげで坂道でアクセルを離してもずり下がらないでいてくれます。このために結局CVTであってもトルコンを介して動力を伝えてクラッチの働きはここに担わせるようになってしまっています。

CVTと従来型AT車の違いは変速機

従来型のトルコン式AT車の変速機構は遊星歯車を使用

正確にいえばトルコンとは特に発進の時に大きな役割を果たすクラッチの役割の部分です。そこから出力される力を車輪にどのように伝えるか、伝え方を変える部分が変速機になります。よく言われるトルコン式ATの場合は遊星歯車という3つのパートを持つ複数の歯車の操作で変速ができるようになっています。これらの歯車は常時噛み合っていますからトルクコンバーターの基本的な構造、流体継手の組合せをクラッチにすることで充分なのです。クラッチにするというよりもシンクロメッシュの代わりにするといった方がいいのかもしれません。

(*注 ホンダのATはこの特許を回避するために多軸の平行軸歯車を使っています。)

いろいろと一度に考えると難しくなってしまいます。まずはこのようにどうしても複雑な仕組みになるトランスミッションの部分とトルクコンバーターは直接は関係ないということを知っておいて下さい。

クラッチレスにするAT車の仕組み

AT車とはクラッチペダルを操作しなくていい車のことです。機械的に繋げたり切断したりするMT車のクラッチとは違いトルコンはそもそも機械的には繋がっていません。回転は充填されたオイルを伝達して伝わる仕組みとして考えられたものです。

大まかにいえば充填されたオイルの中で片方の羽根車(インペラ)がグルグル回って撹拌されて伝わった力で対面する羽根車が回って動力を伝達します。これを流体継手といっています。

アイドリング程度の回転はブレーキで抑えれば伝わらないですし、ちょっとした坂道でずり下がらない程度には伝わります。なかなか都合のいい感じです。

ステーターがトルクを増幅する

その中でトルクコンバーターという言葉が何故出てくるのかといえばただ流体継手の仕組みを作っただけでは車が発進するとき(車輪側が動いていない時)回転を始める力(トルク)が充分得られないからです。

トルクコンバーターはふたつのインペラの間にステーターというパーツを挟むことでトルクを増幅させています。オイルの伝達の流れの方向を変えることで増幅効果が生じています。そしてステータは双方のインペラが等速で回る頃になると役割を終えるようになっています。そういう形状になっているのです。

簡単にスムースに発進、ロスはロックアップ機構で解決

この速度域のあたりから流体継手とは別にエンジンの回転と車輪側への出力を機械的に繋げてしまう仕組みがロックアップ機構で電子制御によってかなり洗練されてきています。

こうしてどのような場合でも流体継手にステータを挟んだトルクコンバータは発進時に機能するクラッチとしてはとても優秀なものです。発進時に自らの絶妙な操作が要求されるMT車のクラッチ操作、高度な電子制御が要求されるDCT(デュアルクラッチトランスミッション)までこだわらなければやはりトルコンは便利で圧倒的な支持をうけるゆえんでもあります。i

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