看了下 http://guides.rubyonrails.org/engines.html 这篇关于 engine 的介绍
目前的疑惑是什么时候应该用 engine，什么时候用 gem ?
Engine is a gem. Rails is also a gem with many dependency gem.
To see demonstrations of other engines, check out Devise, an engine that provides authentication for its parent applications, or Forem, an engine that provides forum functionality. There's also Spree which provides an e-commerce platform, and RefineryCMS, a CMS engine.
换句话说，engine 更像一个模块。
http://stackoverflow.com/questions/23118472/gem-vs-plugin-vs-engine-in-ruby-on-rails 自己找了这篇文章，某种程度上算是解答我的疑惑了吧