How to save unfinished nodes as concepts / draft

I am looking for a way to save nodes regardless if the author saved the node manually. Some sort of auto save functionality similar to Gmail, where unfinished mails get in your concept list.
So I was thinking to run some AJAX on my node create and node edit pages but I don’t really find a way to actually save a node in the database. I am looking at the drupal_form_submit function but don’t see a way to keep the node submitting with a time interval without reloading the page.

So, any help would be appreciated, maybe some of you made a module with similar functionalities in the past and could get me on the right track.

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