Is doing research alone and publishing solo frowned upon by colleagues and superiors in engineering sciences? Research has become more and more complex and difficult to be successfully conducted by a single person. However, there are fields (such as some branches of engineering) in which a researcher can conduct pretty high standard research without wasting anybody's time or resources (for example, using opensource software on his/her own laptop over a couple of months during the weekends). Nevertheless, single author papers are very rare. How is this possible? I am asking because I believe I have produced a pretty interesting manuscript with no sort of help from anyone, but I am afraid it will be frowned upon by my past close collaborators and the PI of the project I am currently working on if I try to publish it.
I think there is no universal answer. There are fields of pure mathematics where single author papers are quite frequent (the majority?) while there are other areas of the sciences (e.g., the biomedical field) that are so broad that nobody has the requisite knowledge and time to do everything themselves.

So it depends on the field you're in. But beyond that, every field has single-author papers, and a single paper isn't going to make others put you in a box: it is, after all, just a single paper. People will turn out to be surprised if you have been in a field (any field!) for ten years, written 30 papers, and 25 of those are single-authored. For a single paper, there really is no rule to go by: If you have a good idea that you've entirely developed yourself, go write it up and get it published!