What is the best repository for Engineering research work in pre-print format?
Update 2020: There is still no widely used preprint server for engineering articles.

There are (now?) a few engineering categories on arxiv, e.g. electrical and computer engineering. Everything that has anything to do with machine learning or data analysis is a good fit, anyway. I have uploaded a few biomedical signal processing papers on arxiv as well. See https://arxiv.org/ for the list of categories.
Very recently, IEEE introduced its own preprint server, &quot;a preprint server for the global technology community&quot;: https://www.techrxiv.org/. I could imagine that with the backing of IEEE, this could become reasonably popular in some time.
Of the three preprint servers I mentioned below in 2016, https://engrxiv.org appears to be the only one that's still reasonably active.
https://www.preprints.org/subject/browse/engineering also appears to be reasonably active.


It appears that there is currently no widely used preprint server for engineering articles. Interestingly, there appears to be a bunch of platforms that have all been launched very recently, and which all target (among others) engineering preprints:

Launched in July 2016, there is now (currently only in a preliminary version) engrXiv, which is a dedicated preprint server for the engineering sciences. It is supported / advocated by the center for open science.
There is the Self-Journal of Science which provides an innovative, open peer review process and also allows for publication of preprints.
Finally, there is The Winnower which also provides a platform for an open peer review process.

Unfortunately, in all of these, activity is currently rather low, probably due to the novelty of these platforms. Time will show whether one of these will become really popular.