I suspect the reason you're having difficult getting answers is because your goal is unclear from your question. The best way to have your work added to an online repository is to submit your work to a recognized engineering journal, such as one of the IEEE societies. If you don't want to support paid publishers, you could consider checking out the PLoS journals.
However, you have to recognize something fundamental about your work; from my experience, people doing serious research aren't trawling the web looking for information that happens to be placed on the internet that just happens to be related to their field of interest. Researchers overwhelmingly use journals to communicate, for the simple reason that journals provide a peer-review process, meaning that the content is vetted for accuracy, quality, and relevance. If you simply want to put your stuff online, get a blog, but if you want others to read it—particularly others recognized as leaders in the field—you will almost certainly want to submit to a journal. Most well-known researchers that I know wouldn't look to "online repositories", if they exist, simply because the signal to noise ratio is guaranteed to be far too high to be worth their time.
tl;dr - I'm not sure of your end goal. Consider submitting your results to be published (and added to the publisher's archive). Also, interesting discussion in comments; read them below.