When I was working as an engineering, I tried looking into this. There is no such thing as a part-time engineering PhD. Yes, sometimes a National Lab or the military may pay for you to get a PhD.
In some schools you can be ABD, and work part-time. Some universities will continue to let you have school resources without paying tuition, so you can finish up the research side of your PhD. Michigan where I got my PhD used to have this open to students. This has changed, so that a student must be continuously enrolled and paying tuition to get a PhD. Even if you could go part-time, it may not be a good idea unless your job really helps you with the PhD.
To get a PhD, you must bite the bullet and go back full time…at least for the first part of the PhD program. I know of no way around this.