I’m curious about the notable exceptions…
It is a tricky thing to do…go full time the 1st/2nd year, pass quals, and then go back part time. You need to find an advisor who is ok with it. I wouldn’t go in and tell profs this is what you have in mind from the start…its kinda of hidden trick that universities don’t want to advertise is available. There are potentially major advantages or major disadvantages to doing this. The disadvantages are that university wants tuition money and won’t let you do this, and they can change the rules midway through your PhD. Another disadvantage is that it could take you 10 years to get a PhD…that is not a good idea.
The major advantages of being able to work, fund yourself as an engineer, and pursue the PhD part-time, is that you are not burdened with TAs, and GRAs that have more do with getting grants than doing real research. Before Michigan changed it rules to prevent students from doing this, I’ve seen students have deep trouble while being funded by my advisor as full time students, but they really succeeded going part-time by finding a job in town, and not relying on the advisor’s support. The TAs and GRAs were an unhelpful distraction.
Also, 10-15 years ago, universities would take profs who were PhD students, and let them finish up there PhDs as they were Profs at new schools. 35 years ago my father was able to do this. I’d even heard about this happening at good engineering schools 15 years ago…I haven’t heard of it happening recently though.