The idea of an 8 hour work day is that there is no idea, we just do this without any sensible excuse for it.

What is referred to here as a side project is actually the persons real life. The job is the side project.

Its a fascinating misconception that we would live to work rather than work to live.

You should make a deal with the engineer that he will do overtime if there is a lot of work to do and he will spend unpaid hours in office if things are slow.

Then you pretend to be interested in his side project.