Basic 4 stroke theory review..

Intake stroke.... piston going down, intake valve open, exhaust closed
Compression stroke....piston coming up, both valves closed.
Power stroke.. piston going down, both valves closed
Exhaust stroke ... piston going up, intake valve closed, exhaust valve open.

There may be some overlap at the margins ... when the piston is at the top of its travel between the intake and exhaust... but diesels have about the least of that so that should not cause any confusion...