Being curious about how diesels work, I found this during a web search, showing exactly what happens in a diesel:

It did answer all my questions except for one. It didn't seem to explain fuel divlivery well enough.

I noticed that the diesel fuel is delivered right at the maximum compressed air. Seems an incredible amount of pressure would be required for the diesel to get into the cylinder. How do they manage that? How much fuel pressure is used in the average diesel engine?