Adam, I disagree with your above statement. (before this)

Im fairly sure the the fuel is injected right as the piston enters TDC. When the piston is at the top of the cylinder the temperature from compression is the highest. Right then the fuel is injected and explodes due to the temperature of compression alone.

In this way the injector firing combines the ideas of a spark plug and fuel entering the system at the same time.

Isnt that why injectors have to pop at such hight pressure? To overcome that which is in the cylinder?