   what is an engine, anyways?
  first, here's my definition: an application which uses acustomizable file system to create a working program.this applies to most game engines: you can change files inBattlefield 1942, Quake, and Unreal to alter these games into "mods" without ever having to recompile the engine (which takes sourcecode, which takes much $$$$).but not to eveything called an "engine". there are "physics" engines and i have seen a few engines that require recompiling and consist of little more than C++ classes with scripting support functions.i would consider these engines "API"'s or libraries even if they are extremly advanced and aimed at game creation.  After all DirectX is not considered an engine, though thin OO wrappers for it are?so what constitutes an engine?   
  An engine is the bit of code you realize you can reuse for your next game after having finished your current one.   ;  Basically, an engine is what powers the car by converting potential chemical energy into mechanical energy by burning fuel, or combining hydrocarbons with oxygen and using the energy netted from the exergonic rxn.  In any other science it's used as an abstraction to describe a similar process, b/c cars are more prolific.   