The second (letter) formerly referred to the fuel type; "D" = diesel, "G" = gasoline. However, since the 1980s, this has changed. Two engine families were introduced using the letter "A" to denote that all the engines in the family had an alloycylinder head. Their latest engines, however, do not follow any previous conventions (e.g. 4M4, 3B2, etc.).
The third (digit) previously denoted the engine family. Five of the "4G" straight-4 engine families had distinct names; "4G1" = Orion, "4G3" = Saturn, "4G4" = Neptune, "4G5" = Astron, and "4G6" = Sirius.
The fourth (digit) is the specific engine model within the family. It is not a guide to its place within that family, nor is it a guide to the capacity of the engine.
There may also be supplementary letters after the initial four characters.... Read More