SGEngine was started in 2009, and never completed.
It was meant as the successor to SGZEngine, and to be in a much more modular format.
This was to be attained by having each major module – Graphics, Sound, etc.. – as a dynamically loadable library which could be replaced at runtime.
While some basic tests were completed, and some parts of the engine written, the whole structure was much more complicated than that of SGZEngine; which was already convoluted.
As such, SGEngine was dropped due to lack of time and from several core mistakes in the design from the outset.
Some SGEngine code is available from GitHub