Electrical energy generators often use commercial diesel engines as a source of mechanical power at a fixed rotational speed. While the frequency of the grid cannot be modified, electric generators demand reciprocating engines to operate not always at their best efficiency range. By the use of a generator capable of running on different speeds and an electronic system that controls the output frequency, it is possible to operate the engines at their best efficiency load points. This paper describes the development of a variable speed diesel fueled electric generator with permanent-magnet high-efficiency synchronous generator. It was used a 1-D computational code to build a map through a mean-value model that combines speed and load to achieve the required power with the best engine efficiency. The generator set was instrumented whilebrake and indicated data were obtained in order validate the computational model.