OsmoGenset is an ultra high density power generator system that combines established designs with a number of advanced technology approaches to reach extremely high levels of efficiency with long MTBF, low noise. A four stroke, spark ignition, piston engine is utilized as the prime power generator in the system. The GreenSight team has built a half dozen JP8 powered engines in this configuration over the last two decades, achieving reliable, high efficiency operation with long run times. An atkinson cycle with mechanical over expansion is utilized to maximize thermal efficiency. A modular crankcase designis utilized that can be configured with cylinder counts between 1 and 6 for modular configurations from 5kW to 30kW. A radial configuration is used to maximize cylinder density and minimize size/weight, as well as optimize cooling. An elegant compact mechanism is used to implement the Atkinson cycle in this modular system. A number of additional efficiency improving design elements are combined to reach the fuel efficiency target of 270g/kWh. A custom designed axial flux halbach array rare earth magnet generator is utilized to maximize system power density and electrical efficiency. This design uses a stackable configuration that enables generator power to be configured from 5-30kW with no unique parts. GreenSight leverages their brushless motor control design experience to produce a high density generator controller using Field-Oriented-Control, which is also capable of starting the prime power generating engine.