Abstract: An engine system includes a plurality of adjacent expansion rotor housings each having a generally cylindrical expansion rotor cavity. The expansion rotor cavity of each expansion rotor housing is substantially aligned with the expansion rotor cavity of each other expansion rotor housing. An elongated shaft extends through the expansion rotor cavity of each expansion rotor housing along a respective longitudinal axis of each expansion rotor cavity. A set of adjacent combustion assemblies is attached to each one of the expansion rotor housings. Each one of the combustion assemblies includes a combustion chamber. An expansion rotor is mounted on the shaft in the expansion rotor cavity of each expansion rotor housing such that the elongated shaft extends through a centroidal axis of each expansion rotor. A fuel delivery system is attached to each one of the combustion assemblies for providing a supply of fuel to each one of the combustion chambers.