The Free Piston Power Pack (FP3) represents a new concept in the design of free piston engines. The FP3 is a free piston engine with an integral generator for electrical power output. Its novel features are an integral compressor and a passive intake valve located in the head of the piston. These improvements eliminate undesirable problems that affect conventional free piston engine designs, such as piston ring wear and the need for an external compressor, and allow a significant increase in power density. The FP3 is designed to meet the highest levels of fuel efficiency and exhaust emissions performance in a compact size for use in hybrid electric vehicles (HEVs). This paper describes the design of the FP3 and gives details of the operation and construction of a prototype.