This work examines the design and operation of a new, small-scale FPE (Free Piston Expander) engine that operates using low temperature waste heat sources to produce useful power output. The FPE is based on a sliding-piston architecture that eliminates challenges associated with MEMS-based rotating systems. A nonlinear lumped-parameter model is derived to study the factors that control the performance of the FPE engine and its unique operating cycle. This basic analysis considers a closed cycle operation of the FPE with low thermal or heat inputs and dimensions on the order of several millimeters. Key system design and operating parameters such as piston mass, external load, and heat input are varied to identify conditions and trends for optimal performance. The model indicated the pressure volume diagram resembles a constant pressure cycle for a certain set of operating conditions but is also condition dependent. Increased heat inputs to the FPE reduced the engine natural or operating frequency while increasing the power output. Thermal efficiencies of the FPE are shown to be predictably low, on the order of 0.2% due to the small heat input and operating temperature gradients associated with waste heat. Key design features are identified that reveal FPE efficiency, operating frequency, and output power are dependent on piston mass, external load, input heat-rate, and duration of heat input. (C) 2016 Elsevier Ltd. All rights reserved.