A resonant heat engine in which the piston assembly is replaced by a sealed elastic cavity is modeled and analyzed. A nondimensional lumped-parameter model is derived and used to investigate the factors that control the performance of the engine. The thermal efficiency predicted by the model agrees with that predicted from the relation for the Otto cycle based on compression ratio. The predictions show that for a fixed mechanical load, increasing the heat input results in increased efficiency. The output power and power density are shown to depend on the loading for a given heat input. The loading condition for maximum output power is different from that required for maximum power density.

This work examines the design and operation of a new, small-scale Free Piston Expander (FPE) 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.

There is a need for the development of large displacement (O (10 −6) m) and force (O (10 −6) N) electrostatic actuators with low actuation voltages (< ±8 V) for underwater bio-MEMS applications. In this paper, we present the design, fabrication, and characterization of a curved electrode electrostatic actuator in a clamped-clamped beam configuration meant to operate in an underwater environment. Our curved electrode actuator is unique in that it operates in a stable manner past the pullin instability. Models based on the Rayleigh-Ritz method accurately predict the onset of static instability and the displacement versus voltage function, as validated by quasistatic experiments. We demonstrate that the actuator is capable of achieving a large peaktopeak displacement of 19.5 µm and force of 43 µN for a low actuation voltage of less than ±8 V and is thus appropriate for underwater bioMEMS applications.

A resonant engine in which the piston-cylinder assembly is replaced by a flexible cavity is realized at the mesoscale using flexible metal bellows to demonstrate the feasibility of the concept. A four stroke motoring technique is developed and measurements are performed to determine parasitic losses. A non-linear lumped parameter model is developed to evaluate the engine performance. Experimentally, the heat transfer and friction effects are separated by varying the engine speed and operating frequency. The engine energy flow diagram showing the energy distribution among various parasitic elements reveals that the friction loss in the bellows is smaller than the sliding friction loss in a typical piston-cylinder assembly. V C 2014 AIP Publishing LLC.

In order to mitigate frictional and leakage losses in small scale engines, a compliant engine design is proposed in which the piston in cylinder arrangement is replaced by a flexible cavity. A physicsbased nonlinear lumped-parameter model is derived to predict the performance of a prototype engine. The model showed that the engine performance depends on input parameters, such as heat input, heat loss, and load on the engine. A sample simulation for a reference engine with octane fuel/air ratio of 0.043 resulted in an indicated thermal efficiency of 41.2%. For a fixed fuel/air ratio, higher output power is obtained for smaller loads and vice-versa. The heat loss from the engine and the work done on the engine during the intake stroke are found to decrease the indicated thermal efficiency. The ratio of friction work to indicated work in the prototype engine is about 8%, which is smaller in comparison to the traditional reciprocating engines. V