Design of a Hot Gas Vane Motor

This paper describes the design of a hot gas vane motor. The motor is intended as a portable direct drive actuator powered by a hot gaseous working fluid as an alternative to battery-powered electromagnetic motors. The objective is thus to convert a maximum amount of enthalpy at the inlet into mechanical rotational output energy in a compact and lightweight package. The described motor incorporates a noncircular stator shape and a geometry of injection and exhaust ports to maximize conversion efficiency. An analytical model is developed to describe the torque output and efficiency of the motor. Simulations are performed to provide insight into the motor behavior and characterize the expected motor performance. INTRODUCTION The state of the art in self-powered small-scale motors consists of either battery-powered electric motors or hydrocarbon-fueled internal-combustion-engines. The state of the art in the former consists of lithium variant (e.g., lithium thionyl chloride) or zinc variant (e.g., nickel zinc) batteries, which power rare earth magnet (e.g., neodymium) brushed or brushless DC or AC motors. This approach provides the advantage of on-demand start/stop and bi-directional, proportional, high-bandwidth, and quiet operation, but has the significant disadvantage of low power density and very low energy density (relative to hydrocarbon-based systems). The state of the art in hydrocarbon-fueled combustion engines include four-stroke, two-stroke, and turbofan engines. These engines provide useful power and energy densities, but do not provide on-demand start/stop operation (i.e., start/stop generally requires an auxiliary starter system), do not provide bi-directional operation, are limited to a small range of torque/speed combinations, are generally low-bandwidth devices, and are extremely loud. This paper describes an alternative to these state of the art propulsion systems that provides comparable energy densities to hydrocarbon-based engines, but additionally provides significantly higher power density (relative to two and fourstroke engines), on-demand start/stop, bi-directional operation, high-bandwidth operation, large dynamic range (i.e., torque/speed range), and quiet operation. Additionally, the efficiency of the proposed system is not adversely affected at small scales, which renders it particularly appropriate for subhorsepower applications. BACKGROUND The proposed motor, called a hot gas vane motor (HGVM), is a variation on a standard rotary vane motor. A standard rotary vane motor incorporates a rotor that is eccentrically located relative to a stator. The rotor includes extendable vanes that provide an increased moment arm on one side of the rotor relative to the other, and thus produce a net torque about the stator when a high-pressure fluid is present. Evidence of the vane motor concept can be found in books early as 1868 [1], and in the U.S. patent database as early as 1924 [2]. These and other standard vane motors utilize cold gas or hydraulic fluid as a working fluid, and thus are not concerned with the thermodynamic efficiency of energy conversion from heat to work. The proposed vane motor utilizes a hot gas generated from an (external) exothermic chemical reaction as the working fluid. Note that [3, 4] similarly proposed the use of a hot gas vane motor, which was develop in the early to mid 1980’s for use as a mechanical auxiliary power unit for starting aircraft engines. These motors