The engines used to power small unmanned aerial systems are often modified commercial products designed for use by hobbyists on small model aircraft at low altitude. For military applications, it is desirable to fly at high altitudes. Maintaining power from the engine at the reduced ambient air pressures associated with high altitudes requires some method of increasing air delivery to the intake manifold. Conventional turbochargers and superchargers are typically very inefficient for the low mass flows associated with small engines. Due to its unique characteristics, a pressure wave supercharger (PWS) can avoid many scaling-related losses. This project designed a small-scale PWS for turbo-normalization of a Brison 95 cc two-stroke engine for a small unmanned aerial vehicle.
A larger PWS called the Comprex®, designed by Brown Boveri Company, was simulated using a quasi-one-dimensional Computational Fluid Dynamics (CFD) code developed at the NASA Glenn Research Center. This code was able to predict the mass flow, temperature ratio, and pressure ratio at each respective port to within 2%, 6% and 18%, respectively when compared to test results. With the code validated for the performance of the Comprex®, the design point and several off-design points of the small-scale rotor were simulated, indicating acceptable performance. The design was then modeled using SolidWorks and the major parts were manufactured via an additive direct metal laser sintering process. A test rig has been built for the purpose of comparing the computational results of the CFD code to the data gathered during testing. Initial simulations indicate that for standard sea level ambient conditions, the scaled pressure wave supercharger will supply air to the intake manifold of the Brison at 38 psia (a pressure ratio of 2.5). This suggests that an inlet pressure of approximately 25 psia could be supplied to the engine if it were operating at 10,000 ft above sea level, where the ambient air pressure is 10.1 psia. Once construction of the test rig is completed, testing will indicate the extent of loss and the real overall performance for the device designed.