Integration of Carbon Fiber Composite Materials Into Air-Cooled Reciprocating Piston Engines for UAV Applications

Abstract

The United States Navy (USN) has shown an interest in the development of small displacement, reciprocating piston diesel engines for Unmanned Aerial Vehicles (UAVs). These engines avoid the logistic challenges of relying on gasoline-fueled UAVs, and have relatively low fuel consumption, but suffer from poor power-to-weight ratios. Reducing weight is of significant importance to UAVs to allow sufficient range and loitering times. Carbon fiber composites offer strength-to-weight benefits over the metal components currently used in piston engines. However, composite components have more restrictive operating temperatures. None of the known previous efforts aimed at integration of composites into engines has focused on air-cooled engines and on their potential benefits for UAVs. A small, single-cylinder air-cooled gasoline engine was chosen as a convenient test platform and surrogate for an air-cooled UAV engine. The crankcase and connecting rod from this engine were redesigned using a high fraction of carbon fiber in order to reduce weight. To develop the designs, steady-state temperature profiles were measured both internally and externally. The steady state temperatures for the crankcase ranged between 93°C and 124°C while the connecting rod ranged roughly between 120°C and 160°C. Carbon fiber composite test specimens were tested for strength at a comparable range of temperatures to demonstrate thermal viability. The tensile and compressive tests showed the carbon fiber to be comparable to all aluminum material properties, if not better. Stress was modeled on the stock engine at the worst-case operating condition and used to design the composite crankcase and connecting rod to ensure sufficient strength to survive at this extreme condition. The modeled stresses resulted in a factor of safety of 1.5 for the connecting rod and 3.8 for the crankcase, although internal adhesion of bearing surfaces to fiber was noted as a problem for the connecting rod in the initial prototype, and ultimately led to premature failure. These composite components were integrated and tested, for various lengths of time, on the engine and indicate a potential weight savings of approximately 80% for the crankcase and 26% for the connecting rod. The composite case failed after approximately 20 minutes (O(104) cycles) of operation at the best torque condition (worst-case in terms of mechanical stress) due to manufacturing deficiency at the corners and base; the connecting rod failed after approximately 2 minutes (O(103) cycles) of worst-case operation due to separation of the bearing surface from fiber. Both sets of failures point to simple remedies for follow-on work. The initial prototypes resulted in an overall weight savings of 8.1%. Using commercially available data for small propeller-driven aircraft, this weight savings could be expected to result in a similar relative improvement in aircraft range.