Development of a two-stroke, two-cylinder, spark-ignition UAV engine
The use of unmanned aerial vehicles (UAVs) for both civil and military applications has increased significantly throughout the world in recent years. The propulsion systems used in locally designed and manufactured, medium-range UAVs for the military are two-stroke internal combustion, spark-ignition (SI) engines that are imported from first world manufacturers at significant cost. This study investigates whether it is possible to design, manufacture and test a two-stroke, two-cylinder, horizontally-opposed, air-cooled, SI engine suitable for UAV application at a lower cost compared to imported products. In order to reduce design effort and assist in mitigating functional risk, appropriate commercially available motorbike components were used in critical areas of the engine. Most of the fundamental engine components, specifically the cylinders, cylinder heads, pistons and connecting rod assemblies, were sourced from a two-stroke Yamaha YFS 200 Blaster quad bike and integrated into the design. To design the remaining crankshaft and crankcase assembly structurally, a theoretical computer-based model was created to determine the forces generated by the engine mechanism, given the chosen cylinder arrangement. Essential input for the theoretical model was obtained experimentally by testing a two-stroke Yamaha DT 175 engine on a dynamometer and capturing the maximum cylinder pressure curves obtained throughout the usable speed range. No thermal, fluid, vibration or detailed structural analysis was done. The design was concluded by producing a complete set of detail drawings and bill of materials for the manufacturing process. Manufacturing of the crankcase required specialised tooling and a competent machinist, while the accurate alignment of the individual component press-fit crankshaft proved arduous. The assembly of the entire engine was, however, quickly accomplished. Once installed in the test cell, the UAV engine was successfully tested on a dynamometer while at full load. The prototype achieved the stated goal. Design refinement is, however, required to reduce the overall mass of the UAV engine, analyse critical components and apply cost-effective manufacturing methods.