The Scaling of Loss Pathways and Heat Transfer in Small Scale Internal Combustion Engines

Prior literature indicates fuel conversion efficiency and normalized power deteriorate increasingly rapidly with decreasing displacement, but does not fully reveal the driving losses. The literature also suggested that increasing losses relax the required fuel anti-knock index (AKI), but offered conflicting conclusions on the performance impact. This comprehensive experimental study of three, 28 cm3 to 85 cm3 displacement, commercial-off-the-shelf (COTS), two-stroke ICEs identified short-circuiting as having the most deleterious impact on COTS engine performance in this size range. Heat transfer losses were comparable to larger engines for displaced volumes greater than 10 cm3. An engine friction model was developed that uses the surface area to volume ratio, speed, and throttle to predict friction losses; a heat transfer model was also validated. The impact of reducing fuel AKI on performance was systematically investigated. The results showed a dependence on engine size; the fuel AKI requirement decreased 20 octane number between 85 cm3 and 28 cm3 displacement. Switching from 98 ON (manufacturer recommended) to 20 ON (JP-8, diesel equivalent) fuel increased power 2 -3 and fuel conversion efficiency by 0.5 -1 at non knock-limited conditions

Integration, Testing and Validation, of a Small Hybrid-Electric Remotely-Piloted Aircraft

Parallel hybrid-electric technology offers a wide variety of new mission capabilities including low-observable loiter operations and increased fuel efficiency for small remotely-piloted aircraft. This research focused on the integration, validation, and testing of a hybrid-electric propulsion system consisting of commercially available components to fabricate a small remotely-piloted aircraft capable of extended low-observable operation. Three novel aspects contributed to the success of the design: optimization of the propulsive components to the integrated system, torque control of the components for additive power, and a one-way bearing/ pulley mechanism (patent pending) mechanically linking the hybrid system components. To the knowledge of the author at the time of publication, this project represents the first functional parallel hybrid-electric propulsion system for a remotely-piloted aircraft. The integration phase entailed the selection, testing, and assembly of components chosen based on prior design simulations. The propulsion system was retrofitted onto a glider airframe with a 12 ft wingspan and a maximum takeoff weight of 35 lbs, also based on the initial design simulations. During the validation and testing phases, results from bench, ground, and flight testing were compared to the design simulations. The designed propulsion system was well matched to the power estimates of the design simulations. Bench and ground tests demonstrated that hybrid mode, electric only mode, combustion only mode, and regeneration mode are fully functional. Comparison of bench test results to an engine only variant of the airframe indicate the HE system is capable of flying the aircraft

In-cylinder Temperature Measurements in a 55-cm\u3csup\u3e3\u3c/sup\u3e Two Stroke Engine via Tunable Laser Absorption Spectroscopy

In-cylinder temperature is a critical quantity for modelling and understanding combustion dynamics in internal combustion engines. It is difficult to measure in small, two-stroke engines due to high operational speeds and limited space to install instrumentation. Optical access was established in a 55 cm3 displacement two-stroke engine using M4 bolts as carriers for sapphire rods to establish a 1.5 mm diameter optical path through the combustion chamber. Temperature Laser Absorption Spectroscopy was successfully used to measure time varying in-cylinder temperature clocked to the piston position with a resolution of 3.6 crank angle degrees at 6000 rpm. The resulting temperature profiles clearly showed the traverse of the flame front and were qualitatively consistent with in-cylinder pressure, engine speed, and delivery ratio. The temperature measurements were compared to aggregate in-cylinder temperatures calculated using the ideal gas model using measured in-cylinder pressure and trapped mass calculated at exact port closure as inputs. The calculation was sensitive to the trapped mass determination, and the results show that using the ideal gas model for in-cylinder temperature calculations in heat flux models may fail to capture trends in actual in-cylinder temperature with changing engine operating conditions