Development of Liquid Fuel Injection System for Small Scale Rotary Engines

*† ‡ § In an attempt to optimize the performance of small scale internal combustion rotary engines for portable power generation, an investigation and development effort for a liquid fuel delivery system is ongoing. Engine operation sets a pair of primary design constraints for liquid fuel delivery systems: fuel mass flow rate and maximum droplet diameter. The fuel mass flow rate requirement is dictated by the stoichiometry, the engine geometry, and the engine residence time as determined by engine (rotational) speed. The droplet diameter constraint is generated from the engine geometry and the residence time. Similarly, the field of use limits operational parameters, such as delivery pressure, subsystem power requirements, and overall system weight. A theoretical prediction has been carried out to determine the required fuel flow rate and initial droplet size for a small-scale rotary engine. For an engine operation of having residence time of 9ms (rotational speed: 10,000RPM), 40mg/sec of methanol with 60µm of initial droplet diameter is required. Guided by these fuel delivery requirements, experimental measurements have been performed using commercially available micro dispensing valves to determine their viability. It was determined that these valves are capable of delivering 10-100mg/sec of fuel with a droplet diameter range of 210-360µm for pressures expected in portable power devices, and commercially available orifice diameters. Several injector parameters, such as the size of orifice, the driving pressure, and the duty cycle, have been varied to determine the effect on mass flow rate and size of droplets to guide future designs. Due to large discrepancy between required initial size of droplets and measurements, the concept of evaporating fuel by external heat source has been adapted and implemented. Using an evaporator, a fuel injector was successfully delivered fuel up to 50mg/sec as mostly vaporized droplets. The results of this study will be applied to develop a fuel delivery system for standalone portable power applications such as micro-scale engines, turbines and potentially fuel cells.