Liquid Fuel Injection System for Mini-Scale Rotary Engines

In an attempt to optimize the performance of small scale internal combustion engines, an investigation and development effort for a liquid fuel delivery system is ongoing. Engine operation sets a pair of primary design constraints, fuel mass flow rate and maximum droplet diameter. The fuel mass flow rate requirement is dictated by the stoichiometry, the engine geometry, and the rotational speed. The droplet diameter constraint is generated from the geometry and the rotational speed. Theoretical prediction has been performed to determine required fuel flow rate and initial size of droplets. Fuel flow rate has been estimated from both engine variables and fuel properties, and droplet size has been determined from the physical time (residence time) of the engine. At the engine rotational speed as 10,000RPM, it required 40mg/sec of methanol with 60μm of initial diameter of droplet. Experimental measurements have been performed to compare with predicted requirements. Micro dispensing valves are used as an injector, and it has ability of delivering 10-100mg/sec with 210-360μm diameters of droplets. Several injector parameters, such as the size of nozzle, the driving pressure, and the duty cycle, have been varied to determine the effect on mass flow rate and size of droplets. 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. With 2nd generation of an evaporator, a fuel injector was successfully delivers up to 50mg/sec with well-mixed 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.