MEMS Liquid Fuel Delivery for Small-Scale Combustion Power Systems

The lack of a compact and efficient power source, spanning the range from10Watts to 1,000Wattslimits the mobility and usage time of personal devicesThis research has explored Commercially-Off-The-Shelf (COTS) and Microelectromechanical System (MEMS) based fuel delivery systems to improve the combustion efficiency of small-scale engines. Researchers at the University of California at Berkeley have developed liquid fuel operated small-scale rotary internal combustion engines, and the performance of such miniature combustion engineis greatly impacted by the performance of the fuel delivery systemOptimization of both mass flow rate and droplet size on the fuel delivering system is important to ensure maximum power output.
Initially, COTS liquid dispenser is characterized and adapted to the Berkeley developed 1.5cm3(S1500) rotary engine. For the optimum combustion process, the S1500 rotary engine requires methanol flow rate of 40mg/sec with droplet size of 60μm at rotational speed of 10,000rpm.While the COTS dispenser does satisfy the flow rate requirement, the injector is not able to produce the required droplet size. The unoptimized COTS fuel delivery system leads to low chemical energy conversion efficiency (4.7%) on the S1500 rotary engine. This low energy conversion efficiency corresponds to the S1500 rotary engine generating only 40.6Watts at 9,600rpm with methanol based glow fuel. Since no currently available COTS fuel delivery system could meet the required fuel droplet and flow rate for optimal combustion, the development of novel MEMS based fuel delivery system has been performed.
The MEMS fuel delivery system has been designed, simulated, and optimized for a compact, high force, low power consumption device. Combining a hole-in-the-wall planar valve with a MEMS electromagnetic (EM) linear actuator provides efficient liquid fuel delivery system for the small-scale engines. Finite Element Method (FEM) analysis has been performed to estimate force fields acting on the MEMS EM actuator, and Genetic Algorithm (GA) has been implemented on the actuator design to maximize the actuation force with the minimum phase area. This optimized MEMS fuel delivery system has been fabricated by surface micromachining. In addition, a unique permalloy electrodeposition method has been developed for trench filling applications by implementing dry film photoresist. The method allows deposition of 100μm thick permalloy without having undesirable side bump growth near the trench edges and successfully prevents the merger of the moving armature and stators.
Experimental measurements have been performed for the MEMS fuel delivery system. The MEMS EM linear actuator has generated the minimum actuation force of 1.34mN with electromotive force of 30AT. The fuel delivery system has delivered a 10mg/sec flow rate at a driving pressure of 14 psi with insignificant leakage. The fuel delivery requirements for S1500 rotary engine with high rotational speed have been achieved by adaption of multiple MEMS fuel delivery systems with reduced width of the microchannel. The maximum fluidic resistance and fluidic resistance ratio of the closed to the open position are determined as 1.83x10^15 Ns/m^5and 1.82x10^2 respectively. These measurements are among the highest recorded metrics compared to the previously reported liquid micro valves. Because of these metrics, the MEMS fuel delivery system is well suited to fuel regulation applications for the engine where the typical fluidic resistance ratio is 100:1.
Carlos Fernandez-Pello
Andrew R. Neureuther
Publication date: 
December 31, 2008
Publication type: 
Ph.D. Dissertation
Park, S. (2008). MEMS Liquid Fuel Delivery for Small-scale Combustion Power Systems. (n.p.): University of California, Berkeley.

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