MEMS Rotary Engine Power System

Special Issue Review MEMS Rotary Engine Power System Non-members A. Carlos Fernandez-Pello*, Albert P. Pisano*, Kelvin Fu*, David C. Walther*, Aaron Knobloch*, Fabian Martinez*, Matt Senesky*, Conrad Stoldt**, Roya Maboudian*, Seth Sanders*, Dorian Liepmann* This work presents a project overview and recent research results for the MEMS Rotary Engine Power System project at the Berkeley Sensor & Actuator Center of the University of California at Berkeley. The research motivation for the project is the high specific energy density of hydrocarbon fuels. When compared with the energy density of batteries, hydrocarbon fuels may have as much as 20x more energy. However, the technical challenge is the conversion of hydrocarbon fuel to electricity in an efficient and clean micro engine. A 10 mm diameter Wankel engine will be shown that has already generated 4 Watts of power at 9300rpm. In addition, the 1mm and 2.4 mm Wankel engines that BSAC is developing for power generation at the microscale will be discussed. The project goal is to develop electrical power output of 90milliwatts from the 2.4 mm engine. Prototype engine components have already been fabricated and these will be described. The integrated generator design concept utilizes a nickel-iron alloy electroplated in the engine rotor poles, so that the engine rotor also serves as the generator rotor. Keywords : MEMS Power Generation, Electroplating, Wankel Engine, Silicon Carbide, DRIE 1. Introduction Due to market changes (increased use of portable computers and phones), there is a tremendous opportunity and need for portable power systems with an increased lifetime. The MEMS Rotary Engine Power System (MEMS REPS) designed to deliver superior specific energy to the consumer would have large commercial potential. Recent breakthroughs in material, micro-combustion, and fabrication techniques make possible the design of such a system. The overall rationale of this research proposal is to develop a micro-rotary engine based, liquid hydrocarbon fueled, portable, power generation system. In order to do this, new MEMS material and fabrication techniques must be developed in parallel with novel MEMS systems. The MEMS Rotary Engine Power System (REPS) project s objective is to develop a liquid hydrocarbon fueled portable power system using a rotary (Wankel) engine as the power source. The selection of liquid hydrocarbon is due to the higher energy density available in liquid hydrocarbons when compared to conventional batteries. The ultimate goal of the MEMS REPS project is a power system capable of producing ~10-100 mw of electrical power. At the heart of the REPS package is the power generation chipset (PGC), consisting of several rotary engines stacked together, integrated electric generator, fuel delivery system, waste heat management, ignition system, and all other ancillary equipment. The PGC is packaged in a low conductivity materially (i.e. aerogel) within an evacuated dewar, in order to thermally isolate the engine. The overall concept of the project with the primary research topics are shown in Figure 1. The project is separated into the separate research avenues necessary to produce the complete MEMS REPS package. * Department of Mechanical Engineering, University of California, Berkeley 6105A Etcheverry Hall, Berkeley, CA 94720-1740 ** University of Colorado, Boulder Research issues include the design and fabrication of the micro-rotary internal combustion engine (including integrated seals, magnetic pole designs, and ignition module), integrated electric generator, and the heat transfer and fluid management system. The power generation unit will be packaged in a thermal isolation chamber to reduce heat losses (an integral thermal management technique for engines of this scale). The integrated micro-rotary engine and electric generator will be placed in a cavity that is evacuated and filled with an aerogel-based material. This insulation material inhibits thermal conduction and blocks radiation. 2. Rotary Engine Development and Fabrication 2.1 Centimeter-Scale Rotary Engine A rotary engine was selected for development as the basis of a MEMS-scale power generation system due to several factors: the planar design of the rotary engine lends itself to MEMS fabrication the rotor controls the timing of the intake and exhaust (no complex valving) the power output is in the form of rotary motion of the shaft. The rotary engine development program is planned in progressive steps leading to the fabrication of a MEMS-scale engine. A series of larger scale mini-rotary engines (referred to as the 10 mm and 13 mm ) have been constructed from steel using electrodischarge machining (EDM) to investigate design and combustion issues as the engine scale is reduced (see Table 1). Table 1. Mini-rotary and micro-rotary specifications. Mini-Rotary Engine Micro-Rotary Engine Rotor Diameter 10 mm 13 mm 1 mm 2.4 mm Engine Depth 3 mm 9 mm 0.3 mm 0.9 mm Displacement 78 mm3 348 mm3 0.08 mm3 1.2 mm3 電学論 E,123 巻 9 号,2003 年 1

Fig. 1. UC Berkeley MEMS REPS research effort and scope. Fig. 2. 10 mm rotary engine. This engine is fabricated from steel using EDM techniques. Fig. 3. Mini-rotary engine test stand. Fig. 4. Net power output of 13 mm mini-rotary engine operating on H 2 -air and using spark ignition (Φ = 0.4 ± 0.04). In addition, an engine test stand was designed and fabricated to test the mini-engine operation (see Figures 2 & 3) (1). The test bench consists of an electric motor / dynamometer, optical tachometer, ignition system (for spark plug use), and flywheel. Spark ignition was used to initially investigate the engine performance. However, actual operation will be by glowplug enhanced auto-ignition. Engine speed is measured using an optical tachometer. The mini-engine is rigidly coupled to the dynamometer via a steel shaft. Positive power output of up to 4 W at 9,300 RPM was achieved with an H 2 -air mixture using both spark and glowplug ignition. Figure 4 shows the 13 mm mini-engine power output as a function of engine speed. The mini-engine power tests successfully proved the basic rotary engine design. The next step is the fabrication of the MEMS micro-rotary engine. 2.2 MEMS-Scale Rotary Engine The MEMS scale 2 IEEJ Trans. SM, Vol. 123, No.9, 2003

Special Issue Review micro-rotary engine is being constructed primarily of Silicon (Si), Silicon Carbide (SiC), and Silicon Dioxide (SiO 2 ). All parts subjected to high temperatures and stresses will be built either using molded SiC (2) or a Si substrate with a thin SiC coating (3). To date, two micro-rotary engines are being fabricated at the UC Berkeley Microfabrication Laboratory: a 0.08 mm 3 displacement engine with a 1 mm rotor diameter, and a 1.2 mm 3 displacement with a 2.4 mm rotor diameter. The smaller 1 mm engine was used to develop the basic MEMS fabrication processes, investigating the fabrication tolerances possible in large-scale MEMS devices. After the fabrication process was proven, the larger 2.4 mm engine has been designed and is being fabricated. The engine fabrication processes consist of multiple mask, deep reactive ion etching (DRIE) with wafer-to-wafer bonding steps. In addition, high precision manual assembly of the engines is also required for final assembly. Research issues being addressed are: DRIE aspect ratio, sidewall straightness, manual assembly, and die-to-die bonding. The 1 mm engine components consist of a housing wafer, rotor, and square shaft (4). The housing wafer features inlet / exhaust channels, the epitrochoidal housing bore, and a spur gear. The rotors and housings have also been coated with a novel low temperature (650ºC) SiC coating (3), which shows excellent structural detail transfer (see Figure 5). The 2.4 mm engine s design and fabrication process has been improved when compared to the 1.0 mm engine. Some design improvements include an integrated apex sealing system, rotor combustion pocket, and waste heat channels (see Figure 6). Changes to the base fabrication process have been introduced to eliminate DRIE overetch and increase overall component tolerance. Fig. 5. MEMS fabricated 1 mm engine components. Rotor diameter is 1 mm and 300 µm thick. All parts have been coated with a conformal 1 µm thick SiC coating. Currently, research is centered on the fabrication of the individual components of the engine, such as the rotor, housing, and square shaft. The overall dimensions of these components and their interaction with each other are an important first step toward an operational power generation system. 3. Integrated Electric Generator The MEMS REPS electric generator is integrated into the micro-rotary engine (see Figure 7). The generator design utilizes a 40%-60% nickel-iron alloy (permalloy) electroplated in the engine rotor poles, so that the engine rotor also serves as the generator rotor. The 40:60 permalloy composition was selected to not only match the thermal expansion coefficient of silicon at elevated temperatures but also offers a relatively high magnetic saturation limit. An external stator structure including a coil and permanent magnet is made from discrete components. The soft magnetic components of the stator use a powder-iron material chosen for its favorable loss properties under AC fields. The generator design has several advantages for use with the rotary engine. The six-pole configuration takes advantage of the triangular symmetry of the Wankel rotor. The use of the rotor for both engine and generator eliminates the need for shaft coupling between two machines. The Ni-Fe poles in the rotor experience only a DC magnetic field, so that there are no eddy currents in the rotor to cause shielding or large losses. The use of discrete components for the stator simplifies the microfabrication of the engine, and allows for a larger volume of copper in the coil to improve efficiency. Finally, placing the permanent magnet at a distance from the combustion chamber eases the issue of degrading magnetic properties at high temperature. Ongoing research efforts for the generator include finite-element modeling of magnetic fields to improve the stator design, refinement of stator materials and design to enable machining with an EDM process, and development of Ni-Fe electroplating techniques. 4. Ancillary Equipment A key consideration in the design of the MEMS REPS is to restrict ancillary equipment energy consumption to less than 10% of the power budget. This constraint leads to the extensive use of passive ancillary sub-systems and sensors. All temperature, pressure, and chemical sensors necessary for engine control are required to be passive. The fuel delivery system will be designed utilizing waste engine heat and a pressurized fuel tank, eliminating the need for micro-pumps. The fuel delivery system is based on the micro-capillary pump loop, which utilizes an evaporator and Coil Permanent Magnet Stator Pole Faces Fig. 6. 2.4 mm engine components. Note the waste heat management channels in the housing and rotor openings for electroplated soft magnetic poles. Partial Stator and Rotor 3D View Fig. 7. Engine Housing Rotor w/ NiFe Poles Powder Iron Permanent Magnet Copper Silicon Nickel Iron Magnetic Path Schematic Integrated electric generator schematic. 電学論 E,123 巻 9 号,2003 年 3

microinertial instruments, microinformation storage systems, and microfluidic systems. He has authored or coauthored more than 80 refereed publications. He has graduated more than 20 Ph.D. students and more than 40 master s students from the University of California at Berkeley. He holds the FANUC Chair of Mechanical Systems and he with the Department of Mechanical Engineering as well as the Department of Electrical Engineering and Computer Science. From 1997 through 1999, he was the Program Manager for Microelectromechanical Systems, Defense Advanced Projects Research Agency. There, he was responsible for a portfolio that rose to more than 83 contracts in MEMS in academia, industry, and government all across the United States. Fig. 8. MEMS capillary loop pump. condenser for electronics cooling (see Figure 8) (1). Glowplugs, rather than spark plugs will be used as the ignition source. Power is supplied to the glowplugs only during engine start-up. After ignition, power to the glowplug is eliminated. Residual heat from the combustion reaction and the high engine compression ratio will sustain engine operation. Significant engine testing will be required to determine the optimal glowplug location for efficient engine operation. In order to reduce power consumption, the heaters must be insulated from the wafer through air cavities and low conductivity materials. (Manuscript received December 31, 2002) References (1) K. Pettigrew, J. Kirshberg, K. Yerkes, D. Trebotich, and D. Liepmann : Performance of a MEMS based micro capillary pumped loop for chip-level temperature control, Technical Digest. MEMS 2001. 14th IEEE International Conference on Micro Electro Mechanical Systems, IEEE. 2001, pp.427-30. Piscataway, NJ, USA (2001) Carlos Fernandez-Pello (Non-member) received a Ph.D. in Engineering Sciences (1975) from the University of California, San Diego. He is currently a Professor at the Department of Mechanical Engineering at the University of California, Berkeley, and Associate Faculty Scientist at the Lawrence Berkeley Laboratory. His research interests are in the fields of combustion, heat and mass transfer, thermodynamics, and MEMS. He is a member of the Spanish National Academy of Engineering and an ASME Fellow, and is on the Editorial Advisory Boards of Combustion Science and Technology, Combustion and Flame, and Progress in Energy and Combustion Science. Albert P. Pisano (Non-member) received the Ph.D. degree in Mechanical Engineering from the Department of Mechanical Engineering, Columbia University, New York, in 1981. In 1983, he joined the University of California at Berkeley, where his research interests are the invention, design, fabrication, modeling, and optimization of microelectromechanical systems such as Dorian Liepmann (Non-member) received the B.S. and M.S. in chemical engineering from the California Institute of Technology, Pasadena, and the Ph.D. degree from the University of California, San Diego. He worked in the Advanced Technology Group at SAIC, La Jolla, CA, on projects for the U.S. Navy. Previously, he was a Member of the Arroyo Center, providing technology and policy analysis for the Army. He joined the Mechanical Engineering Department at the University of California at Berkeley in 1993 and the Bioengineering Department in 1999, in which he is currently Vice Chair. He is an Associate Director of the Berkeley Sensor & Actuator Center. His research interests include microfluiddynamics, mixing, transition, and vortex dynamics. He is currently working on the application of MEMS to biological and medical problems. Roya Maboudian (Non-member) received the Ph.D. degree from the California Institute of Technology, Pasadena. She is currently an Associate Professor in the Department of Chemical Engineering, University of California, Berkeley. Her research interests include the surface physics and chemistry of electronic materials. Her most recent work has focused on the problem of stiction in microelectromechanical systems. She and her group have designed surface processes to reduce adhesion and friction in MEMS. Maboudian is the recipient of several awards, including the Presidential Early Career Award for Scientists and Engineers, NSF Young Investigator award, the Beckman Young Investigator Award, the Alexander von Humboldt Fellowship, and IBM Postdoctoral Fellowship. Seth Sanders (Non-member) received his Ph.D. degree from Massachusetts Institute of Technology in 1989. He is currently an Associate Professor of Electrical Engineering at the University of California, Berkeley. His research interests are in design and control of variable speed electric machine systems, high frequency electrical power conversion, nonlinear circuit and control methods as applied to the power electronics field. Some of his recent work has been on digital control of 4 IEEJ Trans. SM, Vol. 123, No.9, 2003

Special Issue Review high-frequency DC-DC converters, and high-speed electric machine systems as applied in flywheel energy storage systems. He is a recipient of the NSF Young Investigator Award, and prize paper awards from the IEEE Power Electronics Society and the IEEE Industry Applications Society for publications on thin film magnetic components, high-speed electric machinery, and self-sensing magnetic bearings. Conrad Stoldt (Non-member) received his Ph.D. in Physical Chemistry from, Iowa State University in 1999. He currently an Assistant Professor of Mechanical Engineering at the University of Colorado. His research interests include Surface and Interface Physics,Surface Dynamics, Semiconductors, Micromachining and Microelectromechanical Systems (MEMS and Biomaterials. Prof. Stoldt is a member of the American Vacuum Society (AVS) and the Materials Research Society (MRS) David C. Walther (Non-member) received his Ph.D in Mechanical Engineering from the University of California, Berkeley in 1998. He is currently a research engineer at the Berkeley Sensor and Actuator Center. His research interests include thermosciences, MEMS and systems integration. Dr. Walther is a member of the Combustion Institute and the ASME MEMS division. Kelvin Fu (Non-member) received his Ph.D in Mechanical Engineering from the University of California, Berkeley in 2001. He is currently a research engineer at the Berkeley Sensor and Actuator Center. His research interests include DRIE, MEMS and integrated fabrication/assembly. Aaron Knobloch (Non-member) received his B.S. in Mechanical Engineering from Bucknell University in 1998 and his M.S. in Mechanical Engineering from UC Berkeley in 2002. He is currently a Ph.D. graduate student in Mechanical Engineering at the University of California, Berkeley. Fabian Martinez (Non-member) received his B.S. from the University of California, Berkeley in 2000 in Electrical Engineering. He is currently a Ph.D. graduate student in Mechanical Engineering at the University of California, Berkeley. Matt Senesky (Non-member) grew up in Denville, NJ. He received the A.B. and B.E. degrees from Dartmouth College in 1998 and 1999 respectively. He is currently a graduate student in the electrical engineering department at U.C. Berkeley. His research interests include micro-scale power generation, electric machine design and control, and hybrid systems. 電学論 E,123 巻 9 号,2003 年 5