AbstractIII. Test structuresIV. Expected ResultsV. ConclusionApex Seals for a MEMS Rotary Engine Apex Seals for a MEMS Rotary Engine Josh Heppner, Fabian Martinez* Josh Heppner, Fabian Martinez* University of California, Berkeley University of California, Berkeley *44 Hesse Hall, Berkeley, CA 94704, USA *44 Hesse Hall, Berkeley, CA 94704, USA Tel: (510) 643-5282 Fax: (510) 642-1850 Tel: (510) 643-5282 Fax: (510) 642-1850 E-Mail: [email protected]: [email protected] [email protected] Abstract This paper presents possible solutions to the problem of leakage in the MEMS rotary engine currently being developed at U.C. Berkeley. The project is motivated by the challenge to develop a system capable of a higher specific energy than primary batteries. Currently, leakage is the limiting factor in obtaining an engine with an acceptable efficiency. Fluidic analysis has been done to determine a necessary distance of 2 microns between the rotor and the housing of the engine, remembering that the compression ratio only needs to be maintained over the characteristic time, 2 msec for a shaft speed of 40,000 rpm. Mechanical analysis has also been completed to ensure that the apex seals will maintain this minimum distance by integrating a spring-loaded apex design. Test structures will also verify this analysis. I. Introduction Micro internal combustion rotary engines are a potential replacement for batteries in the near future. This comes from the energy density advantage hydrocarbons hold over the current alkaline batteries available, which happens to be 10:1 for an engine with 20% efficiency [1]. A limiting factor, in the development of an internal combustion engine with this efficiency is engine sealing. Without adequate engine sealing within the engine the compression ratio inside the rotary engine plummets causing the efficiency of the engine to do the same [2]. Otto Cycle: 1,11−−=kvOttothrη (1) To ensure that the compression ratio is adequate, it is necessary to implement an engine sealing system capable of high compression ratios which minimizes leakage. Figure 1: Rotary engine rotor with details for apex seal flexure designs: (a) single flexure, (b) bistable flexure, and (c) folded flexure. Engine sealing has been accomplished in the past. Mazda achieved sealing with an elaborate system, which consisted of approximately thirty parts, for their macro scale rotary engine [3]. However, it has been shown that the primary source of leakage in small-scale rotary engines is past the apex seal [2]. This is convenient, since it is necessary to keep systems as simple as possible due to the limits of MEMS fabrication. a) b) ) c) c) The design of the apex seals for the micro engine will address the following issues: structural stiffness, engine fabrication tolerance, leakage rate, and assembly. Another concern is the tolerances of the rotor with respect to the housing (i.e. aspect ratios of both). A determination of the travel of the apex seal will need to be made to account for this tolerance. Fluidic analysis will be performed to determine the necessary gap distance needed to increase engine sealing. Establishing this distance will help prevent additional wear on the apex from over design. Finally, the design of the apex seal will need to be simple with no required assembly. Fabrication The process to be used for this design will be a single mask Deep Reactive Ion Etch (DRIE). The structural layer will be masked and then the rotor will be etched down 300 microns. The minimum line width is 1 µm. The minimum line spacing is >12 µm due to an aspect ratio of 50:1. The aspect ratio will be necessary to determine if the incline of the housing wall matches the incline of the rotor. II. Design The apex seal flexure designs being considered are shown in Figure 1. Of these, the single flexure (Figure 1a) has the simplest geometry, while the folded flexure (Figure 1b) sacrifices some simplicity for design flexibility. The third flexure type incorporates a bistable centrally-clamped parallel-beam adopted from the research of Qiu, Lang, and Slocum of Massachusetts Institute of Technology [4]. A bistable flexure is advantageous because it can potentially benefit the assembly process. Because the geometry of the apex’s tip is constant for the three proposed designs, the fluid analysis will also be constant. In contrast, the mechanical analysis of the apex’s flexures will be unique for each design. 1Mechanical Analysis In a rotary engine, the apex seals maintain contact with the walls of the combustion chamber at all times. To determine this dynamic displacement of the apex tips, the housing is measured to determine the fabrication tolerance. Inaccurate fabrication will cause a variance between the designed housing profile and the fabricated profile. Figure 2 demonstrates a comparison of the mask layout and fabricated engine parts. From this analysis, the required travel of the apex is measured to be less than 10 µm. 2 Friction due to the apex seals will be approximately proportional to the force normal to the engine housing. To estimate the normal force, numerical calculations and finite element models will be made to characterize the proposed designs. The force of the apex seal against the housing wall is defined as kxF = (2) where F is the force, k is the stiffness of the flexure, and x is the deflection. The spring stiffness for the single flexure and folded flexure designs are calculated using unified beam theory [5]. For the single flexure design: 3192lEIk −= (3) For the folded flexure design: ()()132122321333146−+++−+=lllllllEIk (4) where E is the Young’s modulus, l1 & l3 are the long flexures, l2 is the short, connecting flexure (Figure 3), and I is the moment of inertia of the flexure. The moment of inertia is calculated by +++=33222324312sbshbsbhhbI (5) where b is the width of the flexure, h is the height, and s is the aspect ratio of the fabrication process. The stiffness of the bistable flexure will be calculated through finite element modeling. The single and folded flexure stiffnesses will be verified by the same method of finite element analysis. Fluidic Analysis The fluidic analysis of the engine is complex. The Reynolds number, the ratio between inertia and surface effects, for the system is
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