Design, modeling, and analysis of a novel in-plane cantilever apex seal for maintaining high compression ratios in a MEMS-based rotary internal combustion engine are presented. This work is part of an effort to create a portable, MEMS-based Rotary Engine Power System (MEMS REPS) capable of producing power on the order of tens of milliwatts and with an energy density better than that of a conventional battery. A Wankel-type rotary engine is advantageous for a MEMS-based internal combustion engine due to its planar geometry, self-valving operation, and few moving parts. Large scale rotary engines typically incorporate a complex apex and face sealing system composed of many parts and involved assembly. A MEMS-based apex seal system can be incorporated as part of the rotor in order to eliminate manual assembly. The seal system must also have a minimal footprint and closely follow the epitrochoid profile in order to effectively integrate with the other engine systems. Based on these objectives, an integrated in-plane cantilever apex seal system can be integrated into the rotor with a small footprint. The first step in the development of the MEMS REPS is an air-powered expander which can be used to demonstrate electrical generator operation, engine rotation, and apex seal operation. The apex seals discussed here are optimized for use in an air-powered expander.
A performance analysis of this flexure apex seal design is performed, which examines 4 major performance constraints: resonant frequency, strain, pressure, and power dissipation. In addition, the seal design also accounts for fabrication tolerances of thick deep reactive ionetching (DRIE). During operation, dynamic effects due to pressurized gases and mechanical translation may drive the flexures into resonance, leading to galloping of the cantilever tips. Galloping will result in large leakage paths, thereby, reducing the compression ratio. A 0.25% strain limit is imposed to minimize the effect of fatigue on seal performance. Pre-compressed apex seals are used to counteract forces generated on the apex seal due to a pressure differential. The apex seal is also designed to minimize the power dissipated due to frictional losses.
To model the cantilever apex seal, two different loading conditions are examined. One condition is distinguished by point loading at the tip, when contact is made between the seal and housing wall. Another condition is characterized by a distributed loading, due to the changing pressure by both the compression and the combustion events. Analytical models in addition to a finite element analysis were performed.
May 31, 2003
Martinez, F. C. (2003). Apex Seal Design for the MEMS Rotary Engine Power System: Research Project. United States: University of California, Berkeley.