GETI/GLOCOM Platform Joint Seminar

MEMS Rotary Engine Power System: Project Overview and Recent Research Results

Summary by GETI Staff

Prof. Pisano presented 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 talk began with the research motivation for the project, which is the extraordinary high specific energy density of hydrocarbon fuels. When compared with the energy density of batteries, hydrocarbon fuels may have as much as 15x more energy. However, the technical challenge is the conversion of hydrocarbon fuel to electricity in an efficient and clean micro engine. In this project, the Wankel engine, as invented by Professor Wankel of Germany and made famous by the Japanese automobile manufacturer, Mazda, is used as the micro engine design.

The project targets are to integrate the MEMS Rotary Engine and Integrated Power Generator Chipset, Power Conditioning, Logic, and Engine Control, Manual Compressed Air Starter, Electrical Power Output Contacts, and Fuel Reservoir in a 200mm x 180mm x 30mm package and generate electrical power from liquid hydrocarbon fuels. To test the research idea, a 10 mm diameter Wankel engine was fabricated to demonstrate the feasibility of a small-scale engine capable of generating 10-100W of power. When the 10 mm Wankel engine was published in 1998, it became controversial in the combustion community because the combustion chamber was considered too small for combustion to occur and that the quenching distance was much too small. However, the 10 mm engine successfully generated 4 W of power at 9300 rpm. This is the data obtained two years ago.

Several months ago, the research project members put together an even more advanced research concept for 1 mm- and 2.4 mm-diameter MEMS rotary engine power systems, and Prof. Pisano showed some preliminary results. They developed an energy budget for the 2.4 mm Wankel engine. They presume that they can burn octane fuel such that 1.7 W of energy will be generated, of which 1.5 W will be exhausted as waste heat. 30 mW of energy will be recovered to vaporize fuel in the vaporization system such that the fuel vapor will be directly injected into the combustion chamber. They also presume that they can obtain 167 mW of useful torque power at the shaft of the engine. The electrical generator is inside the engine. There is no external generator. The Wankel rotor also has magnetic pole pieces in it. At an 80% efficiency of electrical generation, the 2.4 mm engine should be able to generate 90mW of electricity. 90mW of electricity is not that much if we look at certain applications such as laptop computers. On the other hand, this is a very small engine that would be operated at one-speed, one-power output. Micro power engines would be useful as high energy density power system, however they require a temporary energy storage device to smooth the power demand curve, perhaps as a super-capacitor. The 2.4 mm engine will not have a sparkplug because sparkplugs are power consuming. Prof. Pisano and his colleagues at Berkeley believe that successful micro engines will have compressed air, glowplug or other ignition systems.

Some American companies participate in this research project. ChevronTexaco has promised to develop custom fuels for the micro engine, while Harris Corporation is involved in component integration and packaging and Textron Systems in device/application design requirements and reliability/yield testing. UC Berkeley conducts device fabrication & characterization, auxiliary component fabrication and system integration.

Prototype engine components have already been fabricated at Berkeley, including a square shaft, 2.4 mm Wankel engine rotors, and integrated apex seal to keep the compression ratio high. A 2.4 mm Si Wankel engine is coated with SiC to achieve high resistance and low wear. The research group has also developed a low-temperature SiC deposition process using 1,3-disilabutane (DSB) at 650-850 deg C and achieved film comformality. The rotor has holes to be filled with NiFe soft magnetic material, which will switch the magnetic flux that goes through the engine.

The integrated generator is designed such that coils are placed over and under the rotor filled with NiFe soft magnetic material to create magnetic paths as the rotor rotates. Voltage pulses thus generated will be rectified to provide electric current. The research group is working on the finite element model to confirm the analysis and optimize the design geometry. A generator prototype at 10 mm scale has been constructed, and methods of coil construction for 2.4 mm scale are under investigation.

Prof. Pisano summarized his talk by saying that they have done all the finite element analysis yet have not yet done experiments with the 1.0 and 2.4 mm Wankel engines. However, he has confidence that the theoretical analysis is correct to a few percent. This means that a micro engine designed to have a theoretical compression ratio of 12 (assuming no leakage) is very likely to achieve a real compression ratio of 8.2. Since it is feasible to design the micro engines with a theoretical compression ratio as high as 19, it is therefore likely they will achieve the compression ratio that is almost like the diesel, if the same degradation of the compression ratio is presumed. This means that the overall efficiency of the micro engine may be between 10 and 20%, which is sufficient to provide more than 10x the energy density of batteries.