An Introduction to the Micro-Engine Project

MEMS Engine-Generator Package

The original objective of this project was to develop a MEMS engine-generator. The combination of engine, generator, fuel tank, and control electronics were to deliver 10 W and occupy a volume no greater than 1 cm3. This concept is illustrated above and one should note that the MEMS engine is located at the far left and that it is several times smaller than the fuel tank. The MEMS engine was to operate at 20,000 Hz, have a stroke of 1-4 mm and a "bore" of 1 mm. In addition, the engine would feature planar geometry and be fabricated with MEMS technologies e.g., Deep RIE. Consult U.S. Patent 6,276,313 for more details.

After several failed attempts to construct prototypes, it was determined that state-of-the-art MEMS fabrication technologies lacked the dimensional control necessary for micro-engine parts. Several alternative techniques e.g., LIGA and EDM machining were explored. EDM machining offered the best combination of tolerances and cost. EDM was used to fabricate the steel prototype engine below.

Steel Prototype

Although the tolerance specifications were nearly met, the prototype engines were unusable due to burrs and sliding friction; the planar geometry was subsequently abandoned. Additionally, a suitable linear alternator design was beyond reach. Hence the the engine-generator configuration was also discarded in favor of an engine-compressor configuration. While this is a significant departure from the original concept, compressed air may actually have more applications. For example, a turbine could be used to generate electricity or turn a propeller. In addition, compressed air could be used to drive miniature pneumatic actuators. The current configuration is depicted below.

Micro Combustion

A well-known property of flames is that they cannot propagate through an opening smaller than the quenching distance; the same principle behind the design of spark arrestors and numerous other safety devices. For typical hydrocarbon fuels, this distance is about 2 mm. Micro-engines however, will have characteristic dimensions equal to or considerably smaller than quenching distances. Consequently engine combustion in the traditional sense, e.g., spark ignition, is not feasible. Some alternative combustion strategies include: 1. Catalysts., 2. Elevated or adiabatic walls., and 3. Homogeneous Charge Compression Ignition combustion. This program employs HCCI.

Homogeneous Charge Compression Ignition (HCCI) Combustion

HCCI is a novel engine combustion mode that entails compressing a fuel-air mixture until it explodes. HCCI has the following experimentally verified characteristics: 1. Ignition occurs at several locations in the combustion chamber., 2. An absence of traditional flame propagation., 3. The charge is consumed rapidly., 4. Extremely lean mixtures can be ignited., and 5. Fuel flexibility. Therefore HCCI essentially permits combustion without a flame and quenching is less of a problem.

Free Piston Engines

HCCI depends upon the compression process and fuel oxidation kinetics. Consequently direct ignition control is not possible. Despite this limitation, many efforts to adapt conventional engines to HCCI operation are underway because HCCI has the potential to simultaneously reduce NOx emissions and achieve ``Diesel-like'' fuel economy. Ignition control is also a problem for micro-engines. One possible solution is to employ a variable compression ratio engine configuration e.g., a free piston.

free piston engines have several advantages over traditional i.e. slider-crank, engines. For example, they have fewer moving parts and the compression ratio is an operating parameter. Unfortunately, they are considerably more difficult to design because stable operation depends upon maintaining a "thermodynamic-dynamic balance." Moreover in the case of HCCI, the piston motion is coupled to the combustion process. Consequently few examples of successful free piston engine designs exist. Also, rotational output is difficult to obtain from these engines. Hence these engines are not well-suited for vehicle powerplants. Instead, these devices are typically employed for compressing gases and pumping liquids.

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Last Update: 30 May 2002
Page Author: Hans T. Aichlmayr


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