design and fabrication of a resonant micro reciprocating engine for power generation

8/14/2019 Design and Fabrication of a Resonant Micro Reciprocating Engine for Power Generation

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Design and Fabrication of a Resonant Micro Reciprocating Enginefor Power GenerationICHashimoto, J.Ogawa, T.Toriyama*and SSugiyama

Faculty of Science and Engineering, Ritsumeikan University'Center for Promotion of the CO E Program, Ritsumeikan U niversity

1-1-1 Noji-Higashi, Kusalsu, Shiga 525-8577 Japan

Abstract:Structure and perlormance of a silicon based microreciprocating engine have beeu designed. The Otto cycle withhydrogen fuel has been adopted for the working cyde. Themicro reciprocating engine is composed of opposite-pistonssupported by an elastic spring. Resonant mode of the elasticspring is excited due to combustion pressure to increase thegenerated electric power. Theoretical electric power was foundto be -40mW under the conditions that the compression ratio is5, the m aximum combustion tempera tuv is 850K and the firstresonant frequency of the elastic spring is 610Hz.The microengine can be expected to use as a portable micro powergenerator with high energy density.

~ ~ n d e rBse si)PiStOn (S

Port

1. INTRODUCTION/The miniaturization of heat engine by MEMS technology has Scavengepump Elastic springbeen progressed. Since Epstein and co-workers began 1develop a micro turbine for electric power generation, severalmicro heat engines have been proposed [1-3]. In this paper,we propose a micro reciprocating engine for electric powergeneration. A schematic view of structural concept is shownin Fig.1. The micro engine is composed of opposite-pistonssupported by an elastic spring (hereafter, w e de fine a s piston-

spring system), a cylinder case, a top glass plate, and anelectric power generator. The piston-spring system is areciprocated with resonant mode by receiving an impact 3 0.8?force due to combustion. These motions induce electric 3 0.6power due to the Faradny-Lenlz electromagnetic induction atthe electric pow er generator. 2 0.4

Egursl. Concept of pmpored reciprocatingengine.

1-2 : Isentropiccompression2-3 Constant v d m e heataddition [combustion)3-4 : Isentropicexpansion4-1 :Constant volume heatrejection (scavenging)

2. DESIGNDesign is classified into three items. (I) Working cyclecalculation based on the air-standard OlLo cycle. (11)Resonant mode analysis of the piston-spring system due tothe combustion pressure. Stress and heat transfer analysis ofthe micro engine. (111) Calculation of electric power due tothe Furudny-Lentz electromagnetic induction.2.1 Working cycleIn the air-standard Otto cycle, the thermal efficiency q isdetermined by the compression ratio E and the specific heatratio as 141As shown in Eq. l), the thermal efficiency qv ncreases withincreasing the compression ratio E , Th e micro engine adoptsE = 5 as prototyping, because E < 11 is typically adopted forconventional automobile engines. Using the fundamentalcalculation for the air-standard Ono cycle [4], the pressure-

9 -1- (y)x . (1)

.200 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Volume(mml)Figure 2. Pressurevolumediagram.

volume diagram can be obtained as shown in Fig.2. Th e qv sfound lo be 0.47. Th e maximum temperature and pressure inthe working cycle are 850K and 1.4MPa, respectively. Thepiston displacement is 0.5mm'. Actual thermal efficiencymay be up lo 50% lower than the efficiency calculated by theair-standard cycle. However, the air-standard cycle can beserved as first approximation design. Empirical correctionsmust be applied to lheoretical calculations based on theexperiments. Quenching distance and velocity of combustiongas prevent the down sizing of the heat engines. Therefore,we adopt lhe hydrogen gas as a combustion fuel for the microengine 141. Figure 3shows the schematic of two-stroke Otlacvcle adaoted for the micro enxine.20 03 INTERNATIONAL SYMPOSIUM ONMICROMECHATRONICS AND HUMA N SCIENCE0-7803-8165-3/03/ 17.00 0 2 0 0 3 IEEE. 293


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2.2 Mechanical analysisIn the micro engine, periodic combustion pressure is used toresonate the piston-spring system to increase the relativeelectric power. We defin e that is the natural frequency ofthe piston-spring system, and i is the ignition period. It isassumed that the combustion period coincides with the f .Mechanical vibration of the piston-spring system wasanalyzed to examine the influence of the frequency ratiofif,on the vibration mode. As shown in Fig.4, it is assumed thatthe piston-spring system can be modeled as one-degree offreedom linear vibration system, and the impact force due tothe combustion pressure can be modeled as a periodicallyapplied semi-sine wave pulse. The one-degree of freedomlinear vibration induced by the semi-sine wave pulse can beexpressed as [ 5 ] ,

m i ( t ) + r i ( t ) + k =F ( t ) (2)F(t)-Fslrz(Zn ) O < r < t , ) (3)F ( f ) = O I< < t ) (4)

where, m is mass of the piston-spring system, c is equivalentviscosity attenuation coefficient, k is equivalent springconstant of the piston-spring system, F is the impact forcedue to combustion pressure, f is frequency of the semi-sinewave pulse, I is time, and f, 1/ 2f,), respectively. In theequations, the time interval of the impact force responsecorresponds to 0 < f < tc, and that of the natural vibrationcorresponds to tc < f .In order to determine the equivalent spring constant k and thenatural frequency f. of the piston-spring system, FEManalysis was carried out by MEMCAD4. Results of FEMstress and modal analysis of the piston-spring system areshown in Figs.5 and 6. Displacement of the piston-springsystem is 1.02mm under the combustion pressure of 1.4MPa.The maximum Mises stress of 648MPa initiates in theconnection part behueen the piston and spring. This value islower than the fractu re strength of silicon (-1GPa) [ 6 ] . TheIst, the 2nd and the 3rd natural frequencies of the piston-spring system are 582H z, 3119H; and 3183Hz. respectively(Fig.6).Figure 7 shows the vibration aspect of the piston-springsystem in the case of f i f = 1.05. This result was derivedfrom Eqs. 2) to (4). and FEM modal analysis. In the analysis,the impact force F can be estimated from the cross product ofthe maximum pressure in the working cycle (Fig.2) and crosssectional area of he piston.The frequency of the semi-sine wave pulse fc can beestimated from inverse of combustion interval of hydrogengas, i.e., I , =1/(2f.). Th e viscosity attenuation coefficient isassumed lo be 0.2. In Fig.7, the solid lin e shows the impactforce response region and the dotted line shows the natural

_

To top g b)Compression

(d) Expansion (c) CombustionFigure 3. lbo strolrecycle.

piston-springs y s t emFigure 4. One degree of freedom Linear vibalion system andx m i - d n c wave pulse.

tXFigure 5. E M tress analysisof pislan-spring system

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vibration region. The maximum displacement under theexpansion stroke corresponds to Al and the maximumdisplacement under compression stroke corresponds to AIAs shown in the figure, a steady-state vibration, whichcorresponds to the alternation of expansion by the impactforce response and the compression by the natural vibration,was obtained. This can be achieved by setting the frequencyj 610Hz of the ignition period close to the 1st naturalfrequency f =582Hz f the piston-spring system.Heat transfer from the combustion chamber may induce thethermal expansion and the thermal stress in the cylinder case.In order to estimate the temperalure distribution, the thermalstress and the thermal deformation, FEM analysis was carriedout. As a boundary value, the combustion chamber walltemperature of 8S0K hich corresponds to the design valueof the maximum temperature of the working cycle, wasspecified. The steady-state analysis was applied andconvection such as the exhaust heat does not take intoaccount the calculation.As shown in Fig.8, the average temperature is S12K. histemperature is lower than creep temperature of silicon(673K) [7] The maximum thermal stress is 24.3MPa, andthis value is lower than the fracture strength of silicon at512K [7].The maximum thermal deformation along thetransverse direction of motion of the pislon-spring system is0.04 pm. This value is sufficiently small compared with thedesign clearance between the piston and cylinder case, i.e., 3pm, and does not significantly influence the air leakage andmotion of the pistons.23 lectr ic power calculationAn equivalent magnetic circuit of the electric generator inFig.1 can be used to calculate the electric power due to theFaraday-Lenfz electromagnelic induction. Theelectromagnetic induction voltage Vis function of number ofturns of the coil and time derivative of the magnetic flux @inthe iron core [E]As a result, the generated voltage V was found to be -26mVand corresponding electric power was -40mW.

v = - N a @ l a t . 5 )

3. FABRICATIONFabrication process is classified into two items.(1)Fabrication of the piston and cylinder case. (1I)Packagingof the piston, cylinder case and top glass plate.

In: 582HzFigure 6 .FEM modal analysis of pirlon-spring. system.

ExpansionRmxu2 -

3 151w- omv - l W-200-3m

Time [s]f , / fn= 1 05 Compression

Figure 7. Vibration aspect of piston-spring syslcm.

Ambient TemperaNre300Mr ru Ambient Flow 1X10 IWsl

.Combustion hamber temoeralure 850KM

Figure 8. FEM thermal analysis of cylinder care.

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3.1 .Fabrication of piston and cy linder caseThe piston and cylinder case are fabricated from Handlelayer of SO1 100) wafer (device layer thickness : 100pm,buried oxide thickness : 2pm, handle layer thickness :525pm) using Al mask for ICP-RIE.The fabrication process of the piston is as follows. (Fig.9 0))IC P-R E of device layerEtching the buried oxide using HF (Fig.9(2))

Al evaporation on handle layer (F ig W ))iFig.9i4))

Etching of Al ayer (Fig.9(5))ICP-RIE of handle layer using Al mask (FigX6))

Photolilhoglaphy of piston pattern on Al layer

The fabrication process of the cylinder case is as follows.Al evaporation on handle layerPhotolithoglaphy of cylinder case pattern on Al ayer(Fig.lO(1))(Fig.lO(2))

(Fig.lO(4))Etching of Al layer (Fig.lO(3))ICP-RIE of handle layer using Al maskThe assembly of fabricated piston and cylinder case is shownin Fig.lt.3.2 PackagingAu thermal bonding is used for packaging of the cylindercase and lop glass plate. Figure 12 shows the packagingprocess.

4. EXPERIMENTThe engine operation is demonstrated by supplyingcompressed gas lo the combustion chamber. The schemeticof system used for the engine operation is shown in Fig.13.Experimental principle is as follows.The compressed N2 as (pressure : .4MPa, mass flow rate :13mV n) is supplied from N2 ylinder lo electromagneticvalve. The pulsed Nz gas (pulse period : 50Hz) is injectedinto the combustion chamber from the electromagnetic valve.The reciprocating motion of the piston-spring system isinduced by alternation of intake and exh aust pulsed Nz as atthe combustion chamber (Fig.14). I n Fig.14, a epoxy gluewas used for bonding between the top glass plate and thecylinder case. The displacement of the piston can bemeasured by the laser Doppler vibrometer.

5. CONCLUSIONSA resonant micro reciprocating engine was designed forpower generation, Working cycle calculation, mechanicalanalysis, and electric power calculation were carried out.

(4)PhotolithoQlaphy

@)AI etching

Figurc 9. Fabrication processof piston.

HandleSO1 100)wafer nVC,

1)Al evaporation (4)lCP-RIE

Figurc 10. Fabrication prm- of cylinder c y

igure11.Assembly of piston and cylinder case

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Pressure Top glass piateAdopting hydrogen gas as a fuel and silicon as a structuralmaterial, the theoretical electric power was found lo be-4OmW und er the conditions that compression rat io is 5 , themaximum combustion temperature is SSOK, and naturalfrequency of the piston-spring system is 610Hz. The Aufabrication process based on ICP-RIE bulk micromachiningfor micro engine was established. The engine operation basedon air cycle was demonstrated. As a further work,reciprocating motion of the piston-spring system will berealized by fuel combustion and improvement of sealing.

REFERENCES Figure 12. Packaging racesj.A.H. Epstein, Power MEMS and Microengines,Transducers97,753 (1997 ).M.A.Schmit, Technologies for MicroturbomachineryTransducersOl,2 2001).D.E.Park, Design and Fabrication of MicromachinedInternal Combustion Engine as a Power Source forMicrosystems, MEMS2002,272 2002).H.Yanagihara,Verbrennungskraftmaschinen, Herausgegeben von Rikogakusha Publishing Co 2000).L.Meirovitch, Elements of Vibration Analysis,McCraw-Hill(1986).Sloha nsson, Fracture testing of silicon microelementsin a scanning electron microscope, JAppLPhys., 63,479 9 (1988).T.Namazu. Ylsono and T.Tanaka. Plastic Deformation

rlve svslerq

Ns ylinderleclromagndic Regulalorvalve

Combustionxhaust Piston

i Measurement instrumentiaser DopplerUibrometer. . Flow meter i, ... ...ens ing ignal1f Nanometric Single Crystal Silicon Wire in AFMBending Test at Intermediate Temperatures, J.MEMS,[SI K.Okawa, An Introduction of Permanent MagnetMagnetic Circuit, Sogodenshi syuppan sha (1994).11,125 (2002). Fiyre 13. %hematic of y ~ l e mor engine opsralion.

Intake portllocated on the Combustion chamber)

Exhaust port Exhaust port/

Figure 14. Packaged protolpc enghinc.

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design and fabrication of a resonant micro reciprocating engine for power generation

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