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Microelectronic Engineering 73–74 (2004) 529–534

Design and fabrication of a micro Wankel engineusing MEMS technology

C.H. Lee, K.C. Jiang *, P. Jin, P.D. Prewett

Research Centre for Micro Engineering and Nano Technology, School of Engineering,

The University of Birmingham, Edgbaston, Birmingham B15 2TT, UK

Available online 19 March 2004

Abstract

Hydrocarbon fuels have much higher energy to weight ratios than batteries. A research project is being carried out at

the University of Birmingham to develop microengines to replace batteries. The project will be carried out into two

stages. In the first stage a cryogenic CO2 engine is to be produced, followed in the second stage by the development of a

micro combustion engine. This paper presents the first stage work to develop a micro Wankel engine from the design to

the fabrication of a CO2 microengine. The design of the micro Wankel engine is based on its macro counterpart, but in

the CO2 engine the housing curve has been modified to eliminate the compression stage of the Otto cycle. Finite-element

analysis has been carried out during the design for both cryogenic and combustion engines to ensure that the engines

can withstand the pressure released from their respective energy sources, and that deformation will not cause leakage.

An advanced UV-lithography process has been developed, which can produce ultra thickness and high aspect ratio

engine components. The precision and geometry of the engine components satisfy very strict design requirements. At

present, a CO2 engine has been successfully fabricated and is under test.

� 2004 Elsevier B.V. All rights reserved.

Keywords: Microengine; SU-8; Micro-actuator; Wankel engine; Power MEMS

1. Introduction

In recent years, there has been an increase in

demand for high energy density power units tokeep portable devices running for a long period of

time. This paper describes the design and ongoing

fabrication work of a micro internal combustion

* Corresponding author. Tel.: +44-121-41-44245; fax: +44-

121-4143958.

E-mail address: k.c.jiang@bham.ac.uk (K.C. Jiang).

0167-9317/$ - see front matter � 2004 Elsevier B.V. All rights reserv

doi:10.1016/j.mee.2004.03.030

Wankel engine at the University of Birmingham.

Currently, most portable devices are powered by

batteries which have energy density of around 220

W h/kg, as shown in Table 1. Although more ad-vanced batteries can be used, they are one to two

orders lower than combustive fuels in terms of

energy density, with the latter having energy den-

sity 60� as high as batteries on average. By using a

micro combustion engine, portable devices could

run continuously for extended periods.

There are other universities investigating mi-

cro power generators. MIT have reported the

ed.

Table 1

The energy densities and energy release methods of common energy sources

Energy source Formula Energy density (W h/kg) Release method

Nitrogen N2 110 Cryogen fluid

Methane CH4 200 Cryogen fluid

Carbon dioxide CO2 120 Cryogen fluid

Lead acid battery 30–40 Electricity

Nickel–metal hybride battery 60–80 Electricity

Lithium–sulphur battery (Li–S) 220 Electricity

Iron–titanium hybrid (Fe–Ti–H)

battery

590 Electricity

Methane CH4 15,426 Combustion

Propane C3H8 13,972 Combustion

Gasoline 13,419 Combustion

530 C.H. Lee et al. / Microelectronic Engineering 73–74 (2004) 529–534

development of a six-wafer silicon micro gas tur-bine capable of delivering 10–50 W power in a

combustion chamber, the volume of which is less

than 1 cm3 [1,2]. The combustion chamber sus-

tained a stable hydrogen flame and produced exit

gas temperature in excess of 1600 K. Professor

Fern�andez-Pello at the University of California,

Berkeley, has been developing a rotary engine. A

prototype engine was fabricated using electro-dis-charge-machining (EDM). It has an indicated

output power of 3–40 W. Work continues at

Berkeley to develop a micro rotary engine with

overall dimensions of the order of 1 mm. Whalen

et al. [4] at Washington State University are

working on a heat engine: an external combustion

engine that converts thermal power into mechan-

ical power. Mechanical power is then convertedinto electrical power through a thin-film piezo-

electric membrane generator.

Our objective – a micro Wankel internal com-

bustion engine – has been chosen because most of

its components are in 2D, and therefore amenable

to microfabrication. Many aspects need to be

considered and relevant technology developed in

the microengine project, including design, micro-fabrication, ignition, combustion, timing and fuel

circulation. Due to the complexity of the mic-

roengine development, a two-stage strategy has

been adopted. In the first stage, work is concen-

trated on microfabrication, of a cryogenic engine

to verify the design and the fabrication process.

Combustion issues will be dealt with in the second

stage, viz., ignition, air and fuel mixing, and

combustion within a confined space, where surfaceto volume ratio is high.

2. Design

The initial design of the micro Wankel engine is

described in detail elsewhere [3]. The size of the

combustion chamber must be sufficient to allowsustainable combustion. The overall dimensions of

the designed micro Wankel combustion engine

measure 15� 12.2� 3 mm. This engine has a

practical rotational speed varying from 2,500 to

18,000 rpm and its indicated power output is 12 W

when operating at 17,000 rpm. The engine has a

displacement of 63.5 mm3 and a compression ratio

of 7.2. The Wankel engine operates on the Ottocycle, consisting of four thermodynamic phases:

fuel suction, compression, combustion and ex-

haust. The combustion phase pushes the triangle-

shaped rotor to rotate inside the housing, and

mechanical power is delivered by the moving rotor.

Within a complete rotation, each side of the trian-

gle-shaped rotor performs one Otto cycle, and three

Otto cycles are completed totally by the engine.In the CO2 Wankel engine, the engine housing

has been redesigned. Compared to the conven-

tional Wankel engine, a section of the epitrochoi-

dal curve of the housing has been modified by a

200 lm offset towards the outside, as shown in

Fig. 1. This is to eliminate the compression phase

of the Otto cycle while keeping the expansion

phase. In Fig. 1, the rotor is assumed to be ro-tating anti-clockwise and the fuel is supplied from

Fig. 1. The housing, comparison between the cryogenic and the

combustion engine epitrochoidal curves.

Fig. 2. The rotor with recesses for an increased combustion

chamber.

C.H. Lee et al. / Microelectronic Engineering 73–74 (2004) 529–534 531

the inlet on the cover when the volume of thechamber starts expanding. When the rotor comes

to the original fuel induction phase, no fuel is

supplied here and the modified housing curve

prevents the vacuum of the chamber, so the rotor

can run easily. Effectively, the engine will only

have three expansion phases, without compression

at all. The maximum pressure the engine experi-

ences is 1 MPa when liquid CO2 expands to gas-eous CO2. The material of the cryogenic engine is

SU-8 photoresist because of its reasonably good

mechanical properties [5], and FEA has been car-

ried out for verification of the strength of the SU-8

components.

The design work has also covered the micro

combustion Wankel engine in the second phase. In

this stage of the project, the material of the enginewill be nickel, fabricated using electroforming.

Nickel is chosen because of its high melting point

of 1455 �C, which is sufficiently high to withstand

the temperature from the combustion process, and

its mature electroforming process. Nickel parts

fabricated using an electroforming technique have

demonstrated satisfactory mechanical properties,

with a reported Young�s modulus of 19.9 GPa andyield stress of 810 MPa [6].

As the engine size decreases, surface to volume

ratio increases dramatically, posing one of the main

problems for micro combustion engines. Excessive

heat is lost through the relatively large surface,

resulting in insufficient heat for sustainable com-

bustion. To counter the problem, a modification

has been made to the rotor during the first stage.

The combustion chamber is enlarged by creating a

recess on each side of the triangular rotor. This is

done by separating the rotor into four layers, the

top and bottom layers remain the original convex

shape, while the convex curve has been cut straight

in the middle two layers, referring to Fig. 2. These

recesses increase the volume of the chamber by

1.432 mm3 as well as improving the shape of thecombustion chamber. Taking the constraints of

present UV-lithography technology into account, a

special seal system is incorporated. Supported by a

spring, each of the three seals moves high and low

in a seal slot as the rotor runs within the housing,

keeping the top of the seal in contact with the

housing in all the housing curve except the modi-

fied section, where a small gap between the seal andthe housing exists to prevent vacuum suction.

When the seal is in contact with the housing, it

ensures that the expansion chamber is enclosed,

and leakage can be reduced to minimum.

3. Finite-element analysis

The engine components are expected to experi-

ence a considerable pressure due to the release of

energy from the fuels. The complete engine has been

modelled using Pro/Engineer, and finite-element

Fig. 4. FEA result on the rotor of the combustion engine,

which is made of Nickel. (a) The maximum deformation of

2.873 lm happens at the apex of the rotor, and (b) the maxi-

mum von Mises stress of 63.1 MPa on the seal slots.

532 C.H. Lee et al. / Microelectronic Engineering 73–74 (2004) 529–534

analysis (FEA) has been carried out on all the parts

concerned, using Pro/Mechanica. Emphasis has

been placed on checking the maximum stress and

displacement of the engine parts to verify the design

in prevention of fractures and heavy leakage.

3.1. Cryogenic engine

As mentioned earlier, the maximum pressure

the rotor and the housing are subject to is 1 MPa.

This happens when the liquid CO2 is released from

the tank into the inlet. The FEA result given in

Fig. 3(a) shows the maximum deformation of

5.116 lm happens at the apex of the rotor. Thisdeformation is well below the tolerance between

the seals and the seal slots, and will not affect the

movement of seals in the seal slots. The stress

distribution in Fig. 3(b) shows the maximum von

Mises stress the rotor experiences is 17.44 MPa,

51.3% of the yield stress of SU-8. Hence, the rotor

should be safe from breaking. Similar FEA pro-

cesses have been carried out on the housing andside covers. The results indicate that the maximum

stress and deformation on these components are at

least an order below those of the rotor, and

therefore, the design is valid.

3.2. Internal combustion engine

The maximum pressure the combustion engineexperiences occur immediately after the compres-

Fig. 3. FEA result on the rotor of the cryogenic engine, which

is made of SU-8. (a) A maximum deformation of 5.116 lm on

the left of the slot, and (b) the maximum von Mises of 17.44

MPa at the sealing slot on the right.

sion and ignition. The max pressure is estimated to

be 3 MPa, similar to most internal combustion

engines [7]. Fig. 4(b) shows the stress distribution

of the rotor, where the maximum von Mises stress

is 63.1 MPa located in the two seal slots. This

maximum stress is only 7.8% of the yield stress of

electroplated Ni, which is 810 MPa. The maximumdeformation is 2.873 lm, occurring at the tip of the

seal wall as shown in Fig. 4(a). This deformation

will not affect the movement of the seals since the

tolerance between the seals and the rotor is 15 lm.

The maximum stress and deformation of the

housing is 16.2 MPa and 0.6053 lm, respectively.

4. Fabrication

The main body of the cryogenic engine is con-

structed in six layers, and each of the layers mea-

sures 500 lm thick. The outer two layers are the

front and back cover and the inner four layers

form the housing, the rotor, and gears. The design

requires strict vertical sidewall geometry to enablesmooth movement of the moving parts and pre-

vent the leakage of fluids.

Conventional SU-8 processes often create a

trench with wide top and narrow bottom, known

as the T-shape, which is common to negative

photoresists. The T-shape gets more serious as

thickness increases. In theory, a perfect vertical

Fig. 6. An assembly of the micro Wankel engine, fabricated

using SU-8.

C.H. Lee et al. / Microelectronic Engineering 73–74 (2004) 529–534 533

sidewall could be obtained if UV-light penetrates

the entire layer without losses. However in prac-

tice, UV-light is more or less absorbed when it

travels through a nominally transparent layer.

Though SU-8 is well known for its low UV ab-

sorption, its transparency deteriorates as the layerthickens and UV-light intensity decreases, result-

ing in the T-shape.

An advanced UV-lithography process was de-

veloped at Birmingham to fabricate ultra-thick

SU-8 features [8]. In this process, the optimum

prebake time is used to maintain the very low UV

absorption property of SU-8, and allow the

UV-light to penetrate the SU-8 layer at a uniformintensity. This process fulfills the high thickness

(up to 1000 lm) and strict vertical sidewall re-

quirements of the engine design. The SU-8 mi-

crostructures can be released from the silicon

wafer afterwards.

The SU-8 housing and rotor parts of the mic-

roengine produced have been examined under

SEM, referring to Fig. 5. The fabrication resultssatisfy the design requirements, and are repro-

ducible. The sidewall along the epitrochoid of the

housing is kept between 85� and 90� from the

wafer surface. The images in Fig. 6 show a clear

edge and surface uniformity.

Fig. 6 shows the assembly of the rotor inside

the housing. The housing and the rotor are as-

sembled from layers of SU-8. The complete en-gine is now under test using liquid CO2. The SU-8

fabrication technology will be used to fabricate

mould for electroforming the metal parts in the

second stage.

Fig. 5. SEM pictures of th

5. Conclusions

An ongoing micro Wankel engine development

project has been presented. The energy density of

hydrocarbon fuels shows a big advantage over

batteries. A micro cryogenic Wankel engine and a

micro combustion Wankel engine have been con-

sidered in the design. The housing of the cryogenicengine is modified to eliminate the compression

phase of the thermal cycle. Modifications are also

made to the rotor of the internal combustion en-

gine to increase the volume of the combustion

chamber. The rotor and housing have been anal-

e rotor and housing.

534 C.H. Lee et al. / Microelectronic Engineering 73–74 (2004) 529–534

ysed using finite element analysis on their strength

and deformation, and the results are satisfactory.

The fabrication method of the SU-8 parts has been

established, meeting the strict requirement on the

sidewall of the housing and rotor. The cryogenic

microengine has been assembled and is now undertest. Future work will include the fabrication of

internal combustion parts.

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