Additive Manufactured Functional Prime Mover - ScienceDirect

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ScienceDirect Energy Procedia 110 (2017) 136 – 142

1st International Conference on Energy and Power, ICEP2016, 14-16 December 2016, RMIT University, Melbourne, Australia

Additive manufactured functional prime mover Don Clucas*, Jose Egas University of Canterbury, Christchurch 8140, New Zealand

Abstract

The development cycle of additive manufacturing technology, also known as 3D printing, has reached a stage of industry acceptance. Established multinational 2d printing companies, for example Hewlett Packard, who were not formally associated with additive manufacture (AM) are now actively progressing their own technologies. Associated with this, thousands of application frontiers are being established and whilst the technology opens many paths to the manufacture of parts not previously possible there are also restrictions on what can be made using AM when using traditional design practices. For example an engine cylinder bore can’t currently be manufactured using AM that does not require post machining. Select engine parts, such as gas turbine blades, are being made using AM and the number of parts used in prime movers will increase as the technology develops. With AM proliferation, changes to the practices of prime mover designers and maintenance personnel will be required. This work explores ways engine design can be modified to suit AM. Making a fully functioning Stirling engine made solely by AM was a logical starting point as they use an external heat source and don’t require precision machined parts like valves and injectors. This work presents the design choices used for a making a fully functioning engine using only AM parts that did not require post machining. © Published by Elsevier Ltd. This TheAuthors. Authors. Published by Elsevier Ltd. is an open access article under the CC BY-NC-ND license © 2017 2017The (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility ofthe organizing committee of the 1st International Conference on Energy and Power. Peer-review under responsibility of the organizing committee of the 1st International Conference on Energy and Power. Keywords:Stirling engine; Additive Manufacture; 3D Printing; Prime Mover.

* Corresponding author. Tel.: +64 3 364 2987. E-mail address: [email protected]

1876-6102 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 1st International Conference on Energy and Power. doi:10.1016/j.egypro.2017.03.118

Don Clucas and Jose Egas / Energy Procedia 110 (2017) 136 – 142

1. Introduction 1.1. Additive manufacture At the Rapid 2016 Conference/Expo Hewlett Packard, a multi-national traditional 2D printer and copier manufacturer launched its self-developed “Fusion” industrial additive manufacturing (AM) machine. Also, in September 2016 General Electric purchased two AM equipment companies. Such large and risk averse companies banking on AM indicates an industry acceptance of the fast developing manufacturing process. In 2009 a patent held by Stratasys [1] for Fused Deposition Modelling (FDM) expired, releasing the technology for competitive commercial development. Since then there has been rapid private and commercial development of FDM machines for home and commercial use. Many successful new companies, like MakerGear and Ultimaker, are based on the FDM technology and sell machines costing less than US$2000 from regular consumer outlets. The technology assigned to rapid prototyping a decade ago has become a viable technology for manufacturing end use components. However, mechanical design techniques have not kept up with the rapid technology development and AM is not being used to its full potential. AM offers the ability to make part features that were previously impossible to be manufactured using traditional subtractive technologies. For example, complex high surface area heat exchangers offering far superior performance compared with machined and welded assemblies can now be made in one part. Prime movers, to be used in many applications such as electric power generation, can now benefit from AM. A challenge for both industries is to make the best use of AM but this requires a move away from traditional [2] component design. However, many components designed for these traditional processes can’t currently be made by AM. For example, a sliding piston/cylinder combination can’t currently be manufactured by AM to the required tolerances and surface finish. So either better AM technology is required or alternative designs, like diaphragms in this case, used. 1.2. Additive manufactured engine A challenge set by the authors was to design a fully functioning prime mover manufactured using solely additive manufactured parts. In addition, no parts were to be post processed using machine tools. Internal combustion (IC) engines currently require complex and precision assemblies for the likes of fuel distribution, lubrication systems and valve trains. So current AM technologies could only make a fraction of these assemblies. Stirling Engines use heat transfer to and from an external heat sink and source respectively. As will be shown, it is possible to make the heat exchangers by AM and lose little performance for the chosen engine. An extreme, but possible, application of this technology could be the manufacture of repair components of essential life support systems of a Lunar or Mars outpost. In this application the time for the supply of components from Earth would be excessive [3]. Also, the cost of transporting a full inventory of spares and/or traditional machining equipment would be impractical. Alternatively AM parts using densely stowed media could be used. AM also allows part upgrades by sending digital files from earth. AM jet and rocket engines have been referenced in internet news feeds but none are either fully AM or did not require part post machining [4]. 2. Engine choice 2.1. The Stirling Engine In theory the engine ideal efficiency equals the Carnot cycle [5] but real applications have realisedjust a small fraction of that [5], competing favourably with IC engines. However, as a closed cycle, it offers the advantages of long service life, quiet operation, low emissions and in some cases non-reliance on hydrocarbon fuels. Despite these qualities, prime mover applications have had limited success. During the late 1800’s and early twentieth century non-petroleum and petroleum fueled water pumps and fans were used with moderate application. The use died off with the wide distribution of electric motors and internal combustion engines. Phillips, the Dutch electronics manufacturer, began the development of the modern high power density engine in 1939 [5] but failed to

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have commercial success making prime movers. The modern New Zealand company, WhisperTech Ltd., manufactured the high power density WhisperGenTM for micro Combined Heat and Power (mCHP) [6] but manufacture of the complex high temperature resistant heat exchangers plagued the product development. This was a commercial application that could have benefited from AM.

2.2. Low temperature difference Stirling Engine Stirling engines work by moving a working fluid, often air, from a cold space to a hot space during expansion and the reverse during the compression. The engine is driven by the temperature difference of the hot and cold spaces. A displacer or phased piston motion moves the working fluid. An operating engine, see Fig1., can be made to operate with a low temperature difference between the hot and cold spaces [7]. There are many examples of engines of this design operating from just the heat of a hand or cup of coffee. FDM ABS material has an approximate glass temperature of 70o C making it a candidate hot heat exchanger material for a low temperature difference Stirling Engine (LTDSE). 2.3. Engine configuration Stirling engines can be realised in the three primary configurations, alpha, beta and gamma [8]. Due to the required very low compression ratio of LTDSE engines generally use the gamma configuration, see Fig.1, with a thermally insulating displacer to shuttle the working fluid. Kolin [9] presents many LTDSE designs made by traditional methods and Clucas [10] presented a Bow Tie configuration as an option for AM. An air charged gamma configuration engine was chosen for this project.

Fig. 1. Conventional gamma configuration LTDSE with piston/cylinder and aluminium heat exchanger plates.

3. Available AM technologies AM equipment in the University of Canterbury advance design and manufacturing lab includes: x Stratasys Connex 350 polyjet machine that prints two model materials and one support. This machine can mix the polymer model materials to produce a wide variety of mechanical properties. Rubber like materials of various shore hardiness are possible. The support is scraped and water blasted off the part. x Stratasys Elite FDM machine that lays a bead of ABS polymer in conjunction with a dissolvable support material.


x

TierTime UPbox FDM machine that prints a variety of thermoplastic materials including ABS and nylon. It is a single material printer so the support material must be removed manually which limitspart interior complexity.

Other locally available equipment: x 3DSystems Projet 3500. A very high definition single model material polyjet machine using melt away support. 4. Design choices 4.1. Hot and cold heat exchangers Clucas [11] demonstrated a LTDSE engine that used FDM ABS heat exchangers. However, this engine used a large number of non-AM components such as bearings and screws. An experimental optimisation engine was manufactured to compare the performance of a traditional aluminum flat plate heat exchanger with FDM finned and un-finned plates. The convective heat transfer was determined to be the dominant thermal resistance and it was found that shallow internal fins on the polymer heat exchange surfaces gave near equivalent performance to the aluminum, see Fig.2. This demonstrates a case where AM parts with greater complexity, at little or no extra cost, performed nearly as well as a traditionally manufactured metal part.

Fig. 2. Comparison of metal and polymer heat exchangers.

4.2. Piston vs. diaphragm Sliding piston-cylinder combinations are the norm for LTDSE [7] and occasionally elastomer diaphragms are used for hobby applications. The power produced by small LTDSEs is very low [7] so friction and air leakage must be minimised. Piston-cylinder combinations require very fine tolerances and are often made from ground glass bores and graphite pistons. Diaphragms do not leak and when well designed allow low tolerance parts made by AM. But due to stiffness they can add extra load to the bearings. The engine published by Clucas [11] demonstrated the use of an FDM ABS diaphragm on a LTDSE. FDM material is generally porous so the diaphragm, along with all other FDM manufactured parts sealing the air space, were washed with acetone [12]. An elastomer diaphragm made on the Connex machine was tested but the rubber like material absorbed sufficient energy to prevent the engine operating.

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4.3. Bearings Rotating bearing drag can be one of the largest losses in the engine so for a working engine high quality bearings are essential. The engine presented by Clucas [11] used metal 5mm shaft shielded grease lubricated deep groove ball bearings. These could not be used in the new engine. By using a shaft inertia run down apparatus, see Fig 3a., the friction drag of various shaft/bearing combinations were tested. The shaft with inertia mass was spun up to 700 rpm and let slow due to the bearing drag the rate of speed decline is shown in Fig 3b.A comparison of bearing drag loss for the bearing designs is shown. Non-post machined AM plain journal bearings were known to have considerably higher drag so were not tested. The tests showed polymer rolling element bearings produced on a Project 3500 produced less drag than the precision made metal bearing. Adding light oil increased the drag. The design life of these AM bearings is expected to be low and will be a subject of future research. To minimise bearing loads a lever system was used, see Fig 4., that realised a higher crank radius than a traditional crank-connecting rod arrangement, Fig 1. Consequently, for an equivalent torque the bearing radial load is lower. Also, the LTDSE requires lower flywheel inertia than an internal combustion engine so the bearing load from the flywheel mass is low.

Fig. 3. (a) Inertia bearing drag apparatus; (b) shaft angular velocity run down results.

4.4. Displacer fabrication Traditionally LTDSEs use light weight and thermally insulating polystyrene or polyurethane foam. To obtain a light weight AM manufactured displacer it was designed to be hollow and by using the Elite FDM machine the internal support material was dissolved away. 4.5 Engine Optimisation Due to continuously varying air pressure, flow velocity, density, heat transfer co-efficient and gas temperature simulation and optimisation of the Stirling engine is notoriously difficult and without considerable engine testing the results can’t be relied on. As an alternative to computer simulation, real engine test bed optimisation was performed by using an automated test engine that was programmed to autonomously adjust key variables and record performance data. A post processing search routine was then used to sort the variable data points. Using this method the value of key variables such as compression ratio and phase angle could be established. To enable commissioning adjustment of the key engine variables, such as stroke and phase angle, pinch grip slides were added, see Fig 4.


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5. Results With crushed ice for the heat sink and boiling water for the source the engine, Fig 4., continuously operated as a fully functioning prime mover. The engine reached a peak speed of 75 revolutions per minute.

Fig. 4. Fully 3D printed thermal engine.

6. Conclusions Despite the inaccuracies and poor thermal properties of AM polymer parts, by using non-conventional design techniques a fully functioning thermal prime mover has been made entirely of AM parts requiring no post machining. Whilst the performance was worse than conventional LTDSEs this marks a beginning for further studies of fully AM prime movers. Having proven a low temperature engine of limited power can be made the next phases of the project are to develop a high performance LTDSE and then a functioning fully 3D printed high temperature engine that can produce useful power. References [1] Crump S.S. Apparatus and method for creating three-dimensional objects. 1992. U.S. patent 5121329A. [2] Beyer C. Strategic implications of current trends in additive manufacturing. Journal of Manufacturing Science and Engineering. 2014. 136(6): p. 064701. [3] Leach, N. 3D printing in space. Architectural Design. 2014. 84(6): p. 108-113. [4] NASA Tests Limits of 3-D Printing with Powerful Rocket Engine Check. NASA news 2013. Retrieved from http: //www.nasa.gov/exploration/systems/sls/3d-printed-rocket-injector.html. [5] Hargreaves, C.M., The Phillips stirling engine. New York: Elsevier; 1991. [6] Alexakis A. et al. Experimental and theoretical evaluation of the performance of a Whispergen Mk Vb micro CHP unit in typical UK house conditions. World Renewable Energy Congress. Sweden; 8-13 May; 2011; Linköping; Sweden. 2011. Linköping University Electronic Press. [7] Senft, J.R., An introduction to low temperature differential Stirling engines. Moriya Press; 1996. [8] Walker, G., Stirling engines. Oxford: Clarendon Press; 1980. [9] Kolin, I., Stirling motor. History-Theory-Practice. Dubrovnik: Inter University Center; 1991. [10] Clucas, D. and S. Gutschmidt, ATINER's Conference Paper Series IND2015-1692. [11] Clucas D. Additive Manufacture of a Working Engine. In RAPID, Detroit, MI, USA. 2014.

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[12] Mireles J. et al. Analysis of sealing methods for fdm-fabricated parts. 2011. Technical Report, W.M. Keck Center for 3D Innovation. The University of Texas, El Paso.