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Miniature Free-Piston Homogeneous Charge Compression Ignition Engine

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Miniature Free-Piston Homogeneous Charge Compression Ignition Engine Chemical Engineering Science 57 (2002) 4161–4171 www.elsevier.com/locate/ces Miniature free-piston homogeneous charge compression ignition engine-compressor concept—Part I: performance estimation and design considerations unique to small dimensions H. T. Aichlmayr, D. B. Kittelson, M. R. Zachariah ∗ Departments of Mechanical Engineering and Chemistry, The University of Minnesota, 111 Church St. SE, Minneapolis, MN 55455, USA Received 18 September 2001; received in revised form 7 February 2002; accepted 18 April 2002 Abstract Research and development activities pertaining to the development of a 10 W, homogeneous charge compression ignition free-piston engine-compressor are presented. Emphasis is placed upon the miniature engine concept and design rationale. Also, a crankcase-scavenged, two-stroke engine performance estimation method (slider-crank piston motion) is developed and used to explore the in;uence of engine operating conditions and geometric parameters on power density and establish plausible design conditions. The minimization of small-scale e=ects such as enhanced heat transfer, is also explored. ? 2002 Published by Elsevier Science Ltd. Keywords: Two-stroke engine; Free-piston engine; Homogeneous charge compression ignition; Microchemical; Performance estimation; Micro-power generation 1. Introduction exist: Enhance batteries or develop miniature energy conver- sion devices. Only modest gains may be expected from the This paper is the ÿrst in a two-part series that presents re- former, consequently the latter is being vigorously pursued sults of recent small-scale engine research and development (Peterson, 2001). In particular, miniature engine-generators e=orts conducted at the University of Minnesota. Speciÿ- (Epstein et al., 1997; Yang et al. 1999; Allen et al., 2001; cally, it introduces the miniature free-piston homogeneous Fernandez-Pello, Liepmann, & Pisano, 2001) are considered charge compression ignition (HCCI) engine-compressor especially promising. concept, explores possible operating conditions and con- At ÿrst glance, replacing batteries with miniature straints, establishes potential engine conÿgurations, and engine-generators appears to be an absurd proposition. identiÿes design considerations peculiar to miniature en- Consider however, the enormous energy densities of hydro- gines. It does not, however, address details of the HCCI carbon fuels e.g., 46 MJ=kg. In contrast, the energy density combustion process; this is left to the second paper. of premium lithium batteries is approximately 1 MJ=kg (Yang et al., 1999). Therefore, an engine-generator having a 1.1. Micro-power generation fuel conversion eHciency of only 2.5% would out-perform any battery. Of note, the fuel conversion eHciency of the Considerable e=orts have been directed toward enhancing smallest commercially available model airplane engine portable electronic devices, yet their primary power source (Cox 0.010) is approximately 4% (Yang, 2000). Hence this i.e., batteries, remain essentially unchanged. Batteries en- proposition is sensible. able one to take these devices virtually anywhere, but their intrinsically low energy densities and short lifetimes impose a fundamental limitation. To mitigate it, only two options 1.2. Micro-combustion Despite the clear advantage that a miniature engine- ∗ Corresponding author. Tel.: +1-612-626-9081; fax: +1-612-625-6069. generator has relative to a battery, signiÿcant technical ob- E-mail address: [email protected] (M. R. Zachariah). stacles exist. Some of which include: (1) micro-combustion, 0009-2509/02/$ - see front matter ? 2002 Published by Elsevier Science Ltd. PII: S 0009-2509(02)00256-7 4162 H. T. Aichlmayr et al. / Chemical Engineering Science 57 (2002) 4161–4171 (2) thermal management, (3) gas exchange, (4) friction, 1.4. Homogeneous charge compression ignition (5) sealing, and (6) materials and fabrication limitations. combustion Thus far, work at the University of Minnesota has focused upon micro-combustion. Homogeneous charge compression ignition (HCCI) Micro-combustion could be in;uenced by substantial heat combustion is an alternative engine combustion mode ÿrst and mass transfer to the boundaries. To illustrate, consider identiÿed by Onishi, Jo, Shoda, Jo, & Kato, (1979). Brie;y, the speciÿc heat transfer rate from a stagnant gas occupying HCCI entails compressing a fuel–air mixture until an explo- ˜ a chamber having a volume V and surface area As.Ifone sion occurs. It di=ers from Diesel combustion because the further considers an average heat ;ux and density, then the fresh charge is premixed and it di=ers from SI combustion speciÿc heat transfer rate deÿned by because the charge auto-ignites. Consequently, HCCI has the following experimentally veriÿed (Iida, 1994; Lavy, Q˙ q˙ dA qO˙ A Duret, Habert, Esterlingot, & Gentili, 1996; Gentili, Frigo, As s q˙ = = = ; (1) Tognotti, Patrice, & Lavy, 1997) characteristics: (1) igni- m ˜ dV O V˜ V tion occurs simultaneously at numerous locations within the combustion chamber, (2) an absence of traditional ;ame increases with the surface-area-to-volume ratio, A =V˜. The s propagation, (3) the charge is consumed very rapidly, and surface-area-to-volume ratio meanwhile, is inversely pro- (4) ignition is not initiated by an external event. portional to the characteristic dimension of the chamber. Although HCCI is a relatively recent research topic, the Thus, the speciÿc heat transfer rate increases when the char- rudiments of HCCI are well known. That is, HCCI is a form acteristic dimension decreases. of “knock” combustion and therefore its occurrence depends The phenomenon demonstrated by Eq. (1) is advan- upon chemical kinetics and the compression process (Najt tageous in micro-chemical systems (Jensen, 1999) be- & Foster, 1983). Knock combustion is typically associated cause highly exothermic processes such as catalytic with SI engines and occurs when a pocket of unreacted partial-oxidation reactions (Srinivasan et al., 1997; Hsing, charge i.e., the end gas, auto-ignites before being consumed Srinivasan, Harold, Jensen, & Schmidt, 2000; Quiram, by an advancing ;ame front. When this happens, the end gas Hsing, Franz, Jensen, & Schmidt, 2000; Quiram et al., burns very rapidly and causes the local pressure to rise vio- 1998) and the catalytic oxidation of H (Hagendorf, 2 lently. This in turn, generates pressure waves that transit the Janicke, Schuth,Q Schubert, & Fichtner, 1998) can be con- combustion chamber and initiate phenomena that ultimately trolled. Conversely, this is a disadvantage if one wishes damage engine components. Consequently, avoiding knock to minimize heat loss e.g., from a ;ame, in a micro-sized is a basic design constraint for SI engines and despite con- chamber. siderable research e=orts (Oppenheim, 1984), it is generally satisÿed by restricting compression ratios to modest values 1.3. Micro-internal combustion engines e.g., 9:1. Unfortunately, constraining the compression ratio also limits the fuel conversion eHciency. Several miniature engine programs are underway. Epstein While knock combustion is generally avoided, it has ad- et al. (1997), for instance, are developing a Micro-Gas Tur- vantages over more traditional engine combustion modes. bine Engine at the Massachusetts Institute of Technology. Some of which include: (1) the capability to burn extremely Honeywell International (Yang et al., 1999) is pursuing lean mixtures, (2) no compression ratio limitation, (3) no a 10 W, free-piston homogeneous charge compression ig- external ignition system, (4) unrestricted air intake, and (5) nition (HCCI) engine-compressor. Similarly, Allen et al. a wide variety of fuels may be used. Burning extremely (2001) are developing a free-piston spark ignition (SI) lean mixtures ( ¡0:5) is presently the most appealing engine-generator at the Georgia Institute of Technology. feature because the resulting low combustion temperatures Alternatively, a MEMS rotary engine (Fernandez-Pello yield small NOx emissions. This beneÿt, however, disap- et al., 2001) is under development at The University of pears when the mixture strength is increased (Stanglmaier California, Berkeley. & Roberts, 1999). Additionally, low combustion tempera- Although each of the aforementioned micro-engine pro- tures yield relatively large emissions of unburned hydro- grams have unique advantages and disadvantages, they share carbons and carbon monoxide. Nonetheless, HCCI remains a common problem: ;ame quenching. Each engine proposal attractive because the combination of lean mixtures, unthrot- may then be characterized according to their solution: Waitz, tled air intake, and large compression ratios promise engines Gauba, & Tzeng, 1998 exploit the unique burning proper- with “Diesel-like” fuel economy (Thring, 1989) and small ties of hydrogen and plan to burn hydrocarbons with the as- NOx emissions. sistance of a catalyst; Disseauet al. (2000) employ several Although promising, HCCI is presently impractical. The miniature spark plugs to maximize the amount of charge chief impediment is ignition control—a requirement shared consumed before quenching occurs; and Knobloch et al. by any engine that employs a reciprocating piston. Note (2000) reduce quenching e=ects by heating the walls. Yang that in spark ignition and Diesel engines, ignition con- et al. propose a novel solution: HCCI combustion. trol is achieved by ÿring a spark plug and injecting fuel, H. T. Aichlmayr et al. / Chemical Engineering Science 57 (2002) 4161–4171 4163 respectively. Neither technique,
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