THERMODYNAMIC ANALYSIS OF A SINGLE CYLINDER CRANK-ROCKER ENGINE

Transcription

1 VOL. 11, NO. 0, OCTOBER 016 ISSN Asian Research Plishing Network (ARPN). All rights reserved. THERMODYNAMIC ANALYSIS OF A SINGLE CYLINDER CRANK-ROCKER ENGINE Salah E. Mohammed, M. B. Baharom and A. Rashid A. Aziz Mechanical Engineering Department, Universiti Teknologi Petronas, Bandar Seri Iskandar, Perak, Malaysia Salah_7888@yahoo.com ABSTRACT A thermodynamic model for the simlation of a single crved-cylinder crank-rocker engine which operates on gasoline fel is presented. The thermodynamic model is ased on the single and two-zone heat release models. The general model was expanded to inclde friction losses as a fnction of engine speed and comstion efficiencies at different operating points. The model was developed in MATLAB to predict engine performance and efficiencies on a motorcycle engine. The calclated data is then sed to plot varios thermodynamic parameters and the engine performances with respect to crank angle. The engines volme, indicated cylinder pressre, heat release rate, pressre-volme diagram, and engine rake torqe have een evalated at different crank angle positions. The reslts from the simlation model were plotted and compared with existing data. It was fond that for the same engine capacity, the performance of the crankrocker engine was etter than the slider-crank engine when oth tilized the same injection timing. Keywords: thermodynamic model, crank-rocker engine, slider-crank engine, single-zone model, two-zone model. INTRODUCTION The performance and emissions of internal comstion engines are crcial to hmankind de to the depleting oil reserve and gloal warming. Atomotive sector has contrited largely significant to the gloal air polltion leading to the deterioration of the environment. Therefore, Atomoile companies contine to find and develop innovative technologies in order to increase the engine efficiency and redce emissions. The optimization process to increase the ICEs efficiency is very difficlt ecase the crrent ICEs almost reached its optimm limit. It is also eing installed in millions of vehicle on the road today and this has led to high CO emission. As a reslt, any improvements in engine performance mst incorporate the economic principles and control the emissions. There are a large nmer of technologies now eing developed and evalated which can improve the internal comstion engine, for example these technologies inclde fel comstion strategy, port fel injection, direct fel injection, tro-charging with direction injection system, variale compression ratio engine, variale displacement engine, variale valve timing and lift and etc. Several technological developments related to the improved engine efficiency and redced emissions have een reported y Ford Motor Company [1]. They released a report with the address on environmental concerns. The important focs over recent years has led to varios technological advances in engine control reslting in the increase in efficiency. New technologies sch as variale valve timing have een prodced recently in prodction vehicles. Reference [], has proposed a concept called Variale Valve Actation to predict the otcome of the crrent variale valve timing technology to e camless valve actation which can lead to efficiency improvements. Variale compression and variale displacement engines have een addressed and experimentally investigated y researchers and atomoile manfactrers [3-8]. Several athors [3, 5] have proposed different design techniqes to achieve variale compression ratios. Variale displacement concepts have een analysed in many different scientific plications [5, 8]. The key advantages of variale compression and variale displacement engine are redced the fel consmption and increase the power ot. In general, very few researches look into the possiility of tilizing different engine configration eside the conventional slider-crank engine; despite some concepts of sing new engine configration have een developed to varios extents a long time ago. Several researches have een carried ot in order to convert the conventional reciprocating motion of a piston into rotational motion of a crankshaft [9-11]. Also, several atomotive engineers had attempted to design engines incorporating oscillating crved pistons and some of them patented their work, e.g. L.J. Wolff 199, Monti Farrell 199 and Morgado 006 [1, 13]. The engines were very powerfl t there was high incidence of sealing loss which led to high emission. The proposed design was also very complex. A new configration of an opposed-piston crank rocker engine has een proposed y [14, 15]. It was operating on a and 4-stroke, and each cylinder contained two pistons placed horizontally facing head-to-head and moved in opposition direction to each other. In general, past researches have een condcted in order to improve engine performance in redcing emissions and fel consmption. Althogh several concepts had een developed to varios extents in the past, the progress was still slow. In this paper, a novel crank-rocker engine has een designed and modeled. A single and two-zone heat release models was developed in MATLAB to predict engine performance and efficiencies on the crank-rocker engine. The model was prepared to inclde friction losses as a fnction of engine speed, comstion efficiencies, and temperatre-dependent thermodynamic properties. In two- 139

2 VOL. 11, NO. 0, OCTOBER 016 ISSN Asian Research Plishing Network (ARPN). All rights reserved. zone model, the cylinder contents sch as volme and mass are divided to consist of two regions (zones), rned and nrned zone. CRANK-ROCKER ENGINE CONCEPT The crank-rocker engine introdced in this work is an alternative engine configration. The niqeness of the engine geometry is expected to improve the engine thermodynamic cycles and perform etter than the conventional engines. The main concept of Crank-Rocker Engine was derived from for-ar mechanisms withot any delayed angle (alanced). The most niqe is the zero-alanced angle of the crank-rocker engine where it allows the crank to have a smooth rotary motion. This engine is designed to operate sing a crve piston and the comstion cycle is similar to the OTTO cycle. Figre-1. Single crved-cylinder crank-rocker engine. The configration of the crank-rocker engine as well as the asic parts and shape is shown in Figre-1. As can e seen from the figre, a crved piston assemly travels within a crved engine cylinder. The inpt motion comes from a comstion force acting perpendiclarly at the piston which decays at the end of the rocker stroke. The time taken for the rocker to move from either extreme position to the other shold e eqal to the time taken for the crank to rotate 180 degrees. Similar to the slider-crank engine, the gases pressre prodced from the comstion of fel applies a direct force to the piston which is attached to the rocker tip. This force is transferred to the crank along the connecting rod, and generates sefl mechanical power. Two valves namely the intake and exhasts are attached to the head of the cylinder. A rotating camshaft monted on the engine head (overhead cam) is sed to operate the valves at appropriate time dring the stroke of the piston, operated directly y psh rods. The camshaft is connected to a crankshaft via a chain elt. As the crank rotates throgh 360 0, the rocker will oscillate throgh an angle θ. The engine is intended to e designed to operate sing a single cylinder only for any capacity of the engine reqirements in order to avoid nmeros rotating parts. THERMODYNAMIC MODEL The thermodynamic simlation model of internal comstion engine has long een extensively sed for investigating and analysing the engine performance and giving a good evalation for new developments [16-1]. When investigating the internal comstion engine, the incylinder pressre has always een an important experimental parameter in atomotive research and development, de to the relationship etween the comstion and work processes []. In order to nderstand of how the comstion process propagates throgh the comstion chamer, it is important to relate all parameters sch as cylinder volme change, mass loss in cylinder chamer, and the heat transfer to the cylinder pressre. All of these parameters can e otained sing the first law of thermodynamics. A single zone model has een sed in order to predict the in-cylinder pressre and other parameters. A more accrate thermodynamic analysis wold e to se a mlti-zone model, where the cylinder is divided into a nmer of zones, differing in composition and properties. Each zone is niform in composition and temperatre, and the pressre is the same for all the zones [3, 4]. The model was sed to predict the cylinder gas temperatre and pressre, as well as the performance of the engine. In order to improve and increase the accracy of the single-zone model, the heat transfer, variale specific heat ratios, and friction models were inclded. In this paper will give a more detailed data of the development of the single and two-zone models. Modelling assmptions Some assmptions are adopted to simplify the single-zone model eqations sch as follows: 1. All gases mixtre is ideal gases.. The cylinder contents are assmed niform throghot the cylinder. 3. The comstion is simlated as a release of heat 4. The heat released from the comstion is niformly in cylinder. 5. The separation etween rned and nrned zones is assmed to e very thin and no heat transfer etween the two zones. Single-zone model The simplest approach is to oserve the cylinder contents as a single region or zone [3, 5]. Single-zone models se the Weie fnction to represent the mass fraction rned as a fnction of crank angle and is defined as follows [17]: n1 s X 1 exp a (1) 140

3 VOL. 11, NO. 0, OCTOBER 016 ISSN Asian Research Plishing Network (ARPN). All rights reserved. Where θ = crank angle, θ s = start of ignition, θ = the rn dration, n = Wiee form factor and a = Wiee efficiency factor. The parameters a and n are adjstale parameters sed to fit experimental data. The cylinder gas volme, in a conventional slider-crank engine can e calclated as a fnction of crank angle [17, 4]: V V c V () d Where Vc is the clearance volme which is expressed as: V d Vc (3) Cr Where Vd is the displaced cylinder volme, and Cr is the compression ratio. The engine displacement is given y B V d * S (4) 4 B S = the cylinder ore, = the engine stroke, The crank-rocker engine stroke is not like the crank-slider engine, and it does not depend on the crankshaft radis as previosly sed y the slider-crank engine. The expression is shown elow: S r (5) The crank-rocker stroke is given y S L 41 * (6) Or 1 r S L 41 * * sin (7) L4 Where r is the crank radis, is the rocker throw angle, L 4 is the rocker length; and L 41 is the extended rocker length. Figre shows a diagram of these figres and their relationship to engine geometry. Figre-. Crank-rocker engine geometry and variales. The ideal gas law is defined as: PV MRT (8) By taking the logarithm of oth sides and differentiating with respect to crank angle provides: 1 dp P d 1 dv 1 dt V d T d (9) Rearranging eqation 9 and solving for dp: dp d P dv P dt V d T d (10) The first law of thermodynamics in differential form for an ideal gas with constant specific heat is given y: du dt dq dw mcv (11) d d d d Where Q is the total energy, W = the work, du = the change in internal energy, Cv = the specific heat of the comstion chamer gas. The relationship etween the specific heat, niversal gas constant, and the specific heat ratio is given y: Cv Cv R C C p v 1 1 (1) 141

4 VOL. 11, NO. 0, OCTOBER 016 ISSN Asian Research Plishing Network (ARPN). All rights reserved. The net-work and heat inpt in the spark-ignition engine can e descried form the following eqations [, 3]: dw dv P (13) d d dq dx dqw c * LHV * (14) d d d When sstitting eqation 1 and 13 into eqation 11, the instantaneos change in temperatre is given y: 1 dq dv dt T 1 1 (15) d PV d V d The heat derived from the comstion of fel can e sed in order to compte the change in the gas cylinder pressre. The heat inpt is expressed as [4, 5]: Q in 1 P C * LHV * Vd AF (16) ac RT Finally, the change in the cylinder pressre as a fnction of crank angle is defined as: dp A B C d Where A, B, and defined as follows: P dv A V d 1 X B Qin V d 1 dqw C 1 V d (17) Since all of the variales have een defined, a single-zone model can e developed and estalished. Heat transfer model In general, the heat transfer from the in-cylinder gases to the cylinder wall in spark ignition engines is convection, with only a view percentage from radiation [17]. By sing a Newtonian law, the heat loss to the wall is given y: Q w T w h * A* T (18) Where h T Tw A = convection heat transfer coefficient = temperatre of the cylinder gas = cylinder wall temperatre = exposed comstion chamer srface area Air-fel ratio model Generally, the air-fel ratio can e calclated sing the lamda (λ) reading, the alanced eqation, and stoichiometric reaction etween fel and air. The general, alanced stoichiometric reaction for a HC fel is defined as [5, 6]: C H x y a 3.76N a O 3.76N y xco H O (19) Where x = the nmer of caron atoms in the fel, y = the nmer of hydrogen atoms in the fel, a = constant for alance the eqation. For example, the alanced, stoichiometric reaction for C 8H 18 fel is expressed as: 5 C8H18 O N 3.76N 8CO 9H O (0) Upon sstitting the vales of x, y, and a, the stoichiometric air-fel ratio is fond to e [, 3]: AF stoich a Moleclar. weight Moleclar. weight Finally, the actal air-fel ratio is given y: ac AF stoich air fel (1) AF * () Friction model It well known that friction losses inflence the engine performance (indicated and rake power). Some researchers sch as [17, 7] have sed general linear eqations to calclate FMEP losses as a fnction of RPM. For the spark-ignition motorcycle engine with roller earings, the FMEP loss defined as [7]: L FMEP 50 RPM (3) Where L is engine stroke (m), RPM is engine speed (rev/min). 14

5 VOL. 11, NO. 0, OCTOBER 016 ISSN Asian Research Plishing Network (ARPN). All rights reserved. Two-zone model The two-zone model descries the rned and nrned gas areas and their association to the comstion chamer. Two-zone model can e sed in order to predict the heat transfer and emissions precisely [4]. In order to develop a two-zone model, some modification of the Weie fnction has to e done in order to inclde rned and nrned areas or zone. The rned and nrned masses can e expressed as [7]: m m dx 1 (4) d i m i i m c dx 1 (5) d i m i i m c Where mc is the total mass in cylinder (mass of air + mass of fel) After compting the rned and nrned masses, the volme of rned and nrned mst e otained. Some researchers, [7] have sggested that sing the polytropic relations and in-cylinder pressre-trace in order to calclate the rned and nrned volmes. The nrned volme is given y: V i m V i V i 1 i 1 P P i i 1 1 i (6) Finally, the rned volme can e otained y [4, 7]: V i V i V i (7) B A i Ach BS i (3) RESULTS AND DISCUSSIONS In order to analyse and display the reslts of the crank-rocker engine, a MATLAB code was prepared. The engine performance for a single cylinder, for stroke crank-rocker engine which operate on gasoline was calclated. The reference engine sed in this research is motorcycle engine. The crank-rocker engine specification and dimensions (Tale 1) sed in this model are as follows: Tale-1. The crank-rocker engine main data. Cylinder displacement Cylinder Diameter 10cc 55mm Nmer of Strokes 4 Nmer of Cylinders 1 Stroke 50.6mm Compression Ratio 9.8 Connecting Rod 100mm Rocker and Extendedrocker Length and 138.1mm Throw Angle 1 0 The engine is assmed to operate at 3000 rpm, with an eqivalence ratio of 0.9. The ignition advance of the model engine is considered as 5 0 TDC and the comstion dration is The reslts for the simlation are shown in Figre-3 (a)-(e). By sing the ideal gas law and applying to each zone. The rned and nrned temperatres are given y: i V i i R i P T i (8) m i V i i R i P T i (9) m The rned and nrned areas can e calclated as follows [8]: A i A i 1 X i 0.5 (30) X i 0. X i A i A i 5 (31) Generally, instantaneos heat transfer area is calclated as: 143

6 VOL. 11, NO. 0, OCTOBER 016 ISSN Asian Research Plishing Network (ARPN). All rights reserved. (a) (d) () (c) (e) Figre-3(a-e). The model comparison etween the crankrocker (Ble line) and slider-crank (Red line) engines. (a) Engine Volme, () Cylinder Pressre, (c) Heat Release, (d) Pressre vs Volme, and (e) Brake Torqe Vs Crank Angle. As can e seen form Figre 3a, the two engines have the same volme. Therefore, the method for analysing the displacement of the crank-rocker was correct. Figre 3 shows in-cylinder pressre verss crank angle for oth engines. At 3000 rpm engine speed, the crank-rocker engine prodced higher vale compared to the slider-crank engine. The peak vale was 53. ar and accred at 06 0 CA for crank-rocker, while the peak vale of the slider-crank was ar accred at 03 0 CA. The explanation to this is mainly ecase the comstion process performance of the crank-rocker is etter and the rned fel is good and this phenomenon led to the increase in the cylinder pressre. Heat release rate is one of the main parameters that sed in internal comstion engines. Heat release pattern can explain the comstion process that occrs 144

7 VOL. 11, NO. 0, OCTOBER 016 ISSN Asian Research Plishing Network (ARPN). All rights reserved. inside the cylinder. Higher heat release shows that etter comstion efficiency and higher NOx emission takes place in the process. The heat release characteristics for oth engines as fnction of crank angle is shown in Figre 3c. It is oserved form Figre-3c that the crank-rocker engine prodces higher heat release rate as compared to the slider crank engine de to the increase in the gas temperatre and similar to those of the cylinder. Figre-3d shows the P-V diagram for oth engines. It can e noted that the configration of the crank-rocker engine has improved the thermodynamic cycle. Becase the piston motion of the crank-rocker engine moved faster in the expansion stroke, these led to the decrease in the heat transfer to the walls and hence improve the comstion process. Ths, increasing the incylinder pressre can prodce higher work as well as higher power otpt (see Figre 3e). SUMMARY AND CONCLUSIONS In this paper, a novel crank-rocker engine model was analysed. Simlation on the asis of a single and twozone modelling was condcted in order to otain the cylinder pressre, P-V diagram, torqe and power otpt. The performance of the model was compared against an existing model data for a slider-crank engine. It can e conclded that the crank-rocker engine with some optimization cold improve the engine efficiency. REFERENCES [1] rt.pdf. [] Ronald J. Pierik and James F. Brkhard Design and Development of a Mechanical Valve Actation, SAE Paper [3] Yamin, J.A., Dado, M.H.004. Performance simlation of a for-stroke engine with variale stroke length and compression ratio, Applied Energy 77: [4] Ozcan, H., Yamin, J.A.A Performance and emission characteristics of LPG powered for stroke SI engine nder variale stroke length and compression ratio, Energy Conversion and Management 49: [5] Adams, W.H., Hinrichs, H.G., Pischinger, F., Adamis, P., Schmacher, W., Walzer, P. 00. Analysis of the comstion process of a spark ignition engine with a variale compression ratio, SAE Paper [6] Poliot, H.N., Delameter, W.R., Roinson, C.W A Variale Displacement Spark Ignition Engine, SAE Paper [7] Fredenstein, F., Maki, E. R., Variale Displacement Piston Engine, U. S. Patent #4,70,495, [8] Welsh, H. W., Riley, C. T., The Variale Displacement Engine, An Advanced Concept Power Plant, SAE paper , [9] Ciesio-kiewicz, A., M-yk, P. 00. Dole Pistons Internal Comstion Engine, Jornal of KONES, 1(): [10] Norye, Jan P The Wankel engine: Design Development Applications, Bailey Bros and Swinfen Ltd. Folkestone, UK. [11] Sherman D The Rotary Cl", Atomoile Magazine, [1] n-engine-that-thing-got-a-toroid-in-it/ [13] dal_in.html [14] ListerTS3/TS3.htm. [15] Miros, S. 01. New Concept of a Rocker Engine - kinematic Analysis, Jornal of KONES Powertrain and Transport, 19(3): [16] Fergson C. R Internal Comstion Engines, Applied Thermosciences. New York: John Wiley and Sons. [17] Heywood, J. B Internal Comstion Engine Fndamentals. New York: McGraw-Hill. [18] Sndeep Ramachandran Rapid Thermodynamic Simlation Model of an Internal Comstion Engine on Alternate Fels, IMES. : [19] R. Erahimi, B. Desmet An experimental investigation on engine speed and cyclic dispersion in an HCCI engine Fel, 89 (8): [0] M. Fathi, R.Kh. Saray, M.D. Checkel Detailed approach for apparent heat release analysis in HCCI engines. Fel, 89 (9) 010:

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