THERMAL TO MECHANICAL ENERGY CONVERSION: ENGINES AND REQUIREMENTS Vol. I - Thermodynami Cyles of Reiproating and Rotary Engines - R.S.Kavtaradze THERMODYNAMIC CYCLES OF RECIPROCATING AND ROTARY ENGINES R.S.Kavtaradze Mosow Bauman State Tehnial University, Russia. Keywords: Work, Heat, Reiproating engine, Combined engine, Rotary engine, Thermodynami proess, Reversible proess, Thermodynami yle, Thermal effiieny, Carnot yle, Otto yle, Diesel yle, Trinkler yle, Stirling yle. Contents 1. The main kinds of reiproating engines. 2. Work done by a working fluid in the ylinder of a reiproating engine 3. Working proess and indiator diagram of a four-stroke engine 4. Working proess and indiator diagram of a two-stroke engine 5. Main onepts of thermodynamis 6. Main distintions between atual and thermodynami yles and effiieny of a yle 7. Carnot yle 8. Generalized thermodynami yle of piston and ombined engines 9. Otto yle 10. Diesel yle 11. Trinkler yle 12. Comparative analysis of thermodynami yles of piston engines 13. Combined internal ombustion engines (CICE) 14. Thermodynami yle of CICE with the impulse turbine 15. Thermodynami yle of CICE with onstant pressure in front of the turbine 16. Thermodynami yle of CICE with intermediate ooling of a working fluid 17. Stirling yle 18. About the thermodynami yles of rotary internal ombustion engines (ICE). Glossary Bibliography Biographial Sketh Summary Historial review of the Carnot, Otto, Diesel, Trinkler and Stirling yles is given and pratial realization of these yles in internal ombustion engines is onsidered. 1. The Main Kinds of Reiproating Engines. The engines, in whih heat generated by ombustion of fuel is transformed to mehanial energy, are alled the heat (thermal) engines. Heat engines are divided into two main groups: 1.The internal ombustion engines having pistons, rotary and ombined engines, gas turbines, jet engines et., i.e. all the engines, in whih ombustion and generation of heat takes plae inside the engine; 2. External ombustion engines (steam engines, steam turbines, Stirling engines), whih produe mehanial energy due to the heat provided from the outside. Enylopedia of Life Support Systems (EOLSS)
THERMAL TO MECHANICAL ENERGY CONVERSION: ENGINES AND REQUIREMENTS Vol. I - Thermodynami Cyles of Reiproating and Rotary Engines - R.S.Kavtaradze The basi onfiguration of reiproating or piston engines inludes a ylinder with a piston. Combustion of the fuel takes plae inside the ylinder in the environs of an air (oxidizer). So the formation of a ombustible mixture an be both inside and outside of the ylinder. Historially the first engine with an external mixing was reated in 1876 by a German engineer Nikolaus Augustus Otto (1832-1891). Certainly, there were attempts at reating engines before Otto. One suessful attempt was by a Frenh inventor J. Lenoir (1860), but the redit goes to Otto and his engine is known as Otto s engine. In suh an engine the main part of air and stream of fuel are mixed up before going to the ylinder. The formation of a fuel-air mixture is omplete after entering into the ylinder, where it is ompressed by the piston and at the end of the ompression proess the fuel-air mixture is fired with the help of eletrial spark. The produts of ombustion expand and move the piston, and the mehanial energy so produed in reiproating motion is transmitted to the shaft through a rank-onneting rod mehanism as rotary motion. Thus the Otto engine is a piston drive with external mixing and fored ignition. In 1893 another German engineer Rudolf Diesel (1858-1913) reated essentially a new type of piston engine known as the Diesel engine. In the Diesel engine the air is ompressed by the piston upon entry into the ylinder. The fuel is injeted into the ompressed air at the end of ompression. The fuel-air mixture reated inside the ylinder is self ignited due to the high temperature. Thus Diesel engine is an engine with internal mixing and spontaneous ombustion. This is the essential differene between the Otto engine and the Diesel engine. The proesses of intake, ompression, ombustion-expansion and exhaust making a full yle an be arried out in two strokes of the piston (one rotation of the shaft), or in four strokes of the piston (two rotations of the shaft). In the first ase the engine is named two-stroke, and in seond four-stroke. 2. Work Done by a Working Fluid in the Cylinder of a Reiproating Engine. Let us onsider a losed ylinder with a piston (Figure 1). Gas representing a working fluid takes in the losed volume of the ylinder. After the input of heat ( q ) the gas expands and moves the piston through a distane Δ x = x2-x1, i.e. exeutes mehanial work Δ L= P x, where the fore ( P ), ating on the piston is expressed as the produt of the pressure ( p ) and area of the piston F, i.e. P= pf. Then, Δ L= p Δ V, where Δ V = F Δxis a modifiation of volume as a result of the gas expansion (Figure 1). Let us introdue the definitions of speifi work def L l = m and speifi volume v def = V, m where m - mass of gas (working fluid). Considering infinitesimal inrements gives the differential expression dl = pdv and speifi work exeuted by the gas in the ylinder, is alulated with the help of integral v v 2 l = pdv. (1) 1 Obviously, sine during the ompression of the gas by the piston the initial volume vis 1 more than the final v 2, i.e. dv < 0 and dl < 0. Thus, the work done by the gas (work of Enylopedia of Life Support Systems (EOLSS)
THERMAL TO MECHANICAL ENERGY CONVERSION: ENGINES AND REQUIREMENTS Vol. I - Thermodynami Cyles of Reiproating and Rotary Engines - R.S.Kavtaradze the expansion) is positive, and the work spent on ompressing the gas is negative. In (1) the availability of p indiates the possibility of doing work; however indiation of doing work is the ondition dv 0. Mehanial work has a thermal equivalent. However to express heat by an integral whih is similar to (1), has beome possible only after introdution, in 1865 (1852?) by a German sientist Clausius, of the entropy onept. Entropy is a physial quantity, the modifiation of whih is an indiation of availability of exhange of energy in the form of heat and it is designated by S. Then by analogy with (1) we have: S S 2 q= TdS, (2) 1 where T indiates possibility of heat exhange under the ondition ds 0. Figure1: The Sheme of the ylinder with the piston for definition of mehanial wor From (1) and (2) it is obvious, that it is possible to represent the proesses whih are taking plae in the ylinder (fig.1), both in oordinates V - p, and in oordinates S-T. As the V - p diagram expresses mehanial work, it is frequently referred to as the work diagram, and sine the diagram S-T - shows thermal variables, it is referred to as the entropy diagram or the thermal diagram. 3. Working Proess and Indiator Diagram of a Four-stroke Engine. Figure 2 shows the sheme and indiatory diagram of a four-stroke engine. Indiatory diagram is traditionally the work diagram based on the measurements taken from the working engine or plotted as a result of alulation of in-ylinder proesses. Figure 2a Enylopedia of Life Support Systems (EOLSS)
THERMAL TO MECHANICAL ENERGY CONVERSION: ENGINES AND REQUIREMENTS Vol. I - Thermodynami Cyles of Reiproating and Rotary Engines - R.S.Kavtaradze presents gas pressure p as a funtion of the ylinder volume V. It is shown that the indiatory diagram is a losed urve. If the relationship p = p(v) is onverted into p= p( ϕ), where ϕ is the rankshaft rotation angle, using relationship V = V( ϕ), whih is known from the kinemati analysis, we obtain the developed indiatory diagram. While the piston exeutes reiproating motion, its veloity is zero at the points where its diretion of motion reverses. These points are known as dead enters. As shown in Figure 2 there are two kinds of suh stops: the top and the bottom dead enter, or in abbreviated form TDC and BDC respetively. The first stroke is an intake or filling of the ylinder by a fresh harge. In the ase of the Otto engine it is a fuel-air mixture, and in the ase of the Diesel engine it is air. In the beginning of this stroke the inlet valve 6 is open (outlet valve 7 is losed) and the piston 2 begins moving from TDC to BDC. In the ylinder the pressure drops below pressure p 0 of the environment and beomes equal to p a (line a on Figure 2-b). At the moment of arrival of the piston at the BDC the inlet valve is losed and thus the first stroke is finished. The seond stroke - ompression, during whih the piston 2, moving from BDC to TDC due to losed valves 6 and 7, ompresses the working fluid from full volume V a up to volume V. The pressure in the ylinder inreases from p a to p (line a on Figure 2-b). The third stroke ombustion-expansion takes plae while valves 6 and 7 are losed. In ase of the Otto engine the ignition of the fuel-air mixture takes plae by means of an eletri spark. In the ase of a diesel engine the fuel injetor injets fuel into the ompressed air. The resultant fuel-air mixture is spontaneously ignited due the high temperature of the ompressed air. As a result of expansion the produts of ombustion move the piston from TDC to BDC, i.e. the piston makes a power stroke. During ombustion, in spite of the fat that the piston moves to BDC and volume is inreasing, a sharp ompression from p up to p z (line z on Figure 2-b) takes plae. p z represents the maximum pressure of the yle. Thereafter there is an expansion of the produts of ombustion and the pressure drops from p z to p b (line zb on Figure 2-b). The last, the fourth stroke is arried out due to transition of the piston from BDC to TDC. At this time the exhaust valve 7 is opened and the piston pushes out the gases, whih have filled the ylinder, into the environment with pressure p r (line br on Figure 2-b). Obviously, p > p. After ompletion of this ation there remain ertain amount of the produts of b 0 ombustion, so-alled residual gases, in the ylinder whih, oupy the volume of ompression V (Figure 2-а, b) at pressure p r > p 0 hinder the next proess of filling. However the piston begins to move to BDC and the residual gases expand, their pressure drops up to pr < p0 (line rr' on Figure 2-b) and the intake, the first stroke of the next yle begins. The working proess of the four-stroke engine shematially shown on Figure 2-а, is represented as the losed indiator diagram in Figure 2-b. The diagram onsists of two main parts: 1. The work obtained in the ylinder, representing the differene of work in the proesses of expansion and ompression (positive); 2. Work whih is spent in gas-hanging proesses (of the intake and the exhaust) in the ylinder. It is also known as pumping work and as indiated in Figure 2-b, it is negative. Volume Vh between TDC and BDC, orresponding to the stroke of the piston is alled the working volume. The total volume of the ylinder is V a = V h + V. As the Enylopedia of Life Support Systems (EOLSS)
THERMAL TO MECHANICAL ENERGY CONVERSION: ENGINES AND REQUIREMENTS Vol. I - Thermodynami Cyles of Reiproating and Rotary Engines - R.S.Kavtaradze ombustion takes plae near the TDC, where the volume of the ylinder is not signifiantly different from V, V is also frequently alled as volume of the ombustion hamber. The most important parameter of the working proess of a piston engine is the ompression ratio ε def = Va Vh 1 V = + V, (3) Figure 2: Shemati diagram (a) and indiator diagram (b) f of four- tated engine. The ylinder, 2 piston, 3-onneting rod, 4-rank,5-shaft,6-inlet The value, 7 outlet value Aording to the above disussion, the ompression ratio of Diesel engines is always larger than of Otto engines. It will be lear from the following that the ompression ratio influenes the performane of a piston engine. 4. Working Proess and Indiator Diagram of a Two-stroke Engine. Figures 3(a) and 3(b) show the shemati of a typial two-stroke Diesel engine and the orresponding indiator diagram respetively. The first stroke of ombustion and expansion takes plae when the piston is at TDC. The exhaust valves 6 and 7 are losed (there are no inlet valves in suh sheme!) and the fuel is injeted into the ompressed air at volume V. The proess of ombustion (line z on Figure 3-b) begins spontaneously and the pressure inreases to the maximum value pz of the yle. Next as expansion of the produts of ombustion takes plae, the piston, exeuting a power stroke, ontinues to move to BDC and the pressure in the ylinder dereases to p m (line zm on Figure 3-b). The point m on Figure 3-а orresponds to the moment of opening of the exhaust valves 6 and 7. The pressure ontinues to drop (line mn on Figure 3-b) up to Enylopedia of Life Support Systems (EOLSS)
THERMAL TO MECHANICAL ENERGY CONVERSION: ENGINES AND REQUIREMENTS Vol. I - Thermodynami Cyles of Reiproating and Rotary Engines - R.S.Kavtaradze p n. Coming nearer to BDC the piston opens inlet ports 9, through whih air at pressure p from a superharger 8 enters the ylinder. The air displaes the produts of k ombustion through the outlet valves 6 and 7 while filling the ylinder, i.e. there is a proess of fored filling (line na on Figure 3-b), the piston ontinues to move to BDC, the pressure in the ylinder drops to p a. The seond stroke begins with the movement (of the piston from BDC). The pressurization-filling proeeds (line ak on Figure 3-b) until the piston bloks the inlet windows. The pressure in the ylinder remains during this period almost onstant. At the moment of losing the inlet ports the outlet valves 6 and 7 (point k on Figure3-b) are also losing and the air ompression proess begins. The air already had time to displae the produts of ombustion from the ylinder. Due to ompression (line k on Figure 3-b) the pressure is inreased from p k to p. So, the working yle is arried out for two strokes of the piston (one revolution of the shaft). The sheme of fored filling of the two-stroke engine shown on Figure 3a is alled the diret (valve-slot-hole) sheme and it is not the only sheme. For example, in twostroke Otto engines the onneting rod-hamber sheme of gas-harging is used, in whih the neessary pressure p k forms in the onneting rod hamber, i.e. under the piston, where due to movement towards LDS the fuel-air mixture is ompressed. As the inlet ports open this mixture flows into the ylinder, purges and fills it. In suh a sheme the superharger is not driven by the engine as shown for example in Figure 3-а. In two-stroke engines the atual volume V h and the geometrial volume V h are different. Sine Vh = Vh -Vn, where V n orresponds to the volume oupied by the inlet ports (Figure 3-а, b) and is known as volume loss (on this plot there is no useful stroke of the piston!). Aordingly there are atual ε and geometri ε ratios of ompression, the former is alulated by (3), and the latter by Figure 3: Shemati diagram (a) and indiator diagram (b) two stroked engine. The 1- ylinder, 2-piston, 3- onneting rod, 4-rank,5-shaft, 6 and 7-outlet valves,8- superharger,9-inlet windows,10- injetor Enylopedia of Life Support Systems (EOLSS)
THERMAL TO MECHANICAL ENERGY CONVERSION: ENGINES AND REQUIREMENTS Vol. I - Thermodynami Cyles of Reiproating and Rotary Engines - R.S.Kavtaradze def Va Vh ε = = 1+. (4) V V ε Ψ and ε =, where the fration of the lost volume 1 Ψ in the various engines. 5. Main Conepts of Thermodynamis. def V Ψ= n V h varies from 0.1 to 0.35 The modifiation of the ondition of a thermodynami system haraterized by hanging of ertain parameters, is alled a thermodynami proess. The thermodynami yle is a ontinuous sequene of thermodynami proesses, in whih the working fluid returns to its initial ondition. Obviously, thermodynami yles are represented by losed urves, as the total modifiation of any thermodynami parameter of a working fluid in a yle is equal to zero, i.e. dp = 0, dv = 0, dt = 0, di = 0, ds = 0, du = 0. In eah thermodynami proess the parameters vary in both diretions. The hange of parameters takes plae aording to the proess law. Some parameters may not hange. The thermodynami proesses: isobari, isohori, isothermal proesses orrespond respetively to the onditions: p = onst, v = onst, T = onst. Adiabati proess is a proess, in whih there is no heat exhange between a system and environment; isotropi and isenthalpi proesses take plae at onstant entropy and enthalpy and therefore, adiabati proess is an isentropi proess. The adiabati proess is desribed by the relation pv = onst, where the p isentropi exponent k = is the ratio of speifi heat at onstant pressure to that at v onstant volume. It is onstant for a given working fluid, for example for air k = 1.4. n The thermodynami proess desribed by pv = onst, is said to be polytropi. The polytropi exponent is a onstant. In a thermodynami yle always there are ompression proesses and expansion proesses of a working fluid, and also proesses with add ( dq > 0 ) and remove ( dq < 0 ) heat. Aording to a thermodynami yle thermal energy is onverted into mehanial energy and vie versa. Aording to the seond law of a thermodynamis for fulfillment of a thermodynami yle the availability of two soures of heat with different temperatures is neessary; one hot and the other old. A thermodynami yle in whih greater amount of heat is assoiated with a working fluid at a higher temperature and smaller amount of heat is assoiated with lower temperature, is named diret. The differene between these heats is equal to work, aomplished in a yle. The yle is reversible, if to a working fluid with smaller amount of a heat at lower temperature greater amount of heat at higher temperature is passed. The differene between these heats is equal to the spent work. Thermodynami proess, in whih thermodynami system and environment, interating with it return to initial onditions without any residual modifiations, is named reversible. Otherwise proess is irreversible. If all proesses in a yle are reversible, then the yle is also reversible. For a yle to be irreversible, the irreversibility even of one proess in it is suffiient. The diret thermodynami yles are haraterized by thermal effiieny of a k Enylopedia of Life Support Systems (EOLSS)
THERMAL TO MECHANICAL ENERGY CONVERSION: ENGINES AND REQUIREMENTS Vol. I - Thermodynami Cyles of Reiproating and Rotary Engines - R.S.Kavtaradze yle representing the ratio of work, done in a diret reversible thermodynami yle, to the heat fed into the working fluid from external soures. Similarly, the ratio of the heat removed in a reverse thermodynami yle from a ooled system, to the spent work in this yle, is known as refrigerating fator. An example of a reversible thermodynami yle is the yle the Carnot. - - - Bibliography TO ACCESS ALL THE 30 PAGES OF THIS CHAPTER, Visit: http://www.eolss.net/eolss-sampleallchapter.aspx Carnot S. (1824) Reflexions sur la puissant motrie du feu et sur les mahines a developper atte puissane. Paris: Presses Universitaires de Perpignan. [In this famous book by Sadi Carnot ideal thermodynami yles of the heat and refrigerating mahinery were omprehensively desribed for the first time ever]. Diesel R. (1893) Theorie und Konstruktion eines rationellen Wärmemotors. Berlin: Springer. [The first publiation by Rudolf Diesel desribing the working priniples of the new type engine, whih nowadays is widely known as diesel engine]. Heywood J.B. (1988) Internal Combustion Engine Fundamentals. New York: MGraw-Hill, In. [The omprehensive guide to the Internal Combustion Engine s working proesses]. Kavtaradze R.Z. (2001) Loal Heat Transfer in Internal Combustion Engines. 592 p.p. Mosow: BMSTU Publishing House. [In this book detailed desription of the loal radiation-onvetional in-ylinder heattransfer in internal ombustion engines is provided]. Sass F. (1926) Geshihte des Deutshen Verbrennungsmotorenbaues. Berlin: Springer. [The detailed desription of the Internal Combustion Engine s development up to late 1920-s with examples of designs and operating priniples]. Stodola A. (1928) Leistungsversuhe mit Bühi sher Aufladung. Zeitshrift. VDJ 72 (1928) S. 412 428. [The first sientifi researh of the Internal ombustion Engine superharging system]. Walker G. (1980) Stirling engines. Clarendon Press, Oxford. [This book overs the history, evolution, basi operating priniples and design examples of the Stirling engines]. Biographial Sketh Revaz Kavtaradze (b. 1951) graduated from Georgian Polytehni University (Tbilisi, Georgia) in 1973. He holds a D.S. (Engineering) degree in Internal Combustion Engine design from Bauman Mosow Tehnial University. Currently he is a professor at the Internal Combustion Engines department of BMSTU. Mr. Kavtaradze letured broadly and arried out R&D programs worldwide (Tehnial University of Munih (Germany), Rostok University (Germany), Beijing University of Tehnology (China)). He has extensive experiene and partiular interest in the field of Internal Combustion Engines working proesses and heat transfer. He is an author of numerous monographs and sientifi papers. Enylopedia of Life Support Systems (EOLSS)