Modification of Two Stroke I.C Engine to Reduce Emission and Fuel Consumption

Transcription

1 IACSIT Internatonal Journal of Engneerng and Technology Vol. 2, No.1, February, 2010 Modfcaton of Two Stroke I.C Engne to Reduce Emsson and Fuel Consumpton Jayanth Kumar.T, Arun.M, Murugan.G, Mano.R, Venkatesan.J Abstract Asan ctes face a serous ar polluton problem from two and three-wheeled vehcles that run on two-stroke engnes [3]. Inexpensve two-wheelers form a staggerng 75-80% of the traffc n most Asan ctes. Because two-stroke engnes burn an ol gasolne mxture, they emt more smoke, carbon monoxde, hydrocarbons, and partculate matter than the gas run four-stroke engnes found n newer motorcycles. Makng matters worse, many Asan two-wheelers are converted nto three-wheeled "baby taxs" by addng a sdecar. However the vehcle s not desgned for the extra weght and the engne burns even drter. For small two stroke engne applcaton, t s mportant that the newer concepts/systems should not adversely affect ther exstng advantages, partcularly those of smplcty and low producton and mantenance cost. The present work ams at reducng short crcutng of fresh charge by admttng cooled exhaust gas to pass through reed valves ftted at the upper end of the transfer passage n a crank case scavenged two stroke engne. Ths resulted n apprecable decrease n specfc fuel consumpton and n HC/CO emssons. The major work nvolved n the present work was to desgn and fabrcate alumnum flanges whch accommodate the reed valves. And an arrangement has been made at the upper end of the transfer passage n the engne cylnder to fasten the flange. A heat exchanger was fabrcated n order to cool the exhaust gas at a temperature of about 400 o C to avod the pre-gnton nsde the engne cylnder. The performance of the engne was tested usng eddy current dynamometer. Index Terms EGR (Exhaust gas recrculaton), Alumnum Flange, Reed Valves, Heat Exchanger. I. INTROUCTION Two stroke spark gnton engnes are used manly n two wheeler vehcles whch are meetng the present day ndvdual transportaton needs satsfactorly. Snce there s no dstnct exhaust stroke n these engnes, the responsblty of drvng out the combuston product from cylnder rests wth the fresh charge enterng nto the cylnder. Two stroke S.I engnes n spte of ther advantages lke hgh specfc output, ease of mantenance, they suffer from serous drawbacks lke hgh HC/CO emssons and low brake thermal effcency [2]. It s found that the fuel consumpton and HC emssons of two stroke S.I engnes are more than that of equvalent four stroke S.I engnes. In the crank case scavenged two stroke engnes consderable amount of fresh fuel ar mxture gets short crcuted through the exhaust port durng the scavengng process. Also a lot of exhaust gas gets retaned nsde the cylnder and dlute the fresh charge. The loss of fresh charge through the exhaust port vares from 25 to 40 % of whch comes n to the cylnder dependng upon the operatng condtons and method of scavengng. These factors are responsble for the hgh specfc fuel consumpton and hgh HC/CO emssons. It s evdent that any attempt to flush out the exhaust gas properly wthout losng a consderable amount of fresh charge wll result n an mproved performance of the two stroke SI engne. In the new desgn two reed valves are ftted at the upper end of the transfer ducts connectng the crank case and cylnder as shown n Fg 1. The two reed valves permt exhaust gas to enter the transfer ducts durng the upward movement of the pston due to the partal vacuums created n the transfer ducts and the crank case. Ths result n a complete or at least a partal fllng of the transfer ducts by exhaust gas, A/F mxture s nducted n to the crank case n the conventonal way. urng the downward stroke of the pston the pressure nsde the crank case ncreases and the transfer ports open and the exhaust gas whch s trapped n the transfer ducts enters the cylnder and partcpates n the scavengng. Thereby t reduces the loss of the fresh ar fuel mxture. It s clearly proved that fresh charge loss durng scavengng s the man cause for excess fuel consumpton and hgh HC emsson. Hence attempts have been made to overcome these drawbacks. Prmarly on the consderaton that the scavengng process should be carred out by ar/cooled exhaust gases nstead of A/F mxture. Two reed valves were mounted on the top of the transfer port n a specally desgned alumnum flanges. To these flanges, the cooled exhaust gas lne s ftted. When the pston moves upwards vacuum s created n the crankcase and t leads to the openng of reed valves. Ths result n complete or partal fllng up of transfer port by ar/cooled exhaust gases whch scavenges the burnt products and avods the loss of fresh A/F mxture durng scavengng. Hence t results n mprovement of specfc fuel consumpton and reducton of HC emssons. II. SCAVENGING PROCESS The performance of two-stroke engne depends on the art of scavengng and on port desgn [1]. In ths case the exhaust port s opened near the end of the expanson stroke. Wth pston - controlled exhaust and nlet port arrangement the lower part of the pston stroke s always wasted. Thereby the useful power output whch s about 15% to 40% of the expanson stroke becomes neffectve. The actual percentage vares wth dfferent desgns. Ths early openng of the exhaust ports durng the last part of the expanson stroke s necessary to permt blow down of the exhaust gases and also to reduce the cylnder pressure so that when the nlet port opens at the end of blow down process, fresh charge can enter nto the cylnder. Ths wll scavenge the burnt products. Some of the fresh charge s also lost due to short crcutng. 42

2 IACSIT Internatonal Journal of Engneerng and Technology Vol. 2, No.1, February, 2010 Scavengng s the process of drvng away the products of combuston n the cylnder from the prevous cycle and replacng t wth fresh A/F mxture to be burned n the next cycle. III. MOIFIE ENGINE In modfed engne ar fuel mxture and exhaust gas are suppled n the methods gven below: Prmarly A/F mxture s sent through the carburettor n the conventonal way. When the pston moves from BC to TC the pressure nsde the crankcase s below atmospherc pressure. Ths causes the ar to flow from the atmosphere to the crankcase by the above mentoned ways. When the exhaust gas flows through the reed valves the ar fuel mxture present n the transfer duct as a result of the prevous cycle s forced nto the crankcase and exhaust gas takes ts place. The sequence of events s shown n Fg 1. The flow of ar through the carburettor per cycle for the same throttle postons gets reduced after the modfcatons. Ths s because the suppled exhaust gas wll push the resdual gas n the cylnder thereby fresh A/F. shot crcutng s reduced. urng the downward stroke of the pston, pressure s bult up n the crankcase and the transfer ducts as soon as the nlet port closes and hence the reed valve closes. When the pston descends further the transfer ports open and the exhaust gas n the transfer ducts enters nto the cylnder nstead of the ar fuel mxture as n the conventonal engne. Ths exhaust gas pushes the burned gases out, so that scavengng s done by exhaust gas. The A/F mxture whch follows the exhaust gas s retaned nsde the cylnder to a large extent. Hence short crcutng of fresh A/F mxture can be reduced consderably. because ths volume gets added to the crankcase volume of the unmodfed engne and may affect the scavengng pressure developed to a large extent. The exploded vew of the reed valve assembly s shown n Fg 2. The reed valve assembly are then screwed to the top of the transfer ducts after the fns around that area are removed by mllng and a hole s drlled and tapped sutably. The connectons are made n such a way that the ar through the reed valve perpendcular to the cylnder axs as shown n Fg 3. Another Fg 3 shows the way how reed valve assembles are fxed. Care should be taken to ensure that the reed valve connecton does not extend over the nsde surface of transfer duct wall whch wll obstruct the flow. Fg 2 Reed block desgn V. EXPERIMENTAL SETUP An Eddy Current absorpton dynamometer produces brakng torque usng the prncple of eddy currents nduced n a rotatng metallc dsk, mmersed n a magnetc feld [4]. It s bascally an eddy current brake mounted n trunnon bearngs. Its advantages are mantenance, control, smple constructon and desrable speed-torque characterstcs. The speed-torque characterstcs make the eddy current dynamometer deal for engne testng, and ts versatlty also allows effectve use n testng transmssons, turbnes, electrc motors, gears, pumps and many other machnes. The expermental setup s shown n Fg 3. Fg 1 Engne wth mproved scavengng technque IV. EXPERIMENTAL ETAILS The reed valves are enclosed n specally fabrcated an alumnum alloy contaner. Gaskets are used on ether sde of the reed valve to prevent any leakage. Care should be taken to keep the volume of that porton of the contaner between the engne and the reed valve to a mnmum 43 Fg 3 Expermental setup

3 IACSIT Internatonal Journal of Engneerng and Technology Vol. 2, No.1, February, 2010 VI. HEAT EXCHANGER FOR EXHAUST GAS COOLING When large quanttes of heat are to be transferred, the heat transfer area of the exchanger also becomes large. In a sngle pass heat exchanger ths requrement can be met ether by ncreasng the length of the tube or by decreasng the dameter and ncreasng the number of tubes. Nether of these methods s practcal because there s lmtaton of sze and even the length of the tube cannot be ncreased arbtrarly as large pressure drop would occur wth smaller dameter tubes. These dffcultes lead to the concept of mult pass arrangements. The flud flowng nsde the tube s called the tube flud and the flud flowng outsde the tube s called the shell flud flow condton for a shell and tube type exchanger whch s nether parallel nor counter flow type [5]. The desgn of heat exchanger s shown n Fg 4. Mass flow rate of the fuel n kg/sec s gven by m P ( SFC) (1.1) Mass flow rate of ar n kg/sec s gven by Where, f A ma m f (1.2) F A =Ar fuel rato F Mass flow rate of exhaust gas n kg/ sec s gven by m m m (1.3) g f 15% of Exhaust Gas s re-crculated back nto the engne for scavengng process. Heat lost by exhaust s gven by g pg g1 g2 a m C T T (1.4) Heat ganed by coolng water s gven by m C (1.5) T T w pw w2 w1 From equaton 1.4 and 1.5 t can be nferred that heat lost by the exhaust gas s equal to the heat ganed by the coolng water. T T m C T T m C g pg Tg1 Tg2 m C (1.6) g pg g1 g2 w pw w2 w1 T = + T w2 w1 mw Cpw (1.7) LMT (Log Mean Temperature fference) The log mean temperature dfference s found out usng the equaton gven below. Where, LMT 0 log 0 T T (1.8) g1 w2 0 Tg2 Tw1 (1.9) Heat Transfer The heat transfer from the exhaust gas to the coolng water s found out usng the equaton gven below m C T (1.10) g pg g Heat Transfer Coeffcent Heat transfer coeffcent for exhaust gas s gven by Nu k hg (1.11) The nusselt number for calculatng the heat transfer coeffcent of exhaust gas and coolng water s gven below Nu (Re) (Pr) (1.12) v Re m g v A (1.13) Heat transfer coeffcent for coolng water s gven by h w = N u k o (1.14) (1.15) v o Re (1.16) Overall Heat Transfer Coeffcent The overall heat transfer coeffcent s calculated usng the equaton gven below e e log e (1.17) U h g k hw The amount of heat transfer can also be found out usng the equaton gven below AU LMT (1.18) From equaton 1.19 the area of heat exchanger can be calculated. A (1.19) U LMT A n L (1.20) Length of Heat Exchanger From equaton 1.21the length of the heat exchanger can be calculated A L n (1.21) Thus the length of the heat exchanger was found to be 1.06 m. The outer dameter of the large ppe was assumed to be m and the dameter of the smaller ppe was assumed to be m. The materal of the ppe used s G.I. 44

4 IACSIT Internatonal Journal of Engneerng and Technology Vol. 2, No.1, February, 2010 Fg 4 Heat exchanger desgn VII. NOMENCLATURE SFC Specfc fuel consumpton n kg/bhp.hr P Power n kw C Specfc heat capacty of gas n kcal / kg C T pg g 1 Inlet temperature of the exhaust gas n C T g 2 Outlet temperature of the exhaust gas n C T w 1 Inlet temperature of the coolng water n C T w 2 Outlet temperature of the water n C T g Temperature dfference between exhaust gases n Nu Re Pr v C Nusselt number Renolds Number Prandtl Number Insde ppe dameter of heat exchanger n meters. Velocty n m/sec k Thermal conductvty of flud n kcal / mhr C 3 ensty of exhaust gas kg/ m o A n L m w Outsde dameter of heat exchanger n meters. 2 Area of heat exchanger n m Number of tubes nsde the heat exchanger The length of the heat exchanger n meters. Mass flow rate of the coolng water n kg/ sec C Specfc heat capacty of the coolng water n pw kcal / kg C Effect of Modfed Engne on Brake Thermal Effcency Brake thermal effcency s ncreased at all engne speed than the conventonal engne because the short crculatng of fresh ar fuel mxture s avoded then the unburned fuel s burned nsde cylnder durng scavengng process. But generally the modfed engne s havng hgher brake thermal effcency. The varaton of brake thermal effcency wth engne speed s shown n Fg 7 and Fg 11. At full throttle and 5000 rpm engne speed and at 2.43 hp, the brake thermal effcency wthout modfcaton s 16.51% and wth EGR t s found to be 17.56%. At half throttle and 5000 rpm engne speed and at 1.35 hp the brake thermal effcency wthout modfcaton s 10.02% and wth EGR engne t s found to be 26.19%. Effect Of The Engne Wth EGR On Torque: There s no consderable change n torque than the conventonal engne. The varaton of torque wth engne speed s shown n Fg 5 and Fg 10. Effect Of The Engne Wth EGR On Emsson: There was a reducton of 25% of HC and 35% of CO n the modfed engne, the reason s 1. Burnng of re-crculated unburned fuel partcles nsde the cylnder by mproved scavengng technque. 2. The short crculatng fresh ar fuel mxture s avoded by mproved scavengng technque. The varaton of HC and CO wth engne speed s shown n Fg 8, Fg 9, Fg 13 and Fg 14. At full throttle and 5000 rpm engne speed and at 2.43 hp, the HC emsson wthout modfcaton s found to be 2900 ppm and wth EGR t s 2700 ppm. At full throttle and 5000 rpm engne speed and at 2.43 hp, the CO emsson wthout modfcaton s 2.8% and wth EGR t s found to be 2.5%. At half throttle and 5000 rpm engne speed and at 1.35 hp, the HC emsson wthout modfcaton s found to be 2500 ppm and wth EGR t s found to be 1730 ppm. At half throttle and 5000 rpm engne speed and at 1.35 hp, the CO emsson wthout modfcaton s 2.8% and wth EGR t s 2.3%. HALF TROTTLE CONITION VIII. RESULTS AN ISCUSSION Effect Of Modfed Engne On SFC Specfc fuel consumpton s reduced n all speeds rangng from 20 to 30% than the conventonal engne wthout any modfcaton.the advantage of low specfc fuel consumpton means ncrease n mleage at road condtons. The varaton of SFC wth engne speed s shown n Fg 6 and Fg 12. At full throttle and 5000 rpm engne speed and at 2.43 hp, the SFC wthout modfcaton s kg/bhp.hr and wth EGR t s found to be kg/bhp-hr. At half throttle and 5000 rpm engne speed, the SFC wthout modfcaton s kg/bhp.hr and wth EGR t s found to be kg/bhp.hr. Fg 5 Varaton of torque wth engne speed 45

5 IACSIT Internatonal Journal of Engneerng and Technology Vol. 2, No.1, February, 2010 FULL THROTTLE CONITION Fg 6 Varaton of SFC wth engne speed Fg 10 Engne speed Vs Torque Fg 7 Varaton of brake thermal effcency wth engne speed Fg 11 Varaton of brake thermal effcency wth engne speed Fg 8 Varaton of CO emssons wth engne speed Fg 12 Varaton of SFC wth engne speed Fg 9 Varaton of HC wth engne speed Fg 13 Varaton of CO emssons wth engne speed 46

6 IACSIT Internatonal Journal of Engneerng and Technology Vol. 2, No.1, February, 2010 Fg 14 Varaton of HC wth engne speed IX. CONCLUSION 1) In the modfed engne the SFC value s lower than the conventonal engne n all throttle condtons. 2) The power output s slghtly ncreased than the conventonal engne n all engne speeds because of () preventon of the short crculaton of fresh A/F mxture and () re-burnng of unburned hydrocarbons by means of the mproved scavengng technque. 3) On the emsson sde, the HC and CO emssons are drastcally reduced than the conventonal engne at all the speeds. It can be observed from the analyss that there s about 25 % reducton of HC emsson and % of CO emsson. 4) The thermal effcency for the modfed engne s hgher than the normal engne at all engne speeds. REFERENCE [1] Blar G.P., Kenny R.G. - Further development n scavengng analyss for two stroke cycle engne. SAE Transacton [2] Van Basshuysen, Rchard, and Schäfer, Fred (2004)- "Internal Combuston Engne Handbook," SAE Internatonal. [3] Potera, Carol (2004) - Asa s two stroke engne dlemma. Envron and Health perspect dlemma journal 112(11).A613. [4] Martyr, A; Plnt M (2007)- Engne Testng - Theory and Practce. (Thrd Edton ed.). Oxford, UK: Butterworth-Henemann. [5] Sadk Kakaçand Hongtan Lu (2002) - Heat Exchangers: Selecton, Ratng and Thermal esgn. (2nd Edton ed.). CRC Press. ISBN