BIODIESEL INFLUENCE ON DIESEL ENGINE EMISSION

Transcription

1 4 Mehanial Testing and Diagnosis ISSN , 211 (I), Volume 1, 4-47 BIODIESEL INFLUENCE ON DIESEL ENGINE EMISSION Breda KEGL, Stanislav PEHAN University of Maribor, Maribor, Slovenia, b, 2 Maribor, SLOVENIA breda.kegl@uni-mb.si, stanislav.pehan@uni-mb.si ABSTRACT This paper disusses the influene of the neat biodiesel made from rapeseed oil on the bus diesel engine with injetion M system. Running the diesel engine the relationships among fuel properties, injetion and ombustion harateristis and harmful emission, are investigated. The engine harateristis by experiments and numerial simulation are ompared. The influenes of biodiesel on the injetion pressure, injetion timing, ignition delay, in ylinder gas pressure and temperature, heat release rate, exhaust gas temperatures, harmful emissions, speifi fuel onsumption, and on engine power are analyzed. The results obtained with biodiesel are ompared to those obtained with mineral diesel. Speial attention is given to explanations of higher NO x emission in spite of lower in ylinder gas temperature. Keywords:biodiesel, NO x emission, heat release rate 1. INTRODUCTION In general, biofuels may offer an exellent opportunity to redue some of the harmful emissions without expensive engine modifiations. Sine vegetable oils satisfy the major diesel engine requirements their suitability as alternative fuel has frequently been investigated in reent years. For example, investigations of used orange oil shows that orange oil exhibits an inreased ignition delay, higher ombustion duration, and higher heat release rate, ompared to mineral diesel [1]. This results in reduing HC, CO and smoke emissions meanwhile the NO x emission are higher [2, 3]. Among all biofuels for diesel engines, biodiesel is a very promising fuel beause it is a sulfur-free, non-toxi, oxygenated, renewable, and more than 9% biodiesel an be biodegradable within 21 days [4]. Biodiesel has higher etane number than mineral diesel, no aromatis, almost no sulfur, and ontains more oxygen by weight. Biodiesel fuel with a etane number similar to the diesel fuel produed higher NO x emissions than the diesel fuel. Biodiesel has higher density, visosity, surfae tension, sound veloity, and bulk modulus of elastiity. This affets the fuelling, injetion timing, and fuel spray and

2 Mehanial Testing and Diagnosis, ISSN ,211 (I), Volume 1, onsequently the emission harateristis [5]. On the basis of the physial properties of the biodiesel, it an be summarized that the advantages of the biodiesel are shorter ignition delay due to a higher etane number and an enhaned ombustion proess aused by oxygen in the biodiesel. Biodiesel has lower heating value than mineral diesel and auses some loss of power. Therefore, it is neessary to inrease the fuel amount to be injeted into the ombustion hamber. This will ause longer injetion duration due to hange in the injetion timing. The objetive was to identify as muh as possible the dependenes among fuel properties, the most important injetion and ombustion harateristis, harmful engine emissions, and other engine performanes. Speial attention is devoted to dependenes among injetion timing, in-ylinder gas temperature and NO x emission. 2. NUMERICAL SIMULATION The paper outlines briefly the fundamentals of the employed software AVL BOOST v29.1 and the sub-models hosen for this work. The employed mathematial model is based on the first law of thermodynamis [6], whih an be (for an internal ombustion piston engine) written as dm u dv dq dq w dm (1) f BB p h BB d d d d d Basially, for the high pressure yle this law states that the hange of the internal d m u / d p dv / d, energy in the ylinder is equal to the sum of piston work the onversion of hemial energy to the thermal energy ( dq / d ), heat transfer f ( (dq / d )) W, and the enthalpy flow due to blow-by h (dm / d )). The ( BB BB onversion of hemial energy to the thermal energy represents the heat release. In equation (1) the symbol denotes the angle of the rankshaft rotation, is the mass of the mixture in the ylinder, u is the speifi internal energy, is the in-ylinder pressure, V is the ylinder volume Q is the fuel energy, Q the heat transfer through the liner, f and h m BB the piston and the liner, respetively. With the in-ylinder gas pressure W are the enthalpy and mass of the mixture that esapes through the gap between displaement of one piston, V D determined over the whole yle duration as follows: the gas equation: p (1/ V)m R T where T o p, the indiated mean effetive pressure p (1/ V ) i D CD and the working pi an be. Together with p dv is the in-ylinder gas temperature, equation ( 1) an be solved using the Runge-Kutta method. One the in-ylinder gas temperature is known, the in-ylinder gas pressure an be obtained from the gas equation. For internal mixture preparation it is assumed that the fuel added to the ylinder harge is immediately ombusted, the ombustion produts mix instantaneously with the rest of the ylinder harge and form a uniform mixture, and as a onsequene, the air/fuel ratio of the harge diminishes ontinuously from a high value at the start of ombustion to the final value at the end of ombustion. The flow in the tube is treated as a onedimensional flow. This means that the pressures, temperatures, and flow veloities, obtained from the solution of the gas dynami equations, represent mean values over the (1)

3 42 Mehanial Testing and Diagnosis, ISSN ,211 (I), Volume 1, 4-47 ross-setion of the tubes. The employed software offers several sub-models to aount for several possibilities of flow modeling. In this paper, the options desribed in the following have been seleted by keeping in mind the objetive of a good ompromise between the speed of omputation and auray with respet to the experimentally obtained results. Between the three optional savenging models, available in the employed software, the perfet mixing model is hosen. The energy ontent of the exhaust gases is equivalent to the mean energy ontent of the gases in the ylinder. The gas, leaving the ylinder, has the same omposition as the mixture in the ylinder. For the verifiation of auray and fine tuning of the numerial simulation model, some experimental work is done. 3. EXPERIMENTAL EQUIPMENT AND TEST PROCEDURE The shemati diagram of the engine test bed is presented in Figure 1. The test bed onsists of an engine and eletro-dynamometer Zöllner A-35AC, 3kW, air flow rate meter RMG, fuel onsumption dynami measuring system AVL, UHC analyserratfish, NOxhemoluminisent analyzer Thermoeletron, O2 analyzer Programmeletroni, CO analyzer Maihak, and smoke meter AVL. Using a data aquisition system the instantaneous injetion pressure and needle lift, the instantaneous in-ylinder gas pressure, the temperatures of fuel, ambient air, intake air, ooling water at inflow and outflow of the engine, oil pressure and temperature, and the temperature exhaust gases are measured. Consumption Pump Intake Emissions Water temperature at outflow Fuel tank Brake (torque, speed, power) Air temperature at outlet Air temperature at inlet Oil sump temperature Water temperature at inlet Fig. 1. The engine test bed sheme The main speifiations of the tested engine and fuel injetion system are given in Table 1. The measurements and omputations of engine harateristis were performed at various engine operating regimes. The omparison was done for both tested fuels at several operating regimes at full load, espeially at the peak torque and rated onditions. The fuels under onsideration are: (i) neat mineral diesel (throughout this paper denoted as D2) and

4 Mehanial Testing and Diagnosis, ISSN ,211 (I), Volume 1, (ii) neat biodiesel (throughout this paper denoted as B1 for brevity). D2 onforms to European standard EN 59. B1 is produed from rapeseed by Biogoriva, Slovenia and it onforms to European standard EN A omparison between some measured properties of D2 and B1 are given in Table 2 [5, 7]. Table 1. Test engine and injetion system main speifiations Engine model MAN D 2566 MUM Enginetype 4 stroke, 6 ylinder in line, water ooled Displaement m 3 Compressionratio 17.5 : 1 Bore and stroke 125 mm x 155 mm Max Power 162 kw Injetion model diretinjetion M system Fuelinjetionpump Bosh PES 6A 95D 41 LS 2542 Pump plunger (diameter x lift) 9.5 mm x 8 mm Fuel tube (lengthxdiameter) 124 mm x 1.8 mm Injetionnozzle (numberxnozzleholediameter) 1 x.68 mm Table 2. Diesel and biodiesel properties Fuel D2 B1 2 o C (kg/m 3 ) o C, 3 bar (m/s) o C (mm 2 /s) Surfae 3 o C (N/m) o C, 3 bar (MPa) Calorifivalue (MJ/kg) Cetanenumber 45 > 51 To verify the numerial simulation used for our engine model, presented in Figure 2, the in-ylinder gas pressure, the engine power and the effetive speifi fuel onsumption were measured at various engine operating regimes. Intake tubes Boundary onditions Air Juntion Cylinder Outlet tubes Juntion Boundary onditions Fig. 2. The engine model for numerial simulation

5 44 Mehanial Testing and Diagnosis, ISSN ,211 (I), Volume 1, RESULTS AND DISCUSSION To analyze the influene of fuel properties on bus diesel engine harateristis, the most important injetion and ombustion harateristis, obtained with B1 and D2, are ompared and disussed. Engine performane and harmful emissions are measured at various engine regimes [8]. Beause of a very good agreement between numerial and experimental results, Figure 3 shows the numerially determined engine effetive power and torque and effetive speifi fuel onsumption, only. One an see from Figure 4 that relative NO x emission is pratially the same at all engine speeds exept at very low speed where it dereases with inreasing engine speed. On the other hand, the NO x emission (not relative) generally inreases with higher engine speed. This is partially due to the gas flow motion within ylinder under higher engine speed, whih leads to a faster mixing between fuel and air, and a shorter ignition delay. Consequently, the reation time of eah engine yle is redued ausing an earlier and higher in-ylinder gas temperature peak. By using B1 this effet is even more evident. Figure 5 shows the injetion pressure, needle lift, in-ylinder gas pressure and temperature, and heat release rate by using B1 and D2 fuels at peak torque onditions. Injetion pressure and needle lift are determined experimentally; in-ylinder gas pressure and temperature and heat release rate are obtained by numerial simulation. The differenes in pressure, when B1 and D2 are used, are almost negligible. However, differenes are observed for in-ylinder gas temperature and in heat release rate. The in-ylinder gas temperature and heat release rate are lower by using B1 onsiderably and their peak values are advaned with respet to D2. When using B1, an earlier needle lift (advaned injetion timing) was observed. This is due to higher visosity, bulk modulus and sound veloity of B1. A higher bulk modulus leads to more rapid pressure wave propagation from the pump to the needle nozzle and an earlier needle lift. Higher visosity of B1 leads to redued fuel losses during injetion proess, to faster evolution of pressure and thus to advaned injetion timing Po we r (kw Power Torque Speifi fuel onsumption Fig. 3. Effetive power and torque, injeted fuel mass and speifi fuel onsumption (numeri) D2 B Engine speed (rpm) To r qu e (Nm) Sp e i f i f uel o n s ump t ion (g/

6 Mehanial Testing and Diagnosis, ISSN ,211 (I), Volume 1, Sm o k e (Bo s h Un i t/kw), HC (pp m CO HC Smoke D2 B1 NO x CO (pp m/ kw), x NO (pp m/ kw) Engine speed (rpm) Fig. 4. Harmful relative emissions at full load onditions (experiment) Inj e t ion pr es s ur (bar) e Injetion pressure Needle lift Peak torque ondition D2 B Ne ed l e lif (mm) t In-y l ind er pr es s ur e (bar) Heat release rate Crank angle ( o CA) In-ylinder gas In-ylinder gas temperature Crank angle ( o CA) In-y l ind er temp er at ur e (K Figure 5. Injetion and ombustion harateristis at peak torque onditions

7 46 Mehanial Testing and Diagnosis, ISSN ,211 (I), Volume 1, 4-47 From Figure 6, one an see that ignition delay for B1 is always lower than that of D2. The ignition quality of a fuel is usually haraterized by its etane number. B1 has a higher etane number and therefore exhibits a shorter ignition delay period. This results in lower exhaust gas temperatures and higher engine oolant and oil temperatures. The ignition delay is mainly ontrolled by the mixture temperature, vapor, and oxygen onentration. This is espeially true for the temperature at the point of injetion start. Thus, a shorter ignition delay of B1 is signifiantly influened by higher inylinder gas temperature at the beginning of injetion. However, shorter ignition delay is making the atomization of the biodiesel diffiult due to the higher kinemati visosity and surfae tension of biodiesel. 25 St art of inj e t ion o CA ( BTDC) St art of o mb us t ion o CA ( BTDC) Igni tion del ay o CA) ( Engine speed (rpm) Start of injetion - D2 Start of injetion - B1 Start of ombustion - D2 Start of ombustion - B1 Ignition delay - D2 Ignition delay - B1 Fig. 6. Start of injetion and ombustion, ignition delay at full load onditions 5. CONCLUSION By using experiments and numerial simulation, the relationships among fuel properties and engine emissions through the injetion and ombustion harateristis are analyzed. The findings are as follows: Higher density, visosity, veloity of sound and bulk modulus of B1 and lower vapor ontent ause the advaned injetion timing and higher injetion pressure. The advaned injetion timing auses the inrease of the in-ylinder gas pressure, in-ylinder gas temperature, and an earlier rise of the heat release rate. The higher injetion pressure, higher oxygen ontent and other fuel properties of B1 results in lower smoke and CO emissions and in a slightly higher HC emission. The results show that earlier appearane of temperature and heat release rate maximums prolongs the period with onditions favourable for NO x formation. REFERENCES [1] Purushothaman, K., Nagarajan, G., 29, Performane, emission and ombustion harateristis of a ompression ignition engine operating on neat orange oil, Renew Energy, 34, pp

8 Mehanial Testing and Diagnosis, ISSN ,211 (I), Volume 1, [2] Balat, M., Balat, H., 21, Progress in biodiesel proessing. Appl Energy, 87, pp [3] Park SH, Yoon SH, Lee CS., 21, Effets of multiple-injetion strategies on overall spray behavious, ombustion, and emissions redution harateristis of biodiesel fuel, Appl Energy, [4] Leung, D., Y., C., Wu X, Leung, M., K., H., 21, A review on biodiesel prodution using atalyzed transesterifiation, Appl Energy, 87, pp [5] Kegl, B., 26, Numerial analysis of injetion harateristis using biodiesel fuel, Fuel, 85, pp [6] Ganapathy, T., Murugesan K, Gakkar, P., 29, Performane optimization of Jatropha biodiesel engine model using Taguhi approah. Appl Energy, 86, pp [7] Kegl, B., 28, Effets of biodiesel on emissions of a bus diesel engine. BioresourTehnol, pp. 99: [8] Kegl, B., 26, Experimental Investigation of Optimal Timing of the Diesel Engine Injetion Pump Using Biodiesel Fuel, Energy Fuels, 2, pp