Article AL-QADISIYAH JOURNAL FOR ENGINEERING SCIENCES 12 (2019) 184–192 Contents lists available at http://qu.edu.iq Al-Qadisiyah Journal for Engineering Sciences Journal homepage: http://qu.edu.iq/journaleng/index.php/JQES * Corresponding author. Tel.: +964(0)7814136944. E-mail address: Mohamed.aldawody@qu.edu.iq ( Mohamed Al-Dawody) https://doi.org/10.30772/qjes.v12h3.616 2411-7773/© 2019 University of Al-Qadisiyah. All rights reserved. Computational Combustion and Emission Analysis of Biodiesel in a Variable Compression Ratio Engine Mohamed Al-Dawody a b * and S.K. Bhatti b aFaculty of Engineering – University of Al-Qadisiyah-Iraq bDepartment of Mechanical Engineering - College of Engineering (A) /Andhra University -India A R T I C L E I N F O Article history: Received 15 September 2019 Received in revised form 05 October 2019 Accepted 10 October 2019 Keywords: Soybean biodiesel Performance Emissions CFD analysis A B S T R A C T The aim of the present study is to analyse the combustion characteristics, performance and emission parameters of a variable compression ratio (VCR) diesel engine experimentally and numerically using soybean methyl ester (SME) biodiesel. Initially the engine is fed with diesel to capture the basic data, and then SME was tested as 20 % blend (B20), as 40% blend (B40) and as pure bio-fuel (B100). The experimental investigations are followed by a computational combustion and emissions analysis of diesel engine which is done by using the CFD software (ANSYS FLUENT 13). The combustion, performance and emissions parameters are evaluated by operating the engine at four different compression ratios of 15, 16, 17.5 and 19 and varying the load from 0 kW to 4.4 kW with 1.1 kW step. It is observed that peak pressure is closer to TDC when SME blends is increased. SME blend has earlier combustion start because of the advancement in the injection timing, shorter delay time. Increasing mixing ratio of biodiesel is found to decrease BTE slightly and increases the BSFC. Remarkable decrease in UHC and CO emissions as the ratio of SME is increased due to the complete combustion of biodiesel with presence of more oxygen in the combustion chamber. The measured BSN for B20, B40, and B100 SME was less than that of diesel fuel by 20.44%, 35.78%, and 48.3% respectively. It is inferred from the combustion analysis that as the compression ratio increases from 15 to 19 a decrease in smoke intensity, UHC, and CO, but it increases the emission of NOx. Both turbulent kinetic energy and turbulent dissipation rate were decreased as the percentage of SME increased by 10.84% and 2.01% respectively. The increase in compression ratio from 15 to 19 caused an increase in the peak pressure, density, combustion velocity, turbulence, peak temperature, NOx and a decrease in soot emissions. It can be assessed that the B20 SME is best suited to implement it into diesel engine without any effects. It has been founded from the results that 19 compression ratio has shown good performance and low emissions as compared to other compression ratios. The results obtained from the experimental investigation have been compared with the results of CFD analysis and are found to be in good agreement with each other with just slight deviation. © 2019 University of Al-Qadisiyah. All rights reserved 1. Introduction For more than a century, the reciprocating internal combustion (IC) engines have been a dependable workhorse in a wide variety of applications that require mechanical motive power. From its inception in the late 1800s till date, tremendous progress has been made in the performance, reliability and efficiency. It is a well-known fact that diesel engines have deep impact on the industrial economy, as they are used in heavy trucks, city transport buses, locomotives, electrical generators, farm equipments, underground mine equipments etc., because of their simple, robust construction coupled with high thermal efficiency and specific power output with high fuel economy. Increasingly tough emissions regulations coupled with fast - http://qu.edu.iq/ https://doi.org/10.30772/qjes.v12h3.616 MOHAMED AL-DAWODYA, S.K. BHATTIB /AL-QADISIYAH JOURNAL FOR ENGINEERING SCIENCES 12 (2019) 184–192 185 depleting crude oil resources have caused a stimulation of interest in finding alternative fuels for petroleum-based fuels like diesel and gasoline. For instance, on-road heavy-duty diesel engines face a stiff challenge in meeting the future standards for both NOx and PM emissions. However many researchers directed to search for alternative energy source [1-2]. All over the world, biodiesel is subjected to intensive research work. Biodiesel was found as the best alternate fuel, technically and environmentally acceptable, economically competitive and easily available. It has received much attention in the past decade due to its ability to replace fossil fuels, which are likely to run out within a century. Especially, the environmental issues concerned with the exhaust gases emission by the usage of fossil fuels also encourage the usage of biodiesel, which has proved to be eco- friendly far more than fossil fuels. It can be produced from various vegetable oils, waste cooking oils and animal fats via transesterification process together with methanol as well as KOH as a catalyst. Compared to original diesel fuel, it is reported that biodiesel has greater levels of cetane number, density, viscosity, but at the same time the calorific values of biofuel is 10 to 12 percent less than petroleum diesel [3]. The largest producers of soybeans are: United States, Brazil, Argentina, China and India [4]. The findings of many investigations have emphasized that while a promising reduction in the carbon emissions but at the same the emissions of NOx is increased as compared with neat diesel [5-21]. Although biofuel has a higher cetane number than diesel fuel but the bulk modulus in addition to viscosity have deep impact on the behavior of combustion of the fuel. The higher bulk modulus has been observed to contribute higher NOx production than neat diesel. The compressibility difference could lead to advance the injection timing hence, a greater chance to form NOx is expected [22,23]. The major reasons behind NOx emission are the start of injection, the time of peak heat release and peak temperature [24]. Same findings is reported by [25–28]. This research work is aimed to conduct experiments on a diesel engine powered by neat diesel and different volumetric blends of (SME). The experimental investigation is followed by a computational combustion and emissions analysis of diesel engine is done by using the CFD software ANSYS FLUENT 13 2. Biodiesel Properties The soybean biodiesel under test is supplied from Intech energy systems Pvt. Ltd [29]. The production of soybean biofuel based on transesterification process. As seen in Table 1, all the properties of the biodiesel prepared meet the specifications required by ASTM D 6751-02 and EN14214. The schematic steps of biodiesel production is shown Fig.1 Figure 1 Steps of SME biodiesel production Table 1, Diesel and SME biodiesel [29] Property Diesel B20% SME B40% SME SME Chemical formula C13.77H23.44 C14.97H26.33O0..34 C16.07H28.94O0.68 C19H35O2 Density (kg/m3) 830 841 852 876 Viscosity (cst) 3.0 3.34 3.68 4.25 Calorific value (MJ/kg) 42.5 41.18 39.89 36.22 Flash point (oC) 76 86.8 97.6 130 Cetane number 48 48.69 49.37 51.3 3. Experimental Investigation The Kirloskar TAF-1, single cylinder, air cooled, variable compression ratio and direct injection diesel engine is used in this work. The technical specifications of the engine are given in Table 2. The actual photos of the engine and its attachments are shown in Figs.(2-3). Table 3 shows accuracy of smoke meter and gas analyzer as well along with their range of operation. Nomenclature B20 Blend contains 20 % SME and 80 % diesel [C] Soot concentration in the cylinder B40 Blend contains 40 % SME and 60 % diesel CN Fuel cetane number B100 Blend contains 100 % SME CO Carbon monoxide BP Brake power UHC Unburned hydrocarbons BSFC Brake specific fuel consumption NOx Oxides of nitrogen BSN Bosch smoke number P cylinder pressure BTDC Before top dead center PM Particulate matter 186 MOHAMED AL-DAWODYA, S.K. BHATTIB/AL-QADISIYAH JOURNAL FOR ENGINEERING SCIENCES 12 (2019) 184–192 Table 2, Specifications of engine Table 3, The operating ranges and accuracies Figure 2 The actual photo of the engine test Figure 3 The computer system attached to the test rig 4. Computation Model Analysis In the present work, a standard finite-volume CFD code, ANSYS FLUENT 13 which is frequently used in automotive engineering because of its superior meshing generation, pre- and post- processing, as well as the availability of user supports has been used to simulate the combustion process of variable compression ratio diesel engine working on SME-diesel blends. ANSYS FLUENT 13 solves the three-dimensional Navier-Stokes equations for a diesel engine. A choice of standard k- model is added to for describing turbulence effect. Sub-models are added for, ignition, turbulence-chemistry interactions and pollutant formation prediction. 4.1. Turbulence Model The standard k-  model is a semi-empirical model based on model transport equations. In the derivation of the k- model, the assumption is that the flow is fully turbulent, and the effects of molecular viscosity are negligible (Launder B. et al [30]). The turbulence kinetic energy, k and its rate of dissipation , are obtained from the following transport equations [31]:     kMbk jk t j i i SYGG x k x ku x k t                                 (1)               S k CGCG k C xx u xt bk j t j i i                             2 231 (2) 4.2. Combustion Model Seven combustion species are considered, fuel, N2, O2, CO, H2O, H2 and CO2. The time rate of change of the concentration of species i is given by (Yashuhiro K. et al [32])   cii i YY dt dY / *  (3) (4) The variable f can be considered as a coefficient that explains the effect of turbulence on combustion (Kong SC. et al [33])., hence f is zero , before combustion and equals one when combustion completes. The laminar timescale is derived from a one step kinetic reaction rate as [31]:      RTEOHCA nnl /exp 5.1 2 75.0 22 1     (5) The turbulent time scale is based on the eddy breakup concept and modeled as;  / 2 kC mt  (6) Cm2 model constant for mixing characteristics in the engine. The local mass fraction of each species Yi, is predicted through the solution of a convection–diffusion equation for the mth species i.e., (7) Engine Make Kirloskar TAF-1 Engine Type. 4-Stroke, Diesel Engine Number of Cylinder 1 Bore × stroke 87.5×110 mm Cylinder capacity 0.66 L Compression ratio Variable (15, 16, 17.5, 19) Rated power 4.4 kW , 1500 rpm Maximum torque 28 N.m ,1500 rpm Orifice diameter 0.15 mm Injection timing 20o BTDC Injection pressure 220 bar Emission Range Accuracy CO 0-10 % vol ± 0.2 % 2CO 0-20 % vol ± 1 % UHC 0-20000 ppm ±10 ppm 2O 0-22 % vol ± 0.2 % NOx 0-5000 ppm ±10 ppm Opacity 0-100 % ± 1 % tlc f      iiii i SRJY t Y      MOHAMED AL-DAWODYA, S.K. BHATTIB /AL-QADISIYAH JOURNAL FOR ENGINEERING SCIENCES 12 (2019) 184–192 187 4.3. NOx Model In this work thermal and prompt models have been used [30]. The mechanisms of thermal and prompt NOx, the NO species transport equation is just required: (8) 4.3.1. Thermal NOx Model The principal reactions governing the formation of thermal NOx from molecular nitrogen are as follows: NONNO  2 (9) NOOON  2 (10) NOHOHN  (11) The NO formation rate becomes [30];                                     OHkOk NOk OkNk NOkk NOk dt NOd ff r ff rr f 3,22, 1, 22,21, 2 2,1, 21, 1 1 .2 (12) 4.3.2. Prompt NOx Model The major contribution suggested as the prompt NOx source is from CH and CH2, is given by: NHCNNCH  2 (13) NHHCNNCH  22 (14) The prompt NOx formation rate can be approximately equal to the overall prompt NOx formation rate:        RTEa pr aeFUELNOkf dt NOd / 22 ..  (15) Where f -correction factor which incorporates the effect of fuel type, i.e., number of carbon atoms, and air to fuel ratio which is given by; 32 2.12322.230819.075.4   nf (16)   16 /104.6  a pr pRTk (17) 4.4. Soot Model The one-step model predicted by [32] is used in this analysis (18) soot R -net rate of soot generation, is the balance of soot formation formsoot R , and soot oxidation combsoot R , combsootformsootsoot RRR ,,  (19) The rate of soot formation is given by a simple empirical rate expression: RTEr fuelsformsoot epCR / ,    (20) 4.5. CFD MODEL 4.5.1. Model Geometry and Mesh The CFD analysis investigated for combustion and emissions of SME– diesel blends by replacing the percentage of SME blend. The blended fuel is injected at 220 bars. The inlet temperature of air is considered to be uniform at 300K. The combustion chamber was drawn using GAMBIT. The three dimensional model is based on was a virtual combustion chamber prototype at the end of compression stage. The valves and injector positioning can be understood with the help of Fig.4. Based on the varying compression ratio, the clearance volume changes but as the bore diameter remains the same, the height of the chamber has to be modified and new models have to be created for the analysis purpose. The height of the combustion chamber at the end of compression stroke is given for each compression ratio in Table 4. Application of the boundary conditions and meshing the combustion chamber are done using ICEM CFD by using a tetrahedral element. The mesh size for the model was considered as unity (1) .The total numbers of elements equal to 1157543 see Fig.5. The grid is refined in order to capture the boundary conditions specially in regions closer to nozzle. Figure 4 Positioning of nozzle and valves on cylinder head (dim. in mm) Figure 5 Computational model-grid mesh combustion chamber       NONONONO SYDYY t      ..     sootsoot soot t sootsoot RYYY t                 .. 188 MOHAMED AL-DAWODYA, S.K. BHATTIB/AL-QADISIYAH JOURNAL FOR ENGINEERING SCIENCES 12 (2019) 184–192 Table 4, Height of the chamber at different compression ratio Compression ratio Clearance volume (mm3) Height (mm) 15 47246.608 7.857 16 44096.834 7.333 17.5 40088.031 6.666 19 36747.361 6.111 5. Results and Discussion Fig. 6 presents the pressure of cylinder verse crank angle for neat diesel and biodiesel. The cylinder pressure for biodiesel is lower than diesel by 2.98% due to the difference in the heating value for biodiesel. It is noted that the peak pressure for B100 SME is 78.246 bars at 6o crank angle which comes neat to TDC than ordinary diesel where the maximum pressure was 80.65 bars at 9o crank angle. Fig. 7 shows the full load rate of heat release verse degree crank angle. the value of heat release rate is decreased when biodiesel blends increased. The message from this figure is that the biodiesel blend had an earlier start of combustion. The same observations are reported by [34-36]. Figure 6 Variation of cylinder pressure with crank angle Figure 7 Variation of heat release rate with crank angle BSFC results are presented in Fig.8 for different engine loads. The BSFC decreased as the engine load increased. This is because, the rate of increasing power is greater than consumption of fuel. While BSFC was calculated as 0.537 kg/kW h with SME B20 at 1.1 kW load; it was found as 0.275 kg/kW h at 4.4 kW. The BSFC is increased with the increasing SME percentage due to the higher viscosity and density with lower heating value of biodiesel compared to diesel fuel. At full load, BSFC for SME B20, SME B40, and SME B100 increased by 4.24%, 8.71%, and 14.66% respectively. Fig. 9 displays BTE verse load. From the above discussion, it is concluded that BSFC is increased as the substitution of biodiesel is increased., BTE depends on BSFC and heat quantity as well. In general BTE is reduced as the blend ratio of SME is increased . The result of UHC emissions are illustrated in Fig. 10 for different loads. Typically, (Sayin C. et al [37]). While UHC emission was measured to be 18 ppm with diesel fuel at 0 kW load; it was 62 ppm at 4.4 kW. In Fig. 10 UHC emissions is decreasing with increase the substitution of .biodiesel. This belong to enough quantity of oxygen helps for complete combustion. UHC emissions for SME B20, SME B40, as well as SME B100 decreased by 15%, 27%, and 38.4%. Generally, CO emissions are affected by air–fuel equivalence ratio, fuel type, combustion chamber design, and atomization rate, start of injection timing, injection pressure, compression ratio, engine load and speed. Figure 8 Variation of BSFC with load Figure 9 Variation of BTE with load CO emission results are presented in Fig.11 for different engine loads. Lower CO emission is noticeable at part load while the higher one is noticed at full load. As the load increased, there is an increase in CO emissions and this is typical with all internal combustion engines since air–fuel ratio (A/F) decreases as the load increased (Ramadhas A. et al [38]). While CO emission was measured as 0.054 (% vol) with B20 SME at 0 kW load; it was 0.195 (%vol) at 4.4 kW. CO emission is decreased when biodiesel percentage is increased. The oxygenated nature of biodiesel is responsible for that (Gumus M. [39]). For all engine loads, CO emissions for SME B20, -30 0 30 60 90 Crank angle (degree) -10 0 10 20 30 40 50 H e a t R e le a s e R a te ( k J /m 3 .d e g re e ) 1.1 2.2 3.3 4.4 Brake power (kW) 0.2 0.3 0.4 0.5 0.6 0.7 0.8 B S F C ( k g /k W .h ) 0.0 1.1 2.2 3.3 4.4 Brake power (kW) 0 4 8 12 16 20 24 28 32 36 B ra k e T h e rm a l E ff ic ie n c y ( % ) MOHAMED AL-DAWODYA, S.K. BHATTIB /AL-QADISIYAH JOURNAL FOR ENGINEERING SCIENCES 12 (2019) 184–192 189 SME B40, and SME B100 reduced by 11.36%, 29.1%, and 41.7% compared to that of diesel fuel, respectively. The reduction of NOx emissions are always the target for engine researchers and manufacturers. Three conditions which favor NOx formation are: higher combustion temperature, more oxygen content and faster reaction rate (Hiroyasu et al [40]). Figure 10 Variation of UHC emissions with load Figure 11 Variation of CO emissions with load NOx emissions results are presented in Fig.12 for different engine loads. The NOx emissions increased as the engine load increased, due to the increase in combustion temperature. The emission of NOx increased with the increasing biodiesel blend ratio. The oxygenated nature of SME biodiesel increases NOx emission. Same results is highlighted by (Ozsezen A. et al [41]) and (Lapuerta M. et al [42]). Figure 12 Variation of NOx emissions with load The results of smoke emission were depicted in Fig. 13 for different engine loads. The smoke emission increased with the increase in the brake power. The formation of smoke strongly depends on the engine load. As the load increases, more fuel is injected, and this increases smoke formation (Metin G. et al [43]). While Bosch smoke number (BSN) was measured to be 0.425 with diesel fuel at 4.4 kW, it was 0.3381 for SME B20% at 4.4 kW for 17.5 optimum compression ratio. Figure 13 Variation of BSN with load As illustrated in Fig. 13, the smoke starts to decrease as the percentage of biodiesel increased in the fuel blend. It is mentioned that smoke decreases with high oxygen content in the biodiesel. For all engine loads, the BSN for SME B20, SME B40, and SME decreased by 20.44%, 35.78%, and 48.31%, compared to those of diesel, respectively. Same observations are noted in the result of (Monyem A.[44]). Fig. 14(a-d) shows pressure, velocity, turbulent kinetic energy and dissipation rate contours for B20 SME respectively. Theoretically the pressure is maximum at the core and decreases towards the periphery. But when flame fronts strike the cylinder walls, negative flame fronts are formed resulting in formation of localized high pressure areas near the cylinder walls as shown in Fig. 14(a). Also it is observed that combustion pressure decreases with increase in SME blends. The maximum pressure for neat SME is less than that of diesel by 4.6 % because of the heating values reduction for blended fuel. The base of the combustion chamber is visible in the velocity contour plots. It is evident from Fig.14 (b) that the combustion velocity is higher near the fuel injector and decreases towards the cylinder walls. As the blending ratio of SME increased, the velocity decreased gradually because of higher viscosity and density of soybean biodiesel with respect to diesel fuel. The reduction in the combustion velocity is up to 6.72 % as the blending ratio of SME increased from (0-100) %. Fuel-air mixing and combustion in reciprocating internal combustion engines are highly dependent on the in-cylinder turbulent flow. High turbulence results in improved mixing but the very high values of turbulence can cause knocking. Turbulence in the engine produces kinetic energy also known as turbulent kinetic energy (T.K.E). As seen in Fig.14(c) the T.K.E. maximum at the nozzle region and gradually decreases away from nozzle. T.K.E values reduced as a result of increase in the blend of soybean due to higher viscosity and density values of the blended fuel as compared to conventional diesel fuel. The average T.K.E is reduced by 4% 20% SME, 7.44% for 40% SME and 10.84 neat 100% biodiesel . 0.0 1.1 2.2 3.3 4.4 Brake power (kW) 0 20 40 60 80 U H C ( p p m ) 0.0 1.1 2.2 3.3 4.4 Brake power (kW) 0.00 0.10 0.20 0.30 C O ( v o l % ) 0.0 1.1 2.2 3.3 4.4 Brake power (kW) 0.00 0.10 0.20 0.30 0.40 0.50 0.60 B o s c h s m o k e n u m b e r 0.0 1.1 2.2 3.3 4.4 Brake powre (kW) 0 500 1000 1500 2000 2500 3000 N O x ( p p m ) Experimental results 1500 rpm, 17.5 CR Diesel SME B20 SME B40 SME B100 190 MOHAMED AL-DAWODYA, S.K. BHATTIB/AL-QADISIYAH JOURNAL FOR ENGINEERING SCIENCES 12 (2019) 184–192 Figure 14(a) Pressure contour for B20 SME velocity Figure 14(b) Velocity contour for B20 SME Figure 14(c) Turbulent kinetic energy contour for B20 SME Figure 14(d) Turbulent dispassion rate contour for B20 SME It can be observed from Fig. 14(d) that turbulent dissipation rate (T.D.R) increases in the nozzle region and it highly depends on the T.K.E. It promotes the rate of combustion. By increasing the percentage of SME blends causes a reduction in the rate of dissipation energy by 2 %. Fig. 15(a-b) shows NOx and soot mass fraction contours for B20 SME respectively. It is noticed from Fig. 15(a) that peak NOx value increases near the wall due to the formation of localized high temperature areas near the combustion chamber walls. The NOx emissions for soybean biodiesel blends are higher than that of base line diesel fuel. This is related to the higher combustion temperature and higher oxygen content in the biodiesel as compared to diesel fuel which in turn enhanced NOx formation. Fig. 15(b) shows the soot mass fractions for B20 SME. Soot levels for B20, B40, and B100 are less than diesel by 21.61 %, 36.33 %, and 41.41 % respectively. It is mentioned early that lower C/H ratio together with high oxygen content of biodiesel are the main reasons of reducing soot formation Figure 15(a) NOx emissions for B20 SME Figure 15(b) Soot emissions contour for B20 SME 6. Verification The experimental investigation is compared with CFD analysis as shown in Table 5. The comparison between the two was found in good agreement with a slight difference. MOHAMED AL-DAWODYA, S.K. BHATTIB /AL-QADISIYAH JOURNAL FOR ENGINEERING SCIENCES 12 (2019) 184–192 191 Table 5, Verification of results between diesel and B20 SME Fuel Δ % Peak Pres. Δ % Peak Temp Δ % Veloc ity Δ % T.K.E Δ % T. D.R Δ % NOx Δ % Soot Diesel - - - - - - - B20SME -1.31 +2.27 -1.53 -4 +0.21 + 6 -21.61 B40SME -3.5 +4.50 -3.82 -7.44 -1.22 +14.15 -36.3 SME -4.6 +5.86 -7.61 -10.8 -2 +27.77 -41.41 7. 7. Conclusions 7.1. Conclusions from the Experimental Investigation 1- Increasing load reported remarkable increase in pressure, brake efficiency and smoke while UHC as well as BSFC is decreased. 2- Earlier combustion start for all biodiesel blends. 3- BTE is slightly reduced and fuel consumption is increased when SME percentage increased 4- UHC is reduced with B100 % SME by 38.4% as well as the emission of CO is decreased with B100 % SME 41.7% respectively. 5- The measured BSN for SME blends was less than that of neat diesel. 6- In terms of compression ratio, the test results demonstrated that, BSFC increased with the increased compression ratio. 7.2. 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