Table 1. Engine Specifications
The present work describes a theoretical investigation concerning the performance of a four strokes compression ignition engine, which is powered by alternative fuels in the form of diesel and biodiesel blends. The developed simulation model used to estimate the cylinder pressure, heat release rate, brake thermal efficiency, brake specific fuel consumption and engine out emissions. The simulation model includes Honerberg's equation heat transfer model, Zero dimensional combustion model for the prediction of combustion parameters and NO formation during combustion process using Zeldovich mechanism.Experiments were performed in a single cylinder DI diesel engine fuelled with a blend of pungam methyl ester for the proportion of 10%, 20% and 30% by volume with diesel fuel for validation of simulated results. It was observed that there is a good agreement between simulated and experimental results which reveals the fact that the simulation model developed predicts the performance and emission characteristics of any biodiesel and diesel fuel and engine specifications given as input.
BTHE : brake thermal efficiency
BSFC : brake specific fuel consumption
HRR : heat release rate
PME10 : blend of biodiesel and diesel having 10% biodiesel and remaining diesel
PME20 : blend of biodiesel and diesel having 20% biodiesel and remaining diesel
PME30 : blend of biodiesel and diesel having 30% biodiesel and remaining diesel
There is intense research efforts that are being carried out to identify the potential biodiesel to develop less polluting and more efficient engines. The numerical simulation was preferred to study the combustion behavior and emission of diesel engine because the experimental investigations were time consuming and costly affair. A computer based mathematical model have been proposed in the present work to study and understand the combustion and performance. The various models proposed are one zone and multi zone models.
In the present work the analysis of progressive combustion using zero-dimensional single-zone model is used and assumed the entire combustion chamber is considered as single zone.
In the final step using simplified assumptions, the engine heat transfer can be taken into account by means of empirical equations. Gas exchange processes are introduced in order to analyze the engine intake and exhaust. This analysis is called Actual Cycle Simulation.In this analysis the intake and exhaust processes using fluid dynamic equations, heat transfer and friction are included by means of empirical equations.By means of this step-by-step simulation, the values of pressure, volume and temperature at salient points in the cycle are calculated. Work output as well as thermal efficiency is evaluated. Many researchers have investigated the effects of oxygenated compounds on performance and emissions in a diesel engine [1] S.M. Jameel Basha, K. Raja opal In this paper, an attempt has been made to study the combustion processes in a compression ignition engine and simulation was done using computational fluid dynamic (CFD) code FLUENT. An Axi symmetric turbulent combustion flow with heat transfer is to be modeled for a flat piston 4-stroke diesel engine. The unsteady compressible conservation equations for mass (Continuity), axial and radial momentum, energy, species concentration equations can express the flow field and combustion in axisymmetric engine cylinder. Modeling with variation of initial swirl ratios along with combustion was analyzed in formulating and developing a model for combustion process. Benson [2] is the first person who introduces computers to simulate the flow through engine gas exchange system. John B.Heywood [3] analyzed Heat release rate based on first law of thermodynamics and state equation. He proposed to use more sophisticated models for the gas properties before, during and after combustion with accurate heat transfer model. Many researchers have investigated the effects of biodiesel on performance and emissions in a diesel engine [4-5]. S. Sundarapandian & G. Devaradjane [6]. A theoretical model was developed to evaluate the performance, characteristics and combustion parameters of vegetable oil esters like Jatropha, Mahua and Neem Oil esters. From the investigation, it is concluded that performance of vegetable oil esters such as Jatropha, Mahua and Neem oil esters are in good agreement with diesel performance at optimum engine conditions. Thus the developed model is highly compatible for simulation work with bio diesel as an alternative fuel [7]. Bolem Siva Nageswara Rao1 & V Ganesan the investigation has been carried out using the software STAR-CD. The effect of piston bowl shapes (bath tub, shallow and flat), compression ratios (11.2, 12 and 12.5), different intake port geometries (helical, tangential and straight) and engine speeds (1500, 2000, 2500 and 3000 rpm) on the turbulence kinetic energy and swirl ratio has been studied. From the analysis of the flow field, shallow bowl seems to be preferable from the point of swirl ratio at lower compression ratio, but at higher compression ratio flat bowl shape is preferable [8]. Joginder Singh Kaliravna1, Mangesh Nimbalkar1, Narendra Kumar Jain1 & V.Ganesan through statistical tools it is easy to interpret that what were the values of certain parameters when customers rated that particular engine as good, average or bad. Some of these tools used here are standard deviation (SD), coefficient of variance (COV) and lowest normalized value (LVN). The standard deviation (SD) and lowest normalized value (LNV) of indicative mean effective pressure (IMEP) are determined through the analysis of in-cylinder pressure data of the engine. It has been observed that with second camshaft value of SD and LNV of IMEP is improved significantly which in turn shows improved combustion stability at idling in both fuel modes. [9-10]. Based on the model description a computer program is developed using C program for the diesel cycle simulation to predict the cylinder pressure for the complete cycle and performance parameters can be effectively evaluated. A CFD studies on combustion and emission of diesel engine using Fluent is required to evaluate the emission data. The present work aims to develop a single zone zero dimensional model for DI diesel engines running with biodiesel to predict the engine performance and emission parameters. An experimental work is also conducted in a 4-stroke single cylinder compression ignition engine for comparison with simulated results. It is revealed from the results that the simulation model developed for biodiesel is in very good agreement with experimental results.
This model is based on the first law of thermodynamics is applied. For a closed system, the first law of thermodynamics states that dQ - dW = dE
Where dQ is the heat loss to the chamber walls, dW is the network energy and dE is the change in internal energy.
The state equation is applied for unburned zone consists of pure air.
pV = M Rmol T
where M is the number of kg-mols and V is the Cylinder volume. The volume at each crank angle Vca is calculated using the relation
V(THETA)=vdisp*[cr/ (cr-1)-(1-cosӨ)/2+l/s-1/2*√[(2l/s)^2-sin2Ө]
The fuel injection rate is calculated using the relation,
mfi = Cd (π/4) dn2 (Δ2Pn/f)0.5
Where Pδn is the pressure difference across the injector hole.
Heat transfer coefficient: The heat transfer coefficient between cylinder gases and wall is calculated by Hohenberg's correlation given as:
H1=pow(69626.179688*0.001/1000000,-0.06);
H2=PATM*(VPHT+1.4);
H3=pow(H2,0.8);
H4=pow(TEP,-0.4);
HTC=0.13*H1*H3*H4;
Heat release rate is calculated using wiebe heat release model(zero dimensional combustion model) is based on the exponential rate of the chemical reactions. In this model, it is assumed that all the fuel is injected is injected before the end of ignition delay period itself.
ROHR (dQc/dӨ) = a*(m+1)*(Qav/COMBDUR)^m*[(Ө-SOC)/COMBDUR]^m*exp [-a*(Ө-SOC)/COMBDUR] ^(m+1)]
where: a=6.908; m=2; Qav=QP2;
(Energy Release At Constant Pressure)
QP2= (hrp1+N1*282800)*sf2
Ignition delay time can be calculated as the difference between the time at which combustion starts and the time at which injection starts. Ignition delay is a complicated function of mixture temperature, pressure and equivalence ratio and fuel properties. The correlation proposed by wolfer's relation is used in the model.
Ignition delay = [0.44*exp (4650/AT2)] / (AP2^1.19)
The duration of combustion varies according to the empirical expression given by
Combustion duration = 40+5*(RPM/600-1)+166*(ycc/y-1.1)^2
To calculate the formation of nitric oxide inside each zone, a chemical equilibrium scheme is used to calculate the concentration of various components under equilibrium conditions.
In the present work extended zeldovich mechanism is used for calculation of NO formation. NOx formation is strong dependant on temperature and oxygen concentration.
d(NO)/dt= 6*106/T0.5 exp(-69090/T)[O2]e0.5[N2]e
The equilibrium of oxygen and nitrogen concentration in the burned zone are given by
[O2]e = fO2(P,T) [N2]e= fN2(P,T)
Specification are given in Table 1. A Electrical dynamometer was used to apply the load on the engine. A water rheostat with an adjustable depth of immersion electrode was provided to dissipate the power generated.Tests were carried out at various loads starting from no load to full load condition at a constant rated speed of 1500 rpm. At each load, the fuel flow rate various constituents of exhaust gases such as Hydrocarbon (HC), carbon monoxide (CO) and nitrogen oxides (NOx), were measured with a 5-gas MRU Delta exhaust gas analyzer. The analyzer uses the principle of non-dispersive infrared (NDIR) for the measurement of CO and HC emissions while NOx measurement was by means of electrochemical sensors.
Combustion analysis was carried out by means of an AVL pressure pick-up fitted on the cylinder head and a TDC encoder fixed on the output shaft of the engine. The pressure and the crank angle signals were fed to pentium personal computer. Various combustion parameters like heat release rate, cumulative heat release rate and peak pressure and its accurance were obtained using data acquisition system. The properties of the fuel at various volumetric proportions as specified in Table 2. The engine was first operated with diesel oil to generate the baseline data followed by pungam methyl esters and their blends such as PME10, PME20 and PME30 blends.
Table 1. Engine Specifications
The simulation model is capable of predicting various performance, combustion and emission parameters based on the any engine and any fuel specifications given as input .The developed simulation model can be useful for both diesel and any biodiesel blends. It serves as a versatile tool for a better understanding of the variables involved and their effect on engine performance and also it helps in optimizing the engine design for a particular application, thereby reducing cost and time. Experimental investigation were carried out at different compression ratio, injection timing and injection pressure and its details are mentioned in Table 3. At injector opening pressure of 200bar and 220bar and injection timing of 21°bTDC and 24°bTDC and compression ratio of 17.5:1 and 16:1 were tried for PME10,PME20 and PME30 but from the investigation it was found that the performance was very poor. Further the engine were set to run at higher compression ratio of 19:1,advanced injection timing of 27°bTDC and higher injector opening pressure of 240bar it arrives at the optimum range operating parameters for PME20. It was observed that PME20 it gives better performance for the optimized operating parameters. Finally the theoretical results of cylinder pressure, brake power, thermal efficiency, Specific fuel consumption, and harmful pollutants such as nitric oxide, carbon monoxide, and hydrocarbon are validated with experimental results. The predicted results of performance and emissions have been in close agreement with experimental results. The developed simulation model appears to be a useful tool for analyzing the diesel engine combustion accurately.
Figure 1 shows that the comparison of experimental and predicted cylinder pressure for three different biodiesel blends and diesel fuel with respect to crank angle for various engine operating conditions. The agreement between the predicted values and experimental values of cylinder pressure are satisfactory.
Figure 2 shows the comparison of experimental and predicted heat release rates for diesel and biodiesel blends. From the results it was observed that the heat release rate increases during premixed combustion and decreases during diffusion combustion. The increase in heat release rate is due to the decrease in delay period. As revealed by the figure the combustion model predicts very well the heat release rates in the engine cylinder.
Figure 3 shows the comparison of experimental and predicted brake thermal efficiency for three different biodiesel blends with respect to brake power. The predicted values of brake thermal efficiency for all biodiesel blends are similar experimental brake thermal efficiency.
Figure 4 shows the brake specific fuel consumption predicted by the simulation model and the experimental one. It is seen that the fuel consumption of biodiesel blends is lower than that of diesel fuel operation which is due to better combustion in the cylinder. Due to the better consumption of the fuel BSFC have moderate improvement. The coincidence between the predicted and measured values is good.
Figure 5 shows the calculated and measured CO (in %) with respect to the loads. CO emissions are greatly dependant on the air fuel ratio relative to stoichiometric proportions. It is seen from the figure that the full load emission of CO for diesel is 0.49% and that of biodiesel blend with diesel is 0.2% predicted by the simulation model. The experimental values are 0.51% for diesel and 0.48% for DGM blends with diesel at full load. Due to better combustion of biodiesel blends with diesel, CO emissions present in the exhaust are reduced.
Figure 6 shows the predicted and measured NOx (in ppm) for various loads. The predicted NOx values are slightly higher than the measured ones. The maximum value of NOx is 1245 ppm reached at full load for the DGM in experiment due to high temperatures during combustion. NOx emission is 1115 ppm at full load with diesel and it is increased to 1238 ppm with the addition of diesel- biodiesel blend at full load predicted by the simulation model. The coincidence of simulation and experimental values are found to be good agreement.
Figure 7 shows the comparison of HC with respect to the load predicted by the simulation model and the experimental one. Biodiesel blend addition decreases the HC values with respect to increase of. Hydrocarbon emission is decreased by about 3% predicted by the simulation model. Diesel fuel contains chain and substituted types of aromatic compounds, burns more slowly and produces higher amounts of unburnt load as shown hydrocarbons. The HC emissions of biodiesel blends is reduced due to better combustion of the fuel.
Table 2. Properties of the Different Fuel Blends
Table 3. Operating Parameters Considered in the Present Investigations
Figure 1. Comparison of experimental and predicted cylinder pressure for three different biodiesel blends with respect to crank angle
Figure 2. Comparison of experimental and predicted heat release rate for three different biodiesel blends with respect to crank angle
Figure 3. Comparison of experimental and predicted brake thermal efficiency for three different biodiesel blends with respect to brake power
Figure 4. Comparison of experimental and predicted brake specific fuel consumption for three different biodiesel blends with respect to brake power
Figure 5. Comparison of experimental and predicted carbon monoxide for three different biodiesel blends with respect to brake power
Figure 6. Comparison of experimental and predicted nitric oxide for three different biodiesel blends with respec to brake power
Figure 7. Comparison of experimental and predicted hydrocarbon for three different biodiesel with respect to brake power
In the present work a single zone zero dimensional model for direct injection diesel engine has been developed to predict the performance characteristics.
The predicted results of performance and emissions have been in close agreement with experimental results. The developed simulation model appears to be a useful tool for analyzing the diesel engine combustion accurately.
Thus the developed computer model can predict the various performance and emission parameters of any vegetable oilesters with minimum inputs such as density, calorific value, chemical formula and engine specification.
Moreover the developed model can be considered as an efficient tool to calculate the effect of engine operating parameters such as compression ratio, injection timing and injection pressure.
Finally it is concluded that the predictions revealed fair agreement with experimental results. Hence the developed simulation C-programming model is validated of any given vegetable oil esters at various operating parameters of diesel engine.