Theoretical Analysis of Combustion, Performance and Nox Emission Characteristics of Biodiesel in Compression-Ignition Engine

Transcription

1 Theoretical Analysis of Combustion, Performance and Nox Emission Characteristics of Biodiesel in Compression-Ignition Engine Omodolu T. Mustapha, Christopher C. Enweremadu, and Hilary L. Rutto* Abstract-- In this study, a theoretical model was developed to compute the combustion and performance characteristics of diesel and biodiesel from canola and sunflower oils. The chemical properties of the biodiesel samples were computed using Omega Software. The developed model used the chemical properties of the fuels to compute the desired combustion and performance characteristics. Results showed that diesel has a higher peak pressure and temperature at lower crank angles compared to the biodiesel fuel samples. At higher crank angles starting from about 170 degrees, the peak pressures and temperatures were similar. Diesel has a higher brake power than the biodiesel samples throughout the varied engine speed. The NOx emission of diesel was found to be lower than that of the biodiesel samples at lower crank angles. At higher crank angles of about 170 degrees, the NOx emission of diesel becomes higher than that of the biodiesel samples. Keywords--Biodiesel, Compression-ignition engine, Combustion, Crank angle, Performance, NOx emission I. INTRODUCTION ORLD energy demand has been on the increase partly Wdue to growing industrialization and increase in the number of transportation vehicles. However, about 85% of the energy resources used in the world comes from fossil fuels [1]. It is believed that the supply of fossil fuels at their current rate of use might not last long. Fossil fuels have become a matter of great concern today due to the economic pressures and global warming caused by their usage in power generation and transportation. Prime fossil fuel disadvantages such as crude oil include its high cost and tendency towards depletion. This has led to research into alternative fuels that will be abundant, maximize performance at relatively low cost and be environment-friendly. Biofuels such as vegetable oil has been Omodolu T. Mustapha is with the Department of Mechanical Engineering, Vaal University of Technology, Private Bag X021, Vanderbijlpark 1900, South Africa. Christopher C. Enweremadu is with the Department of Mechanical & Industrial Engineering, University of South Africa, UNISA Science Campus, Florida, Private Bag X6, Florida 1710, South Africa. Hilary L. Rutto* is with the Department of Chemical Engineering, Vaal University of Technology, Private Bag X021, Vanderbijlpark 1900, South Africa. *Corresponding author: Tel.: ; address: hilaryr@vut.ac.za identified as one of these alternative fuels. Neat vegetable oils can be used directly in the internal combustion engines but are not efficient [2]. They possess high viscosities and low volatilities which result in plugging of intricate engine parts, and also distort injector pump fuel spray and nozzle fuel atomization. This leads to incomplete combustion and huge carbon deposits in internal combustion engines. These problems with vegetable oils are associated with their large triglyceride molecules and high molecular mass [3]. To overcome these challenges, vegetable oils have been made to undergo various processes such as; blending with diesel, emulsification (thermal cracking), pyrolysis and transesterification. Transesterification produces a cleaner and better environment-safe fuel known as biodiesel from vegetable oils [2]. Biodiesel is fast becoming an alternative to petroleum diesel due to its ability to reduce pollutant emissions in diesel engines. Therefore, understanding the aspects of biodiesel combustion is important. Diesel engines occupy a prominent role in the present transportation and power generation sectors. Hence, current and future legislation on emissions require engine developers to produce cleaner and more efficient power plant systems. There have been intense research efforts to identify the potential biodiesel to develop less polluting and more efficient engines. One of such is by adopting some modifications to the combustion process. The development of computer technology has encouraged the use of simulation techniques to quantify the effect of the fundamental processes in the engine systems. Diesel engine simulation models have been used for a better understanding of the engine performance, combustion and emission characteristics thereby reducing experimental investigations which are time consuming and costly. Many models, simple and complex; zero dimensional, single zone and multi-zone, use of single and double Wiebe s function, use of Wolfer s relation, etc. have been developed [4-10]. In this study, an attempt has been made to develop a simple thermodynamic model that can predict the engine combustion and performance characteristics with different fuels. Engine performance, emission parameters obtained from the computer program has been compared to validate the experimental results. The model has been used to predict and analyze the cylinder pressure, cylinder temperature, brake power and nitric 238

2 oxide emission characteristics of a diesel engine fuelled with diesel and biodiesel from sunflower and canola respectively. II. THEORETICAL CONSIDERATIONS AND MATHEMATICAL MODELING The approach used to simulating the diesel engine is to evaluate the power output and thermal efficiency for a given speed, ambient air temperature, fuel and air-fuel ratio. To do this, the pressure (p), volume (V), temperature (T) at points such as: intake and exhaust processes (Gas exchange process) 0-1 and 4-1 respectively, compression process 1-2, combustion process 2-3 and expansion process 3-4 as shown in Fig. 1, are expressed numerically by equations which are used in the simulation. A. The Gas Exchange Process A control volume analysis is used to represent the gas exchange processes. The exhaust stroke is represented by (1). (1) where, R = (kj kmol -1 K -1 ); N x is the kilomoles of residual exhaust from the previous cycle in the engine; N a is the kilomoles of air taken in during suction; is the polytropic index of compression; is the specific heat capacity at constant pressure (kj kmol -1 K -1 ) for the reactant N a + N x Since T 1 and r (pressure ratio) are known, then (6) (7) (8) Similarly, the expansion process 3-4 is isentropic. From combustion stroke, the values of T 3, p 3 and V 3 can be calculated. Hence, (9), (10) Also, the intake stroke is represented by (2). Where:, (11) (12) C. The Combustion Process Fig. 1 Diesel Cycle (2) The combustion process is a thermodynamic energyconservation-based model. It is a zero-dimensional (since in the absence of any flow modeling, geometric features of the fluid motion cannot be predicted), progressive combustion model [11]. The zero-dimensional model is based on the first law of thermodynamics. The starting point for the combustion process is chosen as the point where the inlet valve closes. From the energy equation for a closed cycle period: Where (13) is the rate of change of internal energy, Both equations require mass flow rates as shown in (3). B. The Compression and Expansion Processes The compression process 1-2 is analyzed as isentropic. In this case, N x = 0 and T 1 = T a (3) (4) is the rate of heat release, is the rate of heat transfer, is the rate of work transfer. Equation 13 may be rewritten as: Therefore, (14) (5) This relates the temperature to the crank angle at every point and is solved numerically by Runge-Kutta method. The pressure at each crank angle can also be determined from: 239

3 mrt p = v The ignition delay period is obtained by integrating the Wolfer s relation: t ign t ign dt = 1 dt = 1 (15) t(p, T) c 0 q [ E / RT(t) ] { p(t) } exp 0 Where: t is time in seconds; p(t) and T(t) are assumed to vary during compression. D. Heat Release Model Wiebe s heat release pattern is used to model the engine heat release rate. The rate of heat released can be expressed as:. Q = a Where: ( m + 1) m m+1 θ θ 1 θ θ1 exp a (16) θ c θ c is the rate of heat release; a is the parameter representing completeness of combustion; m is the parameter representing the rate of combustion; is the crank angle at any instant and at the start of combustion respectively. E. Heat Transfer Model Heat is transferred to the working fluid at every stage of the cycle; the net work done by the working fluid in one complete cycle is given by: p W = p + V (17) net 2 The pressure change as a result of heat transfer is given by: p hc A( Tw T ) = T p Mc T v (18) where, is the heat transfer coefficient; A is the interior surface area of engine volume; is the interior surface temperature; M is the mass of working fluid; is the working fluid specific heat; is the working fluid temperature. The heat transfer coefficient is calculated using Annand s empirical formula [12]. III. NITROGEN OXIDES EMISSION SIMULATION The formation and destructive reactions of nitrogen oxides in the engine cylinder, unlike other pollutants such as: carbon monoxides, organic compounds and particulates, are not coupled to the primary fuel combustion [13]. The formation of nitrogen oxides is mainly dependent on the cylinder combustion temperature, oxygen concentration and nitrogen concentration. Nitric oxide (NO) and nitrogen dioxide (NO 2 ) are usually grouped together as NOx emissions and NO is the predominant oxide of nitrogen produced [13]. The rate of NO 2 molecules formation is not calculated separately from NO. NO 2 is formed from NO and therefore, the number of moles of NOx can be represented by the number of moles of NO calculated [14]. The mechanism of NO formation has been explained by Zeldovich and based on this mechanism; various equations have been obtained to predict the rate of NO formation [13]. One of such equations is used to predict the rate of NOx formation for the diesel and biodiesel samples. 16 d[no] 6x ,090 = [O ] [N ] exp (19) e 2 e dt T T where, [] e is equilibrium molar concentration IV. METHODOLOGY A. Simulation A thermodynamic model was developed from the first law of thermodynamics and theoretical considerations. The molecular formula of diesel fuel is taken as C 10 H 22. The various properties of the test fuel samples such as their molecular structures, molecular weights and the heat of combustion (see Table I) were determined using Omega Version 1.5 (Petroprogram Company Finland, 2010).Omega is a software program that can be used to determine the physical and chemical properties of numerous fuels. A computer program was developed using Visual C++ for the numerical solution of the equations used in the thermodynamic model. The combustion model used in the software is a progressive combustion, zero-dimensional model. The output values from omega were used in the developed Visual C++ program to generate values for the performance and combustion characteristics of the fuels. For NOx, the maximum temperatures obtained for diesel and biodiesel from the diesel engine simulation (see Table II for engine specifications) was used to generate values used in (19). The stoichiometric combustion equation for diesel and biodiesel was written. Using the JANAF table for each of the combustion products at the maximum temperature produced, and the MATHCAD FIND function, the equilibrium molar concentration of the combustion products were determined theoretically. V. RESULTS AND DISCUSSION The combustion, performance and emission characteristics of diesel and biodiesel fuels were predicted theoretically. The results are presented in Figs

4 Biodiesel 100% Sunflower Average Molecular Weight (g/mol) Carbon TABLE I BIODIESEL CHEMICAL PROPERTIES USING OMEGA SOFTWARE Hydrogen SIMULATED PROPERTIES Oxygen Heating Value (Net) (KJ/Kg) Heating Value (Net) (KJ/Kmol) Carbon Hydrogen Oxygen % Canola Model TABLE II SPECIFICATIONS FOR MERCEDES BENZ OM 364A DIESEL ENGINE ENGINE SPECIFICATIONS Number of cylinders 4 Cylinder bore Piston stroke Vertical, in-line with exhaust gas turbocharger 97.5 mm 133 mm Connecting rod length 230 mm Total piston displacement 3972 cm 2 Compression (dead) space per cylinder 64 cm 2 Compression ratio 16.5:1 Cut-off ratio 3.75 Crank angle for fuel injection Automotive rating Engine gross approximate weight 415 Peak Cylinder Pressure (BAR) Cylinder Pressure (BAR) 87 kw at 20 r/min Fig. 2 Cylinder pressure and Peak Cylinder pressure vs crank angle for the different fuels A. Combustion Characteristics Fig. 2 shows the variation of the cylinder pressure with crank angle at 10 RPM. The cylinder pressure here is the pressure at point 2 which is the end of the compression stroke as shown in Fig. 2. It can be seen that the highest pressure is about bar for the engine cylinders. The three tested fuels have very similar behaviors. The physical properties of the fuel such as density and viscosity influence the cylinder pressure at the end of compression. Since transesterification has brought the density and viscosity of biodiesel close to that of diesel, it might have resulted in the similar nature of the curves. The variation of the peak cylinder pressure with crank angle at 10 RPM is also presented in Fig. 2. The peak cylinder pressure is the pressure at point 3 at the end of the combustion stroke (see Fig. 1). It can be seen from Fig. 2 that sunflower and canola biodiesel have very similar behaviors. Diesel has a higher peak pressure than the biodiesel fuels up to a crank angle of 170 degrees where all three fuels behave similarly. Biodiesel has a lower calorific value and cetane number compared with petroleum diesel. These may be responsible for its lower peak pressure during combustion at lower crank angles. The physical properties of biodiesel such as density and viscosity are improved during transesterification which improve its flow and cause earlier combustion compared with diesel. The presence of oxygen in biodiesel results in its more complete and longer combustion period [15]. At higher crank angles, the ignition delay of diesel fuel is shortened. The shorter combustion period of petroleum diesel and longer combustion period of biodiesel may be responsible for the similar nature of the curve at higher crank angles [16]. Cylinder Temperature (K) Fig. 3 Brake Power (KW) Engine Speed (RPM) Cylinder temperature vs crank angle and Brake power vs engine speed for different fuels In Fig. 3, the variation of the cylinder temperature with crank angle at 10 RPM is presented. The figure shows that diesel has a higher temperature than the biodiesel fuels up to a crank angle of about 173 degrees where the biodiesel fuels possess a slightly higher cylinder temperature than diesel. Pressure is directly proportional to temperature at constant volume. The same factors that affect the peak pressure such as: calorific value, cetane number, density, viscosity, presence of oxygen in biodiesel and ignition delay, may be responsible for the nature of the curves. 241

5 B. Performance Characteristics Fig. 3 also shows the variation of brake power with speed at a crank angle of 164 degrees. Sunflower and canola biodiesel have very similar behaviors except for sunflower biodiesel having a slightly higher brake power than canola biodiesel at 10 RPM. Diesel has a higher brake power than the biodiesel fuels all through the varied engine speed. The lower calorific value and higher molecular weight of biodiesel compared to petroleum diesel might have contributed to its lower brake power. NIitric Oxide (PPM) C. Emission Characteristics 200 Fig. 4 Comparison of nitric oxide emission for different fuels The variation of NOx emission with crank angle at 10 RPM for the fuels tested is presented in Fig. 4. Diesel has a higher nitric oxide emission than the biodiesel fuels up to a crank angle of about 170 degrees where the biodiesel fuels possess a much higher NOx emission than diesel. The amount of oxygen and the cylinder temperature directly affects the nitric oxide emission. The presence of oxygen in biodiesel is responsible for the higher nitric oxide emission of biodiesel than diesel at higher crank angles and hence higher temperatures. Cylinder temperature is higher for biodiesel at higher crank angles. VI. CONCLUSION The theoretical model developed effectively predicted the combustion and performance characteristics of diesel and biodiesel from canola and sunflower oils. The NOx emission of diesel and biodiesel were also effectively computed theoretically. From the results, it is found that diesel has a higher peak pressure and temperature at lower crank angles compared to the biodiesel samples. At higher crank angles of about 170 degrees however, the peak pressures and temperatures are the same for all the tested fuels. Diesel has a much higher brake power than the biodiesel samples throughout the varied engine speed. The NOx emission of diesel is lower than that of the biodiesel samples at lower crank angles. At higher crank angle of about 170 degrees, the NOx emission of diesel becomes higher than that of the biodiesel samples. The higher NOx emission of diesel at higher crank angle and higher temperatures is attributed to the presence of oxygen in biodiesel. REFERENCES [1] OECD. Organisation for Economic Co-operation and Development / International Energy Association, Mobilising Energy Technology Activities of the IEA Working Parties and Expert Groups International [2] S. Bari, C. W. Yu and T. H. Lim, Performance deterioration and durability issues while running a diesel engine with crude palm oil, Proceedings Institution of Mechanical Engineers, Part-D. Journal of Automobile Engineering, 2002, pp [3] A. Murugesan, C. Umarani, R. Subramanian and N. Nedunchezhian, Bio-diesel as an alternative fuel for diesel engines - a review, Renewable and Sustainable Energy Reviews, 2009, vol. 13, pp [4] S. A. 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