CHAPTER 1 INTRODUCTION

Transcription

1 1 CHAPTER 1 INTRODUCTION 1.1 GENERAL Diesel engines are the primary power source of vehicles used in heavy duty applications. The heavy duty engine includes buses, large trucks, and off-highway construction and mining equipments. Furthermore, diesel engines are winning an increasing share of the light duty vehicle market worldwide. The popularity of the diesel engine revolves around its fuel efficiency, reliability, and durability. High compression ratios along with relatively high oxygen concentrations in the diesel combustion chambers are responsible for the good fuel efficiency and low CO and hydrocarbon emissions when contrasted to a comparable gasoline engine. The CO present in residual gas in diesel engine is also less compared with petrol engine. However, these same factors result in high NO x emissions. The various sources of NO x emission to the atmosphere are motor vehicles (49%), electric utilities (25%) and other commertial and residential sourses that burns fuel (19%). The main pollutants from diesel engines are NO x and particulate matter (PM). The particulate matter includes the soot and unburnt hydrocarbon. The mechanism of formations of NO x and PM in the combustion chamber of diesel engines are contradictory and the simultaneous reduction of both is very difficult.

2 2 In this section, the overview of emissions from diesel engines and their harmful effects, the emission norms in India and NO x control strategies are briefly introduced. 1.2 EMISSIONS FROM DIESEL ENGINES The emissions are due to the several non-ideal processes taking place in real combustion and are harmful both to the environment and human health (Timoney et al 2005). Automotive engines, both petrol and diesel, are the major source of urban air pollution. The exhaust gases from the automotive engines is a mixture of unburnt fuel (Hydrocarbon), the product of partial combustion (Hydrocarbon, particulate and carbon monoxide), the product of complete combustion (Carbon dioxide and water vapour), products of high temperatures and pressures generated during combustion (oxides of nitrogen) and the elements of the inlet air that undergo no change during combustion (nitrogen, oxygen, carbon dioxide and water vapour) The components of the exhaust gas mixture which are hazardous to the human life and environment are hydrocarbons, oxides of nitrogen, carbon monoxide and particulate matter (Smith et al 1998) Hydrocarbon Hydrocarbon emission is the consequence of incomplete combustion of the hydrocarbon fuel. The level of unburned hydrocarbon in the exhaust gases is generally specified in terms of the total hydrocarbon concentration expressed in parts per million carbon atom. The fuel which escapes from normal combustion process leads to the emission of unburnt hydrocarbon. In diesel engines, the fuel can escape from combustion process due to two reasons; the fuel-air mixture is too lean or too rich to ignite or to support a propagating flame at the conditions prevailing inside the

3 3 combustion chamber of diesel engine. This fuel then can be consumed only by slower thermal oxidation reaction later in the expansion process. Thus, the hydrocarbon remains unconsumed and unburnt due to the incomplete mixture leads to hydrocarbon emission. The various reasons of HC emission are the fuel injected into the combustion chamber after the ignition delay period at low velocity which does not have enough time to react with oxygen molecule, the nozzle-hole sac volume and spray impingement on combustion chamber wall particularly in small size engines. The hydrocarbon emission indicates the combustion inefficiency and in diesel engine the combustion inefficiency is less than 2 percent Oxides of nitrogen Oxides of nitrogen, both NO and NO 2 grouped as NO x, are formed at high temperature during the combustion process. The major source of nitrogen, to form NO x during the combustion in a diesel engine, is from the intake atmospheric air. Four different mechanisms lead to the formation of nitric oxide in combustion system, namely, thermal NO, prompt NO, fuel NO and N 2 O intermediate route. In diesel engines, the major fraction of NO is formed via the thermal path. In the total NO x concentration in diesel engine 80 to 95% is contributed by thermal mechanism and 5 to 20% is contributed by prompt mechanism. The fuel NO and N 2 O intermediate mechanisms contribute negligible amount of NO in diesel engines Carbon monoxide Carbon monoxide is the product of incomplete combustion when the engine is operated with fuel rich equivalence ratio. CO is not only considered as undesirable emission, but also represents loss of chemical energy. The exhaust of typical spark ignition engine contains 0.2 to 5% CO

4 4 where as the diesel engine exhaust contains very low percentage of CO as the engine is normally operated at lean conditions Particulate matter Diesel particulates consist principally of combustion generated carbonaceous material (soot) on which some organic compounds has become absorbed. Most particulate matter emissions result from incomplete combustion of fuel hydrocarbons; some is contributed by the lubricating oil. The emission rates are typically 0.2 to 0.6 g/km for light duty diesels in an automobile. In larger direct injection engines, particulate emission rates are 0.5 to 1.5 g/kwh. The emission of hydrocarbon and carbon monoxide is more in the spark ignition engine. On the other hand the diesel engine emits more nitric oxide and particulate matter. The mechanism of formation of the engine exhaust emission is governed by the combustion process and chemistry of combustion. The engine-out exhaust gas composition in volume fraction of diesel engine is depicted in Figure 1.1 and the volume fraction of pollutants of diesel engine is shown in Figure % 8.0% 0.2% 9.0% 73.8% N2 O2 H2O CO2 Pollutants Figure 1.1 Volume fractions of exhaust gases in diesel engine (Stiesch 2003)

5 5 85% 4% 4% 1%6% NOx HC CO Soot SO2 Figure 1.2 Volume fractions of pollutants in diesel engine (Stiesch 2003) Smoke emission Soot emissions from a diesel engine are manifested as a visible black smoke. Smoke emission increases with increase in engine load due to overall richer combustion, longer duration of diffusion combustion phase and reduced oxygen concentration. Smoke is measured by measurement of opacity of a column or plume of exhaust gas. 1.3 ENVIRONMENTAL EFFECTS OF EMISSIONS Diesel engines generate undesirable emissions during the combustion process. The emissions exhausted into surrounding pollute the atmosphere and causes the problems like global warming, acid rain, smog, odour and respiratory and other health hazards. The various health effects caused by the internal combustion engine emissions are (i) aggravate heart disorders (ii) affects central nervous system, (iii) reduces oxygen carrying capacity of blood, (iv) Irritation of respiratory tract and coughing (v) eye irritation and (vi) Carcinogenic.

6 6 1.4 EMISSION NORMS IN INDIA Considering the harmful effects of emission from the diesel engines, stringent emission norms and standards are implemented all over the world. The first Indian emission regulations were idle emission limits which became effective in These idle emission regulations were replaced by mass emission limits for both gasoline (1991) and diesel (1992) vehicles, which were gradually tightened during the 1990 s. Since the year 2000, India started adopting European emission and fuel regulations for four-wheeled light duty and heavy duty vehicles. Emission standards for heavy duty and light duty engines are listed in Tables 1.1 and 1.2 respectively (Walsh 1999). Table 1.1 Emission Standards for Diesel Truck and Bus Engines g/kwh Year Reference CO HC NO x PM Euro I * 2005 Euro II Euro III *0.612 for engines below 85 kw Table 1.2 Emission Standard for Light Duty Diesel Engines, g/kwh Year Reference CO HC NO x PM Euro I * 2005 Euro II * for engines below 85 kw

7 7 emission in future. The emission standards will be tightened further to reach near zero 1.5 EMISSION CONTROL STRATEGIES To meet the future emission standard, the NO x and particulate emission from the diesel engine should be reduced significantly. There are three areas in which the research work have been carried out and still in progress are; in-cylinder control of fuel-air mixture preparation and combustion, study of alternate fuels and additives and post-treatment methods. The NO x formation in diesel engine combustion chamber is governed by two important parameters, the temperature of gases in the cylinder during combustion and the availability oxygen to oxidize the nitrogen into NO x. Any method which will reduce the temperature during combustion process and availability of oxygen in cylinder will effectively control the formation of NO x in diesel engines. The in-cylinder control of NO x formation includes: precise control of rate of fuel injection multiple injections of fuel injection time retard controlling the fuel injection pressure optimized air swirl optimized combustion chamber shape number of holes in fuel injector and its location optimum quantity of exhaust gas recirculation The injection of water in the form of diesel-water emulsion as alternate fuel is a possible way of reducing the NO x and PM simultaneously.

8 8 The presence of water in the engine cylinder during combustion reduces the flame temperature by absorbing heat of combustion and reduces the NO x emission. Exhaust gas treatment has the advantage relative to the combustion modification in NO x emissions and it can be retrofitted to existing plants. Exhaust gas treatment includes Selective Non Catalytic Reduction (SNR) and Selective Catalytic Reduction (SCR). The SCR process is more complicated and expensive which uses a surface to catalyze for nitric oxide removal. It requires high upstream pressure to force the flow through the catalyst. The SNR process relies on homogeneous gas phase reaction. The processes are simpler, only injection of the exhaust at a specified temperature. SNR process has shown significant NO removal at fraction of cost of SCR. Currently three processes are utilized for SNR removal of NO, viz., injection of ammonia, injection of urea and injection of cyanuric acid. 1.6 MODELING AND SIMULATION OF COMBUSTION AND EMISSION IN DIESEL ENGINE Performance and emission of diesel engine depends on the combustion behavior of fuel in the combustion chamber. Experimental investigation of the combustion process in a typical diesel engine is a complex, time consuming and costly affair. Mathematical models have been developed by various researchers to understand combustion and subsequently the performance and formation of pollutants. The various mathematical models proposed are Zero dimensional (Single Zone) model, Phenomenological (Multi zone) model and Multi dimensional (CFD) models. Zero dimensional models are based on first law of thermodynamics and are used to analyze performance characteristics of engines. The model computationally efficient but cannot provide insight into local processes such as fuel-air mixing and location of pollutant formation. The computing time is very less for the model. Phenomenological model is more complex than

9 9 thermodynamic model, because the combustion chamber is divided into numerous different zones that are characterized by different temperature and composition. Because of spatial resolution, important sub-process can be modeled and the prediction of heat release rate and exhaust emission as a function of characteristic engine parameters become possible. Computational Fluid Dynamics (CFD) model solves full set of differential equation for species mass, energy and momentum conservation on a relatively fine numerical mesh and also includes sub-models to account for the effect of turbulence. The computation time increases with the complexity of the model. Zero-dimensional model, based on first law of thermodynamics, is used to analyze performance characteristics and NO x formation of diesel engine in this research work. 1.7 OUTLINE OF THE THESIS The various chapters in the thesis are organized as follows: In chapter 1, the various emissions from diesel engine and their harmful effect on environment and human health, the emission norms in India, the various emission control strategies are presented. The modeling and simulation types and their relative significances are also briefly given. In chapter 2, the brief literature review on numerical and experimental study of various researchers on diesel engine combustion and emission using diesel-water emulsion are presented pertaining to the current research. In chapter 3, the scope and objective of this research is explained and the main focus of this research is the study of NO x formation during combustion in diesel engine using single zone model and the effect of engine speed, and compression ratio and water percentage in the diesel-water emulsion on NO x formation. The experimental study is conducted to study the

10 10 performance and emission of diesel engine using diesel-water emulsion as fuel at injection pressures 150, 200 and 250 bar. The effect of NanoXXL fuel additive is also experimentally studied on performance and emission. The effect of water emulsion on kinematic viscosity of lubricating oil is also studied. In chapter 4, the modeling and simulation procedure related to this thesis of various processes in diesel engine are explained with necessary correlations. The modeling of NO x formation for diesel-water emulsion is also developed and fit into the single zone diesel cycle simulation program. A computer program using C language has been developed to obtain the numerical results. In chapter 5, the details of experimental setup, the diesel engine and its instrumentation, the exhaust gas analyzer, and smoke meter are given in detail. The preparation of diesel-water emulsion and experimental procedure are also presented. In chapter 6, the results of computer simulation and experiments are given with exhaustive discussions pertaining to the objective of this research. In chapter 7, the conclusion is made along with the scope for the future work. Use of diesel-water emulsion in diesel engine has a potential to improve the performance and emission characteristics of diesel engine. Switching to emulsified fuel combustion does not require any engine modification or retrofitting.