Diesel motorlarında yanma kanununun bilgisayar modellemesi

Abstract

Heat engines can be classified as the extemal-combustion typfe in which the vorking fluid is entirely separated from the fuel-air mix­ture and the intemal-combustion type in which the working fluid con­sists of the products of combustion of the fuel-air mixture itself. At the present time the reciprocating intemal-combustion engine and the steam turbine are by far the most widely used types, with the gas turbine in wide use only for propulsion of high-speed air­craft. A fundamental advantage of the reciprocating intenal-combus- tion engine over power plants of other types is the absence of heat exchanger in the working fluid stream. The advantages of the reci- procating intemal-combustion engine are of especial importance in the field of land transportation, where small weight ar>d bulk of the engine and fuel are always essential factors. In the study of thermodynai Trics the efficiency of a cyclic process (that is,a process which operates on a given aggregation of materi­als in such a way as to return it to its original state) is defined as - 'w/JQ’(1) vhere: efficiency w : useful work done by the process J : Joule's law coefficientQ': heat which flows into the system during the process Intemal-combustion engines operate by burnjLng fuel in, rather than by adding heat to, the working medium, which is never returned to its original state. In this case, therefore,the thermodynamic de­finition of efficiency does not apply. However, it is convenient to use a definition of efficiency based on a characteristic quan­tity of heat relating to the fuel. The method of determining this value, which is called the heat of combustion of the fuel, is somewhat arbitrary, but it is generally accepted in work with heat engines. Now the efficiency of any heat engine may be defined by the following expression :•0 = N / J H(2)'. I c where N : power Mj.: mass of fuel supplied per unit time H:heat of combustion of a unit mass of fuel c It is obvious that the fuel consumption and the contents of ex­haust gas emissions of an intemal-combustion engine depends on its actual combustion process which still remains as a problem regarding difficulties in predicting the heat release pattern. Since .the combustion of a spark-ignition engine and compression- ignition engine differs widely in the physical aspect of the phenomenon the study of their combustion process is to be held separately.In this thesis general acknowledgements available on the combus­tion process of the Diesel engines (the usual commercial form of the compression-ignition engine) has been firstly discussed then an approach to the modelling of the heat release rate has been performed by using the theory of WIEBE function.The theory of the Wiebe function is based on the Chain Reactions which occurs during the combustion process in a diesel engine and starting from - dN = n .d N(3) e the basic equation of chain reaction theory, where dN : Differential of the amount of the combustible material molecules dN : Differential of the amount of effective nucleus available e in the combustion process during the time interval t, t + dt n : A proportion factor we obtain the expression x = 1- exp [c (riYz )m +(A) which is the most convenient form of a Wiebe function for use in computation of the heat release rate.Since the combustion process in a diesel engine is described by two distinct phases,explosion combustion and diffusion combustion two Viebe functions have been used during the studies of the mathematical modelling. An important quantity of fuel mixture prepared during the ignition delay period leads to the formation of the explosion combustion which is also named as uncontrolled combustion by Ricardo because of its extremely high rate of burning. The rate of burning in this phase directly depends on the ratio of ignition delay to injection period. An increase in this retio by keeping all the other parameters cons­tant causes the rate of burning to rise. The most dangereous situation for a diesel engine in its combustion phase is the deformation of the explosion combustion . into an intermittent type of combustion which is an unwanted form of combustion in an ideal diesel cycle. The second phase in a diesel combustion is the diffusive combustion which is also called controlled combustion. This phase is assumed to start from the maximum pressure point to the tail of combustion. The rate of burning in the diffusive combustion is moderate compared to the explosion combustion and in seme cases so low that some local ex­plosion combustion phases occur : during the second phase because of relatively high rate of mixture prepared at local points. An other phenomenon in the diffusive combustion phase is tail of combustion which causes a decrease in the efficiency of the combustion. The tail of com­bustion mainly forms because of the heterogenities in the richness of mixtures and is considered as an abnormal type of combustion.There are a number of models which involve the rate of heat release in diesel combustion based on some emprical formulations. It is possible to classify these models into three groups such as ; single, double and multi regional combustion . modelling. From these models it is possible to define lyn's, Whitehouse-Way and Grigg’s models as most conventional methods for predicting heat release pattern of a given diesel engine. But, since such models are based on some -emprical formulations they do not suit to perform highly accurate computations of heat release. An algorithm has been established to perform estimation of the Viebe function parameters (m and Y ) for a given diesel engine vith a known pressure and temperature diagrams in the algorithm the least Squares Polinomial Approximation method has been used as a curve-fitting operation during which linearisation of the exponantial form of Wiebe function and the necessity to shift data points (because of few number of data point available) were two main problems to be solved. By analyzing some practical results obtained from the computer program (written in Quick-Basic language) which represents the algorithm it is possible to conclude that :The algorithm proposed is capable to perform the computation of the heat realease pattern of a diesel engine. Some additional work is to be carried out to : Introduce Ignition Delay theory to combustion computations Improve the linearization system of Viebe function to obtain a polinomial form of higher degree.Combine the algorithm with a computer program of a complete cycle simulation of a diesel engine.