Gemi diesel motorlarında silindir modellemesi

Abstract

ÖZET Bu tezde sunulan çalışma, Gemi Diesel Motorlarında Silindir Modellemesi ve bu modellemenin tek silindirli bir motor için özelleştirilmesidir. Önce, bu konuda bugüne kadar yapılan çalışmaları içeren bir kaynak araştırması yapılmış ve bu çalışmalar hakkında kısaca bilgi verilmiştir. Tek boyutlu zamana bağlı homentropik olmayan gaz akımı hakkında bilgi verildikten sonra silindir içerisinde enerji dengelenmesi detaylı bir biçimde incelenmiştir. Boru sistemlerindeki süreksiz basınç ve sıcaklıkları tayin etmede kullanılan ve sayısal bir çözüm yöntemi olan Mesh metod hakkında da bilgi verilmiştir. Bu araştırmada sunulan bilgisayar programında modellemesi yapılan diesel motoru sonsuz büyüklükteki bir kontrol hacminden aldığı temiz havayı yine sonsuz büyüklükteki diğer bir kontrol hacmine egzost olarak göndermektedir. Seçilen Ricardo E6 tipi motorun silindiri içerisindeki basınç, sıcaklık, hacim ve toplam kütle değerleri krank açısı artımları için hesaplanmış ve karşılaştırılmıştır. Burada krank açısı artımlarının seçimi, araştırmacıların isteğine bırakılmış olup, yapılan bu çalışmada beşer derecelik artımlar seçilmiştir. Tezin son kısmında ise elde edilen eğriler hakkında açıklama ve yorumlar yapılmış olup, elde edilen sonuçlar ile bu konuda gelecekte yapılabilecek çalışmalar için öneriler sunulmuştur. vııı

SUMMARY THE MODELLING OF THE CYLINDER IN MARINE DIESEL ENGINES The objective of the work presented in this thesis is the modelling of the gas exchange process in Marine Diesel Engines and specializing the model for an engine which has one cylinder. The classical approach to Marine Diesel Engine design in the form of manual theoretical calculations, prototype manufacturing and experimental investigations can be extremely expensive in terms of money and time. From the point of the engineer, a theoretical model should be the simplest version which will achieve the desired objective. Thus, CAD - CAM design and manufacture investigation programs and simulation models can be of great assistance to the researcher, designer and engine manufacturer. It is given that the power output from an engine depends upon the amount of fuel that will be usefully burnt inside the engine cylinder. This in turn depends on the air which the engine will draw in. In many cases a theoretical model is used to help interpret correctly the results of an experimental ivestigation. A theoretical model can be used as an aid to design and development. Marine Diesel Engines have complex systems of pipes and cylinders. A theoretical model of a complex system requires the assembly of a number of submodels. A typical example is made up of a series of pipes connected to valves, pipe branches and cylinders. The theoretical treatment appropriate for each component will be different but it is necessary to couple these different calculations in order to simulate the overall system. The system consists of unsteady flow in pipes, filling and emptying of cylinders and boundary conditions. The equations for one - dimensional unsteady flow which govern the flow are derived. The treatment is a general one» the equations allow for non - homentropic flow, effects of heat transfer to or from the walls of the duct, gradual changes in cross sectional area of the duct and irreversibilities caused by the influence of the skin friction at the wails of the duct. The homentropic flow is a reversible adiabatic flow in which the entropy of the fluid is constant. IXThe fundamental equations for one - dimensional unsteady flow as follow : 1. Continuity Equation 9u. u dE + 1 ( 3P_ + u ^_) = 0 ax e * dx p at ax 2. Momentum Equation dn +u 3u+J_ 4i+F = 0 at 3x p ^x 3. Energy Equation <`> p c ¦<*>- c -H ¦-#->` `2 <!£¦` i^ The basic principle underlying the nozzle boundary conditions is that while the flow in the pipe connected to the nozzle is unsteady, the flow through the nozzle itself is quasi - steady. That is to say, at any instant of time, the flow through the nozzle is assumed to be the same as it would be under steady flow, with the same conditions of state and velocity at inlet and exit from the nozzle. In other words, quasi - steady flow through a nozzle is that the wave travel time through the nozzle is very short compared with the wave travel time of the pipe to which it is connected. Thus, the steady flow conditions through the nozzle are set up and arranged in a suitable form for inclusion on the characteristic state diagram. Because of simplifying this complex system of Marine Diesel Engines, the exhaust and inlet pipes are removed and this made it more applicable to filling and emptying theory. This theory has been used over the years for calculations on Marine Diesel Engines. In the first part of the thesis, a survey of the relevant available literature is presented. The original part of the presented work is the modelling of the gas exchange process for Marine Diesel Engine which has one cylinder.There are many engineering situations which involve gas exchange between two or more vessels. In certain cases the volumes of some of the vessels change with time, such as the cylinders of a multi - cylinder internal combustion engine, also the effects of heat transfer from the vessel walls to the fluid inside will be important. A common feature of the gas exchange process is, that the flow velocities within the vessels are small and when this condition is satisfied the static and total pressures within the vessel are the same. The vessels which come under this category are described by a variety of names which include vessel, plenum and reservoir. The computer program presented in this thesis intends to simulate a Marine Diesel Engine which draws in its fresh charge from a plenum and exhausts into a second plenum. The mass flow rate through the inlet and exhaust valves are calculated on this basis of quasi - steady assumptions. Acording to this assumption, when the instantaneous axial profiles of thermo dynamic and gas dynamic properties, such as pressure, temperature and velocity at any given instant of time are very close approximations to those for steady flow, with the same inlet conditions and the same overall pressure ratio. This method uses the steady flow energy equation, the continuity equation, the equation representing isentropic flow from the upstream considerations to throat. For subsonic flow at the throat the constant pressure assumption is made. In this work presented, both the exhaust and inlet pipes of a single cylinder are removed and a plenum fitted to the inlet flange on the cylinder head. Quasi - steady flow assumption leaves the one - dimensional steady equations for flow through the nozzle. The mean thermodynamic variables in the cylinder are calculated by means of the Runge - Kutta Method where the increments of the crank angle is constant. In this thesis, the increments of the crank angle is selected 5 crank angle. In the case of non - homentropik flow, there are three characteristics: *, £ and o(. The ?v 's and Ş> 's are disturbance characteristics, and the o< 's are path lines. The path lines are also characteristics. A practical method of performing a numerical solution, by the Mesh Method, to the characteristic problem. In this method the pipe is divided into a number of equal length parts, called meshes. A line of constant time is considered as an initial value line and the calculation carried forward to a later instant in time from this. It is important that the time step should not be too large in this method. XLHowever, in this thesis, a brief summary of the `Method of Characteristics - Mesh Method` is presented. The method of calculating nonhomentropic unsteady flow in a pipe is the `Method of Characteristics - Mesh Method`. This method is used successfully solving different problems encountered in the inlet and exhaust systems of internal combustion engines. Thermodynamics variables in the pipes are calculated accurately by means of the Characteristics Method allowing the increment of the crank angle or time steps to be calculated in the program. In this work presented, a theoretical investigation using the simulation computer program is carried out to predict the effects of different thermodynamics factors such as motor speed, compression ratio, cylinder bore, stroke, opening and closing timings of valves all together, on the fluctuations of pressure, temperature, volume and mass flow rate of the Ricardo E6 type Marine Diesel Engine cylinder for every increment of the crank angle throughout the cycle. The presentation of all the theoretical results. are in graphical form and they are given in the Appendix B. The comparisons of the diagrams are presented in detail in the section 5. A listing of the computer program used is also given. Furthermore, the descriptions of the programs and subroutines are given in the section of descriptions of the program. A data list of the computer program are also presented in this thesis. This program was written in Fortran IV. The detailed conclusions are made at the end of the thesis and some suggestions are put forward for future work. The mean thermodynamic variables in the cylinder will be calculated using other numerical methods and a comparison will be made between two or more numerical methods. The computer program presented in this thesis intends to simulate a Marine Diesel Engine which has a single cylinder. This program will be developed for an engine with inlet and exhaust pipes and multi - cylinders. A comparison will be made on total central processing unit (CPU) using time. The computing time for calculations with using Runge - Kutta Method and the computing time for calculations with one of the numerical solution methods are compared. If the compression ratio is increased while all the variables are kept fixed, the power obtained from the motor increases. As the number of revolution increases, the amount of mass in the cylinder throughout the exhaust stroke increases whereas the condition is just the reverse in the suction stroke. xiiThe computer program presented is comprised of a main program and three subprograms and the results are computed for one cycle. It is possible to analyse the effect of cycle -to the steadiness of results by increasing the number of cycles. During the comparison and interpretation of results it should be noted that dhe discussions are depending on theoretical bases and according to theoretical results. These computed results will be analysed in a wide data range and will be compared with the experimental results, so that, a more definite judgement about the sensitivity of results will be reached. Finally, a list of references related to the topic is also presented. Xlll