İki Zamanlı Bir Gemi Dizel Ana Makinesi'nin Modeli Ve Simülasyonu

Abstract

Dünya ticaretinin büyük bir bölümü deniz taşımacılığı ile gerçekleşmektedir. Ticaret gemilerinin sevkinde kullanılan ana makineler yakıt tüketimi ve güç avantajlarından ötürü genelde iki zamanlı ağır devirli dizel makinelerden oluşmaktadır. Geminin sevki sırasında ana makine veya makine dairesinde meydana gelebilecek önemli bir arıza sonucu operasyonun aksaması veya durması ile sonuçlanabilecek arızaların azaltılabilmesi için ilk olarak makine dairesi içerisindeki en önemli eleman olan ana makinenin uygun koşullarda çalıştırılması ve korunması gerekir. Bu yüzden operasyon sırasında ana makinenin çalışma prensibinin gemi makineleri işletme mühendisleri tarafından iyi anlaşılması çok önemlidir. Ana makine operasyonunda ana makine ile birlikte çalışan; T/C türbin, T/C kompresör, yağlama ve soğutma sistemleri gibi sistem bileşenleri, mevcuttur. Bu sistemlerde veya bileşenlerinde oluşabilecek problemler ana makinenin çalışmasına direk olarak etki eder. Örneğin, iki zamanlı bir ana makine, egzoz, süpürme, ve yeni çevrim için dolgunun silindire alınması işlemini pistonun alt ölü noktaya yakın konumlarında yapar. Kısa süre içerisinde atmosferik basınçlar ile süpürülme ve yeterli dolgunun sağlanması imkansızdır. Bu yüzden turboşarjer sistemi iki zamanlı ağır devirli bir ana makine için elzem bir ekipmandır. T/C sisteminde meydana gelecek bir arıza ana makinenin çalışma koşullarına direkt etki edecektir. Bu tez çalışması, iki zamanlı bir dizel ana makinesinin modellenmesi, T/C kompresörün oluşturduğu hava dolgusu ile hava girişinden itibaren, T/C türbine egzoz gazlarının T/C'i çalıştırmasına kadar olan sürecin modellenmesini kapsamaktadır. Hesaplamarın yapılabilmesi için silindir gazlarının ideal gaz kanunlarına uyduğu, kaçakların veya geri akışların olmadığı gibi kabuller yapılarak modelleme yapılmıştır. Kütle ve enerjinin korunumu ilkesine dayanarak hava silindir içerisine alındıktan sonra püskürtülen yakıtın barındırdığı kimyasal enerjinin silindir gazları (hava, yakıt, yanmış gaz) üzerindeki sıcaklık ve basınç değişimleri tek bölgeli termodinamik model ve Wiebe Yanma Oranları Fonksiyonu kullanılarak modellenmiştir. Ana makine simülasyonu oluşturulması ana makineye bağlı bir çok alt sistemlerin de modellenmesini gerektirdiğinden, çalışma içerisinde ana makine ve alt sistemler bölümlere ayrılmıştır. T/C kompresör, skavenç manifoldu, emme portu, silindir içi prosesleri (sıkıştırma, püskürtme, tutuşma gecikmesi, yanma, genleşme egzoz, dolgu değişimi), egzoz manifoldu, T/C türbini ayrı birer alt sistem olarak düşünülerek modellenmiştir. Alt modellerin sistem içerisindeki görevine göre herbir sistemin model yaklaşımları farklı yapılmıştır. Bu modellerde lineer model; dolgu değişiminde ve soğutmada, ortalama değer modeli; T/C Türbin ve kompresör hesaplarında, fenomenolojik model; silindir içindeki proseslerde, tek bölgeli termodinamik model; yanma olayında, dalgalanmasız modelleme; egzoz prosesinin açıklanmasında kullanılmış, tutuşma gecikmesi, sıkıştırma, genleşme prosesleri ve güç aktarımı, termodinamik ve matematik denklemler ile oluşturulmuştur. Pervaneye iletilecek fren gücünün hesaplanması için sürtünme kuvvetlerini hesaplayan alt program oluşturularak ana modelleme programına yerleştirilmiştir. Tüm bu sistemlerin ve bileşenlerinin birlikte çalışması krank açısı değişimine bağlı olarak dinamik bir şekilde modellenmiştir. Simülasyon, Labview grafiksel programlama dilinde oluşturulmuş, dinamik sistemlerin modellenmesi için kullanılan State Machine (Durum makinesi) yöntemi kullanılarak yazılım mimarisi oluşturulmuştur. Durum belirleyici olarak krank açısı referans alınmış, krank açısının değişimi durumlar arasında geçişi sağlayarak içerisinde bulunan modellerin birbirilerine veri aktarması ile tüm sistemin ve alt sistemlerin birlikte çalışması sağlanmıştır. Simülasyonu oluşturulan ana makine; Sulzer RTA 84C 12 silindirli iki zamanlı ana makinesidir. Silindir çapı, strok uzunluğu, emme portlarının açılma zamanı, biyel kolu uzunluğu gibi ana makineye ait sabit parametreler makine teknik kataloğundan alınmıştır. Değişken durumlardaki T/C kompresör ve skavenç manifoldu basınç değerleri, maksimum basınç oluşma zamanı, T/C türbin bilgileri, devir sayısı ve yakıt tüketimi gibi değerler ise makinenin değişken durumlardaki test verileri alınarak elde edilmiştir. Oluşturulan simülasyonun farklı yüklerde çalıştırılması ile silindir içerisindeki basınç ve sıcaklık değerlerinin değişimi, egzoz manifoldu basınç ve sıcaklık değerleri, T/C Türbini tarafından üretilen güç, skavenç manifoldu basıncı, ve krankşafta aktarılan güç miktarı hesaplanmıştır. Makine operasyonu sırasında makineye verilen yakıt miktarının değiştirilmesi ile ana makinede meydana gelen değişimlerin izlenebileceği bir simülasyon oluşturulmuştur. Bu çalışmadaki modelleme ve bu tez için geliştirilen örnek simülasyonlar, 0531.STZ.2013-2 SANTEZ projesi ile gerçekleştirilecek olan 'Gemi Ana Makine Sümülatörü' projesinde de kullanılacaktır. Bu yüzden programlama mimarisi proje kapsamında geliştirilmesi planlanan simülatör programına da uyum sağlayacak şekilde oluşturulmuştur.

A large part of world trade is carried by sea ways. Main engines which are used as a propulsion unit of merchant ships predominantly consist of two-stroke low speed diesel engines because of lower fuel consumption and power advantages. Transportation of trading goods in a large ship bring economic advantages for both ship owner's and merchants. Large vessels require large marine engines. A ship with its cargo has exceptionally high value; therefore, any potential risk must be taken care timely with expertise and knowledge by marine engineers during their watchkeeping duties. For that reason, the knowledge and expertise of marine engineers have direct effect on the operations which are handled during the operations of ships. The main engine is the most important element of the ship's propulsion system; therefore, it must be operated safely and properly. In case of a failure occurs at the main engine's operation and/or from its components, there are numerous possibilities including damage to the main engine which may impact the navigation of the ship. To avoid the potential risks during navigation, marine engineers must be well-trained before undertaking duty in the engine room. The training practices can be executed via long lasting apprenticeships at ships, as training area. Also, training using simulators is also alternative method of training of marine engineers. The Standards of Training, Certification, and Watchkeeping (STCW'95) is the convention accepted by IMO (International Maritime Organisation) accepted simulators as alternative education tool. The simulation which is generated in this study can not meet the requirements of STCW'95, but it is certainly said that running characteristic of marine engine can be predicted via using the simulation or it helps to trainee to understand changing of operational conditions of marine engine when the required power change from the main engine at the operation. Internal combustion engines produce mechanical power from chemical energy which is contained in fuel. They are categorized according to fuel cycles and operating cycles. Otto cycle, diesel cycle and dual cycle are fuel cycles, and four stroke cycle, two stroke cycle are working cycles of interal combustion engines. Although they are similar in their constitution, two stroke engines can not be modelled with the same architecture as for the four stroke engines due to the difference in their operational characteristics, size and power outputs, and their usage areas. While four stroke engines are generally used in road vehicles, two stroke engines are used in large vessels as they allow for obtaining higher power outputs from the same engine size. Because of their large dimensions, two stroke engines have large components which naturally results in high inertial forces. In this study, diesel cycle and otto cycle approaches were used for mathematical modelling of the two stroke diesel engine in teorically, but it is normally working with dual cycle principle. If the phenomenology of operation of two-stroke diesel engines is reviewed briefly; a power stroke occurs per crankshaft revolution. In one cycle of two stroke diesel combustion, while piston moving upwards, inlet port and exhaust port are closed sequentially by the piston to initiate compression stroke. If exhaust valve exists instead of exhaust port, it is driven by cam shaft. With the piston approaches about Top Dead Centre (TDC) compression stroke ends and fuel is injected via injector in the cylinder. After evaporation and atomization of fuel which is called ignition delay, combustion initiates and the piston is driven down. At the time first lighting is seen in the cylinder fuel injection is kept. After end of the injection, combustion continues for a while about 5-10 crank angle. With the end of the combustion, by the piston moving downwards of the cylinder, expansion stroke initiates and continues until exhaust valve openning time. Between exhaust openning and inlet port openning, burned cylinder gasses flow out to exhaust manifold swiftly (free exhaust), then inlet port opens and remaining residual gases in the cylinder are forced to flow out (forced exhaust) to charge the cylinder with fresh air. Free exhaust, forced exhaust, and fresh air intake processes occur about piston at Bottom Dead Centre (BDC). In this short time these three charging process can not be achieved with atmospheric pressures. Turbo Charger (T/C) is the essential equipment for the two stroke diesel engines to pressurize the scavenge air. The simulation model of marine diesel engine goes off according to phenomenology of two stroke diesel engine which is written above. In this study, after the mathematical methods were established, a simulation was developed using Labview program development environment. For this study, a two stroke turbocharged marine diesel engine, Sulzer RTA 84C model series with 12 cylinder, was used. The power range of the engine extends up to 48600 kW at 102 revolution per minute (RPM). The simulation involves all the processes from air intake via T/C compressor to exhaust gas flow from the T/C Turbine. This range of process also include air receiver, inlet port, cylinder processes (compression, injection, ignition delay, combustion, expansion, exhaust, charging), exhaust valve, exhaust port. Assumptions have been made for computations in the main engine simulation program which are about gas thermodynamics. Such as, fresh air and cylinder gases flowing into the cylinder comply with the ideal gas law, after ignition composition of cylinder gases is uniform, no back flow from the inlet port or exhaust valve, compression stroke and expansion stroke are adiabatic etc. According to conservation of mass and energy law, chemical energy in the fuel is transferred to the cylinder gasses about at the TDC. Temperature and pressure changes of cylinder gases were calculated with the zero dimension thermodynamic model and Wiebe Heat Release Rate model which is widely used in prediction of burn rate. Modelling of main engine system requires modelling of many components included in main engine. Therefore main engine system is divided into sub-components as mentioned earlier. This allowed the sub-components modelling individually by using different methods. Additionally, modelled subcomponents of the engine are called submodels in simulation, futhermore called sub.vi in LabVIEW program. Moreover, main engine and its components or the processes were divided into the sub-processes to allow for having a modular programming. They are consist of cylinder and piston cooling, scavenge and exhaust processes, T/C turbine and compressor, in cylinder and combustion processes. Modelling approaches of submodels are determined by considering the function of the modelled component in the system. For instance, a linear model was used in cooling of the cylinder and piston for modeling the scavenge process. Mean value model was used in calculation of T/C turbine and T/C compressor pressure and mass values. Phenomenological model was used to describe cylinder processes. Thermodynamic model was used in combustion and heat release process. Quasi steady state model was used to describe inlet and exhausting system. On top of all these an ampiric formula was used to estimate ignition delay, and thermodynamic based mathematical model was used to calculate compression and expansion strokes values. Piston and crankshaft friction model was adapted to estimate net torque transmited to the propeller shaft (Brake Power PB). Piston transmits power through a piston rod then connecting rod and crank mechanism. In cylinder processes, many other calculations have been made for estimation of operating conditions such as combustion. For instance the equivalence ratio is a critical issue for it and it is determined by fuel rack position and scavenge air pressure of the charged air by the T/C compressor. The required power which is consumed by the T/C compressor is supplied by the T/C turbine rotation. Via dynamic changes of crank angle the power transmission mechanism submodel calculates the torque which is transmited to propellershaft, and all other pressure and temperature values could be calculated subject to range of one degree crank angle. Some constant parameters of main engine, such as, cylinder bore, piston stroke, number of cylinders, dimension of connecting rod, exhaust valve and inlet ports openning and closing times were taken from technical document of main engine. Also changing condition parameters, such as T/C compressor&turbine pressure, maximum presure in cylinder and its time, mass flows and specific consumption were obtained from test datas. In simulation of the model, engine performance curves have been obtained at Full Ahead, Half Ahead, Slow Ahead, Dead Slow Ahead modes of the operation of the main engine. Pressure and temperature values in cylinder, pressure values in air receiver and exhaust receiver, were compared with the test datas. Because of assuming cylinder gases as ideal gas and adiabatic compression&expansion processes, it is not expected to obtain identical P-V diagrams between simulation cycle and engine cycle of a physical system. Taking into the consideration of previously mentioned assumptions makes it possible that both diagrams display similar behaviour; yet slight variances may still exist at operating values, scavenge air and exhaust pressures, maximum pressures, maximum pressure time and naturally engine output torque due to the lack of unconsidered effects in the modeling. In this study, the results obtained by using the simulation runs demonstrated that the model can perform both steady and dynamic simulation and prediction rate of both heat release and engine performance are with expected fidelity. However, additional effort is required to enhance the fidelity of jacket water cooling and piston oil cooling predictions across a wide range of operating conditions. Ability to simulate real operating proccess of a simulation was the main criterion in this study.