Yüksek devirli gemi diesel motorlarında ısı yayılım modelinin etkisi

Abstract

Yapılan bu çalışma ile amaçlanan çok, silindirli aşırı doldurmalı gemi diesel motorlarının emme ve egsoz manifoldlarıyla birlikte bir bütün olarak modelleyen ve listesi Ek-A ' da verilen bilgisayar programı yardımıyla, farklı ısı yayılım modellerinin 4 veya daha fazla valflı motorlarda (2 emme ve 2 egsoz), motorun işlevsel değerlerine (özellikle güç, basınç, sıcaklık, ortalama efektif basınç, yakıt tüketimi) etkilerini saptamak ve bu yolla motor performansını arttırarak optimum düzeye ulaştırmaktır. Bu çalışmada kullanılan programın teorisi, diesel motorlarının emme ve egzos manifoldlarında meydana gelen zamana bağlı gaz akımının, tek boyutlu, zamana bağlı, gaz zerrecikleri arasında değişken antropiye sahip olarak dalgalar halinde ilerlediği düşüncesine dayanmakta olup, buda enerjinin korunumu, kütlenin korunumu ve momentumun korunumu aksiyomları ile modellenmektedir. Modelleme sonucu elde edilen lineer olmayan hiperbolik kısmi diferansiyel denklemler, Referans [1] ' de detaylı anlatılan Karekteristikler Metodu kullanılarak çözülmekte, çözüm için gerekli sınır koşulları ise Bölüm 5 ' de anlatılan sınır koşulu teorilerinden yararlanılarak temin edilmektedir. Boyutları önceden seçilen yüksek devirli, 16 silindirli, tek pistonlu, direkt püskürtmeli, dört sübaplı, emme ve egzos manifoldlu, aşırı doldurmalı, bir gemi motoru örnek alınarak geliştirilen ve hassasiyeti kanıtlanan bilgisayar programı kullanılarak, söz konusu motora ait orijinal datalar programa uygulanmış, elde edilen termodinamik özellikler ve motora ait işlevsel değerler, programa çeşitli değişik yanma modelleri uygulanması sonucu elde edilen değerlerle karşılaştırılarak güç ve efektif özgül yakıt tüketiminin optimizasyonuna yardımcı olacak bilgiler elde edilmiş, konunun sanayide motorların ön dizayn, dizayn ve üretim safhalarındaki ileriye yönelik yenileştirme ve geliştirme çalışmalarına yardımcı olacağı kanaatına varılmıştır. Yapılan çalışmada çok sayıda yanma modeli uygulanarak incelenmiş, bunlardan seçilen bir bölümüne ait sonuçlar Bölüm 7 ' de irdelenerek Ek-B ' de diyagramlar halinde gösterilmiştir.

Internal combustion engines have found their widest use in marine diesel engine industry, although this type of engines have been utilized in numerous other applications as well. A steady improvement has been recorded in engine design, operation and manufacturing over the years, directed by market forces of the desire to produce higher performance units. In the last three decades not only the environmental constraints and energy crises but the application of the digital computer to the problem have accelerated the engine research, especially engine performance analysis and influenced the development of the marine diesel engines. The analysis of performance enables the research engineer to predict the power output of engines at the design stage ; it is now also possible to postulate models of the combustion process which are capable of calculating all outputs as the prototype construction and experimental studies require high cost and long time in the design of marine diesel engines. When we consider the combustion process in direct injection, multispray systems, generally there is a delay of 9 degrees between the start of injection and start of combustion. The pressure rises rapidly for a few crank angle degrees, than more slowly to a peak value about 5 degrees after TC. Injection continues after the start of combustion. So different rate-of-heat release patterns available for internal combustion engines which they are typical. The heat-release rate diagrams plotted are usually the net heat-release-rate which is the sum of the change of sensible internal energy of the cychnder gases and the work done on the piston. It differs from the rate of fuel energy released by combustion by the heat transferred to the combustion chamber walls. The heat-release-rate diagram shows negligible heat release until toward the end of the compression when Vlll a slight loss of heat during the delay period.(which is due to heat transfer to the walls and to fuel vaporization and heating) is evident. During the combustion process the burning proceeds in three distinguisable stages. in the first stage, the rate of burning is generally high and lasts for only a few crank angle degrees. The second stage corresponds to aperriod of gradually decreasing heat release rate. This is the main heat-release period and generally lasts about 40 degrees. Normally about 80% of the total fuel energy is released in the first two periods. The third stage corresponds to the "tail" of the heat release diagram. The heat release during this period usually amounts to about 20% of the total fuel energy. From studies of rate-of-injection and heat release diagrams, över a range of engine loads, speeds, and injection timings the summary results are achieved. First the total burning period is much longer than the injection period. Second, the absolute burning rate increases proportionally with increasing engine speed; thus on a crank angle basis, the burning interval remains esntially constant. The intend of this study is not only to determine how, different heat release rates affect the functional values of the engine (power, pressure, m e an effective pressure, fuel consumption) and the pressure at the inlet manifold but also to find out the optimum heat release rate diagram without exceeding the construction parameters of the engine. The theoretical fundamentals, which the program used in reference [1] depend on, are based on the principles of conservation of energy mass and momentum with the opinion that the gas flow which occurs time dependantly in the inlet and exhaust manifolds of diesel engines, propagates in wave manner, dimensionally and possessing variable entropy between gas particles. The non-lineer hyperbolic partial differential equations which appear as a result of modeling, are solved by using the method of characteristics which is explained clearly in Reference [1] and the necessary boundary conditions are stated in chapter 5. AH these studies have been carried out by simulating the marine diesel engine with 16 cylinders, 4 valves (2 inlet and 2 exhaust), direct injection, single piston supercharged, inlet and exhaust manifolds, vvhich the configuration scheme in Figüre l Appendix-B. in this study many heat release models are applied and investigated but due to their outputs and results the most appropriate 18 patterns are analyzed. First model is the original case which has been drawn ix and the results have been carried out. in ali cases by using the sam e original engine data calculation has been done using different heat release models keeping the same combustion period. The change in pressure in the cylinder and inlet manifold according to the crank angle, which has been achieved as the result of running the program, has been analyzed and the diagrams performed with original data are given in the Figüre B.3.l- B.3.2, the pressure changes during the closed cycle period due to 18 different heat release models have been demonstrated in Figüre B.10.1-B. 10.12 and same changes which have been plotted on the same axis are given in Figüre B. 13.1. Due to the results of the applications of ali rate-of- burning models total effective power per cylinder, mean effective pressure, specific fuel consumption and their comparative diagrams with tabular data have been demonstrated at the following figures as further work. When we examine the Table 7.1, it can be seen that maximum cylinder pressure appears at the 181*1 condition. in this model although 98% of the fuel is burnt at the first 7 degrees of the crank angle total power value obtained is smaller than the 10tn condition. in the first rate-of-burning model which is the original öne, maximum pressures appear to be at 10 degrees crank angle, whereas it is occurred to be at 7 degrees in the 10*" model. The important point that is observed from the diagrams due to maximum pressures is that, at the first condition which is the original öne, maximum pressure curve is steeper than the l O4*1 model which has the maximum power output. Moreover 10*^ caused more closer maximum pressure values in wider crank angle values. The pressure values in the cylinder during öpen cycle period and at the inlet manifold side are very close each other in ali models especially at the optimum model with the original öne. When we examine the mean effective pressures according to different rate-of-burnıing models, there is 0.50-0.70 bar increase between ltn and 10th model, after 10tn model although there is significant changes occur at the maximum pressures there is no proportional increase occur with respect to maximum pressure increase. From Table 7.2 it can be clearly seen that, with respect to l**1 model after 64*1 model specific fuel consumption decreases by 6-7 (g/kwh), especially this reduction is 8 (g/kwh) at the 10tn model which has been chosen as the optimum model. After 12**1 model the increase of x the rate-of-burning in first 30 degrees crank angle hasn't managed any decrease in specific fuel consumption. When we look to Table 7.3 there is 5%-6% increase in the total power obtained, in the cylinders with respect to original data. After İÜ**1 model although rate-of-burning has increased up to 85%-95% there is some decrease occurred with respect to 10tn position. From ali these outputs and interpretations those conclusions can be achieved which are given below ; i. in the first stage, up to 50% rate-of -burning ratio doesn't effect the maximum pressure ratio as well as 80%-95% rate of burning ratio does, at the first 30 degrees. ii. Increasing rate-of-burning ratio up to 50%-55% at first 22 degrees crank angle value doesn't effect the specific fuel consumption. However when it reaches to 70%-85%, specific fuel consumption decreases effectively, although higher rate-of-burning values has no affect. iii. The exceeding increase in maximum pressure up to certain value doesn't result with total power increase per cylinder. iv. The maximum power output rate-of-burning values corresponds at first stage, up to 22 degrees crank angle 22%, second stage up to 42 degrees 85% and the rest of it 96%. Thus changing the first stage as main heat release period with the second stage, remaining the third as tail heat release period which has small but distinguishable rate of heat release persists throughout much of the expansion stroke, increases the total power output per cylinder. v. The optimum model that has been created caused, 4.5%-5% economy in effective fuel consumption as well as 4.8%-5.6% increase in total power output at the same time. vi. With the optimum rate-of-burning model although total output power increased and specific fuel consumption decreased, per cylinder, there hasn't been any significant pressure rise both in cylinder during öpen and in inlet manifold side. vü. With some further work which can easily be done at this topic, will improve the performance of the machines without changing the design parameters and production lines. xi in order to determine more reliable and supplementary results, it is advisable to the researchers who are interested in this subject to carry out the same study, i. For 6 ported (3 inlet and 3 exhaust), high revolution marine diesel engines with turbocharger unit, ii. For naturally ventilated marine diesel engines, iii. To design the fuel pump that will give the results achieved with this study, iv. To work out the affects of the pressure rise in the cylinders during the closed cycle period on the construction of the machine, v. To analyze the exhaust gas emissions, due to the application of the optimum rate-of-burning model.