Ateşleme sırasının yüksek devirli dizel motor performansına etkileri

Abstract

Bu çalışmada gemi dizel motorunda tasarım değişikliğinin meydana getireceği sonuçların irdelenmesi amaçlanmaktadır. Planlanan tasarım değişikliği, motorun ateşleme sırasında belli kurallara göre tespit edilebilecek bir sistematik çerçevesinde motorun çalışma performansına etkilemiştir. Uyulan kurallar Bölüm 3' de, oluşturulmaya çalışılan sistematik ise Bölüm 6 ' da açıklanmıştır. 16 silindirli, V düzenindeki, 4 subaplı (2 emme ve 2 egzost), direkt püskürtmeli, aşırı doldurmalı, yüksek devirli gemi dizel motoru incelenmek üzere seçilmiş, farklı ateşleme sıraları ve farklı faz açıları altında motorun çalışması, geliştirilmiş bilgisayar programı yardımıyla analitik olarak araştırılmıştır. Her bir tasarım değişikliği, orijinal motor data ve sonuçları ile karşılaştırılmıştır. Karşılaştırmanın yorumları, Sonuçlar ve Öneriler kısmında sunulmuş, aynı zamanda bu yorumlar Ek-B'deki grafiklerle de desteklenmiştir. İncelemeyi mümkün kılan bilgisayar programı, çok silindirli ve aşırı doldurmaya sahip gemi dizel motorlarını modelleyebilmektedir. Bu çalışma, programın ilk kullanımı olmayıp, daha önce de, kaynak araştırması kısmında bahsedilen araştırmacılar tarafından kullanılmış ve gelişmeye tabii tutulmuştur. Programın kısa açıklaması Bölüm 5 'de yapılmıştır. Ek-C'de ise program listesi ve programın kolay anlaşılması için de Ek-A'da akış diyagramı verilmiştir. Belli bir süreçten geçerek şu an ki halini almış olan bu bilgisayar programı, benzer konulardaki çalışmalarda kendini kanıtlamış olan bir yöntemi kullanmaktadır. Yöntemin gerçeğe uygun deneylerle de bundan önce ki araştırmalarda gözlenmiştir. Metoda ait detaylar ve ilgili kaynaklar çalışma içerisinde verilmiştir.

The studies on the internal combustion engines continue to accelerate day by day, though the invention of the engine was a century ago. The reason for so much complicated and intensive work on the internal combustion engines is that they are taking part in the daily life in an increasing importance. Specially in recent years, the constraints brought into action for the internal combustion engines have forced the researchers to design and build the engines which will conform these constraints. Between the costraints adopted, the prohibitions for exhaust gase emissions, noise level and vibrations can be of subject. Naturally the way of conforming these regulations and constraints will lead to designing of engines, less harmful to ecological environment and humans, manufactured of higher quality & longlasting material and also operating on low quality fuel efficiently. However, it should be accepted that if it is considered achieving the ideal solution to above will require several attempts and if, for every attempt a prototype engine is manufactured, one will, for sure, have many non-optimum engines in stock. The cost of such a process will definetely rise to incredible numbers. In this case, either an engine, which is considered to be the best solution with the existing data in hand, will be manufactured or a new way of designing process should be found and this new way of solution is nothing but computer modelling. In this study, it has been tried to find out if it is possible to improve an engine, which is already in use in industry and other fields, by design alteration with a computer simulation programme. x Firing order has been accepted as the main parameter for the modification of existing engine design. The practical importance of firing order can be explained as follows; in modern engines, because of high revolution numbers, even the minör unbalanced masses cause majör inertia forces to occur. These inertia forces, according to the situation, may give rise to intensive vibrations and may give harm to moving parts of the internal combustion engine and also to the foundations. Therefore these forces should be balanced as much as possible and the number of unbalanced forces should be kept to a minimum. However balancing of internal combustion engines is not the only parameter in the selection of the crankshaft arrangement. The irregularity of the power distribution on the engine is also another important point that should be taken into consideration, and the irregularity of the power distribution on the engine is closely tight with firing order. The firing orders of the attempts have been determined according to the following; After the selection of the crankshaft arrangement and placing the cylinders (both banks of the engine) on the crankstar, öne of the cylinders on the TDC (Top Death Center) can be chosen to fire. Then, according to the revolution direction (clockwise ör counterclockwise), second cylinder from the two cylinders that come to the TDC is chosen to fire. As öne of these cylinders is firing, then it means the other is beginning the inlet stroke. This operation is continued until the very first cylinder we have taken to fire reaches TDC önce again. Now we have two choices for cylinders to fire, but öne of the choices has already been fired 360° ago, so it is clear that this cylinder will now begin the inlet stroke and we have take the other cylinder to fire, which has just completed the compression stroke. These firing orders should be checked with respect to their minör harmonics, because minör harmonics of various firing orders may sometimes be of importance regarding to the vibration problem. The majör harmonics are not affected by the firing order, as it is determined already by the number of cylinders only. The computer programme, utilized in the modelling of the sample engine, uses the theoritical fundamentals which are based on the principles of conservation of energy, conservation of mass and conservation of momentum with the opinion that the gas flow, which occurs time dependently in the inlet and exhaust manifolds of engines, propagates in wave manner öne dimensionally and possessing variable antropy between gas particles. The non-linear hyperbolic partial differantial equations appearing as a result of modelling, are solved by the method of characteristics. The study has been carried out by taking the marine diesel engine with 16 cylinders, 4 valves (2 inlet and 2 exhaust), direct injection, supercharged, inlet and exhaust manifolds. During the study, modelling programme has been run with the data given in chapter 6. The obtained numerical results have been transformed into graphics so as to achieve a better understanding of the results. These graphics show the in-cylinder pressure and inlet pipe pressure (of that cylinder) fluctuations. Fluctuations in the graphics are with respect to CA (Crank Angle). For the graphics, specifically 2 and 8 numbered cylinders, belonging to B block, have been chosen. The reason f ör this choice is that pressure fluctuations can better be observed in the far ends of the manifold. The graphics have been prepared in an special form to illustrate the in-cylinder pressure and inlet pipe pressure at the same time. Furthermore the performance parameters (Specific fueloil consumption, power per cylinder) of engine have also been modified into graphical form to compare the performance of each attempt, f ör which the programme has been run. Apart from those, inlet and exhaust air flow rates of cylinders 2 and 8 have been put into tabular form. The attempts numbered 5, 9, 11 and 16 from figures 7.1 and 7.2, are those attempts whose results are closest to that of the original. Of the performance characteristics ( Specific fueloil consumption and power per cylinder), it can be easily seen from figüre 7.1 and 7.2 that these values of attempt 5 are better results than the original. The original values of engine (fueloil consumtion 214.996 gm/kW.h, power 123.69 kW/cyl) are less than attempt 5 (fueloil consumption 214.005 gm/kW.h, power 124.300 kW/cyl). For attempt 11, the performance values arenot better but very close. The fueloil consumption, 215.184 gm/kW.h, is only 0.188 gm/kW.h less and power, 123.619 kW/cyl is only 0.071 kW/cyl worse. For attempt number 9, from figures 7.1 and 7.2, it can be observed that this attempt has the best performance values of the overall study. Really, the specific fueloil consumption 213.965 gm/kW.h and power per cylinder 124.348 kW, characteristics are better than both original and other succesfull attempts. But on the other hand, f rom table 7.1, the inlet and exhaust air flow rates of attempt number 9, for cylinders 2 and 8 arenot that good as previously mentioned characteristics of other attempts. For this attempt, f rom figüre B-8, in-cylinder pressure drops downto 2.25 bar around valve overlap. The lower value of this pressure continues also after valve overlap. For the other attempts; e.g number 13, from table 7.1 it is clear that for both cylinders, the inlet and exhaust air flow rates are very close to original values. But from figüre B-12, during valve overlap, in-cylinder pressure rises suddenly. Same observation is also valid for attempt numbered 18 and 19, from figüre B-18. In- cylinder pressure rises upto 3.75 bar around 270° CA and falls below that after, 60° CA, inlet valve opens. it can be concluded that there is a backpressure event in the latter case. Also high values of in-cylinder pressure becomes effective after valve overlap and causes the exhaust gases escape through valves, specially through inlet valve. This would lead to poor guality combustion air, which is not expected. After the above comments and observations, 3 main results as stated below are concluded; i. Phase angles between cylinder firings are of great importance regarding the pressure fluctuations. Thus, attempt number 9, whose phase angles differ from the original (phase angles 90°, 45°, 90°, 90°, 135°, 135°, 90°) has given more succesfull results. Only öne phase angle is 45°, and others of attempt number 9, are 90° and över 90°. in this case it can be preferred to fire the cylinders in possible maximum intervals. The phase angles and firing order of attempt number 9 can be suggested. ii. As seen f rom table 6.1, neither in attempt number 9 nor in 11 two consecutive cylinders have been fired öne af ter another. This has a regulating effect on the pressure fluctuations and such a firing order can be suggested. iii. In-cylinder pressure should be as low as possible during valve overlap. This is done by providing a continuously dropping in-cylinder pressure and an inlet flowrate which fluctuates constantly about the same value, but above the in-cylinder pressure. in this case the selection of firing order should be done accordingly. At the end of the study followings can be advised to those who wish to work further on similar subjects; i. Making a study by modifying the valve timing together with cylinder firing phase angles to see the effect of firing phase angles in detail, ii. Making a study by modifying the manifold design of existing engine to see the other aspects of pressure fluctuations.