Dizel motor pistonlarında yağ galerisi etkilerinin termal ve yapısal olarak incelenmesi
Investigation of thermal and structural effects of cooling oil gallery in diesel engine pistons
Yazar: ORHAN AYDIN
Danışman: DOÇ. DR. SERTAÇ ÇADIRCI ; DOÇ. HASAN KÖTEN
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Makine Mühendisliği Ana Bilim Dalı / Isı-Akışkan Bilim Dalı
Derece: Yüksek Lisans, 2021
Konu: Makine Mühendisliği = Mechanical Engineering
Dizin: Bilgisayar destekli termal analiz = Computer aided thermal analysis ; Motor pistonu = Motor piston ; Sayısal akışkanlar dinamiği = Computational fluids dynamic ; Türbülanslı akış = Turbulent flow

Özet:
İçten yanmalı motorlarda piston parçası sistemin en önemli parçalarından biri olmaktadır. Teknolojinin gelişmesi ile birlikte bilgisayarların işlem gücünün artması, motorun ve motor alt parçalarının modellenmesi, bilgisayar ortamında sistemin ve alt bileşenlerinin termal, yapısal ve akış analizlerinin yapılabilmesi, son kullanıcı bilgisayarlarında dahi mümkün hale gelmiştir.
Piston parçası, içten yanmalı bir motorda, yanmanın bulunduğu bölgeyle temas halinde olan, dolayısıyla motorda oluşan yüksek basınç ve sıcaklıklara maruz kalan, termal, yapısal ve yorulma dayanımı anlamında incelenmesi en elzem olan parçalardan biridir. Pistonda ortaya çıkan yüksek sıcaklıklar ve gerilmeler piston malzemesinin dayanabileceği üst sınırları geçmemelidir. Ayrıca yorulma dayanımı açısından da oluşabilecek gerilme ve sıcaklıkların optimum düzeyde olması gerekmektedir. Bu nedenlerden dolayı piston parçası tasarım anlamında çok kompleks bir hale gelmektedir. Dizel motorlarda kullanılan alüminyum pistonlarda pistonun üst yüzeyi yanma analizleriyle optimize edilebilmektedir. Pistonun etek ve segman bölgeleri termal ve yapısal analizlerle, ayrıca minimum yağ filmi kalınlığı analizleriyle, belirlenmektedir. Yağ galerisinin boyutları ve konumu da akış analizi, termal ve yapısal analizlerle belirlenmekedir.
Bu çalışmada piston modeli oluşturulmuş ve tasarıma uygun kolbenschmidt tipi yağ galerileri tasarlanmıştır. Tasarım için Siemens NX 10 yazılımı kullanılmıştır. Daha sonrasında yağ galerisinin termal ve akış analizleri Ansys Fluent yazılımı kullanılarak gerçekleştirilmiştir. Pistondaki sıcaklık dağılımı veya sınır koşulları Fluent yazılımında elde edilebildiği için Ansys structural modülü ile termal gerilmeler de elde edilebilmiştir.
Yağ galerisinin pistondaki konumu, sıcaklık dağılımını etkileyen en önemli parametrelerden biri olduğu için, üç farklı eksenel konumda analizler gerçekleştirilmiştir. Daha sonrasında yağa aktarılabilen ısının en yüksek olduğu modelde, galeri üst yüzeyinde iki farklı dalga şekli oluşturularak analizler yapılmıştır. Bu analizler sonucunda yağ galerisinin eksenel konumunun ve galeri üst yüzeyindeki dalga modelinin değişiminin yağa geçen ısı yüzdesine etkisi, oluşan en yüksek sıcaklıklara etkisi, termal ve yapısal gerilmelere etkisi gösterilmiştir.
Yapılan çalışmaların öncesinde altı farklı ağ sayısından bağımsız çözüm ve üç farklı zaman adımından bağımsız çözüm yapılmıştr. En uygun ağ sayısı ve zaman adımı seçilerek analizler gerçekleştirilmiştir.

Abstract:
At an internal combustion engines, the piston part is one of the most important part of the system. With the developing technology, the increase in computing power of the computers enable the modelling of the engine and the subparts of the engine, the thermal, structural and flow analysis of the system and subparts even end user computers.
At an internal combustion engine, the piston part is in contact with the combustion region; consequently, high pressure and temperature occurs at the piston part. The investigation of the thermal, structural and fatigue strength of the piston is crucial. The maximum temperatures and stresses must not excess the endurance limits of the piston material. Additionally, the maximum stresses and temperatures must be opitmum levels for the fatigue life of the piston. Thus, the design of the piston part is so complex. The head surface of the piston can be optimized with the combustion analysis using Converge CFD, Ansys Forte etc. The skirt and ring regions of the piston can be determined with thermal and structural analysis and also using minimum oil film thickness analysis using Ricardo like softwares. The dimensions and position of the oil gallery are determined with the analysis of flow, thermal and structural analysis.
In this graduate thesis, the piston model is created and kolbenschmidt type of oil gallery models are designed in concert with the piston design. For this purpose, Siemens NX 10 software is used for the design. After this, the thermal and flow analysis of the piston are made using the Ansys Fluent 19.2 software. The temperature distribution and the boundary conditions of the piston obtained with Fluent module of the Ansys software. Then, thermal stresses were obtained using structural modulus of the Ansys software.
The position of the oil gallery in piston is one of the most important parameter which effects the temperature distribution. In this graduate thesis, analysis are made at three different axial position of the oil gallery. After that, using the model which is most effective in heat transfer to the oil, two different wavy models at the top surface of the oil gallery are designed and analyse. At the end of these analysis, the effect of the axial position and two different wavy models of the oil gallery to the maximum temperature at the piston, thermal and structural stresses and the heat ratio transferring to the oil are demonstrated.
Before these analysis, some analysis are made at six different mesh number and three different time step due to the obtain independent solutions related to the mesh number and time step. For the independent solution from mesh number, four hundred thousand, five hundred thousand, six hundred thousand, seven hundred thousand, nine hundred thousand and one million mesh number were used. In these analysis, time step were taken as one mili second. At the end of these analysis, one million mesh number is considered sufficient.
After determined the sufficient mesh number, independent time step is investigated. For this purpose, 0.001 second, 0.0008 second and 0.0005 second time steps were analysed. Then, 0.0005 second time step is considered sufficient for the case.
In order to investigate the axial position of the oil gallery in diesel engine piston, three piston models, 123.3 mm, 128.3 and 133.3 mm axial positions from the bottom face of the piston, are created using Siemens NX software. In these models, meshes are created considering more than one million mesh number and the time step is applied as 0.0005 second.
Axial positions of the oil gallery analysis show that the position of the oil gallery in piston is a major subject for heat transfer from piston to cooling oil. In model-1 piston, the maximum temperature is obtained as 612 K, which is 13.5 K lower than reference model piston. The ratio of the heat transfer from piston to oil gallery is observed as %32.48. In structural manner, 138.9 MPa stress and 0.09592 mm deformation is observed.
In model-2 piston, which is 5 mm closer to head of the piston in comparison to model-1, is presented more heat transfer from the piston to the cooling oil. Model-2 has 610 K maximum temperature at the head of the piston, which is 2 K less than Model-1 and 15.5 K less than the referance piston model. The ratio of the heat transfer from piston to oil gallery is obtained ad %34.17, which is more than model-1 because of the closeness of the head of the piston. In this model, maximum 118.73 MPa stress is observed. Also, maximum 0.0932 mm deformation is obtained.
Model-3 has maximum 133.3 mm difference from the bottom face of the oil gallery to the bottom face of the piston. This model has the closest oil gallery to the headt of the piston in the first three piston models. The maximum temperature is 608 K and %35.91 heat transfer ratio is obtained. In structural analyse of the model-3, 119.63 MPa maximum stress and 0.09258 mm maximum deformation is obtained. While the heat transfer is more than model-2, structural effects are not changed much.
Model-4 is the first curved gallery model in these study. It has the same gallery position with model-3; however, it has four vave region at the top region of the gallery. This model is more effective in heat transfer manner compared to model-3 piston. The maximum temperature in the piston falls 606 K. On the other hand, the ratio of the heat transfer from piston to cooling oil rises to %36.6. These thermal results show that the wavy form of the oil gallery effects the heat transfer in good manner. Additionally, in this model, 117.7 MPa maximum stress and 0.09266 mm maximum deformation is obtained. Resultantly, vavy model form improves the heat transfer and decrease the stresses and deformations.
Model-5 is another piston model, which has eight vave form at the top region of the cooling gallery of the piston. The only difference from model-5 to model-4 is the vave number. In thermal manner, 605 K maximum temperature is obtained at this model. Heat transfer ratio from piston to cooling oil is obtained as %37.2. The structural analysis results show that model-5 has maximum 120.1 MPa stress and 0.0921 mm maximum deformation. These results show that, the number of the vave form is increased the heat transfer but also increased the structural stress and deformations.
At a result, the position of the oil gallery in engine pistons have a major effect for the heat transfer. It effects directly the maximum temperatures, temperature distributions and the exit temperature of the cooling oil. Apart from the axial position of the cooling oil gallery of engine piston, vavy formed oil galley models can improve heat transfer and decrease maximum temperature.
Besides the thermal effects, structural effects must be considered when designing an engine piston oil gallery. Oil gallery position and form may be sufficient and reliable in thermal manner. Nevertheless, it may not be good choice for structural manner. The position and form of the oil gallery directly change the geometry of the piston part. Therefore, the thermal and structural effects must be take into account together.