Dizel İçten Yanmalı Motorlarda Silindir İçerisindeki Hava Hareketlerinin İncelenmesi

Abstract

Teknolojinin hızla geliştiği günümüzde, enerji talebi artmakta ve doğal kaynak rezervleri hızla tükenmektedir. Buna ek olarak, çevreye salınan zararlı gaz miktarı da artmaya devam etmektedir. Günlük hayatın her alanında kullanılan içten yanmalı motorlar da bu olumsuz etkilerde önemli bir pay sahibidir. İçten yanmalı motorları kullanan taşıtlardan çevreye salınan NOx, CO2 ve kurum gibi zararlı maddeler doğal yaşamı büyük ölçüde tehdit etmektedir. Bu sebeple, emisyon seviyeleri yasal mevzuatlarla her geçen gün daha sıkı bir şekilde sınırlandırılmakta ve bu durum motor verimliliğini çok kritik bir parametre haline getirmektedir. İçten yanmalı motorlarda verimliliğe etki eden başlıca parametreler; hava kalitesi, yakıt enjeksiyon parametreleri ve hava ile yakıt karışımıdır. Hava yakıt karışımının kalitesi yanma verimini, emisyon seviyesini, yakıt sarfiyatını ve motor verimliliğini doğrudan etkilediği için, yakıt enjeksiyon sistemleri ile hava davranışının doğru bir karşım elde edebilmek amacıyla birlikte tasarlanması oldukça önemlidir. Karışım oluşumu için hava kalitesinin önemi göz önüne alındığında, kullanılan yakıtın yanma karakteristiğine bağlı olarak iki tür hava hareketinden söz edilebilmektedir. Benzinli motorlarda kıvılcım oluşumu gerçekleşmeden önce karışım oluşumunun iyileştirebilmesi amacıyla, hava emiş kanal tasarımı silindir düşey eksenine dik bir eksen çevresinde bir takla hareketi oluşturacak şekilde tasarlanmaktadır. Bunun yanı sıra, dizel içten yanmalı motorlarda ise karışım oluşumunu iyileştirmek amacıyla, silindir düşey ekseni çevresinde bir girdap hareketi oluşturacak şekilde hava emiş kanalı tasarımları geliştirilmektedir. Bu çalışmada, silindir merkezindeki düşey bir eksen çevresinde gerçekleşen dönme hareketi olarak tanımlanan girdap hareketi incelenmiştir. Parçacık Görüntülemeli Hız Ölçeri ve Lazer Doppler Hız ölçeri gibi akış görselleme teknikleri kullanılarak hız vektörleri elde edilerek belirlenebilen bu hareket, aynı zamanda girdap oranıyla da sayısal olarak ifade edilmektedir. Piyasada çeşitli firmalar tarafından geliştirilen sabit rejimli akış koşullarında ölçüm yapan test düzenekleri ile hazır bir silindir kafası veya prototipi kullanılarak girdap oranı belirlenebilmektedir. Fakat, girdap oranı motor geliştirme projelerinin daha ilk safhalarında henüz prototip veya motor parçaları üretilmemişken karar verilen ve bütün yanma odası tasarımını etkileyen çok önemli bir parametredir. Bu sebeple, hava emiş kanal geometrisinin bilgisayar ortamında tasarlanabilmesi ve Hesaplamalı Akışkanlar Dinamiği (HAD) analiz yöntemleri kullanılarak girdap oranının doğru bir şekilde hesaplanabilmesi büyük öneme sahiptir. Bu tez çalışması kapsamında, Ford Otomotiv Sanayi A.Ş. bünyesindeki ölçüm düzeneği kullanılarak üç farklı motor geometrisi ve bir plastik prototip için girdap oranı ölçümleri gerçekleştirilmiştir. Daha sonra Converge isimli ticari HAD analiz kodu kullanılarak, bir simülasyon modeli geliştirilmiştir. Bu model, ölçümleri gerçekleştirilen iki farklı motor geometrisi için oluşturulmuş ve doğrulama çalışmaları yapılmıştır. Simülasyon modelinden ve ölçümlerden elde edilen sonuçlar karşılaştırıldığında iki motor geometrisi için de kütlesel debi değerleri en yüksek %5, girdap oranı değerleri ise en yüksek %10 farkla hesaplanabilmiştir. Bu durum geliştirilen simülasyon modelinin ölçüm düzeneğinin yerine kullanılabilirliğini kanıtlamıştır. İkinci olarak, çalışma kapsamında geliştirilen plastik prototip yaklaşımı kullanılarak PGHÖ test düzeneği ile akış görsellemesi yapılmıştır. Elde edilen hız vektörleri ve akış alanı, simülasyon sonuçlarıyla kıyaslanmış ve sonuçlar değerlendirilmiştir. Yapılan karşılaştırma, kullanılan simülasyon yöntemi ve türbülans modeli sebebiyle zamana bağlı anlık olarak değişen girdap hareketlerinin ve hız alanının ölçüm sonuçlarından farklı hesaplandığını ortaya koymuştur. Bu sebeple, bu çalışmada elde edilen çıktılar ile gerçekleştirilecek daha detaylı bir çalışma için büyük girdap simülasyonu kullanılması önerilmektedir. Son olarak ise, yüzey pürüzlülüğünün ve üretimsel hataların girdap oranı ölçümleri üzerindeki etkileri incelenmiştir. Elde edilen sonuçlar değerlendirildiğinde, üretimsel hataların girdap hareketinin yönünü dahi değiştirebildiği, yüzey pürüzlülüğünün de akış faktörü ve girdap oranı üzerinde önemli etkileri olduğu gözlemlenmiştir.

The purpose of internal combustion engines is the production of mechanical power from the chemical energy contained in the fuel. In internal combustion engines, different from the external combustion engines, this energy is released by burning or oxidizing the fuel. The actual working fluids are the air-fuel mixture before combustion and during the combustion. And work transfer which provides the desired power output occurs between the mechanical components and these working fluids. So that, air-fuel mixture is the main parameter which affects the engine performance in the internal combustion engines. As a result, in-cylinder flow characterization of air in internal combustion engines has always been an important topic has been studied by decades. In the last years, emission levels and the fuel consumption became very strict by regulations especially with the EURO VI and efficiency has become a more crucial term for the internal combustion engines (ICE). Especially, NOx and soot emissions are limited with the EURO VI regulation, and it is known that CO2 emissions will be monitored and limited with the EURO VII regulation in the near future. The key of the satisfying these emission regulations to continue to produce and sell engines is improving the engine efficiency. Since the emission level and the fuel consumption are the results of the combustion efficiency in-cylinder, engine manufacturers expedited the research and development projects to increase the efficiency. The main parameters affecting the engine efficiency are fuel injection parameters, air quality and air-fuel mixture. Since the mixing characteristic effects all the combustion efficiency, emission level, fuel consumption and engine efficiency, fuel injection parameters and the air behaviour should be obviously designed all together to obtain the proper air fuel mixture for the considered combustion chamber. Considering the importance of the air quality for the mixing characteristic, there are two types of the air motion for the ICEs regarding the fuel type. Gasoline engine combustion system relies on the large scale motion around the perpendicular axis to the center axis referred as tumble. On the other hand, the vast majority of modern diesel engine combustion system relies on the application of a large scale charge motion around the center axis of the combustion chamber, a swirling motion, commonly referred as Swirl. Both the swirl and the tumble motions are generated by the intake port and as a result, intake port design becomes a crucial parameter for the engine efficiency. The intake ports of diesel engines can be subdivided into three groups: tangential ports, helical ports, and filling ports. Tangential ports run tangentially at a relatively flat angle into the combustion chamber and thus generate a charge motion around the vertical axis of the cylinder. Tangential ports can be used to generate a relatively high swirl with moderate flow. Using two tangential ports in a 4-V engine is only feasible with turned valve arrangement. Helical ports have a geometry that lets part of the main air flow rotate around the valve stem and thus forces the majority of the drawn in gas mass into the combustion chamber in the direction of the desired swirl. For a parallel valve arrangement, it makes sense to combine a tangential port with a swirl duct. In addition to the options described above for generating a swirl flow with the help of duct geometry, the flow of the air can be forced to rotate around the vertical axis by eccentrically machining the valve seat rings. Besides, masked and shrouded valve arrangements are the alternative designs to obtain the desired swirl inside the cylinder. Although swirl generation in diesel engines has been common practice for many years, the applied techniques for defining, measuring and analysis the characteristics of such charge motion are still relatively crude. These techniques were developed a couple of decades ago, based on the engine applications of that time, and within the boundaries of available technologies. Next to these more classical techniques, new development tools were introduced in the form of numerical simulations. However, even these methodologies often rely on significant simplifications with regard to flow field characterization. As diesel engines have undergone a step development curve in recent years, it is appropriate to reassess current practices for the characterization of in-cylinder flow during combustion system development again, using state of the art numerical and experimental techniques. Laser Doppler Velocimetry (LDV), Hot Wire Anemometer (HWA) and Particle Image Velocimetry (PIV) are the main systems used to investigate the flow behaviour in the cylinder. However, there are a few disadvantages of these techniques; LDV and PIV are non-destructive but also costly measurement techniques since both techniques require optically accessible test specimens. On the other hand, HWA testing is not capable to measure all the flow field considered. Because of the disadvantages of these techniques, a steady-state flow port bench testing method is developed to measure the swirl ratio or tumble ratio in the cylinder. In the present thesis study, in-cylinder flow behaviour is investigated for the diesel internal combustion engines by using state of the art numerical and experimental techniques. The steady-flow port bench testing, PIV measurements and numerical simulation methods are used the determine the flow behaviour inside the cylinder. In addition, endoscopic visualization technique is used to observe the effects of the surface roughness and the manufacturing problems on the flow behaviour for the considered engine structures. The steady-state port bench is used to measure the swirl ratio which includes a honeycomb and a torque meter located under the honeycomb to measure the torque resulting from the swirl motion inside the cylinder. In such experimental set-up, the head is mounted on top of an open ended cylinder. A constant pressure difference across this set-up induces a steady flow of air. The achieved mass flow rate gives an indication of the cylinder head's efficiency. Either a paddle wheel velocimetry or impulse torque meter is used to measure the intensity of the swirling air flow that passes through the cylinder for several fixed positions of the intakes valve. The measurement results are used to calculate a global non dimensional swirl number for which various mathematical definition exist. Such global numbers are commonly used to evaluate combustion system performance as a function of charge motion intensity, either by means of engine dynamometer testing or through numerical simulation. Since the test bench used in this study is a product of FEV which is an engineering company, relevant swirl ratio definition is used for the evaluations. Results are evaulated in terms of repeatability and reproducability first. After that, measurements are conducted for 3 different engine structures and for a plastic prototype. During the tests, a huge discrepancy is observed for one of the cylinder heads and endoscopic camera is used to investigate the cylinder head to understand the surface roughness and manufacturing problems. Endoscopic examination showed that the manufacturing problems can even reverse the swirl direction and also surface roughness has a significant effect on the flow coefficient and swirl ratio. In this study, a numerical methodology is also developed by using the commercial CFD code namely Converge to create a virtual steady-state port bench. Since the swirl ratio is the key parameter defined in the very initial phase of the engine development projects, a validated numerical model becomes really important to built the first time true intake port designs. The main reason of using Converge is about the advantage on grid generation. Converge allows user to generate mesh according to the domain characteristics in terms of velocity, temperature etc. during the simulation by adaptive mesh refinement (AMR) feature. The numerical model solves the Reynolds Averaged Navier Stokes (RANS) equations with the finite volume approach. RNG ( Re-Normalization Group) k-ε turbulence model is used for the numerical simulations. RNG k-ε turbulence model which is a two equation model gives more accurate results at low reynolds number flows and swirly flows due to its additional term in calculation of turbulent stress. The test bench measurements are used to validate the CFD results in terms of accuracy on predicting the mass flow rate and the swirl ratio over the valve lifts. The first step of the validation study is to predict the mass flow rate accurately with numerical simulations, since the swirl ratio has a reverse relation with the square root of the mass flow rate. The validation study is conducted for two different engine structures. The results showed a perfect agreement with the measurements and proved the viability of virtual testing as an important alternative to experiments. Following the port bench measurements and CFD validation study, a PIV test set-up is built to observe the swirling motion visually. In addition to that, a plastic prototype, which can be mounted to both test systems used in this study, is produced with rapid prototyping machine and used for the PIV measurements. Resulting flow field is compared with the CFD solution and discussed in detail. PIV measurements showed that the vortex center is located between two intake valves and the location is predicted accordingly in the CFD simulations. On the other hand, the comparison showed that the steady-state simulation with RANS equations is not capable of simulating the instantaneous time-depent swirl and turbulence effects precisely. For better resolution and accuracy in terms of actual flow field and the velocity distribution, Large Eddy Simulation (LES) can be used for further investigations. Since, Large Eddy Simulation solves the large eddies instead of modelling and gives higher accuracy for the flow distribution on the flow field. As a conclusion, CFD simulations showed a good agreement with the experimental results in terms of mass flow rate and swirl ratio calculations. Mass flow rate values are predicted within %5 error margin and swirl ratio values are predicted within %10 error margin. This proves the viability of numerical method as an important alternative to experiments and the present validated numerical methodology gives chance to design for the first time true and proper intake port geometries to observe the better engine efficiency, fuel consumption and emission level. In addition to that, by using the numerical method, further optimization studies can be conducted by using recently developed optimization algortihms such as, genetic algorithm or design of experiment. Also the plastic prototype approach introduced in this study, can be used to validate the design alternatives before having a production cylinder head or metal prototype.