Otomobil egzoz sistemlerinde kullanılan katalitik konvertörlerin ısıl analizi
Thermal analysis of catalytic converters used on automobiles
Yazar: GÖRKEM TAŞ
Danışman: PROF. DR. MUSTAFA ÖZDEMİR
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Makine Mühendisliği Bölümü / Makine Mühendisliği Ana Bilim Dalı / Isı-Akışkan Bilim Dalı
Derece: Yüksek Lisans, 2015
Konu: Makine Mühendisliği = Mechanical Engineering

Özet:
İçten yanmalı motorlar özellikle ikinci dünya savaşından beri oldukça yaygın bir kullanım alanı bulmaktadırlar. Performans ve verim değerlerinin artırılması için yapılan birçok çalışmanın yanı sıra atmosfere saldıkları zararlı gazlar nedeniyle de birçok çalışmaya konu olmaktadırlar. Bunun nedeni motordan salınan egzoz gazlarının insan sağlığı ve çevreye zararlarının oldukça fazla olması ve her geçen gün küresel ısınma ve çevre kirliliği gibi etmenler dolayısıyla daha fazla fark edilmesidir. İçten yanmalı motorların en yaygın kullanıldığı alan otomobil
endüstrisidir. Dünyada bugün yaklaşık 700 milyon otomobil olduğu ve 2030 yılında bu sayının 1,3 milyara çıkacağı da düşünüldüğünde yapılan bu çalışmaların gerekliliği görülmektedir[5]. Otomobillerin çevresel etkilerini azaltmak için kullanılan teknolojilerden birisi egzoz sistemine entegre edilen katalitik konvertörlerdir. Bu çalışmada içten yanmalı motorlardan salınan zararlı gazları dönüştürerek çevre ortama zararsız bir şekilde atılmasını sağlayan katalitik konvertörlerin ısıl davranışları incelenmektedir. Çalışmaya konu olan katalitik konvertör Ford marka bir ticari araçta kullanılmaktadır. Katalitik konvertör içerisinde oluşan kimyasal tepkimeler incelenerek bir boyutlu ve heterojen tepkime kabulü aracılığıyla yeni bir matematiksel model önerilmektedir. Bu model oluşturulurken literatürde daha önce yapılan çalışmalardan yararlanılmaktadır. Çalışmada 2 aşama bulunmaktadır. İlk aşamada katalitik konvertörde üretilen ısı miktarı hesaplanmaktadır. Bunun için literatürde daha önce yapılmış çalışmalar incelenmiştir. Bir boyutlu matematiksel model kurularak bu modelin değişen koşullar için ürettiği ısı miktarlarını ortaya çıkarması hedeflenmiştir. İkinci aşamada ise katalitik konvertör içerisindeki sıcaklık değişimini radyal ve eksenel olarak hesaplayan bir matematiksel model önerilmektedir. Matematiksel model ortaya çıkarılırken Bessel denklemleri kullanılmaktadır ve sonuç olarak bu model verilen her koordinatta istenen sıcaklık değerini hesaplamaktadır. Çalışmanın daha sonraki kısmında daha önce matematiksel model kullanılarak yapılan hesaplamaların Ford laboratuarlarında alınan ölçümler ile karşılaştırılması ve hesaplanan değerlerin ne ölçüde gerçeğe uygun olduğu yorumlanmaktadır. Sonuç olarak bu matematiksel modeller kullanılarak yapılacak yeni tasarımların test yapılmadan önce istenilen sıcaklıklara çıkıp çıkmayacağının kullanışlı bir yoldan
anlaşılması amaçlanmaktadır.

Abstract:
Today the world's one of biggest problem is environmental pollution. There is huge variety of pollutants caused by men-kind inventions such as ships, factories, passcars, commercial vehicles and they became a must for the daily life. In order to eliminate these pollutants from the nature, a lot of research centers and universities are making studies on those fields. Since automobiles are one of the biggest pollutant factor, there are also many researches and studies going on. Especially in big cities, exhaust gases released by automobiles are crucial for human health. There are millions of vehicles in most of the big cities and those exhaust gases are causing brutal health problems including cancer and premature deaths because the gas is stuck in a small area and recirculation of air is most of the time quite limited.The exhaust gases released from the internal combustion vehicles are also causing environmental damage. NOx is harmful for ozone layer reducing the thickness of it. Also CO2 is the roots cause of green gas effect hence the global warming.Wide use of internal combustion engines was started at early 19th century but those harmful effects were realized by the society after 1950. Some organizations and environmental foundations started to complain about the pollution caused by the vehicles. Therefore those actions pushed governments to act against the pollution from automobiles.Starting from 1950's, emission levels of exhaust gases were decreased. Car manufacturers were requested to produce vehicles which produce less pollution. Therefore car manufacturers started to study on emission reducing systems.First studies were done by US manufacturing companies because US government was the first to release the regulations for emission levels. There was different type of technologies studied to reduce the emissions. However the catalytic converter technology was only started to implement after 1970's because until that time, other technologies were sufficient to comply the emission requirements.What catalytic converters do is to converter the pollutant gases into nonpolluting ones. This is happening by reducing the required activation energy of reactive gases and making them react with O2 at lower temperatures. The effect of activation energy reduction comes from the precious metals on the substrate surface inside catalytic converter.After 1970's, with more restricted emissions, catalytic convertors were started to be used at most of the vehicles. Because catalytic convertors have many advantages compared to other emission control systems. They are not requiring additional energy input, not moving physically and quite effective to remove the pollutants. The disadvantages are there is need space to fit the converter to the vehicle and metals inside the catalytic converter are even more expensive than gold.Of course there are other technologies used as integrated with catalytic converters. The on-board diagnostic systems which is checking and monitoring pollution in exhaust gas, exhaust recirculation system and some others are also used in automotive industry. Therefore while studying the exhaust gases, all those systems are being considered. The performances of catalytic converters also depend on those other parameters.One of the most important parameter for catalytic converters is the catalytic reactions. The efficiency of the reaction taking place in the catalytic converter is a matter of temperature. The more efficient reaction occurs, the more pollutant gas is converted to the non-pollutants therefore this is one of the biggest area for the studies.
Another parameter for catalytic converter is to take a place under the vehicle as small as possible. This is because of limited space under the car due to other required components such as fuel system, driveline components and body components. Therefore it is also aimed to make them smaller and in convenient shape to fit the vehicle.
Also due to chemical reactions in high temperatures inside catalytic converters, they are harmful for the components around. They can heat up the components over their material design temperature and hence make them nonfunctional. This point is also taken into account during the design phase of catalytic converters.
In this study, heat production and heat transfer behavior of a commercial catalytic converter is being investigated. The vehicle is one of Ford light commercial vehicle.First a mathematical model is being investigated to define heat production and heat transfer inside the catalytic converter. After that, temperatures at some points are being compared with some test results and the success of mathematical model is being evaluated.In order to find the heat production inside catalyst, some other studies were investigated. There are other studies which developed the heat produced based on temperature, pressure and mass flow of exhaust gas. The studies are showing that there are many ways to solve the heat transfer equations inside catalytic converter such as 1 dimensional, 3dimensional equations and even numerical methods. What we use is a 1d model to have a convenient way for solution.The heat transfer in radial and longitudinal direction is another matter of study. Temperature is changing at both directions and this is most of the time important for efficiency of chemical reactions inside catalytic converter. In order to have the model, a porous media approximation is being used. The substrate inside the catalytic converter is assumed as a porous media and average heat transfer coefficient and velocity is used to define and solve the equation. This way the central temperature of catalytic converter is used as an input to the model and surface temperature is determined. Then this value is compared with the measured value on the vehicle to define how correct the model is.The last study in the thesis's content is the change of temperature depending on the radial and axial dimensions. In order to find this, Bessel equations are being used. This way the mathematical model is computing the temperature at every point of catalytic converter when radial and axial coordinates are entered.To sum up, a detailed behavior of heat production, heat transfer and temperature distribution for a specific catalytic converter is being investigated. This investigation is a step to provide a convenient way to predict some parameters without doing any tests hence saving time and resources.