Ağır ticari taşıt dizel motoru SCR sisteminin sıcaklık modellemesi
SCR system temperature modelling of a heavy duty vehicle diesel engine
Yazar: OZAN AKBAL
Danışman: PROF. DR. HÜSEYİN ERTUĞRUL ARSLAN
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Makine Mühendisliği Ana Bilim Dalı / Otomotiv Bilim Dalı
Derece: Yüksek Lisans, 2013
Konu: Makine Mühendisliği = Mechanical Engineering
Dizin: Egzoz emisyonları = Exhaust emissions

Özet:
Dünyada nüfus yoğunluğunun artmasıyla birlikte, ulaşım ve nakliye faaliyetlerindeki artış egzoz gazlarının artmasına neden olmaktadır. Birçok ülkenin bu tür egzoz emisyonlarını azaltmayı amaçlayan Kyoto Protokolü'nü imzalaması sebebiyle, özellikle son yıllarda dünyada birçok motor ve taşıt üretici firması egzoz emisyonlarının azaltılması için çalışmalar yapmaktadır. Bu sebeple, hava kirliliğini önlemek amacıyla motorlu araçlara getirilen Euro emisyon standartlarına uygun yeni motor, yeni egzoz sistemleri ve yeni yakıtların kullanımı AB ülkelerinin ardından ülkemizde de zorunlu hale gelmiştir. Türkiye, AB yönetmeliklerine uyum sürecinde daha önce yürürlükte olan emisyon standartlarını AB standartlarına göre yeniden düzenlemiştir.
Euro VI egzoz emisyonu standartlarının önümüzdeki yıllarda uygulanacağının bildirilmesiyle birlikte, ağır ticari taşıt üreticileri, egzoz emisyonlarını azaltıcı çalışmalarına hız vermişlerdir. Bu yeni standartlardaki emisyon limitlerine ulaşabilmek için motora çeşitli eklemeler yapılması, farklı bileşimdeki yakıtların kullanılması ve egzoz gazlarının farklı dönüşümlere uğratılması yolunda çalışmalar yapılmaktadır. Bu çalışmalardan en popüler olanı SCR (Selective Catalytic Reduction) sistemi çalışmalarıdır. SCR sistemi, AdBlue adındaki sulu üre çözeltisini katalitik reaktöre enjekte ederek, azotoksit emisyonlarının havada bulunan zararsız azot gazına ve suya indirgenmesini sağlamaktadır.
AdBlue sıvısının belirli bir sıcaklık değerinin altında donuyor olması sistemin en önemli sorunlarından birisidir. Bu durumu engellemek için, SCR sistemi elektriksel olarak ve motordan gelen sıcak su tesisatı ile ısıtılmaktadır. Sistemin kontrolü ve istenilen emisyon değerlerinin sağlanabilmesi için, sistem komponentlerinin yüzey sıcaklıklarının ve egzoz gazı sıcaklıklarının bilinmesi şarttır. Bu sıcaklık değerleri; ortam sıcaklığı, ortam basıncı, yakıt giriş ve çıkış sıcaklığı, yakıt giriş ve sıcaklık basıncı, motor içindeki havanın sıcaklığı ve nem oranı, motor suyu sıcaklığı, egzoz karşı basıncı, motor yağı sıcaklığı gibi birçok parametreden etkilenmektedir. Dizel motorlarda bulunan bütün SCR komponentlerinin sıcaklık sensörleriyle donatılması çok pahalı bir yöntem olduğundan, sıcaklık sensörleri sayısını minimumda tutmak öncelikli amaçtır. 
Bu tez çalışmasının öncelikli amacı, sıcaklık sensörleri miktarı optimize edilmiş SCR sisteminin ortalama sıcaklık değerlerini yansıtan modelin oluşturulmasıdır. Sistemin sıcaklık modellemesi, fiziksel sıcaklık değerleri ve yapılan ölçümlere göre yapılmaktadır. Modelleme için, değişiklik değerlere göre simulasyon yapılabilen Matlab/Simulink programı kullanılmıştır. Bu tez çalışmasında ayrıca, SCR sistemi ve sistemin sıcaklık modellemesinin yanısıra, diğer emisyon azaltıcı sistemler, dizel motorların genel özellikleri ve emisyonları, yeni egzoz emisyon standartları, ağır ticari taşıt üretici firmaların çalışmaları gibi konulara da değinilmiştir.

Abstract:
Air pollution is a major problem for public health in most cities of the developing world. Researches and studies show that air pollution in developing countries is the reason for lots of deaths and waste of billions of money as medical costs. These loses cause poorer quality of life and decrease effectivity of society in all sectors. Moreover, air pollution is a major environment related health threat to human beings and a risk factor for both acute and chronic respiratory disease. Unfortunately, people have to accept the smell of engine exhaust as a part of everyday life. Therefore, exhaust emission legislations all over the world aim protecting air quality by setting limits for chemicals released from vehicles, requiring newer engines to be less polluting and restricting harmful gas level normally found in outdoor air. 
Over the past ten years, hundreds of studies have been published in the literature demonstrating effects of air pollution which causes serious illnesses like asthma, chronic obstructive pulmonary disease, cardiovascular disease, diabetes, and lung cancer. Lots of people all over the world suffer from these illnesses. Children and elderly people who have weaker immune systems, are at special risk. During the past decade, scientists have also confirmed the relationship between air pollution and cardiovascular disease. 
In theory, you should be able to burn a hydrocarbon fuel like petrol, diesel or gas with air in an engine to produce just carbondioxide and water. The rest of the exhaust would be the nitrogen that came in with the air. Unfortunately the fuels we burn comprise hundreds of differently structured hydrocarbons that burn in different ways and at different rates. This means that in practice the exhaust contains some that were partially burned, some that reacted with others and some that reacted with the nitrogen. If they once released into the air, exhaust emissions are breathed in and transported in the bloodstream to all the body's major organs.
The harmless exhaust products are nitrogen, oxygen and water. Carbondioxide is a nontoxic gas but contributes towards acidification of oceans and one of the most important greenhouse gases. Governments around the world are pursuing policies to reduce carbondioxide emissions to combat global warming. Carbonmonoxide  results from incomplete combustion of fuel. It reduces the ability of blood to carry oxygen and can cause headaches, respiratory problems and death at high concentrations. Nitrogenoxides are produced in any combustion process. Nitrogenoxide emissions are oxidised in the atmosphere and contribute to acid rain. They also react with hydrocarbons to produce photochemical oxidants, which can harm plants and animals. Sulphurdioxide occurs naturally in the crude oil from which petrol and diesel are refined. It forms acids on combustion leading to acid rain and engine corrosion. It also contributes to the formation of ozone and of particulate matter. Sulphur can also adversely affect the performance of catalytic converters and is now removed from both petrol and diesel during the refining process. Hydrocarbons are emitted from vehicle exhausts as unburnt fuel and also through evaporation from the fuel tank, from the nozzle when you fill up and also at stages through the fuel supply chain. They react with nitrogenoxide in sunlight to produce photochemical oxidants like ozone, which irritate the eyes and throat. Benzene occurs in small quantities in petrol and diesel. Benzene is emitted from vehicle exhausts as unburnt fuel and also through evaporation from the fuel system although modern fuel systems are sealed and have carbon canisters to hold the vapours. Benzene is toxic and carcinogenic. It has a suppressive effect on bone marrow and impairs the development of red blood cells. Longterm exposure of benzene can cause leukaemia. Lead accumulates in body systems and is known to interfere with the normal production of red blood cells. This can lead to anaemia which can reduce the body's ability to circulate oxygen and vital nutrients.  After the introduction of unleaded petrol, lead can be eliminated as an exhaust product. Particulate matter is partly burned fuel associated mainly with diesel engines. Some of particulate matters are very small particles that can pass deep into the lungs causing respiratory complaints. Modern diesel cars are fitted with diesel particulate filters to stop these particles passing into the atmosphere.
Diesel fuel is widely used all over the world. It powers heavy duty vehicles like trucks, buses, agricultural equipments and also backup generators. It is also used for many other applications. Diesel engines have historically been more versatile and cheaper to run than gasoline engines or other sources of power. Unfortunately, the exhaust from these engines contains substances that can pose a risk to human health. Diesel exhaust is produced when an engine burns diesel fuel. It is a complex mixture of thousands of gases and fine particles. These include many known or suspected cancercausing substances, such as benzene, arsenic and formaldehyde. It also contains other harmful pollutants, including nitrogen oxides.
Exposure to diesel exhaust can have immediate health effects. Diesel exhaust can damage organs like eyes, nose, throat and lungs and it can also cause coughs, headaches, lightheadedness and nausea. In studies with human volunteers, diesel exhaust particles made people with allergies more susceptible. Exposure to diesel exhaust also causes inflammation in the lungs, which may aggravate chronic respiratory symptoms and increase the frequency or intensity of asthma attacks.
Diesel engines are a major source of fine particle pollution. The elderly people with emphysema, asthma, and chronic heart and lung disease are especially sensitive to fine particle pollution. Additionaly, because children's lungs and respiratory systems are still developing, they are also more susceptible than healthy adults to fine particles. Exposure to fine particles is associated with increased frequency of childhood illnesses and can also reduce lung function in children. Diesel engines also produce nitrogenoxides. Nitrogenoxides can damage lung tissue, lower the body's resistance to respiratory infection and worsen chronic lung diseases, such as asthma. They also react with other pollutants in the atmosphere to form ozone.
European exhaust emission standards first came into force in 1970 and have been tightened up progressively ever since. This means that vehicles are cleaner now then they have ever been and are set to get cleaner still. According to European type approval regulations, vehicles have to pass a collection of safety and environmental tests to be sold in the European Union. Therefore Euro standards effect the construction of the vehicles. Heavy duty vehicle emission standards were first introduced in Europe in 1988. After these years, many countries have developed their own regulations that are aligned in large part with the European standards.
After the first standard in 1970, the next important change came with the introduction of Euro 1 standard. This standard forced catalytic converter usage in gasoline engines to reduce carbonmonoxide emissions. Euro 2 regulation reduced the acceptable limit of carbonmonoxide emissions more. It also reduced hydrocarbons and nitrogenoxides for both petrol and diesel vehicles. Euro 3 modified the test by eliminating the fourty second engine warmup period before the beginning of the emission sampling. It also reduced permitted carbonmonoxide emissions from gasoline and diesel engines. Euro 4 and Euro 5 regulations concentrated more on cleaning up emissions from diesel engines, especially reducing particulate matter and nitrogenoxides. Some Euro 4 diesel engines were equipped with particulate filters, but all diesel cars needed particulate filters to meet the requirements of Euro 5.
Euro VI emission standards were introduced by Regulation No 595/2009. It published on 18 July 2009 in Europa and on 03 August 2011 in Turkey. The new emission limits will become effective in 2014 for all registrations in Europe and in 2016 for all registrations in Turkey. 
Euro VI regulation contains an emission standard of 0,01 g/kWh for particulate matters and 0,4 g/kWh for nitrogenoxides. Compared to the Euro V emission standards, this involves reductions of 67% for particulate matters and 80% for nitrogenoxides. For European countries, the new requirements for particulate matters and nitrogenoxides are the most important in view of European air quality norms. The aim of Euro VI standards of the European Commission is a further reduction of the emissions of vehicles with heavy duty diesel engines. The basic idea is to improve air quality and limit the negative effects of air pollution on human health and nature. Although air quality has improved over the past decade, there are still significant air quality problems in European countries and in Turkey, especially in urban areas and in densely populated regions.
Because new Euro VI regulation is applied at the European level, it will not create trade barriers for European and Turkish manufacturers of vehicles with heavy duty diesel engines. The regulation also states that Euro VI vehicles will eventually be tested according to World Harmonized Transient Cycle (WHTC) and World Harmonized Stationary Cycle (WHSC). In the future, these test cycles must be applied worldwide. The main aim of these two cycles is to reduce the testing costs for manufacturers and to implement worldwide emission requirements. Access to a global market with harmonized norms could improve the competitiveness of European and Turkish manufacturers. Euro VI requirements are also very similar to the new American emission requirements for vehicles with heavy duty diesel engines.
For decreasing exhaust emissions, better after treatment methods are required by heavy duty vehicle manufacturers. Most popular after treatment method is SCR system which means Selective Catalytic Reduction. SCR system injects AdBlue, a mixture of urea and deionized water, into a catalytic reactor in which the urea reduces nitrogenoxide emissions to nitrogen gas and water.
Most important problem with AdBlue is the fact that it freezes at low temperatures. To prevent it from freezing, the components in the SCR system may be heated, both electrically and with hot coolant water from the engine. Due to lack of temperature sensors, it is difficult to define if the AdBlue in all system components are liquid or solid. The problem in that point is installing more sensors is very expensive. 
In this study it is examined if it is possible to estimate the average temperature in different points in SCR system with the use of available temperature sensors. The estimation is done by using measurement datas to determine unknown parameters. The work has been done in a model which is constructed in Matlab/Simulink. In addition to SCR system temperature modelling, SCR system details, other emission reducing systems, general characteristics and emissions of diesel engines, new exhaust emission standards for heavy duty vehicles, studies of popular heavy duty vehicle manufacturers can be found in this thesis work.