Bir dizel motorunda seramik kaplama uygulamaları ve performans analizi
Ceramic coating applications and performance analysis in a diesel engine
Yazar: EKREM BÜYÜKKAYA
Danışman: DOÇ.DR. VELİ ÇELİK
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü
Derece: Yüksek Lisans, 1994
Konu: Makine Mühendisliği = Mechanical Engineering
Dizin: Dizel motorlar = Diesel engines ; Performans analizi = Performance analysis ; Seramik kaplama = Ceramic coating

Özet:
ÖZET Bu çalışmada, tabii emmeli, çift silindirli bir dizel motorun yanma odası elemanlarının (piston, supap ve silindir kapağının) kaplanmasıyla elde edilen performans özelliklerinin, normal motorun ki ile karşılaştırılması esasına dayanmaktadır. öncelikle motorun performans özelliklerini tesbit etmek amacıyla, sabit bir yüke karşılık motor hızı değiştirilerek her devir sayısı için motor karekteristik büyüklükleri, sisteme yerleştirilen ölçüm cihazları ile ölçülmüş, faydalı enerji miktarı, soğutmaya transfer edilen ve egzoz enerji miktarlarını hesaplamak için de bilgisayar programı yapılmış ve alınan değerler grafikler şeklinde sunulmuştur. Doğrudan deneyden alınan değerlerle yapılan mukayese ve bunlara bağlı hesaplamalar sonucunda, seramik kaplı motor ile normal motor için eş özgül yakıt tüketimi eğrileri dört bölgeye ayrılarak çizilmiştir. Bu eğriler üzerinde yapılan incelemelerde, fazla bir fark görülmemesine karşılık kaplamalı motorun bazı noktalarda avantajlı olduğu gözlenmiştir (Pme=3.75 bar, n=1400 d//d). Aslında düşük yük ve devirlerde elde edilebilecek avantajlar son derece önemlidir. Yüksek devir ve tam yük durumlarında aynı gelişme görülmemesine rağmen, düşük ve orta yük ile hızlarda çalışması istenen motorlar için dikkate değer bir yakıt ekonomisi sağlanabilir. Çalışmalar sonunda elde edilen egzoz gazı sıcaklıklarındaki artışlar, seramik ile kaplanmış olan motorun son derece avantajlı olduğunu göstermiştir. Kullanılan kaplama metodununda (Plazma Sprey) uygulanabilirliğini açık bir şekilde ortaya koymuştur. Soğutmaya olan ısı geçişini tesbit edip, soğutma suyu debisi ve sıcaklık farkıda ölçülerek diğer bilinmiyenler hesaplanmıştır. Bunların yanısıra yalıtım kaplamasının sağladığı ısı kayıbı azalması sebebiyle dizayn parametrelerindeki değişmenin motor performansına etkileri araştırılmıştır. Bu çalışmalar ışığında varılan sonuçlar, daha önce yapılan çalışmalar sonucunda elde edilen bulgularla karşılaştırılmış, sağlanan başarılar ve beklentiler bildirilmiştir. ix

Abstract:
CERAMIC COATING APPLICATIONS AND PERFORMANCE ANALYSIS IN A DIESEL ENGINE SUMMARY The major goals of the Adiabatic Engine Project are to remove the water cooling system, reduce heat rejection, and improve thermal efficiency. Elimination of the cooling system includes the radiator, water pump, fan, belts, etc.. Potentially, long term redesign would address engine block and cylinder head cooling passage elimination. The thermal efficiency improvements would be accomplished by turbocompanding the high temperature exhaust gas (converting thermal energy to mechanical energy). Turbocompounding is basically composed of a turbocharger which is geared to the flywheel. In order to accomplish the major goals, effective insulated components (piston, head and liner) are required. In an effort to improve design insulation effectiveness, reliability, and engine performance, a design philosophy is established with the following objectives in mind: - Maximize insulation - Insulate at the combustion surfaces - Minimize design complexity - Maintain reliability - No forced cooling --> Design for potential long term durability - Avoid high alloy steels - Minimize engine design changes - Retain engine performance After recognizing the major project goals and establishing a list of objectives, a strategy is identified as follows: - Insulate at the combustion surface - Use insulating ceramic - Use a high thermal expansion ceramic - Use ceramic-metal assemblies - Retain current structural metals - Retain current combustion chamber Focusing on the overall development goals and establishing a set of objectives is necessary such that a compenent design strategy could be formulated. This process is considered essential in order to limit the number of alternative design approaches which would be addressed. Important factors which influence the strategy that iscoosenincluded such things as: The project is mainly technology development and aplication oriented, use of non-strategic materials, compenents experience temperatures which are higher than most metals are suited for, the project is not restrained by economic trade-off considerations or material costs, and demostration goals and deadlines are specified. Taking into account the overall project goalls, related factors, and established objectives a strategy is established which concentrated on development of ceramic insulating compenents. A low thermal conductivity ceramic material is choosen because it would potentially satisfy the objectives of providing insulation at the combustion chamber surface, provide maximum insulation effectiveness, and reduce the design complexities associated with attaching and forming based structurally equivalent thermal barriers. Although air is a very low thermal conductor and air gaps can be used to provide respectable thermal barriers, the metal portion which is located between the combustion chamber and the air gap will be operating at very high temperatures, experiencing large transient temperature changes, experiencing large thermal expansion distortions, and be acting as a regenerator storing heat energy during the power stroke and releasing energy during the intake stroke. Thermal-barrier coatings (TBCs) have been proposed for use in diesel engines to increase component durability and to function as corrosion barriers. However, most current research is aimed at the use of thermal barrier coatings in producing is to leave more heat in the hot working gas so that the gas can do other work such as turn a turbocharger. Tha main purpose of a thermal barrier coating in an adiabatic engine, therefore, is to reduce heat transfer to components to achieve a direct increase in engine efficiency, not to protect metallic components from degradation. Reduction of heat transfer by eliminating cooling in a turbocharged diesel engine has been estimated to achieve a 1% efficiency increase. Coating the critical components, which are the piston crown and fire deck (cylinder head), with thermal barrier coatings has been estimated to provide an additional 2% increase in efficiency. Exhaust valves and cylinder liners, which support little heat flow, are not considered critical components for thermal barrier coatings. Exhaust valves are, however, candidates for coatings to increase their durability. The conditions that exist in an adiabatic diesel engine are very different from those that exist in gas-turbine engines. Adiabatic diesel engines have a low level of cooling, low average heat fluxes (but very high momentary heat fluxes), and low peak gas temperatures (to 800°C, 1475°F). In contrast, gas turbines operate under conditions of a relatively high steady state heat flux and high temperature. The insulating layer for a low heat flux condition must be thicker to achieve a similar temperature drop as for a high heat flux condition. The greater ceramic coating thickness required in adiabatic diesel engines adds a considerable ammount of thermal expansion mismatch stress to the coating layer due to constraint within the ceramic. The high stresses can cause failure of the thermal-barrier coating even under the relatively mild conditions that exist in diesels. Consequently, it is necessary to grade the coefficiant of thermal expansion of the thick coatings to distribute the stress through the coating thickness. Grading is XIaccomplished by applying a 100% metallic bond coat followed by successive layers ofcoating with decreasing amounts of bond-coat material and increasing fractions of partially stabilized ziconia (PSZ) ceramic. Such a coating has a bulk expansion coefficiant that changes gradually from the substrate to the outer surface. Current work has achieved long thermal barrier coating life by using up to five grading layers from bond coat to top coat to achieve a final coating thickness of approximately 0.25 cm (0.1 in.). This compares to a coating thickness of only 0.00020 to 0.038 cm (0.005 to 0.015 in.) for aircraft gas turbines. The work in diesel thermal barrier coatings is one of the newest areas for thermal barrier coating research and it is still in its early stages. The relatively wide alloable rangeof coating thickness and the low operating temperatures that minimize oxidation concerns has created many options for balancing insulation and durability. Current development includes variations in coating thickness, grading, strength and porosity. It may be appropriate at this point to recognize that the best choice insulating ceramic and the ideal ceramic are two very different materials. If the ideal material were available the design strategy would be modified significantly from that described earlier. The ideal thermal conductivity and a very zero would be most ideal they would also be most ideal they would also be most unrealistic. For a given design, a set of realistic ideal numbers could be defined. The ideal material would have a very low expansion rate which could be determined by evaluation of operating temperature distributions, material expansions, thermal conductivities, elastic modulus, etc... In some design cases the component would be a monolitic ceramic piece and in others it would be a ceramic-metal assembly, all depending on the design restraints which might be influenced by factors which are not necessarily technical in nature. The ideal material would have a tailored elastic modulus to satisfy the design where deflections were important to control, a high elastic modulus would be desirable. If a design included a ceranic component which was not a structural supporting member, but was included only for insulation or wear control purposes, and experienced high degrees of distorsion, then a low elastic modulus material might be desirable. Given the fact that no ideal material is available nor seen in the horizon, a review of the design objectives and previous experience indicated that a material with the following property trends is the most desirable compromise: - Low thermal condutivity - High thermal expansion - Low elastic modulus - High thermal expansion - High use temperature - High strength - High toughness xnAlthough for some components the least potentially complex design could be a monolithic component (entire piston of ceramic, for example), ceramic- metal assembly designs are choosen to provide the least risk in case of a failure associated with the ceramic. In addition, a ceramic material with a relatively high expansion rate, close to that of the metal portion, is desirable in order to keep the attachment design complexity to a minimum. Attachment techniques which are porposed included casting, brazing, and interference press-fit. From experience, reliability has been related to some degree with design complexity. Furthermore, as a result of intrinsic properties of most ceramics, there is a potential for greater high temperature long term durability as opposed to most metals. Finally, in order to limit the amount of engine redesign it is decided that a current production base engine be used and insulated components be integrated inte this engine. The project is not oriented toward development of a new engine which could take advantage of elimination of water cooling passages, etc.. Rather, the project is meant to develop and demostrate insulating technology. In order to reduce the production costs and fuel consumption in automative industry, research and technological development works are under way on this field. The use of ceramic parts in lieu of metal in internal combustion engines has been an important one among these developments. On this subject, several important steps have been taken and mass production of engine parts made of ceramics has been started in the past years. In this study, a naturally aspirated, water cooled, direct injection, and four stroke diesel engine with two cylinder has been tested to determine the performance characteristics by using a hydrolic dynamometer. In this engine, top surface of the piston, part of the cylinder head, exhaust valve surfaces which are in contact with the combustion gases (which forms the upper side of the combustion chamber) have been cooted with a 0.5 mm layer of low-heat conduction ceramic material (MgO+Zr02 PSZ). The thermal expansion coefficient of the coating material and the base material (strata) should be close to each ather in order to have a succesful thermal coating on the base material. Also, the stisfactoriness of the performance of the coating has been tested because the bonding layer, coating technique, type of ceramic coating power, and the coating thickness affect the quality/success of the coating. Before the coating process started, a particular thickness of all the surfaces to be coated has been removed to keep the original design dimensions/sizes of the coated motor parts. Computer simulation results based on a normal engine data have been presented in tables and figures. It is also observed that the amount of heat lost to the cooling water decreases proportionally as the engine speed increases, and that heat loss increases when the mean effective pressure is constant. Exhaust gas temrature and exhaust energy have increased quantitatively and proportionally as the engine speed increased while mean effective pressure did not change. Effective power and the total energy increases as the engine speed increases. xmExpansion and contraction values in intake and exhaust valves and cylinder head have been measured with a sensitivity of 0.01 mm by using a special experimental adaptör equipped with 4 different comparatore to determine the effects of ceramic coatings on engine valve mechanism (springs, valves and connection rod, etc.). Effects of the coating on the opening and closing time of valves, open and overlop periods of valves have also been searched. Increases in surfaces temperatures cause some deviations from the original standart design and operation conditions in the valve mechanism of the engine due to different expansions of the valves and the cylinder head. New design and operation conditions should be set in the light of new experimental outcomes. Performance maps have been drawn with axis of the mean effective pressure, engine speed and the equivalent specific fuel consumption by using experimental results. In these graphs, equivalent specific fuel consumption is a function of mean effective pressure and the engine speed. Both uncoated normal engine and the coated engine have been tested under two different set of value adjustments (conditions) and the results have been compared. For further examination, performance maps have been divided into four seperate regions. These are; 1. Low load, low speed 2. High load, low speed 3. Low load, high speed 4. High load, high speed The region in which minimum specific fuel consumption occurs and the other regions have been evaluated one by one and comparatively. As a result of this evaluation, several important results have been derived such as conditions in which engine operates more economically and an appropriate usage purpose of a particular engine (for stationary purpose or for vehicles). For example, for an engine operated at low speed and low mean effective pressure, the best and the most advantageous operation condition is the first region which has the characteristics of low speed-low mean pressure engine operation. Other ways of getting more benefit from an engine by turbocompounding and turbocharging have also been examined by measuring exhaust gas temperature. There is an increase in the useful energy of adiabatic engine because of revaluation of the high temperature exhaust gases. As a result of experimental measurements, several new arrangements in the cooling system have also been searched due to the decrease in the"heat transfer to the cooling system. There is also a decrease in the temperature of cooling water and wall (cooler side) due to the decrease in the heat loss to the cooling system. Increased gas- side fuel consumption by 4%, and increased engine speed by 1.6% affecting combustion efficiency. There is also an increase in the engine power due to the decrease in the load of cooling system. xivA computer programme which determine the amounts of heat energies to the cooling water, exhaust, and useful power has been prepared. In the test engine, the daesel fuel produced by Ipraş has been used. The closed formula of this fuel is C15H28. The general combustion equation of the fuel is as follows; C15H28+ X Omin(02+ 3.76N2) ^cC02 + hH20 + o02 + c,CO + h,H2 + nN2 + c2C xv