Termal bariyer kaplamanın turbo doldurmalı bir dizel motorunun performansına etkileri /
Yazar: HALİT YAŞAR
Danışman: PROF. DR. VELİ ÇELİK
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Makine Mühendisliği Ana Bilim Dalı
Derece: Doktora, 1997
Konu: Makine Mühendisliği = Mechanical Engineering
Dizin: Dizel motorlar = Diesel engines ; Enerji tasarrufu = Energy saving ; Motor performansı = Motor performance ; Seramik kaplama = Ceramic coating ; Yakıt tüketimi = Fuel consumption

Özet:
ÖZET Otomotiv endüstrisinde maliyetin düşürülmesi ve yakıt sarfiyatının azaltılmasına yöne¬ lik araştırmalar ve teknolojik yenilik çalışmaları sürdürülmektedir. İçten yanmalı bir motorun performansını artırabilmek, toplam yakıt enerjisinin mümkün olan en fazla oranda faydalı enerji haline dönüştürülmesini gerektirmektedir. Bu motorlarda yanma sonucu açığa çıkan enerjinin ancak %30-40 kadarı faydalı işe dönüşmekte, geri kalan enerji ise; motor parçalarını yüksek sıcaklıktan korumak amacıyla yapılmış oian soğutma sistemine, egzoz gazlarına ve diğer kayıplara gitmektedir. Kayıp enerjiyi faydalı hale getirmek; genişleme zamanındaki faydalı işi artırmak, egzoz ve soğutma sistemine giden ısıları azaltmakla sağlanabilir. Bunun için de yanma odasını teşkil eden parçaların ısıl iletkenliği düşük, yüksek çalışma sıcaklığına dayanabilen bir malzemeyle kaplanması, "Düşük Isı Kayıplı Motor" kavramını ortaya çıkarmıştır. Literatürde bu motorlar için, "Adyabatik Motor" kavramı da sıklıkla kullanılmaktadır. Düşük ısı kayıplı motorlarda, soğutma sistemine giden enerjinin azaltılması sonucu, artan silindir sıcaklığı ve tutuşma gecikmesi periyodundaki küçülme ile birlikte özgül yakıt tüketimi de azalmaktadır. Bu çalışmada, seramik kaplamanın türbo doldurmak bir dizel motorunun performan¬ sına etkisi incelenmektedir. Bu amaçla motorun, silindir kapağı, supaplar ve pistonlar termal bariyer amaçlı olarak zirkonya ile 0,5 mm kalınlığında, plazma kaplama tekniği kullanılarak, kaplanmıştır. Daha sonra, normal ve seramik kaplı motor, değişik yük ve devir şartlarında çalıştırılmak suretiyle test edilmiş ve her iki hale ait deneysel veriler hazırlanan bir biigisayar programı yardımıyla analiz edilerek sonuçlar karşılaştırmalı olarak grafikler halinde sunulmuştur. Termal bariyer kaplama sonucu egzoz gaz enerjisi önemli ölçüde artmaktadır. Egzoz enerjisindeki bu artış, motora türbokombine veya rankin ait çevriminin eklenmesiyle faydalı işe dönüştürülebilir. Bu sebeple çalışmada, egzoz gazlarının kullanılabilir enerji (ekserji)'leri de hesaplanarak seramik kaplamanın egzoz gazlarının exerjilerine etkisi de incelenmiştir. Çalışmada motor, türbo doldurucu devre dışı bırakılarak kaplamalı ve kaplamasız olarak test edilerek seramik kaplamanın tabi ernmeii motor çalışmasına etkileri de incelenmiştir. Çalışma sonuçlan, termal bariyer kaplamanın motorun performansını belirgin şekilde iyileştirdiğim göstermektedir. Seramik kaplı motorun püskürtme avansının azaltılması sonucu yakıt ekonomisindeki iyileşmenin daha da arttığı gözlenmiştir. Motorun tabi emmeli olarak çalıştırılması durumunda, seramik kaplama sonucu motor performan¬ sında elde edilen iyileşmenin türbo doldurmalı motora göre çok daha düşük kalmakta ve bazı çalışma şartlarında ise kötüleşme görülmektedir. xviii

Abstract:
EFFECTS OF THERMAL BARR1EK COATING ON A TURBOCHARGED DIESEL ENGİNE PERFORMANCE SUMMARY The quest for increasing the efficiency of an intemai combustion engine has been going on över since the invention of this reiiable workhorse of the automotive world. in recent times, much attention has been focused on achieving this goal by reducing energy iost to the coolant during the power stroke of the cycle. A cursory look at the internai combustion heat balance indicates that the input energy is divided into roughly three equal parts; energy converted to usefui work, energy transferred to coolant and the energy lost to exhaust. This phenomenon, of course, should conforrn to sonıe physicai !aws. The first law of thermodynamics is satisfied as long as energy is conserved and it does not matter how that energy is apportioned between various categories. The second İaw stipuiates that ali the input energy cannot be converted into work; in other words, it is impossible to obtain i 00 percent efficiency, and so some heat to be rejected, preferably at the lovvest possible temperature to achieve highest possible eificiency. The reduction in, ör the elimination of, in-cylinder heat transfer to either the cooiant and/or the environmenî does not violate the second law of thermodynamics and, moreover, according to the first law, has the potential of producing more work. Added to this, another important advantage of the concept is the great reduction in parasitic losses due to the elimination of coofing system, thus increasing the brake horse-power of the engine. The absence of cooîing fans, pumps, radiators, hoses, ete. can facilitate compact design of power plant and considerably enhance its reliability and maintainability. These prospects of improving îhe design and performance have generated impetus to active research on "adiabatic" ör more appropriately. "low heat rejection (LHR)" ör "insulated" engines [50], Öne of the development trends of heat engines is their energy efficiency improvement. in the case of internai combustion piston engines, öne of the ways to achieve this aim is the engine adiabatization. To create suitabie conditions for the thermodynamic cycle in internai combustion engine, it is essential to construct the elements of the combustion chamber from materials of low thermal conductiviîy. Öne of the possible methods to do this to cover the surface of the combustion chamber with a Thermal Barrier Coating. Thermai insuiation thus obtained is supposed to iead, according to the second law of thermodynamics, to engine heat efficiency improvement and fuel consumption reduction. Test data pubiished in specialist literatüre indicates that the effects achieved are İess significant, than were expected from theoretical calcuiations. Exhaust energy rise which accompanies this, xixcan be effectiveiy used in turbocharged engines. Higher tenıperatures in the combustion chamber can also have a positive effect in diese! engines, due to the ignition deiay drop and hardness of engine operation, though an increase of nitric oxide emission may be expected as welî. The efficiency of most commercially avaiiable diesel engine ranges from 38% to 42%. Therefore, between 58% and 62% of the fuel energy content is lost in the form of waste heat. Approxinıateîy 30% is retained in the exhaust gas and the remainder is removed by the cooling, ete. More than 55% of the energy which is produced during the combustion process is removed with cooling water/air and through the exhaust gas. in order to save energy, it is an advantage to protect the höt parts by a thermally insulating layer. This wiiİ reduce the heat transfer through the engine wails, and a greater part of the produced energy can be utilized, involving an increased efficiency. The Low-Heat Rejection (LHR) Engine design promise to meet the increasingly stringent regulations in the areas of fuel economy and permissible emissions levels. The LHR concept uses insulation of the combustion chamber and exhaust passages to reduce, ör possibly, eliminate heat loss during the cîosed portion of the cycle. According to the fırst law of thermodynamics, any energy which is not rejected to the cooSant remains avaiiable to produce useful piston work. According to the second law of thermodynamics, combustion taking place at higher temperature is also more efficient, as irreversibilities decrease with increasing temperature. The reduction in heat losses also results in increased exhaust entalphy. This extra exhaust energy can be utilized to drive a compounding türbine ör a bottoming cycle, thus improving the overalî system efficiency. The changes in the combustion process due to insulation also affect exhaust emissions. Higher gas tenıperatures shouîd reduce the concentration of incomplete combustion products at the expense of increases nitric oxides. However, researches have measured decreases in carbon monoxide, unburned hydrocarbons, and soot oniy under sorne operation conditions. Adiabatic turbocompound diesel engine concept promises reduced brake specific fuel consumption in future diesel engines as a result of three basic design revisions:.Insuîating the combustion chamber..Removing the cooling system..Utüizing the increased energy in the exhaust gases by turbocompounding. The advantages in operating an adiabatic turbocompound diesel engine are:.Reduced specific fuei consumption, « Reduced emission and vvhite smoke,.Multi-Fuel capability,.Reduced noise level, « İmproved reliability,.Smaller size, XX* Lighter weight. A ceramic layer will prevent heat transfer from the hot working medium to the coolant and surroundings. It will contribute to a reduction of the temperature and heat load of the metallic base and will provide corrosion protection. Reduced heat flow implies that a larger fraction of the fuel energy released during the combustion process will be converted into mechanical power and exhaust energy. The question is how much of the energy can be recovered as increased mechanical work. The adiabatic engine name implies no-heat loss engine. It should be noted that this engine is not adiabatic, or without heat loss, in the true thermodynamic sense; however the engine is without conventional forced cooling and strives to minimize heat loss. The adiabatic engine insulates the diese! combustion chamber with high temperature materials to allow "hot" operation with minimized heat transfer. The "hot" or insulated high temperature components include piston, cylinder head, cylinder liner, exhaust valves, and exhaust ports. Additional power and improved efficiency derived from an adiabatic Engine are possible because thermal energy, normally lost to the cooling water and exhaust gas, is converted to useful power through the use of turbomachinery and high temperature materials. Fundamentally, the adiabatic engine is more efficient than a conventional diesel because it converts the fuel heat energy into additional useful output. By greatly reducing lost energy and essentially eliminating the need for a conventional cooling system, the adiabatic engine dramatically improves fuel economy and provides approximately a 40 percent reduction in weight and volume for the same horsepower fuel. Further, from a military system standpoint, the component volume of the total "under armor" propulsion system could be reduced by 40 percent overall. Elimination of the engine cooling system, including cooling fans, radiators, hoses and shrouds would produce a remarkable increase in reliability and maintainability. The engine would not be sensitive to most conventional cooling systems damage and extreme environmental conditions. Fuel economy improvements translate into increased vehicle range and reduced logistics concerns. Specific weight reductions allow improved vehicle response, while less vehicie volume allows reduced armor cover requirements, reduced vehicle weight, and new innovative designs with improved survivability characteristics. The Adiabatic Engine's high temperature operation also gives smoother combustion and a wider range of acceptable fuels. With this engine, the entire philosophy of combat vehicie design becomes far less restrictive. Concerns regarding satisfactory locations for cooling grilles, air passages, and associated equipment are eliminated. The cost of the engine is expected to be equal to or less than its cooled counterpart since engine radiators, cooling fans, water pump, seals, hoses, and costly water jackets would be eliminated. Application of the Adiabatic Engine to commercial activities provides advantages similar to those discussed above and in addition includes improvements in the areas of emissions, simplicity, vehicie aerodynamics, operation cost, and maintenance. ît should be noted that 50% of the commercial and military engine field failures are cooling system related. XXISpecific material requirements and selection methodology for the Adiabatic Engine are somewhat constrained by specific design and application approaches; however, the following properties are representative of important characteristics:. Insulative properties. High expansion coefficients. High temperature capability. High strength. Fracture toughness. High thermal shock resistance. Low-friction and wear characteristics. Low cost Typical desired material properties for an adiabatic diesel engine are listed above:. Temperature Limit (°F) > 1 800. Fracture Toughness, Kic > 8,0. Fiexurai Strength (MPa) > 800. Thermal Conductivity, k, (W/m °C) <0,01. Thermal Shock Resistance, AT, ( °C) > 500. Coefficient of Expansions, (x 1 0"6 / °C) > 1 0 In order to achieve the program's ultimate goals, it is necessary to reduce heat rejection rates by at least 70% for the combustion chamber components, and thus thermal conductivity is an extremely important consideration for an Adiabatic Engine. At the present time, partially stabilized zirconia has been found to be quite desirable for Adiabatic Engine application because of its excellent insulating characteristics, adequate strength characteristics, and thermal expansion characteristics which are relatively close to some metals. Zirconia based thermal barrier coating systems (TBC) were initially developed for high temperature insulation for rocket and turbine engines. When applying this technology to reciprocating engines, such as diesels, one must realize that optimum coating characteristics may be different for turbine and reciprocating engine components. Also, coating failure mechanism can change as TBC morphology and chemistry are changed. One of the primary difficulties of the TBC lies in its endurance problems. With increasing thickness the service life of the TBC decreases. However, thick thermal barriers are of interest to engine manufacturers due to possible improvements in insulation characteristics. Failure arises during engine operation largely because of ceramic coating fatigue. Thermally induced cyclical stresses combine with residual stresses found in the coating system to cause failure. Thermal stress cycles are caused by engine heating and cooling during operation coupled with the mismatch of thermal expansion coefficients of TBC components. The residual stresses arise from complex interaction of grit blasted substrate surface stresses, shrinkage of molten splits of bondcoat metal and zirconia, and stresses arising from temperature transients in the xxnsubstrate and coating during spraying coupled with thermal expansion coefficient mismatch of materials. Lastly, stresses can be induced during elevated temperature operation if TBC components aren't stable at operation times and temperatures. Two primary problems of this class are bond coat oxidation and zirconia destabiiization. Zirconia destabilization and bondcoat oxidation are in general less of a factor in reciprocating engine applications than in turbine and rocket engines due to the lower service temperatures. By selecting oxidation resistant bondcoat materials and sufficiently stabilized zirconia these problems can be avoided [92]. The hot combustion chamber wall temperatures of an Adiabatic Engine provide higher compression charge temperatures with consequent reduction in ignition delay. Short ignition delay is conducive to multi-fuel capability in compression ignition engines. It is important to note that an Adiabatic Engine can be appiied to a wide range of military vehicles, ranging from trucks to main battle tanks. In addition, adiabatic engine technology can be applied commercially to ground vehicles from trucks to automobiles and to such applications as helicopters, winged aircraft, locomotives, marine use etc. Eliminating the conventional liquid cooling system of a diesel engine to conserve energy normally rejected to that heat sink offers promise as a means for improving fuel economy. Such low-heat rejection diesels have generally been advanced for heavy-duty vehicles. The low-heat rejection approach is ill-suited to the conventional gasoline engine because high cylinder-gas temperature promotes combustion knock, but it is ideal for the diesei engine. Among the positive attributes of the low-heat rejection diesel, beyond elimination of a cooling system that includes a coolant pump, radiator, and cooling fan, are the potential for improved fuel economy as a result of decreased heat rejection, and a reduction in the ignition delay period. Shorter ignition delay may allow use of a lower compression ratio, with a consequent decrease in motoring power that could contribute to a higher mechanical efficiency, hence better fuel economy. In an internal combustion engine, the process of heat addition is most important. The performance depends not only on the amount of heat released but also the timing of its release. If, for example, all the heat were released at the beginning of the working stroke (near TDC), ail of it is available to potentially produce work thus giving the highest efficiency; on the other hand, if it were released at the end of the stroke, none of it is useful to do any work in the cylinder, thus giving the lowest (zero) efficiency, but it enhances the availability of the exhaust gases. Therefore, when a comparation is to be made between insulated and non-insulated engine performances, it is imperative that the heat release process be identical for both cases. This may require constant air-fuel ratio and any adjustment or redesign of the combustion chamber and fuel injection system. xxmAnother important factor that affects the engine performance is the heat rejection during the working stroke. If, for instance, ail the heat were to be rejected at the beginning of expansion stroke, no work is possible; on the other hand, if no heat were to be rejected during this stroke, there is a potential for the work to approach isantropic work (which is the maximum possible) by maximizing the indicated mean effective pressure. Of course, this possibility is the main factor that carried the interest in the adiabatic engine research a few year ago. However, all the heat recovered by insulation that would have been lost to coolant will not converted to indicated work. For example, the reduction in heat rejection occurring at the end of the power stroke (near BDC) has little potential to do any work other than increasing the availability of exhaust gases. Siegla et al. [6] have estimated, using the second law of thermodynamics, that about one-third of the reduction in the heat rejection would be converted to work and the rest of the two-thirds would go to increase the avaiiabiiity of exhaust gases. Assuming one-third of the input energy is saved by insulation (which would have gone to coolant otherwise; and most of the energy loss to coolant occurs during the expansion stroke only), the total potential for additional work is about 1 1 percent (1/3 of 1/3) of the input energy. In other words, about 33% improvement in the indicated thermal efficiency may be achieved over the present conventionally cooled engines which are assumed to have about 30% efficiency. In addition, there are savings in the parasitic energy loss due to elimination of the cooling system and the reduction in frictional loss caused by increased oil temperatures in the insulated engine. These savings should increase the brake thermal efficiency of insulated engine further. The utilization of enhanced avaiiabiiity in the exhaust of adiabatic engines by employing turbo-compounding and/or Rankine bottoming cycle would increase the overall efficiency of the adiabatic engine even further. However, all the above potential may not be actually realized in practice due to several reasons. Chief among them is the reduction in the volumetric efficiency for the naturally aspirated insulated engines because of the higher cylinder temperatures. This drop in volumetric efficiency is attributed to be the primary cause for the deterioration in the insulated engine performance in the investigations already reviewed under literature survey on the naturally aspirated engines. Probably this adverse effect would reduce the potential gains of the insulated engine [2]. The purpose of this thesis is to study the effects of thermal barrier coating on the performance of a turbocharged diesei engine. The engine selected for evaluation of the thermal barrier coatings was a four-stroke, direct-injection, six cylinder, turbocharged, intercooled diesei engine. First, this engine was tested, as equipped with a water-cooled intercooler, at different speeds and load conditions without coating. Then, combustion chamber surfaces of the engine were coated with ceramic materials. Cylinder head and valves were coated with a 0,35 mm thickness of CaZrCh over a 0,15 mm thickness of NiCrAl bond coat. The material used on pistons was MgZrCX The coating process was done by using Plasma Spray Technique at Sakarya University Engineering Faculty Plasma Coating Laboratory. Finally, the ceramic coated test engine was again tested at the same operation conditions as the standard engine. The test data of the both cases were analyzed by using a computer program and the results were compared as diagrams and tables. The experimental and xxivcomputational results show that, thermal barrier coating greatly affects performance of a turbocharged diesel engine. The specific fuel consumption and energy transferred to the cooling water were decreased, and energy transferred to the exhaust gases was increased with thermal barrier coating. The results indicate that specific fuel consumption values of insulated engine were 1 to 6% lower than baseline engine. This reduction was especially significant at low load-high speed, medium load-medium speed and medium load-high speed conditions. There was only a little fuel consumption increase at full-load and speed condition. It is a known fact that thermal barrier coating increase cylinder temperatures of diesel engines. Higher cylinder temperatures will cause reduction in ignition delay. For this reason, injection timing of coated engine must be optimized. In the experimental study, the injection timing of the coated engine was lowered from 20 °C A to 1 8 °CA, and the engine was tested. In the decreased injection timing, specific fuel consumption of engine was decreased about %2. The other subject studied was exhaust gas energy recovery. For this reason, availability of exhaust gases were calculated, and thus what amount of this energy can be converted to useful work also studied. By using a turbocompound or a rankine bottoming cycle this energy can be recovered. The exhaust gas availabilities of coated engine were 3,2 to 40 kW, and 3,3-45 kW for uncoated engine. These results indicate that in the case of coated engine, the increase of exhaust gas exergy was between %3 to%21. In the study, another subject studied was to see the effects of thermal barrier coatings on naturally aspirated engine performance. For this aim, the engine was tested as naturally aspirated engine for both coated and uncoated cases. In the case of naturally aspirated engine, specific fuel consumption of coated engine was about two percent lower than that of the uncoated engine at medium-load and speed conditions. At low loads-low speeds and high loads-high speeds, specific fuel consumption of coated engine was increased significantly. It is known that one of the most important result of thermal barrier coating is the reduction in exhaust emissions. For this reason, soot, hydrocarbon, and carbon monoxide emissions were measured and the results were compared with each other. The test results show that the hydrocarbon, particulate and carbon monoxide emissions of the ceramic coated engine were lower than that of the uncoated engine. These reductions, as a result of thermal barrier coating of the engine combustion chamber, were about 35% at carbon monoxide, 40% at hydrocarbon, and 48% at particulate emissions. XXV