Marin uygulaması için benzinli bir motorun soğutma sistemi optimizasyonu
Cooling system optimization of a gasoline engine for marine application
Yazar: ÖZGÜR KAYA
Danışman: YRD. DOÇ. DR. ALİ FUAT ERGENÇ
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Mekatronik Mühendisliği Ana Bilim Dalı
Derece: Yüksek Lisans, 2015
Konu: Gemi Mühendisliği = Marine Engineering ; Makine Mühendisliği = Mechanical Engineering ; Mekatronik Mühendisliği = Mechatronics Engineering

Özet:
Yapılan çalışmanın amacı benzinli bir motorun kendisi ve soğutma sistemi için gerekli hesaplamaları yaparak soğutma sistemini marin uygulamalarında kullanılabilecek şekilde yeniden dizayn etmektir. Bahsedilen değişiklik için motorda hali hazırda soğutma için kullanılan radyatör yerine içinden soğutma suyu geçen bir eşanjör, makine dairesinde egzoz gazlarını soğutacak ek bir eşanjör ve bir de dışarıdan soğutma suyunu basacak ek bir pompaya ihtiyaç vardır. Bu şekilde bilinen benzinli bir motor alınarak marin uygulamalarında kullanılabilecek hale getirilmek amaçlanmıştır. Bu çalışmada gücü, stroğu ve silindir çapı gibi parametreleri bilinen ve şu anda otomobillerde kullanılan 1 lt'lik benzinli bir motor seçilmiştir. İlk etapta motorun bilinen parametrelerinden yararlanarak bilinmeyen çalışma parametreleri her bir çalışma stroğu için hesaplanmıştır. Sonraki adımda ise yakıt ile motora verilen ısı, efektif işin ısısı, soğutucuya verilen ısı ve egzoz gazlarına verilen ısılar hesaplanmıştır. Tüm bu hesaplanan ısılar ve bazı motor verileri motorun maksimum tork ve maksimum güç devirlerini de içeren toplam dört çalışma noktası için tekrarlanmıştır.  Pompa ve eşanjör hesapları maksimum güç noktasına karşılık gelen çalışma devri için gerçekleştirilmiştir. Hesaplanan bu ısılardan yararlanarak motor soğutma suyuna verilen ısı için motor soğutma suyu eşanjörü hesapları ve tasarımı yapılıp gerekli pompa debileri belirlenmiştir. Motor soğutma suyu için tasarlanan eşanjörün iç tarafından dışarıdan alınan soğutma suyu geçerken bu soğutma suyunun dışından motor suyu geçmektedir.  Aynı şekilde egzoz gazlarına verilen ısı için egzoz eşanjörü hesapları ve tasarımı yapılıp gerekli pompa debileri belirlenmiştir. Egzoz gazları için tasarlanan eşanjörün iç tarafından motorun çalışması sonucu ortaya çıkan egzoz gazları geçerken bu gazlardan bir boru ile ayrılan diğer kısımdan ise dışarıdan alınan tatlı su geçerek soğutulma yapılmaktadır. Her iki eşanjördeki sıvı ve gazların soğutulması için dışarıdan tatlı su kaynağından alınan su kullanılmıştır. Motor dairesi kısıtları göz önüne alınarak eşanjörün boru çapları için optimizasyon yapılıp en uygun çap değerleri belirlenmiştir. Ayrıca çalışmamın devamında marinize edilmesi planlanan motorun boyutları bilinen bir teknede ana makina olması durumunda, uygun marin şanzımanı seçilmiş ve  teknenin yapısına ve motoruna uygun pervane boyutları (çap ve hatve vb.) hesaplanmıştır. Bu çalışmada uygulanan hesaplama yöntemi ve adımlar kullanılarak başka bir benzinli motor da aynı şekilde marinize edilebilir.

Abstract:
The objective of this research is, by calculating engine and cooling system base parameters, to redesign the cooling system of a gasoline engine that is aimed to be used for marine applications. 
Although in some cases an air-cooled system can be used for internal combustion engines, the most common usage for vehicle application is to use a radiator to cool the heat of engine to protect the engine and surrounding environment. Heat resulted from the combustion of fuel and friction of engine components is transferred to engine coolant that is forced to flow through and cool the engine block, head and other components. Conventionally engine coolant that is cooled by the air passing through the radiator needs to be cooled with a fluid from a water source via a heat exchanger for internal combustion engine marine applications. To perform this, instead of currently used radiator, a heat exchanger for the heat of cooling water is required.
Another source of heat is the gases that are passing through the exhaust system. Although this heat is not a big issue for engines used for automobile applications since there is a heat convection via the flow of air passing through the engine compartment, in case of indoor usages of internal combustion engine such as in the closed engine department of ships in absence of cooling air, a remarkable amount of heat needs to be cooled to not to cause any damage in engine room. Considering the outstanding heat of exhaust gases, an exhaust heat exchanger that can carry out the heat transfer between the fluid and exhaust gases is required for marine applications. For ships, the available source of coolant fluid is most commonly the water that the ship sails. 
For both engine coolant and exhaust gases heat exchangers, water is drawn from out-er environment via an additional water pump. The flow rate of water pump is defined considering the amount of heat to be rejected from both exhaust gases and engine cooling water heat exchangers. 
By using this method a gasoline engine of a vehicle, currently in use in the market, is modified for a marine application. In this study, a gasoline engine with the known parameters of power, stroke, cylinder bore diameter etc. is preferred. 
Firstly, the unknown working parameters of engine are calculated for each stroke by using know parameters of engine. Working parameters of intake stroke such as pres-sure and temperature of gases at the end of intake process are calculated. Volumetric efficiency that is a measure of charge air and fuel mixture of an engine is defined for this stroke as well.  The mean molar specific heats of fresh mixture, residual gases and working mixture are defined at the end of compression process. Pressure and temperature of gases at the end of compression process are calculated. 
In addition, maximum pressure and maximum temperature of gases for the combustion process are calculated. Finally, temperature and pressure values of gases are calculated for the expansion and exhaust processes.
Secondly, the indicated parameters of working cycle such as theoretical mean indicated pressure, indicated efficiency, and the indicated specific fuel consumption are computed. Additionally, engine performance figures such as mean pressure of mechanical losses, mean effective pressure and mechanical efficiency, effective efficiency and effective specific fuel consumption are defined. 
Determination of all these parameters and carrying out the heat analysis of the engine is repeated for four different operating conditions(engine speeds) separately that are predefined considering the operation with minimum speed providing stable operation of the engine, operation with maximum torque, operation with maximum (rated) power and operation with maximum automobile speed. 
For this specific engine, based on these recommendations and the design specification of this engine, the heat analysis is carried out in succession for n = 1000, 3700, 6000 and 6500 rpms. Engine heat balance is specified by defining the total amount of heat introduced into the engine with fuel, heat equivalent to effective work per second, the heat transferred to the coolant, the exhaust heat and the remaining other heats. 
Furthermore, pump and heat exchangers are designed considering the maximum power condition of the engine. Given the predefined engine coolant-in and engine coolant-out temperatures in heat exchanger and amount of heat to be transferred to the outer environment water, the required mass flow rate of the pump is estimated. 
For the heat transferred to the engine coolant, engine cooling system heat exchanger calculations are conducted and heat exchanger is designed. The temperature of the water drawn from the outer environment is a variable that could not be controlled or changed but to accurately carry out the calculations its average value is known. However the temperature of the water that is discharged back to the environment needs to be within a certain limit that does not harm the environment and ecosystem. Considering these requirements, all limits are defined and engine-cooling heat ex-changer is designed such that the water that is pumped from the outer environment is passing through the pipes of heat exchanger and the engine cooling water is flowing just outside these pipes. The flow type of these two fluids in heat exchanger is a counter-flow and a logarithmic mean temperature difference is considered during the calculations and design of  heat exchanger. The diameter of the inner and outer pipes that both fluids are flowing is predefined and at the end of the calculations depending on the total length of pipes, these diameters are optimized by iteration taking the environment, engine package, engine room restrictions and manufacturability of the pipes into consideration. 
To design the aforementioned heat exchanger heat transfer coefficients need to be determined. Firstly, the type of the flow that is laminar or turbulent needs to be determined for both fluids depending on the Reynolds number as a function of velocity, flow rate, viscosity and diameter of the pipes. After that, depending of the type of flow, diameter of the pipes and thermal conductivity, heat transfer coefficients are defined to be used for heat transfer analysis. At the end, length of pipes are estimated by using the both heat transfer coefficients, temperature difference and wet surface area of heat exchanger.  
Likewise, for the exhaust heat, heat exchanger calculations and design study is carried out and required pump flow rates are estimated. Here the flow rate of exhaust gases is a parameter that is specific to the engine and could not be controlled for an already in production engine. Flow rate of exhaust gases for a specific engine speed is calculated by considering the fuel type and air fuel ratio at this engine speed. The exhaust gases that are produced by the combustion of fuel are passing through the pipes of exhaust heat exchanger and to cool the gases the water pumped from the outer environment is passing outside these pipes. To cool both the engine water and the exhaust gases the water that is pumped from the outer environment is used. Therefore, the total amount of water required for the heats of engine cooling water and exhaust gases needs to be provided by the same unique water pump selected for this specific purpose.
Furthermore, a suitable marine gearbox is selected by taking the maximum and operation speed of the ship end engine. The gear ratio is selected as 2,8 - 1 since the used internal combustion engine is a gasoline engine that has a higher maximum(rated) power engine speed. Also, to define the continuous operation speed of the engine for a ship application, %75 of the maximum engine RPM is recommended and used for this converted light-duty gasoline automotive engine. 
In addition, a small ship that the engine is considered to be used in is selected with its known sizes such as length overall, waterline length, beam overall, waterline beam, hull draft and displacement. The propeller pitch is calculated for an engine speed and power that correspond to the %90 of the maximum engine RPM and related ship speed by taking the slip into account. At the end, the propeller diameter is calculated for an engine speed and power that corresponds to the %95 of the maximum engine RPM. 
In conclusion, with this study the cooling and power train system of an internal combustion automobile engine is converted and optimized to use it for marine applications. Other similar gasoline engines may be modified for marine applications by using the same approach used in this study.