Orta kapasiteli dizel motorlarında atık ısıdan rankine çevrimi ile enerji geri kazanımı
Energy recovery from waste heat of medium duty diesel engine with organic rankine cycle
Yazar: AHMET SİNAN KARTAL
Danışman: PROF. DR. YAKUP ERHAN BÖKE
İstanbul Teknik Üniversitesi / Enerji Enstitüsü / Enerji Bilim ve Teknoloji Ana Bilim Dalı / Enerji Bilim ve Teknoloji Bilim Dalı
Derece: Yüksek Lisans, 2018
Konu: Makine Mühendisliği = Mechanical Engineering

Özet:
Sürdürülebilir bir çevre ve dünya için son yıllarda içten yanmalı motorlarda gerçekleştirilen çalışmaların büyük bir kısmı verim artışı ve zararlı gaz salımlarının azaltılması üzerine yapılmıştır. Bu kapsamda yapılan çalışmalarda yanma enerjisinin büyük bir kısmını oluşturan atık ısıdan enerji geri kazanımı önemli konulardan biridir. Motordaki yanma sonrası oluşan egzoz gazlarındaki atık ısıyı turbo şarj ile, termoelemanlar ile, absorbsiyonlu soğutma sistemleri ile veya Rankine çevrimi ile faydalı işe dönüştürmeyi amaçlayan çalışmalar mevcuttur. Atık ısı geri kazanım sistemlerinden Rankine çevrimi ile enerji geri kazanımının, özellikle gaz salımlarını azaltan yeni salım sistemleri ile atık ısı oranı artan dizel motorlarda gelecek yıllarda ticari uygulamalarda önemli yer tutacağı öngörülmektedir.
Bu çalışmanın ana konusu ticari araçlarda yaygın olarak kullanılan 3,5l hacminde 200 hp gücünde bir dizel motorun egzoz ve EGD (Egzoz Gazları Devir daimi) gazlarından Rankine çevrimi ile enerji geri kazanım potansiyelinin 1-boyutlu analiz yöntemi ile incelenmesidir. Çalışmanın temel amacı Rankine çevrimi ile egzoz gazlarından atık ısı geri kazanım teknolojisinin dizel motorlarında sağlayabileceği güç artışı ve yakıt tüketimindeki azalmayı belirlemektedir.
Çalışmada atık ısı kazanım sistemi olmayan, temel motordan alınan motor çalışma koşullarına bağlı egzoz ve EGD gaz sıcaklık ve debi verileri 1-boyutlu benzetime girdi olarak kullanılmıştır. Benzetim için otomotiv sektöründe yaygın olarak kullanılan, hazır bir paket yazılım olan GT-Suite kullanılmıştır. Birincil olarak sadece egzoz gazlarının, ikincil olarak sadece EGD gazlarının ve üçüncül olarak EGD ve egzoz gazlarının birlikte Rankine çevriminin ısı kaynağı olarak kullanıldığı üç farklı benzetim modeli alternatifi oluşturulmuştur. Yoğuşturucu tarafında ise motor soğutma suyunun çevrim akışkanı olduğu ayrı bir soğutma devresi kullanılacağı öngörülmüştür.
Motor için dokuz adet çalışma noktası belirlenmiş, benzetim modeli alternatiflerinin belirlenen motor çalışma noktaları için analizleri yapılmıştır, çıktılar karşılaştırılarak motorun güç geri kazanımı ve yakıt tüketimine etkileri incelenmiştir. Belirlenen motor çalışma noktaların ortalama çalışma yüzdelerinde ortalama yakıt tüketimindeki azalma hesaplanmıştır. Motorun atık ısısından en fazla yararlanılan durum, egzoz ve EGD gazlarının birlikte ısı kaynağı olarak kullanıldığı sistemde olacağı öngörülmüştür. Bu sistemde güç geri kazanımı belirlenen motor çalışma senaryolarının dokuzuncusunda 2,05 kW'a kadar ulaşmış ve ortalama yakıt tüketiminde ise %2,25 oranında azalma olacağı hesaplanmıştır. Benzetim çıktılarına göre ticarileşme potansiyeli olan, gelecekte seri üretimde kullanılması öngörülen sistem önerileri sunulmuştur.

Abstract:
Issues on the energy are some of the most compelling subjects in the world today. With human's ever increasing need for energy, production must be increased or consumption must be reduced to avoid an unsustainable long-term energy balance.
Modern research and development studies relating to internal combustion engines and vehicle design are largely driven by the persistent need to reduce the global consumption of fossil fuels and the resulting emissions of the greenhouse gas carbon dioxide.
The majority of the energy in the fuel burned by the internal combustion engines used in modern vehicles is lost in the form of waste heat and does not contribute to the propulsion of the vehicle. Three different technologies have been proposed for recovering some of this lost heat and thereby increasing the overall efficiency of combustion engines: the turbocompound, thermoelectric converters, and heat engines based on the Rankine cycle. Turbocompound systems are used in diesel engines for decades and started to be used in gasoline engines in the latest years as well. Thermoelectric converters which generates electricity from temperature difference of exhaust gas and cold sink, were not used a place in automotive industry due to its high application cost and low efficiency. On the other hand, heat engines based on Rankine cycle started to be feasible in medium and heavy duty diesel applications due to increasing waste heat of Euro 6 aftertreatment systems in the latest years. Since, EGR (Exhaust Gas Recirculation), DPF (Diesel Particulate Filter) and SCR (Selective Catalytic Reduction) aftertreatment systems were being used all together with heavy duty Euro 6 diesel engines.
In this thesis, a waste heat recovery system for a medium duty diesel engine is evaluated for utilizing multiple heat sources found in a conventional 3,5 l 200 hp diesel engine. In this type of engine more than 50% of heat energy goes futile. The majority of the heat energy is lost through engine exhaust and cooling devices such as EGR (Exhaust Gas Recirculation) and engine cooling. The potential of recoverable heat recuperation from these devices were studied using thermodynamic analysis. A well-known way of recuperating this heat energy is by employing a Rankine cycle circuit with these devices as heat sources (single loop or dual loop), and thus this thesis is focused on using a Rankine cycle for the heat recovery system.
A simulation model of a Rankine cycle-based heat recovery system is created by using the 1-dimentional flow simulation program GT-Suite, which is widely used in engine-systems development. Measurement data of exhaust and EGR gas temperature and mass flow rate from 3,5 l 200 hp diesel engine which is widely used in medium duty commercial vehicles is used as model input instead of a total engine model which simplify the model complexity and reduce processing time. 
The model of the Rankine cycle consists of a feed pump, which pumps working fluid that comes from a receiver to its evaporation pressure. The heat exchanger is divided into two sections: a master and a slave section. While the slave section represents the heat exchanger in the EGR route, the master part is implemented in the Rankine cycle loop. The expansion device is based on an efficiency-map, which is created using a detailed model of a expander. The condenser (which is also divided into a master and a slave side) condenses the working fluid that has passed through the expander, using water as a coolant.
To determine working fluid of Rankine cycle, not only the thermodynamic properties of fluids but also factors such as global warming effects, flammability, toxicity, price, availability in the market are important. When, environmental factors such as Global Warming Potential (KIP) and Ozone Depletion Potentials (OTP) are also taken into account in fluid selection, using non-flammable R245fa is considered as best alternative for this mobile waste heat recovery system.
As system alternatives three layout is determined according to diversified according to heat sources which are only EGR gas use as evaporator, only exhaust gas use as evaporator, finally EGR and exhaust gas combined use as evaporator. As heat sink cooling water cycle which circulates inside of radiator beside from engine coolant is used in condenser for each system alternatives. Energy recovery from these three system alternatives is simulated according to 9 different engine working scenario. Engine scenarios illustrate low, mid and high torque produce of three engine running rpm.
Heat exchanger for exhaust gas is located at the outet of aftertreatment systems not to effect toxic substances conversion reactions in exhaust gas. Because toxic substances conversion reactions such as NOx to N2 and O2, only occurs at high gas temperature inside exhaust catalyst cells.
For only EGR gas use as evaporator and only exhaust gas use as evaporator layout nine opetation points are specified. First five opetation points indicated refrigerant cycle operation points, following two operation points indicated EGR or exhaust gas inlet and outlet points to evaporator. Final two opetation points indicated cooling water inlet and outlet to condenser. For EGR and exhaust gas combined use as evaporator layout thirteen operation points are specified in similar notation. Simulation results such as, temperature, pressure, entalpy, mass flow rate at each operation points are showed at tables for different working scenarios for each layout. 
New power gain from Rankine cycle is calculated from simulation for each nine engine working senario. The new power gain which is calculated by the difference between output shaft power of the türbine and the input shaft power of the pump, considered to be fed to crankshaft of the engine with a gearbox. In this case, diesel engine supposed to work at a slightly lower power working point which has less fuel consumption. The effect of net power gain to fuel consumption is calculated according to engine's current fuel consumtion map.
Simulation results show that the system which uses only EGR gas as boiler is active and produce power in only three of nine operation points. Beacuse EGR is not active in all operation points, especially at high power scenarios. On the other hand even if exhaust gas temperature is lower than that of EGR gas, waste heat recovery system using exhaust gas as evaporator is active and produce power in all nine operation points. In case of combine usage of exhaust and EGR gas, the largest amount of net power gain is calculated due to high energy input to the system.
In the highest heat recovery system alternative which is EGR, and exhaust gas combine usage, it is possible to recover up to 2,05 kW power at scenario 9 and this may be used as an additional torque in crankshaft. On the other hand, engine scenario using rates are estimated to calculate average fuel consumption. When the fuel consumption decrease at each scenario is averaged according to estimated realization rates, up to 2,25% of average fuel consumption decrease is possible with such an energy recovery system.
The energy recovery rates are lower in single heat source system alternatives compared to exhaust and EGR combined usage case.
It is important to note that, the Rankine cycle's thermal efficiency was between 5-8%, corresponding to 2-5% increase of the engine's power output. On the other hand, Rankine cycle's thermal efficiency is around 10% and engine's power output increase is up to 10% for mobile waste heat recovery applications. Therefore, performance of designed waste heat recovery system is slightly less than the studies in literature. It is considered that cycle efficiency and power output results can be increased with more optimised model and system components.
The studies on waste heat recovery with Rankine cycle have been increasing in latest years and some commercial products to be used are available on the market, more will be on the market in near future. Also, engine and vehicle manufacturers carrying out research and development projects about this issue and especially heavy commercial vehicle manufacturers started to integrate such an energy recovery system in the near future.
In this study, it is aimed to show energy recovery and efficiency increase potential of a medium duty diesel engine with 1-dimensional flow simulation program GT-Suite. This approach yielded a computationally inexpensive system model that allowed simulations to be performed much faster than 3-dimentional computational fluid dynamics simulations while maintaining high numerical robustness. The turbine and pump in the model were map-based, with the maps being functions of the steam conditions and the expander compression ratio. This map based pump and turbine modelling approach allowed to obtain results that are close to actual working conditions.
Finally, there are still potential to improve simulation results with dynamometer tests. The simulations are done in this study show that internal combustion engines still have efficiency improvement potential.