Wankel motoru için çevrim atlatma mekanizması geliştirilmesi
The improvement of skip cycle mechanism for Wankel engine
Yazar: MERVE ÜNGÖR
Danışman: DOÇ. DR. CEMAL BAYKARA
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Makine Mühendisliği Ana Bilim Dalı / Otomotiv Bilim Dalı
Derece: Yüksek Lisans, 2017
Konu: Makine Mühendisliği = Mechanical Engineering

Özet:
Yapılan çalışmada tek rotorlu bir Wankel motoru için çevrim atlatma mekanizması tasarlanmıştır. Tasarım amacının ön plana çıkarılabilmesi için öncelikle Wankel motoru genel hatları ile tanıtılmış, sistemsel ve mekanik açıdan klasik pistonlu motorlardan farklarına değinilmiştir. Özellikle kısmi yüklerde motor karakteristiklerinin şehir içi kullanımda oldukça önemli olduğu bilinmektedir.  Bu tezin amacı çerçevesinde geçmişten günümüze buji ateşlemeli motorlarda kısmi yüklerde verim düşüklüğü ve yüksek yakıt tüketimi tartışılmıştır. Kısmi yük rejiminde verim düşüklüğünün temel nedenlerinden biri olan pompalama kayıplarına yer verilmiştir.
Verim artırmaya yönelik yöntemlerden bahsedilmiş, bunlardan biri olan ve Wankel motorunda ilk kez denenecek olan çevrim atlatma yöntemi tanıtılmıştır. Temel olarak, çevrim atlatma yönteminde güç üretilen dört periyot tamamlandıktan sonra takip eden çevrimde iş üretilmez. Bunu sağlamak için, iş üretilmeyen çevrime gelindiğinde yakıt ve ateşleme kesilir.  Burada temel amaç yakıt tüketimini ve buna bağlı olarak egzoz emisyonlarını azaltmaktır.  Bu yöntemin getireceği yeniliklerin test edilmesi amaçlı çift rotorlu Mazda 13B motoru proje kapsamında yürütülen çalışmalarla tek rotorlu hale getirilmiştir. Çevrim atlatma stratejisinin bu motora uygulanabilmesi için iki mekanizma tasarlanmıştır. İlk mekanizma deney motoru olarak tasarlanmış ve iamaltı gerçekleştirilmiştir. İkinci alternatif tasarımın ise kompakt yapısı nedeni ile otomobillere entegrasyonunun mümkün olacağı düşünülmektedir. Tasarlanan sistemlerde temel mantık, atlatılan çevrimde emme işleminin durdurmak için kullanılacak karmaşık bir supap mekanizması yerine sadece emme kanalına konulacak bir dönen supap yardımı ile emme işlemi durdurmak ve böylece çevrim kontrolü sağlamaktır. Bu amaçla orijinal motorda bulunandan farklı olarak yeni bir döner supap tasarlanmıştır. Supabın mekanizmaya dahil edilmesi ile birlikte motor üzerindeki hareketli parçalar arasında bir bağıntı sağlanarak geliştirilen çevrim atlatma stratejisi anlatılmıştır. Mekanizma iki evrede çalışmaktadır: Supabın açık olduğu durum (normal çevrim) ve supabın kapalı olduğu durum (atlatılmış çevrim). Çalışmalarda Kutlar tarafından 1999 yılında geliştirilen çevrim atlatma stratejisi temel olarak alınmıştır.
Tasarlanan parçalar ve mekanizma montajı Autodesk Inventor  kullanılarak üç boyutlu olarak modellenmiş ve yapısal analizleri ANSYS programında analitik olarak yapılmıştır.

Abstract:
In this study, a skip cycle mechanism has been designed for a single-rotor Wankel engine. Firstly, rotary engines have been introduced and Wankel engine the only rotary piston engine, which is used in many areas in rotary piston machines today is explained in order to put the design forefront. Four stroke working principles has been mentioned and its differences from conventional reciprocating engines are stated from systematical and mechanical aspects.
Especially at partial loads, it is known that the motor characteristics are very important for urban use. Overall aim of this study, the low efficiency and high fuel consumption of spark ignition engine has been discussed from past until today. It is also stated that one of the main reasons of low efficiency in partial load regime is pumping losses. Efficiency improvement methods have been mentioned in the partial load regimes. For this purpose, cylinder and valve deactivation have been mentioned and skip cycling method, which is one of the efficiency improvement methods is introduced for the first time in the Wankel engine. In this method, the basic logic is to stop the fuel and air intake by controlling the intake and exhaust valves in some of the four cycles. In normal cycles, the amount of air-fuel mixture is increased to achieve the same power. In other words, when the engine power is reduced, instead of reducing the amount of fuel-air mixture entering the combustion chamber by reducing the throttle, work is not produced in the other cycle after four times have been completed.
In order to design the skip cycle system for a single-rotor Wankel engine, firstly a prototype engine has been manufactured, followed by an experimental engine.A number of criteria such as being in the market, availability of components, and suitability for design changes have been taken into consideration in the selection of experiment engine. The most suitable option for these purposes is the 13B-Renesis engine manufactured by Mazda. Another important reason for selecting the 13B-Renesis, the only Wankel engine that the exhaust windows are positioned on the side bodies, is more suitable for constructive changes of the body in which the rotor is located. Other important reason, it has its own rotary valve. This engine has been converted into a single rotor within the scope of the project.
The adaptation of the skip cycle method to the Wankel engine requires a series of calculations. The crucial point of mechanism and calculations is the "rotary valve". Rotary valve used in original Wankel engine is a hollow cylindrical structure with a window to transfer air-fuel mixture into the engine. When the engine is under high load and velocity requirements, the valve is turned to the open position and closed at low load and velocity. Thus, this valve works as the open-close mechanism in the original engine. In the newly designed mechanism, the valve performs the new task by turning 360 degrees instead of open-close mechanism. The window geometry should be determined to provide the best performance for the skip cycle strategy, since the air-fuel mixture is transferred into the combustion chamber by means of the valve window. Three important points of this objective are determining the angle of the rotary valve window, the direction of rotary valve rotation and the rotary valve window geometry. The valve window has been shortened in order to fit the auxiliary port. The window angles calculated for determination of the rotary valve window angle are 70, 80, 90 and 110 degrees. Calculations have been made in 0.1 degree intervals of rotor for four different angles of the rotary valve. Due to the geometry of the auxiliary port, the opening direction of the valve has a significant effect on the area scanned window. The calculated values are higher when the valve starts opening in clockwise direction than the counterclockwise. As a result of the calculations, it was decided that the rotary valve should have an 80 degree window, the valve has to be turned clockwise, the window geometry should be rectangular. 
The next step is to carry out studies to adapt the skip cycle strategy to the Wankel engine. The most important information for calculations is the correlation giving the rotation rates of the moving parts of the engine relative to each other. The Wankel engine has three rotating components, an eccentric shaft, a rotor and a rotary valve. As the control of the fuel-air mixture intake into the combustion chamber is provided with the valve, a correlation is provided between the swept areas by the rotor and the swept areas by the valve. Thus, in one cycle the air fuel mixture has been allowed transferring to the combustion chamber, while the other cycle has been skipped. As a result of the calculations, it has been decided that there is a relationship between the rotary valve, the rotor and the eccentric shaft speed. This correlation shows the rotation rates of the moving parts of the engine relative to each other. 
Two mechanisms have been designed to apply the skip cycling method to this engine. The first mechanism has been designed and manufactured as an experiment engine. The first step in designing a mechanism is to define where the drive torque will be applied to move the rotary valve. For this purpose, it has been decided to use the oil-dosing pump. The pump shaft driven by the crankshaft has been extended with a pulley. A gear has been required for the rotary valve to transmit the drive from the pump shaft to the rotary valve. In order to be able to integrate the pulley to rotary valve, a shaft has been added to a valve. An additional connection has been made to the valve inlet in order to fix the shaft which pass through the center of the valve. A pulley has been added to the valve so that the motion can be transmitted. The connection between the two pulleys is provided with a toothed belt by adding a pulley to the valve. The engine does not have a compact design since the drive mechanism is provided externally with the shafts. Therefore, a new mechanism has been designed which will be easy to integrate into the vehicle. 
In this new design, the movement from the pump shaft has been transmitted directly to the end of the valve. For this reason, a number of changes have been made to the valve design. New rotary valve which has its own gear pulley has been designed. A small shaft has been integrated into the other side of the rotary valve so that the rotary valve has been supported from one side. So that, the drive mechanism has not been located on the outside of the manifold. Instead, this mechanism has been located on the compartment of the valve connecting to the engine body. This has been allowed the system to become smaller. The connection between the pulleys has been provided with timing belt. 

The next step is to locate the manifold. It has been inserted into the rotary valve. Due to this connection, there is a gap between the manifold and the rotary valve. For this reason, there is a sealing problem due to manifold rotary valve connection. A non-contact sealing has been required between a stationary element in a rotating part. Since the manifold contains an air-fuel mixture, a gas tightness has also been required here. For these two requirements screwed sealing element has been used. Current screwed sealing element studies have been used to solve this problem. When the fluid moves from the high pressure region to the low pressure region, the sealing element rotates in the appropriate direction. So that the fluid is returned back to the high pressure zone. According to experimental results of these studies, the design with the best sealing performance has been optimized for this project. 
When the pressure between the shaft and the outer part is higher than the maximum pressure, it can be said that there is a transition zone in this region. If the transition zone acts as a barrier and occurs in the sealing zone, this can be shown as a proof that there is an active sealing. In this project different from the references, the moving part is on the outside, the fixed one is inside. For this reason, not the outer part but the inner part has been manufactured with grooves. Precautions have been taken to avoid the effect of vacuum, since there is a movement from high pressure to low pressure. The dimensions of the threads, helical directions and angles have been optimized.
In addition, the manifold must be fixed due to the new design on the rotary valve. For this purpose, the holes positioning two separate bodies on the engine has been used. With the help of this design, the manifold has been fixed to the engine body without interfering the operation of the belt-pulley mechanism. A flange was designed to attach the bearing to the engine body.
ANSYS program has been used to make the strength analysis of the designed parts. First, the analyzes have been made for the rotary valve. The belt forces that provide the valve and pump shaft connection have been calculated and the resultant forces have been entered into ANSYS. Analyzes have been made by fixing both ends of the shaft and the bearing. One of these analyzes is von Mises stress. There are different theories and criteria as to whether any point of a material will have any stress values at that point. For these criteria, compressive or tensile stress is referred to for yield strength or fracture control. The strain-strain curves and values obtained from the tensile or compressive test are used as comparison parameters in these criteria. According to this criterion, when the equivalent stress (von-Mises stress) in the multi-axis condition exceeds the yield stress of the material, the yield strength occurs at that point. Another analyze has been made for deformation.
Designed parts and mechanism assembly have been modeled in three dimensions using Autodesk Inventor and structural analysis has been performed analytically in the ANSYS program.