Wankel motoruna ağırlığı azaltılmış cıvata geliştirilmesi
Leight weight bolt development for wankel engine
Yazar: MEHMET ÇAĞRI ÇUBUKLUSU
Danışman: DOÇ. DR. OSMAN AKIN KUTLAR
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Makine Mühendisliği Ana Bilim Dalı / Otomotiv Bilim Dalı
Derece: Yüksek Lisans, 2024
Konu: Makine Mühendisliği = Mechanical Engineering

Özet:
Wankel motoru tüm motor tipleri arasında güç yoğunluğu bakımından en üstün olandır. Bu özelliği sayesinde günümüzde otomotiv sektöründe elektrikli taşıtlarda menzil uzatıcı olarak kullanımına devam edilmektedir. Ayrıca, havacılık sektöründe de kullanım olanaklarının araştırılması ile ilgili çalışmalar yapılmaktadır.
Wankel 13-B Renesis motorunun konstrüksiyonu incelendiğinde 18 adet uzar cıvatasından kısa olan 14'ünde cıvata kafalarının standart altıgen olduğu, uzun olan 4'ünde ise çanak şeklinde küçük bir boşaltmanın yer aldığı görülmektedir. Bu durum maliyeti artırmadan ve hatta düşürerek ağırlık azaltma çalışması yapma fırsatı vermektedir.
Bu yüksek lisans tez çalışmasında farklı bir anahtar kullanımına ihtiyaç bırakmaksızın, Amerika Birleşik Devletleri'nden getirtilen Wankel 13-B Renesis motorunun altıgen uzar cıvatalarının kafalarına Wankel motorunun piston geometrisinden ve yanma odasının ortasının sol ve sağındaki radyüslerden esinlenerek boşaltma yapılmıştır. Daha sonra bu boşaltmalar ile oluşan yeni tasarım ile uzar cıvataların yarısında CNC makinesinde talaşlı işleme yapılmıştır. Yeni bir cıvata veya saplama geliştirilirken yapılması gereken 3 deney gerçekleştirilmiştir. Bunlar çekme deneyi, kamalı yük deneyi ve burulma deneyidir.
Çekme deneyinde ve kamalı yük deneyinde orijinal uzar cıvatalarla ağırlığı azaltılmış uzar cıvatalardan 3'er adet numune kullanılmıştır. Çekme dayanımı deneyinde bu numunelerin ISO 898-1 şartnamesindeki değerleri sağlayıp sağlamadığına bakılmış, cıvata ve saplamaların en çok kafa altını zorlayan kamalı yük testinde ise kafalarda kopma olup olmadığına bakılmış ve kalıcı şekil değiştirmede farklı numunelerin kalıcı uzama değerleri karşılaştırılmıştır. Burulma deneyi ise yalnızca yeni geliştirilen uzar cıvatalardan 3 tanesine uygulanmış ve bu uzar cıvataların ISO 898-7 şartnamesindeki değerleri sağlayıp sağlamadığına bakılmıştır.
Yeni uzar cıvatalar çekme ve burulma deneylerini başarıyla tamamlamıştır. Kamalı yük deneyinde ise kafalarda kopma hiç yaşanmadığı gibi kalıcı uzamalarda orijinal uzar cıvatalara göre 10 – 20 mikron aralığında deneydeki belirsizlik faktörlerinin sonucu olarak görülebilecek bir farklılık oluşmuştur. Böylelikle yeni tasarım hedeflenen sonuçlara ulaşmış, ticari olarak kullanılabilecek durumda olduğunu kanıtlamıştır.
Yeni uzar cıvataların iki yeni Wankel motoru teknolojisine uygunluğunun kuramsal olarak incelenmesiyle tezin otomotiv programına uygun hale gelmesi amaçlanmıştır. Uygulama bölümünde uzar cıvataların çevrim atlatma sistemli Wankel motoru ile iki zamanlı Wankel motorunun ürettiği kuvvetleri karşılamasına ve bu kuvvetleri karşılarken gerekecek sıkma torklarına bakılmıştır.
Bu çalışmanın bir diğer amacı da taşıtlarda kullanılan pek çok parçada estetik kaygılara önem verilirken bu konuda üzerinde pek durulmamış olan bağlantı elemanlarına da estetik bir görünüm kazandırmaktır. Özellikle çalışırken gösterildiği animasyonlarda ilgililerinin gözlerine de hitap eden Wankel motoru, uzar cıvatalarının bu yönüyle sıradışılığına yeni bir boyut kazandırabilir.

Abstract:
Electricity vehicles doesn't attract many people due to their low range. The charging process also takes very long time. Some of the car manufacturers has come up with serial hybrid systems at which internal combustion engines only charge the battery. Considering the huge amount of petroleum existence in the world, that solution sounds compromising. Mazda MX-30 sport utility vehicle is the only vehicle which uses Wankel engine as a range extender currently. That means the exhaust gas emission issues of Wankel engine has been solved already, however there is still a big room about the iconic engine.
Power density wise, Wankel engine is the best one among all kind of engines. Besides, usage opportunities at aviation industry is still under investigation. The other superior attributes of Wankel with respect to conventional internal combustion engines are volume power density and noise-vibration-harshness. The last is because of less moving parts amount. Not only the moving parts but also the total part amount of Wankel engine is less. This situation provides easier and more economical serial production obviously.
Fasteners of vehicles are called as C-parts and they are usually neglected at weight reduction works. However every drop counts for automotive sector, in other words every gram counts. 70 – 80 gram saving at an engine without any loss of fastening performance may attract almost all of engine design experts. They are spending their time with major issues like combustion efficiency, sealing parts, alternative fuels, alternative mechanisms etc. so some fastener engineers are responsible to convince them about the minor effect of the fasteners. Meanwhile, there is also an application part at this thesis where convenience of the tension bolts for new Wankel engine technologies is proved theoretically.
When Wankel 13-B Renesis engine is reviewed, it is easily realised that it has M10x1,00 18 tension bolts: 14 of them are short, have 265 mm length and 4 of them are long, have 280 mm length. The bolt head of short version is standard hexagon and of long version is hexagon with a little spherical cavity in the middle. This situation gives a chance to reduce weight. Dimensions of the head are according to DIN 931 M10, there is only one exception which gives advantage for weight reduction: Head thickness is as per DIN 931 M12. One of the most practical way to improve power density would change the design of bolt heads. Meanwhile, Wankel range extenders are quiet smaller than double rotor owner 13-B Renesis because range extenders are single rotor. This situation makes this study more valuable for Wankel range extenders rather than conventional use.
At this thesis study, firstly used Mazda 13-B Renesis Wankel engine tension bolt set was ordered from an online shopping site. The first user and seller was at United States of America. It approximately took 2 months for the bolts to arrive at custom in İzmir, Turkey. Then, the hexagon heads have been modified regarding Wankel engine piston geometry and radii of combustion chamber, without need for a different bit. Comparisons with other lightened bolt head designs have been made and all section thicknesses have been determined accordingly. Afterwards half of the bolts have been machined at CNC. The 3 necessary tests during a new bolt or stud development have been done: Tensile test, proof load test and minimum breaking torque test. The tests have been done at Norm Fasteners laboratories. While the company supplies fasteners for almost all global car manufacturers, the measurement equipments are frequently inspected and calibrated. There were also Junker test device at research and development laboratory, however the bolts were too long for the device. This test is not conducted when developing a new product which has a design to be patented.
During tensile test and proof load test, 3 samples of both original and light weight bolts have been used. All bolts have satisfied ISO 898-1 requirement at tensile test. The strength class has been acquired as 10.9 after the test while the results were between 1.040 and 1.200 MPa. The change at head didn't effect tensile strength of the bolt. Waisting was observed at threaded zone for all samples which is expected before the test. During proof load test, the most stressed zone of fasteners were under head and one of the purposes of that test is to check rupture at head. There weren't any issue for any samples. The proof load were applied as 48.200 N. The value is 48.100 N as per ISO 898-1. Plastic elongation difference was around 10 – 20 microns which can be thought as a result of uncertanities. Torsional test has only been applied to light weight samples. The blind hole apparatus for the test were produced at Norm Fasteners workshop due to lack of M10x1,00 apparatus. All of 3 samples has complied with ISO 898-7 requirement. According to this requirement, minimum breaking torque is 102 Nm for 10.9 M10x1,50 bolts and studs and the results were well-above.
New design has accomplished targeted results and proved that it is ready for commercial usage. Approximately %29,52 of head material was removed, while equivalent products offer maximum %27,20 for M10 nominal diameter. It provides weight reduction around 73 gram per engine, less raw material usage and cost reduction. After engineering judgements of Norm Fasteners' experienced designers, it is obvious that the design doesn't require machining process after cold forming. They stated that there would be no request for computer aided cold forming simulation if a customer enquiries for this bolt. Moreover, it is esthetically adventageous because there hasn't been any fasteners so far which recall the engine they used at. Patent application would be convenient for protecting the idea. Since the head design of similar products are confidential properties of the companies, forming a literature summary at this thesis was not possible.
Another importance of that study is about sustainability of the world. Although there will be an extra expenditure for cold forming mold design and manufacture, high quantities during serial production would cause save of raw material which overcomes. Lighter engine also causes less specific fuel consumption.
Although this study has been done with restricted resources, time and samples it is completed with the results which are expected at the initiation. This subject may be carried on with a producibility simulation to prove that there is no need for machining process after cold forming. The depth of socket can be improved to 4,75 mm from 4,50 mm. What is more, edge incavation would be increased hence total weight reduction of bolt head exceed %30 which will make the new bolt by far the best among its equivalents. Another way to improve the study is using the bolts on current Wankel test engines. So, the assembly conditions can be tested. Junker test, which is conducted
to measure loosening of the joint is also partly an alternative for that. Convenience of the experimental setup is important because of the huge length of the bolts.
In conclusion, the design boundaries of the new tension bolt has not been detected yet. It probably has more potential than it has showed during that master thesis and waits the day in the future when it will be discovered.