Taşıt disk ve fren sıvı sıcaklığının tahmini için bir model geliştirilmesi
Developing a model for estimation of vehicle disc and brake liquid temperature
Yazar: YİĞİT DALGA
Danışman: DR. ÖĞR. ÜYESİ ALPER TOLGA ÇALIK
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Makine Mühendisliği Ana Bilim Dalı / Otomotiv Bilim Dalı
Derece: Yüksek Lisans, 2018
Konu: Makine Mühendisliği = Mechanical Engineering
Dizin: Disk fren = Disk brake

Özet:
Fosil enerji kaynakları gün geçtikçe tükenmekte ve enerjiye olan talep sürekli olarak artmaktadır. Toplumda artan çevre bilinci, gerek ulusal olarak gerekse uluslararası komitelerin almış olduğu kararlar ile temiz ve yaşanılabilir bir çevre için çalışmalar yürütülmektedir. Otomotiv üreticileri de çevre duyarlılığı, zaman ve maliyet gibi nedenlerden dolayı yeni tasarımların doğrulama süreçleri için alternatif yöntemler arayışındadır. Artan müşteri beklentileri, hızla değişen teknolojiye ayak uydurma çabası, farklı bir sektörde geliştirilen bir yeniliğin otomotiv sektörüne uygulanması için hızlı, doğruluk payı yüksek ve güvenilir yöntemlere ihtiyaç duyulmaktır.
Artan müşteri beklentileri ile araçların taşıma kapasitelerinde artış meydana gelmekte, bu artış da frenleme sırasında ısıya dönüşecek enerji miktarının artmasına neden olmaktadır. Aynı zamanda yakıt tüketimini düşürmek için aracın dış tasarımında yapılan değişiklikler fren disklerine gelen hava debisini azaltmaktadır. Azalan hava debisi ile diskler üzerinden havaya aktarılacak ısı azalmaktadır. Artan fren disk sıcaklıkları sürücünün ve trafiğin güvenliği olumsuz yönde etkilemektedir.
Bu tezin yazımında geçmiş çalışmalardan, araca etki eden sürtünme kuvvetlerinden, fren sisteminin çalışmasından, taşıtının ve fren sisteminin ısı akışından bahsedilmiş, bir taşıtın fren disk ve fren sıvı çalışma sıcaklıklarını, projenin başlangıç aşamasında hesaplayabilecek bir matematiksel modelin ortaya konulması hedeflenmiştir.
Bu bağlamda matematiksel modelde ihtiyaç duyulan teknik hesaplamalar yapılmış, üretici firmalardan gerekli teknik veriler toplanmış, matematiksel modelin çıktılarının desteklenmesi amacıyla araç testleri ile karşılaştırması yapılmıştır. Bu çalışmalar sonucu oluşturulan matematiksel model sayesinde istenen test çevrimi, taşıt özellikleri ve fren diski için zamana bağlı çalışma sıcaklıkları hesaplanabilmektedir. Yapılan çalışma ile projenin tasarım aşamasında fren disk sıcaklıkları tahmin edilebilmekte, ihtiyaç duyulan değişiklikler parça fiziken üretilmeden yapılabilmekte en uygun çözüm için gerekli tasarım prototip - kalıp maliyetleri oluşmadan tamamlanabilmektedir. Aynı zamanda araçla ilgili sağlanacak ek verilerle fren disk sıcaklıklarının frenleme performansına yapacağı etkiler görülerek regülasyonun sağlanmasını zorunlu kıldığı frenleme performansı için öngörüde bulunulabilecektir. Tez kapsamında öncelikle bir rotadaki disk ve fren sıvısı sıcaklığı doğru şekilde tahmin edilmeye çalışılmış ve gerçek test sonuçları ile kıyaslama yapılmıştır.

Abstract:
Fossil energy sources are running out and the demand for energy is constantly increasing. Increasing environmental awareness in the society, as well as decisions taken by both national and international committees are being carried out for a clean and livable environment. Automotive manufacturers are also looking for alternative methods for verification of new designs due to reasons such as environmental sensitivity, time and cost. Increased customer expectations, a fast-paced effort to adapt to technology, the need for fast, accurate and reliable methods to implement an innovation to automotive industry which developed in a different area.
Increasing customer expectations have led to an increase of the load capacity of vehicles, which increases the amount of energy that will be converted to heat during braking. At the same time, changes on the exterior design of the vehicle to reduce fuel consumption, reduce airflow to the brake discs. With decreasing airflow, the heat transferred to the air through the discs is also reduced. Increased brake disc temperatures affect the driver and traffic safety negatively.
In this study, it is aimed to present a mathematical model that can calculate the operating temperatures of a brake disk and brake fluid of a vehicle, which are mentioned also in previous studies like friction force acting on the brake system, running of the brake system, heat flow of the vehicle and brake system.
In order to calculate accurately the temperatures of the elements in the brake system, the energy generated during braking must be determined with high accuracy. For this, the coast down value is measured and the coefficients in the equation are replaced. The heat is obtained according to the force of the brake in which direction the vehicle must be distributed between the front and rear axle. In the scope of the study, the front wheel with disc brake was chosen. The heat distribution between the disc and the lining was calculated analytically and compared with past works after the heat was calculated on how much the front wheel affected.
During the course of the study, the focus was on the brake disc and the brake fluid. Information about the failure modes of these elements has been given and the problems that can be encountered at high temperatures have been addressed. The boiling point of the brake fluid drops quickly when the moisture is inside it, evaporation seriously affects the braking. For this reason automobile manufacturers are testing vehicles in certain cycles. In the study, the need for testing and how it was performed is explained. One point to note during testing is the placement of measurement equipment on the vehicle. It is known that the measurement location changes the results seriously and different temperatures can occur at different points on the reference part during the test.
Some of the information needed for the mathematical model was determined analytically and some of these were determined from experimental paths. The technical calculations required by a mathematical model with a brake system and other system components were made and necessary technical data were collected from the manufacturers and the mathematical model was compared with the vehicle tests to support the outputs. By means of the mathematical model which is the result of these studies, time dependent operating temperatures for brake disc and brake fluid in desired test cycle can be calculated. The changes required to predict the brake disc temperatures during the design phase of the process can be made without producing parts and the design which needs prototype can be completed without tooling costs with most appropriate solution. At the same time, the vehicle will be provided with additional provisions to predict the braking performance that the brake disc temperatures require to be regulated by observing the effects of braking performance. Within the scope of the thesis, firstly the attempt was estimating the temperature of the disc and the brake fluid temperature accurately in a single cycle and comparing with the actual test results.
In the next stage it has been decided to simplify the model for improvement which seems workimg good in accordance with the test results. It is the desired to make the model simple to apply it to different vehicles and routes. In this context, a second model has been created and is called as simplified brake system temperature estimation model. In the second model, parts which have small effect on the heat distribution in the system are not used. The model was checked with test results and previous model results since it should not be distracted from reality.
There is no thermo-couple or sensor that determines the disc temperature in a majority of existing vehicles. Car manufacturers design the system safely during the design phase and do not use a temperature sensor on the discs. However, the driver can push the vehicle limit beyond the conditions of normal usage and exceed the criteria determined during the design. When considering the high cost of putting sensors on each wheel, car manufacturers are looking for alternative solutions. One of these solutions is the ability to calculate disc and brake fluid temperatures with the help of a mathematical model with high accuracy. However, it should be known that there are many variables during driving. In this master thesis study, the temperatures in a certain and challenging condition are calculated. Future work can be extended by expanding the driving routes, performing the generated mathematical model real time on the vehicle.
Another point is that today's vehicle will leave it's place for autonomous vehicle in the future. Commercial vehicles have the most challenging brake system. Vehicles with high load emit excessive amounts of heat during braking. Commercial vehicles also use auxiliary brake systems for the brake system. Auxiliary braking systems allow the vehicle to use the discs less that means less wearing and overheating. Autonomous heavy commercial vehicles will take the place in our lives after autonomous vehicles. These vehicle's software must decide when to use the primary braking system and the auxiliary braking system. One of the most critical point for decision is the disc temperatures. Heavy commercial vehicles have an air braking system so it is not necessary to calculate the brake fluid temperature. Instead of that, the overheating disc can cause the tires to explode, in further studies on this subject can be studied.
The automotive world imitates its verification in the virtual environment during vehicle development with an ever-increasing rate. This thesis study and similar studies will serve this subject.