3 LT dizel motorun çift kademeli turboşarj sisteminden tek kademeli turboşarj sistemine dönüştürülmesi
Investigation and optimization of the engine parameters when switching to single turbocharger from bi-turbocharger in the 3L V6 engine
Yazar: VEYSEL TEKÇE
Danışman: YRD. DOÇ. DR. HİKMET ARSLAN
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Makine Mühendisliği Ana Bilim Dalı / Isı-Akışkan Bilim Dalı
Derece: Yüksek Lisans, 2015
Konu: Makine Mühendisliği = Mechanical Engineering

Özet:
Otomotiv sektörünü zorlayan ve gelişmesine katkı sağlayan rekabet, maliyet, regülasyon, homologasyon ve müşteri kısıtlamaları her geçen gün artış göstermektedir. İşte bu yüzden içten yanmalı motorlarda gerek performans artışı gerekse yakıt tüketimini azaltmak için yapılan araştırma-geliştirme çalışmaları günümüzde de tüm hızıyla devam etmektedir. Otomotiv endüstrisi daha az yakıtla çalışan motor üzerinde çalışırken, yakıt tüketimini belirleyen önemli faktörlerden biri olan araç ağırlığını azaltma arayışlarına devam etmektedir. Otomobil üreticilerinin yakıt ekonomisini iyileştirmenin yanında emisyonu da azaltmak için araç ağırlığını düşürmelerine yardımcı olan aşırı doldurma sistemini portföylerine dahil etmişlerdir.
1925 yılında ilk defa başarılı bir şekilde içten yanmalı motora uygulanan ve %40 verim artışı sağlayan turboşarj sistemi, günümüzde binek araçların vazgeçilmez bir parçası olmuştur. Turboşarj sistemi başlı başına gelişim göstermiş ve ek üniteler ile verimleri daha yüksek seviyelere çıkarılmıştır. Motordan daha da yüksek verim alınması istendiği durumlarda ise çift kademeli turboşarj sistemleri geliştirilmiş ve motorun yanında kendisine ayrı bir yer edinmiştir.
Çift kademeli turboşarj sistemi turbo gecikmesini azaltması ve yüksek güç sağlaması gibi avantajlarının yanında dezavantajları da vardır. Karışık paket yapısına ve kontrol sistemine sahip olması, tek turboya göre maliyetinin ve basınç kayıplarının yüksek olması; aynı motor için güvenilir ve basit yapıda olan tek kademeli turboşarj sisteminin daha tercih edilir olmasına yol açmıştır.
Bu tez kapsamında çift turboşarj sistemine sahip olan motorun yeni versiyonu, motor maliyetini azaltmak için performans değerlerini koruyacak şekilde tek kademeli turboşarj sistemine dönüştürülmüştür. Ayrıca, motor performans çıkış değerlerini (tork ve güç) sabit tutarak özellikle yakıt tüketimi ve emisyon değerlerinin iyileştirilmesi amaçlanmıştır. Tezde ele alınan motorun önceki versiyonu mevcut olduğu için; silindir hacmi, güç ve tork değerleri motor eski sürümü göz önüne alınarak hedef değerler belirlenmiştir.
Bu çalışmada turboşarj sisteminin çalışma sistemine ve çeşitlerine değinerek bir motor için turboşarj sistemi nasıl şeçildiği adım adım anlatılmıştır. Daha sonra analitik yönlemlerle belirlenen üç kompresör, 1D motor performans simülasyon yazılımı kullanılarak motor ile eşleşen en iyi turboşarj seçilmiştir. Yeni ve eski motorun tek boyutlu analiz sonuçları karşılaştırılarak performans değerlerindeki değişim tartışılmıştır. Müşteri profili göz önüne alınarak yapılan araştırmalara göre en çok kullanılan devir aralığında kompresör veriminin arttığı gözlemlenmiştir. Sayısal yöntemlerle belirlenen yeni turboşarj sistemi özgül yakıt tüketimini %0,9 düşürdüğü ve maliyeti %30 azalttığı müşahede edilmiş ve yapılan testler ile bu hesaplamalar doğrulanmıştır. Yeni turboşarj sisteminde su soğutmalı şaft gövdesi ve kaymalı yatak yerine rulmanlı yatak kullanılması turbo tepki süresini ve yakıt tüketimi azaltan önemli faktörlerdir.
Tez kapsamında yapılan son çalışma ise seçilen turboşarj sisteminin dinamometre testleri ile doğrulanmasıdır. Son bölümde motor dinamometresinin yapısından, prototip motorun teste hazırlık sürecinden ve yapılan motor testlerinden bahsedilmiştir ve turboşarj seçiminin doğruluğu tartışılmıştır.

Abstract:
Competition, cost, regulation, homologation and customer demand that cause to develop and force the automotive industry have been dramatically raised. Therefore the research and development studies on the engine performance improvement, fuel economy and weight reduction are proceed by the engineers till present time. Automotive industry searches the lightweight vehicles and aims to reduce the fuel consumption. All manufacturers have the turbocharged cars in their portfolio due to the benefit on the fuel economy and low emission levels.
Turbocharger, which was applied to the internal combustion engines in 1925 firstly and obtained 40% increase in the efficieny of the engine, is an essential part of today engine. Turbocharger system has been developed from the day it was invented and now it works more efficiently with additional unites. If more power is needed from the same volume engine, then two-stage turbocharger system, which requires a special place on the package, are available in the market. A compact charging assembly with two turbochargers enables charge pressures (absolute) up to 3.8 bar.
Parallel sequential turbocharging system was also designed for the old version of the engine in hand because of the need high power. The boosting system is combination of a primary turbocharger and a secondary turbocharger with gas flow directed to the secondary turbocharger via a valve that is integrated to its turbine housing. Working principle of the parallel sequential turbocharging system consist of 3 modes: single turbo mode, transition turbo mode and bi-turbo mode. Only right hand side primary variable geometry turbocharger spins until 2500 rpm engine speed and insufficient boost problem is eliminated from the system by providing that turbo achieves boost set point pressure all the time by varying turbo vanes according to amount of air flow. After the stated engine revolution is passed, secondary turbocharger becomes active slowly and support to primary turbocharger in order to achieve boost set pressure point. After the transition stage is passed and secondary turbocharger increased and leveled its boost pressure with primary, both of the turbochargers spins and deliver single flow to the intercooler.
Primary turbocharger (VGT) is working all the time and secondary turbocharger (FG) is only activated on at high engine speeds. Boost discontinuity is felt when mono-turbo switching to bi-turbo. Charged air streams are mixed in the intercooler at constant pressure. Before mixing, pressure must be regulated on the secondary side using compressor control and recirculation valves.
Two stage-boosting systems have also disadvanteges beside its advanteges like reducing turbo lag and providing more power. It is hard to package and control two stage boosting system, it has high cost and pressure loss in regards to a single turbocharger. Therefore, more reliable and simple turbocharger was preferred for the new developed engine.
Single turbocharger system was targeted in order to reduce cost and weight mainly by proserving the engine performance of the old generation engine. The engine parameters like tork and power were kept same and improvement on the fuel consumption and emission were also aimed. The single turbocharger engine's installation space requirement is smaller than two stage boosting system with the same power output. Since there exists an old version of the engine, the geometric and performance values were based on it.
Matching study of a turbocharger is one of the most critical phases of the whole turbocharger selection process in terms of performance of part and also engine. Firstly, it is important to understand the working prinsiple of turbocharger and types of it. For that reason that information were given beside the selection process of a turbocharger for an engine in the thesis. Three steps of turbocharger selection are simple analytic method, eliminatain with 1D engine performance analysis software and verification with dyno test. In addition, the vehicle application, which the engine will be applied to should be determined before begining of matching process of turbocharger since the application of vehicle, is the main parameter that can be very crucial in the phase of generation of power and torque curves and also turbocharger matching process.
As a start of matching, basic analytical calculations were performed and elimination of the turbochargers between the portfolios of the suppliers were made. This preliminary matching process gave an idea on the frame size of the turbochargers and shortened the next step of the matching process. This provided to reduce the amount of turbocharger combinations to three, which is tried in 1D analysis. The next step is 1D analysis of engine with determined turbocharger combinations. In this stage, an engine model was prepared in engine performance simulation software, GT-Power, with the all components via library of GT-Power software. Maps of wheels that obtained from turbocharger supplier were employed to engine model and then entire model run at full load. Later, engine analysis performed for the selected compressors and turbine and the one which is more suitable for the engine has been determined. As a result of that process, all parameters like power, torques, BSFC, emission controllability, surge and choke margins, compressor outlet and turbine inlet temperature, turbocharger shaft speed were assessed with best mached turbocharger.
The purpose of the study is to determine and select a turbocharger that will provide the same even better engine performance values than the old version. Old version of the engine was also analysed in GT-Power in order to make a proper comparison. The results showed that the compressor efficiency increased in the speed range where the vehicle used by customer effectively. The cost of the turbocharger system reduced 30% and brake spesific fuel consumption decreased 0,9% compared to ex-generation of the engine. It is thought that these improvements were achived by water cooled bearing housing and ball bearing. Ball bearing systems reduce friction compared to plain bearings and improve transient performance of both the turbocharger and engine. One of the reasons getting lower fuel consumption was obtained from ball bearing due to improved turbo efficiency. It also requires lower oil flow compared to plain bearings.
Final step is the validation of the selected turbocharger system via experimental methods. The tests performed in the engine dynometer and engine performe tests done. It is important to correlate between the analysis results and real test results due to have compentency.
All in all, the study guides to generate an efficient, cheap, durable and high performance turbocharger system for a sport utility car (SUV) which is 3L and 600 Nm.