Dizel motorlarında çift kütleli volan kullanımı ve ilk çalıştırma gürültüsüne etkilerinin incelenmesi

Abstract

Teknolojinin gelişmesiyle birlikte uzun yıllardır kullanılan içten yanmalı motorlar da gelişmekte, son kullanıcının beklentileri de artmaktadır. Otomotiv sektörü de artan beklentilere cevap niteliğinde yeni teknolojiler sunmakta, sürücünün konforunu arttırmayı hedeflemektedir. Bu bağlamda araç içerisindeki ve dışındaki gürültü ve titreşim seviyelerinin minimize edilmesi otomotiv firmalarının önem verdiği bir konu haline gelmiştir. Bir otomobilin gürültü ve titreşimine etki eden parametreler arasında motor ve aktarma organları büyük yer tutar. Dört zamanlı içten yanmalı motorlarda, silindirlerde meydana gelen periyodik patlamalar sonucu krank milinde tork salınımları ve titreşim meydana gelir. Bu salınımlar, motorda üretilen gücün artışı, azalışı oranında değişiklik gösterir. Dizel motorlarında gerek yanma prosesinin farklılığı gerekse yüksek sıkıştırma oranlarının bir sonucu olarak bu titreşimler daha yüksek genliklere sahiptir. Krank milinde meydana gelen bu hız dalgalanmaları ve titreşimin diğer aktarma organlarına iletilmesi, sürüş konforunu azaltmanın yanında parça ömürlerini azaltacak, dayanımı daha yüksek malzemelere ihtiyaç duyulmasına neden olacaktır. Bu titreşim ve hız dalgalanmalarını belli bir seviyede tutabilmek amacıyla, kinetik enerjinin depolanması prensibiyle çalışan, ataleti yüksek silindirik bir parça olan volan kullanılmaktadır. Zorlayıcı emisyon şartları ve düşük yakıt tüketimine sahip otomobiller üretme ihtiyacı, üreticileri aynı gücü ve konforu sağlayacak daha düşük atalete sahip araçlar geliştirmeye itti. Ancak verim artışı gerekliliklerinin meydana getirdiği yüksek düzensizlik ve burulma titreşimleri, şanzıman dişlilerinde yüksek genlikli titreşimlere sebep olmaktaydı. Bu doğrultuda, şanzıman ve motor bağlantısını rijit bir yapıdan elastik yapıya dönüştüren çift kütleli volanlar geliştirilmiştir. Çift kütleli volanlar konvansiyonel volanlardan farklı olarak, sahip oldukları yaylar ile motor tarafındaki düzensizlikleri büyük oranda izole etmekte, motor hareket aktarma sisteminin burulma titreşimi doğal frekansını aracın ağırlıklı olarak çalıştığı motor hızlarının dışında tutmaktadır. Bu sayede çift kütleli volanlar daha yüksek verime sahip motorların üretimine olanak sağlamış, şanzıman kaynaklı takırtı(rattle) gürültülerini ve aracın rezonans frekansında çalıştığında ortaya çıkan gümbürdeme(boom) şikayetlerini ortadan kaldırmış, sürüş konforunu arttırmıştır. Ancak burulma titreşimi doğal frekansının rölanti altı motor hızlarına çekilmesi, motor sisteminde ilk çalıştırma hareketi esnasında doğal frekansa karşılık gelen hızlarda yüksek genlikli titreşimlere sebep olmaktadır. Özellikle yüksek motor düzensizliğine sahip dizel motorlarında sistemin doğal frekansının motorun göreceli olarak yüksek tork sağladığı bölgeye denk gelmesi sonucu meydana çıkan bu titreşimler, motora ilk hareketi sağlayan marş motoru pinyon dişlisi ve birincil volan dişlisinin birbirine vurmasına ve takırtı gürültüsüne neden olmaktadır. Bu tez kapsamında, bir dizel motorda çift kütleli volanların kullanım amacından bahsedilmiş, çift kütleli volanlarda önem arz eden tasarım parametreleri incelenerek farklı tasarım şekilleri paylaşılmıştır. Ardından bir dizel motorun dinamik modeli oluşturulmuş ve bu model verilerinden faydalanarak takırtı gürültüsüne sahip bir dizel motorun volan tasarımının değiştirilmesi sonucu takırtı gürültüsünün giderildiği deneysel çalışmaya yer verilmiştir.

The engine system is one of the major sources of vibration and sound in a vehicle. Due to the nature of internal combustion engines, a constant torque generation is not possible. The engine torque consists of sequential pulses which are generated by combustion in cylinders. In a diesel engine, firing pressures and can be twice as high as in gasoline engines and continue to increase as the size increases which then increase the amplitude of pulses. The torque variations can excite crankshaft and other components' resonances within the engine or the driveline, potentially leading to fatigue failure of the crankshaft, or to important noise or vibration problems, such as gear rattle or driveline boom. Flywheels are rotating mechanical devices that stores the kinetic energy. They have a large moment of inertia and thus resist changes in rotational speed. Dual-mass flywheels (DMF) decouple the crankshaft assembly from the power train. Their working principle is mainly seperating the flywheel mass into a primary engine side and a secondary drive side mass by the elastic and viscoelastic elements between two mass portions. In this thesis as overall, conventional flywheels(single mass flywheel) and dual mass flywheels are compared, dual mass flywheel design parameters are defined and experimental study about start-up event NVH refinement with dual mass flywheel spring characteristic tuning is shared. The first chapter consists of the history of the dual mass flywheel and literature review. Dual mass flywheels have been used since 1980s. They were introduced as a solution for the high vibrations in the gearbox which often cause unexpected failures and rattle noise. Introduction of DMF brought two major benefits; lower hysteresis rates compared to conventional flywheels and significantly lower torsional vibration passing through to transmission. Second chapter includes the basic theory of sound and vibration, NVH measurements and frequency analysis. The Wavelet Analysis which will then be used in experimental study section is explained in this chapter. Basically, Wavelet Analysis reveals the opportunity to locate a frequency content's exact location in time signal. Third chapter mainly focuses on sound and vibration sources in internal combustion engines. The first part is the combustion related vibration and sound contribution. As expected, engine combustion noise is originated from the combustion events which occurs in cylinders. When the fuel is injected to the cylinder chamber where high pressure air exists, the combustion occurs prior to the Forth chapter first starts with main the goal of DMF usage in diesel engines. With conventional type which are single mass flywheels, a powertrain system's first natural frequency of torsional vibration is located in engine speed range of 700-2000 rpm. Although clutch damper implementation would reduce the amplitudes of natural frequency related vibration levels, it deteriorates the post-natural frequency band which correspondes to engine speed of 2000-3000 rpm. Since excessive damping makes a conventional flywheel useless due to having same speed oscillations with engine, damping with clutch is not a complete solution to resolve torsional vibrations. Dual mass flywheels decouples the engine and transmission with the elastic connection between two masses. This reduces the torsional vibration natural frequency of the system and corresponding engine speed range is shifted to sub idle speeds. Although dual mass flywheels extremely lower the vibrations at gearbox, having a natural frequency below idle speed creates a new challenge for controlling the vibration levels at the engine side especially during start-up event. Since an engine needs to sweep sub-idle speed range during start-up, the DMF will reach to maximum wind-up angle position due to the resonance. This may cause high level of vibrations which are not desired at all. A start-up event is kicked off by the starter motor pinion and it stays engaged until engine speeds up to pre-defined speeds. Then starter motor pinion gear is disengaged. The disengagement time may vary based on the system assumptions and engine characteristics. In some cases, when the system starts to oscillate around resonance speed during start-up, a rattle noise occurs. This may be caused by engine sensitivity at that speed which is mainly effected by the combustion process and injection strategy. In previous studies, it is seen that diesel engines are much critical for start-up rattle noise. The thesis includes the case study of a vehicle with 4 cylinder diesel engine which is rated as "poor start-up" due to the metallic noise content during start-up event. However the issue was only reported as metallic which may not be the gear rattle behaivor that is mainly caused by DMF. In this manner, a dynamic model of a powertrain with 4 cylinder diesel engine is created with AVL Excite software. The main goal of creating model was obtaining the torsional vibration natural frequencies of the system. However an advanced powertrain dynamic model is created to ensure representativeness for potential future studies. Then NVH measurements are performed with the complaint vehicle. At first glance, the interior mics were showing different behaivor in driver and passenger sides. Driver side sound pressure levels were slightly higher. Firstly, the interior microphone data is processed and it is found that driver side mic shows slightly higher levels. Due to not having sufficient indications which will point out the root cause of rattle noise, a microphone is placed to the engine compartment. The engine compartment microphone data is processed with Wavelet Analysis method which helps determining the exact timing of rattle noise. When the engine speed and complaint noise peak is plot in the same timeframe, the rattle noise area matches with the first torsional vibration natural frequency of the powertrain. Based on these findings, a reference vehicle with same engine but different flywheel is measured. The only difference between the flywheels was the secondary inertia. The CAE analysis with dynamic model confirmed that the system with lighter secondary inertia(in reference vehicle) has lower natural frequency for the first torsional mode. Engine compartment microphone measurements also showed that the metallic noise does not occur on reference vehicle during start-up. However due to the package constraints, the only achievable option for lowering the natural frequency of complaint powertrain was tuning the arc springs of DMF. The first stage stiffness of DMF which mainly affects the start-up is softened and the second stage which mainly effects the high torque ranges is stiffened to cover same engine power as before. The prototypes have been produced with new springs and NVH measurements were performed. Besides 5dB reduction in rattle specific frequency range of engine compartment microphone sound pressure levels, 3 to 4 dB improvement in overall engine compartment levels and 2 to 3 dB improvement in overall interior noise levels are achieved.