Taşıt motor gürültüsü ve kontrolü

Abstract

Toplumun en büyük kısmını rahatsız eden gürültü trafik gürültüsüdür. İnsan sağlığını korumak, gürültü kirliliğini önlemek için, bu gürültünün azaltılması kanunlarla zorunlu hale getirilmiştir. Trafik gürültüsünün etkin bir biçimde kontrolü de ancak onun ana kaynağı olan motor gürültüsünün kontrolü ile mümkündür. Motor gürültüsünün oluşumu, taşıtın yükü, hızı, kapasitesi gibi faktörlerin yanısıra bakımsız olmasına da bağlıdır. Motor gürültüsünün ana bileşenleri de, yanma gürültüsü, mekanik gürültü, giriş ve çıkış gürültüleridir. Motorun yüzeyinden çıkan ses, silindir basınç değişimlerinden, pistonların vurmalarından ve hareket eden parçaların seslerinden oluşur. Yanma gürültüsü, silindirdeki basınç artış hızlarına bağlıdır. Sıkıştırma stroğunda hızla artan basınç veya ani basınç yükselişi motor gürültüsünün artmasının ana sebebidir. Silindirdeki bu basınç artışının yumuşatılması ile, yani basıncın daha yavaş artması sağlanır ise yanmadan kaynaklanan gürültü kontrol edilebilir. Basınç artışının yumuşatılabilmesi içinde, aşın doldurma, emme havasını ısıtma, ön püskürtme yapma, uygun ön yanma odası kullanılması gibi çareler kullanılmaktadır. Motorun çalışması esnasında motor bloğunun çeşitli yerlerinden, motor bloğuna etkiyen çeşitli kuvvetler vardır. Bunların en önemlileri gaz kuvvetleri ve atalet kuvvetleridir. Bu küvetlerin etkisiyle mekanik gürültüyü oluşturan en büyük bileşen, piston vurması gürültüsü açığa çıkar. Ayrıca yataklardaki ve birbirirle çalışan parçalar arasındaki toleranslar (boşluklar), mekanik gürültüye katkı yapan diğer bir sebeptir. Ayrıca zamanlama dişlisi, sübaplar, püskürtme sistemi, yağ pompası gibi parçaların çalışması da mekanik gürültüye katkıda bulunmaktadır. Motordan çıkan yanmış gazların sebep olduğu egsoz gürültüsü, motorun dönme sayısı ile değişir. Hız azalırsa bütün olaylar yavaşlıyacağı için gürültü basıncı da azalacaktır. Motorun yükü de çıkış basıncını dolayısıyla gürültüyü etkileyecektir. Egsoz gürültüsünün kontrolü de büyük oranda susturucularla yapılmaktadır. Emme gürültüsünün oluşumu aynı egsoz gürültüsü gibidir. Fakat sıcaklık ve basınç farkı büyüktür. Dolayısıyla emme gürültüsü egsoz gürültüsünden yaklaşık 10 dB(A) daha düşüktür. Hava filtreleri bir ölçüde susturucu görevi yaparlar. IX Motor gürültüsünün azaltılması için hem dizayn esnasında, hem de kaplama ve kılıflama, örtme gibi, montaj sırasında tedbirler alınması gerekmektedir. Motor gürültüsünü azaltmak ve engellemek için birçok yöntemler bulunmaktadır, ancak bu yöntemlerin maliyet ve uygulanabilirlikleri, ve motorun diğer özelliklerine ters etki yapmamaları da oldukça önemlidir.

The best definition of noise is disagreeable sound. Intense noise or long stays in a noisy environment can lead to permanent reduction of hearing sensitivity caused by damage to the sensory organs of the inner ear. This type of hearing damage can never be repaired. The major source of environmental noise is the traffic noise. The problem of traffic noise is growing from year to year. Growing urbanization, the rising number of automobiles and trucks and the quantity of traffic have had the effect of increasing noise levels generally. Noise from vehicles is causing very important problems today. Heavy vehicles in particular have recieved much criticism from the people. The rise of noise levels over those of the present levels can be very roughly predicted from estimates of the growth of the motor vehicle numbers, the annual millage covered and the avarege vehicle speeds in urban areas. The main source of vehicle noise is the engine noise. Despite the numerous exciting forces which almost simultaneously exite the engine structure, there is some justification in a simpler way. Since the gas force resulting from combustion tends to be predominant force in most of the engines, the relationship between the gas force characteristics and emitted noise can be used to establish a basic model to identify the effects of fundamental engine designe and operating parameters. The three basic parameters of an engine are speed, size and load. Noise related to engine speed has a number of components: intake and exhaust noise, cooling fan noise, noise emitted by the engine propper and noise from the part of the transmission which rotates at engine speed. Certain operational factors, such as the load, age and general codition of the vehicle and the fuel used, also have an influence on the noise emitted. The predominant noise of the automotive engine lies in the frequency band from 900 to 2000 Hz. which is determined by the vibration of various structure elements of the engine which thus determines the charecteritics of radiated noise. XI It can be shown theoretically that when a force is quickly applied to an elastic structure, vibrations will be set up in the structure with an amplitude depending on the time of force application and the natural periods of structure vibration. In the engine system there are two major forces which are responsible for the engine structure vibration and the emitted noise, namely unidirectional forces and reversible forces. Unidirectional forces are only important in the vicinity of top dead center on the compression strok and are produced from compression and subsequent presssure rise resulting from combustion. In spark ignition engines, noise due to rapid rates of presure rise in the absence of detonation is called engine roughness. Engine roughness usually involves high frequency vibration of the crankshaft- flywheel system in bending. Detonation involves high frequency vibration of the engine structure with a characteristic noise called knock or ping. Many diesel engines produce noise that would be classified as roughness in spark ignition engines. This characteristic is one of those which make diesel engine less attractive than spark ignition engines for use in passenger automobiles. It is evident that noise due to combustion can be reduced eighter the rate of pressure rise or by increasing the natural frequencies of the vibrations involved. The rate of pressure rise with detonation is so high that only the elimination of that form of combustion is effective in eliminating this source of noise. Roughness in spark ignition engines can be controlled to a limitted extend by combustion chamber designe. Control of the rate of pressure rise is an important objective in the design of diesel engines and their injection systems. The effect of injection system on combustion noise; if the injection is retailed, there is a small net reduction in noise for the same smoke level or less increas in noise for a reduction in noise. The effect of turbocharging on combustion noise: By increasing the mass of air trapped in the cylinder at the start of the compression stroke, a greater quantity of fuel can be injected and this tends to increase the peak cylinder pressure with conventional fuel injection equipment. In general turbocharged engines are designed to produce their maximum rated power at a lower speed than the naturally aspirated engine with the same cylinder size. Since noise rise rapidly with speed this has a direct benefit in reducing combustion noise. For controling the rapid rate of pressure rise need to be considered to reduce combustion noise. To reduce the duration of the ignition delay to minimum by; 1. Turbocharging without intercooling, 2. Heating the intake air, XII 3. Increasing the temperature of the surfaces of the cylinder and combustion chamber, 4. Increasing the cetan number of the fuel, 5. Fumigation, this reduces the ignition delay almost completely, and the fumigated fuel is in a mixture too weak to take part in the initial heat release. Reversible forces which change direction are produced by the engine crank mechanism and associated inertia forces. The result of the acceleration of the moving components across the clearances is to impart on contect which occurs the mechanical noise. Since all practical engines have to employ clearances at all bearing surfaces, high frequency excitation can result when the bearing force (which may not have any significant high frequency component of its own) accelerates one half of the bearing surface across the clearance so that it impacts against the other half. These impact, slap and resounance are still classified as mechanical noise sources. The compenents of mechanical noise are piston slap, vibration of the piston, connecting rod, main bearring caps and outer surface of the crankcase. Before the crankcase starts to move showing that the piston, connecting rod and crankshaft are vibrating at their own natural frequencies and thus are contributing to the characteristics of the emitted engine noise.The more the number of cylinder increases, the more the engine noise increases. Mechanical noise also depends on the cylinder size, cylinder pressure, gudgeon pin offset. In addition, crankshaft bearing, timing gears, valve mechanisms and for a diesel engine the injectors are the other sources of mechanical noise. Inlet noise is generated by the interruption of air flow through the inlet passages by the opening and closing of the inlet valves. Inlet noise is load sensitive, increasing by 10-15 dB from no load to full load operation. Where a turbocharger is fitted, noise from the compressor is radiated from the inlet duct. Turbocharger noise is characterised by a pure tone at blade passing frequency and harmonics. Usually evident in the 2-4 kHz. range. The chief difference between exhaust system and intake system is that the magnitude of the pressure change is not as great for the induction system, because we are dealing with considerably more gradual change in pressure as the intake valve opens and the piston descends. Various silencers are used to decrease the induction noise as they are used to decrease the exhaust noise. In silencer design or in a program to improve present silencers, frequency analyses can be made at the peak speeds or periods. Essentially all intake silencers are of the straight -through design, that is using side chambers to control both low frequency and high frequency noise. XIII The exhaust noise makes the second biggest contribution to overall vehicle noise. Exhaust noise is produced by the periodic abrupt release of gas in to the exhaust system as the exhaust valves open and varies considerably between engine types depending on valve configuration and timing. Exhaust noise typically increases 15 dB between no- load and full-load operation. The exhaust noise varies acording to the pressure and the temporature of the gas that is exhausted from the engine. The hot exhaust gas through the pipe and the silencer reaches a high speed of 80 m/s, so it causes bigger flow noise and back pressure. It has necessitated finding an effective method for the reduction of flow noise. There are three noise sources: a gas exhaust noise and a surface radiation noise from both pipes and silencer. For controlling the engine noise, each components needs to be examined and modificated. These modifications can be seen below: Engine: Modification of structure Vibration control; manifold etc. Acoustic shield; cylinder block wall, oil sump Control of standing wave; timing gear cover etc. Exhaust system: Flow noise control; silencer Vibration control; Exhaust piping Intake system: Flow noise control; silencer Vibration control; intake manifolt, Helmutz rezönatör Drive train: Control of gear rattle etc. Cooling system: Flow noise control; cooling fan Overall: Covering and enclosing.. The engineer has three ways to produce a low noise engine: 1. Attention to engine geometry and operating parameters 2. Control of exciting forces (combustion and mechanical) 3. Control of the response of the outer structure to the exciting forces. His tools for this are: mass, stiffness and damping and these are linked with experience measurement and predictive analysis. XIV For the future, the technology should naturally achieve a harmony with nature. The vehicle noise, therefore, should be controlled with some new technologies such as active control and acoustic lens etc. The future vehicle being applied with such new technologies will give a pleasant sound as like as some natural music.