AVL engine noise

972042Meeting Future Demands for QuieterCommercial Powertrain SystemsChristian V. Beidl and Alfred RustAVL LIST GmbH Copyright 1997 Society of Automotive Engineers, Inc.ABSTRACTNoise legislations and the increasing customer demands determine the NVH-development of modern commercial vehicles. In this paper suitable engineering approaches will be discussed.In order to meet the very stringent legislative requirements of the EEC and some other countries refinement of all vehicle noise sources is required. Cost-effective solutions, however, can only be found with low-noise powertrains, thus being able to avoid excessive noise packages on the vehicle. There is increasing demand, because modular systems should be ready to power a variety of different trucks and busses and allow for easy servicability.With this focus on powertrain noise, the paper discusses and outlines the technological developments required to achieve sufficient noise reduction which aims towards a 1m engine noise level of 93 dBA measured in an acoustic test cell under rated conditions.This very tough target for future engines can only be achieved with low-noise design concepts, with special consideration of the diesel injection and combustion system and a perfectly optimized powertrain structure. Potentials, analysis methods and the application of advanced simulation techniques in combination with experimental development and fine-tuning will be described in this paper and can be demonstrated by case studies.Clearly, future demands will also include competitive noise quality inside and outside the vehicle, not only in drive, but also in idle condition. The AVL Annoyance Index can be successfully applied to support the noise quality development.INTRODUCTIONLegislation and market demands drive the development of internal combustion engines. In both fields the requirements were clearly increased in the last years due to tighter legislative limits and higher standards of comfort. And these requirements will continue to increase. This fact will necessitate a more and more holistic approach of engine development. This means, forinstance, that the acoustic development has not only to be focused on legislative noise limits and comfort requirements. It also has to consider the impacts of other requirements (like those of exhaust emissions, fuel consumption, engine performance, engine weight etc) on the engine concept and hence on the engine acoustics already from the very beginning.In the last decades the noise legislation for vehicles was clearly tightened as shown in Fig. 1 for trucks in the EEC. First, the pass-by noise limits were lowered step-by-step. Second, the pass-by noise test procedure for trucks was also partly modified yielding more severe test conditions. In addition to the noise legislation, the non-legislative requirements as mentioned above (market demands, comfort standards) must be taken into account. With trucks, these additional requirements refer mainly to the exterior noise at low idle and to the noise and vibration behaviour in the vehiclecabin.All these noise requirements have to be met with the complete vehicle. The engine which is generally the main noise source of the vehicle, is most influenced by these requirements. Of course, all other sources of vehicle noise like intake system, exhaust system, cooling system, transmission, drive line and tyres need also aFig.1Reduction of the EEC pass-by noise limits for trucks during the last twenty yearsproper noise development and also the complete vehicle requires a final acoustic development and refinement work. However, the engine is still the acoustic kernel of a vehicle, especially of trucks.For engine development purposes it is commonpractice to estimate the 1m engine noise level at rated condition using the so-called vehicle attenuation. It is defined as the difference between the 1 m engine noise level at rated condition and the pass-by noise level and contains, amongst others, the attenuation of the engine noise by the vehicle body [1]*. Based on empirical data of vehicles which do not have a special low-noise package,the vehicle attenuation of heavy duty trucks (≥ 150 kW) is about 12 to 14 dBA and that of medium duty trucks (< 150kW) amounts to 14 to 16 dBA [2]. With the European pass-by noise limits for these vehicle categories (80 dBA and 78 dBA resp., see Fig.1) and the above vehicle attenuation values, the 1m engine noise level is calculated to 92 to 94 dBA or as mean level to 93 dBA. Hence, a level of 93 dBA is nowadays a preferred noise development target for engines which will be used for trucks in EEC countries, since it can be highly expected that the truck will fulfill the noise legislation without major vehicle related measures against engine noise. Fig. 2illustrates this situation and shows that, in general, a noise level around 93 dBA requires shieldingor even enclosures with conventional engines or, if shielding is excluded, new engine designs based on comprehensive low-noise concepts are required.* Numbers in parentheses designate references at end of paper.CONTRIBUTION OF ENGINE TO VEHICLE NOISEThe contribution of the engine noise to the totalvehicle noise depends strongly on the operating condition of the vehicle. At higher road speeds, for instance at speeds above 60 kph, the vehicle noise emission is dominated by the noise arising from the tyre-road contact [3]. Typical conditions of high contribution of engine noise are the low idle condition and the pass-by test operation.Fig. 3 illustrates the contributions of the individual sources to the pass-by noise of a medium duty truck. Before the acoustic development of the vehicle,i.e. at a higher pass-by noise level, the engine noise is clearly dominating yielding a contribution of nearly 70percent. But even if the engine and the vehicle is treated acoustically meeting the current legislative noise limit, the engine noise gives still the major contribution to the pass-by noise.Also with vehicles which yield a bigger noiseattenuation effect by their body such as the pickup truck of Fig. 4, the engine noise is still predominant. The contribution of the engine (engine block plus oilpan)amounts to half of the total pass-by noise of the pickup truck.The noise source ranking shown in Fig. 4 is theresult of the baseline analysis of the pickup truck to be improved. Such a noise source analysis is indispensable for a noise development work, since an efficient improvement strategy can be derived only from the results of a noise source analysis. The fundamental development procedure for the pass-by noise reduction of the pickup truck by 4 dBA is represented in Fig. 5.Fig. 2Relationship between engine noise and pass-by noise legislationFig.3Contributions to the pass-by noise of a medium duty truck with a 6 I engineCONTRIBUTIONS TO ENGINE NOISECommonly, the noise of truck engines isdetermined by the combustion noise and by mechanical noise contributions arising from engine components such as injection system, gear train, piston, crank train and valve train.Exhaust emission legislations like EURO 2 orfuture EURO 3 as well as the demands for low fuel consumption lead to the use of high pressure injection systems with injection pressures up to 1600 bar [4, 5].This trend towards high pressure injection establishes the injection system as the key component of a Diesel engine which has a significant influence not only on the combustion characteristics of the engine, but also on its noise behaviour. The influence on the noise behaviour refers to combustion noise because of high initial injection rate and to mechanical noise because of big pressure gradients yielding high structure-borne noise and because of great torque fluctuations in cam-driven systems yielding strong rattle noise in the gear train.COMBUSTION NOISE - The typical truckengine is a turbocharged and intercooled Diesel engine.For such an engine the operating conditions with high contribution of combustion noise are the low speed and low load conditions (Fig. 6). Especially at low idle the high combustion noise can result in a severe noise quality problem making the engine even unacceptable for the customer. In this case, countermeasures like injection rate shaping are a must. Proven countermeasures are pilot injection and the use of two-spring injectors [1,4,6]. As shown in Fig. 7 the effectiveness of these countermeasures is high in the low speed and low load range which is the same range as mentioned above where combustion noise is predominant. For example,overall engine noise reductions up to 4 dBA can be obtained by such measures at low idle condition [4].Fig. 4 Contributions of main noise sources to the pass-by noise of a pickup truck with a 2.6 l Diesel engineFig.5Pass-by noise development of a pickup truck with a 2.6 l Diesel engineMECHANICAL NOISE - As a consequence ofthe trend to higher peak firing pressures (to 160 bar and even beyond) and higher injection pressures the forces acting on the power train components will increase the mechanical noise with high contributions from injection pump and gear train. In Fig. 8 the noise radiation behaviour of three different heavy duty truck engines is compared. The engines are different in, power output (200versus 300 kW), fuel injection system (in-line pump vs.unit injectors) and location of gear train (flywheel side vs.front side). The average 1m noise levels at rated conditions are 95 and 97 dBA respectively. The results in Fig. 8 show that the engine block (including pistons and crank train), gear train and injection pump are the engine components of highest noise radiation. The dominance of these three engine components is typical of large today’s truck engines. They are also expected to 6-Cylinder Inline Engine, 121 Class, Full Load / Rated Speedrepresent the acoustic key elements of future truck engines. Therefore, these items will be discussed more detailed in the next section.KEY ELEMENTS FOR ENGINE NOISEFUEL INJECTION SYSTEM - Within allimportant noise sources the fuel injection system,particularly a high pressure injection system, holds the key position in view of both combustion and noise. The necessity of high pressure injection for low emission combustion is well known. Its detrimental effect on noise,however, has to be taken into account already in the engine concept. With cam-driven systems (like in-line pumps, distributor pumps, unit pumps, unit injectors), the generation of high pressure occurs intermittently and has therefore multiple effects on the engine noise as illustrated in Fig. 9. First, there is a high noise radiation from the surface of the injection pump, injection lines andFig. 6Contribution of combustion noise to the total noise of a 6 l TCI-DI Diesel engine (derivedfrom separation method as described in [7])Fig.7Reduction of combustion noise by injectionrate shapingFig. 8Noise radiation of truck engines with different design concepts and power outputFig.9Multiple influence of injection system on engine noisenozzle holders. Second, the vibration transfer from the pump and nozzle holders into the engine structure is increased. Third, the excitation of the gear train is also increased. Fourth, and this is valid not only for cam-driven systems, but for any kind of high pressure injection, the combustion noise excitation is clearly higher particularly at low load conditions (e.g. low idle), if special countermeasures are not applied during the combustion development. The overall noise increasing effect of high pressure injection systems depends on the speed/load condition of the engine and can amount up to 10 dBA at certain conditions [5].TIMING GEAR TRAIN - Generally, the main noise contribution of the gear train arises from gear rattle which is induced by high speed fluctuations resulting from torque fluctuations and occurring at certain gear meshes. Hence, the origins of speed fluctuations are all cam-drives (valve train, injection system), auxiliary drives with highly fluctuating torque requirements (air compressor) and the crankshaft. Therefore, cam-driven high pressure injection systems and multi-valve systems lead to higher gear rattle noise. This disadvantage can be compensated by conceptional and detail measures like gear train location at flywheel side, minimization of the extent of the gear train, additional flywheels at certain gear train elements, minimization and control of backlash etc. In Fig. 10, the benefit of the gear train located at the6-Cylinder Diesel Engine, 6l, 3500 rpm, Full Load flywheel side of an engine instead of its front side is demonstrated. Because of the elasticity and the vibration modes of the crankshaft, the angular displacement caused by the torque fluctuation from the crank train is much higher at the front end of the crankshaft than in the vicinity of the flywheel. This fact results in a clear decrease of the impact energy of the rattling gear meshes. The impact energy is approximately halved which corresponds to a reduction of the gear rattle noise by about 3 dBA. The overall effect of engine noise reduction can reach up to 2 dBA [6].ENGINE STRUCTURE - It is a well known fact that a proper design of the engine structure, especially the assembly of cylinder head, engine block and crankshaft including piston and conrod, is a key requirement for low-noise engines. Any potential for structure optimization which is not used can hardly be compensated by other measures.The dynamic behaviour of the engine structure determines the vibration transfer from the locations of noise generation to the outer surface of the engine radiating the engine noise. Therefore, both the selection of the proper design concept and the optimization of the detail design by means of finite element calculation are a must in order to meet the requirements for an excellent dynamic behaviour of the engine structure with low specific engine weight [8].First, the FE-calculation results give very valuable data in the conceptual design phase as demonstrated in Fig. 11, where different bottom end design concepts of smaller in-line Diesel engines (up to a displacement of 1 l per cylinder) are compared in termsFig. 10 Effect of gear train location on rattle noiseFig.11Effect of crankcase bottom design on weight and vibrationof vibration characteristics and weight. The heaviest versions C and G give lowest vibration. If these two versions are not taken into consideration for weight reasons, the full size ladderframe F has certain benefits against the bedplate version B. From experience it is known that these benefits of the ladderframe compared with the bedplate become the, smaller, the larger the engine is.Secondly, given the design concept, thecalculation can provide all the information necessary for low-noise detail design. As an example, Fig. 12 shows the result of a design modification because of a critical vibration response of the block bottom end at 1.25 kHz. In this phase, full size models; are required, where all relevant input forces and nonlinear, effects are considered. On the basis of such accurate models,advanced methods for the direct airborne noise prediction can be applied [8].The engine structure has to have not only anexcellent vibration behaviour, but also must give the possibility to run small clearances in the cranktrain. In particular, low cylinder liner distortion and optimized piston profile are important to enable small piston-to-liner clearances for reducing piston slap noise [10]. However,since piston slap noise is a significant noise component with turbocharged Diesel engines, because of high peak firing pressure, further measures are necessary to controlpiston slap noise. A common measure is piston pin offset,preferrably to the thrust side [6].As already mentioned above, the finite elementcalculation of the engine structure is an important tool also in view of low engine weight, because the aspect of low weight is gaining importance with truck engines too. This tendency leads to the substitution of metal by plastic material for certain engine components like gear train cover, valve cover or even oilpan [10]. In many cases engine components made of plastic material must be carefully designed in order to avoid an increase in noise.However, if highly damped plastic material can be used,the weight reduction effect can be combined with a noise reduction effect as illustrated in Fig. 13.FURTHER POSSIBILITIES OF ENGINE NOISE REDUCTIONThe control of engine noise by the key elementsmentioned above is the main step toward low engine noise levels. However, to achieve low noise levels like 93dBA, further possibilities of noise reduction have to be checked and utilized. Such possibilities are described below and illustrated by some examples.ATTACHED ENGINE COMPONENTS - Theacoustic treatment of attached components like valve cover, oilpan, intake manifold or timing gear cover is very important, since these components represent a big portion of the noise radiating engine surface. Effective kinds of treatment are decoupled mounting of the parts at the engine structure (“vibration isolation”) and the use of highly damped material such as laminated sheet steel or suitable plastic material. Vibration isolation is especially suitable for intake manifolds and valve covers (Fig. 14),whereas highly damped materials are used for oilpans and covers.Fig. 12Calculated structure modification of a 20 l V8truck engineFig. 13Noise radiation from a timing gear coverALTERNATOR AND TURBOCHARGER - Veryoften the cooling fan of alternators causes disturbing noise which can yield a significant noise contribution, if the engine noise is already on a lower level [1]. In most cases this noise contribution is rotational noise of the fan. Such noise can be reduced by improvement of the air flow conditions in the vicinity of the fan or even by conceptional changes of the alternator cooling. Another kind of tonal aerodynamic noise arises from the turbocharger yielding a distinct peak in the very high frequency range of the engine noise spectrum due to the high speed of the turbine. With the high degree of charging of intercooled engines, this peak can contribute essentially to the overall engine noise level. In this case the peak has to be lowered so that its influence on the engine noise level becomes neglectable. Fig. 15 shows such a reduction of turbocharger noise achieved by modification of the charger housing.OILPUMP - Similar to the situation of alternatorand turbocharger noise, the oilpump system can generate noise components which become important and evident only, if the engine noise is on a certain low level. Such noise phenomena can be suppressed, for instance, by adjustment and refinement of the pressure regulating valve as shown in Fig. 16.SHIELDING - Noise shields can be alternativesolutions to noise reduction measures like decoupled valve covers or oilpans made of laminated sheet steel [1].However, a shield can also be the only solution, if a predominant source like a noisy injection pump cannot be treated in another way. The results of such an injection pump shielding are shown in Fig. 17. The highFig. 14Effect of vibration isolated valve cover on the noise of a V10 truck engineFig.15Influence of turbocharger noise on overall engine noise of a 12 l Diesel engine at rated conditionFig.16Reduction of oilpump noise by refinement of the pressure regulating valve of a 12 l Diesel engineFig. 17Effect of injection pump shielding on noise of a 12 l Diesel engine with 250 kW power outputacoustic effectiveness of the shielding as illustrated in Fig.17 is only possible, if some important features are realized. The shield must be attached to the engine structure using vibration isolating mounts, unless the shield is made of a highly damped material like laminated sheet steel. Low-frequency vibration modes of shield panels have to be avoided, since noise quality can suffer considerably from excessive panel vibration. The balance between sound barrier and sound absorption effect must be well matched to the noise source characteristics. Finally, the necessary attenuation effect has to be achieved with the minimum of shielded area for cost and weight reasons. Above all, overheating of the shielded components must be strictly avoided and might require specific development work.RATED SPEED - A further potential for noise reduction is given by decreasing the engine speed. Typically, with a decrease of 20 percent in rated speed a noise level reduction of 1 dBA can be expected. This potential should be considered in the concept phase of a new powertrain, when the basic specifications of engine, transmission and driveline are decided.NOISE QUALITYFuture noise requirements for commercial vehicles will also include competitive noise quality inside and outside the vehicle. The most critical condition for the outside noise quality is low idle with very specific noise components (combustion noise, gear train noise).For the noise quality development work, a quality indicator like the noise annoyance index as described in [11, 12,13] is a very helpful tool. As an example the AVL Annoyance Index results in Fig. 18 demonstrate the engine noise quality improved at low idle.The analysis of the engine noise, by means of this index gives not only an information on the total noise annoyance, but also on its individual contributions like loudness, sharpness and impulsiveness allowing overall and detail comparisons of different engine versions. However, due to the typical ranges indicated in Fig. 18 which are derived from a wide data base, the AVL Annoyance Index can be used also for a general assessment of an engine.For the noise and vibration comfort inside the cabin which is the drivers working environment in commercial vehicles, the quality of the powertrain mounting system is very decisive. For its specific layout and optimisation, simulation tools like the ADAMS code can be used. Specially adapted optimisation routines allow specification of positions and mount stiffnesses and also direct comparison of various mounting concepts such as the transition from a four point to a three point mounting system [14].CONCLUSIONSThe high demands on commercial powertrain systems regarding exhaust emissions, fuel consumption and noise require the careful check of all possibilities of low-noise features taking into consideration the aspects of production, economy and ecology.For this purpose, key steps in the acoustic development of engines have to be set during all phases of development as shown in Fig. 19.In the concept phase, the decisions on the injection system, engine structure concept and gear train preset the engine noise level achievable.Fig.18Noise quality improvement of a 4 l truck engine by injection rate shaping at low idleFig.19Key steps in the acoustic development of truck enginesDuring the design phase, the most decisive steps are the optimization of the dynamic behaviour of the engine structure, the introduction of primary noise reduction measures (like piston pin offset, small clearances etc) and the layout of attached parts as low-noise components.In the experimental development phase, secondary measures (like shields or damping devices) have to be settled after the final tuning of primary measures. In addition, disturbing noise phenomena must be treated to ensure high noise quality. REFERENCES[1]Leipold, F. and Bergmann, H.: Development Stages for Reducing Noise Emissions of the New OM 904 LA Commercial Vehicle Diesel Engine. AVL-Conference “Engine and Environment” ‘96, Graz, Sept. 1996.[2]Albers, P. and Brandl, F.K.: Vehicle Noise Reduction Strategies. SOBRAC Congress, Santa Catarina, Brazil, April 1994.[3]Fingerhut, H.-P.: From Research to Serial Manufacture: MAN SILENT Trucks - Present and Future Requirements. AVL-Conference “Engine and Environment”’96, Graz, Sept. 1996.[4]Bergmann, H., Scherer, F. and Osterwald, H.: The Combustion Development of the New OM 904 LA Diesel Engine of Mercedes Benz Commercial Vehicles. MTZ Motortechnische Zeitschrift 57 (1996) 4, April 1996.[5]Miyashita, N.: Strategy of Engine Noise Reduction for Trucks and Buses. 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