Five dB noise reduction for large medium speed diesel engines
Engine noise has becoming more and more important over years. Especially for maritime industry, the International Convention for the Safety of Life at Sea (SOLAS) launched new noise regulation since July 2014, which demands a maximum acceptable sound pressure level of 110 dB at any individual measurement position in the machinery space. Wartsila has a decade of experience on large medium speed diesel engine noise and vibration research and development. Today's Wartsila is capable to technically reduce the engine noise level by 5 dB without any insulation panel even with increased engine power and higher fuel efficiency. This paper will explain in detail both the structure borne noise and air borne noise from an engine, from the source of excitation, transmission path and the structure noise radiation. Major noisy engine components that contributes the engine room noise level will be addressed, from both theoretical and experimental point of view. This paper will also explain the methodology of noise reduction on different engine noise sources. In the end, other important factors that matter with final engine room noise level will be pointed out. Introduction Wartsila is a global leader in advanced technologies and complete lifecycle solutions for the marine and energy markets. One of its major products is large medium speed engines for shipyards and power plants. The engines being mentioned in this paper refer to W32 type, which has cylinder bore of 32 cm in diameter. Depending on different cylinder configurations ranging from 6 to 20, the weight of W32 in line engines ranges 30 50 tons and V engines ranges 50 90 tons. At full load the engine output is 580 kW per cylinder and nominal speed is 720 or 750 rpm. At operating conditions, such massive machinery can produce significant amount of noise that may easily damage hearing within seconds if no hearing protection is used. The main driving force for research of engine noise reduction is from relevant regulations. For the power plants there are various local occupational health and safety related and environment related regulations. For maritime industry the regulations are mainly from IMO (International Maritime Organization) With the increasing awareness of safety and comfort, the importance of noise has increased dramatically in the past 20 years. In July 2014, IMO has adopted regulation II 1/3 12 of the International Convention for the Safety of Life at Sea (SOLAS) 1974 as amended, concerning protection against noise. The new regulation has become mandatory and it requires a maximum of 110 dB at every individual measurement location 1 m away from the engine surface. This new regulation is much stricter comparing to the old one and it may pose great challenges to shipyards. Wartsila started research work in engine noise and vibration since 1990s. The manpower in the engine acoustics has increased from one to nowadays seven [1] The noise research area covers not only engine noise itself, but also exhaust noise, environmental noise and personnel daily noise exposure. Latest relevant papers can be found [2 5] The pace of development of engine noise reduction solutions has speeded up in the past decade, with great amount of research projects in cooperation with various institutes, mainly in Scandinavian countries. Today's Wartsila is capable of reduce engine noise by more than 5 dB without any insulation panels, even though the engine output has increased by 16% This has made Wartsila extremely competitive in the market where nowadays customers intend to select a manufacture with quieter engines. Meanwhile, shipyards are expected to take the leading role and responsibility of engine room acoustical design which will significantly influence the resulted engine room noise level. 2 Aachen Acoustics Colloquium 2016 Measurement Techniques The SOLAS regulation on machinery room noise requires maximum 110 dB as sound pressure level, which will not only depend on the engine sound power but also the on site room acoustical properties. As engine manufacture, Wartsila is responsible for the engine sound power, which is a unique property of the product. In order to deliver reliable data, the instruments used in Wartisla for standardized sound power reporting meet the requirements of the IEC 1043 and the measurement procedure is based on ISO 9614 "Acoustics Determination of sound power levels of noise sources using sound intensity" Part 2 "Measurement by scanning" Concerning the measurement uncertainty and accuracy, the "Engineering grade" (grade 2) defined in ISO 9614 Part 2 is used. The method of measuring the component of sound intensity normal to a measurement surface which is chosen so as to enclose the engine of which the sound power level is to be determined is specified in house. Surface integration of the intensity component normal to the measurement surface is approximated by subdividing the measurement surface into contiguous segments, and scanning the intensity probe over each segment along a continuous path which covers the extent of the segment. To standardise sound intensity measurement, the engine to be measured is divided into partial sources. Moreover, in most cases the measurements are on purpose of research aiming at localizing the sound sources of an engine, a partial source should be in turn divided into even smaller measurement segments which can completely encloses the engine, Figure 1. Figure 1: Left, partial sources division on a Wartsila V engine; Right, example of detail measurement segments for sound intensity scanning for a Wartsila in line engine. With increasing knowledge gained through standard sound intensity measurements, there have been more and more need for sound source localization for engine product in Wartsila. Scan and paint (S&P) technology has drawn Wartsila's attention for this purpose. Comparing to traditional sound intensity probe which consists of two microphones, the S&P probe consists of one microphone and one velocity sensor, Figure 2. Figure 2: Left, example of traditional sound intensity probe composed by two microphones; Right, example of S&P probe composed by one microphone and one particle velocity sensor. Aachen Acoustics Colloquium 2016 3 The S&P probe is supposed to be used at near field for sound source localization, and at further distance for sound intensity identification. As a relatively new technology in the market, the device was assessed by Wartsila in cooperation with VTT (Technical Research Centre of Finland) in 2015. Measurements were carried out on the top part of a Wartsila in line engine by both the traditional probe and S&P probe. The results are demonstrated in Figure 3. Figure 3: Left, particle velocity level of engine top part at a certain frequency band; Right, A weighted sound power levels comparison between results from traditional and S&P probe. It is found out that this new technology of measuring sound intensity by a probe consisting of one microphone and one velocity sensor is feasible to be used for noise localization and sound intensity measurement on large heavy duty medium speed engines at Wartsila. Engine Mechanical Noise Engine noise is complex in the way that there are multiple excitation mechanisms generate noise and vibration energy that transfers through all engine components giving rise to both structureborne and airborne noise. The typical noise spectrum of a medium speed engine is similar to white noise, i.e. energy distributed in broadband, Figure 4. Figure 4: A weighted sound power level of a certain type of Wartsila diesel engines. 90% of the measured engines have noise level within the grey area. As can be seen from Figure 4, the dominating frequency range is about 500 2k Hz. In this frequency range most of the noise is caused by engine mechanical excitations. From experience, com4 Aachen Acoustics Colloquium 2016 bustion noise is not so important for a large diesel engine because it has relatively low frequency ranges, i.e. below 500Hz, and especially when the engine runs on full load other noise sources will dominate. At higher frequencies above 3k Hz, typically there is turbocharger tonal noise, which can reach over 130 dB at 1 m distance if no silencer is applied. Typically, a diesel engine is noisier than a gas engine. Figure 5 shows an example of a Wartsila dual fuel gen set noise levels on diesel and gas modes. Figure 5: A weighted sound power levels of a certain type of Wartsila dual fuel gen set running on diesel and gas modes. The total noise level of diesel mode is about 4 dB higher than gas mode. The main difference exists in the mechanical noise frequency range, i.e. 500 2k Hz. This is due to the fuel injection process of the diesel mode. When the cam drives the fuel injection pump, high torsional vibration will be generated on the camshaft. This torsional vibration will cause gear teeth impact in the gear train. The energy generated by gear teeth impact will transfer via shaft bearings to engine block and attached structures, thus radiates noise. This phenomenon has been investigated earlier at Wartsila in cooperation with KTH (Royal Institute of Technology) and Lloyd's Register ODS [6] To illustrate the gear teeth impact, the shaft torques were measured and demonstrated in Figure 6. Figure 6: Camshaft and intermediate shaft torques measured on a Wartsila in line engine with 6 cylinders. Red intermediate shaft; Blue camshaft. Aachen Acoustics Colloquium 2016 5 Yellow ovals mark the fuel injection moment for each cylinder. One may also notice that there are sharp negative peaks in the torque measurement results. These peaks marked by blue and red ovals indicate that the gear teeth of the mesh between camshaft and intermediate shaft have traveled through its backlash and are impacting with each other on the non working flank. Before and after gear impacts a loss of contact between the teeth occurs, marked by black ovals. The gear impacts are more