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Reduce the pressure, go for particle velocity

Noise reduction of a large medium speed diesel engine

Engine noise has becoming more and more important over years. Latest regulations for maritime industry demand a maximum acceptable sound pressure level of 110 dB at any individual measurement position in the machinery space. Wärtsilä has a decade of experience on large medium speed diesel engine noise and vibration research and development. Today’s Wärtsilä 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 are addressed. This work also explains the methodology of noise reduction on different engine noise sources.

Extracted from: Saine, K., and Gao, Z., 2016. Five dB noise reduction for large medium speed diesel engines. In Proceedings of Aachen Acoustics Colloquium.

REQUIREMENTS

  • Study the vibro-acoustic behavior of the structures that radiating noise
  • Characterize the engine noise behaviour thoroughly before deciding the most effective way to reduce the engine noise
  • Perform experimental investigation in a large room with high levels of background noise

GOAL

Reduce overall noise radiation of a large diesel engine in an industrial environment

Measurement methodology

The S&P probe is supposed to be used at near field for sound source localization, and at further distance for sound intensity identification. The device was assessed by Wärtsilä in cooperation with VTT (Technical Research Centre of Finland) in 2015. Measurements were carried out on the top part of a Wärtsilä in-line engine by both the traditional sound intensity probe and a p-u probe using Scan&Paint 2D. It was 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 Wärtsilä.

Sound visualization of a large diesel engine

Sound visualization, in particular particle velocity mapping, was used to identify the areas across the engine with a high excitation load. Different noise control measures were applied and evaluated on the main problematic locations following the data extracted from sounds maps.

Engine Structure-borne Noise

The structure-borne noise is considered to be more important than air-borne noise for Wärtsilä diesel engines. One of the components that are effective in noise radiation is the engine side covers for crankcase and cam case. Wärtislä previously used cast aluminium as the side covers. Lately the covers have been updated to thin steel plate achieving a significant noise reduction.The density of steel is about three times as high as the one of aluminium, and the thin steel plate has much lower stiffness than the cast aluminium. These facts have resulted in the critical frequency of the steel plate is much higher than the cast aluminium, thus the radiation efficiency of the steel plate is much lower than the cast aluminium. Furthermore, the new cover frame is still made by cast aluminium and only the plate is made by steel. This has increased the structural impedance mismatch so that some of the vibration energy from the engine block acting on the cover frame dissipates at the contact surface between the plate and the frame.

OUTCOME

To reduce large medium speed diesel engine noise, one should take mechanical noise into serious consideration before focusing on the combustion noise. All mechanical impacts and friction can give rise to mechanical noise, e.g. piston slap, gear train and valve train impacts etc. In case of broad band noise when no obvious resonant peak can be found in the noise spectrum, one may want to study the vibro-acoustic behavior of the structures that radiating the noise. Typically by utilizing designs with low radiation efficiency, which requires high density and low stiffness, and increasing structural impedance mismatch between material layers can improve the noise radiation to a much lower level.

With ever demanding and challenging market, in the past decade Wärtsilä medium speed diesel engines’ output has increased by 20%, while the noise level has dropped significantly without adding any insulation panel that may complicate the maintenance work. Since engine room acoustics plays also an important role, the shipyards are expected to have a leading role in and responsibility for the engine room acoustical design.

Reduce the pressure, go for particle velocity

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