Sound Absorption of Diesel Engine

Sound Absorption of Diesel Engine

Measurement methods

ISO 3744-1994, DIN 45635 part 11

Volvo Penta engine sound power levels are measured in a semi echo-free chamber. The ceiling and walls are covered with sound absorbing wedges. The engine is placed on a hard, reflection floor. By using the chamber, sound propagation equivalent to open field conditions is ensured.

The measurements are taken according to the international standard ISO 3744-1994 for engines with a cylinder volume of 9 liter (2.4 US gals) or more. Measurements of 420-722 engines are taken according to DIN 45635 part 11 or ISO 6798. The sound pressure level is determined by using several microphones arranged in a geometric pattern around the engine. Sound power level is then calculated on the basis of the average sound pressure level. Engine noise is measured during different engine speeds and load conditions. Noise from exhaust, inlet and fan are not included in the noise measurements. This means that the indicated sound power only comprises the noise emitted by the engine structure. Sound data for the entire industrial engine range can be found in the Sales Support Tool, Partner Network.

Diesel Engines Noise

Characteristics

A Volvo diesel engine is a complicated noise source. Dynamic forces cause vibrations and therefore noise emissions. Apart from the actual engine structure, the exhaust and inlet systems along with auxiliary devices contribute to total engine noise level.

Engine noise

Engine noise is the noise emitted from the engine structure. The pie chart below shows how large a relative part of total engine noise emissions given engine components have. Engine noise is created by forces that arise during the combustion process and the mechanical shocks between pistons and liners, valves and gear wheels etc. When a new engine is designed, a great deal of effort is put into minimizing engine noise. The most important means for achieving this is optimizing the injection system, controlling engine block stiffness and special treatment of the gear wheels in the engine timing system. Sound dampening materials can be placed on large noise emitters such as the oil sump and valve cover. Engine noise in an installation is usually reduced by screening off the engine. Such screening can be made even more effective by lining it with absorbent material. Usually, the level of screening is governed by engine cooling capacity. Finally, engine noise is to a large extent dependent on engine speed. For a given power an engine with low rpm is preferable to one with high rpm.

Exhaust Noise

Without a muffler, exhaust noise is the absolute largest source of noise on a diesel engine. High pressure pulses from combustion causes the exhaust flow to oscillate. On a six-cylinder four-cycle

engine, exhaust noise basic frequency is equivalent to three times engine rpm. This means that the muffler in most applications must have a high silencing capability across a wide frequency range.  The two most usual diesel engine muffler types used are:

- Absorbing mufflers

Absorbing mufflers have a chamber that is covered with sound absorbing material. Acoustic energy is transformed into heat in the absorbent material. This type of muffler has its best dampening ability in the medium to high frequency range.

- Expansion mufflers

The inside of the muffler is constructed so that sound is reflected, which reduces the acoustic force transmitted through the muffler. By allowing the pipe to discharge into a chamber and then leading exhaust gasses through several chambers, good sound dampening is achieved across a broad frequency range.

The price paid for good sound dampening in both muffler types is a higher back pressure in the exhaust system. The development of mufflers is often a compromise between acceptable back pressure and necessary noise reduction. Volvo Penta provides both muffler types. Their use ensures large noise reduction and low back pressure

Inlet Noise

As is the case with the exhaust system, pressure variations in the inlet system causes the airflow to oscillate. These pressure variations are of course much lower than those in the exhaust system. On modern turbocharged engines the compressor functions as an effective sound muffler. The pressure waves (noise) are dispersed by the rotating compressor blades. This means that inlet noise does not have any practical significance on Volvo Penta engines.

Auxiliary Noise

The cooling fan is one of an engine's largest noise sources. According to measurements the sound power level increases by 2 to 3 dBA when a fixed fan is used. Because fan noise is relative to engine rpm, it is necessary for the radiator to have a large capacity. An efficient radiator allows fan speed, and thus also noise, to be reduced. On applications where maximum cooling capacity is not a constant requirement, thermostatically or viscously coupled fans may be used. Viscously coupled fans are available as accessories for most Volvo Penta engines for mobile applications. Other equipment such as generators and air compressors have less significance compared to the fan when it comes to noise.

Structure Borne Noise

In many applications engine vibrations that affect the engine bed can cause sound emissions from connected structures. In many installations this can be a bigger problem than noise from the engine itself. By using resilient engine mounts the forces influencing the engine bed are reduced and therefore also frame noise. In the case of insulation for heavy vibrations, the engine bed must be at least 10 times as stiff as the insulator.