A Silencer, a Marine Vessel and a Method of Silencing a

Marine Exhaust

The present invention relates to a silencer for the exhaust of an in-board marine engine, an in-board marine engine, a method of silencing an in-board marine engine and to a boat or marine vessel including such a silencer. The invention is applicable to silencers for use in large marine vessels .

As with almost any engine, significant exhaust noise can be generated in marine engines within a marine vessel by the action of valves on releasing gas etc. and other components of the engine during normal operation. The generated noise passes to the exhaust of the engine and in the absence of any silencer passes out of the marine vessel into the surrounding atmosphere or environment .

In boats there is a desire to reduce the noise generated by the engine. This can be for reasons of passenger comfort and convenience or to comply with statutory and other requirements to limit the noise generated by boats. For marine leisure craft for example, and in particular luxury power boats and yachts lower noise limits have recently been introduced by Directive 2003/44/EC of the European Parliament and the Council of 16 June 2003.

In the case of noise generated by a marine engine used for power generation, this can be particularly important as the generating engine will typically run through the night when passengers may be sleeping or trying to sleep on the boat .

It is known to use silencers in marine exhausts to reduce marine engine exhaust noise. Examples of teachings of this are provided in US-A-4, 607, 723 and United States patent application number US-A-2005/0020152.

According to a first aspect of the present invention, there is provided a silencer for connection to an exhaust outlet of an in-board marine engine, the silencer comprising: a housing defining a Helmholtz resonator chamber and an expansion chamber; and, a baffle arranged within the housing between the expansion chamber and the resonator chamber, the baffle having a first opening for receiving an inlet or an outlet duct and a second opening defining an opening to the resonator chamber the silencer being arranged so that, in use, at least one of an inlet duct and an outlet duct passes through the resonator chamber, wherein the resonator chamber is arranged in use to be substantially free of water.

Embodiments of the present invention provide a simple and robust device suitable for silencing noise from the inboard engine of a large marine vessel. In known systems, such as that disclosed in US-A-2005/0020152, the resonator chamber receives water which varies the tuning of the resonator chamber. By providing a resonator chamber in which no water is present in use, the tuning of the resonator chamber remains substantially fixed during all operating conditions .

Preferably, an inlet duct and an outlet duct are provided on opposite sides of the housing of the silencer.

This provides a particularly advantageous configuration as it means that exhaust gas and water flowing through the silencer can pass easily through the silencer without encountering significant flow resistance.

Preferably, the baffle is selectively moveable, e.g. during production, to vary the tuning of the resonator chamber. This ensures that on fitting, the resonator is tuned to suit the expected frequencies of the engine of the vessel in which the device is fitted. Thus, a single workshop blank can be manufactured whilst enabling use of the device in vessels of different sizes and with different engines .

According to a second aspect of the present invention, there is provided a silencer for connection to an exhaust outlet of an in-board engine of a marine vessel, the silencer comprising: an expansion chamber; and, a resonator chamber, wherein, in use, the expansion chamber is arranged to receive water together with exhaust gas and the resonator chamber is arranged to be substantially free of water.

According to a third aspect of the present invention, there is provided a marine vessel, comprising: an engine for providing motive force or other power generation to the vessel; and a silencer according to the first or second aspect of the present invention connected to an exhaust output of the engine.

Examples of embodiments of the present invention will now be described with reference to the accompanying drawings, in which:

Figure 1 is a plan view of a schematic representation of a silencer for a marine exhaust;

Figure 2 is a longitudinal section through the silencer of Figure 1 ;

Figure 3 is plan view of a baffle for use in the silencer of Figures 1 and 2;

Figure 4 is a schematic view of a boat including a silencer according to Figures 1 and 2; and,

Figure 5 is a schematic representation of a longitudinal section through a silencer.

Figures 1 and 2, show schematically the construction of a silencer 1 according a particular example of an embodiment of the present invention. The silencer 1 includes a main body 6 in which are defined a resonator chamber 8 and an expansion chamber 10, separated by a baffle 12. An engine 22 is shown schematically together with the exhaust tubing 24 leaving the engine 22. A water inlet device 23 is provided in line with the exhaust tubing 24 to mix water with the exhaust gas. This has the effect of cooling the exhaust gas as it passes along the exhaust tubing 24. Although water is the most convenient, any coolant liquid may be used. It therefore is to be understood that the term "water" where used herein encompasses other coolant liquids that may be used.

The main body 6 of the silencer 1 has two walls: a first end wall 14 and a second end wall 16, the baffle 12 being arranged at some point between the end walls 14 and 16. An inlet 4 is provided in the first end wall 14 and an outlet 5 is provided in the second end wall 16. Typically the inlet and outlet are suitably sized holes in the first and second walls 14 and 16, respectively. When fitted in a boat, tubing 2 or an adapter connects the exhaust of the boat engine into the main body 6. The tubing or adapter may be provided as part of the silencer for fitting the silencer in-line with the exhaust tubing from the engine. The adapter may be an integral part of the silencer or an additional separate component. Thus, flexibility may be provided in terms of the size of the tubing to which a standard sized silencer body 6 can be fitted.

An adapter or tail pipe 20 provides a passage for exhaust gas and water to be output from the main body 6 of the silencer. The baffle has two openings - one 11 for receiving the end of the tubing or adapter 2, and another 13 defining an opening to the resonator 8. As with the tubing 2, the tailpipe or adapter may be an integral part of the silencer or may be a separate component .

The resonator 8 is a reactive tuned element that relies on the Helmholtz resonator principle, whereby a mass of air in the neck, defined within opening 13 leading to the chamber 8 of the resonator, is effectively "suspended" on an "air spring" created by the chamber 8, thus providing a one- degree-of-freedom dynamic element. The dynamic element has an inherent resonant frequency, and if excited acoustically, e.g. by an acoustic sound wave, it will return energy 180°

out of phase relative to the incoming acoustic propagation such that sound cancellation occurs. Thus, it has the effect of "silencing" the original acoustic sound wave, which is of course the desired effect. When used herein the term "silencing" (or derivatives thereof) means causing a reduction of the original outlet noise.

The operation of the silencer will now be described in detail. Referring again to Figures 1 and 2, the tubing 2 connects the silencer to the exhaust output of a marine engine. The tubing 2 passes through an opening 4 within the end wall 14 of the silencer.

In the particular example shown, the tubing 2 passes through the opening in the first wall 14, all the way through the resonator chamber 8 and then through the opening 11 in the baffle 12. The tubing 2 terminates at some axial distance downstream from the baffle 12 as shown in Figures 1 and 2. In the examples shown there is a separation between the plane of the baffle 12 and the end of the tubing 2. This ensures that the flow from the inlet or outlet duct does not impinge on the resonator neck opening.

The tail pipe 20 connects to an opening within the second end wall 16 to provide means for discharging the water and exhaust gas to atmosphere or the external environment .

In operation, initially exhaust gas is generated in engine 22 and coupled along exhaust tubing 24, possibly through a turbocharger (not shown) . Typically, the gas will be at about 450 ⁰ C as it leaves the turbocharger. Water is

added via a spray ring 23 which cools the gas immediately to about 30°-60°C, e.g. around 50 ⁰ C. The mixture of water and exhaust gas then passes into the tubing or adapter 2 as shown in Figures 1 and 2.

As well as the physical flow of water and exhaust gas an acoustic wave propagates along the tubing 24 from the engine. As explained above, the acoustic sound wave is generated primarily by the action of the exhaust valves on the gas leaving the cylinders within the engine.

Ordinarily, i.e. in the absence of any silencer, the acoustic wave would propagate all the way along the tubing 24 to the tail pipe and out to atmosphere or the external environment .

The silencer operates as follows. The cooling water, exhaust gas and acoustic wave pass along the tubing 24, through the inlet duct or adapter 2 which extends throughout the length of the resonator chamber 8 and into the expansion chamber 10 where the gas expands. The acoustic wave excites the small disk of air within the opening 13 in the baffle, forcing it to oscillate according to the conventional principles of a Helmholtz resonator. Thus, the resonator 8 functions as a reactive silencer. It is the action of the acoustic wave acting on the opening 13 that causes the resonance to occur. The resonator generates an acoustic wave 180° out of phase with the exciting wave and therefore cancels it out and significantly reduces the noise that eventually leaves the tail pipe 20.

The baffle 12 is fixed with respect to the housing 6 such that the relative sizes of the resonator chamber and

the expansion chamber are defined. The size of the opening 13 is also fixed such that overall the acoustic properties of the silencer are defined. On manufacture the selection of the axial location of the baffle is fixed so as to provide a resonant frequency of the resonator at some desired level, i.e. in dependence on the size of the boat or engine speed and configuration for which it is destined. In a preferred embodiment the baffle may be easily moveable during manufacture or afterwards so as to vary the tuning of the silencer. This is particularly advantageous as it means that a single sized silencer can be optimally tuned for operation at different resonant frequencies.

Figure 4 shows a schematic representation of a marine vessel including a silencer like that of Figures 1 and 2. The vessel includes an in-board engine 25 having an output shaft 27 for driving a propeller. Other power train components or gears etc may be provided but are not shown for reasons of clarity. In the example shown, the engine is a propulsion engine. In an alternative embodiment, the engine is a generator for generating electricity or to satisfy other power requirements of the boat. Such a power- generating engine may well be used at night when passengers are sleeping on the boat. Therefore, it is important in these situations to silence the noise of the engine. The vessel has fitted a silencer 1 of similar construction to that shown in Figures 1 and 2. The vessel may be any sort of marine vessel and is preferably a large marine vessel, i.e. larger than 5 metres in length. Typically for such a vessel the frequency at which the silencer has the greatest effect is within the range of 80Hz to 160Hz. In any event, the parameters of the silencer may be varied during design

and/or manufacture to provide resonance (and therefore optimum silencing capability) at a desired frequency.

For marine vessels such as this the required size of the silencer is typically at least 0.5 metres long. In other words, along its longest external dimension, the silencer is at least 0.5 metres long.

Figure 5 shows a second embodiment of a silencer. As with the example shown in Figures 1 and 2, in this example a resonator chamber 8 is provided as part of the main body 6 of the silencer. A baffle 26 is provided that defines, with the housing 6, the resonator chamber 8 and the expansion chamber 10. An inlet duct 2 is provided for connection to the exhaust outlet of a marine engine. An outlet duct 20 is also provided for connection to the exhaust tailpipe of a boat in which the silencer is arranged.

An opening 28 is provided in the baffle 26 for forming a resonator in the chamber 8. A second opening 30 is also provided in the baffle for allowing the exhaust duct to pass through the resonator chamber and from the resonator chamber into the expansion chamber 10. In this configuration an amount of water will be present in the expansion chamber and the inlet 32 to the outlet duct will be positioned, in use below the water level 34.

In this example the outlet duct passes through the resonator chamber 10. Again, as with the example described above with reference to Figures 1 and 2, such an arrangement allows for a compact arrangement of the silencer. In use, as water and exhaust gas enter the silencer from the inlet

duct 2 the pressure inside the expansion chamber will increase, causing both resonance to occur at the opening of the resonator chamber and also causing an exhaust of liquid and gas through the outlet duct 20.

Embodiments of the present invention have been described with particular reference to the examples illustrated. However, it will be appreciated that variations and modifications may be made to the examples described within the scope of the present invention.