FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
AND
The Patents Rules, 2003
COMPLETE SPECIFICATION
(See section 10 and rulel3)
1. TITLE OF THE INVENTION:
"UNDERHOOD COMPOUND EXHAUST SILENCER"
2. APPLICANT:
(a) NAME: MAHINDRA & MAHINDRA LTD.
(b) NATIONALITY: Indian Company incorporated under the
Companies Act, 1956
(c) ADDRESS: Gateway Building, Apollo Bender, Mumbai - 400001,
Maharashtra, India.
3. PREAMBLE TO THE DESCRIPTION:
The following specification describes the invention and the manner in which it is to be performed.

FIELD OF THE INVENTION:
The present invention relates to an underhood compound exhaust silencer located under the hood or bonnet of a vehicle. The silencer imposes minimal back pressure on the engine exhaust system and attenuates firing frequencies of exhaust gas soundwaves.
BACKGROUND OF THE INVENTION:
In general, silencers are distinguished as reactive (reflective), absorptive (dissipative) and hybrid/ compound (combination of the two types) silencer. A reactive silencer incorporates plurality of expansion and resonator chambers. Reactive silencers are more effective in attenuating lower and medium ranges of frequencies of exhaust sound. An absorptive silencer incorporates plurality of expansion chambers filled with sound absorbing material and perforated tube. It is more effective in attenuating medium and higher ranges of frequencies of exhaust sound. Each of these types of silencers may or may not have a straight through gas flow path; as per design considerations.
Also, exhaust gas sound levels and exhaust gas back pressure on engine are inversely proportional to each other. Hence it is of paramount importance to design an exhaust silencer, which will effectively dampen broader spectrum of sound frequencies with minimum backpressure on engine.
In practice, it is difficult to incorporate all such features in a singular body to be able to attenuate high, medium and low range frequencies of exhaust sounds. Many designs are proposed to attenuate sound frequencies and back pressure on the engine exhaust system. These designs try to obtain an optimal trade-off for the same.
U.S. patent US5033580 describes an "Absorption silencer for internal-combustion engines, especially for off-road vehicles". In order to reduce the noise from the exhaust of an internal-combustion engine, especially for off-road vehicles, it utilizes an absorption silencer comprising: housing made up of a plurality of mountable elements, built from

heat-resisting material and intended to be fitted and fixed in a removable way on the perforated exhaust pipe. Stuffing material of deadening and heat-resisting material is interposed using two annular joints, a front one and a rear one built from partially elastic and highly heat-resisting material such as silicone or viton, and which also ensure the housing seal to the exhaust gases. As such, the absorptive design is unable to effectively attenuate the lower frequencies of exhaust sounds.
WO/2009/087270 proposes an exhaust silencer of a combustion engine, comprising a housing that is to be connected to an exhaust pipe, through which the exhaust gases are arranged to be conducted and which comprises a reactive part and an absorbing part in the flow direction of the exhaust gases. The reactive part is mainly hollow and comprises, at the inlet thereof, a first exhaust pipe for feeding the exhaust gases to the reactive part and, at its outlet, a second exhaust pipe for directing the exhaust gases from the reactive part to the absorbing part, both exhaust pipes being arranged so as to essentially extend inside the reactive part of the housing. The absorbing part comprises an absorbing filling element, which essentially fills the absorbing part of the housing and, at the inlet of the absorbing part is shaped so as to direct the exhaust gases to the space between the filling element and the inner lining of the housing. Although this invention proposes design suitable for multiple ranges of frequencies, it needs customization for suppressing dominant frequencies for a particular engine's exhaust system.
Yet another patent application, having publication number WO/2004/099576, discloses a muffler with enhanced acoustic performance at low and moderate frequencies. It incorporates both, one or more, dissipative silencer elements and one or more reflective elements such as a Helmholtz resonator. It employs dissipative element and resonators for use in such a muffler. As seen, the said application fails to provide relief for higher and specific frequencies of engine exhaust gas firing.
In view of the present state of the art, there is a need of an exhaust silencer having both, reactive and absorptive, capacities incorporated in it and which is well designed to be accommodated in constrained spaces of off-road vehicles, exert minimum back pressure on the vehicle engine and generate minimum self-vibrations. Accordingly, the present

invention effectively attenuates wide ranges of exhaust gas firing frequencies while minimizing the backpressure on vehicle engine.
SUMMARY OF THE INVENTION:
An underhood compound exhaust silencer, having inlet and outlet on the same side, that exerts minimum back pressure on the engine of the vehicle is disclosed herein. The present invention comprises an expansion chamber, two resonator chambers and a dissipation chamber enabled to attenuate a wide band of frequencies of engine exhaust gas sounds.
In an aspect, the present invention attenuates the dominant fundamental tone of the firing frequency, and its first overtone, of engine exhaust and some of the medium and higher range of frequencies of sound waves, by employing the said expansion chamber, two resonator chambers and a dissipative chamber.
In yet another aspect, the present invention generates least gas flow resistance thereby exertingleast exhaust backpressure on engine, which in turn results in reduced engine specific fuel consumption and improvement in overall engine performance.
In a further aspect, the present invention attenuates sound levels associated with exhaust gas arising from engines used with off-road vehicle such as tractor and the likes. Accordingly, the underhood compound exhaust silencer is appropriately located in the constrained underhood spaces of off-road vehicles while reducing self-induced vibrations and hence lasting long.
BRIEF DESCRIPTION OF THE DRAWING:
Fig. 1: illustrates an exposed underhood compound exhaust silencer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT:
The present invention finds its use in constrained underhood/ under bonnet spaces of off-road vehicles. The said exhaust silencer is adapted to make it compound in functionality such that, it balances the exhaust gas flow resistance and exhaust backpressure tradeoff effectively and optimally. Accordingly, the present invention provides least exhaust gas flow resistance thus minimizing the exhaust back pressure on the engine of the vehicle.
In an embodiment of the present invention, as illustrated in FIG.1, the underhood compound exhaust silencer is enclosed between outer shell 3 and end plates 2 and 15. The underhood compound exhaust silencer comprises an expansion chamber 14, first resonator chamber 10, second resonator chamber 7 and a dissipater chamber 5. It further comprises an inlet tube 1, an outlet tube 18 and baffle plates 6, 8 and 11. The expansion chamber 14 is formed between outer shell 3, baffle plate 11 and end plate 15. Inlet pipe 1 passes through end plate 2, baffle plates 6, 8, 11 and opens in expansion chamber 14. Similarly outlet pipe 18 passes through end plate 2, baffle plates 6, 8, 11 and opens in expansion chamber 14. The first resonator chamber 10 formed between baffle plates 8, 11,'outer shell 3and enclosed completely, is connected with the expansion chamber 14 through communicating tube 13. The second resonator chamber 7, formed between baffle plates 6, 8, outer shell 3 and enclosed completely as well, is connected with expansion chamber 14 through communicating tube 12. The said communicating tube 12 opens as opening 9 in the second resonator chamber 7. The dissipative chamber 5, formed between baffle plates 2, 6 and outer shell 3, is filled with a sound absorbing material such as but not limited to glass wool. Inlet tube 1 has perforations 4 on its periphery only within dissipative chamber 5.
As shown in FIG.l, exhaust gas from engine enters through inlet pipe 1, passes through inlet pipe 1 and enters into expansion chamber 14. From expansion chamber 14, exhaust gas enters into outlet pipe 18 and exits from its outlet end. The second resonator chamber 7 and the first resonator chamber 10 are connected to the expansion chamber 14 through the communicating tubes 12 and 13, respectively, to attenuate specific frequencies of the exhaust gas firing sounds according to the engine specifications. The second resonator

chamber 7 is designed to attenuate fundamental engine firing frequency or the fundamental tone of the engine gas firing sound, ranging between 64 - 68 Hz, preferably at 66 Hz. The first resonator chamber 10 is designed to attenuate the first overtone or the first harmonic of fundamental engine firing frequency, ranging between 131 - 135 Hz, preferably at 133 Hz. The dissipative chamber 5 is designed to attenuate the medium to high range of engine exhaust gas firing sound frequencies, which range from 500 to 1500 Hz.
Alternately, the underhood compound exhaust silencer may be further adapted to suitably improve its performance according to specific engine specifications of different makes. End plate 15 can be curved is curved at two ends (instead of flat as shown in FIG.l) to smoothen the gas flow path, thus further reduce, exhaust gas back pressure on engine. Perforations, in any number, can be added on outlet pipe 18 (like those on inlet pipe 1) on its periphery in zig-zag fashion, or any other suited manner, only within dissipative chamber 5.
Hot rolled steel sheets & tubes are used as raw material for the underhood compound exhaust silencer shown in FIG. land is manufactured by conventional fabrication processes viz. metal cutting, forming, welding, etc.
A person skilled in the art may appreciate that the invention, as described here in above, may be suitably modified to attenuate fundamental firing frequency and its second harmonic with minor variations in the overall design and its drawings and description should in no way be considered limiting.

We claim,
1. An underhood compound exhaust silencer, located under the hood of a vehicle, comprising
an expansion chamber (14),
a first resonator chamber (10) enabled to attenuate the first overtone of the fundamental engine firing frequency,
a second resonator chamber (7) enabled to attenuate the fundamental tone of the fundamental firing frequency,
and
a dissipative chamber enabled to attenuate additional high to medium ranges of fundamental firing frequency, wherein,
the said exhaust silencer exerts minimal back pressure on the engine of the said vehicle.
2. An underhood compound exhaust silencer as claimed in claim 1, wherein said compound exhaust silencer is enclosed between outer shell (3) and end plates (2 and 15) and is further divided by intermediate baffle plates (6, 8 and 11) into the said dissipative chamber (5), the second resonator chamber (7), the first resonator chamber (10) and the expansion chamber (14) respectively therein.
3. An underhood compound exhaust silencer as Claimed in claim 1, wherein said compound exhaust silencer comprises an inlet tube (1) as a means for intake of exhaust gases and an outlet tube (18) as a means for exerting out gases.

4. An underhood compound exhaust silencer as claimed in claims2 and 3,wherein exhaust gases enter said compound exhaust silencer through inlet tube (1), pass through intermediate baffle plates (6, 8 and 11), open into the said expansion chamber (14) and subsequently exit through the said outlet tube (18).
5. An underhood compound exhaust silencer as claimed in claim 1, wherein the said first resonator chamber (10), enclosed between baffle plates (11 and 8) and outer shell (3), is connected to the said expansion chamber (14), through communication tube (13), in order, to attenuate the first overtone of the fundamental engine firing frequency.
6. An underhood compound exhaust silencer as claimed in claim 1, wherein the said second resonator chamber (7), enclosed between baffle plates (8 and 6) and outer shell (3), is connected to the said expansion chamber (14), via communication tube (12) through opening (9), in order to attenuate the fundamental engine firing frequency.
7. An underhood compound exhaust silencer as claimed in claim 1, wherein the said dissipative chamber, filled with a noise absorbing material such as, but not limited to, glass wool, has perforations on the periphery of at least one of the tubes from the inlet tube (I) or outlet tube (18) to attenuate medium to high range of engine firing frequencies.
8. An underhood compound exhaust silencer as claimed in claim 1, wherein the gas flow path in the said exhaust silencer is non-obstructed to exert minimal back pressure of the engine of the said vehicle.
9. An underhood compound exhaust silencer as claimed in any of the claims herein above, wherein the end plate (15) is curved at two ends to smoothen exhaust gas (low and hence to reduce back pressure on engine.

10, An underhood compound exhaust silencer as claimed in claim 1, wherein the said inlet tube (1) and the said outlet tube (18) are placed on the same side of the said compound exhaust silencer.