FRONT MUFFLER Technical Field [1] The present invention relates to front mufflers and, more particularly, to a front muffler, which is placed between an exhaust manifold and a catalytic converter of an internal combustion engine and forms an air gap in an internal pipe and a Y-plate to retain the warmth of exhaust gases discharged from the exhaust manifold, thus reducing the time required to activate the catalytic converter, and which has a partition wall in the Y-plate, thus attenuating the operational noise in the engine and increasing engine output. Background Art [2] A muffler device, which is an exhaust device for internal combustion engines, is typically installed in the lower part of a vehicle body and changes harmful components, such as HC, CO, NO , etc., of high temperature and high pressure exhaust gases, which are discharged from an exhaust manifold during the operation of an internal combustion engine of a vehicle, to harmless gases, such as CO , H O, etc. through chemical reactions using a catalyst prior to discharging the exhaust gases to the atmosphere. [3] Exhaust gases from vehicles, which have been recognized as big sources of air pollution, have been severely regulated in recent years, so that vehicles must be equipped with exhaust devices, and the exhaust devices have been actively studied. [4] A conventional muffler device typically comprises a front muffler, a catalytic converter, a center muffler and a main muffler. Exhaust gases from an internal combustion engine are discharged to the front muffler through an exhaust manifold, and, thereafter, sequentially pass through the catalytic converter, the center muffler and the main muffler prior to being discharged to the atmosphere. [5] The exhaust manifold is an exhaust element placed such that, when mixed gases explode in the cylinders of an internal combustion engine and are discharged as exhaust gases from the cylinders, the exhaust gases first meet the exhaust manifold. The exhaust manifold also functions as a collector to collect exhaust gases from the cylinders in one place. [6] To collect exhaust gases from the cylinders of an internal combustion engine in one place using an exhaust manifold, the following design factor must be considered. Described in detail, in a four-cylinder engine which executes suction, compression, explosion, and exhaust strokes, the four cylinders sequentially execute explosion strokes in the order of first, third, fourth and second cylinders, so that the third cylinder discharges exhaust gas after the exhaust stroke of the first cylinder and the second cylinder discharges exhaust gas after the exhaust stroke of the fourth cylinder. During the exhaust stroke of each cylinder, the exhaust valve is opened before the piston has reached bottom dead center. [7] Thus, the exhaust stroke of the third cylinder starts before the exhaust stroke of the first cylinder has completely finished, so that, if the exhaust system is designed such that the exhaust gases from the first and third cylinders are directly combined together, the exhaust gas from the third cylinder may flow back into the first cylinder during the moment that the exhaust valve is temporarily open during the suction stroke of the first cylinder. In other words, although such backflow of the exhaust gases into a cylinder does not occur, if exhaust gases are discharged from two joined cylinders at the same time, the exhaust pipe becomes relatively narrower, such a phenomenon being called "exhaust gas interference" in the related art. [8] Thus, exhaust gases discharged from the cylinders must be sequentially guided to the exhaust pipe. Furthermore, the exhaust manifold has been designed such that the exhaust gases from the first and fourth cylinders are collected separately from the exhaust gases from the third and second cylinders in respective channels and, thereafter, the first collected exhaust gases are secondarily combined together at the rear part of the front muffler. Such a two-stage exhaust gas combining manner has been called a "4-2-1 manner" in the related art. In addition, a "4-1 manner" has been used, in which the exhaust gases from the four cylinders are directly combined together in one stage. The "4-2-1 manner" has been preferably used for vehicles to be driven at high speeds, while the "4-1 manner" has been preferably used for vehicles to be driven under general conditions. [9] To discharge exhaust gases to the atmosphere without causing exhaust gas in¬ terference, ideally one exhaust pipe extends from each of the exhaust ports of respective cylinders to the outside of the engine. However, the provision of the exhaust pipes from respective exhaust ports is undesirably accompanied by an increase in man¬ ufacturing costs, an increase in weight, and a reduction in practicability, thus being less likely to be adapted to practical use. Although the exhaust pipes are provided to extend from respective exhaust ports, if there is an interval between the time when the exhaust gases from the manifolds of different cylinders completely reach a predetermined position, the exhaust gases flowing from a long manifold undesirably meet the exhaust gases flowing from a short manifold, thus causing exhaust gas interference. [10] The most important fact in the design of exhaust systems is that the manifolds must be configured to have similar lengths capable of reducing exhaust gas interference. An exhaust system, which causes exhaust gas interference, undesirably consumes engine power in forcibly discharging exhaust gases to the atmosphere, thus reducing engine output. [11] Furthermore, the exhaust gases just discharged from a cylinder through an exhaust valve have a temperature of about 1000°C and are, thereafter, gradually cooled while the gases pass through the exhaust system. The exhaust gases heat the exhaust ports (the ports around cylinder heads at which the cylinder heads meet the exhaust manifolds) which have been cooled by coolant and engine oil, so that the temperature of the exhaust gases is reduced. Furthermore, while exhaust gases are discharged from a front muffler, the exhaust gases lose heat due to the low temperature front muffler which is cooled by atmospheric air. Thus, the temperature of the exhaust gases is reduced. When the temperature of the exhaust gases is reduced, the volume of the gases is reduced. If the exhaust gases having reduced temperature maintain their original volume, the weight of the exhaust gases must increase. The exhaust gases having increased weight are difficult to circulate or move. [12] When the exhaust gases are cooled, the time required to activate a catalyst to clean the exhaust gases is increased, so that an excessive amount of smoke may be generated at the initial stage of engine startup. [13] Furthermore, when exhaust gases from two exhaust channels are combined together, exhaust gas interference is caused, thus reducing engine output and generating operational noise in the engine. Disclosure of Invention Technical Problem [14] Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a front muffler, which forms therein an air gap to retain the warmth of exhaust gases discharged from an exhaust manifold, thus reducing the time required to activate a catalytic converter, and which is provided therein with a partition wall to attenuate the operational noise of an engine and to increase engine output. Technical Solution [15] In order to accomplish the above-mentioned object, the present invention provides a front muffler to discharge exhaust gases, the front muffler comprising a plurality of internal pipes which communicate with an exhaust manifold of an internal combustion engine; a communication member, through which the internal pipes communicate with each other; and an external plate which forms an air gap around outer surfaces of both the internal pipes and the communication member, wherein the communication member comprises a Y-plate with a partition wall provided in the Y-plate, the Y-plate comprises a first Y-plate and a second Y-plate, which are placed in an upper position and a lower position and assembled with each other, the first and second Y-plates being mounted at front ends thereof to the internal pipes and being combined together at rear parts thereof; and the partition wall comprises a first partition wall which extends along an internal surface of the first Y-plate and a second partition wall which extends along an internal surface of the second Y-plate. [16] Preferably, the first partition wall and the second partition wall are spaced apart from each other by a predetermined distance. [17] Preferably, a partition hole is formed in the rear part of each of the partition walls. Advantageous Effects [18] The present invention is advantageous in that the front muffler forms therein an air gap to retain the warmth of exhaust gases discharged from an exhaust manifold, thus reducing the time required to activate a catalytic converter, and is provided therein with partition walls to attenuate the operational noise of an engine and to increase engine output. [19] Furthermore, the present invention is advantageous in that upper and lower partition walls of the front muffler are spaced apart from each other by a predetermined distance, thus increasing engine torque and engine output while a vehicle runs at a low or middle speed. [20] Furthermore, a partition hole is formed in the rear part of each of the partition walls, thus inducing gradual combining of exhaust gases and preventing thermal degradation of the partition walls caused by rapid combining of the exhaust gases. Brief Description of the Drawings [21] FIG. 1 is a perspective view schematically illustrating a front muffler according to an embodiment of the present invention; [22] FIG. 2 is a partially broken view schematically illustrating the front muffler according to the embodiment of the present invention; [23] FIG. 3 is a view schematically illustrating a Y-plate of the front muffler according to the embodiment of the present invention; [24] FIG. 4 is a plan sectional view schematically illustrating the Y-plate of the front muffler according to the embodiment of the present invention; [25] FIG. 5 is a sectional view taken along the line A-A of FIG. 4; [26] FIG. 6 is a sectional view taken along the line B-B of FIG. 4; [27] FIG. 7 is a graph showing exhaust noise as a function of engine speed for the present front muffler and a conventional front muffler; and [28] FIG. 8 is a graph showing engine torque as a function of engine speed for the present front muffler and a conventional front muffler. Best Mode for Carrying Out the Invention [29] Herein below, a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings. [30] FlG. 1 is a perspective view schematically illustrating a front muffler according to an embodiment of the present invention. FlG. 2 is a partially broken view schematically illustrating the front muffler according to the embodiment of the present invention. FlG. 3 is a view schematically illustrating a Y-plate of the front muffler according to the embodiment of the present invention. FlG. 4 is a plan sectional view schematically illustrating the Y-plate of the front muffler according to the embodiment of the present invention. FlG. 5 is a sectional view taken along the line A-A of FlG. 4. FlG. 6 is a sectional view taken along the line B-B of FlG. 4. FlG. 7 is a graph showing exhaust noise as a function of engine speed for the present front muffler and a conventional front muffler. FlG. 8 is a graph showing engine torque as a function of engine speed for the present front muffler and a conventional front muffler. [31] As shown in FIGS. 1 and 2, the front muffler according to the embodiment of the present invention is placed in the front part of a muffler device which is an exhaust device for internal combustion engines. The front muffler of the present invention is placed between an exhaust manifold (not shown) to discharge exhaust gases from an engine to the atmosphere and a catalytic converter (not shown) to clean the exhaust gases, with an air gap provided in the front muffler. The front muffler comprises an internal pipe 22, a Y-plate 23 functioning as a communication member, an external plate 20, a bellows 30 and a rear pipe 40. [32] The internal pipe 22 comprises two pipes which define an exhaust outlet. The front ends of the two internal pipes 22 communicate with the exhaust manifold through a front flange 10 and define the exhaust outlet, through which exhaust gases from the engine are discharged to the atmosphere. [33] The Y-plate 23 is configured such that the front part of the plate 23 is divided into two channels by a partition wall and the two channels are combined together at the rear part of the plate 23. In the two channels defined in the front part of the plate 23, the two internal pipes 22 are fitted and welded to the plate 23 and are joined together at the rear part of the plate 23. The Y-plate 23 comprises a first Y-plate 23a, which is an upper plate welded to a first partition wall 24a, and a second Y-plate 23b, which is a lower plate welded to a second partition wall 24b. The first and second Y-plates 23a and 23b are placed in upper and lower positions and are assembled with each other through a fitting process, prior to being welded together. [34] The external plate 20 comprises upper and lower parts, which are welded to the outer surfaces of both the internal pipe 22 and the Y-plate 23 using a steel wire mat 21 such that an air gap is formed around the outer surfaces of both the internal pipe 22 and the Y-plate 23. The steel wire mat 21 has a mesh structure, which is a net structure made by densely weaving metal wires together, thus providing a reliable air gap. In the preferred embodiment of the present invention, the steel wire mat 21 is provided around each of the front part of the internal pipe 22 and the junction between the internal pipe 22 and the Y-plate 23. However, it should be understood that many steel wire mats may be placed in many positions to define the air gap without affecting the functioning of the present invention. [35] The bellows 30 comprises a cylindrical bellows pipe, with a steel wire braided to the external surface of the bellows pipe and a cap mounted to each end of the bellows pipe. The bellows 30 is mounted to the rear end of the Y-plate 23 through a welding process. The bellows 30 can attenuate the exhaust vibration and the operational noise generated from the exhaust system of the engine, and can attenuate vibration caused by an uneven road surface while the vehicle is driven on a road, thus allowing passengers to feel comfortable and increasing the effective lifespan of the exhaust system. Thus, the bellows 30 can reduce vibration and impact that may be applied to the muffler device. [36] The rear pipe 40 is fitted into the rear end of the bellows 30 and is welded thereto, and discharges exhaust gases rearwards. A support hanger 41 to hang and support the front muffler in a vehicle body is provided on a sidewall of the rear pipe 40, while a rear flange 50 is welded to the rear end of the rear pipe 40. Two studs 51 to assemble the rear flange 50 with the catalytic converter (not shown) are provided in the rear flange 50 at diametrically opposite positions. [37] As shown in HGS. 3 and 4, the Y-plate 23 comprises a first Y-plate 23a, to the internal surface of which a first partition wall 24a is welded to define an upper partition wall extending downwards. The Y-plate 23 further includes a second Y-plate 2b, to the internal surface of which a second partition wall 24b is welded to define a lower partition wall extending upwards. Thus, the exhaust gases from the two internal pipes 22 can be slowly combined together by the two partition walls 24a and 24b without being quickly combined together in the Y-plate 23 or causing exhaust gas in¬ terference, so that a reduction in output power of the engine due to exhaust gas in¬ terference can be prevented. [38] To cause the exhaust gases to be slowly combined together and thereby prevent the partition walls 24a and 24b from being thermally damaged due to rapid combining of the exhaust gases, a plurality of first partition holes 25a and a plurality of second partition holes 25b each having a predetermined size are preferably formed in the rear parts of respective partition walls 24a and 24b . [39] Furthermore, to cause the exhaust gases to be slowly combined together along the first and second partition walls 24a and 24b, the two partition walls 24a and 24b are preferably spaced apart from each other at a predetermined constant interval. If the interval between the two partition walls 24a and 24b is narrower than 1 mm, the operation effect of increasing the engine torque and the engine output during low/ middle speed driving of the vehicle may be reduced. If the interval between the two partition walls 24a and 24b is wider than 3 mm, it is almost impossible to accomplish the operation effect of increasing the engine torque and the engine output during the low/middle speed driving of the vehicle. Thus, the interval between the two partition walls 24a and 24b is preferably set to 1 ~ 3 mm. [40] The method of manufacturing the external plate 20 and the Y-plate 23 will be described herein below with reference to FIGS. 5 and 6 which are sectional views taken along the respective lines A-A and B-B of FlG. 4. [41] To produce the Y-plate 22 according to the present embodiment, a longitudinal blade-shaped partition wall 24a, having a plurality of first partition holes 25a at the rear part thereof, is welded along the central line of the internal surface of the first Y- plate 23a. In the same manner, a longitudinal blade-shaped second partition wall 24b, having a plurality of second partition holes 25b at the rear part thereof, is welded along the central line of the internal surface of the second Y-plate 23b. [42] Furthermore, the first partition wall 24a is bent in a predetermined direction along an edge thereof so that the first partition wall 24a is provided with a contact surface which contacts the first Y-plate 23a when the first partition wall 24a is welded to the first Y-plate 23a. The second partition wall 24b has a shape equal to the above- mentioned shape of the first partition wall 24a. [43] The first and second Y-plates 23a and 23b, each having a partition wall, are arranged to vertically face each other and are welded together to form a single Y-plate 23. Thereafter, two internal pipes 22 are fitted into the front end of the Y-plate 23 and are integrated with the Y-plate 23 through a welding process. Thus, exhaust gases, flowing from the two internal pipes 22, pass through the Y-plate 23 and are slowly combined without causing exhaust gas interference. [44] In the present invention, the welding process is preferably executed through any of a variety of welding methods, for example, a MIG welding method using welding wires. [45] To form an air gap around the internal pipes 22 and the Y-plate 23, which were welded together, the steel wire mat 21 functioning as a reinforcing material is welded to the internal surfaces of the upper and lower parts of the external plate 20. Thereafter, the upper and lower parts of the external plate 20 are assembled with each other to surround the internal pipes 22 and the Y-plate 23. Furthermore, the front end of the external plate 20 is assembled with the front flange 10 through fitting, while the rear end of the external plate 20 is assembled with the bellows 30 through fitting. Thereafter, the fitted junctions between the front flange 10, the front pipe 22 and the bellows 30 are subjected to a welding process, thus producing important parts of the front muffler. After producing the important parts of the front muffler, both the rear pipe 40 and the rear flange 50 are mounted to the important parts so that a front muffler is produced. In the present invention, the steel wire mat 21 is preferably welded to the external plate 20 through, for example, spot welding. [46] FlG. 7 is a graph showing exhaust noise as a function of engine speed of a front muffler having a partition wall according to the present invention and another front muffler having no partition wall. When comparing the exhaust noise of the front muffler having the partition wall to the exhaust noise of the other front muffler having no partition wall, it is noted that there is little difference in exhaust noise between the front muffler having the partition wall and the front muffler having no partition wall while a vehicle runs at a high speed. However, when the vehicle runs at a middle/low speed, the front muffler having the partition wall reduces the exhaust noise by about 1 ~ 3 dB compared to the front muffler having no partition wall, and reduces air current noise. [47] FlG. 8 is a graph showing engine torque as a function of engine speed of the front muffler having a partition wall according to the present invention and another front muffler having no partition wall. When comparing the engine torque of the front muffler having the partition wall to the engine torque of the other front muffler having no partition wall, it is noted that there is little difference in engine torque between the front muffler having the partition wall and the front muffler having no partition wall while a vehicle runs at a high speed. However, when the vehicle runs at a middle/low speed, the front muffler having the partition wall increases the engine torque by about 2 ~ 6 % compared to the front muffler having no partition wall, thereby increasing the engine output. [48] Although the preferred embodiment of the present invention has been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible without departing from the scope and spirit of the invention as disclosed in the accompanying claims.