COOLING DEVICE OF AUTOMOTIVE ALTERNATOR

Abstract: A cooling device (12) of an automotive alternator (10) comprises a cooling fan (20) of the mixed flow type and including a boss section (22) secured on a rototor shaft (14) of the alternator. The boss section (22) increases in sectional area in the direction from the upstream side of the down­ stream side of the cooling fan in which direction air flows when the cooling fan rotates. A plurality of fins (24) are formed radially on the boss section (22) in a manner that diagonal flow of air stream is generated at a location downstream of the cooling fan. Additionally, a fan-shroud (26) is provided to surround the cooling fan (20) and formed with upstream and downstream openings (26a, 26b) which are respectively located in the vicinity of the upstream and downstream sides of the cooling fan, the downstream opening being larger than the upstream opening. The thus arranged cooling device (12) reduces noise generated due to the rotation of cooling fan, while ensuring a sufficient amount of cooling air.

Description: COOLING DEVICE OF AUTOMOTIVE ALTERNATOR BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a cooling device of an automotive alternator, and more particularly to an improvement in the cooling device from a view point of noise reduction. 2. Description of the Prior Art In connection with alternators used in automotive vehicle engines, they are usually equipped with a cooling device for cooling an alternator body disposed within a housing. The cooling device is conventionally composed of a so-called centrifugal fan which is mounted on a rotor shaft of the alternator to be rotated with the rotator shaft so as to suck air into the alternator casing. However, such centrifugal cooling fan develops considerable noise, thereby increasing total engine noise. SUMMARY OF THE INVENTION A cooling device of an automotive alternator, according to the present invention comprises a cooling fan of the mixed flow type for supplying cooling air into the interior of an alternator housing when rotated. The cooling fan includes a boss section secured on a rotor shaft of the alternator and generally in the shape of a frustum of a cone. The boss section increases in sectional area in the direction from the upstream side toward the downstream side of said cooling fan in which direction air flows when said cooling fan rotates. A plurality of fins are formed radially on the boss section so that diagonal flow of air stream develops at a location downstream of the cooling fan. Additionally, a fan-shroud is disposed to surround the cooling fan and formed with upstream and downstream openings which are respectively located in the vicinity of the upstream and downstream sides of the cooling fan. The downstream opening is larger than the upstream opening. With this alternator cooling device, noise created due to the rotation of the cooling fan can be greatly reduced, while ensuring a sufficient amount of cooling air. BRIEF DESCRIPTION OF THE DRAWINGS The features and advantages of the automotive alternator cooling device according to the present invention will be more appreciated from the following description taken in conjunction with the accompanying drawing in which the same reference numerals designate the same parts and elements, and in which: Fig. 1 is a front view of an alternator equipped with a conventional cooling device; Fig. 2 is a right-side view of the alternator of Fig. 1; Fig. 3 is a side view, partly in section, of an alternator equipped with a preferred embodiment of a cooling device in accordance with the present invention Fig. LI is a rear view of the alternator of Fig. 3; and Fig. 5 is an illustration showing the detail of fins of a fan as the cooling device of Fig. 3. DETAILED DESCRIPTION OF THE INVENTION Referring to Figs. 1 and 2, an automotive alternator 1 provided with a conventional cooling device or fan 2 will be described along with its major shortcomings. The fan 2 is fixedly mounted on a rotor shaft 3 of the alternator 1 and located between alternator housing LI and a pulley 5 through which the rotor shaft 3 is rotated. Accordingly, when the cooling fan 2 rotates together with the rotor shaft 3, air is sucked into the interior of the alternator housing 4 as indicated by broken arrows to cool an alternator body (not shown) located within the casing 4. The cooling fan 2 is of the so-called centrifugal type and provided with twelve or more fins 2a in order to improve its cooling ability. The rotor shaft 3 of the alternator 1 is driven at a rotational speed of about two times engine speed, thereby ensuring an electricity generating ability during low engine speed operation of an engine. As a result, when the engine is operated, for example, at an engine speed of 4000 rpm, the fundamental frequency of noise generated by the cooling fan fins 2a becomes 12 x 2 x r1000 = 1600 Hz 69 in case where the number of the fins is twelve. It is to be noted that this frequency of 1600 Hz is within a particular frequency range (800-2000 Hz) which is critical from a point of view of noise reduction. Furthermore, it will be understood that the noise component of 1600 Hz extremely increases. Therefore, even if engine construction is so improved as to decrease acoustic energy emitted from the engine for the purpose of reducing engine noise, the total engine noise has not been effectively reduced because of the above-discussed considerable noise due to the cooling fan 2 of the alternator 1. In view of the above description of the conventional automotive alternator cooling device, reference is now made tc Figs. 3 to 5, wherein an automotive alternator 10 is provided with a preferred embodiment of a cooling device 12 in accordance with the present invention. The alternator 10 includes a rotor shaft 14 passing through the interior of an alternator housing 16 and extending oppositely outwardly to form opposite extending sections alia, 14b. A pulley 18 is securely mounted on the rotor shaft extending section 14a and driven through a V-belt (not shown) by an engine (not shown) of an automotive vehicle. The cooling device 12 comprises a cooling fan 20 of the so-called mixed flow type wherein air flows or passes, through the fan, diagonally relative to the axis X of the fan. The cooling fan 20 is securely mounted through its boss section 22 on the rotor shaft extending section 14b. The boss section 22 is generally in the shape of a frustum of a cone and gradually increases in cross-sectional area from the upstream side of the fan 20 toward the downstream side of the same, which upstream and downstream sides are relative to the stream of air which is indicated by broken arrows in Fig. 3 and developed when the cooling fan 20 rotates with the rotor shaft 14. A plurality of fins (four fins in this embodiment) are formed on the frustoconical peripheral surface of the boss section 22. The fins 24 are integral with the boss section 22 and formed angularly relative to the axis X of the fan 22 as best shown in Fig. 5. An angle o of each fin 24, relative to an imaginary plane to which the fan axis X is perpendicular, is set, for example, at LIS degrees, and additionally so set that the angle of attack becomes, for example, 45 degrees or more so that air stream at a location downstream of the cooling fan 20 becomes so-called diagonal flow whose flowing direction is diagonal relative the rotor shaft axis X. The angle of attach is preferably 65 degrees. A generally annular fan-shroud 26 is disposed to surround the outer periphery of the cooling fan 20 in a manner that the tip end of each fan fin 24 faces the inner surface of the fan-shroud 26. The fan-shroud 26 is secured to the downstream side of the alternator housing 16 and formed with an upstream side opening 26a located in the vicinity of the upstream side of the cooling fan 20, and a downstream side opening 26b located in the vicinity of the downstream side of the cooling fan 20. As shown, the downstream side opening 26a is larger in sectional area than the upstream side opening 26a. In other words, the downstream periphery of the fan-shroud 26 is widened as compared with the upsteam side periphery of the same. With the thus arranged cooling device 12 of the alternator 10, when the cooling fan 20 rotates with the rotor shaft 14 of the alternator 10, cooling air is sucked and passes through the interior of the alternator housing 16 as indicated by the broken arrows in Fig. 3 to cool an alternator body (not shown) disposed within the casing 16. Air stream having passed through the casing interior becomes diagonal flow and be discharged to be widened toward the end, after passing through the cooling fan 20. Additionally, since the fan-shroud 26 is formed to be widened and the cooling fan boss section 22 is bulged toward the downstream side of the fan 20, the formation of diagonal flow of the discharged cooling air stream can be effectively promoted. Furthermore, since a so-called mixed flow fan is used as the cooling fan 20 in the present invention, it has become possible to take a larger angle of attack (of 45 degrees or more), thereby enabling to obtain a sufficient amount of cooling air even if the number of fins 24 of the fan 20 is smaller. This has been confirmed by the inventors. Accordingly, in the case where the number of the fins 24 is four as in the abovementioned embodiment, the fundamental frequency of noise generated by the cooling fan 20 beocmes EMI7.1 when the engine runs at an engine speed of 4000 rpm. This noise fundamental frequency is considerably lower than the noise frequency range (800-2000 Hz) which is critical in engine noise control, thus effectively achieving total engine noise reduction. In this connection, in conventional alternator cooling devices in which so-called centrifugal fan is used as its cooling fan, a larger angle of attack cannot be taken in order to prevent air stream from breaking away and stalling. Therefore, in order to obtain a sufficient amount of cooling air, it is necessary to increase the number of fins as compared with that in the present invention. The thus increased number of fins greatly contributes to develop noise having frequency within the above-mentioned critical noise frequency range. While the cooling fan 20 has been shown and described as being disposed at the rear end of the alternator casing 16 in the above-discussed embodiment, it will be understood that it may be disposed between the drive pulley 18 and the front end of the alternator casing 16 in which cooling air stream direction is reverse relative to that indicated in Fig. 3. It is preferable that the boss section 22 and the fins 24 are integrally formed as ê one-piece of molded plastics, thereby achieving production cost reduction, weight reduction, while obtaining high ridigity of the cooling fan 20. As will be appreciated from the above, according to the present invention, the mixed flow fan is used as the cooling fan of the alternator, and therefore the number of fins of the cooling fan is reduced as compared with the conventional cooling fans, while obtaining a sufficient amount of cooling air. Therefore, the frequency of wave of condensation and rarefaction generated due to rotation of the cooling fan can be lowered to within a frequency range which is not critical as engine noise, thereby greatly reducing total engine noise containing cooling fan noise.

Claims: WHAT IS CLAIMED IS: 1. A cooling device (12) of an automotive alternator (10) having a rotor shaft (14) and a housing (16), comprising: a cooling fan (20) of the mixed flow type for supplying cooling air into the interior of the housing, when rotated, said cooling fan (20) including a boss section (22) secured on the rotor shaft and generally in the shape of a frustum of a cone, said boss section (22) increasing in sectional area in the direction from the upstream side toward the downstream side of said cooling fan in which direction air flows when said cooling fan rotates, and a plurality of fins (24) formed radially on said boss section so that diagonal flow of air stream develops at a location downstream of said cooling fan; and a fan-shroud (26) disposed to surround said cooling fan (20) and formed with upstream and downstream openings (26a, 26b) which are respectively located in the vicinity of the upstream and downstream sides of said cooling fan, said downstream opening (26b) being larger than said upstream opening (26a). (Figs. 3-5) 2. A cooling device as claimed in Claim 1, wherein each fin (24) of said cooling fan (20) has an angle of attack which is so set as to generate diagonal flow of air stream at the location downstream of said cooling fan. (Figs. 3-5) 3. A cooling device as claimed in Claim 2, wherein the number of said fins (24) is four. (Figs. 3-5) 4. A cooling device as claimed in Claim 1, wherein said fan-shroud (26) is generally annular and secured to the alternator housing (16), said upstream and downstream openings (26a, 26b) being opened upstream and downstream of said cooling fan (20), respectively. (Figs. 3-5) 5. A cooling device as claimed in Claim 1, wherein said boss section (22) and. said fins (24) are integral with each other. (Figs. 3-5) 6. A cooling device as claimed in Claim 5, wherein said boss section (22) and said fins (24) are formed as a one-piece of plastics. (Figs. 3-5)

Inventors: HAYASHI, YOSHIMASA
Applicants: NISSAN MOTOR CO., LTD.
Application year: 1982
Main classification: H02K9
IPC codes: F01P5, F01P1, H02K9