1.  Field of the Invention
The present invention generally relates to a power train, and particularly, it relates to a power train for four-wheel drive vehicles. 
2.  Description of Relevant Art
There has been disclosed a four-wheel drive vehicle power train (hereafter xe2x80x9cfirst conventional power trainxe2x80x9d) in Japanese Patent Application Laid-Open Publication No.  3-224829, another four-wheel drive vehicle power train (hereafter xe2x80x9csecond conventional power trainxe2x80x9d) in U.S. Pat.  No.  5,188,574, and still another four-wheel drive vehicle power train (hereafter xe2x80x9cthird conventional power trainxe2x80x9d) in U.S. Pat.  No.  4,700,800. 
The first conventional power train has a transmission and a power transfer device illustrated in FIG.  1, where they are designated by reference characters 403 and 401, respectively.  The power transfer device 401 includes a front drive train 409+411+413 and a rear transfer train 415.  The front drive train includes a planetary type final reduction gear 409 arranged coaxial to a transversely extending output shaft 405 of the transmission 403, a planetary type center differential gear 411 and a front differential gear 413. 
Drive torque from the transmission 403 is supplied from the output shaft 405, via the final reduction gear 409, to the center differential gear 411, where it is shared to be distributed on the one hand through the front differential gear 413 to left and right front axles 407, and on the other hand through the rear transfer train 415 to rear axles.  The left front axle 407 extends through the transmission output shaft 405. 
The second conventional power train has a transmission and a power transfer device illustrated in FIG.  2, where they are designated by reference characters 503 and 501, respectively.  The power transfer device 501 includes a drive train 509+511+513 and a transfer train 515.  The drive train includes a final reduction gear 509 arranged parallel to an output shaft 505 of the transmission 503, a planetary type center differential gear 511 and a bevel differential gear 513. 
Drive torque from the transmission 503 is supplied from the output shaft 505, via the final reduction gear 509, to the center differential gear 511, where it is shared to be distributed on the one hand through the differential gear 513 to a pair of left and right axles 507, and on the other hand through the transfer train 515 to another pair of axles.  The right axle 507 extends through the transmission output shaft 505. 
The third conventional power train is illustrated in FIG.  3 and has a transmission 603 and a power transfer device 601.  This power transfer device 601 includes a front drive train 611+613 and a rear transfer train 609.  The front drive train includes a planetary type center differential gear 611 and a front differential gear 613. 
Drive torque from the transmission 603 is supplied from the output shaft 605 on the one hand directly to the rear transfer train 609, and on the other hand via the center differential gear 611 to the front differential gear 613, where it is shared to be distributed to left and right front axles 607.  The left axle 607 extends through the transmission output shaft 605. 
In the conventional four-wheel drive vehicle power trains described, their power transfer devices 401, 501, 601 each respectively include a drive train having a center differential gear 411, 511, 611 supplying drive torque to a differential gear 413, 513, 613, where it is differentially distributed to an associated pair of left and right axles 407, 507, 607.  Therefore; in modification for application to a two-wheel drive vehicle, each power train needs a significantly wide re-design covering most of principal components such as differential gears and housings associated with torque transmission lines, as their commonality is low. 
As a result, four-wheel drive vehicles need dedicated components therefor with increase in size and weight. 
Moreover, the first and second conventional power trains have their drive trains elongated along associated axles 407, 507 with undesirable effects on a suspension stroke. 
The present invention has been achieved with such points in view. 
It therefore is an object of the present invention to provide a power train with an increased commonality between a two-wheel drive vehicle and a four-wheel drive vehicle, permitting a down-scaled, light-weighted design. 
To achieve the object, a first aspect of the present invention provides a power train for a vehicle with an engine and a total of four drive wheels, the power train comprising a first power train for driving a first pair of drive wheels, the first power train comprising a transmission connected to the engine, and a first drive train comprising a first final reduction gear connected to the transmission, and a first differential gear connected between the first final reduction gear and the first pair of drive wheels, and a second power train for driving a second pair of drive wheels, the second power train comprising a power transfer train branched from the first drive train between the first final reduction gear and the first differential gear, and a second drive train connected to the second pair of drive wheels. 
According to the first aspect, a first drive train for a four-wheel drive vehicle is employable for a two-wheel drive vehicle and has an increased commonality therebetween. 
According to a second aspect of the invention, the first final reduction gear is connected to an output shaft of the transmission, the first differential gear has an input element thereof driven by the final reduction gear and an output element thereof driving an axle of one of the first pair of drive wheels, and the output shaft and the input element have rotation axes thereof aligned to the axle. 
According to the second aspect, a first drive train for a four-wheel drive vehicle may have an input element eliminated therefrom, permitting a common use to a two-wheel drive vehicle with a flexibility in design to have an increased transverse axle length outside a first power train Further, a reverse application to the four-wheel drive vehicle can be achieved without an undesirable increase in transverse size or weight. 
According to a third aspect of the invention, the power train further comprises a branching element formed on the input element for branching the power transfer train from the first drive train. 
According to the third aspect, an ensured branching is allowed with a simplified supporting structure. 
According to a fourth aspect of the invention, the input element comprises an integral part of a differential casing member of the first differential gear. 
According to the fourth aspect, a first differential gear as well as a branching element has a simplified supporting structure, permitting an increased commonality of a differential casing member and a reduced cost. 
According to a fifth aspect of the invention, the power transfer train includes a drive transfer mechanism connected to the input element for transferring drive torque shared therefrom in a first direction crossing the axle. 
According to the fifth aspect, a first drive train for a four-wheel drive vehicle may be employed, as it is, for a two-wheel drive vehicle. 
According to a sixth aspect of the invention, the drive transfer mechanism comprises a chain transmission mechanism. 
According to the sixth aspect, a drive transfer mechanism has an increased flexibility in spatial arrangement with relaxed criteria for connection accuracy.  Further, a second power train may stand free of noises and vibrations from a first power train. 
According to a seventh aspect of the invention, the power transfer train further includes a transfer path displacing mechanism for displacing a transfer path of the drive torque in a second direction crossing the first direction. 
According to the seventh aspect, a power transfer train permits a drive connection to a second power train to be arranged close to a longitudinal centerline of vehicle, with an improved weight balance. 
According to an eighth aspect of the invention, the transfer path displacing mechanism comprises a drive bevel member rotatable about an axis parallel to the axle and a driven bevel member meshing with one of mutually opposite sides of the drive bevel member near to the transmission. 
According to the eighth aspect, a power transfer train has a reduced longitudinal dimension. 
According to a ninth aspect of the invention, the transfer path displacing mechanism comprises a drive bevel element fixed on a shaft member rotatably supported by a pair of bearings and a: driven bevel element meshing with the drive level element at a point between the pair of bearings. 
According to the ninth aspect, an improved supporting permits an ensured meshing between drive and driven bevel elements. 
According to a tenth aspect of the invention, the power transfer train further includes a coupling adaptive for a differential transmission of the drive torque. 
According to the tenth aspect, drive torque is transmittable to a second drive train in a differential manner, with a light-weighted structure and increased flexibility in layout. 
According to an eleventh aspect of the invention, the second drive train comprises a second final reduction gear connected to the coupling and a second differential gear connected between the second final reduction gear and the second pair of drive wheels. 
According to the eleventh aspect, a differential transmission has a defined differential action. 
According to a twelfth aspect of the invention, the power train further comprises a transfer housing for accommodating the first differential gear and part of the rear transfer train, an internal space defined inside the transfer housing and connected inside the transmission, and a body of lubricant filled in the internal space. 
According to the twelfth aspect, a possible elimination of seal member between a transmission and a transfer housing allows for the more compact design.