FIG. 13 is a constitutional developed view of a conventional power unit 01 which is mounted on a buggy car or the like including respective rotary shafts thereof for example, see JP-A-2004-36835. In the drawing, a left side (indicated by an arrow F) is a front side of a vehicle. The power unit 01 is formed of an integral body constituted of an internal combustion engine 02 and a transmission 03 and includes a crankshaft 04 which extends in the fore-and-aft direction of a vehicle body, a main shaft 05 of the transmission, a counter shaft 06 of the transmission 03, an output shaft 07 and a backward driving changeover intermediate shaft 08. These rotary shafts are arranged in parallel to each other and are rotatably supported by front and rear crankcase members and a power is transmitted to an output shaft 07 from the crankshaft 04 by way of a transmission step. In this example, the internal combustion engine 02 is a single-cylinder internal combustion engine, wherein a piston 010 is connected to the crankshaft 04 by way of a connecting rod 09 and the piston 010 performs the reciprocating movement in a cylinder 011 to generate a power.
A torque converter 012 is mounted on a front end portion of the crankshaft 04. The torque converter 012 is constituted of a pump impeller 013 which is fixed to the crankshaft 04, a turbine runner 014 which is freely rotatable while facing the pump impeller 013 in an opposed manner and a stator 015 which is held by a crankcase member 017 by way of a one way clutch 016. To the turbine runner 014 which is rotatable with respect to the crankshaft 04, a primary driving gear 018 which is rotatable with respect to the crankshaft 04 is integrally joined. A power generated by the internal combustion engine 02 is transmitted to the pump impeller 013 from the crankshaft 04, is transmitted to the turbine runner 014 by way of a working oil and hence, the primary driving gear 018 is driven.
To a front end portion of the main shaft 05 of the transmission, a primary follower gear 019 which is constantly meshed with the primary driving gear 018 is fixed. The rotation of the crankshaft 04 is transmitted to the main shaft 05 of the transmission by way of a primary speed reduction which is constituted of the primary driving gear 018 and the primary follower gear 019. A first hydraulic multi-plate clutch 020 is arranged close to the above-mentioned primary follower gear 019 fixed to the front end portion of the main shaft 05. A clutch outer 021 of the above-mentioned multi-plate clutch 020 is fixed to the main shaft, while a clutch inner 022 is joined to a first-speed driving gear M01 which is freely rotatable with respect to the main shaft 05. To a rear portion of the main shaft 05 which extends while penetrating the center of the first multi-plate clutch 020 and a center hole of the first-speed driving gear M01, a second-speed driving gear M02 and a third-speed driving gear M03 are fixed.
To a front end portion of the counter shaft 06 which is rotatably supported in parallel to the above-mentioned main shaft 05, a first-speed follower gear C01 which is constantly meshed with the above-mentioned first-speed driving gear M01 is fixed. On a center portion of the counter shaft 06, a second-speed hydraulic multi-plate clutch 023 and a third-speed hydraulic multi-plate clutch 026 are mounted. Both of clutch outers 024, 027 of the above-mentioned clutches are fixed to the counter shaft 06. A clutch inner 025 of the above-mentioned second-speed multi-plate clutch 023 is joined to a second-speed follower gear C02 which is freely rotatable with respect to the counter shaft 06, while a clutch inner 028 of the above-mentioned third-speed multi-plate clutch 026 is joined to a third-speed follower gear C03 which is freely rotatable with respect to the counter shaft 06.
On a rear portion of the counter shaft 06, a forward output gear 029 and a backward output gear 030 are mounted in a state that these gears are rotatable with respect to the counter shaft 06. Between both gears, a dog clutch 031 which is fitted on the counter shaft 06 in spline fitting and is movable in the fore-and-aft direction is provided. To a backward driving changeover intermediate shaft 08 which is arranged close to and in parallel to the counter shaft 06 and rotatably, a first intermediate gear 032 which is constantly meshed with the above-mentioned backward output gear 030 and a second intermediate gear 033 which is constantly meshed with an output follower gear 034 described later are fixed, and both of these intermediate gears 032, 033 are interlocked with each other by way of the backward driving changeover intermediate shaft 08.
To the output shaft 07 which is arranged in parallel to the above-mentioned counter shaft, an output shaft follower gear 034 which is constantly meshed with the above-mentioned forward output gear 029 and the second intermediate gear 033 is fixed. An output of the power unit 01 is transmitted to a front wheel from a front end of the output shaft 07 and is transmitted to the rear wheel from the rear end of the output shaft 07.
In the vicinity of the counter shaft 06, a forward and backward changeover device 035 is provided. The forward and backward changeover device 035 is constituted of a shift drum 036 which is rotatably supported in parallel to the counter shaft 06, one shifter 037 which is driven in the fore-and-aft direction along one cam groove which is formed in an outer periphery of the shift drum 036, and a manual rotating device which serves to rotatably drive the shift drum 036. The above-mentioned manual rotating device is constituted of a shift spindle 038, a change lever 039 which is formed on an end portion of the shift spindle 038, a sector gear 040 which is mounted on the shift spindle, and a gear 041 which is integrally formed on a shaft 036a of the shift drum 036 and is meshed with the above-mentioned sector gear 040. The shifter 037 is engaged with the dog clutch 031 and moves the dog clutch 031 in the fore-and-aft direction so as to perform the changeover of the forward driving and the backward driving. Here, an alternating current generator 042 which generates an electric power by the rotation of the crankshaft 04 is mounted on the rear end of the crankshaft 04.
In the above-mentioned transmission 03, the selection of the forward driving and the backward driving is decided based on whether, using the forward and backward changeover device 035, the shifter 037 is moved to the forward output gear 029 side so as to allow the dog clutch 031 to be engaged with the above-mentioned gear 029 or the shifter 037 is moved to the backward output gear 030 side so as to allow the dog clutch 031 to be engaged with the above-mentioned gear 030. The selection of the transmission ratio in the forward driving state is determined based on which one of three multi-plate clutches 020, 023, 026 is selectively connected and which one of the gears M01, C02, C03 is selectively joined to the shaft. Here, when the backward driving state is selected, the first multi-plate clutch 020 is selectively connected so that the first-speed driving gear M01 is selectively joined to the shaft. The operation and the changeover of the hydraulic multi-plate clutch are performed using a hydraulic device not shown in the drawing. FIG. 14 is an automatic transmission manipulation table which arranges the selective manipulations of the clutches and the shifter in the above-mentioned conventional power unit.
However, since the conventional transmission adopts the three-stage automatic transmission, there has been a demand for a multi-stage automatic transmission having a larger number of stages. Further, it has also been a task to reduce the number of parts of a multi-plate clutch having a large weight.
Further, in the conventional transmission, to perform the changeover of the forward driving and the backward driving, a change lever is manually manipulated so as to change over gears, the manipulation becomes cumbersome.