Abstract:
A single-track motor vehicle with an internal combustion engine having at least one cylinder which is arranged in an inclined manner approximately horizontally relative to the driving direction, with at least one intake valve being arranged above at least one exhaust valve. In order to achieve the most compact possible design, the exhaust valve is triggered by way of a rocker lever.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a single-track motor vehicle with an internal combustion engine with at least one cylinder which is arranged in an inclined manner approximately horizontally against the driving direction, with at least one intake valve being arranged above at least one exhaust valve. Single-track vehicles within the terms of the invention are motorcycles or snowmobiles with a drive track through which the propulsion force is transmitted. 
     It is common practice in motorcycles whose engine comprises a crankshaft which is disposed transversally to the driving direction to dispose the cylinder(s) in an upright manner or in a forwardly inclined manner as seen in the driving direction. Generally, the gear is disposed in this case behind the engine and in front of the rear wheel, thus allowing the representation of a favourable power transmission. In motorcycles which are equipped with a low step-through in the form of a motor scooter, there are problems which are difficult to solve in such a design, particularly when engines of higher piston capacity are provided. In particular, an infinitely-variable change-speed gear, which is known as a so-called Variomatic, poses particular packaging problems. 
     Similarly, in snowmobiles with more powerful engines there are problems housing the engine and the gear in the space below the front bonnet. 
     SUMMARY OF PRIOR ART 
     A cross-country motorcycle is known from DE 36 01 273 A which is provided with a cylinder which is arranged rearwardly in a substantially horizontal manner. This arrangement is used in the known solution to achieve the longest possible rear suspension swing arm, thus improving the performance. In the illustrated cross-country motorcycles, an additional load on the front wheel by an engine disposed forwardly as far as possible is advantageous. 
     SUMMARY OF THE INVENTION 
     It is the object of the present invention to house the drive unit of a single-track motor vehicle in a particularly compact and space-efficient manner. In contrast to the aforementioned state of the art, it is intended to provide an engine in a motorcycle which is designed in a particularly short way so as to limit the foot space as little as possible and to achieve the shortest possible wheel base. Similarly, the most compact possible arrangement of the drive unit and a low center of gravity are to be achieved in a similar way in a snowmobile. 
     This is achieved in accordance with the invention in such a way that the exhaust valve is triggered by a rocker lever. In connection with the cylinder which is arranged in a rearwardly horizontal manner this allows a particularly advantageous intake arrangement in a motorcycle, and in particular, a shifting of the cylinder head close to the rear wheel. The triggering of the exhaust valve(s) via the rocker lever further allows reduction in the overall height of the cylinder head in the zone of the closest approach to the rear wheel. In an analogous manner, it is thus possible to achieve a highly compact engine arrangement in a snowmobile, with the cylinder head of the internal combustion engine coming to lie directly before the drive track. 
     It is particularly advantageous in this connection when the intake valve is triggered directly by an overhead camshaft. Due to the higher arrangement of the intake valve it is possible to provide an overhead camshaft without any conflict with the rear wheel. 
     A particularly favourable intake arrangement is achieved when the valves are inclined in a V-shaped manner with respect to one another and when an intake port is guided between the valves to the cylinder. This concerns a so-called reverse tumble arrangement in which a swirl is provided in the combustion chamber whose axis is substantially perpendicular to the cylinder axis. 
     In a particularly favourable embodiment of the invention an infinitely-variable change-speed gear is provided with a first gear shaft which is in connection with the crankshaft, and with a second gear shaft which is held in the cylinder block of the internal combustion engine. A particularly compact arrangement can be achieved with the arrangement of the drive system in accordance with the invention. It is particularly advantageous in this connection when a centrifugal clutch is held on the second gear shaft. Generally, the centrifugal clutch will be provided on the opposite side of the gear. The infinitely-variable change-speed gear can be arranged as a dry-running pulling strap drive or with a thrust link chain which runs dry or in an oil bath. 
     An increase in the driving convenience can be achieved in such a way that at least one mass balancing shaft is integrated in the housing of the internal combustion engine. 
     A particularly compact arrangement of the motorcycle can be achieved when the internal combustion engine is a two-stroke engine with four valves per cylinder. 
     The production of the internal combustion engine can be simplified substantially when a crankshaft axis, an axis of a differential gear shaft and an axis of the gear shaft are disposed in a common plane of division of the internal combustion engine. 
     In the arrangement of the invention as a scooter, it is advantageous when the internal combustion engine is a part of the rear suspension swing arm. This means that the rear wheel is held in a drive set swing arm. 
     The invention will now be explained in greater detail by reference to the embodiments shown in the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a side view of a motorcycle in accordance with the invention; 
     FIG. 2 shows a detail of FIG. 1; 
     FIG. 3 shows a motorcycle according to a further embodiment of the invention; 
     FIG. 4 shows a sectional view of an internal combustion engine of the motorcycle of FIG. 1 in an enlarged scale, and 
     FIG. 5 shows a partial sectional view of a further embodiment of the invention; 
     FIG. 6 shows a snowmobile according to the invention and 
     FIG. 7 a detail of FIG.  6 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the motorcycle of FIG. 1 only the details relevant for the invention are designated and explained below. 
     The motorcycle comprises an arrangement in the form of a motor scooter, which means a relatively low step-through  2  is provided. The internal combustion engine  3  of the scooter  1  is arranged below the seat  4  for the rider who is not shown in detail. The engine concerns a two-cylinder four-stroke internal combustion engine with four valves per cylinder. The cylinders  5  of the internal combustion engine  3  are arranged rearwardly horizontally, with the cylinder axes  6  enclosing an angle α of approx. 12° to the horizontal. In order to achieve the shortest possible wheel base I and to limit the step-through as little as possible, the cylinder head  7  of the internal combustion engine  3  is moved as far as possible towards the rear wheel  8  of the motorcycle  1 . 
     The internal combustion engine  3  is connected in the embodiment of FIG. 1 rigidly with the rear suspension swing arm  40  which is swivellably held about an axis  41  directly in front of the internal combustion engine  3 . Power transmission to the rear wheel is performed via a toothed belt or a chain  42 . 
     FIG. 2 shows the internal combustion engine  3  of the motorcycle of FIG. 1 in a detailed view. The crankshaft axis  43 , the axis  44  of a balancer shaft  45  for the first-order mass balancing which rotates in reverse motion to the crankshaft  10 , and the axis  46  of the gear shaft  18  are disposed in a horizontal plane of division  39  of the internal combustion engine  3 . A pulling strap  15  is wrapped around a pulley  12  and a second pulley  17 . FIG. 2 shows with the unbroken lines a position of the pulling strap  15  at low speed and high ratio of transmission which corresponds to the first gear of a change speed gear. The broken lines show the position of the pulling strap  15  which corresponds to a high gear. FIG. 2 further shows an electric starter  47  and an oilpan  48  which is screwed on from below. The driving torque is transmitted by a driving pinion  22  fastened to the gear shaft  18  on to a toothed wheel  23  which is rigidly connected to a chain wheel  24  which drives the chain  42 . 
     FIG. 2 shows that the intake valves  30  are actuated directly by an overhead camshaft  32 . The exhaust valves  31  are actuated by way of rocker levers  33  which are in operative connection with the camshaft  32 . This allows providing the cylinder head  7  in the zone of the rear wheel  8  with a low arrangement, so that a short overall length is achieved. The intake port  34  is guided in this embodiment from above to the intake valves and receives a throttle valve  37  and an injection nozzle  38 . 
     FIG. 3 shows an embodiment of the invention which differs from the aforementioned variant in such a way that the internal combustion engine  3  is fixedly held in the motorcycle and the rear suspension swing arm  40  is swivellable about an axis disposed behind the internal combustion engine  3 . The motorcycle per se is arranged as a conventional fully encased motorcycle. 
     FIG. 4 shows the detailed arrangement of the internal combustion engine  3 . Pistons  9  are movably arranged in two adjacently arranged cylinders  5  with parallel axes  6 . With the crankshaft  10  of the internal combustion engine there are arranged on the one side the starter and the electric generator  11  and on the other side a pulley  12  for an infinitely-variable change-speed gear  13 . A centrifugal timer  14  is arranged to displace an axially movable pulley half  12   a  with increasing speed in the direction towards an axially fixed pulley half  12   b , so that the roll-off diameter of a pulling strap  15  increases. The pulley  12  is held on a first gear shaft  16  which is formed integrally with the crankshaft  10 . 
     The pulling strap  15  drives a second pulley  17  which is held on a second gear shaft  18 . The pulley consists of an axially fixed pulley half  17   a , against which an axially movable pulley half  17   b  is pressed. The pressing force is produced by a coil spring  19 . The second gear shaft  18  is held in the cylinder block  20  of the internal combustion engine  3  and extends from the pulley  17  directly adjacent to the cylinders  5 ,  6  to the opposite side of the internal combustion engine  3 . The torque of the gear shaft  18  is transmitted by a centrifugal clutch  21  onto a driving pinion  22  which is in connection with a toothed wheel  23 . A chain wheel  24  is provided coaxially to the toothed wheel  23  and rigidly connected with the same in order to transmit the propulsion onto the rear wheel  8 . 
     The embodiment of FIG. 5 shows a detail of a solution with a single-cylinder internal combustion engine. As in FIG. 2, the intake valves  30  are directly actuated by an overhead camshaft  32 . The exhaust valves  31  are actuated by way of rocker levers  33  which are in operative connection with the camshaft  32 . 
     This allows providing the cylinder head  7  in the zone of the rear wheel  8  with a low arrangement, so that a short overall length is achieved. Furthermore, the intake port  34  is guided in the embodiment according to FIG. 5 between the valves  30 ,  31 , so that a reverse-tumble concept is achieved. The valves  30 ,  31  are inclined in a V-shaped manner at an angle of approx. 45° towards one another in order to free up the required space. This allows a particularly simple guidance of the intake air and allows the possibility to provide the carburettor  35  in a laterally swivelled manner next to the rear wheel  8 . The exhaust port  36  is guided downwardly and can thus also be housed in a very compact manner. 
     The present invention allows providing a drive concept for a motorcycle which is arranged in the manner of a scooter with a low overall size and a compact design even when a larger piston capacity of e.g. 500 cm 3  to 900 cm 3  is provided. 
     The snowmobile of FIG. 6 comprises a body  50  on which a seat  4  for the driver and, optionally, a passenger is provided. A drive track  51  is guided by track rollers, deflection rollers  53  and a drive roller  54 . Steering is performed in the conventional manner via a handlebar  55  which acts upon a swivellable ski  56 . 
     The arrangement of the drive unit of the embodiment according to FIGS. 6 and 7 is similar to the embodiment of FIG.  3 . The cylinder  5  of the internal combustion engine  3  is arranged rearwardly in a virtually horizontally manner. Power transmission is performed by an infinitely-variable change-speed gear  13  with a pulling strap  15 . In order to take the requirements of a snowmobile into account, a gear reducer unit (not shown) is interposed between the gear  13  and the drive roller  54  in order to enable the engagement of a reserve gear. As a result of the analogous arrangement in respect to FIGS. 2 and 3, with the valve control of the intake valves  30  via the camshaft  32  directly and the exhaust valves  31  via the rocker levers  33 , the internal combustion engine  3  can be moved in a highly compact manner towards the drive roller  54 .