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TECHNICAL FIELD 
     The invention relates to a drive for a flap provided on a vehicle. 
     Moreover, the invention relates to a pedestrian protection means on a motor vehicle comprising an above mentioned drive. 
     BACKGROUND OF THE INVENTION 
     Today&#39;s automobiles are often provided with drives for the various flaps on the car concerned, including a vehicle door, a trunk lid or gas tank cover and especially an engine hood. Especially with regard to the attachment of engine hoods to the car body, pedestrian protection is to be improved in motor vehicles in that the engine hood is to be made more yielding. For this purpose, thought has been given, for example, to airbags for pedestrians or to a yielding suspension of the engine hood. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention provides a very simple and effective drive that can be used, for example, for a pedestrian protection means comprising the engine hood. The drive proposed is configured in such a way that it raises the engine hood by a defined amount if a pedestrian is detected by an accident sensor. The engine hood is suspended in an elastically yielding manner by this amount so that the impact for the pedestrian is not as hard. The drive is capable of raising the engine hood within a few milliseconds. Moreover, it is capable of being used multiple times for the same purpose without having to go to the repair shop, an important aspect in view of the fact that the accident sensors may not always detect beyond all doubt a collision with a pedestrian but rather, for example, also with objects, for example, a box, which do not cause damage in case of a collision with the vehicle. A pyrotechnical drive device would have to be replaced at great expense after each actuation event that turned out to be unnecessary. 
     The drive proposed is, in particular, intended for use with an engine hood, but can likewise be used as a drive in the form of e.g. a closing means for a vehicle door or for another flap. The drive is distinguished by a simple structure and comprises an electric motor, an actuation shaft that is connected to the flap, and a reduction gear by means of which the rotor of the electric motor is coupled to the actuation shaft. The drive further comprises an energy accumulator by means of which the actuation shaft can be driven independently of the electric motor. The reduction gear is configured in such a way that the actuation shaft is driven in a rotational direction only by the energy accumulator and the electric motor drives the actuation shaft in a rotational direction opposite to the rotational direction called opposite rotational direction, thereby supplying to the energy accumulator an energy that is needed to drive the actuation shaft in the rotational direction. 
     The energy accumulator moves the flap, especially the engine hood, abruptly out of its original position into the desired raised position and the electric motor moves the flap back into the original or starting position against the resistance of the energy accumulator, in order to “arm” the energy accumulator once again. 
     According to one embodiment, the energy accumulator is firmly coupled with the actuation shaft. This means that no complicated coupling mechanisms are provided between the actuation shaft and the energy accumulator and/or the electric motor. 
     According to the preferred embodiment, the reduction gear is configured in such a way that it can be moved into a release position in which the energy stored in the energy accumulator is abruptly released so as to drive the actuator shaft. This means that the electric motor has multiple functions. On the one hand, it moves the flap back into the starting position and, on the other hand, it itself releases the energy accumulator by moving the reduction gear into the release position. In order for this to be possible within an extremely short period of time, the electric motor and the gear have to be designed with as little inertia and loss as possible. For this purpose, the electric motor is a brushless internal rotor that is not very susceptible to dirt deposits and the moving parts of which have a low inertial mass. 
     The reduction gear preferably has a toothed wheel that has no teeth on part of its circumference (toothless area). As soon as the toothless area is rotated so as to be vis-à-vis the toothed counterwheel, which is done by the electric motor, the release position is reached and the toothed counterwheel can rotate freely since the energy accumulator is activated. In the release position, the electric motor is then uncoupled from the actuation shaft. 
     Outside of the release position, the energy accumulator and the rotor of the electric motor are rigidly coupled to each other mechanically by means of the reduction gear. 
     The invention further relates to a pedestrian protection means provided on a vehicle and equipped with a drive as described above. The pedestrian protection means which is proposed has an engine hood and at least one drive to move the engine hood, the energy accumulator raising the engine hood in case of an accident out of an original position into a raised position and holding it in the raised position in an elastically yielding manner. The electric motor then can move the engine hood back into the original position. This means that the energy accumulator likewise has a double function in that, on the one hand, it raises the engine hood and, on the other hand, it ensures the yielding suspension of the engine hood in the raised position, so that the impact of the pedestrian on the engine hood is not so hard. Preferably, the energy accumulator is a spring energy accumulator, especially a spring energy accumulator with a spiral spring. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a perspective view of a pedestrian protection means according to the invention with two drives according to the invention, 
     FIG. 2 shows an exterior view of the drive according to the invention when the hood is closed, 
     FIG. 3 shows an exterior view of the drive when the hood is open, 
     FIG. 4 shows a view of the drive without the cover, 
     FIG. 5 shows an exploded view of the drive according to the invention, 
     FIG. 6 shows a side view of the drive, only the moving parts being shown, 
     FIG. 7 shows a sectional view through the drive along the sectional line VII—VII in FIG. 5, and 
     FIG. 8 shows an enlarged view of the hollow wheel and the spur wheel. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows a pedestrian protection means on a motor vehicle, comprising an engine hood  3  and two drives  5  that raise the engine hood  3  as soon as sensors  7  detect a pedestrian who is struck by the vehicle. The drives  5  are designed identically and are each provided at the rear outer end of the engine hood  3 . Reference is being made to FIG. 6, from which the attachment of the rear end of the engine hood  3  to the moving parts of drive  5  is readily apparent. As an alternative, an additional drive could also be provided on the front end, but this drive would bring about less lifting than the rear drives. Not only can the drives  5  raise the engine hood  3  abruptly, but they can also hold it in this raised position in an elastically yielding manner, i.e. the drives themselves constitute an elastic suspension that provides a specific resistance to a force being exerted from the outside, which strives to bring the engine hood back to its original position. 
     FIG. 2 shows one of the drives  5 , which is very compact in design. The drive has an exterior housing  21  from which at least four electrical connections  9  extend as well as an actuation shaft  11  that is connected to a lever mechanism  13  consisting of levers  15  and  17 . An articulation point  19  serves to create a coupling to the hood. In the area of this articulation point  19 , the hood can be uncoupled from the lever mechanism in order to open the engine hood. FIG. 2 shows the drive when the engine hood  3  is in the original position. The drive  5  can rotate the lever  15  clockwise by 90° within 60 to 70 milliseconds in case of a collision with a pedestrian, thus raising the articulation point by about 55 mm. In the raised position shown in FIG. 3 (also called the holding position), the drive has springy action or a resiliant effect, i.e. when the pedestrian makes contact with the engine hood, the hood is elastically pressed downwards in the direction of the arrow A against the force F of the drive. 
     The drive is shown in greater detail in FIG. 4; it has a housing  21 , an electronic means  23  as well as a direct-current motor without a commutator and configured as an internal rotor, which can also be seen in FIG. 5, where it is designated by the reference numeral  25 . FIG. 4 shows the rotor (armature)  27  and the stator  29 . The rotor  27  is coupled to the actuation shaft  11  by means of a two-stage reduction gear. A first stage of the gear is a toothed belt gear, the toothed belt  30  being coupled to a rotor shaft and a large toothed wheel  32 , whose front face has wedge-shaped elevations  32  that interact with a leaf spring that functions as an return stop  33 , so that the toothed wheel  31  can only be turned in one direction (in FIG. 5 counterclockwise). On the rear of the toothed wheel  31 , as FIG. 6 shows, there is provided a small, formed-on spur wheel  35  that is not completely surrounded by teeth on its outer circumference but rather that has a toothless circumferential area or section  36 . An toothed counterwheel, which is configured as a cup-shaped hollow wheel  37 , can mesh with the spur wheel  35  to form the second stage of the gear. An energy accumulator in the form of a spring accumulator  39  is accommodated inside the hollow wheel  37 . The radial inner end of the spring accumulator  39  is coupled to the actuation shaft  11 , and the radial outer end is coupled to the housing  21 . The hollow wheel  37  is likewise coupled to the actuation shaft  11  so as to be non-rotatable. 
     FIG. 6 shows the drive in its original position. In this position, the first tooth  38  on the circumference and in the clockwise direction beyond the toothless section  37  on the spur wheel  35  is just barely still engaged with the toothed wheel  36 , so that the actuation shaft  11  is firmly coupled to the rotor  27  mechanically via the hollow wheel  37 , the spur wheel  35 , the toothed wheel  31  and the toothed belt  30 . The spring in the spring accumulator  39  is tensioned and strives to drive the actuation shaft  11  in the first, clockwise direction (arrow B). This drive, however, is not possible in the gear position shown here since the return stop  33  is active so that there is no risk that the engine hood will be moved upwards by the energy accumulator during normal driving operations. 
     As soon as a pedestrian is detected, who, according to a vehicle control (not shown), collides with the vehicle, the electric motor  25  is activated so that the rotor  27  moves within just a few milliseconds, thereby moving the toothed wheel  31  together with the formed-on spur wheel  35  in the direction of the arrow A. The spring is still minimally tensioned a bit more until finally, the above-mentioned first tooth on the spur wheel is disengaged from the opposite teeth on the toothed counterwheel in the form of a hollow wheel  37 , so that the two toothed wheels  35 ,  37  are no longer engaged and the reduction gear reaches a release position. In this position, the rotor is uncoupled from the actuation shaft  11  and the spring accumulator  39  can abruptly release its energy and drive the actuation shaft  11  in the direction of the arrow B, thus raising the engine hood. Therefore, the arrow B shows the rotational direction in which the actuation shaft  11  is only driven by the energy accumulator. 
     In the raised position, the engine hood can be pushed down against the force of the spring. 
     After the activation has occurred, the engine hood is automatically moved downwards since the rotor  27  is turned in the same rotational direction as before in order to reach the release position, so that finally, a tooth of the spur wheel  35  engages with the hollow wheel  37  again. Now the rotor  27  is again firmly coupled to the actuation shaft  11  and can turn the latter in the opposite direction of arrow B which opposite direction is called the opposite rotational direction. During this resetting into the position shown in FIG. 6, the spring is tensioned once again so that it can be released again at a later point in time. 
     FIG. 8 shows a few details that have not been mentioned yet, which are advantageous especially during the release of the spring energy. 
     The teeth of the spur wheel  35  and of the hollow wheel  37  are virtually rectangular in order to increase the load-bearing capacity of the teeth. The teeth themselves are only about 0.7 mm deep, of which only about 0.5 mm are engaged, in order to reduce the activation angle and thus the activation time to a minimum. From the original position shown in FIG. 8, a rotation of just 20° on the spur wheel  35 , i.e. 120° on the motor, is needed in order to disengage the teeth of the spur wheel  35  and of the hollow wheel  37  and to release the spring energy. 
     The so-called free-wheeling position, that is to say, the position of the spur wheel  35  when the engine hood  3  is in the raised position, is turned by about 50° in the clockwise direction as compared to the original position shown in FIG.  8 . 
     The so-called start position, that is to say, the position of the spur wheel  35  after which a tooth  53  of the spur wheel  35  is once again engaged with the teeth of the hollow wheel  37 , is at about a 75° rotation in the clockwise direction relative to the original position shown in FIG.  8 . 
     After a rotation of the spur wheel  35  by 360°, the original position is reached once again. 
     Another detail worth mentioning is that the first and the last teeth  51 ,  53  that are engaged, that is to say, the two teeth that delimit the toothless circumferential section  36 , have a very slanted rear tooth flank. In this manner, a collision with the teeth of the hollow wheel  37  is to be largely avoided. The toothless circumferential section  36  extends approximately over an angle of 45°. 
     Between the spur wheel  35  and the toothed wheel  31 , a finger-like, projecting stop  55  is connected to the toothed wheels  31 ,  35  so as to be non-rotatable. The stop  55  serves to fix the position of the spur wheel  35  in the so-called freewheeling position, i.e. when the teeth of the spur wheel  35  and of the hollow wheel  37  are not engaged with each other. The stop is supposed to achieve that the spur wheel  35  and the hollow wheel  37  remain in the free-wheeling position with respect to each other until the drive, that is to say, the actuation shaft  11 , has turned by 90°. For this purpose, the hollow wheel  37  has a shoulder on the end face in the form of a circular cylinder segment. This shoulder is designated with the reference numeral  57 . The shoulder ends after about 90° in an indentation  59 . When the spur wheel  35 , together with the stop  55 , is rotated clockwise by about 50°, so that the stop  55  is in the position shown by the broken lines, then it strikes the shoulder  57 . The spur wheel  35  cannot turn any further and it is in the free-wheeling position. The hollow wheel  37  is driven in the direction of the arrow L. As soon as the indentation  59  has been turned to the stop  55 , the latter can swivel past the shoulder  57  and the spur wheel  35  can continue to turn. 
     All of the toothed wheels shown can be easily made of plastic in order to avoid the need for lubrication of the teeth and to reduce the manufacturing costs. 
     The arrangement shown can also be used with a vehicle door or with an engine hood in order to serve as a closing means, whilst this embodiment should also have a lever arrangement in order to once again engage the spur wheel  35  with the hollow wheel  37  beyond the release position when the electric motor is current-free, so as to make the return stop go into action. As an alternative, it is also possible to provide an energy buffer, e.g. the battery or an auxiliary battery, that causes the spur wheel  35  to engage with the hollow wheel  37  once again beyond the release position. 
     The electric motor is a brushless, highly dynamic direct current motor.

Summary:
The invention relates to a drive for a flap provided on a vehicle, especially an engine hood. The drive comprises an electric motor, an actuation shaft that is connected to the flap, and a reduction gear by means of which the rotor of the electric motor is coupled to the actuation shaft. The drive further comprises an energy accumulator by means of which the actuation shaft can be driven independently of the electric motor. The reduction gear is configured in such a way that the actuation shaft is driven in a rotational direction only by the energy accumulator and the electric motor drives the actuation shaft in an opposite rotational direction, thereby supplying to the energy accumulator an energy that is needed to drive the actuation shaft in the rotational direction. The invention further relates to a pedestrian protection means provided on a vehicle and equipped with such a drive.