Engine brake of an internal combustion engine
BACKGROUND OF THE INVENTION 
     This invention relates to an engine brake of an internal combustion engine for motor vehicles, comprising an exhaust valve controlled by a cam via a follower lever, and a device for leaving the exhaust valve open by a small gap during the compression stroke. 
     DESCRIPTION OF THE PRIOR ART 
     Engine brakes of this type are known and represent retarders which are integrated in the engine. Leaving the exhaust valve open by a small gap during the compression stroke is a known idea which has already been put into practice by companies such as Mack and the Jacobs Manufacturing Company. 
     Mack published their findings in the Feb. 15, 1971 issue of the magazine &#34;Automotive Industry&#34; (pp 49 to 52), in addition to a description in the SAE Paper 710557. The valve concerned in this instance is located on the rocker shaft and controls the oil pressure in two oil bores on the rocker shaft in such a way that a hydraulic element in the rocker arm is lifted continuously during normal operating conditions, gliding over a relief in the base circle of the cam, and that this element becomes hydraulically rigid upon braking, and that the exhaust valve is kept open even when the element is in contact with the base circle of the cam. 
     The exhaust brake designed by Jacobs is described in the SAE Paper No. 387 A. In this design a hydraulic element presses directly against the exhaust valve. The elements and their control mechanism are contained in a rather complex housing which is mounted on the cylinder head. This type of construction is shown in European Patent No. 0 111 232, in combination with an oil pump. The disadvantage of this exhaust brake is the expensive additional component required. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to avoid the disadvantages of the known types of engine brake and to design an engine brake of similar effectiveness, which should be simpler and cheaper, however. 
     According to the invention the support bearing of the follower lever is designed to be adjustable in the direction of the supporting force between two extreme positions, i.e. one corresponding to normal operation and the other one to braking, such that on braking the exhaust valve is lifted by 0.3 to 3 mm, preferably 0.5 to 2 mm, during the phase when the base circle of the cam is in contact with the follower lever. 
     In a further development of the invention the support bearing of the follower lever is provided by a plunger which is located in a housing inserted in the cylinder head, and is permitted to move axially in the direction of the supporting force of the follower lever within a given travel range, and whose end facing away from the support bearing of the follower lever is acted upon by pressurized oil, which will force the plunger into the extreme position corresponding to braking operation. This type of design makes use of the pressurized oil already provided by the internal combustion engine, and thus facilitates control. The control mechanism of this type of engine brake may also be coupled to the ordinary braking system of the vehicle. 
     In a preferred variant of the invention the housing within which the plunger slides axially is inserted in the cylinder head in the direction of the supporting force of the follower lever, where it surrounds a support bearing of this lever, while its end facing away from the support bearing of the follower lever carries a nut which is screwed into an internal thread of the housing and which has an inlet opening for the pressurized oil flowing in through a bore in the cylinder head, for instance. 
     If the engine brake is to be controlled mechanically instead of hydraulically, the support bearing of the follower lever is configured as a threaded part which cooperates with a nut integrated in the cylinder head, and which, together with the support bearing, can assume two extreme positions, one for normal operation and the other one for braking. In this instance it may be of advantage if the threaded part is a screw bolt and if the nut is configured as a flange that is secured against turning on the cylinder head, and if the screw bolt has a pinion rigidly attached to its lower end, which meshes with a toothed rod and whose upper end is configured as a pan serving as the support bearing of the follower lever. 
     According to another variant of the invention the threaded part may also be configured as a screw bolt that is tightly fitted into the cylinder head, for instance by pressing, while the nut screwed onto this bolt may serve as a support bearing for the follower lever and may be adjusted by a control rod attached to it. This variant is characterized by its great simplicity of design. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     Following is a more detailed description of the invention as illustrated by the attached drawings, in which 
     FIG. 1 is a sectional view of an engine brake as described by the invention, 
     FIG. 2 is a sectional view of another variant of the invention along the cylinder axis, and 
     FIGS. 3, 4, 5 give different variants of a detail which may be used with the configurations of both FIG. 1 and FIG. 2. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The cylinder head 1 contains an exhaust port 2 with the exhaust valve 3. This exhaust valve 3 has a conical seat surface 4; the corresponding valve seat in the cylinder head 1 is marked 5. At the end of the valve stem 3&#39; is provided a valve spring retainer 6 holding the valve spring 7, such that the ball-shaped pivot 9 of the follower lever 8 is pressed against the support bearing 10, while the roller 11 of the lever is pressed against the cam 12. In the braking position of the support bearing 10 shown here the roller 11 is in contact with the cam 12 along its base circle 12&#39;, and the exhaust valve 3 is left open by a small gap 13 which is achieved by lifting the pivot 9 of the lever 8 by means of the support bearing 10. 
     The support bearing 10 is axially moveable in the direction of an axis 10&#39;, which is identical with the direction of the supporting force P of the follower lever 8 on the support bearing 10, the upper extreme position shown here corresponding to braking operation and the lower extreme position corresponding to normal operation. 
     In the braking position a valve lift of 0.3 to 3 mm is achieved, preferably 0.5 to 2 mm. This will prevent the air from being compressed, forcing it into the exhaust pipe through the gap 13 of the exhaust valve 3, which acts as a throttle. The energy carried by the air is transformed into heat at the throttle, and is given off partly to the exhaust pipe and partly to the cooling water of the engine. In this way the engine is protected from excessive cooling even during prolonged downhill runs. 
     With this arrangement the braking power may be doubled as compared to a conventional exhaust brake, and no other engine-independent brakes are needed that would be heavy and expensive. 
     In the variant shown in FIG. 2 all parts that are identical with those in FIG. 1 have the same reference numbers. The exhaust valve 3 is actuated from the cam 12 located below the cylinder 14 via the follower lever 8, the tappet push-rod 15 and the rocker lever 16. The pivot 9 of the follower lever 8 rests on the support bearing 10. In the braking position shown here the exhaust valve 3 is slightly lifted from its seat surface 5 at the cylinder head 1, thus forming the gap 13, which may be 0.5 to 2 mm wide. The roller 11 of the follower lever 8 is in contact with the base circle 12&#39; of the cam 12. 
     The crankcase is marked 17, the piston 18 and the connecting rod 19; these parts are not described here any further, however, as they are not vital to the invention. 
     The support bearing 10 may be actuated hydraulically (FIG. 3) or mechanically (FIGS. 4, 5). 
     In the hydraulic variant as presented in FIG. 3 a plunger 21 is provided, which is axially moveable in a housing 20, and whose upper end is shaped as a pan 22 receiving the pivot 9 of the follower lever 8. The housing 20 is inserted into the cylinder head 1 and carries a nut 23 which is fastened to the lower end of the housing by an interior thread 20&#39;, and which serves as the lower stop 21a of the plunger 21 and contains the inlet opening 24 for the pressurized oil fed through bore 25 in the cylinder head 1. The upper stop of the plunger 21 representing the moveable support bearing is marked 21b; the vent bore is referred to as 26. 
     Upon braking pressurized oil is pressed into the bore 25, and the plunger 21 is lifted to its upper stop 21b. During normal operation of the engine the pressure in the bore 25 is reduced and the plunger 21 is pressed downward by the cam 12, or rather the camshaft. In this position of the support bearing the exhaust valve will remain closed during the base circle phase of the cam. 
     The mechanical variant of the moveable support bearing as presented in FIG. 4 provides a threaded part 28 with a pinion 29 at its lower end meshing with a toothed rod 27, and a pan 22 at its upper end receiving the pivot 9 of the follower lever 8. The nut 30 is configured as a flange and is secured on the cylinder head 1 by screws 31 to prevent it from turning. 
     In the mechanical variant of the moveable support bearing as described in FIG. 5 the threaded part 28&#39; is press-fitted into the cylinder head 1 by means of a cylindrical pin 28&#34;. The nut 32 is turned via a lever 33 by the control rod 34 whose ball-joint is shown here. The pan 22 is provided by the upper end of the nut 32.