WO2002042612A1

WO2002042612A1 - Combustion engine comprising an engine brake function

Info

Publication number: WO2002042612A1
Application number: PCT/SE2001/002591
Authority: WO
Inventors: Petter Johnson; Håkan FRANSSON
Original assignee: Scania Cv Ab (Publ)
Priority date: 2000-11-27
Filing date: 2001-11-23
Publication date: 2002-05-30
Also published as: DE10196964T1; SE0004355D0; AU2002218594A1; SE520346C2; SE0004355L

Abstract

The present invention relates to a combustion engine which incorporates an engine braking function. The combustion engine incorporates a camshaft (1) with a peripheral guide surface (2) which includes a first cam (3) which, when a non-compresssion state prevails in the combustion chamber (20), is designed to initiate a control movement for opening the exhaust valve (18), and a second cam (4) which, when a compression state prevails in the combustion chamber (20), is designed to initiate a control movement for opening the exhaust valves (18, 19) against a compression pressure in the combustion chamber (20) so that engine braking is provided, and a motion-transmitting mechanism (5-17) designed to transmit said control movements. The motion-transmitting mechanism (5-17) incorporates a first clearance (11) for eliminating the action of the second cam (4), and a variable-length clearance-adjusting device (13) which, in an engine-braking state, is designed to bridge a second clearance (14) in the motion-transmitting mechanism (5-17.

Description

Combustion engine comprising an engine brake function

BACKGROUND TO THE INVENTION AND STATE OF THE ART

The invention relates to a combustion engine which incorporates an engine brake function according of the preamble to patent claim 1.

So-called compression braking of a vehicle involves the combustion engine's exhaust valves opening when a compression state prevails in a combustion chamber which is usually enclosed in the cylinder and bounded by a piston which is arranged for movement. At least part of the medium pressurised by the piston in the combustion chamber then flows out, which means that some of the energy generated during the piston's compression stroke cannot be recovered during the subsequent expansion stroke. The vehicle thus receives a braking effect. Such a compression brake is mainly used in heavy vehicles as an auxiliary brake to reduce wear on the vehicle's wheel brakes.

SE 470 363 refers to an arrangement for obtaining engine braking of a multi-cylinder combustion engine. In that case a camshaft has a peripheral guide surface with an extra cam which, via a motion- transmitting mechanism, is designed to cause opening of the ordinary exhaust valves when a compression state prevails in the combustion chamber. Such an extra cam has a relatively small height level relative to an ordinary cam which initiates ordinary opening of an exhaust valve. A clearance in the motion-transmitting mechanism then results in no action of the extra cam when engine braking is not desired. When engine braking is desired, a variable-length clearance-adjusting arrangement is activated so that the clearance can be bridged. When the clearance is bridged, the motion-transmitting mechanism also transmits control movements which are initiated by the extra cam, and opening of the exhaust valve against a compression pressure in the combustion chamber takes place. A disadvantage in that case is that the exhaust valve has a longer ordinary stroke during engine braking, when no clearance occurs, than during operation without engine braking, when there is a clearance. The exhaust valve's components have therefore to be dimensioned for the longest valve stroke.

US 4473047 refers to another system for engine braking of a combustion engine. The combustion engine incorporates combustion chambers with two exhaust valves which are opened by a motion- transmitting mechanism during operation of the engine. In this case a separately arranged hydraulic control device opens one of the exhaust valves when engine braking is desired. The arrangement of a separate control device requires a relatively large space, and an extra motion- transmitting mechanism has to be provided to control the movements of the control device.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a combustion engine which has a design which not requires a large space and which gives the exhaust valve a constant stroke length at the time of ordinary opening both with and without engine braking. Another object is to provide a design which makes opening of the exhaust valve possible against a high compression pressure in the combustion chamber without excessive stressing of the camshaft's guide surface.

The first object indicated is achieved with the combustion engine mentioned in the introduction which is characterised by what is indicated in the characterising part of patent claim 1. When the clearance-adjusting device is not activated and engine braking is not taking place, control movements from the camshaft's guide surface have thus always to pass via the first clearance, which incorporates play. The amount of play is such as to totally eliminate the action of the second cam. When engine braking is desired, the clearance-adjusting device is activated so that it bridges the second clearance, and the play in the motion-transmitting mechanism is eliminated. Control movements initiated from the second cam may then be transmitted via the motion-transmitting mechanism in order to open the exhaust valve. A suitably designed motion-transmitting mechanism makes it possible for controlled movements up to a third degree of opening of the exhaust valve to pass via the second clearance, while control movements which initiate a larger degree of opening of the exhaust valve are made to pass via the first clearance. As the control movement for the final ordinary exhaust valve lift is transmitted via the first clearance both with and without engine braking, the same exhaust valve stroke length is obtained in both cases. The motion-transmitting mechanism requires only a small number of extra components as compared with a conventional combustion engine in order to provide engine braking. The extra components also require relatively little space. The control movement may also be transmitted via either the first or the second clearance, resulting in two alternative transmission paths in a subregion of the motion-transmitting mechanism. The possibility is thus created of using a suitable design of the motion-transmitting mechanism to reduce the power transmission from the combustion pressure in the combustion chamber to the camshaft's guide sur ace during an exhaust valve opening phase.

According to a preferred embodiment of the present invention, said motion-transmitting mechanism incorporates a rocker arm which is supported for pivoting about a spindle and the clearance-adjusting device is firmly arranged on the rocker arm. Such a rocker arm constitutes a natural and functional location for fastening the clearance-adjusting device. With advantage, the motion-transmitting mechanism incorporates a first section designed to transmit a control movement from the camshaft's peripheral guide surface to the rocker arm, and a second section designed to transmit the rocker arm's control movements in order to control the position of the exhaust valve, and the first and second clearances are arranged between the rocker arm and the second section of the motion-transmitting mechanism. It is advantageous that the first and second clearances be situated at the transition between the rocker arm and the second section of the motion-transmitting mechanism. The first and second clearances may then be arranged substantially parallel and at different distances from the rocker arm spindle. The second clearance is with advantage arranged at a shorter distance from the rocker arm spindle that the first clearance. This results in a shorter lever from the rocker arm spindle to the second clearance than to the first clearance. Control movements transmitted from the opposite side of the rocker arm via the second clearance are thus provided with a shorter displacement movement but more force than the movements transmitted by the first clearance. The advantage of a relatively short lever from the spindle to the second clearance is that comparatively large forces can be transmitted via the second clearance in order to open the exhaust valve against the action of the compression pressure without the cam surface on the opposite side of the rocker arm spindle being subjected to excessive stresses. A further advantage of such locations for the first and second clearances is that the rocker arm, during its rotation about the spindle, causes a longer displacement movement via the first clearance than via the second clearance. The motion-transmitting components involved are so dimensioned that control movements which initiate exhaust valve opening, during engine braking, up to the third degree of opening pass via the second clearance, while control movements which initiate a greater degree of exhaust valve opening pass via the first clearance. The final exhaust valve control movement up to a first maximum degree of opening is thus always transmitted via the first clearance. The exhaust valve is thus provided, during ordinary opening movements, with the same length of stroke regardless of whether engine braking is taking place or not.

According to another preferred embodiment of the present invention, the combustion chamber incorporates two exhaust valves. It is usual for two exhaust valves to be used per combustion chamber in combustion engines for heavy vehicles. With advantage, one of the two exhaust valves is designed to open in order to allow engine braking. Opening only one instead of two exhaust valves requires less force and results in less stressing of the camshaft's guide surface. With advantage a control movement is transmitted via the first clearance from a stop surface on the rocker arm to a corresponding stop surface on a transverse element which is designed to transmit a substantially similar control movement to both of the exhaust valves. The two exhaust valves are thus provided with a simultaneous and similar ordinary opening movement when engine braking is not taking place. With advantage, a control movement is transmitted via the second clearance from a stop surface on the clearance-adjusting device to a corresponding stop surface on an elongate element which extends movably through the transverse element so as to allow, during its movement, opening of the exhaust valve. Such a movable element results in a simple design which provides exhaust valve opening against a compression pressure in the combustion chamber so that engine braking can be obtained. According to another preferred embodiment of the present invention, the clearance-adjusting device incorporates a hydraulic cylinder. The movements of the hydraulic cylinder may be controlled by hydraulic oil obtained from ducts in the rocker arm and its spindle. Such a hydraulic cylinder may be single-acting. With advantage, the combustion engine incorporates an overload device designed to limit the force which the clearance-adjusting device is allowed to transmit in the second clearance. Such an overload device may incorporate an overpressure valve which opens when the hydraulic pressure is too great. Thus the hydraulic cylinder can only transmit a force up to a certain value via the second clearance, thus ensuring that the camshaft's guide surface is not subjected to excessive pressure. Excessive pressure may otherwise occur in situations where the engine brake is activated at a time when a substantially maximum compression pressure prevails in the combustion chamber. In this case, such an overpressure valve results in the exhaust valve not opening against excessive pressure but opening instead during the camshaft's next turn when a compression pressure more appropriate to valve opening prevails in the combustion chamber. It is desirable, however, for exhaust valve opening to take place when as high a compression pressure as possible prevails in the combustion chamber in order to provide a good braking effect. The measures described above for reducing the transmitted pressure acting on the camshaft's guide surface make it possible for the exhaust valve to open against a higher compression pressure than in the case of conventional engine brakes.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the invention is described below by way of example with reference to the attached drawings, in which: Fig. 1 depicts part of a combustion engine which incorporates components for providing engine braking, and

Fig. 2 depicts schematically in curve form the opening movements of the exhaust valves depicted in Fig. 1 relative to the camshaft's rotational position.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

Fig. 1 depicts a combustion engine which incorporates a camshaft 1 which rotates at a speed which is related to the speed of the engine's crankshaft during operation of the combustion engine. The camshaft 1 incorporates a peripheral guide surface 2 with an ordinary first cam 3 and an extra second cam 4. A roller 5 is designed to roll along the peripheral guide surface 2. The roller 5 is arranged for rotation in a holder 6 which is connected to one end of an arm 7. The other end of the arm 7 is connected to one end of a rocker arm 8. The roller 5, the holder 6 and the arm 7 form part of a first section of a motion-transmitting mechanism designed to transmit a control movement from the camshaft's peripheral guide surface 2 to the rocker arm 8. The rocker arm 8 is supported for pivoting about a transverse spindle 9. The rocker arm 8 has at a second end, on the opposite side from the spindle 9, a stop head 10 with a stop surface 10a. A first clearance 11 in the motion-transmitting mechanism is provided between the stop surface 10a of the stop head 10 and a corresponding stop surface on a transverse element 12. A variable- length clearance-adjusting device in the form of a hydraulic cylinder 13 is firmly arranged on the rocker arm 8. The hydraulic cylinder 13 incorporates a forward stop surface 13a. A second clearance 14 in the motion-transmitting mechanism is provided between the stop surface 13a of the hydraulic cylinder 13 and a corresponding upper stop surface on an elongate element 15. The elongate element 15 is arranged to move through an aperture in the transverse element 12. The hydraulic cylinder 13 is designed to change its length, in an activated state, so that it bridges the second clearance 14 when engine braking is desired. When engine braking is not desired, the hydraulic cylinder 13 is made pressureless and cannot then transmit any pressure force to the elongate element 15. The hydraulic cylinder 13 retracts if it is subjected to loading during a pressureless state, in which case the control movements of the rocker arm 8 are transmitted instead via the first clearance 11. The hydraulic cylinder 13 also incorporates an overpressure valve designed to lead hydraulic oil out from the cylinder 13 if a maximum acceptable pressure is exceeded in the cylinder 13. The overpressure valve, which is not depicted in Fig. 1 , may for example be incorporated in the hydraulic cylinder 13 and include a valve seat with a spring-loaded ball which opens at a certain overpressure. The hydraulic cylinder 13 is activated by supply of hydraulic oil via ducts 16 which extend through the spindle 9 and the rocker arm 8. These ducts 16 also incorporate with advantage a control and check valve arrangement 17. The transverse element 12 incorporates a first releasable connection to an upper portion of a first exhaust valve 18, and a second connection to an upper portion of a second exhaust valve 19. The transverse element 12 and the elongate element 15 form part of a second section of the motion-transmitting mechanism which is designed to transmit control movements of the rocker arm 8 in order to control the position of the exhaust valves 18, 19. Each of the exhaust valves 18, 19 are openable by a displacement movement downwards against the action of its springs, which are not depicted in Fig. 1. The open state of the exhaust valves 18, 19 allows the medium contained in a combustion chamber 20 of the combustion engine to flow out to an exhaust arrangement. The combustion chamber 20 is enclosed in a conventional manner in a cylinder 21 and bounded downwards by a piston 22 which is arranged for movement.

When the combustion engine is running without engine braking, the hydraulic cylinder 13 is not activated. This means that control movements can only be transmitted via the first clearance 11. When the roller 5 runs over the second cam 4, which has a relatively small height level, a corresponding relatively small control movement in the motion- transmitting mechanism is initiated. The arm 7 moves upwards and pivots the rocker arm 8 so as to cause a corresponding downward movement on the opposite side of the rocker arm 8, but the first clearance 11 is so dimensioned that the stop head 10 can in this case not transmit a displacement movement to the transverse element 12.

When the roller 5 runs thereafter over the first cam 3, which has a considerably higher height level than the second cam 4, a considerably larger control movement is initiated in the motion-transmitting mechanism. As the hydraulic cylinder 13 is inactive, here again no control movement is transmitted via the second clearance 14, but when the roller 5 reaches a height level on the first cam 3 which exceeds the height level of the second cam 4, the stop surface 10a of the stop head 10 comes into contact with the stop surface of the transverse element 12 and begins to move the transverse element 12 downwards. At the same time, the transverse element 12 moves the exhaust valves 18, 19 downwards so that they gradually open to a first maximum degree S_Ϊ so that the combustion gases in the combustion chamber 20 flow out. Fig. 2 depicts as an unbroken line the degree of opening s of the exhaust valves 18, 19 as a function of the rotation angle v of the camshaft 1 when control movements are transmitted via the first clearance 11 , which is always the case when engine braking is not taking place. At this stage no opening is initiated by the second cam 4 but the exhaust valves 18, 19 are opened simultaneously by the ordinary cam 3 and follow in a similar manner the unbroken curve so that they reach a corresponding maximum first degree of opening s_!.

When engine braking is desired, the hydraulic cylinder 13 is activated. The length of the hydraulic cylinder 13 changes so that its forward stop surface comes to abut against the stop surface on the elongate element 15. The second clearance 14 is thereby bridged. When the roller 5 runs over the second cam 4, which has a relatively small height level, a corresponding control movement is initiated in the motion-transmitting mechanism. The arm 7 moves upwards and pivots the rocker arm 8 so that a corresponding downward movement takes place on the opposite side of the rocker arm 8. As the hydraulic cylinder 13 is now active and bridging the second clearance, the control movement will be transmitted via the second clearance 14 and move the elongate element 15 downwards. The elongate element 15 opens the first exhaust valve 18 against the action of the previously mentioned spring but mainly against the action of the compression pressure now prevailing in the combustion chamber 20. At this stage a relatively large force is required to open the first exhaust valve 18, but as the second clearance 14 is arranged at a relatively short distance from the spindle 9 of the rocker arm 8, a relatively large force is obtained here without the guide surface 2 of the camshaft 1 being excessively stressed by the roller 5. The first exhaust valve 18 is opened by the second cam 4 to a second degree s₂ represented by the broken line in Fig. 2. The second degree s₂ is thus related to the height level of the second cam 4. At least part of the air compressed by the piston 22 in the combustion chamber 20 can thus flow out. The power generated during the compression stroke of the piston 22 can therefore not be utilised during the subsequent expansion stroke. The result is engine braking. When the roller 5 has passed the second cam 4, the motion-transmitting mechanism receives an opposite movement and the first exhaust valve 18 closes almost completely.

When the roller 5 thereafter runs up onto the first cam 3, which has a considerably greater height level than the second cam 4, a considerably larger movement of the motion-transmitting mechanism is initiated. The arm 7 pivots the rocker arm 8 in relation to the increasing height level on the second cam 4. As engine braking is taking place, the hydraulic cylinder 13 is therefore active and bridges the second clearance 14. This means that the control movement from the first cam 3 will initially be transmitted via the second clearance 14 and move the elongate element 15 downwards at the same time as the first exhaust valve 18 opens. The opening movement of the first exhaust valve 18 is represented by the broken line in Fig. 2. At this stage relatively little force is required to open the exhaust valve 18 when substantially no compression pressure prevails in the combustion chamber 20. As soon as the first cam 3 has transmitted a control movement which exceeds the height level s₂ of the second cam, the stop head 10 comes into contact with the transverse element 12 via the first clearance 11. During the continuing downward movement of the stop head 10, the transverse element 12 moves downwards so that the second exhaust valve 19 opens. The opening movement of the second exhaust valve 19 is thus always transmitted via the first clearance 11. The exhaust valve 19 thus has, both with and without engine braking, an identical opening behaviour represented by the continuous curve in Fig. 2. The first clearance 11 being arranged at a greater distance from the spindle 9 of the rocker arm 8 than the second clearance 14 results, during rotational movement of the rocker arm 8, in a greater downward movement of the transverse element 12 than of the elongate element 15. When the roller 5 reaches a certain height position on the first cam 3, the second exhaust valve 19 will have reached a degree of opening s₃ which is the same as the degree of opening of the first exhaust valve 18. At this stage the transverse element 12 is in contact with the upper portion of the first exhaust valve 18. The transverse element 12 takes over, from the s₃ degree of opening onwards, the continuing opening movement of the two exhaust valves 18, 19 until they reach a maximum s-i degree of opening. When the highest level of the first cam 3 has been passed, the rocker arm 8 moves in an opposite direction and the exhaust valves 18, 19 close gradually in a manner corresponding to that during the opening phase. The transverse element 12 controls the closing movement of the two exhaust valves 18, 19 until the s₃ degree of opening is reached. Thereafter only the closing movement of the second exhaust valve 19 is controlled by the transverse element 12. The second exhaust valve 19 is thus provided with a quicker closing movement than the first exhaust valve 18 which is controlled by the upward movement of the elongate element 15, which is represented by the final broken curve.

As the control movements which cause the si maximum degree of opening are thus always transmitted via the first clearance 11 , the respective stroke lengths of the two valves 18, 19 are the same with and without engine braking. This is an advantage, since the components of the exhaust valves 18, 19 have otherwise to be dimensioned to cope with the requirements which occur at the longest valve stroke.

The present invention is in no way limited to the embodiment depicted in the drawings but may be varied freely within the scopes of the patent claims. For example, the invention may also be applied to combustion engines which have one exhaust valve per combustion chamber. The embodiment described above refers to a control for providing an engine braking function by opening the exhaust valves of one cylinder of the combustion engine, but the invention is of course intended primarily to be applied so as to achieve such opening of the exhaust valves of all the cylinders of the combustion engine. The combustion engine is with advantage a diesel engine designed to power a heavy vehicle such as, for example, a load-carrying vehicle or a bus.

Claims

Patent claims

1. A combustion engine which incorporates an engine braking function whereby the combustion engine incorporates at least one combustion chamber (20) with at least one exhaust valve (18), a camshaft (1) with a peripheral guide surface (2) which incorporates a first cam (3) which is designed, when a non-compression state prevails in the combustion chamber (20), to initiate a control movement for opening the exhaust valve (18) to a first degree (s^, and a second cam (4) which, when a compression state prevails in the combustion chamber (20), is designed to initiate a control movement for opening the exhaust valve (18) to a second degree (s₂) against a compression pressure in the combustion chamber (20) so that engine braking is obtained, and a motion- transmitting mechanism (5-17) which is designed to transmit said control movements and incorporates a first clearance (11) via which control movements, in a non-engine-braking state, are designed to be transmitted so that the action of the second cam (4) is eliminated, characterised in that the motion-transmitting mechanism (5-17) incorporates a variable-length clearance-adjusting device (13) which is designed so that, in an engine-braking state, it bridges a second clearance (14) in the motion-transmitting mechanism (5-17) so that, in the engine-braking state, control movements are transmitted via the second clearance (14) until the exhaust valve (17) opens to a third degree (s₃) which at least corresponds to the second degree (s₂) but is smaller than the first degree (s^, and control movements which initiate opening of the exhaust valve (18) beyond the third degree (s₃) are transmitted via this first clearance (11).

2. A combustion engine according to claim 1 , characterised in that said motion-transmitting mechanism (5-17) incorporates a rocker arm (8) hich which is supported for pivoting about a spindle (9) and that the clearance- adjusting device (13) is firmly arranged on the rocker arm (8).

3. A combustion engine according to claim 2, characterised in that said motion-transmitting mechanism (5-17) incorporates a first section (5-7) designed to transmit a control movement from the peripheral guide surface (2) of the camshaft (1) to the rocker arm (8), and a second section (12, 15) designed to transmit the control movement of the rocker arm (8), in order to control the position of the exhaust valve (18), and that the first clearance (11) and the second clearance (14) are arranged between the rocker arm (8) and the second section (12, 15).

4. A combustion engine according to claim 2 or 3, characterised in that the second clearance (14) is arranged at a shorter distance from the rocker arm spindle (9) than the first clearance (11).

5. A combustion engine according to any one of the foregoing claims, characterised in that the combustion chamber (20) incorporates two exhaust valves (18, 19).

6. A combustion engine according to claim 5, characterised in that one (18) of the two exhaust valves (18, 19) is designed to open in order to allow engine braking.

7. A combustion engine according to claims 2 and 6, characterised in that a control movement is transmitted in the first clearance (11) from a stop surface (10a) on the rocker arm (8) to a stop surface on a transverse element (12) which is designed to transmit a substantially similar control movement to the two exhaust valves (18, 19).

8. A combustion engine according to claim 7, characterised in that a control movement is transmitted in the second clearance (14) from a stop surface (13a) on the clearance-adjusting device (13) to a stop surface on an elongate element (15) which extends movably through the transverse element (12) so as to allow opening of the exhaust valve (18).

9. A combustion engine according to any one of the foregoing claims, characterised in that the clearance-adjusting device incorporates a hydraulic cylinder (13).

10. A combustion engine according to any one of the foregoing claims, characterised in that the combustion engine incorporates an overload device designed to limit the force which the clearance-adjusting device (13) is allowed to transmit in the second clearance (14).