Cam for actuating a valve mechanism of an internal combustion engine

Abstract

Cam for actuating a valve mechanism of an internal combustion engine, having a cam base circle (33), a main cam region (32) and a pre-cam region (31), which is formed between the cam base circle (33) and main cam region and directly precedes a rising section (32a) of the main cam region (32), wherein the pre-cam region (31) has a profile which effects an increase in the cam lift speed followed by a decrease in the cam lift speed followed by a further increase in the cam lift speed.

Description

FIELD OF THE INVENTION

[0001] The invention relates to a cam for actuating a valve mechanism in accordance with the preamble of Patent claim 1, and also to a correspondingly designed valve mechanism. Furthermore, the invention relates to a valve arrangement.

[0002] Hydraulic play adjustment elements (hydraulic valve actuation elements, HVA elements) for valve mechanisms of internal combustion engines are known.

[0003] In a conventional hydraulic valve actuation (HVA) element, for example, a bucket tappet is subjected to load by means of a cam, thereby changing the distance between a piston and an inner housing of the bucket tappet. When the cams are descending, a valve actuation spring presses the piston on which the load has been relieved (with the cam descending) upwards until the tappet bears against the cam or the cam base circle. The increase in space in a working space beneath the pin causes oil to flow out of a storage space through a ball valve into the working space. When the cam is rising, the piston is subjected to load, the ball valve closes and the oil filling in the working space acts as a hydraulically fixed connection. The valve in the interior of the bucket tappet is typically provided with a valve ball, a valve ball spring, a valve seat, a valve cap and a restoring spring, and in the conventional design the valve ball spring applies a load to the valve ball towards the valve seat. In the event of suitable actuation by means of the cam or the bucket tappet, the valve opens or closes.

[0004] Furthermore, there are hydraulic valve actuation elements in which the valve arrangement described comprises a valve which is initially open as the cam starts to rise. Valves of this type are known as freeball valves, since in the load-free state the valve ball can move freely within the high-pressure region. In the event of an appropriate movement of the piston in the bucket tappet, the pressure in the high-pressure region rises, so that oil or hydraulic fluid flows past the valve ball into the oil storage space. When the liquid is at a sufficient velocity, friction or drag forces acting on the valve ball cause the valve to be closed, with the valve ball coming to bear against the valve seat. This blocks fluid communication between high-pressure region and storage space, resulting in the desired nonpositively locking connection (hydraulically fixed connection) for actuation of an engine valve.

[0005] Furthermore, what are known as reverse spring valves, in which a valve spring which presses the valve ball away from the valve seat in the load-free state is provided between the valve seat and the valve ball, are known for better or more reproducible control of a valve ball of this type. In this case too, therefore, the valve is initially open when the cam is descending. A considerable advantage of reverse spring valves of this type is that the ability of the valve to function can be ensured uniformly in various orientations in the space and a very wide range of operating angles.

[0006] However, it should be pointed out that the valve closing operation (to rapidly provide a nonpositively locking connection for actuating the engine valve) is slower than with the freeball valve, since the frictional force which is to be applied by the f lowing hydraulic fluid also has to overcome the force of the spring provided between valve seat and valve ball before the valve will close.

[0007] A reverse spring valve of this type or a hydraulic valve actuation system designed with a valve of this type is known from U.S. Pat. No. 5,758,613.

[0008] Reverse spring and freeball valves produce a greater idle lift than conventional valves. To ensure that the engine valve still opens and closes acceptably, it is necessary to increase the cam ramp height in order to boost the cam lift speed. Hitherto, this higher cam ramp has been designed as a constant-speed ramp. However, a higher constant speed cam ramp lengthens the cam profile, and also a defined constant speed value cannot be exceeded. This resulted in a creeping opening and closing of the engine valve. Overall, reverse spring and freeball valves produce an additional idle lift compared to conventional hydraulic valve actuation elements.

OBJECT OF THE INVENTION

[0009] The invention is based on the object of providing a hydraulic valve actuation element (HVA element) which takes better account of the mode of operation of reverse spring and freeball HVAs than conventional solutions.

SUMMARY OF THE INVENTION

[0010] According to the invention, this object is achieved by a cam for actuating a valve mechanism having the features of Patent claim 1, a corresponding valve mechanism having the features of Patent claim 5 and a valve arrangement having the features of Patent claim 9.

[0011] The configuration according to the invention of a cam for actuating a valve mechanism can be used to effectively reduce idle lift which customarily arose to an increased extent when using freeball or reverse spring HVAS. The provision of a cam of this type entails no additional costs whatsoever compared to the provision of conventional cams.

[0012] Advantageous configurations of the invention form the subject matter of the subclaims.

[0013] The pre-cam region advantageously has a profile comprising a first section for providing a progressive, in particular substantially linear increase in a cam lift speed, a second section, which follows the first section, for providing a decrease in the cam lift speed, a third section, which follows the second section, for providing a substantially constant cam lift speed, and a fourth section, which follows the third section, for providing a further increase in the cam lift speed. In this context, the fourth section represents the transition to the main cam region, by means of which an engine valve can be actuated. According to the invention, the valve ball comes to bear against the valve seat during the third section, with the result that the HVA valve is closed. According to the invention, a very rapid rise in the cam lift speed during the first section allows the cam lift speed to drop or remain constant in the second and third sections, respectively, with the result that overall the actuation times for an engine valve can be improved compared to conventional solutions. It is expedient for the cam lift speed reached in the third section of the pre-cam region to define a value which is acceptable for engine valve opening.

[0014] It has likewise proven expedient for the second section of the pre-cam region to substantially comprise a linear decrease in the cam lift speed. A linear decrease of this nature can be effected in a relatively simple way in design terms.

[0015] It is preferable for the profile of the pre-cam region to be provided in a cam region extending over an angle range .alpha., where a may be from 0.10 to 400.

[0016] It is particularly preferred for a valve mechanism according to the invention to be designed with an HVA valve designed as a reverse spring valve or a freeball valve. The cam profile which is made available in accordance with the invention is able, in particular with valves of this type, to reduce actuation times for an engine valve which are caused by the valve clearance at the HVA valve.

[0017] A valve arrangement according to the invention with an intake valve and an exhaust valve, in which reverse spring valves or freeball valves are fitted only on the intake side, contribute to increasing the torque and power at high engine speeds. According to the invention, a low idle lift can be realized at high engine speeds. However, the idling quality is still maintained, since a relatively large idle lift is formed at an idling speed.

[0018] The provision of reverse spring valves or freeball elements only on the exhaust side contributes to effectively reducing the emissions from an internal combustion engine.

[0019] The provision of reverse spring valves or freeball valves of this type on both the intake side and the exhaust side makes a contribution both to increasing torque and power and to reducing emissions. At the same time, an idling quality which is equal to that of conventional HVA elements can be maintained.

[0020] Overall, in this context it can be concluded that the idle lift of the reverse spring valve or of the freeball valve changes as a function of an engine speed, so that when idling a relatively large idle lift is produced and at high engine speeds relatively little idle lift is produced. This is an extremely desirable state of affairs and can advantageously be taken into account when designing or calculating intersection areas of intake valves and exhaust valves.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The invention will now be described in more detail on the basis of the appended drawing, in which:

[0022] FIG. 1 shows a schematic lateral section view of an HVA valve spring arrangement designed as a reverse spring valve, in which the invention can be deployed in a particularly advantageous way,

[0023] FIG. 2 shows a preferred embodiment of a cam which is provided for actuation of a valve mechanism in accordance with the invention, and

[0024] FIG. 3 shows the lift speed of a tappet acted on by a cam as shown in FIG. 2 at constant angular velocity, plotted against the camshaft angle.

DETAILED DESCRIPTION OF THE DRAWINGS

[0025] FIG. 1 shows a region 100 of a valve mechanism of an internal combustion engine, the valve mechanism being actuated by means of a cam (not shown in this view) of a camshaft of the internal combustion engine. The valve mechanism in turn actuates an engine valve or gas exchange valve, which is likewise not shown in this view.

[0026] The region 100 of the valve mechanism which is illustrated has a piston housing 10 in which a pressure piston 12 is displaceably mounted. In the interior of the piston 12 there is a cavity 14, into which oil at engine oil pressure can be introduced.

[0027] In its underside 12a, the piston 12 has a region formed as a valve seat 16a, which can be acted on by a valve ball 16b. The valve ball 16b and the valve seat 16a, together with a valve spring 18, which is in the form of a reverse spring valve spring, and a restoring spring 9, are part of an HVA valve, which is denoted overall by 20.

[0028] In the load-free state, the valve ball 16b is held at a distance from the valve seat 16a by means of the HVA valve spring 18.

[0029] By means of the HVA valve 20, the cavity 14 is in fluid communication with a high-pressure space 22, into which oil can be introduced at high pressure.

[0030] When the piston 12 moves downwards as a result of being actuated by the cam (not shown), the volume of the high-pressure space 22 is reduced, with the result that oil flows out of the high-pressure space 22 into the cavity 14. In the process, as has already been mentioned, the HVA valve is initially open, since the valve ball 16b is at a distance from the valve seat 16a.

[0031] This flow of oil produces a frictional force on the valve ball 16b, applying a load on the latter in the direction of the valve seat 16a. When the frictional force exceeds the spring force of the HVA valve spring, and if appropriate the force of the weight of the valve ball 16b, the valve ball moves towards the valve seat 16a and reaches the closed position, in which it bears against the valve seat 16a. When the closed position of the HVA valve 20 is reached, a nonpositive lock is produced in the valve mechanism, which leads to the engine valve (not shown) being actuated.

[0032] A cam configuration according to the invention which leads to a particularly favourable lifting movement or lift speed of the cam and of the pressure piston 12, will now be explained in more detail with reference to FIGS. 2 and 3.

[0033] A cam provided on a camshaft (not illustrated in FIG. 2) is denoted by 30 in FIG. 2. The shape of the cam 30 is characterized by a relatively pointed main cam region 32 and the cam base circle 33. The pre-cam region 31, the extent of which is indicated by an angle .alpha. and which is indicated in FIG. 2, is crucial in the context of the present invention. The pre-cam region 31 is formed between the base circle 33 and the main cam region 32 immediately before the rising section 32a of the main cam region 32. Actuation of a tappet 34 results in a lifting movement which can be transmitted in a manner known per se to a piston 12 as illustrated in FIG. 1. Tappet 34 may be designed, in a manner known per se, as a bucket tappet, drag arm, rocker arm or roller tappet. The time derivative of the tappet lift is referred to as the tappet lift speed, which corresponds to the lift speed of the piston 12.

[0034] According to the invention, the cam ramp of the cam 30 is configured in such a manner in the pre-cam region 31 that different characteristic cam lift speed sections result. These sections will now be explained in more detail with reference to FIG. 3.

[0035] First of all, in a first section 1, which follows the cam base circle, the lift speed is progressively increased, initiating a movement of the valve ball 16a, i.e. the critical speed of the hydraulic oil used is reached significantly earlier with this very steep section of a progressive ramp than with conventional systems. This is followed by a section 2 which comprises reducing the lift speed to a value acceptable for the opening of the engine valve. During this phase, the valve ball 16a is moving and generates a further idle lift.

[0036] In the following constant-speed ramp section 3, the valve ball 16a comes into contact with the valve seat 16b and the HVA valve closes. This produces the nonpositive lock in the valve mechanism. The length of the constant-speed ramp at 3 is designed in such a way that mechanical and hydraulic flexibility in the valve mechanism is taken into account.

[0037] Then, in a section 4, the engine valve opens, i.e. after the HVA valve has closed and the valve mechanism has been prestressed.

[0038] Section 4 is followed by the main cam region, which is not illustrated in this figure and is defined by the cam section 32 of the cam 30.

Claims

What is claimed is:

1. A cam for actuating a valve mechanism of an internal combustion engine, having a cam base circle (33), a main cam region (32) and a pre-cam region (31), which is formed between the cam base circle (33) and main cam region (32) and directly precedes a rising section (32a) of the main cam region (32), wherein the pre-cam region (31) has a profile which effects an increase in the cam lift speed followed by a decrease in the cam lift speed followed by a further increase in the cam lift speed.

2. A cam of claim 1, wherein the pre-cam region (31) has a profile comprising a first section (1) for providing a progressive, substantially linear increase in a cam lift speed, a second section (2), which follows the first section (1), for providing a decrease in the cam lift speed, a third section (3), which follows the second section (2), for providing a substantially constant cam lift speed, and a forth section (4), which follows the third section (3), for providing a further increase in the cam speed.

3. A cam of claim 2, wherein the cam lift speed reached in the third section (3) of the pre-cam region (31) defines a value which is acceptable for engine valve opening.

4. A cam, of claim 2, wherein the second section (2) of the pre-cam region (31) substantially comprises a linear decrease in the cam lift speed.

5. A cam of claim 1, wherein the profile of the pre-cam region (31) is provided in a cam region extending over an angle range .alpha., where .alpha. is 0.1.degree. to 40.degree..

6. A valve mechanism for actuating an engine valve, by a cam of claim 1.

7. A valve mechanism of claim 6, characterized by an HVA valve designed as a reverse spring valve.

8. A valve mechanism of claim 7, characterized by an HVA valve designed as a freeball valve.

9. A valve arrangement for an internal combustion engine having an intake valve and an exhaust valve, wherein the intake valve and/or the exhaust valve can be actuated by a valve mechanism of claim 6.