GB2200967A

GB2200967A - Cam

Info

Publication number: GB2200967A
Application number: GB08803315A
Authority: GB
Inventors: Gernot Kumpel
Original assignee: Daimler Benz AG
Current assignee: Daimler Benz AG
Priority date: 1987-02-14
Filing date: 1988-02-12
Publication date: 1988-08-17
Anticipated expiration: 2008-02-12
Also published as: DE3704644A1; IT1219425B; GB8803315D0; FR2610987A1; GB2200967B; DE3704644C2; FR2610987B1; IT8847604A0

Abstract

The rising profile 5 of a cam leads into the descending profile 9 via a section 7, the start 6 of section 7 being further from the cam's axis of rotation 2 than the finish 8 of the section. The bisector 10 of the angle subtended by section 7 at axis 2 makes an angle B with a tangent to the cam's surface of less than 90 DEG . The acceleration @n of a follower is near zero over section 7. <IMAGE>

Description

Cam for controlling a gas exchange valve of a reciprocating piston engine The invention relates to a cam for controlling a gas exchange valve of a reciprocated piston engine by way of a valve actuating element which has a preset sliding surface contour, comprising a leading edge and a trailing edge or flank and a cam peak area having a sliding surface section which minimizes the occurring valve acceleration by way of a preset cam angle of rotation and which, in a first transitional area, is adjacent to the leading edge and, in a second transitional area, leads into a trailing flank.

Figs. 4 and 5 of German Offenlegungsschrift 30 37 652 disclose a so-called "spread cam" in which the peak area of the cam is in the form of a sliding face section (transition circle) which is inserted between a steep leading flank and a steep trailing flank and is concentric with the base circle of the cam. In this way a valve lift can be achieved over a relatively great area length and thus a relatively high load factor can be obtained.

However, it is precisely the flat region of the cam peak, which has a disadvantageous effect, that is particularly if the internal combustion engine is operated in excessive speed ranges (e.g. due to driving errors, such as faulty gear changing or inexpert engine brake operation). In these operating conditions the inertia forces caused by the moving valve control elements can become greater than the valve spring forces counteracting these forces so that already during sliding on the leading cam edge, a cam-operated valve actuating element (e.g.

follower, rocker arm) begins to lift clear of this cam edge and follows F ballistic curve. If this valve actuating element impinges again on the cam after the curve phase, this occurs in a region of constant cam pitch (transitional circle) resulting in extemey high forces which can bring about damage to individual valve control elements and have the effect of exciting, vibrations.

The present invention therefore seeks to provide a cam as defined, by means of which a high degree of protection against damage caused to valve control elements by excessive speeds and also a high load factor are ensured.

According to the present invention there is provided a cam for controlling a gas exchange valve of a reciprocating piston engine by way of a valve actuating element which has a preset sliding surface contour, comprising a leading edge and a trailing edge or flank and a cam peak area having a sliding surface section which minimizes the occurring valve acceleration by way of a preset cam angle of rotation and which, in a first transitional area, is adjacent to the leading edge and, in a second transitional area, leads into the trailing flank, wherein the sliding surface section intersects an imaginary bisector of the angle formed by a straight line run through the first transitional area and the cam axis of rotation and a second straight line run through the second transitional area and the cam axis of rotation, at an angle ( ) deviating from 900, and that the distance between the first transitional area and the cam axis of rotation is greater than that between the second transitional area and the cam axis of rotation.

When the valve actuating element impinges on the sliding face of the cam after the curve phase, all the valve control elements are compressed because of their elasticity, during this compressing action the cam continues to rotate through a specific cam angle of rotation. The design of the cam or sliding face section according to the invention, on which the valve actuating element may impinge in the event of excessive speeds, now causes continuous reduction of the cam pitch during this compression phase so that some of the impact energy is absorbed by the cam, i.e. the impact of the valve actuating element is damped. The resilient valve control elements thus undergo reduced compression and are consequently subjected to a smaller load.

The damping action during impact also results in further spring deflection taking place only to a reduced extent, with a subsequent renewed curve phase.

Because the cam has a sliding surface section which minimises valve acceleration, thereby achieving an opening out of the valve lift curve, valve lifts over great area lengths and thus high load factors can again be achieved in this case without the maximum valve lift having to be increased for the purpose.

The continuity of the transitional zones from the said sliding surface section into the two cam flanks prevents acceleration 'jumps' at these points.

The invention will now be described with reference to the accompanying drawings, in which: Fig. 1 shows a cam according to the invention in a cross-sectional view, Fig. 2 shows in a diagram hN = f (us), and hv = f ( < ) respectively, the valve lift (cam pitch hN) which is predetermined by the cam geometry and the actually occurring valve lift hv as a function of the cam angle of rotation , and Fig. 3 shows in a diagram hN = f ( or), the valve acceleration hN - which is predetermined by the cam geometry - as a function of the cam angle of rotation Fig. 1 shows a radial cam 3 - which rotates about a fixed axis 2 in the direction of arrow 1 - of a gas exchange valve (not shown), the cam having a base circle radius rG.The base circle 4 is followed by a leading cam edge or flank 5 which, in a transitional zone 6, passes into a sliding surface section 7 which is curved in such a manner that, while this section 7 is passed through, the occurring valve acceleration is zero (see also Figure 3).

this sliding surface section 7 is followed at the point 8 by a trailing cam edge 9 which, finally, passes back into the base circle 4 of the cam.

The sliding surface section 7 is inclined in the direction of the trailing edge 9, this section intersection the bisector 10 of the angle formed by the two straight lines 11 and 12 (straight line 11 through axis 2 and transitional zone 6; straight line 12 through axis 2 and transitional zone 8), at an acute angle 6 (g corresponds to the angle between the tangent 13 to the point 14 at which the sliding surface section 7 intersects the bisector 10). The transitional zone 6 thus forms the peak of this asymmetrical cam 3.

In order to avoid acceleration 'jumps' in the two transitional zones 6 and 8, the latter have a continuous path.

Figure 2 shows in a diagram 15 the correlation between the cam pitch hN and the cam angle of rotationcc, characterising the desired valve hN for the valve lift which is predetermined by the cam geometry, and' it also shows the interrelationship between the actual valve lift hv and the cam angle of rotation , the area in which the base circle 4 of the cam 3 ends not being shown for the sake of simplicity because in this case the valve lift is already zero. The individual sliding surface zones 5, 7 and 9 are identified in the diagram 15 by the same reference numerals as in Figure 1.If excessive speeds should occur, a valve actuating element operated by the cam according to the invention can lift clear of the leading cam edge 5 already during the passing of this cam edge and follow a ballistic curve (broken line 16), which means that now the actual valve lift hv is greater than the desired valve hN which is predetermined by the cam geometry. After the curve phase the valve actuating element then impinges again on the cam within the sliding surface on section 7.

At the same time the valve control elements, such as e.g.

the follower, push-rod, rocker arm and valve stem, are deformed as a result of occurring inertia forces so that during this time the actual valve lift hv is less than the desired valve hN (zone 22 in which the broken-line curve lies below the cam pitch curve). However, because the impact of the valve actuating lever occurs in a section in which the cam pitch h decreases in accordance with a liner function (no valve acceleration), its impact is damped, and therefore even after further spring deflection of the compressed valve control elements, lifting of the valve actuating lever clear of the cam sliding surface recurs only in an attenuated form.

0 The associated path of valve acceleration hN which is predetermined by the cam geometry is shown in the diagram 17 hs = f (g) in Figure 3. Again, the zone designations in this case correspond to those in Figure 1. It is not imperative for the section 7 to be designed in the form of a zero acceleration zone because a broadening of the range of valve lift is achieved even in the case of an acceleration path according to the curve 18 shown by dotdash lines in Fig. 3 i.e. even when there is still a slight negative acceleration. The only condition which has to be met is that in each zone to the left and right of the cam peak 6 the sum of all the surfaces formed by the graph hN = f (g) below the abscissa 21 (negative accelerations) be equal to the sum of those above the abscissa 21 (positive accelerations).

A slight remaining acceleration in the zone 7 naturally presupposes a minimally convex curved path (dotdash line 18 in Figure 2) in the valve lift diagram 15 hN = f ( & ) (Figure 2) in the zone 7, i.e. the cam sliding surface (Figure 1) has a slightly greater curvature in the section 7, but the associated view in Figure 1 has been omitted for the sake of clarity.

Claims

1. Cam for controlling a gas exchange valve of -a reciprocating piston engine by way of a valve actuating element which has a preset sliding surface contour, comprising a leading edge and a trailing edge or flank and a cam peak area having a sliding surface section which minimises the occurring valve acceleration by way of a preset cam angle of rotation and which, in a first transitional area, is adjacent to the leading edge and, in a second transitional area, leads into the trailing flank, wherein the sliding surface section intersects an imaginary bisector of the angle formed by a straight line run through the first transitional area and the cam axis of rotation and a second straight line run through the second transitional area and the cam axis of rotation, at an angle (g) deviating from 900, and that the distance between the first transitional area and the cam axis of rotation is greater than that between the second transitional area and the cam axis of rotation.

2. Cam according to claim 1, wherein the two transitional areas run continuously.

3. Cam substantially as described herein with reference to and as illustrated in the drawings.