US2567689A - Cam for operating valves of internal-combustion engines - Google Patents

Description

J. L. H. BISHOP CAM FOR OPERATING VALVES OF INTERNAL-COMBUSTION ENGINES Sept. 11, 1951 2 Sheets-Sheet 1 Filed Feb. 6, 1948 r] /717 Leslie H hf/ 5 [5770 0 Sept. 11, 1951 J. H. BISHOP 2,567,689 CAM FOR OPERATING VALVES OF INTERNAL-COMBUSTION ENGINES Filed Feb. 6, 1948 2 Sheets-Sheet 2 5 h I l J 1 l l I l l l l l l I l l l l l l l x E a Jf E q f K) K) t 1||||||||1|1|||||||||||||| x B 1' C Ji 3 g l L\L \l lllllllll lllllllll ll 11 E fl x Patented Sept. 11, 1951 CAM FOR OPERATING VALVES OF INTERNAL-COMBUSTION ENGINES John L. H. Bishop, Birmingham, England, as-

signor to The Austin Motor Company, Limited, Birmingham, England Application February 6, 1948, Serial No. 6,787 In Great Britain February 24, 1947 2 Claims.

This invention relates to the operation of valves for internal combustion engines, and more particularly to a novel cam design which is particularly suitable for the type of valve gear operated through the medium. of tappet, pushrod and rocker mechanism, but can also be used for side valve operation.

An object of the present invention is to provide a cam having a contour adapted to eliminate or reduce valve spring surge.

A further object of the present invention is to provide a cam for opening and closing the valve without shock.

In the present invention the opening and closing phases have been modified fundamentally. Unlike the convex-flanked cam and mushroom type follower, the straight-flanked cam in con bination with roller type or round nose follower, and the two-sine wave cam of British Patent No. 573,294 wherein the valve commences to open at or near the maximum value of positive acceleration, according to the present invention a cam profile is designed which shall have zero acceleration at the points of opening and closing.

The motion of a cam-operated valve can, for convenience, be described in four intervals. During the first interval the valve is lifted from its seat and accelerated to maximum velocity; during the second interval the valve is decelerated from maximum velocity to zero velocity at the top of the lift, the decelerating force being provided by a valve spring which yieldingly opposes the movement of the valve in its upward motion. During this latter interval reaches its maximum. negative value at the top of the lift.

At the beginning of the third interval the valve enters, at zero velocity, on its return path under the accelerating force of the spring and reaches its maximum negative velocity and zero acceleration, then continuing into the fourth and final interval the valve is decelerated by the cam from maximum negative velocity and finally brought to rest. In designing cams, the mathematical expressions defining the lift curve, or, alternatively the radii determining the geometry of the cam profile are often, but need not necessarily be, symmetrical for the opening and closing side of the cam profile. To those skilled in the art of cam design, in order to minimise the load on the valve spring and to reduce the com-- pression in the mechanism at maximum speed of cam shaft revolutions, it is usual to aim at a maximum positive acceleration of approximately twice the negative acceleration.

the acceleration clearance circle.

A cam for operating a valve of an internal combustion engine, and according to the invention, has its lift curve such that, after taking up clearance, it consists of three half-period sine waves in series, the half-period of the first sine wave accelerating the valve from zero velocity, or some predetermined velocity, to maximum velocity, the second half-period sine wave, commencing at the maximum positive velocity of the first sine wave, decelerating the valve to zero velocity at maximum lift, and again accelerating the valve to maximum negative velocity, the third half-period sine wave decelerating the valve from maximum negative velocity to zero or some predetermined velocity.

It is now quite common practice to arrange a constant velocity ramp at either end of the lift curve for the purpose of taking up tappet clearance, but for the time being, in order better to describe the general principles of the design of the cam profile in accordance with the present invention, it is proposed hereinafter to deal with a side valve cam in its simplest form without ramps, and therefore uncomplicated by the effects of end velocity.

The method of cam design is illustrated by the accompanying drawing and diagrams of which:

Figure 'l is a side view of an actual cam designed according to the invention and showing a conventional tappet and valve spring broken away.

Figure 2 is a graph showing the acceleration curve of the valve lift.

Figure 3 is a graph showing the velocity curve of the valve lift; and,

Figure 4 is a graph showing the valve lift curve.

Referring to Figure 1, a cam Ill is shown engaging the usual tappet [2 which is maintained in contact with the surface of the cam H3 by a spring 14 pressing against the disk l6 and pin l8 which extends diametrically through tappet 12. A indicates the base circle of the cam i9 and B the The part of the cam surface between the points C and D is the opening constant-velocity ramp of the cam surface, the part between the points D and E is the first period of the cam lift from the base circle, the instanta- 'neous radii of the cam surface forming a first half-wave sine curve.

third half-period sine wave. The part between H and J is the closing constant-velocity ramp.

Referring to Figure 2 which includes a frame of reference having a vertical line DY on which are marked off ordinates of acceleration and a horizontal line DX on which are marked ofi angles of cam shaft rotation.

The part of the diagram between D and E I designate the first interval of the valve movement from the base circle. At the point D the acceleration is zero. It rises to a maximum at d and, at the end of the interval, falls again to zero at E. The locus of the acceleration, in this interval, is a half-wave sine curve.

The part of the diagram between E and F I designate the second interval. At the point E the acceleration is zero, as above stated. It rises to a maximum negative acceleration (deceleration) at the point 7 at the end of the interval, the curve being a negative quarter-wave sine curve.

The part of the diagram between F and G I designate the third interval. At the point 1 the acceleration is at maximum negative value and decreases to zero at the end of the interval at G, the curve being a negative quarter-wave sine curve, and is in fact a continuation of the curve of the preceding interval together with which it forms a half-wave negative sine curve.

The part of the curve between G and H which I designate the fourth interval is the facsimile of the curve between D and E and is a half-wave positive sine curve. It rises to a maximum at g and falls to zero at H as the valve closes on its seat.

Referring to Figure 3 the curve there shown is the first integration of the acceleration curve shown in Figure 2, and is a velocity curve. It will be seen that the velocity is at its maximum at the point e at the end of the first interval and becomes zero at F, the end of the second interval. This is the position of the cam when the valve is fuliy open. From F to g the velocity increases in the reverse direction to the end of the third interval and comes to zero at the end H of the fourth interval as the valve closes on its seat. The whole of the curve between D and His made up of three co-sine curves, a middle half-wave co-sine curve covering the second and third intervals and two negative end co-sine curves covering the first and fourth intervals.

Referring to Figure 4 the curve there shown is the second integration of the acceleration curve shown in Figure 2, and the first integration of the velocity curve shown in Figure 3, and it constitutes the valve lift curve. It consists of a first portion included in the first interval between D and E and comprises a half-wave negative sine curve added to an ascending straight line curve shown in dotted lines and merging into a halfwave sin-e curve included in the second and third intervals between e and g, and finally merges into a negative sine wave included in the fourth interval between 9' and H, added to a descending straight line curve shown in dotted lines, the curve ending at the base line at the end H of the fourth interval as the valve closes.

The curve shown in Figure 4 does not, of course, depict the shape of the cam surface, but from it, and in conjunction with the curve shown in Figure 3, the contour of the cam profile can be determined at the several angles of the cam.

Referring again to Figure 2, it will be seen that the amplitude of the acceleration curve in the first interval DE is twice the negative amplitude in the second interval EF, but the angular travel of the cam during the second interval is twice that of the first interval. The area of the figure Delhi is, however, equal to the area of the figure EFf. Likewise the areas of the figures FG and GgI-I are equal. In a cam of this type it is therefore apparent that the time taken in executing the first interval will be approximately half that of the second interval.

The reason for combining the first interval of the lift curve with an ascending straight line curve, is that it is integrated from the first co-sine curve of Figure 3, and would normally commence with a negative curve; hence, to make it positive and to make it merge into the curve of the second interval, it is combined with an appropriate ascending straight line curve. If a ramp is used, the tappet will already have a velocity at the commencement of the first interval, and this can be matched by appropriate adjustment of the slope of the straight line curve.

The various relationships can be mathematically:

Let

m1=number of waves per revolution of camshaft for the first interval.

mz=number of waves per revolution of camshaft for the second and third intervals.

R1=the amplitude of the sine wave of the first interval of the lift diagram.

R2=the amplitude of the sine wave of the second and third intervals of the lift diagram.

n=the constant slope added to the sine wave in the first interval.

y=displacement or lift.

x=angular movement of camshaft in radians.

written Then the general expressions defining the first interva1 are:

Equation to lift is --R1 sin 77L1$+7LZII Equation to eccentricityd :";=m R cos m x+n (2) Equation to instantaneous radiusd y =+m R SlIl mg: (3)

where Eccentricity= Instantaneous radius= w=speed of camshaft rotation in radians per second The general expressions defining the second and third intervals assuming, for convenience, a shift of the frame of reference e, Figure 4, at the commencement of the second phase are:

Equation to lifty=Rz sin max (4) Equation to eccentricityg =m R cos m w (5) Equation to instantaneous radius =-mfilt sin m a; (6)

In the case of symmetrical cams it is usual to calculate values of lift and eccentricity to the top of the lift only. In this case Equations 1 to 6 provide the required data. However, if it is desired to make the cam asymmetrical the fourth :interval can be designed as if it were the first :interval of a new. cam, the coeflicient R1 and n in Equations 1 to 3 being modified to suit any condition of closing ramp velocity required.

From these equations the geometry of the cam is determined. In a practical design the values of lift and eccentricity are calculated from trigonometrical tables for each degree of camshaft travel for the period of the valve opening.

In the case of push rod and rocker operated valves, there will, at high speed, be considerable compression of the parts constituting the valve :gear, and this can be compensated, at any selected speed, by reducing the amplitudes of the sine waves of the lift curve by the calculated or measured amount.

Having fully described my invention, what I claim and desire to secure by Letters Patent is:

1. An operating mechanism comprising a movable member, a coil spring for urging said member in one direction, and a rotary cam having a lifting surface adapted to engage and move said member in the other direction, the contour of said' lifting surface being represented by a lift curve consisting of three functions in series, the first function consisting of a negative half period sine wave combined with a straight line as cending curve, whereby said member is acceler ated from a predetermined velocity to a positive maximum velocity with the acceleration represented by a positive half period sine wave beginning and ending at zero acceleration, the second function consisting of a second half period sine wave, whereby said member is decelerated from said maximum positive velocity to zero velocity at the upper limit of travel and is then accelerated to a maximum negative velocity with the acceleration represented by a negative half period sine wave beginning and ending at zero acceleration, and the third function consisting of a negative half period sine wave combined with a straight line descending curve whereby said member is decelerated from said maximum negative velocity to said predetermined velocity with the acceleration represented by a positive half period wave beginning and ending at zero acceleration.

2. An operating mechanism comprising a movable member, a coil spring for urging said member in one direction, and a rotary cam having a lifting surface adapted to engage and move said member in the other direction, the contour of the major operating portion of said lifting surface being represented by a lift curve consisting of three functions in series, the first function consisting of a negative half period sine wave combined with a straight line ascending curve, whereby said member is accelerated from a predetermined velocity to a positive maximum velocity with the acceleration represented by a positive half period sine wave, the second function consisting of a second half period sine wave, whereby said member is decelerated from said maximum positive velocity to zero velocity at the upper limit of travel and is then accelerated to a maximum negative velocity with the acceleration represented by a negative half period sine wave, and the third function consisting of a negative half period sine Wave combined with a straight line descending curve whereby said member is decelerated from said maximum negative velocity to said predetermined velocity with the acceleration represented by a positive half period sine wave.

JOHN L. H. BISHOP.

REFERENCES CITED The following references are of record in the file of this patent:

Valve Gear Design-Michael C. Turkish. Eaton Manufacturing C0,, Wilcox Rich Division, Detroit, Michigan, 1946.

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