DE4221418A1 - Cam profile for IC engine valve operation - has re-defined profile calculation for area from cam peak to flank. - Google Patents

Abstract

A cam for valve control in a rotating piston engine operates the valve via a flat tappet against the force of a closing spring. The cam profile is determined as: psi =rN/(rG+z(0)) where psi (x) = hydrodynamic reference; rN = cam radius of curvature; rG = pitch circle radius; z = cam lift. In order to determine the characteristic over the critical region 0<=x<=xE, where xE is the cam angle at the end of the cam peak, the following is used: (x) = r(x)/rG + z(x); x = cam angle. The function has the value at x=0 and a maximum at x=xE of = (1+b), where 0.1<=b<=0.3 and is chosen between 0.15<=0.25. The cam profile from the peak is determined by: z(x)=(z(o)+rG).cos square root(1- psi o(1+Kx)x) - rG USE/ADVANTAGE - Cam profile for IC engine valve gear in which pressure from cam point to flank is reduced, without affecting lubrication.

Description

The invention relates to a cam according to the genus Concept of claim 1.

From DE-PS 28 09 915 a cam contour is known, the cams run through the formula

is described.

In the event that the cam tip is assigned the value x ₀ = 0 and if it follows from geometric considerations and continuity conditions that a ₁ = r _G + z (0) and a ₂ = 0, the known cam shape follows the equation

It means:
x ₀ cam angle at the beginning of the range γ ₀ ,
x cam angle starting at x ₀ = 0,
z cam elevation as a function of x,
z (0) cam elevation at the point x = 0,
γ ₀ a selectable value between 0.15 and 0.25,
r _G Radius of the cam base circle.

Such a shaped cam ensures optimal lubrication ratios, but leads to decreasing starting from the cam tip local radii of curvature. Through excessive vibrations The forces on the valve train can cause these small radii of curvature lead to impermissibly high Hertz pressures.

Starting from a cam shape according to the generic term the invention is based, the Hertzpressung the task the cam tip to decrease towards the cam flank, without affecting the lubrication conditions.

This task is solved according to the characteristic features of the claim.

Due to the steadily increasing assessment index γ (x) from the cam tip in the direction of the cam flank, the radius of curvature of the cam shape from the cam tip also increases, thereby avoiding an undesirable increase in the Hertz pressure. The fact that the rating γ (x) is always in a range smaller than γ ₀ = 0.25 means that the lubrication conditions are not adversely affected. This is particularly advantageous in the case of supercharged internal combustion engines, where the gas exchange valves have to be opened against high pressures, so that there is quickly an unacceptable Hertzpressung, which can even lead to failure of the camshaft due to fittings.

Details of the design of a cam according to the invention are shown in drawings. It shows:

Fig. 1 is a schematic representation of a cam in combination with a flat ram,

Fig. 2 is a qualitative acceleration profile of a cam with an indication of the value x _E,

Fig. 3 is a diagram showing the parameters z, p _H , γ (x) as a function of the cam angle x.

In Fig. 1, a cam 1 is indicated, which just runs from a flat tappet 2 . With r _G is a base circle radius and with r _N a radius of curvature of the cam 1 at a cam tip 3 accordingly a cam angle x = 0 ( Fig. 3). Furthermore, the respective cam stroke z, which is based on the cam shape 4 and the maximum cam stroke z (0) = zmax is entered at the point x = 0.

In Fig. 2, a qualitative course of the valve acceleration z '' is shown as a function of the cam angle x. The critical area for the Hertz pressure and the lubrication film formation concerns the area between the cam angle x = 0 and x = x _E. The cam angle x _E represents the end of the cam tip, that is to say the beginning of a transition area to the cam flank. This critical area 0 ≦ x bereit E causes problems with regard to compression and lubrication film formation.

The increased decrease in the Hertz pressure is caused according to the invention by the variation in the range 0 0 x ≦ x _{E of the} assessment index γ ₀ , which is kept constant according to the prior art. This rating index γ (x), which changes with the cam angle x, ensures that the radius of curvature r (x) increases continuously from x = 0 and the value r _N.

While the value γ (0) = γ ₀ = r _N / (r _G + z (0)) also applies to the cam tip at x = 0, the assessment characteristic in the range 0 ≦ x ≦ x _E according to the invention steadily increases according to the law γ (x) = r _(x) / (r _G + z (x)) until it reaches the maximum value γ (x _E ) = γ0 · (1 + b) at X = x _E. However, γ (x) at the point x _E must not exceed 0.25. In the formula for γ (x) r (x) means the radius of curvature at a position x and z (x) the cam stroke at a position x. Since the base circle radius r _G and the maximum cam stroke z (0) = zmax are specified, it follows from the relationship for γ (x) that as the value for γ (x) increases, the radius of curvature begins at x = 0 from the tip of the cam r (x) increases and consequently the Hertzpressure p _{H shown in} FIG. 3 decreases.

In order to achieve the steadily increasing profile of the hydrodynamic reference number γ (x), which can be seen from FIG. 2, the cam stroke z (x) of the equation

are enough. The constant k must be determined iteratively in such a way that when the values z (x _E ) and the radius of curvature r (x _E ) are inserted as a function of the cam shape Z (x) at the point x _E in the formula

results.

If the parameters 0.15 ≦ γ ₀ <0.25 and 0.1 ≦ b ≦ 0.3 are freely chosen, care must also be taken that the maximum value for γ (x) at the point (x _E ) is 0 , Does not exceed 25.

On the one hand, γ (x) does not exceed the value 0.25, but on the other hand, by varying the value γ (x) in the range x ≦ x ≦ x _E, the radius of curvature r (x) increases, in addition to optimal lubrication, the permissible values are exceeded Hertz pressure p _H avoided.

Claims (1)

Cam for controlling a valve in reciprocating piston machines, by means of which the valve is opened via a flat tappet against the force of a closing spring, the cam shape being determined on the basis of the hydrodynamic rating (γ ₀ ), which is derived from the cam radius of curvature (r _N ), the Basiskreisra dius (r _G ) and the cam stroke (z) in the form γ ₀ = r _N / (r _G + z (0)) is formed, characterized in that to determine the cam shape, the hydrodynamic rating in a critical range 0 ≦ x ≦ x _{E of} a continuously increasing function of x in the form obeys that the function value γ (x) at the position x = 0 the value γ ₀ and at the position x = x _E the maximum value γ (x) = γ ₀ · (1 + b), where b is a Pa represents parameter that in the range 0.1 ≦ b ≦ 0.3 and γ _{0 represents} a parameter that can be selected in the range 0.15 ≦ 0.25, but that γ (x) at the point x = x _E must not exceed the value of 0.25, and that the cam shape in the initial state was due to the relationship for the cam stroke is set.
Here mean:
x current cam angle, starting at x = 0,
γ ₀ assessment key at x = 0, selectable between 0.15 and 0.25,
b parameters selectable between 0.1 and 0.3,
k constant that iteratively results from the relationship results in
r (x _E ) radius of curvature of the cam shape at the point x _E ,
z (x _E ) cam stroke at point x _E ,
z (0) maximum cam elevation z _max at x = 0,
r _N radius of curvature at the tip of the cam,
r _G base circle radius,
x _E Cam angle at the end of the cam.