that the gas dynamic forces which are repeatedly exceeding over “mechanics”, it is possible to change the position of a pressure ring in a piston groove. The question arises, but do we need this?
Having some experience and not agreeing with the GOST, we will carry out calculations, according to our theoretical assumptions and our intuition, we will take the size of the radial thickness of the piston compression ring equal to 4.0 mm. According to the gas dynamic scheme (Figure 1), in order to eliminate the negative effect of gas dynamics on the operation of the piston compression ring, it is necessary to equate the axial gas dynamic force F0 acting on the upper end of the ring with the radial gas dynamic force Frad pressing the piston ring working surface to the cylinder wall.
It is necessary to take into account the force of the self-elasticity of the ring, which presses the working surface of the piston ring against the wall of the cylinder Fpr.
In order to balance the gas-dynamic and mechanical systems and ensure the normal operation of the compression (sealing) piston ring, the proposed equality should be fulfilled: Fo = Frad + Fpr.
GOST proposes to accept “minimum elasticity (in the belt) of the ring 14.20 N (1.45 kgf), for the cylinder diameter of 92 mm. This parameter is set within 2.3 … 3.1 kgf, for comparison, in the technical conditions for the upper piston compression ring of the KAMAZ engine (cylinder diameter 120 mm). In the kinematic system of the “cylinder-piston ring-piston”, fundamental changes occur at our will, take the minimum necessary value, for example, Fpr = 6.0 N, that is, 0.6 kgf. Further calculations and related experiments should confirm the validity of these assignments.
To fulfil the proposed equality of forces, it is necessary to equalize the surface areas of the upper end of the piston ring S1 and the inner vertical surface S2, i.e. S1 = S2. Let’s calculate these areas. We took the value of the cylinder diameter of 90 mm, the size of the radial thickness of 4.0 mm, i.e., t = 0.4 cm; the height of the compression ring is denoted by h; operating pressure is Pwork = 80 kg / cm2.
The surface area of the upper end of the compression ring is determined by the formula S1 = π (r12– r22), where:
r1 is the radius of the cylinder, i.e., the outer diameter of the piston ring r1 = 45 mm, or r1 = 4.5 cm;
r2 is the radius of the inner diameter of the piston ring, which is equal to
r2 = r1-t = 45—4 = 41 mm, or r2 = 4.1 cm.
The area of the inner vertical surface of the ring S2 is determined by the formula: S2 = 2 πr2h.
In this formula, we take the height of the piston ring as an unknown quantity, because we have proved that the standards recommend us incorrect data. Let’s try to correct them, for this we equate both areas of these different surfaces of the piston ring π (r12– r22) = 2πr2h. The size of the height of the ring h is unknown, in this equation, it is easily determined by the formula: h = (r12-r22) /2r2. Substitute the values and get: h = (20.25—16.81) /2 × 4.1 = 3.44/8.2 = 0.4195 cm = 4.195 mm.
Define the values of the areas S1 and S2:
S1 = 3.14 (20.25—16.81) = 3.14 × 3.44 = 10.8016 cm2;
S2 = 2 × 3.14 × 4.1 × 0.4195 = 10.801286 cm2.
Now we can accurately calculate the value of the gas-dynamic forces Fo and Frad acting on the piston compression ring intended for the engine cylinder with a diameter of 90 mm. For this, we multiply the value of the maximum working pressure in the cylinder and in the piston groove by the dimensions of certain surface areas:
Fo = Pwork × S1 = 80 kg / cm2 × 10.8016 cm2 = 864.128 kgf;
Frad = Pwork × S2 = 80 kg / cm2 × 10.801286 cm2 = 864.103 kgf.
The difference Frad – F0 = 0.025 kgf, can be negligible, lying within the limits of measurement error.
We can assume that the effect of the gas dynamic forces are balanced, therefore, the negative effect of gas dynamics on the operation of the piston compression ring is neutralized. The working capacity of the piston ring is provided by the force of pressing the working surface of the ring against the wall of the cylinder, that is, by the elasticity of the piston ring. The necessary magnitude of this force can be achieved not only due to the geometric dimensions of the piston ring, but also the determination of the properties of the material from which the piston ring is made and the heat treatment, the gap in the lock of the ring in the free state.
So, we got all the necessary sizes, let’s see how the design will look on the sketch of Fig. 3. The difference from the gas-dynamic scheme shown in Fig. 1 is fundamental, both in form and content. The main thing, to what the taken measures have resulted, is clearing of a compression ring from any overloads connected with gas dynamics. To the piston ring was returned to its elastic qualities, the normal position relative to the flanges of the piston groove and the cylinder wall, normal operability. The form, content (material, thermal operations) and dimensions provided favorable conditions for function execution assigned to the piston compression ring.
The piston compression ring is designed to execution: the following tasks:
– reliable consolidation of the space between the movable piston and the stationary cylinder, excluding any gas-dynamic losses, or reducing them to an insignificant minimum;
– heat transfer from an overheated piston head to a cooled cylinder;
– minimum mechanical losses due to friction of the working surface of the piston ring against the wall of the cylinder.
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