Читать книгу Piston Engines of the New Generation (Without turbo – supercharging) - Anatoly Matveevich Druzhinin - Страница 9

To comparing two engines of the same purpose KAMAZ and MERCEDES, the author – a professional technologist, intuitively (sometimes trusting the designers), was closer the MERCEDES engine. Of course, the importance was not the authority of the firm, but pragmatism, confirmed by many years of searching for the reasons for the low efficiency of the product, the project of which technology is implemented in the metal. Obviously, it is not necessary to convince the designers of the need for difficult searches for a simpler design that is completely would perform the tasks assigned to it.

The main advantage of the German engine, in comparison with the competing engines of “KAMAZ OOO” (Limited Liability Company) " and YaMZ TMZ of “Avtodizel OAO” (Open Joint-stock Company), is the size of its cylinder of 128 mm. The difference is small, only 8 mm, but taking into account the huge operating pressures, the power increases significantly, so the competitor was allowed to provide only 6 cylinders, with all the ensuing positive consequences.

Therefore it would be possible to recommend to domestic trucks of a class KAMAZ and YaMZ to use diameter of the cylinder of 130 mm. “Kostroma MOTORDETAL OAO”(Open Joint-stock Company) is manufactured a similar piston group with a cylinder diameter of 130 mm, for the tractor engines. This measure can be implemented only on condition of fundamental changes in the design of piston devices.

So, back to the calculation of the sealing (compression) piston ring, which could be used for domestic AvtoVAZ models. Considering the strategy of design of similar engines, it was interesting to get acquainted with features of engines of Formula -1. With engine speeds of 18,000 … 22,500 rpm or more, the engine develops power over 750 PS. with a cylinder diameter of 98 mm, a piston stroke of 39.7 mm, fuel consumption of about 60 litres per 100 km.

It is not enough to copy for our engines from AvtoVAZ series, but the design strategy, taking into account the highest class of skill of the mechanics of Formula-1, should be taken into account. Now we have VAZ engines with diameters of cylinders of 76 … 82 mm, three – and four-cylinder.

Let’s finish the interrupted calculation of the effect of gas dynamics on the operation of the compression ring of the VAZ-2190 engine presented above, but already for a virtual engine with the desired initial data.

Based on our research, we can use the maximum cylinder size for VAZ engines. Author is a technologist intuitively suggests that the most preferable can be taken as a basis for further calculations – the diameter of the cylinder is 90 mm. The second, the maximum working pressure is very important for further calculations, which we transfer from the previous calculation of the compression ring of the VAZ – 2190 engine, that is 8 MPa, and for our calculations it is more convenient to operate with 80 kg / cm².

So, we know the size of the outer diameter of the sealing piston ring. Let’s see what size of the inner diameter GOST R 53843—2010 “recommends” to us, “suggesting” the radial thickness of the ring 3.8 ^+0.1_—0.15 mm. Therefore, the internal diameter of the piston ring will be 90.0 – 3.8 = 86.2 mm. GOST offers to take the height of the ring of 2.0 mm. Very elegant ring! Do not prove that the developers ignored the second main task of the compression ring – to transfer heat from the overheated piston head to the cooled cylinder.

Solving this problem with such a “lightweight” piston ring is problematic, since the mass of the transmitting element, i.e. the piston ring, was lost. Due to poor heat transfer between the piston and the cylinder, the author justified the inexpediency of using standard, currently used piston trapezoidal compression rings (for example, on all models of KAMAZ engines) [2].

Nevertheless, the “trapezoidal” piston compression rings continue to be produced by the manufacturer of sets of cylinder-piston group “KOSTROMA-MOTORDETAL”, equipping KAMAZ engines, YAMZ engines and many others. It will be necessary, once again, to prove the absolute axiom shown in Fig. 2, this design does not in any way resemble a piston seal ring and a structural element that, among other things, must provide the best heat dissipation from an overheated piston head to a cooled cylinder.

Figure. 2. A compression ring with a facet at an upper end face in dynamics: 1-A CYLINDER 2-A PISTON; 3-A PISTON RING

Moreover, the initiator of the “twisting”, “wedge-shaped”, and according to our GOST “trapezoidal” piston seal rings, intelligibly explained that the “twisting” rings are obtained as a result of the fact that “… the main axes of inertia formed (after tuck, bevel, facet) of the non-symmetric cross-section of the ring become non-parallel (and correspondingly) non-perpendicular to the working surface, that is, they are located obliquely.

If such ring is compressed to working size, then it doesn’t remain flat in the initial plane, and takes the dish form so that the lower edge comes out stronger, and only it comes to contact with a working surface of the cylinder (fig. 328)” [5]. In this case, the practice confirmed the conclusions of the scientist 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 F_rad 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 F_pr.

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: F_o = F_rad + F_pr.

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, F_pr = 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 P_work = 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 F_o and F_rad 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:

F_o = P_work × S₁ = 80 kg / cm² × 10.8016 cm² = 864.128 kgf;

F_rad = P_work × S₂ = 80 kg / cm² × 10.801286 cm² = 864.103 kgf.

The difference F_rad – 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.