The low efficiency of the engine is unacceptable-evidence of great losses and, as research has shown, most of them occur in the cylinder group.
This work is generally devoted to the research of gas-dynamic, hydraulic and thermodynamic processes impact occurring in the piston cylinder on main technical-economic and ecological showings of the engine.
Sealing Piston Devices
The internal combustion engine is subject to mechanical, gas-dynamic, hydrodynamic and thermodynamic influences that alter the shape and dimensions of its individual elements and parts that are in its natural and working conditions.
The developers who designing new engines, need to know what and how the physical processes affected to the transformation of the construction during the engine operation, which should be reflected in the developed projects.
The developers are not paying enough attention to these physical processes. We will consider in more detail what processes and in what part of the engine, cause changes. And what changes on which engine operation and its efficiency are depend.
The main source of changes in the engine is the cylinder-piston group, in which the main physical and chemical processes occur. They naturally affect to the operation of the engine. The cylinder-piston group determines not only the dimensions of the main engine elements, the connecting rod, the crankshaft and etc., but also the shape of the engine. Lets try to figure this out.
The efficiency of any power device is estimated by the value of the efficiency, tending to unity, depending on the complex of various losses accompanying the operation of this device.
The specifics of internal combustion engines are that, in addition to the normal mechanical losses on the friction of the mobile elements of the kinematic system, there are gas dynamic losses, hydrodynamic losses (motor oil) and thermodynamic losses.
The thermodynamic changes in the engine during its operation have a significant effect on the efficiency of the cylinder-piston group. The thermodynamics changes the shape and dimensions of the cylinder, the piston, the piston rings, and it almost affects on all the processes that occur in the engine.
The engine losses may be objective, because of any physical process occurring within the construction, which is difficult for the developer and user to use during the operation of the product. But they can also be subjective because of the human factor that allows mistakes in the design, manufacture and operation of an energy device.
To significantly increase the efficiency of the engine, it was necessary to identify these errors and propose designs that exclude these losses, or reduce them to a negligible minimum. The example above shows that the opponent of the KAMAZ engine, the losses is much smaller and this naturally affects the efficiency of the engine, its shape and content.
The above losses, in general, should be attributed to the cylinder-piston engine group, which is rightly considered the heart of the motor that determines the technical and economic characteristics and ecological indicators of the engine. Low efficiency of the modern engine, this is, first of all, evidence of imperfection of the CPG engine and the errors committed during its design.
§1. Influence of the gas dynamics on the operation of a piston sealing ring
From the losses considered, it is necessary to highlight, as priority, gas-dynamic losses that affect the main working processes occurring in the combustion chamber and in the engine cylinder, as a result, on the efficiency of the engine.
The researches have shown that when analysis of various losses characteristic of the internal combustion engine and affecting on the value of its efficiency, the main task was to determine the location of each of them identify the priorities and influence on other groups of losses. Two groups of losses mechanical and gas dynamic apply for the first post.
Lets start with the dynamics. The domestic scientists and specialists mechanics, belong indulgently to gas dynamics as to the factor exerting serious impact on work of the ICE. In the best case, the gas dynamics for them is leakage of the working gas, determined for a single piston ring [5].
An analysis of the accuracy and value of the gaps in domestic engines shows that the theoretical calculations of the German scientist in determining the value of leakage of working gas have not found wide application in our country. In the following, we consider some constructions, where the role of leaks is reduced to an insignificant minimum.
Turn to the materials in the domestic textbooks. When the ring is compressed and inserted into the cylinder, it takes a cylindrical shape and exerts a pressure on the cylinder walls equal to 0,05 0,3 MPa (0,5 3 kg / cm2) and more. During operation, the pressure increases of the ring on the walls, since gases penetrating through the gaps between the ring and the piston press the ring against the wall of the cylinder[6].
About the same, in 10 years. The sealing is carried out by pressing the ring against the cylinder wall by the elasticity of the ring and the pressure of the gases. At the moment of flare when the piston is in upper dead center (UDC), the pressure in the groove of the 1st ring is close to the pressure Pz in the cylinder, and in the groove of the 2nd ring it is only 50% of this value. Pressure P3 behind the last ring is much smaller; it is commensurate with the pressure in the crankcase of the engine. In view of the considerable pressure of the rings on the cylinder walls, the majority of the work of friction in the engine (up to 50%, and sometimes up to 60%) falls on the rings, so it is impossible to press the rings with excessive force[7].
The conclusions drawn by scientists on the one hand are quite obvious, and on the other hand inferior, are of a general nature, having little effect on the process of designing the piston rings. There is no answer to the main question: how, and with what efforts does the pressure of working gases in the cylinder act on the sealing piston ring?
The process of constantly changing pressure of intake air into a cylinder, then mixed together with the fuel in the combustion chamber and transfused into working gases, should be considered as a gas dynamic process.
In 2004 for the first time the author published the gas-dynamic scheme of operation of the compression ring of the KAMAZ engine on the basis of an analysis of the established attitude of scientists and specialists of mechanics to gas dynamics [1]. This is how the design and position of the compression ring look without any slopes of the upper end face and chamfers along the inner diameter (Figure 1).
It was necessary to remember and use the known physical law, in the appendix to the given case it can sound as follows:
On the free surfaces of the piston ring (upper end and inside vertical surface) located in the closed space limited to a cylinder wall, a bottom of a piston groove and its upper and lower shelves which is under pressure of working gases, there are forces proportional to the areas of these surfaces
Figure. 1. The gas dynamic scheme of the operation of the compression ring of the KAMAZ engine: 1-A CYLINDER 2-A PISTON; 3-A PISTON RING
Figure. 1. The gas dynamic scheme of the operation of the compression ring of the KAMAZ engine: 1-A CYLINDER 2-A PISTON; 3-A PISTON RING
Breaking through the gap between the piston 2 and the cylinder 1 into the upper piston groove, the working pressure presses the piston ring 3 to the lower shelve of the piston groove with the gas-dynamic force F0, and to the cylinder wall by the radial force Frad and the force of intrinsic elasticity F seal. The calculation of these forces was provided in the authors previous publications.
The most interesting can be seen in this calculation for one of the most popular domestic engines of VAZ-2190, with the following parameters: the maximum pressure of the working gases in the cylinder when the piston is at top dead center, is about Pwork = 80 kg / cm2. The dimensions are in centimetres for convenience of calculations. Diameter of the cylinder is 82 mm = 8.2 cm; the outer radius r1 is 41 mm = 4.1 cm; the inner radius is r238 mm = 3.8 cm; the radial thickness of the ring t = 3.0 mm = 0.3 cm; the height of the upper compression ring h = 1.5 mm = 0.15 cm.
The area of the upper butt is defined by the formula:
S1 = π (r12-r22) = 3.14 (4.123.82) = 3.14 (16.8114.44) = 7.44 cm2.
The area of the inner vertical surface is defined by the formula: S2 = 2 πr2h = 6.28 × 3.8 × 0.15 = 3.58 cm2.
Multiplying the pressure of the working gases by the area values, we get:
Fo = Pwork × S1 = 80 × 7.44 = 595.2 kgf (5.95 kN);
Frad = Pwork × S2 = 80 × 3.58 = 286.4 kgf (2.86 kN).
The conclusion is obvious from the comparison of these two gas-dynamic forces acting on the movable piston ring. Twice the superior axial force reliably pressed the piston ring to the lower flange of the piston groove, depriving the radial force of the working surface of the piston ring from being pressed against the wall of the cylinder.
That is very important to note here. This pattern is observed in all cases where there is an excessive pressure on the piston. About this and not only, we will consider on the different steps of the engine motoring run. But now, running ahead, we can safely predict:
The gas dynamic scheme fundamentally changed the strategy and tactics, the theory and practice of designing internal combustion engines and piston compressors. It optimizes the size, shape and content of the engine and compressor and is reflected in the increase in the efficiency of both with its correct application in calculations.
All justifications are given in the authors work and in this manual. The information were published and patented, brought to the attention of scientists and specialists of the motors. However, producers continue to produce super modern cars equipped with the engines with such significant defects. Lets try again, to spell out in more detail the problem of increasing the efficiency of the internal combustion engines, which is not so difficult to understand, but very important for specialists.
§2. The gas dynamics at the different cycles of the engine operating cycle
The work of (that is most widespread among autotractor internal combustion engines (diameters of cylinders up to 140 mm)), the four-stroke engine consists in effective implementation of all 4 steps of an operating cycle:
admission of a fresh air charge;
compression of the task environment above the piston;
the piston working stroke transforming the huge pressure of working gases to the mechanical work;
release of spent working gases and residues, of the combustion products.
We will consider each of them in the form of a separate project, with its differences and peculiarities. But, as a result, integrated in the integral design of efficient use of the engine.
The working stroke admission (suction) in the cylinder of the engine, or compressor, the estimated amount of fresh air charge.
The task of developers of any constructive element are: define the shape and size of the product, and select the workpiece material based on the conditions in which the element will work. This is a very important stage of the design, from which much will depend on the work of the product. As the initial data, developers only know one size the diameter of the cylinder.
The principal drawback of many domestic designs (including ICE) is that the designers in their calculations take the safety factor not 1.10 or 1.20, as it should be, but 1.50 or 2.0, sometimes more than that. How important this is to the economy are known to most professionals. In the case of mass production, which we are considering, this is simply unacceptable.
Proceeding from the purpose of the working stroke admission, it is necessary to remember, than consolidation between the piston and the cylinder is more reliable, excluding any suction from the crankcase, the extent of discharge of space over the piston is more, the more actively there is a filling of the cylinder with rated quantity of an atmospheric air.
At the beginning of the movement of the piston in the lower position, taking into account the huge speed of displacement of the piston, above it and, accordingly, in the upper piston groove, a certain discharged space is formed. Admission is the only step of an engine motoring run to which influence of the gas dynamic scheme presented on fig. 1 doesnt extend. On this step the gas dynamics is neutral therefore it is possible to approach on other design of pressure rings, proceeding only from the tasks imputed to a step admission.
Making rather difficult calculation of elastic forces of the piston rings, developers shouldnt forget that on all steps of an engine motoring run the sealing (compression) piston ring has to carry out two main task and one compulsory condition:
to condense space between the piston and the cylinder, to provide transfer of heat from a superheated piston crown to the cooled cylinder, at minimum possible mechanical losses on friction.
For the piston ring, which is pressed to the lower flange of the piston groove with the previous stroke, the admission is the relaxation time, one moment. For example, with a piston stroke of 80 mm and a crankshaft rotation speed of 3000 min -1, the piston travel speed is 6 m / s, on the Formula 1 engines the average piston speed is 22.5 m / s. For an extremely short period of time, the piston ring should assume its natural position relative to the piston groove and cylinder wall. The technologists, on this occasion, have the expression: the piston ring shall be installed on the cylinder wall. In the process of moving to the lower dead point by friction of the ring surface of the cylinder, it is shifted to the top flange of the piston groove and is pressed to the cylinder wall with its own elasticity of the ring.
In this case it is worth using the recommendation of the domestic scientist Orlin A.S, therefore, we can take the recommended value of ring pressure on the cylinder walls 0,05 0,3 MPa (0,5 3 kg / cm2) and more [6]. As studies have shown, the expression of the scientist gases are pressed the ring against the wall of the cylinder is not entirely correct with respect to modern piston rings, because they do not correspond to reality. It turns out that they lost their elasticity and were pressed against the lower flange of the piston groove with the superior gas-dynamic force F0.