[0001] The present invention refers to a piston/cylinder assembly of a linear compressor
for cooling with aerostatic bearing arrangement, more particularly to the dimension
relationships of the assembly so as to minimize losses.
Description of the Prior Art
[0002] In general, the basic structure of a cooling circuit comprises four components, namely:
the compressor, the condenser, the expansion device and the evaporator. These elements
characterize a cooling circuit in which a fluid circulates so as to enable the reduction
of the temperature of an internal environment, removing the heat from this medium
and displacing it to an external environment through said elements.
[0003] The fluid that circulates in the cooling circuit generally follows this passage sequence:
compressor, condenser, expansion valve, evaporator and again the compressor, which
characterizes a closed circuit. During the circulation, the fluid undergoes pressure
and temperature variations that are responsible for altering the state of the fluid,
which may be either gaseous or in the liquid state.
[0004] In a cooling circuit, the compressor acts like a heart of the cooling system, creating
the cooling fluid flow along the components of the system. The compressor raises the
temperature of the cooling fluid through the rise in pressure inside it and forces
the circulation of this fluid in the circuit.
[0005] Thus, the importance of a compressor in a cooling circuit is undeniable. There are
various types of compressors applied to cooling systems, and in the field of the present
invention attention will be focused only on the linear compressors.
[0006] Due to the relative movement between the piston and the cylinder, it is necessary
to provide the piston with bearing arrangement. This bearing arrangement consists
of the presence of a fluid in the clearance between the outer diameter of the piston
and the inner diameter of the cylinder, preventing contact between them and the consequent
premature wear of the piston and/or cylinder. The presence of the fluid between said
two components serves also to decrease the friction between them, thus causing the
mechanical loss of the compressor to be lower.
[0007] One of the ways of providing the piston with a bearing arrangement is by means of
aerostatic bearings, which, in essence, consist in creating a gas bearing arrangement
between the piston and the cylinder so as to prevent wear between these two components.
One of the reasons for using this type of bearing arrangement is justified by the
fact that the has a much lower viscous friction coefficient than any other oil, thus
contributing to cause the energy spent in the aerostatic bearing system to be much
lower than that of oil lubrication, thus achieving a better output of the compressor.
One advantage resulting from the use of the cooling gas itself as a lubricating fluid
is the absence of the oil pumping system.
[0008] In figures 1 and 2, it is possible to see that the gas compression mechanism takes
place through the axial and oscillating movement of a piston inside a cylinder. At
the cylinder top is the head, which, in conjunction with the piston and the cylinder,
forms the compression chamber. At the head discharge and suction valves are positioned,
which regulate the entry and exit of gas in the cylinder. In turn, the piston is actuated
by an actuator that remains connected to the linear motor of the compressor.
[0009] The compressor piston actuated by the linear motor has the function of developing
a linear alternating movement, causing the piston movement inside the cylinder to
exert a compression action of the gas admitted by the suction valve, until it is in
a position to be discharged to the high-pressure side through the discharge valve.
[0010] For the correct functioning of an aerostatic bearing arrangement, it is necessary
to use a flow restrictor between the high-pressure region that involves the cylinder
externally and the clearance between the piston and the cylinder. This restriction
serves to control the pressure in the bearing arrangement region and to restrict the
gas flow.
[0011] Between the various possible solutions, it is usual to employ the cooling-circuit
gas itself for providing aerostatic bearing arrangement of the piston. In this way,
the whole gas used in bearing arrangements represents a loss in efficiency of the
compressor, since the gas is diverted from its original function, which is to generate
cold in the evaporator of the cooling system. Thus, it is desirable for the gas flow
rate employed in bearing arrangement to be as low as possible, so as not to impair
the compressor efficiency.
[0012] In order for the functioning of a cooling compressor to be efficient, all the characteristic
losses of this type of equipment should be kept as low as possible, as for example,
mechanical losses (friction between components), electric losses (appearance of parasite
currents, resistance to motor current passage or thermodynamic losses (leakages, flow
of undesirable heat). With regard to gas compression, in order for the efficiency
of the compressor to be high, it is necessary that all the work carried out on the
gas should be employed in the cooling system. For this reason, any type of leakage
or phenomenon that causes loss of gas after the compression of the latter is undesirable.
[0013] Anyway, there will always be leakages, because, in order to provide bearing arrangement,
gas should be present between the cylinder walls and the piston walls. However, the
efficiency logic requires the gas leakages to be kept as low as possible, in order
not to affect the compressor efficiency significantly.
[0014] The main sources of leakages in a compressor are discharge valves and suction valves
and the clearance between piston and cylinder. The clearance between the piston and
the cylinder will be called perimeter clearance hereinafter.
[0015] For a better understanding of the phenomena that cause decrease in the compressor
efficiency, the region between the piston top and the cylinder head is called compression
chamber, and there is where the high pressures on the gas take place. The region that
is between the piston bottom and the cylinder portion opposite the head is called
low-pressure region.
[0016] In linear compressors that make use of aerostatic bearing arrangement, two phenomena
related to loss of gas take place, which will he the object of observation for understanding
the present technology.
Leakage
[0017] The phenomenon leakage is defined by the amount of gas that circulates between the
high-pressure region (above the piston top) and the low-pressure region (below the
piston bottom), through the perimeter clearance. This leakage phenomenon always occurs
when the piston is in the compression phase, i.e., moving toward the head. When this
piston movement takes place, the gas is compressed up to a discharge pressure (Pd)
through the perimeter clearance, throughout the clearance length (Cf), reaching the
suction-pressure region (Ps) located on the opposite side of the compression chamber.
It should be noted that this gas does not come out of the compressor into the cooling
system to play the main role, which is to generate cold.
Irreversibility
[0018] To thermodynamics, irreversibility is a characteristic of all the real processes
and their sources are the dissipative processes. Systems provided with aerostatic
bearing arrangement undergo the irreversibility phenomenon in the compression, caused
by the presence of a small portion of gas in the clearance between the cylinder and
the piston. Irreversibility can be understood as being the loss of energy resulting
from the flow of the small portion of gas into and out of the perimeter clearance.
[0019] Considering the technology of linear compressors provided with bearing arrangement,
a loss of load is always associated to a flow of gas, which inevitably consumes energy,
the compressor being negatively influenced by this irreversibility phenomenon.
The problems
[0020] For a better understanding of the repercussions of the leakage and irreversibility
phenomena, figure 5 shows experimental results that relate the power consumed by the
said two effects as a function of the clearance between piston and cylinder. It should
be noted that the losses due to irreversibility and leakage occur simultaneously.
[0021] The graph in figure 5 does not leave any doubt about the magnitude of the loss of
efficiency, since the variation in dimension between piston and cylinder on the order
of 5µm entails loss of power on the order of 2W-10W, that is, the greater the clearance
in the piston/cylinder assembly, the greater the loss in power associated.
[0022] Therefore, there is no doubt that the technology of linear compressors provided with
aerostatic bearing arrangement needs to have a solution that inhibits the enhanced
loss of energetic efficiency due to the perimeter clearance.
[0023] Thus, at present there are no linear compressors provided with aerostatic bearing
arrangement capable of effectively reducing the loss of efficiency due to the use
of cooling gas for providing the piston with bearing arrangement. In other words,
the present invention manages to achieve a geometric and dimensional relationship
designed for inhibiting the loss of efficiency in providing bearing arrangement by
reducing the specific perimeter clearance, as well as providing a solution of easy
productive implementation, guaranteeing benefits for the final user and, by the result
of better energetic efficiency, for the environment.
Objectives of the Invention
[0024] Therefore, it is an objective of the present invention to minimize the losses of
efficiency that occur on the gas of a linear compressor provided with aerostatic bearing
arrangement.
[0025] It is also an objective of the present invention to provide spacing between the piston/cylinder
assembly, so as to decrease the clearance where there is higher gas density that is
not employed in the cooling process.
[0026] It is a further objective of the present invention to provide a dimensional relationship
and of the form in the piston/cylinder assembly so as to guarantee maximum efficiency
of a linear compressor provided with aerostatic bearing arrangement.
Brief Description of the Invention
[0027] The objectives of the present invention are achieved by means of a piston/cylinder
assembly, the piston being displaceably positioned within the cylinder, the piston
moving between a top dead center and a bottom dead center, wherein there is a perimeter
clearance between the inner wall of the cylinder and the outer wall of the piston
for providing the piston with aerostatic bearing arrangement, wherein the minimum
perimeter clearance occurs in at the upper portion of the piston when the piston is
at its top dead center, and a linear compressor comprising the piston/cylinder assembly
described.
[0028] The objectives of the present invention are also achieved by means of a piston/cylinder
assembly for a linear compressor, the piston being displaceably positioned within
the cylinder, the piston moving between a high-pressure portion and a low-pressure
portion, the high-pressure portion having higher gas density than the low-pressure
portion, a perimeter clearance being defined between the inner wall of the cylinder
and the outer wall of the piston for providing the piston with aerostatic bearing
arrangement with gas, the dimension of the perimeter clearance varying in an inversely
proportional manner with respect to the gas density in the perimeter clearance.
Brief Description of the Drawings
[0029] The present invention will now be described in greater detail with reference to examples
of embodiment represented in the drawings. The figures show:
Figure 1 is a sectional view of a linear compressor provided with aerostatic bearing
arrangement of the prior art.
Figure 2 is a sectional view of a linear compressor provided with aerostatic bearing
arrangement of the prior art showing the gas pressures.
Figure 3 is a sectional view of a linear compressor provided with aerostatic bearing
arrangement of the prior art showing the gas pressures at instant i).
Figure 4 is a sectional vie of a linear compressor provided with aerostatic bearing
arrangement of the prior art showing the gas pressures at instant ii).
Figure 5 is a graph of power loss due to the clearance between cylinder and piston.
Figure 6 is a graph of the pressure profile in the piston/cylinder clearance as a
function of the pressure, position and time.
Figure 7 is a graph of the gas-mass flows in the piston/cylinder clearance in the
top and bottom region of the piston.
Figure 8 is a graph of the gas-mass flows in the piston/cylinder clearance in the
top region of the piston.
Figure 9 is a graph of the gas-mass flows in the piston/cylinder clearance in the
bottom region of the piston.
Figure 10 is a sectional view of a piston/cylinder assembly presenting an efficient
solution.
Figure 11 is a sectional view of a possible embodiment of the piston/cylinder assembly
of the present invention.
Figure 12 is a sectional view of a possible embodiment piston/cylinder assembly of
the present invention.
Figure 13 is a sectional view of a possible embodiment of the piston/cylinder assembly
of the present invention.
Figure 14 is a sectional view of a possible embodiment of the piston/cylinder assembly
of the present invention.
Detailed Description of the Figures
[0030] The present invention proposes a technological advance in the piston/cylinder assembly
of linear compressors with aerostatic bearing arrangement, both in the energetic efficiency
and in the productive process.
[0031] According to the functioning principle of a cooling circuit and as shown in figure
1, preferably, the gas compressing mechanism occurs by the axial and oscillating movement
of a piston 1 inside a cylinder 2. At the head 3, one positions the discharge valve
5 and suction valve 6, which regulate the entry and exit of gas into/out of the cylinder
2. It should be further noted that the piston 1 is actuated by means of an actuator
7 connected to the linear compressor motor, and the latter is not the subject of further
explanations in this document.
[0032] The piston 1 of a compressor, when actuated by the linear motor, has the function
of developing a linear alternating movement, providing a movement of the piston 1
inside the cylinder 2 that exerts a compression of the gas admitted by the suction
valve 6 to the extent in which the gas can be discharged to the high-pressure side
through the discharge valve 5.
[0033] The cylinder 2 is mounted within the block 8, and a cover 9 with the discharge passer
10 and the suction passer 11, which connect the compressor to the rest of the system.
[0034] As said before, the relative movement between piston 1 and cylinder 2 requires the
bearing arrangement of the piston 1, which consists of the presence of a fluid in
the perimeter clearance 12 between the two walls, for the purpose of separating them
during the movement. An advantage of using the gas itself as a lubricating fluid is
the absence of an oil pumping system.
[0035] Preferably, the gas used for the bearing arrangement may be the gas itself that is
pumped by the compressor and sued in the cooling system. In this case, the gas is
diverted, after compression, from the discharge chamber 13, from the cover 9 through
the channel 14, to the pressurized region 15 around the cylinder 2, wherein the pressurized
region 15 is formed by the outer diameter of the cylinder 2 and inner diameter of
the block 8.
[0036] From the pressurized region 15 the gas passes through the restrictors 16, 17, 18,
19 inserted into the cylinder wall 2 toward the perimeter clearance 12 existing between
the piston 1 and the cylinder 2, forming a gas cushion that prevents contact between
the piston 1 and the cylinder 2.
[0037] With a view to restrict the gas flow between the pressurized region 15 and the perimeter
clearance 12, it is necessary to make use of a restrictor 16, 17, 18, 19. This restriction
serves to control the pressure in the bearing-arrangement region and to restrict the
gas flow, since the whole gas used in the bearing arrangement represents a loss of
efficiency of the compressor, since the main function of the gas is to be sent to
the cooling system and generate cold. Thus, it should be pointed out that the gas
diverted to bearing arrangement should be as little as possible, so as not to impair
the efficiency of the compressor.
[0038] In order to maintain the balance of the piston 1 within the cylinder 2, at least
three restrictors 16, 17, 18, 19 are preferably necessary in a given section of the
cylinder 2 and at least two regions of restrictor 16, 17, 18, 19 are necessary on
the cylinder 2. The restrictors should be in such a position that, even with oscillation
movement of the piston 1, the restrictors 16,17, 18, 19 will never be uncovered, that
is, the piston 1 will not come out of the actuation area of the restrictor 16, 17,
18, 19.
[0039] Figure 2 presents information relating to the expressions existing inside the cylinder/piston
1 assembly. The instant of figure 2 corresponds to a gas compression movement effected
by the piston 1. At this instant there is a gas discharge pressure that is much higher
than the pressure existing in the opposite region of the piston 1.
[0040] For a better understanding of the phenomena that entail the decrease in efficiency
of the compressor, the region between the piston 1 top and the cylinder head 3 will
be called high-pressure region. The piston cylinder head 3 will be called low-pressure
region.
[0041] In turn, when the piston 1 top is at the point closest to the cylinder head 3, this
is called top dead center (TDE/PMS) and when the piston 1 top is at the point farthest
from the cylinder head 3 this is called (LDE/PMI). Thus, the piston 1 travels a linear
movement between the top dead end (TDE/PMS) and the lower dead end (LDE/PIM).
[0042] Of course the gas pressure at the moment of compression will be higher in the high-pressure
region. This gas flows to the perimeter clearance 12, defined by the difference between
the piston diameter (Pd/Dp) and the cylinder diameter (Cd/Dc), travelling the whole
length of the clearance (Cf) which, in this case, corresponds to the length of the
piston 1. For a better definition of the invention, for the purpose of the expressions
existing in the perimeter clearance 12, one should understand that the top of the
perimeter clearance 12 and the bottom of the perimeter clearance 12 vary throughout
the clearance (Cf).
[0043] As already demonstrated, the size of the clearances between piston 1 and cylinder
2 entails a loss of efficiency of the compressor in a considerably high relationship.
In order to assess the better solution, one should detect which of the factors leakage
and irreversibility has more influence on the loss of efficiency. For this purpose,
we use theoretical models.
[0044] Anyway, before the explanation on the result of the simulation, it is necessary to
comment a few characteristics on the behavior of a gas. Thus, the heat exchange of
a cooler is based on the "General Equation of the Perfect Gases", which demonstrates
that in a gaseous mass the volumes and pressures are directly proportional to their
absolute temperatures and inversely proportional to each other.
[0045] Additionally, it is necessary to synthesize a few characteristics on the gas flow,
which is established by the perimeter clearance 12:
- as it is the case for any fluid, the gas flow within the clearance exhibits a loss
of load;
- the gas is a compressible fluid, so that the loss of load causes the gas pressure
to vary throughout the clearance and, as a result, its density varies;
- the pressure profile, consequently the gas density, in the perimeter clearance 12
throughout the piston length assumes different forms depending on the instant of the
compression cycle.
[0046] According to the characteristics described, two different instants were considered
for working out the theoretical model. The instant 1 corresponds to figure 3 and occurs
when the piston is at its top dead end. In turn, the instant 2 corresponds to figure
4 and occurs at the moment when the piston 1 is at the beginning of its suction movement.
[0047] Figure 6 shows the pressure profile in the perimeter clearance as a function of the
pressure, position and time of the piston 1 with respect to the cylinder 2. This graph
shows that an oscillation movement cycle of the piston 1 corresponds to the axis X,
and it is possible to identify, around 150 ms, the instants 1 and 2, the dotted line
(see indications i1 and i2). The growing variation at the axis Y corresponds to a
position along the clearance of the cylinder 2 with the piston 1. Finally, the rise
in pressure corresponds to the increase at the axis Z. This graph enables one to consider
that:
- i) at the instant 1 (i1), the pressure profile throughout the piston 1 and the minimum
in the base region of the piston 1; in other words, the pressure at the bottom is
always minimum, regardless of the pressure at the tope of the piston 1;
- ii) at the instant 2 (i2), the pressure profile throughout the perimeter clearance
12 (dotted line) has its maximum value in the central region of the perimeter clearance
12, with the minimum pressure at the bottom and an intermediate pressure at the top
of the perimeter clearance 12.
[0048] The gas mass flow through the perimeter clearance 12 between the piston 1 and the
cylinder 2 behaves, at each moment, in accordance with the pressure profile shown
in figure 6 and the gas density throughout the clearance 12. The diagram in figure
7 shows the mass flows in the bottom and top regions of the piston 1 throughout the
time equivalent to an oscillation of the piston 1, indicating also the instants 1
and 2 (i1 and i2) already mentioned in the graph of figure 6.
[0049] The graph of figure 7 shows that the flow that comes out of the compression chamber
4 corresponds to the negative mass flow, that is, in the top region (TP) or at the
bottom (BP) of the piston 1. A positive flow represents the gas that returns to the
compression chamber 4.
[0050] One can notice that, during the larger part of the time, the mass flow at the top
of the piston 1 is different from the mass flow at the bottom. One can further notice
that, by the bottom region of the perimeter clearance 12, there is a constant leakage
of gas (dotted line of negative values), further that the mass flow thereof varies
a little throughout the oscillation of the piston 1.
[0051] The continuous line that corresponds to the mass flow in the perimeter clearance
12 in the top region of the piston 1 shows that the gas comes out of the compression
chamber 4 and goes into the perimeter clearance 12 during a certain period of time
(negative mass flow - continuous line below the abscissa axis).
[0052] Additionally, at the beginning of the suction motion, the gas that has remained in
the perimeter clearance 12 is returned to the compression chamber 4. Such a pressure,
in the direction opposite the suction pressure (Ps), which goes into the compression
chamber 4 through the suction valve 6, impairs the entry of the gas into the compression
chamber 4, thus interfering with the output of the compressor.
[0053] Examining attentively figures 3 and 4, which correspond to the instants 1 (i1) and
2 (i2), respectively, in the light of the graphs of figures 6 and 7 one can see that
the at the instant 1 (i1) the piston is at the top dead center (PMS), where there
is the highest mass flow (2.8E-10 kg/s) coming out of the compression chamber 4 and
going into the perimeter clearance 12 in the top region of the piston 1, the leakage
through the bottom region of the piston 1 being of 0.04E-10 k/s.
[0054] For the instant 2 the largest flow, of about 1.2E-10 kg/s, takes place in the gas
return in the top region of the perimeter clearance 12 to the compression chamber
4. At the same instant, the leakage through the bottom is on the order of 0.094E-10
kg/s.
[0055] In other words, for both instants 1 and 2, the gas mass flow with high density (GAD)
occurs in the top region of the perimeter clearance 12, the gas flows with low density
(GBD) occurring in the bottom region of the perimeter clearance 12.
[0056] The diagrams of figures 8 and 9 show separately the same curves represented by the
diagram of figure 7. By observing figure 8, which represents the mass flow in the
top region of the piston, one concludes that the gas mass per compressor cycle that
goes into the perimeter clearance 12 is equivalent to the area between the negative
part of the mass flow curve and the abscissa axis (axis xx). In turn, further for
figure 8 the gas mass that returns to the compression chamber 4 through the top of
the diameter clearance 12 is equivalent to the portion of the graph represented above
the abscissa axis.
[0057] The difference between these two amounts of mass, or graphically, the difference
between the areas above and below the abscissa axis of figure 8 corresponds to the
gas mass equivalent to the leakage of gas through the bottom of the piston 1, and
the latter, in turn, is represented by the filled area of the graph in figure 9.
[0058] Therefore, one can conclude that of all the gas that goes into the perimeter clearance.
12 between the piston 1 and the cylinder 2 little will escape through the bottom region
in the form of leakage. The largest part of the gas displaces between the perimeter
clearance 12 and the compression chamber 4.
[0059] Thus, the greatest part of the power lost because of the perimeter clearance 12 existing
between the piston 1 and the cylinder 2 shown in figure 5 comes from the irreversibility
effect, not from the leakage effect.
[0060] The highest gas densities occur in the top region of the piston 1 when the latter
is closest to the head 3, due to the fact that the high pressures in this region are
capable of compressing the gas into a smaller volume.
[0061] On the basis of the identification of the region of the piston-1/ cylinder-2 assembly
responsible for the greatest loss of efficiency of the compression, it is possible
to achieve a solution of high energetic efficiency, which is the focus of the present
invention.
[0062] The way to reduce the irreversibility effect caused by the clearance between the
piston 1 and the cylinder 2 is by keeping the clearance as low as possible, so that
here will be less volume available for the accumulation of gas at high pressure in
the perimeter clearance 12 during the compression phase. In this way, it is possible
to establish a smaller gas flow between the compression chamber 4 and the perimeter
clearance 12.
[0063] However, the decrease of the perimeter clearance 12 between the piston 1 and the
cylinder 2 finds its limits in the pressure limits of the manufacture process (machining
processes) used for making the piston 1 and the cylinder 2.
[0064] As a rule, the perimeter clearance between the piston 1 and the cylinder 2 may be
as follows: the lower the cylindricity errors on the outer surface of the piston 1
and the inner surface of the cylinder 2 the smaller the clearance. At present, this
clearance in cooling compressors is of about a few microns.
[0065] Additionally, it should be noted that the cylindricity error obtained on parts like
pistons 1 and cylinders 2 is dependent upon the length of the cylindrical surfaces,
that is, on the length of piston 1 and cylinder 2. The relationship is established
so that the longer the part length, the greater the cylindricity which it exhibits.
Thus, an option of decreasing the cylindricity error to enable one to reduce the perimeter
clearance 12 might be simply to reduce the length of the piston 1 and /or cylinder
2.
[0066] Figure 10 shows a piston/cylinder assembly with a large clearance in the top region
of the piston 1 due to the high cylindricity error of the cylinder.
[0067] The decrease in length of piston 1 and cylinder 2, however, is not suitable for compressors
that use aerostatic bearings instead of lubricating oil, because they need longer
piston 1 and cylinder 2, so that the aerostatic bearings will provide the necessary
support for the piston 1, preventing contact between the piston 1/cylinder assembly;
otherwise, the assembly would undergo premature wear and, as a result, loss of efficiency.
[0068] The problem to be solved by the present invention is, therefore, one exclusive of
compressors that use aerostatic bearings. On the one hand, there are the difficulties
mentioned in the previous paragraph and, on the other hand, only compressors with
aerostatic bearings have a perimeter clearance 12 through which the cooling gas flows.
[0069] Since it was not possible to reduce the length of the piston 1 and cylinder 2 to
achieve a reduction of the cylindricity errors, due to the questions of stability
and bearing arrangement of the piston 1 in the cylinder 2, a solution has been found
which enables one to achieve the effect of a shorter piston 1 or cylinder 2. Such
a solution results in a decrease in the perimeter clearance 12 between piston 1 and
cylinder 2, without the need to reduce the length of one of the parts of the piston/cylinder
assembly.
[0070] According to what was demonstrated by the results of the theoretical models, the
smallest perimeter clearance 12 possible is all the more necessary and beneficial
the closer to the piston 1 top, that is, the closer to the region of the piston 1
the decrease in the perimeter clearance 1 is carried out, the greater the effect of
reducing irreversibility, since it is in this region that the largest gas-mass flows
that go into and come out of the perimeter clearance 12 take place.
[0071] It is not necessary to reduce the perimeter clearance 12 throughout the clearance
length (Cf), nor during the whole cycle of oscillatory movement of the piston 1, by
rather at the moment when pressures close to the discharge pressure occur in the compression
chamber 4, that is, when the piston 1 is close to the head 3.
[0072] In this regard, the problem of the perimeter clearance 12 can be solved by using
a smaller clearance in the top region of the piston 1 than in the bottom region of
the piston 1.
[0073] Preferably, but not compulsorily, a solution of the present invention for the irreversibility
is by using components (piston and/or cylinder) with a varying cross-section, so as
to create a specific portion in which the clearance will be effectively reduced. These
regions have lengths that are quite shorter than the lengths of the components themselves
and for this reason they will exhibit lower cylindricity errors than those of internal
components.
[0074] Thus, exclusively in these regions the clearance between piston 1 and cylinder 2
can be reduced.
[0075] Figures 11 to 14 show a few possible embodiments of the piston/cylinder assembly
that guarantee better compressor efficiency. The piston 1, due to its smaller bottom
diameter, enables en increase in the clearance at the bottom of the piston/cylinder
assembly and the consequent decrease in the top clearance of the piston 1.
[0076] It should be noted that whatever the solution the clearance in the top portion of
the piston 1 is always smaller than in any other region of the piston/cylinder assembly.
Additionally, the closest to the head 3 the piston 1 is the smaller the perimeter
clearance.
[0077] Figures 11 to 14 show that one can find solutions in which the bottom diameter of
the piston 1 is reduced with respect to the rest of its body (figure 11) The same
result can be achieved through one of more variable sections of the piston 1 and cylinder
2, while achieving a perimeter clearance 12 that is reduced in the top region of the
piston/cylinder assembly.
[0078] Figures 12 and 13 show possible geometrical embodiments of the piston 1/cylinder
2 assembly that make use of two different sections on one of the elements piston 1
or cylinder 2 with the objective of reducing the perimeter clearance 12 as the piston
1 gets close to the cylinder 2 top.
[0079] In figure 12, the piston 1 exhibits two different sections, the section adjacent
the top region of the piston 1 having larger diameter than the region adjacent the
lower portion of the piston 1, that is, the top portion of the piston has larger dimension
than the rest of the piston 1. Thus, as the piston 1 moves to the top of a cylinder
2 that is slightly arched in its longitudinal direction, the diameter clearance 12
reduces to a minimum when the piston 1 is close to the cylinder 2 top. This slightly
ached shape of the cylinder 2 in its longitudinal direction may be defined as a circle-segment
top shape.
[0080] Figure 13 shows a situation analogous to figure 12, but this time it is the cylinder
2 that has two sections provided with different diameters. Naturally, in order to
guarantee a smaller diameter clearance 12, the cylinder 2 undergoes a narrowing in
the section at the portion located closer to the cylinder top (the top portion of
the cylinder 2 has a smaller dimension than remaining portion of the cylinder 2),
which provides the minimum necessary diameter clearance 12.
[0081] Figure 14 shows another of these possible embodiments, which can be achieved by means
of a cylinder 2 that has a frustum-type geometry, wherein the portion of smaller diameter
would be in the top region of the cylinder 2. Thus, as the top of the piston 1 gets
closer to the top of the cylinder 2, the perimeter clearance 12 is reduced.
[0082] The solution of the present invention is, therefore, achieved when one ensures a
relationship in which the dimension of the perimeter clearance 12 varies in an inversely
proportional manner with respect to the density of the gas present in the perimeter
clearance 12.
[0083] Preferred examples of embodiment having been described, one should understand that
the scope of the present invention embraces other possible variations, being limited
only by the contents of the accompanying claims, which includes the possible equivalents.
1. A piston/cylinder assembly, the piston (1) being displaceably positioned inside the
cylinder (2),
the piston moving between a top dead center (TDC/PMS) and a bottom dead center (BDC/PMI),
between an inner wall of the cylinder (2) and an outer wall of the piston 91) there
being a perimeter clearance (12) for aerostatic bearing arrangement (1),
the assembly being
characterized in that:
- there is a minimum perimeter clearance (12) at the top portion of the piston (1)
when the piston (1) is at its top dead center (TDC/PMS).
2. A piston/cylinder assembly according to claim 1, characterized in that the perimeter clearance (12) is variable from the bottom dead center (BDC/PMI) to
the top dead center (TDC/PMS).
3. A piston/cylinder assembly according to claims 1 and 2, characterized in that the closer to the top portion of the piston (1) top the perimeter clearance (12)
is, the smaller it is.
4. A piston/cylinder assembly according to claims 1 to 3, characterized in that the piston (1) has a variable cross-section.
5. A piston/cylinder assembly according to claims 1 to 4, characterized in that the cylinder (1) has a variable cross-section.
6. A piston/cylinder assembly according to claims 1 to 5, characterized in that the top portion of the piston (1) has a dimension larger than the remaining portion
of the piston (1).
7. A piston/cylinder assembly according to claims 1 to 6, characterized in that the top portion of the cylinder (2) has a dimension smaller thant the remaining portion
of the cylinder (2).
8. A piston/cylinder assembly according to claims 1 to 7, characterized in that the piston (1) is conical.
9. A piston/cylinder assembly according to claims 1 to 8, characterized in that the piston (1) has a circle-segment shape.
10. A piston/cylinder assembly according to claims 1 to 9, characterized in that the cylinder (2) has frustum-type geometry.
11. A piston/cylinder assembly according to claims 1 to 10, characterized in that the cylinder (2) has a circle-segment shape.
12. A linear compressor characterized by comprising a piston/cylinder assembly as defined in claims 1 to 11.
13. A piston/cylinder assembly for a linear compressor, the piston (1) being displaceably
positioned within the cylinder (2),
the piston (1) moving between a high-pressure portion (Pd) and a low-pressure portion
(Ps),
the high-pressure (Pd) having higher gas density than the low-pressure portion (Ps),
a perimeter clearance (12) being defined between an inner wall of the cylinder (2)
and an outer wall of the piston (1) for aerostatic bearing arrangement (1) with gas,
the assembly being
characterized in that:
the dimension of the perimeter clearance (12) varies in an inversely proportional
manner with respect to the gas density in the perimeter clearance (12).
14. A piston/cylinder assembly according to claim 1, characterized in that the piston (1) has a variable cross-section.
15. A piston/cylinder assembly according to claims 13 and 14, characterized in that the cylinder (1) has a variable cross-section.
16. A piston/cylinder assembly according to claims 13 to 15, characterized in that the top portion of the piston (1) has a larger dimension than the remaining portion
of the piston (1).
17. A piston/cylinder assembly according to claims 13 to 16, characterized in that the top portion of the cylinder (2) has a smaller dimension than the remaining portion
of the cylinder (2).
18. A piston/cylinder assembly according to claims 13 to 17, characterized in that the piston (1) is conical.
19. A piston/cylinder assembly according to claims 13 to 18, characterized in that the piston (1) has a circle-segment shape.
20. A piston/cylinder assembly according to claims 13 to 19, characterized in that the cylinder (2) is conical.
21. A piston/cylinder assembly according to claims 11 to 20, characterized in that the cylinder (2) has a circle-segment shape.
22. A linear compressor characterized by comprising a piston/cylinder assembly according to claims 13 to 21.
23. A piston/cylinder assembly, the piston (1) being displaceably positioned within the
cylinder (2),
the piston (1) moving between a top dead center (TDC/PMS) and a bottom dead center
(BDC/PMI),
between an inner wall of the cylinder (2) and an outer wall of the piston (1) there
being a perimeter clearance (12) for aerostatic bearing arrangement of the piston
(1),
the assembly being
characterized in that:
- the perimeter clearance (12) is minimum when the piston (1) is at its top dead center
(TDC/PMS) and the closer to the top portion of the piston (1) the perimeter clearance
(12) is, the smaller it is.