Patent Application
20090104048
The invention relates to an axial piston compressor, especially to a compressor for the air-conditioning system of a motor vehicle, in accordance with the preamble of claim 1.
DE 195 14 748 C2 explains the tilting moments which act on a tilt plate in the case of, in principle, all compressors existing in the prior art, and also those compressors that have entered mass production, and which decisively contribute to the tilting behaviour of the tilt plate. The influences acting as moments about the centre of tilt of a tilt plate are, in detail, the following moments, the direction of the particular moments being given in brackets, with (−) denoting down-regulation (in the direction of minimum stroke) and (+) denoting up-regulation (in the direction of maximum stroke):
The moment due to the rotating masses (hereinafter referred to as MSW) generally has a down-regulating action; it is only in the region of very small tilt angles that as a result of, for example, an outwardly displaced location of centre of gravity (Steiner component when calculating the moment of deviation Jyz) an up-regulating moment can be produced in the case of the tilt plate (see DE 195 14 748 C2 in this regard). The component of the moment of deviation Jyz without a Steiner component already predominates, then, in the region of small tilt angles, and as the tilt angle increases the tilt plate has an increasingly down-regulating tilt moment.
Referring to DE 195 14 748 C2, a curve for the masses moved in translation having, as already explained, an up-regulating action is also shown.
Also of interest is the sum of the moments, which is likewise represented in graph form (
From EP 0 809 027 A1 there is known a further drive mechanism which is characterised in that the delivery quantity of the compressor is so compensated by the dynamic behaviour of the drive mechanism of the compressor that the delivery quantity can be kept constant. Specifically, the following is stated therein in this respect: “In order to keep constant the delivery quantity at changing rotation speeds, use can be made of the restoring torque of the wobble plate counteracting its inclined position due to dynamic forces on the co-rotating plate part.”
Building on EP 0 809 027 A1, measures are known from DE 198 39 914 A1 as to how regulation behaviour of such a kind (at least partial compensation of the quantity delivered) can be achieved. It is proposed that the component mass of the tilt plate be so dimensioned in relation to the masses moved in translation that the centrifugal forces of the tilt plate influence its regulation behaviour. Specifically, it says in that respect that the rotating mass of the drive plate is greater than the total mass of all the pistons so that the centrifugal forces occurring during rotation of the drive plate are sufficient to deliberately counteract the pivoting movement of the drive plate in regulating manner and thereby to influence, especially to reduce or to limit, the piston stroke and, accordingly, the quantity delivered.
In the as yet unpublished DE 103 24 393 belonging to the Applicant it is explained why the component mass should not be the preferred parameter for influencing as desired the regulation behaviour of the drive mechanism as a result of changes in the speed of rotation.
The desired regulation behaviour of the compressor is primarily achieved not by means of the component mass of the tilt plate in relation to the masses moved in translation but rather by taking into account the mass moment of inertia of the component unit that is the “tilt plate”, which depends more on its geometry than on the component mass. A central idea is accordingly that, in the case of fluctuations or changes in the speed of rotation, the moment due to masses moved in translation is directly compensated, or even over-compensated, by the moment due to rotating masses.
In the case of such compressors it is desirable to reduce the frequency and intensity of regulatory interventions to a low level. With reference to the prior art and DE 195 14 748 C2 it is extremely disadvantageous that, when there is an increase in the delivery volume due to an increase in the speed of rotation, an additional increase in the delivery volume due to an increase in the tilt angle of the tilt plate is added thereto.
This effect has to be compensated by appropriate regulatory interventions. This is not convenient, is onerous and reduces the efficiency (fuel consumption).
EP 0 809 027 A1 specifies the goal that is to be achieved: keeping constant the delivery quantity.
It can, however, be demonstrated very simply that this is not possible solely by means of the advancing moment (down-regulation) acting on the tilt plate.
The delivery volume is directly proportional to the speed of rotation, that is to say if the speed of rotation doubles, the delivery volume also doubles.
On the other hand, the following equation applies for the tilt plate tilt moment, which is produced by the relevant moment of deviation:
MSW=Jyzω2
Because it is the square of the speed of rotation that influences the tilt moment, the expressed aim of “keeping constant the delivery quantity” cannot be achieved solely as a result of the design or dimensioning of the tilt plate.
The problem of the invention is to provide a compressor having optimised regulation behaviour wherein, at high and highest compressor speeds of rotation, a full-load condition is avoided.
The problem is solved by a compressor having the features in accordance with patent claim 1, preferred developments and embodiments being described in the subordinate claims.
It is accordingly a fundamental point of the invention that the tilting behaviour of the tilt plate acts in a self-limiting manner such that at high speeds of rotation of the compressor, especially at very high speeds of rotation or at the maximum speed of rotation of the compressor, the angle of maximum deflection of the tilt plate is less than the angle of maximum deflection αmax at low speeds of rotation of the compressor. A measure of such a kind rules out the possibility of the compressor's operating with too great a delivery volume at high speeds of rotation, especially at its maximum speed of rotation. Because this measure intervenes in the delivery volume in regulating manner, the number of regulatory interventions required is reduced (tilt plate compressors are usually regulated by means of the pressure difference between the pressure prevailing in the drive mechanism chamber of the compressor and the pressure prevailing on the output side of the compressor, that pressure difference governing the piston stroke and, therefore, the delivery volume of the compressor). This also results in the fact that in particular at moments of increasing speed of rotation, especially in the region of high speeds of rotation, a markedly reduced number of power-consuming regulatory interventions or possibly no regulatory intervention at all is/are required, which particularly in the case of use of a compressor in motor vehicles allows the power at those moments to be directed towards driving the motor vehicle without reduction. This is very advantageous especially in the case of overtaking manoeuvres.
Preferably, a compressor according to the invention is so constructed that both the geometry and also the dimensioning of all parts of the compressor that are moved in translation and of all parts moved in rotation are such that for predetermined tilt angles of the tilt plate, especially between a predetermined minimum tilt angle and a predetermined maximum tilt angle, the moment Mk, ges due to the masses moved in translation is so selected as to be less than the moment MSW due to the moment of deviation, that is to say is so selected as to be less than the moment due to the mass inertia of the tilt plate, that at high speeds of rotation of the compressor, especially at very high speeds of rotation or at a maximum speed of rotation, the angle of maximum deflection of the tilt plate is less than the angle of maximum deflection at lower speeds of rotation of the compressor. Those parts of the compressor which are moved in translation include the pistons and also, where appropriate, the piston rods and sliding blocks or the like. The parts moved in rotation include, basically, the tilt plate and also, where appropriate, one or more members for transmitting its drive. This arrangement constitutes an embodiment of a compressor according to the invention which is simple to produce in structural terms and accordingly economical to manufacture. In addition, of course, the advantages already mentioned hereinbefore are also entirely applicable to the present preferred embodiment, especially the advantage of a low number of regulatory interventions necessary during operation of a compressor according to the invention.
It should be mentioned at this juncture that modern axial piston compressors, especially compressors intended for use in a motor vehicle air-conditioning system, bring about, under partial loading, a coolant mass flow—corresponding as it were to the compressor output—of a magnitude such that a sufficiently high cooling performance is made available. Particularly at high, very high and also at the highest speed(s) of rotation, the compressor therefore has to be down-regulated, the number of regulatory interventions required being very greatly reduced, or no regulatory intervention at all being required, as a result of the present invention. Reference should also be made, moreover, to the safety aspect, which plays an important role especially in the case of compressors which are operated with a high-pressure coolant. The arrangement according to the invention prevents a high speed of rotation and a large compressor piston stroke from occurring at the same time, which may well, in the case of compressors according to the prior art, result—given the (possibly sudden) occurrence of a high speed of rotation and a large stroke—in an increase in compressor pressure such that bursting of the compressor housing ensues.
The compressor according to the invention can comprise at least one arrangement which exerts an adjusting force on the tilt plate. Preferably, said arrangement is associated with the compressor in addition to an arrangement for adjustment or control or regulation of its drive mechanism chamber pressure. This arrangement or these arrangements can function on its/their own as the self-limiting element of the tilt plate but can also be arranged in a compressor wherein the distribution of masses or the distribution of moments of the tilt plate already brings about down-regulation thereof at high speeds of rotation. In this case, an already existing tendency of the compressor, due to the geometry of the tilt plate mechanism, to down-regulate at a high speed of rotation is assisted or reinforced. The at least one arrangement for exerting an adjusting force optionally comprises a resilient element, which can be present especially in the form of a restoring spring, and/or an actuator or an adjusting piston. Alternatively or additionally, a compressor according to the invention can comprise an adjusting member which is dependent on centrifugal force and/or a further spring. It is also feasible for the at least one arrangement for exerting the adjusting force to comprise a throttling location, the cross-section of which can be varied by means of an adjusting member, especially by means of an adjusting piston. The throttling location can be arranged especially on the intake side or on the pressure side of the compressor. If the throttling location is arranged on the intake side of the compressor, it is accordingly possible, as a result, for the density of the intake gas to be regulated or reduced as desired. As an adjustment variable for the adjusting member, especially an adjusting piston, preference is given to the pressure prevailing in the drive mechanism chamber of the compressor and/or the pressure prevailing on the outlet side of the compressor. These adjustment variables are readily accessible. In a further preferred embodiment, the adjusting member is regulated or controlled internally, especially by means of a solenoid or like apparatus. Alternatively, a constant cross-section of the throttling location is also feasible. All the embodiments mentioned hereinbefore are structurally simple measures, which are accordingly economical to produce. Depending on the area of use of the compressor and the desired adjusting force or restoring force, the various measures can be implemented either singly or also in combination with one another in the compressor, as a result of which it is possible to achieve a desired regulation behaviour by simple means.
In a preferred embodiment, the centre of gravity of the tilt plate is located on the tilt axis thereof. The tilt axis of the tilt plate is in turn preferably located on the central axis of the drive shaft. These structural measures ensure quiet running of the compressor with a low degree of wear and with minimal imbalance.
In a further preferred embodiment, the tilt plate is ring-shaped, that is to say is in the form of a tilt ring. This ensures an optimum ratio between the mass of the tilt plate or tilt ring (which is low) and its mass moment of inertia (which is high).
Preferably, the angle of maximum deflection of the tilt plate in the region of low and medium speeds of rotation of the compressor corresponds to an angle αmax, whereas the angle of maximum deflection at the maximum speed of rotation of the compressor corresponds approximately to an angle of αmax/2. Under the terms of the present Application, regions of low and medium speeds of rotation extend up to about half the maximum speed of rotation of the compressor. In the case of compressors of modern design, which reach a maximum speed of rotation of about 8000 to 10,000 revolutions per minute, a range of low and medium speeds of rotation accordingly extends up to about 4000 revolutions per minute, although it should be pointed out that the above-mentioned figures are of course to be regarded as merely being examples.
Preferably, for speeds of rotation from about 4000 to 5000 revolutions per minute upwards, the tilt plate of a compressor according to the invention should no longer be capable of being pivoted up to an/the angle αmax. This ensures optimum loading limitation, which has advantageous effects particularly from the safety aspect. Both the danger of icing-up and also the danger of the compressor housing bursting are markedly reduced compared to compressors according to the prior art.
In a further preferred embodiment, the limitation of the pivot angle becomes greater for increasing speeds of rotation of the compressor. For this embodiment, too, there are obtained advantages, especially when seen from the safety aspect, which are analogous to the above-mentioned embodiments.
The ratio of the moment of deviation in the y direction to the total mass of all parts moved in translation Jy/mk, ges, which include as explained hereinbefore, for example, the pistons and also, where appropriate, the sliding blocks and piston rods, is preferably at least about 1000 mm2. Even more advantageous is a ratio Jy/mk, ges of more than 1500 mm2. The ratio of the moment of deviation in the z direction to the total mass of all parts moved in translation Jz/mk, ges is, in a further preferred embodiment, at least about 2000 mm2; a value of more than 3000 mm2 is even more advantageous. As a result of the large ratio between the moment of deviation and the total mass of the parts moved in translation, the desired regulation characteristic curve is achieved in simple manner.
In accordance with the invention it has also been recognised that a “fitting” spring constant should be selected (this governs the slope of the regulation characteristic curve). In a further preferred embodiment, a restoring spring which exerts a restoring force on the deflected tilt plate has a spring constant of less than about 60 N/mm. Even more advantageous is using restoring springs which have a spring constant of less than 30 N/mm, the moments of the parts moved in translation and in rotation of course being appropriately matched in each particular case. Springs having such spring constants are obtainable at low cost, require little space for installation and, in addition, are subject to a low degree of wear.
In the case of the embodiments described herein, the tilt plate has a directly defined or indirectly defined (by means of the sliding sleeve) minimum and maximum end-stop. The maximum end-stop is not reached in the case of high speeds of rotation and is prespecified in fixed, non-regulatable manner in the embodiments described hereinbefore.
In a further preferred embodiment, a compressor according to the invention has an end-stop which is dependent on the speed of rotation and which limits the angle of maximum deflection of the tilt plate in dependence on the speed of rotation of the compressor. This end-stop which is dependent on the speed of rotation serves as a further measure for ensuring that at high speeds of rotation the angle of maximum deflection of the tilt plate is less than in the case of low speeds of rotation. In this case, for example, a shoulder on the drive shaft is feasible, which shoulder prevents pivoting beyond a predetermined angle. A different arrangement, for example comprising a sleeve displaceably mounted on the drive shaft, or the like, would also be feasible. The end-stop can be controlled, for example, by means of the centrifugal force (which is dependent on the speed of rotation).
It should be pointed out at this juncture that it has been recognised in the context of the invention that the avoidance of outwardly displaced locations of the centre of gravity avoids relatively marked bends in the regulation characteristic curve.
The invention will be described hereinbelow with reference to further advantages and features by way of example and with reference to the accompanying drawings. The drawings show in
For better clarification of the invention, the tilt moment of the pivotable part of the tilt plate arrangement can be expressed in simplified form by means of equations which are relevant to a ring-shaped tilt plate component.
The simplified derivation shown hereinbelow is therefore to be regarded as being given merely by way of example; in the case of a more complex geometry of the tilt plate, the mass moments of inertia and moments of deviation and other variables influenced by the geometry and density would be calculated by means of CAD.
For the derivation of the moment of deviation the following mathematical relationships generally apply (co-ordinate system
The co-ordinate system used herein can be seen from the diagram in
(Note: J3≈2J2)
Aim: Jyz should have a particular magnitude; necessarily increases.
For the moment of deviation, which governs the pivoting movement, the following applies:
Independently of
and also the moment MSW due to the moment of deviation:
In the context of the invention, the following moment ratio should be established by structural means:
MSW≧Mk, ges or
As already mentioned, the (tilting) moment of the tilt ring due to the associated moment of deviation can be deliberately adjusted by means of various parameters (geometry, density distribution, mass, mass centre of gravity) so that
MSW≧Mk, ges
holds true. However, MSW should especially be markedly greater than Mk, ges.
In the context of the equations given, the variables denote the following:
θ rotation angle of the shaft (the considerations above and below being made on the basis of θ=0 for the sake of simplicity)
η number of pistons
R distance from piston axis to shaft axis
ω speed of rotation of shaft
α tilt angle of tilt ring/tilt plate
mk mass of a piston including sliding block (pair)
mk,ges mass of all pistons including sliding blocks
mSW mass of tilt ring
ra external radius of tilt ring
ri internal radius of tilt ring
h height of tilt ring
g density of tilt ring
V volume of tilt ring
βi angle position of piston i
zi acceleration of piston i
Fmi mass force of piston i (including sliding blocks)
M(Fmi) moment due to mass force of piston i
Mk,ges moment due to mass force of all pistons
MSW moment (or advancing moment of tilt ring/tilt plate) due to the moment of deviation (JYZ)
When selecting the parameters for achieving the desired moment ratio, the location of the centre of gravity should, in accordance with the invention, largely have no influence. This means that when calculating the governing moment of deviation, the Steiner component to be taken into account for the mass centre of gravity is basically
yszsm=0
In this case the co-ordinate zs is located in the direction of the shaft axis and the co-ordinate ys is located perpendicular thereto and perpendicular to the axis of tilt xs of the tilt plate (xs is of course also=0).
The moment of deviation J′yz including Steiner component results in
J′
yz
=J
yz
+y
s
z
s
m
In accordance with the derivation given for a tilt plate in ring form, the variables governing the regulation behaviour were calculated. Tables 1 and 2 give the values for two different preferred embodiments of compressors according to the invention.
A sensible design criterion is, as already mentioned, the ratio Jy/mk, ges or Jz/mk, ges, which characterises the moment ratio
MSW≧Mk,ges
and therefore the regulation behaviour of the compressor drive mechanism independently of the coolant (which can be, for example, CO2, R134a, R152a etc.). As can be seen from the two Tables, the preferred embodiments of a compressor according to the invention have respective ratios Jy/mk, ges of more than 1000 mm2 and more than 1500 mm2, whereas the ratio Jz/mk, ges is in both cases clearly above 2000 mm2, and even about 3000 mm2 in the case of the second preferred embodiment.
The regulation characteristic curves (not shown) of a speed of rotation located between 1000 rpm and 8000 rpm lie between the respective curves having the particular suction and compression pressures. As can be seen from
The above-described tendency can be seen from
As an alternative to variation of the spring rate or also possibly of course in addition thereto, the mass inertia of the tilt plate can be increased.
Both of the tilt plate geometries given in Tables 1 and 2 are suitable for obtaining the desired regulation behaviour, as can be seen from the Figures explained hereinbefore.
Finally,
It should be mentioned that the down-regulation of the compressor would also be achievable by means of a regulatory intervention, in which case energy losses also play a large part in addition to the inertia over time which the regulatory intervention involves. In the case of strong acceleration of the vehicle and operation of the compressor under full load, compressor operation initially demands a very high torque (at least until the control corrects this again and the stroke volume is reduced), which is of course extremely undesirable especially during overtaking manoeuvres of the vehicle. In addition, safety is increased in a compressor according to the invention, because malfunctions of the control valve, for example due to dirt accumulation, blockage of the valve seat, icing-up or the like, impair or impede regulatory intervention. A marked pressure increase, especially in the case of high-pressure coolants such as CO2, would very easily become a safety risk in such a case. It is therefore desirable that, at a high speed of rotation, relatively high tilt angles of the tilt plate are not achievable in the first place.
It should furthermore be noted that
It should furthermore be pointed out that the desired regulation characteristic curve (having intercepts of the appropriate curves with the x axis) is basically achieved by suitable selection of MSW and Mk, ges. The fine adjustment of the desired regulation characteristic curve can also be accomplished by other measures such as, for example, suitably selecting the spring constant of the restoring spring.
Finally, reference should be made to
In detail it can be seen from
Although the invention is described using embodiments having fixed combinations of features, it nevertheless also encompasses any further feasible advantageous combinations of those features, as are especially but not exhaustively mentioned in the subordinate claims. All features disclosed in the application documents are claimed as being important to the invention insofar as they are novel on their own or in combination compared with the prior art.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2004 040 042.3 | Aug 2004 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2005/008530 | 8/5/2005 | WO | 00 | 10/13/2008 |
