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Compressor, especially axial piston compressor for a vehicle air conditioning system

Compressor, especially axial piston compressor for a vehicle air conditioning system description/claims

The invention relates to a compressor, especially an axial piston compressor for a vehicle air-conditioning system, having a housing delimiting a drive mechanism chamber, having a cylinder block in which at least one piston is mounted so as to be axially displaceable back and forth, and having a cylinder head having a suction side and a delivery side.

In specific terms, the compressor is a variable-capacity compressor having a swash plate drive or wobble plate drive which is located within the drive mechanism chamber and by means of which the rotary movement of a drive shaft is converted into an axial reciprocating movement of the piston or pistons. The swash plate or wobble plate, also called in general manner a “tilt plate”, is variable in terms of its inclination relative to the drive shaft, which can be coupled to an external motor. The inclination of the “tilt plate” governs the stroke of the piston or pistons. When the pressure in the drive mechanism chamber is relatively low, the inclination of the “tilt plate” is large so that the stroke of the piston or pistons is correspondingly long. When the pressure in the drive mechanism chamber is relatively high, the inclination of the “tilt plate” is small so that the stroke of the piston or pistons is correspondingly short. With respect to the prior art, reference may be made to the following publications: DE 196 11 004 A1 DE 44 41 721 C2 JP 2002/070739 A

Those publications relate in each case to steplessly regulatable compressors having variable adjustment of the piston stroke.

Such compressors are usually constructed in the form of axial piston compressors, with modification of the stroke—as already mentioned—being accomplished by means of a change in the tilt angle of the “tilt plate”. In the process, the position of the lower dead centre of the piston or pistons is changed; the location of the upper dead centre and, as a result, the size of the so-called clearance volume remains unchanged in the idealised case.

In operation of such a compressor, internal leaks and losses usually occur. The main cause thereof is that, in the course of compression of a coolant drawn into the cylinder or cylinders, a so-called partial mass flow usually enters the drive mechanism space of the compressor through the gap between the cylinder and piston. This effect is also known as “blow-by”. Insofar as an oil separator is arranged on the high-pressure side of the compressor, there is a possibility that coolant will, in undesirable manner, enter the drive mechanism chamber by way of the oil return. In order to avoid an undesirable over-pressure in the drive mechanism chamber, there is provided between the drive mechanism chamber and the low-pressure side, or suction side, a fluid connection by way of which leak masses entering the drive mechanism chamber can flow out again. The mentioned fluid connection is usually a connecting bore. The free cross-section of that bore is generally so dimensioned that, even under most unfavourable conditions, no undesirable over-pressure arises in the drive mechanism chamber. Because of the described dependency of the piston stroke on the pressure within the drive mechanism chamber it is usual for the compressor to be externally regulated by influencing the pressure in the drive mechanism chamber. An increase in pressure inside the drive mechanism chamber brings about an effect on the internal force and moment equilibrium of the compressor such that the stroke of the pistons is reduced. The compressor is, as a result, “down-regulated”. The converse occurs in the case of reduction of pressure in the drive mechanism chamber. As a result, the compressor can be “up-regulated”. The corresponding regulating valves in the prior art are electrically controlled. In the process, increasing the pressure within the drive mechanism chamber and, as a consequence, corresponding “down-regulation” of the compressor are accomplished by appropriate opening of a fluid connection between the drive mechanism chamber and the delivery side, or high-pressure side, of the compressor. Arranged in that fluid connection is the mentioned regulating valve, which is preferably electrically controllable. In the process, it should be ensured that the pressure in the drive mechanism chamber does not exceed a predetermined maximum level. For that purpose, a safety fluid connection is provided between the drive mechanism chamber and the suction side of the compressor.

The pressure in the drive mechanism chamber can be adjusted between the high pressure prevailing on the delivery side and the low pressure prevailing on the suction side. The compressor can be up-regulated and down-regulated within those limits. Increasing the pressure in the drive mechanism chamber is of course always accomplished in the prior art in relation to the pressure increase by way of a fluid connection of constant cross-section between the drive mechanism chamber and the suction side of the compressor. In this context it is to be borne in mind that when maintaining the increased differential pressure, owing to the constant cross-section of the mentioned fluid connection, when down-regulating the compressor, that is to say when increasing the differential pressure between the drive mechanism chamber and the suction side, the mass flow flowing out of the drive mechanism chamber becomes continuously and significantly greater. Because that mass flow has to be taken directly from the high-pressure side, it is no longer available in the system for the actual purpose of the compressor, that is to say cooling or heating, and must consequently be regarded as a loss. The mass flow required for down-regulating the compressor is conveyed, almost exclusively by internal compressor means, from the high-pressure side, by way of the regulating valve, to the drive mechanism chamber and from there, through the fluid connection between the drive mechanism chamber and the suction side, back to the suction side, from where is it is again drawn in and compressed. Additional outlay which provides no direct benefit is required for compression of that so-called “regulating mass flow”.

By way of example, FIG. 1 illustrates the above-mentioned behaviour. As the pressure difference between the drive mechanism chamber and the suction side increases (X axis), the mass flow through the fluid connection between the drive mechanism chamber and the suction side increases significantly. In addition to the loss mass flow, the associated inlet and outlet pressures before and after the fluid connection or opening between the drive mechanism chamber and the suction side are also shown, as well as, by way of example, a possible temperature plot at the inlet. All the curve plots are to be regarded merely as examples; however, the basic behaviour for all typical operating points of a compressor for a vehicle air-conditioning system can be recognised. The starting point of the mass flow curve, located at a low pressure difference between the drive mechanism chamber and the suction side and at a corresponding low mass flow, is defined substantially by internal leakage and other factors which will not be described in greater detail here. The free cross-section of the fluid connection between the drive mechanism chamber and the suction side is usually so selected that, for all the operating states that are to be assumed, undesired down-regulation of the compressor does not come about.

Especially in central Europe, with comparatively moderate average annual temperatures and relatively low average atmospheric humidity, air-conditioning systems used in particular in the motor vehicle sector are frequently down-regulated (with the above-mentioned inherent losses caused by the down-regulation). The present invention is intended to provide a simple, efficient and economical solution to that problem in particular.

A further problem besides the energy losses is posed by the loading of the pistons and of the “tilt plate mechanism”. The pressure-related main direction of force in compressors is axial, from the upper side of the piston to the underside of the piston. The opposite loading case (in the direction of the upper side of the piston) occurs, as a result of pressure, significantly only in the case of down-regulation of the compressor, that is to say in the case of an increase in pressure in the drive mechanism chamber above the pressure of the suction side. Therefore, in down-regulated operation it must be ensured that the pressure-related forces acting on the underside of the piston do not exceed a defined level. In this context it must also be borne in mind that, preferably, pistons made of light materials, especially light metal, are to be used, which are advantageous both in energy terms and in regulation terms. The present invention is intended to provide a possible solution under this aspect too.

Consequently, the problem underlying the present invention is to minimise the loss mass flow that occurs during down-regulation, in particular in the case of an externally regulated compressor having any regulation characteristic, and, on the other hand, to provide a safety device which is capable of limiting or reducing the pressure-related forces acting in the direction of the upper side of the piston during the intake process.

In the case of a compressor of the kind mentioned at the beginning, the problem is solved in accordance with the invention by means of the fact that there is provided between the drive mechanism chamber and the suction side a fluid connection in which there is arranged a continuously operating regulating valve by means of which, starting from a predetermined pressure difference between the drive mechanism chamber and the suction side, the fluid connection between the drive mechanism chamber and the suction side is increasingly throttled as the pressure difference further increases and is, in the extreme case, closed completely.

The “constant” opening provided between the drive mechanism chamber and the suction side in the prior art is accordingly replaced, in accordance with the invention, by a “variable opening”, more particularly in regulation terms in such a way that the opening cross-section is increasingly reduced as the differential pressure between the drive mechanism chamber and the low-pressure side, or suction side, increases, as a result of which the loss mass flow can be kept almost constant at the original value.

Preferably, the regulating valve in the fluid connection between the drive mechanism chamber and the suction side opens again in the event of a predetermined excessively high pressure difference between the drive mechanism chamber and the suction side so that damage to or destruction of the pistons is countered. Accordingly, the basic concept is that, in the event of an excessively high pressure difference between the drive mechanism chamber and the suction side, at least one opening which is not active in normal operation comes into effect, through which opening a flow of mass out from the drive mechanism chamber is possible in such a way that the pressure in the drive mechanism drops back to a lower operating pressure. This measure is a safety measure in order to protect the compressor, or drive mechanism chamber, from undesirable over-pressure.

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