DE102004013096A1 - Compressor, in particular axial piston compressor for a vehicle air conditioning - Google Patents

Abstract

Compressor, in particular axial piston compressor, for a vehicle air conditioning system, with a housing defining an engine chamber (17), a cylinder block, in which at least one piston is axially reciprocally displaceable, and a cylinder head comprising a suction and pressure side, wherein between the engine chamber (17) and suction side (18) a fluid connection (12, 13) is provided, in which a control valve (10) is arranged, by means of which from a predetermined pressure difference between the engine chamber (17) and suction side (18) with further increasing pressure difference, the fluid connection between the engine chamber and suction side is increasingly throttled, so as to keep the outflowing mass flow at approximately constant low starting level.

Description

The The invention relates to a compressor, in particular axial piston compressor for one Vehicle air conditioning, with one engine compartment delimiting Casing, a cylinder block in which at least one piston axially back and forth stored herverschieblich, and a comprehensive suction and pressure side Cylinder head.

• – DE 196 11 004 A1
• – DE 44 41 721 C2
• – JP 2002/070739 A

Specifically, it is a variable capacity compressor with swash plate or swash plate drive, which is located within the engine chamber and over which the rotational movement of a drive shaft is converted into an axial reciprocating movement of the piston or. The swashplate or swashplate, also commonly referred to as the "swashplate", may be varied in its inclination relative to the drive shaft which is coupleable to an external motor The inclination of the "swashplate" determines the stroke of the piston or pistons. When the pressure in the engine chamber is relatively low, the inclination of the "swash plate" is large, so that the stroke of the piston or pistons is correspondingly long.When the pressure in the engine chamber is relatively high, the inclination of the "swash plate" is small. so that the stroke of the piston or is correspondingly low. For the state of the art, reference is made to the following documents:

• - DE 196 11 004 A1
• - DE 44 41 721 C2
• - JP 2002/070739 A

These References relate to continuously variable compressors with variable adjustment of the piston stroke.

In usually such compressors are designed as axial piston compressor, wherein the stroke adjustment - how already mentioned - by a change of the Tilting angle of the "swash plate" takes place the position of the bottom dead center of the piston or changes; the Position of top dead center and thus the size of the so-called. Harmful Raums remain idealtypisch unchanged.

At the Operation of such a compressor usually causes internal leakage and Losses on. The main reason for this is that when compacting a in the or the cylinder sucked refrigerant usually a so-called partial mass flow through the gap between cylinder and piston enters the engine room of the compressor. This effect is too known as "blow-by" an oil cutter is arranged on the high pressure side of the compressor, there is the Possibility, that unintentionally refrigerant over the oil return in the engine chamber arrives. To in the engine chamber an undesirable overpressure is to be avoided between the engine chamber and the low-pressure or suction side fluid connection provided over the into the engine chamber incoming leakage masses can escape again. In the mentioned fluid connection it is usually a connection hole. The free one Cross section of this hole is generally dimensioned so that even under the most unfavorable Conditions no unwanted overpressure arises in the engine chamber. Due to the described dependency of the piston stroke from the pressure inside the engine chamber, it is common to use the External compressor to regulate the pressure in the engine chamber being affected. By increase the pressure inside the engine chamber is in the internal forces and Moment equilibrium of the compressor intervened such that the stroke of the pistons is reduced. The compressor is thereby "decelerated." Conversely, it behaves while reducing the pressure in the engine chamber. This can be done the compressor "up." The corresponding Control valves are electrically controlled in the prior art.

there the increase takes place the pressure within the engine chamber and thus a corresponding "Abregeln" of the compressor through targeted opening a fluid connection between the engine chamber and printing or High pressure side of the compressor. In this fluid connection is the mentioned control valve, which is preferably electrically controlled, arranged. there should be ensured that the pressure in the engine chamber does not exceed a predetermined maximum level. To this end is a safety fluid connection between engine chamber and provided the suction side of the compressor.

The pressure in the engine chamber can be adjusted between the high pressure prevailing on the pressure side and the low pressure prevailing on the suction side. Within these limits, the compressor can be off or up. Of course, in the prior art, the increase in the pressure in the engine chamber always takes place in relation to the pressure increase via a fluid connection, which is constant in cross-section, between the engine chamber and the suction side of the compressor. It should be noted that while maintaining the increased differential pressure due to the constant cross-section of said fluid connection when Abregelden the compressor, ie increasing the differential pressure between the engine chamber and suction side of the outflowing mass flow from the engine chamber is steadily and significantly larger. Since this mass flow must be taken directly from the high pressure side, it is in the system for the actual purpose of the compressor, ie cooling or heating, no longer available and must demzufol be regarded as a loss. The required for the Abregelden the compressor mass flow is virtually only inside the compressor from the high pressure side via the control valve in the engine chamber and from there through the fluid connection between the engine chamber and suction side promoted back to the suction side, from where it is sucked again and compacted. For compaction of this so-called "mass flow control" additional effort is required, which creates no immediate benefit.

1 exemplifies the aforementioned behavior. As the pressure difference between the engine chamber and the intake side (X-axis) increases, the mass flow through the fluid connection between the engine chamber and the intake side increases significantly. In addition to the loss mass flow are still the associated inlet and outlet pressures before and after the fluid connection or opening between the engine chamber and suction side and, by way of example, a possible temperature profile at the inlet shown. All curves are to be considered as examples only; However, the basic behavior can be seen for all typical operating points of a compressor for a vehicle air conditioning. The starting point of the mass flow curve, which is located at a low pressure difference between the engine chamber and the suction side and correspondingly small mass flow rate, is essentially defined by the internal leakage and other influences which can not be described here. The free cross-section of the fluid connection between engine chamber and suction side is usually chosen so that it does not come for all assumed operating conditions to an undesirable Abregelreg the compressor.

Especially in Central Europe with relatively moderate Annual average temperatures and relatively lower average Humidity is especially in the field of motor vehicles used air conditioners frequently taxed (with the above-mentioned inherent losses due to the cancellation). Specially for This problem seeks to provide the present invention with a simple, efficient and cost-effective solution Offer.

One Another problem in addition to the energy losses is the burden the piston as well as the "swashplate mechanism" is the pressure-related The main force direction of compressors lies axially away from the piston top to the piston bottom. The opposite load case (in the direction the piston top) occurs due to pressure to a significant extent only when Adjusting the compressor, so when increasing the pressure in the engine chamber above the Pressure on the suction side. It must therefore be ensured in regulated operation that the pressure-induced piston underside forces acting do not exceed the defined level. It must also be on it be respected that preferably pistons made of lightweight materials, in particular Alloy should be used, both energetically as also advantageous in terms of control technology. Also under this aspect the present invention is intended to offer a solution.

Accordingly The present invention is based on the object, specifically with an externally controlled compressor of any control characteristic to minimize the loss mass flow occurring during the regulation and on the other hand to provide a safety device which in the suction process in Limit the direction of the piston top acting pressure-related forces or can reduce.

These Task is according to the invention in a Compressor of the type mentioned solved in that provided between the engine chamber and suction side fluid connection is arranged in which a continuously operating control valve is, by means of from a predetermined pressure difference between Engine chamber and suction side with further increasing pressure difference the fluid connection between the engine chamber and suction side increasingly throttled, in extreme cases completely is closed.

The provided in the prior art "constant" opening between engine chamber and suction side is thus according to the invention replaced a "variable opening", namely Control technology such that the opening cross-section with increasing Differential pressure between engine chamber and low pressure or suction side is increasingly reduced, whereby the loss mass flow approximately constant at the original value can be held.

Preferably, this opens Control valve in the fluid connection between engine chamber and suction side at a predetermined excessive pressure difference between the engine chamber and suction side again, so that damage or destruction of the Piston counteracted. The basic idea is that of at excessive differential pressure between the engine chamber and suction side at least one in normal operation ineffective opening enters into function, by which an outflow of mass from the engine chamber possible is such that the pressure in the engine chamber returns to a lower operating pressure drops. This measure is one Safety around the compressor or the engine chamber against undesirable overpressure to preserve.

Further Details of the invention, in particular constructive type are in the claims 3 ff. Indicated. These details will also be discussed over the course of time Description of the invention, in particular based on exemplary embodiments still referred to separately. In this regard, the attached Drawing referenced.

These shows in:

2 Influence of different temperatures on the loss mass flow;

3 a first embodiment of an inventively designed and arranged control valve between the engine chamber and suction side of a compressor in a schematic representation (normal operating state);

4 the typical Abregelkurve an externally controlled compressor;

5 a possible flow of forces when Abregeln within a mechanical control valve according to the invention;

6 Comparison of the loss mass flows through a fluid connection between the engine chamber and suction side, the opening cross-section is variable by means of the control valve according to the invention; and

7 the embodiment according to 3 with activated safety function.

Considering the loss mass flow for typical operating conditions of a compressor, in particular axial piston compressor for vehicle air conditioning systems, it can be stated that for a Abregelbereich the influence of different temperatures and entry conditions is low. As 2 it can be seen, rather, the pressure difference between the inlet and outlet side of a arranged in a fluid connection between the engine chamber and the suction side control valve is of crucial importance for the outflowing mass flow.

attracts a mean condition is used as a reference in the interpretation, so is for the usual Abregelbereich commonly the maximum deviation of the actually occurring "extreme points" for the usual Slope range less than about 2% (relative).

The fact that the differential pressure between the engine chamber and the intake side is responsible for the loss mass flow can be adjusted with the aid of a mechanical control valve 3 exploit. Accordingly, the control valve comprises 10 a cylinder space 11 who has a lead 12 with the engine chamber of an axial piston compressor on the one hand and via another line 13 on the other hand is fluidly connected to the suction side of the compressor, and within which a low-pressure side closed piston 14 each against the action of an elastic element - here helical compression springs 15 . 16 - And the forces caused by the inlet and outlet pressure forces is slidably mounted back and forth, depending on the on the piston 14 acting pressure difference according to the pressure difference between engine room (in 3 with the reference number 17 indicated) and suction side (in 3 with the reference number 18 indicated) this reduces the effective valve opening between the engine room and suction side more or less, or in extreme cases completely closes. The mentioned fluid passage is defined by the conduits 12 . 13 as well as the cylinder space 11 and pistons 14 , which is for this purpose as an - in 3 upper end face open hollow piston is formed in the jacket 19 at least one axially extending, in particular slot-shaped passage 20 is formed, this passage 20 the suction side 18 or one with the suction side 18 connected and laterally into the cylinder chamber 11 opening fluid line 13 assigned. The interior 21 of the hollow piston 14 stands over the open front 22 with the engine chamber 17 in fluid communication. On the suction side is the piston 14 with a piston bottom 23 locked. At this piston bottom 23 outside is the suction side, ie low pressure. For this purpose, the cylinder chamber 11 below the piston crown 23 over a connecting line 24 with the suction side or the line leading to the suction side 13 connected.

The piston 14 is between two frontally adjacent springs, here helical compression springs 15 . 16 within the cylinder space 11 clamped. The cylinder space 11 is through a corresponding hole in a valve body 25 defined, wherein the bore opening after placement of the helical compression springs together with the piston 14 through a stopper 26 is closed.

The spring elements 15 . 16 are designed and adjusted such that the throttle behavior of the control valve 10 with increasing pressure difference between engine chamber 17 and suction side 18 either linear or progressive, degressive and / or stepped. This also depends on the design of the passageway 20 in the piston 14 from. The slot-shaped passage 20 in the coat 19 of the piston 14 may be formed as axially expanding in one direction either continuously or stepwise widening or tapering slot, depending on the desired control behavior. For the formation of a constant mass flow, a geometry that continuously tapers in the axial direction is to be provided.

The valve body 25 may be part of the compressor housing or a separate component. In training of the piston 14 The springs become plastic 15 . 16 preferably provided integral with the piston as a structural unit, ie shed frontally with the piston material. As already mentioned, the installation of the piston takes place 14 within the cylinder space 11 under bias of the two spring elements 15 . 16 , so that the springs at each operating condition on the piston 14 issue.

The piston 14 is inside the cylinder space 11 fitted with clearance, preferably with a fit of less than 15μm, to keep the mass flow past the piston at a negligible level. To achieve this, additional sealing measures between piston and cylinder wall can be provided.

In 3 is the piston 14 formed with identical faces. It is also conceivable to use instead a differential piston with different sized end faces. It is crucial which forces, in particular differential forces act on the piston. The through the spring elements 15 . 16 In contrast, spring forces are of secondary importance. It gets up in this regard 5 directed.

If the pressure between engine chamber and suction side is balanced, the piston becomes 14 held in a middle position. The in the piston skirt 19 trained passage 20 is then approximately at the level of the suction side 18 leading line 13 ,

Usually, a minimum differential pressure between engine chamber and suction side is required to effect a reduction in the compressor stroke. This minimum pressure difference should be taken into account in the design of the described mechanical control valve. On the one hand, according to the design, the minimum pressure difference referred to should be that in the piston skirt 19 introduced slot-shaped passages 20 if possible be positioned so that the full area of the passage or passages 20 is effective or are. However, a further increase in the differential pressure should as soon as possible result in a successive reduction in the effective opening cross-section of the slot-shaped passage or passages 20 to lead.

The design of the through hole in the piston skirt 19 should be such that internal leakage, or other influences not specified here, at a defined minimum pressure difference between the engine chamber and suction side completely over the piston 14 or the control valve 10 can drain away.

During the Abregelvorgang increases the differential pressure between the engine chamber and suction side. A possible flow of forces of spring and compressive forces in the control valve 10 when increasing the differential pressure and corresponding displacement of the piston 14 (in 3 down) is exemplary in 5 shown. It is also assumed that there is also a rise in the low pressure on the suction side when Abregeln.

The installation position of the control valve 10 is arbitrary, since the dead weight of the piston 14 should be negligible for the scheme. The control valve 10 can be arranged on the cylinder head or cylinder block or taking into account appropriate connections outside the compressor housing.

Again 5 can still be seen, shifts the control piston 14 with increasing differential pressure in the direction of the suction side. This will be the slot-shaped passage 20 in the piston skirt 19 increasingly covered. The opening cross section is correspondingly increasingly reduced. The passage 20 is preferably designed so that after displacement of the piston 14 a residual opening cross-section remains in such a way that an almost constant mass flow is established between the engine chamber and the intake side. It gets up in this regard 6 directed.

Since the outflowing mass flow with the help of the described control valve 10 during the Abregelns can be kept generally at approximately constant low level (see 6 ), in derated operation significantly less loss mass flow is required to adjust the Abregel required differential pressure between the engine chamber and suction side. Thus, relatively less drive power must be expended for the same cooling or heating power. The efficiency is accordingly increased for the entire range of control, with increasing differential pressures between the engine chamber and the suction side, the relative loss mass flow is increasingly reduced.

In the embodiment according to 7 is in the piston skirt 19 above the slit-shaped passage 20 , ie in the direction of the pressure side, still another opening or a so-called. Security slot 27 provided immediately with complete coverage of the lower slot-shaped passage 20 becomes effective, namely when the pressure in the engine room is excessively high and the piston 14 in the control valve 10 is accordingly moved further to the suction side. The security slot 27 So it is effective when the differential pressure between the engine chamber and suction side reaches a predetermined maximum value. Then you can over the security slot 27 the pressure in the Engine chamber effectively and quickly reduced to a lower operating pressure.

Otherwise, the control valve is according to 7 according to the one according to 3 trained and elements based on the 3 already described are in 7 marked with the same reference numerals.

By the described control valve is achieved a further advantage namely the advantage that the mass flow between engine chamber and Compressor suction side is greatly reduced. As a result, the oil mass flow, i.e. the entrained with the gas flow amount of oil accordingly reduced. This has a positive effect on the overall performance as well as the thermal behavior of the compressor and thus a vehicle air conditioning system, and on the other beneficial to the life of the compressor out.

The described control valve leaves to make themselves available as a prefabricated unit. Within the Piston or the cylinder chamber can be further elements integrate, like oil separators, Particle filter or the like ..

The valve body 25 is preferably made of steel, steel alloy, light metal, especially aluminum, or plastic. The same applies to the piston 14 , In training of the piston 14 made of plastic it is possible to use the spring elements 15 . 16 intimately or permanently connected to the piston material, so that piston and spring elements constitute a structural unit, as a whole in the cylinder chamber 11 can be introduced. In training of the piston 14 made of plastic is especially suitable for production by injection molding. Thermosets or thermoplastics can be used. The passageways 20 and 27 can be trained in injection molding in one operation. Slip-optimized plastics can be used, in particular slid-optimized thermosets or thermoplastics.

When using metallic materials for the piston 14 become the passages 20 . 27 preferably formed by laser (laser cutting). This allows any contours or opening cross sections can be achieved. As already mentioned, the slot-shaped passage can 20 in the coat 19 of the piston 14 axially expanding or tapering in one direction either steadily or stepwise, depending on the desired control behavior, wherein the tapered geometry is preferred here. The contour of the passage 20 ultimately also depends on the compressor itself or its operating behavior.

Of course, those with the engine chamber 17 connected fluid line 12 also be arranged so that they are axially in the cylinder space 11 empties. In this respect, it is in the 3 and 7 only schematic schematic sketches. In other words, the inlet opening into the cylinder chamber 11 If necessary, can also through the stopper 26 be formed through. In the area of the mouth of the pipe 13 , which leads to the suction side, a bulge can be provided, which ensures that gas through the passage 20 can also escape when the piston 14 turns around its longitudinal axis. In a similar way, the passage 20 within a recess, in particular annular recess of the piston skirt 19 be positioned. It is also possible in the piston skirt 19 a plurality of passages distributed over the circumference of the piston 20 provided. Preferably, the piston 14 however, secured against rotation within the cylinder space 11 placed.

Furthermore, the connection between the outside of the piston crown 23 and the suction side over the piston 14 be made directly, via trained on the piston or on the cylinder wall longitudinal grooves. Thus, these pressure-communicating grooves are intended to provide fluid communication between the suction side and the space below the piston crown 23 produce. To do this in any axial position of the piston 14 ensure the corresponding grooves are preferably formed on the cylinder wall. Such pressure-communicating grooves also have the advantage of improved axial mobility of the piston 14 , Within the pressure communicating longitudinal or axial grooves, oil entrained by the refrigerant accumulates, which otherwise increases the mobility of the piston 14 within the cylinder space 11 could affect. By means of the mentioned longitudinal or axial grooves, on the one hand, the contact surface between the piston and the cylinder wall is reduced. On the other hand, an oil collection chamber is created so that the oil - especially at low ambient temperatures - the mobility of the piston 14 within the cylinder space 11 is not a hindrance. The danger of the piston 14 within the cylinder space 11 is restricted in its free mobility, is reduced. In addition, it is ensured that on the piston 14 the pressure difference between the engine chamber and suction side is present without having to create an additional connection to the low pressure side.

• – Mittellage bei Druckausgleich zwischen Triebwerkskammer und Saugseite (vollwirksamer Öffnungsquerschnitt des Durchgangs 20 im Kolbenmantel 19)
• – Arbeitspunkt bei anliegender Druckdifferenz zwischen Triebwerkskammer und Saugseite (Öffnungsquerschnitt des Durchgangs 20 ist reduziert)
• – Sicherheitsposition bei ungewollt hoher Druckdifferenz zwischen Triebwerkskammer und Saugseite (nur der Sicherheitsschlitz 27 wird wirksam)

There are three significant positions of the piston 14 :

• - Center position at pressure equalization between the engine chamber and suction side (fully effective opening cross section of the passage 20 in the piston skirt 19 )
• - Working point at applied pressure difference between engine chamber and suction side (Öff cross section of the passage 20 is reduced)
• - Safety position with unintentionally high pressure difference between the engine chamber and suction side (only the safety slot 27 becomes effective)

All Features disclosed in the application documents are considered to be essential to the invention as far as they are individually or in combination with respect to State of the art are new.

10

control valve

11

cylinder space

12

management

13

management

14

piston

15

Helical compression spring

16

Helical compression spring

17

Machine Room

18

suction

19

piston skirt

20

(Slit-shaped) passage

21

inner space

22

open front

23

piston crown

24

connecting line

25

valve body

26

plugs

27

security slot

Claims (15)

Compressor, in particular axial piston compressor for a vehicle air conditioning system, with an engine chamber ( 17 ) limiting housing, a cylinder block in which at least one piston is axially reciprocally mounted, and a suction and pressure side comprising a cylinder head, wherein between the engine chamber ( 17 ) and suction side ( 18 ) a fluid connection ( 12 . 13 ) is provided, in which a control valve ( 10 ) is arranged, by means of which from a predetermined pressure difference between the engine chamber ( 17 ) and suction side ( 18 ) When the pressure difference continues to rise, the fluid connection between the engine chamber and suction side is increasingly throttled, in the extreme case completely closed, in particular the free cross section of the fluid connection is reduced such that the mass flow flowing out of the engine chamber is maintained at a constant low level. Compressor according to claim 1, characterized in that the control valve ( 10 ) at a predetermined higher pressure difference between the engine chamber ( 17 ) and suction side ( 18 ) whose fluid connection opens again, so that a predetermined lower pressure difference can be set. Compressor according to claim 1 or 2, characterized in that the throttle behavior of the control valve ( 10 ) with increasing pressure difference between engine chamber ( 17 ) and suction side ( 18 ) is linear, progressive, degressive and / or stepwise. Compressor according to one of claims 1 to 3, characterized in that the control valve ( 10 ) a cylinder space ( 11 ) associated with the engine chamber ( 17 ) on the one hand and the suction side ( 18 ) On the other hand fluidly connected (lines 12 . 13 ), and within which a piston ( 14 ), in particular in each case against the action of an elastic element ( 15 . 16 ) is mounted reciprocally, wherein, depending on the on the piston ( 14 ) acting pressure difference according to the pressure difference between the engine chamber ( 17 ) and suction side ( 18 ) This more or less wide opens the fluid passage between the engine chamber and suction side, or in extreme cases completely closes. Compressor according to claim 4, characterized in that the piston ( 14 ) of the control valve ( 10 ) is an open at one end side hollow piston, in whose jacket ( 19 ) at least one axially extending, in particular slot-shaped passage ( 20 ), this passage ( 20 ) the suction side or one with the suction side ( 18 ) and laterally into the cylinder space ( 11 ) fluid line ( 13 ), while the interior ( 21 ) of the piston ( 14 ) over its open end face ( 22 ) with the engine chamber ( 17 ) is in fluid communication (line 12 ). Compressor according to claim 4 or 5, characterized in that the suction side ( 18 ) also on the closed end side of the piston ( 14 ) or its piston bottom ( 23 ) is present. Compressor according to one of claims 4 to 6, characterized in that the piston ( 14 ) of the control valve ( 10 ) between two end-side adjacent spring elements, in particular helical compression springs ( 15 . 16 ) within the cylinder space ( 11 ) is clamped. Compressor according to claim 7, characterized in that the spring elements ( 15 . 16 ) integral with the piston ( 14 ) are connected. Compressor according to one of claims 4 to 8, characterized in that the jacket ( 19 ) of the piston ( 14 ) a second passage axially spaced from the first passageway toward the (high) pressure side ( 27 ), which after exceeding a predetermined higher pressure difference between the engine chamber ( 17 ) and suction side ( 18 ) becomes effective and opens or releases the fluid connection between the engine chamber and suction side to reduce the higher pressure difference. Compressor according to one or more of claims 1 to 9, characterized in that in the flow path of the control valve ( 10 ), in particular in the piston ( 14 ) od the same means for the separation of lubricant, particles od. Like. Are arranged. Compressor according to one of claims 1 to 10, characterized in that the piston ( 14 ) of the control valve ( 10 ) made of steel, steel alloy, light metal, in particular aluminum and / or plastic. Compressor according to claim 11, characterized in that when forming the piston ( 14 ) made of plastic, the frontally adjacent spring elements, in particular helical compression springs ( 15 . 16 ) are embedded or embedded in the plastic. Compressor according to one of claims 4 to 12, characterized in that the suction side ( 18 ) associated passage ( 20 ) in the coat ( 19 ) of the piston ( 14 ) within a recess, in particular annular recess, and / or the opening of the side in the cylinder chamber ( 11 ), with the suction side ( 18 ) connected fluid line ( 13 ) within a bulge, in particular annular bulge or annular groove, so that the function of the control valve ( 10 ) also during rotation of the piston ( 14 ) is maintained around its longitudinal axis. Compressor according to one of claims 6 to 13, characterized in that on the outer surface of the piston skirt ( 19 ) and / or on the cylindrical space delimiting cylinder wall, one or more longitudinal grooves are formed, through which a fluid connection between the suction side ( 18 ) and below the piston crown ( 23 ) is maintained part of the cylinder space. Compressor according to one of claims 5 to 14, characterized in that the slot-shaped passage ( 20 ) in the coat ( 19 ) of the piston ( 14 ) of the control valve ( 10 ) extends axially or tapered in one direction either steadily or stepwise, depending on the desired control behavior, in particular tapered towards the engine pressure side either steadily or stepwise, so that the effluent from the engine room mass flow remains substantially constant.