DE102017217449A1 - Charged internal combustion engine with switchable compressor and method for operating such an internal combustion engine - Google Patents

Abstract

The invention relates to a supercharged internal combustion engine with
an intake system (1) for supplying charge air,
an exhaust gas discharge system for discharging exhaust gas, and
at least one compressor (2, 3) arranged in the intake system (1), which is equipped with at least one impeller arranged on a rotatable shaft in a compressor housing, a compressor (2) being connectable as a compressible compressor (2) in a bypass line (4) is arranged, which branches off to form a first node (4a) from the intake system (1) and opens to form a second node (4b) back into the intake system (1).
It is intended to provide a supercharged internal combustion engine of the above type, with which in particular the problem due to the formation of condensate is counteracted.
This is achieved by a supercharged internal combustion engine of the type mentioned, which is characterized in that the switchable compressor (2) is equipped with a discharge opening (5) which is releasable and lockable using a drain valve (5b) and the discharge of the compressor housing collected condensate is used.

Description

• - einem Ansaugsystem zum Zuführen von Ladeluft,
• - einem Abgasabführsystem zum Abführen von Abgas, und
• - mindestens einem im Ansaugsystem angeordneten Verdichter, der mit mindestens einem in einem Verdichtergehäuse auf einer drehbaren Welle angeordneten Laufrad ausgestattet ist, wobei ein Verdichter als zuschaltbarer Verdichter in einer Bypassleitung angeordnet ist, die unter Ausbildung eines ersten Knotenpunktes vom Ansaugsystem abzweigt und unter Ausbildung eines zweiten Knotenpunktes wieder in das Ansaugsystem einmündet.

The invention relates to a supercharged internal combustion engine with

• an intake system for supplying charge air,
• an exhaust gas discharge system for discharging exhaust gas, and
• - At least one arranged in the intake compressor, which is equipped with at least one arranged in a compressor housing on a rotatable shaft impeller, wherein a compressor is arranged as a switchable compressor in a bypass line, which branches off to form a first node from the intake system and to form a second Junction returns to the intake system.

Furthermore, the invention relates to a method for operating such an internal combustion engine.

An internal combustion engine of the type mentioned is used as a motor vehicle drive. In the context of the present invention, the term internal combustion engine includes diesel engines and gasoline engines, but also hybrid internal combustion engines that use a hybrid combustion process, as well as hybrid drives, which in addition to the internal combustion engine comprise an electric motor which can be electrically connected to the internal combustion engine, which receives power from the internal combustion engine or as switchable auxiliary drive additionally delivers power.

In the development of internal combustion engines, efforts are constantly being made to minimize fuel consumption. In addition, the aim is to reduce pollutant emissions in order to be able to comply with future limit values for pollutant emissions.

Internal combustion engines are therefore increasingly equipped with a charge, the charge is primarily a method of increasing performance, in which the charge air required for the engine combustion process is compressed, whereby each cylinder per cycle a larger charge air mass can be supplied. As a result, the fuel mass and thus the medium pressure can be increased.

The charge is a suitable means to increase the capacity of an internal combustion engine with unchanged displacement or to reduce the displacement at the same power. In any case, the charging leads to an increase in space performance and a lower power mass. If the displacement is reduced, then the load spectrum can be shifted to higher loads, where the specific fuel consumption is lower.

Charging thus supports the constant effort in the development of internal combustion engines to minimize fuel consumption, i. to improve the efficiency of the internal combustion engine.

By means of a suitable transmission design, a so-called downspeeding can additionally be realized, as a result of which a lower specific fuel consumption is likewise achieved. Downspeeding exploits the fact that the specific fuel consumption at low speeds is regularly lower, especially at higher loads.

Frequently, an exhaust gas turbocharger is used for charging, in which a compressor and a turbine are arranged on the same shaft. The hot exhaust stream is supplied to the turbine and relaxes with energy release in the turbine, causing the shaft to rotate. The energy emitted by the exhaust gas flow to the turbine and finally to the shaft is used to drive the compressor, which is also arranged on the shaft. The compressor conveys and compresses the charge air supplied to it, whereby a charging of the cylinder is achieved. Advantageously, a charge air cooler is provided downstream of the compressor in the intake system, with which the compressed charge air is cooled before entering the at least one cylinder. The radiator lowers the temperature and thus increases the density of the charge air, so that the radiator to a better filling of the cylinder, i. contributes to a larger air mass. There is a compaction by cooling.

The advantage of an exhaust gas turbocharger in comparison to a - drivable by auxiliary drive - loader is that an exhaust gas turbocharger uses the exhaust gas energy of the hot exhaust gases, while a loader requires the energy required for its drive directly or indirectly from the internal combustion engine and thus, at least as long as the drive power does not come from an energy recovery, adversely affects the efficiency, ie decreases.

If it is not an electric machine, i. electrically drivable charger is regularly a mechanical or kinematic connection for power transmission between the charger and the internal combustion engine is required, which also affects the packaging in the engine compartment.

The advantage of a supercharger over an exhaust gas turbocharger is that the supercharger can always generate and provide the requested boost pressure, regardless of the loader Operating state of the internal combustion engine. This is especially true for a loader that is electrically driven by electric motor and therefore independent of the speed of the crankshaft.

In fact, according to the state of the art, it is difficult to increase the power by means of turbocharging in all engine speed ranges. It is observed a greater torque drop when falling below a certain speed. This torque drop becomes understandable if it is taken into account that the boost pressure ratio depends on the turbine pressure ratio or the turbine output. If the engine speed is reduced, this leads to a smaller exhaust gas mass flow and thus to a smaller turbine pressure ratio or a smaller turbine power. Consequently, the boost pressure ratio also decreases toward lower speeds. This is synonymous with a torque drop.

The internal combustion engine, which is the subject of the present invention, has at least one compressor for charging, wherein in the context of the present invention the term compressor can be subsumed both by a supercharger drivable by means of an auxiliary drive and by a compressor of an exhaust gas turbocharger.

That the internal combustion engine, which is the subject of the present invention, has at least one exhaust-gas turbocharger and / or at least one supercharger drivable by means of an auxiliary drive for charging purposes. According to the invention, a compressor is a switchable compressor which is not operated permanently when the internal combustion engine is in operation, i. is not permanently switched on, but only temporarily, and which is arranged in a bypass line, which branches off to form a first node from the intake system and opens to form a second node back into the intake system.

With targeted design of the charge not only advantages in fuel consumption, i. the efficiency of the internal combustion engine, but also in the exhaust emissions are achieved. Thus, by means of suitable charging, for example in the diesel engine, the nitrogen oxide emissions can be reduced without sacrificing efficiency. At the same time, the hydrocarbon emissions can be favorably influenced. The emissions of carbon dioxide, which correlate directly with fuel consumption, decrease anyway with decreasing fuel consumption.

In order to comply with future limits for pollutant emissions, in addition to the charge but further action is required. Among other things, the development work focuses on reducing nitrogen oxide emissions, which are of particular relevance to diesel engines. Since the formation of nitrogen oxides requires not only an excess of air but also high temperatures, a concept for reducing nitrogen oxide emissions is to develop combustion processes with low combustion temperatures.

Here, the exhaust gas recirculation (EGR), ie the return of combustion gases from the exhaust side to the inlet side, expedient in which with increasing exhaust gas recirculation rate, the nitrogen oxide emissions can be significantly reduced. In this case, the exhaust gas recirculation rate x _{AGR is} determined as x _AGR = m _AGR / (M _AGR + m _air ), where m _{AGR designates} the mass of recirculated exhaust gas and m _air the supplied air. The oxygen provided via exhaust gas recirculation may need to be considered.

In order to achieve a significant reduction in nitrogen oxide emissions, high exhaust gas recirculation rates are required, which can be on the order of x _AGR ≈ 60% to 70% and more. Such high recirculation rates regularly require cooling of the recirculated exhaust gas.

Also, the internal combustion engine according to the invention can be equipped with an exhaust gas recirculation, is returned to the exhaust gas from the Abgasabführsystem in the intake system.

As part of the exhaust gas recirculation can form condensate. Two scenarios are relevant.

On the one hand, condensate can form when recirculated hot exhaust gas in the intake system collides with cool fresh air and is mixed. The exhaust gas cools, whereas the temperature of the fresh air is raised. The temperature of the mixture of fresh air and recirculated exhaust gas, i. the charge air temperature is below the exhaust gas temperature of the recirculated exhaust gas. As part of the cooling of the exhaust gas previously contained gaseous substances contained in the exhaust gas, in particular water, condense when the taut temperature of a component of the gaseous charge air flow is exceeded. It comes to a condensation in the free charge air flow, often impurities in the charge air form the starting point for the formation of condensate droplets.

On the other hand, condensate may form when hot exhaust gas or the charge air impinges on the inner wall of the intake system, since the wall temperature is usually below the dew temperature of the relevant gaseous components.

The problem of condensate formation is exacerbated with increasing recirculation rate, since with the increase in the amount of recirculated exhaust gas, the proportions of the individual exhaust gas components in the charge air inevitably increase, in particular the proportion of water contained in the exhaust gas.

Even without exhaust gas recirculation condensate can form in the intake system, in particular, if a charge air cooler is provided in the intake system for cooling the compressed charge air. Previously still gaseous in the charge air contained liquids, especially water, can condense when the taut temperature of a component of the gaseous charge air flow is exceeded.

Condensed water can accumulate in the intake system, the radiator and / or compressor and suddenly released and fed to the cylinder of the internal combustion engine, whereby not only a serious failure of the operation of the internal combustion engine, but also irreversible damage can be caused.

Critically, the formation of condensate is particularly upstream of a compressor impeller because it damages the blades of the compressor impeller, i. of the compressor, can come. The latter is associated with a reduction in the efficiency of the compressor. However, condensate and condensate droplets also lead to an increased noise emission in the intake system and are therefore generally undesirable.

Against the background of the above, it is an object of the present invention to provide a supercharged internal combustion engine according to the preamble of claim 1, with which the problems known from the prior art can be overcome and with which in particular the problem due to condensation is counteracted.

A further sub-task of the present invention is to show a method for operating such an internal combustion engine.

• - einem Ansaugsystem zum Zuführen von Ladeluft,
• - einem Abgasabführsystem zum Abführen von Abgas, und
• - mindestens einem im Ansaugsystem angeordneten Verdichter, der mit mindestens einem in einem Verdichtergehäuse auf einer drehbaren Welle angeordneten Laufrad ausgestattet ist, wobei ein Verdichter als zuschaltbarer Verdichter in einer Bypassleitung angeordnet ist, die unter Ausbildung eines ersten Knotenpunktes vom Ansaugsystem abzweigt und unter Ausbildung eines zweiten Knotenpunktes wieder in das Ansaugsystem einmündet,

die dadurch gekennzeichnet ist, dass

• - der zuschaltbare Verdichter mit einer Ablassöffnung ausgestattet ist, welche unter Verwendung eines Ablassventils freigebbar und versperrbar ist und dem Ablassen von im Verdichtergehäuse gesammeltem Kondensat dient.

The first subtask is solved by a supercharged internal combustion engine

• an intake system for supplying charge air,
• an exhaust gas discharge system for discharging exhaust gas, and
• - At least one arranged in the intake compressor, which is equipped with at least one arranged in a compressor housing on a rotatable shaft impeller, wherein a compressor is arranged as a switchable compressor in a bypass line, which branches off to form a first node from the intake system and to form a second Node returns to the intake system,

which is characterized in that

• - The switchable compressor is equipped with a discharge opening, which is releasable and lockable using a drain valve and serves to drain condensate collected in the compressor housing.

The shiftable compressor of the internal combustion engine, which is the subject of the present invention, has a drain port for discharging condensate collected in the compressor housing, i. be able to discharge or dispose of.

When the internal combustion engine is in operation, the discharge opening is kept locked using the drain valve and that at least as long as the switchable compressor is switched on and contributes to the generation of a requested boost pressure or is used. An open drain opening would preclude or make pressure build-up in the compressor, so the drain opening is only released using the drain valve, if the switchable compressor is turned off. Then, condensate collected in the compressor housing can be discharged via the opened drain opening. To support this discharge of condensate, a pump can be provided and used, which is at least temporarily activated and operated with open drain port.

The collected condensate can be introduced, for example, into the Abgasabführsystem and there are at least partially evaporated due to the high temperatures or fed via the intake system to the cylinders of the internal combustion engine. The collected condensate can also be supplied to a liquid cooling of the internal combustion engine or stored in a container and disposed of, for example, as part of a maintenance

The internal combustion engine according to the invention solves the first object underlying the invention, namely to provide a supercharged internal combustion engine according to the preamble of claim 1, with which can cope with the problems known from the prior art and with the particular problem of condensation is counteracted.

Further advantageous embodiments of the supercharged internal combustion engine are discussed in connection with the subclaims.

Embodiments of the supercharged internal combustion engine in which a charge air cooler is arranged in the bypass line upstream of the switchable compressor are advantageous.

A charge air cooler cools the charge air supplied to the switchable compressor and is particularly advantageous if the charge air is already pre-compressed upstream of the charge air cooler and thus upstream of the switchable compressor as part of a preceding compression. Then the precompressed charge air is intermediately cooled in the course of a two-stage compression, i. cooled between the two compressors and before entering the switchable or switched compressor.

Also advantageous for the reasons mentioned above, embodiments of the supercharged internal combustion engine, in which a further compressor is arranged in the intake upstream of the first node.

Another compressor could be designed as a permanently operated compressor. Then a single-stage charging of the internal combustion engine using this additional compressor would be possible without the switchable compressor would have to be used to generate the boost pressure. Or the charge air supplied to the internal combustion engine would be charged as part of a two-stage compression.

Embodiments of the supercharged internal combustion engine in which a further charge air cooler is arranged in the intake system downstream of the second node point are advantageous.

The further intercooler cools the compressed charge air before entering the cylinders of the internal combustion engine and because of its arrangement in the intake system both in the cases in which the charge air is compressed in one stage using the switchable or connected compressor or another compressor, as well as in the cases in which the charge air is compressed in several stages with the addition of the switchable or connected compressor.

Embodiments of the supercharged internal combustion engine in which the switchable compressor is an electrically drivable compressor are advantageous.

A compressor which can be electrically driven by means of an electric motor is independent of the rotational speed of the crankshaft of the internal combustion engine, for which reason a boost pressure requested, irrespective of the operating state of the internal combustion engine, can be generated at any time. can be generated and made available on demand. Incidentally, reference is made to the already mentioned in connection with an electrically driven compressor advantages and statements made.

The switchable compressor to perform as an electrically driven compressor, moreover, has the advantage that the required for the operation of the switchable compressor electrical power can be easily provided, optionally as regenerative energy, i. as energy derived from an energy recovery. The latter could not be readily ensured in a permanently operated compressor, since then the energy required for the operation would be much larger.

Embodiments of the supercharged internal combustion engine in which the further compressor is a compressor of an exhaust gas turbocharger are advantageous. An exhaust-gas turbocharger comprises a turbine arranged in the exhaust-gas removal system and a compressor arranged in the intake system.

With regard to the advantages of an exhaust gas turbocharger, reference is made to the statements already made in connection with the charging. To counteract a torque drop at low speeds, embodiments may be advantageous in which two or more exhaust gas turbochargers are provided, which are arranged for example in series or in parallel or.

Advantageous embodiments of the supercharged internal combustion engine, in which an exhaust gas recirculation is provided, which includes a return line, which branches off to form a third node of the Abgasabführsystem and opens into the intake system to form a fourth node.

If an exhaust gas recirculation is provided, embodiments of the supercharged internal combustion engine may be advantageous in which the return line opens downstream of a planned compressor in the intake system.

In this so-called high-pressure EGR, the exhaust gas is introduced into the intake system downstream of the compressor or compressors. In order to provide or to ensure the pressure gradient between the exhaust-gas removal system and the intake system required for a return, the exhaust gas is preferably taken from the exhaust-gas removal system upstream of the associated turbine during an exhaust-gas turbocharging. The high-pressure EGR has the advantage that the exhaust gas does not pass through the compressor or compressors and therefore none before the recirculation Exhaust after-treatment, for example in a particulate filter, must be subjected. Deposits in the compressor, which change the geometry of the compressor, in particular the flow cross-sections, and thus worsen the efficiency of the compressor, are not to be feared. Condensation takes place, if at all, downstream of a compressor which heats the charge air supplied to it as part of the compression.

When operating an internal combustion engine with turbocharging and simultaneous use of a high-pressure EGR, a conflict may arise when the recirculated exhaust gas is taken from the Abgasabführsystem upstream of the turbine and is no longer available for driving the turbine.

With an increase in the exhaust gas recirculation rate, the exhaust gas flow introduced into the turbine simultaneously decreases. The reduced exhaust gas mass flow through the turbine causes a smaller turbine pressure ratio, whereby the charge pressure ratio also decreases, which is equivalent to a smaller compressor mass flow. In addition to the decreasing boost pressure, additional problems with the operation of the compressor with regard to the surge limit can occur. Disadvantages can also arise in the pollutant emissions, for example, with regard to the formation of soot in diesel engines during acceleration.

Concepts are required that ensure sufficiently high boost pressures with high exhaust gas recirculation rates.

Therefore, embodiments of the supercharged internal combustion engine may also be advantageous, in which exhaust gas recirculation is provided, which comprises a return line, which opens into the intake system upstream of a designated compressor. In an exhaust gas turbocharging, this return line preferably branches off the exhaust gas removal system downstream of the turbine.

Advantageous embodiments of the supercharged internal combustion engine, in which the discharge opening is arranged in the installed position of the switchable compressor at the geodetically lowest point of the compressor housing. Of relevance is the interior of the compressor housing, because inside the condensate collects and regularly on an outer circumference around the at least one impeller.

Then, the discharge opening in the interior of the compressor, where condensate collects, adjoins adjacent areas that are higher in geodesic terms than the discharge opening.

In this context, embodiments of the supercharged internal combustion engine in which the areas of the bypass line, which lie upstream and downstream of the compressor interior or of the exhaust opening, are geodetically higher are advantageous.

Embodiments of the supercharged internal combustion engine in which a discharge line adjoins the discharge opening are advantageous.

In this context, embodiments of the supercharged internal combustion engine in which a pump is provided in the discharge line are advantageous.

To support the discharge of condensate, a pump may be advantageous, which is at least temporarily activated and operated with open drain opening.

The second sub-task underlying the invention, namely to disclose a method for operating an internal combustion engine of a type described above, is achieved by a method which is characterized in that the discharge opening is kept locked using the discharge valve, as long as the switchable compressor is switched on and operated is to generate a boost pressure.

What has already been said for the internal combustion engine according to the invention also applies to the method according to the invention. Different embodiments of the internal combustion engine according to the invention may accordingly require different process variants, for which reference is made to the corresponding explanations.

Advantageous are process variants in which the drain opening is released using the drain valve, if the switchable compressor is turned off in order to drain condensate collected in the compressor housing can.

For operating a supercharged internal combustion engine, in which the discharge opening adjoins a discharge line in which a pump is provided, process variants are advantageous, which are characterized in that the pump is activated and operated to support the discharge of condensate.

Advantageous in this context are process variants in which the pump is activated to assist in the discharge of condensate as soon as a first predefinable time interval Δt _{1 has} elapsed after the release of the discharge opening or the switch-off of the switchable compressor. The period Δt ₁ is used to collect Condensate after switching off the switchable compressor. The condensate should be given enough time to collect in the compressor housing of the shut-off compressor.

Process variants in which the pump is operated to support the discharge of condensate only for a second predefinable time interval Δt ₂ are advantageous. The operation of the pump is limited in time or regulated in order to minimize the drive power of the pump and to reduce the friction loss of the internal combustion engine.

Advantageous are process variants in which collected condensate is disposed of.

In this context, process variants in which collected condensate is introduced into the intake system and disposed of with exhaust gas to be recirculated are advantageous.

Also advantageous in this context are process variants in which collected condensate is introduced into the exhaust gas removal system and thus disposed of.

Process variants are advantageous in which no collected condensate is disposed of in the warm-up phase or after a cold start of the internal combustion engine.

• 1 schematisch ein Fragment des Ansaugsystems einer ersten Ausführungsform der Brennkraftmaschine.

In the following the invention is based on an embodiment according to 1 described in more detail. Hereby shows:

• 1 schematically a fragment of the intake system of a first embodiment of the internal combustion engine.

1 schematically shows a fragment of the intake system 1 a first embodiment of the internal combustion engine. The internal combustion engine has an intake system 1 for supplying charge air and via a Abgasabführsystem for discharging the exhaust gases (not illustrated sets).

The internal combustion engine is equipped for the purpose of charging with an exhaust gas turbocharger, which is arranged in the Abgasabführsystem turbine and in the intake system 1 arranged compressor 3 . 3a includes.

In addition, downstream of this compressor 3 . 3a a switchable compressor 2 provided and indeed in a bypass line 4 , forming a first node of the intake system 1 downstream of the compressor 3 . 3a the exhaust gas turbocharger branches off and forming a second node 4b back into the intake system 1 empties.

The switchable compressor 2 is an electrically driven compressor 2a which can be activated at any time, if necessary, in order to provide or generate a requested charge pressure.

In the bypass line 4 is a charge air cooler 6 upstream of the switchable compressor 2 arranged to be in the compressor 3 . 3a the turbocharger pre-compressed charge air before entering the switched compressor 2 to cool. Downstream of the switchable compressor 2 in the intake system 1 downstream of the second node 4b is another intercooler 7 arranged, which the cooling of the switched-in compressor 2 compressed charge air is used or bypassing the switchable compressor 2 the cooling of the compressor 3 . 3a the exhaust gas turbocharger compressed charge air.

The switchable compressor 2 is at its geodesically lowest point with a drain opening 5 equipped, which using a drain valve 5b can be released or blocked and serves to drain condensate collected in the compressor housing.

To the drain opening 5 closes a drain line 5a in which a pump 5c is provided, which is the discharge of condensate when the discharge opening 5 supported and at least temporarily activated and operated.

LIST OF REFERENCE NUMBERS

1

intake system

2

Compressor, switchable compressor

2a

electrically driven compressor

3

another compressor

3a

Compressor of an exhaust gas turbocharger

4

bypass line

4a

first node

4b

second node

5

drain hole

5a

drain line

5b

drain valve

5c

pump

6

Intercooler

7

further intercooler

Δt ₁

first predetermined period of time

Δt ₂

second predetermined period of time

Claims (15)

A supercharged internal combustion engine comprising: - an intake system (1) for supplying charge air, - an exhaust gas discharge system for discharging exhaust gas, and - at least one compressor (2, 3) arranged in the intake system (1) and having at least one in a compressor housing on a rotatable shaft arranged impeller, wherein a compressor (2) is arranged as a switchable compressor (2) in a bypass line (4) which branches off to form a first node (4a) from the intake system (1) and forming a second node (4b) again in the intake system (1), characterized in that - the switchable compressor (2) is equipped with a discharge opening (5) which is releasable and lockable using a drain valve (5b) and serves to drain condensate collected in the compressor housing , Charged internal combustion engine after Claim 1 , characterized in that in the bypass line (4) upstream of the switchable compressor (2) a charge air cooler (6) is arranged. Charged internal combustion engine after Claim 1 or 2 , characterized in that in the intake system (1) upstream of the first node (4a), a further compressor (3) is arranged. Supercharged internal combustion engine according to one of the preceding claims, characterized in that in the intake system (1) downstream of the second node (4b), a further intercooler (7) is arranged. Supercharged internal combustion engine according to one of the preceding claims, characterized in that the switchable compressor (2) is an electrically drivable compressor (2a). Charged internal combustion engine according to one of Claims 3 to 5 , characterized in that the further compressor (3) is a compressor (3a) of an exhaust gas turbocharger. Supercharged internal combustion engine according to one of the preceding claims, characterized in that an exhaust gas recirculation is provided, which comprises a return line which branches to form a third node of the Abgasabführsystem and opens to form a fourth node in the intake system (1). Supercharged internal combustion engine according to one of the preceding claims, characterized in that the discharge opening (5) in the installed position of the switchable compressor (2) is arranged at the geodetically lowest point of the compressor housing. Supercharged internal combustion engine according to one of the preceding claims, characterized in that adjoining the discharge opening (5) is a discharge line (5a). Charged internal combustion engine after Claim 9 , characterized in that in the discharge line (5a), a pump (5c) is provided. Method for operating a supercharged internal combustion engine according to one of the preceding claims, characterized in that the discharge opening (5) is kept locked using the discharge valve (5b) as long as the switchable compressor (2) is switched on and operated to generate a boost pressure , Method according to Claim 11 , characterized in that the discharge opening (5) using the discharge valve (5b) is released if the switchable compressor (2) is turned off in order to drain condensate collected in the compressor housing can. Method according to Claim 12 for operating a supercharged internal combustion engine, in which the discharge opening (5) is followed by a discharge line (5a) in which a pump (5c) is provided, characterized in that the pump (5c) is activated and operated to aid in the discharge of condensate becomes. Method according to Claim 13 , characterized in that the pump (5c) is activated to support the discharge of condensate as soon as a first predetermined period of time .DELTA.t _{1 has} elapsed after switching off the switchable compressor (2). Method according to Claim 13 or 14 , characterized in that the pump (5c) is operated to support the discharge of condensate only for a second predetermined period .DELTA.t ₂ .

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