DE19629015C2 - Device and method for cooling an exhaust gas stream of an internal combustion engine and its use - Google Patents

Description

The invention relates to a cooling device for cooling one in one Exhaust gas recirculation system of an internal combustion engine, in particular a diesel engine, led exhaust gas flow according to the preamble of claim 1, on a Internal combustion engine with an exhaust gas recirculation system and with a cooling device for Cooling an exhaust gas flow carried in the exhaust gas recirculation system, and to a method for operating the internal combustion engine.

It is known to equip internal combustion engines with an exhaust gas recirculation system. At An internal combustion engine with exhaust gas recirculation is the air that is generated when the machine is operating through gas inlet valves in the cylinders of the engine to help the combustion of a fuel in the cylinders to allow a mixture of fresh air and the Exhausts the cylinders after each combustion process through gas exhaust valves leave. The exhaust gas recirculation system is used to at least a part of the Collect cylinder-originating exhaust gases and mix them with those of the cylinders provide fresh air to be supplied.

Depending on the proportion of the exhaust gas in the supplied air, the Combustion processes in the cylinders and thus the operating behavior of the Optimize the internal combustion engine. A return of exhaust gases leads to one Reduction of the oxygen and an increase in the carbon dioxide content in the cylinders and thus to a slower combustion and a lowering of the Combustion peak temperatures. These effects can be used to to achieve various beneficial effects. For example, in diesel engines a reduction in the pressure gradient dp / dα (nailing) and  a reduced ignition delay with a low partial load of the internal combustion engine Furthermore, the combustion processes can be influenced so that the origin various undesirable pollutants, such as nitrogen oxides, with the exhaust gas usually released to the environment.

Known in connection with multi-cylinder internal combustion engines Exhaust gas recirculation systems can be differentiated according to which of the cylinders Exhaust gas recirculation system donor cylinders are d. H. Combustion gases to that with the Exhaust gas recirculation system contribute to recirculated exhaust gas flow, and which cylinders are so-called receiver cylinders of the exhaust gas recirculation system, d. H. supplied with air be mixed with exhaust gas from the recirculated exhaust gas stream.

DE 44 14 429 C1 describes a multi-cylinder internal combustion engine with an exhaust gas recirculation system discloses, in which the exhaust gas recirculation system comprises an exhaust gas return line, which part the exhaust gases of all cylinders collect and one with the gas inlet valves of all cylinders connected, fresh air-carrying manifold. In this case all cylinders are both donor cylinder and receiver cylinder of the exhaust gas recirculation system.  

DE 43 31 509 C1 describes a multi-cylinder internal combustion engine Exhaust gas recirculation system known in which a first, with fresh air without the addition of exhaust gases supplied group of cylinders exclusively as donor cylinders of the exhaust gas recirculation system acts, while the group of the remaining cylinders exclusively as the receiver cylinder of the Exhaust gas recirculation system is designed. This configuration of the exhaust gas recirculation system has the advantage that the recirculated exhaust gas stream contains a particularly low proportion of soot. This reduces the wear on the cylinders due to soot deposits.

Finally, from US 4,249,382 is a multi-cylinder internal combustion engine Exhaust gas recirculation system is known in which at least one cylinder as the exhaust gas cylinder Exhaust gas recirculation system is designed. The exhaust gas flows through this exhaust gas cylinder first a conventional cooling device for cooling the exhaust gas and then either alone or in certain proportions mixed with the exhaust gas from the other cylinders fed back to the intake side of the engine where possible via a manifold is evenly distributed over the inlets of all cylinders. The use of a conventional heat exchanger for exhaust gas cooling, however, entails both high costs - as well as maintenance, which is a disadvantage for the entire internal combustion engine is.

To keep the thermal load of an internal combustion engine with exhaust gas recirculation system so low to keep as possible, the exhaust gas recirculation system is usually with a Cooling device equipped for cooling the recirculated exhaust gas flow. Known Try concepts for cooling devices in connection with exhaust gas recirculation systems to take into account the problem that the exhaust gases from internal combustion engines, in particular of diesel engines, acid components and particles with a strong tendency to adhesion on surfaces containing progressive damage to the exhaust gas-carrying parts of the Exhaust gas recirculation system caused by corrosion and particle deposits and so the Limit the lifetime of the exhaust gas recirculation system. Farther  there is the problem that none of the known materials have the best possible resistance against corrosion and particle deposits over the entire temperature range brings about the recirculated exhaust gas flow in an exhaust gas recirculation system is usually cooled, for example from a temperature above 500 ° C to one Temperature below 100 ° C.

Taking account of the material problems mentioned are in DE 44 14 429 C1 Process for cooling diesel engine exhaust gases in an exhaust gas recirculation system proposed in which method the cooling of the hot exhaust gas flow in is carried out at least two successive steps, the exhaust gas flow is cooled to a predetermined temperature in each cooling step, and one to device based on this method, in which for cooling of Diesel engine exhaust gases in an exhaust gas recirculation line with respect to at least two Exhaust gas flow series heat exchangers are provided, each Heat exchanger is adapted to a predetermined temperature range. The Cooling the exhaust stream in successive stages allows everyone Heat exchanger material for use in a predetermined temperature range adapt, d. H. with regard to corrosion and particle adhesion. Specifically, in DE 44 14 429 C1 for cooling a 500-700 ° C hot exhaust gas flow one Preamble of claim 1 corresponding two-stage cooling device with a first Heat exchanger stage for temperatures above 250 ° C and a second Heat exchanger stage proposed for temperatures below 250 ° C, being as Materials for the components in contact with the exhaust gas flow Heat exchanger high temperature resistant steels in the case of the high temperature range assigned heat exchanger and with polytetrafluoroethylene (also called Teflon®) coated titanium in the case of the intended for the lower temperature range Heat exchanger are selected. The material selection regarding that for the lower one Temperature range certain heat exchanger has the advantage that titanium is special is resistant to the aqueous acids that are carried along with the exhaust gas stream and have a particularly corrosive effect on the surfaces on which they condense. Since the condense aqueous acids at temperatures below 250 ° C, is particularly the for the lower temperature range certain heat exchangers by the acid parts of the Exhaust gas flow claimed. The coating with polytetrafluoroethylene, which is used for the exhaust gases and the acid components contained therein are permeable, also sets the Particle adhesion down. A disadvantage of this two-stage cooling device can be seen that the heat exchanger intended for the lower temperature range  constructively complex cooling stage, which provides the desired resistance to condensates aqueous acids through the use of a particularly expensive material.

The invention has for its object a structurally simple, if possible To create inexpensive to produce cooling device for cooling one in one Exhaust gas recirculation system of an internal combustion engine exhaust gas flow suitable and is as resistant as possible to the aqueous acids contained in the exhaust gas stream.

This object is achieved by the entirety of the features of claim 1.  

In the cooling device according to the invention, the through-channel has the second Cooling device has a cooling zone, which acts as a relaxation zone for the exhaust gas flow is trained on. The exhaust gas stream is expanded when entering the relaxation zone and cools down to temperatures below the condensation temperature of the aqueous acids. A through channel with a relaxation zone The cooling zone can be implemented, for example, as a tube with a suitable throttle point. With a suitable design of the relaxation zone, the cooling of the exhaust gas stream is just because of the relaxation intense enough to make up a substantial part of the watery To allow acids to condense in the passage of the second cooling device.

Since the aqueous acids at relatively low condensation temperatures less than 100 ° C condense is the predetermined temperature range over which the exhaust gas flow in Through channel of the second cooling device can be cooled, relatively narrow. The Cooling device according to the invention therefore has the advantage that the cooling stage, the is mainly burdened by the acid components of the exhaust gas flow, not must necessarily be designed as a heat exchanger, but with constructive less complex and less expensive means can be realized.

The passage channel of the second cooling device preferably consists of ceramic material or at the interfaces that come into contact with the exhaust gas flow Ceramic coating. Ceramic material ensures the desired Resistance to aqueous acids and is inexpensive, for example compared to Titanium. It is also conceivable to add additional coatings from suitable materials to the Attach surfaces of the passageway of the second cooler to For example, to minimize the adhesion of particles.

In a development of the cooling device according to the invention is a discharge device for removing condensates of the aqueous acids from the through-channel of the second discharge device provided. For example, the through channel be designed so that the aqueous acids preferentially collect at a point where they flow out of the through-channel through a drainage channel or by suction can be removed.

Further advantageous design options of the cooling device according to the invention can be seen in connection with the exhaust gas recirculation system of an internal combustion engine, with which the cooling device should cooperate advantageously. As is well known, for a  suitable pressure drop within the exhaust gas recirculation system along the exhaust gas flow be taken care of so that the exhaust gas flow into the receiver cylinder of the exhaust gas recirculation system penetrates with the intensity that to optimize the running in the internal combustion engine Combustion processes is needed. Furthermore, the  Amount of exhaust gas, which ideally should be fed to the receiver cylinders, depends from the current load of the internal combustion engine.

Further advantages of the cooling device according to the invention now result from a suitable coordination of the flow resistances of the through channels of the first and / or the second cooling device to the pressure, hereinafter called the inlet pressure, with which the exhaust gas flow from the donor cylinders is fed into the exhaust gas recirculation system becomes. The inlet pressure is particularly dependent on the time at which the Gas outlet valves of a donor cylinder are opened, or by the corresponding crank angle, based on the top dead center of the piston of the corresponding cylinder. In general, the smaller the pressure, the greater the pressure Crank angle is at which the gas outlet valve of the donor cylinder in question is opened becomes.

The cooling device according to the invention is particularly advantageous in connection with a Exhaust gas recirculation system of a multi-cylinder internal combustion engine, in which a first group of one or more cylinders exclusively as the receiver cylinder of the Exhaust gas recirculation system and a second group of one or more cylinders are designed exclusively as donor cylinders of the exhaust gas recirculation system. In this The exhaust gas flow can be configured under particularly high inlet pressure Exhaust gas recirculation system can be supplied by means of a variable valve control Gas outlet valves of the cylinders of the second group with a correspondingly smaller one Crank angle compared to the gas outlet valves of the cylinders of the first group be opened. Through an additional modulation of the opening time of the gas outlet valves the cylinder of the second group, the inlet pressure can be varied additionally, for example depending on the current load of the internal combustion engine.

This configuration of the exhaust gas recirculation system is particularly advantageous with regard to mentioned preferred embodiment of the cooling device according to the invention, in which the passage channel of the second cooling device has a cooling zone which is called Relaxation zone is formed for the exhaust gas flow. Education is the relaxation zone in the through channel with increased flow resistance and thus associated with an increased pressure loss along the through-channel. This Pressure loss can, however, be compensated for by an increased inlet pressure of the exhaust gas flow be by using the variable valve control, the gas outlet valves of the cylinders of the second group with a correspondingly small crank angle in the range of, for example  Open 80 ° -100 ° after the top dead center (TDC) of the piston. The possibility, the inlet pressure of the exhaust gas flow is particularly high by means of the variable valve control to choose also opens up the possibility of the flow resistance of the through channel to choose the first cooling device particularly large. This gives the advantage of being first cooling device to choose a heat exchanger that is particularly compact.

This configuration is particularly suitable for diesel engines. Since the donor cylinder of the Exhaust gas recirculation system combustion technology from the receiver cylinders of the Exhaust gas recirculation system are separated and supplied with fresh air without the addition of exhaust gas, is a particularly low contamination of the engine with combustion residues, for example with soot particles, and a particularly low pollutant emission can be achieved.

In the following an embodiment of the cooling device according to the invention a diesel engine with exhaust gas turbocharger and an exhaust gas recirculation system based on the Drawing explained in more detail.

The drawing shows a multi-cylinder diesel engine 1 with an exhaust gas turbocharger 9 and an exhaust gas recirculation system 20 . The diesel engine 1 comprises two functionally different groups of cylinders which are separate in terms of distribution technology: a first group of cylinders 2 , each with at least one gas inlet valve 2 'and with at least one gas outlet valve 2 ''each, and a second group of cylinders 3 , each with at least one gas inlet valve 3 '. and each with at least one gas outlet valve 3 ''. In the present case, the second group includes, for example, only one cylinder 3 . Within the scope of the invention, a plurality of the cylinders 3 could also have belonged to the second group, provided that the gas inlet valves 3 ′ and the gas outlet valves 3 ″ have the same connections with regard to a gas inlet into the cylinders 3 and a gas outlet from the cylinders 3 .

The exhaust gas turbocharger 9 comprises a turbine 7 and a compressor 11 driven by the turbine 7 . With the compressor 11 , fresh air can be drawn in via the fresh air supply 10 and - compressed and cooled - can be supplied to the cylinders 2 and 3 via the compressor outlet line 12 and the two suction pipes 13 and 14 , into which the compressor outlet line 12 branches. Each gas inlet valve 2 'or the gas inlet valve 3 ' is correspondingly connected to the intake pipe 13 or your intake pipe 14 . A charge air cooler for cooling the air supplied to cylinders 2 and 3 is not shown in FIG. 1; it can be designed conventionally and is therefore of no interest in this context.

The turbine 7 is driven by exhaust gases from the cylinders 2 . It is connected via the exhaust gas line 5 to the gas outlet valves 2 ″ of the cylinder 2 and has an outlet line 8 for the driving exhaust gases. A soot filter 6 in line 5 limits contamination of the turbine 7 and the environment by the exhaust gases.

The exhaust gas recirculation system 20 connects the gas outlet valve 3 ″ to the intake manifold 13 for the cylinders 2 by means of the exhaust gas return line 21 . Via the exhaust gas recirculation line 21 formed from exhaust gases from the cylinder 3 in the exhaust gas stream, the intake manifold 13 can be introduced. The exhaust gas recirculation system 20 comprises a first cooling device 25 with a through channel 26 and a second cooling device 30 with a through channel 31 . The passage channels 26 and 31 are connected to the exhaust gas return line 21 in such a way that an exhaust gas flow guided in the exhaust gas return line 21 flows through the two passage channels one after the other.

The cooling device 25 is designed as a heat exchanger, the through-channel 26 being able to be cooled with a cooling medium guided in the cooling line 27 . The passage 31 of the cooling device 30 , as symbolically shown in FIG. 1, is designed as a tube with a constriction at a throttle point 32 and with a relaxation zone 33 . The relaxation zone 33 represents a cooling zone, especially since an exhaust gas stream expanding through the throttle point 32 into the relaxation zone 33 cools down. A removal device 34 is provided for removing liquids condensing in the relaxation zone. The discharge device comprises a drainage channel which opens into the relaxation zone 33 at a point at which condensates preferentially collect and through which condensates can flow off or be removed by suction.

During operation of the internal combustion engine shown in Fig. 1 gases flow in the lines 5, 8, 10, 12, 13, 14 and 21 in the area marked by arrowheads directions. A recirculated exhaust gas flow flows out of the cylinder 3 along the exhaust gas return line 21 into the intake manifold of the cylinders 2 . The pressure conditions in the lines 13 , 14 and 21 are set up so that the intake manifold 14 only carries fresh air, while the recirculated exhaust gas in the intake manifold 13 , mixed with fresh air, flows in the direction of the gas inlet valves of the cylinders 2 . Cylinder 3 is therefore exclusively the donor cylinder of exhaust gas recirculation system 20 , while cylinders 2 act exclusively as receiver cylinders of exhaust gas recirculation system 20 .

The cooling capacities of the heat exchanger 25 and the cooling device 30 are dimensioned such that the exhaust gas flow within the through-channel 26 to temperatures above and in a second cooling step in the region of the relaxation zone 33 of the through-channel 31 of the second cooling device 30 to temperatures below the condensation temperature contained in the exhaust gas stream cools aqueous acids. The acids transported with the exhaust gas stream therefore begin to condense downstream of the throttle point 32 and can be disposed of with the discharge device 34 .

The through channels 26 and 31 are adapted to the operating conditions by a suitable choice of the materials from which they are made, in such a way that their damage due to the action of the exhaust gas flow at predetermined temperatures occurs at the temperatures to which they are exposed during the operation of the internal combustion engine 1 Keep boundaries.

Any material which is resistant to aqueous acids and from which a correspondingly shaped tube can be produced or with which a correspondingly shaped, not necessarily acid-resistant tube can be coated is suitable for producing the through-channel 31 . A suitable material for producing the through-channel is, for example, ceramic.

The cooling device shown in Fig. 1 comprises two cooling stages. It is also conceivable to cool the exhaust gas flow in three or more stages and to adapt each cooling stage individually to the operating conditions which are characteristic of it by suitable choice of material.

The gas inlet valves 2 'and 3 ' and the gas outlet valves 2 '' and 3 '' can be opened with a valve control (not shown in FIG. 1) at predetermined crank angles for each cylinder, in each case based on the top dead center of the piston of a cylinder 2 or 3 or be closed. A variable valve control is advantageous, which makes it possible to open the gas outlet valves 3 ″ at a smaller crank angle, for example 80 ° -100 ° after TDC, than the gas outlet valves 2 ″. This results in the above-mentioned advantages with regard to the design of the cooling devices 25 and 30 , based on the possibility of increasing the inlet pressure of the exhaust gas flow by opening the gas outlet valves 3 'at a relatively small crank angle.

The device shown in Fig. 1 is a specially selected example. A diesel engine is shown. However, the cooling device according to the invention can also be used in connection with other internal combustion engines with variable valve control. Furthermore, Fig. 1 shows a preferred configuration of an exhaust gas recirculation system, in which a first group of cylinders are formed exclusively as a receiver cylinder and a second group of cylinders solely as an exhaust cylinder of the exhaust gas recirculation system. However, the cooling device according to the invention is also suitable for exhaust gas recirculation systems which are connected to cylinders which are both exhaust gas cylinders and receiver cylinders of the exhaust gas recirculation system.

Reference list

1

Diesel engine

2nd

Cylinder, from the group of the first cylinders

2 '

Gas inlet valve, for first cylinders

2nd

'' Gas outlet valve, for first cylinders

3rd

Cylinder, from the group of second cylinders

3 '

Gas inlet valve, for second cylinders

3rd

'' Gas outlet valve, for second cylinders

5

Exhaust pipe for the first cylinder

6

Soot filter

7

turbine

8th

Outlet pipe

9

Exhaust gas turbocharger

10th

Fresh air supply

11

compressor

12th

Compressor discharge line

13

Intake manifold for the first cylinder

14

Intake manifold for second cylinders

20th

Exhaust gas recirculation system

21

Exhaust gas return line

25th

first cooling device: heat exchanger

26

Through channel of the first cooling device

27

Cooling pipe

30th

second cooling device

31

Through channel with throttle and relaxation zone

32

Throttling point

33

Relaxation zone

34

Removal device for aqueous acids.

Claims (7)

1. Cooling device for cooling an exhaust gas flow conducted in an exhaust gas recirculation system ( 20 ) of an internal combustion engine ( 1 ), in particular a diesel engine, with a first cooling device ( 25 ) and a second cooling device ( 30 ) downstream of the first flow direction for the exhaust gas flow, wherein each cooling device comprises a passage channel ( 26 , 31 ) for the exhaust gas flow, the exhaust gas flow along each passage channel ( 26 , 31 ) can be cooled over a predetermined temperature range, the cooling capacities of the cooling devices ( 25 , 30 ) being adjustable in such a way that the exhaust gas flow can be adjusted by means of the the first cooling device ( 25 ) can be cooled to temperatures above and by means of the second cooling device ( 30 ) to temperatures below the condensation temperature of the aqueous acids contained in the exhaust gas stream, and the through-channels ( 26 , 31 ) are adapted to the predetermined temperature ranges, the through-channel ( 31 ) the second cooling device is designed to be resistant to the aqueous acids, characterized in that the through-channel ( 31 ) of the second cooling device ( 30 ) has a cooling zone which is designed as a relaxation zone ( 33 ) with a throttle point ( 32 ) for the exhaust gas flow provided upstream thereof. 2. Cooling device according to claim 1, characterized in that the through-channel ( 31 ) of the second cooling device ( 30 ) is made of ceramic material or that the through-channel ( 31 ) of the second cooling device ( 30 ) has an interface formed from ceramic material to the exhaust gas flow. 3. Cooling device according to claim 1 or 2, characterized in that the relaxation zone ( 33 ) is designed as a tube. 4. Cooling device according to one of claims 1-3, characterized in that a discharge device ( 34 ) for discharging condensates of the aqueous acids from the through-channel ( 31 ) of the second discharge device ( 30 ) is provided. 5. Cooling device according to one of claims 1-4, characterized in that the first ( 25 ) cooling device ( 30 ) is designed as a heat exchanger. 6. internal combustion engine, in particular diesel engine, with a first and a second group of one or more, in each case a gas outlet (2 ', 3') having cylinders (2, 3), a device for actuating the gas outlet valves at predetermined crank angles, an exhaust gas recirculation system ( 20 ) for an exhaust gas flow which can be discharged from the gas outlet valves ( 3 ''), the cylinders ( 3 ) of the second group being designed as exhaust gas cylinders of the exhaust gas recirculation system ( 20 ) and the cylinders ( 2 ) of the first group being designed as receiver cylinders of the exhaust gas recirculation system ( 20 ), and with a cooling device for cooling the exhaust gas flow according to one of claims 1-5. 7. A method of operating an internal combustion engine according to claim 6, characterized in that the exhaust valves ( 3 '') of the cylinders ( 3 ) of the second group are opened at a smaller crank angle than the exhaust valves ( 2 '') of the cylinders ( 2 ) of the first Group.