The present invention relates to a system for an exhaust gas re ^¬ circulation, an engine, the use of a system, a method for ex ^¬ haust gas recirculation and a diesel exhaust gas composition ac ^¬ cording to the independent claims.

The external recirculation of (cooled) exhaust gas (EGR) into the air intake of a Diesel engine as a way to reduce NO _x emis ^¬ sions has been known and implemented for many years in automo ^¬ tive application. For large marine 2-stroke Diesel engines this technology has still to be developed. It is thus generally known to recirculate exhaust gas from an exhaust outlet of an engine back to the air inlet of the engine to optimise the combustion.

The use of residual fuel oil containing up to 3.5%m sulphur re ^¬ sults in the presence of sulphuric acid (H ₂ SO ₄ ) in the exhaust gas, coming from the combustion products SO _x and ¾0. The recir ^¬ culation of acidic exhaust gas causes corrosion on engine parts. Furthermore the combustion of residual fuel produces more par ^¬ ticulate matter (PM) than distillate fuel. Hence, the recircula ^¬ tion of exhaust gas highly loaded with particulate matter poses the risk of excessive fouling of engine parts which are in con ^¬ tact with the exhaust gas.

The patent application DE 10 2012 009 319 discloses a system for exhaust gas recirculation for a marine diesel engine. The ex ^¬ haust gas of the system is used to power a compressor which presses exhaust gas back into the inlet of the engine. This sys ^¬ tem uses recirculated gas but needs a turbocharger in the ex- haust gas cleaning duct. Hence, the turbocharger cannot be used without the exhaust gas passing through the cleaning device.

In DE 10 2012 009 315 an exhaust gas cleaning device is dis ^¬ closed, in which the exhaust gas can either power a second tur ^¬ bine or be recirculated through a cleaning device. There is no possibility to simultaneously let exhaust gas power the second turbine and be recirculated.

The document WO 2011/141631 discloses an arrangement for exhaust gas recirculation and turbocharging in which two turbocharges are arranged in series after one another. A turbocharging in two steps in series is more complex than a parallel turbocharging.

The document DE 10 2012 009 314 discloses a combustion engine with exhaust gas recirculation, in which the exhaust gas is ei ^¬ ther used for powering a compressor or recirculated through a cleaning device. There is no possibility to adapt the amount of cleaning to the situation needed.

In DE 103 31 187 an engine with exhaust gas recirculation is disclosed, in which heat of the exhaust gas is used to power the exhaust gas recirculation device. There is no disclosure on the amount of exhaust gas that is recycled and an adaption to the exhaust situation.

WO 94/29587 discloses an exhaust gas recirculation for a large supercharged diesel engine in which the exhaust cleaning device is arranged in series with a turbocharger. This leads to allow efficiency factor, since the exhaust gas coming from the engine is first cleaned and then let to the turbocharger. The air then arriving to the turbocharger already lost a significant amount of energy. It is an object of the present invention to avoid the disad ^¬ vantages of the prior art and in particular to create a system for exhaust gas recirculation, an engine, a method for exhaust gas recirculation and a diesel exhaust gas composition which allows to specifically adapt the exhaust gas composition to the desired values.

The object is achieved by a system, an engine, a method and an exhaust gas composition according to the independent claims.

In particular, the object is achieved by a system for exhaust gas recirculation, which is arrangeable between an exhaust outlet and an air inlet of an engine. The engine preferably is a two stroke engine. The system comprises a first turbocharger and a first functional duct between exhaust outlet and air inlet and a second turbocharger and a second functional duct between ex ^¬ haust outlet and air inlet, wherein the first and the second turbochargers are separate and arranged in parallel. Preferably, the first and the second functional ducts are separate and ar ^¬ ranged in parallel. The system further comprises an exhaust gas cleaning device arranged in an air duct arranged between exhaust outlet and air inlet. Additionally, the system comprises a con ^¬ trol unit for controlling the functional status of the system. The air duct is arranged at least partially and parallel to the second functional duct and the control unit is configured such that it controls a first entry valve arranged in the air duct upstream the exhaust gas cleaning device and a second entry valve arranged upstream the second turbocharger in a way that both entry valves can simultaneously assume an at least partial ^¬ ly open position. A system for exhaust gas recirculation with two entry valves being able to assume a simultaneously at least partially open po ^¬ sition enables the switching on of an exhaust gas recirculation independent from the engine power. Additionally, it is possible to use two turbochargers in parallel and additionally clean part of the exhaust gas and recirculate it. Hence, such a system ena ^¬ bles an adaption of the exhaust gas composition according to the standards needed in an easy way.

The exhaust outlet is arranged at the outlet of one or more cyl ^¬ inders of a combustion engine. The air inlet of the engine pref ^¬ erably comprises a scavenge air receiver.

A functional duct according to the invention uses exhaust gas for powering a turbocharger and leads fresh air to the air inlet of an engine. An air duct according to the invention leads ex ^¬ haust gas from exhaust outlet to the air inlet of an engine.

The entry valves can assume an at least partially open position so that the air duct and the second functional duct are able to be used in parallel. The control unit is able to control the opening of the two valves from completely closed to completely open independent of the other valve respectively. Furthermore, the position of the valve can be adapted to the situation.

The first functional duct can comprise a first cooler downstream of the first turbocharger and preferably a first water mist catcher downstream the first cooler.

The implementation of a cooler downstream a turbocharger leads to a higher effectiveness factor of the engine, since the cooled air has a higher density compared to warmer air. A water mist catcher dries the air and hence leads to less corrosion in the combustion chamber.

The exhaust gas cleaning device can comprise a scrubber and at least a second water mist catcher.

The scrubber can be used to clean the exhaust gas and in combi ^¬ nation with a water mist catcher which catches the water which the scrubber introduced into the exhaust gas.

The scrubber can be a combined scrubber for gas collection and particulate matter removal.

The scrubber has to clean the recirculated exhaust gas from SO ₂ , particulate matter, SO ₃ and H ₂ SO ₄ in order to prevent fouling and corrosion on downstream components. There are various scrubber technologies known for exhaust gas scrubbing but no commercially available product exists in a compact form for high pressure ap ^¬ plications. There are dry and semi-dry scrubbers available, which are very efficient regarding gas removal. Furthermore, dry scrubbers are large and heavy, which makes it almost impossible to add a dry scrubber on a marine engine. For particulate matter removal other scrubber types are most efficient, such as wet or electro-cyclone scrubbers. If different scrubber types are com ^¬ bined, the auxiliary equipment for the scrubbers should use the same medium or detergent. This leads to an economic integration of a combined scrubber.

Due to the water produced during the combustion process and due to the required cooling of the recirculated exhaust gas to ap ^¬ proximately 30 - 35 °C condensation will occur in the air duct. The condensate has to be collected, treated and discharged or stored. Hence, on-board equipment for the condensate or scrubber water treatment has to be installed. In case of a wet scrubber the equipment for condensate treatment additionally can be de ^¬ signed and used for the scrubber water treatment. Additional scrubber detergent handle equipment is not necessary. Since wet scrubbers are able to remove both gaseous pollutants and partic ^¬ ulate matter at high efficiency, wet scrubbers combine the above-mentioned requirements and are the most preferred scrubber type in case no combination is wanted or possible.

Wet scrubbers need scrubber water handling devices such as neu ^¬ tralisation unit, water supply and sludge tank or water separa ^¬ tion unit. The washing liquid can be either sea water (open-loop system) or fresh water (closed-loop system) with an appropriate level of alkalinity for acid neutralization.

Possible scrubbers are plate tower scrubber, spray tower scrub ^¬ ber or ejector venturi scrubber for SO ₂ absorption and/or venturi scrubber or multiple-venturi scrubber for particulate matter removal .

Downstream the second water mist catcher there can be provided a pressure elevation device such as a blower or a compressor.

This blower can be electrically or mechanically driven and pref ^¬ erably with a controllable speed. This variability offers a rel ^¬ atively simple way to adjust the exhaust gas flow and the pres ^¬ sure elevation, depending on the engine load point and tuning requirements .

By using a pressure elevation device the system for exhaust gas recirculation can be used for a two-stroke engine. Without a pressure elevation device the system for exhaust gas recircula ^¬ tion can only be used for four-stroke engines. The second functional duct can comprise a second cooler down ^¬ stream the second turbocharger.

The use of a second cooler enhances the efficiency factor of the engine, since cooled air is more dense than warm air directly out of the exhaust gas.

A first valve can be arranged directly downstream the second turbocharger .

A first valve directly downstream the second turbocharger leads to the possibility to completely cut off the second turbocharger and only use the exhaust gas recirculation in a duct for part of the exhaust gas.

The term directly is to be understood such that no other device is ranged between turbocharger and valve which has an influence on the air out of the turbocharger. Of course there has to be a line between valve and turbocharger which can have different length or diameters.

The air duct can comprise a third cooler arranged upstream the exhaust gas cleaning device.

A third cooler enhances the efficiency factor of the engine, since the air, which is going through the exhaust gas cleaning device, is also cooled.

The second cooler and the third cooler can be a combined cooler arranged downstream the second turbocharger and downstream the first valve in the air duct. A combined cooler only needs one component and is easy to in ^¬ stall. The air duct can comprise a combined line valve down ^¬ stream the combined cooler. The combined line valve preferably is a three-way valve.

A combined line valve enables the choice of leading the cooled exhaust gas or fresh air through the scrubber or to bypass a scrubber. Hence, the amount of pollutants can be adapted accord ^¬ ing to the situation needed.

The exhaust gas cleaning device can comprise a bypass duct be ^¬ tween combined line valve and the second water mist catcher to bypass the scrubber.

A bypass enables to bypass the scrubber with the air and thereby obviates the use of the scrubber or the passing of air through the scrubber when it is not in use.

This enhances the longevity of the whole system.

Downstream the third cooler to three-way valves can be arranged in series and the air duct is connected to the second functional duct by two connection lines, wherein the first connection line starts at the first three-way valve and connects the second functional duct directly upstream the second cooler at a first connection point. The second connection line starts at a second three-way valve and connects directly downstream the second cooler at the second connection point. A first check valve can be arranged upstream a first connection point and a second check valve can be arranged downstream the second connection point.

Such an arrangement enables on the one hand a combined operation of the first turbocharger, the second turbocharger and the ex- haust gas cleaning in the air duct. Additionally, the second turbocharger can be cut out and only the air duct can be fed with exhaust gas that is completely clean. For this case it is advantageous, that the first and the second cooler can be ar ^¬ ranged in series and hence reaches a higher cooling performance than just one cooler. Furthermore, such an arrangement is ex ^¬ tremely reliable in its application.

The two three-way valves downstream the third cooler can be com ^¬ bined in one valve, preferably a flap.

The combination of two valves in one leads lower production costs and an optimise use of the room for installation of the complete system.

A post-connection point valve, preferably a flap, can be ar ^¬ ranged downstream the second connection point.

A post-connection point valve leads to the possibility to lead a complete air or exhaust gas respectively, through the cleaning system of the exhaust gas recirculation system.

A third water mist catcher can be arranged downstream the post- connection point valve.

A third water mist catcher in the second functional duct leads to dryer air that is fed into the air input of the engine. This leads to lower corrosion.

Alternatively, downstream the third cooler two three-way valves can be arranged in series and the air duct can be connected to the second functional duct by two connection lines. The first connection line starts at the first three-way valve and connects the second functional duct at a third three-way valve upstream the second cooler and the second connection line starts at the second three-way valve and connects at a fourth three-way valve directly downstream the second cooler.

As previous one, this arrangement leads to the possibility to either use the second turbocharger or the air duct or a combina ^¬ tion of both. This leads to a very flexible system which can be adapted to the situation needed.

A pre-water-mist-catcher can be arranged downstream the third cooler .

Such a pre-water-mist-catcher may enhance the efficiency of the scrubber and thereby leads to a cleaner recirculated exhaust gas .

A pre-scrubber can be arranged downstream the first entry valve.

A pre-scrubber leads to a more efficient cleaning of the exhaust gas and may help to keep the third cooler's surface clean.

The recirculated exhaust gas has to be cooled down to the scav ^¬ enge air temperature before being mixed with the scavenge air. The target temperature is in the range of 30-35°C at ISO condi ^¬ tions. If a wet scrubber is used with 40% recirculation rate about 40% of the total exhaust gas energy would be dissipated in the scrubber water if no heat exchanger is used upstream of the scrubber. Due to the high temperature level (350-500°C) of the pre-turbine exhaust gas an exhaust gas recirculation system de ^¬ sign including a heat exchanger is beneficial in regard of waste heat recovery, (e.g. the recovered energy can be used for oper ^¬ ating a fresh water generator.) Two types of heat exchanger, also mentioned as cooler, can be used: Firstly, a dry heat exchanger: The outlet temperature of the exhaust gas is above the dew point of water vapour, no con ^¬ densate is accrued due to the cooling process. In order to pre ^¬ vent deposit build up on the cooler surface a relatively high gas velocity through the heat exchanger is required which re ^¬ sults in an increased pressure loss across the heat exchanger. Secondly, a wet heat exchanger: The temperature of the exhaust gas at the cooler outlet is intentionally below the dew point of water vapour. The condensate cleans the cooler pipes permanent ^¬ ly. An additional pre-scrubber where e.g. water is injected up ^¬ stream of the heat exchanger may be installed in order to in ^¬ crease the cleaning effect by an increased condensate flow.

On the one hand the recovered energy is larger with a wet heat exchanger than with a dry heat exchanger as the outlet temperature is not limited by the dew point temperature. On the other hand the heat exchanger selection may be restricted by the scrubber type and its requirements for the exhaust gas condition at the scrubber inlet, i.e. in regard of saturation and loading with water droplets. The heat exchanger material will also de ^¬ pend on the operation regime dry/wet.

A third water mist catcher can be arranged downstream the fourth three-way valve.

A third water mist catcher removes humidity from the air and thereby leads to less corrosion in the engine when the air is introduced into the cylinder. Any of the before mentioned systems can comprise a mixing device in which air is mixable out of the functional and / or the air duct .

The mixing device can be an active or a passive mixing device and is used to mix the recirculated exhaust gas and the fresh air out of the turbochargers such that the composition of the air that is introduced into the engine is uniform and has no peaks of pollutants, particulate matter or oxygen.

The passive mixing device is just composed out of a space where the air is able to mix itself, where an active mixing device ac ^¬ tively mixes the air by means of for example a stirring device.

The exhaust gas cleaning device can be combined with a scavenge air unit.

The combination of both devices leads to a better use of the room available.

The scavenge air unit comprises at least a cooler and at least a water mist catcher. The exhaust gas cleaning device usually com ^¬ prises a scrubber and optionally another water mist catcher.

The cooler can be used as scavenge air cooler and / or as ex ^¬ haust gas cooler in one device. This further leads to optimised costs of the system.

The object is further achieved by an engine, preferably a two- stroke engine, which comprises a system for exhaust gas recircu ^¬ lation as previously described. Such an engine is flexibly adaptable to the exhaust gas regula ^¬ tions of the surroundings.

The object is further achieved by the use of a system as previ ^¬ ously described for upgrading of an engine of a marine vessel. By upgrading an engine of a marine vessel with a system as pre ^¬ viously described the exhaust gas regulations can even be achieved by ships that are already in operation.

The object is further achieved by a method for exhaust gas re ^¬ circulation, preferably in a marine vessel; preferably using a system as previously described comprising the following steps: a. using at least part of exhaust gas of a combustion engine for actuation of a first turbine of a first turbocharger in a first functional duct and compressing air in a first compres ^¬ sor and convey the compressed air to an air inlet of a com ^¬ bustion engine;

b. using at least part of exhaust gas of a combustion engine for actuation a second turbine of a second turbocharger in a sec ^¬ ond functional duct and compressing air in a second compres ^¬ sor and convey the compressed air to an air inlet of a com ^¬ bustion engine;

c. using a third part of exhaust gas of a combustion engine in an air duct for recirculating the gas and cleaning the exhaust gas between exhaust gas outlet and air entice of a com ^¬ bustion engine in an exhaust gas cleaning unit;

d. a control unit at least controls the amount of exhaust gas which is going through the second turbine and the air duct in a way that a first entry valve in the air duct and a second entry valve upstream the second turbine can both assume an at least partially open position. Such a method for exhaust gas recirculation enables the recircu ^¬ lation and cleaning of part of the exhaust gas even when the en ^¬ gine is fully working at 100% without the need of using an ex ^¬ haust waste gate.

Hence, exhaust pollutant values can be achieved directly and more easy compared to systems where the engine power has to be reduced before exhaust gas cleaning can be applied.

The exhaust gas in the air duct can be let through a pre- scrubber downstream the first entry valve.

A pre-scrubber downstream the first entry valve leads to a pre- cleaning of the exhaust gas and to higher efficiency of the ex ^¬ haust gas cleaning.

The exhaust gas can be let through at least one, preferably two, coolers downstream the entry valve.

The cooling of the exhaust gas leads to a lower density of the exhaust gas and to a higher efficiency of the engine when rein ^¬ troduced into the engine. The use of two coolers enables an adaption to the amount of exhaust gas let trough the exhaust gas cleaning .

The exhaust gas can be let through a scrubber downstream the cooler .

The scrubber cleans the exhaust and can be a combined scrubber as previously described.

The scrubber can be bypassed by the compressed air downstream the cooler. When no exhaust gas cleaning is needed the scrubber can be by ^¬ passed and is thereby conserved and the lifetime for use of the scrubber is enhanced.

The exhaust gas can be let through at least one water mist catcher downstream the cooler and / or downstream the scrubber.

The application of a water mist catcher removes water from the air or exhaust gas and thereby reduces corrosion in the engine.

The exhaust gas is mixed with compressed outside air before be ^¬ ing recirculated into the air inlet.

The invention is in the following described in embodiments by means of figures.

Schematic overview of a first embodiment of the system; Schematic overview of an operating condition for 0-100% engine load and 0% exhaust gas recirculation of the first embodiment;

Schematic overview of an operating condition for 0-60% engine load and 10% exhaust gas recirculation of the first embodiment;

Schematic overview of an operating condition for 60- 100% engine load and 10% exhaust gas recirculation of the first embodiment;

Schematic overview of an operating condition for 0-100% engine load and 40% exhaust gas recirculation of the first embodiment;

Schematic overview of a second embodiment of the sys ^¬ tem; Fig. 7 Schematic overview of an operating condition for 0-60% engine load and 10% exhaust gas recirculation of the second embodiment;

Fig. 8 Schematic overview of an operating condition for 60-

100% engine load and 10% exhaust gas recirculation of the second embodiment;

Fig. 9 Schematic overview of an operating condition for 0-100% engine load and 0% exhaust gas recirculation of the second embodiment;

Fig. 10 Schematic overview of an operating condition for 0-100% engine load and 40% exhaust gas recirculation of the second embodiment;

Fig. 11 Cross-sectional view of an engine with space for a sys ^¬ tem for exhaust gas recirculation;

Fig. 12 Cross-sectional view of an exhaust gas recirculation system according to the second embodiment in EGR mode; Fig. 13 Cross-sectional view of an exhaust gas recirculation system according to the second embodiment in non-EGR mode ;

Fig. 14 3-dimensional view of an exhaust gas recirculation sys ^¬ tem according to the second embodiment in EGR mode;

Fig .15 3-dimensional view of an exhaust gas recirculation sys ^¬ tem according to the second embodiment in non-EGR mode;

Fig. 16 Cross-sectional view of an exhaust gas recirculation system according to the second embodiment in EGR mode;

Fig. 17 Cross-sectional view of an exhaust gas recirculation system according to the second embodiment in non-EGR mode ;

Fig. 18 3-dimensional view of an exhaust gas recirculation sys ^¬ tem according to the second embodiment in EGR mode;

Fig. 19 3-dimensional view of an exhaust gas recirculation sys ^¬ tem according to the second embodiment in non-EGR mode; Cross-sectional view of an exhaust gas recirculation system according to the first embodiment in a first op erational mode;

Cross-sectional view of an exhaust gas recirculation system according to the first embodiment in a second operational mode;

Cross- sectional view of Fig. 20 or 21 at A-A;

3-dimensional view of an exhaust gas recirculation sys tem according to the first embodiment in a first opera tional mode;

3-dimensional view of an exhaust gas recirculation sys tem according to the first embodiment in a second oper ational mode;

Fig. 1 shows a schematic overview of a first embodiment of the system 1. The system 1 for exhaust gas recirculation is arranged between an exhaust outlet 2 and an air inlet 3. The exhaust gas out of the exhaust outlet 2 is partially led into a first turbo- charger 4 in which the exhaust gas powers a turbine and fresh air is drawn into a compressor, which is driven by the turbine of the first turbocharger 4. The compressed air is led to a first cooler 12 and then through the first water mist catcher 13. From the first water mist catcher 13 the compressed air is led to the mixing device 37. Another part of the exhaust gas out of exhaust outlet 2 is led through the second turbo charger 6 where the exhaust gas drives a turbine similar to the first tur ^¬ bo charger 4. Upstream the turbine of the second turbo charger 6 a second entry valve 11 is arranged. The fresh air compressed by the compressor of turbo charger 6 is led through the first valve 18 and further through second cooler 17 to the fourth three-way valve 33. Downstream the fourth three-way valve 33 a third water mist catcher 29 is arranged. The dried air is then led to the mixing device 37. The third part of the exhaust gas is let from exhaust outlet 2 through a first entry valve 10 into third cool ^¬ er 19. Downstream third cooler 19 two three-way valves 22a, b are arranged in series. Downstream the two three-way valves 22a, b the exhaust gas cleaning device 8 is arranged. The exhaust gas cleaning device 8 comprises a scrubber 14 and downstream the scrubber 14 a second water mist catcher 15. Downstream the second water mist catcher 15 a pressure elevation device 16 is ar ^¬ ranged. The pressure elevation device in this embodiment is a blower. The air out of the blower is led into mixing device 37. The first three-way valve 22a is connected to the third three- way valve 32 by connection line 23a. The third three-way valve 32 corresponds to the first valve 18 in this embodiment. The second three-way valve 22b is connected to the fourth three-way valve 33 by connection line 23b. The advantages of this design are laid out in the description regarding figures 2 - 5.

Fig. 2 shows a schematic overview of an operating condition for

0 - 100% engine load and 0% exhaust gas circulation of the em ^¬ bodiment shown in Fig. 1. In this operational mode both turbo chargers 4, 6 are operating at 100% of their capability. The first turbo charger 4 is powered by approximately 60% of the ex ^¬ haust gas while the second turbo charger 6 is powered by approx ^¬ imately 40% of the exhaust gas. Downstream both turbochargers 4, 6 a cooler 12, 17 is arranged respectively. Downstream every cooler 12, 17 water mist catchers 13, 15 are arranged. The com ^¬ pressed air out of both turbo chargers 4, 6 are led to a mixing device 37, where the air is mixed before being introduced into the air inlet 3. In this operational mode of the system 1 the first functional duct 5 is built by the first turbo charger 4, the first cooler 12 and the first water mist catcher 13. The second functional duct 7 is built by a second turbo charger 6, the second cooler 17 and the second water mist catcher 15. Up ^¬ stream the second turbo charger 6 a second entry valve 11 is ar ^¬ ranged which can assume any position between fully open and fully closed. Hence, the amount of exhaust gas led through the sec ^¬ ond functional duct 7 is controllable depending on the engine load. The position of the second entry valve 11 is controlled by a control unit (not shown) . In this operational mode the system

1 fulfils the requirements of the TIER II limits.

Fig. 3 shows a schematic overview of an operation condition for 0 - 60% engine load and 10% exhaust gas recirculation of the first embodiment shown in Fig. 1. In this operational state the first functional duct 5 with its components first turbocharger 4, first cooler 12 and first water mist catcher 13 is operative as shown in Fig. 2. Contrary to the operational state of Fig. 2, the exhaust gas recirculation is active in this operational state. Approximately 60% of the exhaust gas is led through the first turbo charger 4 and powers the turbo charger 4 and with it a first functional duct 5. The remaining exhaust gas is led through air duct 9, which is composed from a first entry valve 10, a third cooler 19, a scrubber 14, a water mist catcher 15 and pressure elevation device 16. The exhaust gas is then led from the pressure elevation device 16 to mixing device 37, to be remixed with fresh air from the first functional duct 5. The three-way valves 22a, b only allow exhaust air to flow through the air duct 9. At least the position of first entry valve 10 is controlled by a control unit (not shown) and can be controlled between completely open and completely closed.

Fig. 4 shows a schematic overview of an operating condition for 60 - 100% engine load and 10% exhaust gas recirculation of the first embodiment according to Fig. 1. In this operational state both functional ducts 5, 7 and the air duct 9 are operational. As already described regarding Fig. 2 and 3 the first functional duct 5 is fed with approximately 60% of the exhaust gas which powers the turbine of the first turbo charger 4 and the com ^¬ pressed air out of the first turbo charger 4 is led through first cooler 12 and first water mist catcher 13 into mixing de ^¬ vice 37. The remaining exhaust gas is led through functional duct 7 and air duct 9. The composition of functional duct 7 cor ^¬ respond to the functional duct 7 of Fig. 2 while the composition of air duct 9 correspond to the air duct 9 of Fig. 3. To control the amount of exhaust gas going through air duct 9 and function ^¬ al duct 7 first entry valve 10 of air duct 9 and second entry valve 11 of functional duct 7 are controlled by a control unit (not shown) . In this operational mode it is possible to partial ^¬ ly recirculate the exhaust gas under a 100% engine load which leads to a fulfilment of the TIER II requirements. Fig. 5 shows a schematic overview of an operating condition for 0 - 100% engine load and 40% exhaust gas recirculation of the first embodiment according to Fig. 1. In this embodiment the second turbo charger 6 (see Fig. 1) is cut off. The first func ^¬ tional duct 5 is operating as already described regarding Fig. 2 - 4. The air duct 9 is extended by second cooler 17 by means of all for three-way valves 22a, 22b, 32 and 33. Hence, the exhaust gas is led from exhaust outlet 2 through first entry valve 10 to third cooler 19 and then redirected by three-way valve 22b through connection line 23a to third three-way valve 32 and fur ^¬ ther through a second cooler 17. Downstream the second cooler 17 the exhaust gas is led through the fourth three-way valve 32 through connection line 32b to three-way valve 22a and further on through the exhaust gas cleaning device 8 which is composed from scrubber 14 and second water mist catcher 15. Downstream the exhaust gas cleaning device 8 the air is led through pres ^¬ sure elevation device 16 and fed into mixing device 37. After being mixed with fresh air out of first functional duct 5 the mixed air is led through the air inlet 3. In this configuration the system 1 is able to fulfil the TIER III criteria under 100% engine load.

Fig. 6 shows a schematic overview of a second embodiment of the system 1. The system 1 of the second embodiment comprises a first functional duct 5 comprising a first turbo charger 4, a first cooler 12 and a first water mist catcher 13. The com ^¬ pressed fresh air out of the first functional duct 5 the is led into mixing device 37 before entering the engine (not shown) in air inlet 3. Deviating from the first embodiment according to Fig. 1, the first functional duct 5 further comprises an exhaust waste gate 39, which enables the wasting of exhaust gas without using it for powering a turbo charger or for recirculating it. The second functional duct 7 comprises a second entry valve 11, which is arranged upstream second turbo charger 6. The com ^¬ pressed air out of second turbo charger 6 is led through first valve 18 into second cooler 17. Downstream second cooler 17 a combined line valve 20 in form of a three-way valve is arranged. Out of combined line valve 20 a first line leads to scrubber 14 and into water mist catcher 15. Furthermore, out of combined line valve 20 a bypass duct 21 bypasses scrubber 14 and leads directly to water mist catcher 15. The exhaust gas is led from exhaust outlet 2 to first entry valve 10 into cooler 17. Hence, cooler 17 is a combined cooler for the second functional duct 7 and the air duct 9. Furthermore, the second water mist catcher 15 is shared between air duct 9 and second functional duct 7. Downstream the second water mist catcher 15 a pressure elevation device 16 is arranged directly upstream mixing device 37. In mixing device 37 recirculated exhaust gas and compressed fresh air is mixed and then fed into air inlet 3. Furthermore, the second embodiment comprises a non-return valve 40 for cases when the pressure of the air led out of second water mist catcher 15 is sufficient for direct introduction into the air inlet 3. The operational modes of the second embodiment according to this figure is described in Fig. 7 - 10.

Fig. 7 shows a schematic overview of an operating condition for 0 - 60% engine load and 10% exhaust gas recirculation of the second embodiment according to Fig. 6. The first functional duct 5 is composed as described regarding Fig. 1 - 6 out of first turbo charger 4, first cooler 12 and first water mist catcher 13. Approximately 60% of the exhaust gas out of exhaust outlet 2 is fed into turbo charger 4 for powering the compressor. The remaining exhaust gas is fed from exhaust outlet 2 through first entry valve 10 into second cooler 17. Downstream second cooler 17, the combined line valve 20 is in a position to lead the ex- haust gas through scrubber 14 and water mist catcher into blower 16. Downstream blower 16 the exhaust gas is led into mixing de ^¬ vice 37 and further to air inlet 3. This configuration leads to the fulfilment of the TIER II requirements.

Fig. 8 shows a schematic overview of an operating condition for 60 - 100% engine load and 10% exhaust gas recirculation of the second embodiment according to Fig. 6. This operating condition corresponds to the operating condition of Fig. 7 apart from the exhaust gate 39 which enables to directly waste super fluent ex ^¬ haust gas to the atmosphere under higher engine load. The air duct 5 is not capable of handling the remaining exhaust gas flow at these loads due to turbocharger capacity limitations. This configuration leads to the fulfilment of the TIER II criteria.

Fig. 9 shows a schematic overview of an operating condition for 0 - 100% engine load and 0% exhaust gas recirculation of the second embodiment according to Fig. 6. Approximately 60% of the exhaust gas is led from exhaust outlet 2 through first turbo charger 4. The compressed air out of turbo charger 4 is led through first cooler 12 and first water mist catcher 13 into mixing device 37. This first functional duct 5 corresponds to the first functional duct 5 of the first embodiment shown in Fig. 1. Since in this embodiment the exhaust gas is not recircu ^¬ lated approximately 40% of the exhaust gas is used to power tur ^¬ bo charger 6. The amount of exhaust gas used for powering turbo charger can be controlled by second entry valve 11 which is con ^¬ trolled by a control unit (not shown) . The air compressed by turbo charger 6 is further led to first valve 18 and second cooler 17. After cooling the air in second cooler 17 the air is further let through combined line valve 20 and bypass duct 21 to bypass the cleaning device. Downstream bypass duct 21 the air is let through second water mist catcher 15 and due to its high pressure level the air can directly be fed into air inlet 3 without any need for further pressure elevation. In this opera ^¬ tional mode the requirements of the TIER II can be achieved.

Fig. 10 shows a schematic overview of an operating condition for 0 - 100% engine load and 40% exhaust gas recirculation of the second embodiment according to Fig. 6. The first functional duct 5 of this operational mode corresponds to the first functional duct of Fig. 9. To recirculate the exhaust gas, the exhaust gas is led from exhaust outlet 2 through first entry valve 10 into cooler 17 and further to combined line valve 20. The combined line valve 20 is in a position to lead the exhaust gas to scrub ^¬ ber 14 and second water mist catcher 15. Hence, this embodiment only requires first turbo charger 4. The first entry valve 10 is controlled by a control unit (not shown) . This operational mode leads to a fulfilment of the TIER III criteria.

Fig. 11 shows a cross-sectional view of an engine 38 with space for a system 1 for exhaust gas recirculation. The space for the system 1 for exhaust gas recirculation has to be integrated into the available space of an engine 38. The integration further leads to low pressure losses across the exhaust gas recircula ^¬ tion system 1. The following design proposal shown in Fig. 12 - 24 are based on the embodiments of Fig. 1 or 6 and adapted to the space available shown in Fig. 11.

Fig. 12 shows a cross-sectional view of an exhaust gas recircu ^¬ lation system according to the second embodiment shown in Fig. 6 in exhaust gas recirculation mode. The general design consists of two compartments: an outer compartment as scavenge air com ^¬ partment 41 and an inner compartment comprising the scrubber 14. Out of second functional duct 7 and/or air duct 9 the exhaust gas is led through second cooler 17. Downstream the second cool- er 17 combined line valve 20 is constructed as two flaps, which can pivot around a pivoting point 42. The combined line valve 20 is in an open position which allows exhaust gas recirculation. The exhaust gas is led through the scrubber 14. The direct gas flow through the scavenge air compartment 41 is blocked and the exhaust gas is guided through the scrubber 14 through venturi nozzles 45. Washing liquid spray nozzle 43 are located in the venturi nozzle throat for highest particulate removal efficien ^¬ cy. After venturi nozzle 45 the exhaust gas flow is direct up ^¬ wards into the gas scrubber 14 by the round scrubber compartment shape. At the lowest point of the scrubber a drain can be posi ^¬ tioned. The exhaust gas is passing the gas scrubber 14 in an up ^¬ stream flow. The scrubber 14 is designed as a plate scrubber. The plates can be simple perforated plates, sieve plates, im ^¬ pingement plates, bubble-cut plates or valve plates or a combi ^¬ nation thereof. If the gas removal efficiency is not sufficient additional packing material may be placed in between the plates for increasing contact surface area. On the top of the plates the washing liquid is poured into the gas scrubber 14 and is flowing driven by gravity downwards through the plates in coun ^¬ ter current flow to the exhaust gas. Above the scrubber plates the exhaust gas is guided into the longitudinal centre of the scrubber compartment. The exhaust gas is then entering into a diagonal downward channel 44 (see Fig. 14) . After leaving this channel the exhaust gas is entering the scavenge air compartment 41 and is passing through water mist catcher 15 in order to remove the injected washing liquid. The cleaned exhaust gas has then to be processed to the pressure elevation device 16 (not shown) and the mixing device 37 (not shown) .

Fig. 13 shows a cross sectional view of an exhaust gas recircu ^¬ lation system 1 according to the second embodiment (shown in Fig. 6) in non-EGR mode. Without any exhaust gas recirculation the combined line valve 20 in form of flaps is in a position that closes the access to the scrubber 14. Compressed air coming from the turbo charger 4 (not shown) is entering through cooler 17. The compressed air is then directly flowing through a scavenge air compartment 41 and water mist catcher 15. Downstream the water mist catcher 15 the air is further led back to the air inlet 3 (not shown) .

Fig. 14 shows a three-dimensional view of an exhaust gas recir ^¬ culation system according to the second embodiment in EGR-mode (see Fig . 12 ) .

Fig. 15 shows a three-dimensional view of an exhaust gas recir ^¬ culation system according to the second embodiment in a non EGR- mode according to Fig. 13.

Fig. 16 shows a cross sectional view of an exhaust gas recircu ^¬ lation system according to the second embodiment in EGR-mode. This embodiment corresponds to the embodiment shown in Fig. 12. The only difference is the geometry of the flaps, which corre ^¬ spond to the combined line valve 20. The combined line valve 20 in this embodiment is not combined in one flap as shown in Fig. 12 but comprises two separate flaps. The advantage of such a de ^¬ sign is that this solution needs less space for rotating the flaps. Hence, the scrubber compartment can take more space in such an embodiment .

Fig. 17 shows a cross sectional view of an exhaust gas recircu ^¬ lation system according to the second embodiment in a non-EGR- mode . The embodiment is based on the embodiment shown in Fig. 13 with the same difference as between Fig. 12 and 16. The combined line valve 20 needs less space. Fig. 18 shows a three-dimensional view of an exhaust gas recir ^¬ culation system according to the second embodiment in EGR-mode . The three-dimensional view of Fig. 18 corresponds to the cross sectional view of Fig. 16. The combined line valve 20 comprises two separate flaps and therefore requires less space compared to the embodiment shown in Fig. 14.

Fig. 19 shows a three-dimensional view of an exhaust gas recir ^¬ culation system according to the second embodiment in non EGR- mode. The embodiment of Fig. 19 corresponds to the cross sec ^¬ tional view of Fig. 17 and has the only difference compared to the embodiment shown in Fig. 15 that the combined line valve 20 comprises two separate flaps. The advantages are already dis ^¬ cussed regarding Fig. 18.

Fig. 20 shows a cross sectional view of an exhaust gas recircu ^¬ lation system according to the first embodiment (shown in Fig. 1) in a first operational mode. The first operational mode com ^¬ prises a low exhaust gas recirculation rate (approximately 10%) and a reduced compressed air flow (approx. 30%) from turbo- charger 6 or about 40% exhaut gas recirculation rate at 0 - 25% engine load or a cut-off exhaust gas recirculation. The exhaust gas recirculation system generally comprises the scavenge air compartment 41 and the exhaust gas cleaning device 8. The ex ^¬ haust gas is entering the scrubber 14 at the top. The exhaust gas is passing the special exhaust gas recirculation cooler 19 and optionally a pre-water-mist-catcher . The cooled exhaust gas is then entering the scrubber 14. The scrubber 14 consists of two sections, a particulate scrubber and a gas scrubber. The particulate scrubber is based on a venturi scrubber principle. In particular multiple venturi nozzles (shown in Fig. 22) are arranged horizontally. In order to achieve highest particulate removal efficiency down to the submicron range of about 0,04pm multiple water spray nozzles 43 (see Fig. 22) are arranged in the venturi nozzles 45. After the venturi nozzles 45 the exhaust gas flow is directed upwards by the round geometry (see Fig. 22) through the plate scrubber 14. In the third water mist catcher 29 water droplets are removed from the cleaned exhaust gas be ^¬ fore it is then introduced into the air inlet 3 (not shown) . In parallel compressed intake air from turbocharger 6 is entering the scavenge air compartment 41 where it is cooled in the cooler 17 and where water droplets are removed by the third water mist catcher 29. The exhaust gas is then led through third water mist catcher 15 and then introduced into air inlet 3 (not shown) .

Fig. 21 shows a cross-sectional view of an exhaust gas recircu ^¬ lation system according to the first embodiment (shown in Fig. 1) in a second operational mode. The second operational mode comprises 40% exhaust gas recirculation rate. In this operation ^¬ al mode the third cooler 19 and the second cooler 17 are through flown by the exhaust gas in series. For this purpose, the flaps 46 and 47 are put into a horizontal position. Turbocharger 6 is cut-off, hence there is no compressed intake air flow. Down ^¬ stream the second cooler 17 the exhaust gas is led to the ex ^¬ haust gas cleaning device 8. For this purpose the flaps 47 are closed .

Fig. 22 shows a cross-sectional view of Fig. 20 or 21 respec ^¬ tively at A-A. The exhaust gas enters through third cooler 19, the special exhaust gas recirculation cooler. The exhaust gas then passes through venturi nozzles 45 which are equipped with spray nozzles with washing liquid 43. Due to the round shape of the bottom the exhaust gas flow is directed upwards and the ex ^¬ haust gas is entering the gas scrubber. By the round shape of the injected washing liquid in the venturi nozzles can be sepa ^¬ rated already at the lowest position into a drain pipe (not shown) to the water treatment system. In order to achieve maxi ^¬ mum gas removal efficiency a large contact surface area between the gas and washing liquids and a long residence time are key factors. This can be realized by a plate scrubber which consists of multiple horizontal plates. These plates can be simple perfo ^¬ rated plates, sieve plates, impingement plates, bubble-cut plates or valve plates or a combination thereof. If the gas re ^¬ moval efficiency is not sufficient additional packing material can be placed in between the plates for increased contact sur ^¬ face area. On top of the plates the washing liquid is poured in ^¬ to the gas scrubber 14 and is flowing driven by gravity downwards through the plates and countercurrent flow to the exhaust gas. Downstream the gas scrubber the exhaust gas is passing a water mist catcher 15. The air is then processed to a pressure elevating device (not shown) .

Fig. 23 shows a three-dimensional view of an exhaust gas recir ^¬ culation system according to the first embodiment in the first operational mode shown in Fig. 20. The scrubber compartment cor ^¬ responds to the scrubber shown in Fig. 22.

Fig. 24 shows a three-dimensional view of an exhaust gas recir ^¬ culation system according to the first embodiment in the second operational mode shown in Fig. 21. The scrubber compartment cor ^¬ responds to the scrubber compartment shown in Fig. 22.