DK201300615A1 - A large slow running turbocharged two-stroke internal combustion engine with crossheads and exhaust gas recirculation - Google Patents

Abstract

A large slow-running two-stroke uniflow combustion engine (l) o f the crosshead type. The engine (l) comprises a plurality of cylinders (2) that are each connected to a scavenge air receiver (11) and to an exhaust gas receiver (6), a turbocharger (7) with a turbine (T) receiving exhaust gas from the exhaust gas receiver (6) and a compressar (C) providing pressurized scavenge air, a scavenge air path for leading the pressurized scavenge air from the outlet of the compressar (C) of the turbocharger (7) to the scavenge air receiver (11) and an exhaust gas recirculation path for recirculating at least a portion o f the exhaust gas from the cylinders ( 2) or from the exhaust gas receiver ( 6) to the scavenge air receiver (11). The scavenge air path has a split-point ( 2 3) where a blow path ( 2 6) branches off. The engine further comprises a blower (30) receiving any exhaust gas from the exhaust gas recirculation path and receiving any branched off scavenge air from the blow path (26) and the blower (30) forces any exhaust gas received and scavenge air received towards the scavenge air receiver (11).

Description

A LARGE SLOW RUNNING TURBOCHARGED TWO-STROKE INTERNAL COMBUSTION ENGINE WITH CROSSHEADS AND EXHAUST GAS RECIRCULATION

FIELD OF THE INVENTION

The present invention relates to large slow running turbocharged two-stroke internal combustion engines with crossheads and exhaust or combustion gas recirculation. Further, the invention relates to a method of operating a large slow running turbocharged internal combustion engine with crossheads and exhaust or combustion gas recirculation.

BACKGROUND ART

Large slow running two-stroke internal combustion engines with crosshead are typically used in propulsion systems of large ships or as prime mover in power plants. These engines have a crosshead disposed between the piston and the crankshaft.

Emission requirements have been and will be increasingly difficult to meet, in particular with respect to NOx levels.

Presently, a number of theoretical options exist to reduce NOx formation in large slow running turbocharged internal combustion engine with changes to be applied to the engine process, in particular the following: • Exhaust- or combusted Gas Recirculation (EGR) • Use of water emulsified fuel. • Humidification of the Fresh Charge, i.e. Scavenge Air Moisterization - (SAM). • Selective Catalyst Reactor Method (SCR)

Exhaust Gas Recirculation (EGR) is a measure known to assist in small fast running diesel engines to reduce N0X emissions. However, there are up to now very few commercially operating large two-stroke diesel engines that use exhaust gas recirculation. The reason is that it has proven to be a challenge to implement exhaust gas recirculation in a large two-stroke diesel engine.

One reason that it has proven to be so challenging to implement exhaust gas recirculation in a large two-stroke diesel engine is the fact that these engines are generally operated with heavy fuel oil which has a high content of sulfur. Consequently, the sulfur content of the exhaust gas is much higher than in smaller diesel engines which are operated on fuel oil having no or low sulfur content. A high concentration of sulfur greatly reduces the choice of cleaning methods and apparatus since most of them will not be able to tolerate the sulfur and sulfuric acid levels present in the exhaust gases of a large two-stroke diesel engine burning heavy fuel with high sulfur content.

One technique for cleaning the exhaust gases off large two-stroke diesel engines that has been able to resist these high sulfur levels is wet scrubbing. This technique in the exhaust gas is passed through a so-called wet scrubber that uses water as a scrubbing solution.

Another challenge has been the amount of power required to transport the recirculated exhaust gases from the exhaust gas receiver into the scavenge air flow. In a large two-stroke diesel engine, the scavenge air pressure is typically up to about 0.3 bar higher than the pressure in the exhaust gas receiver of large two-stroke diesel engines. Thus., A blower or other means is required for forcing the recirculated exhaust gases from the exhaust gas receiver into the scavenge air system. In a large bore 12 or 14 cylinder two stroke diesel engine, such as the MAN B&W 12K98MC-C engine the power required to operate such a blower would be close to 0.5 MW. This is a significant amount of energy to use on an exhaust gas system and an electric motor to operate a blower with such large of power requirements is extremely expensive.

Thus, the first cost for the machinery e.g. wet scrubber and blower, in an exhaust gas recirculation system for large diesel engines amounts to a substantial sum, due to the dimensions of these parts.

Large slow-running two-stroke combustion engines require an auxiliary blower to assist the compressor of the turbocharger in obtaining a sufficiently high scavenge air pressure in the scavenge air receiver when the engine is operating in medium to low load conditions, e.g. Below 25-45% maximum continuous rating of the engine, depending on engine and turbocharger characteristics. Thus, a dedicated auxiliary blower is provided in existing large slow-running internal combustion engines for ensuring operation during low to medium load conditions.

DISCLOSURE OF THE INVENTION

Against this background, it is an object of the present invention to provide a large slow-running two-stroke uniflow combustion engine of the crosshead type and EGR with a more simple, reliable and less expensive construction.

This object is achieved by providing a large slow-running two-stroke uniflow combustion engine of the crosshead type, the engine comprising a plurality of cylinders each connected to a scavenge air receiver and to an exhaust gas receiver, a turbocharger with a turbine receiving exhaust gas from the exhaust gas receiver and a compressor providing pressurized scavenge air, a scavenge air path for leading the pressurized scavenge air from the outlet of the compressor of the turbocharger to the scavenge air receiver, an exhaust gas recirculation path for recirculating at least a portion of the exhaust gas from the cylinders or from the exhaust gas receiver to the scavenge air receiver, the scavenge air path having a split point where a blow path branches off, a blower receiving any exhaust gas from the exhaust gas recirculation path and receiving any branched off scavenge air from the blow path and the blower forces any exhaust gas received and scavenge air received towards the sc turn off the air receiver.

By providing blower that receives both scavenge air from branched off blow path and recirculated exhaust gas it becomes possible to use a single blower for two purposes, namely to assist the turbocharger during low to medium load conditions and to provide the flow of recirculated exhaust gas during EGR operation. This combined use of the blower simplifies the construction of the engine and reduces costs. Having only one blower as opposed to two with different functionality saves material and space.

In an embodiment the scavenge air path comprises piping, a scavenge air cooler upstream of the split point, a nonreturn valve downstream of the split point to avoid back flow during operation of the blower and a mixer chamber downstream of the non-return valve. The non-return valves prevent backflow at low load - while the blower is both supporting the EGR operation and as an auxiliary blower for the turbocharger. During this dual function the gas' mixing is efficiently performed in the blower - before the mixed gas enters to the mixer chamber.

At higher loads - with no need for aux blower support - the mixer chamber allows for the recirculated exhaust gas to be mixed with the main flow of scavenge air that enters the mixing chamber from the normal air passage through a non-return valve.

In an embodiment of an outlet the blower is connected to an inlet of the mixer chamber and an outlet of the mixer chamber is connected to the scavenge air receiver via a non-return valve.

In an embodiment the exhaust gas recirculation path comprises piping and an EGR cooler.

In an embodiment the auxiliary blow path extends from the split-point to the inlet of the blower and comprises a blow control valve to control and balance the flow in the EGR flow path and the flow in the scavenge air path.

In an embodiment the engine further comprises a nonreturn valve in the blow path to avoid backflow during operation of the blower.

In an embodiment the engine further comprises a nonreturn valve upstream of an inlet of the blower that receives flow of exhaust gas from the EGR flow path and the non-return valve is configured to avoid backflow during normal operation of the engine - with no EGR support .

In an embodiment the engine further comprises an electronic control unit configured to maintain a given scavenge air pressure and a given oxygen content or C02 content of the gas in the scavenge air receiver for a given EGR rate.

In an embodiment the electronic control unit receives a signal representative of the oxygen content or of the C02 content and a signal representative of the scavenge air pressure and the electronic control unit is configured to control the position of the blow control valve and EGR blower flow. .

In an embodiment the flow path that leads recirculated exhaust gas to the inlet of the blower includes an EGR control valve and the electronic control unit is configured to control the position of the EGR control valve.

In an embodiment of the blower, the compressor of the turbocharger assists in reaching a desired scavenge air pressure when required and assists in obtaining a required flow of recirculated exhaust gas when required.

The object above is also achieved by providing a method of operating a large slow-running two-stroke uniflow combustion engine of the crosshead type, the engine comprising a plurality of cylinders each connected to a scavenge air receiver and to an exhaust gas receiver , a turbocharger with a turbine receiving exhaust gas from the exhaust gas receiver and a compressor providing pressurized scavenge air, a scavenge air path for leading the pressurized scavenge air from the outlet of the turbocharger compressor to the scavenge air receiver, an exhaust gas recirculation path for recirculating at least a portion of the exhaust gas from the cylinders or from the exhaust gas receiver to the scavenge air receiver, a blower receiving any exhaust gas from the exhaust gas recirculation path and receiving any branched off scavenge air from the blow path , using the blower to force any exhaust gas received and scavenge air received by the blower to the scavenge air receiver.

In an embodiment the method further comprises operating the engine with EGR continuously, and using the blower to force the EGR flow and using the blower to assist the compressor of the turbocharger to obtain a scavenge air pressure during low load conditions and further to assist the turbocharger for combustion chamber cooling at high load.

The object above is also achieved by providing a large slow-running two-stroke uniflow combustion engine with crossheads comprising a plurality of cylinders each connected to a scavenge air receiver and to an .exhaust gas receiver, a turbocharger with a turbine receiving exhaust gas from the exhaust gas receiver and a compressor providing pressurized scavenge air, a scavenge air path for leading the pressurized scavenge air from the outlet of the compressor of the turbocharger to the scavenge air receiver, an exhaust gas recirculation path for recirculating at least a portion of the exhaust gas from the cylinders or from the exhaust gas receiver to the scavenge air receiver, a blower for forcing an EGR flow into the scavenge air receiver, a sensor for providing a signal representative of the 02 level or of the C02 level in the scavenge air receiver. An electronic control unit in receipt of the signal from the sensor and the electronic control unit ECU is configured to control the EGR flow in response to the signal from the sensor.

Further objects, features, advantages and properties of the large slow running turbo-charged two-stroke combustion engine and method according to the invention will become apparent from the detailed description.

LETTER DESCRIPTION OF THE DRAWINGS

In the following detailed portion of the present description, the invention will be explained in more detail with reference to the exemplary embodiments shown in the drawings, in which:

FIG. 1 is a diagrammatic depiction of an engine'according to an example embodiment,

FIG. 2 is a detailed view of an electronic control unit and control valves of the engine of FIG. 1, and FIG. 3 is a diagrammatic depiction of an engine according to another example embodiment.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The following detailed description of the large slow-running turbocharged two-stroke combustion (diesel) engine with crossheads according to the invention will be described by the exemplary embodiments.

The construction and operation of large slow running turbocharged two-stroke combustion engines with crossheads is as well-known and should not require further explanation in the present context. Further details regarding the operation of the exhaust gas systems are provided below.

FIG. 1 shows a first example embodiment of a large slow-running turbocharged two-stroke internal combustion (Diesel) engine 1 of the uniflow type. The engine 1 may e.g. used as the main engine in an ocean going vessel or as a stationary engine for operating a generator in a power station. The total output of the engine may, for example, range from 2,000 to 110,000 kW.

The engine is provided with a plurality of cylinders 2 arranged besides one another in line. Each cylinder 2 is provided with an exhaust valve 3 associated with its cylinder cover. The exhaust channels can be opened and closed by the exhaust valve 3. A crosshead 4a and connecting rod 4b of the engine connecting the piston rod to a large end of the crankshaft is also shown. The exhaust bends 5 connect to an exhaust gas receiver 6. The exhaust gas receiver 6 is disposed in parallel to the row of cylinders 2. From the exhaust gas receiver 6 at least a substantial portion of the exhaust gases are guided. of a turbocharger 7 via an exhaust conduit (there may be more than one turbocharger 39). Another portion of the exhaust gas is recycled and enters an exhaust gas recirculation flow path that will be described in greater detail further below. The exhaust gases are disposed into the atmosphere downstream of the turbine of the turbocharger 7 via outlet 9a, 9b or outlet 9c through an exhaust gas bypass 54.

The turbocharger 7 also includes a compressor C which is connected to a fresh air intake 10a. The compressor C delivers pressurized scavenge air to a scavenge air receiver 11 via a scavenge air flow path which in an example embodiment includes a cooler 12 and a water mist catcher 13. The water mist catcher 13 removes any water that condenses when the scavenge air is cooled. From the water mist catcher the scavenge air path continues via a non-return valve 24 to a mixer chamber 25. The mixer chamber 25 also receives recirculated exhaust gas from the exhaust gas recirculation path which will be described in greater detail further below.

The scavenge air flow path includes a split point 23 downstream of the water mist catcher 13 and upstream of the non-return valve 24. A blow conduit 26 extends between the split point 23 and the inlet of a blower 30. The blow conduit 26 allows a portion or all of the scavenge air to lead to the inlet of the blower 30. The blow conduit 26 includes a non-return valve 27 and a blow control valve 28.

The scavenge air enters the mixer chamber 25 from the split point via the non-return valve 24 or via the blow conduit 26 and blower 30.

In the mixer chamber 25 the scavenge air that enters via non-return valve 24 mixes with recirculated exhaust gas in EGR operation and the outlet of the mixer chamber 25 connects to the scavenge air receiver 11 via a non-return valve 31. The scavenge air path is part of a scavenge air unit 36 which includes an example embodiment of the scavenge air cooler 12, the water mist catcher 13, the split-point 23, the non-return valve 24, the mixer chamber 25 and the non-return valve 31st

The scavenge air or scavenge air mixed with recirculated exhaust gas is passed from the scavenge air receiver 11 to the scavenge ports 14 of the individual cylinders 2.

An exhaust gas bypass 54 is provided to handle fast slow down operations of the engine and / or will be used with an exhaust gas bypass matched turbocharger at high load operation to avoid turbocharger overloading. A cylinder bypass 47 may be added in order to reduce the pressure difference between the exhaust receiver 6 and scavenge air receiver 11. When hot air is bypassed from turbocharger 7 compressor outlet C to the exhaust receiver 6 - less power will be needed by the blower 30 -to overcome the reduced differential pressure. The bypass inlet 61 to the exhaust receiver 6 - is kept away from the exhaust gas unit 37 inlet 62 - in order not to reduce the exhaust gas concentration of C02 guided into the exhaust gas recirculation unit 37.

The exhaust gas receiver 6 is also connected to an exhaust gas recirculation path at outlet 62, which forms part of an exhaust gas recirculation unit 37. The exhaust gas recirculation unit 37 may be located on the engine or in the vicinity of the engine . The recirculation path includes a pre scrubber 15 followed by an EGR cooler 16 which is followed by a scrubber 17 which is followed by a water mist catcher 18 which is followed by as gas suction chamber 19. The scrubbers 15, 17 are in an embodiment wet scrubbers where exhaust gases are brought into contact with scrubbing liquid droplets and steam, which are atomized and evaporated into the exhaust gas from one or more nozzles. The scrubbing liquid and steam are normally water based. The scrubbing liquid droplets and steam absorb particles such as catalytic fines, fuel residues, soot particles and gases such as sulfur compounds and thereby clean the gas. Besides cleaning the exhaust gas the wet scrubbing action also cools the exhaust gas. In the EGR cooler 16 the gas is cooled and water may condense in the EGR cooler 16. The water mist catcher 18 (separating means) collects the condensed water and also tiny water droplets that may be carried from the scrubbers by the gas flow. The water collected in the water mist catcher 18 is discarded via an outlet drain (not shown) and can be reused after a rinsing process.

The exhaust gas unit 37 may be used in two fundamentally different ways: • In Normal operation • In EGR operation

Further details regarding the operation of the exhaust gas unit 37 are provided below.

In Normal operation:

The exhaust gas unit 37 acts as a normal air cooler unit -with closed valve 43, open valves 41 +42 and turbocharger 39 in operation. Normal air is passed through the exhaust gas unit 37 - without scrubber support and finally guided through the non return valve 29 to the scavenging receiver 11 at medium and high load. At low load the air will pass through the conduit 21 and the blower 30 before the air enters the mix chamber 25 and pass through the non-return valve 31 to the scavenging air receiver 11. Thus the exhaust gas recirculation unit 37 now acts as a normal air cooler unit. A shutoff valve 41 and a shutoff valve 42 are used to connect or disconnect the second turbocharger 39. When the second turbocharger 39 is active, it's compressor C delivers the compressed air directly to the EGR cooler 16 and the pre-scrubber 15 is bypassed. For this purpose, a shutoff valve 43 in a conduit 62 extending between the exhaust gas receiver 6 and the pre-scrubber 15 is closed. Operation without EGR may be chosen in case the basic engine without EGR is emission Tier 2 matched.

The blower 30 can be used at low load as an auxiliary blower when the scavenging air pressure provided by turbochargers 7 and 39 together is not sufficient to ensure proper combustion. In such a case, air passes from the EGR cooler 16 through an inactive scrubber 17, the water mist catcher 18, the gas suction chamber 19, the conduit 21, through the blower 30, to the mixing chamber, through the non-return valve 31 on its way to the scavenge air receiver 11.

In EGR operation

The exhaust gas unit 37 acts as an exhaust gas cleaning device with open valve 43, closed valves 41 +42 and turbocharger 39 inactive. Exhaust gas is passed through opening 62 from the exhaust receiver 6 - through the scrubber 15. In the pre scrubber 15 the gas is cooled and cleaned before the water and gas is guided through the EGR cooler 16 for further cooling. The flow continues through the scrubber 17 for further cleaning and to capture a majority of the scrubber water from the exhaust gas. The cleaned and cooled exhaust gas pass through an additional water mist catcher 18 before it enters the suction chamber 19. If pressure in the suction chamber 19 is lower than the pressure in the scavenging air receiver 11, the non return valve 29 is closed. An outlet of the gas suction chamber 19 is connected to a conduit 21 which leads to the inlet of the blower 30. The conduit 21 includes a non-return valve 20 and an EGR control valve 22. The non-return valve 20 prevents back flow during blower operation.

The blower 30 receives exhaust gas from the EGR flow path and branched off scavenge air from the blow path (conduit) 26. The blower 30 is powered by an electric drive motor or by a hydraulic drive motor or steam turbine or exhaust gas expander and serves to increase the pressure of the exhaust gas and / or the scavenge air it receives to thereby force it into the scavenge air receiver 11 via the non-return valve 31. The operation of the blower (blower speed or / and diffuser position) is controlled by the electronic control unit (ECU) shown in FIG. 2nd

The electronic control unit receives a signal representative of the oxygen (02) level (content) or of the carbon dioxide (C02) level (content) of the gas in the scavenge air receiver 11 from a sensor 51 and a signal representative of the pressure in the scavenge air receiver 11 from a pressure sensor 52. As shown in FIG. 2., the electronic control unit issues a control signal to the blow control valve 28 in conduit 26 and / or to the EGR control valve 22 in conduit 21. The electronic control unit is configured to ensure that the engine operates with a given scavenge air oxygen or C02 content (by EGR ratio) and with sufficient scavenge air pressure. The engine 1, under the control of the electronic control unit, can flexibly change between operation with different EGR ratios. If the engine is a ship engine this could be advantageous in order to meet geographically varying exhaust gas requirements, such as NOx output limits. Examples of such requirements are the IMO NOx Tier-II and Tier-Ill regulations.

The blow control valve 28 may throttle scavenge air coming from the split point - assuming that the pressure drop from the turbo charger compressor C to the inlet of the blower 30 is less than the pressure drop from the exhaust gas receiver 6 over the EGR module 37 to the inlet of the blower 30 (this is typically the case). The throttling of the blow control valve 28 will balance the flows from the scavenge air module 36 and the EGR module 37 and thus control the EGR ratio.

The total mixed mass flow is controlled by the speed of the blower 30 and / or by the diffuser position of the blower 30 under command of the electronic control unit.

The flow through conduit 27 is matched such that with the blow control valve 28 fully open the required EGR rate is given for high EGR flow at load just before normal auxiliary blower setpoint. (Tier III mode at approximately 25-30% load)

For Tier III mode - which has a lower NOx limit - the EGR rate is increased by blower flow in order to arrive at the desired oxygen content in the scavenge air receiver 11. At low load also the conduit 26 is included to support the turbocharger 7. The blow control valve 28 may be throttling to obtain the correct scavenging air pressure and oxygen content in the scavenge air receiver 11

Simplified setup:

FIG. 3 illustrates another example embodiment which is similar to the embodiment above, but with a simpler setup. In this embodiment the EGR system is running continuously. This embodiment simplifies the construction and uses components in a dual configuration, such as the EGR cooler in always wet running condition. The wet cooler surface also functions as a scrubber. Accordingly, now scrubber 17 is needed. This embodiment does not have EGR valves 43 and 22 since the blower is continuously running and flow is fully controlled over the mode and load range with the EGR blower 30 flow. One non-return valve 20 is included to avoid backflow to the EGR unit 37, which may be relevant during low load operation or blower failure. However, there is no second turbocharger nor cut out valves 41 +42 nor conduit with non-return valve 29 connecting the EGR unit 37 directly to the scavenge air receiver 11 because there is no need for such components. The turbocharger 7 may be matched according to different matching strategies that will depend on the application of the conduit 54. In the simplest setup the turbocharger can be matched without the conduit 54. Further the embodiment does not have the exhaust gas bypass 54 since it is Assume that the engine control system will handle a slow-down operation fast enough to avoid over speeding of the turbocharger 7. The embodiment uses a single blower or more blower in parallel 30 for auxiliary blowing and for EGR blowing. The conduit 47 may be aborted in case EGR blower 30 is matched accordingly. In this embodiment the EGR cooler 16 will operate continuously in a wet running condition. The wet cooler surface functions simultaneously as a scrubber and accordingly no separate scrubber 17 is required. No throttling / control valve 22 is required in the EGR conduit 21 since the blower 30 is continuously operating and the .flow is fully controlled over the mode and load range with the blower 30 and the control valve 28.

The table below shows examples of possible modes of operation for the embodiment of Fig. 3:

Mode 2: is fail safe mode - in case of blower failure so mode 3 or 4 cannot be used. In the mode is the load. restricted by the turbocharger 7 maximum capacity.

Mode 3 is a low EGR mode to meet for example IMO Tier II emission requirements. The engine can be operated in a load range of 0 to 110%. The blower 30 is used to provide a low flow of recirculated exhaust gas, i.e. a sufficient flow of recirculated exhaust gas to arrive at an EGR ratio that results in NOx emission cycle value below the Tier II threshold. For example, the EGR ratio is in the range between 0 and 15%. In this mode the blow control valve 28 is closed when the engine load is medium to high (e.g. above 25%) and there is no need to assist the compressor C of the turbocharger 7 in achieving the required scavenge air pressure. When the load is low, for example between 0 and 25% the blow control valve 28 is open or throttled and the blower 30 has a double function since it also assists the compressor C of the turbocharger 7 to achieve the required scavenge air pressure.

Mode 4 is a high EGR mode to meet for example IMO Tier III emission requirements. The engine 1 can be operated in a load range of 0 to 110%. The blower 30 is used to provide a significant flow of recirculated exhaust gas, i.e. a sufficient flow of recirculated exhaust gas to arrive at an EGR ratio that results in NOx emission cycle value below the strict Tier III threshold. For example, the EGR ratio could be in the range of 30 to 40%. In this mode, the blow control valve 28 is closed when the engine load is medium to high, for example above 25% and there is no need to assist the compressor C of the turbocharger 7 in achieving the required scavenge air pressure. When the load is low, for example between 0 and 25% the blow control valve 28 is open or throttled and the blower 30 has a double function since it also assists the compressor of the turbocharger to achieve the required scavenge air pressure.

The teaching of this invention has numerous advantages. Different embodiments or implementations may yield one or more of the following advantages. It should be noted that this is not an exhaustive list and there may be other advantages which are not described herein.

One advantage of teaching this application is that it provides for a more simple construction of the gas exchange system of a large slow running two-stroke combustion engine with exhaust gas recirculation. Another advantage is that it provides for more flexible operation of a large slow running two-stroke combustion engine with exhaust gas recirculation that allows for fuel optimized operation in the IMO Tier II emission mode at part load. However, obvious is the reduction in number of components - in order to reduce the final cost of this application.

Although the teaching of this application has been described in detail for purposes of illustration, it is understood that such detail is solely for that purpose, and variations can be made therein by those skilled in the art without departing from the scope of the teaching of this Application .. Example may be scaling the principle to large engine - where more turbochargers and exhaust gas systems may be needed in parallel to cover the flow requirements. Same scaling consideration can be made on the blower application - where typically more than one blower will be installed either due to flow capacity requirement or just by the simple redundancy requirement for the aux blower operation.

It should also be noted that there are many alternative ways of implementing the teaching of this invention. For example, the blower for increasing the pressure of both recirculated exhaust gas and of scavenge air is retrofitted to an existing engine. Either being an integrated engine system or as a standalone exhaust gas unit - located in proximity to the engine - and connected to the engine with equal pipe conduits.

In another example embodiment the large slow-running two-stroke uniflow combustion engine 1 with crossheads comprises a plurality of cylinders 2 each connected to a scavenge air receiver 11 and to an exhaust gas receiver 6, a turbocharger 7 with a turbine T receiving exhaust gas from the exhaust gas receiver 6 and a compressor C providing pressurized scavenge air, a scavenge air path for leading the pressurized scavenge air from the outlet of the compressor C of the turbocharger 7 to the scavenge air receiver 11, an exhaust gas recirculation path for recirculating at least a portion of the exhaust gas from the cylinders 2 or from the exhaust gas receiver 6 to the scavenge air receiver 11, a blower 30 for forcing an EGR flow into the scavenge air receiver, a sensor 51 for providing a signal representative of the 02 level or of the C02 level in the scavenge air receiver 11. An electronic control unit ECU in receipt of the signal from the sensor 51 and the electronic control u nit ECU is configured to control the EGR flow in response to the signal from sensor 51.

In another example embodiment the large slow-running two-stroke uniflow combustion engine with crossheads comprises a plurality of cylinders 2 each connected to a scavenge air receiver 11 and to an exhaust gas receiver 6, a turbocharger 7 with a turbine T receiving exhaust gas from the exhaust gas receiver 6 and a compressor C providing pressurized scavenge air, a scavenge air path for leading the pressurized scavenge air from the outlet of the compressor C of the turbocharger 7 to the scavenge air receiver 11, an exhaust gas recirculation path for recirculating at least a portion of the exhaust gas from the cylinders 2 or 'from the exhaust gas receiver 6 to the scavenge air receiver 11, a blower 30 to forcing an EGR flow into the scavenge air receiver, a sensor 51 for providing a signal representative of the 02 level or of the C02 level in the scavenge air receiver 11. An electronic control unit ECU in receipt of the signal from the sensor 51 and the electronic control un the ECU is configured to control the EGR flow in response to the signal from the sensor 51.

When the 02 level is measured the electronic control unit ECU is configured in an embodiment to increase the EGR ratio when the 02 level in the scavenge air receiver 11 is above a threshold and possibly to decrease the EGR ratio when the 02 level in the scavenge air receiver 11 is below a particular threshold.

When the C02 level is measured the ECU electronic control unit is configured in an embodiment to decrease the EGR ratio when the C02 level in the scavenge air receiver 11 is above a threshold and possibly to increase the EGR ratio when the C02 level in the scavenge air receiver 11 is above a particular threshold.

In a variation of this embodiment, the electronic control unit ECU is configured to control the EGR ratio by adjusting the speed of the blower 30.

The term "comprising" as used in the claims does not exclude other elements or steps. The term "a" or "an" as used in the claims does not exclude a plurality.

The electronic control unit may fulfill the functions of several means recited in the claims.

The reference signs used in the claims shall not be construed as limiting the scope.

Although the present invention has been described in detail for purposes of illustration, it is understood that such detail is solely for that purpose, and variations may be made therein by those skilled in the art without departing from the scope of the invention.

Claims (15)

1. A large slow-running two-stroke uniflow combustion engine (1) with crossheads, said engine (1) comprising: a plurality of cylinders (2) that are each connected to a scavenge air receiver (11) and to an exhaust gas receiver (6), a turbocharger (7) with a turbine (T) receiving exhaust gas from said exhaust gas receiver (6) and a compressor (C) providing pressurized scavenge air, a scavenge air path for leading said pressurized scavenge air from the outlet of the compressor (C) of said turbocharger (7) to said scavenge air receiver (11), an exhaust gas recirculation path for recirculating at least a portion of the exhaust gas from said cylinders (2) or from said exhaust gas receiver (6) to said scavenge air receiver (11), said scavenge air path having a split-point (23) where a blow path (26) branches off, a blower (30) receiving any exhaust gas from said exhaust gas recirculation path and receiving any branched off scavenge air from said blow path (26) and said blower (30) forces any exhaust gas received and scavenge air received towards said scavenge air receiver (ID-

2. An engine (1) according to claim 1, wherein said scavenge air path comprises piping, a scavenge air cooler (12) upstream of said split point (23), a non-return valve (24) downstream of said split-point (23) to avoid back flow during operation of the blower (30) and a mixer chamber (25) downstream of said non-return valve.

2. An engine (1) according to claim 1, wherein said scavenge air path comprises piping, a scavenge air cooler

3. An engine (1) according to claim 2, wherein an outlet of said blower (30) is connected to an inlet of said mixer chamber (25) and an outlet of said mixer chamber (25) is connected to said scavenge air receiver via a non-return valve (31).

4. An engine (1) according to any one of claims 1 to 3, wherein said exhaust gas recirculation path comprises piping and an EGR cooler (16).

5. An engine (1) according to any one of claims 1 to 4, wherein said blow path (26) extends from said split-point (23) to the inlet of said blower (30) and comprises a blow control valve (28) to control the flow in the blow path.

6. An engine (1) according to any one of claims 1 to 5, further comprising a non-return valve (20) upstream of an inlet of the blower (30) that receives flow of exhaust gas from the EGR flow path and said non-return valve (20) is configured to avoid backflow during operation or failure of the blower (30).

7. An engine (1) according to any one of claims 1 to 6, further comprising an electronic control unit (ECU) configured to maintain a given scavenge air pressure and a given oxygen content level or C02 content level of the gas in the scavenge air receiver (11) for a given EGR rate .

8. An engine (1) according to claim 7, wherein said . electronic control unit (ECU) receives a signal representative of the oxygen content or of the C02 content and a signal representative of the scavenge air pressure and wherein said electronic control unit (ECU) is configured to control the position of said blow control valve (28).

9. An engine (1) according to claim 8, wherein said electronic control unit (ECU) is configured to control the scavenge air pressure and the oxygen content of the C02 content in the scavenge air receiver by controlling the speed of the blower 30 and by keeping the blow control valve (28) open when the scavenge air pressure delivered by the compressor of the turbocharger is insufficient during low load engine operation and by closing the blow control valve (28) when the scavenge air pressure delivered by the compressor of the turbocharger is sufficient during to medium to high load engine operation.

10. An engine (1) according to claim 8, wherein the flow path that leads recirculated exhaust gas to the inlet of the blower (30) includes an EGR control valve (22) and wherein said electronic control unit (ECU) is configured to control the position of said EGR control valve (22).

11. An engine (1) according to any one of claims 1 to 10, wherein said blower (30) assists the compressor of the turbocharger in reaching a desired scavenge air pressure when required and assists to obtain a required flow of recirculated exhaust gas when required.

12. A method of operating a large slow-running two-stroke uniflow combustion engine (1) of the crosshead type, said engine (1) comprising: a plurality of cylinders (2) that are each connected to a scavenge air receiver (11) and to an exhaust gas receiver (6), a turbocharger (7) with a turbine (T) receiving exhaust gas from said exhaust gas receiver (6) and a compressor (C) providing pressurized scavenge air, a scavenge air path for leading said pressurized scavenge air from the outlet of the compressor (C) of said turbocharger (7) to said scavenge air receiver (11), an exhaust gas recirculation path for recirculating at least a portion of the exhaust gas from said cylinders (2) or from said exhaust gas receiver (6) to said scavenge air receiver (11), a blower (30) receiving any exhaust gas from said exhaust gas recirculation path and receiving any branched off scavenge air from said blow path (26) , using said blower (30) to force any exhaust gas received and scavenge air received by said blower to said scavenge air receiver (11).

13. A method according to claim 14, said method further comprising operating said engine (1) with EGR continuously, and using said blower (30) to force the EGR flow and using said blower to assist the compressor of turbocharge -(7) in obtaining a scavenge air pressure only during low load conditions.

14. A large slow-running two-stroke uniflow combustion engine (1) of the crosshead type, said engine (1) comprising: a plurality of cylinders (2) that are each connected to a scavenge air receiver (11) and to an exhaust gas receiver (6), a turbocharger (7) with a turbine (T) receiving exhaust gas from said exhaust gas receiver (6) and a compressor (C) providing pressurized scavenge air, a scavenge air path for leading said pressurized scavenge air from the outlet of the compressor (C) of said turbocharger (7) to said scavenge air receiver (11), an exhaust gas recirculation path for recirculating at least a portion of the exhaust gas from said cylinders (2) or from said exhaust gas receiver (6) to said scavenge air receiver (11), a blower (30) for forcing an EGR flow into the scavenge air receiver, a sensor (51) for providing a signal representative of the 02 level or of the C02 level in the scavenge air receiver (11), an electronic control unit (ECU) in receipt of said signal from said sensor (51) and said electronic control unit (ECU) being configured to control the EGR flow in . response to the signal from said sensor 51.

15. An engine (1) according to claim 14, wherein said electronic control unit (ECU) is configured to control the EGR ratio by adapting the speed of said blower (30).