DK179038B1

DK179038B1 - A two-stroke internal combustion engine with a SCR reactor located downstream of the exhaust gas receiver

Info

Publication number: DK179038B1
Application number: DKPA201570704A
Authority: DK
Inventors: Morten Vejlgaard-Laursen
Original assignee: Man Diesel & Turbo Filial Af Man Diesel & Turbo Se Tyskland
Priority date: 2015-11-02
Filing date: 2015-11-02
Publication date: 2017-09-11
Also published as: JP2017110632A; CN107035518B; CN107035518A; DK201570704A1; KR20170051367A; KR101897708B1; JP6266730B2

Abstract

A two-stroke internal combustion engine (1) has a SCR reactor (11) located downstream of an exhaust gas receiver (6). A regeneration flow path (17) is connectable to the SCR reactor for regenerating catalyst material in the SCR reactor. At least one cylinder out of the plurality of engine cylinders is adapted to deliver to the regeneration flow path (17) a first flow of exhaust gas at a higher temperature than the temperature of the exhaust gas in the exhaust gas receiver (6).

Description

The present invention relates to a two-stroke internal combustion engine comprising a plurality of cylinders having combustion chambers, and an exhaust gas system, which exhaust gas system comprises an exhaust gas receiver for receiving exhaust gas from the plurality of cylinders, at least one turbocharger and at least one SCR reactor located downstream of the exhaust gas receiver, where the individual cylinder has a cylinder wall with scavenge air ports and a cylinder cover with an exhaust valve at an exhaust duct extending to the exhaust gas receiver in the exhaust gas system, and where a regeneration flow path is connectable to the SCR reactor for regenerating catalyst material in the SCR reactor. DK 177462 B1 discloses such an engine, in which the exhaust gas received has a central duct separated from an outer area with swirling gas flow, and the central duct is used to evaporate a reductant for use in the SCR reactor.

In general, two-stroke internal combustion engines are used as propulsion engines in vessels like container ships, bulk carriers, tankers and gas carriers. Two-stroke internal combustion engines are also used as prime movers in stationary power stations where the internal combustion engine drives a generator supplying an electricity grid with power. Environmental issues are relevant to both propulsion engines and stationary prime movers. The two-stroke internal combustion engines of the present invention are typically large in-line engines having a power of at least 400 kW per cylinder. These two-stroke engines are typically of the crosshead type, and they can operate with high efficiency (low specific fuel oil consumption) on fuel oils of poor quality, such as heavy fuel oil and heavy fuel oil containing sulphur, or on fuel gas and pilot oil, where the pilot oil is a fuel oil containing sulphur.

The SCR reactor (SCR is Selective Catalytic Reduction) is a standard component used for exhaust gas treatment by which the NOx generated during the combustion process in the cylinders can be reduced to a low level before the exhaust gas leaves the chimney and is released to the environment. In the SCR reactor, the NOx is reduced catalytically to nitrogen and water by adding ammonia as a reducing agent. Further details on this are described in the paper ‘Emission Project Guide’, 2nd Edition, March 2014, MAN Diesel & Turbo, Denmark.

The operating temperature of the SCR reactor is influenced by the temperature of the exhaust gas flowing to the SCR reactor for being cleaned and of the pressure of the exhaust gas in the SCR reactor and of the fuel sulphur content. If the exhaust gas temperature is too low (such as below 320 °C at 2.5 bara and a sulphur content of 3 %), sulphuric acid is neutralized by ammonia and forms a sticky product, ammonium bisulphate, called ABS, which may deposit on the catalyst material in the SCR reactor. If the exhaust gas temperature is above a maximum temperature, such as 550 °C, the catalyst material may begin to sinter. If the temperature is above 350 °C deposited ABS may dissolve and thus the catalyst material regenerates. EP 2 216 523 A1 discloses that an SCR reactor cleaning the exhaust gas from a large marine diesel engine when the temperature of the exhaust gas is equal to or below 300 °C. In order to regenerate the catalyst material a power producing auxiliary diesel engine, which is a four-stroke engine different from the internal combustion engine, is used to provide exhaust gas at a higher temperature of about 350 °C, and this exhaust gas is directed to a selected section of the SCR reactor in order to regenerate the catalyst material in this section. JP H5-285343 discloses a two-stroke internal combustion engine having an exhaust gas temperature in the range from 230 °C to 280 °C and operating on fuel containing from 2% to 5% sulphur, and the sulphur content increases the formation of ABS. An SCR reactor is located downstream of a turbocharger. The SCR reactor is divided into compartments and has in each compartment a gate valve that can block the inflow of exhaust gas from the turbocharger, and a control valve to open for hot gas taken out from the exhaust passage between the exhaust gas receiver and the turbocharger, viz. upstream of the turbocharger where the exhaust gas temperature is higher than downstream of the turbocharger.

An object of the present invention is to improve the possibility for regeneration of catalyst material in the SCR reactor, such as when the two-stroke internal combustion engine is operating at less than full engine load.

With a view to this, the initially mentioned two-stroke internal combustion engine according to the present invention is characterized in that at least one cylinder out of the plurality of cylinders is adapted to deliver to the regeneration flow path a first flow of exhaust gas at a higher temperature than the temperature of the exhaust gas in the exhaust gas receiver.

The entire two-stroke internal combustion engine operates at an engine load set by the engine governor, and the engine power delivered at that engine load is the result of summation of the power developed by the individual cylinders. If all cylinders on the engine deliver to the regeneration flow path, such as by diverting the initial, most hot portion of the exhaust gas to the regeneration flow path, then the engine governor automatically compensates for this. When the at least one cylinder out of the plurality of cylinders delivers exhaust gas at higher temperature to the regeneration flow path, then this may influence the power delivered by this or these cylinders, however the other cylinders in the plurality of cylinders compensate for this as the engine governor maintains the set engine load. When the at least one cylinder delivers exhaust gas at a higher temperature than the exhaust gas from the other cylinders, it is possible to supply the regeneration path with exhaust gas at a higher temperature than the average temperature of the exhaust gas in the exhaust gas receiver. The delivery of exhaust gas at a high temperature may occur also when a two-stroke internal combustion engine operates at part load, such as at 50% engine load (50% of MCR, Maximum Continuous Rating, which is 100% engine load). This possibility of obtaining sufficiently hot exhaust gas from at least one cylinder is very advantageous when the two-stroke internal combustion engine is a propulsion engine in a vessel, because the vessel typically sails at reduced speed - and thus at low engine load - when the vessel is in vicinity of a harbour, and in these waters emission requirements are strict and the SCR reactor should be in operation. It is also possible to operate the two-stroke internal combustion engine at part load, such as 75% of MCR, for extended periods of service because regeneration of the SCR reactor is possible due to the high temperature exhaust gas delivered by the single cylinder.

In ordinary, prior art operation of the two-stroke internal combustion engine all cylinders have the same operating cycle. This is also possible for the two-stroke internal combustion engine according to the present invention, and then only the hottest portion of the exhaust gas from the single cylinder is directed to the regeneration flow path. However, in an embodiment of the present invention the plurality of cylinders is divided into a first group comprising said at least one cylinder and a second group comprising the remaining cylinder(s), and this allows the operating cycle of the at least one cylinder in the first group to be different from the operating cycle of the cylinder or cylinders in the second group. When regeneration is required, the at least one cylinder in the first group is set to a modified operation cycle resulting in higher exhaust gas temperature. When regeneration is completed the at least one cylinder returns to the ordinary operating cycle, and then all cylinders have the same operating cycle until the next regeneration process. Of course it is also possible that the at least one cylinder permanently has an operating cycle different from the other cylinders, but this is not preferred because the specific fuel oil consumption, SFOC, of the two-stroke internal combustion engine will typically be lower when all cylinders have the same operating cycle.

In an embodiment the operating cycle of the at least one cylinder is adapted by injecting additional fuel. The additional fuel will increase the combustion gas temperature in the at least one cylinder, and thus the exhaust gas from the at least one cylinder will have increased temperature. If the at least one cylinder delivers more power then consequently the engine governor will regulate the other cylinders to deliver less power by reducing the fuel amount injected in the other cylinders.

In a further development the operating cycle of the at least one cylinder is adapted by opening the exhaust valve earlier in the engine cycle that the opening of the exhaust valves in cylinders in the second group. Normally, the exhaust valve is opened near the end of the working stroke and in the latter part of the working stroke both the pressure and the temperature in the combustion chamber are reduced. The early opening of the exhaust valve in the at least one cylinder thus results in that the exhaust gas from the at least one cylinder has higher pressure and higher temperature. Another effect is that the at least one cylinder will deliver less power, and consequently the engine governor regulates the other cylinders to deliver more power.

In another further development the operating cycle of the at least one cylinder is adapted by closing the exhaust valve earlier in the engine cycle than the closing of the exhaust valves in cylinders in the second group. The early closing may cause incomplete scavenging and thus higher temperatures as not all combustion gas has been replaced by cool scavenge air.

In an embodiment a control device at the at least one cylinder is adapted to separate the exhaust gas into a first portion of pure combustion gas and a second portion of combustion gas and scavenge air, said first portion being the first flow of exhaust gas delivered to the regeneration flow path. The pure combustion gas flowing out of cylinder, when the exhaust valve opens, has a high temperature. The exhaust gas has a lower temperature when the scavenge air reaches the exhaust valve, and just before the scavenging is terminated the exhaust gas is a mixture of combustion gas and scavenge air with a large proportion of scavenge air and consequently a comparatively low temperature of the exhaust gas. In the exhaust gas receiver all the exhaust gas is mixed, and the average temperature of the exhaust gas in the exhaust gas receiver is lower than the temperature of the pure combustion gas in the first flow of exhaust gas. As an example, when operating at 25% engine load the temperature of the exhaust gas in the exhaust gas receiver can be 280 °C, whereas the pure combustion gas in the first flow of exhaust gas can have a temperature of 700 °C. The single cylinder can have the same operating cycle as the other cylinders or a different operating cycle as described in the above.

In an embodiment the control device controls an adjustable gate in the exhaust duct of the single cylinder, which gate has an open position in which exhaust gas is directed into the regeneration flow path, and a closed position in which exhaust gas is directed to the exhaust gas receiver. This embodiment has the advantage that the adjustable gate can be positioned in the exhaust passage downstream of the exhaust valve, and the cylinder cover on the at least one cylinder in the first group can be of the same design as the cylinder covers on the other cylinders in the second group. In an alternative embodiment the control device controls an additional exhaust valve in the at least one cylinder, which additional exhaust valve is positioned at an end of the regeneration flow path. However, this alternative embodiment entails a modification of the cylinder cover.

In another embodiment the at least one cylinder is located at an end of the two-stroke internal combustion engine, and the exhaust duct from the at least one cylinder extends to an end area of the exhaust gas receiver, and the regeneration flow path is connected to the end area of the exhaust gas receiver. The regeneration flow path is supplied with exhaust gas from the at least one cylinder via the end area of the exhaust gas receiver. The exhaust gas receiver can be provided with an inner plate located in between an outlet opening for the exhaust duct from the at least one cylinder and an outlet opening for the exhaust duct from the cylinder adjacent to the at least one cylinder. The inner plate creates a barrier between exhaust gas from the adjacent cylinder and the exhaust gas from the at least one cylinder so that the regeneration flow path is supplied with the exhaust gas from the at least one cylinder.

In an embodiment the at least one cylinder is a single cylinder. In this embodiment the fist group includes one cylinder and the second group includes the remaining cylinders, their number being the number of cylinders in the plurality of cylinders minus the single cylinder.

In an embodiment the SCR reactor is located upstream of the turbocharger which brings the advantage of a higher temperature level as the exhaust gas has not yet been expanded in the turbine of the turbocharger.

In an embodiment the SCR reactor has an outlet connected with a control valve, which control valve is a pressure control valve. In normal operation of the engine the control valve is in fully open position. When regeneration is to be performed, the control valve can be set in a partially open position with such a small opening area that a pressure drop occurs across the control valve, and at the same time the pressure increases in the SCR reactor and this results in higher temperature in the SCR reactor because the temperature in the exhaust gas decreases when the exhaust gas expands to lower pressure.

Examples of embodiments of the invention are described in further detail in the following with reference to the highly schematic drawings, on which

Fig. 1 illustrates a first embodiment according to the present invention,

Fig. 2 illustrates a second embodiment according to the present invention,

Fig. 3 illustrates a third embodiment according to the present invention, and

Fig. 4 illustrates a diagram of how exhaust gas temperature varies with the engine load. A two-stroke internal combustion engine 1 has a plurality of cylinders 2, such as from 4 to 15 cylinders. In the illustrated embodiment the engine has six cylinders 2. The engine can e.g. be of the make MAN Diesel & Turbo and the type ME or MC, or of the make Wårtsilå, or of the make Mitsubishi. The cylinders can have a bore in the range of e.g. 25 to 120 cm, preferably from 35 to 98 cm. The two-stroke internal combustion engines used as main propulsion engines is of the crosshead type and typically have a speed indicated as rpm in the range from 20 to 260 rpm, typically 55 to 195 rpm. These engines are named low speed engines. The low speed is required for transferring the propulsion thrust via the propeller to the water in the wake of the vessel with high efficiency.

Each cylinder 2 has a reciprocating piston in the cylinder. The cylinders are typically of the uniflow scavenging type with scavenge air ports located at the lower end area of the cylinder and an exhaust valve 3 located at the top of the cylinder mounted in a cylinder cover 4. The exhaust valve opens and closes an exhaust duct 5 extending to an exhaust gas receiver 6. A turbocharger 7 has a compressor part delivering pressurized inlet air to a scavenge air receiver 8. The scavenge air receiver 8 is connected to an inlet air chamber located at the lower end of each cylinder so that scavenge air ports in the cylinder wall is supplied with scavenge and inlet air. A turbine part of the turbocharger is supplied with exhaust gas and after expanding the exhaust gas in the turbine part the exhaust gas flows onwards in the exhaust system and ends in a chimney where the exhaust gas is released as indicated by arrow 1 0.

When the two-stroke internal combustion engine is operated on heavy fuel oil, and in particular when operated on heavy fuel oil containing sulphur, or on fuel gas and fuel oil as pilot oil, or on any other fuel containing sulphur, it can be desirable to clean the exhaust gas in an SCR reactor 11.

In the different embodiments, the same reference numerals are used for details of the same type and function. In the embodiment of Figs 1 and 3 the SCR reactor is installed in the exhaust gas system downstream of the turbocharger, and in the embodiment of Fig. 2 the SCR reactor is installed in the exhaust gas system upstream of the turbocharger. The exhaust gas has a higher temperature upstream of the turbocharger than downstream of the turbocharger, because the exhaust gas is expanded in the turbine part. A first control valve 13 can open for exhaust gas flow to the SCR reactor when the exhaust gas is to be cleaned, and a urea supply 14 adds urea to the exhaust gas upstream of the SCR reactor. The first control valve can be closed and a second control valve 15 in a by-pass line 12 can be opened and then the exhaust gas is not passed through the SCR reactor. A third control valve 16 is located on the downstream side of the SCR reactor in between the SCR reactor and the by-pass line 12.

Fig. 4 illustrates how the average exhaust gas temperature in the exhaust gas receiver varies with the engine load. At full engine load (100% of MCR) the exhaust gas temperature is high, but when the two-stroke internal combustion engine is operated at part load, such as a load in the range of 25 % to 80 % of MCR the exhaust gas temperature is lower. The curves a, b and c in Fig. 4 illustrated different lay-outs of the two-stroke internal combustion engine, and it s seen that the trend is the same, namely that the exhaust gas temperature is lower when the engine load is lower. When the SCR reactor is cleaning exhaust gas having a temperature below 320 °C, ammonium bisulphate deposit on the catalyst material in the SCR reactor, and the catalytic cleaning effect to the reactor is reduced. As ABS builds up on the surface of the catalyst material, the pressure drop across the SCR reactor increases. It is thus possible to measure this pressure drop and when a predetermined limit value is exceeded than a regeneration process is initiated. It is also possible to perform a regeneration process at certain intervals, instead of monitoring the pressure drop.

The regeneration process can be performed on the whole SCR reactor, or on a section thereof, if the SCR reactor has internal separations dividing the SCR reactor in two or more sections so that one section can be regenerated while the other sections are in operation cleaning exhaust gas. When the SCR reactor has sections, each section has valves like the first control valve 13 for closing off the supply of exhaust gas, and a control valve for opening for flow connecting to a regeneration flow path 17. It is also possible to have several SRC reactors installed in parallel, and then regenerate one of the SCR reactors while another is in operation.

The regeneration flow path 17 has a fourth control valve 18 that can open and close for flow in the regeneration flow path. In the embodiment of Fig. 1 the regeneration flow path is connected to the exhaust duct 5 downstream of the exhaust valve and upstream of the exhaust gas receiver. This embodiment can be supplemented with a control device for the single cylinder delivering a first flow of exhaust gas at high temperature. In Fig. 1 the single cylinder is the cylinder at the left hand end of the engine, but the regeneration flow path can be installed at any of the cylinders. The control device separates the exhaust gas into a first portion of pure combustion gas, which first portion is directed to the regeneration flow path 17, and into a second portion of combustion gas and scavenge air, which second portion is directed to the exhaust gas receiver 6. This can be done by operating an adjustable gate in the exhaust duct. Alternatively, the single cylinder can have an additional exhaust valve that is opened first and then closed before or when the exhaust valve 3 opens. The additional valve is connected to the regeneration flow path.

In the embodiment of Fig. 3, the regeneration flow path 17 is connected to an end area of the exhaust gas receiver, and an inner plate 19 is located inside the exhaust gas receiver. The inner plate 19 ensures that primarily exhaust gas from the single cylinder at the end of the engine is supplied to the regeneration flow path 17.

The first flow of exhaust gas at higher temperature can also be obtained by modifying the operating cycle of the single cylinder. This can involve injection of additional fuel into the single cylinder, preferably at the end of the fuel injection process, or a modification of the timing of the exhaust valve so that the exhaust valve of the single cylinder opens before the exhaust valves in the other cylinders or closes before the exhaust valves in the other cylinders. Fuel injection and the operation of the exhaust valve is preferably controlled electronically, so the opening or closing signal to the exhaust valve actuator is simply adjusted to a different timing in the engine cycle. When additional fuel is to be injected the engine governor controls the injection system of the single cylinder to inject the additional amount of fuel.

As an example, the first flow of exhaust gas at higher temperature can have a temperature in the range of 680 °C to 750 °C at entry into the regeneration flow path 17, and when the pressure of the exhaust gas is expanded to about 1 bar abs pressure the temperature is in the range of 450 °C to 470 °C, which is sufficiently hot for performing regeneration.

In the above-mentioned embodiments the at least one cylinder is embodied as a single cylinder, but more than one cylinder may be de signed like the single cylinder and form part of a first group of cylinders, and in this case the remaining cylinders, if any, form part of a second group, in which the cylinders are not designed with a possibility for delivering exhaust gas at a higher temperature to the regeneration flow path.

Details of the various embodiments described can be combined into further embodiments within the scope of the patent claims.

Claims

A two-stroke internal combustion engine comprising a plurality of cylinders (2) with combustion chambers and an exhaust gas system, comprising an exhaust gas receiver (6) for receiving exhaust gas from the plurality of cylinders, at least one turbocharger (7) and at least one SCR reactor (11) located downstream of the exhaust gas receiver, the individual cylinder (2) having a cylinder wall with flush air ports and a cylinder cover (4) with an exhaust valve (3) and an exhaust duct (5) extending to the exhaust gas receiver (6). the exhaust gas system and wherein a regeneration flow path (17) can be connected to the SCR reactor (11) to regenerate catalyst material in the SCR reactor, characterized in that at least one cylinder is provided among the plurality of cylinders (2) to supply to the regeneration flow path (17) a first flow of exhaust gas at a higher temperature than the temperature of the exhaust gas in the exhaust gas receiver (6).

A two-stroke internal combustion engine according to claim 1, wherein the plurality of cylinders (2) is divided into a first group comprising at least one cylinder and a second group comprising the remaining cylinder or cylinders.

A two-stroke internal combustion engine according to claim 2, wherein the operating cycle of the at least one cylinder is adapted to be different from the operating cycle of the cylinders of the other group.

A two-stroke internal combustion engine according to one or more of claims 1 to 3, wherein the operating cycle of the at least one cylinder is adapted to inject additional fuel.

A two-stroke internal combustion engine according to one or more of claims 2 to 4, wherein the operating cycle of the at least one cylinder is arranged to open the exhaust valve (3) earlier in the motor cycle than the opening of the exhaust valves (3) in the cylinders of the second group.

A two-stroke internal combustion engine according to one or more of claims 2 to 4, wherein the operating cycle of the at least one cylinder is arranged for closing the exhaust valve (3) earlier in the motor cycle than the closing of the exhaust valves (3) in the cylinders of the second group.

A two-stroke internal combustion engine according to one or more of claims 1 to 6, wherein a control device at the at least one cylinder is arranged to divide the exhaust gas into a first portion of clean exhaust gas and a second portion of exhaust gas and rinsing air, wherein said first portion constitutes the first stream of exhaust gas supplied to the regeneration flow path (17).

A two-stroke internal combustion engine according to claim 7, wherein the control device controls an adjustable port in the exhaust duct (5) of the at least one cylinder, which has an open position in which exhaust gas is directed into the regeneration flow path (17) and a closed position in which exhaust gas is fed to the exhaust gas receiver (6).

The two-stroke internal combustion engine of claim 8, wherein the control means controls an additional exhaust valve in the at least one cylinder, said additional exhaust valve being located at one end of the regeneration flow path (17).

The two-stroke internal combustion engine according to one or more of claims 2 to 6, wherein the at least one cylinder is located at one end of the two-stroke internal combustion engine and the exhaust duct from the at least one cylinder extends to an end region of the exhaust gas receiver, and wherein the regeneration flow path (17) is connected to this end region of the exhaust gas receiver.

A two-stroke internal combustion engine according to claim 10, wherein the exhaust gas receiver is provided with an inner plate (19) located between an outlet opening for the exhaust duct from the at least one cylinder and an outlet opening for the exhaust duct from the neighboring cylinder to the at least one cylinder.

A two-stroke internal combustion engine according to one or more of claims 1 to 11, wherein the at least one cylinder is a single cylinder.

A two-stroke internal combustion engine according to one or more of claims 1 to 12, wherein at least one SCR reactor (11) is located upstream of the turbocharger (7).

A two-stroke internal combustion engine according to one or more of claims 1 to 13, wherein the SCR reactor (11) has an outlet connected to a third control valve (16), the third control valve being a pressure control valve.