EXHAUST VALVE FOR A LARGE SIZED TWO STROKE DIESEL ENGINE, PROCESS FOR REDUCTION ON NOX-FORMATION IN SUCH AN ENGINE AND SUCH ENGINE

Abstract

For reducing the NOx-emission of a large sized two stroke diesel engine having at least one combustion room (3), a reciprocating piston (4) and an exhaust opening (12) controlled by an exhaust valve (13) at each work cyclus a small volume of burnt gas is retained in the combustion room (3) and so added to the fresh air for the next combustion. For achieving this retention of burnt gas the underside of the valve disc (15) off the exhaust valve (13) is provided with a shallow concave face (17) building a basin-like collection room, whose depth is within a range of 2-10% of the outer diameter of the valve disc (15).

Description

The invention relates according a first idea to an exhaust valve for a large sized two stroke diesel engine, according a further idea to a process for reduction of NO_x-formation in such a large sized two stroke diesel engine, and according a still further idea to such a large sized two stroke diesel engine. Also manufacturing methods as well as use of an inventive exhaust valve as well as of a such engine are scoped.

It is generally known in the art that peaks of combustion temperature and herewith the generation of NO_x can be reduced when some burnt gas is added to the new air filling of the combustion room. For this purpose it has been proposed already to recirculate a part of the exhaust gas (DE 101 16 643 C2). But there is a risk that not only exhaust gas but also scavenge air reaches the exhaust channel so that the exhaust gas is diluted with scavenge air what leads to a higher oxygen content of the recirculated exhaust gas and therefor to a higher NO_x-formation Further substantial disadvantages herewith are that an expensive and complex recirculation device is needed, and that such device is occupying hardly-available space near the engine.

The GB 222 7055 A describes a two stroke diesel engine with an exhaust opening controlled by an exhaust valve. The underside of this exhaust valve is concave and makes a symmetric cavity bordered by the rim. The reason for this is here that the fuel supply tube and injection nozzle are integrated in the exhaust valve. Conserving some burnt gas in the manner of a nest within the rim area of the concave underside of the exhaust valve for reduction of NO_x-formation isn't mentioned. The same is true also for the dimension of the cavity on the underside of the exhaust valve.

Starting from this prior art it is an object of the invention to create an improved exhaust valve suitable for reduction of NO_x-formation as well as to create an improved method for manufacturing it.

A second object of the invention is to create an improved process for reduction of NO_x-formation.

Further objects of the invention are to design an improved large sized two stroke diesel engine as well as to create an improved method for manufacturing or remanufactoring it.

The first object is solved by the exhaust valve according claim 1 or one of the manufacturing claims 17, 19-22 respectively.

The second object is solved by the process according claim 12.

The further objects are solved by the diesel engine according claim 23, 24 or 33 respectively or one of the manufacturing claim 35 or 41 respectively.

By the superposed teaching it is achieved, that the right amount of burnt gas to add to the air for the next combustion is conserved in the combustion room in the manner of a nest of exhaust gas built within the rim area of the underside of an exhaust valve controlling an exhaust opening arranged coaxially with the combustion room. The burnt gas retained in the combustion room is not diluted with fresh air so that the content of oxygen is very low. Therefor just a minor volume of retained gas is enough for an effective reduction on NO_x-emission as well as for a favorable temperature distribution in the exhaust valve material. Further means of the said cavity the mass of the exhaust valve and therefor also the heat transfer to the gas within this combustion room is reduced.

Useful developments and advantageous embodiments of the superposed ideas as well as manufacturing methods there of are mentioned in the subclaims.

In the following one working example of the invention shown in the drawing is described.

In the drawing shows:

FIG. 1 a schematic vertical section through a large sized two stroke diesel engine of inventive type,

FIG. 2 schematically, the upper area of one cylinder of FIG. 1 with an opened exhaust valve according to the invention,

FIG. 3 the arrangement of FIG. 2 with closed exhaust valve and

FIG. 4 a section view along the line IV/IV in FIG. 3.

Diesel engines of the type as shown in FIG. 1 normally are used for ship propulsion or for driving big stationary power plants. Everywhere in this description and in the related claims directional terms like up, down, side, etc. are referring to the principal relative positions of the piston, cylinder, combustion room, and exhaust valve as shown in FIG. 1 of a standing engine with vertical cylinder axis. At other orientation directional terms are to be interpreted accordingly.

The diesel engine 1 shown in FIG. 1 is a large sized two stroke diesel engine of the crosshead type. For one such engine normally more cylinders are provided often as a single-row I-arrangement although other arrangements also are known, e.g. V-, X-, Y-arrangements. One of these cylinders 2 can be seen in the section view of FIG. 1. Each cylinder 2 contains a combustion room 3 which downwards is delimited by a reciprocating piston (assembly) 4 by means of which the volume of the combustion room can be increased/decreased.

The piston assembly 4 is connected with a crosshead assembly 5 via a piston rod 6. The crosshead assembly 5 comprises a swingable bearing construction of a known type, here shortly called crosshead pin 7. The piston rod 6 is fastened to the piston assembly and to the crosshead assembly 5. The crosshead assembly 5 is connected to a crankshaft 8 by means of a connecting rod 9 which has one end swingably connected at the crosshead pin 7 and has an other end rotatably connected to a crankshaft pin as known in the art.

The cylinder 2 is provided in its lower area with air inlet slots 10. The air inlet slots 10 are opened and closed relative to the combustion chamber by the reciprocating piston 4. Through the air inlet slots 10 when opened, air can be fed into the combustion room 3 for it's scavenging and filling. Throughout this description and the related claims, in this connection, “air” more than normal air from ambient also means such air possibly having other gassy component(s) mixed-in to e.g. control the overall oxygen content of “air” fed into the combustion chamber. In the upper area of the cylinder 2 fuel injection valve(s) 11 is/are positioned for injection of fuel into the combustion room 3 when the piston 4 is in the region of its top dead center.

Burning of the injected fuel is causing a downward movement of the piston 4. After that, the burnt gas is exhausted through an exhaust opening 12 positioned at the upper end of the combustion room 3, preferably coaxially. This exhausted burnt gas normally is called exhaust gas. The exhaust opening 12 is controlled by an assigned exhaust valve 13. The exhaust valve 13 can be designed as a built valve or as an one piece valve. The exhaust valve can be manufactured according to various methods whereof some are scoped by claims. Among benefits from such manufacturing methods are re-use of new possibly surplus other valves or valve parts as well as beneficial re-use of useable parts/areas of else worn valves of compatible design. Thus manufacture/reconditioning of an inventive exhaust valve is made more economical and environmental friendly by reuse of both previous results of costly processing efforts and of material, then simultaneously both leaving less waste material and requiring less virgin material. In each case an inventive exhaust valve has a valve shaft 14 and a valve disc 15 arranged at the lower end of the valve shaft 14. In connection with large sized two stroke diesel engine 5 the diameter of the valve disc 15 should be 100 mm at least. Also bigger diameters of for example 160-200 mm or even larger would be possible. In the region of the lower end of the valve shaft 14 above the valve disc 15, propeller blades 16 can be provided for rotating the exhaust valve 13 by the passing exhaust gas as is only indicated in FIG. 2, where propeller blade are shown without their normal “twist”.

One 360° revolution of the crankshaft 8 wherein one full ascent and full descent of the piston 4 is completed defines one work cycle. During each work cycle a compression of the filling of the combustion room 3, an injection of fuel, a burning of the injected fuel and the air filling of the combustion room 3, an opening of the exhaust valve 13 and exhausting of burnt gas (socalled exhaust gas) as well as an opening and closing of the air inlet slots 10 are carried out, whereby the combustion room 3 is scavenged and filled with new air. The air inlet slots 10 might be so inclined, that the air fed into the combustion room 3 is rotated around a central axis of the combustion room 3.

During the scavenging of the combustion room 3 some of the burnt gas is retained in the combustion room 3. This burnt gas conserved in the combustion room 3 has a low oxygen content which causes reduction of the peak combustion temperature and therefor reduction of the level of generated NO_x. In this way the NO_x-emission of the diesel engine 1 can be reduced. Also the material temperature distribution in the exhaust valve and especially in it's valve disc 15 can be improved in this way.

For this purpose the underside of the valve disc 13 is provided with a shallow rotationally symmetric concave face 17 as can be seen from FIGS. 2 and 3. This rotationally symmetric concave face 17 leads to a cavity 18, surroundingly bordered by the valve disc's rim area and being open towards the combustion room 3.

The function of this cavity 18 is to build a basin-like collection room for retention of burnt gas when the exhaust valve 13 is opened as is schematically shown in FIG. 2. During the compression stroke of the piston 4 the collected and retained burnt gas can build up a nest 19 of burnt gas on the underside of the exhaust valve 13. The upper side of the piston 4 is in this example provided with a rotationally concave face 20 opposite to the concave underside of the exhaust valve 13. This assist the nest formation mentioned above as can be recognized from FIG. 3. Of course a flat piston top surface can be used under circumstances.

During activation of the fuel injection valves 11 the fuel at least partly is injected in the nest 19 of burnt gas conserved in the combustion room 3 as is indicated by spraying lines 21 in FIGS. 3 and 4. The spray direction is inclined against the radial direction so that most of the fuel is injected into the fresh air rotating according arrow 22 in FIG. 4 and a small part hits the nest 19 of retained burnt gas. Since this burnt gas has a low oxygen content the combustion is retarded and therefor peaks of combustion temperature and NO_x-formation are reduced.

The concave face 17 of the underside of the exhaust valve 13 is so designed that the maximum of the rise relative to an imaginary directly supporting planar face 23, indicated in FIG. 2, is within the range of 2% to 10% of the outer diameter of the valve disc 15, which can be 100 mm-160 mm or bigger as mentioned above. Preferably this range can be 5%-7% and highly preferably 6%-6.5% of the outer diameter of the valve disc 15. This dimensioning of the rise of the concave face 17 results in a sufficient volume of conserved burnt gas. The basin-like cavity 18 can further be so designed that it's volume is within the range of 0.5%-3% of the volume of the combustion room 3 in it's mostly diminished condition, that is when the piston 4 has reached it's top dead center—the exhaust valve 13 of course at the same time being fully raised to seal the combustion room 3 towards the exhaust opening 12. Preferably this range can be 1%-2% and highly preferable 1.6%-1.7% of the said minimum volume of the combustion room 3.

Said volume of the cavity 18 would lead to an enlargement of the total volume of the combustion room 3. For avoiding such enlargement, compensation means are provided. For achieving the wished compensation the top level of the piston 4 can be raised accordingly compared to an arrangement with an uncarved exhaust valve. This can be done by providing the upper face (part) of the piston 4 with a top layer of corresponding thickness. According an other possibility which is to prefer a shims layer of corresponding thickness can be inserted between two supporting areas assigned to the piston assembly 4. In the preferred embodiment shown in FIG. 1 a shims layer 24 is shown inserted between mutually supporting areas of the piston rod 6 and of the crosshead pin 7, respectively.

Manufacturing of a large sized two stroke diesel engine scoped by the invention generally is performed by mounting of at least one inventive exhaust valve to any other part or assembly of parts being/to be part of a said engine. The latter method is highly relevant as submounting to major assemblies before final assembling of the engine is very common for this size of engines often used as sole source for propulsion power in a ship. Also mounting of a shims layer between two supporting areas assigned to a piston assembly being/to be delimiting a combustion room also being/to be delimited by an inventive exhaust valve can be comprised in such manufacturing. Preferably during the manufacturing a shims layer is inserted between a present crosshead pin structure and it's corresponding piston rod to co-function at same combustion room with an inventive exhaust valve.

In most cases a new large sized two stroke diesel engine is built by the manufacturing, but a used such engine can be brought to a scoped state by retrofitment to an existing large sized two stroke diesel engine of else known type, of at least one inventive exhaust valve to replace a conventional exhaust valve which of course previously was removed from said engine. Preferably inventive exhaust valves then are installed for all the engine's cylinders, that an else for environmental reasons “forbidden”/“dead” engine cheaply can be “revived”, possibly as an original-MAN B&W-brand-“refurbished” engine, to yield many more years of effective service e.g. when installed in a ship then being acceptable to the current IMO-rules. (IMO=International Maritime Organization, a United Nations body issuing maritime regulations, “TIER”s, for emissions upper limits, etc.)

A special type of retrofitment or remanufacturing occurs when at an existing large sized two stroke diesel engine already having inventive exhaust valves mounted, one or more of these exhaust valves are replaced by an other inventive exhaust valve having a shape of it's valve head being different from the replaced exhaust valve. The purpose of such replacement then is to obtain a better reduction of the NO_x-formation. Reasons herefor can be shift to use of other fuel for a time period of substantial length, that a new optimization is required for an engine mounted in a ship, that this ship still can fulfil e.g. IMO-requirements to low emissions to get access to coastal areas and the haurbours lying there. Such retrofit also can be relevant when said engine, maybe still with same fuel-type, for a period of time is foreseen to function at a typical power output level deviating more than 15% from a recently used other typical power output level, e.g. because a raised level of fuel prizes makes it beneficial to reduce the nominal cruising speed from e.g. 90% of max. engine power to say 60% of the max. engine power, often resulting in only a few knots reduced sailing speed. Remanufacturing of an inventive large sized two stroke diesel engine by replacement of a worn exhaust valve with an inventive exhaust valve is another beneficial posibility of manufacture of a such large diesel engine yielding a possibility to revitalize an else worn out engine.

For all such engine manufacturing it is foreseenable to use inventive exhaust valves provided by any mentioned or claimed method of manufacturing of inventive exhaust valves. Common to all such methods are processing to an exhaust valve from a specific state. Such processing of course is to be understood in a broad sense and can therefore comprise e.g. milling, turning, grinding, polishing or other material removal methods, but also material combining, adding or compacting methods as welding, depositing methods like sintering, metalspraying, pressing, forging etc. or known combinations thereof, all of which methods are well known to the person skilled in the art.

Further, an inventive exhaust valve embodiment of a given external disc diameter may be manufactured in a number of almost similar variants which are very difficult to effectively destinguish/identify by simple human visual observation of the shape of the cavity in the valve's underside, or by a simple measuring procedure. Thus a pronounced risk has been reported, for faulty selection of a thus unsuited inventive exhaust valve for later being mounted to function in an inventive large sized two stroke diesel engine. Consequently said engine afterwards with such faulty selected and installed inventive exhaust valve will malfunction, possibly to a degree that access to some “restricted pollution” coastal/harbour zones may be denied for the vessel which is propelled by the malfunctioning engine. Therefore, and due to the possibly large amount of details required to fully document an inventive exaust valve, such valve preferably is made easily identifiable by data stored at various occasions to contactlessly be read from at least one tag of said valve, preferably a tag of a RFID-type, comprising stored data to unambiguously define said valve.

Preferably at least one tag is further loaded with data specifying details to authenticate said valve, such authenticating data preferably also comprising manufacturing-, (IMO-)approval-, historic and possible or planned future use-data for said valve.

Preferably at least one said tag is located within the ¾ of the valve overall axial length opposed the valve disc end. Hereby is secured that at least this tag, normally being an electronic device sensitive to high temperatures, can be given a position shielded from being swept and destroyed by the very hot exhaust gas leaving the combustion chamber, when the exhaust valve is opened. Instead then the tag of said exhaust valve can be exposed for reading by non-contacting means positioned above the combustion room for said exhaust valve.

Normally such reading is performed before/at engine startup to supply parameters/data to the engine's control system, but reading of a such tag by non-contacting means to collect data from the at least one such said exhaust valve can also be performed during the functioning of an inventive large sized diesel engine. Hereby a person who could be an inspector from IMO or a ship insurance company, could verify the installed exhaust valve to allow continued function of the engine, data for such accept possibly also being read into the tag at the same time, without for this purpose having to stop the engine.

In the description above preferred embodiments of the invention are explained. But the invention is not restricted to this. The protected scope of the invention is specified in the following claims.

Claims

1. Exhaust valve designed for controlling an exhaust opening (12) of a large sized diesel engine of the two stroke type and having a valve disc (15) arranged at the lower end of a valve shaft (14), whereby the valve disc's (15) underside to face a combustion room (3) is provided with a basin designed as a rotationally symmetric cavity (18), being surroundingly bordered and being downwardly open, characterized in that for retention of burnt gas, the maximum of rise of the concavely shaped underside of the valve disc (15) relative to an imaginary planar face (23) directly supporting the valve disc (15), is within the range of 2-10% of the outer diameter of the valve disc (15).

2. Exhaust valve according claim 1, characterized in that the maximum of rise of the concavely shaped underside of the valve disc (15) relative to an imaginary planar face (23) directly supporting the valve disc (15) is within the range of 5-7% of the outer diameter of the valve disc (15).

3. Exhaust valve according claim 2, characterized in that the maximum of rise of the concavely shaped underside of the valve disc (15) relative to an imaginary planar face (23) directly supporting the valve disc (15), is within the range of 6-6.5% of the outer diameter of valve disc (15).

4. Exhaust valve according claim 1 or 2, characterized in that the outer diameter of the valve disc (15) of the exhaust valve (13) is at least 100 mm.

5. Exhaust valve according claim 4, characterized in that the outer diameter of the valve disc (15) of the exhaust valve (13) is at least 160 mm.

6. Exhaust valve according claim 1 or 2, characterized in that the valve disc (15) having the cavity (18) designed as the basin for retention of burnt gas is provided with a protection layer on the valve disc's underside.

7. Exhaust valve according claim 1 or 2, characterized in that it is designed as a built valve whereby the valve disc (15) is applied to the valve shaft (14).

8. Exhaust valve according claim 1 or 2, characterized in that the valve shaft (14) is provided with propeller blades (16).

9. Exhaust valve according to one or more of previous claims, characterized in being identifiable by data contactlessly read from at least one tag of said valve, preferably a tag of a RFID-type, comprising stored data to unambiguously define said valve.

10. Exhaust valve according to claim 9, characterized in

11. Exhaust valve according to claim 9 or claim 10, characterized in

12. Process for reduction of the NOx-formation in a large sized diesel engine (1) of the two stroke type having at least one combustion room (3) with an exhaust opening (12) coaxially arranged within and controlled by an exhaust valve (13) which is realized according any of the foregoing claims, whereby in the combustion room (3) at each work cycle a fuel injection, a combustion of fuel, an ejection of burnt gas, a scavenging and filling with fresh air as well as a compression of the filling of the combustion room (3) are carried out, whereby some burnt gas is added to the air before combustion, and whereby at least some of the burnt gas added to the air is conserved in the combustion room (3) in the manner of a nest (19) of burnt gas built within the rim area of the underside of the said exhaust valve (13) being concavely shaped on the underside.

13. Process according claim 12, characterized in that the fresh air fed to the combustion room (3) is rotated around a central axis.

14. Process according claim 12 or 13, characterized in that at fuel injection the fuel at least partly is injected from outward of the nest (19) into the nest (19) of burnt gas built on the underside of the exhaust valve (3).

15. Process according claim 12 or 13, characterized in that the exhaust valve (3) is rotated around it's axis, during time when not being in a fully closed position.

16. Process according claim 12 or 13, characterized in that most of the fuel is injected into the fresh air.

17. Manufacturing of an exhaust valve according to one or more of the claims 1-11, characterized in said valve being made as a new said valve from new elements.

18. Manufacturing of an exhaust valve according to claim 17, characterized in said exhaust valve initially was made and identified as a finished new exhaust valve not comprising the characterizing feature of claim 1,

19. Manufacturing of an exhaust valve according to one or more of the claims 1-11, characterized in said valve being made by further processing of a used valve not hitherto having comprised the characterizing feature of claim 1, to obtain a state being scoped by one or more of the claims 1-11.

20. Manufacturing of an exhaust valve according to one or more of the claims 1-11, characterized in said valve being made by reconditioning of a worn exhaust valve at least earlier (possibly still) having comprised the characterizing feature of claim 1, to obtain a functional state being scoped by one or more of the claims 1-11.

21. Manufacturing of an exhaust valve according to one or more of the claims 1-11, characterized in said valve being made by special processing of an existing exhaust valve, such special processing comprising the steps of: removal of a valve shaft part area from the residual valve disc part area, andattachment of an other valve shaft part area to the residual valve disc part area,

22. Manufacturing of an exhaust valve according to one or more of the claims 1-11, characterized in said valve being made by special processing of an existing exhaust valve, such special processing comprising the steps of: removal of a valve disc part area from the residual valve shaft part area, andattachment of an other valve disc part area to the residual valve shaft part area,

23. Large sized diesel engine of the two stroke type having a combustion room (3) delimited by a reciprocating piston (4), characterized in having at least one exhaust valve (13) according any of the foregoing claims 1-11.

24. Large sized diesel engine of the two stroke type having a combustion room (3) delimited by a reciprocating piston (4), characterized in having at least one exhaust valve according to claim 11, andthe tag of said exhaust valve being exposed for reading by non-contacting means positioned above the combustion room for said exhaust valve.

25. Large sized diesel engine according to claim 24, characterized in that the else added combustion room (3) volume caused by the cavity (18) on the underside of the valve disc (15) is compensated by raising the top face level of the piston (4), compared to an arrangement with an uncarved valve disc.

26. Large sized two stroke diesel engine according claim 25, characterized in that a shims layer (24) is provided between two supporting areas assigned to the piston assembly (4), for a compensating elevation of the piston top face.

27. Large sized two stroke diesel engine according claim 25 having a crosshead (5), the pin of which is connected with the piston assembly (4) via a piston rod (6) supported on the crosshead (5) construction, characterized in that a shims layer (24) is inserted between the crosshead pin (7) and the piston rod (6).

28. Large sized diesel engine according to claim 24 or 25, characterized in that the volume of the cavity (18) on the underside of the valve disc (15) is within the range of 0.5-3.0% of the volume of the combustion room (3) when the exhaust valve is in closing position and the piston (4) has reached the top dead center.

29. Large sized diesel engine according claim 28, characterized in that the volume of the cavity (18) on the underside of the valve disc (15) is within the range of 1-2% of the volume of the combustion room (3) when the exhaust valve is in closing position and the piston (4) has reached the top dead center.

30. Large sized diesel engine according claim 29, characterized in that the volume of the cavity (18) in the underside of the valve disc (15) is within the range of 1.6%-1.7% of the volume of the combustion room (3) when the exhaust valve is in closing position and the piston (4) has reached the top dead center.

31. Large sized diesel engine according claim 24 or 25, characterized in that the piston (4) has a concave upper side (20) opposite to the concave underside (17) of the exhaust valve (13).

32. Large sized diesel engine according claim 24 or 25, characterized in that in the combustion room (3) fuel injection valves (11) are located out of the concave underside (17) of the exhaust valve (13).

33. Large sized diesel engine of the two stroke type, characterized in having the combustion room (3) of at least one of it's cylinders operated by a process according claim 12.

34. Large sized diesel engine of the two stroke type according to claim 33, characterized in to at least one said cylinder's combustion room having mounted an exhaust valve according to claim 11, andthe tag of at least one such said exhaust valve during functioning of said large sized diesel engine is read by non-contacting means to collect data from the tag.

35. Manufacturing of a large sized two stroke diesel engine to accord to one or more of the claims 23 to 34, characterized in mounting of at least one exhaust valve according to one or more respectively relevant of the claims 1 to 11, to any other part or assembly of parts being/to be part of a said large sized two stroke diesel engine according to one or more of the claims 23 to 34.

36. Manufacturing of a large sized two stroke diesel engine according to claim 35, characterized in mounting of at least one shims layer between two supporting areas assigned to a piston assembly being/to be delimiting a combustion room also being/to be delimited by an exhaust valve according to one more of the claims 1 to 11.

37. Manufacturing of a large sized two stroke diesel engine according to claim 35 or 36, characterized in insertion of a shims layer between a present crosshead pin structure and it's corresponding piston rod to co-function at same combustion room with an exhaust valve according to one more of the claims 1 to 11.

38. Manufacturing of a large sized two stroke diesel engine according to one or more of the claims 35 to 37, characterized in said large sized two stroke diesel engine is built as a new engine.

39. Manufacturing of a large sized two stroke diesel engine according to one or more of the claims 35 to 37, characterized in retrofitting to an existing large sized two stroke diesel engine not being scoped by any previous claim, of at least one exhaust valve according to one or more of claims 1 to 11, to replace an exhaust valve not scoped by any present claim, which latter valve previously was removed from said engine.

40. Manufacturing of a large sized two stroke diesel engine according to one or more of the claims 35 to 39, characterized in at an existing large sized two stroke diesel engine being scoped by one or more previous claims, replacing at least one exhaust valve according to one or more of claims 1 to 11, with an other exhaust valve according to one or more of claims 1 to 11 but having a shape of it's valve head being different from the exhaust valve to replace, to better reduce the NOx-formation, preferably when said engine for a period of time is foreseen to function at a typical power output level deviating more than 15% from a recently used other typical power output level.

41. Remanufacturing of a large sized two stroke diesel engine according to one or more of the previous claims, characterized in replacement of a worn exhaust valve with an exhaust valve according to one or more of the claims 1-11.