WO2024260549A1

WO2024260549A1 - A method of operating a four-stroke multi-cylinder piston engine and a four-stroke internal combustion piston engine

Info

Publication number: WO2024260549A1
Application number: PCT/EP2023/066808
Authority: WO
Inventors: Christer Hattar; Kenneth HELLMAN
Original assignee: Wärtsilä Finland Oy
Priority date: 2023-06-21
Filing date: 2023-06-21
Publication date: 2024-12-26

Abstract

A method of operating a four stroke multi-cylinder piston engine, in which engine (10) the cylinders have a fixed geometric compression ratio, by combusting a first fuel, the method being characterized in that air is compressed by a single-stage compressor (28) of a turbo charger (26) coupled with the engine (10), a combustion chamber of each cylinder (12) is charged with the compressed air and the first fuel according to working cycle and firing order, a second fuel is provided into the cylinder to be fired and becomes ignited, the first fuel is ignited by the second fuel, and is combusted as lean burn combustion, exhaust gas produced by combustion is led to the single-stage turbine (30) of the turbo charger (26), in which exhaust gas is expanded producing work for operating the compressor, exhaust gas is led to a NOx reduction catalysator (38) where NOx emissions are reduced, and when running the engine (10) at power below 75 % of its nominal maximum power, and when at least 30% of heat released in the combustion is from ammonia as the first fuel, the exhaust gas is cooled prior to leading exhaust gas to the NOx reduction catalysator (38). Invention relates also to a four stroke multi-cylinder internal combustion piston engine (10) configured to combust a first fuel comprising ammonia and second fuel.

Description

A method of operating a four-stroke multi-cylinder piston engine and a four-stroke internal combustion piston engine

Technical field

[001] The present invention relates to a method of operating a four-stroke multicylinder piston engine, in which engine the cylinders have a fixed geometric compression ratio, by combusting fuel comprising ammonia.

[002] The present invention relates to a four-stroke internal combustion piston engine configured to combust ammonia.

Background art

[003] Environmental aspects accelerate use of non-hydrocarbon fuels also in larger internal combustion piston engines suitable for prime movers for ships and land-based power plants. Such engines have been traditionally relying on diesel process where liquid fuel, such as light fuel oil or marine diesel oil is injected into combustion chamber and diffusion combusted after being compression ignited. In certain situations, especially in case of 4-stroke engines, the engines are run with variable speeds and power levels requiring the engines to have good transient response and simultaneously maintaining high efficiency with low emission levels. In marine applications such needs arise typically from harbour manoeuvrability and in land-based applications from capability of the engine powered generators to react on changes in the requirements of electric power grid. These requirements become even more challenging in multi fuel engines running with different fuels. Particularly the present demand of decreasing carbon emissions the design of an engine face new challenges and practises and known process behaviour relating to common conventional carbon fuels such as diesel and natural gas cannot be considered directly usable in combustion process of e.g. ammonia and hydrogen.

[004] An object of the invention is to provide a method of operating a four stroke multi-cylinder piston engine by combusting fuel comprising ammonia in which the operation with various engine loads is efficient and produce low exhaust gas emissions. Disclosure of the Invention

[005] Objects of the invention can be met substantially as disclosed in the independent claims and in the other claims describing more details of different embodiments of the invention.

[006] A method of operating a four stroke multi-cylinder piston engine according to the invention, in which engine the cylinders have a fixed geometric compression ratio, by combusting a first fuel, comprises following steps wherein: a) air is compressed by a single-stage compressor of a turbo charger coupled with the engine, b) a combustion chamber of each cylinder is charged with the compressed air and the first fuel according to working cycle and firing order, c) a second fuel is provided into the cylinder to be fired and becomes ignited, d) the first fuel is ignited by the second fuel, and is combusted as lean burn combustion, e) exhaust gas produced by combustion is led to the single-stage turbine of the turbo charger, in which exhaust gas is expanded producing work for operating the compressor, f) exhaust gas is led to a NOx reduction catalysator where NOx emissions are reduced, and g) when running the engine at power below 75 % of its nominal maximum power, and when at least 30% of heat released in the combustion is from ammonia as the first fuel, the exhaust gas is cooled prior to leading exhaust gas to the NOx reduction catalysator.

[007] This manner the four stroke multi-cylinder piston engine having fixed geometric compression ratio, can be operated at low load conditions by combusting ammonia as its main fuel and a second fuel as a pilot fuel as lean burn combustion, while simultaneously NOx emissions are at low and acceptable level. When decreasing the power of the engine below 75% of its nominal maximum power, particularly when the engine is run by combusting the first fuel comprising at least 30% of ammonia, the present invention provides advantageous circumstances of efficient combustion and NOx emissions abatement. [008] According to a further aspect of the invention the first fuel comprises ammonia at least 40% and when running the engine at power below 75 % of its nominal maximum power, the exhaust gas is cooled prior to leading exhaust gas to the NOx reduction catalysator.

[009] According to a still further aspect of the invention the first fuel comprises ammonia at least 50% of total delivered heat value and may according to this aspect be called as a main fuel.

[0010] More precisely, the internal combustion piston engine has its geometric compression ratio fixed to a setting at which compression ignition and combustion of diesel fuel is effectively usable as well. The fixed geometric compression ratio is within a range of 11 to 17, advantageously 14 to 15.

[0011] According to a further aspect of the invention the exhaust gas is cooled prior to leading the exhaust gas to the NOx reduction catalysator at least when the engine is run at power below 40% of its nominal maximum power and when at least 30% of heat released in the combustion is from ammonia as the first fuel.

[0012] According to a still further aspect of the invention the exhaust gas is cooled prior to leading the exhaust gas to the NOx reduction catalysator at least when the engine is run at power below 50% of its nominal maximum power and when at least 30% of heat released in the combustion is from ammonia as the first fuel. When decreasing the power of the engine below 50% of its nominal maximum power, particularly when the engine is run by combusting the first fuel comprising at least 50% of ammonia, and the second fuel functioning as a pilot fuel, as lean burn combustion having lambda value between 1 ,5 - 2,2, exhaust gas engine exit temperature tends to increase more intensively, and thus cooling of the exhaust gas is further intensified to adapt the temperature of the exhaust gas according to demand set by the NOx reduction catalysator and the following exhaust gas channel.

[0013] According to a still further aspect of the invention the exhaust gas is cooled prior to leading the exhaust gas to the NOx reduction catalysator at least when the engine is run at power below 50% of its nominal maximum power and when at least 50% of heat released in the combustion is from ammonia as the first fuel. When decreasing the power of the engine below 50% of its nominal maximum power, an even when the engine is run by combusting the first fuel comprising at least 50% of ammonia, and the second fuel functioning as a pilot fuel, as lean burn combustion having lambda value between 1 ,5 - 2,2, exhaust gas engine exit temperature tends to increase, and thus cooling of the exhaust gas is further intensified to adapt the temperature of the exhaust gas according to demand set by the NOx reduction catalysator and the following exhaust gas channel.

[0014] According to an aspect of the invention exhaust gas is cooled by leading a controllable amount of air to the exhaust gas. The air needs to be at lower temperature than that of the exhaust gas. Mixing the air at lower temperature with the exhaust gas provides a straightforward and effective way of the cooling the exhaust gas, which then will become a mixture of exhaust gas and the air at reduced temperature.

[0015] According to an aspect of the invention the air is compressed charge air of the engine, which can be taken from upstream or downstream position to an air cooler device arranged to engine charge air channel. Both locations provide advantageous effect because the air is at suitable temperature also upstream to such air cooler device.

[0016] According to an aspect of the invention cooling air is delivered to the exhaust gas by an auxiliary blower which is configured to provide air to the exhaust gas. Such auxiliary blower is provided with a dedicated drive means, like electric motor, which is controllable independently from the engine. Pressure level of the exhaust gas at location downstream the turbine of the turbo charge low enough to use a simple blower to deliver the air.

[0017] According to and aspect of the invention the exhaust gas is cooled by combination of cooling with air delivered to the exhaust gas by an auxiliary blower and cooling with compressed charge air of the engine. This provides an effect that e.g. at low load operations available capacity of the compressor of the turbo charger can be used for providing charge air to the engine and the air for cooling the exhaust gas is pressurized and delivered by the auxiliary blower. The blower can deliver 0 - 100 % of the needed air, while remaining portion is obtained from the compressor of the turbo charger.

[0018] According to an aspect of the invention exhaust gas is cooled by transferring heat to a heat transfer medium in a heat exchanger arranged between the turbine of turbocharger and the NOx reduction catalysator. This way the capacity of the turbo charger is not used for cooling the exhaust gas.

[0019] According to and aspect of the invention the exhaust gas is cooled by combination of cooling the exhaust gas with air delivered to the exhaust gas by an auxiliary blower, cooling with compressed charge air of the engine and by transferring heat to a heat transfer medium in a heat exchanger. This provides even more versatile solution for multiple running options.

[0020] According to an aspect of the invention when running the engine by combusting the first fuel comprising ammonia as main fuel and the second fuel as pilot fuel at power below 75 % of its nominal maximum power turbocharger is operated to produce charge pressure less than 5 bar(g). This way lambda value is controlled to more advantageous level for desired operation window of the engine.

[0021] Preferably, when running the engine at power below 75 % of its nominal maximum power, lambda value in the lean burn combustion is less than or equal to 2.2. Preferably the lambda value is less than or equal to 2.2 and more than 1 ,5. This result in operation with good efficiency and low nitrogen oxide emissions.

[0022] According to a further aspect of the invention when running the engine by combusting the first fuel comprising ammonia as main fuel and the second fuel as pilot fuel at power below 50 % of its nominal maximum power turbocharger is operated to produce charge pressure less than 2 bar(g). This way lambda value is controlled to more advantageous level for desired operation window of the engine at low load operation.

[0023] According to an aspect of the invention the second fuel is arranged to the cylinder by direct injection of fuel into combustion chamber at the end part of the compression stroke. In this case the fuel is ignited by compression ignition.

[0024] According to another aspect of the invention the second fuel is gaseous fuel, such as hydrogen or other non-carbon fuel, which is arranged to the cylinder along with combustion air. In respect to arranging the second fuel to the cylinder, the term cylinder includes a prechamber space in connection with the cylinder. Ignition of gaseous second fuel may be accomplished by a spark or other source of ignition.

[0025] The invention is particularly advantageous for use in an engine operating within the range from 250 rev/min to about 1200 rev/min, having its bore diameter at least 200 mm and/or nominal power at least 150 kW/cylinder. Further the invention is particularly advantageous when running the engine within the power range of 15-75% of its nominal maximum power and when at least 30% of heat released in the combustion is from ammonia as the first fuel.

[0026] The most important benefit of using ammonia as the fuel arises from the fact that it does not contain any carbon being a truly non-carbon fuel. Ammonia is also one of the most used and thus available chemicals in the world. On the other hand, ammonia has a relatively low calorific value, rather narrow flammability limit, high minimum ignition energy and high self-ignition temperature together with low flame speed I flame propagation. Despite of these properties in combination the present invention provides effective combustion with small emissions with varying loads and speeds. By means of the invention it is possible to operate four stroke multi-cylinder piston engine with fixed geometric compression ratio by combusting ammonia with high efficiency and low emissions, particularly low NOx and N2O emissions and ammonia slip.

[0027] According to an aspect of the invention the method comprises operating the engine by combusting a first fuel, the method being characterized by a) air is compressed by a single-stage compressor of a turbo charger coupled with the engine, b) a combustion chamber of each cylinder is charged with the compressed air and the first fuel according to working cycle and firing order, c) a second fuel is provided into the cylinder to be fired and becomes ignited d) the first fuel is ignited by the second fuel, and is combusted as lean burn combustion in the combustion chamber, e) exhaust gas produced by combustion is led to the single-stage turbine of the turbo charger, in which exhaust gas is expanded producing work for operating the compressor, f) exhaust gas is led to a NOx reduction catalysator where NOx emissions are reduced, and g) when running the engine at power below 75 % of its nominal maximum power, and when at least 30% of heat released in the combustion is from ammonia as the first fuel, the exhaust gas is cooled prior to leading exhaust gas to the NOx reduction catalysator, and the engine is run during engine’s starting phase by combusting second fuel with compression ignition, wherein

I. air is compressed by a single-stage compressor of a turbo charger coupled with the engine,

II. a combustion chamber of each cylinder is charged with compressed air according to working cycle and firing order,

III. a predetermined amount of the second fuel is injected into the combustion chamber, ignited by compression ignition and combusted in the combustion chamber,

IV. exhaust gas produced by combustion is led to the single-stage turbine of the turbo charger in which exhaust gas is expanded, where exhaust gas temperature is decreased to turbine exit temperature, producing work for operating the compressor,

V. exhaust gas is led to a NOx reduction catalysator at the temperature of turbine exit temperature, where NOx emissions are reduced by catalytic reactions.

[0028] It has been noticed that when operating a four stroke multi-cylinder piston engine, in which engine the cylinders have a fixed geometric compression ratio, by combusting ammonia such that ammonia is ignited by providing a second, pilot fuel into the charge, and combusted as lean burn combustion in the combustion chamber, exhaust gas temperature will rise from that of full power operation, which is compensated by cooling the exhaust gas outside the engine, between the turbocharge and NOx catalysator. It has been found that when practising the method at power less than 75% of its nominal maximum power, exhaust gas temperature will rise to an undesired level. More particularly at power below 50% of nominal maximum power, the cooling effect on exhaust gas prior to leading exhaust gas to the NOx reduction catalysator is increased according to an aspect of the invention.

[0029] Ending of starting phase of the engine means fulfilling at least one of the following conditions: the engine speed has raised to its nominal speed, engine’s operation allows coupling the engine to mechanical load, engines temperature (coolant, lubricant, exhaust) is at adequate level.

[0030] According to an aspect of the invention the method comprises operating the engine by combusting a first fuel , and when running the engine at power below 75 % of its nominal maximum power, a) air is compressed by a single-stage compressor of a turbo charger coupled with the engine, b) a combustion chamber of each cylinder is charged with compressed air and the first fuel according to working cycle and firing order, c) a second fuel is provided into the cylinder to be fired and becomes ignited, d) the first fuel is ignited by the second fuel, and is combusted as lean burn combustion in the combustion chamber, e) exhaust gas produced by combustion is led to the single-stage turbine of the turbo charger, in which exhaust gas is expanded producing work for operating the compressor, f) exhaust gas is led to a NOx reduction catalysator where NOx emissions are reduced, and g) the exhaust gas is cooled prior to leading exhaust gas to the NOx reduction catalysator, and when running the engine at power at or above 75 % of its nominal maximum power, wherein

I. air is compressed by the single-stage compressor of the turbo charger coupled with the engine,

II. the combustion chamber of each cylinder is charged with compressed air and the first fuel according to working cycle and firing order,

III. the first fuel is ignited by providing a second fuel into the charge, wherein the share of the first fuel is less than 50% and the share of the second fuel is equal to or more than 50 % of total heat flow contained in the fuel provided, and the fuel is combusted in the combustion chamber, d) exhaust gas produced by combustion is led to the single-stage turbine of the turbo charger, in which exhaust gas is expanded producing work for operating the compressor, where turbine the temperature of the exhaust gas is decreased to turbine exit temperature, and e) exhaust gas is led to from the turbine to a NOx reduction catalysator at the temperature of turbine exit temperature, where NOx emissions are reduced.

[0031] Since by using the method according to the invention results in substantially low exhaust gas temperature, requirements to the catalyst for NOx reduction to N2 are much lower, which in practice means less expensive equipment. Additionally, lower exhaust gas temperatures make it possible to construct exhaust channels and catalyst encapsulating and support systems from regular materials rather than high heat resistant materials.

[0032] Suitable catalysts for use in connection with the invention are for example either oxide of base metals such as vanadium or tungsten. Reduction of NOx emissions is based on selective catalytic reduction (SCR) which means of converting nitrogen oxides with the aid of a catalyst into diatomic nitrogen (N₂), and water (H₂O). A reductant, typically anhydrous ammonia (NH3), aqueous ammonia (NH4OH), or a urea (CO(NH₂)₂) solution may be added to a stream of flue or exhaust gas and is reacted onto a catalyst.

[0033] In the scope of the invention following first fuel compositions are feasible. First fuel composition 1 :

Ammonia, 100%

First fuel composition 2:

Ammonia, at least 50%

Remaining portions consist of the second fuel

Fist fuel composition 3:

Ammonia, 80 - 90%

Remaining portions consist of the second fuel

First fuel composition 4:

Ammonia, 50 - 100%

Hydrogen, 0 - 50%

First fuel composition 5:

Ammonia, 50 - 100%

Natural gas, 0 - 50%

First fuel composition 6:

A mixture of ammonia, a non-carbon fuel, comprising ammonia at least 50% of the mixture [0034] In the scope of the invention following first fuel compositions are feasible.

Second fuel composition 1 :

Diesel fuel

Second fuel composition 2:

Any combination of natural gas, hydrogen, syngas, methanol

[0035] In this context the term diesel fuel means any one of the following fuels: bio diesel fuel, synthetic diesel fuel, light fuel oil, marine diesel fuel, a fuel capable of compression ignition in the circumstances in the cylinder of the engine at the end part of compression stroke, or a combination of these fuels.

[0036] Actual combination of the first fuel and the second fuel may comprise any combination of the first fuel compositions and the second fuel compositions.

[0037] According to a specific embodiment of the invention an internal combustion piston engine is configured for operating in lean burn NH3-mode when ammonia is the main fuel in the combustion and alternatively in Diesel-mode, where the engine is operated by diesel fuel. The engine comprises a fixed geometric compression ratio, but its effective compression ratio may be changed based on active operation mode. Effective compression ratio may be controlled e.g. by controlling timing of valves opening and closing, as well as by controlling charge pressure. Lean burn NH3 -mode refers to operating a four stroke multicylinder piston engine by combusting the first fuel comprising ammonia having 50% share of the fuel, with lambda value 1 ,5 - 2,2, which first fuel is ignited by providing a second fuel into the charge as a pilot fuel.

[0038] The pilot fuel may be directly injected liquid fuel, such as diesel fuel, ignited with compression ignition. Injection of the pilot fuel may be practised directly to the cylinder or to a prechamber in connection with the cylinder. The pilot may be also gaseous fuel which is brought into combustion air prior to leading to a cylinder of the engine, or directly to the cylinder which is then ignited by a separate source of ignition, such as a spark. The second fuel may also be provided to a prechamber arranged in connection with the cylinder of the engine. [0039] According to this particular example the NH3-mode is practised so that the second fuel is diesel fuel, which is arranged to the cylinder by direct injection of diesel fuel into combustion chamber at the end part of the compression stroke. This may be referred to as a pilot injection. The second fuel is ignited by compression ignition which in turn ignites the ammonia.

[0040] Advantageously the share of the diesel fuel in pilot injection is 4 to 30% of the main fuel when running less than 75% nominal maximum power of the engine. The percentage is calculated from heat value of first fuel administered to a cylinder during one cycle.

[0041] According to an aspect of the invention the ammonia mode is practised so that the second fuel is hydrogen, or other gaseous non-carbon fuel, which arranged to the cylinder along with combustion air. The second fuel may be a mixture of gaseous fuel and liquid fuel to adapt the fuel for autoignition. The second fuel is ignited by compression ignition or with assistance of external energy, like a spark, functioning as a source of ignition, which in turn ignites the ammonia.

[0042] Advantageously, the share of the hydrogen is 4 to 10% of the main fuel in full load operation. The percentage is calculated from mass of ammonia administered to a cylinder during one cycle.

[0043] In this context the term “geometric compression ratio” is a ratio of the volume of the space in the cylinder above the piston at the point of bottom dead center (sum of a displacement volume and a clearance volume) to the volume present above the piston when the piston is at top dead center (clearance volume).

[0044] In this context the term lean burn combustion means combustion with excess of air compared to stoichiometric air-fuel ratio considering the first and the second fuels. Lambda value denotes a ratio of actual air-fuel ratio present in a charge to stoichiometric air-fuel ratio.

[0045] In this context the term nitrogen oxide or NOx refers to compositions formed by nitrogen and oxide, inter alia nitric oxide (NO), nitrogen dioxide (NO2), nitrogen trioxide (NO3), or nitrous oxide (N2O) or mixtures thereof. [0046] In this context any expression of fuel share or percentage share is from total heat flow contained in the fuel or fuel mixture provided.

[0047] The exemplary embodiments of the invention presented in this patent application are not to be interpreted to pose limitations to the applicability of the appended claims. The invention is applicable to difference engine configurations such as in-line or V-engines. The verb “to comprise” is used in this patent application as an open limitation that does not exclude the existence of also unrecited features. The features recited in dependent claims are mutually freely combinable unless otherwise explicitly stated. The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims.

Brief Description of Drawings

[0048] In the following, the invention will be described with reference to the accompanying exemplary, schematic drawings, in which

Figure 1 illustrates an internal combustion piston engine according to an embodiment of the invention,

Figure 2 illustrates an internal combustion piston engine according to another embodiment of the invention, and

Figure 3 illustrates an internal combustion piston engine according to still another embodiment of the invention.

Detailed Description of Drawings

[0049] Figure 1 depicts schematically an in-line internal combustion piston engine 10 depicting embodiments of the invention. The engine comprises several cylinders 12 each being provided with one or more, typically two intake 12 valves and exhaust valves 16. There an air receiver 18 arranged through which compressed combustion is led to the cylinders. Each intake channel in the engine is provided with a gas admission valve 20 upstream the intake valve 14. Respectively the engine is provided with a direct injection valve 22 in connection with each cylinder 12. According to an operational aspect of the injection valves, the direct injection valves 22 are configured to inject liquid fuel such as diesel fuel as so-called pilot fuel. According to another operational aspect of the injection valves, the direct injection valves 22 are configured to inject liquid fuel as a main fuel enabling the engine to be operated also in diesel mode.

[0050] Further, the engine 10 comprises an exhaust gas manifold 24 which is connected to each cylinder of the engine via the exhaust gas valves 16. The engine has fixed geometric compression ratio. The engine is provided with a turbo charger 28 which comprises a compressor 30 and a turbine 30. The compressor 28 and the turbine are in mechanical force transmission with each other. The compressor 28 is in connection with the air receiver 18 at its outlet via a charge air channel 19, and there is an air cooler i.e. a so called after cooler 32 arranged between the compressor 28 and the receiver 18 for cooling compressed charge air. The turbine is in connection with the exhaust gas manifold 24 via an exhaust gas channel 25. An outlet of the turbine 30 is in connection with a NOx reduction catalysator 38. The turbine 30 is preferably provided with a so-called waste gate 40, which may be external to the turbine or integrated to the turbine.

[0051] As an embodiment of the exhaust gas cooling means, the engine 10 is provided with an air bypass line 34 which extends between the charge air channel 19 and the exhaust gas channel 25. The air bypass channel is provided with an air bypass valve 36, by means of which a controllable amount of compressed charge air may be led to the exhaust gas. Preferably the air bypass line 34 is connected to the charge air channel between the air cooler 32 downstream the air cooler 32.

[0052] The engine is provided with an engine controller system 100. The engine controller system comprises a computer in which executable instructions, such a computer program, may be executed, an access to databases may be arranged so as to cause the engine to carry out a method according to the invention. The controller system 100, among its other functionalities, is arranged in data transfer communication at least with the air bypass valve 36, but preferably also with the waste gate 40 and/or intake valve control system 15. Compressor 28 can be operated by controlling the waste gate 40. Effective compression ratio can be affected by controlling the waste gate 40 and/or timings of the intake valves 15.

[0053] Figure 2 discloses schematically a combustion piston engine 10 depicting other embodiments of the invention relating to cooling of the exhaust air. In addition, or alternative to having the air bypass line 34 and the air bypass valve 36, the engine is provided with a heat exchanger 42 in the exhaust gas line 25 between the NOx catalysator and the turbine 30 for cooling the exhaust gas by transferring heat to a heat transfer medium in the heat exchanger 42. In addition, or alternative to having the direct injection valves 22 arranged in connection with each cylinder 12, the engine is provided with a second gas admission valves 21 upstream the intake valve 14 in each intake channel in the engine. The second gas admission valve 21 is configured to administer second fuel into the combustion are alternatively or additionally to providing the second fuel directly to the combustion chamber. The second gas admission valve 21 is advantageously configured for hydrogen admission instead of direct injection liquid fuel. The controller system 100, among its other functionalities, is arranged in data transfer communication at least with the air bypass valve 36, but preferably also with the waste gate 40 and/or intake valve control system 15, as well as with the heat exchanger 42 for controlling the cooling power of the heat exchanger. In other respect the engine shown in the figure 2 corresponds to that shown in the figure 1.

[0054] Figure 3 discloses schematically a combustion piston engine 10 depicting other alternative embodiments of the invention relating to cooling of the exhaust air. In addition, or alternative to having the air bypass line 34 and the air bypass valve 36, the engine 10 is provided with an auxiliary blower 44 which is configured to transfer air from the surroundings of the engine to the exhaust gas line at a location between the turbine 30 and the NOx reduction catalysator 38. The blower 44 is provided with a dedicated drive means 46, like electric motor, which is controllable independently from the engine. By means of the auxiliary blower 44 the air for cooling the exhaust gas is pressurized and delivered to the exhaust gas line 25 independently from operation of the engine and the turbo charger.

[0055] The controller system 100, among its other functionalities, is arranged in data transfer communication at least with the air bypass valve 36, but preferably also with the waste gate 40 and/or intake valve control system 15, as well as the motor 46 of the blower. In other respect the engine shown in the figure 3 corresponds to that shown in the figure 2.

[0056] While the invention has been described herein by way of examples in connection with what are, at present, considered to be the most preferred embodiments, it is obvious to the skilled person that, along with the technical progress, the basic idea of the invention can be implemented in many ways. The invention and its embodiments are thus not limited to the examples and samples described above but they may vary within the contents of patent claims and their legal equivalents. The details mentioned in connection with any embodiment above may be used in connection with another embodiment when such combination is technically feasible.

Claims

1. A method of operating a four stroke multi-cylinder piston engine, in which engine (10) the cylinders have a fixed geometric compression ratio, by combusting a first fuel, the method being characterized in that a) air is compressed by a single-stage compressor (28) of a turbo charger (26) coupled with the engine (10), b) a combustion chamber of each cylinder (12) is charged with the compressed air and the first fuel according to working cycle and firing order, c) a second fuel is provided into the cylinder to be fired and becomes ignited, d) the first fuel is ignited by the second fuel, and is combusted as lean burn combustion, e) exhaust gas produced by combustion is led to the single-stage turbine (30) of the turbo charger (26), in which exhaust gas is expanded producing work for operating the compressor, f) exhaust gas is led to a NOx reduction catalysator (38) where NOx emissions are reduced, and g) when running the engine (10) at power below 75 % of its nominal maximum power, and when at least 30% of heat released in the combustion is from ammonia as the first fuel, the exhaust gas is cooled prior to leading exhaust gas to the NOx reduction catalysator (38).

2. A method of operating a four stroke multi-cylinder piston engine (10) according to claim 1 , characterized in that the fixed geometric compression ratio is within a range of 11 to 17.

3. A method of operating a four stroke multi-cylinder piston engine (10) according to claim 1 , characterized in that cooling of the exhaust gas is practised prior to leading the exhaust gas to the NOx reduction catalysator (38) when the engine (10) is run at power below 50% of its nominal maximum power, and at least 50% of heat released in the combustion is from ammonia as the first fuel.

4. A method of operating a four stroke multi-cylinder piston engine (10) according to claim 1 , 2, 3 or 4, characterized in that exhaust gas is cooled by leading a controllable amount of air to the exhaust gas.

5. A method of operating a four stroke multi-cylinder piston engine (10) according to claim 4, characterized in that the air is delivered to the exhaust gas by an auxiliary blower (44).

6. A method of operating a four stroke multi-cylinder piston engine (10) according to claim 4, characterized in that exhaust gas is cooled by leading a controllable amount of compressed charge air (34,36) to the exhaust gas between the turbine of turbocharger (26) and the NOx reduction catalysator (38).

7. A method of operating a four stroke multi-cylinder piston engine (10) according to claim 1 , 2, or 3, characterized in that exhaust gas is cooled by transferring heat to a heat transfer medium in a heat exchanger (42) arranged between the turbine of turbocharger (26) and the NOx reduction catalysator (38).

8. A method of operating a four stroke multi-cylinder piston engine (10) according to claim 1 or 2, characterized in that when running the engine (10) at power below 75 % of its nominal maximum power turbocharger (26) is operated to produce charge pressure less than 5 bar(g).

9. A method of operating a four stroke multi-cylinder piston engine (10) according to claim 1 or 2, characterized in that when running the engine (10) at power below 50 % of its nominal maximum power turbocharger (26) is operated to produce charge pressure less than 2 bar(g).

10. A method of operating a four stroke multi-cylinder piston engine (10) according to anyone of the preceding claims, characterized in that when running the engine (10) at power below 75 % of its nominal maximum power, lambda value in the lean burn combustion is less than or equal to 2,2.

11. A method of operating a four stroke multi-cylinder piston engine (10) according to anyone of the preceding claims, characterized in that the second fuel is arranged to the cylinder (12) by direct injection (22) into combustion chamber at the end part of the compression stroke and is ignited by compression ignition.

12. A method of operating a four stroke multi-cylinder piston engine (10) according to anyone of the preceding claims 1 to 10, characterized in that the second fuel is gaseous fuel which is arranged to the cylinder along with combustion air and is ignited by a source of ignition.

13. A method of operating a four stroke multi-cylinder piston according to anyone of the preceding claims, characterized in that the engine (10) is run during engine’s starting phase by combusting second fuel with compression ignition, wherein

I. air is compressed by a single-stage compressor (28) of a turbo charger (26) coupled with the engine (10),

II. a combustion chamber of each cylinder (12) is charged with compressed air according to working cycle and firing order,

III. a predetermined amount of the second fuel is injected into the combustion chamber, ignited by compression ignition, and combusted in the combustion chamber,

IV. exhaust gas produced by combustion is led to the single-stage turbine (30) of the turbo charger (26), in which exhaust gas is expanded producing work for operating the compressor (28) where exhaust gas temperature is decreased to turbine exit temperature,

V. exhaust gas is led to a NOx reduction catalysator (38) at the temperature of turbine exit temperature, where NOx emissions are reduced by catalytic reactions.

14. A method of operating a four stroke multi-cylinder piston engine (10) according to anyone of the preceding claims running the engine (10) at power below 75 % of its nominal maximum power, and when running the engine (10) at power equal to or more than 75 % of its nominal combusting the first fuel, I . air is compressed by the single-stage compressor of the turbo charger coupled with the engine (10),

II. a combustion chamber of each cylinder is charged with compressed air and the first fuel according to working cycle and firing order,

III. the first fuel is ignited by providing a second fuel into the charge, wherein the share of the first fuel is less than 50% and the share of the second fuel is equal to or more than 50 % of total heat flow contained in the fuel provided and the fuel is combusted in the combustion chamber,

IV. exhaust gas produced by combustion is led to the single-stage turbine of the turbo charger, in which exhaust gas is expanded producing work for operating the compressor,

V. exhaust gas is led to a NOx reduction catalysator where NOx emissions are reduced.

15. A four stroke multi-cylinder internal combustion piston engine (10) configured to combust a first fuel comprising ammonia and second fuel, comprising: a) fixed geometric compression ratio, b) first fuel feeding system configured to provide first fuel into combustion chamber of each cylinder of the engine (10), c) second fuel feeding system configured to provide second fuel into combustion chamber of a cylinder, d) single stage turbocharger, e) NOx catalysator arranged downstream of the turbocharger, f) exhaust gas cooling means arranged between the NOx catalysator and the turbocharger, g) an engine (10) controller system, which comprises executable instructions, which, when executed by the computer controller system of the engine (10), cause the engine (10) to carry out a method of any one of the preceding claims.