WO2017207864A1

WO2017207864A1 - Method for reducing hydrocarbon emissions and piston engine

Info

Publication number: WO2017207864A1
Application number: PCT/FI2016/050390
Authority: WO
Inventors: Alberto Cafari; Martin AXELSSON; Karl André Sebastian AHLSKOG
Original assignee: Wärtsilä Finland Oy
Priority date: 2016-06-01
Filing date: 2016-06-01
Publication date: 2017-12-07
Also published as: KR20180136554A; EP3464867A1; EP3464867B1; KR101970116B1; CN109312695B; CN109312695A

Abstract

In the method for reducing emissions of unburned and partially burned hydro-carbons in a piston engine (1) that is operated using gaseous fuel as a main fuel, gas molecules of the fuel are electrically charged with a predetermined polarity before being introduced into combustion chambers (15) of the engine (1), and electric charge with the same polarity is applied to walls of the combustion chambers (15).

Description

Method for reducing hydrocarbon emissions and piston engine Technical field of the invention

The present invention relates to a method for reducing emissions of unburned and partially burned hydrocarbons in a piston engine in accordance with the preamble of claim 1 . The invention also concerns a piston engine as defined in the preamble of the other independent claim.

Background of the invention

Control of hydrocarbon emissions of internal combustion engines is important, since hydrocarbon emissions have adverse health and environmental effects. As an example of a harmful effect, methane is a strong greenhouse gas. Emission regulations often refer to total hydrocarbon (THC) emissions or to non- methane hydrocarbons (NMHC). Reduction of hydrocarbon emissions of internal combustion engines is a challenging task, since many potential measures have an adverse effect especially on NO_x emissions. In gas engines, lean burn technology provides a solution for reducing NO_x emissions, but THC emissions are a remaining challenge. There are different mechanisms that contribute to THC emissions. One such mechanism relates to a thermal boundary layer in the combustion chamber. In the vicinity of the walls of the combustion cham- ber, rapid heat exchange towards the walls of the combustion chamber has a negative effect on the combustion. As a result, fuel oxidation is ineffective causing THC emissions.

Summary of the invention An object of the present invention is to provide a method for reducing emissions of unburned and partially burned hydrocarbons in a piston engine that is operated using gaseous fuel as a main fuel. The characterizing features of the method according to the invention are given in the characterizing part of claim 1 . Another object of the invention is to provide an improved piston engine that comprises means for introducing gaseous fuel into an intake duct. The charac- terizing features of the engine are given in the characterizing part of the other independent claim.

In the method according to the invention, gas molecules of the fuel are electrically charged with a predetermined polarity before being introduced into com- bustion chambers of the engine, and electric charge with the same polarity is applied to walls of the combustion chambers.

The engine according to the invention comprises means for electrically charging the gaseous fuel with a predetermined polarity before the fuel is introduced into combustion chambers of the engine, and means for applying electric charge with the same polarity to the wall of each combustion chamber.

Because of the electrical charging of the fuel and the combustion chamber walls, the combustion chamber walls repel the gas molecules of the fuel. Fuel concentration near the combustion chamber walls, where the combustion is not as effective as in the other parts of the combustion chambers, is thus low- ered. THC emissions caused by a thermal boundary layer are thus reduced.

According to an embodiment of the invention, both the fuel and the walls of the combustion chambers are positively charged.

According to an embodiment of the invention, the electric charge is applied to an anti-polishing ring of each cylinder of the engine. Alternatively or in addition, the electric charge can be applied to the cylinder liner of each cylinder of the engine. If the electric charge is applied only to the anti-polishing rings, the charged parts are easier to electrically isolate from the rest of the engine. On the other hand, by charging the whole cylinder liner, it is ensured that the concentration of fuel in the vicinity of the combustion chamber walls is kept lower during the whole combustion process.

According to an embodiment of the invention, the walls of combustion chambers are electrically charged using a high-voltage source. The high-voltage source can be, for instance, a linear output transformer. The voltage can be, for instance, in the range of 50 to 100 kV. According to an embodiment of the invention, the fuel is electrically charged with an ionizer that is arranged in a fuel feed system upstream from gas ad- mission valves of the cylinders. For example a magnetic field, photoionization or microwaves can be used for electrically charging the fuel.

The engine can be operated using a lean air/fuel mixture.

According to an embodiment of the invention, the engine is provided with an own ionizer for each cylinder of the engine. However, it is also possible to provide the engine with one ionizer that is common to several or all cylinders of the engine.

Brief description of the drawings Embodiments of the invention are described below in more detail with reference to the accompanying drawings, in which

Fig. 1 shows schematically a piston engine,

Fig. 2 shows a cylinder of the engine in accordance with an embodiment of the invention, and Fig. 3 shows a cylinder of the engine in accordance with another embodiment of the invention.

Description of embodiments of the invention

In figure 1 is shown schematically a piston engine 1 . The engine 1 is a large in- ternal combustion engine, such as a main or an auxiliary engine of a ship or an engine that is used at a power plant for producing electricity. The cylinder bore of the engine 1 is at least 150 mm. The rated power of the engine 1 is at least 100 kW/cylinder. The engine 1 comprises a plurality of cylinders 2. In figure 1 , four cylinders 2 are shown, but the engine 1 can comprise any reasonable number of cylinders 2, which can be arranged, for instance, in line or in a V- configuration. The engine 1 is a gas engine or a dual-fuel or multi-fuel engine that can be operated using two or more alternative fuels. If the engine 1 is a dual-fuel or multi-fuel engine, it can be operated in at least one gas mode, in which a gaseous fuel is used as a main fuel. The expression "main fuel" refers to a fuel that is responsible for the major part of heat release in the cylinders 2 of the engine 1 . Preferably, at least 90 % of the heat that is released in a cylinder 2 is released by combustion of the main fuel. The gaseous fuel can be, for instance, natural gas, biogas or associated gas from an oil drilling site. The engine 1 can also have a liquid fuel mode, in which a liquid fuel is used as a main fuel. The liquid fuel can be, for instance, light fuel oil or heavy fuel oil. In a gas mode, the engine 1 is operated according to the Otto-cycle. In a liquid fuel mode, the engine 1 can be operated according to the Diesel-cycle.

In the gas mode, the gaseous main fuel can be ignited either by spark plugs or with the aid of pilot fuel injection. In the embodiment of the figures, the engine 1 is provided with a pilot fuel injection system. The engine 1 also comprises a gas feed system for introducing gaseous fuel into the cylinders 2 of the engine 1 . The engine 1 could also comprise a further fuel injection system for introducing liquid main fuel into the cylinders 2 of the engine 1 .

The gas feed system comprises a gas admission valve 9 for each cylinder 2 of the engine 1 . The gas admission valves 9 are used for controlling the supply of the gaseous fuel into the cylinders 2. The gas admission valves 9 are connected to a control unit 6, which controls opening and closing of the gas admission valves 9. The gaseous fuel is not introduced directly into the cylinders 2, but each gas admission valve 9 is configured to introduce the gaseous fuel into an intake duct of the engine 1 close to the intake valves of the cylinder 2. The gaseous fuel is thus introduced into the cylinders 2 of the engine 1 during the intake stroke. The gaseous fuel is mixed with the intake air to form a homogenous mixture. The mixture is preferably a lean mixture containing more air than is needed for complete combustion of the fuel that is introduced into the cylin- ders 2. Part of the gaseous fuel can be introduced into a prechamber for forming a richer mixture, which facilitates ignition of the fuel/air mixture. Each cylinder 2 can be provided with a prechamber valve for controlling the supply of the gaseous fuel into the prechamber. When a cylinder 2 is operated using the gaseous fuel as the main fuel, Otto combustion process is utilized. In the em- bodiment of the figures, liquid pilot fuel is used for igniting the gaseous fuel. Alternatively, the mixture of air and the gaseous fuel could be ignited by means of spark plugs. The gaseous fuel is supplied to the cylinders 2 of the engine 1 from a gas tank 10 via a gas supply pipe 1 1 . The gas source could also be, for instance, a pipe line or an industrial process producing combustible gas. The gas feed system is provided with a main gas valve 12 for controlling the pres- sure of the gas that is supplied to the gas admission valves 9 of the cylinders 2.

The pilot fuel injection system shown in figure 1 is a common-rail system. The pilot fuel injection system comprises a pilot fuel injector 3 for each cylinder 2 of the engine 1 . Each pilot fuel injector 3 can be individually controlled. The pilot fuel injectors 3 are connected to a control unit 6 for controlling the timing and duration of pilot fuel injection. The pilot fuel injection system comprises a low- pressure pump 4 and a high-pressure pump 5. The low-pressure pump 4 supplies fuel from a fuel tank 7 to the high-pressure pump 5. The high-pressure pump 5 supplies the fuel into a fuel rail 8 at a pressure that is suitable for direct fuel injection into the cylinders 2 of the engine 1 . The pressure in the fuel rail 8 can be, for instance, in the range of 800 to 3000 bar. Each fuel injector 3 is connected to the same fuel rail 8. The pilot fuel injection system could also comprise more than one low-pressure and/or high-pressure pumps 4, 5. In- stead of or in addition to a fuel rail that is common to all pilot fuel injectors 3, each pilot fuel injector 3 could be provided with an own fuel accumulator. The pilot fuel injection system could also comprise several fuel rails 8. The pilot fuel injection system or parts of the pilot fuel injection system could also be used for introducing liquid main fuel into the cylinders 2 of the engine 1 . Figure 2 shows schematically one cylinder 2 of the piston engine 1 of figure 1 . The cylinder 2 is formed by a cylinder liner 20, which is inserted into an engine block. A piston 14 moves in a reciprocating manner in the cylinder 2 between bottom dead center and top dead center. A cylinder head 16 closes the top dead center end, i.e. the upper end of the cylinder 2. The piston 14 delimits to- gether with the cylinder head 16 and the walls of the cylinder liner 20 a combustion chamber 15, in which the combustion of the fuel takes place. Intake air, and in the gas mode also the gaseous main fuel, is introduced into the cylinder 2 via an intake duct 13. The gas admission valve 9 is arranged to introduce the gaseous main fuel into the intake duct 13. An intake valve 17 is arranged to open and close fluid communication between the combustion chamber 15 and the intake duct 13. Exhaust gas is discharged from the cylinder 2 via an exhaust duct 18. Fluid communication between the combustion chamber 15 and the exhaust duct 18 can be opened and closed by an exhaust valve 19. Figure 2 also shows the pilot fuel injector 3, which can be used for injecting liquid pilot fuel directly into the combustion chamber 15. In a gas engine, a significant part of the total unburned or partially burned hydrocarbon (THC) emissions is created close to the walls of the combustion chambers 15. In a thermal boundary layer that is located adjacent to the walls of the combustion chamber 15, the rapid heat exchange rate towards the com- bustion chamber walls disturbs combustion, and as a result THC emissions are formed. In accordance with the invention, this phenomenon is decreased by applying an electric charge with the same polarity to the gas molecules of the fuel and the walls of the combustion chambers 15. The combustion chamber walls and the fuel thus repel each other, which lowers the fuel concentration in the thermal boundary layer. As a result, the fuel is more effectively combusted and less THC emissions are formed.

The walls of the combustion chambers 15 are charged with a high-voltage source 24 which creates a DC voltage. The voltage applied to the walls of the combustion chambers can be, for instance, in the range of 50 to 100 kV. In the embodiment of figure 2, each cylinder liner 20 is connected to the positive terminal of the DC voltage source 24. Each cylinder liner 20 is thus positively charged. However, the combustion chamber walls could also be negatively charged if the fuel is negatively charged. The cylinder liners 20 are electrically isolated from the engine block and the cylinder heads 16 with insulation mate- rial 26.

Before the gaseous fuel is introduced via the gas admission valves 9 into the intake duct 13 and further into the combustion chambers 15, the gas molecules of the fuel are electrically charged, i.e. ionized. For charging the fuel, an ionizer 25 is arranged upstream from the gas admission valves 9. In the embodiment of the figures, each cylinder 2 of the engine 1 is provided with an own ionizer 25. However, also an ionizer 25 that is common to a group of cylinders 2 or all the cylinders 2 of the engine 1 could be used.

In case the fuel is positively charged, a free electron and a cation are created in the ionization process. The energy needed for removing the most loosely bound electron from each gas molecule is called ionization energy. The energy can be brought into the process in many different ways, and the operation of the ionizers 25 can thus be based on many different principles. At least magnetic fields, microwaves or photoionization can be utilized. For example, in photoionization the electron is removed from a molecule when a photon col- lides with the molecules. The different operating principles can be implemented in many different ways. For instance a non-thermal plasma technology can be utilized in the ionization process. In case natural gas is used as the gaseous fuel, methane cations (CH₄ ⁺) are formed. As the cations enter the combustions chambers 15, they are repelled by the positively charged cylinder liners 20. The concentration of the fuel is thus lower in the vicinity of the walls of the combustion chambers 15.

Figure 3 shows another embodiment of the invention. The cylinder liner 20 of figure 3 differs from the cylinder liner 20 of figure 2 in that it is provided with a recess 21 for receiving an anti-polishing ring 22. The recess 21 is located at the upper end of the cylinder liner 20. The anti-polishing ring 22 is a sleeve-like element, which is used for removing carbon deposits from the piston 14. The inner diameter of the anti-polishing ring 22 is smaller than the inner diameter of the cylinder liner 20, but larger than the outer diameter of the piston 14 in the area above piston rings 23. The lower end of the anti-polishing ring 22 is at such a height that when the piston 14 is at top dead center, the lower end of the anti-polishing ring 22 is above all the piston rings 23 of the piston 14, but the upper end of the piston 14 is above the lower end of the anti-polishing ring 22. The anti-polishing ring 22 can thus scrape carbon deposits from a piston crown above the piston rings 23. In the embodiment of figure 3, the whole cylinder liner 20 is not electrically charged. Instead, the DC voltage source 24 is connected to the anti-polishing ring 22. An electrical insulation 26 is arranged between the anti-polishing ring 22 and the cylinder liner 20. Only the anti-polishing ring 22 is thus electrically charged. An advantage of this embodiment is that the whole cylinder liner 20 does not need to be electrically isolated from the engine block. Because most of the combustion takes place close to top dead center, a sufficient reduction of THC emissions can be achieved even with the embodiment of figure 3.

It will be appreciated by a person skilled in the art that the invention is not limited to the embodiments described above, but may vary within the scope of the appended claims.

Claims

1 . A method for reducing emissions of unburned and partially burned hydrocarbons in a piston engine (1 ) that is operated using gaseous fuel as a main fuel, characterized in that gas molecules of the fuel are electrically charged with a predetermined polarity before being introduced into combustion chambers (15) of the engine (1 ), and electric charge with the same polarity is applied to walls of the combustion chambers (15).

2. A method according to claim 1 , wherein both the fuel and the walls of the combustion chambers (15) are positively charged. 3. A method according to claim 1 or 2, wherein the electric charge is applied to an anti-polishing ring (22) of each cylinder (2) of the engine (1 ).

4. A method according to any of claims 1 to 3, wherein the electric charge is applied to the cylinder liner (20) of each cylinder (2) of the engine (1 ).

5. A method according to any of the preceding claims, wherein the walls of combustion chambers (15) are electrically charged using a high-voltage source

(24).

6. A method according to claim 5, wherein the high-voltage source (24) is a linear output transformer.

7. A method according to any of the preceding claims, wherein the voltage applied to the walls of the combustion chambers (15) is in the range of 50 to

100 kV.

8. A method according to any of the preceding claims, wherein the fuel is electrically charged with an ionizer (25) that is arranged in a fuel feed system upstream from gas admission valves (9) of the cylinders (2). 9. A method according to any of the preceding claims, wherein the fuel is electrically charged using a magnetic field, photoionization or microwaves.

10. A method according to any of the preceding claims, wherein the engine (1 ) is operated using a lean air/fuel mixture.

1 1 . A piston engine (1 ) comprising means (9) for introducing gaseous fuel in- to an intake duct (13), characterized in that the engine (1 ) comprises means (25) for electrically charging the gaseous fuel with a predetermined polarity before the fuel is introduced into combustion chambers (15) of the engine (1 ), and means (25) for applying electric charge with the same polarity to the wall of each combustion chamber (15). 12. An engine (1 ) according to claim 1 1 , wherein the means (24) for charging the combustion chamber walls is a high-voltage source.

13. An engine (1 ) according to claim 12, wherein the high-voltage source (24) is a linear output transformer.

14. An engine (1 ) according to any of claims 1 1 to 13, wherein the means (25) for charging the fuel comprises an ionizer.

15. An engine (1 ) according to claim 14, wherein the engine (1 ) is provided with an own ionizer (25) for each cylinder (2) of the engine (1 ).

16. An engine (1 ) according to claim 14 or 15, wherein the operation of the ionizer (25) is based on a magnetic field, photoionization or microwaves.