EP1608856A1

EP1608856A1 - Self-igniting internal combustion engine

Info

Publication number: EP1608856A1
Application number: EP04724291A
Authority: EP
Inventors: Alois Raab; Martin Schnabel
Original assignee: DaimlerChrysler AG
Current assignee: Mercedes Benz Group AG
Priority date: 2003-04-03
Filing date: 2004-03-30
Publication date: 2005-12-28
Also published as: DE10315149A1; US20060037563A1; WO2004088109A1; JP2006522262A

Abstract

The invention relates to a method for operating an internal combustion engine consisting in directly injecting fuel in a combustion chamber as pre-injection main injection and optionally post-injection, with the aid of an injector nozzle provided with several injecting borings,, the pre-injection being preferably carried out in a cadenced manner. The aim of said invention is to reduce a combustion temperature and to avoid increased pressure in the combustion chamber. For this purpose, a certain quantity of water is introduced in the combustion chamber during or and after the pre-injection.

Description

Internal combustion engine with auto-ignition

The invention relates to a method for operating an internal combustion engine with auto-ignition, in particular a diesel internal combustion engine, according to the preamble of claim 1.

In direct-injection internal combustion engines with auto-ignition, the heterogeneous type of combustion control inevitably results in very high pressures and high combustion temperatures due to the auto-ignition of the injected fuel in the combustion chamber, which in particular produces high NOx emissions. Furthermore, the fuel-rich zones result in considerable amounts of soot particles, some of which are oxidized at the high temperatures present. In order to avoid the disadvantages of such a heterogeneous type of combustion control, a modern homogeneous internal combustion engine strives for a combined homogeneous / heterogeneous mode of operation with which an improved combustion is to be achieved.

A method is known from EP 509 372 B1, in which a gaseous main fuel and a liquid secondary fuel are used, the ignition of the main fuel being initiated with the secondary fuel. The liquid secondary fuel is injected into the combustion chamber as a mixture of water and fuel in the form of a pilot injection. The mixing of water with the liquid fuel serves to enable the pilot injection to initiate ignition of the main fuel and the volume of the mixture injected by the pump to be chosen in such a way that atomization can be designed precisely by means of the injection nozzle.

EP 459 083 B1 discloses a method for operating an internal combustion engine, in which water and diesel fuel are used, which are introduced into the combustion chamber of the internal combustion engine by means of a fuel / water injection device in such a way that fuel in an amount of between 5% is initially used during an injection. or more and 75% or less of a total fuel injection amount, then a predetermined amount of water and finally the remaining fuel are injected. In this water / diesel internal combustion engine, both the fuel and the water are injected into the combustion chamber via a single fuel injection valve, so that a rise in temperature of a flame is suppressed in order to minimize the generation of NOx emissions.

According to the current state of the art, it is difficult to control the combustion described above, since the pressure increase in the combustion chamber depends on the fuel portions of the pre-injection and main injection, the ignition timing of the pre-injection, the ignition timing of the main injection, and the injection timing of the pre-injection and main injection.

It is therefore the object of the invention to create a method for operating an internal combustion engine with auto-ignition, in which complete combustion and a good distribution of the fuel in the combustion chamber are designed in such a way that high combustion chamber pressure increases and high combustion temperatures are avoided.

This object is achieved according to the invention by a method with the features of claim 1.

The method according to the invention is characterized in that during suction, compression and / or explosion expansion stroke for cooling a mixture present in the combustion chamber, a liquid with a high evaporation enthalpy is introduced into the combustion chamber, so that an increase in pressure in the combustion chamber is reduced and, if necessary, an ignition timing of the pre-injection or main injection is delayed. Thus, a maximum temperature is still reduced. The cooling liquid is preferably introduced into the combustion chamber with a high evaporation enthalpy during the intake and / or compression stroke. Furthermore, the ignition timing of the pre-injection and / or main injection can be delayed depending on the fuel quantity of the pre-injection. Due to the liquid introduced into the combustion chamber and serving as a cooling medium, fuel-side cooling is carried out, with which the ignition timing of the main injection is delayed, so that an optimal center of gravity of the combustion is achieved and a high pressure rise in the combustion chamber is reduced. Exhaust gas recirculation is preferably carried out in order to further reduce the exhaust gas emissions formed, in particular the formation of NOx.

In an embodiment of the invention, the liquid is introduced into the combustion chamber before or after the start of the pre-injection. Through an almost simultaneous injection of the cooling liquid, the presence of the cooling liquid during the pre-injection due to the high enthalpy of vaporization of the liquid achieves the desired cooling effect before the ignition of the homogeneous mixture, which is formed by the early pre-injection. This delays the ignition timing of the pre-injection, reduces the pressure rise in the combustion chamber and lowers the temperature level.

According to a further embodiment of the invention, the liquid is introduced into the combustion chamber after the pre-injection has ended. The introduction of the cooling medium serving liquid takes place in this case after the ignition of the homogeneous mixture or after the ignition of the homogeneous mixture, whereby the pressure increase in the combustion chamber and a maximum temperature are reduced.

According to a further embodiment of the invention, the introduction of the liquid into the combustion chamber is ended before the end of the main injection of the fuel. The introduction of the cooling medium influences both the combustion of the homogeneous premix and the combustion of the heterogeneous part of the main injection, so that the pressure rise is reduced and the temperature level is reduced. This enables the start of injection of the injections and the pressure curve of the combustion to be optimized.

In a further embodiment of the invention, the liquid is introduced into the combustion chamber in the form of a quantity of water. This removes heat from the fuel or the mixture in the combustion chamber without changing the composition of the mixture. This is expedient and expedient in particular in a combustion process with pre-injection, main injection and possibly post-injection, since the injection times and fuel quantities of the partial injections carried out are regulated depending on the operating point. It is nevertheless conceivable that, according to the invention, another liquid with a comparable high enthalpy of vaporization is used instead of the introduction of water. Alternatively, a second fuel can be introduced which has a vaporization enthalpy that is comparable to that of water.

In a further embodiment of the invention, the amount of water is mixed with the fuel during the pre-injection and / or the main injection within the injection device in such a way that the water is introduced into the combustion chamber in the form of a fuel / water emulsion. Consequently the desired cooling effect is ensured, since the water and fuel are mixed before the fuel is injected into the combustion chamber.

According to a further embodiment of the invention, the amount of water is introduced into the combustion chamber by means of an additional injection device. By introducing the water with a separate injector, high fuel injection pressures can be achieved without having to take into account the use of water in the fuel injection device.

According to a further embodiment of the invention, the amount of water is mixed with the fuel during the pre-injection and / or the main injection within the injection device such that the water in the form of a fuel / water / fuel stratification or fuel / water stratification or water / fuel stratification - is introduced into the combustion chamber.

In a further embodiment of the invention, the pre-injection is carried out in a compression stroke range of approximately 150 ° KW to 30 ° KW before top dead center. The pre-injection is preferably clocked. Due to the early pre-injection and any timing of the pre-injection, the basic mixture consisting of fuel, air and possibly exhaust gas is homogenized to a greater extent, so that a subsequent or simultaneous introduction of the amount of water can achieve targeted cooling.

According to a further embodiment of the invention, the main injection and optionally the post-injection are carried out in succession around top dead center in a range from 20 ° KW before top dead center to 40 ° KW after top dead center. In a further embodiment of the invention, the pressure of the fuel introduced into the combustion chamber is changed during an injection process. This is intended to avoid wetting the combustion chamber walls with fuel. The injection pressure is preferably varied as a function of the operating point and / or according to a back pressure prevailing in the combustion chamber, so that the fuel wall wetting is minimized.

According to a further embodiment of the invention, the pre-injection is carried out in a clocked manner, a fuel cloud of a fuel jet generated during an injection cycle being displaced or shifted laterally during the pre-injection by means of a swirl movement formed in the combustion chamber, so that during a subsequent injection cycle the newly injected fuel jets do not enter the fuel cloud the previous injection stroke penetrate. This is intended to avoid wetting the combustion chamber walls with fuel and to achieve a greater homogenization of the pre-injected fuel quantity.

Further characteristics and combinations of characteristics result from the description. Specific exemplary embodiments of the invention are shown in simplified form in the drawings and are explained in more detail in the description below. Show it:

1 shows a schematic cross section through a direct-injection internal combustion engine with auto-ignition,

2 shows a diagram for a schematic cylinder pressure curve during the combustion of a homogeneous mixture of the internal combustion engine according to FIG. 1 without the use of a cooling medium and / or exhaust gas recirculation,

3 shows a schematic illustration of the fuel injection times of the combustion according to FIG. 2, 4 shows a diagram for a schematic cylinder pressure curve of the internal combustion engine from FIG. 1 during the combustion of a homogeneous mixture with exhaust gas recirculation and water injection,

5 shows a diagram for a schematic cylinder pressure curve of a homogeneous / heterogeneous combined combustion of the internal combustion engine according to FIG. 1 without the use of a cooling medium and / or exhaust gas recirculation,

6 shows a schematic illustration of a fuel injection strategy for the combustion according to FIG. 5,

7 shows a schematic illustration of a fuel injection strategy of a homogeneous / heterogeneous combined combustion with a water injection of the internal combustion engine according to FIG. 1,

8 shows a schematic illustration of a second exemplary embodiment of the combustion according to FIG. 7,

9 shows a schematic illustration of a third exemplary embodiment of the combustion according to FIG. 7,

10 shows a schematic illustration of a fourth exemplary embodiment of the combustion according to FIG. 7,

FIG. 11 shows a schematic illustration of a fifth exemplary embodiment of the combustion according to FIG. 7, and

FIG. 12 shows a diagram for a cylinder pressure curve of a homogeneous / heterogeneous combined combustion of the internal combustion engine according to FIG. 1 with a water injection. 1 shows an internal combustion engine 1 in which a crankshaft 2 is driven by a piston 5 guided in a cylinder 9 via a connecting rod 4. A combustion chamber 8 is formed in the cylinder 9 between the piston 5 and a cylinder head 10, which preferably comprises a piston recess 6 which is introduced into the piston crown 7. When a crank 3 of the crankshaft 2 rotates clockwise on a crank circle 11, the combustion chamber 8 becomes smaller, the air enclosed in it being compressed. The gas exchange in the combustion chamber 8 takes place via gas exchange valves, not shown, which are arranged in the cylinder head 10.

When top dead center 12 of crank 3, hereinafter referred to as TDC, is reached, the end of compression is reached, at which combustion chamber 8 assumes its smallest volume. The current position of the piston 5 is determined by the crank angle φ in relation to TDC. A multi-hole injection nozzle 13 is arranged almost centrally in the cylinder head 10, being controlled by an electronic control unit 16 of an engine control system via a signal line 15 and an actuator 14, for example a piezoelectric or a hydraulic actuator.

The internal combustion engine 1 works according to the 4-stroke principle. A cylinder pressure curve of a homogeneous combustion of the internal combustion engine 1 with auto-ignition is shown in FIG. 2, the corresponding fuel injection times being shown in FIG. 3. In a first intake stroke or intake stroke, the piston 5 moves in a downward movement from the top dead center 12 to a bottom dead center UT. In this case, combustion air is supplied to the combustion chamber 8 via an inlet duct (not shown). A certain amount of exhaust gas from a previous work cycle is preferably admixed to the combustion air supplied to the combustion chamber 8 by an exhaust gas recirculation valve. In a second compression stroke or compression stroke, the piston 5 moves in an upward movement from bottom dead center UT to an upper ignition dead center ZOT, fuel being injected into the combustion chamber 8 filled with compressed air shortly before ZOT. In a subsequent expansion stroke, the piston 5 moves to bottom dead center UT, with the exhaust gases then being pushed out of the combustion chamber 8 in a further extension stroke. According to FIG. 3, the time of the fuel injection can be between 150 ° KW and 30 ° KW before ZOT. The fuel ignites through compression heat before ZOT, whereby the focus of combustion is clearly before OT. The focus of the combustion is the piston position or the crank angle, at which a 50% conversion of the fuel mass involved in the combustion has taken place. According to FIG. 2, there is a combustion with a very large increase in pressure, which leads to pressure fluctuations and poor noise behavior. Due to the unfavorable position of the combustion or the disadvantageous focus of the combustion, poor efficiency is achieved. If the time for homogenization is too short, additional high NOx emissions occur.

Different ignition takes place depending on the pre-injection quantity. If the pre-injection quantity is so small that the mixture becomes leaner, the ignition only takes place when the main injection quantity is injected, ie the main injection serves as an ignition jet. If the pre-injection quantity is large enough and the mixture does not lean out, the pre-injection quantity is ignited. With compression ratios between 12 and 21 and normal temperature boundary conditions of the intake air temperature, component temperature etc., the ignition takes place well before TDC, which results in a poor combustion center of the pre-injection quantity. Furthermore, the sudden combustion of the mixture leads to high pressure increases and, as a result, to pressure fluctuations. Influence on the ignition and the pressure increase of the pre-injection component, the main injection proportion and their maximum pressures and temperatures is achieved according to the invention by using a cooling liquid, for example water, and preferably in combination with exhaust gas recirculation.

According to the invention, two injection strategies are preferred. In the first variant, the introduction of a cooling medium, preferably water or a second fuel with a high enthalpy of vaporization, can be carried out before the ignition of the homogeneous mixture, as a result of which the ignition timing is delayed and the pressure rise is reduced. In the second variant, the cooling medium is introduced after the homogeneous mixture has ignited, which also results in a reduction in the pressure increase.

Fig. 4 shows a cylinder pressure curve in the combustion of a homogeneous mixture, in which a shift in the start of ignition and a reduction in the pressure rise is achieved by means of fuel cooling by injecting water in combination with exhaust gas recirculation, so that a shift in the center of gravity of the combustion after TDC while avoiding knocking combustion is achieved. Combustion control by means of EGR and the use of cooling effects on the fuel are carried out.

6 shows an injection strategy of a combustion combined from a homogeneous component and a heterogeneous component. Here, a pre-injection VE is first carried out in a range between 150 ° KW and 30 ° KW before TDC, after which a main injection around top dead center, preferably between 30 ° KW before TDC and 30 ° KW after TDC, takes place. 5 shows the pressure curve of such a combustion from a homogeneous and heterogeneous fraction.

In order to optimize a homogeneous / heterogeneous combined combustion according to the combustion shown in FIG. 5, a Water injection made so that a cylinder pressure curve according to FIG. 12 takes place. The aim is to shift the start of ignition of the pre-injection quantity, to shift the center of gravity of the pre-injection combustion and to reduce the pressure increase. The maximum combustion chamber temperature is also reduced.

It is conceivable that another liquid with a comparatively high evaporation enthalpy is used instead of water. Alternatively, a second fuel can be introduced instead of water injection, which likewise has a vaporization enthalpy that is comparable to that of water.

FIG. 7 shows a first embodiment of such a fuel / water injection strategy for the internal combustion engine 1 to achieve a combustion chamber pressure curve according to FIG. 12. In the compression stroke, part of the fuel is first injected into the combustion chamber 8 as a pre-injection, this pre-injection being able to be carried out in the intake and / or compression stroke. A water injection WE is started shortly after the start of the pre-injection VE, which is ended before the end of a main injection HE. The pre-injection carried out ensures good distribution of the fuel in the combustion chamber, so that a homogeneous fuel / air mixture mixed with the injected water is formed. The use of water injection delays the onset of combustion for the pre-injected fuel quantity and reduces the pressure increase, so that the center of gravity of the combustion is shifted to later. Without the use of the amount of water, the center of gravity of the combustion according to FIG. 5 would be too early, as a result of which the efficiency of the internal combustion engine 1 deteriorates. The strong pressure increase also leads to poor noise behavior without the use of water and / or EGR. The fuel pre-injection VE preferably takes place between 150 ° KW and 30 ° KW before TDC. On- finally, in a region around the top ignition dead center ZOT, a further quantity of fuel is introduced into the combustion chamber 8 as a main injection HE. The main injection HE preferably takes place between 20 ° KW before TDC and 30 ° KW after TDC. The water injection preferably takes place between 150 ° KW before TDC and 20 ° KW after TDC. Furthermore, after the main injection HE, a small amount of fuel can be injected as a post-injection at a later time.

According to the first embodiment, the injection of the amount of water lowers the temperature level and slows down the vaporization of the pre-injected fuel, so that a later start of ignition is achieved. ^' The advantage of this injection strategy is that a combined homogeneous / heterogeneous combustion with auto-ignition is guaranteed in the entire map. As a result, the proportions of the pre-injection and the main injection can be varied depending on the load. Furthermore, the injection times of the homogeneous part and the injection times of the heterogeneous part can be selected depending on the load and speed.

In a second embodiment of the fuel injection strategy according to FIG. 8, the injection of the water quantity WE is started only after the pre-injection VE has ended, so that the water quantity is introduced only after the ignition of the homogeneous mixture.

According to a third embodiment, the amount of water WE is mixed with the fuel during the pre-injection VE and during the main injection HE within the injector 13 such that the water with the fuel is injected into the combustion chamber 8 as a fuel-water emulsion in accordance with the injection strategy shown in FIG. 9. The aim of this injection strategy is to ensure the desired cooling effect, so that a shift in the start of ignition and a reduction in the pressure increase the pre-injection VE, and the temperature level of the pre-injection and main injection is reduced. The pre-injection VE of the fuel water emulsion takes place between 150 ° KW and 30 ° KW before TDC. The main injection HE of the fuel water emulsion is carried out between 20 ° KW before TDC and 30 ° KW after TDC.

It is conceivable that, according to a fourth embodiment, the amount of water WE is only added to the pre-injection VE within the injection device, so that, according to FIG. 10, a fuel water emulsion or a fuel / water stratification is introduced into the combustion chamber in the form of a pre-injection. The pre-injection VE of the fuel water emulsion or a fuel / water stratification takes place between 150 ° KW and 30 ° KW before TDC. The main fuel injection HE is carried out between 20 ° KW before TDC and 30 ° KW after TDC.

Furthermore, it is conceivable that, according to a fifth embodiment, the water quantity of the main injection HE is admixed within the injection device, so that the fuel water emulsion or water / fuel stratification according to FIG. 11 is introduced into the combustion chamber as a main injection HE. The pre-injection VE of the fuel takes place between 150 ° KW and 30 ° KW before TDC. The main injection HE of the fuel water emulsion or water / fuel stratification is carried out between 20 ° KW before TDC and 30 ° KW after TDC.

According to the invention, in all embodiments, a water / fuel stratification can be carried out in such a way that the amount of water is mixed with the fuel during the pre-injection and / or the main injection within the injector in such a way that the water in the form of a fuel / water / fuel stratification or Force- Material / water stratification or water / fuel stratification is introduced into the combustion chamber.

According to a further embodiment, the pressure of the fuel introduced into the combustion chamber is changed during an injection process. Here, for example, the injection pressure of the pre-injection VE can be at a lower level than the injection pressure of the main injection HE. This avoids wetting the combustion chamber walls with fuel, particularly during the pre-injection. Preferably, there is still a higher fuel pressure during the main injection than during an optional post-injection.

In order to achieve an intensive homogenization of the pre-injected fuel quantity, according to a further preferred embodiment, a fuel cloud of a fuel jet generated during an injection stroke is displaced or shifted laterally during the pre-injection by means of a swirl movement formed in the combustion chamber, so that during a subsequent injection stroke the newly injected fuel jets do not the fuel cloud of the previous injection stroke penetrate. This achieves optimal homogenization of the pre-injection quantity, which has a positive effect on the pressure increase and thus improves the center of combustion and the noise behavior. If a small pre-injection quantity is used, the better leaning out of the mixture by the swirl can influence the ignition timing of the pre-injection by means of the main injection (ignition jet).

The invention is based on a method for operating an internal combustion engine with auto-ignition, in which the fuel is injected directly into the combustion chamber as a pre-injection and main injection and, if necessary, as a post-injection by means of a fuel nozzle with a plurality of injection bores, the pre-injection preferably being clocked follows. In order to optimally design the combustion, a liquid serving as a cooling medium, for example water, is introduced into the combustion chamber during the intake and / or compression stroke, so that an increase in pressure in the combustion chamber is reduced and, if necessary, an ignition timing of the pre-injection is delayed. The liquid introduced into the combustion chamber provides cooling on the fuel side, with which the ignition timing of the pre-injection is delayed and the pressure rise is reduced, so that an optimal center of gravity of the combustion is achieved. Exhaust gas recirculation is preferably carried out in order to further reduce the exhaust gas emissions formed, in particular the formation of NOx. If the fuel quantity of the pre-injection is designed in such a way that the pre-injection quantity does not ignite due to an emaciation of the premix, then the ignition timing of the mixture and the pressure increase due to the injected liquid in the combustion chamber during ignition are influenced by means of the main injection made as an ignition jet.

Claims

claims

1. Method for operating an internal combustion engine with auto-ignition, in which

Fuel is injected directly into a combustion chamber by means of an injection device, which preferably has a multi-hole nozzle, in such a way that part of the fuel in the intake and / or compression stroke is first injected into the combustion chamber as a pre-injection, and further fuel as a main injection and optionally as a post-injection is injected into the combustion chamber at a later time, characterized in that a liquid with a high enthalpy of vaporization is introduced into the combustion chamber during the intake, compression and / or expansion stroke for cooling a mixture present in the combustion chamber, so that a pressure increase in the combustion chamber is reduced and, if necessary, an ignition timing of the pre-injection or main injection is delayed.

2. The method according to claim 1, characterized in that the liquid is introduced into the combustion chamber before or after the start of the pre-injection.

3. The method according to claim 1 or 2, characterized in that the liquid is introduced into the combustion chamber after completion of the pre-injection.

4. The method according to any one of claims 1 to 3, characterized in that the introduction of the liquid into the combustion chamber is ended before the end of the main injection of the fuel.

5. The method according to any one of claims 1 to 4, characterized in that the liquid introduced into the combustion chamber is an amount of water.

6. The method according to any one of claims 1 to 5, characterized in that the amount of water is mixed with the fuel during the pre-injection and / or the main injection within the injection device such that the water is introduced into the combustion chamber in the form of a fuel / water emulsion ,

7. The method according to any one of claims 1 to 5, characterized in that the amount of water is introduced into the combustion chamber by means of an additional injection device.

8. The method according to any one of claims 1 to 5, characterized in that the amount of water is mixed with the fuel during the pre-injection and / or the main injection within the injection device such that the water in the form of a fuel / water / fuel stratification or fuel / Water stratification or water / fuel stratification is introduced into the combustion chamber.

9. The method according to any one of the preceding claims, characterized in that the pre-injection is carried out in a compression stroke range of approximately 150 ° KW to 30 ° KW before top dead center.

10. The method according to any one of the preceding claims, characterized in that the main injection and optionally the post-injection are carried out in succession around top dead center in a range from 20 ° KW before top dead center to 40 ° KW after top dead center.

11. The method according to any one of the preceding claims, characterized in that the pressure of the fuel introduced into the combustion chamber is changed during an injection process.

12. The method according to any one of the preceding claims, characterized in that the pre-injection is carried out clocked, wherein during the pre-injection by means of a swirl movement formed in the combustion chamber, a fuel cloud of a fuel jet generated during an injection cycle is displaced or shifted laterally, so that in a subsequent injection cycle the newly injected fuel jets do not penetrate into the cloud of fuel from the previous injection stroke.