WO2020255647A1

WO2020255647A1 - Device and method for controlling a temperature of a prechamber included in an ignition device of an internal combustion engine

Info

Publication number: WO2020255647A1
Application number: PCT/JP2020/020777
Authority: WO
Inventors: Naoki YONEYA; Henning SAUERLAND; Masayuki Saruwatari
Original assignee: Hitachi Automotive Systems, Ltd.
Priority date: 2019-06-19
Filing date: 2020-05-26
Publication date: 2020-12-24
Also published as: CN113939646A; JP2022538012A; JP7225438B2; DE102019208930A1; CN113939646B

Abstract

The present subject matter relates to a control unit and a method for controlling the temperature of a fuel-fed prechamber of an ignition device included in an internal combustion engine. In order to avoid inadmissible high temperatures of the prechamber, a targeted injection is introduced into the main combustion chamber during the compression stroke and/or into the prechamber depending on the prechamber temperature. Introducing an injection into the main combustion chamber at the end of the compression stroke can reduce the temperature of the gas mixture in/around the prechamber and therefore help to control the prechamber temperature at moderate increased temperatures without negatively affect the particle emissions. Hence, a cooling injection directly into the prechamber, which may cause drawbacks regarding particle emissions, is only necessary when the prechamber temperature is strongly increased.

Description

DEVICE AND METHOD FOR CONTROLLING A TEMPERATURE OF A PRECHAMBER INCLUDED IN AN IGNITION DEVICE OF AN INTERNAL COMBUSTION ENGINE

The present subject-matter relates to a control unit and a method for controlling the temperature of a fuel-fed prechamber of an ignition device included in an internal combustion engine.

In order to improve the thermal efficiency of an internal combustion engine, it is known to use an ignition device comprising a fuel-fed prechamber, particularly in case of a gasoline or a gas engine. The high ignition energy of such an ignition device is provided by a precombustion taking place in the prechamber. This precombustion is initiated by injecting a small amount of fuel into the prechamber and igniting the resulting air-fuel mixture therein. Since the prechamber is connected to the main combustion chamber via small orifices, the combustion inside the prechamber results in multiple reactive jets, which enter from the prechamber into the main combustion chamber and ignite the air-fuel mixture therein. These reactive jets usually expand throughout the entire main combustion chamber so that a plurality of ignition spots is provided enabling a reliable ignition even of very lean air-fuel mixtures.

However, the use of a precombustion as explained above can lead to an increase of particle emissions. For example, a fuel injection into the prechamber under hot conditions of the prechamber may result in a deposition of residuals on the surface of the prechamber injector. The deposition of residuals can lead to an increase of particle emissions. Further, a fuel injected into the main combustion chamber can autoignite on the hot outer surface(s) of the prechamber and thermal damage of the engine can be caused. The temperature in the main chamber increases with increasing engine load, the temperature of the prechamber increases due to heat transfer from the main chamber to the prechamber, too. Therefore, hot conditions can be expected, e.g., during high load conditions of the engine.

In order to avoid thermal damages of the prechamber and the engine, e.g., during high load conditions, it was proposed to suppress autoignition on the hot prechamber surface and to avoid thermal damage of the prechamber by applying cooling measures. In this context it was proposed to decrease the temperature in the prechamber by an additional injection of fuel into the prechamber after the combustion has taken place. The vaporization of the fuel of the additional injection cools the inside of the prechamber.

Patent Literature 1 (JP 2018-91267 A) describes a control device for an internal combustion engine comprising a fuel-fed prechamber, wherein the temperature of the injector inside the prechamber is reduced by performing a second/additional injection into the prechamber after the end of the prechamber combustion.

However, as described above, performing an additional/second injection into the prechamber can lead to increased/too high particle emissions, especially under high load conditions of the engine.

JP 2018-91267 A

An object of the described subject matter is to realize a control unit and a method for effectively cooling the prechamber without increasing the particle emissions of a combustion engine.

The above-described object is solved by the subject matter according to the independent claims. Further preferred developments are described by the dependent claims.

The control unit for controlling an internal combustion engine may control an injection/an injection of an amount of fuel into a prechamber and/or into a main combustion chamber of the internal combustion engine depending on a temperature of the prechamber. This injection is preferably an additional injection which may take place after the fuel injection which serves the combustion. The additional injection may be called “cooling injection” in the following and, generally, the term “injection” shall refer to the cooling injection, especially if it is used without indicating anything else.

In other words, the control device may perform a control which includes to “decide” whether fuel shall be injected (for cooling) into the prechamber or into the main combustion chamber. The “decision” may be taken depending on the temperature of the prechamber. It may be also possible to perform a combined injection which injects fuel into the main combustion chamber and into the prechamber.

With regard to the terms which will be used in the following for the mode/state of the ignition device comprising the prechamber fuel injector, it is noted that a mode or an operation during which fuel is injected into the prechamber may be named “active mode” and a mode or an operation during which fuel is injected into the main combustion chamber may be named “passive mode”.

The internal combustion engine may have at least one cylinder, at least one main combustion chamber (briefly: “main chamber”), at least one intake port, at least one main fuel injector and at least one ignition device which may ignite an air-fuel-mixture inside the main combustion chamber. The ignition device may comprise a spark plug, a prechamber fuel injector and a prechamber which may be connected to the main combustion chamber via at least one orifice in a prechamber wall.

If the temperature of the prechamber is higher than a predetermined temperature, the control unit may control the main fuel injector to perform an injection (cooling injection) into the main combustion chamber during a compression stroke of the internal combustion engine. Preferably, said predetermined temperature may have a value in the range of 100°C to 250°C. The predetermined temperature or the value/range of values thereof may be set depending on the location in/on the prechamber at which the temperature is determined. Even more preferably the range may be set to be between 150°C and 200°C.

In the above described case, the ignition device is operated in the passive mode so that (substantially) no fuel is (directly) injected into the inside of the prechamber during the cooling injection.

As explained above, an additional/cooling injection of fuel into the prechamber can lead to undesirably high particle emissions during high load conditions of the engine. This is avoided by the present subject-matter which uses an additional fuel injection into the main combustion chamber (briefly: “main chamber”) for cooling the prechamber. Specifically, if the cooling injection into the main chamber is performed while the piston is moving upward, at least a fraction of the injected fuel may flow into the prechamber and vaporize therein while another (larger) fraction of the fuel may vaporize in the main combustion chamber. The vaporization of the fuel cools the prechamber. Preferably, the start of the cooling injection into the main combustion chamber may be initiated at the end of the compression stroke (which may be named “late injection”), when the temperature of the cylinder is increased because of a preceding compression. This may lead to a fast fuel vaporization which can effectively lower the mixture temperature inside the prechamber as well as around the prechamber. Preferably, the cooling injection is finished before ignition timing because, otherwise, the injected fuel may be prevented from entering into the prechamber due to the increasing pressure therein.

The above described flow/transport of the injected fuel from the main chamber into the prechamber can be enhanced if the cooling injection is initiated by the control unit when the pressure in the prechamber is lower than the pressure in the main combustion chamber. Said pressure difference between the main chamber and the prechamber may support the transport of small fuel droplets and cooled air-fuel mixture into the prechamber.

To determine the pressure in the main combustion chamber and in the prechamber, pressure sensors may be provided in both chambers. With regard to the prechamber, as described above, a measuring spark plug may be used which may include a pressure sensor next to/close to/in the vicinity of the central electrode. It may be also possible to install a small stand-alone pressure sensor inside the prechamber. For measuring the pressure in the main combustion chamber a commonly known pressure sensor for permanent use can be used.

Alternatively, it may be also possible to determine the optimal crank angle region, in which the cooling injection should be performed, during an engine testing phase. This testing phase can include all relevant operating points and the optimal crank angle values determined thereby may be stored in the control unit as characteristic curves or maps.

Furthermore, the control unit may adjust the fuel amount to be injected into the main combustion chamber for cooling depending on the temperature of the prechamber. If a multiple cooling injection should be performed, which is preferably possible within the framework of the present subject-matter, the amount of fuel to be injected during the cooling injection may be adapted to the reaction of the prechamber temperature on a first cooling injection into the main chamber. For example, the amount of fuel injected during a second cooling injection may be decreased in case the prechamber temperature goes down after the first cooling injection or it may be increased in case the prechamber temperature further goes up or stays constant after first cooling injection.

Specifically, for example, if the prechamber temperature does not decrease after the first (late) cooling injection including, e.g., 1% of the entire fuel amount injected into the main combustion chamber (the entire amount may include the fuel amount which is injected for the purpose of the combustion and the fuel amount of cooling injection(s)), the respective fuel amount may be increased to 1.5% or 2% of the entire fuel amount. If the prechamber temperature still remained constant, the injected fuel amount may be further increased until a limit for the late cooling fuel injection is achieved which may be dependent on hydrocarbon and/or particle emissions. The maximum fuel amount injected into the main combustion chamber for cooling during the compression stroke may preferably be in the range of 4% to 10% of the entire fuel amount and most preferably in the range of 5% to 7% thereof. Further for example, in case the prechamber temperature goes down but does not fall below a predetermined temperature (threshold), the amount of fuel injected for cooling the prechamber may be reduced depending on the temperature gradient. Or, if the prechamber temperature rapidly decreases, the injected fuel amount may be decreased by a high value, such as 1% of the entire fuel mass, whereas, if the prechamber temperature slowly decreases, the injected fuel amount may remain constant or may be decreased only by a small value, such as 0.3% of the entire fuel amount.

Moreover, the control unit may control the prechamber fuel injector to perform a cooling injection into the prechamber depending on the temperature of the prechamber. For example, the cooling injection into the prechamber may be performed if the cooling injection(s) into the main combustion chamber do(es) not lead to a decrease in temperature of the prechamber and/or when the prechamber temperature exceeds another/second temperature threshold at which thermal damages could be caused. Under such (emergency/exceptional) circumstances the ignition device may be switched to the active mode and a small amount of fuel may be injected into the prechamber regardless of a possible increase of the particle emissions in order to avoid a thermal damage of the prechamber or even an engine damage because of uncontrolled autoignition provoked by the hot surface of the prechamber. The amount of fuel injected into the prechamber may preferably be in a range of 1% to 5% of the entire fuel amount and most preferably in a range of 2% to 4%. The second temperature threshold to initiate an injection into the prechamber (active mode) may preferably be higher than the first threshold and it may be in the range of 200°C to 500°C depending on the location in/on the prechamber at which the temperature is determined. Most preferably it may be in the range of 300°C to 400°C.

By starting the active mode, the (late) cooling injection into the main combustion chamber may be fully stopped or a continuous transition from passive to active mode may be performed. In the latter case, the amount of fuel injected into the main combustion chamber at the end of the combustion stroke may be continuously or stepwise decreased wherein the amount of fuel injected into the prechamber may be continuously or stepwise increased at the same time. It may also be possible to combine active and passive mode by injecting a first amount of fuel into the main combustion chamber and a second amount directly into the prechamber.

Furthermore, the control unit may determine the temperature of the prechamber based on a measured temperature and/or based on characteristic parameters stored in the control unit as characteristic curves or maps.

Preferably, the prechamber temperature may be measured by using at least one temperature sensor which is permanently applicated inside the prechamber or on the prechamber wall. For example, a temperature sensor installed inside the prechamber can measure the gas temperature therein. Alternatively or in addition, the temperature of the prechamber wall can serve as reference for the prechamber temperature. In this case, the temperature sensor may be applied, for example, on the lower surface of the prechamber wall facing the main combustion chamber.

Alternatively or in addition, the pressure inside the prechamber may be measured by a pressor sensor, e.g., integrated into the spark plug. Measuring the pressure rise in the prechamber may allow for estimating the heat release therein and thus providing an estimated gas temperature inside the prechamber and a good correlation to the temperature of the prechamber wall.

Moreover, the temperature sensor or the pressure sensor may only be provided during a testing phase in which engine operations are carried out at all relevant boundary conditions. During the testing phase, the gas temperature in the prechamber and/or the prechamber wall temperature and/or the pressure inside the prechamber can be measured, for example, at different engine load and speed, different spark timing, different air-fuel ratio, different intake air temperature and pressure, different EGR rates, different valve timings and so on. The characteristic prechamber temperature determined in the testing phase may be stored in the control unit as characteristic curves or maps depending on parameters which are continuously measured or calculated by the control unit

Furthermore, the characteristic prechamber temperature may be determined by executing a CFD simulation taking all possible boundary conditions at the relevant engine operation points into account. The temperature values determined by the simulation may also be stored in the control unit as characteristic curves or maps.

Further, the subject matter may include a method for controlling an internal combustion engine having at least one cylinder, at least one main combustion chamber, at least one intake port, at least one main fuel injector, at least one ignition device for igniting an air-fuel-mixture inside the main combustion chamber and at least one control unit, wherein the ignition device may comprise a spark plug, a prechamber fuel injector and a prechamber, connected to the main combustion chamber via at least one orifice in a prechamber wall, and wherein a fuel amount to be injected (for cooling) into the prechamber and/or into the main combustion chamber may be controlled by the control unit depending on a temperature of the prechamber.

According to the described subject matter, if the temperature of the prechamber is higher than a (first) predetermined temperature, a cooling injection into the main combustion chamber may be performed by the main fuel injector during a compression stroke of the internal combustion engine in order to transport at least a part of the injected fuel into the prechamber and to cool the latter by the latent heat of said fuel.

Preferably, the cooling injection into the main combustion chamber during the compression stroke may be performed by the main fuel injector, when the pressure in the prechamber is lower than the pressure in the main combustion chamber to support the fuel transport into the prechamber.

Furthermore, the fuel amount of the cooling injection into the main combustion chamber during the compression stroke may be adjusted by the main fuel injector depending on the temperature of the prechamber. This allows for adapting the fuel amount to be injected according to the development of the prechamber temperature after performing a first cooling injection into the main combustion chamber.

In case that further increasing the fuel amount of the cooling injection introduced into the main combustion chamber does not lead to a decrease of the prechamber temperature or in case a second temperature threshold of the prechamber temperature is exceeded, an injection into the prechamber may be performed by the prechamber fuel injector depending on the temperature of the prechamber.

Further, the subject matter may include an internal combustion engine having at least one cylinder, at least one main combustion chamber, at least one intake port, at least one main fuel injector and at least one ignition device which may ignite an air-fuel-mixture inside the main combustion chamber, wherein the ignition device may comprise a spark plug, a prechamber fuel injector and a prechamber, connected to the main combustion chamber via at least one orifice in a prechamber wall, and the internal combustion engine may comprise the control unit as described above.

Further, the subject matter may include a computer program product storable in a memory comprising instructions which, when carried out by a computer or a computing unit, cause the computer to perform the above described method or aspects thereof, as well as a computer-readable [storage] medium comprising instructions which, when executed by a computer, cause the computer to carry out said method or aspects thereof.

Summarizing, the control unit and the method for controlling the temperature of a fuel-fed prechamber of an ignition device included in an internal combustion engine provide effective cooling of the prechamber at high engine load without increasing the particle emissions. This is achieved by performing a targeted cooling injection of fuel into the main combustion chamber and/or into the prechamber depending on the prechamber temperature.

In the following the subject matter will be further explained based on at least one preferential example with reference to the attached exemplary and schematic drawings, wherein:

Figure 1 depicts a schematic view of a cylinder of an internal combustion engine comprising an ignition device having a fuel-fed prechamber; Figure 2 depicts a schematic view of the ignition device; Figure 3 (Figures 3a － 3c) illustrates schematically different fuel injection modes depending on the prechamber temperature; Figure 4 (Figure 4a － 4c) shows schematically the amount of injected fuel during the engine cycle depending on the prechamber temperature; Figure 5 illustrates exemplary pressure curves determined in the prechamber and the main chamber as well as the relating pressure difference; Figure 6 depicts a flow chart as an example for the claimed control method.

Figure 1 shows schematically an exemplary cylinder 100 of an otherwise unspecified internal combustion engine which may have more than one cylinder 100. The engine may, for example, have two, three, four, six, eight or less/more cylinders 100. The engine comprises at least one piston 2 driven via a connecting rod 3 by a crankshaft (not depicted) for repeated reciprocal movement in the cylinder 100 to define the main combustion chamber therein.

An (air) intake port 4 with an intake valve 6 as well as an exhaust port 5 with an exhaust valve 7 are connected to the main combustion chamber 1. Ambient air is drawn into the main combustion chamber 1 through the intake port 4. Exhaust gases are discharged from the combustion chamber 1 via the exhaust port 5. An ignition device 10 comprising a spark plug 10a, a prechamber fuel injector 10b and a prechamber 10c is attached to the internal combustion engine. A temperature sensor (not depicted) may be disposed in/on the prechamber (10c).

The spark plug 10a of the ignition device 10 may be electrically connected to an ignition coil (not depicted). The spark plug 10a in combination with the ignition coil form the spark ignition device which preferably offers a variable spark duration or multi-spark ignition. The internal combustion engine may have one or more ignition device 10. Preferably, it has at least one ignition device(s) 10 per cylinder 100. The ignition device 10, or at least parts thereof, is connected to the inside of the main combustion chamber 1 so that reactive jets (depicted in dotted lines) can be introduced into the main combustion chamber 1. Furthermore, a direct fuel injector 8, or at least parts thereof, is joined to the inside of the main combustion chamber 1 which allows to inject fuel therein. The direct fuel injector 8 may preferably be an electrohydraulic fuel injector or a piezoelectric fuel injector. Additionally, a port fuel injector 9 is connected to the intake port 4 of the cylinder 100. The high-pressure fuel supply of the direct fuel injector 8 and the high- or low-pressure fuel supply of the port fuel injector 9 are not depicted. The main fuel injection may be either performed by the direct main fuel injector 8 or the port main fuel injector 9 or may be divided between both injectors.

A control unit 11 for controlling the ignition device is further shown in Figure 1. The control unit 11 is electrically connected to the ignition device 10, the direct main fuel injector 8 and/or the port main fuel injector 9 and controls the multiple units/injectors/actuators. The control unit 11 may, for example, be the engine control unit (ECU).

The control unit 11 may also be any other control unit, and signal line connections between the control unit 11 and the controlled units may differ from the example of Figure 1. For example, there may be a plurality of control units 11 which may control subgroups of the controlled units, e.g. one control unit 11-1 may control only the ignition device 10, another control unit 11-2 may control only fuel injectors 8, 9 and so on. Even further, if there is a plurality of control units 11, these control units 11 may be interconnected with each other hierarchically or in another way. Alternatively, there may be one single control unit 11 which includes all the control functions of the multiple actuators.

Furthermore, at least one temperature sensor, which is not shown, may be arranged inside the prechamber 10c or on the prechamber wall 10d of the ignition device 10 (see Fig. 2). This temperature sensor allows for monitoring a characteristic prechamber temperature in order to timely initiate prechamber cooling measures to prevent engine damage.

Further, pressure sensors which are not shown may be disposed, e.g., in the wall of the main combustion chamber 1 and/or in the prechamber 10c of the ignition device 10. Measuring the pressure within the main combustion chamber 1 and/or the prechamber 10c allows for realizing a feedback combustion control and may also improve the prechamber cooling measures by providing additional information regarding the conditions in both chambers.

In Figure 2 a schematic view of the ignition device 10 is depicted. The ignition device 10 comprises a fuel injector 10a, a spark plug 10b and a prechamber 10c. The prechamber 10c is divided off the main combustion chamber 1 by a prechamber wall 10d in which orifices 10e are disposed to introduce the reactive jets generated by the prechamber combustion into the main combustion chamber 1.

Furthermore, at least one temperature sensor which is not shown may be arranged inside the prechamber 10c and/or on the prechamber wall 10d. A temperature sensor disposed in the prechamber in a way that allows for measuring the gas temperature therein may firstly enable extrapolating the component temperature of the prechamber therefrom and may secondly be useful to derive information about the quality of the prechamber combustion. Alternatively or in addition, a temperature sensor attached on the prechamber wall 10d can deliver the relevant temperature. In this case, the temperature sensor may be applied, for example, on the lower surface of the prechamber wall 10d, close to the orifices 10e, since at this location the hottest temperature is expected. This would allow to get early information about a temperature increase of the prechamber wall. It may equally be possible to dispose a temperature sensor on the prechamber wall which is located distant from the tip of the prechamber 10c, in order to protect the sensor against damage due to excessive temperature.

Alternatively or in addition, it may also be possible to dispose at least one temperature sensor on the spark plug 10b, for example on the center electrode and/or on the ground electrode of the spark plug. Alternatively or in addition, at least one temperature sensor may be arranged on the tip of the prechamber fuel injector 10a. A temperature sensor at this position could particularly provide information about increased particle emissions to be expected due to a prechamber injection.

The shape of the prechamber 10c is not limited to the shape shown in Figure 2 but can be designed in many different shapes, such as hemispherical, conical or cylindrical shapes or combinations thereof. Further, the number, the geometry and the position of the orifices 10e in the prechamber wall 10d are not limited to the example shown in Figure 2. The prechamber 10c may comprise a plurality of orifices 10e which are disposed at different positions in the prechamber wall 10d and provided with different diameters. The prechamber injector 10a may be connected to the high-pressure fuel supply or the low-pressure fuel supply of the engine (not depicted) or may be connected to a separate fuel supply (not depicted) in order to inject a different fuel as injected into the main combustion chamber 1. The spark plug 10b may be electrically connected to an ignition coil (not depicted) which may be included into the ignition device 10 or located at another place of the engine remote to the ignition device 10. Preferably, there may be one ignition coil for each ignition device 10 but a single ignition coil for multiple ignition devices 10 may also be possible.

Figures 3a － 3c illustrate schematically different fuel injection modes depending on the prechamber temperature T_PC. Fig. 3a shows the case in which the prechamber temperature T_PC remains below a (first) temperature threshold T₁ so that no prechamber cooling is necessary and only one main injection (serving the combustion) into the main combustion chamber 1 during the intake stroke is performed. Alternatively or in addition, the main injection serving the combustion or at least a part of the main injection may be introduced into the intake port 4. In Fig. 3b a situation is shown, when the prechamber temperature T_PC is between the first and the second predetermined temperatures (thresholds) T₁ and T₂. In this case, a cooling injection into the main chamber 1 is carried out at the end of the compression stroke or, more precisely, at a defined crank angle before firing top dead center FTDC. The appropriate injection timing is related to the ignition timing which will be explained in connection with Fig. 5. Furthermore, Fig. 3c depicts a case in which the prechamber temperature T_PC exceeds the threshold T₂ so that a cooling injection into the prechamber 10c is performed shortly before firing top dead center FTDC in addition to the main chamber injection (serving the combustion) during the intake stroke.

Figure 4a － 4c schematically show the different amounts of injected fuel during the engine cycle depending on the prechamber temperature T_PC. In case the prechamber temperature T_PC remains below a first temperature threshold T₁, the entire fuel amount is injected into the main combustion chamber 1 during the intake stroke serving the combustion. Alternatively or in addition, the entire fuel amount serving the combustion or at least a portion thereof may be injected into the intake port 4. If the prechamber temperature T_PC rises above the threshold T₁, a part of the entire fuel amount introduced into the main chamber 1 or the intake port 4 is injected during the compression stroke for achieving a cooling effect. As schematically depicted in Fig. 4b, said fuel amount for cooling increases with increasing prechamber temperature T_PC. Besides cooling the prechamber 10c, at least a portion of the fuel injected late in the compression stroke also contributes to the heat release in the main chamber 1. Therefore, as schematically depicted in Fig. 4a, the amount of fuel to be injected during the intake stroke can be reduced when increasing the fuel amount during the compression stroke. As schematically illustrated in Fig. 4c, in case the prechamber temperature T_PC exceeds the second threshold T₂, a cooling injection into the prechamber 10c is performed. The dotted lines depicted in Fig. 4c illustrate the amount of fuel needed to be injected into the prechamber 10c if no cooling injection in the main combustion chamber 1 during the compression stroke was performed.

In Figure 5 exemplary pressure curves measured in the prechamber 10c and the main chamber 1 as well as the resulting pressure difference are depicted. It can be derived from Figure 5, that the late cooling injection introduced in the main combustion chamber 1 should be performed before the combustion inside the prechamber 10c has started. Otherwise the injected fuel was prevented from entering into the prechamber 10c due to the increasing pressure therein. Figure 5 shows that the period for a late injection into the prechamber starts at a certain angle before firing top dead center FTDC and ends when the pressure difference between the main chamber pressure p_cyl and the prechamber pressure p_prechamber drops. This may be the case when the combustion in the prechamber 10c starts after ignition has been performed by the spark plug 10b. Therefore, it may be also possible to determine the latest possible injection timing in relation to the ignition timing. Further, in order to effectively cool the prechamber 10c, the cooling injection into the main chamber 1 should be started as late as possible. Hence, the start of the cooling injection may be determined depending on the latest possible end of said injection and the amount of fuel to be injected.

To determine the pressure in the main chamber 1 and the prechamber 10c, pressure sensors may be applicated in both chambers. Regarding the prechamber 10c, a measuring spark plug may be used which includes a pressure sensor next to the central electrode. It may be also possible to use a small pressure sensor applicable inside the prechamber 10c. For measuring the pressure in the main combustion chamber 1 a commonly known pressure sensor for permanent use can be applied.

Alternatively, it may be also possible to determine the ideal period for a late cooling injection into the main chamber 1 within an engine testing phase. In this testing phase all relevant operating points may be taken into account and the determined optimal crank angle ranges for a late injection into the main combustion chamber 1 may be stored in the control unit 11 as characteristic curves or maps.

In Figure 6 a flow chart showing an example for the claimed control method is depicted. As long as the prechamber temperature T_PC stays in an allowable range, the fuel mass to be injected into the prechamber m_{f_PC} as well as the fuel mass to be injected into the main combustion chamber during the compression stroke m_{f_MC_CS} are set to zero (S100, S101) and the temperature thresholds T₁ and T₂ are checked regularly every time step Δt. The time step Δt for monitoring the prechamber temperature T_PC may preferably be in the range of 10 ms to 1000 ms and most preferably in the range of 50 ms to 500 ms. In case the prechamber temperature T_PC exceeds the second threshold temperature T₂, the prechamber fuel mass m_{f_PC} is increased by the predefined value Δm_{f_PC} per time step Δt until it reaches a predefined limit m_{F_PCmax} (S102)_.The predefined value Δm_{f_PC}may preferably be in the range of 0.1% to 1% of the entire fuel mass and most preferably in the range of 0.3% to 0.7% of the entire fuel mass. Further, the predefined limit m_{F_PCmax} may be 5% of the entire fuel mass and most preferably 4% of the entire fuel mass. The second threshold temperature T₂ may preferably be in the range of 200°C to 400°C depending on the location in/on the prechamber on which the temperature is determined and most preferably in the range of 250°C to 350°C.

If the prechamber fuel mass m_{f_PC}exceeds the limit m_{F_PCmax} and the prechamber temperature T_PC still remains above the threshold T₂, the engine power P is decreased by a predefined value ΔP (S103), which may preferably be in the in the range of 1 kW to 10 kW and most preferably in the range of 2kW to 5kW. As long as the engine power P does not fall below a predefined threshold P_min, the power reduction is performed after every time step Δt. In case the engine power falls below the threshold P_min, an error is detected (S104), which may, e.g., result in operating the engine in limp home mode or stopping it completely. In this context, the engine power limit P_min may preferably be in the range of 85% to 98% of the rated power and most preferably in the range of 90% to 95% of the rated power.

In case the prechamber temperature T_PC does not exceed the temperature threshold T₂ but exceeds the temperature threshold T₁, the fuel mass to be injected for cooling into the main combustion chamber during the compression stroke m_{f_MC_CS} is increased by the value Δm_{f_MC_CS}every time step Δt until it reaches a predefined limit m_{F_MC_CSmax} (S105). The predefined value Δm_{f_MC_CS}may preferably be in the range of 1 to 20% of the entire fuel mass and most preferably in the range of 10% to 15% of the entire fuel mass. Further, the predefined limit m_{F_MC_CSmax} may be in the range of 10% to 50% of the entire fuel mass and most preferably in the range of 20% to 40% thereof.

If said limit m_{F_MC_CSmax} is exceeded the late cooling injection in the main combustion chamber 1 is stopped by setting m_{f_MC_CS} to zero (S106) and the prechamber cooling injection is switched on by increasing the amount of fuel injected into the prechamber m_{f_PC} by the value Δm_{f_PC} (S102)_.In the following steps the same procedure as described for the condition T_PC > T₂ is executed. In this context, instead of setting m_{f_MC_CS} to zero, it may also be possible to perform a continuous transition from the late main chamber injection to the prechamber injection by continuously or stepwise decreasing the amount of fuel injected into the main combustion chamber 1 and increasing the amount of fuel injected into the prechamber 10c in parallel. Another possibility may be a combination of both cooling injections by introducing a first amount of fuel into the main combustion chamber 1 and a second amount of fuel directly into the prechamber 10c.

In general, all features of the different embodiments, aspects and examples, which are described herein, and which are shown by the Figures, may be combined either in part or in whole. The herein described subject-matter shall also entail these combinations as far as it is apparent to the person skilled in the art without applying inventive activity.

It should also be noted that the description and drawings merely illustrate the principles of the proposed methods, devices and systems. It will thus be appreciated that those skilled in the art will be able to devise various arrangements that, although not explicitly described or shown herein, embody the principles of the claimed subject matter and are included within its spirit and scope.

Again summarizing, the present subject-matter avoids inadmissible high temperatures of the prechamber by introducing a targeted cooling injection into the main combustion chamber during the compression stroke and/or into the prechamber depending on the prechamber temperature. Introducing a cooling injection into the main combustion chamber at the end of the compression stroke helps to control the prechamber temperature without increasing the particle emissions. The upward movement of the piston transports at least a part of the injected fuel into the prechamber where it can vaporize and cool the latter. Hence, a cooling injection directly into the prechamber, which may cause increasing particle emissions, can be avoided by the present subject-matter for an extended operational range of the engine and is only performed when the temperature of the prechamber becomes too high.

1 ... main combustion chamber
2 ... piston
3 ... connecting rod
4 ... intake port
5 ... exhaust port
6 ... intake valve
7 ... exhaust valve
8 ... direct main fuel injector
9 ... port main fuel injector
10 ... ignition device
10a ... spark plug
10b ... prechamber fuel injector
10c ... prechamber
10d ... prechamber wall
10e ... orifice
11 ... control unit
100 ... cylinder

Claims

Control unit (11) for controlling an internal combustion engine having at least one cylinder (100), at least one main combustion chamber (1), at least one intake port (4), at least one main fuel injector (8, 9) and at least one ignition device (10) configured to ignite an air-fuel-mixture inside the main combustion chamber (1),
wherein the ignition device (10) comprises a spark plug (10a), a prechamber fuel injector (10b) and a prechamber (10c) connected to the main combustion chamber (1) via at least one orifice (10e) in a prechamber wall (10d), and
wherein the control unit (11) is configured to control an injection of fuel into the prechamber (10c) and/or into the main combustion chamber (1) depending on a temperature of the prechamber (10c).
Control unit (11) according to claim 1, wherein,
when the temperature of the prechamber (10c) is higher than a predetermined temperature,
the control unit (11) is configured to control the main fuel injector (8) to perform an injection into the main combustion chamber (1) during a compression stroke of the internal combustion engine.
Control unit (11) according to at least one of the preceding claims 1 to 2, wherein
the control unit (11) is configured to control the main fuel injector (8) to perform an injection into the main combustion chamber (1), when the pressure in the prechamber (10d) is lower than the pressure in the main combustion chamber (1).
Control unit (11) according to at least one of the preceding claims 1 to 3, wherein
the control unit (11) is configured to adjust the fuel amount of the injection into the main combustion chamber (1) depending on the temperature of the prechamber (10d).
Control unit (11) according to at least one of the preceding claims 1 to 4, wherein,
the control unit (11) is configured to control the prechamber fuel injector (10b) to perform an injection into the prechamber (10c) depending on the temperature of the prechamber (10c).
Control unit (11) according to at least one of the preceding claims 1 to 5, configured to determine the temperature of the prechamber (10c) based on a measured temperature and/or based on characteristic parameters stored in the control unit as characteristic curves or maps.
Method for controlling an internal combustion engine having at least one cylinder (100), at least one main combustion chamber (1), at least one intake port (4), at least one main fuel injector (8, 9), at least one ignition device (10) configured to ignite an air-fuel-mixture inside the main combustion chamber (1) and at least one control unit (11),
wherein the ignition device (10) comprises a spark plug (10a), a prechamber fuel injector (10b) and a prechamber (10c) connected to the main combustion chamber (1) via at least one orifice (10e) in a prechamber wall (10d), and
wherein a fuel injection into the prechamber (10c) and/or into the main combustion chamber (1) is controlled by the control unit (11) depending on a temperature of the prechamber (10c).
Method according to claim 7, wherein,
when the temperature of the prechamber (10c) is higher than a predetermined temperature,
an injection into the main combustion chamber (1) is performed by the main fuel injector (8) during a compression stroke of the internal combustion engine.
Method according to at least one of the preceding claims 7 to 8, wherein
an injection into the main combustion chamber (1) is performed by the main fuel injector (8), when the pressure in the prechamber (10d) is lower than the pressure in the main combustion chamber (1).
Method according to at least one of the preceding claims 7 to 9, wherein
the fuel amount of the injection into the main combustion chamber (1) is adjusted by the control unit (11) depending on the temperature of the prechamber (10c).
Method according to at least one of the preceding claims 7 to 10, wherein,
an injection into the prechamber (10c) is performed by the prechamber fuel injector (10b) depending on the temperature of the prechamber (10c).
Internal combustion engine including the at least one control unit of at least one of the preceding claims 1 to 6.
A computer program product storable in a memory comprising instructions which, when carried out by a computer, cause the computer to perform a method according to at least one of the method claims 7 to 11.