Method for operating an internal combustion engine with gas as fuel and internal combustion engine for carrying out such a method

Abstract

Method for operating an internal combustion engine (1) with at least one cylinder (2) and at least one intake line (3) for supplying the at least one cylinder (2) with fresh air or fresh mixture, in which the gas serving as fuel for the internal combustion engine (1) is introduced into the at least one intake line (3) either by means of intake manifold injection (5) or by means of direct injection (6) into the at least one cylinder (2), wherein the intake manifold injection (5) is used at low and medium loads T and the direct injection (6) is used at high loads T, so that starting from the intake manifold injection (5) at low loads, a change is made to the direct injection (6) if the load T orque a predeterminable load T orque,up exceeds with T orque ≥ T orque,up .

Description

[0001] The invention relates to a method for operating an internal combustion engine having at least one cylinder and at least one intake line for supplying the at least one cylinder with fresh air or fresh mixture, wherein gas is used as fuel.

[0002] Furthermore, the invention relates to an internal combustion engine for carrying out such a method.

[0003] In the context of the present invention, the term internal combustion engine includes in particular spark-ignition gasoline engines.

[0004] The German patent application DE 10 2009 013 589 A1 describes an arrangement and a method for controlling an engine with two different fuel nozzles.

[0005] It is proposed to use intake manifold injection and direct injection simultaneously, i.e., at the same engine map point / load point and across the entire engine map. Furthermore, according to the teaching of DE 10 2009 013 589 A1, fuel can be introduced into a cylinder during a combustion cycle using either intake manifold injection or direct injection. The amount of fuel introduced into the cylinder is distributed between the two different injection systems, namely intake manifold injection and direct injection.

[0006] For this purpose, each cylinder of the internal combustion engine is equipped with intake manifold injection and direct injection.

[0007] The amount of fuel for port injection is determined based on driver needs, engine speed, and engine temperature to achieve a specific engine torque. The engine speed and load can be used to determine when port injection is desirable.

[0008] The German patent application DE 10 2008 001 111 A1 relates to a method and a device for operating an internal combustion engine in which each cylinder is equipped with at least two injection systems, for example an intake manifold injection and a direct injection.

[0009] However, the at least two injection systems can both involve intake manifold injection or direct injection.

[0010] German patent application DE 198 53 799 A1 describes a method for mixture formation in a spark-ignition internal combustion engine with fuel injection. The amount of fuel to be injected into a cylinder is distributed between two different injection systems, namely intake manifold injection and direct injection, with the distribution being regulated or controlled depending on the load.

[0011] German patent application DE 10 2006 007 279 A1 relates to a compression-ignition internal combustion engine and a method for operating this internal combustion engine. Fuel is injected multiple times per combustion cycle, at least once directly into the cylinder and, if necessary, into the intake air stream.

[0012] Due to the limited resources of fossil fuels, in particular the limited deposits of mineral oil as a raw material for the production of fuels for the operation of internal combustion engines, alternative fuels are increasingly being used to operate internal combustion engines.

[0013] The market share of combustion engines using alternative fuels is steadily increasing. This trend is being reinforced and supported by legislation, particularly through tax incentives and statutory regulations.

[0014] For example, in addition to gasoline as the traditional gasoline fuel, gasoline engines also use liquefied petroleum gas (LPG), a propane / butane mixture, also known as autogas (LPG), compressed natural gas (CNG), primarily methane, or hydrogen (H2), ethanol, or fuel blends of gasoline and ethanol. Within the scope of the present invention, a gas is used as fuel for the internal combustion engine.

[0015] Since different fuels have different physical and chemical properties, the internal combustion engine must be designed for the specific fuel used.

[0016] This requires adjusting the internal combustion engine's operating parameters, such as ignition timing and injection timing. The timing, boost pressure, coolant temperature, injection duration, charge air quantity, and / or similar parameters, as well as design parameters such as the compression ratio, can and often are designed for operation with a specific fuel.

[0017] The fuel-specific design of the compression ratio, for example, takes into account the fact that different fuel types have different knock resistances, which are indicated by the octane numbers RON and MON, respectively. This means that a specific compression ratio that allows trouble-free operation of the internal combustion engine with a specific fuel can, with another fuel, lead to knocking or self-ignition before the actual ignition point, which frequently results in knocking combustion. Since knocking and self-ignition must be prevented to avoid damage to the internal combustion engine, the fuel used and its knock resistance must be taken into account when designing the internal combustion engine.

[0018] The fuel used also influences the design of the fuel supply system of the internal combustion engine, in particular the injection system, which serves as an auxiliary unit of the internal combustion engine for the introduction of the fuel.

[0019] There are basically two concepts for fuel injection: intake manifold injection and direct injection.

[0020] With intake manifold injection, the gasoline engine operates with a homogeneous fuel-air mixture, which is prepared by external mixture formation, in which fuel is introduced into the intake air in at least one intake manifold. The desired power is adjusted by changing the cylinder's fuel quantity, so that the operating principle of the gasoline engine—unlike the diesel engine—is based on quantity control.

[0021] Load control is usually achieved by means of a throttle valve in the intake manifold. By adjusting the throttle valve, the pressure of the intake air downstream of the throttle valve can be reduced more or less. The further the throttle valve is closed, i.e. the more the intake manifold is blocked, the greater the pressure loss of the intake air across the throttle valve and the lower the pressure of the intake air behind the throttle valve and before entering the cylinder. With a constant combustion chamber volume, the air mass, i.e. the quantity, can be adjusted in this way via the pressure of the intake air. This type of load control proves to be particularly disadvantageous in the part-load range because low loads require severe throttling and a significant reduction in the pressure of the intake air.

[0022] In order to reduce the described throttling losses, i.e. charge exchange losses, various concepts have been developed.

[0023] One approach to de-throttling the gasoline engine's operating process is based on adopting technical features of the traditional diesel engine process, which is characterized by air compression, an inhomogeneous mixture, auto-ignition, and quality control, in which the load is controlled via the amount of fuel injected directly into at least one cylinder. Throttling of the intake air is eliminated by design.

[0024] The injection of fuel directly into the cylinder's combustion chamber is considered a suitable measure for significantly reducing fuel consumption, even in gasoline engines. Efficiency improvements can be achieved with stratified mixture formation through dethrottling, so that quality control is used within certain limits. Furthermore, by utilizing the evaporation enthalpy of the fuel evaporating directly in the combustion chamber, an internal cylinder cooling effect is created, which enables a further increase in efficiency by increasing the compression ratio. This effect also leads to a reduction in fuel consumption with homogeneous direct injection. Homogeneous direct injection, like intake manifold injection, operates with a homogeneous fuel-air mixture.

[0025] The disadvantage of direct injection is that comparatively little time is available for the injection of the fuel, the mixture preparation in the combustion chamber, namely the preparation of the fuel, possibly by evaporation, and the mixing of air and fuel, as well as the ignition of the prepared mixture.

[0026] Direct-injection gasoline engines are therefore significantly more sensitive to changes and deviations in mixture formation, especially during injection, and during ignition than conventional gasoline engines. The inhomogeneity of the mixture and its cyclical variability make reliable and defined ignition of the fuel-air mixture difficult.

[0027] As can be seen from the above, both injection concepts, intake manifold injection and direct injection, have both advantages and disadvantages.

[0028] As with the use of gasoline as the traditional gasoline fuel, when using gas as an alternative fuel, it must be considered on a case-by-case basis and decided whether to use port fuel injection or direct fuel injection.

[0029] Against this background, it is an object of the present invention to provide a method for operating an internal combustion engine having at least one cylinder and at least one intake line for supplying the at least one cylinder with fresh air or fresh mixture, in which method gas is used as fuel for the internal combustion engine and with which the most advantageous operation of the internal combustion engine can be realized at all operating points.

[0030] A further sub-object of the present invention is to provide an internal combustion engine for carrying out such a method.

[0031] The first sub-task is achieved by a method for operating an internal combustion engine with at least one cylinder and at least one intake line for supplying the at least one cylinder with fresh air or fresh mixture, in which the gas serving as fuel for the internal combustion engine is introduced into the at least one intake line either by means of intake manifold injection or by means of direct injection into the at least one cylinder, wherein the intake manifold injection is used at low and medium loads T and the direct injection at high loads T, so that starting from the intake manifold injection at low loads, a change is made to the direct injection if the load T orque a predeterminable load T orque,up exceeds with T orque ≥ T orque,up .

[0032] The method according to the invention uses both injection methods, ie both intake manifold injection and direct injection, and thus creates the possibility of using the injection method suitable for this operating point at each operating point or of selecting and applying the injection method which is more advantageous according to a current objective.

[0033] The gas serving as fuel can be introduced into the intake manifold, for example, by means of intake manifold injection in order to achieve favorable fuel consumption values ​​during partial load operation. With regard to thorough mixing, i.e., homogenization, of the fuel-air mixture, intake manifold injection offers advantages over direct injection when small amounts of fuel, i.e., gas, are to be introduced. Further advantages arise, in particular, when the gas is introduced into the intake manifold in the gaseous phase. The latter, along with further process variants, will be described in connection with the preferred embodiments.

[0034] Direct injection of the gas into at least one cylinder, on the other hand, is effective in increasing or achieving maximum performance.

[0035] In practice, a change in injection method is usually achieved by using different maps for intake manifold injection on the one hand and direct injection on the other. This change can involve, in particular, a change in the ignition timing or injection start, but also an adjustment of the injection duration.

[0036] The method according to the invention solves the first sub-problem underlying the invention, namely to demonstrate a method with which - using gas as fuel for the internal combustion engine - the most advantageous operation possible can be realized at all operating points of the internal combustion engine.

[0037] Further advantageous variants of the method according to the invention are discussed in connection with the subclaims.

[0038] Embodiments of the method are advantageous in which, starting from the intake manifold injection at low loads, the system switches to the direct injection only when the load T orque a predeterminable load T orque,up and for a predefined period of time Δt up is greater than this specified load T orque,up .

[0039] The introduction of an additional condition for switching to direct injection is intended to prevent too frequent or hasty changes in the operating parameters, in particular a transition to direct injection when the load T orque only briefly the specified load T orque,up and then falls again or by this specified load T orque,up fluctuates, without exceeding this load limit justifying a switch to direct injection.

[0040] In this context, it should be noted that the direct injection system is sensitive to changes and deviations in mixture formation. The sensitivity of the system increases with decreasing load, since smaller amounts of fuel are injected at low and medium loads, making it more difficult to provide a reproducible fuel-air mixture. Direct fuel injection in the partial load range can therefore lead to increased misfires or misfiring. The latter leads to the unburned fuel being expelled from the cylinder via the exhaust pipe, which is considered critical with regard to pollutant emissions.

[0041] The additional condition according to the process variant in question ensures that direct injection is only used if the boundary conditions for direct injection of the gas are present or suitable and a sufficiently good mixture formation can be guaranteed.

[0042] Embodiments of the method are advantageous in which, starting from direct injection at high loads, the system switches back to intake manifold injection if the load T orque a predeterminable load T orque,down falls below with T orque ≤ T orque,down .

[0043] With regard to improved mixture preparation, it is advantageous to switch to intake manifold injection at partial load in order to provide the cylinders with a well-mixed, homogeneous fuel-air mixture and to ensure reliable ignition of the mixture.

[0044] Also advantageous in the present context are embodiments of the method in which, starting from direct injection at high loads, the system only switches back to intake manifold injection if the load T orque a predeterminable load T orque,down and for a predefined period of time Δt down is smaller than this given load T orque,down .

[0045] The above condition for changing the injection method is intended to help avoid too frequent or hasty changes in the operating parameters. Reference is made to the time period Δt up The proposed approach allows for an appropriate response to scenarios where the load T orque only briefly below the specified load T orque,down falls and then rises again or fluctuates around the specified load.

[0046] Embodiments of the process are advantageous in which the gas used as fuel for the internal combustion engine is LPG. LPG has the advantage that it is in the liquid phase even at low pressures of approximately 5 or 6 bar and ambient temperature and can be stored, whereas natural gas must be specifically cooled to be liquefied.

[0047] Embodiments of the method in which the gas used as fuel for the internal combustion engine is hydrogen (H2) are also advantageous.

[0048] Embodiments of the method are advantageous in which the gas serving as fuel for the internal combustion engine is introduced into the at least one intake line in liquid phase by means of intake manifold injection.

[0049] Liquid injection into the intake manifold proves to be not quite as effective as direct liquid injection into the cylinder during full-load operation, since the heat of vaporization does not take effect directly in the cylinder, which can result in higher gas temperatures in the cylinder, which have a detrimental effect on knocking behavior.

[0050] Nevertheless, the combination of intake manifold injection of liquid fuel at full load and intake manifold injection of gaseous fuel during partial load is an advantageous and comparatively cost-effective process variant.

[0051] The liquid gas introduced into the intake air evaporates, removing the heat of vaporization from the intake air stream. Consequently, the temperature of the air supplied to the cylinders drops, and with it the temperature of the cylinder charge as a whole. This reduces the risk of knocking and the occurrence of unwanted autoignition in the gas-air mixture.

[0052] In addition, lowering the temperature of the cylinder fresh charge leads to a lower thermal load on the internal combustion engine and to a lower concentration of nitrogen oxides (NO x ) in the exhaust gas, ie reduced nitrogen oxide emissions.

[0053] However, the reduction in temperature also increases the density of the air, so that the injection of the gas in liquid form contributes to a better filling of the combustion chamber with air, whereby the increase in the air mass supplied to the combustion chamber may have to be counteracted by greater throttling.

[0054] Since the effects described above cause increased throttling in the partial load range, i.e., may require a greater pressure reduction to reduce the intake air volume, it may be advantageous to introduce the gas into the intake line in the gaseous phase rather than in the liquid phase. In the gaseous phase, the introduced gas has a significantly larger volume, which is why a throttle element provided in the intake line can be opened further without increasing or causing the air volume drawn in via the intake line to increase. This has significant advantages with regard to the desired dethrottlement of the internal combustion engine.

[0055] In addition, there is no cooling of the intake air flow, since evaporation of the introduced gas and thus the resulting temperature reduction are eliminated.

[0056] Therefore, embodiments of the method in which the gas serving as fuel for the internal combustion engine is introduced into the at least one intake line in gaseous phase by means of intake manifold injection are particularly advantageous.

[0057] When the fuel is injected into the intake manifold in gaseous form, a large part of the intake air is displaced by the gaseous fuel, i.e. substituted by it, so that the internal combustion engine can be operated in a much more dethrottled manner in partial load operation than with liquid injection into the combustion chamber.

[0058] During full-load operation, however, gaseous fuel injection is disadvantageous because the volumetric displacement of the gas impedes cylinder filling, significantly reducing the volumetric efficiency and the maximum achievable power. Furthermore, with gaseous fuel injection, no heat of vaporization is extracted from the fuel-air mixture, as would be the case with liquid injection. Due to the lack of internal cylinder cooling, higher mixture temperatures result in the combustion chamber. These high temperatures lead to an increased tendency to knock, necessitating a reduction in ignition timing at full load, which reduces engine efficiency.

[0059] Therefore, during full-load operation, liquid fuel injection, particularly directly into the cylinder to utilize the maximum internal cylinder cooling effect, is advantageous.

[0060] Consequently, embodiments of the method are advantageous in which the gas serving as fuel for the internal combustion engine is introduced into the at least one cylinder in liquid phase by means of direct injection.

[0061] Direct injection of the gas into at least one cylinder offers advantages over manifold injection with regard to the maximum achievable power. At high loads, the comparatively large amount of fuel, i.e., gas, can be introduced into the at least one cylinder more easily, i.e., with less effort, in liquid form than in the gaseous phase. Otherwise, the injection duration required to introduce the entire fuel quantity could be disproportionately or unacceptably long.

[0062] However, embodiments of the method may also be advantageous in which the gas serving as fuel for the internal combustion engine is introduced into the at least one cylinder in the gaseous phase by means of direct injection.

[0063] The second sub-objective underlying the invention, namely to provide an internal combustion engine for carrying out a method of the aforementioned type, is achieved by an internal combustion engine with at least one cylinder and at least one intake line for supplying the at least one cylinder with fresh air or fresh mixture, which is characterized in that both an intake manifold injection and a direct injection are provided for introducing the gas serving as fuel, wherein the intake manifold injection serves to introduce the gas into the at least one intake line and the direct injection serves to introduce the gas into the at least one cylinder.

[0064] What has already been said regarding the method according to the invention also applies to the internal combustion engine according to the invention, which is why reference is generally made here to the statements made above regarding the method. The various method variants require a corresponding internal combustion engine.

[0065] Fuel supply systems for internal combustion engines with intake manifold injection include, as essential components, the fuel tank, the injection nozzles and a fuel pump for delivering the fuel and generating the necessary injection pressure, as well as a fuel supply line that connects the various components from the fuel tank to the injection nozzles.

[0066] Fuel supply systems for direct-injection internal combustion engines also have—in addition to the low-pressure fuel pump—a second pump, namely a high-pressure pump. While the high-pressure pump ensures the required high injection pressures, the actual fuel pump serves as a so-called pre-feed pump for filling the high-pressure pump. The fuel supply line located between the pre-feed pump and the high-pressure pump is called the low-pressure line, whereas the section of the fuel supply line that runs between the high-pressure pump and the injectors forms the high-pressure fuel line.

[0067] Direct-injection diesel engines also often have a special injection system, namely a so-called common rail injection system, in which all cylinders of the internal combustion engine are supplied with fuel via a common high-pressure fuel line - the so-called common rail - with the individual injection into the individual cylinders being carried out or controlled, for example, by means of piezoelectric actuators.

[0068] If the internal combustion engine is equipped with an engine control system, embodiments are advantageous in which the engine control system is adapted in such a way that the gas serving as fuel is introduced either by means of intake manifold injection or by means of direct injection, depending on the load T.

[0069] In the following, the invention is described with reference to Fig. 1 is described in more detail. Here: Fig. 1 schematically shows an embodiment of the spark-ignition internal combustion engine.

[0070] Fig. 1 schematically shows an embodiment of the spark-ignition internal combustion engine 1 in which gas is used as fuel.

[0071] It is a three-cylinder in-line engine 1 in which the three cylinders 2 are arranged along the longitudinal axis of the cylinder head, ie in series.

[0072] An exhaust line 4 is provided for discharging the hot combustion gases, and an intake line 3 is provided for supplying the three cylinders 2 with fresh air or fresh mixture. To adjust the load, a throttle valve 8 is provided in the intake line 3, which is controlled or regulated by the engine control unit 7.

[0073] For direct injection 6 of fuel into cylinders 2, i.e., to create a direct injection system 6, each cylinder 2 is equipped with its own injector, with gas serving as the fuel and being injected. The injectors are activated, i.e., controlled, via a control line by the engine control system 7. The injected fuel quantity is used to adjust the air ratio λ.

[0074] The internal combustion engine 1 is further equipped with an intake manifold injection 5, with which gas is introduced into the intake line 3 upstream of the cylinders 2.

[0075] The intake manifold injection 5 is used at low and medium loads T, whereas the direct injection 6 is used at high loads T. Starting from an intake manifold injection 5 at low loads, the system switches to a direct injection 6 if the load T orque a predeterminable load T orque,up exceeds. Reference symbol 1 internal combustion engine 2 cylinders 3 intake line 4 exhaust pipe 5 Intake manifold injection 6 Direct injection 7 Engine control 8 Throttle valve LPG liquid gas, liquefied petroleum gas CNG natural gas, compressed natural gas LNG natural gas, liquid natural gas H2 hydrogen T orque torque, load T orque,down preset lower load T orque,up preset upper load Δt down Predeterminable minimum time period for falling below T orque,down Δt up predeterminable minimum time period for exceeding T orque,up

Claims

[1] Method for operating an internal combustion engine (1) with at least one cylinder (2) and at least one intake line (3) for supplying the at least one cylinder (2) with fresh air or fresh mixture, in which the gas serving as fuel for the internal combustion engine (1) is introduced into the at least one intake line (3) either by means of intake manifold injection (5) or by means of direct injection (6) into the at least one cylinder (2), wherein the intake manifold injection (5) is used at low and medium loads T and the direct injection (6) is used at high loads T, so that starting from the intake manifold injection (5) at low loads, a change is made to the direct injection (6) if the load T orque a predeterminable load T orque,up exceeds with T orque ≥ T orque,up . [2] Method according to claim 1, characterized bythat starting from the intake manifold injection (5) at low loads, the system switches to the direct injection (6) only when the load T orque a predeterminable load T orque,up and for a predefined period of time Δt up is greater than this specified load T orque,up . [3] Method according to claim 1 or 2, characterized by that starting from the direct injection (6) at high loads, the system switches back to the intake manifold injection (5) if the load T orque a predeterminable load T orque,down falls below with T orque ≤ T orque,down . [4] Method according to claim 3, characterized by that starting from the direct injection (6) at high loads, the system only switches back to the intake manifold injection (5) if the load T orque a predeterminable load T orque,down and for a predefined period of time Δt down is smaller than this given load T orque,down . [5] Method according to one of the preceding claims, characterized by that the gas used as fuel for the internal combustion engine (1) is LPG. [6] Method according to one of claims 1 to 4, characterized by that the gas used as fuel for the internal combustion engine (1) is natural gas (CNG, LNG). [7] Method according to one of claims 1 to 4, characterized by that the gas used as fuel for the internal combustion engine (1) is hydrogen (H2). [8] Method according to one of the preceding claims, characterized by that the gas serving as fuel for the internal combustion engine (1) is introduced in the liquid phase into the at least one intake line (3) by means of intake manifold injection (5). [9] Method according to one of claims 1 to 7, characterized by that the gas serving as fuel for the internal combustion engine (1) is introduced in the gaseous phase into the at least one intake line (3) by means of intake manifold injection (5). [10] Method according to one of the preceding claims, characterized by that the gas serving as fuel for the internal combustion engine (1) is introduced in the liquid phase into the at least one cylinder (2) by means of direct injection (6). [11] Method according to one of the preceding claims 1 to 9, characterized by that the gas serving as fuel for the internal combustion engine (1) is introduced in the gaseous phase into the at least one cylinder (2) by means of direct injection (6). [12] Internal combustion engine (1) for carrying out a method according to one of the preceding claims, having at least one cylinder (2) and at least one intake line (3) for supplying the at least one cylinder (2) with fresh air or fresh mixture, characterized bythat both an intake manifold injection (5) and a direct injection (6) are provided for introducing the gas serving as fuel, wherein the intake manifold injection (5) serves to introduce the gas into the at least one intake line (3) and the direct injection (6) serves to introduce the gas into the at least one cylinder (2). [13] Internal combustion engine (1) with engine control (7) according to claim 12, characterized by that the engine control (7) is adapted in such a way that the gas serving as fuel is introduced depending on the load T either by means of intake manifold injection (5) and by means of direct injection (6).

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