Method for operating an internal combustion engine in emergency mode

Abstract

Method for operating an internal combustion engine in an emergency driving mode, wherein a pressure limiting valve (160) which fluidly connects a high-pressure region (120) of a fuel supply device (100) to a low-pressure region (130) is opened when a pressure of the fuel in the high-pressure region exceeds a trigger pressure value (p0), wherein a lower idle speed of the internal combustion engine is set to an increase value (n0, n1, n2).

Description

[0001] The present invention relates to a method for operating an internal combustion engine in emergency operation and a computing unit for carrying out the method. State of the art

[0002] The invention relates to fuel supply systems for internal combustion engines that inject high-pressure fuel directly into cylinders via injectors. The fuel is supplied to the injectors via a high-pressure line, which is equipped with a pressure relief valve. Such fuel supply systems can be used in both gasoline and diesel engines.

[0003] In fuel supply systems for internal combustion engines, a high-pressure pump compresses the fuel to a high pressure level. The compressed fuel fills the high-pressure line (e.g., the common rail), which is constantly pressurized during engine operation and supplies the injectors (=injection valves) of the individual cylinders of the internal combustion engine (possibly via connected branch lines).

[0004] For controlled operation of the combustion engine, a suitable fuel pressure must be maintained in the high-pressure line. Different approaches to pressure control are known. This can be achieved either on the high-pressure side via a pressure control valve (DRV) on the high-pressure line or on the intake side (low-pressure side) via a metering unit (ZME) integrated into the high-pressure pump or provided as a separate component. Both solutions are known as two-position systems. The actual value for the control can be provided by a pressure sensor (e.g., rail pressure sensor RDS).

[0005] Systems are known in which the high-pressure line is equipped with a pressure-limiting valve (PLV) to prevent overpressure situations that can occur, for example, when the fuel balance (i.e., the difference between the amount of fuel pumped into the high-pressure line and the amount of fuel discharged from the high-pressure line) is positive. Systems with a pressure-limiting valve are conveniently controlled on the intake side. A pressure-limiting valve is described in DE 198 22 671 A1.

[0006] DE 197 31 995 B4 discloses a method for controlling an internal combustion engine, particularly one with a common rail system, in which the fuel pressure in a reservoir is regulated by a pump. The transmission behavior of the pressure regulator is adjusted depending on the available pressure build-up, the engine speed, and / or the control deviation.

[0007] DE 195 48 280 A1 discloses a method for controlling an internal combustion engine with high-pressure injection, particularly with a common rail system. In the event of a failure in the pressure control system in the high-pressure range, the fuel pressure is controlled by influencing the fuel flow in the low-pressure range.

[0008] DE 103 23 874 A1 discloses a method for operating an internal combustion engine with a fuel pressure accumulator. The target pressure in the accumulator is achieved either by regulating the fuel flow to the high-pressure pump via a volume flow control valve (first operating mode) or by releasing pressure from the accumulator (second operating mode).

[0009] WO 2012 / 040756 A1 discloses a method for a pressure relief valve in common rail heavy fuel oil injection systems. A valve piston that can be moved in the valve carrier, which sits on a valve seat in the closed state and is held closed by a compression spring, can be moved against the spring force via a bolt and an actuating element to enable a purge function.

[0010] In order to avoid damage to the fuel supply system and / or the combustion engine and to prevent it from being forced to shut down in the event of a pressure measurement fault, an emergency driving mode can be provided. This type of emergency driving mode usually involves increasing the pressure in the high-pressure line until the pressure relief valve opens (and remains open) so that the subsequent line pressure is roughly known. It can be estimated in particular from the characteristics of the pressure relief valve. However, situations can arise in which the quantity balance becomes negative, e.g. due to increased leakage, which would then lead to the pressure relief valve closing and make emergency driving impossible. To prevent this, a maximum permissible injection quantity can be specified in order to always keep the quantity balance positive.Depending on the system, this can significantly limit emergency driving capability, as the available engine torque is significantly restricted or even reduced to zero, depending on the high-pressure flow balance. If the required or requested load torque (hereinafter referred to as torque demand) exceeds the available torque (hereinafter referred to as engine torque), the combustion engine stalls.

[0011] Based on this state of the art, it is desirable to provide a way to prevent the pressure relief valve from closing during emergency driving and to keep the engine torque sufficiently high so that, in particular, every torque requirement can be met during emergency driving. Disclosure of the invention

[0012] According to the invention, a method for operating an internal combustion engine in emergency operation is proposed with the features of patent claim 1. Advantageous embodiments are the subject of the subclaims and the following description. Advantages of the invention

[0013] The invention presents a possibility of keeping the engine torque sufficiently high during emergency driving mode so that, in particular, a torque request for emergency driving mode can also be met. The invention makes use of the knowledge that the torque request in emergency driving mode and the engine torque are speed-dependent, whereby the basic rule is that the torque request and engine torque also increase with increasing engine speed. In this case, the case can arise that the engine torque is only greater than the torque request above a threshold speed. Increasing the lower idle speed to an increase value that corresponds at least to this threshold speed now means that the engine torque can meet the torque request across all speeds (although now only to a limited extent) and that the pressure relief valve remains open across all speeds.The invention makes it possible to prevent the pressure relief valve from closing unintentionally during emergency operation, in particular by limiting the permissible injection quantity. On the other hand, the increased lower idle speed leads to a higher minimum delivery rate for a typically speed-dependent high-pressure pump. Both measures shift the flow balance toward the positive. This prevents undefined and thus potentially critical engine operation. A discrepancy between torque demand and engine torque caused by limiting the injection quantity, which would lead to stalling of the combustion engine, is prevented by the idle speed increase.

[0014] According to a practical embodiment, the boost value can be fixed, i.e., the idle speed is set to the fixed boost value in emergency operation. The boost value is expediently specified depending on the combustion engine configuration and selected such that the engine torque can reliably meet the torque requirement across all speeds and / or that the minimum delivery rate is reliably sufficient to keep the pressure relief valve open. In this case, worst-case tolerance levels of the components (particularly leakage) and worst-case operating conditions are expediently taken into account. For example, such a fixed boost value can be determined empirically on a test bench.

[0015] According to a preferred embodiment, a dependence of the threshold speed on the fuel temperature is taken into account. It has been shown that the engine torque depends on the fuel temperature, with the engine torque generally decreasing as the fuel temperature rises. This is essentially because, as the fuel temperature rises, the possible pump delivery rate decreases due to increasing leakage quantities. Likewise, in injectors subject to leaks, the leakage quantity and thus the system demand increase. Both effects lead to a reduction in the maximum injectable quantity, which, in systems with an already marginal quantity balance, has a direct impact on the achievable engine torque, as well as to a negative shift in the quantity balance. It is therefore proposed to specify the increase value as a function of temperature.This now advantageously allows, in cases where the combustion engine could deliver a sufficiently high engine torque and / or a sufficient minimum delivery rate even at low engine speeds at the actual fuel temperature, to prevent an increase in idle speed and a related restriction of the possible engine speed range.

[0016] The fuel temperature is preferably measured using a suitable temperature sensor (e.g., in the low-pressure circuit of the main filter or in the high-pressure pump) and provided for the process. Derivation from other temperature signals (e.g., cooling water and / or engine oil temperature) is also possible.

[0017] According to a further preferred embodiment, the increase value is determined in emergency mode. For this purpose, the threshold speed is preferably determined in emergency mode, which is preferably done by varying the idle speed and simultaneously monitoring the opening state of the pressure relief valve. If the pressure relief valve is detected as closing, the currently selected idle speed is below the threshold speed. To restore emergency mode, the pressure relief valve must be reopened after closing.

[0018] Preferably, the increase value is set to the determined threshold speed. This measure allows the tolerance level of the actually installed components and the actual operating conditions to be taken into account. The idle speed is increased only as much as absolutely necessary. In the case of emergency operation with the pressure relief valve open, the idle speed is increased to a level that – based on the high-pressure flow balance due to the tolerance level of the actually installed components – allows the pressure relief valve to remain safely open and sufficient engine torque to be maintained.

[0019] According to a first embodiment, the idle speed is initially set to an upper increase value at which the pressure relief valve remains reliably open across all speeds. This advantageously takes into account worst-case tolerance levels of the components (leakage) and worst-case operating conditions. The idle speed is then gradually reduced until closure of the pressure relief valve is detected. The last idle speed before closure is then advantageously used as the increase value.

[0020] According to a second embodiment, the idle speed is initially set to a medium increase value at which the pressure relief valve remains open in most cases across all speeds. In this case, medium tolerance levels of the components and medium operating conditions are expediently taken into account. If no closing of the pressure relief valve is detected, the idle speed is then gradually reduced until closing of the pressure relief valve is detected. The last idle speed before closing is then expediently used as the increase value. If, on the other hand, closing of the pressure relief valve is already detected at the medium increase value, the idle speed is then gradually increased until no closing of the pressure relief valve is detected. The first idle speed after closing is then expediently used as the increase value.

[0021] It goes without saying that the upper and / or middle increase value can also be specified depending on the fuel temperature.

[0022] According to a third embodiment, the idle speed is initially not changed, i.e., the increase value is initially the normal idle speed. If closure of the pressure relief valve is detected here, the idle speed is subsequently increased gradually until no closure of the pressure relief valve is detected. The first idle speed after closure is then expediently used as the increase value.

[0023] Preferably, the closing of the pressure relief valve is detected based on the line pressure. This is useful when pressure measurement to the extent described below is still possible. If the pressure relief valve closes, the high-pressure quantity that the pressure relief valve has kept open until then is immediately available as an excess quantity. This causes a pressure increase in the high-pressure line, which is evident as an increase in the actual pressure after a kink (i.e. after a gradient jump). After a short time, the actual pressure also leaves an expected tolerance band of the emergency driving pressure (threshold value exceeded). Both measures can be used individually or together to detect the closing of the pressure relief valve. This is particularly advantageous since a pressure sensor is already present in the high-pressure line.

[0024] Preferably, the pressure relief valve is equipped with measuring devices that can determine the opening state.

[0025] A computing unit according to the invention, e.g. a control unit of a motor vehicle, is configured, in particular in terms of programming, to carry out a method according to the invention.

[0026] Implementing the method in software form is also advantageous, as this results in particularly low costs, especially if an executing control unit is also used for other tasks and is therefore already available. Suitable storage media for the computer program include floppy disks, hard disks, flash memory, EEPROMs, CD-ROMs, DVDs, and others. Downloading a program via computer networks (internet, intranet, etc.) is also possible.

[0027] Further advantages and embodiments of the invention will become apparent from the description and the accompanying drawings.

[0028] It is understood that the features mentioned above and those to be explained below can be used not only in the combination specified in each case, but also in other combinations or on their own, without departing from the scope of the present invention.

[0029] The invention is illustrated schematically in the drawing using exemplary embodiments and is described in detail below with reference to the drawing. Short description of the drawings Fig. 1 shows a block diagram of the essential elements of a fuel supply device on which the invention may be based. Fig. 2 shows a pressure curve for the case of an opening pressure relief valve. Fig. 3 shows a torque request in emergency mode as well as several engine torques for different fuel temperatures. Embodiment(s) of the invention

[0030] In Fig. 1, the essential elements of a fuel supply system are shown as a block diagram and designated 100. The fuel supply system 100 comprises a high-pressure region 120 and a low-pressure region 130, in each of which fuel is present at a different pressure. In the high-pressure region, for example, a pressure of 1,500 bar - 2,000 bar is typical, whereas in the low-pressure region, a pressure of up to 10 bar can prevail.

[0031] Components of the high-pressure area 120 are essentially a high-pressure line 150 (e.g. the so-called common rail) and the injectors 151, 152 and 153 for metering the high-pressure fuel into one or more cylinders (not shown) of an internal combustion engine.

[0032] Among other things, to regulate the high pressure (line pressure), a computing unit designed as an engine control unit 170 is provided, which controls an actuating element 110 for controlling the line pressure P with a control signal A. The actuating element 110 can be a pressure control valve (DRV), which connects the high-pressure region 120 to the low-pressure region 130, and / or a controllable high-pressure pump, which pumps the fuel from the low-pressure region 130 to the high-pressure region 120. By appropriately controlling an electromagnetic valve provided on the high-pressure pump (so-called metering unit ZME), the pumped quantity and thus the pressure in the high-pressure region can be controlled. The low-pressure region 130 (for example in the fuel tank, main filter, or in the high-pressure pump) is equipped with a temperature sensor 162, which measures the temperature of the fuel.

[0033] A pressure sensor 140 (e.g., a so-called rail pressure sensor RDS) detects the current value P of the pressure in the high-pressure region, also referred to here as line pressure. A corresponding signal from pressure sensor 140 is sent to control unit 170. Depending on various other signals not shown, the control unit calculates control signals for actuating injectors 151, 152, and 153. These injectors meter a specific amount of fuel to the combustion engine at a specific time, depending on the respective control signal. The injectors are connected to the low-pressure region 130 via return lines 154, through which excess fuel flows away. The figure shows only three injectors and three cylinders. However, the described procedure can be used with any number of injectors and / or cylinders.

[0034] Furthermore, a pressure relief valve 160 is provided, which connects the high-pressure area 120 with the low-pressure area 130 via a return line 161. Normally, this valve is closed, and the connection is interrupted. If the pressure in the high-pressure area 120 (i.e., the line pressure) rises above a trigger pressure value (e.g., 2,000 bar), the pressure relief valve 160 opens, and the line pressure drops to a holding pressure (e.g., 800 bar).

[0035] In Fig. For further illustration, a pressure curve in the high-pressure line 150 is shown against time t. In Fig. Figure 2 shows the case of an opening pressure relief valve. The line pressure curve is labeled 201.

[0036] At time t0, a fault in the line pressure measurement, in particular a failure of the line pressure sensor, is detected. To enable emergency operation, at time t1, the setpoint for the line pressure is increased to a value that is above the trigger pressure of the pressure relief valve. As a result, the delivery rate of the high-pressure pump, in particular, is set to the maximum value, which is represented by curve 203. The line pressure 201 increases.

[0037] The line pressure rise continues until the trigger pressure p0 is exceeded and the pressure relief valve opens. The line pressure then drops to the holding pressure p1.

[0038] In Fig.For further illustration, Figure 3 shows a rough schematic of several torque curves M plotted against engine speed n. An engine torque (i.e., the torque delivered by the engine) in normal operation is designated by 301. A torque request (i.e., the torque to be delivered by the engine to meet the demand) in emergency operation is designated by 302. 303, 304, and 305 denote engine torques in emergency operation for different fuel temperatures, for example, 30°C, 60°C, and 80°C. A lower idle speed is designated by n0. All of these curves can be determined, for example, on an engine test bench.

[0039] It is clear that the engine torque 303 is sufficient to meet the torque requirement 302. Therefore, an increase in engine speed is not necessary in emergency operation.

[0040] It also becomes clear that the engine torque 304 is only sufficient to satisfy the torque request 302 above a higher speed threshold n1. Increasing the lower idle speed n0 to an increase value that corresponds at least to the speed threshold n1 now results in the engine torque being able to satisfy the torque request across all possible speeds (i.e., only above the increase value).

[0041] It also becomes clear that the engine torque 305 is only sufficient to satisfy the torque request 302 above an even higher speed threshold n2. Increasing the lower idle speed n0 to a boost value that corresponds at least to the speed threshold n2 now results in the engine torque being able to satisfy the torque request across all possible speeds (i.e., above the boost value).

[0042] Preferably, the control unit 170 is programmed to permit emergency operation of an internal combustion engine when normal operation is not possible due to a malfunction in the high-pressure measurement. A typical emergency operation is characterized in that the line pressure is adjusted using the pressure relief valve 160 by forcing the opening of the pressure relief valve 160. For this purpose, preferably upon detection of a fault in the line pressure sensor 140, the actuator 110 is controlled such that the line pressure increases. In particular, it is provided that the high-pressure pump is controlled such that it delivers the maximum possible amount (so-called pressure increase). In addition, it can be provided to briefly switch off the injections after an applicable waiting time (so-called pressure shock) in order to achieve a rapid increase in the line pressure above the trigger pressure value.

[0043] If the pressure relief valve opens successfully, a holding pressure (so-called emergency pressure) is established within the pressure relief valve (e.g., 800 bar). The emergency pressure can be adjusted by appropriately designing the pressure relief valve and usually corresponds to approximately half of the normal line pressure (nominal pressure, e.g., 1600 bar). If the pressure relief valve fails to open successfully, the resulting pressure level corresponds to the equilibrium between the incoming and outgoing high-pressure flow. The resulting pressure is then above the nominal pressure and below the trigger pressure value.

[0044] The control unit 170 is also programmed to limit the injection quantity per working cycle in emergency operation in order to always keep the quantity balance positive.

[0045] Control unit 170 is further programmed to increase the idle speed n0 during emergency operation, preferably taking the fuel temperature into account, in order to prevent a torque undercoverage caused by the restriction of the injection quantity. The invention preferably allows for an idle speed increase to be carried out only to the extent necessary.

[0046] If there is still a possibility of measuring line pressure or if the pressure relief valve 160 is equipped with measuring devices that indicate its opening state, a further preferred embodiment of the invention provides for the determination of a suitable increase value that corresponds as closely as possible to the current threshold speed. For this purpose, according to a preferred embodiment of the invention, the idle speed is initially set to an upper increase value at which the pressure relief valve remains reliably open across all speeds. The idle speed is then gradually reduced until a closing of the pressure relief valve is detected (based on the line pressure or the measuring devices). The last idle speed before closing is then used as the increase value.

Claims

[1] Method for operating an internal combustion engine in an emergency driving mode, wherein a pressure limiting valve (160) which fluidly connects a high-pressure region (120) of a fuel supply device (100) to a low-pressure region (130) is opened when a pressure of the fuel in the high-pressure region exceeds a trigger pressure value (p0), wherein a lower idle speed of the internal combustion engine is set to an increase value (n0, n1, n2). [2] Method according to claim 1, wherein the boost value (n0, n1, n2) is fixed. [3] Method according to claim 1, wherein the increase value (n0, n1, n2) is predetermined as a function of a parameter of the low-pressure region (130). [4] Method according to claim 3, wherein the increase value (n0, n1, n2) is predetermined as a function of a temperature of the fuel in the low-pressure region (130). [5] Method according to claim 4, wherein the increase value (n0, n1, n2) is predetermined as a function of a temperature of the fuel in a fuel tank, a fuel filter or a fuel pump. [6] Method according to claim 4 or 5, wherein the temperature of the fuel is measured by means of a sensor (162) or derived from another measured value. [7] Method according to one of the preceding claims, wherein the increase value (n0, n1, n2) is determined in emergency operation. [8] Method according to claim 7, wherein the idle speed varies, an opening state of the pressure relief valve (160) is monitored and an idle speed value is used as the increase value (n0, n1, n2) at which no closing of the pressure relief valve (160) is detected. [9] Method according to claim 8, wherein the opening state of the pressure relief valve (160) is determined from a pressure curve in the high-pressure region (120). [10] Method according to claim 8 or 9, wherein a closing of the pressure relief valve (160) is detected when the pressure in the high pressure region (120) increases and / or exceeds a threshold value. [11] Method according to one of the preceding claims, wherein in the emergency driving mode, the amount of fuel injected per working cycle from the high-pressure range by means of at least one injector (151, 152, 153) into at least one cylinder of the internal combustion engine is limited. [12] Method according to one of the preceding claims, wherein the boost value (n0, n1, n2) is determined from a curve of an engine torque (303, 304, 305) and a torque request (302) over a rotational speed (n) of the internal combustion engine. [13] Method according to claim 12, wherein a speed threshold is used as the boost value (n0, n1, n2), wherein the engine torque (303, 304, 305) for speeds above the speed threshold is greater than the torque request (302). [14] Computing unit (170) configured to carry out a method according to any one of the preceding claims. [15] Computer program with program code means which cause a computer or a corresponding computing unit to carry out a method according to one of claims 1 to 13 when executed on the computer or the corresponding computing unit. [16] A machine-readable storage medium having stored thereon a computer program according to claim 15.

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