Method of operating an internal combustion engine close to a combustion limit in the low power range

Abstract

The invention relates to a method for operating an internal combustion engine, in particular a hydrogen engine, having a plurality of cylinders, by determining a combustion air ratio of the internal combustion engine, comparing the determined combustion air ratio with a predetermined lower limit value and an upper limit value corresponding to the combustion limits, to control the operation of the internal combustion engine by: a) adjusting (10) cylinder-specific injection quantities of fuel based on a rail pressure signal of the combustion engine, as soon as the determined combustion air ratio corresponds at most to the lower limit value, in order to achieve equalisation of the injection quantities of the cylinders, b) adjusting (12) the cylinder-specific injection quantities of fuel based on the speed of the combustion engine, as soon as the determined combustion air ratio in a transition range close to the combustion limit is above the lower limit value and below the upper limit value, in order to take into account cycle fluctuations of the cylinders, c) switching off (14) at least one cylinder and adjusting the cylinder-specific injection quantities of fuel distributed to the remaining cylinders according to measure a) or measure b), as soon as the determined combustion air ratio has reached the upper limit value and thus the combustion limit, wherein the respective ignition angle is kept in an efficiency-optimised range as part of all of measures a), b) or c).

Description

[0001] Description

[0002] Title:

[0003] Method for operating an internal combustion engine near a combustion limit in the low power range

[0004] The present invention relates to a method for operating an internal combustion engine, in particular a hydrogen engine, having a plurality of cylinders, and to a control device for controlling an internal combustion engine, in particular a hydrogen engine, having a plurality of cylinders.

[0005] State of the art

[0006] The operation of hydrogen combustion engines with multiple cylinders at low loads, and especially during idle operation, is challenging. A combustion air ratio significantly greater than 1 is desired to avoid nitrogen oxide emissions and improve fuel consumption and engine responsiveness. However, an excessive increase in the combustion air ratio can lead to potential combustion stability problems. Throttled operation, cylinder suppression, or corresponding control of a diverter valve are known to be used for this purpose, although these measures cannot be clearly distinguished from one another. The combustion limit, i.e.The range beyond which combustion instabilities can occur due to an excessively high combustion air ratio cannot be accurately detected during engine operation, and measures to prevent this range can therefore usually only be initiated based on boundary conditions determined during engine calibration. Disclosure of the invention.

[0007] An internal combustion engine operated with such a lean homogeneous combustion process offers the advantage - unlike a stoichiometric combustion process - of being able to implement rapid torque interventions by adjusting the fuel quantity, without having to take the inertia of an air system into account. This allows torque-effective interventions via ignition control, which are associated with efficiency disadvantages, to be reduced to a minimum. When controlling torque via the fuel quantity, however, it must be ensured that the so-called combustion limits of the fuel-air mixture are observed, i.e. that an excessively high combustion air ratio is avoided. Once this combustion limit is reached, the fuel quantity cannot be reduced any further without generating combustion instabilities. Consequently, the torque must be regulated using additional measures, some of which impair efficiency.

[0008] Consequently, the object of the invention is to propose a method and a system with which an improved idle control can be achieved while largely avoiding ignition angle interventions and making the best possible use of combustion limits.

[0009] The object is achieved by a method having the features of independent claim 1. Advantageous embodiments and further developments can be found in the subclaims and the following description.

[0010] A method for operating an internal combustion engine, in particular a hydrogen engine, with a plurality of cylinders is proposed. The method comprises determining a combustion air ratio of the internal combustion engine, comparing the determined combustion air ratio with a predetermined lower limit and an upper limit corresponding to the combustion limit, and regulating the operation of the internal combustion engine by torque-effective measures in accordance with the following control cascade: a) adjusting cylinder-specific fuel injection quantities based on a rail pressure signal of the internal combustion engine as soon as the determined combustion air ratio corresponds at most to the lower limit in order to achieve equalization of the injection quantities of the cylinders, b) adjusting the cylinder-specific fuel injection quantities based on the speed of the internal combustion engine,as soon as the determined combustion air ratio lies above the lower limit and below the upper limit in a transition range close to the combustion limit in order to take account of cycle fluctuations of the cylinders, c) switching off at least one cylinder and adjusting the cylinder-individual injection quantities of fuel distributed among the remaining cylinders according to measure a) or measure b) as soon as the determined combustion air ratio has reached the upper limit and thus the combustion limit, whereby within the framework of all measures a), b) or c) the respective ignition angle is kept in an efficiency-optimised range.

[0011] The operation of the internal combustion engine by a method according to the invention in a low load range is characterized by a prioritization of torque-effective interventions within the framework of the specified control cascade in order to largely avoid efficiency-reducing measures. The execution of the individual possible interventions depends on a determined combustion air ratio of the engine, which is designated by and indicates the mass ratio of air to fuel relative to the respective stoichiometric ratio. In this context, it should be noted that a combustion limit must be observed as the upper limit of the combustion air ratio, above which combustion instabilities could occur due to an excessively high air proportion. A lower limit with a sufficient safety margin from the combustion limit is specified; a sensible selection of the lower limit will be discussed further below.If the determined combustion air ratio lies far enough below the combustion limit and at most at the lower limit, the method according to the invention regulates the fuel supply to the individual cylinders based on the rail pressure signal, which is previously identified with measure or priority a). The combustion air ratio is therefore richer or lower or equal to the lower limit. The function implemented in a control unit adjusts the supplied fuel quantity from cylinders with a fuel quantity that deviates from the engine cylinder average to the cylinder average level. In particular, if the supplied fuel quantity is too low, a cylinder-individual combustion air ratio is too high, which consequently also lies closer to the combustion limit.

[0012] In this process, it is conceivable to evaluate a minimum rail pressure during injection at the cylinders. For cylinders where a higher minimum rail pressure is observed during injection than the average for the other cylinders, the activation duration of the corresponding injector could be increased until the minimum rail pressure is slightly lower or slightly higher than the previously calculated cylinder average.

[0013] A combustion air-ratio band could be defined around the previously calculated engine cylinder average, within which the combustion air-ratio should lie after a corresponding correction of the cylinder's fuel quantity. This band could be characterized by a certain percentage lower or higher fuel injection quantity than the engine average, which could be approximately 10%. The ignition angles are still maintained within an efficiency-optimal range.

[0014] However, if the combustion air ratio is close to the combustion limit, i.e. the upper limit value but still below it, the invention regulates the fuel supply based on the engine speed signal within this transition range close to the combustion limit. During this process, the engine speed signal is evaluated and high cycle fluctuations in individual cylinders at the combustion limit are compensated for, in particular by increasing the injection quantity to the level of the other engine cylinders. This is marked b) above. When the engine speed is recorded using a speed sensor, rotational irregularity in the engine can be determined, allowing conclusions to be drawn about the cycle fluctuation in each individual cylinder. If a cylinder exhibits increased cycle fluctuation, the fuel supply to this cylinder is increased in order to increase the distance to the combustion limit at this cylinder.Since, in addition to proximity to the combustion limit, a possible difference in the fuel quantity delivered per cylinder can also be a relevant cause for cycle fluctuations, a comparison of the differences between cylinders in a rail pressure signal as a measure of the actual fuel quantities delivered and the speed irregularity as a measure of the deviations due to both possible causes can be used to differentiate between the two possible causes. This way, all cylinders are brought to the combustion limit as best as possible, taking into account all influences on the combustion limit, such as the quantity dispersion of the individual fuel metering valves and any uneven distribution of the cylinder charge.

[0015] Only when the combustion limit is reached does selective cylinder deactivation occur to maintain a greater distance from the combustion limit for each cylinder, while simultaneously distributing the fuel quantity required to achieve the target torque to the remaining active cylinders. As long as the combustion air ratio is not at the combustion limit in this operating mode, control should continue to be purely by adjusting the fuel supply, with a distinction being made between a) and b) depending on the resulting combustion air ratio.

[0016] In an advantageous embodiment, the method further comprises: d) reducing the engine speed if, after step c), the combustion air ratio of individual cylinders reaches at least 95% of the upper limit. To maintain torque, the individual filling of the individual cylinders can be increased and consequently a greater distance from the combustion limit can be achieved. The engine's own speed must be taken into account in this case. In an advantageous embodiment, the method further comprises: e) adjusting the ignition angle out of the efficiency-optimal range if, after step d), the combustion air ratio of individual cylinders reaches at least 95% of the upper limit. The adjustment of the ignition angle of individual cylinders is only carried out when the previous measures named under a) to d) are no longer successful. Combustion could become less efficient as a result.

[0017] Steps a) to e) are each to be understood as measures that must be implemented if the combustion engine is too close to or directly at the combustion limit. They are to be evaluated from a) to e) in descending order of priority, meaning that the individual measures can be implemented one after the other until operation sufficiently away from the combustion limit is possible.

[0018] The lower limit can be determined as the difference between the upper limit and a subtrahend s, where the subtrahend s is formed by adding together the combustion air ratio influences. Symmetrical tolerance contributions are assumed. The combustion air ratio influences can include an upper boost pressure tolerance of an air system, a lower shot-to-shot variation of an injector injection mass (approximately 5%), and an upper cylinder air charge deviation from an engine mean value.

[0019] In addition, a plausibility check can be carried out using an air ratio range currently calculated in an air system model to ensure that the engine is actually currently operating in the range close to the combustion limit (this could be particularly relevant if cylinder deactivation is active), so that a clear assignment of speed irregularity to proximity to the combustion limit is possible.

[0020] The invention further relates to a control unit for controlling an internal combustion engine, in particular a hydrogen engine, with a plurality of cylinders, comprising a rail pressure sensor input, a speed sensor input, a computing unit, and a plurality of control outputs, wherein the computing unit is designed to carry out the method described above, to determine a combustion air ratio of the internal combustion engine and to compare it with a predetermined lower limit value and an upper limit value, and wherein the control unit is designed to cascade-regulate the operation of the internal combustion engine by generating and providing control signals at the control outputs as follows: a) adjusting cylinder-specific injection quantities of fuel based on a rail pressure signal as soon as the determined combustion air ratio corresponds at most to the lower limit value,b) adjusting the cylinder-individual fuel injection quantities based on the engine speed as soon as the determined combustion air ratio is above the lower limit and below the upper limit, c) switching off at least one cylinder and adjusting the cylinder-individual fuel injection quantities distributed among the remaining cylinders according to a) or b) as soon as the determined combustion air ratio reaches the upper limit, wherein the control unit is designed to keep the respective ignition angle in an efficiency-optimized range during the aforementioned measures a), b) and c).

[0021] In an advantageous embodiment, the control unit is designed to d) reduce the speed of the engine if, after carrying out c), the combustion air ratio of individual cylinders reaches at least 95% of the upper limit value.

[0022] In an advantageous embodiment, the control unit is designed for e) adjusting the ignition angle from the efficiency-optimal range; after carrying out d), the combustion air ratio of individual cylinders should reach at least 95% of the upper limit value.

[0023] In an advantageous embodiment, the control unit is configured to determine the lower limit value as the difference between the upper limit value and a subtrahend s, wherein the subtrahend s is formed from the addition of combustion air ratio influences. In an advantageous embodiment, the control unit is configured to execute a simulation of the internal combustion engine, wherein the determination of the combustion air ratio is carried out based on the simulation.

[0024] Further measures improving the invention are presented in more detail below together with the description of the preferred embodiments of the invention with reference to figures.

[0025] Examples of implementation

[0026] It shows:

[0027] Figure 1 is a schematic representation of a method for operating an internal combustion engine;

[0028] Figure 2 shows a block-based representation of a control strategy;

[0029] Figure 3 shows an example of two different operating states and corresponding control;

[0030] Figure 4 is a schematic representation of a system for operating an internal combustion engine.

[0031] Figure 1 shows a method 2 for operating an internal combustion engine, in particular a hydrogen engine with multiple cylinders. The method 2 begins with determining 4 a combustion air ratio, after which this is compared with a predetermined lower limit and an upper limit 6. Subsequently, the operation of the internal combustion engine is controlled 8 by a priority-controlled strategy, the execution of which depends on the previous comparison with the limit values. In descending order of priority, the control includes: a) adjusting 10 cylinder-specific fuel injection quantities based on a rail pressure signal as soon as the determined combustion air ratio corresponds at most to the lower limit; b) adjusting 12 the cylinder-specific fuel injection quantities based on the engine speed as soon as the determined combustion air ratio is above the lower limit and below the upper limit;c) Deactivating 14 at least one cylinder and adjusting 10 or 12 the cylinder-individual fuel injection quantities distributed among the remaining cylinders according to a) or b) as soon as the determined combustion air ratio reaches the upper limit, whereby the respective ignition angle is maintained in an efficiency-optimal range; d) Reducing 16 the engine speed if, after step c), the combustion air ratio of individual cylinders reaches at least 95% of the upper limit; or e) Adjusting 18 the ignition angle from the efficiency-optimal range if, after step d), the combustion air ratio of individual cylinders reaches at least 95% of the upper limit. Determining 4 the combustion air ratio can comprise executing 20 a (simplified) simulation model of an internal combustion engine. In the simplest case, the simulation model can be implemented as a lookup table.in which experimentally or theoretically determined combustion air ratios are stored and retrievable.

[0032] Figure 2 shows a block-based representation of a control strategy. In block 22, a plausibility check is performed to determine whether a determined combustion air ratio roughly corresponds to a simulated combustion air ratio or is close to the combustion limit. This can be used as an enable condition for the subsequent process.

[0033] The comparison 6 takes place in a block 24, which is connected to a setpoint for the combustion air ratio Ä S0n and current, cylinder-specific lower limit values ​​for a current air charge in the cylinders. This results in deviations which are evaluated in a subsequent block 26 and used to prioritize active measures for operating the combustion engine. These include, as previously explained, adjusting 10 and 12 cylinder-specific injection quantities, deactivating 14 at least one cylinder, reducing 16 the engine speed, and adjusting 18 the ignition angle. The cylinder-specific lower limit values ​​are formed in block 28 from a base value 30 for a current combustion limit and combustion air ratio influences. The latter are formed by a cylinder-specific analysis 32 of the irregularity of the engine speed or cylinder pressure signal over several cycles and a comparison with a rail pressure signal, as well as subjected to an operating point-dependent and cylinder-specific weighting 34.

[0034] Figure 3 shows two examples, I and II, of different operating states of an engine with six cylinders ZI to Z6, as well as the corresponding control system. In example I, the air / fuel ratio exceeds the lower limit and falls below the upper limit, so that the combustion engine is operating close to the combustion limit. Therefore, the speed signal is primarily analyzed and compared with the rail pressure signal. This is repeated over several cycles. The analysis is labeled block 40. An examination of the engine speed reveals speed irregularity, i.e., a cycle fluctuation in which cylinders Z2 and Z6 are noticeable due to a momentary drop in speed (see diagram 36). The rail pressure (see diagram 38) behaves the same for all cylinders Z1-Z6, so it can be assumed that the cylinders are evenly filled.The measure taken here is a (load-dependent) reduction 42 of the combustion limit for cylinders Z2 and Z6 as an adjustment of the control system; none of the measures a) to e) are carried out.

[0035] In Example II, the lower limit is exceeded, so the rail pressure signal is primarily evaluated. The rail pressure signal is abnormal in both cylinders Z2 and Z6. As a countermeasure, the cylinder-specific injection quantities are increased based on a rail pressure signal-dependent control (measure a)).

[0036] Finally, Figure 4 shows an exemplary representation of a control unit 44 for controlling an internal combustion engine, in particular a hydrogen engine, with multiple cylinders. The control unit has a rail pressure sensor input 46, a speed sensor input 48, a computing unit 50, and a plurality of control outputs 52. Furthermore, the control unit 44 has a plurality of optional cylinder pressure inputs 54. The computing unit 50 is coupled to the inputs 46, 48, and 54 as well as the control outputs 52 and is designed to determine a combustion air ratio of the internal combustion engine and to compare it with a predetermined lower limit and an upper limit.The control unit 44 is further designed to regulate the operation of the internal combustion engine by generating and providing control signals at the control outputs 52 for a) adjusting cylinder-specific fuel injection quantities based on a rail pressure signal at the rail pressure sensor input 46 as soon as the determined combustion air ratio corresponds at most to the lower limit value, or b) adjusting the cylinder-specific fuel injection quantities based on the speed from the speed sensor input 48 as soon as the determined combustion air ratio is above the lower limit value and below the upper limit value, or c) switching off at least one cylinder and adjusting the cylinder-specific fuel injection quantities distributed among the remaining cylinders according to a) or b) as soon as the determined combustion air ratio reaches the upper limit value.The control unit 44 is designed to maintain the respective ignition angle in a), b), and c) within an efficiency-optimized range. As previously described, measures d) and e) can also be carried out by the control unit 44 according to the determined combustion air ratio.

Claims

Claims 1. Method (2) for operating an internal combustion engine, in particular a hydrogen engine, with a plurality of cylinders, comprising: determining (4) a combustion air ratio of the internal combustion engine, Comparing (6) the determined combustion air ratio with a predetermined lower limit and an upper limit corresponding to the combustion limit, Control (8) of the operation of the internal combustion engine by means of torque-effective measures in accordance with the following control cascade: a) Adjusting (10) cylinder-specific fuel injection quantities based on a rail pressure signal of the internal combustion engine as soon as the determined combustion air ratio corresponds at most to the lower limit in order to achieve equalization of the injection quantities of the cylinders, b) Adjusting (12) the cylinder-specific fuel injection quantities based on the speed of the internal combustion engine as soon as the determined combustion air ratio is above the lower limit and below the upper limit in a transition range close to the combustion limit in order to take cycle fluctuations of the cylinders into account, c) Switching off (14) at least one cylinder and adjusting the cylinder-specific fuel injection quantities distributed among the remaining cylinders according to measure a) or measure b),as soon as the determined combustion air ratio has reached the upper limit and thus the combustion limit, whereby the respective ignition angle is kept in an efficiency-optimal range within the framework of all measures a), b) or c). Method (2) according to claim 1, the method (2) further comprising: d) reducing (16) the speed of the engine if, according to measure c), the combustion air ratio of individual cylinders reaches at least 95% of the upper limit value. Method (2) according to claim 2, the method (2) further comprising: e) adjusting (18) the ignition angle from the efficiency-optimal range if, according to measure d), the combustion air ratio of individual cylinders reaches at least 95% of the upper limit value. Method (2) according to one of the preceding claims, further comprising determining the lower limit value as the difference between the upper limit value and a subtrahend s, wherein the subtrahend s is formed from an addition of combustion air ratio influences. Method (2) according to one of the preceding claims, wherein the determination (4) of the combustion air ratio is carried out using a simulation model.Control unit (44) for controlling an internal combustion engine, in particular a hydrogen engine, with a plurality of cylinders, comprising: a rail pressure sensor input (46), a speed sensor input (48), a computing unit (50), and a plurality of control outputs (52), wherein the computing unit (50) is designed to determine a combustion air ratio of the internal combustion engine and to compare it with a predetermined lower limit value and an upper limit value corresponding to the combustion limit, and wherein the control unit (44) is designed to carry out torque-effective measures in accordance with the following control cascade by generating and providing control signals at the control outputs (52):. a) adjusting (10) cylinder-specific fuel injection quantities based on a rail pressure signal of the internal combustion engine as soon as the determined combustion air ratio corresponds at most to the lower limit value in order to achieve equalization of the injection quantities of the cylinders, b) adjusting (12) the cylinder-specific fuel injection quantities based on the speed of the internal combustion engine as soon as the determined combustion air ratio is above the lower limit value and below the upper limit value in a transition range close to the combustion limit in order to take cycle fluctuations of the cylinders into account, c) switching off (14) at least one cylinder and adjusting the cylinder-specific fuel injection quantities distributed among the remaining cylinders according to measure a) or measure b) as soon as the determined combustion air ratio has reached the upper limit value and thus the combustion limit, wherein the control unit (44) is designed towithin the scope of all measures a), b) or c), to keep the respective ignition angle in an efficiency-optimized range. Control unit (44) according to claim 6, wherein the control unit (44) is designed to: d) reduce (16) the engine speed if, after carrying out c), the combustion air ratio of individual cylinders reaches at least 95% of the upper limit value. Control unit (44) according to claim 7, wherein the control unit (44) is designed to: e) adjust (18) the ignition angle from the efficiency-optimized range if, after carrying out d), the combustion air ratio of individual cylinders reaches at least 95% of the upper limit value. Control unit (44) according to one of claims 6 to 8, wherein the control unit (44) is designed to determine the lower limit value as the difference between the upper limit value and a, Subtrahend s, wherein the subtrahend s is formed from an addition of combustion air ratio influences. Control unit (44) according to one of claims 6 to 9, wherein the control unit (44) is designed to simulate the combustion engine, whereby the combustion air ratio is determined based on the simulation.

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