Method for operating a gaseous fuel engine

Abstract

A method for operating a gaseous fuel internal combustion engine comprising a fuel rail, a turbocharger, and an exhaust line with an after treatment device; combustion events include injection of a fu

Description

Technical field The present invention generally relates to a method for operating a gaseous fuel engine, in particular a hydrogen internal combustion engine. Background Art Hydrogen-fueled internal combustion engines have garnered significant attention as a sustainable alternative to conventional fossil-fueled engines, owing to their potential for zero carbon emissions during combustion. Hydrogen engines are typically operated lean, i.e. with a relatively high air-to-fuel ratio, such that hydrogen fuel is heavily diluted with excess air or inert gases. This approach has the benefit of significantly reducing the formation of nitrogen oxides (NOx), which are harmful pollutants. However, the lean operation and heavy dilution of hydrogen present notable challenges. The resulting air-fuel mixture is characterized by a lower energy density and combustion temperature, which, while beneficial for NOx reduction, results in reduced flame propagation speed and lower thermal energy release. These effects lead to “cold” fuel conditions that may impair engine performance during transient operation, such as rapid accelerations or load changes. Under these dynamic conditions, the engine struggles to respond swiftly, exhibiting issues such as incomplete combustion, delayed power delivery, and potential misfires. This trade-off between minimizing NOx emissions and ensuring robust transient performance creates a need for an innovative method to optimize hydrogen engine operation. The present invention addresses this gap by providing a method that enhances transient operation without compromising the emission benefits of lean, diluted hydrogen combustion. Object of the invention It is an object of the present invention to provide a method for operating a gaseous fuel engine which enhances transient operation without compromising the emission benefits of lean operation. General Description of the Invention This object is achieved by a method for operating a gaseous fuel internal combustion engine as claimed in claim 1. The invention relates to method for operating a gaseous fuel internal combustion engine. The engine comprising a fuel delivery system with a fuel rail configured to inject fuel in respective engine cylinders, a turbocharger and an exhaust line with at least one exhaust after treatment device. Combustion events are performed to generate torque, by which a fuel quantity is injected into a respective engine cylinder based on main fuel command that is determined from a main lambda value and a quantity of available air. According to the invention a post injection strategy is performed to influence exhaust line temperatures, the post injection strategy comprising performing at least one of a postl event and a post2 event. The postl event is an injection event that is performed during a power stroke after the main injection event, by injecting a fuel amount that is based on a postl fuel command, in order to increase the enthalpy of exhaust gas flowing through the turbocharger. The postl fuel command is determined from the difference between a combustion fuel quantity and the main fuel command, whereby the combustion fuel quantity is determined from a postl lambda value- smaller than the main lambda value - and the quantity of available air. The post2 event is an injection event that is performed during the exhaust stroke, by injecting a fuel amount that is based on a post2 fuel command, in order to increase the heat in exhaust line. The post2 fuel command is determined from a value QPost2A that is determined from the difference between a total fuel quantity and the previously injected combustion fuel quantity, the total fuel quantity being determined from a post2 lambda value- smaller than the main lambda value - and a quantity of available air. The invention thus provides a post injection strategy that may implement one or two post injections, depending on enabling conditions, to provide heat downstream of the engine. The fuel quantities for the post injection events are determined from specific lambda values (i.e. postl lambda and post2 lambda) and take into account previously injected fuel quantities (for the same cylinder and cycle). For ease of description, the terms postl and post2 are used as prefix to qualify parameters that relate to the postl event and post2 event, respectively, such as e.g. fuel quantity, injection timing, and lambda numbers. The invention has been developed for hydrogen fuel engines, but may be applicable to other gaseous fuel engines. The proposed post injection strategy can be applied over the engines, i.e. to all cylinders, in accordance with their respective cycles. Advantageously, the postl lambdal value is determined from a postl lambda mapping in function of engine speed and load. The post2 lambda value may also be determined based on a post2 lambda mapping in function of engine speed and load. In particular, the postl lambda mapping and post2 lambda mapping may be calibrated to be richer (i.e. smaller lambda number) than the lambda targets (main lambda) that are typically used in the context of the determination of the main fuel mass. In embodiments, the postl events are performed if postl enabling conditions are satisfied. The postl enabling conditions may comprise: the engine being in a transient operational state; and optionally, at least one of: fuel rail pressure being within a predefined range, coolant temperature being within a predefined range, and exhaust temperature being within a predefined range. Preferably, if the postl enabling conditions are not satisfied then the postl fuel command is set to zero. In embodiments, the first post injection event starts at a timing which corresponds to a crank angle for which a ratio between a fuel rail pressure and a cylinder pressure exceeds a predetermined threshold, plus an additional offset. Preferably, the additional offset is determined from a calibrated mapping relating said additional offset to engine speed and / or engine load. In embodiments, the postl event ends at the latest at a timing which corresponds to an opening of an exhaust valve, plus an additional offset. This offset is calibratable and is generally constant over the operating range. In embodiments, the postl fuel command is capped based on a maximum first post injection value. The maximum first post injection value is determined based on a maximum first post injection pulse width. The first post injection pulse width is determined based on a difference in timing between the start of the first injection event and the end of the first injection event. In embodiments, if the postl fuel command is smaller than a minimum first post injection threshold, the postl fuel command (is set to 0, and / or no first post injection event is triggered. In embodiments, the second post injection fuel command is determined from a minimum value between a mapped fuel amount corresponding to a calorific energy required to heat up an exhaust-gas treatment system and the value Qpost2A. In embodiments, post2 events are performed if post2 enabling conditions are satisfied, wherein the post2 enabling conditions comprise: a temperature of exhaust after treatment device being below a predetermined threshold; and optionally, at least one of: fuel rail pressure being within a predefined range, coolant temperature being within a predefined range, and exhaust temperature being within a predefined range In embodiments, the second post injection event starts at the earliest at the timing which corresponds to an opening of an exhaust valve, plus an additional offset; whereby the second post injection event ends at the latest at the timing which corresponds to a closure of an exhaust valve, plus an additional offset. In embodiments, the post2 injection fuel command is capped based on a maximum post2 injection value. The maximum post2 injection value is determined based on a maximum post2 injection pulse width. The post2 injection pulse width is determined based on a difference in timing between the start of the second injection event and the end of the second injection event. In embodiments, if the post2 fuel command is smaller than a minimum post2 injection threshold, the post2 fuel command is set to 0, and / or no post2 event is triggered. Advantageously, one or more of the main fuel command, the postl fuel command and the post2 fuel command may be stored independently in a memory of a control unit. Computational functions may then access the different fuel quantities they require. For instance, some functions need to consider only the main fuel mass, while others may also need to consider the first post injection mass. The parameters of the main injection, namely main lambda, main fuel command and related injection timing can be determined from any appropriate strategy. In embodiments, at least during transient engine operation, the main lambda is obtained by computing a raw lambda from the current airflow rate (Qair) and a main fuel demand. The raw lambda is then capped to a maximum deviation with respect to a standard lambda value; and further capped between minimum and maximum lambda values that are calibrated in function of engine speed and load. This lambda strategy applied to the main lambda allows changing the fuel rate faster than the air rate, reducing the time to meet the power demand. This strategy, known as fast lambda strategy, is explained in WO2024 / 061765. These and other aspects of the invention are also recited in the appended dependent claims. According to a further aspect, the invention further concerns a control unit comprising instructions which, when executed, cause the control unit to carry out the method according to the present disclosure. According to still a further aspect, the invention concerns a computer readable medium comprising instructions which, when executed by a control unit comprising a processor, cause the control unit to carry out the method according to the present disclosure. Brief Description of the Drawings Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings in which: FIG.1 is a flow diagram of an embodiment of the present method; FIG.2 is a diagram representing the timing of the injection events (i.e. plotted vs crank angle); FIG.3 is plot representing cylinder pressure, exhaust valve stroke and intake valve stroke vs. crank angle. Fig.4 is a diagram of a strategy for determining the main lambda value. Description of Preferred Embodiments The present invention relates to fuel control strategies for internal combustion engines operating with gaseous fuels. The invention will be described in the following with respect to a hydrogen ICE. It comprises a fuel delivery system that typically comprises a fuel tank containing pressurized fuel (e.g. hydrogen at pressures of up to 350 or 700 bar), a fuel rail and a plurality of fuel injectors for selective fuel injection into the engine. The fuel injectors may be coupled directly to the fuel rail via so-called sockets, or indirectly via tubes. The fuel rail is fed from the tank via a supply line that typically integrates pressure regulation means, a shut off valve, a filter, and possibly other functionalities such as a pressure relieve valve, a purge valve. Conventionally, the engine comprises an engine block with a plurality of cylinders with associated reciprocating pistons mechanically coupled to a crankshaft. At least one fuel injector is provided per cylinder to inject fuel to be combusted in the respective combustion chamber and generate torque. The fuel injectors are configured for direct injection (DI) (fuel is introduced directly into the cylinder I combustion chamber). Each cylinder comprises at least one intake valve for admitting fresh air and at least one exhaust valve for discharging combustion gases. Depending on the engine design, there may be more than one intake, respectively exhaust valve per cylinder. In the following, the expression intake valve is used in singular, but should be understood to mean the one or more intake valves of a respective cylinder. Introduction of fuel in a given cylinder, i.e. cylinder fueling, is performed during an injection event, by applying a drive signal to the fuel injector to activate an electromechanical actuator, e.g. a solenoid actuator, to cause the injector to open during a predetermined time period. Much simplified, injection control strategies use mappings (known as calibrated flow curves) that relate the fuel quantity to the injector actuation time that is referred to as pulse width, PW. To perform an injection event, a drive pulse is applied during a time period PW to discharge a corresponding fuel amount. Conventionally, injection control strategies are programmed in the Engine Control Unit (ECU) that receives various signals indicating the state of the engine from various sensors, and is, inter alia, configured to determine a fuel quantity to be injected and a corresponding timing of injection. More specifically, the ECU is configured to determine fuel quantities to be injected to achieve a given demand (typically a torque demand but could be engine speed demand), and subsequently determine the corresponding injector control signals (injector actuation PW). The injection control strategies typically determine fuel quantities to be introduced in each cylinder, i.e. per cylinder combustion cycle. Typically, a fuel demand Qd is determined from an input torque demand Td, and a corresponding desired air mass Md is determined based on the fuel demand Qd and for a predetermined lambda setpoint. At least one charge air parameter is subsequently adjusted based on said desired air mass Md. The term “charge air parameter” herein refers to a parameter that effects the amount of air entering the engine. For example, adjusting at least one charge air parameter may involve adjusting at least one of a throttle position and a variable gate of a turbocharger. The torque demand Td represents the sum of torque demands comprised of driver torque demand (direct via accelerator pedal or indirect via cruise control, e.g.) and torque demands of other engine / vehicle components). A fuel calculation function determines a fuel command, i.e. the fuel quantity to be injected into the next scheduled injection event (upcoming combustion cycle in respective cylinder). The fuel command is determined on the basis of the fresh air flow Qair and a lambda setpoint. The fresh air flow, also referred to as quantity of available air Qair, is the current or actual air mass available in the cylinder and can be estimated based on the intake manifold pressure, temperature, and volumetric efficiency. In the context of the invention, the fuel calculation function is thus configured to determine the fuel quantity to generate torque. This is the fuel command and is here referred to as the main fuel command Qmain. The lambda set point that is applied for the determination of the main fuel command Qmain is referred to as the main lambda value Amain. As will be understood, the determination of Qmain and Amain can be based on a variety of strategies. The above is only disclosed for the sake of exemplification and should not be construed as limiting. < Post fuel injection strategy > The method according to the invention implements an injection strategy that implements post fuel injection after the main injection event. As will be explained, depending on enabling conditions, one or two post injections may be carried to influence the temperature in the exhaust. The main injection event (injection of Qmain) is the torque producing event, as explained above. The proposed post injections are not designed to produce torque, and are not a sub-portion of the torque producing fuel quantity Qmain. In the following the invention is described with respect to the combustion cycle operated on one cylinder. The invention is of course generally applicable over all of the engine cylinders, provided that the enabling conditions are met. FIG.1 shows a flowchart of an embodiment of the present injection strategy, while FIG.2 is a graphical representation of the relative timing of the respective injection events. In general, the method according to the present disclosure can be implemented by hardware and / or software. In practice, it may conveniently be implemented by a control unit comprising a processor and a memory, such as e.g. the ECU. In such case the memory may contain instructions that, when executed by the ECU, cause the latter to carry out the present method. Classically for a four-stroke engine, the combustion cycle of a cylinder is divided in four phases depending on the position of the piston. In FIG.2, the first phase from the left (540-0°) corresponds to a compression stroke, during which air in the cylinder is compressed while fuel is injected (for a direct-injection engine). The second phase (0-180°) is a power stroke, during which the fuel-air mixture is burned, thereby generating power and exhaust gas in the combustion chamber. The third phase (180°-360°) is an exhaust stroke, during which the exhaust valves are open and exhaust gas are evacuated. The third phase is followed by a fourth and last phase (360°-540°, not represented), which corresponds to an intake stroke, during which fresh air is introduced in the cylinder. In the engine, the crank angle (angular position of the crankshaft) is known in real time, classically by means of a magnetic sensor that is associated with a toothed wheel rotating with the crankshaft. As shown on Fig. 2, the injection strategy first performs the torque-generating main injection event, whereby the injector is operated to inject a fuel quantity based on the main fuel command Qmain. The main injection event is conventionally triggered at a predetermined timing (crank angle) with respect to the firing TDC (noted TDC1 on FIG.2). The main injection event is typically performed during the compression stroke, but may end shortly after firing TDC. The inventive method defines a post injection strategy that may involve one or two post injections that depend on enabling conditions. < First post injection event - Postl > A first post injection event, referred to as postl event, is designed to increase the enthalpy of the exhaust gas in the turbine of the turbocharger. Such postl is particularly desirable during transients such as acceleration phases, or more generally in case an increase torque demand is detected. The postl events are therefore implemented if enabling conditions are satisfied. The postl enabling conditions comprise: the engine being in a transient operational state. Preferably, the enabling conditions include a number of environmental conditions such as: the fuel rail pressure being within a predefined range, the engine coolant temperature being within a predefined range, and the exhaust temperature being within a predefined range. As will be understood, the term transient operational state refers to a temporary condition where an engine or system is transitioning from one stable state to another. The transient operational state may particularly be due to a variation in torque demand, i.e. an increase or decrease of torque demand, leading to a torque transient. The variation in torque demand can be determined by any appropriate approach. For example, a variation in torque demanded can be determined by comparing a torque difference (Torque(t1) - Torque(tO)) to a threshold, or by comparing a torque ratio (rate of change in torque: Torque(t1 ) / Torque(tO)) to a threshold. Hence, a variation in torque demand may be considered to be present where the difference in torque between the current timepoint and a previous time point (determined as a difference or a ratio) exceeds a predefined threshold. The postl event is performed during the power stroke after the main event, whereby the injector is operated to inject a fuel quantity based on a postl fuel command Qpostl. The postl event is not intended to generate torque (although it may generate negligible torque) and is accordingly positioned at a certain distance from the firing TDC, but is provided to generate hot exhaust gas. This additional heat provided by the postl event increases the exhaust gas enthalpy in the turbine, driving the turbocharger with more energy, resulting in more air being fed to the engine. This is particularly useful during transient operation to ensure the engine has enough air to respond to the increased torque demand. An additional spark event may be triggered once the second post injection event has begun to ensure the injected fuel bums inside the cylinder. Postl fuel quantity determination As described above and as shown on FIG.1, the main fuel command Qmain is conventionally determined from the main lambda value Amain and the air quantity Qair. Again, the available air quantity Qair is the current or actual air mass available in the cylinder and can be estimated based on the intake manifold pressure, temperature, and volumetric efficiency. The main lambda value Amain is a design lambda value applied for the computation of Qmain. Any appropriate strategy to define Amain can be used, one of which is explained in detail below. The main fuel command Qmain is determined from the following equation: Qmain = ;------------ ( Eq. 1 ) I f LUC IL A A CD ' ' ' Kstoich Where AFRstoich is the stochiometric air-fuel ratio in mass for hydrogen (34.3:1). This main fuel command Qmain is sent as instruction to the injector driving module. To determine the first post injection fuel command Qposti, a combustion fuel quantity Qcomb is first determined from the fresh air quantity Qairand a postl lambda value Apost 1 . The combustion fuel quantity Qcomb is determined from the equation: Qcomb = ;---—---- (Eq.2) Aposti *AFRst0 ich The first post injection fuel command Qposti is then determined from the difference between the combustion fuel quantity Qcomb and the main fuel command Qmain, namely Qposti Qcomb Qmain (Eq.3) The postl lambdal value is determined from a postl lambda mapping in function of engine speed and load. Hence, when Aposti is smaller (i.e. richer) than the Amain, Qcomb >Qmain and Qposti > 0. If no first post injection is required, e.g. outside of transient engine operation, Aposti and Qcomb may be ignored and Qposti is set to zero. This value for the postl fuel command Qposti may then be clamped between 0 and a maximum threshold based on injection timing, as will be described below. Qposti values smaller than a minimum threshold may also be rounded down to zero to prevent injector inaccuracies for low injection volumes, thereby effectively canceling the injection. Postl timing The injection timings for the postl event may be determined as follows. Referring now to FIG.3, the cylinder pressure (line 10), the lift of the exhaust valve (line 12), and the lift of the intake valve (line 14) valves are plotted vs. crankshaft angle (CA) for a given cylinder. The first firing top dead center (TDC1), exhaust valve opening (EVO), exhaust valve closing (EVC), and the second non-firing top dead center (TDC2) are also indicated in FIG.3. An authorized injection window AIW1 is defined for the first post injection event, within which the entirety of the postl event occurs. For the postl injection, injected fuel should bum within the cylinder. Accordingly, AIW1 ends before the exhaust valve opens. Furthermore, the postl event is determined to account for the in-cylinder pressure generated by the combustion of the fuel Qmain injected during the main event. In particular, it is desirable that the pressure in the fuel rail be greater than the cylinder pressure to prevent backflow. Accordingly, the start of the authorized window AIW is advantageously positioned after a timing for that the pressure ratio Praii / Pcyi exceeds a predetermined threshold, represented by line Pth on fig. 3, where Praii is the pressure (measured) in the fuel rail and Pcyi is the in-cylinder pressure (estimated). Praii is typically available of the fuel delivery system, since the fuel rail is conventionally equipped with a pressure sensor. Pcyi can be estimated from the amounts of air and fuel, angles of valve closing, engine compression ratio and combustion timing and efficiency. The boundaries of AIW1 are thus defined by the angular timings corresponding to the moment the predetermined pressure threshold Pth is reached, and to the exhaust valve opening EVO. These boundaries are preferably set with some safety offset, e.g. 5 to 10°, and may be used to define the timings for the start of injection or end of injection. The safety offset Offsl may be calibrated to depend on engine speed and / or engine load. Offs2 may be a calibratable offset, fixed over the engine operating range. As previously mentioned, Qposti may be limited by a maximum threshold based on injection timing. In embodiments, this maximum threshold is set to the maximum fuel mass that can be injected while fitting in AIW1. More specifically, the maximum angular duration of the injection is first determined, i.e. the distance between the injection timing and the furthest window boundary, depending on injection timing reference (start or end). Then this angular duration is converted into a time-based duration based on engine speed. Finally, the maximum fuel mass that can be injected is determined based on this time-based duration and the current rail pressure. This may be done using flow model mappings (calibrated maps similar to flow curves). < Second post injection event - Post2 > The second post injection event, post2 event, is performed during the exhaust stroke, whereby the injector is operated to inject a fuel quantity based on a post2 fuel command Qpost2. The second post injection is introduced while the combustion chamber is open and generates no torque whatsoever. Instead, the gaseous fuel is evacuated through the exhaust valves of the cylinder towards the exhaust-gas treatment system. The gaseous fuel may then be ignited therein to generate heat. This is particular useful to accelerated the light-off of an exhaust after treatment device. The implementation of the post2 event may advantageously be subject to enabling conditions, which may include: a temperature of an exhaust after treatment device being below a predetermined threshold; and optionally environmental conditions such as fuel rail pressure being within a predefined range, coolant temperature being within a predefined range, and exhaust temperature being within a predefined range. In other words, post2 events are particularly desirable to the heat-up exhaust after treatment device (e.g. a catalytic device) in a so-called light-off phase, or to maintain the temperature of the exhaust after treatment device above a predetermined temperature (e.g. in cold ambient conditions) Indeed, an exhaust-gas treatment device typically requires to be above a certain temperature before being fully operational. Post2 injections performed during the exhaust stroke and the subsequent ignition of the injected fuel are able to quickly increase the temperature of the exhaust-gas treatment system, thereby decreasing the time needed for the exhaust-gas treatment system to heat up. This is particularly useful at engine start up, when the exhaust-gas treatment system is cold. POST2 fuel quantity To determine the second post injection fuel command QpOst2, a fuel mass Qpost2OL is first determined based on the temperature of the exhaust-gas treatment system. QPost2OL can be read from a map that is calibrated to define fuel amounts corresponding to calorific energy required to heat up an exhaust-gas treatment system. In parallel, a total fuel quantity Qtotai is determined from the fresh air quantity Qairand a further lambda number: post2 lambda value Apost2. The total fuel quantity Qtotai is determined from the equation: ««« = x ., (Eq-4) ^post2^A^ ^stoich The post2 fuel command Qpost2 is then defined as the smallest value between the absolute open loop mass Qpost2OL, and the difference between the total fuel quantity Qtotai and the combustion fuel quantity Qcomb, i.e. Qpost2 MIN[QpOst2OL> Qtotai Qcomb^ (Eq5) If no post2 event is required, e.g. when the exhaust treatment system is sufficiently hot, APost2 and Qtotai may be ignored, and Qpost2 is set to zero. This value for the second post injection fuel command Qpost2 may then be clamped between 0 and a maximum threshold based on injection timing, as will be described below. Second post injection fuel commands Qpost2 values smaller than a minimum threshold may also be rounded down to zero to prevent injector inaccuracies for low injection volumes, thereby effectively canceling the injection. Post2 timing An authorized injection window AIW2 for the post2 event, within which the entirety of the post2 event occurs, is also defined. For the post2 event, injected fuel should bum in the exhaust treatment system. Accordingly, AIW2 starts after the exhaust valve opens, and ends before the exhaust valve closes. The boundaries of AIW2 are thus defined by the angular timings corresponding to exhaust valve opening EVO and to the exhaust valve opening EVC. These boundaries are preferably set with some safety offset, e.g. 5 to 10°, and may be used to define the timings for the start of injection or end of injection. These safety offsets Offs3, Offs4 may be calibrated to depend on engine speed and / or engine load. As previously mentioned, the post2 fuel command Qpost2 may be limited by a maximum threshold based on injection timing. In embodiments of the invention, this maximum threshold is set to the maximum fuel mass that can be injected while fitting in AIW2. The maximum fuel mass that can be injected while fitting in AIW2 may be computed in a similar manner as the maximum fuel mass that can be injected while fitting in AIW1, as described above. Further implementation details In embodiments, the main lambda may be determined according to a so-called fast lambda strategy that is applied at least during transient engine operation. Such fast lambda strategy is explained in WO2024 / 061765, which is incorporated herein by reference. This is only an example and other approaches may be applicable. In summary, the main lambda may be obtained by computing a raw lambda from the current air flow rate Qair and a main fuel demand. The main fuel demand corresponds to the fuel amount that is classically determined from the torque demand. This raw lambda is then capped to a maximum deviation with respect to the standard lambda value; and further capped between minimum and maximum lambda values that are calibrated in function of engine speed and load. For the sake of exemplification, such main lambda determination is explained with respect to Fig.5. The diagram is simplified and there could be intervening functions. Conventionally, a torque structure module receives torque demands from various components, for example direct torque demand from the driver (accelerator pedal) or indirect torque demand via cruise control, torque demands from the transmission system, from driving dynamics, or torque demands related to specific components (catalytic converter, HVAC, etc.). The torque structure module coordinates these various demands and generates a global torque demand Td. A desired fuel mass Qd (also referred to as fuel demand) is then determined to meet the desired torque demand Td, typically by calculation based on IMEP (Indicated Mean Effective Pressure), cylinder volume and combustion efficiency coefficients. A desired air mass Md is computed based on the desired fuel mass Qd and taking into account a desired lambda read from a mapping Lstd in function of engine speed and load. The throttle and turbocharger gate positions are adjusted on the basis of the desired air mass. The Lstd mapping defines lambda number calibrated for a lean combustion, which is thus designed for rather steady state operation. As shown, Lstd is not directly used as set for the determination of the fuel command Qmain, but the set point is Amain. Qmain is determined from Qair and Amain aS explained with respect to Fig.1, by implementing Eq.1. In brief, Amain is determined in a module that receives as input the current air Qair and the standard lambda value Lstd. First, a so-called raw lambda Lraw is computed based on the desired fuel mass Qd and on the actual air flow rate Qair: ^raw QairF QiyAFRstoich (Eq.6) Lraw is then capped to a maximum deviation with respect to the standard lambda value. Namely, the value Lraw is compared to a value corresponding to a predetermined deviation ratio Rdev with respect to Lstd, which is used as maximum deviation ratio. If Lraw exceeds the maximum value of the predetermined deviation ratio Rdev applied to Lstd, then Lraw is bound (limited) to the max value. The predetermined deviation ratio Rdev is conveniently dependent on the difference in torque demand. In practice, Rdev may be mapped in function of the difference in torque demand. It is thus possible to vary Rdev with the amplitude of difference in torque demand. Furthermore, it is therewith possible, to set Rdev to zero to forbid variations from Lstd, where the difference in torque demand is zero or very small. Hence in fact Lfast may be computed all the time but configured such that it cannot vary from Lstd where the difference in torque demand reflects steady state operation. Lraw is then further capped between minimum and maximum values that are calibrated in function of engine speed and load. This can be done using a table defining: - A maximum limit Lmax to avoid too “lean” combustion in order to keep 5 acceptable combustion stability; - A lower limit Lmin to avoid too rich combustion to control Nox emissions and abnormal combustions; where Lmax and Lmin depend on engine speed and load. Hence if Lraw is within the range [Lmin; Lmax] then the value Lraw is used as fast 10 lambda value, i.e. as setpoint. This can be written as LFAST=Lraw. If Lraw exceeds Lmax, then Lfast takes the value Lmax (LFAST=Lmax) If Lraw is below Lmin, then Lfast takes the value Lmin (LFAST=Lmin) The so determined value of Amain is then applied for the determination of the fuel command Qmain. 15

Claims

1. Method for operating a gaseous fuel internal combustion engine, the engine comprising a fuel delivery system with fuel rail configured to inject fuel in respective engine cylinders, a turbocharger and an exhaust line with at least one exhaust after treatment device;wherein combustion events are performed to generate torque, by which a fuel quantity is injected into a respective engine cylinder based on main fuel command (Qmain) that is determined from a main lambda value (Amain) and a quantity of available air (Qair);wherein a post injection strategy is performed to influence exhaust line temperatures, said post injection strategy comprising performing at least one of:an injection event, referred to as postl event, that is performed during a power stroke after said main injection event, by injecting a fuel amount that is based on a postl fuel command, in order to increase the enthalpy of exhaust gas arriving at the turbocharger, the postl fuel command (Qposti) being determined from the difference between a combustion fuel quantity (Qcomb) and the main fuel command (Qmain), whereby the combustion fuel quantity (Qcomb) is determined from a postl lambda value (Aposti), that is smaller than the desired main lambda value (Amain), and the quantity of available air (Qair); andan injection event, referred to as post2 event, that is performed during the exhaust stroke, by injecting a fuel amount that is based on a post2 fuel command, in order to increase the heat in exhaust line, wherein the post2 fuel command (Qpost2) is determined from a value (Qpost2A) determined from the difference between a total fuel quantity (Qtotai) and the previously injected cylinder fuel quantity (Qcomb), the total fuel quantity (Qtotai) being determined from a post2 lambda value (Apost2), that is smaller than the main lambda value (Amain), and the quantity of available air (Qair).

2. Method according to any of the preceding claims, wherebythe postl lambdal value is determined from a postl lambda mapping in function of engine speed and load; and / orthe post2 lambda value is determined from a post2 lambda mapping in function of engine speed and load;3. Method according to any of the preceding claims, whereby when both postl event and post2 event are performed, the post2 lambda value is smaller than the postl lambda value.

4. Method according to any of the preceding claims, whereby the main fuel command (Qmain) is computed from Qmniv =----—----.\ / I V IJIUUI Tl -j, a c dKstoich5. Method according to any of the preceding claims, whereby the combustion fuel quantity (Qcomb) is computed from Qcomb = ----—----.kpostl^FRstoich.

6. Method according to any of the preceding claims, whereby postl events are performed if postl enabling conditions are satisfied, wherein the postl enabling conditions comprise: the engine being in a transient operational state; and optionally, at least one of: fuel rail pressure being within a predefined range, coolant temperature being within a predefined range, and exhaust temperature being within a predefined range;and preferably if said postl enabling conditions are not satisfied then the postl fuel command is set to zero.

7. Method according to any of the preceding claims, whereby the first post injection event starts at a timing which corresponds to a crank angle for which a ratio between a fuel rail pressure and a cylinder pressure exceeds a predetermined threshold, plus an additional offset.

8. Method according to the previous claim, whereby the additional offset is determined from a calibrated mapping relating said additional offset to engine speed and / or engine load.

9. Method according to any of the preceding claims, whereby the postl event ends at the latest at a timing which corresponds to an opening of an exhaust valve, plus an additional offset.

10. Method according to claims 7 or 8 and according to claim 9, whereby the postl fuel command (Qposti) is capped based on a maximum first post injection value;whereby said maximum first post injection value is determined based on a maximum first post injection pulse width;whereby said first post injection pulse width is determined based on a difference in timing between the start of the first injection event and the end of the first injection event.

11. Method according to any of the preceding claims, whereby if the postl fuel command (Qposti) is smaller than a minimum first post injection threshold, said postl fuel command (Qposti) is set to 0, and / or no first post injection event is triggered; orwhereby if the post2 fuel command (QPost2) is smaller than a minimum post2 injection threshold, said post2 injection fuel command (QPost2) is set to 0, and / or no post2 event is triggered.

12. Method according to any of the preceding claims, whereby a post injection spark event is triggered after the start of the postl event to ignite fuel delivered during the postl event.

13. Method according to any of the preceding claims whereby the second post injection fuel command (QPost2) is determined from a minimum value between a mapped fuel amount (Qpost2OL) corresponding to a calorific energy required to heat up an exhaust-gas treatment system and said value (Qpost2A).

14. Method according to any of the preceding claims, whereby the total fuel quantity (Qtotai) is computed from Qtotal = ---------.^post2^A^ ^stoich15. Method according to any of the preceding claims, whereby post2 events are performed if post2 enabling conditions are satisfied, wherein the post2 enabling conditions comprise: a temperature of exhaust after treatment device being below a predetermined threshold; and optionally, at least one of: fuel rail pressure being within a predefined range, coolant temperature being within a predefined range, and exhaust temperature being within a predefined range.

16. Method according to any of the preceding claims, whereby the second post injection event starts at the earliest at the timing which corresponds to an opening of an exhaust valve, plus an additional offset;and whereby the second post injection event ends at the latest at the timing which corresponds to a closure of an exhaust valve, plus an additional offset.

17. Method according to the previous claim, whereby the post2 fuel command (QPost2) is capped based on a maximum post2 injection value;whereby said maximum post2 injection value is determined based on a maximum post2 injection pulse width;whereby said post2 injection pulse width is determined based on a difference in timing between the start of the post2 event and the end of the post2 event.

18. Method according to any of the preceding claims, wherein at least during transient engine operation, the main lambda is obtained by computing a raw lambda from the current air flow rate Qair and a main fuel demand; and the raw lambda is capped to a maximum deviation with respect to the standard lambda value; and further capped between minimum and maximum lambda values that are calibrated in function of engine speed and load.

19. A control unit comprising instructions which, when executed, cause the control unit to carry out the method according to any of the preceding claims.

20. A computer readable medium comprising instructions which, when executed by a control unit comprising a processor, cause the control unit to carry out the method of any one of claims 1 to 18.IntellectualPropertyOfficeApplication GB2501519.9Search report under Section 17 of the Patents Act 1977Date search completed: 22 July 2025Claims searched: 1-20International classificationSubclass and subgroup Valid from F02D19 / 02 01 / 01 / 2006 F02D41 / 00 01 / 01 / 2006 F02D41 / 02 01 / 01 / 2006 F02D41 / 14 01 / 01 / 2006Field of searchWorldwide search of patent documents classified in the following areas of the IPC:F02DDatabases used in the preparation of this search report:SEARCH-PATENTDocuments considered to be relevantPatent literatureCategory Relevant Document of relevanceclaimsY 1,2, 4-6, 9, 12, 19, 20 US 2015075487A1 (GLUGLA et al), See whole document with reference to fuel injection during the exhaust stroke to increase exhaust gas enthalpy for reducing turbo lag. Y 1,2, 4, 5, 14-16, 19, 20 US 2015167576A1 (GLUGLA et al), See whole document with reference to multiple fuel injections in a gaseous fuel engine to increase heat in the exhaust line. Y 1,2, 4-6, 9, 12, 14-16, 19, 20 WO 2024061765 A1 (BORGWARNER), See whole document with reference to operating a hydrogen combustion engine with a fast lambda strategy. Non-patent literature Category Relevant claims Document of relevanceCategoriesLetter or symbolDescriptionX Document indicating lack of novelty or inventive step.Y Document indicating lack of inventive step, if combined with anotherdocument of the same category.& Member of the same patent family.A Document indicating technological background.Letter or symbol Description P Document published on or after the priority date but before the fling date of the present application. E Earlier application published on or after the filing date of the present application.

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