Method for operating an indirect injection gaseous fuel engine

Abstract

A method for controlling indirect injections for a hydrogen powered internal combustion engine comprises determining a fuel demand QD from an input torque demand TD; determining 18 an initial fresh ai

Description

Technical field The present invention generally relates to a method for operating a gaseous fuel internal combustion engine, more specifically to a method of controlling indirect injections for a hydrogen powered internal combustion engine. Background Art The transition to cleaner and more sustainable energy sources has prompted a strong interest in hydrogen as a potential alternative fuel for combustion engines. Hydrogen combustion engines (H2-ICE) offer the prospect of significantly reducing greenhouse gas emissions and contributing to a more environmentally friendly transportation and industrial landscape. As hydrogen gains traction as a viable fuel, the precise control of torque in hydrogen combustion engines emerge as crucial factors in achieving optimal performance, efficiency, and emissions reduction. Today, hydrogen fueled engines are developed I designed based mainly on the knowledge and know-how gained with gasoline engines, making required adaptations and new developments. As is known, gasoline / gas operated engines are conventionally controlled based on an ‘air lead’ approach, where lambda is mainly constant (stoichiometric), the air charge being adjusted in function of load and then the fuel being computed from the fresh air flow to meet the lambda setpoint. This control of the air fuel mixture is required for optimal operation of the catalytic converter. One drawback of this conventional approach, when applied to hydrogen combustion engines, is a lag in torque response, when the driver depresses the accelerator pedal to request a rapid change of torque. This lag is due to a combination of elements: boost lag from the turbocharger running with low enthalpy (lean lambda setpoint); lambda demand depends on actual torque to get consistent combustion setpoints (Air / spark / injection timing demand). A prior strategy developed by the present inventors is shown in Figure 1 and relates to a method of operating a hydrogen combustion engine which alleviates the above mentioned drawback by computing a so-called fast lambda value, which is based on the current air flow rate and the current fuel demand. However, with gaseous fuels such as hydrogen, estimating air flow rates in indirect injection systems is more challenging than in direct injection systems. Indeed, in indirect injection systems, fuel is injected into the intake manifold I upstream of intake valves (also known as PFI application - for Port Fuel Injection), rather than directly into the combustion chamber where the air and fuel mixture ignites. As hydrogen density is very low, it occupies a considerable volume in the intake manifold (PFI application), which must be considered to accurately estimate the air flow rate. Technical problem It is an object of the present invention to provide a method of controlling indirect injections for a hydrogen powered internal combustion engine which overcomes the aforementioned drawbacks. This object is achieved by a method as claimed in claim 1. General Description of the Invention The present invention proposes a method for controlling injection in a hydrogen powered internal combustion engine, wherein hydrogen is indirectly injected into the engine. The method comprises the steps of: - determining a fuel demand Qd from an input torque demand Td; - determining an initial fresh air flow Mf, k based on a previous specific gas Constant Rmixing, kj - determining a fast lambda value 1 Fast based on the initial fresh airflow Mf, kJ - determining an updated specific gas constant Rmixing, k+1 based on the fast lambda ZFast; - determining an updated fresh air flow MF,k+i based on the updated specific gas constant Rmixing, k+1 J - determining a fuel quantity Qf based on the updated fresh air flow Mf, k+1; - performing an injection event by injecting said final fuel quantity in at least one cylinder. This method has been developed to address lambda management during torque transients in indirect injected (PFI) engines. It is thus advantageously applied in case of a variation in torque demand. Compared to prior art strategies, the invention allows for an accurate trapped air estimation of PFI engine running with variable lambda setpoint, as is the case in H2-ICE engines, and thus for an improved combustion lambda control. Further benefits of the inventive method are the reduction of combustion issues (knock, pre-ignition) and pollutant emissions (NOx). The fast lambda value AFast is a lambda number that is used in torque transient and typically computed based on the current airflow rate Mf and advantageously further on current fuel demand Qd. In steady state, for a given Torque Demand, a desired air mass Md is typically determined based on the fuel demand Qd and on a standard lambda number (lambda desired), and at least one charge air parameter is adjusted based on said desired air mass Md. The fuel quantity Qf to be injected is then determined based on current (intake) air flow Mf rate and on a lambda setpoint corresponding to the standard lambda number (lambda desired). In case of a variation in torque demand, the lambda setpoint is then the fast lambda value XFast. In embodiments, the steps of determining a fast lambda value XFast', Determining an updated specific gas constant Rmixing, k+1; and Determining an updated fresh air flow Mf, k+i may be repeated one or more time for improving convergence. In embodiments, the fast lambda value Xfast is further determined based on the fuel demand Qd. In embodiments, the fast lambda value Xfast is clamped to an interval centered on a desired lambda value XD. The width of said interval may be determined from a difference in torque demand and the desired lambda value XD. In embodiments, XFast is clamped between a minimum lambda value Xmin and a maximum lambda value that are calibrated in function of engine speed and engine load. For example, Amin may be comprised between 1.5 and 1.8, and is comprised between 2.2 and 3.5. Preferably, the fresh air flow Mf, k (air mass) is computed based on a specific gas constant Rmixing.k and is derived from, resp. is done according to, the following formula: . . Pintake * ^cylinder * Effvol = —T------- 1 intake ^mixing,k where Pintake is the gas pressure in the intake port, VcyUnder is the volume of a cylinder, Effvot is the volumetric efficiency, Tintake is the temperature in the intake port, and RmiXing,k is the specific gas constant. Preferably, the specific gas constant RmiXing,k is computed based on fast lambda \ast and derived from, resp. is done according to, the following formula: R n _ _________________ ^mixing,k n ao A Past where R is the ideal gas constant, Mair is the molar mass of air, MH2 is the molar mass of hydrogen, and XFast is the fast lambda value. Brief Description of the Drawings A preferred embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is flowchart illustrating a method for operating a direct injection system using a fast lambda value; and Figure 2 is flowchart illustrating an embodiment of the method according to the present invention. Description of Preferred Embodiments Figure 1 shows a flowchart representing the method for operating a direct injection system with a fast lambda value; this method has been described by the present inventors in patent application GB 2213718.6, which is herein incorporated by reference. Reference sign 10 designates a torque structure module that 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, from the gearbox or torque demands related to specific components (e.g. accessory torque). The torque structure module 10 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. At 12 a desired air mass Md is computed based on the desired fuel mass Qd and on a desired lambda XD. The throttle and turbocharger gate positions are adjusted on the basis of the desired air mass Md. That is, in steady state, the predetermined lambda value is a desired Lambda number XD. The fuel calculation module 14 determines a final fuel mass Qf, i.e. the fuel quantity to be injected into the engine for the upcoming combustion cycle (total fuel for all cylinders) and used in the engine management scheme. The final fuel mass Qf is determined on the basis of the fresh air flow Mf and for a given lambda setpoint corresponding toAD. The method of Fig.1 applies a strategy whereby a different lambda value A is used for the calculation of the final fuel mass in case of a change in torque demand. Roughly speaking, desired lambda XD values are calibrated to be applied during steady state engine conditions, i.e. when little to no increase or decrease of torque demand Td occurs. Desired lambda XD values typically correspond to a lean air / fuel ratio. For example XD may be in the range of 3.5 to 2.5 at low-mid load and down to 1.6 at full load (this is however only an example and rather depends on the engine). When the driver presses the pedal, or in case of deceleration (i.e. where the engine is no longer in steady state operation), there is a difference (i.e. a variation) in torque demand Td. A difference in torque demand Td can generally be determined by comparing the new torque demand (e.g. Td) to a previous reference value of torque demand. In case a variation in torque demand is present, then a different lambda (module 16) referred to as fast lambda value XFast, which is different from the desired lambda XD, is used (as setpoint) for the fuel global mass calculation 14. The comparison can e.g. be done by subtraction or by computing a ratio, the result of which can then either be compared to a threshold or directly used as input to influence the value XFast. In embodiments, the new torque demand is compared to a moving average of torque demand (for previous combustion events or cycles). The fast lambda value XFast is computed based on the desired fuel mass Qd, the fresh air flow Mf (determined by module 18) and the desired lambda XD (module 20). More specifically, a quantity Xraw may be computed as: Xraw =------------ [Eq. 1] raW QD-AFRstoichH2 L J where AFRstoichH2 is the mass stoichiometric ratio of fuel to hydrogen (34.33 : 1). This Xraw is then subsequently clamped to an interval based on the desired lambda XD, the engine speed and the engine load to obtain the fast lambda value XFast. The width of the clamping interval may depend on the difference in torque demand. For example, a table may be defined having: - A maximum limit Lmax to avoid too “lean” combustion in order to keep 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 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 final fuel mass Qf is then determined in module 14 based on the fresh airflow Mf and the fast lambda value AFast (used as lambda setpoint). The final fuel mass Qf represents the fuel quantity to be injected into the engine for the upcoming combustion cycle (total fuel for all cylinders) and used in the engine management scheme. <lnvention> Whilst the above method enables to address transient load operations in direct injection engines, it requires some adjustments to properly function with indirect injection engines. Indeed, both the final fuel mass Qf and the fast lambda value AFast are determined based on the fresh air flow Mf. However, with gaseous fuel such as hydrogen, estimating air flow rates in indirect injection systems is more challenging than in direct injection systems. Indeed, as hydrogen density is very low, it occupies a considerable volume in the intake manifold (PFI application), which must be considered to accurately estimate the air flow rate. The present invention thus proposes an improvement of the method of Fig.1, specifically adapted for indirectly injected hydrogen combustion engines, i.e. where hydrogen is introduced in the fresh air flow upstream of the cylinder. Hence, the functions modules operate in the same / similar manner, unless specified otherwise. As will be seen, the functional flow chart of Fig.2 is mostly identical to that of Fig. 1, except for the new module 22 relating to the Gas constant Rmixing. Again, to recap, to generally operate a combustion event a predetermined fuel quantity Qf needs to be injected in an engine cylinder. Therefore, a fuel demand Qd is determined from an input torque demand Td. Module 12 determines a corresponding desired air mass Md based on the fuel demand Qd and on a desired lambda value XD, whereby at least one charge air parameter is typically adjusted based on said desired air mass Md. The predetermined fuel quantity Qf based on current (intake) airflow Mf rate and on a lambda setpoint Lset. In steady state, Lset corresponds to the desired lambda value XD. However, in case of a variation in torque demand, the lambda setpoint Lset will change to take the value of lambda fast AFast. Furthermore, Qf is then determined based on an updated fresh air mass. The fresh air flow Mf (air mass) can generally be computed from the following formula: _ Pintake*^cylinder*^ffvol [Eq. 2] Tintake Rmlxln9 where Pintake is the gas pressure in the intake port, VcyUnder is the volume of a cylinder, Effvot is the volumetric efficiency, Tintake is the temperature in the intake port, and Rmixing is the specific gas constant. This air mass calculation is carried out by module 18 based on Eq.2. For direct injection, the composition of the gas volume admitted (trapped) into the cylinder is pure air, hence R=287 [J / kg.K] is constant. However, for indirect injection, the flow admitted into the cylinder via the inlet valves is a mix of air and hydrogen, the proportion of which is given by the lambda value 1. In this case, the specific gas constant R of this mixing is defined by the following formula: Rmixing = [Eq. 3] where Mair is the molar mass of air and MH2 is the molar mass of hydrogen. Hence the lambda value A depends on the fresh air flow Mf, which itself depends on the specific gas constant Rmixing, which itself depends on the lambda value A. In other words, for indirect injection engines, these quantities are highly mathematically coupled and thus difficult to accurately determine. The prior strategy of Fig.1 is thus modified by implementing modules 18 and 22 to determine the fresh air flow Mf and the specific gas constant Rmix, to take into account the volume occupied by hydrogen in the case of PFI engine. To overcome this difficulty and accurately approximate these quantities, the inventive methods comprise the following steps in respect of an injection event: a) determining a fuel demand Qd from an input torque demand Td; b) determining an initial fresh air flow Mf, k based on a previous specific gas Constant Rmixing, kj c) determining a fast lambda value 1 Fast based on the initial fresh air flow MF,k; d) determining an updated specific gas constant Rmixing, k+1 based on the fast lambda ZFast; e) determining an updated fresh air flow Mf, k+1 based on the updated specific gas constant Rmixing, k+1 J f) determining a final fuel quantity Qf based on the updated fresh airflow MF,k+i. So, in step b) an initial value of Mf is computed taking into account a previous value of Rmixing. The previous value of Rmixing is a known / stored value previously determined, typically for the previous injection event (e.g. 180° of CA before, i.e. for the previously fueled cylinder). The computation of Mf is done using Eq.2, with actual values of pressure and temperature (i.e. currently determined / measured). In step c) the value of fast lambda XPast is determined based on initial Mf and Qd (module 16). At step d) an updated value of Rmixing is computed, and next an updated value of Mf is computed with Eq.2 based on the updated Rmixing and the actual intake pressure and temperature values (same as in step b). Next, the fuel quantity QF is determined by module 14 based on updated Mf and fast lambda XFast. Subsequently, (step g) an injection event is performed by injecting said final fuel quantity Qf in the respective engine cylinder. The inventive method enables to minimize possible errors and avoid running “too rich” when the torque demand increases. Hence the present method / strategy is beneficial in case of a variation in torque demand. In practice, steps c), d) and e) may be repeated at least one time, to improve convergence. The terms ‘previous’, as in previous specific gas constant Rmixing, k, refers to an existing value, previously determined (and available in memory), for example for the previous cylinder in fueling order or for a previous determination for upcoming cylinder to be fueled according to fueling order. The present method is typically implemented by a control unit, e.g. an engine control unit, by means of appropriate software and hardware. In particular the control unit 5 may comprise a data processing system storing instructions which, when executed by the system, cause the latter to perform the steps of the present method.

Claims

1. A method for controlling indirect injections for a hydrogen powered internal combustion engine, the method comprising the steps of:- determining a fuel demand Qd from an input torque demand Td;- determining an initial fresh air flow Mf, k based on a previous specific gas Constant Rmixing, kj- determining a fast lambda value 1 Fast based on the initial fresh air flow MF,k;- determining an updated specific gas constant Rmixing, k+1 based on the fast lambda 1 Past ;- determining a updated fresh air flow Mf, k+1 based on the updated specific gas constant Rmixing, k+1 J- determining a fuel quantity Qf based on the updated fresh air flow Mf, k+1;- performing an injection event by injecting said fuel quantity into an engine cylinder.

2. The method according to any of the preceding claims, wherein the steps of:- Determining a fast lambda value 1 Fast;- Determining an updated specific gas constant Rmixing, k+1;- Determining an updated fresh air flow Mf, k+1;are repeated at least one time.

3. The method according to any of the preceding claims, wherein the fast lambda value 1 fast is further determined based on the fuel demand Qd.

4. The method according to any of the preceding claims, wherein the fast lambda value 1 fast is clamped to an interval centered on a desired lambda value 1D.

5. The method according to the preceding claim, wherein a width of said interval is determined from a difference in torque demand and the desired lambda value 1D.

6. The method according to any of the preceding claims, wherein 1 Fast is clamped between a minimum lambda value and a maximum lambda value lmnx that are calibrated in function of engine speed and engine load.

7. The method according to the preceding claim, wherein Xmin is comprised between 1.5 and 1.8, and 1 max is comprised between 2.2 and 3.5.

8. The method according to any of the preceding claims, wherein said fuel demand Qd, respectively said fuel quantity Qf, are determined in respect of an upcoming injection event in a respective cylinder.

9. The method according to any of the preceding claims, wherein computing a fresh air flow Mf, k based on a specific gas constant RmiXing,k is derived from the following formula:. . Pintake * ^cylinder * Effvol = —T------- 1 intake ^mixing ,kwhere Pintake is the gas pressure in the intake port, VcyUnder is the volume of a cylinder, Effvot is the volumetric efficiency, Tintake is the temperature in the intake port, and RmiXing,k is the specific gas constant.

10. The method according to any of the preceding claims, wherein computing a specific gas constant Rmixing, k based on the fast lambda is derived from the following formula:R^mixing,k n 47A PastFastwhere Rmixing, k is the specific gas constant, R is the ideal gas constant, Mair is the molar mass of air, MH2 is the molar mass of hydrogen, and 1 Fast is the fast lambda value.

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