Method of operating an internal combustion engine powered with gaseous fuel

Abstract

A method of operating an internal combustion engine (ICE) powered with gaseous fuel, particularly hydrogen, whereby the engine comprising a fuel tank assembly 12 containing pressurized fuel, a gas supply line 18 connects the tank assembly to a fuel rail 16 delivering gaseous fuel to a gas injector(s) 20. A pressure regulator (22) integrated in the gas supply line reduces the upstream supply gas pressure (Psupply) to a controllable output pressure to achieve a predetermined rail pressure. The method comprises the steps of: monitoring (S10) the supply gas pressure (Psupply) upstream of the pressure regulator, determining (S12) a maximum gas consumption (Cmax) within the gas rail based on the supply gas pressure and on a target rail pressure (PTR) and limiting (S16) engine torque based on said maximum gas consumption. The maximum gas consumption may be determined from a mapping that relates the target rail pressure and supply gas pressure and may take into account the supply gas temperature. Preferably limiting engine torque involves computing a gas quantity per injection event(s) based on engine speed and on the number of engine cylinders. An ICE and computer program are also claimed.

Description

Technical field The present invention generally relates to the field of internal combustion engines and more specifically to the operation of such engines powered with gaseous fuels, in particular hydrogen. Background Art For automotive applications, hydrogen engines are considered as a promising alternative to gasoline or diesel engines. Indeed, emissions from hydrogen internal combustion engines consist mainly of water and do not comprise nearly as much pollutants as those from traditional engines. In conventional engines (diesel / gasoline), the fuel delivery system typically comprises a liquid fuel tank with a low-pressure pump, a high-pressure pump, a fuel rail and a plurality of gas injectors. The high-pressure pump allows precisely controlling the fuel pressure in the fuel rail. On gas combustion engines however, the gas is stored at high-pressure in a gas tank, and the operating pressure, namely rail pressure, is always lower than the tank pressure. As a result, unlike liquid fuels, the flow rate of hydrogen in the engine is directly influenced by the pressure in the gas tank. This may cause some difficulties in engine control when the tank pressure becomes low and the consumption is high. Technical problem It is an object of the present invention to provide an improved method of controlling a gaseous fuel powered internal combustion engine. General Description of the Invention The invention is based on the inventor’s observation that, unlike liquid fuels where a pump is used to maintain a constant pressure in the fuel rail, the pressure in fuel rails of hydrogen I gas combustion engines depends on the gas pressure in the storage tank. Indeed, engine operating gas pressures are always lower than the tank pressure. To ensure a controllable system behavior, the operating gas pressure needs to stay above a predetermined value. But this can be difficult when tank pressure becomes low and consumption is high. This problem is solved by a method of operating an internal combustion engine powered with gaseous fuel as claimed in claim 1. Accordingly, the invention relates to a method of operating an internal combustion engine powered with gaseous fuel, the engine comprising a tank assembly containing pressurized fuel, a gas supply line connecting the tank assembly to a fuel rail delivering gaseous fuel to at least one gas injector arranged to inject gaseous fuel into an engine cylinder, and a pressure regulator integrated in the gaseous fuel supply line and configured to reduce the upstream gas pressure to a controllable output pressure to achieve a predetermined rail pressure. The inventive method comprises the steps of - monitoring the supply gas pressure upstream of the pressure regulator; - determining a maximum gas consumption Cmax within the gas rail based on the supply gas pressure and on a target rail pressure; and - limiting engine torque based on said maximum gas consumption Cmax. The inventive method hence proposes an engine control strategy wherein a maximum gas consumption Cmax value is determined under the current upstream and downstream conditions. Cmax represents the maximum gas flow rate through the fuel rail, and hence determines the maximum available gas amounts that can be delivered by the injectors. Engine operation is then controlled to limit engine output torque based Cmax. In practice, the torque limitation can be performed by acting on fuel quantities (e.g. by using maximum gas quantity threshold), or by acting on torque. This latter option is easier to implement since in engine control strategies fuel quantities are normally determined from the torque demand. The target rail pressure is a predetermined pressure value to be achieved in the fuel rail. The target rail pressure can correspond to an operational demand rail pressure for a given operating point, as read from conventional control strategies, or may represent a safety operating pressure, which may correspond to the demand rail pressure corrected by a safety coefficient or offset, i.e. typically lower than the nominal demand rail pressure. The maximum gas consumption Cmax may be determined from a mapping that relates the target rail pressure and supply gas pressure, and preferably further taking into account the supply gas temperature. Indeed, the gas flowrate through a regulating valve (such as the pressure regulator) is proportional to the orifice crosssection of the valve and depends on the difference between upstream and downstream pressures, here represented by supply gas pressure (i.e. generally the pressure in the tank assembly) and target rail pressure, respectively. This mapping can be experimentally calibrated by testing carried out on a fuel rail with predetermined through-bore forming a leakage path; or alternatively can be simulated. Limiting engine torque may involve computing a gas quantity per injection event based on engine speed and on the number of engine cylinders. That is a fuel quantity is computed that corresponds to the maximum consumption rate (i.e. Cmax) and taking into account the operating conditions, i.e. the engine revolution speed (e-RPM). This gas quantity can be expressed in any desirable unit and computed per cylinder (injection event per cylinder) or per engine cycle (i.e. total fuel mass for all cylinders). As alluded above, since torque is related to injected fuel quantity, one possibility is to limit torque at the level of the fuel structure by using limiting the injected amount to a maximum fuel quantity determined from Cmax. In preferred embodiments, limiting engine torque involves determining a maximum allowable torque Tmax based on the maximum gas consumption Cmax, and controlling the engine to keep engine output torque below the maximum allowable torque Tmax. Here, gas quantity per injection event(s) and determined based on Cmax is used to determine a corresponding torque value, which is the maximum allowable torque Tmax. This determination of Tmax can in particular be involve the use of a mapping relating engine torque and gas quantity (as conventional in the art); the mapping can be expressed as gas quantities per injection event (per cylinder) or for an entire engine cycle (i.e. for all injection events / all cylinders). To allow for a certain range of operating pressures in the fuel rail, Conventionally, the pressure regulator may be electronically controlled, which allow for range of operating pressures, e.g. between 2 and 45 bar. In embodiments, the supply gas pressure is the pressure in the tank assembly. In embodiments, the tank assembly comprises a mechanical pressure reducer configured to deliver gaseous fuel into the gas supply line at a fixed pressure, in particular between 45 and 50 bar. The above method may be performed in real time by a control unit (e.g. engine control unit) comprising software and / or hardware configured to perform the herein disclosed algorithms and functions. According to another aspect, the invention relates to an internal combustion engine comprising a plurality of cylinders and a fuel delivery system including a tank assembly containing pressurized fuel, a gas supply line connecting said tank assembly to a fuel rail delivering gaseous fuel to gas injectors arranged to inject gaseous fuel in the respective cylinders, a pressure regulator being integrated in the gaseous fuel supply line and configured to reduce an upstream gas supply pressure to a controllable output pressure to achieve a predetermined rail pressure; and a control unit configured to perform a method according to the present disclosure. It remains to be noted that whereas the present invention has been developed in the context of hydrogen combustion engines, it is however likewise applicable to engines operating with other gaseous fuels, e.g. CNG, biogas, methanol, syngas, etc. Brief Description of the Drawings Preferred embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings in which: Fig. 1 is a principle diagram of gas delivery system fora hydrogen powered internal combustion engine; Fig. 2 is a flowchart of an embodiment of the present method; Fig. 3 is a functional diagram implementing the method of Fig.2. Description of Preferred Embodiments Fig.1 schematically depicts a gas delivery system 10 as used in internal combustion engines operated with gaseous fuel. The fuel delivery system comprises a fuel tank assembly 12, which comprises one or several fuel tanks storing gaseous fuel, e.g. hydrogen or CNG, at high pressure Piank, e.g. up to 350 or 700 bar. The fuel tank assembly 12 is connected to a fuel rail 16, or gas rail, via a supply line 18. The gas rail 16 is typically formed as a tubular member that is connected to the supply line 18 and comprises a plurality of outlet ports to which gas injectors 20 are coupled (here four injectors). The gas rail 18 comprises an internal chamber that forms a storage volume for gas before flowing to the gas injectors 20 (here four). The gas injectors 20 in turn are arranged to discharge gas into respective engine cylinders (not shown). Conventionally, the gas injectors 20 are electromagnetically controlled e.g. by means of a solenoid (not shown). When energized, their respective valve group is opened and gas is discharged into the cylinder. In practice, the tank assembly 12 and fuel rail 16 are equipped with pressure and temperature sensors. In the following, Piank and Tiank correspond to measured pressure and temperature of the gas in the tank assembly 12. PRaii corresponds to measured gas pressure in the rail 16. The gas delivery system 10 includes pressure regulation means configured to reduce the pressure of the stored gas and regulate the working pressure in the fuel rail. Conventionally, the fuel tank assembly 12 may include one or more tanks and a pressure reducer (e.g. mechanical pressure regulator) such that gas is released at a lower, generally constant pressure into the supply line, for example around 45 to 50 bar. A controllable pressure regulator 22 (typically electronic pressure regulator) is advantageously integrated in the gaseous fuel supply line and configured to reduce the upstream gas pressure Pin (pressure upstream of / at inlet of regulator 22) to a controllable (adjustable) output pressure Pout, e.g. between 2 and 40 bars. Since downstream of the pressure regulator 22 the fuel supply line 18 communicates with the fuel rail 16, we typically have Pout= PRaii- As is known in the art, in operation, injection events operated by actuating the fuel injectors 20 permit to deliver controlled gas quantities into the engine in order to achieve a given torque demand. The injectors 20 draw gas from the gas rail 16. In use, the pressure regulator 22 is operated to control the downstream gas pressure Pout, i.e. also in the gas rail 16, and maintain the pressure in the gas rail at predetermined operating pressures. Conventionally, desired operating rail pressures, referred to as demand rail pressure Pdr, are associated with fuel demands (e.g. in maps). Conventionally, injection control strategies are operated by an engine control unit, which may comprise software and / or hardware configured to perform the injection control strategies and other algorithms and function for engine operation. In particular the engine control unit may comprise one or more processors in operational communication with ROM and RAM memories, as well as with input sensors and output actuators, and a communication interface. It may be noted that for good system operation, it is desirable to guarantee a stable demand rail pressure Pdr. The demand rail pressure Pdr is typically defined by calibration (depending e.g. on engine design, engine operating point). Indeed, properly maintaining Ppaii at the demand rail pressure Pdr is of relevance because the rail pressure level affects injector opening, gas jet characteristic and combustion, as well as and injected flow rate. As will be understood, if gas consumption by the injectors is higher than the possible flow through the filling valve (i.e. pressure regulator 22), then the rail 16 will discharge and the pressure will decrease. < Inventive method > In order to avoid such situation, the present invention provides a control strategy, an embodiment of which will now be described with reference to Fig. 2. According to the inventive method, the pressure upstream of the pressure regulator 22 is determined at step S10. This upstream pressure is preferably measured by the tank pressure sensor. It can be read continuously or with a predetermined frequency. On the high-pressure side, there may be other pressure regulators as explained. Hence, the pressure upstream pressure regulator 22 can be referred to as Psuppiy. However, since the pressure source is the tank assembly 12, in practice one may assume that Psuppiy=Prank. Next, at S12, a maximum gas consumption, noted Cmax, is determined based on the supply gas pressure Psuppiy and on a target rail pressure, Ptr. Cmax represents a flow rate through the fuel rail, it may be expressed as a mass per time unit. According to known physics principles, the gas flowrate through a regulating valve is proportional to the orifice cross-section of the valve and depends on the difference between upstream and downstream pressures. If the flow rate is higher, the greater the pressure drop will be. Conversely, if the flow rate is lower, the pressure drop will also be lower. In the present case, the flow rate through regulator 22 depends on the upstream and downstream pressures. The upstream pressure is here Psuppiy. The downstream pressure Pout can be easily measured with the rail pressure sensor, hence PouT=PRaii. As the tank assembly 12 empties due to engine consumption, and when the supply pressure Psuppiy reaches levels close to or within the range of the rail operating pressures, the pressure difference through the pressure regulator 22, and hence the flowrate, is much reduced compared to situation where Psuppiy (Ptank) is at 100 bar and more. The present method thus determines a maximum gas consumption Cmax, in view of the upstream gas pressure Psuppiy and on a target rail pressure Ptr. The target rail pressure Ptr is a desired / target value in gas rail 16. The target rail pressure Ptr can correspond to an operating demand rail pressure Pdr for a given operating point, as read from conventional control strategies, or correspond to a safety operating pressure, which may correspond to the demand rail pressure Pdr corrected by a safety coefficient or offset, i.e. typically lower than the nominal demand rail pressure Pdr. Typically, Cmax can be read from a mapping in function of Psuppiy and Ptr, which may be a calibrated table, but could also include calculation using equations etc. Such calibrated table can be obtained by experimentation, using a fuel rail with a predetermined through bore forming a gas leak path, and measuring the flow rate at fixed Psuppiy and Praii. The maximum gas consumption Cmax can be expressed in any appropriate unit, e.g. mass per time unit, in particular g / s or kg / hour. Next, at step S14, a gas quantity per injection event (Qinjmax) is computed from Cmax and injection speed, noted inj_speed. The injection speed represents the number of injection events per time unit, which depends on the engine speed (crankshaft speed) and number of engine cylinders. The parameter of maximum consumption Cmax is hence here converted into an actual fuel quantity per engine cylinder Qinjmax, for an upcoming combustion cycle (per respective cylinder). Finally, at step S16 a maximum allowable Torque Tmax is determined from the fuel quantity Qinjmax determined at S14. Fuel to torque conversion is conventional in the art and generally involves mappings. This step S16 hence involves converting the fuel quantity representing the max. fuel amount per cylinder into Tmax, typically using a mapping (fuel mass; torque). This maximum allowable Torque Tmax is then used by the control unit to limit engine output torque. <Example> Fig. 3 shows a functional diagram of an embodiment of the present method, implementing the strategy of Fig.2. Psuppiy and Ptr are input into a first map M1 that outputs a gas flowrate value (e.g. in g / s), the map having been calibrated for the flow characteristic of the pressure regulation means. This flow rate value is corrected by a correction coefficient in function of the gas tank temperature Trank, read from map M2: the result is the maximum gas consumption Cmax (also expressed in g / s). This value Cmax is then converted in a mass per cylinder (or per injection event), noted Qinjmax. Therefore, Cmax is divided by the injection speed (engine speed in RPM multiplied by the number of injectors divided by 2 [1 injection event for 2 engine revolutions]) and multiplied by 60 (conversion in seconds - and 1000 (conversion in mg). Let us suppose that Cmax = 4 g / s, with engine speed= 4000 RPM in a 4-cylinder engine (i.e. 4 injectors). The injection speed is 133 injections per second, whereby Qinjmax = 30 mg per injection. The same Cmax at 2000 RPM leads to Qinjmax = 60 mg. These masses are then converted into corresponding torque values Tmax, as explained above in relation to step S16.

Claims

1. A method of operating an internal combustion engine powered with gaseous fuel, the engine comprising a fuel tank assembly (12) containing pressurized fuel, a gas supply line (18) connecting said tank assembly to a fuel rail (16) delivering gaseous fuel to at least one gas injector (22) arranged to inject gaseous fuel into an engine cylinder, a pressure regulator (22) integrated in the gas supply line (18) and configured to reduce the upstream, supply gas pressure (Psuppiy) to a controllable output pressure to achieve a predetermined rail pressure,the method comprising the steps of:- monitoring (S10) the supply gas pressure (Psuppiy) upstream of the pressure regulator (22);determining (S12) a maximum gas consumption Cmax within the gas rail (16) based on the supply gas pressure (Psuppiy) and on a target rail pressure (Ptr);- limiting (S16) engine torque based on said maximum gas consumption Cmax.

2. The method according to claim 1, wherein said maximum gas consumption Cmax is determined from a mapping that relates the target rail pressure (Ptr) and supply gas pressure (Psuppiy).

3. The method according to claim 2, wherein said mapping further takes into account the supply gas temperature (TSUppiy).

4. The method according to any of the preceding claims, wherein limiting engine torque involves computing a gas quantity per injection event(s) based on engine speed and on the number of engine cylinders.

5. The method according to any of the preceding claims, wherein limiting engine torque involves determining a maximum allowable torque Tmax based on said maximum gas consumption Cmax, and controlling the engine to keep engine output torque below said maximum allowable torque Tmax.

6. The method according to claim 5 depending on claim 4 , wherein said maximum allowable torque Tmax is determined from a mapping relating engine torque and gas quantity.

7. The method according to any of the preceding claims, wherein said pressure regulator is electronically controlled.

8. The method according to any of the preceding claims, wherein said supply gas pressure (Psuppiy) is the pressure in the tank assembly.

9. The method according to any of the preceding claims, wherein said tank assembly comprises one ore more tanks configured to contain pressurized fuel.

10. The method according to any of the preceding claims, wherein said tank assembly comprises a mechanical pressure reducer configured to deliver gaseous fuel into said gas supply line (18) at a fixed pressure, in particular between 45 and 50 bar.

11. An internal combustion engine comprising a plurality of cylinders and a fuel delivery system including a tank assembly (12) containing pressurized fuel, a gas supply line (18) connecting said tank assembly to a fuel rail (16) delivering gaseous fuel to gas injectors (22) arranged to inject gaseous fuel in the respective cylinders, a pressure regulator (22) being integrated in the gaseous fuel supply line (18) and configured to reduce an upstream gas supply pressure (Psuppiy) to a controllable output pressure to achieve a predetermined rail pressure; and a control unit configured to perform a method according to any one of the preceding claims.

12. A computer program comprising instructions which, when the program is executed by a computer, cause the computer to carry out the method of any of claims 1 to 10.

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