SE538869C2

SE538869C2 - A method for operating a compression ignited engine and suchan engine

Info

Publication number: SE538869C2
Application number: SE1550559A
Authority: SE
Inventors: Tabatabai Ali; Carlgren Magnus; Nordin Mikael
Original assignee: Scania Cv Ab
Priority date: 2015-05-04
Filing date: 2015-05-04
Publication date: 2017-01-17
Also published as: SE1550559A1; DE102016004791A1

Description

A METHOD FOR OPERATING A COMPRESSION IGNITED ENGINE AND SUCH AN ENGINE TECHNICAL FIELD OF THE INVENTION The present invention relates to a method for operating a compression ignited engine having at least one cylinder and for each cylinder a fuel injection nozzle configured to inject fuel into said cylinder and connected to a pressurized fuel accumulator, said method comprising the steps of a) determining a second position at which a start of a main injection of fuel into said cylinder in an operation cycle thereof is to be requested with respect to the position of a piston in said cylinder, b) determining a first position at which a pilot injection of fuel into said cylinder with a time gap before said main injection is to be requested with respect to the position of a piston in said cylinder, and c) determining whether a separation of said first and second positions of said pilot injection and main injection, respectively, is considered to be large or not, as well as a compression ignited engine according to the preamble of the appended independent claim directed thereto.

The invention is not restricted to any particular use of a said engine but relates to injection of fuel into cylinders in compression ignited engines designed for any type of use, such as industrial applications, in grinding machines and all types of motor vehicles, although the invention is particularly applicable to utility vehicles, especially wheeled utility vehicles, such as trucks or lorries and buses, and the invention will for that sake hereinafter at some places be discussed for that field of use for illuminating the invention but accordingly not in any way restricting it thereto.

The position at which a start of a main injection of fuel into a cylinder in an operation cycle in a compression ignited engine is requested is dependent upon various operation parameters of the engine. The reason for requesting a "late" main injection may for example be a desire to raise the temperature of the exhaust gases resulting from the combustion of said fuel in the engine or for influencing the amount of emissions resulting from the combustion to decrease. "Late" means in this context that the piston in said cylinder has at least passed the upper dead point ending a compression stroke thereof.

A said pilot injection of fuel is in such an engine carried out for different reasons, of which one is to ensure that the fuel injected in a said main injection is really combusted when there is a risk that that would not happen without such a pilot injection. A said pilot injection may not be made to late for ensuring that the pilot injected fuel will be combusted. Said first position has for that sake to be located at a certain distance before the position at which a said piston reaches said upper dead point in a compression stroke of that piston. When a said "late" main rejection is requested this means that the separation of said first and second positions of the pilot injection and the main injection will be large resulting in an imminent risk that the fuel injected according to said main injection will not be combusted even if a combustion of pilot injected fuel takes place would this combustion not be important enough.

BACKGROUND ART US 2008 243358 discloses a method of the type defined in the introduction addressing the problem just described by increasing the amount of fuel injected in a said pilot injection when the main injection is late. Said increase is based upon a correction value for the position of the main injection.

SUMMARY OFTHEINVENTION The object of the present invention is to provide a method and an engine of the type defined in the introduction being improved in at least some aspect with respect to such methods and engines already known.

This object is with respect to the method obtained by providing such a method with the features listed in the characterizing part of the appended patent claim 1.

By measuring the fuel pressure inside said fuel accumulator and comparing the value of said pressure measured with a pre-established table for the type of fuel injection nozzles to which said fuel injection nozzle belongs stating the minimum amount of fuel in the form of opening time of said nozzle to be ordered for a certain fuel pressure in a said fuel accumulator for ensuring that the pilot injection of fuel will result in a sufficient combustion of pilot injected fuel for a said large separation and determining said minimum amount for said value of the pressure measured, it will be ensured that a proper pilot injection of fuel will automatically always be activated in the case that said separation of said pilot injection and main injection is large and that the fuel injected according to said main injection will really be combusted and no misfiring occur due to uncombusted fuel. Thus, it is ensured that always a sufficient amount of pilot injected fuel is introduced in the combustion chamber of the cylinder for ensuring that the main injected fuel will be combusted. Accordingly, said pre-established table has been provided by a calibration of a said engine with respect to which amount is the minimum amount allowed for a pilot injection at a certain fuel pressure in said accumulator for the type of injection nozzles in question. Such a calibration may then be carried out for one engine and then said pre-established table be used for a plurality of identical engines.

According to an embodiment of the invention said determining in step c) is carried out by comparing said first and second positions, determining the separation thereof and determining that a large separation thereof exists if said separation exceeds a predetermined separation value. Making the determination of the existence of a large separation dependent upon the value of this separation constitutes an advantageous way of ensuring that a sufficient amount of fuel is injected in a said pilot injection for ensuring that the main injected fuel will be combusted under the conditions prevailing.

According to another embodiment of the invention said determining in step c) is carried out by comparing a said second position of a requested start of injection of a main injection of fuel with respect to the position of a piston in said cylinder with a predetermined position of a such start of injection and determining that said separation is to be considered to be large if said second position of said start of injection is after said predetermined position of a start of injection. This way of determining whether a said separation is considered to be large may also form a suitable base for the use of a said pre-established table for ensuring that the main injected fuel will certainly be combusted as a consequence of a sufficient combustion of pilot injected fuel, i.e. a sufficient temperature raise and pressure increase resulting from a combustion of the pilot injected fuel.

According to another embodiment of the invention said first position at which a pilot injection of fuel into said cylinder is requested is located before said piston of the cylinder in a compression stroke thereof reaches the upper dead point ending that stroke.

According to another embodiment of the invention said predetermined position considered in step c) is after an upper dead point of said piston of the cylinder ending a compression stroke thereof.

According to another embodiment of the invention the requested position of a start of a main injection is in step a) determined based upon values of operation parameters of the engine, and according to another embodiment of the invention said values of operation parameters is considered when determining said requested position is at least one of number of revolutions per time unit of a crank shaft of said engine and/or temperature of air injected into said cylinder and/or temperature of engine cooling liquid and/or pressure of turbo charged air and/or fuel pressure and/or amount of inert gas.

The object of the present invention is with respect to the engine obtained by providing an engine according to the independent engine claim. The advantages of such an engine appear clearly from the above discussion of the method according to the invention and embodiments thereof.

The invention also relates to a computer program having the features of claim 10, a computer program product having the features of claim 11, an electronic control unit having the features of claim 12 and a motor vehicle according to claim 13.

Other advantageous features as well as advantages of the present invention appear from the description following below.

BRIEF DESCRIPTION OF THE DRAWINGS With reference to the appended drawings, below follows a specific description of an embodiment of the invention cited as an example.

In the drawings: Fig 1 is a schematic view of an internal combustion engine with a common rail fuel injection system upon which the method according to the present invention is applicable, Fig 2 is a very simplified view illustrating positions of a pilot injection and a main injection when a separation thereof is considered to be large, Fig 3 is a flow chart illustrating the steps carried out in a method according to an embodiment of the invention, and Fig 4 is a schematic view illustrating an electronic control unit for implementing a method according to the invention.

DETAILED DESCRIPTION OF AN EMOBODIMENT OF INVENTION The use of a compression ignited engine according to the present invention in a motor vehicle will now by way of example be described, and a drive train 1 of a motor vehicle is for that sake schematically illustrated in Fig 1. The drive train 1 comprises a compression ignited engine 2, for example a diesel engine, which is connected to driving wheels (not shown) of the vehicle through a clutch 3 and a gearbox 4.

The engine comprises a plurality, more exactly four, cylinders 5 schematically indicated. However, an engine according to the invention may have any suitable number of cylinders. Fuel is fed to the cylinders by means of a fuel injection system 6. This fuel injection system 6 comprises a number of electrically controlled injection nozzles 7. Each cylinder of the engine 2 is associated with an own such injection nozzle 7. The injection nozzles 7 are connected to a pressurized fuel accumulator 8 in the form of a so-called common rail, which constitutes an accumulator for accumulating high pressure fuel to be fed to the injection nozzles. The fuel accumulator 8 is supplied with pressurized fuel through a high pressure pump 9, which receives fuel from a fuel tank 10 of the vehicle 11. The injection nozzles 7 are connected to the fuel accumulator 8 through fuel ducts 12 so as to inject fuel accumulated in the fuel accumulator 8 into the respective cylinder.

Opening times for valves associated with the injection nozzles 7 and by that of the injection nozzles are controlled by a control unit 13, such as an electronic control unit. The control unit 13 is configured to determine a second position at which a start of a main injection of fuel into each cylinder in an operation cycle of the cylinder is to be requested with respect to the position of a piston in said cylinder. This requested position of start of a main injection will by the control unit 13 be determined based upon values of operation parameters of the engine, such as number of revolutions of a crank shaft of the engine, temperature of air injected into the cylinder, temperature of engine cooling liquid and pressure of turbo charged air.

The control unit 13 is also configured to determine a first position at which a pilot injection of fuel into said cylinder with a time gap before said main injection is to be requested with respect to the position of the piston in said cylinder and to determine whether a separation of said first and second positions of said pilot injection and main injection, respectively, is considered to be large or not. The engine further comprises means 14 configured to measure the fuel pressure inside the fuel accumulator 8 and send information thereabout to the control unit 13. The engine also comprises means 19 included in the control unit 13, configured to compare the value of the pressure measured by said means 14, in the case that the control unit has determined that said separation is to be considered to be large, with a pre-established table for the type of fuel injection nozzles to which the fuel injection nozzle 7 belongs stating the minimum amount of fuel in the form of opening time of a said nozzle 7 to be ordered for a certain fuel pressure in a said fuel accumulator ensuring that the pilot injection of fuel will certainly result in a sufficient combustion of pilot injected fuel for a said large separation. The control unit will by this determine said minimum amount for said value of the pressure measured and control the fuel injection nozzle to inject at least said minimum amount of fuel in said pilot injection of fuel. Fig 2 illustrates schematically the compression stroke A and the combustion stroke B of a piston in an operation cycle of a cylinder and the location of an upper dead point UDP of the piston separating these strokes. A second position at which a start of a main injection of fuel into said cylinder is requested is indicated at P2, and it is shown that this main injection is rather late by being after said upper dead point of the piston with respect to rotation of a crank shaft of the engine. This results in a large separation of a first position Pi of a pilot injection, since this has to be carried out not later than at least some degrees before the UDP. This large separation means that a combustion of pilot injected fuel taken place before said main injection of fuel will not automatically result in a guarantee of a combustion of the main injected fuel, but it has to be ensured that at least a certain minimum amount of fuel is injected in said pilot injection for raising the temperature and increasing the pressure sufficiently thereby for ensuring a later combustion of the main injected fuel. This is also according to the present invention achieved by said fuel pressure measurement and using the pre-established table obtained through calibration of the engine with respect to the minimum amount of fuel to be injected for a specific said fuel pressure for the type of injection nozzles used. Fig 3 illustrates a flow chart of a method according to an embodiment of the present invention carried out for a compression ignited engine of the type shown in Fig 1. The method is carried out for each operation cycle of each cylinder of the engine and starts with a step Si of determining a second position of a request to start a main injection of fuel into the cylinder. A first position of a requested pilot injection of fuel into the cylinder with a time gap before said main injection is then determined in step S2. It is after that in a step S3determined whether a separation of said first and second positions is large, whereupon in step S4 the fuel pressure inside said fuel accumulator is measured. It is in step S5 asked if said separation is determined to be large. If the answer to this question is "yes" the value of the pressure measured is in Step Se compared with a pre-established table stating how to obtain a minimum amount of fuel in a pilot injection for different such pressure values resulting in a sufficient combustion of that fuel for a said large separation, whereupon in a step S7 the fuel injection nozzle of the cylinder is controlled to inject said minimum amount of fuel in a pilot injection at said first position of the piston. If the answer to the question put in step S5is "no" the fuel injection nozzle is in step Ss controlled to inject a predetermined amount of fuel in a pilot injection. This step Ss may as an alternative involve a decision that no pilot injection is necessary to carry out.

Computer program code for implementing a method according to the invention is with advantage included in a computer program which can be read into the internal memory of a computer, e.g. the internal memory of an electronic control unit of a motor vehicle. Such a computer program is with advantage provided via a computer program product comprising a data storage medium which can be read by a computer and which has the computer program stored on it. Said data storage medium is for example an optical data storage medium in the form of a CD ROM disc, a DVD disc etc., a magnetic data storage medium in the form of a hard disc, a diskette, a cassette tape etc., or a flash memory or a memory of the ROM, PROM, EPROM or EEPROM type. Fig. 4 illustrates very schematically an electronic control unit 13 comprising an execution means 15, e.g. a central processor unit (CPU), for execution of computer software. The execution means 15 communicates with a memory 16, e.g. of the RAM type, via a data bus 17. The control unit 13 comprises also a non-transitory data storage medium 18, e.g. in the form of a flash memory or a memory of the ROM, PROM, EPROM or EEPROM type. The execution means 15 communicates with the data storage medium 18 via the data bus 17. A computer program comprising computer program code for implementing a method according to the invention, e.g. in accordance with the embodiment illustrated in Fig. 3, is stored on the data storage medium 18.

The invention is of course in no way restricted to the embodiments described above, since many possibilities for modifications thereof are likely to be obvious to one skilled in the art without having to deviate from the scope of invention defined in the appended claims.

The fuel injected into the cylinders of the engine may be any compression ignited fuel.

Claims

1. A method for operating a compression ignited engine (2) having at least one cylinder (5) and for each cylinder a fuel injection nozzle (7) configured to inject fuel into said cylinder and connected to a pressurized fuel accumulator (8), said method comprising the steps of a) determining a second position at which a start of a main injection of fuel into said cylinder in an operation cycle thereof is to be requested with respect to the position of a piston in said cylinder, b) determining a first position at which a pilot injection of fuel into said cylinder with a time gap before said main injection is to be requested with respect to the position of a piston in said cylinder, and c) determining whether a separation of said first and second positions of said pilot injection and main injection, respectively, is considered to be large or not, characterizedin that it comprises the step of d) measuring the fuel pressure inside said fuel accumulator (8) and when it in step c) is determined that said separation is to be considered to be large the steps e) and f) of e) comparing the value of said pressure measured in step d) with a pre-established table for the type of fuel injection nozzles (7) to which said fuel injection nozzle belongs stating the minimum amount of fuel in the form of opening time of said nozzle to be ordered for a certain fuel pressure in a said fuel accumulator for ensuring that the pilot injection of fuel will result in a sufficient combustion of pilot injected fuel for a said large separation, and determining said minimum amount for said value of the pressure measured, and f) controlling the fuel injection nozzle (7) for the cylinder (5) to inject fuel according to said pilot injection of at least said minimum amount determined in step e).

2. A method according to claim 1,characterizedin that said determining in step c) is carried out by comparing said first and second positions, determining the separation thereof and determining that a large separation thereof exists if said separation exceeds a predetermined separation value.

3. A method according to claim 1,characterizedin that said determining in step c) is carried out by comparing a said second position of a requested start of injection of a said main injection of fuel with respect to the position of a piston in said cylinder (5) with a predetermined position of a such start of injection and determining that said separation is to be considered to be large if said second position of said start of injection is after said predetermined position of a start of injection.

4. A method according to any of the preceding claims,characterizedin that said first position at which a pilot injection of fuel into said cylinder (5) is requested is located before said piston of the cylinder in a compression stroke thereof reaches the upper dead point ending that stroke.

5. A method according to claim 3,characterizedin that said predetermined position considered in step c) is after an upper dead point of said piston of the cylinder (5) ending a compression stroke thereof.

6. A method according to any of the preceding claims,characterizedin that the requested position of a start of a main injection is in step a) determined based upon values of operation parameters of the engine.

7. A method according to claim 6,characterizedin that said values of operation parameters considered when determining said requested position is at least one of number of revolutions per time unit of a crank shaft of said engine and/or temperature of air injected into said cylinder and/or temperature of engine cooling liquid and/or pressure of turbo charged air and/or fuel pressure and/or amount of inert air.

8. A compression ignited engine comprising at least one cylinder (5) and for each cylinder a fuel injection nozzle (7) configured to inject fuel into said cylinder and connected to a pressurized fuel accumulator (8), said engine further comprising a control unit (13) for controlling said fuel injection nozzle and configured to determine a second position at which a start of a main injection of fuel into said cylinder in an operation cycle thereof is to be requested with respect to the position of a piston in said cylinder, to determine a first position at which a pilot injection of fuel into said cylinder with a time gap before said main injection is to be requested with respect to the position of a piston in said cylinder and to determine whether a separation of said first and second positions of said pilot injection and main injection, respectively, is considered to be large or not, and means (14) configured to measure the fuel pressure inside said fuel accumulator (8),characterizedin that the engine further comprises means (19) configured to compare the value of said pressure measured, in the case that the control unit (13) has determined that said separation is to be considered to be large, with a pre-established table for the type of fuel injection nozzles (7) to which said fuel injection nozzle belongs stating the minimum amount of fuel in the form of opening time of said nozzle to be ordered for a certain fuel pressure in a said fuel accumulator (8) ensuring that the pilot injection of fuel will certainly result in a sufficient combustion of pilot injected fuel for a said large separation, and determine said minimum amount for said value of the pressure measured, and that said control unit (13) is configured to control the fuel injection nozzle (7) to inject at least said minimum amount of fuel determined by said means in said pilot injection of fuel.

9. An engine according to claim 8,characterizedin that it is a diesel engine.

10. A computer program comprising a computer program code for causing a computer to implement a method according to any of claims 1-7 when the computer program is executed in the computer.

11. A computer program product comprising a non-transitory data storage medium (18) which can be read by a computer and on which the program code of a computer program according to claim 10 is stored.

12. An electronic control unit comprising an execution means (15), a memory (16) connected to the execution means and a non-transitory data storage medium (18) which is connected to the execution means (15) and on which the computer program code of a computer program according to claim 10 is stored.

13. A motor vehicle, especially a wheeled vehicle such as a truck or bus,characterizedin that it comprises an engine according to claim 8 or 9.