This application claims priority to British Patent Application No. 0919309.5, filed Nov. 3, 2009, which is incorporated herein by reference in its entirety.
The technical field relates to a method for determining an index representing a crank angle at which a given fuel mass fraction has been burnt into a cylinder of an internal combustion engine, in particular of a Diesel engine.
It is known to control the injection of fuel in each cylinder of a Diesel engine using a closed-loop control of a parameter representative of the fuel combustion in the engine cylinders, in order to stabilize the combustion and reduce polluting emission. One of the mostly used parameter in controlling the combustion of a Diesel engine is an index which represents the crank angle, at which a given mass fraction of the fuel injected in the cylinder during an engine cycle has been burnt. As a matter of fact, said index typically indicates the crank angle at which 50% of the injected fuel mass has been burnt into the cylinder, so that it is generally referred as MFB50 (Mass Fraction Burnt 50%).
The determination of such index requires the ECU to sample the pressure within the cylinder during an engine cycle, in order to acquire an in-cylinder pressure curve. The pressure is sampled by means of a pressure sensor set inside the cylinder, typically integrated in the glow plug associated to the cylinder itself.
The ECU uses the in-cylinder pressure curve for calculating a curve representing the heat release rate during said engine cycle, according to the equation:
Where Q represents the heat, P represents the in-cylinder pressure, V represents the volume of the combustion chamber defined by the piston within the cylinder, k is the specific heat ratio (the ratio between the specific heat constants for constant pressure and constant volume processes) and a represents the crank angle.
The heat release rate curve is then integrated by the ECU according to the equation:
in order to achieve a curve representing the cumulative heat release during the engine cycle.
At this point, the ECU determines the minimum and the maximum value of the cumulative heat release curve, and uses the given fuel mass fraction (50% in the case of MFB50 determination), for calculating a target value of the cumulative heat release between said minimum and maximum values, according to the equation:
Tv=mv+f(Mv−mv) (3)
Where Tv is the target vale, my and My are respectively the minimum and maximum value of the cumulative heat release curve, and f is a fraction corresponding to the given fuel mass fraction.
Finally, the ECU finds the goal point of the cumulative heat release curve which corresponds to the target value Tv, and assumes as index the crank angle corresponding to the goal point.
A drawback of this method is that the sampled in-cylinder pressure curve may be affected by some noises due to pressure sensor wiring, or to electrical interferences between the pressure sensor and other components of the engine system, such as for example the glow plug and the actuator of the injector. These noises manifest themselves in form of variations of the pressure curve, which locally deviates from the expected trace and rapidly returns to it.
It follows that the pressure rate dP/dα in the neighborhood of said noises is quite high and therefore, according to equation (2), it produces an unexpected fluctuation in the heat release rate curve having high amplitude. Such fluctuation is further magnified if the pressure noise is located in a portion of the pressure curve corresponding to a phase of the engine cycle in which also the combustion chamber volume rate dV/dα is high. According to equation (3), each unexpected fluctuation of the heat release rate curve produces in turn a fake spike in the cumulative heat release curve, which can stick out from the expected trace either upward or downward. These fake spikes can imply several problems in the determination of the MFB50, as well as of any other index representing a crank angle at which a given quantity of injected fuel mass has been burnt into the cylinder.
A first problem consists in that the vertex of a fake spike could actually be the minimum or the maximum value of the cumulative heat release curve. In this case, the presence of the fake spike introduces an error in calculating the target value Tv, which results in a deviation of the determined index with respect to the real one.
Even if the target value Tv was correct, a second problem consists in that a fake spike could have one or more points corresponding to the target value Tv. In this case, it is generally not possible for the conventional system to effectively distinguish the goal point of the cumulative heat release curve from the points belonging to the fake spike, so that said conventional system could find a wrong goal point, which inevitably returns an index different from the real one.
In view of the foregoing, at least one object of the present invention is to solve, or at least to positively reduce the above mentioned drawbacks, in order to achieve an index which is more reliable than that provided by the conventional system. At least another object of the present invention is to meet the above mentioned object with a simple, rational and inexpensive solution. In addition, other objects, desirable features, and characteristics will become apparent from the subsequent summary and detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background.
A method is provided for determining an index representing the crank angle at which a given fuel mass fraction has been burnt in a cylinder of an internal combustion engine during an engine cycle, wherein said determining method generally comprises the steps of: sampling the pressure within the cylinder during the engine cycle, using the pressure samples for determining the heat release rate curve during the engine cycle, using said heat release rate curve for determining the cumulative heat release curve during said engine cycle, determining a minimum value and a maximum value of said cumulative heat release curve, using the given fuel mass fraction for calculating a target value of the cumulative heat release between said minimum and maximum values finding a goal point of the cumulative heat release curve which corresponds to said target value, and assuming the crank angle corresponding to said goal point as the index.
The method provides for determining an opening angle within the crank angular range corresponding to compression stroke of the engine cycle, determining a closing angle within the crank angular range corresponding to the expansion stroke of the engine cycle, using said opening and closing angles for delimiting between them a first angular window, and limiting the determination of the minimum and maximum values of the cumulative heat release curve within said first angular window. In this way, the fake spikes which are eventually located outside said first angular window are disregarded, and do not affect the determination of minimum and maximum value.
According to an embodiment of the invention, in case of Diesel engine, the opening angle shall be as late as possible but before the start of the first fuel injection, and the closing angle shall be as early as possible but after the end of the last fuel injection. In case of Spark ignited engine, the opening angle shall be as late as possible but before the spark angle, and the closing angle shall be as early as possible but after the spark angle.
The method further provides for determining a lower point of the cumulative heat release curve which corresponds to the determined minimum value, determining a upper point of the cumulative heat release curve which corresponds to the determined maximum value, and limiting the finding of the goal point within the portion of the cumulative heat release curve which is comprised between said lower and upper point. In this way, the fake spikes which are eventually located outside the considered portion of the cumulative heat release curve do not affect the finding of the goal point, even if such fake spikes have one or more points corresponding to the target value.
According to another embodiment, the finding of the goal point comprises the step of: determining a lower point of the cumulative heat release curve which corresponds to the determined minimum value, determining a upper point of the cumulative heat release curve which corresponds to the determined maximum value, evaluating the points of the cumulative heat release curve in sequence from the lower point towards the upper point, determining the first point of the sequence which corresponds to the target value, and assuming such first point as the goal point. This finding procedure is based on the assumption that the cumulative heat release curve is monotonic and increasing from the minimum to the maximum value.
The assumption is theoretically valid, since the portion of the cumulative heat release curve between the minimum and maximum values generally comprises the combustion phase of the fuel in the cylinder, so that it is not plausible for the heat release to decrease in this phase. By applying the finding procedure in question, it is effectively possible to reduce the ECU computing load and the operating time for achieving the goal point, because the step of evaluating the cumulative heat release curve can be interrupted after the detection of the first point corresponding to the target vale, to thereby disregarding all the other.
Moreover, the finding procedure in question returns always a single goal point, even if a fake spike is located in the portion of the cumulative heat release curve between the minimum and maximum, to at least avoiding any uncertainty in the decision about which point should be considered as the right one.
According to another embodiment of the invention, the method comprises the steps of determining a lower point of the cumulative heat release curve which corresponds to the determined minimum value, determining a upper point of the cumulative heat release curve which corresponds to the determined maximum value, determining the crank angle corresponding to said lower point of the cumulative heat release curve, determining the crank angle corresponding to said upper point of the cumulative heat release curve, and checking whether the crank angle corresponding to said lower point precede the crank angle corresponding to said upper point or not.
If the crank angle corresponding to lower point does not precede the crank angle corresponding to upper point, it means that the determined cumulative heat release curve is wrong, since it is not theoretically plausible that the heat release decreases during the fuel combustion. In this latter case, the method preferably provides for aborting the normal determination of the index and for performing instead a special procedure. Such special procedure can assign to the index a default crank angle, or can assign to the index the crank angle at which the same given fuel mass fraction has been burnt in the cylinder during a previous engine cycle.
According to another embodiment of the invention, the method further comprises the step of determining an intermediate angle within the first angular window and within the crank angular range corresponding to the expansion stroke of the engine cycle, using said intermediate angle and the closing angle of the first angular window for delimiting between them a second angular window, determining a not positive threshold for the heat release rate, evaluating the portion of the heat release rate curve comprised within said second angular window, and checking whether at least one point of said portion of the heat release rate curve corresponds to a value beneath said not positive threshold or not.
If one or more points of said portion of the heat release rate curve correspond to values beneath said not positive threshold, it means that a fake spike is located within the second angular window and therefore that the determination of the index is probably affected by an error. In fact, a fake spike in the cumulative heat release curve always comprises a sharp heat release increase which is followed or anticipated by a sharp heat release decrease. To any heat release decrease there are corresponding negative values of the heat release rate. Therefore, a fake spike located in the second angular window manifest itself with at least a negative value of the heat release rate. However, the second angular window corresponds to the combustion phase of the fuel within the cylinder, and it is not theoretically plausible to have a negative value of the heat release rate in this phase. In this latter case, the method preferably provides for aborting the normal determination of the index and for performing instead a special procedure. Such special procedure can assign to the index a default crank angle, or can assign to the index the crank angle at which the same given fuel mass fraction has been burnt in the cylinder during a previous engine cycle.
According to an embodiment of the present embodiment, the intermediate angle which defines the second angular window is comprised between the TDC angle between the compression stroke and expansion stroke and the closing angle of the first angular windows.
A wider method is also provided for controlling an internal combustion engine. The index determination method of the invention is repeated for each engine cycle during the engine functioning.
The methods according to the invention can be realized in the form of a computer program comprising a program-code to carry out all the steps of the methods of the invention and in the form of a computer program product comprising means for executing the computer program.
The computer program product comprises, according to a preferred embodiment of the invention, a control apparatus for an internal combustion engine, for example an engine microprocessor based controller ECU, in which the program is stored so that the control apparatus defines the invention in the same way as the method. In this case, when the control apparatus execute the computer program all the steps of the method according to the invention are carried out.
The methods according to the invention can be also realized in the form of an electromagnetic signal, said signal being modulated to carry a sequence of data bits which represent a computer program to carry out all steps of the methods of the invention.
The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and:
The following detailed description is merely exemplary in nature and is not intended to limit application and uses. Furthermore, there is no intention to be bound by any theory presented in the preceding background or summary or the following detailed description.
The embodiments of the invention are hereinafter disclosed with reference to a four-stroke Diesel engine. A method is provided for determining an index (MFB50 in the present embodiment) representing the crank angle at which a given fuel mass fraction has been burnt into a cylinder of the Diesel engine, during an engine cycle. In a four-stroke Diesel engine, each engine cycle is performed during two crankshaft rotations)(720°), which correspond, to four strokes of the piston into the cylinder: intake stroke, compression stroke, expansion stroke and exhaust stroke.
The fuel is injected into the cylinder during an injection phase which is performed across the top dead center (TDC) of the piston between compression and expansion stroke. Accordingly, the fuel combustion occurs approximately during the same phase or slightly later. The determination of the above mentioned index provides for sampling the pressure within the cylinder during the engine cycle.
The pressure is sampled by means of a pressure sensor set inside the cylinder, typically integrated in the glow plug associated to the cylinder itself. The pressure samples are used for determining the in-cylinder pressure curve ICPC during the engine cycle, as shown in
The heat release rate curve HRRC is calculated according to the equation:
where Q represents the heat, P represents the in-cylinder pressure, V represents the volume of the combustion chamber defined by the piston within the cylinder, k is the specific heat ratio (the ratio between the specific heat constants for constant pressure and constant volume processes) and α represents the crank angle.
The heat release rate curve HRRC is then used for determining the cumulative heat release curve CHRC during the same engine cycle, as shown in
For sake of simplicity, the in-cylinder pressure curve ICPC, the heat release rate curve HRRC and the cumulative heat release curve CHRC, are shown within a crank angular range comprised between −80° and +80°, wherein 0° corresponds to the crank angle at with the piston is at the TDC between combustion stroke and expansion stroke of the engine cycle.
At this point, the determination of the index provides for determining the minimum value my and the maximum value My of the cumulative heat release curve CHRC, and using the given fuel mass fraction f (50% in the present embodiment), for calculating a target value Tv of the cumulative heat release between said minimum and maximum values, according to the equation:
Tv=mv+f(Mv−mv)
Finally, the determination of the index provides for finding the goal point GP of the cumulative heat release curve CHRC which corresponds to said target value Tv, and assuming as index the crank angle I corresponding to said goal point GP.
The embodiments of the present invention improves this determining method, in order to return an index I which is less affected by noises on in-cylinder pressure curve ICPC, which can be generated by pressure sensor wiring or by electrical interferences between the pressure sensor and other components of the engine system, such as for example the glow plug and the actuator of the injector.
For example, the in-cylinder pressure curve ICPC can be affected by a noise N1 located at the beginning of the compression stroke, as shown in
The in-cylinder pressure curve ICPC can be also affected by a noise N2 located at the end of the compression stroke, as shown in
In order to disregards fake spikes such as FS1 and FS2, the method provides for limiting the determination of the minimum value my and maximum values My of the cumulative heat release curve CHRC within a first angular window FAW, as shown in
Preferably, the opening angle OA shall be as late as possible but before the start of the first fuel injection, and the closing angle CA shall be as early as possible but after the end of the last fuel injection. According to an embodiment of the invention, the opening angle OA and/or the closing angle CA and/or the width of the first angular window FAW, can be regulated on the base of one or more engine operating parameters, such as for example engine speed and engine load. As a matter of fact, the opening angle OA and/or the closing angles CA and/or the width of first angular window FAW, can be empirically evaluated during a calibration activity, to thereby being memorized in data sets or maps which respectively correlate the opening angle OA, the closing angle CA and the width of first angular window FAW, to said one or more engine operating parameters. Afterwards, these empirically determined data sets or maps can be used in the method of the invention, for determining the opening angle OA and/or the closing angle CA and/or the width of first angular window FAW, on the base of the actual values of said one or more engine operating parameters.
As shown in
In order to solve this drawback (see
According to the method, the determination of the lower point LP and the upper point UP can also be used to perform a plausibility check of the cumulative heat release curve CHRC. As a matter of fact (see
If the crank angle LPA corresponding to lower point LP does not precede the crank angle UPA corresponding to upper point UP, it means that the determined cumulative heat release curve is wrong, since it is not theoretically plausible that the heat release decreases during the fuel combustion. In this latter case, the method preferably provides for aborting the normal determination of the index and for performing instead a special procedure.
Such special procedure can assign to the index I a default crank angle, or can assign to the index I the crank angle at which the same given fuel mass fraction has been burnt in the cylinder during a previous engine cycle.
As shown in
As a matter of fact (see
As shown in
In order to meet this purpose (see
If one or more points of said portion of the heat release rate curve HRRC correspond to values beneath said not positive threshold TH, it means that a heat release decrease has been happened within the second angular windows SAW. Since the second angular windows SAW corresponds to the fuel combustion phase, it is not plausible to have a heat release decrease in this phase. It follows that such heat release decrease must be due to a fake spike FS5 and that the determination of the index is probably affected by an error.
In this latter case, the method preferably provides for aborting the normal determination of the index and for performing instead a special procedure. Such special procedure can assign to the index I a default crank angle, or can assign to the index I the crank angle at which the same given fuel mass fraction has been burnt in the cylinder during a previous engine cycle.
According to another embodiment of the invention, the intermediate angle IA which defines the second angular window SAW, is comprised between the crank angle 0° (corresponding to the TDC between the compression stroke and expansion stroke of the engine cycle) and the closing angle CA of the first angular windows FAW. According to yet another embodiment of the invention, the intermediate angle IA and/or the not-positive threshold TH can be regulated on the base of one or more engine operating parameters, such as for example engine speed and engine load. As a matter of fact, the intermediate angle IA and/or the not-positive threshold TH can be empirically evaluated during a calibration activity, to thereby being memorized in data sets or maps which respectively correlate the intermediate angle IA and the not-positive threshold TH to said one or more engine operating parameters.
Afterwards, these empirically determined data sets or maps can be used in the method of the invention, for determining the intermediate angle IA and/or the not-positive threshold TH on the base of the actual values of said one or more engine operating parameters.
While at least one exemplary embodiment has been presented in the foregoing summary and detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration in any way. Rather, the foregoing summary detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope as set forth in the appended claims and their legal equivalents.
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