GB2513296A

GB2513296A - Method of operating a compression ignition engine

Info

Publication number: GB2513296A
Application number: GB1303803.9A
Authority: GB
Inventors: Claudio Monferrato; Claudio Ciaravino; Gennaro Merlino
Original assignee: GM Global Technology Operations LLC
Current assignee: GM Global Technology Operations LLC
Priority date: 2013-03-04
Filing date: 2013-03-04
Publication date: 2014-10-29
Also published as: GB201303803D0

Abstract

A method of operating a compression ignition engine under cut-off conditions, such as release of the accelerator pedal. The method performs a control procedure that comprises the steps of: regulating the pressure within the fuel rail (170, fig.1) to a predetermined value (Pref); setting a desired value (Qdes) of a fuel quantity to be injected into the cylinÂder (125, fig.1 ); determining an energizing time (ET0) based on the desired fuel quantity value ((Qdes) and the fuel rail pressure value (Pref); and performing an empirical test that energises the fuel injector (160, fig.1) for the determined time (ET0) to perform a fuel injection during an engine cycle; measuring the pressure within the engine cylinder during the engine cycle in which the fuel injection occurs; and using the measured pressure to calculate a value (FCcal) of a parameter indicative of the fuel combustion within the engine cylinder . The measured value of this parameter can be regarded as an index of the performance of the fuel injector.

Description

METHOD OF OPERATING A COMPRESSION IGNITION ENGINE

TECHNICAL FIELD

The present invention generally relates to a method of operating a compression ignition engine, typically a compression ignition engine of a motor vehicle, such as for example a Diesel engine.

BACKGROUND

It is known that Diesel engines generally comprise an engine block defining one or more cylinders, each of which accommodates a piston coupled to rotate a crankshaft. A fuel and air mixture is sequentially disposed in each of the cylinders and ignited, resulting in hot expanding exhaust gasses causing reciprocal movement of the pistons and rotation of the crankshaft. The fuel is provided by a fuel injection system that comprises at least one fuel injector per cylinder. The fuel injectors receive the fuel at high pressure from a fuel rail, which is in fluid communication with a high pressure fuel pump that increases the pressure of the fuel received from a fuel source.

The fuel injectors are electromechanically operated valves that open a passage between the fuel rail and the respective cylinder when they are energized with an electric tension, and automatically close that passage when the energizing tension ceases. As long as the passage is open, the fuel contained in the fuel rail is injected at high pressure inside the cylinder. Therefore, the fuel quantity introduced in the cylinder per fuel injection de-pends mainly on the pressure within the fuel rail and on the time the fuel injector remains open, which is usually referred as energizing time.

The fuel injectors are generally controlled by an engine control unit (ECU), which is con-figured to operate a plurality of fuel injections per engine cycle in each of the cylinders, according to a multi-injection pattern that includes at least one main-injection and several small quantity injections, such as for example pilot-injections, after-injections and post-injections. The main-injection is generally provided for injecting the most of the torque forming fuel, namely of the fuel that is responsible for the movement of the piston,

I

whereas the small quantity injections are performed for different purposes.

By way of example, a pilot-injection is provided for injecting a small quantity of fuel be- fore the main injection is performed, in order to reduce vibrations, to optimize fuel con-sumption and to reduce pollutant emissions, in particular the emissions of particulate matter (PM).

ECU usually operates the small quantity injections using a feed-forward control strategy that provides for the ECU to determine a quantity of fuel to be injected during the fuel in-jection, to measure the pressure within the fuel rail, to determine the energizing time necessary to get the desired fuel quantity under the measured fuel rail pressure, and then to operate the fuel injector accordingly. The energizing time is generally determined by means of a map (i.e. a look-up table) which correlates different values of the fuel quantity and of the fuel rail pressure to corresponding values of the energizing time. This correlation map is conventionally determined during an experimental activity performed on a test bench using an internal combustion engine having nominal characteristics, and then it is memorized in a memory system of the ECU5 of all the internal combustion en-gines of the same kind.

However, the performance of a fuel injection system is normally different from an internal combustion engine to another, even if these engines are of the same kind, so that the use of a nominal correlation map may cause small quantity injections to produce unrelia-ble effects.

These unreliable effects are caused mainly by a different behavior of the fuel injectors and/or of other components of the injection system with respect to the nominal ones, due for example to production spread or aging of these components, which may lead the fuel injectors to inject different quantities of fuel for the same energizing time. Other unrelia- ble effects may be due to the composition of the fuel, especially to the percentage of Bi- odiesel, or more precisely of fatty acid methyl esters (FAME), in the conventional hydro-carbon-based diesel fuel. In fact, different percentages of Biodiesel impact on both The physical characteristics (e.g. density and viscosity) and the chemical characteristics (e.g. lower heating value -LHV) of the fuel. As a consequence, for a given energizing time, different physical characteristics of the fuel may cause the fuel injector to inject different quantities of fuel, and even if the fuel quantities are the same, different chemical charac-teristics of the fuel may still have an effect on the combustion.

For these and other reasons, it is currently known to periodically evaluate the actual per-formance of the fuel injection system of the internal combustion engines. This evaluation is performed when the engine is operating under a cutoff condition, typically when an ac-celerator pedal associated to the internal combustion engine is completely released, so that no fuel injections should normally occur. Under this condition, the ECU operates one fuel injector at the time to inject a small quantity of fuel. A small quantity of fuel is regard- ed as a quantity of fuel whose combustion inside the cylinder is unable to effectively pro-duce torque at the crankshaft, so that the driver cannot perceive it. As a matter of fact, a small fuel quantity is generally smaller than 3 mm3 of fuel. Once the fuel injection has been performed, the ECU measures a parameter indicative of the effect produced by that fuel injection. The measured value of this parameter may be regarded as an index of the performance of the fuel injector and may be used for different purposes. For example, the measured value of the parameter may be compared with an expected value thereof, namely a value that would be measured if the entire system were operating under nomi-nal conditions, and the difference between these two values may be used to determine a correction to be applied to the energizing time provided by the correlation map that con-trols the small fuel injections during normal running.

The parameter that is currently involved in this kind of evaluations is the acceleration of the crankshaft, which is measured by processing the signals coming from a crankshaft position sensor. However, the signals produced by the crankshaft position sensor are usually heavily affected by noises and other sort of disturbances coming from various devices associated to the internal combustion engine, such as electrical load, actuators and so forth, or even from the external environment, such as for example vibrations due to the roughness of the road. For these reasons, the signals coming from the crankshaft position sensor generally need strong analog and/or digital filtrations to allow the ECU to get information about the effect of the small fuel injections, which inevitably increases the complexity and the cost of the electronic systems and it is not always sufficiently accu-rate.

In view of the above, it is an object of an embodiment of the present invention to provide a strategy for evaluating the performance of a fuel injection system of an internal com-bustion engine, whose outcomes may be more accurate and reliable than those provided by conventional strategies.

Another object is to provide an evaluation strategy that may be advantageously em- ployed to determine corrections to be applied to the energizing times provided by the cor-relation map that is involved in the feed-forward control of the small fuel injections.

Still another object is to provide an evaluation strategy that may be advantageously used to determine the content of fatty acid methyl esters (FAME) in the fuel supplied to the in-ternal combustion engine.

A further object is to achieve the above mentioned goals with a simple, rational and ra-ther inexpensive solution.

SUMMARY

These andlor other objects are attained by the features of the embodiments of the inven-tion as reported in the independent claims. The dependent claims recite preferred and/or especially advantageous features of the embodiments of the invention.

In particular, an embodiment of the invention provides a method of operating a compres-sion ignition engine under cutoff conditions, wherein the engine comprises a fuel rail in fluid communication with at least a fuel injector associated to an engine cylinder, and wherein the method provides for performing a control procedure that comprises the steps of: -regulating the pressure within the fuel rail to a predetermined value, -setting a desired value of a fuel quantity to be injected into the engine cylinder, -determining an energizing time on the basis of the desired fuel quantity value and the predetermined fuel rail pressure value, and -performing an empirical test that provides for: -energizing the fuel injector for the determined energizing time to perform a fuel injection during an engine cycle, -measuring the pressure within the engine cylinder during the engine cycle in which the fuel injection occurs, -using the measured pressure to calculate a value of a parameter indicative of the fuel combustion within the engine cylinder.

In this way, the calculated value of the combustion parameter may be regarded as an in-dex of the performance of the fuel injector, which is much more reliable than the index obtained by conventional strategies, because it is advantageously determined from a physical quantity (i.e. the in-cylinder pressure) that is directly affected by the fuel injec-tion, rather than a physical quantity (i.e. the crankshaft acceleration) that is less sensitive to the fuel injection and that can be heavily affected by many noises and disturbances, as it happens in the conventional strategies.

According to an aspect of the invention, the desired value of the fuel quantity may be a small value.

As mentioned above, a small fuel quantity is regarded as a fuel quantity whose combus-tion does not have an effective impact on the torque at the crankshaft, for example a fuel quantity whose value is smaller than 3 mm3. The advantage of this solution is that the driver does not perceive that a fuel injection is occurring and that the control procedure is underway, while the accelerator pedal is released and the internal combustion engine is under cutoff conditions.

According to an aspect of the invention, the energizing time may be determined through a first correlation map that receives the desired fuel quantity value and the predeter-mined fuel rail pressure value as inputs, and that yields as output a corresponding value of the energizing time.

This first correlation map may be determined during an experimental activity performed on a test bench using a nominal internal combustion engine, and then it can be memo-rized in a memory system of the ECU5 of all the internal combustion engines of the same kind, so that the energizing time can be advantageously retrieved with very low corn puta-tional effort.

The first correlation map may be the same map which is involved in the determination of the energizing time for the small quantity injections (such as pilot injections, after injec-tions, post injections, etc.) during the normal operation of the engine. In this way, the control procedure may be performed taking advantage of some of the routines already implemented in the ECU, without any further software or hardware requirements for de-termining the energizing time.

According to a first embodiment of the invention, the control procedure may comprise the additional step of calculating a difference between the calculated value of the combustion parameter and an expected value thereof.

The expected value of the combustion parameter may be regarded as the value that would be calculated if the internal combustion engine were supplied precisely with the desired fuel quantity value. As a consequence, the difference between the calculated value and the expected value of the combustion parameter may be advantageously rep-resentative of the difference between the real quantity of fuel that has been injected and the desired quantity thereof.

According to an aspect of this first embodiment, the expected value of the combustion parameter may be advantageously determined on the basis of the desired fuel quantity value and the predetermined fuel rail pressure value.

For instance, the expected value of the combustion parameter may be determined through a second correlation map that receives the desired fuel quantity value and the predetermined fuel rail pressure value as inputs, and that yields as output a correspond-ing expected value of the combustion parameter.

This second correlation map may be determined during an experimental activity per-formed on a test bench using a nominal internal combustion engine, and then it can be memorized in a memory system of the ECUs of all the internal combustion engines of the same kind, so that the expected value of the combustion parameter can be advanta-geously retrieved with very low computational effort.

According to another aspect of the first embodiment, the control procedure may provide for: -repeating the empirical test using the same predetermined rail pressure value but a dif- ferent energizing time, until the difference between the calculated value and the ex-pected value of the combustion parameter is smaller than a predetermined threshold, so as to get a corrected energizing time that meets that condition, and then -calculating a difference between the corrected energizing time and the initially deter-mined energizing time.

In this way, the corrected energizing time may represent the energizing time that should be applied to the fuel injector to actually inject the desired quantity of fuel, so that the dif-ference between the corrected energizing time and the initially determined one may be the correction to be applied (added) to the initially determined energizing time, if the fuel injector is requested to inject that desired fuel quantity under the corresponding fuel rail pressure value.

According to an aspect of the invention, the control procedure may provide for memoriz-ing the difference in a correction map that correlates said difference to the predetermined rail pressure value and to the desired fuel quantity value.

Thanks to this solution, the memorized difference can be easily retrieved and used to correct the initially determined energizing time, whenever the fuel injector is requested to actually inject the corresponding fuel quantity under the corresponding fuel rail pressure value.

In order to obtain a complete correction map, the method may provide for repeating the control procedure using different predetermined values of the fuel rail pressure and dif-ferent desired value of the fuel to be injected.

This correction map may be advantageously used to correct the operation of the fuel in-jector during the normal running of the engine.

In particular, it can be used for adjusting the small quantity injections (such as for exam-ple the pilot injections, the after injections and the post injections) that in normal running are controlled according to feed-forward strategies.

Since the corrections memorized in the correction map are determined by monitoring a combustion parameter related to the in-cylinder pressure rather than the crankshaft ac- celeration, it is advantageously possible to achieve a higher accuracy of the small quanti- ty adjustments, thereby obtaining benefits in terms of combustion noises, injector disper-sion and emissions repeatability engine to engine. Small quantity adjustments performed using a combustion parameter related to the in-cylinder pressure may also be faster than using the crankshaft acceleration.

In order to always keep the correction map up to date, the entire operating method de- scribed above may be repeated periodically, for example every 10000 km run by the ve-hicle.

It should be observed that the accuracy of the first embodiment of the invention generally depends on whether the fuel supplied to the engine may be assumed to always have ap-proximately the same quality (i.e. it is stable) or not. If the fuel is stable, as it happens in many countries particularly in Europe, the first embodiment of the invention may be ex-tremely accurate. If not, the accuracy may decrease but it may still be acceptable as long as the fuel quality variation range is not too wide.

According a second embodiment of the invention, which is particularly useful when the quality of the fuel cannot be assumed to be stable, the control procedure may comprise the step of using the calculated value of the combustion parameter to determine a con-tent of fatty acid methyl ester (FAME) in the fuel.

This second embodiment of the invention is possible because the percentage of FAME (Biodiesel) impacts on density, viscosity and lower heating value of the fuel. As a conse-quence, for a given quantity of fuel that is injected in the cylinder, different percentages of Biodiesel have different effects on the combustion and then on the in-cylinder pres-sure, so that it is possible to establish a reliable correlation between the pressure-based combustion parameter and the blend of Biodiesel and conventional hydrocarbon-based diesel in the fuel.

Knowing the blend of Biodiesel and conventional hydrocarbon-based diesel in the fuel it is advantageously possible to adjust accordingly the operation of many actuators and devices of the engine, thereby improving the engine efficiency and thus reducing the fuel consumption and the pollutant emissions.

According to an aspect of this second embodiment, the content of fatty acid methyl ester may be determined by means of a mathematical function that receives as input the cal-culated value of the combustion parameter and yields as output a corresponding content of fatty acid methyl ester.

This mathematical function may be determined during an experimental activity performed on a test bench using a nominal internal combustion engine, and then it can be memo-rized in a memory system of the ECUs of all the internal combustion engines of the same kind1 so that the content of FAME can be advantageously determined with a minimum of 1 5 computational effort.

It should be observed that this second embodiment of the invention is reliable if the injec-tor is actually able to inject precisely the desired quantity of fuel. To do so, it may be necessary to implement a learning method of the injector drifts, in order to be able to compensate them. This method may be one of those that are conventionally known, but it can also be a method according to the first embodiment of the present invention, pro-vided that a proper discrimination strategy is implemented to discriminate the effects on the combustion parameter due to the injector drifts from those due to the fuel quality. In this regard, a discrimination strategy may be devised considering that the effects due to the injector drift have different time constants and accuracy requirements.

According to another aspect of the invention, the combustion parameter involved in the empirical test (for either the first embodiment of the invention or for the second embodi- ment of the invention) may be a maximum of a heat release rate due to the fuel combus-tion.

The use of the maximum of the heat release rate is advantageous because it provides reliable and repeatable results.

Alternatively, the combustion parameter involved in the empirical test (for either the first embodiment of the invention or for the second embodiment of the invention) may be a maximum of a ratio between the pressure measured in the engine cylinder during the engine cycle in which the fuel injection occurs, and a pressure measured within the same engine cylinder during an engine cycle in which no fuel injections occurred.

The method performed using this pressure ratio may be more reliable than that per- formed using the maximum of the heat release rate, because the pressure ratio is inde-pendent from pressure sensor gain error.

The method according to all the embodiments of the invention can be carried out with the help of a computer program comprising a program-code for carrying out all the steps of the method described above, and in the form of a computer program product comprising the computer program. The method can be also embodied as 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 method.

Another embodiment of the invention provides an apparatus for operating a compression ignition engine under cutoff conditions, wherein the engine comprises a fuel rail in fluid communication with at least a fuel injector associated to an engine cylinder, and wherein the apparatus comprises means for performing a control procedure which include: -means for regulating the pressure within the fuel rail to a predetermined value, -means for setting a desired value of a fuel quantity to be injected into the engine cylin-der, -means for determining an energizing time on the basis of the desired fuel quantity value and the predetermined fuel rail pressure value, and -means for performing an empirical test which include: -means for energizing the fuel injector for the determined energizing time to perform a fuel injection during an engine cycle, -means for measuring the pressure within the engine cylinder during the engine cycle in which the fuel injection occurs, -means for using the measured pressure to calculate a value of a parameter indicative of the fuel combustion within the engine cylinder.

This embodiment of the invention has basically the same advantages of the method dis- closed above, in particular that of obtaining an index of the performance of the fuel injec-tor which is much more reliable than the index obtained by conventional strategies According to an aspect of the invention, the means for setting the desired value of the fuel quantity may be configured for setting a small value.

In this way, the driver is not aware that a fuel injection is occurring and that the control procedure is underway, while the accelerator pedal is released and the internal combus-tion engine is under cutoff conditions.

According to an aspect of the invention, the means for determining the energizing time may be configured to determine the energizing time through a first correlation map that receives the desired fuel quantity value and the predetermined fuel rail pressure value as inputs, and that yields as output a corresponding value of the energizing time.

This aspect has the advantage that the energizing time can be advantageously retrieved with very low computational effort.

According to a first embodiment of the invention, the means for performing the control procedure may additionally include means for calculating a difference between the calcu-lated value of the combustion parameter and an expected value thereof.

The difference between the calculated value and the expected value of the combustion parameter may be representative of the difference between the real quantity of fuel that has been injected and the desired quantity thereof.

According to an aspect of this first embodiment, there may be means for determining the expected value of the combustion parameter on the basis of the desired fuel quantity value and the predetermined fuel rail pressure value.

For instance, the means for determining the expected value of the combustion parameter may be configured to determine the expected value of the combustion parameter through a second correlation map that receives the desired fuel quantity value and the predeter- mined fuel rail pressure value as inputs, and that yields as output a corresponding ex-pected value of the combustion parameter.

This aspect of the invention has the advantage that the expected value of the combus-tion parameter can be advantageously retrieved with very low computational effort.

According to another aspect of the first embodiment, the means for performing the con- trol procedure may be configured to repeat the empirical test using the same predeter-mined rail pressure value but different energizing times, until the difference between the calculated value and the expected value of the combustion parameter is smaller than a predetermined threshold value, so as to get a corrected energizing time that meets that condition, and then to calculate a difference between the corrected energizing time and the initially determined energizing time.

More particularly, the means for performing the control procedure may be configured to memorize the difference in a correction map that correlates said difference to the prede-termined rail pressure value and to the desired fuel quantity value.

In order to achieve a complete correction map, the means for performing the control pro- cedure may be configured to repeat the entire control procedure using different prede- termined values of the fuel rail pressure and different desired value of the fuel to be in-jected.

According a second embodiment of the invention, the means for performing the control procedure may comprise means for using the calculated value of the combustion param-eter to determine a content of fatty acid methyl ester (FAME) in the fuel.

This second embodiment is advantageous because, knowing the blend of Biodiesel and conventional hydrocarbon-based diesel in the fuel, it is possible to adjust accordingly the operation of many actuators and devices of the engine, thereby improving the engine ef-ficiency and thus reducing the fuel consumption and the pollutant emissions.

According to an aspect of this second embodiment, the means for determining the con-tent of fatty acid methyl ester may be configured to determine the content of fatty acid methyl ester by means of a mathematical function that receives as input the calculated value of the combustion parameter and yields as output a corresponding content of fatty acid methyl ester In this way the content of FAME can be advantageously determined with a minimum of computational effort.

According to another aspect of the invention, the means for using the measured pressure to calculate a value of a parameter indicative of the fuel combustion may be configured to calculate a maximum of a heat release rate due to the fuel combustion.

Alternatively, the means for using the measured pressure to calculate a value of a pa-rameter indicative of the fuel combustion may be configured to calculate a maximum of a ratio between the pressure measured in the engine cylinder during the engine cycle in which the fuel injection occurs, and a pressure measured within the same engine cylin-der during an engine cycle in which no fuel injections occurred.

The method performed using this pressure ratio may be more reliable than that per- formed using the maximum of the heat release rate, because the pressure ratio is inde-pendent from pressure sensor gain error Still another embodiment of the invention provides an automotive system comprising a compression ignition engine equipped with a fuel rail in fluid communication with at least a fuel injector associated to an engine cylinder, and an electronic control unit configured to operate the engine under cutoff conditions by performing a control procedure, to per-form this control procedure the electronic control unit being configured to: -regulate the pressure within the fuel rail to a predetermined value, -set a desired value of a fuel quantity to be injected into the engine cylinder, -determine an energizing time on the basis of the desired fuel quantity value and the predetermined fuel rail pressure value, and -perform an empirical test, to perform which the electronic control unit is configured to: -energize the fuel injector for the determined energizing time to perform a fuel injection during an engine cycle, -measure the pressure within the engine cylinder during the engine cycle in which the fuel injection occurs, -use the measured pressure to calculate a value of a parameter indicative of the fuel combustion within the engine cylinder.

This embodiment of the invention has basically the same advantages of the method dis- closed above, in particular that of obtaining an index of the performance of the fuel injec-tor which is much more reliable than the index obtained by conventional strategies.

According to an aspect of the invention, the electronic control unit may be configured to set a small value as the desired value of the fuel quantity.

According to an aspect of the invention, the electronic control unit may be configured to determine the energizing time through a first correlation map that receives the desired fuel quantity value and the predetermined fuel rail pressure value as inputs, and that yields as output a corresponding value of the energizing time.

According to a first embodiment of the invention, the electronic control unit may be addi- tionally configured to calculate a difference between the calculated value of the combus-tion parameter and an expected value thereof.

According to an aspect of this first embodiment, the electronic control unit may be con-figured to determine the expected value of the combustion parameter on the basis of the desired fuel quantity value and the predetermined fuel rail pressure value. -For instance, the electronic control unit may be configured to determine the expected value of the combustion parameter through a second correlation map that receives the desired fuel quantity value and the predetermined fuel rail pressure value as inputs, and that yields as output a corresponding expected value of the combustion parameter.

According to another aspect of the first embodiment, the electronic control unit may be configured to repeat the empirical test using the same predetermined rail pressure value but different energizing times, until the difference between the calculated value and the expected value of the combustion parameter is smaller than a predetermined threshold, so as to get a corrected energizing time that meets that condition, and then to calculate a difference between the corrected energizing time and the initially determined energizing time.

More particularly, the electronic control unit may be configured to memorize the differ-ence in a correction map that correlates said difference to the desired fuel quantity value and to the predetermined fuel rail pressure value.

In order to achieve a complete correction of the first correlation map, the electronic con-trol unit may be configured to repeat the control procedure using different predetermined values of the fuel rail pressure and different desired value of the fuel to be injected.

According a second embodiment of the invention, the electronic control unit may be con-figured to use the calculated value of the combustion parameter to determine a content of fatty acid methyl ester (FAME) in the fuel.

According to an aspect of this second embodiment, the electronic control unit may be configured to determine the content of fatty acid methyl ester by means of a mathemati-cal function that receives as input the calculated value of the combustion parameter and yields as output a corresponding content of fatty acid methyl ester.

In this way the content of FAME can be advantageously determined with a minimum of computational effort.

According to another aspect of the invention, the electronic control unit may be config-ured to calculate, as combustion parameter value, a maximum of a heat release rate due to the fuel combustion.

Alternatively, the electronic control unit may be configured to calculate, as combustion parameter value, a maximum of a ratio between the pressure measured in the engine cylinder during the engine cycle in which the fuel injection occurs, and a pressure meas-ured within the same engine cylinder during an engine cycle in which no fuel injections occurred.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described, by way of example, with reference to the accompanying drawings.

Figure 1 schematically illustrates a motor vehicle equipped with a full hybrid powertrain.

Figure 2 is a schematic view of an internal combustion engine belonging to the power-train of figure 1.

Figure 3 shows a flowchart illustrating a learning method according to an embodiment of the present invention.

Figure 4 is a diagram showing an in-cylinder pressure curve during an engine cycle.

Figure 5 is a diagram showing the heat release rate (HRR) curve corresponding to the in-cylinder pressure curve of figure 4.

Figure 6 is a diagram showing three HRR curves determined for different value of the in-jected fuel quantity.

Figure 7 is an enlargement of figure 6.

Figure 6 is a diagram showing two in-cylinder pressure curves measured respectively during an engine cycle in which is performed a fuel injection and during an engine cycle in which is no fuel injections are performed.

Figure 9 is a diagram showing the curve of a ratio PR between the pressure curves of figure 6.

Figure 10 is a diagram showing three PR curves determined for different value of the in-jected fuel quantity.

Figure 11 shows a flowchart illustrating a detection method of the FAME percentage in the fuel to another embodiment of the present invention.

Figure 12 is a diagram showing three HRR curves determined for different value of the FAME percentage in the fuel.

Figure 13 is an enlargement of figure 12.

Figure 14 is a diagram showing three PR curves determined for different value of the FAME percentage in the fuel.

DETAILED DESCRIPTION

Some embodiments may include an automotive system 100, as shown in Figures 1 and 2, that includes an internal combustion engine (ICE) 110 having an engine block 120 de-fining one or more cylinders 125 having a piston 140 coupled to rotate a crankshaft 145.

A cylinder head 130 cooperates with the piston 140 to define a combustion chamber 150.

A fuel and air mixture (not shown) is disposed in the combustion chamber 150 and ignit-ed, resulting in hot expanding exhaust gasses causing reciprocal movement of the piston 140. The fuel is provided in each of the combustion chambers 150 by at least one fuel injector 160 and the air through at least one intake port 210. The fuel is provided at high pressure to all the fuel injectors 160 from a fuel rail 170 in fluid communication with a high pressure fuel pump 180 that increases the pressure of the fuel received from a fuel source 190. Each of the cylinders 125 has at least two valves 215, actuated by a cam- shaft 135 rotating in time with the crankshaft 145. The valves 215 selectively allow air in-to the combustion chamber 150 from the port 210 and alternately allow exhaust gases to exit through at least one exhaust port 220. In some examples, a cam phaser 155 may selectively vary the timing between the camshaft 135 and the crankshaft 145.

The air may be distributed to the air intake port(s) 210 through an intake manifold 200.

An air intake pipe 205 may provide air from the ambient environment to the intake mani-fold 200. In other embodiments, a throttle body 330 may be provided to regulate the flow of air into the manifold 200. In still other embodiments, a forced air system such as a tur-bocharger 230, having a compressor 240 rotationally coupled to a turbine 250, may be provided. Rotation of the compressor 240 increases the pressure and temperature of the air in the intake pipe 205 and manifold 200. An intercooler 260 disposed in the intake pipe 205 may reduce the temperature of the air. The turbine 250 rotates by receiving ex-haust gases from an exhaust manifold 225 that directs exhaust gases from the exhaust ports 220 and through a series of vanes prior to expansion through the turbine 250. This example shows a variable geometry turbine (VGT) with a VGT actuator 290 arranged to move the vanes to alter the flow of the exhaust gases through the turbine 250. In other embodiments, the turbocharger 230 may be fixed geOmetry and/or include a waste gate.

The exhaust gases exit the turbine 250 and are directed into an exhaust system 270.

The exhaust system 270 may include an exhaust pipe 275 having one or more exhaust aftertreatment devices 280. The aftertreatment devices may be any device configured to change the composition of the exhaust gases. Some examples of aftertreatment devices 280 include, but are not limited to, catalytic converters (two and three way), oxidation catalysts, lean NO traps, hydrocarbon adsorbers, selective catalytic reduction (SCR) systems, and particulate filters. Other embodiments may include an exhaust gas recircu- lation (EGR) system 300 coupled between the exhaust manifold 225 and the intake man- ifold 200. The EGR system 300 may include an EGR cooler 310 to reduce the tempera-ture of the exhaust gases in the EGFt system 300. An EGR valve 320 regulates a flow of exhaust gases in the EGR system 300.

The automotive system 100 may further include an electronic control unit (ECU) 450 in communication with one or more sensors and/or devices associated with the ICE 110.

The ECU 450 may receive input signals from various sensors configured to generate the signals in proportion to various physical parameters associated with the ICE 110. The sensors include, but are not limited to, a mass airflow and temperature sensor 340, a manifold pressure and temperature sensor 350, an in-cylinder pressure sensor 360, coolant and oil temperature and level sensors 350, a fuel rail pressure sensor 400, a camshaft position sensor 410, a crankshaft position sensor 420, exhaust pressure and temperature sensors 430, an EGR temperature sensor 440, and a position sensor 445 for an accelerator pedal 446. Furthermore, the ECU 450 may generate output signals to various control devices that are arranged to control the operation of the ICE 110, includ-ing, but not limited to, the fuel injectors 160, the throttle body 330, the EGR Valve 320, the VGT actuator 290, and the cam phaser 155. Note, dashed lines are used to indicate communication between the ECU 450 and the various sensors and devices, but some are omitted for clarity.

Turning now to the ECU 450, this apparatus may include a digital central processing unit (CPU) in communication with a memory system 460 and an interface bus. The memory system 460 may include various storage types including optical storage, magnetic stor- age, solid state storage, and other non-volatile memory. The interface bus may be con-figured to send, receive, and modulate analog and/or digital signals to/from the various sensors and control devices. The CPU is configured to execute instructions stored as a program in the memory system 460, and send and receive signals to/from the interface bus. The program may embody the methods disclosed herein, allowing the CPU to car-ryout out the steps of such methods and control the ICE 110.

In particular, the ECU 450 is generally configured to operate each fuel injector 160 to perform controlled fuel injections. The fuel injections may include main injections and small quantity injections, such as for example pilot-injections, after-injections and post-injections. To perform small quantity injections, the ECU 450 normally implements a feed-forward control strategy that provides for determining a quantity of fuel to be inject-ed during the fuel injection, measuring the pressure within the fuel rail 170, determining an energizing time, and then for operating the fuel injector 160 accordingly.

The energizing time may be determined by means of a first correlation map which corre-lates different values of the fuel quantity and of the fuel rail pressure to corresponding values of the energizing time. This first correlation map may be determined during an ex- perimental activity performed on a test bench using an internal combustion engine hav-ing nominal characteristics, and then it may be memorized in the memory system 460 of the ECU 450.

However the performance of the fuel injection system of the ICE 110 can be affected by production spread and aging of the injectors 160, as well as of other components of the engine, so that the fuel quantity actually injected by each fuel injector 160 during a small quantity injection may deviate from the desired one. To compensate for this deviation, the ECU 450 may implement a learning method for determining corrections to be applied to the energizing times memorized in the first correlation map, in order to be able to ad-just the quantity of fuel that is actually injected during the small quantity injections.

The learning method is performed for each fuel injector 160 when the ICE 110 is operat-ing under cutoff conditions. A cutoff condition is met when the accelerator pedal 446 is completely released, so that no fuel injections are normally performed inside the engine cylinder 125. While the ICE 110 is operating under a cutoff condition, the learning meth-od may perform the control procedure represented in the flowchart that is illustrated in figure 3.

This control procedure firstly provides for regulating the pressure within the fuel rail 170 so that it is substantially equal to a predetermined value Pref (block 500). To do so, the ECU 450 may adjust the fuel pump 180 or, more precisely, a fuel metering unit associat-ed to the fuel pump 180.

The control procedure further provides for setting a desired value Qdes of a fuel quantity to be injected (block 505). The desired fuel quantity value Qde$ may be a small value, so as to determine a small fuel injection. By way of example, the desired fuel quantity value Qdes may be smaller than 3 mm3. In this way, the resulting fuel injection is not effectively torque forming.

The desired fuel quantity value Qdes and the fuel rail pressure value Prs may be fed as inputs to the above mentioned first correlation map, which yields as output a correspond-ing value El0 of the energizing time (block 510).

At this point, while the fuel rail pressure is kept at the predetermined value Pref, the con-trol procedure provides for performing an empirical test which is globally indicated with 515 in figure 3.

The empirical test 515 provides for energizing one of the fuel injectors 160 for the deter- mined energizing time ET0 (block 520), thereby performing a single small quantity injec-tion inside the respective engine cylinder 125 during an engine cycle.

The empirical test 515 further provides for measuring the pressure within this engine cyl-inder 125 during the engine cycle in which this small quantity injection is performed (block 525), so as to get a pressure curve that represents the variation of the in-cylinder pressure caused by the fuel injection and its combustion. The pressure within the cylin- der 125 may be measured by means of the in-cylinder pressure sensor 360 accommo-dated therein.

The pressure curve is then used to calculate the value FC1 of a parameter indicative of the fuel combustion within the engine cylinder 125 (block 530). In this way, the combus- tion parameter value FC1 may be regarded as a reliable and effective index of the per-formance of the fuel injector 160.

At this point, the control procedure of figure 3 may provide for calculating a difference 0 between the calculated value FC1 and an expected value FCexp of the same combustion parameter (block 535). The expected value FCexp of the combustion parameter may be regarded as the value that would be calculated if the internal combustion engine were supplied precisely with the desired fuel quantity value Qdes. In this way, the difference D between the calculated value FC1 and the expected value of the combustion pa-rameter may be representative of the difference between the real quantity of fuel that has been injected and the desired quantity Qdes The expected value FCexp of the combustion parameter may be advantageously deter-mined on the basis of the desired fuel quantity value Qdes and the predetermined fuel rail pressure value Pm,. For instance, the expected value FCexp of the combustion parameter may be determined through a second correlation map that receives the desired fuel quantity value Qd and the predetermined fuel rail pressure value Pref as inputs, and that yields as output a corresponding expected value FCexp of the combustion parameter This second correlation map may be determined during an experimental activity per-formed on a test bench using a nominal internal combustion engine, and then it can be memorized in the memory system 460 of the ECU 450, so that the expected value FCexp of the combustion parameter can be advantageously retrieved with very low computa-tional effort.

At this point, the control procedure may provide for checking whether the absolute value of the difference D is smaller than a predetermined threshold value D, (block 540). The threshold value Drn may be extremely small, so that the checking block 540 may yield positive only if the calculated difference D is almost zero.

If the checking block 540 yields negative, the control cycle may provide for varying the energizing time (block 545) and for repeating the empirical test 515 using an energizing time ET that is different than the previous one.

In this way, the empirical test 515 is repeated, using each time the same rail pressure value Pr but progressively varying the energizing time El, until the difference D be- tween the calculated value FC1 and the expected value FCexp of the combustion param-eter is smaller than the predetermined threshold value D. When the difference 0 is smaller than the predetermined threshold value D(h, namely when the calculated value FC1 of the combustion parameter is almost equal to the ex-pected value FC8, the energizing time ET* that meets this condition may be regarded as the energizing time that should be applied to the fuel injector 160 to actually inject the desired quantity of fuel Qd9.

At this point, the control procedure may provide for calculating a difference AET between the corrected energizing time ET* and the initially determined energizing time ET0 (block 550), which difference AET may be regarded as the correction to be applied (added) to the initially determined energizing time ET0, in order to allow the fuel injector 160 to actu-ally inject the desired fuel quantity value Qdes.

For this reason, the control procedure may provide for memorizing the difference AFT (block 550), for instance in a correction map wherein said difference aEl is correlated to the desired fuel quantity value Q and to the predetermined fuel rail pressure value Prer.

In this way, the difference 4-i-may be advantageously retrieved and used to operate the fuel injector 160, whenever a fuel quantity Qdes is requested under a fuel rail pressure P rer.

In order to obtain a complete correction map, the learning method may provide for re-peating the control procedure illustrated in figure 3 using different predetermined values Prer of the fuel rail pressure and different desired value Qder of the fuel to be injected.

In order to always keep the correction map up to date, the entire learning method de-scribed above may be performed periodically, for example every 10000 km run by the automotive system 100.

It should be understood that the learning method described above has to be performed for each fuel injector 160 individually, in order to get a dedicated correction map for each one of them.

Turning now to the combustion parameter involved in the empirical test 515, this parame-ter may be a maximum of a heat release rate HRR caused by the combustion of the fuel injected quantity. The HRR may be calculated with the following formula: HRR = k -1 (kp ci V + Vdp) where p is the pressure measured inside the cylinder 125, V is the cylinder volume swept and k is the polytrophic coefficient.

As a consequence, starting from an Ui-cylinder pressure curve like the one shown in fig-ure 4, it is possible to determine a corresponding HHR curve like the one shown in figure 5, and then the maximum HRRmax of the HHR curve may be easily calculated.

To use the maximum HRRmax of the HHR as combustion parameter is advantageous be-cause it has a good correlation with the actual fuel injected quantity. This can be proved looking at the figures 6 and 7, which represent the HRR curves obtained during an en-gine test bench for different values of the injected fuel quantity. In particular, the curve A represents the HRR curve obtained for an injection of 1.4 mm3, the curve B represents the HRR curve obtained for an injection of 1.5 mm3, and the curve C represents the HRR curves obtained for an injection of 1.6 mm3. These curves clearly show that, knowing the maximum HRRmax of the HHR, it is possible to detect small quantity injections with great accuracy.

Alternatively, the combustion parameter involved in the empirical test 515 may be a max-imum PRm of a ratio PR between the pressure Pmer measured in the engine cylinder 125 during the engine cycle in which the fuel injection has been performed, and a pressure Pmot measured within the same engine cylinder 125 during an engine cycle in which no fuel injections occurred: PR = 1fuei Pmor Starting from pressure curves Pfuel and Pm0 like those represented in figure 8, it is possi-ble to determine the corresponding curve of the pressure ratio PR like the one shown in figure 9, and then the maximum PRm of a ratio PR can be easily calculated. In order to have a more robust evaluation1 PRm may be calculated as the value of the PR curve in correspondence of the minimum of the PR first derivative.

To use the maximum PRmax of the ratio PR as combustion parameter is advantageous because it has a good correlation with the actual fuel injected quantity. This can be proved looking at figure 10, which represents the PR curves obtained during an engine test bench for different values of the injected fuel quantity. In particular, the curve E rep-resents the PR curve obtained for an injection of 1.4 mm3, the curve F represents the PR curve obtained for an injection of 1.5 mm3, and the curve G represents the PR curves ob-tained for an inlection of 1.6 mm3. These curves clearly show that, knowing the maximum PRmax of the PR curve, it is possible to detect small quantity injections with great accura-cy.

The learning method performed using the maximum PRmax of the ratio PR as combustion parameter may be more reliable than the same learning method performed using the maximum HRRmax of the HRR, because the pressure ratio PR is independent from the gain error of the pressure sensors 360.

It should be observed that the accuracy of the learning method described above depends on whether the fuel supplied to the ICE 110 may be assumed to always have approxi-mately the same quality (i.e. it is stable) or not. If the fuel is stable, as it happens in many countries particularly in Europe, the learning method can be extremely accurate. If not, the accuracy decreases but it may still be acceptable as bAg as the fuel quality variation range is not too wide.

However, it may happen sometimes that the quality of the fuel supplied to the ICE 110 is not stable but varies widely from an oil company to another. That may be the case when the oil companies provide a fuel which is a blend of conventional hydrocarbon-based diesel fuel and Biodiesel, or more precisely fatty acid methyl esters (FAME). Sometimes, the percentage of Biodiesel in this blend may vary in a wide range, up to a maximum of 30% of Biodiesel in the fuel. These fuels are generally indicated with the letter B followed by the percentage of Biodiesel. For example, a fuel containing a 5% of Biodiesel is usual-ly referred as B5, a fuels containing a 10% of Biodiesel is usually referred as BiD, and so forth. The percentage of Biodiesel may have an impact on both the physical characteris-tics (e.g. density and viscosity) and the chemical characteristics (e.g. lower heating value -LH'vO of the fuel, thereby affecting the operation of the engine.

S For this reason, the ECU 450 may implement a method for detecting the quality of the fuel that is actually supplied to the ICE 110. This detection method is performed while the ICE 110 is operating under cutoff conditions and may be embodied as the control proce-dure represented in the flowchart that is illustrated in figure 11.

This control procedure firstly provides for regulating the pressure within the fuel rail 170 so that it is substantially equal to a predetermined value Pref (block 600). To do so, the ECU 450 may adjust the fuel pump 180 or, more precisely, a fuel metering unit associat-ed to the fuel pump 180.

The control procedure further provides for setting a desired value Ques of a fuel quantity to be injected (block 605). The desired fuel quantity value Qdes may be a small value, so as to determine a small fuel injection. By way of example, the desired fuel quantity value Qdes may be smaller than 3 mm3. In this way, the resulting fuel injection is not effectively torque forming and advantageously it may not be perceived by the driver of the automo-tive system 100.

The desired fuel quantity value Qdes and the fuel rail pressure value Pref may be fed as inputs to the above mentioned first correlation map, which yields as output a correspond-ing value ET0 of the energizing time (block 610).

At this point, while the fuel rail pressure is kept at the predetermined value Pre the con-trol procedure provides for performing an empirical test which is globally indicated with 615 in figure 3.

The empirical test 615 basically provides for energizing one of the fuel injectors 160 for the determined energizing time ET0 (block 620), thereby performing a single small fuel injection inside the respective engine cylinder 125 during an engine cycle.

The empirical test 615 further provides for measuring the pressure within this engine cyl-inder 125 during the engine cycle in which this small fuel injection has been performed (block 625), so as to get a pressure curve that represents the variation of the in-cylinder pressure caused by the fuel injection and its combustion. The pressure within the cylin- der 125 may be measured by means of the in-cylinder pressure sensor 360 accommo-dated therein.

The pressure curve is then used to calculate the value FCcai of a parameter indicative of the fuel combustion within the engine cylinder 125 (block 630). In this way, the combus- tion parameter value FC1 may be regarded as a reliable and effective index of the per-formance of the fuel injector 160.

According to the embodiment of figure 11, the empirical test 615 may then provide for us-ing the calculated value FC of the combustion parameter to determine a percentage 6% of fatty acid methyl ester (FAME) in the fuel (block 635), namely the percentage of the so called Biodiesel.

This is possible because the percentage of FAME impacts on density, viscosity and low-er heating value of the fuel. As a consequence, for a given quantity of fuel that is injected in the cylinder 125, different percentages of FAME have different effects on the combus-tion and then on the in-cylinder pressure.

More particularly, the percentage of FAME B% may be determined by means of a math- ematical function that receives as input the calculated value FC1 of the combustion pa-rameter and yields as output a corresponding percentage B% of FAME.

This mathematical function may be determined during an experimental activity performed on a test bench using a nominal internal combustion engine, and then it can be memo-rized in the memory system 460 of the ECU 450, so that the percentage B% of FAME can be advantageously determined with a minimum of computational effort.

Knowing the percentage of FAME blended in the conventional hydrocarbon-based diesel fuel, it is advantageously possible to adjust accordingly the operation of many actuators and devices of the ICE 110, thereby improving the engine efficiency and thus reducing the fuel consumption and the pollutant emissions.

It should be observed that the detection method described above may be performed us- ing only one fuel injector 160 and performing just one empirical test. The detection meth- od may be repeated after every refill of the fuel source 190, in order to keep the calculat-ed percentage B% of the FAME in the fuel continuously up to date.

It should however be observed that the detection method can be reliable if the injector is actually able to inject precisely the desired quantity of fuel Qd05. To do so, it may be necessary that the ECU 450 implements a learning method of the injector drifts in or- der to be able to compensate them. This method may be one of those that are conven- tionally known, but it can also be the learning method that has been previously de- scribed, provided that a proper discrimination strategy is also implemented to discrimi-nate the effects on the combustion parameter due to the injector drifts from those due to the fuel quality. In this regard, a discrimination strategy may be devised considering that the effects due to the injector drift have different time constants and accuracy require-ments.

Turning now to the combustion parameter involved in the empirical test 615, also in the present case this parameter may be a maximum HRRmax of an heat release rate HRR caused by the combustion of the fuel injected quantity. As already explained, the HRR may be calculated with the following formula: HRR = 1< -1 (kpdv + Vdp) where p is the pressure measured inside the cylinder 125, V is the cylinder volume swept and k is the polytrophic coefficient.

As a consequence, starting from an in-cylinder pressure curve like the one shown in fig-ure 4, it is possible to determine a corresponding HHR curve like the one shown in figure 5, and then the maximum HRRmax of the HHR curve may be easily calculated.

To use the maximum HRRmax of the HEIR as combustion parameter is advantageous be-cause it has a good correlation with the percentage of FAME in the fuel. This can be proved looking at figures 12 and 13, which represent the HRR curves obtained during an engine test bench for the same fuel injected quantity but different percentages of FAME in the fuel. In particular, the curve H represents the HRR curve obtained for a 85 fuel, the curve I represents the HRR curve obtained for a B15 fuel, and the curve L represents the HRR curves obtained for a 830 fuel. These curves clearly show that, knowing the maxi- mum HRRmaX of the HHR, it is possible to detect small variations of the FAME percent-age in the fuel.

It is possible to demonstrate that these results have an acceptable repeatability for each level of FAME in the fuel (possible errors are around the mean values, so that the differ- ent levels may be distinguished with mathematic method). As a consequence, it is possi-ble to determine an acceptable linear relationship and trend line equation that correlates different values of the HRRmax to corresponding percentages of FAME in the fuel.

Alternatively, also in this case, the combustion parameter involved in the empirical test 615 may be a maximum PR of a ratio PR between the pressure measured in the engine cylinder 125 during the engine cycle in which the fuel injection has been per-formed, and a pressure Pmot measured within the same engine cylinder 125 during an engine cycle in which no fuel injections occurred: PR = fuel mo t Starting from pressure curves Pruel and Pmct like those represented in figure 8, it is possi-ble to determine the corresponding curve of the pressure ratio PR like the one shown in figure 9, and then the maximum PRrnax of a ratio PR can be easily calculated. In order to have a more robust evaluation, PRmax may be calculated as the value of the PR curve in correspondence of the minimum of the PR first derivative.

To use the maximum PRm of the ratio PR as combustion parameter is advantageous because it has a good correlation with the percentage of FAME in the fuel. This can be proved looking at figure 14, which represents the PR curves obtained during an engine test bench for the same value of the fuel injected quantity but different values of the per-centage of FAME in the fuel. In particular, the curve M represents the PR curve obtained for a 65 fuel, the curve N represents the PR curve obtained for a B15 fuel, and the curve R represents the PR curves obtained for a B30 fuel. These curves clearly show that, knowing the maximum PRmax of the ratio PR, it is possible to detect small variations of the FAME percentage in the fuel.

The detection method performed using the maximum PRma, of the ratio PR as combus-tion parameter may be more reliable than the same detection method performed using the maximum HRRm8x of the HRR, because the pressure ratio PR is independent from the gain error of the pressure sensors 360.

In conclusion, with regard to the two methods described above, we would like to highlight the ECU 450 may be configured to perform only the learning method, to perform only the detection method, or to perform both the learning method and the detection method.

In the first case, the learning method is particularly accurate if the fuel quality is stable, otherwise it may still be acceptable provided that the fuel quality does not vary in a too wide range. In the second case, the detection method is accurate if other strategies are implemented to correct the injector drifts. In the third case, both the methods may be ac-curate provided that some measures are taken, such as for example if the detection method is repeated at every refill, the calculation of the percentage of FAME is made every time taking into account the percentage of FAME calculated at the previous refill, and the learning method is also repeated at every refill, in order to adapt the combustion to the actual fuel properties and to compensate for the fuel injector drift over the time.

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 forgoing summary and detailed description will provide those skilled in the art with a convenient road map for implementing at least one 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 in their legal equivalents.

REFERENCES

automotive system internal combustion engine 120 engine block cylinder cylinder head camshaft piston 145 crankshaft combustion chamber cam phaser fuel injector fuel rail 180 fuel pump fuel source intake manifold 205 air intake pipe 210 intake port 215 valves 220 exhaust port 225 exhaust manifold 230 turbocharger 240 cdmpressor 250 turbine 260 intercooler 270 exhaust system 275 exhaust pipe 280 aftertreatment devices 290 VGT actuator 300 exhaust gas recirculation system 310 EGR cooler 320 EGR valve 330 throttle body 340 mass airflow and temperature sensor 350 manifold pressure and temperature sensor 360 in-cylinder pressure sensor 380 coolant and oil temperature and level sensors 400 fuel rail pressure sensor 410 camshaft position sensor 420 crankshaft position sensor 430 exhaust pressure and temperature sensors 440 EGR temperature sensor 445 accelerator pedal position sensor 446 accelerator pedal 450 ECU 460 memory system 500 block 505 block 510 block 515 empirical test 520 block 525 block 530 block 535 block 540 block 545 block 550 block 555 block 600 block 605 block 610 block 615 empirical test 620 block 625 block 630 block 635 block Prer predetermined value of the fuel rail pressure Qdes desired value of a fuel quantity to be injected El0 energizing time FC calculated value of a combustion parameter FCexp expected value of a combustion parameter D difference between FC1 and FCexp D(h threshold value of the difference D ET energizing time ET* corrected energizing time ET difference HRRm2x maximum of the heat release rate A HRR curve B HRR curve C HRR curve PRrnax maximum pressure ratio PR E PRcurve F PR curve G PR curve H HRR curve I HRR curve L HRR curve M PR curve N PR curve R PR curve

Claims

CLAIMS1. A method of operating a compression ignition engine (110) under cutoff conditions, wherein the engine (110) comprises a fuel rail (170) in fluid communication with at least a fuel injector (160) associated to an engine cylinder (125), and wherein the method pro-vides for performing a control procedure that comprises the steps of: -regulating the pressure within the fuel rail (170) to a predetermined value (P1), -setting a desired value (Qdes) of a fuel quantity to be injected into the engine cylinder (125), -determining an energizing time (El0) on the basis of the desired fuel quantity value (Ques) and the predetermined fuel rail pressure vaiue (Prer), and -performing an empirical test that provides for: -energizing the fuel injector for the determined energizing time (El0) to perform a fuel injection during an engine cycle, -measuring the pressure within the engine cylinder during the engine cycle in which the fuel injection occurs, -using the measured pressure to calculate a value (FC1) of a parameter indicative of the fuel combustion within the engine cylinder (125).
2. A method according to claim 1, wherein the desired value (Qdes) of the fuel quantity is a small value.
3. A method according to claim 1 or 2, the energizing time (ET3) is determined through a first correlation map that receives the desired fuel quantity value (Odes) and the predetermined fuel rail pressure value (Prer) as inputs, and that yields as output a corre-sponding value of the energizing time.
4. A method according to any of the preceding claims, wherein the control procedure comprises the additional step of calculating a difference (D) between the calculated value (FC1) of the combustion parameter and an expected value (FCexp) thereof.
5. A method according to claim 4, wherein the expected value (FCexp) of the combus-tion parameter is determined on the basis of the desired fuel quantity value (Odes) and the predetermined fuel rail pressure value (Pier).
6. A method according to claim 5, wherein the expected value (FCexp) of the combus-tion parameter is determined through a second correlation map that receives the desired fuel quantity value (Odes) and the predetermined fuel rail pressure value (Pier) as inputs, and that yields as output a corresponding expected value of the combustion parameter.
7. A method according to any of the claims from 4 to 6, wherein the control procedure provides for: -repeating the empirical test using the same predetermined rail pressure value (Pref) but a different energizing time (El), until the difference (D) between the calculated value (FC) and the expected value (FCexp) of the combustion parameter is smaller than a predetermined threshold (0th), so as to get a corrected energizing time (ET*) that meets that condition, and then -calculating a difference (AEJ-) between the corrected energizing time (ET*) and the ini-tially determined energizing time (ET3).
8. A method according to claim 7, wherein the control procedure provides for memo-rizing the difference (Ar) in a correction map that correlates said difference (AET) to the predetermined rail pressure value (%) and to the desired fuel quantity value (Odes).
9. A method according to any of the preceding claims, which provides for repeating the control procedure using different predetermined values (Pret) of the fuel rail pressure and different desired value (Odes) of the fuel to be injected.
10. A method according to claim 1, wherein the control procedure comprises the step of using the calculated value (FCcai) of the combustion parameter to determine a content (8%) of fatty acid methyl ester in the fueL
11. A method according to claim 10, wherein the content (B%) of fatty acid methyl es- ter is determined by means of a mathematical function that receives as input the cacu-lated value (FC,) of the combustion parameter and yields as output a corresponding content of fatty acid methyl ester.
12. A method according to any of the preceding claims, wherein the combustion pa-rameter is a maximum (HRRmax) of an heat release rate due to the fuel combustion.
13. A method according to any of the claims from I to 11, wherein the combustion pa-rameter is a maximum (PRmax) of a ratio between the pressure measured in the engine cylinder (125) during the engine cycle in which the fuel injection occurs, and a pressure measured within the same engine cylinder (125) during an engine cycle in which no fuel injections occurred.
14. A computer program comprising a computer code suitable for performing the method according to any of the preceding claims.
15. A computer program product on which the computer program of claim 14 is stored.