Abstract:
A method for inducing the regeneration of a particulate filter associated with an exhaust gas emitting engine while being operated in a vehicle in variable driving conditions. The method includes turning on, for a time period, a heating device in physical contact with the filter subject to the following conditions: i) an average pressure of the exhaust gas at the entrance of the filter is greater than a predetermined value; and ii) an instantaneous pressure of the exhaust gas at the entrance of the filter is at a high improbable value. The time period for such heating device to stay turned on is sufficiently long to reach the particulate ignition temperature and for such instantaneous pressure to get from such high improbable value to a significantly lower value. The average pressure is obtained by averaging a plurality of pressure data sensed at the entrance of the filter.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a national phase application based on PCT/EP2009/054350, filed Apr. 10, 2009 the content of which is incorporated herein by reference, which claims the priority of PCT/IT/2008/000541, filed Aug. 8, 2008. 
     FIELD OF THE INVENTION 
     The present invention relates to the filtering of exhaust gas emissions. Particularly, the present invention relates to the regeneration of filters of particulate emissions produced by combustion processes, like those occurring in a diesel engine. 
     BACKGROUND OF THE INVENTION 
     With the term “exhaust gas” it is intended a flue gas that is produced as a result of the combustion of fuels, such gasoline/petrol, diesel, fuel oil or coal. The ever increasing diffusion of power plants, industrial process plants, and motor vehicles in the world, has urgently led to the study of possible solutions for reducing the harmful effects of the exhaust gases on the environment and on the man. 
     Indeed, although the largest part of most exhaust gases is relatively harmless nitrogen, water vapor (exception made for pure-carbon fuels), and carbon dioxide (with the exception of hydrogen as fuel), a relatively small part thereof is formed by undesirable toxic substances, such as carbon monoxide, hydrocarbons, nitrogen oxides, partly unburnt fuel, and particulate matter. Generally speaking, with the term “particulate matter” (briefly referred to as “PM”) it is intended solid or liquid particles suspended in a gas. In an exhaust gas, such as the exhaust gas produced by a diesel engine, the main fraction of PM is composed of very small particles, mainly consisting of impure carbon particles (in jargon, also referred to as “soot”). Because of their small size, said particles, when inhaled, may easily penetrate deep into the lungs. The rough surfaces of these particles make it easy for them to bind with other toxins in the environment, thus increasing the hazards of particle inhalation. The discharge amount of PM becomes large in a diesel engine using a gas oil as a fuel or a direct-injection type gasoline engine recently coming into wide use. 
     A solution for removing (or at least reducing) the PM emissions of an exhaust gas produced by fuel combustion, e.g., in a vehicle engine, provides for the use of a particulate filter. Making reference to the exhaust gas produced by a diesel engine, a particulate filter—in this case, referred to as Diesel Particulate Filter (DPF)—is a device arranged in an exhaust gas emission path of the diesel engine for receiving the exhaust gas and retain the PM included thereinto. 
     A conventional DPF may consist of a cylindrical body made of porous material, such as silicon carbide (SiC), with a first base (upstream side) receiving the flow of the exhaust gas produced by the engine. Such DPF has a honeycomb structure, with a plurality of exhaust gas flowing channels extending in parallel to the longitudinal direction of the cylindrical body, from the upstream side body to a downstream side, corresponding to a second base of the cylindrical opposite to the first one. These channels are alternatively plugged at either the upstream side or the downstream side to form a checker pattern. The exhaust gas (including PM) hits the first surface, and is forced to flow through the channels of the DPF that are not plugged at the upstream side. Thanks to the porosity properties of the SiC, the PM included in the exhaust gas is blocked by the walls of said channels, and remain confined in the DPF, while the rest of the exhaust gas (essentially free of PM) crosses the walls, passes into the adjacent channels and exits from the DPF, for being outputted outside the vehicle through exhaust pipes. 
     While disposable DPFs exist, the majority of the current DPFs are designed to be subjected to cleaning operations for removing from the DPF the PM accumulated with the use. Particularly, said cleaning operations, also known as filter regeneration operations, may provide for burning off the accumulated PM, providing heat to the accumulated PM in such a way that the latter reaches its burning temperature—which, for a PM made of carbon particles, is around 600-650° C. 
     According to a first method known in the art, the filter regeneration is performed in a “passive” way, with the DPF that is brought to the burning temperature of the PM by exploiting the heat of the exhaust gas itself Since however it is very difficult to reach the burning temperature of 600-650° C. by simply using the heat of the exhaust gas only, a fuel additive is mixed together with the fuel, which lowers the burning temperature of the PM of about 300° C. According to the latter method, if the temperature of the exhaust gas reaches 300-350° C. and is maintained for a certain time, the PM collected in the DPF burns for spontaneous combustion, emptying the channels thereof However, even in presence of the fuel additive, bringing the temperature of the exhaust gas to said temperature may be quite difficult, especially for those vehicles equipped with small engines, that frequently run in urban areas at a low speed, lift trucks, operative vehicles that work for long period at the idling condition, and the like. This involves an undesired build-up of PM within the DPF, which even after few hours of operations may occlude the channels of the DPF, with a consequent power off of the vehicle. In view of these reasons, the passive filter regeneration is quite inefficient. 
     According to a second method known in the art, the filter regeneration is performed in an “active” way, with additional heat that is supplied to the DPF by an external source, to reach the burning temperature of the PM in an easiest way. For example, the DPF may be provided with proper heating devices, that are periodically activated to heat the exhaust gas before it enters into the channels of the DPF. Also according to this solution, the fuel can be mixed with proper fuel additives, for lowering the burning temperature of the PM and facilitating the filter regeneration. For example, the heating devices may be implemented with a spiral-shaped resistance, or with ceramic or metal glow plugs positioned in the proximity of the upstream side of the DPF. Said heating devices are controlled by a suitable control unit in such a way as to heat the exhaust gas at predetermined times for favoring the trigger of the burning of the PM included in the DPF. 
     The European Patent application EP 990777A1 discloses a regeneration system for an exhaust gas cleaning device disposed in an exhaust emission path of an internal combustion engine. The regeneration system comprises an exhaust gas cleaning honeycomb filter and a heating means for the filter. The filter is a checkered SiC honeycomb filter having a given cell structure, and the heating means is a heater or a glow plug when using a fuel containing fuel additive. 
     The European Patent application EP 1582714A1 discloses a system and a method for regenerating particulate filters. Particularly, estimated values of an amount of accumulated particulate matter on a particulate filter are obtained before initiation of a forceful regeneration operation of the particulate filter. A maximum operating time period of the regeneration operation is set based on the estimated values of the amount of accumulated particulate matter. The particulate filter is regenerated in the regeneration operation by performing post fuel injection in the diesel engine during each exhaust stroke of the diesel engine to supply fuel to the particulate filter and thereby to remove particulate matter from the particulate filter through use of combustion heat of the supplied fuel upon combustion of the supplied fuel. 
     The French patent 2771449B1 discloses a further method and device for the regeneration of a particulate filter. Particularly, heat is provided to local portions of the particulate filter when the counter pressure upstream of the filter exceeds a predetermined threshold value. Said predetermined threshold value is chosen to be lower than the counter pressure for which the particulate burns by self combustion at the average temperature of the exhaust gas inside the particulate filter. The predetermined threshold value depends on the load of the engine and its Revolutions-Per-Minute (RPM). In particular, heat is provided to the particulate filter when the RPM of the engine are low, and particularly when they are lower than or equal to 50% of the maximum RPM of the engine. 
     SUMMARY OF THE INVENTION 
     The Applicant has observed that known solutions for performing the particulate filter regeneration are often not efficient, expensive, energy consuming for the vehicle, and not always useful. Indeed, with many solutions already known in the art, the burning of the PM retained by the filter may be triggered when it is not really necessary, or even it may not succeed. 
     As a matter of fact, the solutions for performing the particulate filter regeneration often need a deep interaction with a number of vehicle&#39;s components (e.g. the engine, for deriving a number of parameters related to the operation of the engine itself), so that the addition of a particulate filter system, including a regeneration system of the filter, particularly in a vehicle which was not originally equipped with the same (i.e. a retrofit), is hardly accomplishable due to the inherent complexity of such solutions. 
     The Applicant has observed that it would be desirable to perform the activation of the regeneration operations only when necessary, for improving the efficiency of the regeneration process without wasting the vehicle energy budget. 
     The Applicant has also observed that the conditions for the activation of the particulate filter regeneration should be as simple as possible, particularly in order to provide the possibility to a vehicle to be equipped with a particulate filter system as a retrofit system (thereby improving its exhaustion emission category) without a substantial amount of needed space in the vehicle and/or installation time. 
     The Applicant has also observed that a vehicle may behave in a very different manner from other vehicles in terms of produced PM per time unit, in dependence of many factors, such as, e.g., the type of engine, the conditions of use, the traveled path, etc. 
     Additionally, even considering the same vehicle, its behavior typically changes during its life, with an impact on the produced PM. The Applicant has thus observed that a filter regeneration system should contemplate the possibility of tracking, as far as the activation of the regeneration operations is concerned, the changes in the behavior of the vehicle. 
     The Applicant has found that different operating conditions of a vehicle can be detected based on few sensed data relating exhaust emissions of its engine. The Applicant has also found that it is possible to set, and possibly adjust, one or more thresholds in order to activate the particulate filter regeneration operations based on the detected operating conditions of the vehicle, preferably based on a statistical analysis of the sensed data, so as to adapt the activation of the regeneration operations to the vehicle type and/or actual use, with a positive effect on the resulting efficiency of the filter regenerations. 
     In this regard, the Applicant has observed that the pressure of the exhaust emissions sensed at the input of the particulate filter is a very useful parameter for assessing when the activation of the particulate filter regeneration operations should be performed. 
     However, the Applicant has also observed that the pressure of the exhaust emissions sensed at the input of the particulate filter is actually a largely fluctuant parameter, particularly when a vehicle is driven in urban areas, so that a simple activation of the filter regeneration triggered by a generically “high” value of instantaneous pressure at the input of the filter (as suggested, for example, in the above mentioned document FR 2771449) does not lead to an effective solution of the problem of when the activation of the filter regeneration should be performed. Indeed, the Applicant has found that such simple solution does not work, because a threshold for the activation is hardly settable. 
     In particular, after a number of tests it has been found that in many cases the regeneration was activated when not needed, due to the fact that a high pressure of the exhaust gases was just obtained because the engine was working at high RPM, and not because the filter was full of accumulated soot. In other cases, the filter actually needed regeneration, but a high instantaneous pressure due to a working of the engine at high RPM leaded to an excessive heat removal in the filter, and finally to a failure of the soot combustion. 
     Differently, it has been found that by performing an average of the pressure values of the exhaust emissions sensed during a certain time interval (e.g. 30 minutes) at the input of the particulate filter, an indication of the actual load of PM in the particulate filter could be derived: in particular, it has been found that a low average pressure may indicate that it is still not necessary to start the particulate filter regeneration operations, since the PM included in the particulate filter is too low. In other words, it has been found that an averaging of the pressure values sensed during a certain time interval (e.g. 30 minutes) may be used as a quite precise indication of whether the particulate filter needs regeneration. In spite of the fact that the instantaneous pressure value at a certain time can largely fluctuate, the average pressure is a slow varying parameter, which is indicative of the degree of soot accumulation in the filter. 
     With regards to the instantaneous pressure, i.e. the pressure value which is sensed (at the input of the particulate filter) at a certain time instant, the Applicant has surprisingly found that low values of such pressure do often fail to activate the filter regeneration. 
     On the other hand, still surprisingly, it was found that high values of instantaneous pressure were occasionally associated with the desired activation of the filter regeneration. 
     Subsequent statistical considerations, made on series of pressure values collected in a substantial number of time intervals, leaded to the conclusion that “exceptionally high” values of the instantaneous pressure may be used as “trigger” of the activation of the regeneration operations once the average pressure value indicates that the particulate filter is ready for being regenerated. 
     This result is surprising, because, indeed, the value of the instantaneous pressure is related to the flow of the exhaust gas at the input of the particulate filter. Since the higher the flow rate of the flow, the higher the heat removal rate from the particulate filter, one would normally anticipate that the heating device used for the activation of the regeneration should be turned-on when the flow is as small as possible. To the contrary, as already anticipated, the Applicant has observed that after the occurrence of high peaks of instantaneous pressure there is a very high probability that the instantaneous pressure (and correspondingly the flow of exhaust gas at the input of the particulate filter) falls. It is believed that such fall is due to the fact that, with all probability, the engine RPM decreases to a substantially lower regime. Thus, substantially every time a high peak of instantaneous pressure is detected, the flow of exhaust gas is likely to subsequently decrease to a condition which is favorable for the combustion of the PM. 
     It has particularly been found that the decrease of the engine RPM to a lower regime (and consequently of the instantaneous pressure at the input of the filter) is likely to occur in a time window, which is compatible with the propagation of the soot combustion in the whole filter, after the turning-on of the heating device(s) activating the regeneration. 
     The Applicant has thus found that the problem of activating the regeneration of the particulate filter can be solved by treating the pressure sensed at the input of the particulate filter in a separate manner, i.e. as a slowly varying parameter obtained by averaging on a number of collected pressure data for a correlation with the quantity of soot in the particulate filter, and as a largely fluctuating instantaneous parameter to be used as a trigger for the activation of the regeneration once the average pressure value indicates that the quantity of soot is high enough to be removed. In particular, such trigger corresponds to a high improbable value of the instantaneous pressure, i.e. an “exceptionally high” value. Typically, such high improbable value corresponds to a very high engine RPM regime. 
     An aspect of the present invention relates to a method for inducing the regeneration of a particulate filter associated with an exhaust gas emitting engine while being operated in a vehicle in variable driving conditions. 
     Another aspect of the present invention relates to a regeneration system for inducing the regeneration of a particulate filter associated with an exhaust gas emitting engine that operates in a vehicle in variable driving conditions. 
     A further aspect of the present invention relates to a filter system for filtering particulate matter present in exhaust emissions of an engine of a vehicle. 
     A still further aspect of the present invention relates to a vehicle including an exhaust gas emitting engine and a particulate filter system. 
     The invention, in one or more of the above aspects, comprises one or more of the following preferred features. 
     At least one heating device is disposed substantially in punctual contact with the filter at its gas entrance so as to cause a local heating thereof. The at least one heating device is turned on when the activation of the regeneration starts. 
     At least the pressure of the exhaust emissions at the input of the particulate filter is sensed. The turning on of the at least one heating device is conditioned to a comparison of the sensed data with at least one respective threshold. 
     Particularly, an average pressure is obtained from a number of sensed pressure values. The average pressure should be higher than a respective minimum threshold, which is related to the load of PM in the particulate filter. 
     Moreover, preferably, when the average pressure is higher than its respective minimum threshold, the at least one heating device is activated when the instantaneous pressure is higher than a further respective minimum threshold. 
     The minimum threshold for the instantaneous pressure is preferably set to a value sufficiently high to be reached by the instantaneous pressure only during the high peaks thereof 
     More particularly, the minimum threshold for the instantaneous pressure is set to a high improbable pressure value, capable of being reached in presence of high peaks of instantaneous pressure only. 
     It is believed that, to the purposes of the present invention, the engine RPM value can be used, in combination with the average pressure as described above, instead of the instantaneous pressure as a trigger for the activation of the filter regeneration. However, the need to work with the average pressure over a time window implies the detection of the instantaneous pressure. Hence, the instantaneous pressure is a parameter readily available as a matter of course. Moreover, working with pressure values makes easy to install the particulate filter regeneration system in a vehicle with no needed interface with its engine. 
     The PM combustion process triggered by locally heating the particulate filter with the heating device is not instantaneous, but requires an amount of time to propagate in the whole extent of the particulate filter. For this purpose, the at least one heating device is turned on for a time period sufficiently long for the instantaneous pressure to get from such value higher than the minimum threshold to a significantly lower value. 
     The value of said pressure threshold could be adjusted based on an operating condition of the vehicle. The operating condition can be detected based on the received sensed data. 
     With the proposed method, the efficiency of the regeneration process of the particulate filter can be substantially improved, both in terms of power consumption and reliability. 
     In particular, it has been found that the proposed solution allows a better tuning of the activation of the regeneration operations, such that the heating device is activated only when the conditions for successful filter regeneration are favorable. 
     Thus, the occurrence of unnecessary activations of the heating device is reduced, so as to avoid excessive waste of electrical power. 
     Moreover, the performances of the regeneration can be dynamically and autonomously improved based on the actual vehicle condition and the actual driving condition. In particular, the operations performed by the proposed filter regeneration method and/or system are based on threshold values that can dynamically evolve based on the actual vehicle conditions and the actual driving condition. 
     According to a preferred embodiment of the present invention, a collection of last received data is kept (e.g. it is stored in a memory of a control unit of the regeneration system). 
     Particularly, according to an embodiment of the present invention, a collection of the more recent received data of the exhaust emissions instantaneous pressure (sensed at the input of the particulate filter) is kept. 
     Moreover, the adjustment of the pressure threshold may be performed by determining said high improbable value of the instantaneous pressure based on a statistical analysis of the data in said collection. 
     Particularly, said statistical analysis may be performed by obtaining a statistical distribution of the data in the collection. The value of said high improbable value can be determined based on a predetermined percentile of said statistical distribution. 
     The setting of the instantaneous pressure threshold can thus advantageously be performed in a simple way, without excessive calculation complexity. 
     Preferably, said predetermined percentile is at least higher than a 90th percentile of the statistical distribution. In such way the peaks of the instantaneous pressure reached during use of the vehicle in the period corresponding to the collected data can be easily identified. 
     According to an embodiment of the present invention, the at least one heating device is kept activated for a time period sufficiently long for the instantaneous pressure to get from such value higher than the high improbable value to about one third of said high improbable value. 
     In a further embodiment, the adjustment of the pressure threshold can be performed by assessing a result of a filter regeneration based on the received data after the occurrence of a previous filter regeneration. The value of said pressure threshold can be adjusted based on the assessed result. A fine tuning of the regeneration process can thus be performed. 
     Preferably, said assessing the result of a filter regeneration based on the data of the exhaust emissions average pressure includes assessing if the average pressure of the exhaust emissions after a previous filter regeneration is lower than a further predetermined threshold value. 
     According to a further embodiment of the present invention, a number of successful regenerations and a number of unsuccessful regenerations could be quantified based on assessed results of consecutive filter regenerations. 
     For example, the threshold value on the average pressure can be increased by a first amount if said number of unsuccessful regenerations has reached a first predetermined limit or can be decreased by a second amount if said number of successful regenerations has reached a second predetermined limit. 
     This adjustment of the average pressure threshold provides another fine tuning of the activation of the regeneration operations. 
     Preferably, the values of said first and second amounts depend on the difference between the value of the exhaust emissions average pressure after the filter regeneration and the threshold value previously set. 
     The Applicant has observed that the temperature of the exhaust emissions sensed at the output of the particulate filter is another very useful parameter for assessing when activating the particulate filter regeneration operations in an efficient way. 
     Particularly, activating the heating device when the temperature of the exhaust gas is too low may obstacle the triggering of the burning of the PM. This is a great drawback, since the activation of the heating device consumes a non-negligible electric power, which in this case is entirely wasted. Wasting the already limited vehicle battery power is highly undesirable. 
     For this purpose, a preferred embodiment of the present invention provides for conditioning the activation of the heating device based on a comparison on received data relating to the instantaneous temperature sensed at the output of the particulate filter with a minimum and a maximum threshold. This is performed in order to asses—if the temperature of the particulate filter is sufficiently high for allowing the propagation of the PM combustion through the whole particulate filter, and/or if it is lower than a maximum value, above which the activation of the heating device would be unnecessary, since the PM would burn for spontaneous combustion. 
     According to an embodiment of the present invention, a collection of last received data regarding the instantaneous temperature of the exhaust emissions of said vehicle engine is kept (e.g. it is stored in a memory of a control unit of the regeneration system). 
     The setting of the value of at least one among the lower instantaneous temperature threshold and the higher instantaneous temperature threshold can be performed based on a statistical analysis of the data in said collection. 
     Preferably, said statistical analysis is performed by obtaining a statistical distribution of the data in the collection. 
     In an embodiment of the invention, the value of at least one among the lower instantaneous temperature threshold and the higher instantaneous temperature threshold are determined based on at least one predetermined percentile of said statistical distribution. 
     The setting of the lower and/or the higher instantaneous temperature threshold(s) can thus advantageously be performed in a simple way, without excessive calculation complexity. 
     In particular, the value of the lower instantaneous temperature threshold can be set on the basis of a first predetermined percentile of said statistical distribution. 
     Furthermore, the value of the higher instantaneous temperature threshold can be set on the basis of a second predetermined percentile of said statistical distribution. 
     According to an embodiment of the present invention, said first predetermined percentile is at least higher than a 70th percentile of a first statistical distribution of received data related to an operating condition of the vehicle being unfavorable for the regeneration of the particulate filter. 
     Preferably, said first predetermined percentile is at least higher than a 95th percentile of the statistical distribution. 
     According to an embodiment of the present invention, said second predetermined percentile is at least lower than a 45th percentile of a second statistical distribution of received data related to an operating condition of the vehicle being favorable for the regeneration of the particulate filter. 
     Preferably, said second predetermined percentile is at least lower than a 30th percentile. 
     According to an embodiment of the present invention, the value of the lower instantaneous temperature threshold is not lower than a minimum temperature required for performing the regeneration of the particulate filter. 
     According to a further embodiment of the present invention, the value of the higher instantaneous temperature threshold is not higher than a maximum temperature for which the regeneration of the particulate filter is self-performed without the need of the activation of the heating device. 
     The above mentioned minimum and maximum temperatures can be set as default threshold parameters for the instantaneous temperature at the installation of the particulate filter, to be then adjusted according to preferred embodiments of the invention. 
     Furthermore, the Applicant has observed that the instantaneous temperature sensed at the output of the particulate filter is also a useful parameter for assessing when deactivating the heating device, avoiding waste of the vehicle battery. Particularly, once the burning of the PM has been sufficiently propagated across the particulate filter, the heating device may be deactivated, since—at this time—the burning of the PM is self-sustained, and is capable of continuing without the need of the additional heat provided by the heating device. The Applicant has found that said condition of self-sustained burning may be detected from the occurring of a relatively high increasing rate of the instantaneous temperature sensed at the output of the particulate filter. 
     In view of the above, a preferred embodiment of the present invention provides for turning off the heating device at a time corresponding to such a relatively high increasing rate of the instantaneous temperature sensed at the output of the particulate filter due to the combustion of the PM. 
     In order to perform the above mentioned activities, the regeneration system typically includes a control unit adapted to turn on for a time period the at least one heating device substantially in punctual contact with the particulate filter at its gas entrance if the average pressure of the exhaust gas at the gas entrance of the particulate filter is greater than a predetermined value, and if the instantaneous pressure of the exhaust gas at the gas entrance of the particulate filter is at a high improbable value. 
     Furthermore, the system includes a pressure sensor for sensing the pressure of the exhaust gas at the input of the particulate filter. 
     In this way, the control unit may receive sensed data relating to the sensed pressure, keep a collection of last received data, and determine and/or adjust said predetermined value and/or said high improbable value based on a statistical analysis of the data in the collection. 
     According to an embodiment of the present invention, the system further includes a temperature sensor for sensing the temperature of the exhaust gas at the output of the particulate filter. 
     In this way, the control unit may receive sensed data relating to the sensed temperature, keep a collection of last received data, and determine and/or adjust higher and/or lower instantaneous temperature thresholds to be used for conditioning the activation of the heating device. 
     According to an embodiment of the present invention, the particulate filter is a SiC filter. The filter may have a honeycomb structure. 
     According to an embodiment of the present invention, the at least one heating device comprises a number of glow plugs. 
     In view of the above, the filter regeneration system of the present invention needs very few additional hardware resources to be installed on the vehicle. 
     Indeed, only the addition of a filter regeneration apparatus including a temperature sensor after the particulate filter, glow plugs directly contacting the particulate filter (and preferably, its input base), and an electronic control unit can be advantageously provided for the installation of the particulate filter. 
     Consequently, the proposed filter regeneration system may be advantageously installed in whichever vehicle type, without having to drastically modify the fabrication process of the vehicle itself Thus, the proposed filter regeneration system is particularly adapted to be sold as an aftermarket product, i.e., as an accessory to be installed as a retrofit in an already operating vehicle. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features and advantages of the present invention will be best understood by reading the following detailed description of some embodiments thereof, given purely by way of a non-limitative example, to be read in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a possible application of a filter regeneration control system according to an embodiment of the present invention; 
         FIG. 2  illustrates a sectional view of a vehicle exhaust device included in the system of  FIG. 1 ; 
         FIG. 3  illustrates an exemplary structure of a control unit included in the system of  FIG. 1 ; 
         FIG. 4A  depicts a flow chart illustrating the main operations performed by the filter regeneration control system of  FIG. 1 ; 
         FIG. 4B  depicts a diagram showing an exemplary evolution in time of the instantaneous pressure during the vehicle operation; 
         FIG. 4C  depicts two diagrams showing exemplary evolutions in time of the instantaneous pressure and of the instantaneous temperature during some operations performed by the filter regeneration control system in the flow chart of  FIG. 4A ; 
         FIG. 5A  depicts a flow chart illustrating the operations performed by the control unit of  FIG. 1  for determining instantaneous pressure threshold values to be used for the operations performed by the filter regeneration control system in the flow chart of  FIG. 4C ; 
         FIG. 5B  depicts an exemplary statistical distribution of a instantaneous pressure log; 
         FIG. 6  depicts a flow chart illustrating the operations performed by the control unit of  FIG. 1  for determining average pressure threshold values to be used for the operations performed by the filter regeneration control system in the flow chart of  FIG. 4C ; 
         FIG. 7A  illustrates experimental results obtained through an exemplary temperature test campaign; 
         FIG. 7B  depicts a flow chart illustrating the operations performed by the control unit of  FIG. 1  for determining temperature threshold values to be used for the operations performed by the filter regeneration control system in the flow chart of  FIG. 4C , and 
         FIG. 7C  depicts exemplary statistical distributions of instantaneous temperatures used for performing the operations of the flow chart of  FIG. 5B . 
     
    
    
     DETAILED DESCRIPTION 
     With reference to the drawings,  FIG. 1  illustrates in terms of schematic blocks a possible application of a filter regeneration control system according to an embodiment of the present invention. 
     More particularly, in the scenario illustrated in  FIG. 1 , a vehicle  100 , for example a car or a van, is equipped with an engine  105 , for example of the diesel type, which generates kinetic energy for moving the vehicle  100 . The vehicle  100  further includes a fuel tank  110  adapted to store fuel, such as gas oil; the fuel stored in the fuel tank  110  is supplied to the engine  105  through a fuel pipe  115 , for being combusted in the engine  105  in order to generate the kinetic energy that causes the vehicle  100  to move. 
     The combustion process of the fuel occurring in the engine  105  produces exhaust gas, which is evacuated to the outside of the vehicle  100  through a vehicle exhaust device  120 ; particularly, the vehicle exhaust device  120  comprises a Diesel Particulate Filter (DPF)  125  provided with an exhaust pipe. The vehicle exhaust device  120 , and in particular the DPF  125  attenuate the acoustic emissions generated by the engine  105  of the vehicle  100 ; if desired, the acoustic emissions generated by the vehicle  100  may be further reduced by providing the vehicle with a muffler, arranged to receive exhaust gas from the DPF  125 . The vehicle exhaust device  120  further includes a heating device  127  contacting an input surface of the DPF  125  for locally heating portions of said input surface of the DPF  125 , in order to promote the local triggering of the burning of PM in said portions of the DPF, as will be described in greater detail in the following of the present description. 
     As already discussed in the introduction of the present description, the purpose of the DPF  125  is to retain the PM included in the exhaust gas generated by the engine  105 , in such a way that the (filtered) exhaust gas being outputted into the air is less harmful for the human health and for the environment in general. 
     A more detailed description of the vehicle exhaust device  120  according to an embodiment of the present invention will be now provided with reference to  FIG. 2 . Particularly,  FIG. 2  illustrates a sectional view of the vehicle exhaust device  120  taken along a direction II-II parallel to a longitudinal axis LA of the vehicle exhaust device  120  of  FIG. 1 . 
     The DPF  125  comprises a generically cylindrical body made of porous material, such as silicon carbide (SiC), with a first base BI (also referred to as “input base”) receiving the flow of the exhaust gas—denoted in the figure with the reference FLin—produced by the engine  105 . The DPF  125  has a honeycomb structure, with a plurality of exhaust gas flowing channels FLCH extending in parallel to the longitudinal direction of the cylindrical body from the input base BI to a second base BO (“output base”) of the cylindrical body opposite to the input one. The exhaust gas flowing channels FLCH are alternatively plugged at either the input base BI or the output base BO by means of respective closure elements PE to form a checker pattern. 
     Without entering into details known to those skilled in the art, a SiC DPF having a honeycomb structure can be obtained starting from a SiC paste in the following way. Briefly, a number of DPF portions having the length of the desired final DPF are firstly formed by means of extrusion operations performed on the SiC paste; during this step, also the various exhaust gas flowing channels FLCH are generated through extrusion. At this point, one end of each exhaust gas flowing channel FLCH of each DPF portion is plugged. Finally, the various DPF portions are cemented to each other, in order to form the DPF  125 . It has to be appreciated that a DPF obtained with such method presents on both its bases cementing lines (not illustrated in the figure) between each pair of adjacent DPF portions that has been cemented to form the final DPF. For example, in case the DPF  125  has been obtained starting from four DPF portions, the bases of the final DPF will present four different sections, delimited by respective cementing lines. It has to be appreciated that the presence of more than one portion cemented to each other allow to obtain a more stable and resistant structure with respect to the case of a monolithic DPF. 
     The DPF  125  is inserted (in jargon, “canned”) into a protective container  205 , for example a metallic (e.g., made of stainless steel) cylinder. The protective container  205  has an input aperture CBI corresponding to the input base BI of the DPF  125  and an output aperture CBO corresponding to the output base BO of the DPF  125 . An input lid structure  210  is connected to the protective container  205  for covering the input aperture CBI, while an output lid structure  215  is connected to the protective container  205  for covering the output aperture CBO. The input lid structure  210  is provided with a pipe union member  220  adapted to receive the flow FLin of the exhaust gas produced by the engine  105 . 
     In operation, the vehicle exhaust device  120  receives exhaust gas from the engine  105 . The flow FLin of the exhaust gas is collected by the pipe union member  220  of the input lid structure  210 , and is conveyed toward the input base BI of the DPF  125 . In this way, the exhaust gas is forced to flow through those exhaust gas flowing channels FLCH that are not plugged at the input base BI, until reaching the plug elements PE. As a consequence, since said exhaust gas flowing channels FLCH are blocked by the plug elements PE, the exhaust gas is forced to cross the later walls, so as to reach the adjacent exhaust gas flowing channels FLCH—which are not plugged at the output base BO—and exit from the DPF  125 . Particularly, thanks to the porosity properties of the SiC forming the walls of the gas flowing channels FLCH, the PM included in the exhaust gas remains confined in the DPF  125 , while the (filtered) exhaust gas crosses said walls and flows through the channels FLCH that are not plugged at the output base BO. The exhaust gas exiting the output base BO forms an output flow of (filtered) exhaust gas—denoted in the figure with the reference Flout—which is evacuated, through a pipe union member  225  on the output lid structure  215 , to the output of the vehicle  100 . 
     The heating device  127  for heating the flow FLin of the exhaust gas generated by the engine  105  comprises at least one, preferably a plurality of glow plugs GLP, for example including a glow plug GLP for each section of the input base BI. Each glow plug GLP has an end that is provided with a heating element HE capable of reaching temperatures of the order of 900-1000° C. when electrified due to electrical resistance. 
     According to an embodiment of the present invention, the glow plugs GLP forming the heating device  127  are positioned in the input lid structure  210  in such a way that the heating elements HE are in physical contact with, i.e., in punctual abutment to, the (corresponding section of the) input base BI of the DPF  125 . Particularly, the input lid structure  210  is provided with threaded bushings  230  adapted to accommodate the glow plugs GLP with an angle σ with respect to the longitudinal axis LA of the DPF  125 , so as not to obstacle the flow FLin of exhaust gas. In order to improve the efficiency of the filter regeneration process obtainable with the proposed structure, the glow plugs GLP are screwed in the respective threaded bushings  230  to an extent such that the heating element HE of each glow plug GLP results in contact with the surface of the input base BI in any operative condition. 
     In order to be activated for promoting the triggering of the burning of PM in the DPF  125 , the glow plugs GLP need to be properly supplied with electrical power. For this purpose, the vehicle  100  further comprises a supply block  135  including relays supplied in turn by a vehicle battery  140 , as illustrated in  FIG. 1 . 
     The vehicle exhaust device  120  is provided with sensors adapted to monitor some physical quantities related to the condition of the DPF  125  and the exhaust gas flowing therethrough. 
     Particularly, a pressure sensor PRS is positioned within the input lid structure  210 , in the proximity of the input base BI of the DPF  125 . As will be described in greater detail in the following of the present description, the pressure sensor PRS allows to obtain useful information regarding the PM accumulated in the DPF  125 , based on the measurement of the “counter-pressure” generated by the flow FLin of exhaust gas when passing through the DPF  125 . 
     Moreover, a temperature sensor TES, such as a thermocouple or a thermistor, is positioned within the output lid structure  215 , in the proximity of the output base BO of the DPF  125 . As will be described in greater detail in the following of the present description, the temperature sensor TES allows to obtain useful information regarding the temperature of the DPF  125  based on the temperature of the flow FLout outputted by the DPF  125 . 
     Referring back to  FIG. 1 , the vehicle  100  further includes an additive tank  145 , which is used to store a fuel additive adapted to be mixed with the fuel in such a way as to alter some of its chemical/physical attributes; particularly, as already explained in the introduction of the description, the fuel additive is mixed with the fuel for lowering the burning temperature of the PM included in the gas exhaust generated by the combustion of the fuel. For this purpose, the fuel additive is injected into the fuel pipe  115  by means of a clutch  150  connected to a pipe coming from the additive tank  145 . 
     The operations performed by the filter regeneration control system according to an embodiment of the present invention are managed by an electronic control unit, globally denoted in  FIG. 1  with the reference  160 . The control unit  160  is coupled with the vehicle exhaust device  120 , and in particular with the pressure sensor PRS and the temperature sensor TES. As will be described in the following of the present description, the control unit  160  processes the information received from the sensors PRS, TES, and as a result of said processing determines when the heating device  127  has to be turned on for favoring the triggering of the filter regeneration process in an efficient way, and accordingly drives the supply block  135  for activating the glow plugs GLP. 
     In  FIG. 3 , an exemplary structure of the control unit  160  is schematically shown in terms of functional blocks. Several functional units are connected in parallel to a data communication bus  305 . In particular, a processing unit  310 , typically comprising a microprocessor or a microcontroller or any other suitable data processing unit, controls the operation of control unit  160 ; a working memory  315 , for example volatile registers, is directly exploited by the processing unit  310  for the execution of programs and for temporary storage of data, and a Read Only Memory (ROM)  320  stores a basic program for the bootstrap of the control unit  160 , as well as other relevant data to be preserved when the control unit  160  is off. The control unit  160  is coupled with the pressure sensor PRS and the temperature sensor TES through respective circuit interfaces  325 ,  330 , which include conditioning circuits (such as sample-and-hold circuits, A/D converters, amplifiers, buffers and the like) adapted to treat the signals generated by the sensors PRS, TES for making them interpretable by the processing unit  310 . Using specific algorithms and performing operations (such as calculation of mean values of certain physical quantities) that will be described in the following of the present description, the processing unit  310  processes data received from the sensors PRS, TES, and accordingly drives the supply block  135  for activating the glow plugs GLP through a supply driver circuit  335 . 
     It is pointed out that some of the components shown in  FIG. 3  may be absent in some specific types of control units, and/or additional units may be provided, depending on the specific implementation of the present invention. 
     According to an embodiment of the present invention, the filter regeneration process (and, in particular, the timing of the glow plugs GLP activation) is managed in a more efficient way exploiting information parameters deducible from the physical quantities (pressure, temperature) monitored by the pressure sensor PRS and the temperature sensor TES. 
     Very useful information parameters are obtainable from the pressure monitored by the pressure sensor PRS. Particularly, a first useful parameter obtainable from physical quantity monitored by the pressure sensor PRS is the load of PM that has accumulated in the DPF  125  during the use of the vehicle. Generally speaking, the higher the PM load, the higher the counter-pressure generated by the flow FLin of exhaust gas when passing through the DPF  125 , because of the obstruction of the gas flowing channels FLCH. Moreover, it has to be appreciated that the higher the load of PM in the DPF  125 , the higher the combustible material available within the filter to be burned during the regeneration. Thus, the efficiency of the regeneration process may be also favored by a DPF  125  that is highly loaded. As disclosed in the foregoing, an efficient way to quantify the PM load accumulated in the DPF  125  provides for calculating the average value of the counter-pressure values sensed by the pressure sensor PRS, hereinafter referred to as PRm; the average pressure value is calculated over a number NS of samples of the pressure values sensed by the pressure sensor PRS. For this purpose, the interface  325  of the control unit  160  may sample the value monitored by the pressure sensor PRS at a specific sample rate, e.g., every ten seconds, storing the sampled values into the working memory  315 , so as to create a log of pressure values, including a number NS of samples, which may depend on the storage capability of the working memory  315 ; for example, the number NS of samples in the pressure values log may be  200 . The average counter pressure PRm is then calculated by the processing unit  310  using the pressure values in the sample log. 
     Based on the pressure values sensed by the pressure sensor PRS it is also possible to derive an indication of the flow rate of the exhaust gas. The flow rate of the exhaust gas, and particularly that of the flow FLin at the input of the DPF  125  depends on the load of the engine  105  and on its Revolutions-Per-Minute (RPM), which depend in turn on the current driving condition of the vehicle  100 . The flow rate of the flow Flin negatively affects the triggering of the combustion of the PM and that of the regeneration process, since the higher the flow rate of the flow Flin, the more heat is removed from the DPF  125 . The flow rate of the flow Flin can be monitored based on the instantaneous value PRi of the counter-pressure generated by the flow Flin, which directly depends thereon. The instantaneous value of the pressure PRi is stored in the working memory  315  of the control unit  160 , and is continuously updated as new sample values are received from the pressure sensor PRS through the interface  325 . 
     Another useful parameter is the DPF temperature. The temperature of the DPF  125  depends on the temperature of the exhaust gas, the flow rate thereof and the thermal inertia of the DPF itself As a result of several experimental proofs, even in presence of glow plugs GLP that heat localized portions of the DPF  125 , the Applicant has observed that the remaining portions of the DPF  125  need to be at a temperature at least higher than a lower threshold temperature Tlow (of the order of 150° C.), otherwise the regeneration process (i.e., the burning of the PM) would not be triggered in a sufficient way. Particularly, if the DPF  125  is at temperatures lower than Tlow, even if the glow plugs GLP were be successful in triggering the burning of the PM accumulated near the glow plugs GLP, the combustion of the PM would not propagate toward the rest of the DPF  125 , even in presence of an additive in the fuel. The higher the DPF temperature with respect to the lower threshold temperature Tlow, the higher the probability that the combustion of the PM propagates through the whole DPF  125 . Since a DPF made of SiC has a high thermal inertia, a good way for obtaining a stable measure of the DPF temperature consists in monitoring the temperature of the flow FLout—hereinafter referred to as Tfl—outputted by the DPF itself In this way, the monitored temperature Tfl results to be essentially independent from any undesired rapid fluctuations introduced by any external factor (unlike the temperature of the flow Flin, which varies quickly); in other words, thanks to the thermal inertia of the DPF  125 , the control unit  160  does not have to perform any average operation, being sufficient to store in the working memory  315  and continuously updating a sample of the temperature value Tfl provided by the interface  330  coupled with the temperature sensor TES. 
     As better described later, according to an embodiment of the present invention the regeneration process of the DPF  125  is managed in an efficient way taking into account the values of at least two of the three parameters previously introduced, and particularly the average pressure PRm and the instantaneous pressure PRi. As will be described in the following of the present description, according to an embodiment of the present invention the activation of the heating device  127  (i.e., of the glow plugs GLP) is conditioned to the fact that the values of the parameters PRm and PRi exceeds predetermined thresholds, and/or fall in respective ranges; moreover, for further improving the efficiency of the regeneration process, it is possible to condition the activation of the heating device  127  to the fact that the value of the instantaneous temperature Tfl falls in a respective range, too. 
     More particularly,  FIG. 4A  depicts a flow chart  400  illustrating the main operations performed by the filter regeneration control system according to an embodiment of the present invention—and in particular by the control unit  160 —on a vehicle  100 . As described in the following, according to an embodiment of the present invention two different types of filter regeneration process are envisaged, and particularly an automatic filter regeneration process, totally managed by the filter regeneration control system during the operation of the vehicle  100 , and an emergency regeneration process—performed with the active involvement of the driver, or a technician of a service center—to be performed in case the automatic filter regeneration process has not given satisfactory results. 
     The control unit  160  firstly checks whether the vehicle  100  is running (block  405 ), for example according to a signal taken from the alternator (not shown in the figures) of the engine  105  of the vehicle  100 . Alternatively, the control unit  160  may assess whether or not the engine  105  is turned on by detecting the presence or absence of the flow Flin of exhaust gas generated by the engine  105  when turned on; this can be achieved by monitoring the instantaneous pressure PRi. 
     Having assessed that the engine  105  is turned on (block  405 , exit branch “Y”), the control unit  160  performs a first control (“pressure alarm control”) on the values of the average pressure PRm and the instantaneous pressure PRi. Particularly, this control is performed for determining if the condition of PM load of the DPF  125  are so serious (from a PM occlusion point of view) to require a manual emergency regeneration process. 
     Particularly, the pressure alarm control (block  410 ) provides for a joint control on both the average pressure PRm and the instantaneous pressure PRi. If the value of the instantaneous pressure PRi remains within a range defined by a minimum alarm instantaneous pressure LAPRi (lower threshold) and a maximum alarm instantaneous pressure HAPRi (higher threshold) for a predetermined alarm time interval Δtall, and at the same time the value of the average pressure PRm belongs to a range defined by a minimum alarm average pressure LAPRm (lower threshold) and a maximum alarm average pressure HAPRm (higher threshold), then the control unit  160  assesses that the DPF  125  is excessively loaded by PM, and that the DPF  125  needs a (manual) emergency regeneration process. Both the maximum alarm instantaneous pressure HAPRi and the maximum alarm average pressure HAPRm may be set to the maximum pressure value that the pressure sensor PRS can sense (full-scale value). Since, as mentioned in the foregoing, the average pressure PRm is directly related to the load of PM accumulated in the DPF  125 , the minimum alarm average pressure LAPRm is advantageously set to a value corresponding to a high load of PM accumulated in the DPF  125 , but sufficiently low not to completely compromise the vehicle  100  operation (i.e., a value that ensures that the DPF  125  is not so loaded by PM that the engine  105  cannot efficiently run). Since also the instantaneous pressure PRi is related to the load of PM (the counter-pressure generated by the flow FLin of exhaust gas when passing through the DPF  125  depends on the PM accumulated in the DPF  125 ), also the minimum alarm instantaneous pressure LAPRi is advantageously set to a value corresponding to a relatively high load of PM accumulated in the DPF  125 ; in this way, a double check is made before deciding that the emergency regeneration has to be performed. 
     If the previous conditions are satisfied (exit branch “Y” of block  410 ), the filter regeneration control system signals an alarm condition; for example, the control unit  160  notifies the alarm condition to the vehicle driver through a corresponding signaling light on the dashboard (not shown in the figures). The vehicle driver should react to the signaled alarm condition by taking the steps necessary for performing the emergency regeneration process (block  415 ). 
     The emergency regeneration process may for example provide that the vehicle driver drives the vehicle  100  at a relatively high speed possible for about 10 minutes (compatibly with the traffic conditions and the speed limit restraints). 
     In this way, the temperature of the DPF  125  reaches a temperature favorable for the combustion of the PM included therein. At this point, the vehicle&#39;s march is arrested, and the engine  105  is set to the idling condition. Then, the driver manually controls the activation of the heating device  127 , for example by means of a suitable switch (not shown in the figures) on the cockpit of the vehicle  100  connected to the control unit  160 . The control unit  160  then drives the supply block  135  through the supply driver circuit  335 , in such a way as to turn on the glow plugs GLP of the heating device  127 . After a time that may depend on the glow plug type (for example, 20 seconds), the temperature of the heating elements HE contacting the input base BI of the DPF  125  reaches a substantially steady temperature of about 900° C. The PM accumulated in the DPF  125  in the proximity of the heating elements thus reaches the burning temperature for spontaneous combustion (for example, 350° C. in presence of fuel additive). Since the DPF  125  has been previously brought to a sufficiently high temperature by driving the vehicle  100  at high speed for about 10 minutes, the combustion of the PM propagates through the whole DPF filter  125 . 
     After a predetermined amount of time, sufficient for allowing the average pressure PRm to adapt to the new condition of the DPF  125  after the activation of the glow plugs GLP (for example, 30 minutes), the control unit  160  checks whether the emergency regeneration has been successful or not (block  420 ). Particularly, the control unit  160  controls if the value of the average pressure PRm after the emergency regeneration still falls within the range defined by the minimum alarm average pressure LAPRm and the maximum alarm average pressure HAPRm or not. 
     In the affirmative case (exit branch “Y” of block  420 ), the control unit  160  decides that emergency regeneration was not successful, since the value of the average pressure PRm is still too high (this meaning that the PM included in the DPF  125  is still too high); the control unit  160  then increments a regeneration failure counter Nreg (block  425 )—used to count the number of unsuccessful emergency regenerations—and checks if said counter has reached or not a maximum regeneration failure number MAXNreg (block  430 ). In the affirmative case (exit branch “Y” of block  430 ), the DPF  125  needs to be unmounted from the vehicle  100  for being substituted or fixed at a maintenance service center (block  435 ), since performing other emergency regenerations would be useless. On the contrary, if the regeneration failure counter Nreg is still lower then the maximum regeneration failure number MAXNreg (exit branch “N” of block  430 ), the operation flow returns to block  405  for performing again the pressure alarm control. 
     If instead the value of the average pressure PRm after the emergency regeneration has been assessed to be lower than the minimum alarm average pressure (exit branch “N” of block  420 ), the control unit  160  decides that the emergency regeneration has been successful; indeed, the fact that the value of the average pressure PRm is lower than the minimum alarm average pressure LAPRm means that the PM included in the DPF  125  has been burned out to a sufficient extent. In this latter case, the regeneration failure counter Nreg is reset to zero (block  440 ), and the emergency regeneration procedure is terminated (the operation flow returns to block  405 ). 
     During the normal operation of the vehicle  100 , i.e., when the DPF  125  is not so occluded by PM to an extent such as to bring the filter regeneration control system in an alarm condition, the control unit  160  manages control operations for determining the when to activate the heating device  127  (i.e., the glow plugs GLP), in such a way that an efficient regeneration process is triggered. 
     Particularly, the control unit  160  monitors the current values of the average pressure PRm and the instantaneous pressure PRi, checking if said values are all within respective value ranges (block  445 ), and then activating or not the glow plugs GLP based on the result of said check. 
     Particularly, if:
         the value of the instantaneous pressure PRi is higher than a minimum instantaneous pressure LPRi (lower threshold), and   the value of the average pressure PRm is higher than a minimum average pressure LPRm (lower threshold),
 
the control unit  160  determines that the glow plugs GLP can be activated for attempting a regeneration (exit branch “Y” of block  445 ), otherwise the glow plugs are not activated (exit branch “N” of block  445 , returning to block  405 ).
       

     Preferably, according to a further embodiment of the present invention, an optional check is performed on the instantaneous temperature Tfl too. Particularly, in said embodiment the glow plugs GLP are activated when the above-mentioned conditions on the instantaneous pressure PRi and the average pressure PRm are fulfilled, and at the same time if the value of the instantaneous temperature Tfl remains within an interval defined by a minimum instantaneous temperature LT (lower threshold) and a maximum instantaneous temperature HT (higher threshold) for a predetermined time interval Δt(T) 
     When the conditions above are fulfilled, it is likely that the conditions of the DPF  125  are such as to ensure the triggering of a probable efficient regeneration process if the glow plugs GLP are activated at that moment. Particularly, in order to be in a good condition for an efficient regeneration process, which ensures at the same time not to unnecessarily activate the glow plugs GLP and wasting battery electric power, the average pressure PRm should be higher than a minimum value (LPRm), since a too low average pressure PRm indicates that it is still not necessary to regenerate the DPF  125  and/or that the quantity of PM in the DPF  125  is not sufficient for guaranteeing an efficient PM combustion. The check on the instantaneous pressure PRi is made in order to derive an indication of whether, at the moment the glow plugs will be activated, the engine RPM is likely to be high or low; moreover, the instantaneous pressure PRi gives a useful confirmation about the DPF  125  overload condition. The value of the instantaneous pressure PRi is related to the flow Flin of the exhaust gas at the input of the DPF  125 . Since the higher the flow rate of the flow Flin, the higher the heat removal rate from the DPF  125 , the glow plugs GLP should be activated when the flow FLin is as small as possible. However, based on experimental trials, the Applicant has found that, especially in the urban driving condition, i.e., in the condition in which the filter regeneration favored by the glow plugs activation is mostly necessary, after relatively high peaks of instantaneous pressure PRi, corresponding to relatively high flow Flin generated by a sudden increase of the engine RPM, the instantaneous pressure PRi (and correspondingly the flow Flin) falls (because the engine RPM decrease). Thus, every time a relatively high peak of instantaneous pressure PRi is detected, i.e., when the value of the instantaneous pressure PRi has exceeded the minimum instantaneous pressure LPRi, it is likely that the flow FLin of exhaust gas will subsequently decrease during the time interval in which the glow plugs GLP are turned-on, a condition favorable for the combustion of the PM. 
     The optional check on the instantaneous temperature Tfl may be performed for assessing if the temperature of the DPF  125  is sufficiently high for allowing the propagation of the PM combustion through the whole DPF  125  (Tfl&gt;LT), and at the same time if it is lower than a maximum value (HT), above which the activation of the glows plugs GLP would be unnecessary, since the PM would burn for spontaneous combustion. 
     In case the control unit  160  has assessed that the conditions are suitable for the activation of the heating device  127 , it drives the supply driver circuit  335  to activate the glow plugs GLP (block  450 ) for an amount of time, referred to as activation interval Ton. It has been found that a duration of the activation interval Ton set in the control unit  160  in order to be sufficiently compliant with the need of at least locally initiating the PM combustion, advantageously allows the instantaneous pressure PRi to fall from the value higher than the minimum instantaneous pressure LPRi that has triggered the activation of the glow plugs GLP to a lower value corresponding to a low flow Flin of exhaust gas, which is favorable to the propagation of the combustion of the PM in the DPF  125 . 
     More particularly, the combustion of the PM in the DPF  125  is not an instantaneous event, since the burning of the PM initially occurs only in the portions of the DPF  125  close to the glow plugs GLP; as a consequence, the burning of the PM requires an amount of time to propagate through the whole DPF  125 . Taking into account this consideration, the Applicant has observed that the “slowness” of the burning propagation can be advantageously exploited for guaranteeing with a good probability that the regeneration process is performed when the flow FLin of the exhaust gas produced by the engine  105  is sufficiently low, when there is no substantial obstacle to the propagation of the combustion of the PM to the whole filter. 
     When the condition on the average pressure PRm is fulfilled—and thus the value thereof is higher than the minimum average pressure LPRm—, it means that a regeneration of the DPF  125  is recommended, and the load of PM accumulated therein is sufficient for guaranteeing a good PM combustion. 
     It has to be appreciated that the value of the average pressure PRm increases with a substantially slow rate, being calculated by means of an average over the NS samples of the pressure values collected by the control unit  160  through the pressure sensor PRS. Unlike the average pressure PRm, the instantaneous pressure PRi (and thus the flow FLin of the exhaust gas) can vary rapidly in time, depending on the load of the engine  105  and on its Revolutions-Per-Minute (RPM), which depend in turn on the current driving condition of the vehicle  100 . Thus, having assessed through the fulfilment of the condition on the average pressure PRm that the DPF  125  needs a regeneration, the event that actually triggers the activation of the glow plugs GLP is the fulfilment of the condition on the instantaneous pressure PRi. Particularly, substantially as soon as the instantaneous pressure PRi reaches the minimum instantaneous pressure LPRi, the control unit  160  drives the supply block  135  for activating the glow plugs GLP; since the burning of the PM is not instantaneous, but requires an amount of time to propagate through the whole DPF  125 , the glow plugs GLP are kept activated during the activation interval Ton, in such a way as to leave to the instantaneous pressure PRi the time to fall to a significantly lower value, corresponding to a low flow Flin of exhaust gas. As will be described in detail in the following of the present description, setting the minimum instantaneous pressure LPRi to a value sufficiently high (statistically improbable) to be reached by the instantaneous pressure PRi only during the high peaks thereof that are generated by a sudden increase of the engine RPM, and considering that every time a relatively high peak of instantaneous pressure PRi is detected, it is likely that the flow FLin of exhaust gas will subsequently decrease, it is possible to guarantee with a good probability that the regeneration process be successfully performed. The duration of the activation interval Ton is properly set in such a way to be sufficiently sure that the glow plugs GLP are still activated when the flow FLin of exhaust gas has reached a low level. Thus, the choice of activating the glow plugs GLP when the instantaneous pressure PRi has a high value, which at a glance would seem unfavorable for the outcome of the regeneration process, is really advantageous, since at least a portion of the activation interval Ton necessary for the instantaneous pressure PRi to reach a low value corresponding to a low flow FLin of exhaust gas may correspond to the time needed for the burning of the PM to propagate in the whole extent of the DPF  125 . Moreover, it has to be appreciated that the activation interval Ton may include at least the time spent by the glow plugs GLP after their activation to reach a temperature sufficiently high to trigger the combustion of the PM. 
     Returning back to a flow chart  400  illustrated in  FIG. 4A , the control unit  160  then checks if the regeneration process has been successful or not. Particularly, after a predetermined amount of time, sufficient for allowing the average pressure PRm to adapt to the new condition of the DPF  125  after the activation of the glow plugs GLP (for example, 30 minutes), the control unit  160  controls if the value of the average pressure PRm after the regeneration is still higher than the minimum average pressure LPRm or not (block  455 ) 
     If the value of the average pressure PRm after the regeneration is lower than the minimum average pressure LPRm (exit branch “N” of block  455 ), the control unit  160  determines that the regeneration process has been successful, while in the opposite case, the regeneration process has not been successful (exit branch “Y” of block  455 ). In both cases, after the regeneration process the control unit  160  again checks if it is necessary a manual emergency regeneration process (returning to block  405  and then to block  410 ). However, in case the control unit  160  has determined that the regeneration process has been successful, the regeneration failure counter Nreg is reset to zero (block  460 ). According to an embodiment of the present invention, the control unit  160  may perform further operations directed to optimize the values of the minimum average pressure LPRm for improving the efficiency of the filter regeneration process based on information collected by the control unit  160  after the glow plugs have been activated. A description of said operations will be provided in the following of the description (particularly, in connection to  FIG. 6 ). 
     All the operations that have been described with reference to  FIG. 4A  may be performed by the control unit  160  by executing a sequence of instructions of a firmware stored in the control unit  160  itself—for example, in the ROM  320 —and executed by the processing unit  310 . 
       FIG. 4B  depicts a diagram  465  showing an exemplary evolution in time of the instantaneous pressure PRi during the vehicle  100  operation. Particularly, the abscissa of the diagram  465  indicates the time, while the ordinate indicates the value of the instantaneous pressure PRi. The instantaneous pressure PRi has an oscillating trend, following the RPM of the engine  105 , with an average value that increases in time (due to the increase of the PM load in the DPF  125 , that causes a constant increase of the average pressure PRm). After a relatively high peak in the instantaneous pressure PRi—like the one indicated in  FIG. 4B  with the reference  470 —, the flow FLin of exhaust gas decreases significantly. 
     In order to describe in greater detail the evolution in time of the regeneration process, and the relationship occurring between the activation of the glow plugs GLP and the instantaneous pressure PRi according to an embodiment of the present invention, reference will be now made to  FIG. 4C . 
     Particularly,  FIG. 4C  depicts a first diagram  475  showing an exemplary evolution in time of the instantaneous pressure PRi and a second diagram  480  showing an exemplary evolution in time of the instantaneous temperature Tfl during the activation of the glow plugs GLP and the triggering of the regeneration process of the DPF  125 . For example, the evolution in time of the instantaneous pressure PRi shown in the diagram  475  may correspond to a portion of that shown in the diagram  465  illustrated in  FIG. 4B , and particularly a portion including the high peak  470 . 
     It is supposed that at the time corresponding to the beginning of the diagrams  475  and  480  the average pressure PRm is higher than the minimum average pressure LPRm, meaning that the DPF  125  needs a regeneration, and the load of PM accumulated therein is sufficient for guaranteeing a good PM combustion. As previously described, in this condition the event that triggers the activation of the glow plugs GLP is the fulfilment of the condition on the instantaneous pressure PRi. 
     As soon as the instantaneous pressure PRi reaches the minimum instantaneous pressure LPRi (time t 0 ), the control unit  160  drives the supply block  135  for activating the glow plugs GLP. As will be described in the following of the present description, the minimum instantaneous pressure LPRi is set by the control unit  160  to a value sufficiently high to be reached by the instantaneous pressure PRi only during the high peaks thereof, like the peak identified with the reference  470 . In the example at issue, the instantaneous pressure PRi equals the minimum instantaneous pressure LPRi close to the upper portion of the peak  470 , and particularly briefly before the top thereof. 
     After the activation of the glow plugs GLP at the time t 0 , the temperature thereof starts to increase, reaching a substantially steady temperature at a subsequent time t 1  that depends on the glow plug type. For example, a typical glow plug GLP may reach a steady temperature of about 900° C. in about 20 seconds. 
     Based on the results of several experimental proofs, the Applicant has observed that the high peaks of instantaneous pressure PRi typically have a quite short duration, and particularly much lower than the time needed for the glow plugs GLP to reach the steady temperature after the activation. For example, in the urban driving condition, a relatively high peak of instantaneous pressure PRi that corresponds to a relatively high flow Flin generated by a sudden increase of the engine RPM, may have a duration in time of the order of 2-5 seconds. In the example at issue, the instantaneous pressure PRi falls from the value corresponding to the top of the peak  470  to about one third of said value in about 2 seconds (time t 2 ). This means that, by properly setting the minimum instantaneous pressure LPRi, the glow plugs GLP are activated few seconds before the reaching of a local minimum of the instantaneous pressure PRi, which corresponds to a low flow Flin of exhaust gas. In other words, when the DPF  125  is in a condition favorable for the combustion of the PM, with the proposed solution the glows plugs GLP are already activated, and the regeneration process may be performed correctly with a high probability. 
     The PM in the DPF  125  initially starts to burn locally, in the portions of the DPF  125  close to the glow plugs GLP. The time corresponding to the beginning of the local burning (time t 3 ) depends on several factors, like the actual temperature of the flow Flin, the load of PM, and the presence of additives added to the fuel and remaining in the exhaust gases. The time t 3  corresponding to the beginning of the local burning may occur before the time t 1  at which the glow plugs GLP reach the steady temperature, but likely after the time t 2  at which the instantaneous pressure PRi reaches a local minimum. In the example at issue, the local burning begins at a time t 3  equal to about 10-18 seconds after the time to. 
     As time goes by, the burning of the PM propagates, until substantially involving the whole extent of the DPF  125  (time t 4 ). Also the time t 4  at which the burning of the PM can be considered to have substantially propagated through the whole DPF  125  depends on several factors, like the actual temperature of the flow Flin, the load of PM and the size of the DPF  125  itself. In the example at issue, said time t 4  occurs about 30-35 seconds after the time t 0 . 
     A good parameter that can be used for assessing if the regeneration process has been correctly triggered, i.e., if the burning of the PM has occurred substantially in most of the DPF  125  is the instantaneous temperature Tfl sensed by the temperature sensor TES, i.e., the temperature of the flow outputted by the DPF  125 . Thanks to the thermal inertia of the DPF  125 , the instantaneous temperature Tfl starts to significantly increase only a non-negligible time period after the time t 3  corresponding to the beginning of the local burning. In the example at issue, the instantaneous temperature Tfl starts to increase few second before the time t 4  at which the burning of the PM has reached the whole extent of the DPF  125 . 
     In order to avoid any unnecessary waste of battery electric power, once the burning of the PM has been sufficiently propagated across the DPF  125 , the glow plugs GLP may be deactivated, since at this time, the burning of the PM is auto-sustained, and is capable of continuing without the need of the additional heat provided by the glow plugs GLP. 
     According to an embodiment of the present invention, the glow plugs GLP are turned off by the control unit  160  at a time t 5  corresponding to a relatively high increasing rate of the instantaneous temperature Tfl (i.e., when the slope of the curve depicting the instantaneous temperature Tfl is sufficiently high, exceeding a predetermined threshold slope). Indeed, for the reasons previously described, at this time t 5  it is possible to asses—with a high probability that the burning of the PM has been sufficiently propagated across the DPF  125 , and that the burning is able to continue without the need of the additional heat provided by the glow plugs GLP. In the example at issue, the time t 5  at which the glow plugs GLP are turned off occurs few seconds after the time at which the burning of the PM is involving the whole extent of the DPF  125 , and particularly about 40 seconds after the time t 0 . 
     In other words, according to an embodiment of the present invention, the glow plugs GLP are kept activated for an activation interval Ton starting from a time t 0  close to the occurrence of the top of an instantaneous pressure PRi peak, and terminating at a time t 5  corresponding to a relatively high increasing rate of the instantaneous temperature Tfl. 
     In order to improve the efficiency, according to an embodiment of the present invention, the filter regeneration system performs the above described controls on the current values of the instantaneous pressure PRi, the average pressure PRm and the instantaneous temperature Tfl using threshold values that are optimized for the specific vehicle  100  and for its driving conditions. 
     According to an embodiment of the present invention, the threshold values used by the control unit  160  for assessing when to activate the glow plugs GLP are adjustable. Preferably, the threshold values used by the control unit  160  for assessing when to activate the glow plugs GLP are automatically determined by the control unit  160  itself during the vehicle  100  operation, fitting them to the particular driving conditions of the moment. 
     Regarding in particular the instantaneous pressure, a way for improving the efficiency of the filter regeneration process is to dynamically adjust the value of the minimum instantaneous pressure LPRi based on the vehicle  100  condition (particularly, based on the driving condition). 
     During the operation of the vehicle  100 , the control unit  160  dynamically sets the minimum instantaneous pressure LPRi as described in the schematic flowchart  500  of  FIG. 5A . 
     As soon as the engine  105  of the vehicle  100  is started, the control unit  160  begins to collect samples of the instantaneous pressure PRi (block  510 ). As the control unit  160  collects PRi samples, a pressure sample log—including a number PNS of samples (for example,  200 )—is progressively filled to generate a statistical distribution of instantaneous pressures PRi. An example of such a distribution is shown in  FIG. 5B  with the reference DP(i), wherein the abscissa indicates the possible values of the instantaneous pressure PRi that can be sampled during the operation of the vehicle  100 , and the ordinate indicates the frequency of samples for each pressure value. 
     The control unit  160  sets the value of the minimum instantaneous pressure LPRi using the information obtainable from the distribution DP(i) corresponding to the actual pressure sample log (block  520 ). More particularly, the control unit sets the minimum instantaneous pressure LPRi to a high improbable pressure value corresponding to a relatively high percentile (e.g., the 90th, and preferably the 95th percentile) of the distribution DP(i), in such a way to select a pressure corresponding to a high peak. Alternatively, the control unit  160  may firstly select from the PNS samples of the pressure sample log, those samples that correspond to instantaneous pressures PRi higher than the average pressure PRm for generating a corresponding further statistical distribution of instantaneous pressures PRi, and then set the value of the minimum instantaneous pressure LPRi to a pressure value corresponding to said relatively high percentile of the further statistical distribution. 
     Then, the pressure sample log is updated with new, incoming PRi samples (block  530 ), and the process is reiterated (returning to block  510 ). Particularly, the control unit  160  updates the sample log with a number (possibly one) of new PRi samples, removing therefrom a corresponding number of the oldest PRi samples, and recalculates the minimum instantaneous pressure LPRi on the basis of the distribution obtained with the new sample log. 
     In order to further improve the efficiency of the proposed filter regeneration system, the possibility may also be contemplated that, in operation, the control unit  160  varies the value of the percentile of the distribution DP(i) used for setting the minimum instantaneous pressure LPRi, so as to better fit the expected driving condition of the vehicle  100 . 
     According to an embodiment of the present invention, also the value of the minimum average pressure LPRm may be dynamically determined by the control unit  160  in an automatic way to improve the filter regeneration process based on the vehicle  100  operating condition (and in particular based on the DPF  125  condition). 
     Making reference to the flow chart  600  illustrated in  FIG. 6 , the operations which are shown in the flow chart  600  are performed during the vehicle  100  operation, and integrate the operations of the control unit  160  shown in the flow chart  400  of  FIG. 4A . For this reason, the blocks corresponding to those shown in the  FIG. 4A  are denoted with the same references, and their explanation is omitted for the sake of brevity 
     The control unit  160  stores, for example in the ROM  320 , default values for the minimum average pressure (block  605 ). Particularly, the minimum average pressure LPRm may be instead initially set to a default minimum average pressure DLPRm corresponding to a relatively high value, in such a way to avoid that, at the beginning of the vehicle  100  operation, the glow plugs GLP are unnecessarily activated even when the PM accumulated in the DPF  125  is low (indeed, the value of the average pressure PRm is directly related to the PM blockage of the DPF  125 ). 
     At this point, the control unit  160  performs the normal operations previously described in connection with  FIG. 4A  for determining whether to activate the glow plugs GLP (blocks  445 ,  450 ), and then checks if the regeneration process has been successful or not, by controlling if the value of the average pressure PRm after the regeneration is still higher than the minimum average pressure LPRm or not after a predetermined amount of time (block  455 ). 
     In case the value of the average pressure PRm is still higher than the minimum average pressure LPRm (exit branch “Y” of block  455 ), meaning that the regeneration process has not been successful, the control unit  160  increments a regeneration fault counter NRf, for example stored in the working memory  315  (block  610 ). At this point, the control unit  160  checks if the value of said regeneration fault counter NRf has reached or not a preset regeneration fault threshold TNRf (block  620 ). If the regeneration fault counter NRf results to be lower than the regeneration fault threshold TNRf (exit branch “N” of block  620 ), no further additional operation is performed on the minimum average pressure LPRm, and the operation flow of the control unit  160  returns to block  445 . If instead the regeneration fault counter NRf reaches the value of the regeneration fault threshold TNRf (exit branch “Y” of block  620 ), it means that a number TNRf of consecutive activations of the glow plugs GLP has not been successful. In this case, the minimum average pressure LPRm is increased (for example by a predetermined amount, and until a predetermined maximum value, e.g., corresponding to the default minimum average pressure DLPRm), in such a way that the following activation of the glow plugs GLP will be conditioned to a higher average pressure PRm (block  625 ). This means that the regeneration process will be facilitated with respect to the previous cases, since the activation of the glow plugs will occur with a higher load of PM to be combusted. After having increased the minimum average pressure LPRm, the regeneration fault counter NRf is reset to zero, and the operation flow of the control unit  160  returns to block  445 . 
     In case instead after the activation of the glow plugs GLP the value of the average pressure PRm has fallen below the minimum average pressure LPRm (exit branch “N” of block  455 ), meaning that the regeneration process has been successful, the control unit  160  increments a regeneration successful counter NRs, for example stored in the working memory  315  (block  630 ). The control unit  160  then checks if the value of said regeneration successful counter NRs has reached or not a preset regeneration successful threshold TNRs (block  635 ). If the regeneration successful counter NRs is lower than the regeneration successful threshold TNRs (exit branch “N” of block  635 ), no further additional operation is performed on the minimum average pressure LPRm, and the operation flow of the control unit  160  returns to block  445 . If instead the regeneration successful counter NRs has reached the value of the regeneration successful threshold TNRs (exit branch “Y” of block  635 ), it means that a number TNRs of consecutive activations of the glow plugs GLP has been successful. In this case, the minimum average pressure LPRm is decreased (for example by a predetermined amount, and down to a predetermined minimum value), in such a way that the subsequent activation of the glow plugs GLP will be conditioned to a lower average pressure PRm (block  640 ). This means that the subsequent activation of the glow plugs will occur with a lower load of PM to be combusted, since it is likely that the previous activations of the glow plugs GLP have been performed with an excessive PM load. After having decreased the minimum average pressure LPRm, the regeneration successful counter NRs is reset to zero, and the operation flow of the control unit  160  returns to block  445 . 
     According to a further embodiment of the present invention, the procedure for dynamically determining the minimum average pressure LPRm can be further improved, by conditioning the extent of the increments/decrements of the minimum average pressure LPRm to the variation of the average pressure PRm after the activation of the glow plugs GLP. Particularly, based on the extent of the variation of the average pressure PRm after the activation of the glow plugs GLP, it is possible to quantify the extent to which the regeneration process has been successful/unsuccessful. For example, if the activation of the glow plugs GLP gives as a result only a little decrease of the average pressure PRm, it means that said activation has triggered the combustion of only a little fraction of the PM included in the DPF  125 . If instead the activation of the glow plugs GLP has given as a result a greater decrease of the average pressure PRm, it means that said activation has triggered the combustion of a greater fraction of the PM included in the DPF  125 . Based on these considerations, instead of increasing/decreasing the minimum average pressure LPRm by fixed amounts, as previously described with reference to  FIG. 6 , the increments/decrements may be a function of the variation of the average pressure PRm. For example, the control unit  160  may calculate the variation of the average pressure PRm by subtracting the value of the average pressure PRm after an activation of the glow plugs GLP from the value of the average pressure PRm before the activation of the glow plugs GLP, and then using said value for weighing preset increment and decrement amounts. 
     According to an embodiment of the present invention, the minimum instantaneous temperature LT and the maximum instantaneous temperature HT, as well as the other thresholds set for the average and instantaneous pressure, may be determined starting from preliminary experimental results, obtained through test campaigns performed on the field, for example on a test vehicle corresponding to the vehicle  100  on which the system is intended to be installed. 
       FIG. 7A  illustrates the experimental results obtained through an exemplary test campaign. Samples of the instantaneous temperature Tfl of the DPF  125  are collected by monitoring the test vehicle in two distinct operating conditions, namely a “urban” condition, which is unfavorable for the natural PM combustion, in which the test vehicle is driven essentially in a urban scenario, at relatively low speeds and with frequent stops, and a “highway” condition, favorable for the PM combustion, wherein the test vehicle is driven essentially continuously at relatively high speeds, with few stops. 
       FIG. 7A  shows a diagram  700  in which the abscissa indicates the instantaneous temperature Tfl of the DPF, and the ordinate indicates the frequency of samples taken during the test campaign. The samples statistical distribution corresponding to the urban condition—denoted in  FIG. 7A  with reference  710 —is centered around a low temperature value (for example, 120° C.), while the samples statistical distribution corresponding to the highway condition—indicated in  FIG. 7A  with the reference  720 —is centered around a higher temperature value (for example, 370° C.). This is a reasonable outcome, since in the urban condition the vehicle  100  is subjected to low stresses, and the engine  105  essentially works at low RPM, while in the highway condition the engine works at higher RPM. 
     In order to derive useful values for the temperature thresholds, i.e. the minimum instantaneous temperature LT and the maximum instantaneous temperature HT, the statistical distributions  710  and  720  are compared with two temperature threshold, namely a low temperature threshold ALT corresponding to the minimum temperature of the DPF  125  that allows the PM combustion to propagate through the DPF  125  once triggered by the glow plugs GLP (e.g., 150° C.), and a high temperature threshold AHT corresponding to the temperature above which the PM burns for self combustion and the activation of the glow plugs GLP is no longer necessary for triggering the PM combustion (e.g., 350° C.,). 
     Comparing such two low and high temperature thresholds ALT and AHT with the distributions  710  and  720 , it is possible to infer that the low temperature threshold ALT corresponds to a relatively high percentile of the distribution  710  (for example, higher than the 70th percentile and preferably equal to approximately the 95th percentile), while the high temperature threshold AHT corresponds to a relatively low percentile of the distribution  720  (for example, lower than the 45th percentile and preferably equal to approximately the 30th percentile). 
     In a practical case (like during the operation of the control unit  160 ), said statistical distribution of the temperature value samples will generally differ from both the distribution  710  and the distribution  720 , for example because before reaching a highway, the vehicle  100  first travel in a scenario similar to that corresponding to the urban condition, represented by the distribution  720 . An example of a more close-to-reality statistical distribution is indicated in  FIG. 7A  with the reference  730 . It can be observed that the distribution  730  is less regular, being more spread across a wider range of temperature values extending from those corresponding to the distribution  710  to those corresponding to the distribution  720 . 
     Based on statistical computations, it is possible to deduce the percentiles of the distribution  730  that correspond to the minimum instantaneous temperature LT and the maximum instantaneous temperature HT using the percentiles corresponding to the temperature thresholds ALT, AHT of the distributions  710  and  720 , respectively, with a good approximation. For example, starting from a low temperature threshold ALT corresponding to the 95th percentile of the distribution  710  and from a high temperature threshold HLT corresponding to the 45th percentile of the distribution  720 , a good approximation may give as a result a minimum instantaneous temperature LT corresponding to the 30th percentile and a maximum instantaneous temperature HT corresponding to the 70th percentile of the distribution  730 . 
     However, the Applicant has observed that even if the temperatures of the distribution  720  lower than but relatively close to that for which the PM burns for self combustion (i.e., 350° C.) are favorable to the PM combustion, and thus would in principle be favorable for the activation of the glow plugs GLP, said range of temperature corresponds to a condition in which no great advantage can be taken by the activation of the glow plugs GLP. Particularly, the temperatures of the distribution  720  that are lower than but relatively close to the temperature for which the PM burns for self combustion may correspond to a condition in which the engine  105  is working at high RPM, which in turn corresponds to a condition in which the flow rate of the flow Flin is relatively high. As already mentioned in the present description, a high flow rate Flin negatively affects the triggering of the combustion of the PM and that of the regeneration process, since the higher the flow rate of the flow Flin, the more heat is removed from the DPF  125 . 
     In this regard, it has to be appreciated that in those driving conditions wherein the vehicle is driven essentially continuously at relatively high speeds—such as when the vehicle is being driven in a highway environment—, the engine RPM may remain high for a relatively long period of time. Unlike in the urban driving condition, wherein after relatively high peaks of instantaneous pressure PRi the instantaneous pressure PRi (and correspondingly the flow rate of the flow Flin) is likely to fall because the engine RPM decrease, in e.g. highway driving conditions the instantaneous pressure PRi (and, correspondingly, the flow rate of the flow Flin) may not fall to a low value for a long period of time. Thus, in highway driving condition, by activating the glow plugs GLP as soon as the instantaneous pressure PRi has exceeded the minimum instantaneous pressure LPRi, the combustion of the PM may not be correctly triggered, since it is likely that the flow rate of the flow FLin will not subsequently decrease during the time interval in which the glow plugs GLP are turned-on. 
     Therefore, according to an embodiment of the present invention, it may be useful not to activate the glow plugs GLP when the temperature of the exhaust gas is lower than but relatively close to the temperature for which the PM burns for self combustion, because such a temperature value may correspond to a condition in which the engine  105  works at high RPM, corresponding in turn to a condition in which the flow rate of the flow Flin is relatively high. 
     As a consequence, according to an embodiment of the present invention, the maximum instantaneous temperature HT may be calculated using a lower percentile of the distribution  720 , for example the 30th percentile or a lower percentile. Thus, even if in this way a range of temperatures which are in principle favorable for the PM combustion (under the temperature point of view) are “discarded”, and the activation of the glow plugs GLP is performed at lower temperatures, it is assured that the glow plugs GLP are activated in a more convenient condition, with a flow rate Flin that is not too high. 
     According to an embodiment of the present invention, the results obtained from the test campaign are then used by the control unit  160  for dynamically adjusting the minimum instantaneous temperature LT and the maximum instantaneous temperature HT based on the driving condition of the vehicle  100 . 
     Reference is now made to the flow chart  740  depicted in  FIG. 7B , illustrating the operations performed by the control unit  160 , together with a diagram  745  shown in  FIG. 7C , in which the abscissa indicates exemplary values of the instantaneous temperature Tfl of the DPF  125  that may be sampled during the operation of the vehicle  100 , and the ordinate indicates the frequency of samples for each instantaneous temperature value. 
     The control unit  160  stores as configuration parameters, for example in the ROM  320 , the low and high temperature thresholds ALT, HLT, to be used as default temperature threshold values in case the temperature threshold values calculated using the samples taken during the actual vehicle run results unsuitable, as will be described in the following (block  750 ). 
     When the engine  105  of the vehicle  100  is started, the control unit  160  starts collecting samples of the instantaneous temperature Tfl (block  755 ). As the control unit  160  collects instantaneous temperature Tfl samples, a temperature sample log—including a number TNS of samples (for example,  200 )—is progressively filled to generate a first distribution of instantaneous temperatures Tfl—indicated in  FIG. 7C  with the reference D(i). As can be observed in  FIG. 7C , the distribution D(i) is similar to the distribution  710  corresponding to the urban condition obtained with the preliminary test campaign ( FIG. 7A ). A reason for this is that, at the beginning of any travel, the vehicle  100  is in a condition similar to that represented by the distribution  710  (at the beginning of the travel, the vehicle  100  starts with the engine  105  that is cold, and probably travels in a urban scenario, or at least in a scenario that does not allow reaching high speeds for long periods). 
     Then, the control unit  160  calculates a tentative minimum instantaneous temperature tLT and a tentative maximum instantaneous temperature tHT, using the percentiles calculated according to the experimental results and stored in the memory  320  as part of the configuration parameters, and applying said percentiles to the distribution D(i). Making reference to the example at issue, the tentative minimum instantaneous temperature tLT is calculated taking the 30th percentile of the distribution D(i), while the tentative maximum instantaneous temperature tHT is calculated taking the 70th percentile of the distribution D(i) (block  760 ). Said tentative values are indicated in the diagram  745  of  FIG. 5C  with the references tLT(i) and tHT(i). 
     At this point, the control unit  160  sets the minimum instantaneous temperature LT to the maximum value between the tentative minimum instantaneous temperature tLT and the low temperature threshold ALT, while the maximum instantaneous temperature HT is set equal to the minimum value between the tentative maximum instantaneous temperature tHT and the high temperature threshold AHT (block  765 ). In this way, it is ensured that a tentative minimum instantaneous temperature tLT lower than the low temperature threshold ALT is discarded, since it corresponds to a temperature Tfl lower than the minimum one sufficient for allowing the PM combustion to propagate through the DPF  125  once triggered by the glow plugs GLP (e.g., 150° C.); similarly, it is ensured that a tentative maximum instantaneous temperature tHT higher than the high temperature threshold AHT is discarded, since it corresponds to a temperature Tfl higher than the maximum one for which the activation of the glow plugs GLP is required for activating the PM combustion (e.g., 350° C.) and beyond which the PM burns for self combustion). 
     Thus, the tentative minimum instantaneous temperature tLT(i) obtained with the exemplary distribution D(i) will be discarded—being lower than the low temperature threshold ALT—, while the tentative maximum instantaneous temperature tHT(i) will be maintained—being lower than the high temperature threshold AHT. 
     Preferably, if both the tentative minimum instantaneous temperature tLT(i) and the tentative maximum instantaneous temperature tHT(i) are lower than the low temperature threshold ALT, the control unit  160  may set the minimum instantaneous temperature LT to the low temperature threshold ALT and the maximum instantaneous temperature HT to the high temperature threshold AHT. Alternatively, in case both the tentative minimum instantaneous temperature tLT(i) and the tentative maximum instantaneous temperature tHT(i) are lower than the low temperature threshold ALT, the control unit  160  may maintain the minimum instantaneous temperature LT and the maximum instantaneous temperature HT that were calculated in a previous step. 
     Then, as the vehicle travel proceeds, the temperature sample log is updated with new Tfl samples (block  770 ), and the process is reiterated (returning to block  560 ). Particularly, the control unit  160  updates the sample log with a number (possibly equal to one) of new Tfl samples, removing therefrom a corresponding number of the oldest Tfl samples, and recalculates the tentative minimum instantaneous temperature tLT and the tentative maximum instantaneous temperature tHT using the same percentiles used before but applying them to the new distribution—indicated in  FIG. 7C  with the reference D(i+1). 
     In the example of  FIG. 7C , the new distribution D(i+1) is wider, for example because the engine  105  is now working at a higher temperature, and the vehicle  100  is traveling at higher speeds. Making still reference to the exemplary distribution D(i+1) of  FIG. 7C , the new tentative minimum instantaneous temperature—indicated in  FIG. 5C  with the reference tLT(i+1) has overtaken the low temperature threshold ALT, while the new tentative maximum instantaneous temperature—tHT(i+1) in  FIG. 7C  is now higher than the high temperature threshold HLT. Thus, the minimum instantaneous temperature LT will be set equal to the value t 1 T(i+1), while the maximum instantaneous temperature HT will be now equal to the high temperature threshold AHT. 
     Preferably, if both the tentative minimum instantaneous temperature tLT(i+1) and the tentative maximum instantaneous temperature tHT(i+1) are higher than the high temperature threshold AHT, the control unit  160  may set the minimum instantaneous temperature LT to the low temperature threshold ALT and the maximum instantaneous temperature HT to the high temperature threshold AHT. Alternatively, in case both the tentative minimum instantaneous temperature tLT(i+1) and the tentative maximum instantaneous temperature tHT(i+1) are higher than the high temperature threshold AHT, the control unit  160  may maintain the minimum instantaneous temperature LT and the maximum instantaneous temperature HT that were calculated in a previous step. 
     All the operations that have been described with reference to  FIGS. 7B and 7C  are performed by the control unit  160  together with the operations described with reference to  FIG. 4A . 
     Thus, according to an embodiment of the present invention, instead of having fixed temperature threshold values for assessing if the instantaneous temperature Tfl of the DFP  125  is at a favorable level for activating the glow plugs GLP, said thresholds are advantageously determined by the control unit  160  dynamically, so as to fit the actual driving condition of the vehicle  100 . 
     According to another embodiment of the present invention, the minimum and maximum instantaneous temperature thresholds LT, HT may be set by the control unit  160  using predetermined relationships—e.g., determined during the test campaign and stored in the memory  320  as part of the configuration parameters—which establish the values of the minimum and maximum instantaneous temperature thresholds LT, HT on the basis of where the statistical distribution of the samples of instantaneous temperature is located with respect to the low temperature threshold ALT and the high temperature threshold AHT. The position of the statistical distribution of the samples of instantaneous temperature may be determined by observing which temperature value corresponds to a first percentile of the statistical distribution, relatively higher than the 50th percentile thereof, e.g., the 70th. According to this embodiment, the values of the minimum and maximum instantaneous temperature thresholds LT, HT are set by the control unit  160  according to a relationship that provides the values for the minimum and maximum instantaneous temperature thresholds LT, HT using said first percentile. For example, this relationship may be implemented through a relationship table stored in the memory  320 , which includes, for each value (or for each range of values) that said first percentile may assume in a predetermined range including the low temperature threshold ALT and the high temperature threshold AHT, a corresponding pair of predetermined values for the minimum and maximum instantaneous temperature thresholds LT, HT. Additionally, instead of having fixed, predetermined minimum and maximum instantaneous temperature thresholds LT, HT values, the table may provide—for some values that said first percentile may assume—further percentiles to be calculated again on the statistical distribution of the samples of instantaneous temperature for obtaining the minimum and/or the maximum instantaneous temperature thresholds LT, HT. A possible example of said relationship table, wherein the first percentile is the 70th of the statistical distribution, the low temperature threshold ALT is equal to 150° C. and the high temperature threshold AHT is equal to 350° C., is illustrated in the following: 
                                     1st percentile (70th)   LT   HT                   140° C.   150° C.   99th       160° C.   70th   95th       240° C.   40th   80th       260° C.   30th   70th       340° C.   20th   60th       360° C.   10th   350° C.                    
According to this example, if the 70th percentile of the statistical distribution of the samples of instantaneous temperature is equal (or close) to 140° C. (that is lower than the low temperature threshold ALT, which is 150° C.), the control unit  160  directly sets the minimum instantaneous temperature threshold LT to the low temperature threshold ALT (150° C.), while sets the maximum instantaneous temperature threshold HT to the 99th percentile of the statistical distribution of the samples of instantaneous temperature; if instead the 70th percentile is equal (or close) to 160° C., the control unit  160  sets the minimum instantaneous temperature threshold LT to the 70th percentile of the statistical distribution, while sets the maximum instantaneous temperature threshold HT to the 95th percentile of the statistical distribution, and so on.
 
     According to a still further embodiment of the present invention, the control unit  160  may be capable of inferring, from the statistical distribution of the samples of instantaneous temperature, the vehicle driving condition, and accordingly set the minimum and maximum instantaneous temperature thresholds LT, HT. Exploiting the percentiles calculated as described above, according to the experimental results, and stored in the memory  320  as part of the configuration parameters, the control unit  160  may establish whether the actual statistical distribution of the samples of instantaneous temperature it has collected substantially corresponds to the distribution  710  or to the distribution  720 . For example, in order to establish whether the actual statistical distribution substantially corresponds to the urban distribution  710 , the control unit  160  may calculate the tentative minimum instantaneous temperature tLT, using the 30th percentile stored as configuration parameter: if the calculate tentative minimum instantaneous temperature tLT is significantly lower than the low temperature threshold ALT (e.g., 150 ° C.), then the control unit  160  may decide that the vehicle is being driven in a urban environment, for which the statistical distribution  710  applies; the control unit  160  may in this case take a high percentile, e.g. higher than the 70th, and preferably the 95th of the actual statistical distribution, and calculate the tentative minimum instantaneous temperature tLT. If the calculated tentative minimum instantaneous temperature tLT is lower than the low temperature threshold ALT, the control unit  160  sets the minimum instantaneous temperature tLT equal to the low temperature threshold ALT. If the calculated tentative minimum instantaneous temperature tLT falls within a temperature range defined by the low temperature threshold ALT and a minimum instantaneous temperature upper limit (e.g., 200° C.), the minimum instantaneous temperature tLT is set equal to the calculated tentative minimum instantaneous temperature tLT. If instead the calculated tentative minimum instantaneous temperature tLT is higher than the minimum instantaneous temperature upper limit (this may occur in case the actual statistical distribution is centered around a low temperature, such as 120°-150° C., but has an extended upper tail that corresponds to relatively high temperature values), the minimum instantaneous temperature tLT is set equal to said minimum instantaneous temperature upper limit. 
     Similarly, the control unit  160  may calculate the tentative maximum instantaneous temperature tHT, using the 70th percentile stored as configuration parameter: if the calculate tentative minimum instantaneous temperature tHT is significantly higher than the high temperature threshold AHT (e.g., 350 ° C.), then the control unit  160  may decide that the vehicle is being driven in a motorway environment, for which the statistical distribution  520  applies. The control unit  160  may in this case take a low percentile, e.g. lower than the 45th, and preferably the 30th of the actual statistical distribution, and calculate the tentative maximum instantaneous temperature tHT. If the calculated tentative maximum instantaneous temperature tHT is higher than the high temperature threshold AHT, the control unit  160  sets the maximum instantaneous temperature tHT equal to the high temperature threshold AHT. If the calculated tentative maximum instantaneous temperature tHT falls within a temperature range defined by the high temperature threshold AHT and a maximum instantaneous temperature lower limit (e.g., 250° C.), the maximum instantaneous temperature tHT is set equal to the calculated tentative maximum instantaneous temperature tHT. If instead the calculated tentative maximum instantaneous temperature tHT is lower than the maximum instantaneous temperature lower limit (this may occur in case the actual statistical distribution is centered around a high temperature, such as 300°-350° C., but has an extended lower tail that corresponds to relatively low temperature values), the maximum instantaneous temperature tHT is set equal to said maximum instantaneous temperature lower limit. 
     In conclusion, the filter regeneration system according to the embodiments of the invention herein described allows to substantially improve the efficiency of the regeneration process of a DPF for a vehicle, both in terms of power consumption and reliability. Particularly, it has been found that, with the proposed solution, the heating device is activated only when the conditions for a successful filter regeneration are favorable. In this way, the occurrence of unnecessary activations of the heating device are reduced, so as to avoid excessive waste of electrical power. 
     Another advantage provided by the proposed solutions is that the filter regeneration system herein described needs very few additional hardware resources to be installed on the vehicle, being necessary only the addition of a filter regeneration apparatus including the temperature sensor after the DPF, an input lid structure provided with glow plugs directly contacting the DPF input base, and an electronic control unit (the pressure sensor being normally already mounted in all the vehicles including a particulate filter). 
     In view of the above, the proposed filter regeneration system may be advantageously installed in whichever vehicle type, without having to drastically modify the fabrication process of the vehicle itself. 
     Moreover, the condition on the average pressure PRm of the block  445  can be modified so as its value has to fall in a range defined by the minimum average pressure LPRm and a maximum average pressure HPRm (higher threshold), wherein said maximum average pressure HPRm may be set, for example, to the minimum alarm average pressure LAPRm. In this way, if the average pressure PRm exceeds said maximum average pressure HPRm, it means that the DPF  125  includes too much PM, and an emergency regeneration needs to be performed. 
     Even if in the present description reference has been made to a system for regenerating particulate filters that filter the PM emissions generated by a diesel engine, the concepts of the present invention can be applied to other types of engines, like the gasoline ones. 
     Naturally, in order to satisfy local and specific requirements, a person skilled in the art may apply to the solution described above many modifications and alterations. Particularly, although the present invention has been described with a certain degree of particularity with reference to preferred embodiment(s) thereof, it should be understood that various omissions, substitutions and changes in the form and details as well as other embodiments are possible; moreover, it is expressly intended that specific elements and/or method steps described in connection with any disclosed embodiment of the invention may be incorporated in any other embodiment as a general matter of design choice. 
     For example, the control unit may be adapted to vary, when in operation, the value of one or both of the percentiles used for setting the minimum and maximum instantaneous temperatures, so as to even better fit the expected driving condition of the vehicle  100 .