Method and apparatus for controlling the regeneration of at least one particulate filter installed in a diesel engine.

0001This invention relates to a method and apparatus for controlling the regeneration of at least one particulate filter installed in a diesel engine of non-supercharged or supercharged type.

0002As is well known, triggering and controlling the combustion of particles which have accumulated within a particulate filter (also known familiarly as a trap) are still the two major problems to be solved in the development of an efficient filtration system for the exhaust gas of a diesel engine. In this respect, the periodical process of oxidising the trapped carbon particles, commonly known by the term regeneration, is necessary in order to maintain the back pressure on the engine exhaust at an acceptable level. In the same manner, regeneration control is necessary to prevent excessive temperature being reached during particulate combustion, with possible destruction of the filter.

0003Regeneration is effected in various ways, but generally by operating thermal energy generators (burners etc.) when it is considered that the clogging of the filter has reached a suitable level between a maximum and a minimum limit.

0004The maximum limit is determined, as stated, by the need to avoid both excessive back pressure on the engine exhaust, with unacceptable deterioration in performance, and too high temperature during regeneration, which could lead to breakage of the filter.

0005More critical is the required minimum amount of particulate which has to be accumulated before regeneration can be initiated. If the quantity of particulate trapped in the filter is small there would certainly be no excess temperature, but in this case combustion would not be complete as the propagation of the flame front, for example in the last part of a ceramic filter, is sustained by the thermal energy released by the combustion of the carbon particles of the particulate.

0006If often happens that regeneration is triggered spontaneously when the quantity of particulate has not yet reached the aforesaid minimum level, so that combustion is not sustained. Thus at the end of regeneration there still remains particulate in the last part of the filter, this particulate then being added to that which collects during the next accumulation period. The initiation of the next regeneration on the basis of an average clogging evaluation could lead to irreversible damage to the filter. This is because the particulate will have accumulated to a larger extent in certain zones of the filter, with consequent greater development of thermal energy deriving from the combustion of the particulate.

0007A further drawback of current regeneration control systems is that the evaluation of the filter clogging is done by instruments the measurements of which could be affected by systematic errors or by errors consequent on particular environmental conditions such as the temperature at which the reading is taken. It is apparent that this situation can lead to the regeneration being initiated either early or late, leading to all the aforementioned consequences.

0008The object of the present invention is to provide a method and apparatus for controlling the regeneration of particulate filters for diesel engines which obviate the aforesaid drawbacks of current control systems.

0009Said object is attained according to the present invention by a method for controlling the regeneration of at least one particulate filter installed in a diesel engine, characterised by comprising the following operations in sequence: - defining an upper clogging limit for said filter, on reaching which the regeneration is initiated; - periodically monitoring the level of clogging of said filter caused by the progressive accumulation of particulate in the filter; - comparing the last calculated clogging level with the previous level; - if the clogging level has undergone a reduction not resulting from intentional regeneration, defining a new clogging limit lower than the previously defined clogging limit.

0010Said object is further attained according to the present invention by an apparatus for controlling the regeneration of at least one particulate filter installed in a diesel engine, characterised by comprising: - regeneration means for said filter; - means for measuring quantities related to the clogging of said filter by said particulate; - means for processing said measured quantities in order to periodically calculate successive clogging levels of said filter consequent on progressive accumulation of said particulate; - means for memorizing an upper clogging threshold for said filter, and arranged to activate said regeneration means each time the calculated clogging level exceeds said threshold; - means for comparing the last clogging level calculated by said processing means with the previously calculated clogging level; - control means which, if said comparison means note a reduction in the clogging level which has not resulted from intentional regeneration, update the value of said threshold contained in said memory means by means of a new clogging limit, the value of which is less than the previously memorized clogging limit.

0011The present invention will be more apparent from the description of a preferred embodiment thereof given hereinafter by way of non-limiting example with reference to the accompanying drawings in which: <ul id="ul0001" list-style="none"><li>Figure 1 is a simplified block diagram of an apparatus constructed in accordance with the present invention and shown in one example of application;</li><li>Figure 2 is a flow diagram representing a sequence of operations effected by the apparatus of Figure 1 under the control of a processing unit of the apparatus;</li><li>Figure 3 shows the relationship between two quantities measured by respective instruments of the apparatus of Figure 1 under different operating conditions; and</li><li>Figure 4 shows two examples of the action of the apparatus of Figure 1 under two different operating conditions.</li></ul>

0012In Figure 1, the reference numeral 10 indicates overall an apparatus for controlling the regeneration of two filters 1 and 2 for the particulate present in the exhaust gas of a diesel engine 3. The filters 1 and 2 are substantially connected in parallel and are interposed between an exhaust gas collection pipe 5 disposed immediately downstream of the engine 3 and a venturi device 6 which is connected to a further pipe 7 carrying the exhaust gas.

0013The pipe 5 comprises two branches 11 and 12 which lead respectively to the filters 1 and 2 and into which there are connected solenoid valves 13 and 14 activated in opposition by an an electrical signal reaching a terminal 15 and which, in the case only of the solenoid valve 13, is negated by an inverting gate 16. each filter 1, 2 (consisting for example of a monolithic ceramic element) is provided with a respective burner (with its control system) 21, 22 of conventional type.

0014Each burner 21, 22 is connected both to a first terminal 23, 24 from which it receives a corresponding activation signal, and to a second terminal 25, 26, to which it transmits a corresponding control signal usable to indicate whether the burner and relative control system are active or not.

0015A first pressure difference sensor 31 is installed across the filters 1 and 2, a second pressure difference sensor 32 is installed between the outlets of the filters and the venturi device 6, and a third pressure difference sensor 33 is installed between the inlets to the filters 1 and 2 (upstream of the solenoid valves 13 and 14) and the venturi device 6.

0016The sensors 31, 32, 33 are enclosed, together with a temperature sensor 34, within a container 35 to keep them all at the same temperature (read by the sensor 34). The sensors 31, 32, 33, 34 generate respective signals which are fed to corresponding terminals 41, 42, 43, 44.

0017The apparatus 10 also comprises a processing and computing unit 50 (conveniently provided with a microprocessor) having a plurality of input and output terminals indicated by the same reference numerals as said terminals, to which they are connected in a manner not shown.

0018The unit 50 comprises a further five outputs connected to respective indicator lamps 51, 52, 53, 54, 55 which are activated respectively in the following case: lamp 51: filter 1 connected lamp 52: filter 2 connected lamp 53: filter 1 undergoing regeneration lamp 54: filter 2 undergoing regeneration lamp 55: fault situation.

0019Figure 3 shows the relationship between the pressure difference Δp2 measured across the venturi device 6 and the pressure difference Δp1 measured across the filter 1 or 2, for three different decreasing clogging levels Ia, Ib, Ic of particulate trapped in the filter 1 or 2. In reality, the relationship is not linear as shown in Figure 3, in that the straight line segments shown in this figure are obtained by a multiple linear regression operation on a large number of measurements. It is in any event reasonable to consider that the clogging level of each filter 1 or 2 can be evaluated with good approximation simply be measuring the said values Δp1 and Δp2.

0020Figure 4 shows the variation in clogging with time for two operating conditions.

0021The first condition can be considered as the norm, in that the clogging increases progressively with time to provide a monotone increasing function which intercepts the maximum regeneration threshold Smax at time tr to initiate intentional regeneration at that moment.

0022The second condition involves spontaneous regeneration at time t1 which, as specified hereinafter, results in the exhaust gas flow being switched to the other filter for a time T (which terminates when spontaneous regeneration or intentional regeneration takes place in said other filter), the regeneration threshold being reduced to the value S1 (less than Smax), and subsequent intentional regeneration at time tr1, with reinstatement of the value Smax as regeneration threshold.

0023The operation of the apparatus 10 is described hereinafter in detail with reference to Figure 2.

0024The operation starts with a block 60 which reads the data provided by the sensors 41, 42, 43, 44, after which comes a comparison block 61 which checks if the temperature T measured by the sensor 34 is greater than a predetermined minimum value Tm.

0025If the result is negative, the procedure returns to the input of block 60 to prevent processing of the data provided by the other sensors, which are not accurate for low temperatures.

0026If the result is positive, the program passes to a comparison block 62, which checks if the absolute value of the subtraction of Δp1 and Δp2 from Δp3 is greater than a small predetermined value indicated by A. If the result is positive, this signifies that the data provided by at least one of the sensors 41, 42, 43 is in error, so that the program passes to a block 63 which indicates a fault in the system and lights the lamp 55. If the result is negative, the program passes to the evaluation of clogging (for example of the filter 1) by block 64.

0027Block 64 and the next block 65 calculate the clogging of the filter 1 at times N-1 and N to provide two values indicated by IN-1 and IN. A block 66 is then reached to check if the last calculated clogging value (IN) is greater than the preceding (IN-­1).

0028If the result is positive, this signifies that there has been a further accumulation of particles between the two calculation times, and it is therefore necessary to check whether the maximum clogging value (Smax) has been exceeded, and thus initiate regeneration, or to again execute the clogging calculation as described heretofore. This is done by passing from block 66 to block 67, in which N is put equal to N+1 and setting a counter K to 0, to then pass to a block 68 for comparing IN with Imax, and then to a block 69 for initiating regeneration of the filter 1 (by activating the burner 21 and lighting the indicator lamp 53), or again to the input of block 64.

0029If the result is negative, this signifies that partial spontaneous regeneration of the filter 1 has occurred, and the program passes to a block 70 which sets the counter content K to K+1. Depending on the effective value assumed by the parameter K (variable for example from 1 to n), a plurality of comparison blocks 71, 72, ....79 are traversed to reach a respective command block 81, 82,....89, which progressively lowers the value of the regeneration limit threshold to S1,S2,....Sn respectively. If the value of K is n+1, the progress passes directly to a block 90 for initiating regeneration of the filter 1 in the same manner as described with reference to said block 69, to then pass to a block 90a which reinstates Smax as the regeneration threshold value.

0030The program passes from the blocks 81,.....89 and from the block 90a to a block 91, which zeroes the value relative to the clogging of the filter 1. The block 91 is also reached from the block 69 after passing through a block 69a which reinstates Smax as the regeneration threshold.

0031The program passes from block 91 to a block 92, which causes a signal to be fed to the terminal 15 of the unit 50 to switch over the solenoid valves 13, 14 and consequently deviate the exhaust gas from the engine 3 to the filter 2.

0032At this point, a comparison block 93 checks whether the filter 2 is connected and remains in a waiting state passing through a timer block 94 to execute the same check periodically.

0033If the result of the check is negative, the program returns to the input of the computing block 64 to repeat the aforesaid cycle. In a manner not illustrated but parallel to the aforesaid, the unit 50 also checks constantly whether the time period which has passed from the last regeneration command lies between a minimum value and a maximum value, and if it does not the unit provides a fault signal for example to light the indicator lamp 55.

0034The advantages of the method and apparatus of the present invention are apparent from an examination of their characteristics.

0035Firstly, the progressive lowering of the intervention threshold eliminates the problems caused by spontaneous regeneration. Arranging the sensors in the container 35 and enabling the data provided by them to be used only when the temperature reaches an optimum value means that the sensors can be used over that part of their characteristic curve of greatest accuracy, with undoubted advantages from the point of view of most favourable overall management of regeneration. In addition the presence of the third sensor 33 enables any casual or systematic errors in the measurement of the various pressure differences to be identified, so further improving reliability.

0036Connecting the filters 1 and 2 in parallel enables the same sensors and the same venturi device to be used for both, so significantly limiting cost.

0037Finally, checking the time period which has passed since the last regeneration enables operating faults which may have escaped the previous checks to be detected. For example, a too frequent request for regeneration could not be realistic as it is not possible for the particulate to accumulate in the filter in such quantity in a short time. It could instead be the case that the filter is really clogged but the apparatus is unable to detect this because the clogging calculation can give results which are lower than the true results if the engine 3 operates at low r.p.m., the measurement across the venturi device 6 then possibly being in considerable error.

0038Finally, it is apparent that modifications can be made to the aforesaid method and apparatus but without leaving the scope of the present invention.

0039For example the engine could be configured with a single particulate filter provided with a by-pass, or could comprise several filters connected in parallel, provided the aforesaid concept is respected.