Method and device for determining the load condition of particle filters

A method and a device for determining the load condition of a particle filter (10) used in the exhaust gas system (11) of a diesel engine (12) employed in particular in a motor vehicle, wherein PA1 a pressure value (.DELTA.P.sub.filter, P.sub.abs.pre-filter or P.sub.rel.pre-filter) and a temperature value (t.sub.m,filter of the exhaust gas volume flow in the particle filter (12) are measured; PA1 the engine speed (n) proportional to the volume flow is measured; PA1 an actual characterizing value is calculated considering these measurement values; and PA1 a comparison between actual characteristic value (IK) and limit characteristic value (GK) is performed for initiating a regeneration process when the difference (DI) is sufficiently small.

FIELD OF THE INVENTION 
The invention relates to a method of determining the load condition of a 
particle filter used in the exhaust gas system of a diesel engine employed 
in particular in a motor vehicle, and to a device for performing such 
method. 
BACKGROUND OF THE INVENTION 
The particle filters which are used increasingly for cleaning the exhaust 
gas in diesel engines and which, apart from filtering out other harmful 
gas constituents of the engine exhaust gas, serve in particular for 
filtering out soot particles contained in the exhaust gas, must be 
subjected to frequent cleaning (regeneration) for retaining their 
operability. In this respect especially the thermal regeneration of 
particle filters has turned out to be effective, in which the soot 
particles contained in the particle filter are ignited and burnt by the 
introduction of high-temperature heating gases (of approx. 600.degree. C. 
to 900.degree. C.). 
For carrying out a thermal regeneration of particle filters, there are in 
essence three methods known, namely stationary regeneration, alternating 
regeneration and full flow regeneration. In case of a stationary 
regeneration, burning out of the particle filter takes place, while the 
vehicle engine is at a standstill by means of a heating means provided for 
this purpose and being independent of the engine. Alternating regeneration 
renders possible a thermal regeneration of the particle filter while the 
vehicle is in operation. To this end, two particle filters are connected 
in parallel, and in alternating manner one particle filter has engine gas 
flowing therethrough while a heating gas heated by the engine-independent 
heating means flows through the other particle filter, which is 
disconnected from the exhaust gas system, during the time of thermal 
regeneration. With full flow regeneration, in which the regeneration also 
takes place during operation of the vehicle, the particle filter is 
disposed permanently in the exhaust gas flow which is subjected during the 
time of regeneration to a heating gas flow which is produced by the 
engine-independent heating means and mixed with the engine exhaust gas and 
together with the latter is introduced into the particle filter for 
obtaining the afore-mentioned gas temperature necessary for regeneration. 
Irrespective of the choice of the regeneration method performed, a thermal 
regeneration of course has to be carried out only when a certain load 
condition of the particle filter has been reached, in which either the 
filter has lost its effectiveness or in which the exhaust gas back 
pressure produced by the clogged filter has a disadvantageous effect on 
the engine power, or in which the filter, by further loading thereof, 
would be thermally destroyed during the next regeneration due to the heat 
set free in the combustion of soot. 
As practical possibilities of continuously monitoring the load condition of 
a particle filter during operation are not yet known so far, certain fixed 
operational intervals are set by the manufacturers of particle filters, 
defining when a thermal regeneration is to be carried out. As the actual 
load condition is not known, the regeneration intervals are selected such 
that also under extreme operating conditions of the engine, such as 
frequent short-distance operation with extreme soot formation in the 
engine exhaust gases, there is provided sufficient security in the 
intervals for guaranteeing that the regeneration definitely can take place 
in due time before occurrence of the afore-described harmful effects. The 
regeneration of a particle filter in the case of operation of the engine 
with low soot formation thus necessarily takes place at a time at which 
such regeneration actually would not be necessary. 
One has started to adapt the regeneration intervals in consideration of the 
predominantly prevailing operating conditions, such as e.g. short-distance 
or long-distance operation, the particular conditions of use of a particle 
filter, but even this refined pattern of the regeneration intervals turns 
out to be too coarse in practical application. 
The generally known relationship between the pressure drop in a flow medium 
when flowing through a filter or the pressure increase in front of the 
filter, respectively, and the degree of clogging or loading of the filter, 
which as such holds only for filters having a constant flow therethrough, 
cannot be used alone for determining the load condition of a particle 
filter because of the predominantly varying operation of a combustion 
engine. Rather, for example the volume flow through the filter must be 
taken into consideration, which changes depending on the engine speed. 
SUMMARY AND OBJECTS OF THE INVENTION 
It is the object of the invention to provide a method and a device 
rendering possible a simple determination of the actually existing load 
condition of a particle filter in consideration of the particular engine 
operation conditions. 
According to the invention, a method is provided for determining the load 
condition of a particle filter used in an exhaust gas system of a diesel 
engine, particularly a diesel engine employed in a motor vehicle. The 
method includes measuring at least one thermodynamic quantity of the 
exhaust gas volume flowing through the particle filter, the quantity being 
specific for the filter and measuring a quantity that is specific for the 
engine and proportional to the volume flowing through the engine. A 
characteristic value is calculated based on each of the two measurement 
values. A limit characteristic value is determined and an actual 
characteristic value is determined. The actual characteristic value is 
compared to the limit characteristic value in order to initiate a 
regeneration process when the difference between the actual characteristic 
value and the limit characteristic value is sufficiently small. 
In the method according to the invention, the measurement of at least one 
thermodynamic quantity that is specific for the filter and defines the 
condition of the exhaust gas volume flow, and its association with a 
measured quantity that is specific for the engine and proportional to the 
volume flow, leads to the creation of a simple relationship between 
measurement values, which allows the determination of an actual 
characteristic value defining the load condition of the particle filter. 
By comparison with an empirically determined limit characteristic value, 
the deviation of the actual characteristic value from the limit 
characteristic value can be ascertained as a difference so that a 
regeneration process can be initiated, when the difference, which by 
definition may be a quantity between zero and an arbitrary value, is 
sufficiently small. 
The limit value may be predetermined e.g. as a limit characteristic line 
ascertained during testing stand tests in consideration of various load 
conditions and volume flows. As a rule, the admissible limit value is 
defined depending on the extent of the admitted torque drop arising as a 
consequence of the exhaust gas back pressure caused by the particle filter 
in the exhaust gas system. 
It turns out to be particularly reliable to ascertain the actual 
characteristic value when two quantities specific for the filter, namely a 
pressure value and a temperature value, as well as the engine speed as the 
value that is specific for the engine are measured. 
It is advantageous to use the pressure upstream of the particle filter, 
i.e. the pre-filter pressure, or the pressure drop across the particle 
filter, i.e. the differential pressure measured at the particle filter, as 
the pressure value of the exhaust gas volume flow and to use the average 
volume flow temperature in the particle filter as the temperature value of 
the exhaust gas volume flow. 
The invention further provides for the determination and processing of 
additional measurement values. This allows, in a particularly advantageous 
manner, the determination of the load condition of a particle filter 
disposed in a diesel engine equipped with an exhaust gas charging system. 
In addition to the specific filter quantities, namely the pressure in 
front of the particle filter, the pressure drop across the particle filter 
and the temperature of the exhaust gas volume flow in the particle filter, 
specific quantities of the charging volume flow, namely the temperature 
and the pressure of the charging volume flow, are taken into consideration 
as well in determining the actual characteristic value of the load 
condition. 
When a sufficiently small difference between an actual characteristic value 
and a corresponding limit characteristic value is ascertained, preferably 
a display means is activated which, when the method according to the 
invention is used e.g. in a motor vehicle, informs the vehicle driver of 
the critical load condition of the particle filter. The driver then may 
initiate a regeneration of the particle filter according to one of the 
regeneration methods described hereinbefore. 
A further possibility consists in coupling the display means with a start 
means for automatically initiating one of the afore-described regeneration 
methods, or in triggering the start means directly upon determination of a 
critical difference value, without prior display of the value. 
When a display means is provided, it turns out to be particularly 
advantageous to display, either continuously or in predetermined time 
intervals, the actual characteristic value ascertained in the method 
according to the invention, starting from an initial characteristic value 
having a predetermined difference from the limit characteristic value, in 
order to thereby make the progression of the load condition visible from 
the outside, so that the necessity of performing a particle filter 
regeneration is recognizable in advance. 
The device according to the invention for determining the load condition of 
a particle filter used in the exhaust gas system of a diesel engine 
employed in particular in a motor vehicle includes a differential pressure 
measuring means disposed in the region of the particle filter for sensing 
a pressure drop in the exhaust gas volume flow across the particle filter 
(a filter differential pressure) or a pressure measuring means disposed in 
front of the particle filter in measuring the pre-filter pressure. A 
temperature measuring means is provided for measuring the temperature of 
the exhaust gas volume flow in the particle filter. An engine quantity 
measuring means is provided for sensing a quantity that is specific to the 
engine and proportional to the volume flow. This quantity specific to the 
engine is preferably the engine speed. Computer means are provided for 
forming an actual characteristic value based on the measurement value 
sensed, by the differential pressure measuring means or the pressure 
measuring means, as well as the temperature measuring means and the engine 
quantity measuring means, as well as for comparing the actual 
characteristic value to a predetermined limit characteristic value. The 
device according to the invention can additionally comprise a display 
means which serves to display the reaching of the limit characteristic 
value and/or the difference between actual characteristic value and limit 
characteristic value as ascertained by the comparison in said computer 
means. 
When the display means is designed such that it indicates merely that the 
limit characteristic value has been reached, this constitutes the signal 
for the vehicle driver to perform a regeneration process of the particle 
filter. When the difference between actual and limit characteristic values 
is indicated, the vehicle driver has the possibility of obtaining 
information continuously on the progression of the load condition and, to 
a certain extent, of determining himself the moment for performing a 
regeneration. 
A modified embodiment of the device according to the invention can be 
provided, which permits a particularly advantageous application in 
determining the load condition of a particle filter disposed in the 
exhaust gas system of a charged diesel engine. In addition to the 
measuring means for measuring quantities specific for the filter, namely a 
differential pressure measuring means, a pressure measuring means for 
measuring means for measuring the pre-filter pressure and a temperature 
measuring means as well as a measuring means for ascertaining a quantity 
that is proportional to the volume flow, and specific for the engine, this 
modified embodiment is provided also with measuring means for measuring 
specific quantities of the charging volume flow, namely a temperature 
measuring means sensing the temperature of the charging volume flow, and a 
pressure measuring means sensing the pressure of the charging volume flow. 
Both with the device according to the invention not involving a charged 
engine, which is particularly suited for use with an aspirating diesel 
engine, and with the modified device according to the invention, which is 
particularly suited for use with a charged diesel engine, it is possible 
to provide in addition to the display means, or instead of the same, a 
starting means which can be actuated depending on the difference between 
actual and limit characteristic values and effects starting of a 
regeneration means, in particular a burner means for thermal regeneration. 
The method as well as the device according to the invention for determining 
the load condition of a particle filter will be elucidated in more detail 
hereinafter with reference to the drawings. 
The various features of novelty which characterize the invention are 
pointed out with particularity in the claims annexed to and forming a part 
of this disclosure. For a better understanding of the invention, its 
operating advantages and specific objects attained by its uses, reference 
is made to the accompanying drawings and descriptive matter in which 
preferred embodiments of the invention are illustrated.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows a particle filter 10 disposed in an exhaust gas system 11 of a 
diesel engine 12 operated as an aspirating engine. The particle filter 10 
is connected on its entrance side via flange connections 13, 14 to a 
pre-chamber 15 and on its discharge side to a reducer 16. The pre-chamber 
15 is connected to the diesel engine 12 via an exhaust pipe 17. The 
reducer 16 merges on its downstream side with an exhaust pipe 18 through 
which the engine exhaust gases are discharged towards the free end of the 
exhaust gas system 11. All air flow 22 supplied to the diesel engine 12 
through an intake line 21 is burnt in the system, schematically shown in 
FIG. 1 as the diesel engine 12, while fuel is supplied to the latter. The 
exhaust gases produced are discharged through the exhaust pipe 17, the 
pre-chamber 15, the particle filter 10, the reducer 16 and the following 
exhaust pipe 18 as well as through further exhaust gas system components, 
not shown, and are discharged into the open air as exhaust gas flow 23 
after the soot components as well as other harmful components have been 
substantially filtered out from the exhaust gas flow. 
For regeneration of the particle filter 10, there is provided a combustion 
chamber 19 which serves to heat a gas flow fed to said chamber through a 
supply means 20. For regeneration, the gas flow introduced through the 
supply means 20 is heated in the combustion chamber 19 for obtaining the 
exhaust gas temperature necessary for regeneration, and is mixed with the 
exhaust gas flow flowing through exhaust pipe 17 into pre-chamber 14. 
Being able to determine the load condition of the installed particle 
filter, various measuring means are provided in the system shown in FIG. 
1. In the region of the particle filter 10 there is provided a 
differential pressure measuring means 24 having two measuring sensors 25, 
26 in the embodiment shown herein. Measuring sensor 25 is provided for 
measuring the pressure in the exhaust gas volume flow prior to entering 
the particle filter, and measuring sensor 26 is provided for measuring the 
pressure in the exhaust gas volume flow after exit thereof from the 
particle filter. The differential pressure measuring means 24 in total 
measures in known manner the differential pressure existing between the 
installation sites of the measuring sensors 25, 26 i.e. the pressure drop 
across the particle filter 10. In addition thereto, the measuring sensor 
25 disposed upstream of the particle filter 10 serves to provide a 
description of the condition of the exhaust gas when entering the filter. 
As a further measuring means in the region of the,particle filter 10, there 
is provided a temperature measuring means 27 having measuring sensors 37, 
38 and adapted to determine the average temperature of the exhaust gas 
volume flow in the particle filter 10. Finally, there is provided a speed 
measuring means or tachometer 28 connected to the diesel engine 12. 
It has been found out that the relationship illustrated in FIG. 2 exists 
between the quotient of the filter differential pressure 
.DELTA.p.sub.filter determined by the differential pressure measuring 
means 24 and the average filter temperature t.sub.m determined by the 
temperature measuring means 27 as well as the speed of the diesel engine 
12 which is proportional to the volume flow and determined via the speed 
measuring means 28. The upper graphical representation of the two 
representations shown in FIG. 2 indicates the limit characteristic value 
for the load condition of the particle filter, which is defined to be 
constant by the gradient of the graph. This limit characteristic value GK 
may be ascertained e.g. in tests performed on a testing stand, the limit 
characteristic value for the load condition of the particle filter being 
fixed e.g. depending on the admissible power loss of the diesel engine as 
a result of clogging of the particle filter with soot particles and the 
like. 
The second, lower graph shows the unloaded condition of the particle 
filter, as compared to the maximum admissible load condition represented 
by the upper graph. 
Due to the relationship found out between the thermodynamic quantities 
.DELTA.p.sub.filter and the engine speed n, as shown in FIG. 2, it is thus 
possible, by comparing an operating point defined by the quantities 
mentioned with the admissible load GK, to find out whether the maximum 
admissible load condition has been reached or how far the actual load 
condition of the particle filter defined by the operating point is away 
from the admissible load condition. 
The relationship with the filter differential pressure .DELTA.p.sub.filter 
as load indication means, as shown in FIG. 2 and further below in FIG. 5, 
holds also when, instead of the filter differential pressure 
.DELTA.p.sub.filter, the absolute pressure or the relative to atmospheric 
pressure in front of the filter are utilized as load indication means. 
An actual characteristic value IK defining the actual load condition, i.e. 
the operating point of the particle filter, is shown in exemplary manner 
in FIG. 2. As shown furthermore in FIG. 2, there is an actual difference 
DI present at this operating point of the particle filter between the 
limit characteristic value GK and the actual characteristic value IK. The 
maximum load condition thus has definitely not yet been reached in this 
point. This means that a regeneration of the particle filter need not be 
started yet. This needs to be done only when the actual characteristic 
value IK is equal to the limit characteristic value GK. As it may also 
turn out to be advantageous in some cases to initiate the regeneration 
process already before the maximum admissible load condition defined by 
the limit characteristic value GK has been reached, or at least, before 
occurrence of the maximum load condition, to obtain information on the 
imminent occurrence of the maximum load condition, it turns out to be 
advantageous, in particular when the currently present load condition is 
shown on a display means, to define a tolerance region (shown in hatched 
manner in FIG. 2) across a nominal difference DS between the limit 
characteristic value GK and the actual characteristic value IK. In case DI 
is equal to DS, regeneration of the particle filter can be initiated or 
the necessity for a soon required regeneration of the particle filter can 
be displayed, respectively. 
FIG. 3 shows by way of a flow diagram a possible manner of performing the 
method of determining the load condition of a particle filter disposed in 
the exhaust gas system of a diesel engine operated as aspirating engine. 
The thermodynamic quantities .DELTA.p.sub.filter and t.sub.m,filter as 
well as the engine speed n proportional to the volume flow, which have 
been ascertained by the measuring means 24, 27, 28 shown in FIG. 1, are 
supplied to a computer means 29. The computer means 29 first performs a 
calculation of the actual characteristic value from the quotient of the 
filter differential pressure .DELTA.p.sub.filter and the product of the 
engine speed n and the average filter temperature t.sub.m,filter. 
Subsequent thereto, the actual difference DI between the calculated actual 
characteristic value GK is established and displayed. 
According to the representation of FIG. 3, after calculation of the actual 
difference DI, an examination is made whether the actual difference DI is 
smaller than a predetermined nominal difference DS. In case this is so, 
the regeneration means is put into operation thereafter, i.e. the 
combustion chamber 19 shown in FIG. 1 is put into operation. 
It is of course also possible to perform the comparison not after 
calculation of the actual difference DI, but to compare the actual 
characteristic value IK directly to the limit characteristic value and to 
initiate the regeneration process upon detection that the characteristic 
values are identical or that the limit characteristic value GK is 
exceeded. 
FIG. 4 shows a particle filter 10 disposed in an exhaust gas system of a 
diesel engine 12 provided with an exhaust gas charger 30. The components 
of the exhaust gas system 11 depicted in FIG. 4 and identical to those of 
FIG. 1 have the same reference numerals as in FIG. 1. As a difference from 
the system shown in FIG. 1, the system of FIG. 4 has the exhaust gas 
charger 30 disposed upstream of the diesel engine 12. The exhaust gas 
charger 30 consists in essence of a compressor 31 disposed in the intake 
line 21 and driven via an exhaust gas turbine 32 coupled to the compressor 
31 and disposed in exhaust pipe 17. An intermediate cooling means 33 may 
optionally be provided in intake line 21 between the compressor 31 of the 
exhaust gas charger 30 and the entrance to the diesel engine 12. 
In addition to the measuring means already provided in the system according 
to FIG. 1, namely the differential pressure measuring means 24, the speed 
measuring means 28 and the temperature measuring means 27, the system 
illustrated in FIG. 4 is provided with further measuring means. In intake 
line 21, immediately upstream of the entrance to the engine, there are 
provided on the one hand a pressure measuring means 34 for measuring the 
pressure in the volume flow at the engine entrance P.sub.ME and a 
temperature measuring means 35 for measuring the temperature t.sub.ME of 
the volume flow 22 entering the diesel engine 12. In a simplified 
embodiment, the temperature measuring means 35 may also be omitted. In 
addition thereto, a pressure measuring means 36 measuring the pre-filter 
pressure P.sub.pre-filter disposed in the entrance region of the exhaust 
gas volume flow into the particle filter 10, in the present embodiment in 
the pre-filter chamber 15. The pressure measuring means 36 of course may 
also be formed by the measuring sensor 25 of the differential pressure 
measuring means 24, so that the pressure measuring means 36 so to speak 
forms part of the differential pressure measuring means 24. 
The diagram shown in FIG. 5 illustrates the established linear relationship 
between the filter differential pressure .DELTA.p.sub.filter and the 
average filter temperature t.sub.m,filter as well as the volume flow 
introduced into the diesel engine. Due to the compression of the volume 
flow introduced into the diesel engine 12, as effected by the exhaust gas 
charger 30, the quantity proportional to the volume flow shown in FIG. 5 
along the abscissa is not solely dependent on the speed (as shown in FIG. 
2), but is also determined by the quotient of the engine entrance pressure 
P.sub.ME and the product of engine entrance temperature t.sub.ME and 
pre-filter pressure P.sub.pre-filter. 
As for the rest the same relationships hold in FIG. 5 as those depicted in 
FIG. 2 with respect to the limit characteristic value GK, the actual 
characteristic value IK, the nominal difference DS and the actual 
difference DI. 
FIG. 6 shows by way of a flow diagram a possible manner of canning out a 
method of determining the load condition of a particle filter disposed in 
the exhaust gas system of a diesel engine provided with an exhaust gas 
charger. The measuring values ascertained by the measuring means shown in 
FIG. 4, namely the engine speed n, the filter differential pressure 
.DELTA.p.sub.filter, the average filter temperature t.sub.m,filter, the 
engine entrance pressure P.sub.ME, the engine entrance temperature 
t.sub.ME and the pre-filter pressure p.sub.pre-filter, are first fed to 
the computer means 29. Thereafter, the calculating operation shown in FIG. 
6 is carried out for calculating the actual characteristic value IK. 
The further sequence shown in FIG. 6 corresponds to that shown in FIG. 3 
after calculation of the actual characteristic value IK, so that reference 
is made to the description of FIG. 3 for further elucidation. 
While specific embodiments of the invention have been shown and described 
in detail to illustrate the application of the principles of the 
invention, it will be understood that the invention may be embodied 
otherwise without departing from such principles.