Method of and an apparatus for controlled regeneration of a diesel soot filter

With a method of controlled regeneration of a diesel soot filter of a diesel engine the diesel soot filter is divided into a plurality of sections which are connected in parallel in the exhaust gas tract and into at least one of which the influx of exhaust gas is blocked in response to the exhaust gas temperatures measured downstream of the diesel soot filter. When the maximum admissible exhaust gas temperature downstream of the diesel soot filter is exceeded this section, consequently, is shut off in part or even completely from the exhaust gas stream so that the mass flow rate through the corresponding filter section is reduced or even cut off, while the mass flow rate through the remaining sections is increased in a way so as to again control the regeneration or even discontinue it.

FIELD OF THE INVENTION 
The invention relates to a method of and an apparatus for controlled 
regeneration of a diesel soot filter of a diesel engine. 
BACKGROUND OF THE INVENTION 
It is known that diesel soot filters having a porous wall permit gaseous 
components of the exhaust gas to pass, while they prevent the passage of 
solid particles which result, above all, from coagulation of carbon 
molecules and heavy hydrocarbons. 
The soot particles deposited in the diesel soot filter and, in the course 
of operation, progressively clogging the diesel soot filter can be 
decomposed by thermal (burning), catalytic, or other processes to 
regenerate the diesel soot filter. 
Thermal regeneration with the engine running, e.g. during operation of a 
vehicle equipped with the same, may cause an uncontrolled rise of the 
reaction speed of the combustion of the particles collected in the diesel 
soot filter under stochastic conditions when the regeneration is released 
automatically. That may lead to an inadmissible temperature increase and 
result in partial or total fusion or destruction of the diesel soot 
filter. 
It has been suggested to measure the temperature of the exhaust gas 
downstream of the diesel soot filter (DE 38 06 219 A1) to avoid 
uncontrolled combustion. If a predetermined maximum admissible temperature 
is exceeded in this case valves upstream and downstream of the diesel soot 
filter are actuated by means of an electronic unit so that all of the 
exhaust gas will be discharged through a bypass circumventing the diesel 
soot filter. Thus the diesel soot filter, temporarily, is disconnected 
entirely from the exhaust gas stream. 
Although operation through the by pass makes up only a small percentage of 
the overall operating time of the engine, muffling equipment is need to 
dampen the annoying noise which is created during the bypass operation. 
Moreover, it is undesirable from the ecological point of view to discharge 
unscrubbed exhaust gases without filtering into the atmosphere even for a 
short time, as occurs in bypass operation. 
It is likewise known (CH 663 253 A) to divide a particle filter in the 
exhaust gas tract of an internal combustion engine, especially one with 
exhaust gas supercharging, into two sections connected in parallel, namely 
one thermally well insulated "high temperature flow zone" and one "low 
temperature flow zone". By means of a control flap, exhaust gas will flow 
through the low temperature flood only at low engine load and through both 
flow zones at high engine load, by corresponding adjustment of the control 
flap. The results hereof, apart from frequent particle combustion, is that 
the exhaust gas supercharger connected downstream of the exhaust gas tract 
always is fed quickly with exhaust gas of sufficient temperature in order 
to prevent the so-called "turbo hole" upon depression of the accelerator 
pedal. However, protection against uncontrolled burn off of deposited 
particles in the flow zones cannot be achieved by the measure according to 
the Swiss patent. 
SUMMARY OF THE INVENTION 
It is the object of the invention to indicate a method and an apparatus to 
regenerate diesel soot filters in diesel engines with which effective 
protection against damage or destruction of the diesel soot filter can be 
achieved in simpler manner than before and under avoidance of the 
inconveniences of bypass operation. 
A method according to claim 1 and an apparatus according to claim 2 serve 
to solve that problem. 
According to the invention, the diesel soot filter is divided into a 
plurality of sections which are connected in parallel in the exhaust gas 
flow and of which at least one is adapted to be shut off totally or in 
part from the exhaust gas stream in response to the particular exhaust gas 
temperature prevailing at the outlet of the respective section. 
While the reaction in the isolated section is reduced or interrupted, the 
mass flow rate through the remaining section or sections not disconnected 
of the diesel soot filter is increased. Hereby the reaction speed of the 
thermal regeneration in these remaining sections is reduced. 
If the maximum admissible temperature should be surpassed also in one of 
the remaining sections not shut down of the diesel soot filter then this 
section, too, will be disconnected from the exhaust gas stream. This is 
continued in accordance with the invention until the mass flow rate 
through at least the last remaining section of the diesel soot filter has 
risen to such a degree that the reaction which was started will be broken 
off or "extinguished". 
In this manner effective protection is provided against damage or 
destruction of the diesel soot filter by uncontrolled combustion of the 
deposited particles, making a bypass and thus the emission of unfiltered 
exhaust gas into the atmosphere during bypass operation dispensable as 
well as expensive muffling equipment. 
Advantageous modifications of the invention are indicated in the subclaims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows a four cylinder diesel engine 1 in the exhaust gas tract 3 of 
which diesel soot filters 2, 4, and 6 are installed, with no bypass being 
provided to bridge the diesel soot filters. These three diesel soot 
filters are installed parallel to each other in the exhaust gas tract 3 
and comprise a common inlet 5 from which individual inlet pipe ends 5a, 
5b, 5c are branched off, each opening into an associated diesel soot 
filter 2, 4, 6, and individual outlet pipe ends 7a, 7b, 7c which each 
start from a diesel soot filter 2, 4, 6 and are joined in a common outlet 
7. 
In the inlets 5a, 5b, 5c, a throttling member 10, 12, 14 each--in other 
words as many throttling members as diesel soot filters 2, 4, 6--are 
provided. Also, thermocouples 9, 11, 13 are provided in the outlets 7a, 
7b, 7c for individually detecting the exhaust gas temperatures of the 
three diesel soot filters. Their output signals are input into an 
electronic unit 100 which controls the throttling members. The throttling 
members 10, 12, 14 here are designed as throttle flaps. 
From time to time, soot particles deposited in the diesel soot filters are 
regenerated by combustion due to a temperature increase during operation 
or by means of exhaust gas throttling. In the case of enforced 
regeneration all the throttling members 10, 12, 14 are actuated 
simultaneously by the electronic unit 100. 
That diesel soot filter 2, 4, or 6 behind which the exhaust gas temperature 
measured exceeds a given temperature is shut off from the exhaust gas 
stream by blocking the corresponding throttling member 10, 12, or 14 for 
protection against uncontrolled burning during regeneration. The closing 
of the respective inlet 5a, 5b, or 5c takes place totally or partly by 
means of the corresponding throttling member 10, 12, or 14 upon an ON/OFF 
signal or an analog actuating signal emitted by the electronic unit 100. 
This unit 100 receives the output signals of the thermocouples 9, 11, 13 
as input signals. 
The reaction speed of the combustion started of the soot accumulated in the 
diesel soot filter at constant exhaust gas temperature is a function of 
the mass flow rate m. The dependence of the reaction speed w of the 
combustion on the mass flow rate m is illustrated qualitatively in the 
form of a diagram in FIG. 2. If the temperature downstream of the diesel 
soot filter 2 loaded with a quantity G of particles (FIG. 1) exceeds the 
maximum admissible exhaust gas temperature this filter 2 is blocked partly 
or even totally by actuation of the corresponding throttle flap 10. And, 
as a consequence, the mass flow is reduced under m.sub.1. Now the main 
exhaust gas flow rate is passed through the diesel soot filters 4 and 6. 
Accordingly, the mass flow rate m through these filters 4 and 6 is 
increased to above m.sub.2. This means that the reaction speed w in all 
filters is reduced, as may be gathered from the diagram of FIG. 2. In the 
event that the exhaust gas temperature surpasses the maximum admissible 
value downstream another filter, e.g. downstream of filter 4, this filter 
is throttled or closed in the same manner as filter 2 previously. This 
leads to a further increase of the mass flow rate m through the filter 6 
in which, therefore, the reaction speed is reduced by such an amount that 
a reaction possibly initiated will be broken off or "extinguished". 
Normal operation of the system is resumed when the exhaust gas temperature 
downstream each diesel soot filter 2, 4, 6 drops below the maximum 
admissible value. 
It is a prerequisite for full protection against uncontrolled combustion of 
the amount of particles deposited in the diesel soot filter that the mass 
flow rate in the last filter causes break-off or "extinction" of the 
reaction already when the diesel engine is running idle. 
Thus the volume and number of diesel soot filters are determined in advance 
for a particular diesel engine. 
FIG. 3 illustrates an arrangement of a six cylinder diesel engine 
comprising four diesel soot filters 16, 18, 20, 22. Here, too, a 
throttling member 24, 26, 28, 30 each in the corresponding inlets, not 
designated here, and a thermocouple 32, 34, 36, 38 each in the 
corresponding outlets, not designated here, are associated with each 
diesel soot filter. 
The individual shutoff of the diesel soot filters from the exhaust gas 
stream is effected stepwise by means of the electronic unit, not shown 
here, in the same manner as with the embodiment according to FIG. 1, with 
at least one diesel soot filter being left in operation in the final 
stage. 
The throttling members may be embodied by throttle flaps or poppet valves. 
FIG. 4 shows a diesel soot filter with two inlets 50, 51 and consisting of 
a monolith. The exhaust gas is introduced through inlet 51 which does not 
incorporate a throttling member into section 52 of the soot filter. 
Exhaust gas introduced through the inlet 50 flows through section 57. This 
inlet 50 is provided with a throttling member 40. Corresponding 
thermocouples T1 and T2 are installed centrally downstream of the outlet 
cross sections of both sections 52 and 57. 
Protection against uncontrolled combustion is warranted by actuation of the 
throttling member at the inlet 50, regardless of which one of the two 
thermocouples T1 or T2 recorded an inadmissible temperature. By virtue of 
the dimensioning of the tubular inlet 51, the section 52 is selected to be 
so small that the resulting mass flow during idle running already is so 
great that the combustion of the deposited soot is delayed or even 
interrupted. 
Other than with the embodiments shown in FIGS. 1 and 3, the one illustrated 
in FIG. 4 (and also the embodiments according to FIGS. 5 and 6) can do 
with but one throttling member 40, whereby the number of movable and, 
therefore, sensitive parts is reduced and the structure simplified. 
Surprisingly, the omission of the throttling member for section 52 does 
not affect the forced regeneration by exhaust gas throttling. 
Regeneration by particle burnoff is forced by the closing of the only 
blocking member 40. Thereby, an increase is obtained of the exhaust gas 
counterpressure and, accordingly, of the temperature upstream of both 
sections of the diesel soot filter. 
Protection against uncontrolled burning usually becomes effective at low to 
medium rotational speeds or at low load. Under these circumstances the 
driver does not notice any deterioration of the driving characteristics 
due to the increased counterpressure caused by the reduced outlet area 
since he can push down further on the accelerator pedal to achieve the 
desired performance. If, on the other hand, the protection should occur in 
the range of maximum rotational speed or performance the protection 
process can be interrupted by a kickdown switch. As is known, protection 
against uncontrolled burning in this operating range is taken over by the 
cooling effect of the exhaust gas stream. By analogy, throttling that has 
commenced can be cancelled if the driver should demand full power. 
FIG. 5 shows a diesel soot filter which includes two sections, namely a 
bigger section 67 and a smaller one 62, and no blocking members in the 
inlets 60, 61. A single blocking member is embodied by a thermostat 64 
which is disposed in the common outlet passage of the two sections 62, 67 
and through which pass the exhaust gas streams through the sections 62, 
67. When the thermostat expands it closes the outlet 63 of the greater 
section 67 at a predetermined exhaust gas temperature. With this 
embodiment, monitoring of the temperatures at the filter outlet and, 
therefore, an electronic control unit become superfluous since the 
thermostat 64 automatically takes over the protection against uncontrolled 
burning of both sections 62, 67 when the given exhaust gas temperature is 
reached. 
With this embodiment the thermostat 64 consists of a closed corrugated tube 
which expands axially under elevated internal pressure. It is provided 
with a suitable filling of a substance (such as sodium) which will 
evaporate when a certain temperature is reached, thereby causing a quick 
rise of the internal pressure. Thereby, the thermostat 64 is extended 
elastically and the outlet 63 becomes blocked. 
A variant of the embodiment according to FIG. 5 is presented in FIG. 6 
where small and big sections 72, 77 of a filter 71 are formed as two 
sections of a monolith 72. Exhaust gas flows through the sections via a 
common inlet 70. As with the embodiment according to FIG. 5, a thermostat 
74 in the form of an axially expandable corrugated tube is arranged in the 
outlet 73 of the great section 77 in such a way that it will automatically 
close the outlet 73 thereof when a predetermined temperature is reached.