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BACKGROUND OF THE INVENTION
The invention relates to an apparatus for fixing the composition of the gas
content of internal combustion engine cylinders. A system is known in
which the air ratio .lambda. pertaining to the aspirated fresh air and
optimal for normal conditions is set in accordance with load and rpm by
means of the open or closed loop controlled adjustment of an exhaust
feedback valve and is governed with the aid of air quantity measurement
(German Offenlegungsschrift No. 24 09 774). However, in this apparatus, a
quantity of exhaust gas, not clearly definable, mixes in with the
aspirated fresh-air flow, the extent depending on pressure and temperature
of the fresh air and the exhaust and also on the state of the engine and,
if it is present, the supercharger. Large deviations from the normal state
result, particularly at extremes of ambient temperature and because of
differing ambient pressure relating to elevation above sea level. It is
also known that the creation of toxic substances during combustion in the
cylinder is substantially determined by the flame velocity and the peak
combustion temperature. These variables are dependent, in turn, on the
total cylinder content, among other factors. In the known system, these
variables cannot be sufficiently attended to and/or controlled.
OBJECT AND SUMMARY OF THE INVENTION
Accordingly, it is a principle object of the invention to be able to set
optimum exhaust feedback rates, with respect to toxic substances, within
broad limits, despite continuously changing environmental conditions. The
toxic substances may be determined, for example, by the performance of a
test prescribed by law.
Various embodiments of the invention as disclosed and claimed herein
provide particularly simple methods of determining the toal cylinder
content. In addition, points of intervention are given in the control
system of a Diesel engine in which the desired ration of fresh air to
exhaust gas can be set in a particularly simple manner.
The invention will be better understood as well as further objects and
advantages thereof become more apparent from the ensuing detailed
description of the preferred embodiments taken in conjunction with the
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic representation of a Diesel engine with its
measurement locations and possible points of intervention in accordance
with the invention;
FIG. 2 is a block diagram of the control device of the invention for the
fuel, fresh-air, and exhaust gas metering in an internal combustion
engine; and
FIG. 3 is a block diagram of a first embodiment for detecting the total
cylinder content;
FIG. 4 is a block diagram of a second embodiment for detecting the total
cylinder content;
FIG. 5 is a block diagram of a third embodiment for detecting the total
cylinder content;
FIG. 6 is a block diagram of a fourth embodiment for detecting the total
cylinder content;
FIG. 7 is a block diagram of a fifth embodiment for detecting the total
cylinder content; and
FIG. 8 is a block diagram of a sixth embodiment for detecting the total
cylinder content.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Turning now to FIG. 1, an internal combustion engine with autoignition is
shown schematically, together with the essential circuitry. The engine
itself is identified by the reference numeral 10, and associated with it
are an air intake manifold 11, an exhaust line 12, and an injection pump
13 for supplying fuel to the various injection valves (not shown). Located
in the air intake manifold 11, is an exhaust feedback control element 14
which comprises a valve (similar to an engine valve) and which can be
continuously adjusted by a preferably electromagnetic control member.
Thus, to a variable extent, the control element 14 opens up a cross
section from an exhaust feedback line 15 to the air intake manifold 11.
If the pressure ratios dictated by engine conditions do not permit
sufficient exhaust gas to be fed back only by means of opening the valve
14, then a throttle valve located upstream in the air intake manifold 11
can be articulated onto the valve 14 which simultaneously throttles the
fresh-air flow while the valve 14 opens. As a result, it is possible to
feed back the desired quantity of exhaust gas.
Upstream of the exhaust feedback control element 14, there is an air flow
rate meter 17 and upstream from it, an air filter 18. Downstream of the
exhaust feedback control element 14, following a supercharger 77, which
may be present, there is at least one transducer 16 for the degree of
total cylinder filling.
A control device 20 is coupled on the input side with a driving pedal 21,
which furnishes a desired fuel signal QK. The control device 20 is also
coupled on the input side with the air flow rate meter 17, a fuel quantity
meter 22 and, via inputs 23 and 24, with transducers for rpm, temperature,
and so forth. In addition, the control device 20 receives the measurement
results from the cylinder content meter or transducer 16. On the output
side, the control device 20 is connected with a fuel pump 25, and also
with the exhaust feedback control element 14.
During combustion in the cylinders of an internal combustion engine, the
higher the combustion temperature is and the more oxygen is present in the
cylinder, the more toxic NO.sub.x components are created. A reduction of
the oxygen in the partial-load range only by means of throttling the fresh
air, however, would result in a higher peak combustion temperature because
of the reduced filling volume in the cylinders, because at the same level
of energy conversion, the heat capacity of the cylinder contents would
drop. The portion of the fresh air withheld from the cylinder by
throttling must therefore be replaced by exhaust gas which is
substantially poorer in oxygen, in order to maintain the heat capacity of
the cylinder contents to the greatest possible extent. This enables a
reduction to the minimum required value of the oxygen component with
respect to hydrocarbons and soot, without the temperature of combustion
rising to a critical extent. Only in this manner can a considerable
reduction in NO.sub.x be attained with only a small increase in the
emission of hydrocarbons and smoke.
In the event of lower initial air densities at high intake temperatures
and/or low air pressure at higher elevations, the large cylinder filling
volume which is also desired in order to improve emissions in the
partial-load range (exhaust-feedback range) both in the suction engine and
in an engine having a supercharger, and which is present under normal
conditions, is no longer attained. Although the air ration pertaining to
the fresh air can be held constant up to a certain limit by means of a
reduced exhaust feedback, higher peak combustion termperatures are
attained, at the same level of energy conversion, as a result of the
reduced total content of the cylinders. These higher peak temperatures
cause a consequent increase in the toxic NO.sub.x component of the
exhaust, and a drop in the hydrocarbon component.
The original distribution of toxic substances in the exhaust can be
approximately attained again by varying the air ratio .lambda. pertaining
to the fresh air in accordance with the total cylinder filling detected by
measurement techniques.
Another possible means of maintaining the original charge even with a
reduced density in aspirated air, is in controlling a supercharger which
then is necessarily oversized for normal conditions in such a manner that
the total cylinder charge remains constant. In order to do so, the total
cylinder filling must again be detected by measurement techniques, at
least indirectly.
The status prevailing in the intake manifold before the inlet valves, for
instance, permits a conclusion as to the level of the total cylinder
filling. To this end, a measurement location indicated by the transducer
16 is provided as shown in FIG. 1, at which a conventional, absolute
pressure meter or transducer detects the pressure and a temperature meter
similarly detects the temperature of the aspirated gas mixture, so that
its density and, using the known stroke volume, the total cylinder filling
can be ascertained.
In accordance with this indirect signal for the total cylinder filling, an
appropriately altered set-point value for air quantity is set by means of
the control device 20 via the exhaust feedback control element 14, in
order to make the ratio of exhaust gas to fresh air dependent on the
particular total cylinder filling at the time. This adaptation of the
mixture ratio of fresh air to exhaust gas enables optimal operation of the
engine with respect to the toxic substances in the exhaust, and a
predetermined upper limit of NO.sub.x emissions can be maintained.
One possible embodiment of the control device 20 is shown in the block
circuit diagram of FIG. 2. Elements in FIG. 2 corresponding to those of
FIG. 1 are given the same reference numerals.
The driving pedal 21 acts upon a driving pedal position transducer 30, at
whose output 31 there is a desired-fuel signal QK.sub.w. This desired-fuel
signal QK.sub.w is delivered to a first input 32 of a minimum-value
selection stage 33 having a further input 34, whose output 35 in turn is
coupled with a quantity-characterizing stage 36. This stage 36 associates
a particular signal value from the output of the minimum-value selection
stage 33 with a particular fuel quantity value, which is subsequently fed
via a comparison point 37 to a quantity controller 38 with a final control
element 39. The output of this final control element 39 is linked in turn
with the comparison point 37 for the purpose of feedback of the direct or
indirect quantity signal.
While the units or stages numbered 33, 36, 38 and 39 are part of the "fuel
side" of the controller, further stages represent the corresponding "air
side." The output 31 of the driving pedal position transducer 30 is linked
with a multi-dimensional characterizing stage 40, which has three inputs
41, 42 and 43 and one output 44. This multi-dimensional performance graph
is basically a computer circuit, in which one output variable is computed
in the manner of the invention on the basis of more than two input
variables. Performance graphs which have more than two input variables are
virtually impossible to obtain in practice in their complete form.
Accordingly, the attempt is made to realize the multi-dimensional
performance graph 40 by means of a combination of performance graphs each
having at the most two input variables (one exemplary embodiment is shown
in FIG. 7), or to represent it by means of algebraic operations. An rpm
signal from the input 23 of the governor is present at the input 41. The
desired-fuel signal from the driving pedal position transducer 30 is
present at the input 42 and the output signal of a detection stage 45 for
the total cylinder filling is present at the input 43. The input values
for the detection stage 45 are a pressure signal and a temperature signal
from the measurement transducer 16 in the air intake manifold 11 of the
internal combustion engine 10.
At the output 44 of the characterizing stage 40, a set-point value for air
quantity QLs appears, which is fed further to a comparison point 47. The
next element is an air quantity controller 48 with a subsequent
position-control characterizing stage 49 for the position of the exhaust
feedback control element 14 of the subject of FIG. 1.
Following this characterizing stage 49, in turn, is a complete closed-loop
control circuit for the setting of this control element 14 having a
controller 50 and the position feedback device 51. The position of the
exhaust feedback control element 14 determines the fresh-air flow in the
intake manifold 11, which is detected by means of the air flow rate meter
17 and is the input value both for a limitation characterizing stage 53
and for the comparison point 47. An rpm signal is fed to the limitation
characerizing stage 53 via a further input 54. On the output side, this
limitation characterizing stage 53 is connected with the input 34 of the
minimum-value selection stage 33.
The mode of operation of the arrangement illustrated in FIG. 2 is as
follows:
Depressing the driving pedal 21 on the part of the operator of the vehicle
equipped with this engine effects a corresponding desired-quantity signal
for fuel at the output 31 of the driving pedal position transducer 30.
This is because in autoignition engines, the power output is primarily
determined by means of the quantity of fuel injected. This desired
quantity is passed on via the minimum-value selection stage 33, which
serves to set a maximum quantity limit, to the quantity characterizing
stage 36 and the subsequent quantity controller furnishes the desired
quantity. Parallel to this, a particular desired fuel quantity at a
desired .lambda. ratio produces an appropriate set-point signal for the
fresh-air quantity via the characterizing stage 40. This air quantity is
dependent on the rpm, among other factors, which affects the output signal
of the characterizing stage 40 via the input 41.
The selected set-point value for fresh air is compared with the measurement
signal for the fresh-air quantity from the air flow rate meter 17. The
control deviation is processed in the controller 48 and converted via the
position-control characterizing stage 49 into a corresponding set-point
value for the exhaust-feedback control element 14, whose position is
controlled via the controller 50 and the feedback device 51. On the basis
of this actual value for fresh air, the highest permissible fuel quantity
is also signalled to the minimum-value selection stage 33. Thus, in the
arrangement of FIG. 2, a required fresh-air quantity is determined in
principle on the basis of a desired fuel quantity via a predetermined
.lambda. value, and this fresh-air quantity is set via the exhaust
feedback control element 14 acting as a mixing device. The rest of the
aspirated volume then comprises the exhaust gas, which because of the
given pressure conditions flows into the intake manifold 11 of the
internal combustion engine 10 via the exhaust feedback line 15 opened up
by the exhaust feedback control element 14.
The output signals of the detection stage 45 for the total cylinder filling
also affect the characterizing stage 40 in the sense that all the
variables which can be detected by measurement techniques and which
determine the combustion behavior in the cylinders have an effect on the
set-point value for air quantity. This is particularly true with respect
to clean exhaust at partial load.
Accordingly, on the basis of signals for driving pedal position, rpm, and
total cylinder filling, an optimum fresh-air quantity is ascertained with
respect to toxic exhaust substances in the arrangement of FIG. 2. The
exhaust feedback control element 14 acting as a mixing device dispenses
this fresh-air quantity and simultaneously mixes in the maximum possible
exhaust feedback quantity for the particular pressure conditions
prevailing at that time. By way of example, FIG. 2 shows the generation of
a signal for total cylinder filling via a pressure and a temperature
signal.
The following FIGS. 3-8 show various methods for furnishing and processing
signals for the total cylinder filling. Thus, FIG. 3 shows a system in
which the total quantity of gas aspirated by the engine 10 is measured by
means of a total gas-quantity meter 60 in the intake manifold 11
subsequent to the entry of the exhaust feedback line. The total-quantity
measurement signal is converted in the computer stage 79, by means of an
rpm signal, into a total cylinder filling signal Qs and this signal is
delivered to the input 43 of the characterizing stage 40. In FIG. 3, the
block 61 represents a "mixture formation system" GBS and contains the
remaining stages known in FIG. 2.
According to FIG. 3, the total-gas-quantity meter 60 mentioned there can be
realized, for instance, by means of a baffle plate, following the
principle of the known L-Jetronic device. However, this measurement method
has the disadvantage, in contrast to the combined pressure-temperature
measurement method for the total cylinder charge, that soot particles from
the exhaust can stick to the baffle plate which in the course of time can
adversely affect the measurement.
FIG. 4 shows an arrangement which has an exhaust feedback quantity meter
64, the output signal QARF.sub.ist (that is, the actual value for exhaust
feedback quantity) of which, again converted in a computer stage 79 to the
quantity per stroke, is added in an addition stage 65 to the actual-value
air quantity signal already present from the mixture formation system 61.
The further processing of the signal takes place as in the apparatus of
FIG. 3.
FIG. 5 corresponds to what is shown in FIG. 2. The total cylinder filling
Qs is determined from a measurement of temperature and absolute pressure
in the intake manifold 11. The measurement signals from the pressure
transducer 67 and temperature transducer 68 are delivered to the detection
stage 45 for the total cylinder filling and there, from both input signal,
one output signal is furnished pertaining to the total filling Qs. This
arrangement has the advantage that the intake manifold temperature
.theta.s (temperature at the onset of compression) can also be used to
determine the optimum exhaust feedback rate by carrying the temperature
signal .theta.s on, via a broken line 69, to the characterizing stage 40.
When there are no very strict requirements of precision, it is also
conceivable that beyond the intake manifold temperature, only the intake
manifold overpressure compared with ambient pressure need be detected and
this acts as a correction signal on the set-point quantity value. This is
sufficient particularly when an ambient pressure correction is already
available and only an improved adaptation to the dynamic conditions of the
supercharger and the exhaust feedback control element is needed.
FIG. 6 shows a simplified, and thus, less precise, variant of the system of
FIG. 5. Here, only the intake manifold pressure is detected by means of a
pressure inducer 67 and delivered to the characterizing stage 40.
The subjects of FIGS. 7 and 8 are apparatuses in which the processing of
the signals is modified and simplified. It is here assumed that the stages
45, that is, the detection stages for the total cylinder filling, contain
one of the systems shown in FIGS. 3-6 for the detection of the total
cylinder filling.
In many cases, a four-dimensional characterizing stage 40 is not necessary
in order to obtain sufficient precision.
In the system of FIG. 7, accordingly, the four-dimensional characterizing
stage 40 is subdivided into three-dimensional characterizing stages. In a
first characterizing stage 70, the optimum set-point air quantity for
normal conditions, QL.sub.soll is determined. This quantity is corrected
in a subsequent correction characterizing stage 71 in accordance with the
total cylinder filling.
Frequently, the optimum exhaust feedback rate is dependent only on the
load, that is, on the driver's intention QK.sub.w, but not on the rpm.
Should this be the case, then the four-dimensional characterizing block 40
can easily be reduced to three dimensions. FIG. 8 shows an arrangement
which has been simplified in comparison with that of FIG. 7, in which it
is assumed that the optimum set-point air quantity per stroke is
independent of the rpm and the normal set-point air-quantity signal
QL.sub.soll can be corrected by means of a simple arithmetical calculation
to the true set-point air-quantity signal QL.sub.soll. Then the
characterizing stage 70 in the arrangement of FIG. 7 is reduced to a
characterizing stage 75, whose output signal QL.sub.soll can be linked in
a subsequent stage 76 in the following manner, by way of example:
QL.sub.soll =QL*.sub.soll [1+K(Qs-Qs.sub.normal)]
wherein Qs.sub.normal is the known cylinder filling, presumed to be
constant, under normal conditions, and wherein K is a system constant.
Beyond the systems described, in which a set-point air quantity is
furnished on the basis of a desired fuel quantity and the ration of fresh
air to exhaust gas is determined accordingly, a variation in the metered
fuel quantity is also conceivable in accordance with the total cylinder
filling. However, this variation can be undertaken only within narrow
limits, because of the virtually direct relationship between the quantity
of fuel injected and the engine power output.
The foregoing relates to preferred embodiments of the invention, it being
understood that other embodiments and variants thereof are possible within
the spirit and scope of the invention, the latter being defined by the
appended claims.
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