Open-loop/closed-loop control system for an internal combustion engine

An open-loop/closed-loop control system for adjusting the air/fuel mixture of an internal combustion engine 12 exhibits an oxygen probe (.lambda.-probe) 14 which is exposed to the exhaust gas of the internal combustion engine 12 and which emits an output signal which represents a measure of the air ratio .lambda.. The control system also has a basic memory 10 for storing fuel-metering times which are used for precontrolling the internal combustion engine 12 to a predetermined air ratio .lambda., a desired-value memory 18 for storing desired values of the air ratio and a closed-loop control device 20 which, in depedence on an output signal of the .lambda.-probe 14 measured and on an associated desired value read out of the desired-value memory 18, corrects the particular fuel-metering time read out of the basic memory 10 and the desired-value memory 18 stores the inverse value of the air ratio .lambda.. The fuel-metering time read out of the basic memory 10 is multiplicatively combined with the corresponding inverse value of the air ratio .lambda. read out of the desired-value memory 18. A conversion device 16 converts, with the aid of an at least approximately known probe-characteristic relationship between the output signal of the .lambda.-probe 14 and the air ratio .lambda., the output signal into a corresponding inverse value of the air vario .lambda.. A fast and accurate control is achieved by means of a simple linear closed-loop control device by taking into consideration the linear relationship between the inverse value of the air ratio .lambda. and the fuel quantity (fuel-metering time).

FIELD OF THE INVENTION 
The invention relates to an open-loop/closed-loop control system for 
adjusting the air/fuel mixture of an internal combustion engine. 
BACKGROUND OF THE INVENTION 
Such systems have a .lambda.-probe which is exposed to the exhaust gas of 
the internal combustion engine and which emits an output signal which 
represents a measure of the air ratio .lambda.. In particular, a 
.lambda.-probe is used, the characteristic of which has an essentially 
jump-like behavior in the region of .lambda.=1 (Nernst-type 
.lambda.-probe). Furthermore, the open-loop/closed-loop control system has 
a basic memory, a desired-value memory and a closed-loop control device. 
In the basic memory, fuel-metering times (for example, injection times for 
the injection valves of the internal combustion engine) are stored in 
dependence on operating parameters of the internal combustion engine and 
in the desired-value memory, desired values of the air ratio .lambda. are 
stored in dependence on operating parameters of the internal combustion 
engine. The closed-loop control device corrects the fuel-metering time 
read out of the basic memory in dependence on an output signal of the 
.lambda.-probe measured and on a corresponding desired value read out of 
the desired-value memory. 
Low-pollutant vehicles are usually operated with a three-way catalytic 
converter arranged in the exhaust gas of the internal combustion engine. 
In order to ensure the optimum conversion rate of the catalytic converter, 
it is necessary that an air ratio of .lambda.=1 is almost exactly 
maintained, that is the air ratio .lambda. may only fluctuate by a 
particular permissible amount around the value of .lambda.=1 (so-called 
catalytic converter window). In actual closed-loop control systems, 
control is frequently effected not exactly to .lambda.=1 but to 
.lambda..congruent.1 (for example .lambda.=0.998). In the text which 
follows, the term ".lambda.=1 control" will still be used for reasons of 
simplification, this term also being intended to encompass 
.lambda..congruent.1. 
If the arrangement of a catalytic converter is omitted, a further 
possibility for reducing particular pollutant components of the exhaust 
gases of an internal combustion engine consists in operating the internal 
combustion engine in the lean range (.lambda.&gt;1). Thus, a large decrease 
of the nitrogen monoxides (NOx) contained in the exhaust gas is achieved, 
for example, with an air ratio of .lambda.=1.4. The carbon monoxide 
content (CO) of the exhaust gas is already very low at air ratios from 
.lambda.=1. However, there is an increase in the hydrocarbon content (HC) 
of the exhaust gas with large air ratios (from .lambda..congruent.1.1). 
However, the driving characteristic of the internal combustion engine 
stands in the way of increasing the air ratio .lambda. and the possible 
reduction in the pollutant components. To achieve an adequate driving 
characteristic of the internal combustion engine in any operating phase, 
it is necessary to enrich the air/fuel mixture in particular operating 
phases (for example idling, full load) by increasing the fuel quantity 
added so that values of the air ratio .lambda. occur which, under certain 
circumstances, are less than 1. 
To be able to reliably cover such a wide control range 
(.lambda..congruent.0.9 to 1.4) by closed-loop control techniques, it is 
necessary in accordance with the solutions available in the prior art to 
use several controllers or to achieve a switch-over between individual 
control ranges by means of elaborate circuit measures. A closed-loop 
control device for the mixture composition of an internal combustion 
engine with switchable control ranges for .lambda.=1 range and lean range 
is known from U.S. Pat. No. 4,594,984, in which the .lambda.=1 control is 
effected by means of a two-position controller and lean control is 
effected either via an altered desired value of the two-position 
controller or with the aid of a constant controller. 
The invention is based on the object of improving an open-loop/closed-loop 
control system for setting the air/fuel mixture, particularly for a 
control in the lean range. 
SUMMARY OF THE INVENTION 
The open-loop/closed-loop control system according to the invention is 
characterized by the fact that the desired-value memory stores the inverse 
value of the air ratio .lambda.. In dependence on the operating parameters 
of the internal combustion engine, the fuel-metering time read out of the 
basic memory for precontrolling the internal combustion engine for a 
predetermined air ratio .lambda. is multiplicatively combined with the 
associated inverse value of the air ratio .lambda. read out of the 
desired-value memory to obtain a fuel-metering time which is adapted to a 
change in the predetermined air ratio .lambda.. A .lambda. control is 
superimposed on the precontrol, in order to take into consideration the 
influence of interfering variables. For this purpose, the 
open-loop/closed-loop control system according to the invention has a 
conversion device which converts, with the aid of a probe-characteristic 
relationship between the output signal of the .lambda.-probe and the air 
ratio .lambda., the output signal into a corresponding inverse value of 
the air ratio .lambda.. A control deviation is supplied to the closed-loop 
control device of the open-loop/closed-loop control system according to 
the invention and this deviation is determined on the basis of the 
difference of inverse values of the air ratio .lambda. read out of the 
desired-value memory in dependence on operating parameters of the internal 
combustion engine and the associated inverse values of the air ratio 
determined as actual values by the conversion unit on the basis of the 
output signal of the .lambda.-probe. 
Compared with the known systems, the open-loop/closed-loop control system 
according to the invention has the advantage that, for example with a 
control in the lean range (.lambda..congruent.0.9 to 1.4), only one 
closed-loop control device is necessary in the entire range and additional 
elaborate circuit measures are avoided. The known closed-loop control 
systems control to the air ratio .lambda. and vary the fuel-metering time 
in proportion to the control deviation. In reality, however, there is a 
non-linear relationship between the air ratio .lambda. and the fuel 
quantity added. Thus, the air ratio .lambda. is proportional to the 
inverse value of the fuel quantity and, conversely, the fuel quantity 
added is proportional to the inverse value of the air ratio .lambda.. With 
a control to .lambda.=1, a relatively small error is obtained with 
proportional fuel metering if the control deviation is kept sufficiently 
small since the air ratio .lambda. is approximately identical with its 
inverse value in this range. To use such a closed-loop control device in 
the entire lean range, however, leads to considerable errors due to the 
non-linear relationship between the air ratio .lambda. and the fuel 
quantity in fuel metering in the lean range. These errors are avoided in 
the open-loop/closed-loop control system according to the invention by 
controlling for the inverse value of the air ratio .lambda.. The 
open-loop/closed-loop control system according to the invention has the 
advantage that the control is linear in the entire .lambda. range to be 
controlled since the conversion device supplies the inverse value of the 
air ratio .lambda. to the closed-loop control device and that the output 
signals of the .lambda.-probe are not directly used for controlling as is 
usually done. Independently of the magnitude of the respective desired 
value, a particular percentage control error relative to the desired value 
corresponds to the same actuating variable so that the gain of the 
controller can be selected independently of the desired value. 
In a preferred embodiment of the invention, the memories (basic memory, 
desired-value memory), the closed-loop control device and the conversion 
unit are functional units of a microcomputer. It is particularly 
advantageous to store the fuel-metering times, the desired values of the 
air ratio .lambda. and the probe-characteristic relationship between the 
output signal of the .lambda.-probe and the air ratio .lambda. in 
characteristic fields which are addressed by means of the operating 
parameters of the internal combustion engine.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
The open-loop/closed-loop control system according to the figure has a 
basic memory 10 from which fuel-metering times T.sub.LKF are read for 
precontrolling an internal combustion engine (ICE) 12. The rotational 
speed n and a load characteristic L of the internal combustion engine 12 
are used as input parameters for the basic memory 10. Depending on the 
existing sensor device, the throttle flap position of the internal 
combustion engine, the pressure in the intake pipe of the internal 
combustion engine or the air mass drawn in by the internal combustion 
engine can be used as load characteristic. 
The open-loop/closed-loop control system also has a .lambda.-probe 14, a 
conversion unit 16, a desired-value memory 18 and a closed-loop control 
device 20. The closed-loop control device 20 has a timing unit 20.1 and a 
correction device 20.2. Furthermore, a switch-over device 22 and a 
control-enable device 24 are provided. 
The desired-value memory 18, which is, like the basic memory 10, 
addressable via the rotational speed and a load characteristic of the 
internal combustion engine, is subdivided into three sections. These 
sections are: a section in which the inverse values of the desired air 
ratio .lambda. for .lambda. greater than and less than 1 are stored; a 
section in which the desired inverse value of the air ratio .lambda.=1 is 
stored for a control with catalytic converter; and a section in which 
desired inverse values of the air ratio .lambda. are stored for 
controlling the internal combustion engine 12 in particular operating 
phases (for example start-up phase, acceleration phase, deceleration 
phase). The switch-over device 22 is supplied with the engine temperature 
T.sub.W, the rate of change of a load characteristic dL/dt and the 
information whether there is a catalytic converter in the exhaust gas of 
the internal combustion engine. The switch-over device 22 drives, via a 
switch 22.1, on the basis of the magnitudes stated, the associated section 
in which the inverse value of the air ratio .lambda. is stored as desired 
value and determines from which of the three sections the desired inverse 
values of the air ratio .lambda. are read out. 
The basic memory 10 is advantageously constructed as a characteristic field 
for fuel-metering times for an open-loop/closed-loop control to 
.lambda.=1. Such a characteristic field is measured and tested for many 
vehicles. These fuel-metering times are usually set on a test stand. 
The fuel-metering times T.sub.LKF read out of the basic memory 10 are 
multiplicatively combined with the inverse values of the air ratio 
.lambda. which are read out of the desired-value memory in accordance with 
the position of the switch 22.1 of the switch-over device 22 and which, at 
the same time, represent correction factors (MFK). This results in the 
fuel-metering time T.sub.LKF *. If the internal combustion engine 12 has 
not yet reached its operating temperature or if the internal combustion 
engine 12 is in an unstable phase (acceleration, deceleration), the 
fuel-metering time T.sub.LKF * is used for precontrolling the internal 
combustion engine 12. 
If the internal combustion engine 12 has reached its normal operating 
temperature and is operating in a stable mode, that is the amount of the 
rate of change of a load characteristic is less than a predetermined 
value, then the control-enable device 24 closes a switch 24.1 and the 
fuel-metering time T.sub.LKF * is multiplicatively superposed by a 
correction factor FALK output by the closed-loop control device 20, which 
results in the fuel-metering time T.sub.E. The determination of the 
correction factor FALK is explained in greater detail in the following. 
Initially, the .lambda.-probe 14 arranged in the exhaust gas of the 
internal combustion engine 12 outputs an output signal U.sub.S which is 
supplied to a conversion unit 16. Using an at least approximately known 
probe-characteristic relationship between the output signal of the 
.lambda.-probe 14 and the air ratio .lambda., the conversion unit 16 
determines the corresponding inverse value of the air ratio .lambda.. This 
current inverse value of the air ratio .lambda. is supplied to a 
comparator 26 as actual value. At the same time, a corresponding inverse 
value of the air ratio .lambda., read out of the desired-value memory 18, 
is present as desired value at the comparator 26. The difference between 
actual value and desired value of the air ratio .lambda. is supplied as 
control error to the timing unit 20.1 of the closed-loop control device 
20. The subsequent correction device 20.2 then determines the correction 
factor FALK. 
A jump-like change in the air ratio .lambda. with relatively large 
deviations of the desired value from the actual value, and thus a 
jump-like change in the fuel-metering time, results in a jump-like change 
in the torque of the internal combustion engine. The driver of an internal 
combustion engine notices this as a jolting behavior of the vehicle. This 
jolt is quite desirable in an acceleration process. However, a jolt 
produces negative sensations if a jump-like change (increase) in the air 
ratio .lambda. into the lean range occurs in deceleration phases. Thus, 
for example, a jump towards the lean mixture of the air ratio of 
approximately 20% (for example .lambda. desired old=1.2, .lambda.desired 
new=1.3) entails a drop in power of approximately 10 to 15%. In order that 
this power drop does not occur suddenly, the desired 1/.lambda. value is 
slowly lowered from the old desired 1/.lambda. value to the new desired 
1/.lambda. value at a predetermined rate of lowering by means of a 
closed-loop down-control unit 27 in a preferred further development of the 
open-loop/closed-loop control system according to the invention. The rate 
of lowering has been selected to be a few percent change in desired value 
per second. 
To increase the control accuracy, it is of advantage to filter out 
higher-frequency components of the probe signal which have their cause, 
for example, in a spread of the air/fuel mixture from cylinder to cylinder 
or in other interference signals, by means of a filtering device in order 
to prevent the probe signal from becoming "noisy". 
In a particularly advantageous embodiment, all memories and devices of the 
open-loop/closed-loop control system are functional units of a 
microcomputer within an electronic control device. In this connection, it 
has been found to be advantageous to additionally provide a parameter 
adjusting device by means of which the parameters of a closed-loop control 
device having, for example, PID characteristics, can be varied. This makes 
it possible to use the electronic control device with the same 
configuration both for a .lambda.=1 control and for a lean control. If a 
.lambda.-probe of the Nernst type is provided, that is the output signal 
of the .lambda.-probe exhibits a jump-like behavior in the region of 
.lambda.=1, the closed-loop control device in a .lambda.=1 control must 
exhibit a high rate of control in order to maintain a predetermined narrow 
catalytic converter window. This may lead to a deterioration in comfort 
with respect to the driving behavior since the control parameters for 
maintaining the catalytic converter window must be adjusted in such a 
manner that the closed-loop control device is operating close to its limit 
of oscillation. However, such a high rate of control, that is an operation 
of the closed-loop control device in the vicinity of its stability limit, 
is not required with a lean control because the probe signal exhibits a 
constant behavior in the lean range. The parameter adjusting device 
provides the possibility of optimally adjusting the closed-loop control 
device to the actual control concept (.lambda.=1 control, lean control). 
When a Nernst-type .lambda.-probe is used, the output signal of the 
.lambda.-probe is of a low magnitude in the lean range (approximately 100 
to 30 mV). With the measuring devices used today in motor vehicle 
engineering, it is therefore necessary to amplify the output signal in the 
lean range (for example gain=7). The output signal is amplified by a 
factor of 4 to 5 in the range of .lambda.=1 and it is not necessary to 
amplify the output signal in the rich range (.lambda.&lt;1). Considering this 
background, it is particularly advantageous to subdivide the conversion 
unit into three sections, namely a section for controlling in the range of 
.lambda.=1 (for example between .lambda.=0.97 and .lambda.=1.03), a rich 
range (for example .lambda.&lt;0.97) and a lean range (for example 
.lambda.&gt;1.03). This reduces the computing time needed for determining the 
inverse .lambda.-value from the measured output signal of the 
.lambda.-probe.