Optimization control for gas engines with a three-way catalyst

The invention relates to a process for controlling the fuel-air ratio for a gas engine which is provided with one or more .lambda. (1) sensors and a three-way catalyst, as well as a control system for adjusting the fuel-air ratio by means of a control element on the basis of the signal of a first .lambda. (1) sensor, comprising stepwise adjusting the .lambda. (1) value at regulated intervals, at constant load of the engine, and measuring the corresponding activity of the catalyst, determining the corresponding measuring signals of the .lambda. (1) sensor, and determining the value of the measuring signal at which the activity of the catalyst considerably increases or decreases, followed by adjusting the value of the control signal of the .lambda. (1) sensor which is maintained as the desired control value on the basis of the latter measuring signal.

FIELD OF THE INVENTION 
The present invention relates to an optimization control for gas engines 
with a three-way catalyst and more in particular to the use of such an 
optimization control during the operation of gas engines which are 
provided with a .lambda. sensor and a controlled three-way catalyst. 
BACKGROUND 
Total energy plants with gas engines show a higher emission of noxious 
components than other electricity generators. In order to drastically 
reduce these emissions (NO.sub.x and unbrined components), a three-way 
catalyst can be used. It is necessary for the proper functioning of the 
three-way catalyst that the gas-air ratio of the mixture supplied to the 
gas engine is kept constant. The gas-air ratio can be indicated by the air 
factor .lambda.. 
The object of a .lambda. control is to keep the composition of the gas-air 
mixture supplied to the engine very close to the optimum working point of 
the three-way catalyst by making use of a closed control circuit. The 
maximum deviation from the working point is determined by the so-called 
.lambda. window. Within this window are all the values of .lambda., for 
which it applies that the emission of all the noxious exhaust gas 
components remains below the maximum limits fixed for these components. 
The lower and upper limits of the .lambda. window are determined by an 
exhaust gas component which at a lower or higher .lambda. value exceeds 
the maximum emission permissible for this component. In case of a properly 
functioning three-way catalyst, the emission of carbon monoxide (CO) is 
decisive of the lower limit, and the emission of nitrogen oxides 
(NO.sub.x) is decisive of the upper limit of the .lambda. window. This is 
shown in FIG. 1. 
Earlier research and experiences from demonstration projects show that 
problems arise with the existing techniques to keep within the .lambda. 
window. By ageing and wear of the sensor, the air factor gradually 
changes. The three-way catalyst then no longer works at its optimum 
working point, and the emissions of noxious substances impermissibly 
increase. The applicant has done research to solve this problem. This 
research has led to the development of an optimization method for 
periodically determining the value of the .lambda. signal which 
corresponds to the optimum value of the fuel-air ratio for the proper 
functioning of the three-way catalyst. 
In the Netherlands, in contrast to other countries such as Germany, 
Austria, and Switzerland, few gas engines with a three-way catalyst are 
used. Much more use is made of another NO.sub.x limiting technique, namely 
the lean mixture gas engine. Although the potency of these lean mixture 
techniques is high, the use of particularly catalytic cleaning methods 
seems unavoidable, if the emission requirements are made more stringent in 
the future. The branch in which uses with three-way catalysts can be 
expected soonest is the greenhouse horticulture. 
In this branch not only the generated electricity and heat, but also the 
combustion gases of a total energy plant can be used for CO.sub.2 
fertilization, provided these gases do not contain too many noxious 
components. Apart from saving energy, this is also of economic advantage. 
For CO.sub.2 fertilization the NO.sub.x emission must be considerably 
lower then the present legal requirement of 140 g/Gj. Besides, limiting 
values are also imposed on other noxious components. A three-way catalyst 
is satisfactory here, as has been demonstrated both in applicant's 
laboratory and in practice, but exact control of air factor .lambda. by 
means of a .lambda. sensor is then necessary. 
A .lambda. sensor comprises a small sheet of a ceramic material consisting 
of zirconium dioxide (ZrO.sub.2) stabilized by means of yttrium oxide 
(Y.sub.2 O.sub.3), provided on both sides with thin platinum electrodes 
permeable to gas. One of these electrodes comes into contact with the 
exhaust gases. This electrode functions as a small catalyst. The other 
electrode is in contact with the ambient air and serves as reference 
electrode with respect to the oxygen concentration. 
It should be observed that only at temperatures above ca. 300.degree. C. is 
the electrical resistance of the ceramic material sufficiently low for 
practical use. At this temperature the time lapsed between the moment when 
changes occur in the gas-air mixture and the moment of change of the 
.lambda. signal is still in the order of seconds, however. This reaction 
time of the .lambda. sensor largely depends on the temperature. At a 
temperature of 600.degree. C. this is reduced to less than 50 ms. 
A three-way catalyst converts hydrocarbons, carbon monoxide, and nitrogen 
oxides having a high to very high conversion efficiency into substances 
that are not or less noxious. Here it is necessary that the gas-air 
mixture burned in the engine has an air factor which is only slightly 
different from 1. In practice, therefore, engines of which the exhaust 
gases are cleaned by a three-way catalyst are referred to as .lambda.=1 
engines. This practically stoichiometric gas-air mixture must be 
maintained as the optimum working point under all the operating 
conditions. This stringent requirement cannot even be satisfied by the 
most advanced fuel systems without a feedback control. It is therefore 
necessary to use a so-called .lambda. control. 
In practice, the .lambda. control operates as follows. Depending on the 
composition of the exhaust gases, the sensor produces a signal, and 
depending thereon, the fuel-air ratio is corrected. 
The .lambda. sensor is mounted at a location in the exhaust gas system 
where all the exhaust gases pass. 
The width of the .lambda. window is determined by the emission of noxious 
exhaust gas components by the gas engine before the three-way catalyst 
(more emission.fwdarw.narrower .lambda. window), the conversion efficiency 
of the three-way catalyst for each of the noxious components separately, 
largely depending on the degree of ageing (more ageing.fwdarw.narrower 
.lambda. window), and the limits of the maximum emission permissible for 
each of the components (more stringent requirements.fwdarw.narrower 
.lambda. window). When the .lambda. window becomes narrower, it is 
necessary to control (even) more accurately. When the .lambda. window has 
become very narrow, it is better for computer control to have a bit too 
little gain and too large a time constant. Thus overshoot is prevented and 
the emission keeps within the limits. 
Optimization of the .lambda. control is necessary, because both the 
.lambda. sensor and the three-way catalyst is susceptible to ageing. Owing 
to ageing of the three-way catalyst, the .lambda. window changes, and 
owing to ageing of the .lambda. sensor, the .lambda. signal no longer 
corresponds to the desired .lambda. value within the .lambda. window. 
Optimization is necessary to ensure low emissions of noxious components 
for a longer period of time. 
Ageing of the .lambda. sensor and the three-way catalyst depends on the 
specific use, the process quantities of the use, the number of service 
hours, the type of catalyst, the size of the catalyst, the type of sensor, 
the oil consumption of the engine, etc. The ageing process will therefore 
be different in each situation. 
The ageing of the .lambda. sensor manifests itself in general as the 
gradual decrease of the sensor voltage with an increasing number of 
service hours at a constant gas-air ratio and constant engine conditions. 
In the past the applicant already researched this ageing behavior of 
.lambda. sensors. In general, when used in a gas engine, a .lambda. sensor 
can stand a long time of use (&gt;10,000 h). In order to prevent thermal 
damage to the catalytically active outer layer of the ceramic material, 
the temperature of the sensor may not rise too much. For a longer period 
of time the maximum temperature of the .lambda. sensor may not exceed ca. 
800.degree. C. Higher temperatures lead to damage to the catalytic outer 
surface of the ceramic material and thus to accelerated ageing. 
Another cause of ageing is contamination of the catalytic surface at the 
outside of the ceramic material. 
The .lambda. signal gradually decreases by ageing. The .lambda. control is 
designed so as to keep the measured .lambda. signal equal to the desired 
adjusted .lambda. signal. Consequently, owing to the ageing of the 
.lambda. sensor the gas-air mixture will be adjusted increasingly richer. 
This is diagrammatically shown in FIG. 2. With the lapse of time the real 
.lambda. is no longer within the .lambda. window of the three-way 
catalyst. Thus the emission of specific components of the exhaust gas 
becomes too high. This ageing is clearly shown in FIG. 3 for a practical 
situation. 
It is clearly visible that at a fixed .lambda. the measured .lambda. signal 
after, for instance, 5501 service hours, is lower by about 50 mV than 
after 619 service hours. This difference is practically independent of the 
.lambda. value. The characteristic of a .lambda. sensor gradually shifts 
parallel downwards in the graph with the lapse of time. This voltage drop 
due to ageing of the .lambda. sensor is different for each situation and 
each sensor. In case of specific .lambda. sensors, immediately after being 
put into use, the phenomenon may occur that the .lambda. signal increases 
during the first service hours, before the effect of gradual ageing 
occurs. In this situation, too, optimization is necessary. 
FIG. 2 shows that at a .lambda. sensor signal of 400 mV the fuel-air ratio 
practically does not change despite ageing of the sensor. In petrol cars 
equipped with a .lambda. control and a three-way catalyst use is made of 
this reference point. This reference point is very regularly sought, and 
then the correcting element is so controlled (on a time base) that a 
richer mixture is formed. For gas engines this reference point is too 
remote from the .lambda. sensor signal, which results in low emissions. 
Very regularly seeking the reference point results in (too) high NO.sub.x 
emissions, as a result of which this control system cannot be used in 
stationary gas engines. This control system is known from U.S. Pat. No. 
4,526,001. 
As in the case of the above ageing of the .lambda. sensor, the service life 
of the three-way catalyst also largely depends on the exhaust gas 
temperature prevailing in the catalyst (thermal ageing process) as well as 
on the type and the concentration of fuel additions. Ageing of a catalyst 
means that the total active surface area is reduced. Consequently, the 
conversion efficiency decreases for all the components. This leads to an 
increasing emission of the noxious exhaust gas components as compared to 
the situation for a new catalyst. Hence the .lambda. window becomes 
narrower, since the emission of noxious exhaust gas components increases 
on ageing, while the requirements for the maximum limits of course remain 
unchanged. A narrower .lambda. window requires a more accurate adjustment 
of .lambda.. FIG. 5 shows the effect of the ageing of the three-way 
catalyst on the emission of the different exhaust gas components. It is 
clearly visible that the emission levels of C.sub.x H.sub.y, CO, and 
NO.sub.x rise, and that, moreover, the .lambda. value at which the 
conversion is optimal shifts to the richer side. 
The above clearly shows that there is a need for a system for more or less 
continuously readjusting or optimizing the .lambda. signal, in relation to 
the factual emission. 
SUMMARY OF THE INVENTION 
The invention is based on the surprising insight that it is readily 
possible to correct the adjustment of the optimum fuel-air ratio for the 
gas engine on the basis of the signal of the .lambda. sensor by means of 
the activity of the catalyst. 
The invention therefore relates to a process for controlling the fuel-air 
ratio for a gas engine which is provided with one or more .lambda. sensors 
and a three-way catalyst, as well as a control system for adjusting the 
fuel-air ratio by means of a control element on the basis of the signal of 
a first .lambda. sensor, comprising stepwise adjusting the .lambda. value 
at regular intervals, at constant load of the engine, and measuring the 
corresponding activity of the catalyst, determining the corresponding 
measuring signals of the .lambda. sensor, and determining the value of the 
measuring signal at which the activity of the catalyst considerably 
increases or decreases, followed by adjusting the value of the control 
signal of the .lambda. sensor which is maintained as the desired control 
value on the basis of the latter measuring signal. 
The invention also relates to a process for generating heat and/or 
electricity by means of a gas engine, which process also comprises the 
above control.

The conversion process in the three-way catalyst directly depends on the 
air factor .lambda. of the gas-air mixture. The catalyst has the highest 
conversion efficiency for all the noxious components at a .lambda. of 
about 0.995. As described, both the .lambda. characteristic of the 
.lambda. sensor and the optimum .lambda. value of the three-way catalyst 
are influenced by ageing. The optimization method is directed to the 
periodic readjustment of the gas-air ratio such that the three-way 
catalyst can operate at the working point at which the conversion of the 
noxious components is optimal. 
The optimization according to the invention occurs on the basis of a fixed 
reference point which is largely independent of the ageing of sensor 
and/or catalyst. During the optimization the system is first brought into 
equilibrium in the fuel-rich range, for instance .lambda.=0.989, and then 
.lambda. is increased stepwise, for instance by steps of not more than 
0.001, while bringing about the equilibrium. As soon as the optimum range 
has been traversed and an excess of oxygen enters the gas, a large 
increase in activity first occurs, followed by a large decrease of 
activity in the range of .lambda.&gt;1,000. 
Before the activity of the catalyst decreases, a sudden increase of voltage 
of the signal of the .lambda. sensor takes place after the catalyst. This 
may serve as reference point to correct the control value. However, it is 
also possible to control at the NO content or the O.sub.2 content after 
the catalyst. Because the NO.sub.x consists mainly of NO, an NO monitor 
may be used to correct the control value. For simplicity's sake, use is 
preferably made of a control on the basis of the signal of the .lambda. 
sensor after the catalyst. Within the scope of the invention, there is a 
great change of the activity, when the temperature changes more than 10 mV 
at a .DELTA..lambda. of 0.002. 
The following explanation of the invention is made chiefly on the basis of 
measurements of the .lambda. signal after the catalyst, but it will be 
clear that these controls are also possible on the basis of the other 
methods. 
During the optimization the aim is to select the .lambda. value such that 
the conversion efficiency is optimal for all the noxious components. A 
high conversion efficiency is connected with a high temperature in the 
catalyst. The optimization process could be based on the maximum catalyst 
temperature. It turns out that, in practice, optimization on the basis of 
the maximum temperature in or after the catalyst does not always proceed 
in a reliable manner, as a result of which the optimum working point of 
the catalyst is not found. 
It is important during the optimization to have a fixed reference point 
that can always be detected and does not shift on ageing of sensor and 
catalyst. Such a point can be found by determining the trend of the 
.lambda. sensor after the catalyst. Important is the point at which the 
signal suddenly increases or decreases. 
FIG. 5 relates to a gas engine in a test arrangement. In this figure the 
signals of the .lambda. sensors before and after the catalyst and the 
NO.sub.x signal are plotted against the fuel-air ratio (.lambda.). In the 
tests the fuel-air ratio is varied from small to great. At a small 
.lambda. there is a clear CO emission after the catalyst (&gt;500 ppm). In 
the tests the .lambda. is increased by steps of about 0.001. Just before 
there is NO.sub.x emission after the catalyst, an increase in activity 
takes place in the catalyst. This results in a sudden considerable 
increase of the signal of the .lambda. sensor after the catalyst. When the 
signal decreases, there is NO.sub.x emission after the catalyst. The point 
of reaching the highest value of the sensor signal after the catalyst is 
the point of beginning release of free oxygen after the catalyst and of 
beginning increase of the NO.sub.x content after the catalyst. 
The increase of the signal of the .lambda. sensor can be used as reference 
point. The reference point may also be the maximum value, a decreasing 
value or a combination thereof. A breakpoint can be used to adjust the 
control value. The fuel-air ratio of the gas engine can be controlled at 
the signal after the .lambda. sensor. This may be done, for instance, on 
the basis of the value found just before a defined increase of the sensor 
signal has taken place. It is also possible to determine, when a 
breakpoint is detected, the sensor signal before the catalyst. This may be 
a measured value, an average value of a number of measurements or a 
progressive average. This may then be controlled at, if required after a 
correction. The advantage of using two sensors is, among other things, 
that in case of failure of one sensor a change-over to the other sensor 
can be effected. This increases the reliability of the control. 
Because of the highly fluctuating character of the .lambda. signal and in 
order to consider the trend of the sensor characteristic, use is 
preferably not made of the momentaneous .lambda. signal, but of the 
progressive average of preferably at least 5, but more in particular about 
10 successive optimization values of the .lambda. signal. In order to 
obtain a good history of the signal, the .lambda. is sent to the rich 
range by means of the control valve. Subsequently, by controlling the 
control valve by small defined steps, a leaner gas-air mixture is 
proceeded to, until the activity of the catalyst first considerably 
increases and then considerably decreases. Between the controls of the 
valve there is a wait to express the effect of the .lambda. variation on 
the activity of the catalyst. The waiting time depends on the buffer 
action of the catalyst and can last for minutes. 
The new desired .lambda. signal is obtained by adding a specific number of 
millivolts to or subtracting them from the progressive average of the 
.lambda. signal in order to obtain the correct control value. The research 
into the ageing of .lambda. sensors revealed that the characteristic of 
the sensor signal in the .lambda. window practically does not change but 
shifts in its entirety. In case of an aged or non-aged sensor, adding or 
subtracting a number of millivolts will then result in a similar .lambda. 
variation. The new .lambda. signal which is to be controlled at, must then 
correspond to the air factor at which the emission of the critical exhaust 
gas components is minimal. It will be clear that the determination of this 
optimum .lambda. signal can be carried out more accurately by reducing the 
step amount of .lambda. during the optimization process. 
Perhaps unnecessarily, it should be observed that this optimization method 
only works if the power of the total energy plant is not changed during 
optimization. The fact is that modulation of the power results in a change 
of the temperature of the exhaust gases from the gas engine and thus in 
the three-way catalyst as well. It is also possible that a sudden change 
of gas pressure may change the composition of the gas in and after the gas 
engine. This disturbs the optimization process. If a sensor observes a 
defined change upstream of the catalyst, then the optimization process 
will be interrupted. A defined change may be a considerably increasing or 
decreasing .lambda. signal or a temperature change of the exhaust gases. 
Modulation between two optimizations is of course no problem. 
As shown is FIGS. 6 and 7, the NO.sub.x emission may increase to high 
values during the optimization process. These values are much higher than 
the maximum value permissible for CO.sub.2 fertilization. However, when 
determining this maximum value, it has been assumed that it must be 
possible to permanently expose the crop to this value. However, at a 
relatively short exposure time (in this experiment ca. 10 minutes) the 
crop can stand higher concentrations. The exposure to NO.sub.2 can be 
taken as an instance: for 1 hour the crop can be exposed to 1.9 ppm 
NO.sub.2, which corresponds to 225 ppm in a stoichiometric gas engine and 
a dilution up to 800 ppm CO.sub.2 in the greenhouse. This short increase 
of the emissions due to optimization gives no rise to concentrations of 
noxious substances unacceptable to the crop in the greenhouse atmosphere. 
In general, optimization need not take more than ca. 30 minutes. 
FIG. 8 shows the considerable increase and decrease of temperature due to a 
changed conversion in the catalyst. In contrast to the momentaneous 
.lambda. sensor signal, the periodically determined progressive average 
shows no fluctuations. 
The control of the gas-air ratio is connected with an optimization method 
which must ensure that the air factor of the gas-air mixture always keeps 
within the .lambda. window, despite ageing of the .lambda. sensor and the 
three-way catalyst. Thus the emissions of particularly CO and NO.sub.x 
remain below the maximum permissible values. If the exhaust gases are used 
for CO.sub.2 fertilization, the .lambda. window on the rich side is not 
determined by the CO emission, but by the NH.sub.3 emission. This is 
measured at a new catalyst. In this situation, too, the optimization 
control can be used and too high emissions of CO, NO.sub.x, NH.sub.3, and 
C.sub.2 H.sub.4 are avoided. 
After each change of power, optimization must preferably be effected. The 
fact is that a change of power is connected with a change of the .lambda. 
signal, even if the gas-air ratio is not changed. In spite of the fact 
that use is made of a heated .lambda. sensor, there is likely to be a 
connection with the temperature of the exhaust gases flowing past. 
Besides, dependence on the pressure in the exhaust system is not 
impossible. The fact is that the transport of the oxygen ions in the 
.lambda. sensor is determined by, among other things, the temperature of 
the ceramic material of the .lambda. sensor. Optimization after a change 
of the power is particularly important for a very aged catalyst in which 
the .lambda. window has become very narrow. 
In general, it may be said that at a constant load optimization must be 
effected with some regularity. The frequency thereof is dependent on, 
among other things, the age of the sensor, the age of the catalyst, and 
the specific use. In a new sensor, optimization must be effected more 
frequently, for instance once a day. When the sensor has aged, after about 
one month, it is sufficient to optimize once a week. If the use is not 
very critical, this frequency may even be reduced. However, when used in 
combination with an aged catalyst, the frequency must be increased again.