Method and device for regulating the NO.sub.x emission of an internal combustion engine

Method and device for regulating the NO.sub.x emission of an internal combustion engine. It has been found that the NO.sub.x emission is dependent on the efficiency of the engine. That is to say the NO.sub.x emission can be optimized by determining and controlling the efficiency. This can be performed by making the measured and determined efficiencies equal to one another by altering the air/fuel ratio.

The present invention relates to a method for regulating an internal 
combustion engine, comprising the adjusting of the air/fuel ratio as a 
function of the operating conditions of said engine, the output of said 
engine being measured. 
Two systems are known in the prior art for controlling the emission of 
nitrogen oxides. 
In the first system, direct measurements are performed in the cylinder or 
cylinder head. Said measurements relate to temperature measurements. It 
is, after all, known that bonding of oxygen to nitrogen is dependent to an 
appreciable extent on the combustion temperature of the mixture in the 
cylinder. 
This method has the advantage that it functions particularly well, but it 
has the disadvantage that the devices for determining the temperature are 
particularly sensitive and have a limited service life. 
In addition, such devices require appreciable interventions in the 
combustion engine, which means that subsequent incorporation is impossible 
or is possible only by applying special effort. 
In the case of fairly small cylinders and cylinder heads, it is not 
possible to measure sufficiently accurately. 
The other group of methods for reducing the nitrogen oxide component is 
based on the so-called indirect measurement. That is to say derivatives of 
the combustion temperature are used to control the engine. 
Thus, it is known that a leaner mixture, that is to say a mixture 
containing a greater excess of air, gives a lower combustion temperature. 
In addition, a hotter mixture gives a higher combustion temperature. It 
has also been found that there is a relationship between, on the one hand, 
the engine output and, on the other hand, the pressure and temperature in 
the inlet duct (see, for example, European Patent 0 259 382). 
In some of the methods described above, .lambda.-sensors, which are 
susceptible to ageing, are used. If the quality of the gas, that is to say 
the calorific value, varies, it has been found that, for equal .lambda. 
(air/fuel ratio) a varying NO.sub.x emission occurs. In addition, for an 
unduly lean mixture, it is known that output instability occurs, that is 
to say the engine no longer runs evenly. 
As a result of internal alterations in the engine which alter the cylinder 
charge, such as, for example, a change in the valve clearance or in the 
outlet back pressure, the determination of the pressure in the inlet 
manifold becomes an unreliable method. It has also been found that the 
determination of the mixture temperature correction is difficult in 
practice because the temperature of the mixture drawn in is difficult to 
control. 
The object of the present invention is to provide a method and a device 
which does not have said disadvantages and in which the NO.sub.x emission 
can be limited according to requirement with the aid of the indirect 
method using means which are known in the prior art and which can be 
controlled well. 
This object is achieved in the case of a method described above, which 
method comprises the determination of the desired efficiency of said 
engine under said operating conditions, the measurement of the efficiency 
under said operating conditions and the alteration of the air/fuel ratio 
of the mixture in such a way that the determined and measured efficiencies 
are equal, the air/fuel ratio being reduced if the measured efficiency is 
higher than the determined efficiency. 
Surprisingly, it has been found that there is a relationship between the 
mechanical efficiency of a combustion engine and the combustion 
temperature and, as stated above, the combustion temperature is of 
importance for the NO.sub.x emission. That is to say, if the efficiency is 
kept constant at a certain output, the NO.sub.x emission will also be 
constant. In this connection, it is unimportant whether parameters, such 
as the inlet temperature, inlet pressure, gas composition, vary. As long 
as the efficiency remains constant, the NO.sub.x emission will not change. 
Even if the mixture strength changes, it has been found that, at constant 
efficiency, the NO.sub.x emission remains unchanged. This means that, for 
varying composition of the gas and/or other parameters, such as delivery 
pressure and delivery temperature, a constant emission can be achieved 
with the regulation according to the invention. 
If the engine is a stationary engine which continuously rotates at the same 
speed, the efficiency varies as a function of the loading. If, however, 
the engine is operated at varying speed and varying loading, the 
efficiency can be read, according to the invention, from a table which is 
determined on the basis of the characteristic diagram of said engine. 
It is possible to combine the method described above with regulating 
systems for keeping the speed and/or the output of the engine constant. 
Because changes in air/fuel composition, to which the invention relates, 
will generally proceed less quickly than changes in speed and/or output, 
preference is given to performing responses to such changes relatively 
quickly to regulate the engine speed or output and relatively slowly to 
vary the air/fuel ratio. The efficiency of the engine can be measured in 
all the ways known in the prior art. 
One method comprises the arithmetical determination of the desired gas flow 
from measurements on the engine. Such measurements may comprise, for 
example, the measurement of the air flow. A so-called `hot wire flow 
meter` can be used for this purpose. In this case, air flows over a heated 
wire and the change in resistance due to cooling of the wire by the air 
moving over the wire is recorded and converted into an air flow. It is 
also possible to determine these quantities from the mixture flow. 
In this connection, the desired gas flow is determined by measuring various 
parameters of the combustion engine, also known under the name 
`speed/density method`. 
In the speed/density method, the temperature and pressure of the mixture 
are determined in the inlet duct of the combustion engine, as well as the 
volumetric efficiency and the stroke volume of the latter. 
Assuming that the desired air/fuel ratio is known, the gas flow can be 
calculated either from the air flow determined or from the mixture flow. 
Proceeding from the insight that the mechanical efficiency is critical for 
the NO.sub.x emission, this means that, according to the invention, it is 
no longer important to know the air/fuel ratio or the mixture temperature 
and gas quality. 
The mechanical efficiency can be reproduced in the form of a formula as 
follows. 
##EQU1## 
where P=output in kW 
Q.sub.gn =normalized gas flow in nm.sup.3 /sec 
H.sub.u =net calorific value of the gas in kJ/nm.sup.3. 
If the engine is operated at constant output and speed, this means that 
both the mechanical efficiency and P are constant. It then follows from 
formula (I) that Q.sub.gn .times.H.sub.u is constant. 
If, for example, the gas quality becomes higher, the gas flow should, 
according to the above formula, be reduced accordingly to keep the output 
and the mechanical efficiency constant with the same quantity of air 
delivered. 
In order to verify the theory on which the present invention is based, some 
measurements have been performed and a test arrangement has been made. 
FIG. 1 shows an example of the regulating system according to the present 
invention; 
FIG. 2 shows efficiency and No.sub.x emissions for various air-fuel ratios; 
FIG. 3 shows No.sub.x emissions versus power for different fuels; and 
FIG. 4 shows No.sub.x emissions and efficiency for different air 
temperatures.

In FIG. 1, 1 shows a combustion engine provided with an inlet duct 2 having 
a throttle valve 3 for the mixture flow which is installed therein and 
which is regulated by a regulating member 4. Indicated by 9 is a `hot wire 
flow` meter for the air flow, which is connected to the control system 6. 
Gas is delivered via a line in which there is a metering valve 12 which is 
controlled by a motor 5 which receives its signals from control system 6. 
A gas flow meter is indicated by 13. Combustion engine 1 is provided with 
an outlet 7 and with a device with which, inter alia, the output of the 
engine can be determined, such as a generator 8. The output 
power--measuring output of the generator 8 is also connected to control 
system 6. The gas line 16 is connected to a gas reservoir 10. Present in 
the inlet duct 2 downstream of the throttle valve 3 are a temperature 
sensor and a pressure gauge which are connected to control system 6 via 
line 15 and 14, respectively. 
A device for measuring the instability of the engine is shown by 11. If, 
for any reason, the combustion cannot be optimized within certain limits 
as a result, for example, of the complete or partial failure of a 
cylinder, this instability meter 11 will emit a signal and disable the 
regulating system and continue to run at the basic setting. 
If the abovementioned regulating system is used to drive generator 8 at 
constant speed and output, the following will occur if the gas quality 
changes: 
If, for example, gas having a higher calorific value is supplied, throttle 
valve 3 will be closed slightly by a quick regulation to control both the 
output and the speed as a first response aimed at keeping the engine speed 
and the output delivered constant. 
In the position before the delivery of the slightly enriched gas, the 
mechanical efficiency is determined in control system 6 on the basis of 
the table stored therein. Q.sub.gn is determined on the basis of either 
the quantity of air delivered as determined by flow meter 9 or by the 
`speed/density method` dependent from the signals from the temperature 
sensor and pressure gauges in the inlet channel. In this connection, use 
is made of the stoichiometric .lambda. of the outlet gas. 
After the closing of the gas valve, the output will be recalculated 
assuming the same stoichiometric .lambda. (which is now incorrect). 
Because the Q.sub.gn has now decreased, a difference will arise between 
the calculated output and the measured output, that is to say, for 
unchanged efficiency, the calculated output is lower than the measured 
output in the case of enrichment. 
As a result of subsequently changing the air/fuel ratio, that is to say of 
limiting the quantity of gas in this case, the output of the engine will 
tend to fall, as a result of which the throttle valve 3 is opened. 
The result of this measure is that Q.sub.gn returns to the old value and 
the .lambda.-value is changed. That is to say the same efficiency is 
maintained with a changed air/fuel ratio. As indicated above, it has been 
found, surprisingly, that a change in the efficiency amounts, in fact, to 
a change in the NO.sub.x emission. This is illustrated by reference to 
FIG. 2 for various air/fuel ratios. The measurements shown therein have 
been performed on a `lean burn engine` having an output of 210 kW. 
It will be understood that the regulation according to the present 
invention occurs not only on changing the composition of the fuel 
delivered, but also if the temperature of the inlet air or the temperature 
of the gas is altered. 
FIG. 3 shows how approximately the same NO.sub.x emission can always be 
obtained by adjusting the quantity of gas delivered with the aid of the 
regulating system according to the present invention within a wide output 
range for the same engine if different fuels are supplied. The gas 
originating from the gas main had an H.sub.u of 31.6 MJ/M.sup.3 while the 
bottled gas had an H.sub.u of 38.4 MJ/M.sup.3. It is clear from this graph 
that the regulating system according to the present application gives 
optimum NO.sub.x values regardless of the type of gas. 
Finally, FIG. 4 comprises in turn a comparison of the NO.sub.x emission and 
the efficiency. If the temperature is increased, the NO.sub.x emission 
appears to increase, which also applies to the efficiency. 
Although the invention has been described above in combination with a 
quick-acting regulating system for keeping the output and speed constant, 
it will be understood that it can be coupled to any other regulating 
system and can also be obtained with various speeds and outputs. These and 
other changes which are obvious to the person skilled in the art are 
deemed to be within the scope of the appended claims.