Device for detecting the air-fuel ratio of an internal combustion engine

A device for detecting the air-fuel ratio of the mixture supplied to an internal combustion engine of the type including an exhaust gas recirculation system for recirculating a part of the exhaust gas into an intake passage. The device comprises means for detecting the thermal conductivity of the exhaust gas, an electric circuit having a battery which supplies an electric current to said thermal conductivity detecting means and means for measuring the changes of the electric resistance of said thermal conductivity detecting means.

This invention generally relates to a device for detecting the air-fuel 
ratio of the mixture supplied to an internal combustion engine and, more 
particularly, to such a device having means for detecting the thermal 
conductivity of the exhaust gas. 
Presently, the problem with respect to the exhaust gas discharged from 
motor vehicles is significant and various methods have been proposed for 
reducing the poisonous components contained in the exhaust gas, such as 
Hydrocarbons, CO, NO.sub.x, etc. Based on these methods, it is already 
known in the art to use as a catalytic converter, a so called three way 
catalytic converter which reduces three components at the same time. In 
such a catalytic converter the unburnt mixture in the exhaust gas is 
reacted and the deoxidation of oxide NO.sub.x occurs simultaneously, under 
a condition wherein the fresh air-fuel weight ratio of the mixture 
introduced to the internal combustion engine is controlled so as to be 
close as possible to a theoretical value and the rate of oxygen contained 
in the exhaust gas is maintained within a suitable range. That is to say, 
the oxidation and the deoxidation of the exhaust gas are carried out at 
the same time and, thus, the poisonous components, such as HC, CO and 
NO.sub.x, are simultaneously reduced by the three way catalytic converter. 
Such method, however, requires that the air-fuel ratio of the mixture 
introduced to the internal combustion engine be controlled so as to be as 
close as possible to the theoretical value and, for this purpose, it is 
necessary to provide means for accurately detecting the air-fuel ratio of 
the mixture. 
It is conventionally well known to those skilled in the art to detect the 
air-fuel ratio of the mixture by measuring the concentration of oxygen 
contained in the exhaust gas by means of an oxygen sensor (namely, O.sub.2 
sensor) provided in the exhaust passage. This is because it is difficult 
to directly detect the air-fuel ratio at the suction side of the internal 
combustion engine. The oxygen sensor, however, is not satisfactory with 
regard to accuracy and durability. 
Accordingly, it is the principal object of the present invention to provide 
a new and improved air-fuel ratio detecting device which is satisfactory 
with regard to accuracy and durability. 
Another object of the present invention is to provide an air-fuel detecting 
device in which the air-fuel ratio of the mixture is detected in the 
exhaust side of the internal combustion engine in the same way as the 
conventional oxygen sensor, but which has means for measuring the changes 
of the thermal conductivity of the exhaust gas.

Referring now to the drawings, FIG. 1 illustrates an internal combustion 
engine having a device for detecting the air-fuel ratio of the mixture 
according to this invention. The air clarified by an air cleaner 2 is 
mixed with a fuel in an intake passage 3 and the air-fuel mixture is 
introduced to an internal combustion engine body 1. After the combustion 
of the mixture, the exhaust gas is discharged through an exhaust passage 
4, which has therein a three way catalytic converter 5 for treating the 
exhaust gas in the manner described above. This internal combustion engine 
comprises an exhaust gas recirculation system and is, thus, provided with 
an exhaust gas recirculation passage 6, generally called an E.G.R. line. 
This E.G.R. line 6 takes out a part of the exhaust gas from the exhaust 
passage 4 and recirculates it to the intake passage 3. A fuel supplying 
installation, not shown in the drawings, is any one of the conventionally 
well known types and can be either of a carburetor type or a fuel 
injection type. 
In the embodiment shown in FIG. 1, the E.G.R. line 6 is provided with a 
thermal conductivity detecting member 7, such as for example a hot-wire 
resistance, which changes its value in accordance with the changes of the 
thermal conductivity of the exhaust gas in the E.G.R. line 6. The 
resistance value is amplified by an amplifier 8 and transmitted to a 
measuring device 9. 
In the embodiment of the present invention shown in FIG. 2, an air passage 
10 is formed around the exhaust gas recirculation passage 6. The 
temperature of the air in the air passage 10 is maintained the same as 
that of the exhaust gas in the exhaust gas recirculation passage 6 by any 
way, not shown, known in the art. The means for detecting the thermal 
conductivity of the exhaust gas consists of four hot-wire resistance 
elements, two of them R.sub.1 and R.sub.3 being located in the exhaust 
passage 4 and the remaining two elements R.sub.2 and R.sub.4 being located 
in the air passage 10. These four hot-wire resistance elements are 
connected to each other and constitute a bridge electric circuit as shown 
in FIG. 3. Under normal conditions, that is to say in the atmospheric 
conditions, each of the hot-wire resistance elements R.sub.1, R.sub.2, 
R.sub.3 and R.sub.4 shows an equal resistance value. 
The input of the bridge circuit, as shown in FIGS. 2 and 3, is connected to 
a battery 11 through an adjustable resistance 12 and the output thereof is 
connected to a measuring device 9 through an amplifier 8. Therefore, when 
there is no difference between the thermal conductivities of the exhaust 
gas in the exhaust passage 4 and the air in the air passage 10, the 
hot-wire resistance elements R.sub.1, R.sub.2, R.sub.3 and R.sub.4 
indicate the same resistance values and, thus, the output electric power 
of the bridge circuit is zero. 
However, if the values of the hot-wire resistance elements R.sub.1 and 
R.sub.3 change while the values of the elements R.sub.2 and R.sub.4 remain 
constant, due to a difference between the thermal conductivities of the 
exhaust gas and the air, a small electric current is generated at the 
output of the bridge circuit. The small electric current is amplified by 
the amplifier 8 and measured by the measuring device 9. In this way, the 
values of the hot-wire resistance elements R.sub.1 and R.sub.3 located in 
the exhaust passage 4 change in accordance with the condition of the 
exhaust gas, namely with the changes of the thermal conductivity thereof. 
Now, such phenomenon is described that the thermal conductivity of the 
exhaust gas changes. FIG. 4 is a diagram showing the relationship of the 
thermal conductivity (.lambda.) of the exhaust gas to the air-fuel ratio 
of the mixture. It is apparent from the diagram that the thermal 
conductivity of the exhaust gas abruptly increases when the air-fuel ratio 
of the mixture displaces to the rich region from the theoretical air-fuel 
ratio. The reason why the thermal conductivity increases is that a large 
amount of hydrogen is contained in the exhaust gas when the air-fuel ratio 
of the mixture is in the rich region and, thus, the hydrogen is reacted 
and then the thermal conductivity (.lambda.) of the exhaust gas increases 
to a large extent. Contrary to this, smaller amounts of hydrogen exsist in 
the exhaust gas when the air-fuel ratio of the mixture is in the lean 
region. Thus, the existence of hydrogen in the exhaust gas results 
increasing the thermal conductivity to a large extent, so that the 
hot-wire resistance elements R.sub.1 and R.sub.3 change their resistance 
values. Therefore, the bridge circuit serves as a detector for physically 
detecting the existence of hydrogen in the exhaust gas. That is to say, if 
the air-fuel ratio of the mixture changes from the theoretical value, the 
electric balance of the bridge circuit is lost. For this purpose, the 
hot-wire resistance elements R.sub.1, R.sub.2, R.sub.3 and R.sub.4, shown 
in FIGS. 2 and 3, must be made from some materials that are sensitive to 
the thermal conductivities. It is preferable that the hot-wire resistance 
elements R.sub.1 and R.sub.3, located in the exhaust passage 4, be coated 
with anti-corrosive material, such as a ceramic, so as not to be corroded 
by the exhaust gas. 
As described above, a device for detecting the air-fuel ratio of the 
mixture according to this invention, which is satisfactory in accuracy and 
durability, is obtained. The air-fuel ratio of the mixture supplied to an 
internal combustion engine can be controlled as closely as possible to a 
theoretical air-fuel ratio by using such a device together with a fuel 
supplying installation.