Field of the Invention:
The present invention relates to a device for detecting an air/fuel ratio of a fuel mixture being supplied to an engine of a motor vehicle fitted with a Compressed Natural Gas (CNG) kit or a Liquefied Petroleum Gas (LPG) kit by probing exhaust gas resulting from combustion of the air fuel mixture. The present invention also relates to a system employing a device for detecting an air/fuel ratio of a fuel mixture being supplied to the engine of the motor vehicle fitted with a LPG/CNG kit for controlling an air/fuel ratio of the fuel mixture being combusted.
Background of the Invention:
Up until now a numerous types of exhaust gas sensors have been developed for the purpose of measuring the oxygen concentration of exhaust gas resulting from combustion of a fuel mixture within an engine so as to detect the air/fuel ratio of the fuel mixture.
As would be known to a person skilled in the art, there are two different types of exhaust gas sensors commonly used in the art, namely:
(a) a device commonly referred to as oxygen sensor; and
(b) a device commonly referred to as air/fuel ratio (A/F) sensor.
It may be observed that the terms "oxygen sensor" and "A/F sensor" are not interchangeably used in the specification (although the industry may some times do so).
The oxygen sensor, made up of zirconia, platinum electrodes and a heater, is of the oldest style and has a narrow range of oxygen detection. Coming to the operation of the oxygen sensor, the oxygen sensor generates a voltage signal based on the amount of oxygen in exhaust stream compared to the atmospheric oxygen. The greater the difference in oxygen content between the exhaust stream and atmosphere, the higher the voltage signal. Thus, when exhaust stream's oxygen content is high, oxygen sensor voltage output is low, while when exhaust stream's oxygen content is low, oxygen sensor voltage output is high. An exhaust stream containing low oxygen content is commonly referred to as "rich mixture" and an exhaust stream containing high oxygen content is commonly referred to as "lean mixture". The amplitude of the voltage signal produced by the

oxygen sensor in respect of a rich mixture is in the range of 0.6 to 1.0 volts and the amplitude of the voltage signal produced by the oxygen sensor in respect of a lean mixture is in the range of 0.4 to 0.1 volts. At an air/fuel ratio of 14.7:1 (which is considered as the stoichiometric mixture), the oxygen sensor voltage output is approximately 0.45 volts. The output from the oxygen sensor is provided to an analog to digital (A/D) converter. The output of the A/D converter is fed to a control mechanism (commonly referred to as Oxygen Sensor based Closed Loop Fuel Feedback Control System) which controls rate of addition of fuel to air.
The air/fuel ratio (A/F) sensor is similar to the oxygen sensor. Though, the A/F sensor appears similar to the oxygen sensor, it is constructed differently and has different operating characteristics. The advantage of using the A/F sensor is that it has the ability to detect air/fuel ratios over a wide range. This is accomplished in the A/F sensor by:
• operating a heater at approximately 650°C (as opposed to about 400°C, which the temperature of operation of the oxygen sensor); and
• providing the output in the form of changes in current (amperage) in relation to the amount of oxygen in the exhaust stream (as opposed to generating a varying voltage signal based on the amount of oxygen in the exhaust stream).
The A/F sensor is designed such that when a stoichiometric mixture is combusted in the engine, there is no current flow. A rich mixture which leaves very little oxygen in the exhaust stream produces a negative current flow and a lean mixture which has more oxygen in the exhaust stream produces a positive current flow.
As an A/D converter cannot produce a digital signal (for supplying to the control mechanism) by taking differences in the current flow as input, a converter circuit is provided to convert the current flow changes to voltage output changes. The output of the said converter circuit is thereafter provided to the A/D converter as input. The A/D converter's output is fed to a control mechanism (commonly referred to as A/F Sensor based Closed Loop Fuel Feedback Control System) which controls rate of addition of fuel to air.

The converter circuit detects a change in flow direction and strength of the current and generates a voltage signal relatively proportional to exhaust oxygen content. A typical converter circuit produces a voltage output having amplitude of about 3.3 volts when there is no current flow (i.e. under stoichiometry operation). The converter circuit produces a voltage output having amplitude below 3.3 volts when there is a negative current flow i.e. for a rich mixture, and produces a voltage output having amplitude above 3.3 volts when there is a positive current flow i.e. for a lean mixture. As the voltage signal thus produced by the converter circuit is proportional to the change in the air/fuel ratio, the same could be converted into a digital form and a digital signal thus produced can be used by the control mechanism. This allows the control mechanism to more accurately judge the exact air/fuel ratio under a wide variety of conditions and quickly adjust the amount of fuel to the stoichiometric point.
Thus, it can be observed that although oxygen sensor provides a directly measurable voltage signal, its sensitivity is lower than the sensitivity of the A/F sensor. As A/F sensor does not produce a directly measurable signal, it is necessary to include additional circuit to make the output of the A/F sensor easily measurable. Even after incorporation of the additional circuit (in the form of a conversion circuit), the voltage signal thus produced by the conversion circuit is not identical to that produced by the oxygen sensor. More particularly, if we consider a situation where a rich mixture is supplied to the engine, the oxygen sensor produces a high voltage signal (having amplitude in the range of 0.6 to 1.0V) while the A/F sensor produces a low voltage signal (having amplitude less than 3.3V). Similarly, if we consider a situation where a lean mixture is supplied, the oxygen sensor produces a low voltage signal (having amplitude in the range of 0.1 to 0.4 V) while the A/F sensor produces a high voltage signal (having amplitude greater than 3.3 V).
Because of the aforesaid differences in the nature of signals being produced by the A/F sensor and the oxygen sensor, use of a common type of controller with both types of exhaust gas sensors is not possible. More particularly, it is not possible to provide the output of an A/F sensor to a controller which is configured to receive output from an oxygen sensor and vice-versa.

Furthermore, the use of both of the aforesaid distinct types of exhaust gas sensors and the associated controller mechanisms are well known only in respect of a motor vehicle which runs on a single fuel (either a petrol or diesel). However, when the motor vehicle is constructed (originally) or modified (retrofitted) to enable the same to run on a plurality of fuels, for example, petrol and CNG or petrol and LPG, the manner of utilizing the sensor to detect the air/fuel ratio for an air-CNG mixture and an air-petrol mixture (or alternatively air-LPG mixture and air-petrol mixture) is not known.
Hence, there exists a need to look into the aforesaid aspect of how to effectively provide a device for detecting an air/fuel ratio of a fuel mixture being supplied to an engine of a motor vehicle fitted with a CNG kit by probing exhaust gas resulting from combustion of the air fuel mixture.
Object of the Invention:
The object of the present invention is to provide a device for detecting an air/fuel ratio of a fuel mixture being supplied to an engine of a motor vehicle retrofitted with a LPG/CNG kit by probing exhaust gas resulting from combustion of the air fuel mixture.
Another object of the present invention is to provide a device for detecting an air/fuel ratio of an air-CNG mixture and an air-petrol mixture (or alternatively air-LPG mixture and air-petrol mixture) being supplied to an engine of a motor vehicle.
Summary of the Invention:
Accordingly, the present invention provides a device for detecting an air/fuel ratio of a fuel mixture being supplied to an engine of a motor vehicle fitted with a LPG/CNG kit by probing exhaust gas resulting from combustion of the air fuel mixture, said device comprising A/F sensor located in the motor vehicle providing an output in the form of a change in current in relation to the amount of oxygen in the exhaust stream, a relay mechanism receiving the output of the A/F sensor and providing the same to a first circuit in presence of a change over signal and to a second circuit in the absence of the change over signal.

The present invention also provides a system employing the device as described above for detecting an air/fuel ratio of a fuel mixture being supplied to the engine of the motor vehicle fitted with a LPG/CNG kit for controlling an air/fuel ratio of the fuel mixture being combusted.
Brief Description of the Accompanying Drawings:
In order that the invention may be readily understood and put into practical effect,
reference will now be made to exemplary embodiments as illustrated with reference to
the accompanying drawings, where like reference numerals refer to identical or
functionally similar elements throughout the separate views. The figures together with a
detailed description below, are incorporated in and form part of the specification, and
serve to further illustrate the embodiments and explain various principles and advantages,
in accordance with the present invention where:
Figure 1 represents the configuration of the device constructed in accordance with the
present invention;
Figure 2 illustrates the construction of the relay mechanism used in the device of the
present invention;
Figure 3 illustrates the configuration of the 1st circuit contained in the device of the
present invention;
Figure 4 illustrates the device used for generating reference voltage VRef-1; and
Figure 5 illustrates an alternative construction of 2nd circuit that can be contained in the
device of the present invention.
Skilled artisans will appreciate that elements in the drawings are illustrated for simplicity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the drawings may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
Detailed Description of the Preferred Embodiments:
Before describing in detail embodiments that are in accordance with the present invention, it should be observed that the embodiments reside in providing a device that effectively detects an air/fuel ratio of a fuel mixture being supplied to an engine of a

motor vehicle fitted with a LPG/CNG kit by probing exhaust gas resulting from combustion of the air fuel mixture. Although in the following paragraphs the Inventors have used the terminology CNG while describing the detailed working of the invention, it may be understood that the device of the present invention would work equally efficiently when a vehicle is fitted with a LPG kit.
More particularly, the present invention provides a device for detecting an air/fuel ratio of an air-CNG mixture and an air-petrol mixture being supplied to an engine of a motor vehicle. The device of the present invention utilizes one (or one set) of A/F sensor(s) to detect the air/fuel ratio of the air-CNG mixture and the air-petrol mixture being supplied to the engine of the motor vehicle.
Accordingly, the device has been represented by appropriate conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having benefit of the description herein.
The terms "comprises", "comprising", or any other variations thereof, are intended to cover a non-exclusive inclusion, such that a device or system or apparatus that comprises a list of parts or components does not include only those parts or components but may include other parts/components expressly listed or inherent to such device or system or apparatus. Simply put, one or more elements in a system or apparatus proceeded by "comprises... a" does not, without more constraints, preclude the existence of other elements or additional elements in the system or apparatus.
In an embodiment, as illustrated in figure 1, the present invention provides a device (100) for detecting an air/fuel ratio of air-fuel mixture being supplied to an engine (not depicted) of a motor vehicle (not depicted) fitted with a LPG/CNG kit (not depicted) by probing exhaust gas resulting from combustion of the air-fuel mixture, said device comprising:

an A/F sensor (10) located in the motor vehicle for providing an output in the form of a
change in current in relation to the amount of oxygen in the exhaust stream;
a first circuit (30) for receiving the output of the A/F sensor (10) as input and taking a
first action to detect air/fuel ratio based on the same;
a second circuit (40) for receiving the output of the A/F sensor (10) as input and taking a
second action to detect air/fuel ratio based on the same; and
a relay mechanism (20) for providing the output of the A/F sensor (10) to the said first
circuit (30) in presence of a change over signal and to the second circuit (40) in the
absence of the change over signal.
With reference to figure 2, it can be observed that the relay mechanism (20) is configured to generate a false signal to be provided to the second circuit (40) in presence of the change over signal and similarly, generate a false signal to be provided to the first circuit (30) in the absence of the change over signal.
More particularly, the false signal thus produced by the relay mechanism and provided to the second circuit (40) corresponds to output produced by the A/F sensor (10) for a stoichiometric air-fuel mixture. Also, the false signal thus produced by the relay mechanism and provided to the first circuit (30) corresponds to output produced by the A/F sensor (10) for a stoichiometric air-fuel mixture.
As it is conventionally known, a vehicle operating on CNG and Petrol comprises a change over mechanism which enables the driver to switch from one type of fuel (petrol) to the other type of fuel (CNG) and vice-versa. In some instances, the change over mechanism may be configured to automatically enable change over from petrol to CNG (for example, after a predetermined amount of time after the vehicle is started) and from CNG to Petrol (for example, when the CNG level has fallen below a predetermined amount). When such a change over occurs, the change over mechanism is configured to generate a change over signal. It can be observed from figures 1 and 2 that in presence of the change over signal and the ignition "ON" signal (represented by the symbol "IGN" in figure 2), the relay mechanism (RL or 20) breaks contact. Thus, current is forced to flow through the shunt resistor Rsh1 which forms part of the first circuit. On the other hand, in

the absence of the change over signal, the relay mechanism maintains the contact, due to which the current flows through the shunt resistance RSh2 which forms part of the second circuit. Thus, it can be observed that when the relay mechanism breaks contact (i.e. in presence of the change over signal), no electrical current flows through the second circuit and more particularly through the shunt resistance (RSh2) and hence, it effectively produces a false signal which the A/F sensor produces upon receipt of stoichiometric air-fuel mixture. Similarly, in the absence of the change over signal, the relay mechanism is configured to disallow current flow through the first circuit and more particularly through the shunt resistance (RSh1) and hence, it effectively produces a false signal which the A/F sensor produces upon receipt of stoichiometric air-fuel mixture.
It can be observed from figures 1 and 3 that the first circuit (30) is configured to detect air/fuel ratio of a fuel mixture comprising air-compressed natural gas (CNG). The first circuit (30) comprises a shunt resistor (RSh1) located in an electrical path between a positive and negative terminal of the A/F sensor (10) for enabling voltages V1 and V2 to be sensed and a comparator (C1) for comparing the difference between the voltages V1
and V2 thus sensed and a generate a signal indicative of the air/fuel ratio of air-CNG mixture being supplied to the engine of the motor vehicle.
In a furthermore embodiment of the present invention, the voltage V1 is added to a first reference voltage VREF1 and provided to the comparator (C1) and the voltage V2 is added to a second reference voltage VREF2 and provided to the comparator.
In another embodiment of the present invention, a value of the reference voltage VREFI is in the range of 3.69 to 3.71 and a value of the reference voltage VREF2 is in the range of 3.24 to 3.26. A sample circuit which can be used for producing the reference voltage VREFI is shown in figure 4. A similar kind of circuit with appropriate values for the resistors could be used for producing the reference voltage VREF2-
In yet another embodiment of the present invention, the signal thus generated by the comparator (C1) is clipped (by for example, diodes D1 and D2) so as to maintain the maximum value below 1 volts and the minimum value above 0.1 volts.

In still another embodiment of the present invention, the signal indicative of the air/fuel ratio of air-CNG mixture thus generated by the first circuit (30) is a varying voltage signal based on the amount of oxygen in the exhaust stream.
In a further embodiment of the present invention, amplitude of the voltage signal thus generated by the first circuit (30) is in the range of 0.6 to 1.0 volts in respect of a rich air-CNG mixture, 0.4 to 0.1 volts in respect of a lean air-CNG mixture and approximately 0.45 volts in respect of stoichiometric air-CNG mixture.
It can be observed from figure 1 that the second circuit (40) is configured to detect air/fuel ratio of a fuel mixture comprising air-petrol.
In another embodiment of the present invention, the signal indicative of the air/fuel ratio of air-petrol mixture thus generated by the second circuit (40) is a varying voltage signal based on the amount of oxygen in the exhaust stream.
In yet another embodiment of the present invention, amplitude of the voltage signal thus generated by the second circuit (40) is below 3.3 volts in respect of a rich air-petrol mixture, above 3.3 volts in respect of a lean air-petrol mixture and approximately 3.3 volts in respect of stoichiometric air-petrol mixture.
In an embodiment of the present invention and as depicted in figure 1, the second circuit (40) comprises a shunt resistor (RSh2) located in an electrical path between a positive and negative terminal of the A/F sensor (10) for enabling voltages V3 and V4 to be sensed and a comparator (C2) for comparing the difference between the voltages V3 and V4 thus sensed and a generate a signal indicative of the air/fuel ratio of air-petrol mixture being supplied to the engine of the motor vehicle. The voltage V3 is added to a third reference voltage VREF3 and provided to the comparator (C2) and the voltage V2 is added to a forth reference voltage VREF4 and provided to the comparator (C2). The value of the reference voltage VREF3 is in the range of 6.29 to 6.31 and a value of the reference voltage VREF4 is in the range of 2.99 to 3.01. The sample circuit shown in figure 4 with appropriate values for the resistors could be used for producing the reference voltages VREF3 and VREF4.

Some of the alternate constructions of the second circuit are shown in U.S. Patent No. 6,844,734, one of which is shown in figure 5.
Although not specifically depicted, it can be observed that the output of the first circuit shown in figure 1 can be taken and provided to an appropriate A/D converter. The digital output of the A/D converter can be provided to a control mechanism whose construction could be similar to the Closed Loop Fuel Feedback Control System that operates on the basis of an output from an oxygen sensor. Also, the output of the second circuit (either as shown in figure 1 or as shown in figure 5 or any other alternative one) can be taken and provided to an appropriate A/D converter. The digital output of the A/D converter can be provided to a control mechanism that operates on the basis of an output from an A/F sensor.
Thus, the present invention also teaches a system for controlling an air/fuel ratio of fuel mixture being combusted in an engine of a motor vehicle fitted with a CNG kit, said system comprising: a device for detecting an air/fuel ratio of air-fuel mixture being supplied to the engine by probing exhaust gas resulting from combustion of air-fuel mixture; a first controlling circuit for controlling air/fuel ratio of fuel mixture being combusted in an engine based on the air/fuel ratio detected by the first circuit; and a second controlling circuit for controlling air/fuel ratio of fuel mixture being combusted in an engine based on the air/fuel ratio detected by the second circuit. The device for detecting the air/fuel ratio of the air-fuel mixture is constructed in accordance with the aforesaid teachings.
The present invention provides a device for detecting an air/fuel ratio of a fuel mixture being supplied to an engine of a motor vehicle fitted with a LPG/CNG kit by probing exhaust gas resulting from combustion of the air fuel mixture, said device comprising an oxygen sensor located in the motor vehicle providing an output in the form of a change in voltage in relation to the amount of oxygen in the exhaust stream, a relay mechanism receiving the output of the oxygen sensor and providing the same to a first circuit in

presence of a change over signal and to a second circuit in the absence of the change over signal.
Although not illustrated, the present invention also provides a device (100) for detecting
an air/fuel ratio of air-fuel mixture being supplied to an engine (not depicted) of a motor
vehicle (not depicted) fitted with a LPG/CNG kit (not depicted) by probing exhaust gas
resulting from combustion of the air-fuel mixture, said device comprising:
an oxygen sensor located in the motor vehicle for providing an output in the form of a
change in voltage in relation to the amount of oxygen in the exhaust stream;
a first circuit for receiving the output of the oxygen sensor as input and taking a first
action to detect air/fuel ratio based on the same;
a second circuit for receiving the output of the oxygen sensor as input and taking a
second action to detect air/fuel ratio based on the same; and
a relay mechanism for providing the output of the oxygen sensor to the said first circuit in
presence of a change over signal and to the second circuit in the absence of the change
over signal.

WE CLAIM:
1. A device for detecting an air/fuel ratio of air-fuel mixture being supplied to an
engine of a motor vehicle fitted with a CNG kit by probing exhaust gas resulting
from combustion of the air-fuel mixture, said device comprising:
an A/F sensor located in the motor vehicle for providing an output in the form of a change in current in relation to the amount of oxygen in the exhaust stream;
a first circuit for receiving the output of the A/F sensor as input and taking a first action to detect air/fuel ratio based on the same;
a second circuit for receiving the output of the A/F sensor as input and taking a second action to detect air/fuel ratio based on the same; and
a relay mechanism for providing the output of the A/F sensor to the said first circuit in presence of a change over signal and to a second circuit in the absence of the change over signal.
2. The device as claimed in claim 1, wherein the relay mechanism is further configured to generate a false signal to be provided to the second circuit in presence of the change over signal.
3. The device as claimed in claim 1, wherein the relay mechanism is further configured to generate a false signal to be provided to the first circuit in the absence of the change over signal.
4. The device as claimed in claim 2, wherein the false signal provided to the second circuit corresponds to output produced by the A/F sensor for a stoichiometric air-fuel mixture.
5. The device as claimed in claim 3, wherein the false signal provided to the first circuit corresponds to output produced by the A/F sensor for a stoichiometric air-fuel mixture.
6. The device as claimed in claim 1, wherein the first circuit is configured to detect air/fuel ratio of a fuel mixture comprising air-compressed natural gas (CNG), and

the first circuit comprises oa shunt resistor located in an electrical path between a positive and negative terminal of the A/F sensor for enabling voltages V1 and V2 to be sensed and a comparator circuit for comparing the difference between the voltages V1 and V2 thus sensed and a generate a signal indicative of the air/fuel ratio of air-CNG mixture being supplied to the engine of the motor vehicle.
7. The device as claimed in claim 6, wherein the voltage V1 is added to a first reference voltage VREFI and provided to the comparator and the voltage V2 is added to a second reference voltage VREF2 and provided to the comparator.
8. The device as claimed in claim 7, wherein the value of the reference voltage VREFI is in the range of 3.69 to 3.71 and a value of the reference voltage VREF2 is in the range of 3.24 to 3.26.
9. The device as claimed in claim 6, wherein the signal thus generated by the comparator is clipped so as to maintain the maximum value below 1 volts and the minimum value above 0.1 volts.
10. The device as claimed in claim 6, wherein the signal indicative of the air/fuel ratio of air-CNG mixture thus generated by the first circuit is a varying voltage signal based on the amount of oxygen in the exhaust stream.
11. The device as claimed in claim 10, wherein amplitude of the voltage signal thus generated by the first circuit is in the range of 0.6 to 1.0 volts in respect of a rich air-CNG mixture, 0.4 to 0.1 volts in respect of a lean air-CNG mixture and approximately 0.45 volts in respect of stoichiometric air-CNG mixture.
12. The device as claimed in claim 1, wherein the second circuit is configured to detect air/fuel ratio of a fuel mixture comprising air-petrol.
13. The device as claimed in claim 12, wherein the signal indicative of the air/fuel ratio of air-petrol mixture thus generated by the second circuit is a varying voltage signal based on the amount of oxygen in the exhaust stream.

14. The device as claimed in claim 13, wherein amplitude of the voltage signal thus generated by the second circuit is below 3.3 volts in respect of a rich air-petrol mixture, above 3.3 volts in respect of a lean air-petrol mixture and approximately 3.3 volts in respect of stoichiometric air-petrol mixture.
15. A device for detecting an air/fuel ratio of a fuel mixture being supplied to an engine of a motor vehicle retrofitted with a CNG kit by probing exhaust gas resulting from combustion of the air fuel mixture, said device comprising:
an A/F sensor located in the motor vehicle providing an output in the form of a change in current in relation to the amount of oxygen in the exhaust stream; and
a relay mechanism receiving the output of the A/F sensor and providing the same to a first circuit in presence of a change over signal and to a second circuit in the absence of the change over signal.
16. A system for controlling an air/fuel ratio of fuel mixture being combusted in an
engine of a motor vehicle fitted with a CNG kit, said system comprising:
a device for detecting an air/fuel ratio of air-fuel mixture being supplied to the engine by probing exhaust gas resulting from combustion of air-fuel mixture;
a first controlling circuit for controlling air/fuel ratio of fuel mixture being combusted in an engine based on the air/fuel ratio detected by the first circuit; and
a second controlling circuit for controlling air/fuel ratio of fuel mixture being combusted in an engine based on the air/fuel ratio detected by the second circuit;
wherein the device for detecting an air/fuel ratio of air-fuel mixture being supplied to the engine comprises:
an A/F sensor located in the motor vehicle for providing an output in the form of a change in current in relation to the amount of oxygen in the exhaust stream,
a first circuit for receiving the output of the A/F sensor as input and taking a first action to detect air/fuel ratio based on the same;
a second circuit for receiving the output of the A/F sensor as input and taking a second action to detect air/fuel ratio based on the same; and

a relay mechanism for providing the output of the A/F sensor to the said first circuit in presence of a change over signal and to a second circuit in the absence of the change over signal.