HK1068409A - Automatic calibration mode for carbon dioxide sensor - Google Patents

Description

Automatic calibration mode of carbon dioxide sensor

Priority

The present application claims priority from U.S. patent application entitled "carbon dioxide sensor automatic calibration mode" filed on 27/4/2001 and having application number 09/842,622, the disclosure of which is incorporated herein by reference. This application relates to an application entitled "method of detecting ventilation of a burner in an undesirable space" filed on 7.1.1998 and having application number 09/004,142, the disclosure of which is incorporated herein by reference.

Technical Field

The present invention relates generally to the field of sensor technology. More particularly, the present invention relates to the development of various Infrared (IR) gas sensor application technologies related to carbon dioxide sensing, particularly to measurement/control of Exhaust Gas Recirculation (EGR) of diesel engines.

Background

In order to obtain any kind of information required, measurements of physical parameters have to be made. Devices that allow these measurements are broadly categorized as sensors. The term "sensor" encompasses a wide range of technologies and devices that respond to physical stimuli (i.e., light, heat, sound, pressure, magnetism, or specifically motion) and transmit the resulting pulses, typically for measurement or manipulation.

Sensors are widely used in many different applications. Some may be as simple as thermocouple direct measurements or as complex as all-weather imaging systems. No matter how complex the sensor is, its interaction with the physical environment produces a signal that ultimately produces the desired information.

In various instances, sensor technology has become the basic starting technology (enablinggttechnology). The large number of applications has prompted a rapid increase in interest in sensors, for example, the analysis of selected compounds in blood, where sensors may be of great public interest.

In addition, sensors are particularly important in safety-related areas, ranging from evaluation of aircraft integrity to monitoring of fire safety. Common research and techniques in these different applications include interpretation of spectral signals of quantities of interest, such as concentration, temperature or thermal properties.

For example, market demand for gas measurement platforms that measure carbon dioxide concentration levels is promoting an increase in carbon dioxide technology activity as it is applied in part to understanding and detecting ventilation and Indoor Air Quality (IAQ). Regulations and standards governing building ventilation, such as ASHRAE (American Society of Heating Air Conditioning and refrigeration Engineers) standards 62-99, have established minimum volumes based on each person's outdoor Air demand.

One of its applications is to use this parameter to detect population numbers, since each person typically exhales a predictable amount of carbon dioxide. An increase or decrease in carbon dioxide concentration may indicate ingress or egress to an indoor area.

In addition, because the outdoor carbon dioxide concentration is very low and remains constant, indoor measurements can also provide a dynamic measure of the population of the indoor, and the amount of low concentration outdoor air to be introduced to dilute the contaminant concentration. Thus, carbon dioxide measurements in a space can be used to detect and control the rate of ventilation of each person in the space.

Thus, although carbon dioxide is not a direct measure of indoor air quality, it has the potential to be an excellent measure of effective ventilation (mechanical ventilation plus infiltration). Generally, the higher the carbon dioxide concentration, the lower the ventilation. In other words, when the indoor carbon dioxide concentration is very high (i.e., above 1800ppm) and the ventilation is low (below 7 cfm/person), these conditions can cause the pollutants to accumulate, causing irritation and discomfort.

The apparatus for determining the level of carbon dioxide concentration is a powerful device that can be used in home, office, school and other commercial settings. However, its viable applications are limited by manufacturing and other costs, as well as health, safety, quality, and other issues.

For example, in health and safety applications, oxygen sensors have been used to detect the consumption of oxygen. However, oxygen sensors are not only expensive, but often require periodic replacement or recalibration. Therefore, there is a need for an inexpensive, alternative method of detecting oxygen consumption.

In the automotive industry, there is also an increasing demand for carbon dioxide sensor technology that improves the quality, safety and comfort of automobiles. For example, it is well known that the concentration of carbon dioxide in the combustion air entering the engine can be used to determine the amount of exhaust gas in the combustion air that is recycled to the engine. This is because the concentration of carbon dioxide in the engine exhaust is much higher than the concentration of carbon dioxide in the atmosphere (i.e., 9% versus 350-550 ppm).

However, conventional sensing methods for various gases in an engine employ in situ sensors that are directly exposed to the gas stream to be detected. Exposing these types of sensors to harsh engine environments, particularly high temperatures, can impair the quality and results of the detection. Thus, there is a need for an alternative detection method to determine the concentration of carbon dioxide that can withstand the harsh environment of the engine while obtaining accurate measurements.

Equally important to drivers in the automotive industry is the installation of sensors in automotive products, which helps to prolong human life and improve safety. For example, it is desirable to detect the presence of a person in the trunk of a vehicle to avoid inadvertent or accidental closure causing death of the person.

In the field of sensor recalibration, there is an increasing demand for sensors having the features of automatic calibration mode performance, which have a fast recalibration time and can provide stable, error-free readings.

Therefore, there is a need for an inexpensive sensor technology control device for use as an indicator of a carbon dioxide concentration level characteristic. In addition, there is also a need for a control device that is suitable (i.e., can be standardized) for many different applications.

Disclosure of Invention

The above needs and others are met, to a great extent, by the present invention, which includes a highly reliable method of determining the concentration level of a gas, such as carbon dioxide.

More specifically, implementations of the present invention utilize a gas measurement criterion based on optically measuring the rate of change of carbon dioxide concentration and its deviation. Because of the inert nature of carbon dioxide, optical methods are the most accurate and reliable methods for measuring carbon dioxide, and carbon dioxide rarely reacts chemically with other gases, making it difficult to measure reliably with sensors that rely on physical or chemical reactions.

In one aspect, the invention employs a method of measuring oxygen consumption that utilizes the rate of change of carbon dioxide as a surrogate indicator of oxygen consumption or oxygen displacement in the air. Oxygen consumption can be measured in one of two examples.

In the first instance, if oxygen is replaced by carbon dioxide, the natural result is that the carbon dioxide must be raised to a very high concentration to replace a significant amount of oxygen (e.g., greater than 30,000ppm or 3% CO)₂)。

Conversely, in the second example, if oxygen is replaced by another gas, the result is that the concentration of carbon dioxide will eventually decrease to normal atmospheric levels below 350-450 ppm. For example, if the rate of decrease in carbon dioxide concentration drops below 300ppm in 24 hours or less, then the decrease could reasonably indicate oxygen consumption and trigger a warning or control.

Given a known spatial volume, a more accurate control level can be established. Likewise, the rate of change of carbon dioxide may also be utilized if it exceeds the normal rate of generation by the occupant that can be expected.

In another aspect, the present invention provides a remote carbon dioxide sensor in an automobile or diesel engine to measure and control Exhaust Gas Recirculation (EGR) to the diesel engine. With ERG technology, the emissions of certain pollutants, such as Nitrogen Oxides (NOX), can be reduced to meet EPA or other environmental requirements.

In order to maintain optimum operating conditions of the engine while reducing emissions of NOX and the like, the ratio of exhaust gas recirculated into the engine intake to external fresh air introduced into the engine must be relatively constant. Maintaining this ratio can be difficult because engine operating speeds and corresponding engine combustion air demands are constantly changing.

This ratio may vary with the design of the engine, but the ratio of EGR to fresh air is typically about 20-25%. Since the outside air has a very low concentration and the engine exhaust has a very high concentration (i.e., 9-12% by volume), the concentration of carbon dioxide in the plenum may provide an indication of the mix ratio of outside air and EGR and may be used to maintain the proper EGR ratio. The method employs a sampling method to determine the concentration of carbon dioxide.

In this method, a sampling conduit is installed in the pre-combustion mixing chamber of the engine and directs the gas to a remote carbon dioxide sensor. The chamber is the area where the exhaust gas mixes with the outside air before entering the engine for combustion. The chamber may be internal to the engine or may be attached to the engine as part of an accessory.

The sampling conduit is long enough to allow the gas sample to be additionally cooled, thereby bringing the temperature of the sampled gas below 50 degrees celsius, but not so long as to significantly delay the response time. Gas sampling of the pre-combustion gas mixing chamber can be accomplished by a pressure differential of the chamber (e.g., 1 atmosphere or greater) with ambient pressure of the engine.

Optionally and alternatively, the sampling method can remove particles for control, which may require filtration, if desired. In addition, reducing the temperature of the sample allows for less complexity and cheaper carbon dioxide sensor technology to be employed, as components and calibrations that run in high temperature environments are eliminated.

This aspect of the invention also has another feature and advantage if conditional sampling is employed, as it makes the measurement of other gases, such as NOX, easier to measure optically at temperatures below 50 degrees celsius.

In a third aspect, the present invention uses the rate of change of the carbon dioxide concentration level to indicate the presence of a person in the trunk of a vehicle. There are several other sources of carbon dioxide that must be considered as these sources may affect the accurate carbon dioxide sensor readings. Two major sources are: (1) carbon dioxide exhaust gas leaking into the trunk of the automobile; and (2) carbon dioxide deliberately injected into the trunk of the vehicle for the purpose of deliberately activating the sensor; perhaps to automatically open the trunk of the car.

To prevent both of the above situations from occurring, it is preferred that the configuration of the carbon dioxide sensor of the present invention and its alarm/control logic identify the rate of change of carbon dioxide when one or more persons are trapped in the trunk of the vehicle.

The amount of carbon dioxide produced by a person is a function of the amount of activity and the size of the person's body. Thus, the action/control point in the control logic may be calculated based on rate of change data, including, for example, a possible range of carbon dioxide production rates for a given area or activity level. The effect/control point in the control logic may also be calculated based on the rate of change data, including, for example, the volume of the trunk of the vehicle; an assumed gas leakage rate of the car trunk; and age of the occupant.

Preferably, these calculations provide a range of rates of change of carbon dioxide within which the occupant's presence in the trunk of the vehicle can be determined with reasonable accuracy. If the measured rate of change in the trunk of the vehicle falls within the calculated range, the desired control or alarm scheme may be activated. Control or alarm schemes may include indicator lights, buzzers, opening of the trunk of the car, flashing lights, sounding horns, etc. If a suspected carbon dioxide concentration is detected that is outside a desired or predetermined detection range, an alternative indicator may be activated.

In a fourth aspect, the present invention provides an automatic calibration mode for a carbon dioxide sensor. Calibration is based on the use of zero gas (zero gas) for calibration. The method can be used for other infrared sensors besides carbon dioxide sensors.

Assuming that the carbon dioxide concentration in the atmosphere is generally 350ppm or more and the carbon dioxide concentration in the space is gradually changed, the carbon dioxide sensor according to this aspect of the invention is designed to recognize a unique carbon dioxide concentration change rate, which represents a zero point calibration procedure. Once this unique pattern is verified, the sensor is triggered to enter calibration mode and reset its calibration based on the carbon dioxide concentration to be measured in calibration mode.

In a preferred embodiment, the unique pattern may comprise a significant drop in sensor readings of 200-300ppm or greater in about 15 to 30 seconds. Another unique pattern may include a steady reading over the next 30 seconds, which indicates a constant flow of calibration gas to the sensor. Optionally, the modality activates a calibration mode of the sensor. As long as the carbon dioxide concentration remains constant during calibration (e.g., 1 to 5 minutes), the sensor itself can be calibrated to zero based on the same gas being measured.

There has thus been outlined, rather broadly, the more important features of the invention in order that the detailed description thereof that follows may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will form the subject matter of the claims appended hereto.

In this respect, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein, as well as the abstract, are for the purpose of description and should not be regarded as limiting.

As such, those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention.

Drawings

FIG. 1 is a graph of carbon dioxide concentration versus time for a rate of change method according to the present invention.

FIG. 2 is a flow diagram of decision logic for determining whether a person is present in a space in accordance with an aspect of the present invention.

FIG. 3 is a logic flow diagram of the decision to determine oxygen consumption conditions using the bias of the rate of change method of the simulation model of the present invention.

FIG. 4 is a flow chart of logic for determining an automatic calibration mode for a carbon dioxide sensor in accordance with another aspect of the present invention.

FIG. 5 is a perspective view of an engine compartment employing a carbon dioxide sensor to measure and control recirculation of exhaust gas to a diesel engine in accordance with another aspect of the present invention.

Detailed Description

Carbon dioxide is present in the atmosphere, but is typically present at very low concentrations, i.e., from 350ppm to 450 ppm. Humans are the major contributors to carbon dioxide in buildings, and if ventilation in a space is poor, humans can contribute sufficient carbon dioxide to raise its concentration up to 3000 ppm. The concentration level of carbon dioxide may be higher, but such high concentrations are not normal under normal circumstances.

Certain characteristics of the carbon dioxide concentration in the space may be indicative of health-and/or safety-hazardous conditions. These features include the achievement of an absolute threshold level, a significant increase in carbon dioxide concentration occurring with another activity, or a rate of change of carbon dioxide concentration over a period of time.

The gas measurement technology of the invention adopts the concept of change rate to measure the concentration of gas in space, and can be applied to various different actual lives. This rationale is always valid whether rate of change methods are used as the only standard or a combination of rate of change and other measurement techniques.

One aspect of the invention is to use the rate of change as the only criterion, exemplified in the home. For example, for most of the average population living in a residence, simulation studies have shown that human contributed carbon dioxide concentrations will result in carbon dioxide concentration change rates of less than 5ppm per minute.

In contrast, similar simulation studies have shown that if smoke from the furnace stack (typically containing 90,000 to 120,000ppm of carbon dioxide) is discharged directly into the building, it will contribute 20-100 ppm or more of carbon dioxide concentration rise per minute. Thus, depending on the location of the measurement and the volume of the enclosed space, the rate of rise of the carbon dioxide concentration is an accurate indication of the presence of combustion fumes or the like.

It is noted that a controlled combustion process, such as a furnace, will provide a relatively stable rate of change of carbon dioxide concentration during operation. This is in contrast to uncontrolled combustion, which steadily increases the rate of change of carbon dioxide concentration due to the increase in the area of uncontrolled combustion.

Depending on the volume of the space, the rate of rise of the carbon dioxide concentration may also accurately indicate a combustion leak. For example, a carbon dioxide concentration increase of more than 30 to 50ppm per minute indicates leakage of furnace or fireplace combustion products into the living space of the house. Such measurements can typically be made in the living space of a house, in a furnace room, in a utility room, or in a garage beside a house. Alternatively and optionally, a carbon dioxide concentration rise rate threshold of about 25ppm per minute may be used as a criterion to indicate improper venting.

According to a first aspect of the invention, the rate of change of the carbon dioxide concentration is used as an accurate criterion for indicating the presence of one or more persons in the space. The criteria may also be used to detect the approximate number of people contained in the closed container, with potential stowaway personnel. In addition, if the space is the trunk of a car, it is another advantageous application of the invention to avoid death of people trapped in the trunk of the car by mistake or other reasons.

More specifically, to characterize the rate of change of carbon dioxide concentration, the present application derives a simulation model that takes into account a number of factors in calculating the rate of change of carbon dioxide over a period of time, including the volume of the space, the air leakage from the space, and, if any, other sources of carbon dioxide that may enter the space.

Another set of factors considered by the simulation model is the human carbon dioxide production rate. The rate of carbon dioxide production in a person is related to the size, including age, and activity level of the person. For example, an adult (i.e., 14-65 years of age) typically produces carbon dioxide fixed at about 0.25 liters per minute in a resting mode. When a person is hidden in a narrow space, the carbon dioxide production rate may be in this range. For a person locked in the trunk of a car, the activity level will be higher and therefore the production rate will be in the range of 0.5 to 1.5 litres per minute.

The increase in carbon dioxide over time is taken into account in the calculation of the rate of change method employed in the simulation model of the present invention, for example: (1) the number of people in the space, and their expected age and activity level; (2) a volume of space; and (3) any natural or mechanical ventilation method that can dilute the space concentration with outside air. These relationships follow the following equations:

C_t＝[(C_t-1+N/V-C_oA)(1-A/V)+C_OA

in the formula:

C_tcarbon dioxide concentration at time t (ppm);

C_t-1carbon dioxide concentration (ppm) at the previous time;

C_OAcarbon dioxide concentration in the outside air (ppm);

n-carbon dioxide production rate (cfm) of the occupant;

v ═ volume in space (cu.ft.);

a is the volume of dilution air entering the space.

Using the above equation, a plot of the rate of change for a series of possible/desired situations, including the presence of one or more people in space, can be obtained. When the rate of change data for one or all of the above scenarios is determined, a range of carbon dioxide concentration values may be given to indicate the likely level of carbon dioxide change in a given trunk or enclosure of the vehicle.

For example, the range may be obtained by calculating the lower and upper limits of the carbon dioxide concentration with respect to changes in volume of space, activity level, ambient carbon dioxide concentration and/or ventilation. The carbon dioxide concentration values were plotted against time to obtain FIG. 1. The region between the upper and lower threshold values is described as an alarm region, wherein one or more alarm schemes are activated when the carbon dioxide concentration level falls within the region.

Referring now to FIG. 2, a decision logic flow diagram is depicted illustrating a process for determining the presence of a person in space using the simulation model rate of change method of the present invention.

As discussed previously, the process begins with the determination of a recognized leak condition in the space (step 20 or S20). In one embodiment, the space comprises a trunk of an automobile. In another embodiment, the space may include a container or the like. For example, if the space is the trunk of an automobile, then a leak of carbon dioxide exhaust gas into the trunk of the automobile is determined.

Once a "normal" leak is measured, the next step is to measure or calculate all other suitable carbon dioxide leaks into the space (S22). For example, other suitable leakage situations may include intentional injection of carbon dioxide into the trunk of the vehicle, as carbon dioxide may be intentionally injected to activate the sensor to open the trunk of the vehicle. Other such possible situations should also be determined.

To avoid these and other possible situations from occurring or to prevent unnecessary false alarms due to these possible situations, it is preferred that the warning/control logic is configured to include these possible and/or undesirable situations. Essentially, the warning/control logic is installed to identify a rate of change of carbon dioxide concentration representative of one or more trigger values (S24), such as when one or more persons are trapped in the trunk or enclosed space of the vehicle.

In a preferred embodiment, the identifying means is implemented using one or more action/control points calculated from the rate of change data. The rate of change data includes calculations based on, for example, car trunk volume information; the assumption of the air leakage rate of the automobile trunk; carbon dioxide production rate (e.g., breath data per minute); a possible range of carbon dioxide production rates assuming a range or activity level (e.g., low-high activity level expiration/minute data); and age of the occupant (e.g., adult-child).

These calculations preferably provide a carbon dioxide rate of change criterion for determining with reasonable accuracy the rate of change of carbon dioxide in the space (S26) and ultimately determining whether a person is present in the trunk or enclosed space of the vehicle.

If the carbon dioxide concentration change rate level in the space falls outside the calculated range (i.e., the trigger range) (S28), the process is repeated. However, if the rate of change of carbon dioxide concentration falls within the trigger range, one or more control or alarm schemes are activated (S30).

In a preferred embodiment, an alarm is activated if a rate of change of carbon dioxide of at least about 50ppm per minute is detected in the trunk or enclosed space of the automobile. The alarm may be an indicator light.

The rate of change trigger value required to generate the alarm signal within the required time period is variable and closely related to the volume of space. For example, a control or alarm scheme may be used such that an alarm is not triggered when only the rate of change of carbon dioxide is detected, but is activated once the rate of change of carbon dioxide is detected for a plurality of consecutive time periods. For example, the possibility of false alarms may be further reduced by not triggering an alarm signal unless the rate of change of carbon dioxide is detected for two or more or even more (e.g., five or more) time periods.

Alternatively and optionally, different alarm signals are generated for different trigger values. For example, if a rate of change greater than about 5ppm per minute is detected, a primary alarm signal is generated; generating a secondary alarm signal if a rate of change greater than about 10ppm per minute is detected; if a rate of change greater than about 25ppm per minute is detected, a three-level alarm signal is generated. Each alarm signal can adopt the forms of a buzzer, opening a car trunk, flashing light, sounding a horn and the like.

Audible alarm signals are also valuable in explaining why an alarm signal is generated. For example, if a suspected carbon dioxide rate of change value is detected outside the detection range of the trunk of the vehicle, a very low-sound alarm signal is activated indicating that a low carbon dioxide concentration is present.

In another aspect of the present invention, the rate of change concept described above is also used to indicate the condition of oxygen consumption. Reference is now made to FIG. 3, which is a flow chart of a process for detecting oxygen consumption using the bias of the rate of change method of the present invention. The concept herein is to employ a rate of change of carbon dioxide concentration that is raised above or lowered below the indicated oxygen displacement level.

The process begins with the determination of all relevant carbon dioxide leaks in the space (S40). The space includes various closed spaces such as a room in a house, an office or commercial building, a concert hall, and the like. The related leakage cases are user-definable and situation-definable.

The next step is to determine the level of the rate of change in carbon dioxide concentration indicative of oxygen substitution (S42). For example, if oxygen is replaced by carbon dioxide, the carbon dioxide must be raised to a very high concentration (e.g., > 30,000ppm) to visibly indicate that a significant amount of oxygen has been replaced.

Conversely, it is alternatively preferred that the carbon dioxide concentration if reduced at a rate below the normal atmospheric level of 350 to 450ppm is indicative of oxygen being replaced by other gases. Any consistent and sustained reduction in concentration below these levels indicates that carbon dioxide is displaced. In this case, carbon dioxide serves as a surrogate indicator of the amount of oxygen consumed or displaced in the space.

The next step is to determine the rate of change of the carbon dioxide concentration in the space (S44). It should be noted that the above steps need not be performed strictly in the order discussed, as long as the various determinations are performed before step 46(S46), step 46 being to query whether the rate of change of the carbon dioxide concentration in the space is above or below the level of the rate of change of carbon dioxide.

An alert/control logic may also be configured to identify a carbon dioxide rate of change range or a threshold level indicative of oxygen substitution. The rate of change data may include the following: information of the volume of space; assumption of air leakage rate; carbon dioxide production rate assuming a range or activity level; and age of the occupant.

Also, as previously discussed, upon detection of an undesired rise or fall, a warning or control alarm is optionally activated (S48). The warning or alarm may be in the form of a visual or audible signal.

For known spatial volume information, a more accurate control level can be established. The spatial volume is a key factor in the rate of change calculation. Likewise, the rate of change of carbon dioxide concentration may also be used if it exceeds the normal rate of change produced by humans.

In yet another aspect, the present invention provides an automatic calibration mode for a carbon dioxide sensor. Automatic calibration of carbon dioxide or other infrared sensors is based on calibration using a zeroing gas. Referring to fig. 4, the concept is to inject a zero-setting gas into the sensor (S52) and look for a significant drop in carbon dioxide concentration (S54) below the given or desired operating baseline of the sensor (S50) (atmospheric carbon dioxide concentrations are generally unlikely to be below 350 ppm). Preferably, the zeroing gas is not the gas normally tested by the sensor, such as nitrogen.

The significant drop can be determined as desired. In a preferred embodiment, the decrease may be determined as a decrease of 200ppm or more over a period of 5 minutes. In other words, if the drop exceeds 200ppm within the specified or desired time (S56), the sensor is configured with a microprocessor-driven switch that is closed to automatically recalibrate the sensor. Essentially, a significant drop in the sensor reading within 5 minutes would indicate a zeroed gas, and the sensor would recalibrate the reading as a zero point.

In another embodiment, the significant drop can be provided as a sensor reading in the range of about 200 to 300ppm or greater in about 15 to 30 seconds. Alternatively and optionally, the trigger condition may include the reading stabilizing within the following 30 seconds, indicating a constant flow of calibration gas to the sensor. This way the calibration mode of the sensor can optionally be activated. If the carbon dioxide concentration remains stable during calibration (e.g., 1-5 minutes), the sensor itself can be recalibrated to zero based on the same gas to be measured.

A diagnostic procedure may also optionally be performed to determine the stability of the reading over a period of time, such as 1 minute. Preferably, the deviation in measurement readings should not exceed the background noise of the sensor.

In another aspect of the present invention, the rate of change of carbon dioxide concentration can be measured in the cycle gas of the engine pre-combustion gas mixing chamber 10. Fig. 5 illustrates a simplified gas mixing chamber 10 that includes those components that facilitate the practice of the present invention.

A sampling duct 12 is mounted in the pre-chamber 10 of the engine, leading gas from the pre-chamber 10. In the air mixing chamber 10, the exhaust gas and the outside air are mixed before being introduced into an engine (not shown) for combustion. Also, the plenum 10 may be disposed within the engine or attached to the engine as an accessory.

The length of the sampling tube 12 is important. In a preferred embodiment, the sampling conduit 12 is long enough to allow the sampled gas to cool additionally, such that the temperature of the sampled gas is below 50 degrees celsius, but not too long, thereby causing a significant response time delay. The sampled gas passes through conduit 12 by means of a gas mixing chamber 10 (i.e. 1 atmosphere or greater) and the atmospheric pressure differential surrounding the engine.

A carbon dioxide sensor 14 is disposed on the sample tube 12 remote from the gas mixing chamber 10. With this arrangement, the carbon dioxide concentration in the precombustion gas supplied to the engine can be accurately measured despite the harsh engine environment.

Alternatively and optionally, this arrangement can be used to remove particles from the air when desired, which may require filtration. Additionally, if the sample temperature can be reduced, less complex and therefore less expensive non-temperature sensitive carbon dioxide sensor technology can be used, as components and calibrations that operate in high temperature environments are not required.

Moreover, in the case of using conditional sampling, this arrangement of the invention makes it easier to measure other gases, such as Nitrogen Oxides (NOX), whose temperature is lower than 50 degrees celsius, which is easier to measure optically. That is, the calculation of the concentration of NOx in the exhaust gas may be determined based on the measured NOx in plenum 10 and the percentage of Exhaust Gas Recirculation (EGR) introduced into plenum 10 calculated from the concentration of carbon dioxide. In other words, maintaining a certain amount of EGR percentage, the concentration of NOx can be maintained no higher than a certain concentration.

It should be appreciated that in the measurement of the rate of change of carbon dioxide concentration, there is no need to measure absolute concentration. The emphasis is on the relative change in concentration. To reduce costs, it is contemplated that non-dispersive infrared (NDIR) sensors may be used as carbon dioxide sensors in practicing embodiments of the present invention. NDIR detectors respond well to carbon dioxide, with better average sensitivity and longer life than electrochemical detectors.

Thus, a carbon dioxide detector that relies on a proportional technique may also be used to determine when the rate of change of carbon dioxide exceeds the desired concentration. A description of the proportional technique used in NDIR is found in US 5,026,992, the disclosure of which is incorporated herein by reference.

The above description and drawings are only for the purpose of illustrating preferred embodiments for achieving the objects, features and advantages of the present invention, and are not to be construed as limiting the invention thereto. It should be apparent to those skilled in the art that further modifications and variations can be made to the actual concepts described herein without departing from the spirit and scope of the invention as defined by the following claims.

For example, it should be possible to design a simple device for implementing the invention that includes various logic options and/or is programmable to reset or modify the logic options. Also, the alarm signal may take any of a variety of forms, such as an audible or visual warning.

Or the alarm signal is an electrical signal transmitted to a device that is responsive to the alarm signal. Thus, for example, the generation of an alarm signal may trigger another event, such as shutting down the engine of the vehicle or changing the operation of the vehicle engine. Alternatively, another device or process may be triggered to operate to remove air particles.

The many features and advantages of the invention are apparent from the detailed specification, and thus, it is intended by the appended claims to cover all such features and advantages of the invention which fall within the true spirit and scope of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described, and accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.

Claims (16)

1. An automatic calibration mode for a carbon dioxide sensor, the mode comprising:

(a) determining a baseline carbon dioxide concentration value for sensor operation;

(b) injecting a zeroing gas into the sensor;

(c) determining whether at least one drop in the carbon dioxide concentration value occurs below a baseline value within a predetermined time period; and

(d) when the carbon dioxide concentration drop occurs, activating recalibration of the sensor.

2. The calibration mode according to claim 1, further comprising:

a predetermined rate of change of carbon dioxide concentration is identified.

3. The calibration mode according to claim 2, further comprising:

once the predetermined rate of change is identified, the calibration of the sensor is reset based on the measured carbon dioxide concentration.

4. The calibration mode according to claim 1, wherein the step of determining a baseline carbon dioxide concentration value at which the sensor operates is user definable.

5. A calibration mode according to claim 1, wherein said step of determining a decrease in carbon dioxide concentration within a given time period is user definable.

6. Calibration mode according to claim 1, wherein said drop is 200ppm within a time period of 5 minutes.

7. Calibration mode according to claim 1, wherein said drop comprises a range of 200-300ppm within a time period of 15-30 seconds.

8. A calibration mode according to claim 1, wherein a drop in the sensor reading during the 5 minute time period is indicative of zero gas, and recalibrating the sensor to bring the reading to zero.

9. A calibration mode according to claim 1, wherein said step of activating recalibration comprises resetting calibration of the sensor using a microprocessor-driven switch.

10. The calibration mode according to claim 1, further comprising:

within another predetermined time period, a diagnostic routine is performed to determine the stability of one or more sensor readings.

11. The calibration mode according to claim 1, further comprising:

when the step of activating recalibration occurs, an alarm scheme is activated.

12. The calibration mode according to claim 11, wherein said activating an alarm scheme comprises activating at least one of a visual and an audible alarm.

13. The calibration mode according to claim 11, wherein said step of activating an alarm scheme comprises activating at least one of a visual and audible alarm when a sensor recalibration occurs alone.

14. A calibration mode according to claim 11, wherein said step of activating an alarm scheme comprises activating at least one of a visual and an audible alarm when the reading stabilizes for 30 seconds after said drop, said steady reading indicating a constant flow of calibration gas to said sensor.

15. A calibration mode according to claim 1, wherein the carbon dioxide concentration readings to be measured in said sensor do not vary by more than the background noise of said sensor.

16. Calibration mode according to claim 1, wherein said zeroing gas is nitrogen.