Method for detection of emissions levels during extended engine speed controlled operation

A method for detection of emissions levels during extended engine speed controlled operation is provided. The method includes monitoring mass airflow passing through the engine while operating the engine. The method further includes adjusting mass airflow responsive to engine speed to maintain a desired engine speed. The method further includes shutting down the engine when engine mass airflow becomes higher than a predetermined mass airflow threshold.

FIELD

The present application relates to a system for detecting emissions levels of an engine vehicle in an extended speed controlled operation.

BACKGROUND

Carbon monoxide (CO) and carbon dioxide (CO2) emissions can accumulate when a vehicle operates at extended speed controlled conditions, for example at idle speed, in an enclosed environment. Oxygen (O2) is involved in combustion reactions that produce CO and CO2, and both are emitted from an exhaust tailpipe. As these concentrations increase, the engine may begin to act like an exhaust gas recirculation system (EGR), taking in higher concentrations of CO and CO2 via the intake manifold.

A method for detecting CO is described in U.S. Pat. No. 5,333,703, wherein the vehicle includes cabin and external CO sensors. When the sensors detect a predetermined maximum carbon oxide threshold of CO, the engine can be disabled if the vehicle is in neutral or park mode.

However, CO sensors present an additional cost in the manufacturing of a vehicle. In contrast, the subject application presents a low-cost, or even no-cost, solution for estimating O2, CO, and CO2 concentrations when the engine is in extended speed controlled conditions.

A method for detection of emissions levels during extended engine speed controlled operation is provided. The method includes monitoring mass airflow passing through the engine while operating the engine. The method further includes adjusting mass airflow responsive to engine speed to maintain a desired engine speed. The method further includes shutting down the engine when engine mass airflow becomes higher than a predetermined mass airflow threshold.

By using an airflow sensor, such as an air meter, in the intake manifold of an engine, O2 concentration, CO concentration, and CO2 concentration (herein referred to as [O2], [CO], and [CO2]) may be estimated. A mass airflow increase during extended operation of an engine under speed controlled conditions (e.g., engine idle speed) indicates a decrease in intake [O2]; that is, as the engine seeks to achieve stoichiometric conditions for combustion in a reduced [O2] situation, a request to increase mass airflow to the engine is executed. Using predetermined relationships between at least mass airflow rate, engine power, [CO2], [CO], and [O2], concentrations of these constituent gases may be estimated. Thus, when concentration of one or more constituent gases exceeds a predetermined maximum carbon oxide threshold or becomes less than a predetermined minimum oxygen threshold, a method for disabling the engine can be employed.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1is a block diagram of an embodiment of a system for detecting engine emissions. The system includes a vehicle drivetrain and an electronic controller that receives mass airflow and throttle parameters and sends commands to various components of the system, based on parameters received.FIG. 2is a schematic view of an example cylinder of a direct injection engine with an electronic valve actuation system, showing further details of a cylinder of the engine ofFIG. 1, for example.

FIG. 3Ashows an exemplary flowchart illustrating an embodiment of a method for engine idle speed control and detection of mass airflow and automatic engine shut-off conditions during extended idling conditions. Further,FIG. 3Bdetails a continuation of the flowchart ofFIG. 3Aillustrating steps for estimation of constituent gas concentrations including carbon dioxide concentration (herein referred to as [CO2]), carbon monoxide concentration (herein referred to as [CO]), and oxygen concentration (herein referred to as [O2]). The constituent gas concentrations are estimated and compared to predetermined maximum carbon oxide thresholds and a predetermined minimum oxygen threshold to determine if the engine should be shut off. As is described with respect to the graphs ofFIG. 4andFIG. 5, an elevated mass airflow during engine idling mode indicates [CO2] is elevated. An elevated [CO2] is associated with reduced [O2]; in an enclosed space, the intake manifold will take in this elevated [CO2] and begin to act like an EGR circuit. Accordingly, the electronic controller may send a command for increased mass airflow to pass through the intake manifold as the engine seeks adequate [O2] for combustion in a cylinder.

The predetermined mass airflow threshold, predetermined maximum carbon oxide threshold, and/or predetermined minimum oxygen threshold may be determined by looking up a value in a prestored map of values relating mass airflow to [CO2], [CO], and/or [O2]. In one example, mass airflow may be correlated with [CO2] such that it may be determined if [CO2] is above a predetermined maximum carbon oxide threshold, based on measured mass airflow.

The values in the prestored map described above may be estimated values. Alternately, the predetermined maximum carbon oxide threshold and the predetermined minimum oxygen threshold may by computed with an estimator algorithm, taking other parameters, such as engine load, into account.

Referring toFIG. 1, the figure schematically depicts a system100for controlling a vehicle including an engine170while operating in extended speed controlled conditions. This system may include an internal combustion engine170, further described herein with reference toFIG. 2, which may output engine torque to a torque converter172coupled to a transmission174. The transmission174may be a manual transmission, an automatic transmission, or combinations thereof. Transmission174is shown coupled to vehicle wheels176.

Further, the engine170may include an intake manifold including a mass airflow sensor178or otherwise coupled to a mass airflow sensor178, which sends a mass airflow measure to an electronic controller180. The system may include an electronic controller180which may include a map182of constituent gas concentrations and mass airflow such that constituent gas concentrations may be estimated by mass airflow. The map182may include values accounting for engine power output. The electronic controller180may determine a predetermined mass airflow based on the map182, and compare actual mass airflow to the predetermined mass airflow threshold. The electronic controller180may also include a timer184to measure a time period of mass airflow; in one example, this may be included in the determination of a predetermined mass airflow threshold. Further, the system100may include an alarm186configured to initiate based on measured mass airflow and estimated constituent gas concentrations.

Thus, the electronic controller180may be configured to compare measured mass airflow to a predetermined mass airflow threshold and configured to compare estimated constituent gas concentrations to a predetermined maximum carbon oxide threshold or a predetermined minimum oxygen threshold. The electronic controller180may be further configured to initiate the alarm186and shut down the engine170if one or more of the predetermined mass airflow threshold, the predetermined maximum carbon oxide threshold, or the predetermined minimum oxygen threshold is met.

Further still, engine speed is received at the electronic controller180. To maintain engine idle speed, the electronic controller180can generate and send a throttle command to a throttle188based on current measured throttle angle received at the electronic controller180.

In another embodiment, the vehicle may be a hybrid engine vehicle, indicated by the dashed lines. The hybrid engine vehicle may include an energy conversion device190(e.g., an electric motor) coupled to the engine170. Further, the hybrid engine vehicle may include an energy storage device192(e.g., a battery), which may store energy to drive the energy conversion device190coupled to the transmission174. Hybrid propulsion embodiments may include full hybrid systems, in which the vehicle can run on just the engine, just the energy conversion device (e.g. motor), or a combination of both. Assist or mild hybrid configurations may also be employed, in which the engine is the primary torque source, with the hybrid propulsion system acting to selectively deliver added torque, for example during tip-in or other conditions. Further still, starter/generator and/or smart alternator systems may also be used.

The exemplary hybrid propulsion system is capable of various modes of operation. In an example full hybrid implementation, the propulsion system may operate using an energy conversion device190(e.g., a motor) as the torque source propelling the vehicle. In another mode, for example when the battery is being charged, engine170may be turned on and thus act as the torque source powering the vehicle wheels176. Alternately, if the battery is being charged and the vehicle is operating under extended speed controlled conditions, the engine170may be providing energy to a generator, such as a generator built into a vehicle or a portable generator, as some examples. In this case, the engine170may operate under low load conditions, such as in engine idle speed mode, as one example. In another example, the hybrid vehicle may be a plug-in vehicle, and the engine may operate under high load conditions, for example powering a generator and/or battery which is in turn, supplying power to a house, for example. In such a case, the engine170may be operating at speeds higher than engine idle speed.

Referring now toFIG. 2, this schematic view shows one cylinder of a multi-cylinder engine, as well as the intake and exhaust path connected to that cylinder. Internal combustion engine170is shown inFIG. 2as a direct injection gasoline engine with a spark plug; however, engine170may utilize port injection exclusively or in conjunction with direct injection. In an alternative embodiment, a port fuel injection configuration may be used where a fuel injector is coupled to intake manifold43in a port, rather than directly to combustion chamber29.

Engine170includes combustion chamber29and cylinder walls31with piston35positioned therein and connected to crankshaft39. Combustion chamber29is shown communicating with intake manifold43and exhaust manifold47via respective intake valve52and exhaust valve54. While one intake and one exhaust valve are shown, the engine may be configured with a plurality of intake and/or exhaust valves.FIG. 2merely shows one cylinder of a multi-cylinder engine, and each cylinder has its own set of intake/exhaust valves, fuel injectors, spark plugs, etc.

In some embodiments, intake valve52and exhaust valve54may be controlled by electric valve actuators (EVA)55and53, respectively. Valve position sensors50and51may be used to determine the position of the valves such as for example, fully opened, fully closed, or another position in between.

In some embodiments, combustion cylinder29can be deactivated by at least stopping the supply of fuel supplied to combustion cylinder29for at least one cycle. During deactivation of combustion cylinder29, one or more of the intake and exhaust valves can be adjusted to control the amount of air passing through the cylinder. In this manner, engine170can be configured to deactivate one, some or all of the combustion cylinders, thereby enabling variable displacement engine (VDE) operation.

Engine170is further shown configured with an exhaust gas recirculation (EGR) system configured to supply exhaust gas to intake manifold43from exhaust manifold47via EGR passage130. The amount of exhaust gas supplied by the EGR system can be controlled by EGR valve134. Further, the exhaust gas within EGR passage130may be monitored by an EGR sensor132, which can be configured to measure temperature, pressure, gas concentration, etc. Under some conditions, the EGR system may be used to regulate the temperature of the air and fuel mixture within the combustion chamber, thus providing a method of controlling the timing of combustion by autoignition.

Engine170is also shown having fuel injector65coupled thereto for delivering liquid fuel in proportion to the pulse width of signal FPW from electronic controller180directly to combustion chamber29. As shown, the engine may be configured such that the fuel is injected directly into the engine cylinder, which is known to those skilled in the art as direct injection. Distributorless ignition system88provides ignition spark to combustion chamber29via spark plug92in response to electronic controller180. Universal Exhaust Gas Oxygen (UEGO) sensor76is shown coupled to exhaust manifold47upstream of catalytic converter70. The signal from sensor76can be used to advantage during feedback air/fuel control in a conventional manner to maintain average air/fuel at stoichiometry during the stoichiometric homogeneous mode of operation.

FIG. 2further shows engine170configured with an aftertreatment system comprising a catalytic converter70and a lean NOx trap72. In this particular example, temperature Tcat1of catalytic converter70is measured by temperature sensor77and temperature Tcat2of lean NOx trap72is measured by temperature sensor75. Further, gas sensor73is shown arranged in exhaust manifold47downstream of lean NOx trap72, wherein gas sensor73can be configured to measure the concentration of NOx and/or02in the exhaust gas.

In some embodiments, the engine may include a fuel vapor purging system for purging fuel vapors to the combustion chamber. As one example, fuel vapors originating in fuel tank160may be stored in fuel vapor storage tank164until they are purged to intake manifold43via fuel purge valve168. Fuel vapor purge valve168may be connected to electronic controller180. Furthermore, the position of the fuel vapor purge valve may be varied by the control system to provide fuel vapors to the combustion chamber during select operating conditions.

Electronic controller180is shown inFIG. 2as a conventional microcomputer including: microprocessor102, input/output ports104, and read-only memory106, random access memory108, keep alive memory110, and a conventional data bus. Electronic controller180is shown receiving various signals from sensors coupled to engine170, in addition to those signals previously discussed, including: engine coolant temperature (ECT) from temperature sensor112coupled to cooling sleeve114; a pedal position sensor119coupled to an accelerator pedal; a measurement of engine manifold pressure (MAP) from pressure sensor122coupled to intake manifold43; a measurement (ACT) of engine air charge temperature or manifold temperature from temperature sensor117; and an engine position sensor118from a Hall effect sensor sensing crankshaft39position. In some embodiments, the requested wheel output can be determined by pedal position, vehicle speed, and/or engine operating conditions, etc. In one aspect of the present description, engine position sensor118produces a predetermined number of equally spaced pulses for a revolution of the crankshaft from which engine speed (RPM) can be determined.

Storage medium read-only memory106can be programmed with computer readable data representing instructions executable by microprocessor102for performing the methods described below as well as other variants that are anticipated but not specifically listed.

In some embodiments, electronic controller180can be configured to control operation of the various systems described above with reference toFIG. 1. For example, the energy storage device192may be configured with a sensor that communicates with electronic controller180, thereby enabling a determination to be made of the state of charge or quantity of energy stored by the energy storage device192. In another example, electronic controller180or other controller can be used to vary a condition of the energy conversion device190and/or transmission174. Further, in some embodiments, electronic controller180may be configured to cause combustion chamber29to operate in various combustion modes, as described herein. The fuel injection timing may be varied to provide different combustion modes, along with other parameters, such as EGR, valve timing, valve operation, valve deactivation, etc.

Combustion in engine170can be of various types/modes, depending on operating conditions. In one example, spark ignition (SI) can be employed where the engine utilizes a sparking device, such as spark plug coupled in the combustion chamber, to regulate the timing of combustion chamber gas at a predetermined time after top dead center of the expansion stroke. In one example, during spark ignition operation, the temperature of the air entering the combustion chamber is considerably lower than the temperature required for autoignition. While SI combustion may be utilized across a broad range of engine torque and speed it may produce increased levels of NOx and lower fuel efficiency when compared with other types of combustion.

Another type of combustion that may be employed by engine170uses homogeneous charge compression ignition (HCCI), or controlled autoignition (CAI), where autoignition of combustion chamber gases occurs at a predetermined point after the compression stroke of the combustion cycle, or near top dead center of compression. Typically, when compression ignition of a pre-mixed air and fuel charge is utilized, fuel is normally homogeneously premixed with air, as in a port injected spark-ignited engine or direct injected fuel during an intake stroke, but with a high proportion of air to fuel. Since the air/fuel mixture is highly diluted by air or residual exhaust gases, which results in lower peak combustion gas temperatures, the production of NOx may be reduced compared to levels found in SI combustion. Furthermore, fuel efficiency while operating in a compression combustion mode may be increased by reducing the engine pumping loss, increasing the gas specific heat ratio, and by utilizing a higher compression ratio.

In compression ignition operation mode, it may be desirable to exercise close control over the timing of autoignition. The initial intake charge temperature directly affects the timing of autoignition. The start of ignition is not directly controlled by an event such as the injection of fuel in the standard diesel engine or the sparking of the spark plug in the spark ignited engine. Furthermore, the heat release rate is not controlled by either the rate or duration of the fuel-injection process, as in the diesel engine, or by the turbulent flame propagation time, as in the spark-ignited engine.

Note that autoignition is also a phenomenon that may cause knock in a spark-ignited engine. Knock may be undesirable in spark-ignited engines because it enhances heat transfer within the cylinder and may burn or damage the piston. In controlled compression ignition operation, with its high air-to-fuel ratio, knock does not generally cause degradation of the engine because the diluted charge keeps the rate of pressure rise low and the maximum temperature of the burned gases relatively low. The lower rate of pressure rise mitigates the damaging pressure oscillations characteristic of spark ignition knock.

In comparison to a spark ignition engine, the temperature of the charge at the beginning of the compression stroke typically may be increased to reach autoignition conditions at or near the end of the compression stroke. It will be appreciated by those skilled in the art that numerous other methods may be used to elevate initial charge temperature. Some of these include: heating the intake air (heat exchanger), keeping part of the warm combustion products in the cylinder (internal EGR) by adjusting intake and/or exhaust valve timing, compressing the inlet charge (turbo-charging and supercharging), changing the autoignition characteristics of the fuel provided to the engine, and heating the intake air charge (external EGR).

During HCCI combustion, autoignition of the combustion chamber gas may be controlled to occur at a desired position of the piston or crank angle to generate desired engine torque, and thus it may not be necessary to initiate a spark from a sparking mechanism to achieve combustion. However, a late timing of the spark plug, after an autoignition temperature should have been attained, may be utilized as a backup ignition source in the case that autoignition does not occur.

Note that a plurality of other parameters may affect both the peak combustion temperature and the required temperature for efficient HCCI combustion. These and any other applicable parameters may be accounted for in the routines embedded in engine electronic controller180and may be used to determine optimum operating conditions. For example, as the octane rating of the fuel increases, the required peak compression temperature may increase as the fuel requires a higher peak compression temperature to achieve ignition. Also, the level of charge dilution may be affected by a variety of factors including both humidity and the amount of exhaust gases present in the intake charge. In this way, it is possible to adjust engine parameters to compensate for the effect of humidity variation on autoignition, i.e., the effect of water makes autoignition less likely.

In one particular example, autoignition operation and combustion timing may be controlled by varying intake and/or exhaust valve timing and/or lift to, for example, adjust the amount of residual trapped gasses. Operating an engine in HCCI using the gas trapping method can provide fuel-efficient combustion with extremely low engine out NOx emissions.

However, the achievable HCCI window of operation for low engine speed and/or low engine load may be limited. That is, if the temperature of the trapped gas is too low, then HCCI combustion may not be possible at the next combustion event. If it is necessary to switch out of HCCI and into spark ignition mode during low load in which temperatures may fall too low, and then to return back into HCCI operation once conditions are acceptable, there may be penalties in engine emissions and fuel economy and possible torque/NVH disruption to the driver during each transition. Therefore, in one embodiment, a method that enables additional operation in HCCI or other limited combustion mode at high or low speeds and loads is described herein utilizing an alternative torque source, such as an energy conversion device/generator. Furthermore, extending the low load limit of HCCI operation, for one or more cycles, to obtain increased benefit from HCCI operation may be desirable.

While one or more of the above combustion modes may be used in some examples, still other combustion modes may be used, such as stratified operation, either with or without spark initiated combustion.

As discussed above, a hybrid propulsion system may be operated in a variety of different modes. Various inputs may be used to select from among the different modes, and/or to control operation of the hybrid propulsion system while operating in a given mode. Example inputs include engine speed, vehicle speed, requested torque, catalyst temperature, manifold pressure, air/fuel ratio, catalyst temperature and/or status of aftertreatment systems, throttle position, accelerator pedal position, requested power, adaptively-learned drive behavior, operating temperature conditions, humidity, etc., status of climate controls, PIP, state of charge (SOC) in hybrid-electric vehicle, etc.

Referring now toFIG. 3A, an example method300for engine operation during extended speed controlled conditions (e.g., extended idle conditions) including monitoring the mass airflow passing through the engine is illustrated. The method may include adjusting mass airflow responsive to engine speed to maintain a desired engine idle speed and shutting down the engine when engine mass airflow becomes higher than a predetermined mass airflow threshold.

Specifically, if it is determined that the engine speed is in engine idle mode at312, the method may include determining if a mass airflow sensor, located in the intake manifold43, for example, is degraded at313. If the answer is yes at313, the routine may end. If the answer is no at313, the method may further include detecting a mass airflow at an intake passage of the engine via the mass airflow sensor at314. As another example, mass airflow at an intake passage of the engine may be measured by measuring throttle angle and, accordingly, the predetermined mass airflow threshold may be a throttle angle threshold.

The predetermined mass airflow threshold FTHis determined at316and the detected mass airflow is compared to FTHat318. The method may further include initiating an alarm at320if the detected mass airflow exceeds the predetermined mass airflow threshold FTH. A timer is initiated at322. The timer may measure duration of the state of the electronic controller in which it has been determined that mass airflow is above a first predetermined mass airflow threshold FTH.

Thus, if it is determined that a time period from the initiation of vehicle alarm has exceeded a predetermined time threshold TTH324, a command to execute engine shut-off326is sent to the engine170and the timer is reset328. In this way, the engine may be shut down when engine mass airflow becomes higher than a predetermined mass airflow threshold wherein the predetermined mass airflow threshold is measured over a time period. Alternately, the engine may be shut down when the engine mass airflow becomes higher than the predetermined mass airflow threshold, and the predetermined mass airflow threshold may be computed as a cumulative mass airflow over a time period. If the time since alarm initiation has not exceeded a predetermined time threshold TTH324, the routine ends. This step may be useful for preventing premature engine shut-off if mass airflow increases transiently, for example.

In this example, if mass airflow does not exceed the predetermined mass airflow threshold FTHat318, engine speed may be maintained within a predetermined engine speed range (e.g., engine idle speed range) by adjusting one or more of the mass airflow, fuel pulse width, fuel pulse timing, and/or valve timing. In one example, mass airflow and fuel amount are increased in response to decreases in engine idle speed and mass airflow and fuel amount are decreased in response to increases in engine idle speed. It may be appreciated that mass airflow adjustments may be made by adjusting the throttle angle.

Specifically, it is determined if the actual engine speed NEis greater than the desired engine speed NOat330. If the answer is yes, mass airflow may be decreased by decreasing mass airflow via adjustments to the throttle angle and/or by decreasing fuel injection amount at332. If the answer is no at330and NEis less than NO, mass airflow may be increased by increasing the throttle angle and/or by increasing the fuel injection amount at334.

In an alternate procedure, mass airflow may be detected at314and the routine may proceed toFIG. 3Bwhich illustrates example steps of the method300including estimating constituent gas concentrations based on mass airflow and comparing constituent gas concentrations to predetermined maximum carbon oxide thresholds and the predetermined minimum oxygen threshold. In this example, constituent gas concentrations may include [CO2], [O2], and/or [CO]. In one example, the method may include correlating an increase in mass airflow to an increase in [CO2] and a decrease in [O2] in ambient air.

For example, an estimate of [CO2] is made at336based on mass airflow, by accessing values in a prestored map of mass airflow and [CO2], for example. If [CO2] exceeds a predetermined maximum carbon oxide threshold, C1, at338the routine proceeds to step320. If the answer is no at338, the routine ends. Similarly, [O2] may be estimated at340by accessing values in a prestored map of mass airflow and [O2], for example. If [O2] is below a predetermined minimum oxygen threshold, C2, at342the routine proceeds to step320. In this example, [CO] may be estimated at344, by accessing values in a prestored map of mass airflow rate and [CO], for example. If [CO] exceeds a predetermined maximum carbon oxide threshold, C3, at346the routine proceeds to step320. In one example, if the answer is no at steps338,342, and346, the routine ends. In another example, if the answer is yes for at least one of the steps338,342, or346, the routine proceeds to step320. Thus, in one example, the method may include initiating an alarm at320if an alarm criterion is met wherein an alarm criterion is one or more of the [CO2] concentration greater than the predetermined maximum carbon oxide threshold and the oxygen concentration less than the predetermined minimum oxygen threshold. Further, the method may include shutting down the engine if an estimate of [CO2] and/or the estimate of [CO], based on mass airflow measured at the intake manifold, are greater than the predetermined maximum carbon oxide threshold or if the estimate of [O2] based on mass airflow measured at the intake manifold is less than a predetermined minimum oxygen threshold.

Further still, a timer may be initiated at322to measure duration of the state of the electronic controller180in which it has been determined that at least one of the constituent gas concentrations is greater than the predetermined maximum carbon oxide thresholds (e.g., C1, C3) or is less than a predetermined minimum oxygen threshold (e.g., C2)

Thus, the alarm may be initiated and/or the engine may be shut down if the estimate of carbon dioxide concentration and/or carbon monoxide concentration based on mass airflow measured in the intake manifold exceeds the predetermined maximum carbon oxide threshold. Further, the predetermined maximum carbon oxide threshold may be computed over a time period. Alternately, the alarm may be initiated and/or the engine may be shut down if the estimate of oxygen concentration based on mass airflow measured in the intake manifold is less than the predetermined minimum oxygen threshold wherein the predetermined minimum oxygen threshold is computed over a time period. As an additional alternate, it may be appreciated that the engine may be shut down if the estimate of carbon dioxide concentration based on mass airflow measured in the intake manifold exceeds the predetermined maximum carbon oxide threshold wherein the predetermined maximum carbon oxide threshold is computed as a cumulative carbon dioxide concentration over a time period. Likewise, the engine may be shut down if the estimate of oxygen concentration based on mass airflow measured in the intake manifold is less than the predetermined minimum oxygen threshold. Alternately, the predetermined minimum oxygen threshold may be computed as a cumulative oxygen concentration over a time period. It may be appreciated that the maximum carbon oxide threshold may include different maximum thresholds for carbon monoxide concentration and carbon dioxide concentration.

The relationships between mass airflow, [O2], and [CO2] are illustrated inFIG. 4and changes based on engine power output are further described inFIG. 5. Thus, a prestored map of mass airflow and maximum carbon dioxide thresholds may be developed based on these relationships, as one example.

FIG. 4depicts changes in mass airflow (expressed as a percentage of baseline mass airflow during engine idle mode), [O2], and [CO2] through the intake manifold43of an engine170in engine idle mode in a closed environment. In this example, as time progresses, [O2] decreases because the engine continues to output CO2 through the exhaust tailpipe in the absence of adequate ventilation. As a result of the decreased [O2], the electronic controller180may request a greater mass airflow to the engine170to meet stoichiometric [O2] demands and thus to achieve a desired air-fuel ratio and maintain idle engine speed. In this case, at approximately 450 minutes, mass airflow reaches a maximum while [O2] has concurrently decreased. It may be appreciated that the mass airflow may reach a maximum value earlier or later than depicted depending on, for example, engine load, temperature, etc. In the application described herein, the predetermined maximum carbon oxide thresholds and the predetermined minimum oxygen threshold for engine automatic shut-off may be configured such that they are below this mass airflow maximum.

FIG. 5shows the changing relationship between engine power and mass airflow rate through the intake manifold43as a function of [CO2]. It is known that, without CO2 in the environment, there is a base mass airflow rate (solid line) through an intake manifold43. As [CO2] increases, the slope of this line increases as indicated (dashed lines). The slope increase is one measurement by which elevated [CO2] may be detected. Predetermined curves, such as the lines illustrated, based on engine power output, may be stored in the electronic controller180or may be determined by an algorithm.

From the graphs, it may be appreciated that the predetermined mass airflow threshold may be computed such that the predetermined mass airflow threshold may increase as engine output power increases, to account for the increased mass airflow that flows through the intake passage of the engine at higher power output levels. Further, curves accounting for other factors such as ambient temperature, exhaust output, etc., may be created and stored in the electronic controller180and these may be accounted for prior to initiating the alarm and/or automatic engine shut-off.