Reducing engine misfires due to charge air cooler condensate using in-cylinder enrichment and positive valve overlap

Abstract

Engine process, which includes the following: Burning a rich air-fuel ratio and adjusting the valve timing to increase positive valve overlap in response to an increase in air mass flow, where the combustion of the rich air-fuel ratio and the adjustment of the valve timing continue to be based on a condensate concentration in an intercooler, wherein the burning of the rich air-fuel ratio and the adjustment of the valve timing to produce the positive valve overlap respond to a request to increase the air mass flow rate above a threshold rate when the condensate concentration is above a threshold concentration, furthermore, comprehensively increasing the positive valve overlap of an intake valve and an exhaust valve to keep an exhaust gas mixture close to stoichiometry during the combustion of the rich air-fuel ratio.

Description

[0001] Turbocharged and supercharged engines can be configured to compress ambient air entering the engine to increase power. Compressing the air can cause an increase in air temperature, which is why an intercooler (IC) can be used to cool the heated air, thereby increasing its density and further raising the engine's potential power. Ambient air from outside the vehicle flows over the IC to cool intake air moving through its interior. Condensation can form inside the IC when the ambient air temperature decreases or in humid or rainy weather conditions when the intake air is cooled below the dew point of water. Condensation can collect at the bottom of the IC, in its internal passages, and in the cooling turbochargers.When torque increases, such as during acceleration, the increased air mass flow can remove the condensate from the intercooler, causing it to be sucked into the engine and increasing the likelihood of engine misfire and / or combustion instability.

[0002] Document DE 10 2013 209 027 A1 discloses methods and systems for coordinating camshaft control and forced air supply, utilizing positive valve overlap. Document DE 10 2010 022 949 A1 describes a method for calculating exhaust gas temperature, which includes the effect of forced air on engine operation. Document DE 10 2013 111 448 A1 discloses a method for purging condensate from an intercooler. Document DE 10 2013 218 284 A1 describes a condensate dispersion element for intercoolers and the control of condensate in an intercooler.

[0003] Other attempts to address engine misfires caused by condensate absorption involve preventing condensate buildup. However, the inventors of the present invention have recognized potential problems with such methods. In particular, while some methods may reduce or slow down condensate formation in the intercooler, condensate can still accumulate over time. If this buildup cannot be stopped, the absorption of condensate during acceleration can cause combustion instability and engine misfires. Another method to prevent engine misfires caused by condensate absorption involves capturing and / or draining the condensate from the intercooler. While this may reduce condensate concentrations in the intercooler, it moves the condensate to another location or reservoir that may be subject to other condensate-related problems, such as freezing and corrosion.

[0004] In one example, the problems described above can be addressed by adjusting combustion parameters to improve combustion stability during periods of increased airflow when condensate forms in the intercooler. Specifically, during periods of increased airflow when the condensate concentration in the intercooler exceeds a certain threshold, a rich air-fuel mixture can be used, while also increasing the valve overlap between the intake and exhaust valves. This improves combustion stability while condensate enters the engine from the intercooler, reducing the likelihood of engine misfires and / or unstable combustion.

[0005] As an example, an engine controller, in response to a request to increase the air mass flow rate above a threshold when the condensate concentration exceeds a threshold, might lower the air-fuel ratio and increase valve overlap. Lowering the air-fuel ratio might involve increasing the amount of fuel injected into an engine cylinder for combustion. Increasing valve overlap might involve extending the duration during which an intake and exhaust valve are open simultaneously. The valve overlap is increased in such a way that the exhaust mixture is maintained near stoichiometrically during the combustion of the rich air-fuel ratio.The air-fuel ratio and valve overlap can be reduced to base concentrations if one or more of the condensate concentration and / or air mass flow decrease below their respective threshold values.

[0006] It is understood that the above brief outline is presented to introduce, in simplified form, a selection of concepts that are described in more detail in the full description. It is not intended to identify any important or essential features of the claimed subject matter, the scope of which is defined exclusively by the claims that follow the full description. Furthermore, the claimed subject matter is not limited to implementations that resolve any disadvantages mentioned above or in any part of this disclosure. Fig. Figure 1 is a schematic diagram of an exemplary engine system that includes an intercooler. Fig. Figure 2 shows a flow diagram of a procedure for adjusting an air-fuel ratio and valve control in response to a condensate concentration in an intercooler and an air mass flow rate. Fig. Figure 3 shows a flowchart illustrating a method for determining the amount of condensate within an LLK according to an embodiment of the present disclosure. Fig. Figure 4 shows a flow diagram of a procedure for determining an air-fuel ratio and an extent of valve overlap based on a condensate concentration in an intercooler and an air mass flow rate. Fig. Figure 5 shows a graphical example of adjusting an air-fuel ratio and valve control based on a condensate concentration in an intercooler and an air mass flow rate.

[0007] The following description concerns systems and methods for adjusting combustion parameters to increase combustion stability under conditions when condensate formed in an intercooler (IC) enters an inlet of an engine system, such as the one in Fig. This can occur in the engine system shown in section 1. During conditions of increased air mass flow, if the condensate concentration in the intercooler exceeds a threshold concentration, the air-fuel ratio and valve timing may need to be adjusted to increase combustion stability. Fig. Section 2 provides an exemplary procedure for determining when the air-fuel ratio should be reduced and valve overlap increased in response to the condensate concentration and the air mass flow rate. The condensate concentration or amount in the intercooler can be determined by a Fig. 3. The procedure presented will be determined. Fig. Figure 4 shows an exemplary procedure for determining the air-fuel ratio and the degree of valve overlap based on the condensate concentration and the air mass flow rate. Finally, at Fig. Five exemplary adjustments of the air-fuel ratio and valve control based on condensate concentration and air mass flow are shown.

[0008] Fig. Figure 1 schematically shows aspects of an exemplary engine system 100 with an engine 10. In the illustrated embodiment, the engine 10 is a turbocharged engine coupled to a turbocharger 13, which contains a compressor 14 driven by a turbine 16. In particular, fresh air is introduced into the engine 10 along the inlet passage 42 via an air filter 12 and flows to the compressor 14. The flow rate of ambient air entering the intake system through the inlet passage 42 can be controlled, at least partially, by adjusting a throttle valve 20. The compressor 14 can be any suitable intake air compressor, such as an engine-driven or shaft-driven supercharger compressor. In the engine system 10, however, the compressor is a turbocharger compressor mechanically coupled to the turbine 16 via a shaft, with the turbine 16 being driven by the expanding engine exhaust.In one embodiment, the compressor and the turbine can be coupled within a twin-scroll turbocharger. In another embodiment, the turbocharger can be a variable geometry turbocharger, in which the turbine geometry is actively varied as a function of the engine speed.

[0009] As in Fig. As shown in Figure 1, the compressor 14 is coupled to the throttle valve 20 via an intercooler (IC) 18. The IC can be, for example, an air-to-air or air-to-water heat exchanger. The throttle valve 20 is coupled to the engine intake manifold 22. From the compressor, the hot, compressed charge air enters the inlet of the IC 18, cools as it flows through the IC, and then exits to flow through the throttle valve to the intake manifold. Ambient air from outside the vehicle can enter the engine 10 through a grille at the front of the vehicle and pass over the IC to assist in cooling the charge air. Condensation can form and accumulate in the IC when the ambient air temperature decreases, or during humid or rainy weather conditions when the charge air is cooled below the dew point of water. If the charge air contains recirculated exhaust gases, the condensate can become acidic and corrode the intercooler housing.Corrosion can lead to leaks between the charge air, the atmosphere, and potentially the coolant in the case of water-to-air intercoolers. Additionally, condensate can accumulate at the bottom of the intercooler and then be drawn into the engine all at once during periods of increased airflow, such as acceleration (or tip-in), increasing the likelihood of engine misfire. As discussed herein with reference to the... Fig. By performing 2-5, combustion parameters such as air-fuel ratio and valve control can be adjusted during periods of increased air mass flow, thus increasing combustion stability and reducing engine misfire events.

[0010] At the in Fig. In the embodiment shown in Figure 1, the pressure of the air charge within the inlet manifold is detected by a manifold air pressure (MAP) sensor 24. A compressor bypass valve (not shown) can be coupled in series between the inlet and outlet of the compressor 14. The compressor bypass valve can be a normally closed valve configured to open under selected operating conditions to release excess boost pressure. For example, the compressor bypass valve can open during decreasing engine speed conditions to prevent compressor pumping.

[0011] The intake manifold 22 is coupled to a series of combustion chambers 30 via a series of intake valves (not shown). The combustion chambers are further coupled to an exhaust manifold 36 via a series of exhaust valves (not shown). In the illustrated embodiment, a single exhaust manifold 36 is shown. However, in other embodiments, the exhaust manifold may contain multiple exhaust manifold sections. Configurations with multiple exhaust manifold sections allow exhaust from different combustion chambers to be directed to different locations in the engine system.

[0012] One or more fuels, such as gasoline, alcohol-fuel mixtures, diesel, biodiesel, compressed natural gas, etc., can be supplied to the combustion chambers (e.g., cylinders) 30. Fuel can be supplied to the combustion chambers via a fuel injector 66. In the illustrated example, the fuel injector 66 is configured for direct injection, although in other embodiments the fuel injector 66 may be configured for intake port injection or throttle body injection. Furthermore, each combustion chamber can contain one or more fuel injectors with different configurations so that each cylinder can receive fuel via direct injection, intake injection, throttle body injection, or combinations thereof. Combustion in the combustion chambers can be initiated by spark ignition and / or compression ignition.The amount of fuel injected into the combustion chambers 30 by the fuel injector 66 can be adjusted to achieve a desired air-fuel ratio (A / F). For example, the air-fuel ratio can be adjusted based on a condensate concentration in the intercooler and / or an air mass flow rate (e.g., mass airflow). Procedures for this adjustment are described below. Fig. 2-5 discussed.

[0013] Exhaust gas from the exhaust manifold 36 is directed to the turbine 16 to drive the turbine. If reduced turbine torque is desired, some exhaust gas can be diverted bypassing the turbine through a wastegate (not shown). The combined flow from the turbine and the wastegate then flows through an exhaust aftertreatment device 70. In general, one or more exhaust aftertreatment devices 70 can include one or more aftertreatment catalysts configured to catalytically treat the exhaust gas stream, thereby reducing the amount of one or more substances in the exhaust gas stream. For example, an aftertreatment catalyst can be configured to capture NOx from the exhaust gas stream when the exhaust gas stream is lean and to reduce the captured NOx when the exhaust gas stream is rich.In other examples, an exhaust aftertreatment catalyst may be configured to disproportionate NOx or to selectively reduce NOx with the aid of a reducing agent. In still other examples, an exhaust aftertreatment catalyst may be configured to oxidize residual hydrocarbons and / or carbon monoxide in the exhaust stream. Various exhaust aftertreatment catalysts with such functionality may be arranged in intermediate layers or otherwise within the exhaust aftertreatment stages, either separately or together. In some embodiments, the exhaust aftertreatment stages may include a regenerable soot filter configured to capture and oxidize soot particles in the exhaust stream. The treated exhaust gas from the exhaust aftertreatment device 70 may be discharged wholly or partially into the atmosphere via an exhaust duct 35. An exhaust gas sensor 128 is shown coupled to the exhaust duct 35 upstream of the exhaust aftertreatment device 70.The sensor 128 can be any suitable sensor to provide an indication of the air-fuel ratio in the exhaust gas, such as a broadband oxygen probe or UEGO (Universal or Wide-Range Exhaust Gas Oxygen), a dual-state oxygen sensor or EGO, a HEGO (heated EGO), a NOx, HC or CO sensor.

[0014] Depending on the operating conditions, a portion of the exhaust gas from the exhaust manifold 36 upstream of the turbine 16 can be recirculated to the inlet manifold 22 downstream of the compressor 14 via an EGR passage 51, through the EGR cooler 50, and the EGR valve 22. This enables high-pressure exhaust gas recirculation (HP-EGR). In some embodiments, low-pressure exhaust gas recirculation (LP-EGR) can also be enabled in addition to HP-EGR, whereby a portion of the treated exhaust gas is recirculated from the exhaust manifold 36 downstream of the turbine 16 to the inlet manifold 22 upstream of the compressor 14 via a low-pressure EGR passage, an EGR cooler coupled therein, and an EGR valve (not shown). The EGR valve 52 can be opened to allow a controlled amount of cooled exhaust gas into the intake manifold for desired combustion and exhaust gas purification performance. The relatively long EGR flow path in engine system 10 ensures excellent homogenization of the exhaust gas into the intake air charge.Furthermore, the arrangement of EGR tap and mixing points provides very effective cooling of the exhaust gas for increased available EGR mass and improved performance.

[0015] Each combustion chamber (e.g., cylinder) 30 can be operated by one or more valves. In the present example, each cylinder 30 contains a corresponding intake valve 62 and an exhaust valve 64. The engine system 100 further includes one or more camshafts 68 for actuating the intake valve 62 and / or exhaust valve 64. In the illustrated example, the intake camshaft 68 is coupled to the intake valve 62 and can be activated to actuate the intake valve 62. In some embodiments, where the intake valves of several cylinders 30 are coupled to a common camshaft, the intake camshaft 68 can be activated to actuate the intake valves of all coupled cylinders.

[0016] The intake valve 62 can be activated between an open position, which allows intake air into the corresponding cylinder, and a closed position, which essentially blocks intake air from entering the cylinder. The intake camshaft 68 can be included in intake valve activation systems 69. The intake camshaft 68 includes an intake cam 67, which has a cam nose profile for opening the intake valve 62 for a defined intake duration. In some embodiments not shown, the camshaft can include additional intake cams with an alternative cam nose profile that allows the intake valve 62 to open for an alternative duration (also referred to here as a cam profile switching system). Based on the nose profile of the additional cam, the alternative duration can be longer or shorter than the defined intake duration of the intake cam 67.The nose profile can influence the cam lift height, cam duration, and / or cam timing. A controller may be able to switch the intake valve duration by moving the intake camshaft 68 longitudinally and switching between cam profiles.

[0017] In the same way, each exhaust valve 64 can be activated between an open position, which allows exhaust gas to escape from the corresponding cylinder, and a closed position, which essentially retains gas within the cylinder. It is understood that although only the intake valve 62 is shown to be activated by a cam, the exhaust valve 64 can also be activated by a similar exhaust camshaft (not shown). In some embodiments where the exhaust valve of several cylinders 30 is coupled to a common camshaft, the exhaust camshaft can be activated to actuate the exhaust valves of all coupled cylinders. As with the intake camshaft 68, if included, the exhaust camshaft can include an exhaust cam with a cam nose profile for opening the exhaust valve 64 for a defined exhaust duration.In some embodiments, the exhaust camshaft may further include additional exhaust cams with an alternative cam nose profile that allows the exhaust valve 64 to open for an alternative duration. The nose profile may influence the cam lift height, cam duration, and / or cam timing. A controller may be able to switch the exhaust valve duration by moving the exhaust camshaft longitudinally and switching between cam profiles.

[0018] It is understood that the intake and / or exhaust camshafts can be coupled to subsets of cylinders, and that multiple intake and / or exhaust camshafts can be present. For example, a first intake camshaft can be coupled to the intake valves of a first subset of cylinders, while a second intake camshaft can be coupled to the intake valves of a second subset of cylinders. Similarly, a first exhaust camshaft can be coupled to the exhaust valves of a first subset of cylinders, while a second exhaust camshaft can be coupled to the exhaust valves of a second subset of cylinders. Furthermore, one or more intake and exhaust valves can be coupled to each camshaft. The subset of cylinders coupled to the camshaft can be based on their position along an engine block, their firing order, the engine configuration, and so on.

[0019] An intake valve activation system 69 and an exhaust valve activation system (not shown) may further include piston rods, rocker arms, tappets, etc. Such devices and features can control the activation of the intake valve 62 and the exhaust valve 64 by converting the rotary motion of the cams into a translational motion of the valves. As previously discussed, the valves may also be activated by means of additional cam nose profiles on the camshafts, the cam nose profiles being able to provide varying cam lift, cam duration, and / or cam timing between the different valves. However, if desired, alternative camshaft arrangements (overhead-operated and / or piston rod) could be used. Furthermore, in some examples, the cylinders 30 may have more than one exhaust valve and / or intake valve.In further examples, each of the exhaust valve 64 and the intake valve 62 of one or more cylinders can be activated by a common camshaft. Even further, in some examples, some of the intake valves 62 and / or exhaust valves 64 can be activated by their own independent camshaft or other device.

[0020] The 100 engine system can include variable valve timing (VVT) systems, such as the 80 system for variable cam timing (VCT). A variable valve timing system can be configured to open an intake valve for a first duration and an exhaust valve for a second duration. The first and second durations can be based on engine operating conditions. For example, the first and second durations can be adjusted based on the air mass flow and the condensate concentration in the intercooler.

[0021] The VCT system 80 can be a dual independent variable camshaft timing system to modify the intake and exhaust valve timing independently. The VCT system 80 can include an intake camshaft adjuster coupled to the common intake camshaft 68 to modify the intake valve timing. Similarly, the VCT system can include an exhaust camshaft adjuster coupled to a common exhaust camshaft to modify the exhaust valve timing. The VCT system 80 can be configured to advance or retard the valve timing by advancing or retarding the camshaft timing and can be controlled by the controller 38. The VCT system 80 can be configured to vary the control of the valve opening and closing events by varying the relationship between the crankshaft position and the camshaft position.For example, the VCT system 80 can be configured to rotate the intake camshaft 68 independently of the crankshaft, thus advancing or retarding the valve timing. In some embodiments, the VCT system 80 can be a cam torque-activated device configured to rapidly vary the cam timing. In some embodiments, the valve timing, such as intake valve closing (IVC) and exhaust valve closing (EVC), can be varied by a continuously variable valve lift (CVVL) device.

[0022] The valve / cam control devices and systems described above can be hydraulically driven, electrically activated, or a combination thereof. In one example, the position of the camshaft can be changed via cam phase adjustment by an electric actuator (e.g., an electrically activated cam adjuster) with an accuracy exceeding that of most hydraulically actuated cam adjusters. Signal lines can send control signals to the VCT system 80 and receive cam control and / or cam selection measurements from it.

[0023] By adjusting the VCT system 80, the position of the intake camshaft 68 can be adjusted to thereby control the opening and / or closing of the intake valve 62. By varying the opening and closing of the intake valve 62, the degree of positive valve overlap between the intake valve 62 and the exhaust valve 64 can be varied. For example, the VCT system 80 can be adjusted to advance or retard the opening and / or closing of the intake valve 62 relative to a piston position.

[0024] During engine operation, a cylinder piston gradually moves downwards from top dead center (TDC), striking bottom dead center (BDC) at the end of the power stroke. The piston then returns to its top position at TDC at the end of the exhaust stroke. The piston then moves again to BDC during the intake stroke, returning to its original top position at TDC by the end of the compression stroke. During cylinder combustion, an exhaust valve can open precisely when the piston strikes the bottom of the power stroke. The exhaust valve can then close when the piston completes the exhaust stroke, remaining open until at least one subsequent intake stroke has begun. In this way, an intake valve can open at or before the start of an intake stroke and remain open until at least one subsequent compression stroke has begun.

[0025] Based on the control differences between exhaust valve closing and intake valve opening, the valves can be operated with negative valve overlap, whereby for a brief period after the end of the exhaust stroke and before the beginning of the intake stroke, both the intake and exhaust valves are closed. This period, during which both valves are closed, is referred to as negative (intake-to-exhaust) valve overlap. In one example, the VCT system can be adjusted so that negative intake-to-exhaust valve overlap control can be a standard cam position of the engine during cylinder combustion.

[0026] Alternatively, the valves can be operated with positive valve overlap, whereby for a brief period before the end of the exhaust stroke and after the beginning of the intake stroke, both the intake and exhaust valves can be open. This period during which both valves can be open is referred to as positive (intake-to-exhaust) valve overlap. As explained herein, the VCT system 80 can be adjusted to increase the degree of positive valve overlap during selected engine operating conditions. In particular, the intake camshaft position can be adjusted to advance the intake valve timing. Consequently, the intake valve can open earlier, before the end of the exhaust stroke. As such, the duration for which both valves are open can be increased, resulting in greater positive valve overlap.For example, positive valve overlap can be increased by moving the intake camshaft from a position with some positive valve overlap to a position with more positive valve overlap. Another example is increasing positive valve overlap by moving the intake camshaft from a position with negative valve overlap to a position with positive valve overlap. In one example, the VCT system can be adjusted so that negative intake-to-exhaust valve overlap control can be the standard cam position for the engine during a cold start.

[0027] It is understood that while the above example suggests increasing positive valve overlap by advancing the intake valve timing, in alternative examples positive valve overlap can be increased by adjusting an exhaust camshaft to retard the exhaust valve closing. Furthermore, each of the intake and exhaust camshafts can be adjusted to vary the positive valve overlap by varying both the intake and exhaust valve timing.

[0028] Fig. Figure 1 also shows a controller 38, which can be an electronic control system of the vehicle in which the engine system 10 is installed. In embodiments where at least one inlet or outlet valve is configured to open and close according to an adjustable control, the adjustable control can be controlled via the electronic control system to regulate the amount of exhaust gas present in a combustion chamber during ignition. The electronic control system can also be configured to command the opening, closing, and / or adjustment of various other electronically activated valves in the engine system as required to implement any of the control functions described herein. These valves can include, for example, control valves, compressor bypass valves, wastegates, EGR valves, shut-off valves, and various reservoir inlet and outlet valves.The controller can also adjust the fuel quantity and the injection control of the fuel injectors. As such, the controller can adjust the VCT system and the air-fuel ratio. To evaluate operating conditions in conjunction with the control functions of the engine system, the controller can be operationally coupled to several sensors located within the engine system. These may include flow sensors, temperature sensors, pedal position sensors, pressure sensors, an airflow mass sensor, etc. In particular, a pedal position sensor 134 is shown coupled to an accelerator pedal 130 to detect a force exerted by a vehicle operator 132. The controller 38 can use data from these various sensors to estimate other engine operating conditions. For example, the controller 38 can, as with respect to... Fig. As discussed in sections 2-3 below, estimate the condensate concentration in the intercooler.

[0029] As described above, shows Fig. Figure 1 is a non-limiting example of an internal combustion engine. It is understood that, among other things, the engine may have more or fewer combustion cylinders, control valves, throttles, and compression devices in some embodiments. Exemplary engines may have cylinders arranged in a "V" configuration. Furthermore, a common intake camshaft may control the intake valves for a first set of cylinders on a first bank, while a second intake camshaft may control the intake valves for a second set of cylinders on a second bank. That is, a common camshaft of a cam activation system (e.g., a VCT system) may be used to control the valve operation of a group of cylinders.

[0030] In the engine system described above, the airflow through the intercooler can increase during periods of increased air mass flow. If the air mass flow rises to a sufficiently high level, the increased airflow through the intercooler can remove condensate from the intercooler and force it into the engine cylinders. Combustion instability and engine misfires can occur if a sufficient amount of condensate enters the engine cylinder at once. Thus, there may be a threshold air mass flow rate that evacuates (e.g., flushes) condensate from the intercooler to the engine intake. Similarly, there may be a threshold concentration of condensate that can cause engine misfires and / or unstable combustion if it is drawn into the engine. An increased air mass flow can occur in response to a spike in torque demand, such as during a tip-in.Tip-in can be indicated by an increase in pedal position and / or an increase in throttle opening. For example, the air mass flow can increase during an acceleration event.

[0031] Combustion stability can be improved during condensate purging by adjusting combustion parameters. These parameters can include the air-fuel ratio and valve timing. For example, fuel injection can be enriched to increase combustion stability. Enriching the fuel injection involves increasing the amount of fuel injected into the cylinders for combustion, thereby reducing the air-fuel ratio. The extent of the increase in injected fuel can be based on the amount of condensate in the intercooler. For instance, a larger amount of condensate might require a lower air-fuel ratio and therefore a larger amount of fuel injected into the cylinder for stable combustion. The extent of the increase in injected fuel can also be based on the required air mass flow rate.For example, if the air mass flow rate increases to a higher rate, the air-fuel ratio for stable combustion may decrease to a lower level.

[0032] To maintain a near-stoichiometric exhaust mixture, a quantity of compressed intake air, referred to here as blow-through air or gas, can be directed from the intake manifold to the exhaust manifold. The amount of blow-through air can increase for a decreasing air-fuel ratio. Blow-through air can be generated by increasing the positive valve overlap of an intake and an exhaust valve. As discussed above, positive valve overlap is generated when both the intake and exhaust valves are open simultaneously. This allows air to move through the intake valve into the combustion cylinder and out of the exhaust valve. The amount of blow-through air can be increased by increasing the degree of positive valve overlap.To deliver a desired amount of blow-through air through the engine cylinders, the VCT system can be adjusted, in one example, from a first position with no positive valve overlap to a second position with increased positive valve overlap. In another example, the VCT system can be adjusted from a first position with some positive valve overlap to a second position with even more positive valve overlap. Increasing the positive valve overlap also improves combustion stability by scavenging residual exhaust gas from the cylinders. Thus, positive valve overlap can be used to enhance combustion stability during scavenging and to maintain near-stoichiometric exhaust gas flow.

[0033] The combustion of the rich air-fuel ratio and adjustment of the valve timing with increased positive valve overlap can continue until the mass airflow rate drops below the threshold rate and / or the condensate concentration in the intercooler drops below the threshold concentration. When one or both of these events occur, condensate may no longer enter the combustion cylinders. Thus, reducing the air-fuel ratio and increasing the positive valve overlap for improved combustion quality may no longer be necessary. As such, in response to the condensate concentration dropping below the threshold concentration and / or the mass airflow rate dropping below the threshold rate, the controller may revert the air-fuel ratio and positive valve overlap to baseline or currently requested values.This may include reverting the VCT and / or the fuel injection quantity to preset values.

[0034] In this way, the system of Fig. 1. A method for burning a rich air-fuel ratio and adjusting the valve timing to increase positive valve overlap in response to an increase in mass airflow. The burning of the rich air-fuel ratio and adjustment of the valve timing can further be based on a condensate concentration in a condensate accumulator (CAC). Furthermore, the burning of the rich air-fuel ratio and adjustment of the valve timing to generate positive valve overlap can respond to a requirement to increase the mass airflow rate above a threshold rate when the condensate concentration exceeds a threshold concentration. In an example, burning the rich air-fuel ratio involves increasing the amount of fuel injected into an engine cylinder. The degree of increase in the injected fuel quantity can be based on the condensate concentration and / or the mass airflow.The method can further include increasing the positive valve overlap of an intake and an exhaust valve to maintain a near-stoichiometry exhaust mixture during combustion at a rich air-fuel ratio. Increasing the positive valve overlap can involve increasing the duration for which both the intake and exhaust valves are open. For example, increasing the duration for which both the intake and exhaust valves are open can involve advancing the opening of the intake valve and / or retarding the closing of the exhaust valve.The procedure may further include increasing the air-fuel ratio from a rich air-fuel ratio and decreasing the positive valve overlap in response to a decrease in the air mass flow rate below a threshold rate and / or a decrease in the condensate concentration below a threshold concentration. Further details of this procedure are given below with reference to the [reference to be added]. Fig. 2-5 presented.

[0035] Fig. Figure 2 shows a flowchart of a procedure 200 for adjusting an air-fuel ratio and valve control in response to a condensate concentration in an intercooler and an air mass flow rate. A controller, such as the one in Fig. The controller 38 shown in Figure 1 may contain instructions stored on it for executing Procedure 200. Procedure 200 begins at Figure 202 by estimating and / or measuring engine operating conditions. Engine operating conditions may include engine speed and load, vehicle speed, pedal position (PP), throttle position, intercooler temperature and pressure, engine temperature, mass airflow rate, air-fuel ratio, VVT, etc. The procedure at Figure 204 involves determining whether there is a request to increase the mass airflow. For example, an increase in mass airflow may be a response to a tip-in. The tip-in may be indicated by an increase in pedal position (PP) and / or throttle opening.If there is no request to increase the mass airflow, the procedure proceeds to 206 to maintain the engine operating conditions (including valve timing and air-fuel ratio). However, if there is a request to increase the mass airflow, the procedure proceeds to 208 to determine whether the mass airflow request increases the mass airflow by more than a threshold. This threshold can be a threshold rate or a level of the mass airflow. If the mass airflow request is not greater than the threshold rate, the controller maintains the engine operating conditions at 206. However, if the mass airflow increases or will increase (due to the request) above the threshold rate, the procedure proceeds to 210.

[0036] At 210, the method can determine the condensate concentration in the intercooler based on engine operating conditions. For example, at 212, and as further with the model at Fig. As described in section 3, the rate of condensation within the intercooler is based on the ambient temperature, the intercooler outlet temperature, the air mass flow rate, the EGR, and the humidity. This allows the amount or concentration of condensate in the intercooler to be calculated. In another example, section 214 maps a condensation value to the intercooler outlet temperature and a ratio of intercooler pressure to ambient temperature. Alternatively, the condensation value can be mapped to the intercooler outlet temperature and the engine load. The engine load can be a function of the air mass, torque, accelerator pedal position, and throttle position, thus providing an indication of the airflow velocity through the intercooler.For example, a moderate engine load, combined with a relatively cool intercooler outlet temperature, can indicate a high condensation formation value due to the cool surfaces of the intercooler and the relatively low intake airflow velocity. The map may also include a modifier for ambient temperature.

[0037] Again with reference to Fig. At point 216, the procedure determines whether the intercooler condensate concentration (e.g., the amount of condensate in the intercooler) exceeds a threshold concentration. As described above, the threshold concentration may be based on the amount of condensate that could cause engine misfires or unstable combustion if it were taken up by the engine all at once. If the intercooler condensate concentration does not exceed the threshold concentration, the controller at point 218 can maintain the requested or preset air-fuel ratio and the requested or preset valve timing (e.g., VVT). However, if the intercooler condensate concentration exceeds the threshold concentration, the procedure proceeds to point 220 to decrease the air-fuel ratio and increase the positive valve overlap of the intake and exhaust valves.Details on determining the air-fuel ratio and positive valve overlap, and making the corresponding adjustments to the fuel injection and VVT systems, are described in . Fig. 4 presented.

[0038] At 222, the procedure determines whether the mass airflow has decreased below the threshold rate and / or the intercooler condensate concentration has decreased below the threshold concentration. If neither of these conditions is met, the controller maintains the adjustments for the reduced air-fuel ratio and reduced valve timing at 224. If one or more of the conditions at 222 are met, the controller can revert the air-fuel ratio and valve timing to currently requested or original (e.g., preset) settings. Thus, the conditions at 222 may indicate that the increased combustion stability is no longer required.

[0039] Fig. Figure 3 illustrates a method 300 for estimating the amount of condensate stored in an intercooler. Based on the amount of condensate in the intercooler relative to a threshold value, an air-fuel ratio and valve control during periods of increased air mass flow can be determined using the values ​​obtained at Fig. 2 and Fig. The 4 methods shown can be adjusted.

[0040] The procedure begins at 302 by determining the engine operating conditions. These can include, as previously explained at 202, ambient conditions, intercooler conditions (intake and exhaust temperatures and pressures, flow rate through the intercooler, etc.), mass airflow, MAP, EGR flow, engine speed and load, engine temperature, boost pressure, etc. Next, the routine at 304 determines whether the ambient humidity (humidity) is known. In one example, the ambient humidity might be known based on the output of a humidity sensor connected to the engine. In another example, the humidity might be inferred from a downstream UEGO sensor, or it might be obtained from an infotronic system (e.g., internet connection, a vehicle navigation system) or a rain / windshield wiper sensor signal.If the humidity is unknown (for example, if the engine does not contain a humidity sensor), the humidity can be set to 100% at 306. In an alternative embodiment, the humidity can be estimated based on inferred conditions such as intercooler efficiency and windshield wiper speed. However, if the humidity is known, the known humidity value, as provided by the humidity sensor, can be used as the humidity setting at 308.

[0041] The dew point of the intake air can be determined using the ambient temperature, pressure, and humidity. This dew point can be further influenced by the amount of EGR in the intake air (e.g., the EGR may have a different humidity and temperature than the ambient air). The difference between the dew point, the pressure at the intercooler outlet, and the intercooler outlet temperature indicates whether condensation forms inside the intercooler, and the air mass flow rate can influence how much condensation actually accumulates within the intercooler. At 310, an algorithm can calculate the saturation vapor pressure at the intercooler outlet as a function of the temperature and pressure at the intercooler outlet. The algorithm then calculates the mass of water at this saturation vapor pressure at 312.Finally, the condensation rate at the intercooler outlet is determined at 314 by subtracting the water mass at the saturation vapor pressure condition at the intercooler outlet from the water mass in the ambient air. By determining the time interval between condensate measurements at 316, procedure 300 can determine the amount of condensate within the intercooler since a last measurement at 318. The current amount of condensate in the intercooler is calculated at 322 by adding the condensate value estimated at 318 to the previous condensate value and then subtracting any condensate losses since the last routine (i.e., an amount of condensate removed, for example, via purge routines) at 320. Condensate losses can be assumed to be zero if the intercooler outlet temperature was above the dew point. Alternatively, at 320, the removed amount of condensate can be modeled or empirically determined as a function of the air mass and integrated down with each software task loop.with each run of routine 300).

[0042] Fig. Figure 4 shows a flowchart of a procedure 400 for determining an air-fuel ratio and a degree of valve overlap based on a condensate concentration in an intercooler and an air mass flow rate. The procedure begins at 402 by determining the air-fuel ratio based on the condensate concentration determined at 210 in procedure 200. For example, the controller can determine the reduced air-fuel ratio based on the degree by which the intercooler condensate concentration exceeds the threshold concentration. In one example, the air-fuel ratio can decrease as the condensate concentration increases. Determining the air-fuel ratio can also involve determining the amount of fuel injected into the engine cylinders for combustion. To decrease the air-fuel ratio, the amount of fuel injected into the cylinders can be increased.Thus, the extent of the increase in the amount of fuel injected can be based on the condensate concentration and / or the air mass flow rate. For example, more fuel can be injected, thereby reducing the air-fuel ratio, to increase combustion stability at higher condensate concentrations and air mass flow rates. In this way, the amount of fuel injected into the cylinders can increase with increasing condensate concentration and increasing air mass flow rates.

[0043] Returning to 402, after the controller determines the new air-fuel ratio, the procedure proceeds to 404 to determine the amount of forced air required to keep the exhaust near stoichiometric. A stoichiometric exhaust mixture can be one that contains no unburned fuel. As such, if an air-fuel ratio decreases, a greater amount of forced air may be required to burn all of the injected fuel during combustion. Thus, at 404, the controller can determine the amount of forced air required to maintain a stoichiometric exhaust for the reduced air-fuel ratio determined at 402.

[0044] In the 406, the controller determines the valve timing required to deliver positive valve overlap for a specified amount of blow-through air. In one example, delivering positive valve overlap might involve increasing it from negative or no positive valve overlap to a slightly higher positive valve overlap. In another example, delivering positive valve overlap might involve increasing it from a slightly higher positive valve overlap to a significantly higher positive valve overlap. Increasing positive valve overlap could involve increasing the duration for which an intake and exhaust valve of the engine are open. This allows more blow-through air to be delivered for a longer duration while the intake and exhaust valves are open simultaneously.In one example, increasing the duration for which both the intake valve and the exhaust valve are open involves advancing the opening of the intake valve and / or retarding the closing of the exhaust valve.

[0045] Adjusting the opening and closing of the intake and exhaust valves can be controlled by a variable valve timing system, such as the VCT (Variable Cam Timing) system described above. In the case of 408, the procedure involves adjusting the intake and exhaust cams of the VCT system to provide the valve timing specified in 406. The controller can also adjust the fuel injection to deliver the specified air-fuel ratio while supplying blow-through air.

[0046] In this way, the air-fuel ratio can be reduced, and the positive valve overlap of an intake and exhaust valve can be increased in response to an air mass flow rate exceeding a threshold rate and a condensate concentration in an intercooler exceeding a threshold concentration. Reducing the air-fuel ratio can involve decreasing it from a first ratio to a second ratio, where the second ratio decreases with increasing condensate concentration. Increasing the positive valve overlap can involve increasing it from a first value to a second value, where the second value is based on the amount of blow-through air required to maintain the air-fuel ratio of an exhaust mixture close to stoichiometry.The amount of blown air can increase as the second ratio decreases. Furthermore, increasing the positive valve overlap from the first value to the second value can involve adjusting a variable cam system from a first position with no positive valve overlap to a second position with increased positive valve overlap. In one example, increasing the positive valve overlap from the first value to the second value involves adjusting a variable cam system from a first position with some positive valve overlap to a second position with more positive valve overlap than the first position.If the increased combustion stability is no longer required, the process may continue to involve increasing the air-fuel ratio from the second ratio to the first ratio and decreasing the valve overlap from the second value to the first value in response to the condensate concentration falling below the threshold concentration and / or the air mass flow falling below the threshold rate.

[0047] Fig. Figure 5 shows a graphical example of adjusting an air-fuel ratio and valve timing based on a condensate concentration in an intercooler and an air mass flow rate. Specifically, the graphic shows changes in pedal position (PP) at curve 502, changes in air mass flow at curve 504, changes in intercooler condensate concentration at curve 506, changes in the degree of valve overlap of a VVT system at curve 508, changes in the combustion air-fuel ratio at curve 510, and changes in the exhaust air-fuel ratio at curve 512. At curve 508, the degree of valve overlap is shown relative to a line 514, which denotes no overlap (e.g., the intake and exhaust valves are not open at the same time).Any degree of overlap above line 514 represents positive valve overlap, while any degree of overlap below line 514 represents negative valve overlap. Adjustments to the air-fuel ratio in response to condensate concentration and air mass flow can be adjustments to the combustion air-fuel ratio. As shown in curve 510, an air-fuel ratio smaller than a stoichiometric ratio 516 can be a rich air-fuel ratio (e.g., a lower air-fuel ratio). As shown in curve 512, the exhaust air-fuel ratio can be maintained close to a stoichiometric ratio 518.

[0048] Before time t1, the air mass flow rate is below a threshold rate T1 (curve 504), and the intercooler condensate concentration is below a threshold concentration T2 (curve 506). The VVT ​​may be set such that there is a negative degree of valve overlap (curve 508). Furthermore, the air-fuel ratios of the combustion and exhaust gas may be around their respective stoichiometric ratios 516 and 518 (curves 510 and 512). Immediately before time t1, the condensate concentration in the intercooler rises above the threshold concentration T2. ​​At time t1, a tip-in may occur, as indicated by an increase in pedal position (curve 502). In response, the air mass flow rate rises above the threshold rate T1 (curve 504).In response to the increase in air mass flow above threshold rate T1 and the rise in condensate concentration above threshold concentration T2, the controller reduces the air-fuel ratio. The extent of the decrease 528 in the air-fuel ratio is based on the extent of the increase 520 in the intercooler condensate concentration above threshold concentration T2. ​​Furthermore, in response to the increase in air mass flow above threshold rate T1 and the increase in condensate concentration above threshold concentration T2 at time t1, the controller increases the extent of valve overlap (curve 508). The extent of the increase 520 in positive valve overlap can be based on the extent of the decrease 528 in the combustion air-fuel ratio.The extent of the increase 524 in the positive valve overlap provides sufficient blow-through air to keep the air-fuel ratio of the exhaust gas close to the stoichiometric ratio 518 (curve 512).

[0049] At time t2, the intercooler condensate concentration drops below the threshold concentration T2. ​​Consequently, the VVT ​​and the air-fuel ratio for combustion are returned to preset or requested values. At time t3, the tip-in period ends, and the air mass flow rate drops below the threshold rate T1. At time t4, the pedal position increases further. The air mass flow rate rises above the threshold rate T1 (curve 504); however, the intercooler condensate concentration remains below the threshold concentration T2. ​​Thus, at time t4, the valve timing and the air-fuel ratio are maintained. The air mass flow rate drops below the threshold rate T1 at time t5, while the pedal position decreases.

[0050] Between time t5 and time t6, the intercooler condensate concentration rises above the threshold concentration T2. ​​At time t6, a tip-in occurs, as indicated by the increase in pedal position (curve 502). Consequently, the air mass flow rate rises above the threshold rate T1 (curve 504). In response to the slope of the air mass flow rate above the threshold rate T1 when the condensate concentration is above the threshold concentration T2, the controller increases the positive valve overlap (curve 508) and decreases the air-fuel ratio of the combustion (curve 510). The magnitude of the increase 522 in the intercooler condensate concentration at time t6 is greater than the magnitude of the increase 520 in the intercooler condensate concentration at time t1. As such, the extent of the reduction 530 in the air-fuel ratio at time t6 is greater than the extent of the reduction 528 in the air-fuel ratio at time t1.This also results in the magnitude of the increase 526 during positive valve overlap at time t6 being greater than the magnitude of the increase 524 during positive valve overlap at time t1. The amount of blow-through air supplied at time t6 maintains the air-fuel ratio at the stoichiometric ratio 518. At time t7, the tip-in ends and the air mass flow rate drops below the threshold rate T1 (curve 504). As a result, the VVT ​​and the air-fuel ratio of the combustion are returned to their currently requested or preset values.

[0051] In one example, the controller reduces the air-fuel ratio (e.g., the combustion air-fuel ratio) and increases the positive valve overlap during a first condition (as shown at time t1 and time t6) when the air mass flow rate is above the threshold rate and the condensate concentration in the intercooler is above the threshold concentration. In another example, the controller maintains the air-fuel ratio and the positive valve overlap during a second condition (shown at time t4) when the air mass flow rate is below the threshold rate and / or the condensate concentration is below the threshold concentration. As described above, increasing the positive valve overlap, as shown at time t1 and time t6, can involve advancing the opening of an intake valve and / or retarding the closing of an exhaust valve.As shown at time t6, the decrease in the air-fuel ratio increases with increasing condensate concentration in the intercooler. Furthermore, the increase in positive valve overlap increases with decreasing air-fuel ratio and increasing condensate concentration in the intercooler. In one example, as shown at time t7, the air-fuel ratio and positive valve overlap are restored to their respective base values ​​in response to the air mass flow rate falling below the threshold rate. In another example, as shown at time t2, the air-fuel ratio and positive valve overlap are restored to their respective base values ​​in response to the condensate concentration falling below the threshold concentration.

[0052] In this way, combustion stability can be increased in response to increased airflow when the condensate concentration in an intercooler exceeds a threshold concentration. Specifically, burning a rich air-fuel ratio can improve combustion stability under conditions where intercooler condensate can be absorbed by the engine. Adjusting the valve timing to increase the degree of positive valve overlap can increase the amount of blow-through air passing through the engine cylinders during the rich air-fuel combustion period. Increasing the blow-through air volume can help maintain an exhaust mixture close to stoichiometry. Furthermore, increasing the degree of positive valve overlap can further enhance combustion stability.In this way, combustion stability can be increased during periods of potential condensate absorption, thereby reducing engine misfire events.

[0053] Note that the exemplary control routines contained herein can be used with various engine and / or vehicle system configurations. The specific routines described herein can represent one or more of any number of processing strategies, such as event-driven, interrupt-driven, multitasking, multithreading, and the like. As such, various actions, operations, or functions shown may be performed in the sequence shown, in parallel, or in some cases, omitted. Likewise, the processing sequence is not necessarily required to achieve the features and benefits of the embodiments described herein, but is presented for the convenience of illustration and description. One or more of the actions or functions shown may be performed repeatedly, depending on the specific strategy used.Furthermore, the described actions can graphically represent a code that is to be programmed into the computer-readable storage medium in the engine control system.

[0054] It is understood that the configurations and routines disclosed herein are exemplary and that these specific embodiments are not to be considered limiting, as numerous variations are possible. For example, the above technology can be applied to V-6, I-4, I-6, V-12, four-stroke boxer, and other engine types. Furthermore, one or more of the various system configurations can be used in combination with one or more of the described diagnostic routines. The subject matter of this disclosure includes all novel and non-obvious combinations and partial combinations of the various systems and configurations and other features, functions, and / or properties disclosed herein. Key to symbols

[0055] Fig. 1 42 ADMISSION 70 EXHAUST PURIFICATION DEVICE 35 ON THE ATMOSPHERE Fig. 2 202 Estimating and / or measuring engine operating conditions 204 Request for an increase in air mass flow? NO 206 Maintaining engine operating conditions 208 Air mass flow requirement > Threshold? 210 Determining the condensate concentration in the intercooler ( Fig. 3) 212 Rate model based on ambient temperature, intercooler outlet temperature, mass flow, EGR and humidity 214 Shown in relation to intercooler outlet temperature and ratio of intercooler pressure to ambient pressure 216 LLK condensate concentration > threshold? 218 Maintaining the requested air-fuel ratio and valve control 220 Reducing the air-fuel ratio and increasing the valve overlap ( Fig. 4) 222 Air mass flow < threshold or intercooler condensate concentration < threshold? 224 Maintaining the air-fuel ratio and valve timing 226 Restoring air-fuel ratio and valve control to requested / original settings Fig. 3 302 Estimating and / or measuring engine operating conditions (e.g. intercooler conditions, ambient conditions, EGR, BP, Ne, load, gain, etc.) 304 Known moisture levels? NO YES 306 Setting the humidity to 100% 308 Adjusting the humidity to a known humidity level 310 Calculating the saturation vapor pressure at the intercooler outlet based on estimated conditions and humidity 312 Calculating the mass of water at saturation vapor pressure 314 Calculating the condensation rate based on the calculated amount of water 316 Determining Δt between measurements 318 Determining the amount of condensate (since the last measurement) 320 Updating the condensate value by adding it to the previous condensate value and subtracting losses (amount of condensate removed) 322 Calculating the current amount of condensate in the intercooler end

Claims

[1] Engine process comprising the following: Burning a rich air-fuel ratio and adjusting the valve timing to increase positive valve overlap in response to an increase in air mass flow, where the combustion of the rich air-fuel ratio and the adjustment of the valve timing continue to be based on a condensate concentration in an intercooler, wherein the burning of the rich air-fuel ratio and the adjustment of the valve timing to produce the positive valve overlap respond to a request to increase the air mass flow rate above a threshold rate when the condensate concentration is above a threshold concentration, furthermore, comprehensively increasing the positive valve overlap of an intake valve and an exhaust valve to keep an exhaust gas mixture close to stoichiometry during the combustion of the rich air-fuel ratio. [2] Motor method according to claim 1, wherein the increase in air mass flow is based on a tip-in. [3] Engine method according to claim 1, wherein the combustion of the rich air-fuel ratio includes increasing the amount of fuel injected into an engine cylinder. [4] Engine method according to claim 3, wherein the extent of the increase in the amount of fuel injected is based on the condensate concentration and / or the air mass flow. [5] Engine method according to claim 1, wherein increasing the positive valve overlap involves extending a duration for which both the inlet valve and the exhaust valve are open. [6] Engine method according to claim 5, wherein extending the duration for which both the inlet valve and the exhaust valve are open includes pre-adjusting the opening of the inlet valve and / or retarding the closing of the exhaust valve. [7] Engine method according to claim 1, further comprising increasing an air-fuel ratio from the rich air-fuel ratio and decreasing the positive valve overlap in response to a reduction in the air mass flow below a threshold rate and a decrease in the condensate concentration below a threshold concentration. [8] Engine method comprising the following: Reducing an air-fuel ratio and increasing a positive valve overlap of an intake valve and an exhaust valve in response to an air mass flow rate exceeding a threshold rate and a condensate concentration in an intercooler exceeding a threshold concentration, furthermore, comprehensively reducing the air-fuel ratio from a first ratio to a second ratio, with the second ratio decreasing with increasing condensate concentration, further comprehensively increasing the positive valve overlap from a first value to a second value, the second value being based on a blow-through quantity, which is required to keep the air-fuel ratio of an exhaust gas mixture close to stoichiometry. [9] Motor method according to claim 8, wherein the amount of air passed through increases with decreasing second ratio. [10] Engine method according to claim 8, wherein increasing the positive valve overlap from the first value to the second value involves adjusting a variable cam control system from a first position without positive valve overlap to a second position with increased positive valve overlap. [11] Engine method according to claim 8, wherein increasing the positive valve overlap from the first value to the second value involves adjusting a variable cam control system from a first position with some positive valve overlap to a second position with more positive valve overlap than the first position. [12] Engine method according to claim 8, further comprising increasing the air-fuel ratio from the second ratio to the first ratio and decreasing the positive valve overlap from the second value to the first value in response to the condensate concentration falling below the threshold concentration and / or the air mass flow falling below the threshold rate. [13] Engine method comprising the following: During a first condition, when an air mass flow rate is above a threshold rate and a condensate concentration in an air charge cooler is above a threshold concentration, reducing an air-fuel ratio and increasing a positive valve overlap to maintain an exhaust gas mixture close to stoichiometry during combustion; and During a second condition, when the air mass flow rate is below the threshold rate and / or the condensate concentration is below the threshold concentration, maintaining the air-fuel ratio and positive valve overlap. [14] Engine method according to claim 13, wherein increasing the positive valve overlap includes pre-adjusting the opening of an intake valve and / or retarding the closing of an exhaust valve. [15] Engine method according to claim 13, wherein the reduction of the air-fuel ratio increases with increasing condensate concentration in the charge air cooler. [16] Engine method according to claim 15, wherein the increase in positive valve overlap increases with decreasing air-fuel ratio and increasing condensate concentration in the charge air cooler. [17] Engine method according to claim 13, further comprising restoring the air-fuel ratio and the positive valve overlap to their respective base values ​​in response to the air mass flow falling below the threshold rate and / or the condensate concentration falling below the threshold concentration.

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