Optimized combustion control of an internal combustion engine equipped with exhaust gas recirculation

An EGR equipped internal combustion engine is controlled to maximize the beneficial effects and minimize the detrimental effects of EGR on engine operation. Specifically, at least one parameter indicative of the O2 concentration in the intake mixture and/or at least one parameter indicative of the H2O concentration in the intake mixture is monitored, and the monitored parameter is relied on to control one or more aspects of engine operation by open loop adjustment of other control strategies and/or by a separate closed loop control strategy. These controls are applicable to virtually any engine, and are particularly beneficial to lean burn engines such as diesel (compression ignition) engines, spark ignited natural gas engines, and dual fuel or other compression ignited natural gas engines. The engine may be equipped with either actively controllable EGR or passive and uncontrolled EGR.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to internal combustion engines and, more particularly, relates to a system and method for controlling operation of an engine equipped with an exhaust gas recirculation (EGR) system.

2. Discussion of the Related Art

Countries worldwide are implementing ever-stricter emission(s) standards for diesel and other internal combustion engines. Past and some current standards for oxides of nitrogen (NOx), hydrocarbon (HC), and particulate emissions have been met through various improvements to engine design, advancements in fuel injection equipment and controls, etc. However, many of these techniques are incapable of meeting stricter emission standards that are being implemented or will soon be implemented by the United States and many other countries. Exhaust gas recirculation (EGR) is therefore becoming an increasingly important weapon in the war against emissions.

EGR systems have been used for decades to reduce NOx emissions and, as now developed, have been successfully applied to modem gasoline engines to meet past and current emission regulations. Because of the tightening NOx standards for compression ignition (diesel) engines, EGR systems are currently being investigated for application to diesel engine emission systems for reduction of NOx. However, application of EGR systems to diesel and other lean burn engines presents several distinct challenges. For instance, the direct recirculation of hot exhaust gases to the air intake system of a diesel engine increases air intake manifold temperature, increasing hydrocarbon emissions and particulate levels due to deterioration in the combustion process. In addition, soot and other particulates in the EGR system accumulate in the aftercooler and other components of the engine's intake and exhaust systems, decreasing the effectiveness of those components and shortening their useful lives. Moreover, unlike in a throttled otto cycle engine, an unthrottled diesel engine often experiences an insufficient differential pressure across the EGR line to generate an EGR flow sufficient to obtain an optimal EGR mass fraction in the air/EGR mixture inducted into the engine.

Some of the problems associated with attempting to reduce emissions in a diesel engine through EGR, and proposed solutions to them, are discussed, e.g., in U.S. Pat. No. 5,440,880 to Ceynow, U.S. Pat. No. 5,806,308 to Khair, and U.S. Pat. No. 6,301,887 to Gorel. For instance, the Gorel patent discloses a so-called low pressure EGR system for a turbocharged diesel engine. The Goret EGR system includes an exhaust particulate filter that is located downstream of and in fluid communication with the outlet of the turbocharger turbine for removing particulate matter from the exhaust gases. It also includes a low-pressure EGR line that extends from an inlet located within the main exhaust particulate filter to an outlet located upstream of the turbocharger compressor and downstream of the engine's air filter. An EGR valve, an EGR cooler, and an EGR return are located in series within the EGR line. In addition, an EGR pick-up unit is located at the inlet of the EGR line within the main particulate filter. It has an internal particulate filter to remove particulates from the EGR stream.

Solutions proposed by the Gorel patent and others solve some of the problems discussed above to the extent that it is now possible to implement a practical EGR system in a diesel engine on either an original equipment manufacturer (OEM) or an aftermarket basis. However, the controls of prior EGR equipped engines do not take full advantage of EGR when attempting to reduce emissions or otherwise optimize combustion control.

For instance, an increasingly popular technique for reducing emissions is to optimize engine operation based on excess air or “lambda.” Lambda is usually defined as the ratio of total air available for combustion during a particular combustion cycle to that required for stoichiometric combustion, i.e., that required to burn all of the fuel during that cycle. If lambda drops below a minimum threshold, the reduced oxygen level in the combustion chamber increases NOxand other emissions to unacceptable levels. On the other hand, if lambda rises above a maximum acceptable threshold, misfire can occur, resulting in excessive, unwanted emissions and sharply decreased thermal efficiency. Optimum lambda varies with speed, load, and other factors. Characteristics that are controlled to optimize lambda include fuel supply quantity, charge pressure or manifold absolute pressure (MAP), and air charge temperature (ACT).

Of course, oxygen is the only reactive constituent of air. The remaining constituents, principally nitrogen, are largely inert. Lambda based controls assume that the oxygen concentration in the combustion mixture is equal to the oxygen concentration in the ambient atmosphere, i.e., 21% on a mole fraction basis, and then base their calculations on that assumption. This assumption is incorrect in EGR equipped engines. The recirculated exhaust gases contain little or no oxygen and, when mixed with ambient air, produce an intake mixture that has substantially less oxygen on a mole fraction basis than ambient air. Lambda based controls therefore overestimate the reactability of the combustion mixture, leading to inaccurate calculations and resultant inferior controls. Other standard combustion control strategies similarly fail to adequately take the oxygen concentration reducing effects of EGR into account.

EGR is also relatively heavily laden with water vapor, which is a major combustion product. The mixing of EGR with intake air therefore introduces substantial quantities of water vapor into the resultant intake mixture. This water vapor introduction has two effects, one potentially beneficial and one potentially harmful, neither of which has been adequately addressed by the prior art.

First, the inventors have discovered that the water vapor in the intake mixture can have the same effect as water injection, which is widely-used in diesel engines to reduce the flame temperature in the combustion chamber for NOxreduction purposes. No known system takes this effect into account when adjusting engine operating characteristics such as ignition timing and lambda. Nor does any known system actively control engine operation to obtain a specific desired water vapor concentration dependent parameter in the intake mixture.

Second, under some engine operating conditions, the water vapor may condense after it is mixed with ambient air. This condensation can lead to accelerated corrosion of downstream components of the air intake system. Some systems attempt to prevent condensation by removing at least some water vapor from the exhaust stream in an aftercooler located upstream of the air/EGR mixing device. However, as should be apparent from the preceding paragraph, the removal of more water vapor than is required to avoid condensation results in reduced NOxreduction effectiveness due to the unnecessarily low moisture concentration of the intake mixture.

In light of the foregoing, it should be apparent that the need has arisen to optimize the combustion control of an internal combustion engine based on the actual oxygen concentration in the intake gas stream or a parameter indicative of the oxygen concentration.

As also should be apparent from the foregoing, the need has additionally arisen to take advantage of the water content in an EGR stream to reduce NOxemissions, preferably while still preventing condensation in the engine's air intake system.

SUMMARY OF THE INVENTION

In accordance with a preferred aspect of the invention, an improved method for controlling an engine equipped with an EGR system includes measuring a parameter of a constituent of an air/EGR intake mixture; calculating, based on the measuring step, a parameter indicative of a partial pressure of the constituent in the intake mixture; and adjusting at least one engine operating characteristic based on the calculating step.

The calculating step may comprise calculating a parameter that depends on a partial pressure of oxygen in the mixture, such as excess oxygen ratio (EOR). The parameter can then be used to adjust other engine control operations on an open loop basis or as a separate, closed loop control in which the determined value of the parameter is compared to a desired value, and in which the adjusting step is performed based on the comparison. In this case, the adjusting step may comprise adjusting at least one of a fuel supply timing, a fuel supply quantity, EGR flow, ignition timing, manifold absolute pressure (MAP), and air charge temperature (ACT). The characteristic(s) adjusted will vary with, e.g., the result sought and the operating characteristics of the controlled engine.

Instead of or in addition to basing controls on oxygen measurements, the measuring step may comprise measuring at least one parameter that is dependent on the concentration of moisture in the intake mixture. In this case, the calculating step may comprise determining a value of a parameter indicative of water vapor partial pressure in the intake mixture and comparing the calculated value of the parameter to a desired value of the same parameter. The adjusting step may then be performed based on the comparison. It may comprise adjusting at least one of ignition timing, fuel supply quantity, and EGR flow.

The moisture measurements may also be used to prevent condensation in the engine's intake system. In this case, the measured parameter preferably is indicative of one of 1) the ambient relative humidity and 2) the relative humidity of the intake mixture. The engine can then be controlled, based on this measurement, to avoid condensation in the engine's air intake system. For instance, the adjusting step may comprise adjusting at least one of 1) a temperature of the intake mixture and 2) the moisture concentration of the intake mixture. The moisture concentration can most easily be adjusted by adjusting the setting of an exhaust gas recirculation valve controlling EGR flow to the intake system of the engine.

In accordance with another aspect of the invention, an EGR equipped engine is provided that implements a method having some or all of the foregoing aspects. The engine may be a lean burn engine such as a diesel engine, a spark ignited gas engine, or a compression ignited pilot-fueled gas engine. Many controls described above can be performed even if the engine is equipped with a passive EGR system lacking an actively settable exhaust gas recirculation valve.

Other aspects and advantages of the invention will become apparent to those skilled in the art from the following detailed description and the accompanying drawings. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the present invention, are given by way of illustration and not of limitation. Many changes and modifications could be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Pursuant to preferred embodiments of the invention, an EGR equipped internal combustion engine is controlled to maximize the beneficial effects and minimize the detrimental effects of EGR on engine operation. Specifically, at least one parameter indicative of the O2concentration in the intake mixture and/or at least one parameter indicative of the H2O concentration in the intake mixture is monitored, and the monitored parameter is relied on to control one or more aspects of engine operation. For instance, for O2dependent control, the O2concentration in the intake gas stream can be monitored, and an excess oxygen ratio (EOR) or another oxygen partial pressure dependent parameter can be derived from the resultant data. The derived parameter can be used for open loop adjustment of another control strategy and/or can be used as the basis for a separate, closed loop control strategy designed to optimize that or a related parameter. As another example, a relative humidity or other sensor can monitor the H2O concentration in the intake mixture, and the monitored parameter or another parameter derived from it can be used 1) to avoid condensation in the engine's air intake system, 2) as the basis for open loop adjustment of another control strategy to take the parameter into account, and/or 3) as the basis for a separate closed loop control strategy for controlling that parameter to obtain a desired effect. These controls are applicable to virtually any engine, and are particularly beneficial to lean burn engines such as diesel (compression ignition) engines, spark ignited natural gas engines, and dual fuel or other compression ignited natural gas engines. The engine may be equipped with either actively controllable EGR or passive and uncontrolled EGR.

The theory behind the control strategies discussed in the preceding paragraph and application of that theory to specific engines will now be discussed in turn.

Combustion and emissions are dependent upon the oxygen concentration in the intake mixture. Combustion control therefore can be performed most effectively if it is based directly on oxygen measurements rather than indirectly through air measurements. This fact is confirmed graphically inFIGS. 1-3, which illustrate the effects of O2concentration changes in a Caterpillar model C12 dual fuel engine.FIG. 1illustrates the relationship between 1) “O2partial pressure” in an intake mixture, and 2) NOxand HC emissions, where “O2partial pressure” is defined as the partial pressure of oxygen in the intake mixture to the total pressure.FIG. 1illustrates that both NOxand HC emissions vary with O2partial pressure changes, with the optimal tradeoff at 100% load and 1880 rpm engine speed. Referring toFIG. 2, both NOxand HC emissions also vary with “gas lambda,” which is defined as the ratio of effective air available for combustion of natural gas to that required for stoichiometric combustion in the combustion chamber. The effective air available takes into consideration of EGR mass fraction within the mixture of recirculated exhaust gas and fresh intake air.FIG. 2indicates that, for a given gaseous fuel quantity, gas lambda is proportional to and varies directly with O2partial pressure.FIG. 3illustrates that both NOxand HC emissions also vary with EGR mass fraction. The decrease in NOxemissions can be attributed partly to increase in water vapor content in the gas stream, as discussed in more detail below.

Armed with the data partially tabulated in one or more ofFIGS. 1-3, it is possible to perform combustion control to optimize an O2partial pressure dependent parameter for NOxand/or HC emissions for prevailing speed and load conditions or at least take that parameter into account when optimizing another parameter, such as lambda, to obtain a desired effect, such as a desired NOxemission level, a desired HC emission level, or a desired brake specific fuel consumption (BSFC). A convenient and intuitive O2partial pressure dependent parameter, but by no means the only possible parameter, usable as a basis for this control is excess oxygen ratio (EOR). EOR is defined as the ratio of the actual oxygen in the intake mixture to the oxygen required for a stoichiometric combustion. The EOR can be determined on a cylinder-by-cylinder, cycle-by-cycle basis by 1) first determining the mass of oxygen in the combustion chamber using the detected O2partial pressure and the known supply valve opening period, or the known volumetric efficiency and cylinder displacement, then 2) determining the actual O2/fuel ratio using the determined mass of oxygen and the known fuel injection quantity, and then 3) determining EOR from the determined O2/fuel ratio and known or determined fuel quality information. EOR is an attractive basis for combustion control because it provides a more direct and more accurate indication of the available oxygen for combustion in the mixture than lambda.

Possible mechanisms for optimizing combustion based on EOR or another measured O2partial pressure dependent parameter in different engines are described generally below in connection with the flowchart of FIG.5and more specifically below with respect to the engine schematics ofFIGS. 6 and 7, respectively.

Water vapor partial pressure also affects combustion, and combustion control therefore can be optimized based on a water vapor partial pressure dependent parameter. Referring again toFIG. 3, NOxemissions decrease with increased EGR mass fraction throughout the speed and load range of the engine. This reduction results not only from the reduced excess oxygen content of the intake mixture, but also because of the flame temperature reduction effect of the increased water vapor in that stream as a result of EGR. It has also been discovered that lambda and water vapor partial pressure in the exhaust gases are inversely related. The curve10ofFIG. 4reveals that, in a typical natural gas engine, lambda decreases from about 2.0 to 1.0 as the partial pressure of water vapor in the exhaust gases increases from a mole fraction of about 0.10 to about 0.19. This relationship permits an engine equipped with EGR to be run relatively rich and yet maintain relatively low NOxemission levels by retaining a relatively high water vapor partial pressure in the intake mixture, resulted from an increased EGR mass fraction. Hence, a target lambda or corresponding target EOR could be adjusted downwardly in the presence of a relatively high water vapor partial pressure. Other combustion control strategies could be adjusted to take H2O partial pressure into account in a similar manner. The engine could also be actively controlled to maintain the water vapor partial pressure or a parameter dependent directly on it at a target level required to obtain a desired effect, such as a desired NOx emission level at prevailing speed and load conditions. Mechanisms for affecting these controls are described below in conjunction with the description of FIG.5.

High water vapor concentrations can, however, lead to condensation under some operating conditions, leading to accelerated corrosion of air intake system components. This potential problem can be avoided by monitoring the relative humidity in the mixture and taking active measures to prevent the relative humidity from exceeding 100%. These measures could include taking steps to increase air charge temperature, reduce water vapor concentration, or both. An example of these measures is described in detail below in conjunction withFIGS. 5A-5C.

3. Construction and Operation of Practical Embodiments

The techniques described above can be employed on a variety of different engines using many different control strategies. They are particularly (but not exclusively) beneficial to lean burn engines equipped with cooled, low pressure EGR. Two different types of lean burn engines to which the technique is applicable are illustrated inFIGS. 6 and 7, respectively. Both engines are controllable using specific implementations of the generic strategy illustrated inFIGS. 5A-5C, which will be discussed both in conjunction with the engine of FIG.6and the engine of FIG.7.

A. Application to Diesel Engine

Referring now toFIG. 6, a diesel engine20to which the invention is applicable includes a number of cylinders22(only one of which is shown), an intake system24supplying an air/EGR combustion mixture to the cylinders22, an exhaust system26, a fuel injection system28, an EGR system30, and other components (not shown) commonly found on a compression ignition engine such as intake and exhaust valves. The fuel injection system28includes a source of diesel fuel and at least one electronically controlled fuel injector per cylinder22. The intake system24comprises an air intake manifold32having a split outlet connected to the various cylinders22and an inlet connected to an air filter34by an intake passage36. The exhaust system26comprises an exhaust manifold38having a split inlet coupled to the various cylinders and an outlet coupled to a diesel particulate filter40by an exhaust passage42. The illustrated engine20is a turbocharged engine having a compressor44located in air intake passage36upstream of the intake manifold32and a turbine46located in the exhaust passage42downstream of the exhaust manifold38. A passive or controllable air charge cooler48and a controllable turbo air bypass (TAB) valve50are also provided in the air intake system24for further treating and/or controlling the flow of compressed air to the air flow intake manifold32. The TAB valve50is located in a TAB line51bypassing the compressor44.

In accordance with a preferred embodiment of the invention, the EGR system30is configured to recirculate a portion of the exhaust gases through an EGR line52having an inlet in fluid communication with an outlet of the turbine46and an outlet in fluid communication with the intake passage36upstream of the compressor44. The EGR system30includes, from upstream to downstream end, an EGR cooler54, an EGR valve56, and an EGR filter58, all located in the EGR line. The EGR valve56may be a completely passive valve that relies on operation of the remaining EGR system components for its setting, hence providing a totally passive and uncontrolled EGR control system. Preferably, however, it is controllable at least to the extent that it can be electronically shut off upon demand in order to halt EGR and thus prevent condensation in the intake system24. In more sophisticated systems, it may alternatively be a variable/orifice EGR valve electronically settable to actively control EGR flow to the intake system24as described in more detail below.

The outlet of the EGR line52discharges into the EGR inlet of a venturi60that is disposed in the intake passage36upstream of the compressor inlet. The venturi60also has a fresh air inlet that receives ambient air from the air filter34and a mixture outlet that discharges the air/EGR mixture to the compressor inlet. A preferred venturi suitable for drawing EGR into the incoming air stream and mixing it with the air stream is described in co-pending and commonly assigned patent application Ser. No. 10/1 93,257, filed Jul. 11, 2002, the contents of which are hereby incorporated by reference in their entirety. The preferred particulate trap (if present), EGR cooler, and EGR filter are also described in the '257 application.

A controller62is also provided for controlling operation of the fuel injectors, the TAB valve50, the EGR valve56(if the EGR valve is actively controlled), and possibly other components of the engine. The controller62receives signals indicative of O2partial pressure in the intake mixture, water vapor concentration in the intake mixture, intake mixture temperature or air charge temperature (ACT), intake mixture pressure or manifold absolute pressure (MAP), speed, load, and possibly additional data. MAP is monitored by a sensor located in or near the intake manifold. The MAP sensor may be part of a block or module64that also measures ACT at the same location. O2partial pressure indicative data pressure preferably is supplied by an O2concentration sensor66located in the intake passage36at the compressor inlet. Placing the sensor66in this location permits the controller62to calculate O2partial pressure using data from the O2sensor and the MAP sensor. The water vapor dependent parameter data preferably is obtained from a sensor arrangement generally denoted68. The sensor arrangement68preferably monitors relative humidity and temperature in the intake passage36at the compressor inlet. Alternatively, the ambient relative humidity could be measured. The H2O partial pressure and dew point can then be calculated from this data and/or data from the ACT sensor in the module64. Speed, load, and any other data desired for combustion control is supplied via known sensors, collectively denoted70in FIG.6.

The controller62can then control the TAB valve50, the EGR valve56, and/or fuel injectors based on the monitored and calculated parameters so as to optimize performance characteristics such as NOxreduction, HC reduction, and condensation prevention. The control preferably is implemented on a cylinder-by-cylinder, cycle-by-cycle basis. A routine that can be programmed into the controller62for this purpose is illustrated schematically inFIGS. 5A-5C, with the main routine being illustrated at80inFIG. 5A and H2O and O2control subroutines being illustrated at100and120inFIGS. 5B and 5C, respectively. It should be understood that the term “routine” is used herein to designate any computer implemented strategy and is not necessarily limited to the performance of operations by executing programmed codes.

Referring toFIG. 5A, the main routine80proceeds from START in Block82to a data acquisition Block84, where the controller62reads MAP, ACT, O2concentration, relative humidity or another parameter indicative of H2O concentration, speed, and load from the sensors64,66,68, and70. The routine80then proceeds to Block86, where the controller62determines an O2partial pressure dependent parameter and an H2O partial pressure dependent parameter that will serve as the basis for combustion control in the subroutines100and120ofFIGS. 5B and 5C. The controller62also determines the dew point from the measured or determined relative humidity and the measured temperature in the intake passage36.

Next, in Block88, the routine80determines whether the mixture temperature is less than the dew point. The data for the mixture temperature may come from the sensor68, the ACT sensor in the module64or, most preferably, may comprise the lower of the readings provided by the two sensors. If the answer to the inquiry is YES, the controller62takes active steps to increase ACT and/or reduce the H2O concentration sufficiently to prevent condensation in the intake system24. For instance, it may adjust the EGR valve56setting to decrease the percentage of recirculated exhaust gases. The controller62could also simply close the EGR valve56. The routine80then returns to Block88and repeats the operations of Blocks88and90until the temperature as measured by the sensor64and or the sensor68is less than the detected or determined dew point.

The routine80then proceeds to Block92to execute the H2O control subroutine100of FIG.5B. Subroutine100proceeds from START in Block102to Block104, where the subroutine100effects open-loop adjustment of one or more characteristics of the fuel and/or supply system air to take the beneficial reduction effects of the EGR water vapor in the exhaust stream into account. As indicated above, this control may be based on any parameter that is directly indicative of the water vapor content in the intake mixture. H2O partial pressure is preferred. The controller62preferably controls the engine20to make it run richer and increase ignition delay in the presence of higher H2O partial pressures. Hence, if a relatively low H2O partial pressure is detected, the controller62may control the fuel injectors to adjust diesel fuel injection quantity and/or control the TAB valve50to increase MAP in order to produce a leaner mixture. It may also retard the start of injection to compensate for a shortened ignition delay. Conversely, if the determined H2O partial pressure is relatively high, the controller62will typically control the fuel injectors to increase diesel fuel injection quantity and/or control the TAB valve50to reduce MAP to produce a richer mixture. It may also advance the start of fuel injection to compensate for an increased ignition delay. Some of these adjustments could be implemented by adjusting an existing lambda control procedure and/or by adjusting the EOR routine120described below to take the determined H2O partial pressure into account when selecting an optimal lambda or an optimal EOR. For instance, the target EOR as reflected by the inquiry Block126of the subroutine120ofFIG. 5Cmay be revised downwardly at high H2O partial pressures. The controller62would then adjust the fuel and/or air supply and/or EGR devices to obtain the revised target EOR. The magnitude of target revision is engine specific. It preferably is selected from a map that correlates the magnitude of revision for a full range of H2O partial pressures against a full range of engine speed and load conditions for a particular engine operating parameter, such as NOxreduction. Similar strategies could be used to adjust a lambda optimization routine, a fuel injection timing optimization routine, etc.

If the engine22is capable of adjusting the H2O partial pressure in the intake system24, the controller62can also effect closed loop optimization of H2O partial pressure or a parameter indicative of it to achieve a desired effect. For instance, if the EGR valve56is an actively controllable valve, it is possible to adjust the setting of the EGR valve56in a closed loop fashion to maintain the H2O partial pressure at a target or desired value that achieves a desired NOxemission level under prevailing speed, load, and fuel supply conditions. Hence, referring again toFIGS. 5 and 6, the routine100compares the determined H2O partial pressure or related parameter to the desired parameter in Block106, and the controller62adjusts the EGR valve56to vary the H2O concentration in the intake mixture in Block108. The subroutine100then returns to Block106and repeats Blocks106and108until the determined H2O parameter equals the desired H2O parameter. The subroutine100then returns to main in Block110.

Referring again toFIG. 5A, the routine80proceeds to Block94to execute the subroutine120either in series with or parallel with execution of the subroutine of FIG.5B. Execution of the subroutine120ofFIG. 5Ceffects O2partial pressure based control using a strategy similar to that utilized to effect H2O partial pressure based control. Hence, the subroutine120proceeds from Start in Block122to Block124, where it effects open-loop adjustment of combustion control based on a determined O2partial pressure dependent parameter. The O2partial pressure dependent parameter may be the O2partial pressure itself, the oxygen/fuel ratio, or, most preferably, the excess oxygen ratio (EOR). For instance, the controller62may control the fuel injectors to advance injection timing in the presence of a high EOR in order to compensate for the increased ignition delay resulting from the high EOR. Instead of or in addition to this open-ended control, the subroutine120may affect closed loop control of one or more engine operational characteristics to maintain an O2partial pressure dependent parameter at a desired value that optimizes a performance characteristic. Hence, in Block126, the subroutine120first queries as to whether the determined O2parameter equals the desired parameter and, if not, proceeds to Block128, where it adjusts engine operation to alter that parameter. In the preferred case in which the O2parameter is EOR, the same techniques that are used to adjust lambda may be used to adjust EOR. Indeed, at zero EGR, lambda equals EOR, and EOR control is identical to lambda control. One known lambda optimization technique, relying primarily on adjusting the setting of a TAB valve to adjust MAP or ACT to vary lambda, is described in detail in U.S. Pat. No. 6,273,076, the subject matter of which is hereby incorporated by reference. In addition to MAP adjustment, oxygen concentration, and therefore, EOR, can be adjusted even more directly by adjusting the setting of the EGR valve56. EOR could also be adjusted by adjusting fuel injection quantity through control of the fuel injectors. The desired BOR value for any particular engine operating under a given set of speed and load conditions will depend upon the engine performance characteristic or characteristics sought to be optimized. Typically, and for the purposes of the present example, the desired EOR can be considered to be that which strikes the ideal balance between emissions and fuel economy at prevailing rpm, load, ACT, and skip fire conditions. This “ideal balance” may vary depending upon whether the designer is primarily concerned with maximizing fuel economy or with minimizing emissions. It may also take the H2O based optimization routine of FIG. SB into account.

The closed loop control of Blocks126and128is repeated until the subroutine120determines in Block128that the determined EOR or related parameter at least approximately equals the desired parameter, at which time the subroutine returns to the main routine in Block130. The main routine80then proceeds to Return in Block96, and the process is repeated.

B. Application to Gas Engine

The combustion control strategy described above is also applicable to either spark ignited natural gas engines or dual fuel or other compression ignited natural gas engines. One such engine220is illustrated schematically in FIG.7. It contains most the same components as the corresponding diesel engine20ofFIG. 6, with components of the engine220ofFIG. 7corresponding to components of the engine ofFIG. 6being incremented by200. The engine220therefore includes:a number of cylinders222;an intake system224including an air filter234, an intake passage236, an intake manifold232, an air cooler248, a TAB valve250, and a venturi260;an exhaust system226including an exhaust manifold238, a particulate trap240, and an exhaust passage242;an EGR system230including an EGR passage252, an EGR cooler254, and an EGR valve256;a turbocharger including a compressor244and a turbine246; anda controller262receiving signals from sensors264,266, and270and controlling all electronically controlled components of the engine220.
The engine220ofFIG. 7differs from the engine20ofFIG. 6primarily in the following respects:The diesel fuel supply system228ofFIG. 6is replaced with a fuel supply system that includes a gaseous fuel supply system and an ignition source for the gaseous fuel. The gaseous fuel supply system typically will consist of an LPG or CNG source and supply valves for selectively supplying natural gas to the individual cylinders222of the engine220from that source on demand. If the engine220is a spark-ignited engine, the ignition source228will include one or more spark plugs for each cylinder to gas. If the engine220is a dual fuel or other compression ignited gas engine, the ignition source will include diesel fuel injectors of the type commonly employed by such engines and a corresponding source of diesel fuel; andThe particulate trap240is relocated at of the EGR passage inlet, and an oxidation catalyst280is placed in the exhaust outlet leading to the atmosphere. Oxidation catalyst280may be specially formulated for a lean-burn natural gas engine to reduce methane and non-methane hydrocarbons emissions.

The controller262engine ofFIG. 7controls operation of the various engine components using the same routine80illustrated inFIGS. 5A-5C. Referring again toFIGS. 5A-5C, the specific application of that routine to the engine ofFIG. 7differs from its application to the engineFIG. 6only in that:Gas fuel quantity rather than diesel fuel quantity are adjusted in Blocks104and124.In order to adjust ignition timing, pilot fuel injection timing or spark timing are adjusted in Blocks104and124rather than adjusting diesel fuel injection timing.

It can thus be seen that O2partial pressure and H2O partial pressure can be used to optimize combustion control of a variety of engines equipped with EGR. This control possibility considerably widens the range of applications for a relatively simple, inexpensive, reliable passive and uncontrolled and cooled EGR system while still meeting NOxand HC emission targets demanded by government regulations. It also can be used to prevent condensation in a variety of different EGR equipped engines in a manner that retains at least partial access to the benefits of EGR under most engine operating conditions. Also, as should be apparent from the above, the O2dependent control and H2O dependent control can be used either separately or together in a variety of different EGR equipped engines. To the extent that they might not be apparent from the above, the scope of variations falling within the scope of the present invention will become apparent from the appended claims.