METHOD AND SYSTEM FOR CONTROLLING FUEL CONSUMPTION

Abstract

Methods and systems are provided to improve fuel efficiency when treating knocking by adjusting ignition timing (retarding) and direct injection of a fluid based on engine operating conditions and the composition of the injected fluid. One or more engine parameters, such as EGR, VCT, boost pressure, and throttle position, are coordinated with the direct injection to reduce torque and EGR transients.

Description

Field of InventionThe present application relates to methods and systems for controlling the use of various fuels in an internal combustion engine.General State of the Art and Brief Description of the InventionAlternative fuels have been developed to mitigate the increasing prices of conventional fuels and reduce exhaust emissions. For example, alcohol and alcohol-containing fuel mixtures have been recognized as attractive alternative fuels, particularly for automotive applications. Different engine systems may be used with alcohol fuels using different engine technologies and injection technologies. Further, various approaches may be used to control such alcohol-operated engines to take advantage of the charge air cooling effect of the high octane alcohol fuel to particularly address engine knock. For example, engine control methods may include adjusting boost or spark control depending on alcohol fuel and various other engine operating conditions.Engines may be configured with exhaust gas recirculation (EGR) systems to redirect at least some exhaust gas from an engine exhaust to an engine intake. For example, an EGR valve may be actuated to adjust an amount of exhaust gas recirculated to the engine intake. While providing desired engine dilution, such EGR systems may improve engine performance by reducing knock, throttling losses, heat transfer losses, and NOx emissions.However, the inventors herein have recognized that during engine operating conditions, when a sudden increase or decrease in the desired amount of EGR occurs, EGR transients may be generated. Transient control of EGR is difficult because of the delay between the actuation time of an EGR valve and the corresponding change in dilution in the cylinder. For example, in response to a sudden tip-in from a low to medium load, an EGR valve may be adjusted to increase a provided amount of EGR. However, until the EGR gases mix with intake air throughout the intake manifold and the amount of EGR at the cylinders reaches the new desired amount, engine efficiency may deteriorate. Similarly, in response to a sudden tip-out from a medium load to a light load, an EGR valve may be adjusted to decrease a provided amount of EGR. However, until the amount of EGR premixed with air in the intake manifold is consumed by the cylinders, the actual EGR level in the cylinders is higher than desired and the stability and efficiency of engine combustion may be deteriorated.DE 60 2004 013 388 T2 discloses a combustion control apparatus and a combustion control method for an engine, by means of which a premix combustion can be carried out, which enables a sufficient reduction in the amount of NOx and smoke in a relatively large operating range.DE 199 02 349 A1 discloses a water injection quantity control system for a fuel and water injected engine, which is arranged for injecting two liquids, i.e. fuel and water, into a combustion chamber.DE 699 36 081 T2 discloses a compression ignition engine arranged to self-ignite a premixed charge of fuel and air to achieve reduced emissions while maintaining fuel economy. Also disclosed is an internal combustion engine operable in a premixed charge compression ignition mode.The objective technical object to be achieved can be seen in eliminating or at least reducing the disadvantages of the prior art. The object is achieved by the subject matters of the independent claims.In one example, the above issues may be at least partially addressed by a method of operating an engine with EGR. In one embodiment, during an increase in EGR flow from a first amount to a second, higher amount, the method includes increasing water direct injection to a first water injection amount more quickly and then decreasing the water injection amount to a second amount less than the first amount more slowly. The method includes adjusting an engine load at which water is directly injected into a cylinder based on the EGR flow.In one example, an engine with a turbocharger may be configured to provide boosted air charge, and an exhaust gas recirculation (EGR) passage for diverting at least some exhaust gas from the engine exhaust to the engine intake. In one example, the EGR passage may be configured to provide low pressure (LP) EGR, wherein exhaust gas is recirculated from the exhaust downstream of a turbocharger turbine to the inlet upstream of a turbocharger compressor. In an alternative embodiment, the EGR passage may be configured to provide high pressure EGR (HP) wherein the exhaust gas is recirculated from the exhaust upstream of the turbocharger turbine to the inlet downstream of the turbocharger compressor. The engine may also be configured with a direct injector for directly injecting knock control fluid into an engine cylinder. In one example, the injected fluid may be water. In alternative examples, the injected fluid may be an alcohol-gasoline fuel mixture or an alcohol fuel such as ethanol or methanol, or a mixture of one or more of these fuels with water. Herein, the inherent octane effects and / or the charge cooling effects and / or the dilution effects of the direct injected fluid may be used to treat cylinder knock, reduce engine NOx emissions, and / or provide at least some engine dilution.A desired amount of engine dilution may be determined based on engine operating conditions. For example, the desired amount of dilution may be based on engine speed / load conditions, knock probability, exhaust temperature, exhaust purification system temperature, etc. As such, the desired dilution may be largely provided by EGR under some engine conditions. Thus, based on the desired amount of dilution, an amount of EGR that may provide the desired engine dilution may be determined.In response to a sudden increase or decrease in the desired amount of dilution, an EGR valve in the EGR passage may be actuated to increase or decrease the amount of EGR provided. To compensate for EGR transients that may occur due to the delay between actuation of the EGR valve and the change in engine dilution at the cylinder, an amount of knock control fluid, e.g., water directly injected into the cylinder (e.g., from a first amount) may be increased to provide the required difference in engine dilution substantially immediately. As such, the amount of direct injected water may be adjusted to reflect an amount required to treat engine knock at the prevailing engine operating conditions and also treat dilution to reduce EGR transient control issues. Herein, by injecting water directly (direct water injection) into the cylinder, substantially immediate vaporization of the injected water to steam may be achieved, thereby providing a more rapid change in engine dilution as compared to actuation of the EGR valve.As EGR begins to operate and starts up to provide the desired dilution, direct water injection may be decreased or shut down. For example, after the direct water injection increases from the first amount, the amount of water injected may be gradually decreased to a second amount. The second amount may reflect an amount of water required to treat knock only after the desired amount of EGR has been increased. Thus, in one example, the injected second amount of water may be below the first amount.To address sudden transients that may arise due to EGR ramp-up, such as sudden tip-in or tip-out by the engine operator, the rate of water injection amount decrease may be adjusted to be below the rate of increase in response to the presence of EGR transients. That is, if the desired amount of EGR is not available, direct water injection may be rapidly increased to provide the desired dilution immediately. However, even after the desired amount of EGR is available, water direct injection may be gradually decreased, so that sudden unexpected EGR transients are better treated.To address sudden transients that may result during EGR shutdown, for example due to sudden pedal tip-in or tip-out by the engine operator, the amount of dilution with EGR during steady-state operation may be limited. For example, at medium load, the total desired dilution may be high, but achieving this dilution with EGR may not be feasible because it is impossible to quickly reduce the current EGR at the cylinder in the event of a subsequent tip-in or tip-out, which may result in a sudden decrease in the desired dilution. Under these conditions, the overall desired dilution may be achieved with a combination of EGR plus direct water injection. If a sudden decrease occurs at the desired dilution, it can be achieved by rapidly eliminating the direct water injection to the cylinder.As such, to address knock issues, the direct injected amount of water may be maintained above a lower threshold and below an upper threshold. Thus, in one example, when EGR enters a transition, the direct injected amount of water may be reduced to the lower threshold. Thereafter, further EGR transients may be treated with some VCT (variable cam timing) retard at least temporarily. In another example, when EGR enters a transition, the direct injected amount of water may be increased to the upper threshold. Thereafter, additional EGR transients may be treated with some VCT advance at least temporarily. In alternative embodiments, an amount of boost, throttle displacement, and / or an amount of spark advance may be used to compensate for dilution and torque transients.It should be appreciated that while the illustrated example is illustrated with reference to water as the direct injected knock control fluid, in alternative examples, such as where the injected knock control fluid is an alcohol mixture, the amount and timing of the direct injection may be adjusted based on the type of fluid injected. In particular, the amount may be based on a combination of the inherent octane effect, dilution effect, and charge air cooling effect of the injected knock control fluid. For example, if the injected fluid has a higher dilution effect, a larger amount of fluid may be injected to compensate for EGR transients. In another example, if the injected fluid has a low dilution effect, but a high charge cooling and / or octane effect, a lesser amount of fluid may be injected to control knock and nothing may be injected to compensate for EGR transients.In one example, the combination of effects from the molar or volumetric composition of the injected fluid may be inferred. For example, if the injected fluid is a mixture containing an alcohol fuel, the molar composition may be based on the volumetric fractions of the fluids contained in the fuel mixture, as well as their molecular weights and densities. Thus, as the alcohol content of the injected fluid increases, the inherent octane effect and the charge cooling effects increase, so that a direct injection amount for knock control can be decreased. Similarly, as the water content of the injected fluid increases, the dilution effect increases, so that the direct injection amount for dilution can be increased, and a smaller subsequent amount of VCT and / or EGR dilution may be required.It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not intended to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Moreover, the claimed subject matter is not limited to implementations that solve any disadvantages mentioned above or in any part of this disclosure.Brief Description of the DrawingsFIG. 1 shows an embodiment of an engine combustion chamber; FIG. 2 is a map illustrating variations in fuel economy loss and torque ratio with varying spark retard; FIG. 3 shows a map with which a displacement threshold may be identified by comparing spark retarded fuel economy losses to ethanol fuel injection retarded fuel economy losses; FIG. 4 shows a map comparing retarded spark losses to ethanol fuel injection losses for various cost functions; FIGS. 5A-B show a high level flow chart depicting a routine that may be implemented for retarding an amount of spark retard and fuel injection to treat engine knock; FIG. 6 shows a high level flow chart for adjusting engine dilution and engine combustion speed based on availability of knock control fluid; FIG. 7 shows a map that allows the spark retard and high octane fuel injection threshold point to be adjusted based on engine speed-load conditions; FIG. 8 shows a high level flow chart depicting a routine that may be implemented to retard the threshold point for use in spark retard and high octane fuel injection based on various engine operating condition limits; FIG. 9 shows a map depicting an example use of spark retard and direct injection of an ethanol fuel to treat knock, in accordance with the present disclosure; FIGS. 10-11 show maps illustrating example spark retard and direct injection adjustments to a knock control fluid based on the amount of fuel injected; FIG. 12 is a map illustrating example changes in direct injection (water direct injection) of water in response to EGR transients; FIG. 13 shows a map depicting example throttle displacements in response to control (relative to IVC) of water direct injection of water.DETAILED DESCRIPTIONThe following description relates to systems and methods for improving fuel usage efficiency in multi-fuel engines, such as the engine of FIG. 1 In response to engine knock, an amount of spark retard and injection of a knock control fluid may be used to treat knock. In particular, spark may be retarded to a predetermined amount (such as predetermined timing or threshold point) until it may be more advantageous (e.g., fuel saving) to use retarding spark and beyond which it may be more advantageous to inject knock control fluid to treat engine knock. As shown in FIGS. 2-5, a controller may adjust the use of spark retard and the use of direct injected and / or port injected fuels or knock control fluids to treat knock based on a cost function selected by the driver. The use may further be based on a combination of inherent octane content, dilution effect and charge air cooling effect of the available knock control fluid or fluids, for example as a conclusion from a composition (e.g. molar or volumetric) of the injected knock control fluid. As shown in FIGS. 7-8, under selected engine operating conditions (such as speed-load conditions), engine constraints (such as torque, temperature, and emissions constraints) may be determined and the predetermined timing may be correspondingly advanced. As shown in FIG. 9, responsive to knock, the controller may use spark retard to the predetermined timing to treat knock, after which the controller may use injection of a knock control fluid to treat knock. As shown in FIGS. 10-11, the adjustments between the use of spark retard and the injection of knock control fluid may also be based on the amount of knock control fluid injected, as compared to injector pulse width injection constraints. As shown in FIGS. 6 and 12, additional engine adjustments may be made based on the dilution effect of the injected knock control fluid to better coordinate the engine dilution benefits and knock suppression benefits of the injected fluid. Further, as shown in FIG. 13, throttle adjustments may be made based on a timing of the direct injection relative to intake valve closing to better compensate for torque transients. In this way, by improving the use of retarding spark timing and injecting fuel or knock control fluid in treating knock, knock control fluid may be used more reasonably while improving engine performance.FIG. 1 shows an embodiment of a combustion chamber or cylinder of engine 10. engine 10 may receive control parameters from a control system including controller 12 and input from vehicle operator 130 via input device 132. In this example, the input device 132 includes an accelerator pedal and a pedal position sensor 134 for generating a proportional pedal position signal PP. Cylinder (also herein "combustion chamber") 14 of engine 10 may include combustion chamber walls 136 with piston 138 positioned therein. The piston 138 may be coupled to a crankshaft 140 such that reciprocating motion of the piston is translated into rotational motion of the crankshaft. The crankshaft 140 may be coupled to at least one drive wheel of the passenger vehicle via a transmission system. Further, a starter motor may be coupled to crankshaft 140 via a flywheel to enable a starting operation of engine 10.Cylinder 14 may receive intake air via a series of intake air passages 142, 144, and 146. The air intake passage 146 may communicate with other cylinders of the engine 10 in addition to the cylinder 14. In some embodiments, one or more of the intake passages may include a supercharger such as a turbocharger or a supercharger. For example, FIG. 1 shows an engine 10 configured with a turbocharger, including a turbocharger compressor 174 disposed between intake passages 142 and 144, and an exhaust turbine 176 disposed along the exhaust passage 148. Turbocharger compressor 174 may be at least partially powered by exhaust turbine 176 via a shaft 180, where the supercharger is configured as a turbocharger. However, in other examples, such as when engine 10 is equipped with a supercharger, exhaust turbine 176 may optionally be omitted, and turbocharger compressor 174 may be driven by mechanical input from an electric motor or the engine. A throttle valve 20 including a throttle plate 164 may be provided along an intake passage of the engine to vary the flow rate and / or pressure of intake air supplied to the engine cylinders. For example, throttle valve 20 may be disposed behind turbocharger compressor 174, as shown in FIG. 1, or alternatively may be provided in front of turbocharger compressor 174.Exhaust passage 148 may receive exhaust gases from other cylinders of engine 10 in addition to cylinder 14. An exhaust gas sensor 128 is shown coupled to the exhaust passage 148 before an emission control device 178. The sensor 128 may be selected from among various suitable sensors to provide an indication of an air-fuel ratio in exhaust gas, such as a linear lambda sensor or a universal or wide-range exhaust gas oxygen (UEGO), a two-state lambda sensor or an EGO (as shown), a heated EGO (HEGO), NOx, HC, or CO sensor, for example. The emission control device 178 may be a three-way way catalyst (TWC), NOx trap, various other emission control devices, or combinations thereof.The exhaust temperature may be measured by one or more temperature sensors, not shown, disposed in the exhaust passage 148. Alternatively, exhaust temperature may be inferred based on engine operating conditions such as speed, load, air-fuel ratio (AFR), spark retard, etc. Further, exhaust temperature may be calculated by one or more exhaust gas sensors 128. It should be appreciated that the exhaust temperature may alternatively be estimated by any combination of temperature estimation methods listed herein.Each cylinder of engine 10 may include one or more intake valves and one or more exhaust valves. For example, cylinder 14 is shown including at least one intake assist valve 150 and at least one exhaust assist valve 156, located in an upper region of cylinder 14. In some embodiments, each cylinder of engine 10, including cylinder 14, may include at least two intake assist valves and at least two exhaust assist valves disposed at an upper region of the cylinder.Intake valve 150 may be controlled by controller 12 by cam actuation via cam actuation system 151. Similarly, exhaust valve 156 may be controlled by controller 12 via cam actuation system 153. Cam actuation systems 151 and 153 may each include one or more cams and may use one or more of CPS (cam profile switching), VCT (variable cam timing), WT (variable valve timing), and / or VVL (variable valve lift) systems that may be actuated by controller 12 to vary valve operation. Operation of the intake valve 150 and the exhaust valve 156 may be determined by valve position sensors and / or camshaft position sensors 155, 157, not shown. In alternative embodiments, the intake and / or exhaust valves may be controlled by electric valve actuation. For example, cylinder 14 may alternatively include an intake valve controlled via electric valve actuation and an exhaust valve controlled via cam actuation including CPS and / or VCT systems. In still further embodiments, the intake and exhaust valves may be controlled by a common valve actuator or system or a VVT actuator or system. As elaborated with reference to FIGS. 6 and 12, cam control may be adjusted (by advancing or retarding the VCT system to adjust engine dilution in coordination with EGR flow and / or direct injection of knock control fluid, thereby reducing EGR transients and improving engine performance.The cylinder 14 may have a compression ratio that is the ratio of the volumes when the piston 138 is at bottom dead center to top dead center. Conventionally, the compression ratio is in the range of 9:1 to 10:1. however, in some examples where different fuels are used, the compression ratio may be increased. This can be done, for example, when higher octane fuels with a higher latent enthalpy of vaporization are used. The compression ratio may also be increased when direct injection is used for its effect on engine knock.In some embodiments, each cylinder of engine 10 may include a spark plug 192 for initiating combustion. Ignition system 190 may provide spark to combustion chamber 14 via spark plug 192 in response to an spark advance signal SA from controller 12 under selected operating modes. However, in some embodiments, spark plug 192 may be omitted, such as when engine 10 may initiate combustion via auto-ignition or by injecting fuel, as may be the case with some diesel engines.In some embodiments, each cylinder of engine 10 may be configured with one or more injectors to provide knock control fluid thereto. In some embodiments, the knock control fluid may be a fuel, wherein the injector is also referred to as a fuel injector. As a non-limiting example, cylinder 14 is shown including fuel injector 166. Fuel injector 166 is shown coupled directly to cylinder 14 to directly inject fuel proportional to the pulse width of signal FPW received from controller 12 via electronic driver 168. In this way, fuel injector 166 delivers what is known as direct injection (also referred to as "DI" below) of fuel in combustion cylinder 14. While shown as a side injector in FIG. 1, injector 166 may also be located over the piston, such as near the position of spark plug 192. Such a position may improve mixing and combustion when operating the engine with an alcohol-based fuel due to the lower volatility of some alcohol-based fuels. Alternatively, the injector may be positioned at the top and near the intake valve to improve mixing. Fuel may be supplied to fuel injector 166 from a high pressure fuel system 8 including fuel tanks, fuel pumps, and a fuel rail. Alternatively, fuel may be supplied at a lower pressure via a single stage fuel pump, and then the timing of direct fuel injection during the compression stroke may be more limited than when a high pressure fuel system is used. Further, although not shown, the fuel tanks may include a pressure transducer that sends a signal to the controller 12. It should be appreciated that in an alternative embodiment, the injector 166 may be an intake port injector that delivers fuel into the intake port upstream of the cylinder 14.It will also be appreciated that in one embodiment, the engine may be operated by injecting the variable fuel or knock control fluid mixture via a single direct injector; in alternative embodiments, the engine may be operated by using two injectors (a direct injector 166 and an intake port injector) and varying the relative injection amount of each injector.Fuel may be delivered from the injector to the cylinder during a single stroke of the cylinder. Further, the distribution and / or relative amount of fuel or knock control fluid supplied from the injector may vary with operating conditions such as the air charge temperature, as described below. Further, for a single combustion event, multiple injections of the delivered fuel may be performed per stroke. The plurality of injections may be performed during the compression stroke, the intake stroke, or any corresponding combination thereof.As described above, FIG. 1 shows only one cylinder of a multi-cylinder engine. As such, each cylinder may similarly include its own set of intake / exhaust valves, fuel injector(s), spark plug, etc.Fuel tanks in fuel system 8 may contain fuel or knock control fluids of different qualities, such as different compositions. These differences may include a different alcohol content, a different water content, a different octane number, a different heat of vaporization, different fuel mixtures, different water contents, and / or combinations thereof, etc. In one example, knock control fluids with different alcohol contents may include a fuel that is gasoline, the other being ethanol or methanol. In another example, the engine may use gasoline as a first substance and an alcoholic fuel mixture such as E85 (which is about 85% ethanol and 15% gasoline) or M85 (which is about 85% methanol and 15% gasoline) as a second substance. Other alcohol fuels may be a mixture of alcohol and water, a mixture of alcohol, water and gasoline, etc. In yet another example, both fuels may be alcohol blends, where the first fuel may be a gasoline alcohol blend having a lower alcohol ratio than a gasoline alcohol blend of a second fuel having a larger alcohol ratio, such as E10 (which is about 10% ethanol) as the first fuel and E85 (which is about 85% ethanol) as the second fuel. In addition, the first and second fuels may also differ in other fuel qualities, such as difference in temperature, viscosity, octane number, latent enthalpy of vaporization, etc.In addition, fuel characteristics of the fuel or knock control fluid stored in the fuel tank may vary frequently. In one example, a driver may refill the fuel tank with E85 on a day, closest with E10, and closest with E50. The variations in tank refill from day to day may thus result in frequently varying fuel compositions, thereby affecting the fuel composition delivered by the injector 166.Although not shown, it should be appreciated that the engine may further include one or more exhaust gas recirculation passages for diverting at least a portion of the exhaust gas from the engine outlet to the engine inlet. As such, by recirculating some exhaust gas, engine dilution may be affected, which may improve engine performance by reducing engine knock, peak cylinder combustion temperatures and pressures, throttling losses, and NOx emissions. The one or more EGR passages may include an LP EGR passage coupled between the engine inlet upstream of the turbocharger compressor and the engine outlet downstream of the turbine and configured to provide low pressure EGR (LP). The one or more EGR passages may further include an HP EGR passage coupled between the engine inlet downstream of the turbocharger compressor and the engine outlet upstream of the turbine and configured to provide high pressure EGR (HP). In one example, an HP EGR flow may be provided during conditions such as absence of boost provided by the turbocharger, while an LP EGR flow may be provided during conditions such as in presence of turbocharger boost and / or when an exhaust temperature is above a threshold. The LP EGR flow through the LP EGR passage may be adjusted via an LP EGR valve, while the HP EGR flow through the HP EGR passage may be adjusted via an HP EGR valve, not shown.Controller 12 is shown in FIG. 1 as a microcomputer, including microprocessor unit 106, input / output ports 108, an electronic storage medium for executable programs and calibration values, shown as read only memory chip 110 in this particular example, random access memory 112, random access memory 114, and a data bus. Controller 12 may receive various signals from sensors coupled to engine 10, in addition to those signals previously discussed, including measurement of inducted mass air flow (MAF) from mass air flow sensor 122; engine coolant temperature (ECT) from temperature sensor 116 coupled to cooling sleeve 118; a profile ignition pickup signal (PIP) from Hall effect sensor 20 (or other type) coupled to crankshaft 140; throttle position (TP) from a throttle position sensor; and manifold absolute pressure signal (MAP) from sensor 124. An engine speed signal RPM may be generated by the controller 12 from the signal PIP. Manifold absolute pressure signal MAP from a manifold pressure sensor may be used to provide an indication of vacuum or pressure in the intake manifold. Still other sensors may include fuel level sensors and fuel composition sensors coupled to the fuel tank or tanks of the fuel system.Storage medium read-only memory 110 may be programmed with computer readable data representing instructions executable by a processor 106 for carrying out the methods described below as well as other variants that are anticipated but not specifically listed.During selected engine operating conditions, such as at low engine speeds and under high load conditions, engine knock may occur. In one example, engine knock may be treated by retarding spark control, also referred to herein as retarding spark. By retarding spark control from MBT, peak cylinder pressure and temperature may be reduced, thereby reducing the occurrence of knock. Retarding spark from the MBT, however, also reduces engine thermal efficiency and torque output. Thus, to meet the driver demand torque when retarding spark, the air and fuel flow may be increased to compensate for torque loss. Thus, the additional air and fuel compensation results in reduced fuel economy. The resulting fuel economy penalty SPARK FE_LOSS may be calculated as follows: where TR is the torque ratio. The change in fuel economy loss is represented in map 200 of FIG. 2 by curve 202. As shown by curve 204, the torque ratio is highest when the spark is at MBT, but decreases as the spark is retarded.Instead of retarding spark, engine knock may be treated by injecting knock control fluid into the cylinder. The knock control fluid may be a fuel having a highly effective octane content such as an ethanol fuel. As such, the effective octane content of an injected fuel may be comprised of various components representing different knock mitigation characteristics of the fuel, such as a combination of the inherent octane number of the fluid, a dilution effect of the fluid, and an evaporation effect of the fluid. For example, an ethanol fuel mixture (such as E85) may be directly injected into the cylinder to take advantage of the inherently high octane number of the fuel (inherent fuel octane component) and the charge air cooling effect of the ethanol fuel (evaporating octane component). However, due to the lower heating value (LHV)) of ethanol as compared to gasoline, a larger volume of fuel may need to be burned to achieve the same energy output. Thus, despite the improved thermal efficiency of operating closer to MBT spark control, there is a reduction in volumetric fuel economy (miles per gallon). The resulting fuel economy penalty ETHF FE_LOSS may be calculated as follows: where E is the ethanol content in mass percent in the direct injected fuel, D is the mass percent of the direct injected fuel required to alleviate knock, LHV ETH and LHV GAS are the lower heating values of the fuels, and ρ ETH and ρ GAS are the densities of the fuels.Under knock conditions, a controller may determine whether to retard the spark by a required amount and accept the thermal efficiency and fuel economy loss associated with the amount of spark retard, or whether to leave the spark at MBT and directly inject a required amount of knock control fluid (such as injection of an ethanol fuel) and accept the volumetric fuel economy loss associated with the ethanol injection. For example, an engine controller may be configured to compare the fuel economy penalty from direct injection of an ethanol fuel (such as E 85) with the fuel economy penalty from spark retard as follows:In one example, based on a map such as map 300 of FIG. 3, the degradation in fuel economy from retarded spark ( 302) may be compared to the degradation in fuel economy from ethanol injection ( 304). Based on the comparison, a timing or threshold point 306 may be determined. As such, threshold point 306 (also referred to herein as a costability point or shift point) may represent a predetermined amount of retard or timing (such as in crank angle degrees or retard degrees of spark), after which increasing injection of the ethanol fuel may provide a fuel economy benefit over retarded spark, and before retarding spark may provide a fuel economy benefit over increasing injection of the ethanol fuel when handling engine knock. Thus, in response to engine knock, an engine controller may first retard spark control to the predetermined amount of retard. After the spark control reaches the predetermined timing, the controller may increase an amount of knock control fluid (or fuel) directly injected into the cylinder to suppress engine knock while maintaining the spark control at the predetermined amount of retard (i.e., at the predetermined timing). That is, the use of retarding spark timing and increasing direct injection of knock control fluid may be switched at this point.As such, the predetermined timing may be based on engine operating conditions (such as engine speed and load conditions) and further based on an operator selected cost function. As stated above, these selected cost functions may include fuel economy, CO 2- emissions, and / or price effectiveness.The predetermined timing may be further adjusted based on the effective octane content of the injected fuel. As such, the injected knock control fluid may include one or more of gasoline, ethanol, methanol, windshield wiper fluid, other alcohols, water, and combinations thereof. In one example, when the direct injected fuel is an ethanol-fuel mixture, the effective octane content of the injected fluid may be based on the alcohol content of the fuel, and thus the threshold point may vary based on the alcohol content of the fluid. For example, the predetermined timing may be retarded from MBT as the alcohol content of the injected fuel increases. Thus, an ethanol fuel mixture with a lower amount of ethanol (such as E10 having about 10% ethanol) may have a lower threshold point (i.e., a relatively lower amount of retard) than an ethanol fuel mixture with a larger amount of ethanol (such as E85 having about 85% ethanol).In an example where the ethanol fuel is E85 and the selected cost function is volumetric fuel economy (miles per gallon), the predetermined timing (306) may be 11 degrees of spark retard. During knock conditions, as octane demand increases, spark retard may be used to treat knock up to 11 degrees of spark retard, as the loss of heat efficiency associated with spark retard up to this point is lower than the volumetric loss of fuel economy associated with E 85. After spark is retarded to 11 degrees, increasing injection of E85 may provide a lower loss of fuel economy than spark retard. Thus, to address further knock, spark ignition control may be maintained 11 degrees retarded from MBT while increasing an initiated E85 amount to meet the knock address demand.While the above example uses volumetric fuel economy as the cost function in determining the predetermined amount of retard to address engine knock, it should be appreciated that other cost functions may be used in alternative examples. Alternative cost functions may include, for example, exhaust emissions (such as CO 2- emissions) and price effectiveness (such as operating costs in miles per dollar). In an example where the cost function is price effectiveness, fuel prices of the injected fluid may be considered. Thus, a cost loss ETH DOLLAR_LOSS resulting from ethanol fuel injection, may be calculated as follows:In another example where the cost function is CO 2- emissions, CO 2- losses ETH CO2_LOSS resulting from an ethanol fuel injection may be calculated as follows: where C GAS and C ETH are the CO 2- amounts produced by each fuel in kg CO 2 produced per GJ of energy released from the fuel. This CO 2- cost function may be calculated in various ways, such as CO 2- total tailpipe emissions, CO 2- fossil-origin tailpipe emissions, or CO 2- emissions for the entire fuel life (borehole to wheels).Thus, based on the driver selected cost function in response to engine knock, an engine controller may determine whether to use a retarded spark or a knock control fluid injection (e.g., an ethanol fuel) after comparing the retarded spark related cost function loss to the ethanol direct injection. That is, if the cost function is price effectiveness, a threshold point may be determined as follows:Similarly, if the cost function is CO 2- emissions, a threshold point may be determined as follows:In general, the cost function loss comparison may be determined as follows:FIG. 4 shows a map 400 of curves generated from Expressions (3), (6), (7) and (8). Specifically, map 400 shows late spark retard from MBT on the x-axis and cost function loss comparisons (i.e., a difference between a loss due to late spark retard and a loss due to ethanol fuel injection for the selected cost function as a percentage) on the y-axis. Curve 406 (dashed line) shows a comparison of losses when the cost function is volumetric fuel economy, curve 404 (dash-dot line) shows a comparison of losses when the cost function is CO 2- emissions, and curve 402 (solid line) shows a comparison of losses when the cost function is price effectiveness.In either case, a corresponding predetermined amount of spark retard or timing (or threshold point) may be determined where the curve crosses zero. In the case of curve 404, the threshold point occurs at 408, while for curve 406, the threshold point occurs at 410, where if it is below the threshold point, it may be more advantageous to retard spark, while above the threshold point, it may be more advantageous to retard spark and increase ethanol fuel injection. In the event that a curve is always above zero, such as shown with reference to curve 402, it may be increasingly advantageous to inject the ethanol fuel directly instead of retarding spark.While the examples of FIGS. 3-4 are shown with reference to direct injection of the ethanol fuel E85, it should be understood that this is not meant in a limiting sense and that other knock control fluids may be used in alternative examples. These may include, for example, various alcohol fuel mixtures having a higher or lower alcohol content or fluids or fuel mixtures having different inherent octane, dilution or vaporization properties, compressed natural gas (CNG), water, methanol, washer fluid (which is a mixture of about 60% water and 40% methanol), etc.As such, in addition to the predetermined amount of spark retard, an amount of knock control fluid directly injected into a cylinder may also be adjusted based on the effective octane content of the injected fluid (or fuel) to treat knock. For example, the adjustment may include a decrease in the amount of knock control fluid directly injected as the effective octane content of the fluid increases. As mentioned previously, the effective octane content of a given fluid may be a combination of the inherent octane content of the fluid (or octane number), a dilution effect of the fluid (based on the inert content of the fluid such as water), or an evaporation effect of the fluid (based on the heat of vaporization of the fluid). In one example, when the injected fluid is CNG, the effective octane content of the fuel may be based only on the octane number of the CNG fuel because the CNG may have substantially no charge air cooling effect (i.e., no evaporating octane component) or dilution effect (i.e., no inert component). In another example where the injected fluid is an ethanol fuel mixture, the effective octane content of the fuel may be based on the inherent octane component of the fuel as well as an evaporating octane component because of the charge air cooling effect of ethanol, but there may be no dilution effect octane component. In yet another example, when the injected fluid is water, the effective octane content of the fluid may be based on the dilution effect and the charge air cooling effect of only water. In one example, the octane content of the injected fluid may be based on the molar composition of the fluid. Thus, the predetermined timing and the amount of directly injected fluid fuel can be adjusted on the molar composition of the injected fuel. However, in an alternative example, the octane content of the injected fluid may be based on the volumetric composition of the fluid.The inherent octane component of a fuel may be evaluated for research octane number (RON) and / or engine octane number (MON). In the case of fuel blends, alcohols having a small molecular weight, such as ethanol and methanol (which have higher RON and MON values) are added to the gasoline to improve the RON value of the gasoline blend. The addition of even a small amount of alcohol (for example, 10 percent by volume) may result in a large and non-linear increase in the RON value of the mixed fuel, thus complicating a determination of the change in the RON value for the gasoline fuel due to the addition of the alcohol. In some cases, a "mixed RON (bRON)" value may be used to describe the effects of adding small amounts of alcohol to gasoline in mixed fuels. The bRON value may be calculated based on the volumetric composition of alcohol and gasoline in the mixture as follows: wherein X v,alc is the volume fraction of alcohol in the fuel mixture, RON blend is the RON value of the alcohol-gasoline fuel mixture, RON base is the RON value of the base gasoline, and bRON v,alc is the mixed RON value of alcohol in the base gasoline based on the volumetric content. However, since the bRON value depends on the concentration of alcohol added to the base gasoline, the RON value of the base gasoline, and the hydrocarbon composition of the base gasoline, there may not be a single bRON value for a given alcohol, thereby limiting the usefulness of the bRON approach.In another approach, the inherent octane component of a mixed fuel may be calculated based on the molar composition of the fuel. Compared to the volumetric composition approach based on the volume of alcohol and gasoline in the liquid state fuel mixture (i.e., as prepared and typically measured), the molar composition reflects the number of molecules of alcohol and gasoline in the mixture. As such, the molar composition may be more relevant for describing the gas phase chemical reactions occurring with vaporized fuel in the engine cylinder. Further, in the gaseous state, the volumetric composition may be about equal to the molar composition. The molar composition of a fuel mixture may be calculated similar to the volumetric composition (as described in Equation 9), except that the alcohol content is calculated on a molar basis (as described below in Equation 10)) using the density and molecular weight of the alcohol (i.e., known physical properties of the alcohol) and the density and average molecular weight of the gasoline (as measured or estimated). Since the density and molecular weight of gasoline do not have much variance for commercial gasoline, estimates for such calculations may be sufficiently precise. Thus, for a fuel blend, the bRON value may be calculated based on the molar composition of alcohol and gasoline in the blend as follows: wherein X is m,alc the molar fraction of alcohol in the fuel blend, RON is blend the RON value of the alcohol-gasoline fuel blend, MW is base the average molecular weight of the base gasoline, MW is alc the molecular weight of the alcohol, ρ is base the density of the base gasoline, ρ is alc the density of the alcohol, and bBRON is m,alc the mixed RON value of alcohol in the base gasoline based on the molar content. It is understood that although Equations (9)-(11) are shown in the context of mixed RON values, the same may be applied to RON and MON values. As such, an RON value calculated based on the molar composition of a mixed fuel may have a substantially linear relationship with the alcohol content of the fuel. Thus, mixed RON values calculated on a mole basis may show little to no dependence on the alcohol concentration in the fuel or on the RON value of the base gasoline. In addition, there may be considerably less variation in the mixed RON molar value of a mixed fuel as compared to its mixed RON volumetric value. Further, the resulting molar mixed RON of an ethanol-gasoline fuel mixture may be substantially equal to the mixed RON for pure ethanol, while the molar mixed RON of a methanol-gasoline fuel mixture may be substantially equal to the mixed RON for pure methanol.A simplified formulation for estimating RON or MON for an alcohol additive to a gasoline composition may be determined as follows: wherein X is m,alc the mole fraction of alcohol in the fuel mixture, X is v,alc the volume fraction of alcohol in the fuel mixture, RON is blend the RON value of the alcohol-gasoline fuel mixture, RON is base the RON value of the base gasoline, RON is alc the RON value of the pure alcohol (e.g., 109 for ethanol and methanol), and RON is ga the alcohol specific liquid volume ratio, The molecular weights and densities of the alcohol and generally gasoline comprise (e.g. 0.400 for ethanol or 0.25 for methanol). Alternatively, equations (12) and (13) may be combined as follows:Thus, in one example, such an approach for determining the inherent octane component (and thus the effective octane content) of a mixed fuel may be determined more accurately and reliably, which in turn may utilize the predetermined amount of spark retard or the predetermined timing (or threshold point) for the use of spark retard and increasing direct fuel injection in treating knock. For example, based on engine operating conditions, a knock feedforward probability and a RON value required to treat knock may be determined. The mixture calculation may then be used to determine an amount of directly injected knock control fluid or fuel (e.g., ethanol) that may be required to meet the RON requirement. In another example, in a multi-fuel vehicle operating with a fuel added with ethanol, an amount of ethanol in the mixed fuel may be estimated from feedback from a oxygen sensor (such as an air-fuel ratio estimating oxygen sensor). This in turn allows the RON value of the fuel to be predicted and the threshold point to be used for spark control and fuel injection. The same may also be used as feedforward information to allow adaptation of a spark control strategy after a refueling event, but before knock sensor feedback is used. By adjusting the amount of knock control fluid or direct injected fuel and the predetermined timing or threshold point based on the molar composition of the injected fluid (e.g., using the mixed RON octane number of the fuel as determined by a molar approach), octane benefit calculations from the alcohol fuel injection may be linearized and simplified. However, in alternative embodiments, the calculations may be based on the volumetric composition of the injected fluid.In some engines, the fluid injected to treat knock may also be one of a second fluid coupled to a second auxiliary fluid tank. Examples of second knock control fluids may include water, methanol, ethanol, higher octane gasoline, washer fluid, or combinations thereof. In one example, when the injected fluid is water, the effective octane content of the fluid may contain only an evaporating octane component and a diluting effect octane component. In another example, where the direct injected fluid is washer fluid, the effective octane content of the fuel may include an inherent fuel octane component, an evaporating octane component, and a dilution effect octane component. In still further examples where EGR is used, only one dilution effect octane component may be included.As such, to increase the air charge cooling effect of the injected knock control fluid, direct injection of the fluid may be performed in response to knock. Here, by directly injecting knock control fluid into the cylinder, evaporative cooling of the fluid is enhanced and the effective octane number of the fluid injected into the cylinder can be rapidly increased, thereby reducing knock margin. Based on the amount of knock control fluid directly injected into the cylinder, an amount of fuel injected via the intake passage may be adjusted such that the total amount of fuel supplied via the fuel injectors meets stoichiometric requirements.The octane number Oct req required to treat knock may be calculated as follows: where X is the direct injected fraction of knock control fluid and (1-X) is the injected fraction of the fuel inlet passage, Oct DI is the effective octane value of the knock control fluid supplied by the direct injectors, and Oct PFI is the effective octane value of the knock control fluid supplied by the suction passage injectors. In equation (15), X may be resolved to determine the minimum fraction of directly injected knock control fluid that may be required to treat knock as follows:As stated above, the effective octane content (or effective octane number) of a knock control fluid supplied by a direct injector and / or an intake port injector may be determined as a combination of the plurality of octane components or effects as follows: and wherein Oct INH, Oct EVAP and Oct DIL are the inherent vaporizing and diluting effect octane components for the directly injected (prefix DI) or intake port injected (prefix PFI) knock control fluids or fuels, and further wherein: wherein A and B are calibrateable constants, E DI is the ethanol content in the direct injection fuel system in mass percent, and E PFI is the ethanol content in the port injection fuel system in mass percent. By calculating the effective octane content of each fluid and solving equations (15) and (16) with these values, the ratio of direct fluid injection and port injection for treating knock can be determined.By retarding spark timing to a predetermined amount of spark retard and increasing a direct injected amount of knock control fluid to suppress engine knock after spark timing reaches the predetermined amount of spark retard while maintaining spark retard at the predetermined amount of spark retard or the predetermined timing, the amount of knock control fluid used to decrease knock may be reduced while volumetric fuel economy and range of a vehicle may be increased.Referring now to FIGS. 5A-B, an example routine 500 is shown for adjusting a predetermined spark timing (or threshold point) until which a retarded spark timing may be used to address knock and after which an amount of knock control fluid directly injected into the cylinder may be increased to address engine knock. By using at least some spark retard to address knock before an amount of knock control fluid directly injected is increased to address knock, substantial volumetric fuel economy benefits may be achieved.At 502, engine operating conditions may be estimated and / or measured. These may include, for example, engine speed, desired torque, MAP, BP, ECT, catalyst temperature, intake air temperature, spark timing, boost, etc. At 504, a feedforward probability for engine knock may be determined based on the estimated engine operating conditions. At 506, a desired engine dilution may be determined based on the estimated engine operating conditions as well as the determined knock feedforward probability. At 508, and as further elaborated with reference to FIG. 6, a knock control fluid injection, VCT, and EGR amount may be determined to provide the desired dilution. Specifically, based at least on the dilution effect of the injected knock control fluid, an amount of engine dilution provided by the injection may be determined and corresponding EGR and VCT adjustment may be performed. At 510, an amount of knock control fluid or fuel available in the engine fuel system may be determined. At 512, and as further elaborated at FIG. 6, the position of a charge motion control valve in the engine intake may be adjusted to thereby adjust an engine combustion speed based on the availability of knock control fluid in the fuel system. At 514, an effective octane content of each fuel or knock control fluid in the engine fuel system may be determined. In particular, for multi-fuel vehicles, the effective octane content of each of the plurality of fuels (such as primary and secondary fuels or knock control fluids) in the fuel system of the engine may be determined. As stated above, the effective octane content of each fuel or knock control fluid may be calculated based on a combination of the inherent octane component (or fuel octane number) of the evaporating octane component or the effect and the dilution octane component or the effect of each fluid. In one example, one or more of these components or effects may be based on the alcohol content of the knock control fluid.At 516, a cost function selected by the operator may be determined. The cost function may include, for example, volumetric fuel economy (miles per gallon), exhaust emissions, miles per dollar, etc. In one example, the vehicle operator may prefer fuel economy over exhaust emissions. In another example, the operator preferences may be weighted. For example, a customer may weight a preference for fuel economy more than a preference for low exhaust emissions. Operator preferences and inputs may be received from the vehicle operator through an active display device such as a display on the vehicle instrument panel for interacting with the vehicle operator or a control panel on the vehicle display. Alternatively, operator cost function preferences may be inferred based on a previous driving behavior, for example, non-aggressive drivers may be considered to prefer fuel economy.At 518, a threshold point may be determined based on the operator selected cost function, estimated engine operating conditions, estimated probability of engine knock, and effective octane content of available fuels or knock control fluids. Specifically, the predetermined timing until which spark may be retarded (i.e., the predetermined amount of spark retard) and a ratio of usage of spark retard and direct injection of a knock control fluid may be determined to cumulatively provide an octane amount for treating engine knock.For example, a degree of spark retard may be correlated with a change in octane number by 1 (1 ON). In another example, where E 85 is direct injected fuel and gasoline is port injected fuel, the direct injection of E 85 may provide a greater octane number than a corresponding port injection of gasoline. For example, a direct injection of E85 of 0.28% may correlate with a change in 1 ON, while a 100% direct injection of E85 may correlate with about 140 RON of port injected gasoline.At 520, it may be determined if further adjustments to the threshold point are required. As embodied with reference to FIGS. 7-8, under some conditions, such as certain engine speed / load conditions, there may be engine constraints that may require the threshold point to be adjusted such that a greater or lesser amount of spark retard is used and accordingly an adjustment in knock control fluid injection amount is also made. For example, during conditions where the requested torque may be (at least temporarily) limited or exhaust temperature may be increased, or particulate matter emissions may be increased, or engine pre-ignition may occur, etc., the threshold point may be adjusted. If it is determined that threshold point adjustments are required, then at 522 and as executed in FIG. 8, the routine may adjust the predetermined timing. In particular, the predetermined threshold may be adjusted under these conditions (thereby retarding an amount of spark advance and adjusting a knock control fluid injection amount used to reduce knock), although there may be a transient loss in the operator selected cost function due to the adjustment to address the engine limitations mentioned.In one example, the predetermined timing may be stored in a look-up table in memory of the controller, in which a predetermined amount of spark retard and an amount of direct knock control fluid injection may be tabulated for a given speed-load condition and a given knock control fluid. As such, the lookup table readings may have been previously calculated using maps, such as the maps of FIGS. 3-4, where cost function losses due to spark retard are compared to cost function losses for various fuel or fluid combinations. Further adjustments to the predetermined timing and direct injection may also be tabulated for selected engine speed-load windows, such as those shown in the map of FIG. 7.If no threshold adjustments are required at 520, or after threshold adjustments are completed at 522, then an octane number required to treat the expected knock may be calculated at 524. As previously stated, knock may be treated by retarding spark timing and / or increasing an octane number of knock control fluid injected into the cylinder. As such, a larger octane number may be required in the cylinder if spark is maintained at MBT, while a smaller octane number may be required in the presence of retarded spark. Therefore, by retarding an amount of spark timing and an amount of knock control fluid injected into the cylinder, an effective octane value of the cylinder may be adjusted to treat knock.At 526, based on the predetermined (and adjusted) timing and further based on the octane value required to treat the expected knock, an amount of spark retard and a knock control fluid injection profile for the engine may be determined. The knock control fluid injection profile may include an amount of a first fuel or a knock control fluid for direct injection and an amount of a second fuel for port injection into the cylinder. For example, a first amount of fluid (such as a first fuel) directly injected into a cylinder may be based on the molar (or volumetric) composition of the injected fluid and a dilution effect of the injected fluid. Then, a second amount of fluid (such as a second fuel) injected into the cylinder via the intake passage may be based on the first amount of fluid injection. At 528, it may be determined if engine knock occurs, i.e., if feedback knock occurs. In one example, the feedback knock may be determined using a knock sensor coupled to the engine. If feedback knock has not occurred, then at 530, the controller may proceed to operate the engine by retarding the engine to the predetermined timing and injecting fuel or knock control fluid according to the profile previously determined at 526.If feedback knock is determined, then at 532, it may be determined whether spark is still prior to the predetermined timing, i.e., whether more spark retard is possible. If yes, that is, if spark retard is not limited, the controller may treat feedback knock by continuing to retard spark retard at 534 to the predetermined timing. As such, this may represent a relatively faster and more immediate feedback knock reduction approach. In one example, after the feedback knock has been treated, spark timing may be reset to the original value (i.e., the value responsive to the feedforward knock probability) while a direct fuel injection is initiated slowly. In comparison, if the adjustments are after the predetermined timing, that is, if spark retard is limited, then at 536, the controller may treat feedback knock by maintaining the spark retard at the predetermined timing while increasing an amount of fuel or knock control fluid injected into the cylinder. As such, this may represent a relatively slower approach to reducing feedback knock. As further elaborated with reference to FIGS. 10-11, the adjustments may also be made based on the amount of fuel or fluid required to treat knock and injector pulse width limitations compared to that amount. Regardless of the adjustments made at the predetermined timing, at 538 and as further elaborated at FIGS. 6 and 12, dilution limits of the engine may be adjusted based on the predetermined amount of retard (or predetermined timing), the amount of knock control fluid / fuel injected, and at least the dilution effect of the injected fluid to compensate for EGR and torque transients.In one example, an engine may be configured with direct injection of E85 and port injection of gasoline. Based on engine operating conditions, a knock probability may be determined and the predetermined timing may be determined as 11 degrees spark retard. To address the expected feedforward knock, the controller may use a setting of 5 degrees spark retard and 10% ethanol direct injection. That is, the settings may be before the predetermined timing. In response to feedback knock, the controller may immediately address knock by gradually retarding spark control from 5 degrees to the predetermined timing of 11 degrees. After the spark control reaches 11 degrees of spark retard, further feedback knock may be treated by maintaining the spark control at 11 degrees retard while increasing direct injection from E 85 from 10% to 12%. Alternatively, if feedback knock is reduced after using 11 degrees spark retard, the controller may advance the spark retard to 5 degrees (i.e., reduce the amount of spark retard) and increase the E85 direct injection to 12%.In an alternative example, knock feedforward probability may be treated based on engine operating conditions by adjusting 11 degrees spark retard and 8% ethanol direct injection. That is, the settings may be beyond the predetermined timing.Herein, in response to feedback knock, spark retard may be maintained at the predetermined amount of spark retard, here 11 degrees of spark retard, while E85 injection is increased from 8% to 12%.In this way, engine knock may be treated by retarding to a predetermined timing of a threshold point, and after the retarding of the spark has reached the threshold timing, the controller may treat further knock by maintaining the retarding of the spark and increasing injection of knock control fluid into the cylinder, thereby providing the required effective octane value to treat engine knock.An example use of retarding spark timing and direct injection of an ethanol fuel to treat knock is shown in map 900 of FIG. 9. Map 900 illustrates an octane value along the x-axis required to treat knock, an amount of spark retard along a first y-axis 901, and an amount (in %) of direct injection of an ethanol fuel (here, E 85) along a second y-axis 902. In the depicted example, a predetermined timing 904 may be determined based on engine operating conditions, the molar composition, and combination of the inherent octane effect, the dilution effect, and the vaporization effect of E85, and the operator-selected cost function of fuel economy. In this example, the predetermined timing 904 may correspond to 11 degrees of spark retard. Thus, as the octane value required to treat knock increases, the spark may be retarded (dashed line 906) to reduce knock, as the fuel economy loss associated with the thermal efficiency penalty of the retarded spark may be less than the volumetric fuel economy loss associated with the reduced energy content of E 85. The spark retard may be used until a predetermined amount of spark retard or timing 904 is reached. For example, in response to knock, the spark may be retarded to 5 degrees of spark advance, and as knock continues, the spark may be retarded to 11 degrees of spark advance. After a predetermined timing 904 is reached, the volumetric fuel economy penalty associated with E 85 may be lower than that associated with retarding spark. Thus, after a predetermined timing 904 is reached, the spark may be maintained at 11 degrees of retard and the direct injection of E 85 may be increased (solid line 908) to meet the additional octane requirement. As stated above, the predetermined amount of spark retard or timing and amount of directly injected knock control fluid may be adjusted based on a composition (e.g., molar or volumetric composition) of the injected fluid. In the illustrated example, the injected fluid is a mixed fuel that includes a first fuel and a second fuel. Thus, here, the molar composition of the mixed fuel is based on the volumetric fraction, the molecular weight, and the density of each of the first and second fuels in the mixed fuel. For example, as shown, the adjustments may include retarding the predetermined timing further away from MBT as the mole fraction of the alcohol (here, ethanol) in the mixed fuel increases. Similarly, the amount of fluid directly injected can be decreased as the molar fraction of alcohol in the mixed fluid increases.Referring now to FIG. 6, an example routine 600 for adjusting one or more engine operating parameters to thereby adjust an engine dilution and an engine combustion speed is shown. In particular, an engine operating parameter may be adjusted to thereby adjust engine dilution based on at least the dilution effect of knock control fluid directly injected in response to engine knock as well as the availability of the knock control fluid.At 602, a desired percent dilution may be determined based on the estimated engine operating conditions. In one example, a higher percent dilution may be desired as engine load increases to reduce the likelihood of engine knock and improve engine NOx emissions, but subject to a combustion stability limit. As such, engine dilution may be affected by one or more of the following: an amount of EGR, VCT, and the dilution effect characteristics of a direct injected knock control fluid. At 604, an amount of knock control fluid (such as water, ethanol, methanol, alternative fuels, etc.) available in the engine fuel system may be determined. In one example, this may include estimating a fuel level from the fuel level sensor coupled to the fuel tank of the selected fluid.At 606, it may be determined if the available amount of knock control fluid exceeds a threshold. In one example, the threshold may be based on an amount of fluid required to treat knock (e.g., a minimum amount required to treat knock). If the amount of fluid is above a threshold, then at 608, a volume of knock control fluid required to provide the desired engine dilution may be determined.At 610, a volume of knock control fluid required to treat a feedforward probability of knock may be determined. As stated above, the amount of knock control fluid injected may be based on engine speed and engine load and other operating conditions, and may be further based on a combination of an inherent octane effect, a dilution effect, and an evaporation effect of the injected fluid. At 612, the maximum volume required to provide the desired engine dilution and the volume required to treat knock may be injected into the cylinder. At 614, a CMCV coupled to the engine intake upstream of the injectors may be adjusted (e.g., opened) to decrease an engine combustion speed while increasing injection of the knock control fluid. In particular, by adjusting (e.g., opening) the CMCV at higher engine loads to decrease an engine combustion speed at the higher engine loads, peak cylinder pressure and the rate of pressure increase may be reduced to allow engine operation at higher loads. At the same time, knock resulting from the reduced engine combustion rate may be advantageously treated by increasing direct injection of the knock control fluid while adjusting the CMCV such that the engine combustion speed decreases. As such, when the CMCV is opened and the knock control fluid is injected, spark timing may be maintained at a predetermined amount of spark retard. The predetermined amount of spark retard may also be based on engine load and the combination of the inherent octane effect, the dilution effect, and the vaporization effect of the injected fluid.In one example, a controller may determine whether the CMCV is opened at high engine loads based on the availability of the knock control fluid. For example, the adjustment may include opening the CMCV while the engine is operating at high engine loads if the availability of the knock control fluid (e.g., by closing from a fuel level) exceeds a threshold. The adjustment may be further based on the composition of the injected knock control fluid. For example, the CMCV may be opened while the engine is operating at the higher engine load as the alcohol content of the (sufficiently available) knock control fluid increases.If the available amount of knock control fluid does not exceed the threshold, then at 618, the available amount of knock control fluid is directly injected. Then, at 620, the CMCV is adjusted (e.g., closed) to increase the engine combustion speed because sufficient levels of knock control fluid are not available for treating knock resulting from a decrease in the engine combustion speed. In this way, by adjusting operation of the CMCV, particularly at high engine loads, an engine combustion speed may be coordinated with knock suppression operations based on availability of a knock control fluid.At 616 and 622, one or more engine operating parameters may be adjusted based on CMCV adjustment and / or direct injection of a knock control fluid to maintain engine torque and a desired engine dilution. In one example, one or more of the following may be adjusted based on injection: throttle opening, an amount of EGR, VCT, spark advance, valve lift, and / or engine boost. The engine operating parameter may be adjusted based at least on the dilution effect of the injected fluid. In one example, the adjusted engine operating parameter may include an amount of EGR (or EGR flow). Herein, when the injected fluid has a higher dilution effect, the adjusting may include increasing the amount of direct injected fluid based on the dilution effect of the fluid and decreasing the amount of EGR based on the increased amount of injected fluid. When the injected fuel has a lower dilution effect as compared, the amount of direct injected fluid may be decreased while the amount of EGR is maintained. In this way, the dilution provided by the EGR may be coordinated with the dilution provided by the injected knock control fluid.In another example, the adjusted engine operating parameter may be variable cam timing (VCT). Herein, when the injected fluid has a higher dilution effect, the adjustment may include increasing the amount of direct injected fluid based on the dilution effect of the fluid and using less "internal EGR" via VCT based on the increased amount of injected fluid. As such, the VCT displacement required to reduce this dilution and provide less "internal EGR" may be based on the VCT configuration of the engine (e.g., whether it is only intake, only exhaust, twice equal, or twice independent). Thus, based on the VCT type, under some conditions, the VCT may be retarded to reduce dilution, while under other conditions based on the VCT type, the VCT may be advanced to reduce dilution. In comparison, if the injected fuel has a lower dilution effect, the amount of direct injected fluid may be decreased while maintaining "internal EGR" via VCT.In yet another example, the adjusted engine operating parameter may be an amount of valve lift, wherein the adjustment may include, when the injected fluid has a higher dilution effect, increasing the amount of direct injected fluid based on the dilution effect of the fluid and decreasing an amount of dilution due to the valve lift based on the increased amount of injected fluid. When the injected fuel has a lower dilution effect as compared with the injected fuel, the amount of direct injected fluid can be decreased while the amount of dilution due to the valve lift is maintained. Still other combinations of EGR, VCT, and valve lift adjustments may be possible.An alternative engine operating parameter may also be adjusted. For example, engine charging may also be adjusted based on the CMCV adjustment. In one example, if the knock control fluid level level is lower than the threshold, then the CMCV may be closed (to increase combustion speed and avoid knock) while engine boost is reduced. In this case, the maximum engine torque or boost may be reduced to avoid an excessively high cylinder pressure and / or an excessively high rate of pressure increase. In another example, if the amount of knock control fluid is below the threshold, the CMCV may be closed while spark is retarded.At 624, it may be determined whether any changes in the fuel injection to spark retard ratio have been made as previously carried out in FIG. 5B (at 528) in response to feedback knock. If no, the routine may end. If adjustments were made to the ratio, then at 626, an amount of knock control fluid injected may be limited and / or one or more of VCT, EGR, valve lift, spark timing, boost, etc., may be retarded to maintain desired engine dilution and torque output based on the change in the fuel injection to spark ratio.In this way, by adjusting whether the CMCV is opened at high engine loads based on the availability of a knock control fluid and adjusting knock control fluid injection based on the CMCV opening, CMCV operations may be adjusted based on the availability of a knock control fluid while compensating for torque transients with an engine operating parameter such as boost and compensating for dilution transients with an alternative engine operating parameter such as VCT, EGR, or valve lift. Thus, during a first condition, when the amount of knock control fluid is above a threshold, the CMCV is opened at higher engine loads while the injection of the knock control fluid is increased. In comparison, during a second condition, when the amount of knock control fluid is below the threshold, the CMCV is closed at higher engine loads while engine boost is reduced. Alternatively, the CMCV may be closed and the spark may be retarded.Referring now to FIG. 7, a map 700 is shown for further retarding the threshold point or predetermined timing between usage of spark retard and direct fuel injection. In particular, map 700 identifies regions (indicated herein as regions 701- 706) based on speed-load windows where the threshold point may be adjusted at an increased or decreased predetermined amount of spark retard and corresponding decreased or increased direct injection of a fuel or knock control fluid to address alternative engine limitations.In the first region 701 of the map, characterized by average load (BMEP) and average speed conditions, the threshold point may remain unadjusted after determination in FIGS. 5A-B. That is, in region 701, a retarded spark timing and fuel injection ratio may be used to treat knock based on engine operating conditions, the octane content of the fuel, and the operator selected cost function, as carried out in FIGS. 5A-5B.Region 702 may be characterized by high speed and high load conditions. In this area, the threshold point for exhaust temperature and particulate restrictions may be adjusted. In particular, exhaust temperatures may be increased during high load and high speed conditions. The retarding action of spark may result in additional unburned fuel in the exhaust gas under such conditions, which may further increase the exhaust temperature. This may further increase particulate emissions. In comparison, the use of ethanol fuel injection may provide the benefits of providing both the charge air cooling effect of ethanol in reducing the elevated exhaust temperatures and lower particulate emissions of ethanol fuels (as compared to gasoline). In the case of a boosted engine, the reduction in exhaust temperatures may also help reduce a turbine inlet temperature, thereby reducing turbocharger durability issues and minimizing enrichment related to increased turbine inlet temperatures. Thus, in region 702, the threshold point may be adjusted to retard the predetermined amount of spark displacement used to treat knock and correspondingly increase the amount of knock control fluid directly injected to treat knock. As such, this may result in a transient drop in volumetric fuel economy, but this may be accepted in light of exhaust temperature and particulate constraints.Region 703 may be characterized by very high speed and load constraints. In this region, exhaust temperature and particulate restrictions may be more limited than those discussed above for region 702. Thus, to limit greatly increased exhaust temperatures and particulate emissions, the predetermined amount of spark retard may be further reduced and knock control fluid injection may be further increased. In one example, substantially no spark retard may be used and knock may be treated substantially or fully using knock control fluid injection. Further, knock may be treated more aggressively by at least temporarily operating the cylinders with a rich injection of the knock control fluid.The region 704 may be characterized by low speed, high load conditions. Under such conditions, the engine may be susceptible to pre-ignition combustion events. In this area, spark advance restriction may be treated by adjusting the threshold point such that the used predetermined amount of spark retard is reduced while an amount of knock control fluid injection is increased. In one example, substantially no spark retard and substantially all knock control fluid injection may be used to reduce the likelihood of pre-ignition.The region 705 may be characterized by low speed, medium load conditions. Under such conditions, cooled EGR may be limited. For example, there may be a delay in achieving the desired amount of cooled EGR. Herein, the cooled EGR constraint may be treated by adjusting the threshold point such that the predetermined amount of spark retard is increased while decreasing an amount of injected knock control fluid. Alternatively, if spark retard is limited, injection of knock control fluid, particularly one with a higher dilution effect (such as water), may be increased. Additionally, multiple compression stroke injections of knock control fluid may be used for EGR management. Engine dilution adjustments based on coordination of EGR flow with direct injection of knock control fluid with a higher dilution effect (such as water) are further detailed herein with reference to FIG. 12.The region 706 may be characterized by low speed, low load conditions. Under such conditions, torque may be limited. In particular, during tip-in (e.g., from a light load to a medium load), the use of spark retard may cause torque transients such that driver demand torque is not met. Thus, to reduce the occurrence of torque transients, the threshold point may be adjusted such that an amount of spark retard used is decreased while an amount of knock control fluid injection (such as a direct injected amount of water) is increased. Additionally, multiple compression stroke injections of the fluid may be used for torque management. In boosted engines, the use of increased knock control fluid injection may also assist in treating turbo lag issues. For example, the use of increased knock control fluid injection during low speed, low load conditions may enable a rapid build-up of turbocharger boost and improve turbocharger transient torque control.Referring now to FIG. 8, an example routine 800 for adjusting the threshold point (i.e., predetermined timing) to use spark retard and direct injection of a knock control fluid based on various engine operating limitations is shown.At 802, an amount of spark retard and an amount of knock control fluid injection may be determined based on engine operating conditions and the predetermined threshold point (or predetermined amount of retard) as previously carried out in FIGS. 5A-B. At 804, it may be determined if there are any torque constraints. In one example, torque constraints may be present during low speed, low load conditions, as shown in region 706 of FIG. 7. At 806, the torque restrictions may be treated by adjusting the threshold point such that an amount of spark retard is reduced while an amount of knock control fluid injection is increased. For example, spark timing may be advanced to MBT when engine torque is below a requested (e.g., driver requested) torque. Additionally, multiple compression stroke fuel injections may be used to address torque transients under such conditions. For example, a number of compression injections may be increased when engine torque is below the requested torque.At 808, it may be determined if any exhaust temperature constraints are present. In one example, elevated exhaust temperatures may be at high and very high speed and load conditions, as shown in regions 702 and 703 of FIG. 7. Under such conditions, the use of retarded spark may result in increased exhaust temperatures. In contrast, a directly injected knock control fluid may provide an air charge cooling effect that reduces knock and spark retard resulting in lower exhaust temperatures. Problems with increased exhaust temperatures may also lead to turbocharger problems due to increased turbine inlet temperatures. At 810, exhaust temperature restrictions may be treated by adjusting the threshold point such that an amount of spark retard is reduced while an amount of knock control fluid injection is increased. Specifically, spark timing may be advanced to MBT when exhaust temperature exceeds an upper threshold, while spark may be retarded further away from MBT when exhaust temperature falls below a lower threshold. Under some conditions, such as very high speed and load conditions, substantially no spark retard may be used (i.e., spark control may be maintained at MBT), while substantially all octane requirement may be met by knock control fluid injection. In an example where the injected fuel is the ethanol fuel E85, exhaust temperature constraints may be treated with the ethanol fuel by at least transient rich operation. Further, the increased direct injection of knock control fluid may be provided using multiple compression stroke injections.At 812, it may be determined whether there are any particulate matter (PM) restrictions. In one example, increased particulate emissions may occur during high and very high speed and load conditions, such as shown in regions 702 and 703 of FIG. 7. Under such conditions, the use of spark retard may increase an amount of unburned fuel in the exhaust gas, resulting in increased PM emissions. In contrast, ethanol fuels can inherently produce lower PM concentrations. The PM constraints may be treated at 810 by adjusting the threshold point such that an amount of spark retard is reduced while an amount of knock control fluid injection is increased. Specifically, the ignition timing may be advanced toward MBT as the exhaust particulate emission concentration increases (e.g., exceeds a threshold).At 814, it may be determined if there is a probability of pre-ignition. In one example, pre-ignition may occur during low speed, high load conditions, such as shown in region 704 of FIG. 7. Under such conditions, the use of spark retard may increase the incidence of pre-ignition. In contrast, ethanol fuels may produce a charge air cooling effect that may reduce the occurrence of pre-ignition. Thus, at 810, the spark advance constraint may be treated by adjusting the threshold point such that an amount of spark advance is reduced to late while an amount of knock control fluid injection is increased. In one example, the likelihood of pre-ignition may be inferred from the knock intensity. Herein, the adjusting may include advancing the spark control to MBT when the knock intensity exceeds a pre-ignition threshold.At 816, it may be determined if there are cooling EGR constraints. In one example, cooled EGR may be restricted during low speed, medium load conditions, such as shown in region 705 of FIG. 7. Under such conditions, injection of a secondary fluid, such as an ethanol fuel, water, or washer fluid, may produce a dilution effect that can rapidly provide the desired dilution while limiting cooled EGR. In particular, under low speed, medium load conditions, if the level of cooled EGR is not as high as requested, then by injecting knock control fluid, the desired dilution may be provided substantially immediately while increasing EGR flow. Knock control fluid injection adjustments in coordination with the EGR flow to provide a desired dilution are further detailed herein with reference to FIG. 12. Thus, at 818, an amount of spark retard may be increased if spark retard is not already limited. However, if spark retard is already limited, then the cooler EGR constraint may be treated by maintaining spark retard and increasing an amount of knock control fluid (such as water) injected into the cylinder.At 820, it may be determined if the engine cooling system is a smart cooling system. As such, smart cooling systems may be configured to provide multiple engine temperature settings based on multiple engine coolant temperature (ECT) settings. If so, then at 822, the controller may adjust the smart cooling system such that the engine cooling system is operated with a first, lower engine temperature setting during spark retard (i.e., before the threshold point) and with a second, higher engine temperature setting during direct injection (i.e., after the threshold point). By using a lower ECT setting at spark retard and a higher ECT setting with fuel injection, engine performance may be improved.At 824, the controller may operate the engine to address knock by using retarded spark to the threshold point and increasing direct injection of knock control fluid after the spark is retarded to the threshold point while maintaining spark timing at the threshold point.Referring now to FIGS. 10 and 11, example spark retard adjustments and knock control fluid direct injection increases responsive to knock are shown. In particular, the examples illustrate adjustments based on a direct injected amount of knock control fluid in relation to the pulse width of the direct injector.In map 1000 of FIG. 10, changes in spark timing retard (spark retard) are shown in plot 1002, while changes in a direct injected amount of knock control fluid are shown in plot 1004. During a first condition, at t 1, in response to an indication of knock (shown by an arrow), spark timing may be retarded (i.e., spark retard may be increased) to a predetermined timing 1003 (i.e., to a predetermined amount of retard). At the same time, the direct injected amount of knock control fluid may be maintained. At t 2, after the spark timing reaches the predetermined amount of retard or the predetermined timing 1003, the spark timing may be maintained at the predetermined amount of retard while increasing a direct injected amount of knock control fluid to suppress engine knock. As such, the amount of direct injection may be determined based on engine operating conditions. For example, in the first state depicted in FIG. 10, the amount of direct injection required to treat knock at t 2 may be a first (higher) amount 1008. Here, the first extent 1008 may be greater than the smallest pulse width 1006 of the direct injector. At t 3, responsive to additional knock, the amount of direct injected fuel may be further increased to suppress engine knock while maintaining engine spark timing at the predetermined amount of retard.Referring now to FIG. 11, map 1100 shows changes in spark retard (retarded spark) at plot 1102, while changes in a direct injected amount of knock control fluid are shown at plot 1104. During a second condition at t 1, responsive to the indication of knock, as with the first condition shown in FIG. 10, spark timing may be retarded to a predetermined timing 1003 (i.e., spark retard may be increased). At the same time, the direct injected amount of knock control fluid may be maintained. At t 2, after the spark timing reaches the predetermined timing 1003, the spark timing may be maintained at the predetermined timing while increasing a direct injected amount of knock control fluid for suppressing engine knock. In the second condition shown in FIG. 11, the amount of direct injection required to treat knock at t 2 may be a second (lower) amount 1108. Here, the second extent 1108 may be less than the smallest pulse width 1006 of the direct injector. As such, it may not be possible to provide the second smaller amount of direct injection that is less than the pulse width of the injector. Thus, at t 2, the direct injector may be constrained and may inject an amount corresponding to the smallest pulse width of the injector. That is, the injector may inject an amount greater than the amount required to treat knock (i.e., greater than 1108). Thus, to compensate for the difference, when the required injection amount is smaller than the smallest pulse width of the direct injector, the spark may be advanced from the predetermined amount of retard while increasing the amount of knock control fluid directly injected into the engine. The ignition advance may be adjusted based on a difference between the required injection amount and the pulse width of the injector. For example, the amount of spark advance may be increased or an amount of spark retard may be decreased as a difference between the amount of knock control fluid that needs to be directly injected to treat knock and the smallest pulse width of the injector increases. Then, at t 3, in response to additional knock, spark may again be retarded to the predetermined timing 1003. At t 4, after spark timing reaches the predetermined timing, further knock may be treated by increasing the amount of knock control fluid directly injected to suppress engine knock while maintaining spark timing at the predetermined timing.Referring now to FIG. 12, map 1200 shows an example adjustment to direct water injection of water (or knock control fluid containing some water or other non-combustible fluid) with the EGR flow to provide a desired dilution. By adjusting the water direct injection in coordination with the EGR flow, a desired dilution may be provided while reducing EGR transient control issues. Map 1200 shows changes in direct injected water amount at plot 1202, changes in EGR flow at 1204, changes in engine dilution at 1206, and changes in VCT at 1208. It should be appreciated that in an alternative example, while the illustrated example is shown using water as the knock control fluid, the knock control fluid may be ethanol, methanol, another alcohol, wiper fluid, or combinations thereof.Prior to t 1, a desired dilution may be determined based on engine operating conditions (such as an engine speed and load condition). Based on the desired dilution, an EGR flow, a VCT setting, and a water injection amount may be determined. In one example, at t 1, a sudden increase in engine dilution and EGR flow may be requested. This increase in EGR flow may be requested during pedal tip-in (which in turn results in throttle tip-in), for example. Accordingly, at t 1, an amount of EGR flow may be increased. In one example, the EGR flow may be an LP EGR flow received through an LP EGR passage coupled between the engine intake upstream of a turbocharger compressor and the engine exhaust downstream of a turbocharger turbine. However, in alternative examples, the EGR flow may be an HP EGR flow received through an HP EGR passage coupled between the engine inlet behind a turbocharger compressor and the engine exhaust upstream of a turbocharger turbine. The LP EGR flow may be increased by actuation of an LP EGR valve in the LP EGR passage (plot 1204). However, from the time the LP EGR valve is actuated and the time the increased EGR flow is achieved and obtained at the cylinder, there may be a delay. In particular, the delay in mixing EGR gases with intake air to achieve the desired EGR amount / flow and the arrival of the mixed EGR flow at the cylinder may result in EGR transients, which may temporarily degrade engine efficiency. Herein, to compensate for the EGR transients, at t 1, during an increase in LP EGR flow (such as from a first amount of EGR to a second, higher amount of EGR), a direct water injection may be increased to a first water injection amount. Herein, the substantially immediate vaporization of the direct injected water in the cylinder may provide a substantially immediate dilution effect that compensates for the EGR transients and allow the desired dilution to be provided substantially immediately. The amount of direct water injection may be based on the amount of EGR in the EGR flow. Then, the direct water injection may be decreased to a second water injection amount less than the first water amount. As shown in plot 1202, the rate of increase of water direct injection may be faster than the rate of decrease of water direct injection. By adjusting and coordinating water direct injection and EGR flow, the desired dilution may be provided immediately.At t 2, responsive to a decrease in the requested dilution, the EGR flow (e.g., LP EGR flow) may be reduced. In one example, the reduction in LP EGR flow may be requested during a pedal tip-out. Here, also during the reduction in the LP-EGR flow, the direct water injection may also be reduced (to allow an immediate drop in the dilution) after the water injection amount may be gradually increased.In one example, it may be desirable to maintain the direct injected amount of water within an upper threshold 1201 and a lower threshold 1203 due to knock concerns. Thus, during the increase or decrease in LP EGR flow, a VCT may be adjusted based on the amount of water injection and further based on the EGR flow. For example, during the reduction in LP EGR flow initiated at t 2, when the required water injection amount is below the lower threshold 1203, the water injection amount may be reduced to and held there for only a period between t 2 and t 3 (as shown by dotted segment 1207), while VCT timing is retarded for the same period (between t 2 and t 3) (curve 1208). Herein, by adjusting the VCT based on the direct injection amount, the direct water injection may be maintained at a threshold required to maintain knock while the desired engine dilution is provided at least in part by the VCT. In an alternate example not shown, the VCT may be advanced during an increase in LP EGR flow when the required water injection amount is above the upper threshold. Herein, the direct water injection may be maintained at the upper threshold amount for a period of time (e.g., temporarily) while the VCT is advanced for the same period. It should be appreciated that the VCT displacement used (e.g., VCT retard or advance displacement) may be based on the VCT type. Thus, if the VCT is of a first type, VCT advance may be used, and if the VCT is of a second type, VCT retard may be used to achieve the desired dilution effect.A timing with reference to engine load of the increasing direct water injection in response to an increase in EGR flow may be adjusted based on the EGR flow and engine torque. The adjustment may include, for example, increasing water direct injection at lower engine loads when EGR flow is below a threshold and engine torque is lower, and reducing water direct injection when EGR flow exceeds the threshold. For example, as shown at t 4, in response to engine load being below a threshold and EGR flow being below a threshold, a direct injected amount of water is gradually increased. Herein, at the lower engine load, water direct injection of water may be increased in anticipation of an increase in engine load (such as due to sudden tip-in) and in anticipation of EGR transients resulting from the delay in increasing EGR flow, although to a lesser extent, to provide at least a portion of engine dilution that may be requested during the expected increase in engine load. In an alternate example not shown, at higher engine loads (i.e., when engine load is above a threshold and EGR flow is above a threshold), a reduction in engine load (such as due to sudden tip-out) may be expected. Further, in expectation of EGR transients resulting from a delay in reducing EGR flow, water direct injection of water may be reduced, although to a lesser extent, to at least partially reduce a portion of the engine dilution decrease that may be requested during the expected decrease in engine load. In either case, the rate of increase in water injection may be faster than the rate of decrease in water injection to also better compensate for EGR transients. Thus, load balancing can be effectively performed in expectation of load changes.Herein, water direct injection of water may include an amount of water that is directly injected as well as a rate of water direct injection. As such, the amount of direct water injection may be based on the amount of EGR in the EGR flow. Water direct injection may also be adjusted based on the presence of an HP EGR flow. It should be understood that while the example shown is described in the context of direct water injection, this is not intended to be limiting, and in alternative examples, the direct injected fluid may be knock control fluid such as ethanol, methanol, other alcohols, water, gasoline, windshield wiper fluid, and combinations thereof. At this time, the amount and timing of the direct water injection may be adjusted based at least on the dilution effect of the injected fluid.In this way, by better anticipating changes in engine dilution based on engine load conditions and adjusting an amount of water directly injected into the engine cylinder in coordination with an EGR flow, EGR transient control problems may be better anticipated and treated. By adjusting an operating parameter such as VCT, spark advance, or boost, knock may be further treated based on the amount of water injected while also providing the desired engine dilution.Referring now to FIG. 13, example direct injection timing adjustments relative to intake valve closing timing (IVC) are shown with throttle adjustments to compensate for torque transients. Specifically, an amount of knock control fluid (such as fuel, water, ethanol, etc.) may be directly injected into an engine based on engine operating conditions, while an engine throttle position may be adjusted based on the knock control fluid injection amount and the injection control. In a non-limiting example, the knock control fluid includes water, although in alternative embodiments, a different knock control fluid may be used. Map 1300 shows a timing of direct injection of water at plot 1301. In particular, at 1302 and 1303, two example injections are shown. As such, each injection control is illustrated in the context of its relationship to IVC. Corresponding throttle displacements are shown in curve 1304.At 1302, a first direct water injection of water is shown. The direct water injection may be adjusted based on knock, desired engine dilution, and / or EGR transients. As such, water direct injection may include adjustments to a timing and / or amount of water direct injection. In the example shown, the first injection timing may be such that substantially all water is injected before IVC. Thus, substantially all of the injected water may evaporate and occupy the space of the air charge in the cylinder. Herein, to compensate for a reduced volumetric efficiency resulting from the cylinder filling effect of the injected water, the engine throttle opening may be adjusted to a first, greater amount, the amount based on the amount and timing of the injected water to achieve the desired torque. While the example shown shows the entire first injection timing before IVC, in an alternative example, at least a portion of the first injection timing may be before IVC.In comparison, a second direct water injection is shown at 1303. In the example shown, the second injection timing may be later than the first injection timing such that substantially all water is injected after IVC. That is, the second injection timing is more retarded from IVC than the first injection timing. Thus, at the time air is trapped in the cylinder (IVC), a smaller portion of the injected water may take up the space of the air charge in the cylinder, thereby producing a less impact on volumetric efficiency. To obtain the desired torque with higher volumetric efficiency, the engine throttle may be adjusted to a second, lesser amount of opening, the amount based on the amount and timing of direct water injection.The timing and amount of knock control fluid (here water) injected may be adjusted relative to IVC based on engine operating conditions. Engine operating conditions may include knock, desired engine dilution, and / or EGR transients. Thus, in one example, injection timing may be retarded from IVC when volumetric efficiency is not a constraint and maximum water evaporation and mixing is desired. In another example, injection timing may be retarded from IVC when volumetric efficiency becomes a restriction on injected water. In yet another example, injection timing may be retarded from IVC as knock increases or as desired engine dilution increases. It will be appreciated that as the timing of injection changes in relation to IVC, the time available for water evaporation before IVC may vary, thereby changing the amount of water that evaporates before IVC and occupies the space of the air charge trapped in the cylinder. As such, the relationship may be linear on a molar basis only. In either case, a throttle adjustment may be made based on the expected amount of cylinder charge.While the example shown is described in the context of direct water injection, this is not intended to be limiting, and in alternative examples, the direct injected fluid may be a knock control fluid such as ethanol, methanol, other alcohols, water, gasoline, and combinations thereof. Herein, the injected amount of knock control fluid may be adjusted based on a combination of the inherent octane value, the dilution effect, and the vaporization effect of the injected fuel. For example, an amount of knock control fluid injected may be decreased as the vaporization effect of the injected fluid increases. In another example, the amount of fluid injected may be adjusted based on a molar composition or alcohol content of the injected fluid. Additionally, the engine throttle position may be further adjusted based on the dilution effect of the injected fluid.Although not shown in FIG. 13, additional adjustments, such as one or more of an amount of VCT and EGR, may be adjusted responsive to the amount of water direct injection to provide a desired engine dilution. In one example, the amount of VCT and / or EGR may be decreased as the amount of knock control fluid injected increases. In this way, torque and EGR transients may be better anticipated and treated.In this way, by adjusting a threshold point between using spark timing retard and water direct injection of knock control fluid, fluid usage may be more reasonable and fuel economy benefits may be obtained. By adjusting the threshold point based on an alternative cost function, such as lower operating costs or lower exhaust emissions, engine performance may be maintained while providing the benefits of the selected cost function. By coordinating EGR, throttle, VCT, and boost adjustments based on timing, amount, and type of knock control fluid injected, EGR transients and torque transients may be reduced while handling knock.Note that the example control and estimation routines included herein may be used with various system configurations. The specific routines described herein may represent one or more of a number of processing strategies such as event-driven, interrupt-driven, multi-tasking, multi-threading, and the like. As such, various acts, operations, or functions shown may be performed in the sequence shown, in parallel, or in some cases omitted. Likewise, the order of processing is not necessarily required to obtain the features and advantages of the embodiments described herein, but is provided for ease of illustration and description. One or more of the illustrated acts, functions, or operations may be repeatedly performed depending on the particular strategy being used. Further, the described operations, functions, and / or acts may graphically represent code to be programmed into the computer readable storage medium in the control system.It should be understood that the configurations and routines disclosed herein are exemplary in nature and that these specific embodiments are not to be considered in a limiting sense because numerous variations are possible. For example, the above technology can be applied to V-6, I-4, I-6, V-12, 4-clock boxer and other engine types. The subject matter of the present disclosure includes all novel and non-open combinations and sub-combinations of the various systems and configurations, and other features, functions, and / or characteristics disclosed herein.The following claims particularly emphasize certain combinations and sub-combinations that are considered novel and non-obvious. These claims may refer to "a" element or "a first" element or the equivalent thereof. Such claims should be understood to include inclusion of one or more such elements, with two or more such elements not being required or excluded. Other combinations and sub-combinations of the disclosed features, functions, elements, and / or characteristics may be claimed through a modification of the present claims or the presentation of new claims in this or a related application. Such claims, whether broader, narrower, equal or different in scope from the original claims, are also intended to be included within the subject matter of the present disclosure.

Claims

A method of operating an engine (10) having an exhaust gas recirculation passage coupled between an engine intake and an engine exhaust, comprising: adjusting an engine load at which water is directly injected into an engine cylinder (14) via a water direct injection based on an exhaust gas recirculation flow, the adjustment including water direct injection at lower engine loads when the exhaust gas recirculation flow is below a threshold.The method of claim 1, wherein the direct water injection includes an amount of water injected and a rate of direct water injection.The method of claim 2, wherein the amount of direct water injection is based on the amount of exhaust gas recirculation in the exhaust gas recirculation flow.The method of claim 3, wherein the adjusting further includes decreasing the rate of direct water injection when the exhaust gas recirculation flow exceeds the threshold.The method of claim 4, wherein the rate of lowering the water injection is below a rate of increase of the water direct injection, the rate of increase of the water direct injection being when the exhaust gas recirculation flow increases from a first amount to a second amount.The method of claim 1, wherein the exhaust gas recirculation flow is a low pressure exhaust gas recirculation flow and the exhaust gas recirculation passage is a low pressure exhaust gas recirculation passage coupled between the engine inlet upstream of a turbocharger compressor (174) and the engine exhaust downstream of a turbocharger turbine.The method of claim 6, wherein the engine (10) further includes a high pressure exhaust gas recirculation passage coupled between the engine inlet downstream of the turbocharger compressor (174) and the engine exhaust upstream of a turbocharger turbine, and wherein the adjustment is further based on the presence of a high pressure exhaust gas recirculation flow.The method of claim 7, wherein the adjusting includes increasing an engine load at which the water is injected in the presence of a high pressure exhaust gas recirculation stream.The method of claim 8, wherein variable cam control is adjusted based on the exhaust gas recirculation flow and the direct water injection.A method of operating an engine (10) having a low-pressure EGR passage coupled between an engine inlet upstream of a turbocharger compressor (174) and an engine exhaust downstream of a turbocharger turbine, comprising: during an increase in low-pressure EGR flow from a first amount to a second, higher amount, increasing water direct injection to a first water injection amount and then decreasing water direct injection to a second water injection amount below the first amount.The method of claim 10, wherein increasing the water direct injection is faster than decreasing the water direct injection.The method of claim 11, wherein increasing the low pressure EGR flow occurs during a tip-in.The method of claim 11, further comprising, during a decrease in low pressure EGR flow from the second amount to the first amount, decreasing water direct injection to a third water injection amount and then increasing water direct injection to a fourth water injection amount above the third amount.The method of claim 13, wherein the low pressure exhaust gas recirculation flow reduction occurs during a tip-out.The method of claim 13, further comprising adjusting variable cam control based on the first and second water injection amounts during the increase of the low pressure EGR flow and adjusting variable cam control based on the third and fourth water injection amounts during the decrease of the low pressure EGR flow.The method of claim 15, wherein the adjusting during the increase in low pressure EGR flow includes advancing the variable cam timing when the second water injection amount is above a first threshold, and during the decrease in low pressure EGR flow, includes retarding the variable cam timing when the third water injection amount is below a second threshold.The method of claim 14, wherein a timing is adjusted responsive to an engine load of the increasing direct water injection based on the low pressure EGR flow and the engine torque, the adjustment including increasing the direct water injection at lower engine loads below an engine load threshold when the EGR flow is below a threshold and the engine torque is lower.An engine system comprising: an engine (10) having an engine inlet and an engine exhaust; a turbocharger having a turbocharger compressor (174) and a turbocharger turbine coupled between the engine inlet and the engine exhaust; a low pressure exhaust gas recirculation passage coupled between the engine inlet upstream of the turbocharger compressor (174) and the engine exhaust downstream of the turbocharger turbine; a high pressure exhaust gas recirculation passage coupled between the engine inlet downstream of the turbocharger compressor (174) and the engine exhaust upstream of the turbocharger turbine; a variable cam timing mechanism for adjusting the timing of one or more cams; a direct injector for injecting knock control fluid into an engine cylinder (14), the knock control fluid including water; and a control system having computer readable instructions to, during an increase in exhaust gas recirculation flow, increase a direct fluid injection amount based on engine load and a desired engine dilution and fluid composition; during a decrease in exhaust gas recirculation flow, decrease a fluid injection amount based on engine load and exhaust gas recirculation flow and fluid composition; and during the increase and decrease of exhaust gas recirculation flow, adjust the variable cam control based on the fluid injection amount and fluid composition.The engine system of claim 18, wherein the exhaust gas recirculation flow includes a low pressure exhaust gas recirculation flow through the low pressure exhaust gas recirculation passage and / or a high pressure exhaust gas recirculation flow through the high pressure exhaust gas recirculation passage.

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