Method to optimize engine operation using active fuel management

Abstract

A method for operating an internal combustion engine includes providing a vehicle having an internal combustion gasoline engine including multiple cylinders and wherein the engine is capable of running on at least one of a plurality firing fractions, providing a vacuum offset (Offset.sub.vac) to adjust airflow capacity for each of the plurality of firing fractions, determining a torque capacity of each of the plurality firing fractions and a plurality of available firing fractions that provides at least enough torque capacity to accommodate a current torque requested (T.sub.req), determining a plurality of viable firing fractions of the plurality of available firing fractions, and determining and implementing an optimal firing fraction of the viable firing fractions if the optimal firing fraction provides enough fuel economy benefit over a current firing fraction.

Claims

1. A method for operating an internal combustion engine, the method comprising: providing a vehicle having an internal combustion gasoline engine including multiple cylinders and wherein the engine is capable of running on at least one of a plurality of firing fractions; providing a vacuum offset (Offset.sub.vac) to adjust airflow capacity for each of the plurality of firing fractions; determining a torque capacity of each of the plurality of firing fractions and a plurality of available firing fractions that provides at least enough torque capacity to accommodate a current torque requested (T.sub.req), and wherein determining the torque capacity of each of the plurality of firing fractions comprises: determining a net torque capacity (T.sub.net) of the engine; determining a maximum brake torque (T.sub.FF) for each firing fraction; and determining a minimum firing fraction that produces at least enough brake torque T.sub.FF to accommodate the current torque request T.sub.req; determining a plurality of viable firing fractions of the plurality of available firing fractions; and determining and implementing an optimal firing fraction of the viable firing fractions if the optimal firing fraction provides enough fuel economy benefit over a current firing fraction.

2. The method of operating an internal combustion engine of claim 1 wherein providing the vacuum offset (Offset.sub.vac)) to adjust airflow capacity for each of the firing fractions further comprises: increasing Offset.sub.vac if an intake manifold vacuum (Vac) is less than a first predetermined threshold for a period of time (T); decreasing Offset.sub.vac if an intake manifold vacuum (Vac) is greater than a first predetermined threshold for a period of time (T) and an engine load is high; and maintaining a current Offset.sub.vac.

3. The method of operating an internal combustion engine of claim 1 wherein determining the net torque capacity (T.sub.net) of the engine further comprises determining the T.sub.net as a function of engine speed (RPM), maximum torque cam position, barometric pressure, Vac, Offset.sub.vac, temperature, and humidity.

4. The method of operating an internal combustion engine of claim 1 wherein determining the maximum brake torque (T.sub.FF) for each firing fraction further comprises determining T.sub.FF by the equation:
T.sub.FF=T.sub.net*FF+T.sub.friction wherein T.sub.friction is a constant torque loss due to friction losses of the engine.

5. The method of operating an internal combustion engine of claim 1 wherein determining a plurality of viable firing fractions of the plurality of available firing fractions further comprises: determining a new engine speed EngSpd.sub.new and a transit engine speed EngSpd.sub.transit for one of the plurality of available firing fractions; determining a minimum engine speed EngSpd.sub.min of the one of the plurality of available firing fractions; determining a maximum engine speed EngSpd.sub.max of the one of the plurality of available firing fractions, and wherein EngSpd.sub.max is the highest of a current engine speed EngSpd.sub.current, EngSpd.sub.new, and EngSpd.sub.transit; determining a net torque T.sub.netES.sub.min and T.sub.netES.sub.max for each of EngSpd.sub.min and EngSpd.sub.max; determining a torque limit T.sub.limit as the minimum of TnetES.sub.min and TnetES.sub.max; assigning the one of the plurality of available firing fractions as a viable firing fraction if a brake torque limit of a firing fraction T.sub.brklim is greater than the requested brake torque T.sub.brkreq in addition to a hysteresis and if T.sub.limit is greater than a requested net torque T.sub.netreq in addition to the hysteresis; and assigning the one of the plurality of available firing fractions as a nonviable firing fraction if the brake torque limit of the firing fraction T.sub.brklim is not greater than the requested brake torque T.sub.brkreq in addition to the hysteresis or if T.sub.limit is not greater than a requested net torque T.sub.netreq in addition to the hysteresis.

6. The method of operating an internal combustion engine of claim 1 wherein determining and implementing an optimal firing fraction of the viable firing fractions if the optimal firing fraction provides enough fuel economy benefit over a current firing fraction further comprises: determining a most fuel efficient of the plurality of viable firing fractions FF.sub.best; determining a fuel efficiency of the current firing fraction FF.sub.current; determines a ratio of a fuel efficiency Effratio of the most fuel efficient firing fraction FF.sub.best to the efficiency of the current firing fraction FF.sub.current; maintaining the FF.sub.current if the Effratio is greater than a first threshold ratio TH1; switching to the FF.sub.best if the Effratio is less than a second threshold ratio TH2; maintaining the FF.sub.current and determining the most fuel efficient of the plurality of viable firing fractions FF.sub.best if the Effratio is less than a first threshold ratio TH1 and greater than a second threshold ratio TH2.

7. The method of operating an internal combustion engine of claim 6 wherein maintaining the FF.sub.current if the Effratio is greater than a first threshold ratio TH1 further comprises maintaining the FF.sub.current if the Effratio is greater than 98.5% and switching to the FF.sub.best if the Effratio is less than a second threshold ratio TH2 further comprises switching to the FF.sub.best if the Effratio is less than 95%.

8. A method for operating an internal combustion engine, the method comprising: providing a vehicle having an internal combustion gasoline engine including multiple cylinders and wherein the engine is capable of running on at least one of a plurality firing fractions; providing a vacuum offset (Offset.sub.vac) to adjust airflow capacity for each of the plurality of firing fractions providing a vacuum offset (Offset.sub.vac) to adjust airflow capacity for each of the firing fractions by: increasing Offset.sub.vac if an intake manifold vacuum (Vac) is less than a first predetermined threshold for a period of time (T); decreasing Offset.sub.vac if an intake manifold vacuum (Vac) is greater than a first predetermined threshold for a period of time (T) and an engine load is high; and maintaining a current Offset.sub.vac.; determining a torque capacity of each of the plurality firing fractions and a plurality of available firing fractions that provides at least enough torque capacity to accommodate a current torque requested (T.sub.req) by: determining the net torque capacity (T.sub.net) of the engine; determining the maximum brake torque (T.sub.FF) for each firing fraction; and determining a minimum firing fraction that produces at least enough brake torque T.sub.FF to accommodate a current torque request T.sub.req; determining a plurality of viable firing fractions of the plurality of available firing fractions; and determining and implementing an optimal firing fraction of the viable firing fractions if the optimal firing fraction provides enough fuel economy benefit over a current firing fraction.

9. The method of operating an internal combustion engine of claim 8 wherein determining the net torque capacity (T.sub.net) of the engine further comprises determining the T.sub.net as a function of engine speed (RPM), maximum torque cam position, barometric pressure, Vac, Offset.sub.vac, temperature, and humidity.

10. The method of operating an internal combustion engine of claim 8 wherein determining the maximum brake torque (T.sub.FF) for each firing fraction further comprises determining T.sub.FF by the equation:
T.sub.FF=T.sub.net*FF+T.sub.friction wherein T.sub.friction is a constant torque loss due to friction losses of the engine.

11. The method of operating an internal combustion engine of claim 8 wherein determining a plurality of viable firing fractions of the plurality of available firing fractions further comprises: determining a new engine speed EngSpd.sub.new and a transit engine speed EngSpd.sub.transit for one of the plurality of available firing fractions; determining a minimum engine speed EngSpd.sub.min of the one of the plurality of available firing fractions; determining finds the maximum engine speed EngSpd.sub.max of the one of the plurality of available firing fractions, and wherein EngSpd.sub.max is the highest of a current engine speed EngSpd.sub.current, EngSpd.sub.new, and EngSpd.sub.transit; determining a net torque T.sub.netES.sub.min and T.sub.netES.sub.max for each of EngSpd.sub.min and EngSpd.sub.max; determining a torque limit T.sub.limit as the minimum of T.sub.netES.sub.min and T.sub.netES.sub.max; assigning the one of the plurality of available firing fractions as a viable firing fraction if the brake torque limit of the firing fraction T.sub.brklim is greater than the requested brake torque T.sub.brkreq in addition to the hysteresis and if T.sub.limit is greater than a requested net torque T.sub.netreq in addition to a hysteresis; and assigning the one of the plurality of available firing fractions as a nonviable firing fraction if the brake torque limit of the firing fraction T.sub.brklim is not greater than the requested brake torque T.sub.brkreq in addition to the hysteresis or if T.sub.limit is not greater than a requested net torque T.sub.netreq in addition to the hysteresis.

12. The method of operating an internal combustion engine of claim 8 wherein determining and implementing an optimal firing fraction of the viable firing fractions if the optimal firing fraction provides enough fuel economy benefit over a current firing fraction further comprises: determining the most fuel efficient of the plurality of viable firing fractions FF.sub.best; determining the fuel efficiency of the current firing fraction FF.sub.current; determines a ratio of the fuel efficiency Effratio of the most fuel efficient firing fraction FF.sub.best to the efficiency of the current firing fraction FF.sub.current; maintaining the FF.sub.current if the Effratio is greater than a first threshold ratio TH1; switching to the FF.sub.best if the Effratio is less than a second threshold ratio TH2; maintaining the FF.sub.current and determining the most fuel efficient of the plurality of viable firing fractions FF.sub.best if the Effratio is less than a first threshold ratio TH1 and greater than a second threshold ratio TH2.

13. The method of operating an internal combustion engine of claim 8 wherein maintaining the FF.sub.current if the Effratio is greater than a first threshold ratio TH1 further comprises maintaining the FF.sub.current if the Effratio is greater than 98.5% and switching to the FF.sub.best if the Effratio is less than a second threshold ratio TH2 further comprises switching to the FF.sub.best if the Effratio is less than 95%.

14. A method for operating an internal combustion engine, the method comprising: providing a vehicle having an internal combustion gasoline engine including multiple cylinders and wherein the engine is capable of running on at least one of a plurality firing fractions; providing a vacuum offset (Offset.sub.vac) to adjust airflow capacity for each of the plurality of firing fractions providing a vacuum offset (Offset.sub.vac) to adjust airflow capacity for each of the firing fractions by: increasing Offset.sub.vac if an intake manifold vacuum (Vac) is less than a first predetermined threshold for a period of time (T); decreasing Offset.sub.vac if an intake manifold vacuum (Vac) is greater than a first predetermined threshold for a period of time (T) and an engine load is high; and maintaining a current Offset.sub.vac.; determining a torque capacity of each of the plurality firing fractions and a plurality of available firing fractions that provides at least enough torque capacity to accommodate a current torque requested (T.sub.req) by: determining the net torque capacity (T.sub.net) of the engine; determining the maximum brake torque (T.sub.FF) for each firing fraction by the equation:
T.sub.FF=T.sub.net*FF+T.sub.friction wherein T.sub.friction is a constant torque loss due to friction losses of the engine; and determining a minimum firing fraction that produces at least enough brake torque T.sub.FF to accommodate a current torque request T.sub.req; determining a plurality of viable firing fractions of the plurality of available firing fractions; and determining and implementing an optimal firing fraction of the viable firing fractions if the optimal firing fraction provides enough fuel economy benefit over a current firing fraction.

15. The method of operating an internal combustion engine of claim 14 wherein determining the net torque capacity (T.sub.net) of the engine further comprises determining the T.sub.net as a function of engine speed (RPM), maximum torque cam position, barometric pressure, Vac, Offset.sub.vac, temperature, and humidity.

16. The method of operating an internal combustion engine of claim 14 wherein determining a plurality of viable firing fractions of the plurality of available firing fractions further comprises: determining a new engine speed EngSpd.sub.new and a transit engine speed EngSpd.sub.transit for one of the plurality of available firing fractions; determining a minimum engine speed EngSpd.sub.min of the one of the plurality of available firing fractions; determining finds the maximum engine speed EngSpd.sub.max of the one of the plurality of available firing fractions, and wherein EngSpd.sub.max is the highest of a current engine speed EngSpd.sub.current, EngSpd.sub.new, and EngSpd.sub.transit; determining a net torque T.sub.netES.sub.min and T.sub.netES.sub.max for each of EngSpd.sub.min and EngSpd.sub.max; determining a torque limit T.sub.limit as the minimum of T.sub.netES.sub.min and T.sub.netES.sub.max; assigning the one of the plurality of available firing fractions as a viable firing fraction if the brake torque limit of the firing fraction T.sub.brklim is greater than the requested brake torque T.sub.brkreq in addition to the hysteresis and if T.sub.limit is greater than a requested net torque T.sub.netreq in addition to a hysteresis; and assigning the one of the plurality of available firing fractions as a nonviable firing fraction if the brake torque limit of the firing fraction T.sub.brklim is not greater than the requested brake torque T.sub.brkreq in addition to the hysteresis or if T.sub.limit is not greater than a requested net torque T.sub.netreq in addition to the hysteresis.

17. The method of operating an internal combustion engine of claim 14 wherein determining and implementing an optimal firing fraction of the viable firing fractions if the optimal firing fraction provides enough fuel economy benefit over a current firing fraction further comprises: determining the most fuel efficient of the plurality of viable firing fractions FF.sub.best; determining the fuel efficiency of the current firing fraction FF.sub.current; determines a ratio of the fuel efficiency Effratio of the most fuel efficient firing fraction FF.sub.best to the efficiency of the current firing fraction FF.sub.current; maintaining the FF.sub.current if the Effratio is greater than a first threshold ratio TH1; switching to the FF.sub.best if the Effratio is less than a second threshold ratio TH2; maintaining the FF.sub.current and determining the most fuel efficient of the plurality of viable firing fractions FF.sub.best if the Effratio is less than a first threshold ratio TH1 and greater than a second threshold ratio TH2.

18. The method of operating an internal combustion engine of claim 17 wherein maintaining the FF.sub.current if the Effratio is greater than a first threshold ratio TH1 further comprises maintaining the FF.sub.current if the Effratio is greater than 98.5% and switching to the FF.sub.best if the Effratio is less than a second threshold ratio TH2 further comprises switching to the FF.sub.best if the Effratio is less than 95%.

Description

DRAWINGS

(1) The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way.

(2) FIG. 1 is a depiction of a powertrain of a vehicle in accordance with an aspect of the present invention;

(3) FIG. 2 is a top view schematic of an internal combustion engine, in accordance with an aspect of the present invention;

(4) FIG. 3 is a side view schematic of an internal combustion engine, in accordance with an aspect of the present invention;

(5) FIG. 4 is a top level flow chart depicting a method of controlling an engine of a vehicle, in accordance with an aspect of the present disclosure;

(6) FIG. 5 is a flow chart depicting a sub-routine of controlling an engine of a vehicle, in accordance with an aspect of the present disclosure;

(7) FIG. 6 is a flow chart depicting a sub-routine of controlling an engine of a vehicle, in accordance with an aspect of the present disclosure;

(8) FIG. 7 is a flow chart depicting a sub-routine of controlling an engine of a vehicle, in accordance with an aspect of the present disclosure, and

(9) FIG. 8 is a flow chart depicting a sub-routine of controlling an engine of a vehicle, in accordance with an aspect of the present disclosure.

DETAILED DESCRIPTION

(10) The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses.

(11) With reference to FIG. 1, an exemplary powertrain is generally indicated by reference number 10. The powertrain 10 includes an engine 12, a transmission 14, a driveshaft and rear differential 16, drive wheels 18, and a powertrain control module 20. The engine 12 is an internal combustion engine that supplies a driving torque to the transmission 14. Traditionally, an internal combustion engine is identified by the number of cylinders it includes and in what configuration the cylinders are arranged. The engine 12 shown is a V8 configured engine 12 as the engine 12 includes eight cylinders arranged in a V configuration. The transmission 14, capable of several forward gear ratios, in turn delivers torque to the driveshaft and rear differential 16 and drive wheels 18.

(12) Turning now to FIGS. 2 and 3, the engine 12 is illustrated and described in greater detail. The engine 12 as a system is a combination of multiple sub-systems operating in a coordinated manner managed by the powertrain control module 20 to convert combustion into mechanical work. For example, the engine 12 may include a fuel delivery system 22, an ignition system 24, an air intake system 26, a power conversion system 28, an exhaust system 30, and a valvetrain system 32, among other subsystems. More particularly, the power conversion system 28 includes a plurality of pistons 34, connecting rods 36, cylinders 38, and a crankshaft 40. Each piston 34 is disposed in one of the cylinders 38 with the piston 34 pinned to an end of a connecting rod 36 with the other end of the connecting rod 36 pinned to an offset journal of the crankshaft 38. The top side of the piston 34 and the cylinder 38 form a combustion chamber 42.

(13) The air intake system 26 includes a plurality of air ducts 44 and a throttle valve 46. The throttle valve 46 controls the amount of airflow passing into the air intake system 26 while the air ducts 44 direct incoming air to be used in the combustion process into the combustion chamber 42.

(14) The valvetrain system 32 includes an intake valve 48 and an exhaust valve 50 in each cylinder 38 and a mechanism (not shown) for actuating the intake valve 46 and exhaust valve 48. The intake valve 48 opens to allow communication between the air ducts 44 of the air intake system 26 and the combustion chamber 42. In the present example, there is only one intake valve 48 and one exhaust valve 50 in each combustion chamber 42. However, valvetrain systems 32 having more than one intake valve 48 or exhaust valve 50 in each cylinder 38 may be considered without departing from the scope of the present invention.

(15) The fuel delivery system 22 includes a pressurized fuel source or fuel pump 52, fuel lines 54, and fuel injectors 56. The fuel pump 52 is disposed in the fuel tank (not shown) located elsewhere in the vehicle. The fuel pump 52 pressurizes the fuel lines 54 which deliver pressurized fuel to the fuel injectors 56. The fuel injectors 56 are disposed in the air ducts 44 of the air intake system 26 proximate the intake valve 48. The fuel injectors 56 may also be located in the combustion chamber 42 wherein the fuel is injected directly into the combustion chamber 42.

(16) The ignition system 24 includes spark plugs 58, ignition coils 60, and ignition wires 62. A single spark plug 58 is disposed in each of the combustion chambers 42. An ignition coil 60 is disposed electrically between the powertrain control module 20 and each of the spark plugs 58. The powertrain control module 20 sends a low voltage electric signal to the ignition coils 60 where the signal is stepped to a high-voltage signal required to create a spark and then sent to the spark plugs 58 through the ignition wires 62. Alternatively, an individual coil can be placed directly on top of each of the spark plugs 58 thus eliminating the high-voltage ignition wires 62.

(17) The exhaust system 30 collects exhaust gases from the combustion process in the combustion chamber 42 and directs the gases through a series of aftertreatment mechanisms such as catalytic converters and mufflers (not shown). Some of the exhaust gases can be diverted back to the intake system for improved combustion and fuel economy.

(18) The powertrain control module 20 is electronically connected to at least the engine 12 and transmission 14 and is preferably an electronic control device having a preprogrammed digital computer or processor, control logic, memory used to store data, and at least one I/O peripheral. The control logic includes a plurality of logic routines for monitoring, manipulating, and generating data. The powertrain control module 20 controls the operation of each of the engine 12 and transmission 14. The control logic may be implemented in hardware, software, or a combination of hardware and software. For example, control logic may be in the form of program code that is stored on the electronic memory storage and executable by the processor. The powertrain control module 20 receives the output signals of several sensors throughout the transmission and engine, performs the control logic and sends command signals to the engine 12 and transmission 14. The engine 12 and transmission 14 receive command signals from the powertrain control module 20 and converts the command signals to control actions operable in the engine 12 and transmission 14. Some of the control actions include but are not limited to increasing engine 12 speed, changing air/fuel ratio, changing transmission 14 gear ratios, etc, among many other control actions.

(19) For example, a control logic implemented in software program code that is executable by the processor of the powertrain control module 20 includes control logic for implementing a method of operating the engine 12 in an active fuel management mode or method 100. The active fuel management method 100 is initiated to improve fuel consumption by cutting off fuel delivery to and deactivating selected cylinders while torque demand on the engine is less than the maximum torque available from the engine. The selected cylinder may change from one crankshaft rotation to the next. In this manner, multiple firing patterns may be developed. The firing pattern is derived from a firing fraction. Each firing fraction has a particular torque capacity associated with that firing fraction and compared to the total torque available from the engine 12. A torque ratio is equivalent to the torque capacity available when the engine 12 is operating at a particular firing fraction divided by the total torque available from the engine 12.

(20) The active fuel management method 100 control logic, for example, includes a routine having several method steps as shown in FIG. 4 as a flowchart. The several active fuel management method 100 steps each further include a sub-routine as part of the active fuel management method 100 and are illustrated in flowchart form in FIGS. 5-7. For example, a first step 200 of the active fuel management method 100 is a first sub-routine for capacity adaptation 210 in which a vacuum offset variable (Off.sub.vac) is adjusted to change the vacuum request used for engine capacity to accommodate a load. If the capacity of the air intake system is not adjusted then an undesirable situation may occur wherein the engine does not deliver the desired torque. A second step 300 of the active fuel management method 100 is for determining torque capacity by firing fraction and produces a minimum firing fraction that is capable of providing enough torque to accommodate the current torque requested from the engine 12. A third step 400 of the active fuel management method 100 for determining which of the available firing fractions that achieve noise and vibration specifications. A fourth step 500 of the active fuel management method 100 selects the most optimal firing fraction of the viable firing fractions from the third step 400 and determines if the optimal firing fraction provides enough fuel economy benefit over the current firing fraction to make the change to the new optimal firing fraction.

(21) Referring now to FIG. 5, a flow chart depicting a first sub-routine 202 for capacity adaptation for the first step 200 of the active fuel management method 100 for operating an embodiment of the powertrain 10 is illustrated and will now be described. The first sub-routine 202 for capacity adaptation includes a first step 204 for deciding if the intake manifold vacuum (Vac) is less than a first predetermined threshold for a period of time (T) greater than a second predetermined threshold and if the firing fraction is stable or not changing to another firing fraction. If the outcome of the first step 204 is positive, then the first sub-routine for capacity adaptation 202 continues to a second step 206 for increasing Offset.sub.vac. However, if the outcome of the first step 204 is negative, then the first sub-routine for capacity adaptation 202 continues to a third step 208. The third step 208 of the first sub-routine for capacity adaptation 202 decides if the load on the engine is high and Vac is greater than the first predetermined threshold for a period of time (T) greater than the second predetermined threshold. If the outcome of the third step 208 is positive, then the first sub-routine for capacity adaptation 202 continues to a fourth step 210 of decreasing Offset.sub.vac. However, if the outcome of the third step 208 is negative, then the first sub-routine for capacity adaptation 202 ends without changing Offset.sub.vac.

(22) Referring now to FIG. 6, a flow chart depicting a second sub-routine 302 for determining torque capacity by firing fraction for the second step 300 of the active fuel management method 100 for operating an embodiment of the powertrain 10 is illustrated and will now be described. The second sub-routine 302 for determining net torque capacity for all active cylinders by firing fraction includes a first step 304 for finding the engines 12 net torque capacity (T.sub.net). T.sub.net is a function of engine speed (RPM), maximum torque cam position, barometric pressure, Vac, Off.sub.vac (from the first sub-routine 202), temperature, and humidity. A combination of inputted variables, calibration tables of coefficients, and imbedded equations is used to arrive at the engine net torque capacity T.sub.net. A second step 306 finds the maximum brake torque T.sub.FF for each firing fraction (FF);
T.sub.FF=T.sub.net*FF+T.sub.friction
where the T.sub.friction is a constant torque loss (thus a negative value) due to the various friction losses in the engine 12. A third step 308 determines the minimum firing fraction FF.sub.min that produces at least enough torque T.sub.FF to accommodate the current torque request T.sub.req.

(23) Referring now to FIG. 7, a flow chart depicting a third sub-routine 402 for determining the viable firing fractions for the third step 400 of the active fuel management method 100 for operating an embodiment of the powertrain 10 is illustrated and will now be described. The third sub-routine 402 for determining the viable firing fractions includes a first step 404 for finding a new engine speed EngSpd.sub.new and a transit engine speed EngSpd.sub.transit for a particular firing fraction that satisfies the minimum torque requirements as determined by the second subroutine 302. EngSpd.sub.new is the transmission input speed for a particular torque request and firing fraction in addition to the expected slip from a torque converter of the transmission 14 (from look-up tables). EngSpd.sub.transit is the transmission input speed for a particular torque request, firing fraction, and transient slip (from look-up tables). A second step 406 finds the smallest or minimum engine speed EngSpd.sub.min of a firing fraction which is the minimum of the current engine speed EngSpd.sub.current, EngSpd.sub.new, and EngSpd.sub.transit. A third step 408 finds the highest or maximum engine speed EngSpd.sub.max of a firing fraction which is the highest of the current engine speed EngSpd.sub.current, EngSpd.sub.new, and EngSpd.sub.transit. A fourth step 410 finds the net torque limit that meets noise and vibration requirements for the EngSpd.sub.min (T.sub.netES.sub.min=f(EngSpd.sub.min, transmission 14 gear ratio)) and the net torque for the EngSpd.sub.max (T.sub.netES.sub.max=f(EngSpd.sub.max, transmission 14 gear ratio)) using look-up tables. A fifth step 412 determines the torque limit T.sub.limit as the minimum of T.sub.netES.sub.min and T.sub.netES.sub.max. A sixth step 414 determines if T.sub.limit is greater than the requested net torque T.sub.netreq in addition to a hysteresis. If the sixth step 414 results in the positive, then the seventh step 416 determines if the brake torque limit of the firing fraction T.sub.brklim is greater than the requested brake torque T.sub.brkreq in addition to the hysteresis. If the seventh step 416 results in the positive, then the firing fraction is flagged as viable in the eighth step 418. If either the sixth step 414 or the seventh step 416 results in the negative, then the firing fraction is flagged as not viable in the ninth step 420. A tenth step 422 restarts the third subroutine 402 for a new firing fraction until a number of viable firing fractions are found to be viable or all firing fractions have been evaluated.

(24) Referring now to FIG. 8, a flow chart depicting a fourth sub-routine 502 for selecting the firing fraction for the fourth step 500 of the active fuel management method 100 for operating an embodiment of the powertrain 10 is illustrated and will now be described. The fourth sub-routine 502 for selecting the firing fraction includes a first step 504 of determining the most fuel efficient of the viable firing fractions FF.sub.best. The second step 506 determines the fuel efficiency of the current firing fraction FF.sub.current. The third step 508 determines a ratio of the fuel efficiency Effratio of the most fuel efficient firing fraction FF.sub.best to the efficiency of the current firing fraction FF.sub.current. A forth step 510 determines if the Effratio is greater than a first threshold ratio TH1 (for example TH1=99.5%). If the fourth step results in the positive then the FF.sub.best is not sufficiently more efficient and the current firing fraction is kept the same in the fifth step 512. If the fourth step 510 results in the negative then a sixth step 514 is executed. The sixth step 514 determines if the Effratio is less than a second threshold ratio TH2 (for example, TH2=95%). If the sixth step 514 results in the positive then the FF.sub.best is sufficiently more efficient and the firing fraction is changed to the most efficient of the viable firing fractions FF.sub.best from the first step 504 in step seven 516. If the sixth step 514 results in the negative then an eighth step 518 and a ninth step 520 are executed. The eighth step 518 counts a number N of loops. A ninth step 520 determines if N loops have been completed. If the ninth step 520 results in the positive then step seven 516 changes the firing fraction to the most efficient of the viable firing fractions FF.sub.best from the first step 504. If the ninth step 520 results in the negative then step five 512 keeps the firing fraction the same and the fourth subroutine 502 is run again. Rerunning this routine ensures that efficiency improvements are consistent and avoids excessive firing fraction transitions so that changing to the firing fraction with the Effratio that is between the two threshold limits TH1, TH2 will actually benefit the fuel efficiency of the engine 12.

(25) The description of the invention is merely exemplary in nature and variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.