System and method for adjusting fuel mass for minimum fuel injector pulse widths in multiple fuel system engines

1. A control system for an engine, the control system comprising:

a fuel mass determination module configured to, independent of a position of a throttle of the engine, determine a first minimum fuel mass that is capable of being delivered by a first fuel system of the engine based on either (i) an amount of air per cylinder of the engine and a speed of the engine, or (ii) a first minimum pulse width of a first fuel injector of a cylinder of the engine and a flow rate of the first fuel injector,

wherein

the engine comprises the first fuel system, a second fuel system, and a plurality of fuel injectors,

the plurality of fuel injectors have respective minimum pulse widths,

the minimum pulse widths include the first minimum pulse width,

each of the minimum pulse widths is (i) based on a respective point of injection and a type of fuel injected by a corresponding one of the plurality of fuel injectors, and (ii) refers to a minimum ON time per injection of a corresponding one of the plurality of fuel injectors,

the plurality of fuel injectors comprise the first fuel injector;

a mass fraction determination module that determines first minimum and maximum mass fractions based on the first minimum fuel mass and a total fuel mass for the cylinder of the first fuel injector; and

a fuel injector control module that (i) limits a first mass fraction to be greater than the first minimum mass fraction and less than the first maximum mass fraction, and (ii) controls the first fuel injector of the engine based on the limited first mass fraction such that an actual fuel injector pulse width of the first fuel injector is greater than or equal to the corresponding one of the minimum pulse widths.

2. The control system of claim 1 , wherein the fuel mass determination module further determines a second minimum fuel mass corresponding to the second fuel system of the engine based on either (i) the amount of air per cylinder of the engine and the speed of the engine, or (ii) a minimum pulse width of a second fuel injector of the cylinder of the first fuel injector and a flow rate of the second fuel injector.

3. The control system of claim 2 , wherein the mass fraction determination module further determines second minimum and maximum mass fractions based on the second minimum fuel mass and the total fuel mass for the cylinder of the first fuel injector.

4. The control system of claim 3 , wherein the fuel injector control module further (i) limits a second mass fraction to be greater than the second minimum mass fraction and less than the second maximum mass fraction, and (ii) controls the second fuel injector of the engine based on the limited second mass fraction.

5. The control system of claim 4 , wherein:

the first and second fuel injectors are associated with the first and second fuel systems, respectively;

the first fuel system provides a first fuel to the first fuel injector;

the first fuel injector is configured for direct fuel injection;

the second fuel system provides a second fuel to the second fuel injector;

the second fuel injector is configured for port fuel injection; and

the first fuel is gasoline and wherein the second fuel is compressed natural gas (CNG).

6. The control system of claim 2 , wherein the fuel mass determination module determines the first and second minimum fuel masses based on the amount of air per cylinder and an engine speed.

7. The control system of claim 6 , wherein the fuel mass determination module further includes a lookup table that maps the amount of air per cylinder and an engine speed to the first and second minimum fuel masses.

8. The control system of claim 2 , wherein the fuel mass determination module determines:

the first minimum fuel mass based on the minimum pulse width of the first fuel injector and the flow rate of the first fuel injector; and

the second minimum fuel mass based on the minimum pulse width of the second fuel injector and the flow rate of the second fuel injector.

9. A method for controlling an engine, the method comprising:

independent of a position of a throttle of the engine, determining a first minimum fuel mass that is capable of being delivered by a first fuel system of the engine based on either (i) an amount of air per cylinder of the engine and a speed of the engine, or (ii) a first minimum pulse width of a first fuel injector of a cylinder of the engine and a flow rate of the first fuel injector,

wherein

the engine comprises the first fuel system, a second fuel system, and a plurality of fuel injectors,

the plurality of fuel injectors have respective minimum pulse widths,

the minimum pulse widths include the first minimum pulse width,

each of the minimum pulse widths is (i) based on a respective point of injection and a type of fuel injected by a corresponding one of the plurality of fuel injectors, and (ii) refers to a minimum ON time per injection of a corresponding one of the plurality of fuel injectors,

the plurality of fuel injectors comprise the first fuel injector;

determining first minimum and maximum mass fractions based on the first minimum fuel mass and a total fuel mass for the cylinder of the first fuel injector;

limiting a first mass fraction to be greater than the first minimum mass fraction and less than the first maximum mass fraction; and

controlling the first fuel injector of the engine based on the limited first mass fraction such that an actual fuel injector pulse width of the first fuel injector is greater than or equal to the corresponding one of the minimum pulse widths.

10. The method of claim 9 , further comprising determining a second minimum fuel mass corresponding to the second fuel system of the engine based on either (i) the amount of air per cylinder of the engine and the speed of the engine, or (ii) a minimum pulse width of a second fuel injector of the cylinder of the first fuel injector and a flow rate of the second fuel injector.

11. The method of claim 10 , further comprising determining second minimum and maximum mass fractions based on the second minimum fuel mass and the total fuel mass for the cylinder of the first fuel injector.

12. The method of claim 11 , further comprising:

limiting a second mass fraction to be greater than the second minimum mass fraction and less than the second maximum mass fraction; and

controlling the second fuel injector of the engine based on the limited second mass fraction.

13. The method of claim 12 , wherein:

the first and second fuel injectors are associated with the first and second fuel systems, respectively;

the first fuel system provides a first fuel to the first fuel injector;

the first fuel injector is configured for direct fuel injection;

the second fuel system provides a second fuel to the second fuel injector;

the second fuel injector is configured for port fuel injection; and

the first fuel is gasoline and wherein the second fuel is compressed natural gas (CNG).

14. The method of claim 10 , further comprising determining the first and second minimum fuel masses based on the amount of air per cylinder and an engine speed.

15. The method of claim 14 , further comprising using a lookup table that maps the amount of air per cylinder and an engine speed to the first and second minimum fuel masses.

16. The method of claim 10 , further comprising determining:

the first minimum fuel mass based on the minimum pulse width of the first fuel injector and the flow rate of the first fuel injector; and

the second minimum fuel mass based on the minimum pulse width of the second fuel injector and the flow rate of the second fuel injector.

17. The control system of claim 1 , wherein:

the first minimum mass fraction is the first minimum fuel mass divided by the total fuel mass of the cylinder of the first fuel injector;

the first maximum mass fraction is a first maximum fuel mass divided by the total fuel mass of the cylinder of the first fuel injector; and

the first limited mass fraction is a mass of a first fuel provided via the first fuel injector divided by the total fuel mass.

18. The control system of claim 3 , wherein the mass fraction determination module is configured to determine the total fuel mass based on the amount of air per cylinder and an air/fuel ratio.

19. The control system of claim 4 , wherein:

the first minimum mass fraction is the first minimum fuel mass divided by the total fuel mass of the cylinder of the first fuel injector;

the first maximum mass fraction is a first maximum fuel mass divided by the total fuel mass of the cylinder of the first fuel injector;

the first limited mass fraction is a mass of a first fuel provided via the first fuel injector divided by the total fuel mass;

the second minimum mass fraction is the second minimum fuel mass divided by the total fuel mass of the cylinder of the first fuel injector;

the second maximum mass fraction is a second maximum fuel mass divided by the total fuel mass of the cylinder of the first fuel injector;

the second limited mass fraction is a mass of a second fuel provided via the second fuel injector divided by the total fuel mass; and

the total fuel mass is equal to a sum of the mass of the first fuel and the mass of the second fuel.

20. The control system of claim 4 , wherein the fuel injector control module is configured to (i) minimize the limited first mass fraction while not reducing a pulse width of the first fuel injector to less than the minimum pulse width of the first fuel injector, and (ii) compensate for the minimizing of the limited first mass fraction by increasing the limited second mass fraction such that the total fuel mass is provided to the cylinder of the first fuel injector.