Fuel systems having reduced bypass flow

1. A fuel system for an engine, comprising:

a fuel circuit;

a primary fuel pump in fluid communication with the fuel circuit configured to pump insufficient fuel flow to the engine for running the engine at least one engine speed or speed range, wherein the primary fuel pump is mechanically driven and mechanically connected to the engine such that the primary fuel pump is configured to pump fuel as a function of engine speed;

a supplemental fuel pump in fluid communication with the fuel circuit that is configured to selectively pump fuel to the engine at least during the at least one engine speed or speed range to supplement fuel flow from the primary fuel pump to provide sufficient total fuel flow to the engine for running the engine at all engine speeds, wherein the supplemental fuel pump is not mechanically connected to the engine, wherein the supplemental fuel pump is operatively connected to a turbine disposed in fluid communication with the fuel circuit downstream of the primary fuel pump such that the turbine is configured to be driven by fuel flow from the primary fuel pump, wherein the fuel circuit further comprises at least two parallel branches downstream of the primary fuel pump, wherein the turbine is in fluid communication with a first branch of the at least two parallel branches; and

a control valve disposed in fluid communication with a second branch of the at least two parallel branches, wherein the control valve is configured to selectively allow and/or meter fuel flow therethrough to create a bypass path around the turbine to selectively allow and/or meter flow through the turbine, and thus control a speed of the supplemental fuel pump, wherein the control valve is configured to shut off flow in the second branch to control fuel flow to the turbine, wherein the primary fuel pump is connected via the first branch to the turbine without a valve therebetween to provide uninterrupted flow to the turbine, and wherein the first branch and the second branch both lead to a combustor of the engine.

2. The fuel system of claim 1 , wherein the primary fuel pump and the supplemental fuel pump are positive displacement pumps.

3. The fuel system of claim 1 , wherein the fuel circuit includes two parallel pump branches, wherein the primary fuel pump and the supplemental fuel pump are connected in parallel by the two parallel pump branches, wherein the primary fuel pump is disposed on a first branch of the two parallel pump branches, and wherein the supplemental fuel pump is disposed on a second branch of the two parallel pump branches.

4. The fuel system of claim 1 , wherein the fuel circuit includes a return loop for returning excess fuel flow to the primary fuel pump and/or supplemental fuel pump, wherein the return loop includes a pressure release valve for allowing flow at a predetermined pressure.

5. The fuel system of claim 1 , further comprising a boost pump in fluid communication with the fuel circuit between a fuel tank and the primary and supplemental fuel pumps, the boost pump configured to provide a boost pressure to the primary fuel pump and the supplemental fuel pump.

6. The fuel system of claim 5 , wherein the boost pump is driven on a common shaft with the primary fuel pump.

7. The fuel system of claim 1 , wherein the supplemental fuel pump is driven by an electric motor.

8. The system of claim 7 , further comprising a valve disposed upstream or downstream of the supplemental fuel pump to selectively prevent bypass flow through the supplemental fuel pump when not in use.

9. The system of claim 1 , wherein the engine speed or speed range is a takeoff engine speed or speed range.

10. The system of claim 1 , wherein the engine speed or speed range is a startup or windmill engine speed or speed range.

11. The system of claim 1 , further comprising a control module configured to control the supplemental fuel pump to track a predetermined engine fuel flow requirement to provide a total flow substantially equal to or greater than the predetermined engine fuel flow requirement.

12. A method for providing sufficient fuel flow to an engine, comprising:

pumping a first fuel flow with a primary fuel pump to the engine, the first fuel flow being insufficient for running the engine at least one engine speed or speed range; and

pumping a second fuel flow with a supplemental fuel pump to supplement the first fuel flow to provide sufficient flow to the engine for running the engine at the at least one engine speed or speed range, wherein pumping the first and the second fuel flow includes driving a turbine connected to the supplemental fuel pump with fuel flow from the primary fuel pump to drive the supplemental fuel pump to provide more fuel flow to the engine than pumped by the primary fuel pump alone; and

controlling a control valve on a parallel branch from the turbine to meter an amount of fuel flow through the turbine to control a speed of the supplemental fuel pump to control an amount of total fuel flow, wherein the control valve is configured to shut off fuel flow in the second branch to control fuel flow to the turbine, wherein the primary fuel pump is connected via the first branch to the turbine without a valve therebetween to provide uninterrupted fuel flow to the turbine, and wherein the first branch and the second branch both lead to a combustor of the engine.

13. The method of claim 12 , further comprising controlling total flow to track a predetermined engine fuel flow requirement.

14. The method of claim 12 , further comprising controlling a speed of an electric motor connected to the supplemental fuel pump to drive the supplemental fuel pump to provide more flow to the engine than pumped by the primary fuel pump alone.