Control of piston trajectory in a free-piston combustion engine

1. A method performed by a programmed computer system for controlling displacement of a free-piston assembly, the method comprising:

a) determining a force to be effected on the free-piston assembly based at least in part on a current position of the free-piston assembly relative to a target position, and an estimated amount of work to be performed on the free-piston assembly between the current position and the target position;

b) effecting displacement of the free-piston assembly based on the force; and

repeating a) and b) during a stroke of the free-piston assembly.

2. The method of claim 1 , wherein the estimated amount of work is to be performed by at least a driver section on the free-piston assembly.

3. The method of claim 2 , wherein determining the force is further based on a pressure associated with the driver section when the free-piston assembly is at the current position.

4. The method of claim 1 , wherein the estimated amount of work is to be performed by at least a combustion section on the free-piston assembly.

5. The method of claim 4 , wherein determining the force is further based on a pressure associated with the combustion section when the free-piston assembly is at the current position.

6. The method of claim 1 , wherein determining the force based on the current position relative to the target position comprises determining the force based on a difference between the current position and the target position.

7. The method of claim 1 , where the target position is an apex position.

8. The method of claim 1 , wherein determining the force to be effected on the free-piston assembly is further based on a kinetic energy of the free-piston assembly when the free-piston assembly is at the current position.

9. A system comprising:

a free-piston assembly comprising a translator;

a stator, wherein the translator and the stator form a linear electromagnetic machine (LEM), and wherein the translator is configured to move linearly within the stator; and

a control system coupled to the LEM and configured to:

a) determining a force to be applied to the free-piston assembly based at least in part on a current position of the free-piston assembly relative to a target position and an estimated amount of work to be performed on the free-piston assembly between the current position and the target position;

b) effecting displacement of the free-piston assembly based on the force; and

repeating a) and b) during a stroke of the free-piston assembly.

10. The system of claim 9 , further comprising a driver section in contact with the free-piston assembly, wherein the estimated amount of work is to be performed by at least the driver section on the free-piston assembly.

11. The system of claim 10 , wherein the control system is further configured to determine the force based on a pressure associated with the driver section when the free-piston assembly is at the current position.

12. The system of claim 9 , further comprising a combustion section in contact with the free-piston assembly, wherein the estimated amount of work is to be performed by at least the combustion section on the free-piston assembly.

13. The system of claim 12 , wherein the control system is further configured to determine the force based on a pressure associated with the combustion section when the free-piston assembly is at the current position.

14. The system of claim 9 , wherein determining the force based on the current position relative to the target position comprises determining the force based on a difference between the current position and the target position.

15. The system of claim 9 , where the target position is an apex position.

16. The system of claim 9 , wherein the control system is further configured to determine the force to be effected on the free-piston assembly based on a kinetic energy of the free-piston assembly when the free-piston assembly is at the current position.

17. The system of claim 9 , wherein determining the force to be applied to the free-piston assembly comprises determining the force to be applied by the LEM.

18. A method performed by a programmed computer system for controlling displacement of a free-piston assembly, the method comprising:

using one of a position-force control and a repetitive adaptive control to determine a position-force trajectory of a free-piston assembly based at least in part on operating conditions; and

when it is determined to use position-force control:

determining the position-force trajectory for displacing the free-piston assembly without regard to a previously determined position-force trajectory, and

effecting displacement of the free-piston assembly based on the position-force trajectory,

when it is determined to use repetitive adaptive control:

determining the position-force trajectory for displacing the free-piston assembly based at least in part on a previously determined position-force trajectory, and

effecting displacement of the free-piston assembly based on the position-force trajectory.

19. The method of claim 18 , wherein determining the position-force trajectory for displacing the free-piston assembly without regard to the previously determined position-force trajectory is based at least in part on a current position of the free-piston assembly relative to a target position and an estimated amount of work to be performed on the free-piston assembly between the current position and the target position.

20. The method of claim 18 , further comprising determining whether to use position-force control or repetitive adaptive control based on whether the operating conditions are sufficiently steady, wherein:

if the operating conditions are sufficiently steady the programmed computer system uses the repetitive adaptive control; and

if the operating conditions are sufficiently unsteady the programmed computer system uses the position-force control.