Energy storage and conversion in free-piston combustion engines

1. A method of controlling a free-piston engine, the method comprising:

causing, using processing circuitry, a free-piston assembly to achieve a first position profile between a first apex position and a second apex position and a second position profile between the second apex position and a third apex position, wherein an expansion stroke occurs from the first apex position towards the second apex position, and a compression stroke occurs from the second apex position towards the third apex position;

causing, using the processing circuitry, at least one force profile acting on the free-piston assembly to oppose a motion of the free-piston assembly during each of the expansion stroke and the compression stroke;

causing, using the processing circuitry, storage of sufficient energy in a driver section during the expansion stroke to perform the subsequent compression stroke; and

causing, using the processing circuitry, output of net electrical power over each of the expansion stroke and the compression stroke.

2. The method of claim 1 , wherein:

the free-piston assembly is a first free-piston assembly, the free-piston engine comprising a second free-piston assembly facing the first free-piston assembly;

the first free-piston assembly and the second free-piston assembly are arranged to move along a common axis; and

the at least one force profile comprises a first force profile acting on the first free-piston assembly and a second force profile acting on the second free-piston assembly.

3. The method of claim 1 , wherein the net electric power to be outputted during the expansion stroke and during the compression stroke is the sum of all electrical power inputted and outputted over the expansion stroke and during the compression stroke.

4. The method of claim 1 , wherein causing the at least one force profile acting on the free-piston assembly to oppose the motion of the free-piston assembly during both the expansion stroke and the compression stroke comprises:

causing the at least one force profile acting on the free-piston assembly to oppose the motion of the free-piston assembly during a first portion of the expansion stroke and during a first portion of the compression stroke; and

causing the at least one force profile acting on the free-piston assembly to apply no force to the free-piston assembly during a second portion of the expansion stroke and during a second portion of the compression stroke.

5. The method of claim 1 , wherein the first apex position is a top dead center position, wherein the second apex position is a bottom dead center position, and wherein the third apex position is the top dead center position.

6. A free-piston engine system, comprising:

a cylinder comprising a reaction section;

at least one free-piston assembly in contact with the reaction section;

at least one driver section in contact with the at least one free-piston assembly, wherein the at least one driver section is configured to store energy from the at least one free-piston assembly, and wherein the at least one driver section is configured to receive a gas;

at least one linear electromagnetic machine for directly converting between kinetic energy of the at least one free-piston assembly and electrical energy; and

a control system configured to:

control flow of the gas supplied to the at least one driver section; and

cause the at least one driver section to store at least a sufficient amount of energy from the at least one free-piston assembly during an expansion stroke to perform a subsequent stroke based at least in part on the gas without net electrical energy input.

7. The free-piston engine system of claim 6 wherein the control system is a mechanical control system.

8. The free-piston engine system of claim 6 , wherein the driver section comprises a gas spring, the free-piston engine system further comprising at least one port for controlling the gas to be supplied to the gas spring.

9. The free piston engine system of claim 8 , wherein the at least one port comprises an active valve configured to open and close the at least one port, and wherein the control system is configured to control the active valve.

10. The free piston engine system of claim 8 , wherein the at least one port comprises a passive valve configured to open and close the at least one port.

11. The free piston engine system of claim 10 , further comprising a mechanical spring coupled to the passive valve, wherein the mechanical spring is configured to bias the passive valve to a closed position, and wherein the passive valve is configured to open based on a pressure of the gas spring.

12. The free piston engine system of claim 8 , wherein the at least one port is maintained at atmospheric pressure.

13. The free piston engine system of claim 8 , further comprising a manifold coupled to the at least one port, wherein the control system is further configured to cause the gas to be supplied to the gas spring by controlling a pressure in the manifold.

14. The free piston engine system of claim 13 , wherein the control system comprises a compressor configured to supply the gas to the manifold.

15. A driver system comprising:

a gas spring operatively coupled to a linear electromagnetic machine;

at least one port configured to provide a gas to the gas spring; and

a control system configured to control a flow of the gas through the at least one port to store an amount of energy in the driver section during a compression of the gas spring, wherein the amount of energy is sufficient to perform a subsequent expansion of the gas spring, and wherein the linear electromagnetic machine outputs net electrical energy over the subsequent expansion.

16. The driver system of claim 15 , wherein the at least one port comprises an active valve configured to open and close the at least one port, and wherein the control circuitry is configured to control the active valve.

17. The driver system of claim 15 , wherein the at least one port comprises a passive valve configured to open and close the at least one port.

18. The driver system of claim 15 , wherein the linear electromagnetic machine outputs net electrical energy over the subsequent compression of the gas spring.

19. The driver system of claim 18 , wherein the gas comprises one of a pressurized gas and an atmospheric pressure gas.

20. The driver system of claim 15 , wherein the gas spring is operatively coupled to the linear electromagnetic machine by a free piston assembly, wherein the gas spring is in contact with the free piston assembly, and wherein the linear electromagnetic machine is configured for directly converting between mechanical energy of the free-piston assembly and electrical energy.