Control of piston trajectory in a free-piston combustion engine
Various embodiments of the present disclosure are directed towards free-piston combustion engines. As described herein, a method and system are provided for displacing a free-piston assembly to achieve a desired engine performance by repeatedly determining position-force trajectories over the course of a propagation path and effecting the displacement of the free-piston assembly based, at least in part, on the position-force trajectory. In a dual-piston assembly free-piston engine, synchronization of the two piston assemblies is provided.
1. A method performed by a programmed computer system for controlling displacement of a free-piston assembly, the method comprising:
a) determining a current position of the free-piston assembly and a current pressure in a combustion section in contact with the free-piston assembly;
b) determining a position-force trajectory for displacing the free-piston assembly based at least in part on the current position, the current pressure, and a desired engine performance;
c) effecting displacement of the free-piston assembly based on the position-force trajectory; and
repeating a), b), and c) during a stroke of the free-piston assembly.
2. The method of claim 1 , wherein determining the position-force trajectory for displacing the free-piston assembly is without regard to a previously determined trajectory.
3. The method of claim 1 , wherein the current pressure comprises a measured pressure.
4. The method of claim 1 , wherein the current pressure comprises an estimated pressure.
5. The method of claim 1 , wherein the current pressure corresponds to the current position, the method further comprising determining the current pressure based on a pressure at a previous position, a volume of gas at the previous position, and a volume of gas at the current position.
6. The method of claim 1 , further comprising determining the current pressure based on a force balance model applied to the free-piston assembly.
7. The method of claim 1 , further comprising determining the current pressure based on an energy balance model.
8. The method of claim 1 , wherein the current pressure is indicative of a combustion section pressure, the method further comprising determining the current pressure based on a current pressure of a gas spring, a previously calculated force, a current acceleration of the free-piston assembly and a mass of the free-piston assembly.
9. The method of claim 1 , wherein determining the position-force trajectory is further based on an estimated amount of work associated with moving the free-piston assembly from the current position to a target position.
10. The method of claim 9 , wherein the estimated amount of work is determined based on the current position and the current pressure.
11. The method of claim 1 , wherein the desired engine performance comprises a desired apex position.
12. The method of claim 1 , wherein the desired engine performance comprises a desired synchronization between the free-piston assembly and another free-piston assembly.
13. A control system for controlling displacement of a free-piston assembly, the system comprising:
control circuitry to:
a) determine a current position of the free-piston assembly and a current pressure in a combustion section in contact with the free-piston assembly;
b) determine a position-force trajectory for displacing the free-piston assembly based at least in part on the current position, the current pressure, and a desired engine performance;
c) control displacement of the free-piston assembly based on the position-force trajectory; and
repeat a), b), and c) during a stroke of the free-piston assembly.
14. The control system of claim 13 , wherein the current pressure comprises an estimated pressure.
15. The control system of claim 13 , wherein the current pressure corresponds to the current position, and wherein the control circuitry further determines the current pressure based on a pressure at a previous position, a volume of gas at the previous position, and a volume of gas at the current position.
16. The control system of claim 13 , wherein the current pressure is indicative of a combustion section pressure, and wherein the control circuitry further determines the current pressure based on a current pressure of a gas spring, a previously calculated force, a current acceleration of the free-piston assembly and a mass of the free-piston assembly.
17. The control system of claim 13 , wherein the control circuitry further determines the position-force trajectory based on an estimated amount of work associated with moving the free-piston assembly from the current position to a target position.
18. The control system of claim 13 , wherein the desired engine performance comprises a desired apex position.
19. The control system of claim 13 , wherein the desired engine performance comprises a desired synchronization between the free-piston assembly and another free-piston assembly.
20. A control system for controlling displacement of a free-piston assembly, the system comprising:
control circuitry to:
a) determine a current position of the free-piston assembly and a current pressure in a combustion section in contact with the free-piston assembly;
b) determine a position-force trajectory for displacing the free-piston assembly based at least in part on the current position, the current pressure, and a desired engine performance, wherein the control circuitry determines the position-force trajectory for displacing the free-piston assembly without regard to a previously determined trajectory;
c) control displacement of the free-piston assembly based on the position-force trajectory; and
repeat a), b), and c) during a stroke of the free-piston assembly.