Systems and methods for transient control of a free-piston engine

Patent metadata

Field	Value
Publication number	US-10202897-B2
Application number	US-201414784243-A
Country	US
Kind code	B2
Filing date	Apr 15, 2014
Priority date	Apr 16, 2013
Publication date	Feb 12, 2019
Grant date	Feb 12, 2019

Abstract

A free-piston (“FP”) engine is a type of internal combustion engine with no crankshaft, so that its piston trajectory is no longer constrained by the mechanical linkage. FP engines have a high potential in terms of energy saving given their simple structure, high modularity and high efficiency, among other attributes. One of the technical barriers that affect FP engine technology is a lack of precise piston trajectory control. For example, the presence of a transient period after a single combustion event can prevent the engine from continuous firing. The present subject matter provides a control scheme that can utilize a reference and control signal shifting technique to modify the tracking error and the control signal to reduce the transient period.

First claim

What is claimed is:

1. A free-piston engine, comprising:
an oscillating piston travelling along an actual piston trajectory;
a controller configured to:
store a predetermined reference trajectory for controlling oscillation of the piston,
compare the actual piston trajectory of the oscillating piston to the stored predetermined reference trajectory,
locate a first local extremum for the actual piston trajectory, the first local extremum corresponding to a first actual reversal point for the oscillating piston,
record a first time instant corresponding to the first local extremum,
detect a first combustion event of the piston by comparing the first time instant to a predetermined first reference time index, the predetermined first reference time index corresponding to a first reference reversal point corresponding to the first actual reversal point, and
apply a first reference shift to the stored predetermined reference trajectory upon detecting the first combustion event to digitally shift the stored predetermined reference trajectory by a time interval to create a first shifted reference trajectory, the time interval corresponding to the difference between the first time instant and the predetermined first reference time index;
an engine cylinder for slidably receiving the piston; and
a force transducer having a linear actuator coupled to the piston, wherein the controller is coupled to the force transducer and configured to control the oscillation of the linear actuator of the force transducer to influence the piston oscillation such that the actual piston trajectory approximates the stored predetermined reference trajectory prior to the first combustion event and the first shifted reference trajectory following the first combustion event.

2. The free-piston engine of claim 1 , wherein the first combustion event is detected when the difference between the first time instant and the predetermined first reference time index exceeds a predetermined threshold that corresponds to the stored predetermined reference trajectory.

3. The free-piston engine of claim 1 , wherein the controller is further configured to:
monitor acceleration of the piston following the first time instant.

4. The free-piston engine of claim 3 , wherein the controller is further configured to:
monitor acceleration of the piston through a plurality of non-combustion piston cycles;
compare acceleration of the piston following the first time instant to prior piston accelerations for a non-combustion cycle; and
validate the detection of the first combustion event found by the difference between the first time instant and the predetermined first reference time index if the piston acceleration following the first time instant exceeds the piston accelerations of the non-combustion cycles.

5. The free-piston engine of claim 3 , wherein the controller is further configured to:
apply a correction to the first shifted reference trajectory corresponding to the acceleration of the piston;
wherein the correction is one of modifying amplitude of the first shifted reference trajectory, modifying magnitude of the first shifted reference trajectory and combinations thereof.

6. The free-piston engine of claim 1 , further comprising:
a sensor configured to detect the first combustion event within the engine cylinder and notify the controller of the detected first combustion event.

7. The free-piston engine of claim 6 , wherein the sensor is configured to detect combustion by monitoring at least one of intake port pressure, exhaust port pressure, and location of the piston.

8. The free-piston engine of claim 6 , wherein the sensor is configured to:
detect the strength of the first combustion event based on at least one of pressure generated by combustion, piston velocity following combustion, piston acceleration and combinations thereof;
wherein the controller is configured to control the force transducer in a subsequent piston cycle to compensate for the detected strength.

9. The free-piston engine of claim 8 , wherein the controller is further configured to:
control the force transducer to compensate for the detected strength, and determine whether to influence a position of the piston in association with the first shifted reference trajectory by comparing the detected strength to a strength associated with the first shifted reference trajectory.

10. The free-piston engine of claim 1 , wherein the controller is configured to:
locate a second local extremum for the actual piston trajectory, the second local extremum corresponding to a second actual reversal point;
record a second time instant corresponding to the second local extremum;
detect a second combustion event by comparing the second time instant to a second reference time index, the second reference time index corresponding to a second reference reversal point corresponding to the second actual reversal point; and
apply a second reference shift to the first shifted reference trajectory upon detecting of the second combustion event to create a second shifted reference trajectory, the second reference shift corresponding to the difference between the second time instant and the second reference time index.

11. The free-piston engine of claim 10 , further comprising:
a sensor configured to monitor cylinder pressure of the engine cylinder for at least two combustion events;
wherein the controller is further configured to:
compare a second cylinder pressure of the second combustion event to a cylinder pressure of the first combustion event, wherein the second combustion event is subsequent to the first combustion event: and
update the second reference shift by a factor corresponding to the difference between the cylinder pressure and the second cylinder pressure.

12. The free-piston engine of claim 1 , further comprising an energy storage device coupled to the force transducer to store energy generated by movement of the piston.

13. The free-piston engine of claim 12 , wherein the force transducer comprises a linear alternator and the energy storage device comprises a battery.

14. The free-piston engine of claim 12 , wherein the force transducer comprises a hydraulic pump, and the piston is connected with a plunger of the hydraulic pump and the hydraulic pump is coupled with the energy storage device.

15. The free-piston engine of claim 14 , wherein the energy storage device comprises a high pressure fluid source connected with the hydraulic pump on one side thereof while a low pressure fluid source is connected with another side of the hydraulic pump.

16. The free-piston engine of claim 15 , wherein the controller is coupled to at least one valve configured to place high pressure fluid stored in the high pressure source in fluid communication with at least one hydraulic chamber of the hydraulic pump.

17. The free-piston engine of claim 16 , wherein a control valve can place either of the low pressure fluid source and the high pressure fluid source to a plurality of chambers of the hydraulic pump.

18. The free-piston engine of claim 1 , wherein the controller is further configured to:
record an error signal corresponding to the difference between the actual piston trajectory and the first shifted reference trajectory; and
generate a control signal for the force transducer to minimize the error signal.

19. The free-piston engine of claim 18 , wherein the controller is further configured to:
apply the first reference shift to the error signal and the control signal.

20.

Assignees

• Univ Minnesota

Inventors

• Sun Zongxuan
• Li Ke

Classifications

• F02D2200/0406

  Intake manifold pressure · CPC title

• F01B11/00

  Reciprocating-piston machines or engines without rotary main shaft, e.g. of free-piston type · CPC title

• F02D2200/1012

  Engine speed gradient · CPC title

• F02D35/023

  by determining the cylinder pressure · CPC title

• F02D41/1448

  the characteristics being an exhaust gas pressure · CPC title