Methods and systems for free piston engine control
Motion control of a hydraulic free-piston engine is achieved in order to enable advanced combustions such as low temperature combustion. To accomplish this, an active controller acts as a virtual crankshaft, which causes a piston to follow a reference trajectory using energy from a storage element. Given the periodic nature of free-piston engine motion, an advanced controller of the present invention is preferably of robust repetitive type that is capable of tracking periodic reference signals.
1. A free piston engine comprising:
a piston movably provided within an engine cylinder for providing a combustion chamber on one side of the piston and another engine chamber on an opposite side of the piston, the piston being operatively connected with a load device; and
an active control system that is operatively connected with an energy storage device for controlling piston trajectory during a firing mode of the engine by coordinating cylinder combustion and piston load, the active control system comprising an active repetitive controller including desired piston trajectory information and a discretized dynamic model of the free piston engine, the active control system being represented as
y
(
k
)
=
B
(
q
-
1
)
A
(
q
-
1
)
u
(
k
)
(
2
)
u ( k )= C ( q −1 )[ r ( k )− y ( k )] (3)
where k is a discrete step index, q −1 is the one step delay operator, u(k) and y(k) are an input and output of a dynamic model of a free piston engine, r(k) is a desired motion profile and C(q −1 ) is the active repetitive controller which can be described as:
C
(
q
-
1
)
=
R
(
q
-
1
)
q
-
N
1
-
q
-
N
,
(
4
)
where R(q −1 ) is a stable filter based on the dynamic model of the free piston engine,
the active control system also comprising an operative connection between the active repetitive controller and at least one engine chamber that is provided adjacent to the piston for transmitting a control signal for controlling a desired trajectory of the piston within the engine cylinder according to a predetermined trajectory from the desired piston trajectory information based at least partially upon actual piston trajectory information and a model dependent stroke by stroke updated calculation of the control signal, and the active control system further comprising at least one engine sensor for determining at least one of piston position, combustion chamber pressure, engine chamber pressure, combustion chamber temperature, and engine chamber temperature for trajectory tracking by the repetitive controller so that that piston motion, including piston displacement, velocity, and acceleration will follow the predetermined trajectory and the desired piston trajectory information can be varied on a stroke by stroke basis of the piston relative to the engine cylinder.
2. The free piston engine of claim 1 , wherein the combustion chamber is operatively connected with an engine power control that provides a desired power output signal, as such may be programmed or dynamically electrically provided to the engine power controller, to a fuel injection controller for creating a desired combustion within the combustion chamber.
3. The free piston engine of claim 1 , wherein the piston is connected with a plunger of a hydraulic pump that is operatively connected with an energy storage device.
4. The free piston engine of claim 3 , 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.
5. The free piston engine of claim 3 , wherein the active control system further comprises at least one controllable valve that allows high pressure fluid as stored within the high pressure source to be selectively connected to at least one hydraulic chamber of the hydraulic pump in real-time by providing control signals to the control valve.
6. The free piston engine of claim 5 , wherein control signals are provided to the control valve from the repetitive control.
7. The free piston engine of claim 6 , wherein the control valve can selectively connect either of the low pressure fluid source and the high pressure fluid source to either of a plurality of chambers of the hydraulic pump under the control of the active control system for defining a desired piston trajectory.
8. The free piston engine of claim 7 , wherein the active control system is also functional to cause a motoring mode of the free piston engine using energy from the energy storage device selectively to cause movement to the hydraulic pump which is thus transferred to the piston of the free piston engine.
9. The free piston engine of claim 1 , wherein the piston is connected with a component of a linear alternator and the energy storage device comprises a battery.