Abstract:
An internal combustion free-piston pump-engine provides pressurized fluid for a hydrostatic powertrain. The free-piston is reciprocably mounted in a piston bore and transfer combustion pressure directly into pressurized hydraulic fluid. Fast, computer controlled hydraulic two-way valves initiate the stepwise pressurization of the smaller face, the larger face, or both faces of the hydraulic end of the free-piston, controlling the piston movement and especially its top-end and bottom end positions. The control concept can be applied to a two-stroke, or four-stroke compression or spark ignition engine. A pressure wave charger, transfers the energy of the exhaust pressure wave into an air intake pressure wave. A cylindrical piston is reciprocally mounted in a piston bore of a housing. The exhaust pressure wave moves the piston from the top-end position to the bottom-end position, against the pressure of the intake air and the force of a return spring. The spring returns the piston back to the top-end position, with the support of collapsing exhaust pressure, after the charger piston releases the exhaust before reaching the bottom-end position. A fuel injector apparatus includes a fuel pump, a fuel accumulator/common rail, an injector valve and an injector ring assembly with injection nozzles at the inner circumference. The oscillating fuel pump piston converts hydraulic pressure directly into pressurized fuel. The nozzles are manufactured into the axial face of a first ring, covered and sealed by means of a second ring of the assembly.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]    This application is a divisional of copending application Ser. No. 09/408,046, filed on Sep. 29, 1999. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Technical Field  
           [0003]    This invention relates generally to free-piston type internal combustion engines, compressors or pumps, and in particular, to innovations which improve the controllability and efficiency of the free-piston engine or pump and reduce the toxic emissions, the weight and the size of such engines.  
           [0004]    2. Background Art  
           [0005]    Although advantageous in applications where pressurized fluid is used to transmit the energy, the simple concept of free-piston internal combustion engines or pumps, transferring the chemical energy of a combustible fuel direct into mechanical energy of pressurized hydraulic fluid, is rarely utilized due to the inability to control their operating characteristics, and in particular, the top and bottom end positions of the piston, sufficiently.  
           [0006]    Free-piston Control  
           [0007]    In one known free-piston engine, disclosed in U.S. Pat. No. 4,791,786, the hydraulic piston control mechanism of the free-piston requires three hydraulic control surfaces in axial direction to control the top-end position and the bottom-end position somewhat sufficiently. The free-piston has a combustion end and a hydraulic end, consisting of a plunger with one outer control surface acting in opposite direction to the combustion forces, and a piston with one larger control surface acting also in the opposite direction and a smaller control surface acting in the same direction as the combustion force. During the compression stroke, pressurized fluid at the outer plunger surface advances the free-piston toward the top-end position while the chamber at the larger piston surface draws fluid in from a reservoir. The smaller piston surface, acting in opposite direction and being smaller than the plunger surface, is permanently pressurized with hydraulic fluid and provides a buffer function at the end of the compression stroke in top-end position by depressurizing the plunger surface. During the expansion stroke, all three control surfaces are exposed to pressurized fluid, advancing the fluid, drawn in during the compression stroke, to the accumulator. The bottom-end position of the free-piston can be obtained by readjusting the position after a stop.  
           [0008]    In this prior art free-piston engine, the top-end position is determined by the balance of the buffer force, depending on the fluid pressure in the accumulator, and the dynamic mass forces of the heavy free-piston, depending on its velocity. The bottom-end position is determined by the balance of combustion and dynamic piston mass forces versus hydraulic forces, and depends on the insufficiently controllable, variable velocity of the piston (rpm.) and the fluid pressure in the accumulator. During a cycle interruption, the bottom-end position can be corrected.  
           [0009]    The variations in top-end position and bottom-end position are too high to allow for an overall sufficient control of the compression ratio and combustion conditions, reducing the efficiency and increasing the amount of toxic emissions. Furthermore, the requirement for three hydraulic control surfaces increases the cost and size of the free piston engine and reduces the efficiency of the free-piston engine.  
           [0010]    In another known free-piston engine, which is disclosed in U.S. Pat. No. 5,556,262, the hydraulic piston control mechanism consists of four control surfaces in axial direction to control the top-end position and the bottom-end position. The free-piston has a hydraulic end, consisting of a compression section, having a larger and a smaller control surface, and a pump section, also having a larger and a smaller control surface in which the larger surfaces are acting in opposite direction to the combustion end of the free-piston assembly.  
           [0011]    During the compression stroke, the smaller control surface of the compression section is in communication with the fluid reservoir and the larger control surface is pressurized with fluid from a compression (bouncing) accumulator, advancing the free-piston toward the top-end position, while the pump section of the hydraulic end draws fluid in from the reservoir. During the expansion stroke, the hydraulic section, controlled by non-return valves, advances the fluid to the pressure accumulator, while the pressure conditions in the compression section remain unchanged. The bottom-end position is determined by decreasing combustion forces and increasing hydraulic forces. The hydraulic section has no noticeable influence in the control of the free-piston.  
           [0012]    The top-end position is determined by the balance of nearly constant hydraulic forces and mass forces of the free-piston, varying with the velocity of the free-piston, and the compression forces acting in opposite direction. The bottom-end position is controlled by the mass forces of the free-piston in addition to the combustion pressure and the increasing hydraulic forces. Increased accuracy of the bottom-end position is obtained with increasing hydraulic losses to brake the free-piston. The compression ratio, which determines efficiency and combustion conditions as well as the amount of toxic emissions, can only be controlled by changing the pressure in the compression (bouncing) accumulator. However, this results in loss of energy and is very time consuming. Moreover, the need for four control surfaces and the requirement of an additional accumulator increase expense, and require additional space and reduce the efficiency of the free-piston engine.  
           [0013]    Charge Mechanism  
           [0014]    The utilization of exhaust gas energy increases the efficiency and reduces weight and size by increasing the specific power output, resulting in a smaller engine with less heat and friction losses.  
           [0015]    In U.S. Pat. No. 5,261,797, there is disclosed a pressure wave charger (pulse pressure booster) which consists of a compressor, driven by an exhaust turbine or the crankshaft, and a booster, having a spring loaded booster piston, which is reciprocally mounted in a piston bore of a booster housing. The ingress of fresh air from the compressor to one chamber at the first end of the booster piston is controlled by a non-return valve. The egress to the combustion chamber is controlled by a rotary valve. A chamber at the second end of the booster piston, opposite to the first end, is in communication with the sump of the two-stroke combustion engine, controlled by a valve. A second chamber, being in communication with the exhaust port of the combustion engine, can be arranged at the second end of the booster piston, further increasing the pressure of the pressurized air from the compressor in the booster chamber.  
           [0016]    Starting in the top-end position after the ignition, the combustion piston advances toward the bottom-end position, compressing the air in the sump while the compressor charges the first booster chamber with fresh, pressurized air. Near the bottom-end position of the combustion piston, the exhaust port and the valve to the sump of the engine open and provide pressure to their respective chambers at the second end of the booster piston in opposition to the compressed air. Pressure, sufficient to overcome the forces of the compressed air and a spring at the opposite side of the booster piston, will advance the piston toward the top-end position and increase the pressure of the compressed fresh air being forced through the intake port into the combustion chamber. Due to the spring force, the booster piston will return in its original position when the ports of the combustion engine are closed during the compression stroke.  
           [0017]    The charge apparatus is complex and the overall efficiency low due to friction, leakage and larger amounts of compressed air, not participating on the combustion process.  
           [0018]    Fuel Injection Apparatus  
           [0019]    Known fuel injection systems provide fuel of increasingly higher pressure levels to a single, centrally or nearly centrally located fuel injector with one or several closely spaced nozzles to provide improved conditions for a more efficient combustion with reduced toxic emissions. The higher injection pressure provides an improved air-fuel mixture for a more efficient and cleaner combustion/lower toxic emissions, but the increased injection pressure consumes part of the gain in efficiency of the combustion process.  
           [0020]    It is therefore, an object of the invention to provide a simplified hydraulic control mechanism for the free-piston, an efficient and robust extraction of energy from the exhaust gas and a simplified fuel injection apparatus which allows a more efficient and cleaner combustion.  
           [0021]    Another object of the invention is to reduce known technical shortcomings of prior art free-piston engines or pumps, in particular, the limited ability to control the top-end and bottom-end positions of the piston in an inherently simple and efficient manner.  
         SUMMARY OF THE INVENTION  
         [0022]    The present invention provides a novel hydraulic pressure-step force control mechanism for moving a piston or plunger of any apparatus, and for example, for moving the free-piston of an internal combustion engine, a pump, or the like, which hydraulic pressure-step force control mechanism results in a more accurate control of the top and bottom end positions of the piston. In addition, the invention provides a simple exhaust pressure wave charger. Moreover, the invention provides an improved fuel injection apparatus which introduces fuel into the combustion chamber under high pressure conditions and provides a more even distribution over the entire combustion chamber while preventing concentration of jets of atomized fuel within the combustion chamber, resulting in faster, more even combustion characterized by higher efficiency and reduced toxic emissions and soot.  
           [0023]    In accordance with the invention there is provided a free-piston internal combustion pump-engine which includes a housing including a piston bore and at least one free-piston, mounted in the piston bore for reciprocating movement between a bottom-end position and a top end position. The free-piston has a drive end and a combustion end. The drive end of the free-piston cooperates with the piston bore to define a first chamber and a second chamber. The combustion end of the free-piston cooperates with the piston bore to at least partially define a combustion chamber. A control system produces pressure control forces for moving the free-piston between the bottom-end position and the top-end position during a compression stroke, the control system supplying pressurized fluid to the piston bore at the drive end of the free-piston for applying a pressure control force to the drive end of the free-piston to move the free-piston toward the top-end position. The control system varies the supply of pressurized fluid to the piston bore to thereby vary the pressure control force applied to the free-piston at different times during the compression stroke. The free-piston is moved toward the bottom-end position by an expansion pressure within the combustion chamber during an expansion stroke, causing the pressurized fluid to be extracted from the piston bore as the free-piston is moved toward the bottom-end position during the expansion stroke.  
           [0024]    In one embodiment of the invention, the combustion end of the free-piston has an outer combustion face and an inner bounce face, and the drive end has two control surfaces, an inner face and an outer face, acting in opposition to the combustion face, to control the top-end and bottom-end positions of the free-piston. At the beginning of the compression stroke, when starting the engine or pump, both hydraulic control surfaces are in fluid communication with a medium pressure accumulator, resulting in a small differential hydraulic piston force. A valve shift depressurizes one of the control surfaces, acting in opposition to the other control surface, increasing the hydraulic piston force in the middle portion of the stroke. In response to a further valve shift, the pressure supply is shifted from the medium pressure accumulator to a high pressure accumulator which results in sufficient force to advance the free-piston into its top-end position. During ignition, the increased pressure in the combustion chamber drives the free-piston toward its bottom-end position. During the expansion stroke, the sequence of valve actuation is reversed and delayed in time to allow for the extraction of the combustion energy. During the expansion stroke, the air in the bounce chamber at the opposite side of the combustion chamber is compressed. The timing of the fast-acting, two-way valves, which control the pressure stages at the hydraulic control piston, and therefore the end positions, is preferably determined by an electronic powertrain controller, considering the operating conditions (e.g., hydraulic and compression pressure, piston velocity, etc.) needed to control the free-piston engine-pump effectively.  
           [0025]    In accordance with another aspect of the invention, the exhaust pressure wave is transformed into an intake pressure wave, divided by a pressure wave separator, to charge the combustion chamber with pressurized fresh air. This provides higher power density of the engine and improves the scavenging and fuel mixture process. The remaining exhaust energy is extractable at the outlet of the pressure wave charger.  
           [0026]    In one embodiment, the pressure wave charger comprises a charge piston having an exhaust end and an air-intake end, reciprocally mounted in a piston bore. The piston bore has an exhaust end chamber in communication with the exhaust port of the combustion engine and the charger exhaust (muffler) port. The charge exhaust port is opened at the end of the charger piston stroke. The piston bore has an intake chamber in communication with an air intake port of the combustion chamber and an air intake (air filter). The exhaust side is pressurized by the exhaust gas pressure wave from the combustion chamber, advancing the charge piston, against the forces of the compressing intake air and a bias structure, toward the bottom-end position of the charger piston, filling the combustion chamber with new compressed air. The bias structure returns the charger piston, with the support of the collapsing exhaust pressure wave, back into the top-end position during the compression stroke of the combustion engine, while the charger piston draws fresh air to the chamber at the air-intake end.  
           [0027]    The free-piston engine or pump further includes a novel fuel injection apparatus which provides a finer atomization and a more equal distribution of fuel within the ignition chamber. This results in more uniform air/fuel ratio and combustion, reducing the amount of toxic emissions and soot, and reducing the ignition delay, thereby improving efficiency.  
           [0028]    In one preferred embodiment, the fuel injection apparatus comprises a novel fuel pump including a fuel injector mechanism and a novel nozzle structure which defines a fuel conduction channel and one or more nozzles. The fuel pump includes a fuel piston mounted in a bore of the fuel pump housing for reciprocating movement within the bore for increasing the pressure of the combustion fuel being injected into the combustion chamber. A second fuel pump can be added to provide uninterrupted fuel supply from the fuel injection pump.  
           [0029]    In one preferred embodiment, the nozzle structure includes a fuel injection ring which defines one or more fuel conduction channels, for providing communication between the fuel injector and a plurality of micro-slots which define a plurality of nozzles at the inner, circumferential face of the fuel injection ring which is disposed to encompass the periphery of the ignition chamber. The fuel conduction channels and the micro-slots are preferably configured as depressions, arranged at an axial face of the fuel injection ring, the depressions being covered and sealed by a second section of the injection ring, allowing long and very narrow slots for increased atomization. A comparable function can be obtained by forming a thin shim-like layer on a substrate, incorporating the nozzle arrangement in form of a discontinuities formed in the layer, between the two parts of the fuel injection ring. The circumferential arrangement of slots/nozzles prevents the concentration of jets of atomized fuel and provides a more even distribution over the whole ignition chamber. The assembly of more than one multi channel/micro-slot arrangement allows for differently timed injection or different combustion ingredients. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0030]    The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with the further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings, wherein like reference numerals identify like elements, and wherein:  
         [0031]    [0031]FIG. 1 is a simplified representation of a free-piston engine and hydraulic control system in accordance with the invention.  
         [0032]    [0032]FIG. 1A is a simplified representation of a further embodiment of a free-piston engine in accordance with the invention.  
         [0033]    FIGS.  2 A- 2 I are simplified representations of the hydraulic piston control mechanism, showing the operation states of the cycle-valves, over a complete work cycle of a single piston, two-stroke engine.  
         [0034]    [0034]FIG. 3 is a piston force diagram, indicating hydraulic piston force as a function of stroke during one complete cycle of the piston control mechanism as shown in FIGS.  2 A- 2 I.  
         [0035]    [0035]FIG. 4 is a cross section view of one embodiment of a free-piston engine of the system shown in FIG. 1.  
         [0036]    [0036]FIG. 5 shows a static engine start arrangement, including a hydraulic starter valve, for the free-piston engine shown in FIGS.  2 A- 2 I.  
         [0037]    [0037]FIG. 6 is a cross section view of the fuel injection apparatus taken along the line  6 - 6  of FIG. 1.  
         [0038]    [0038]FIG. 7 is a cross section view, similar to that of FIG. 6, of a further embodiment of a fuel injection apparatus for the system of FIG. 1.  
         [0039]    [0039]FIG. 8 is an enlarged fragmentary perspective view of one embodiment of a nozzle structure for the fuel injection apparatus shown in FIG. 6.  
         [0040]    [0040]FIG. 9 is an enlarged fragmentary perspective view of another embodiment of a nozzle structure for the fuel injection apparatus shown in FIG. 6.  
         [0041]    [0041]FIG. 10 is an enlarged fragmentary perspective view of a further embodiment of a nozzle structure for the fuel injection apparatus shown in FIG. 6.  
         [0042]    [0042]FIG. 11 illustrates a free-piston internal combustion engine in which the hydraulic end of the piston provides the function of a “driver” or piston movement coordinator. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0043]    Referring to the drawings, and in particular to FIG. 1, the novel hydraulic pressure-stepwise force control mechanism is described with reference to an application in a free-piston internal combustion engine, shown in FIG. 1, for moving the free-piston of the engine. In one preferred embodiment, the internal combustion engine  1  is a two-stroke compression or spark ignition engine including a single cylinder  3  with two opposed pistons  4  and  4 ′, and which is pressure wave charged. However, the piston force control mechanism can also be applied to four-stroke compression or spark ignition engines. The free-piston engine includes a novel fuel injection apparatus. Although the invention is described with reference to an internal combustion engine, the invention has other applications including pumps, compressors and the like. Moreover, the stepwise force control of the piston or plunger is also advantageous for applications of a piston or plunger for force and/or speed control, and in particular, in those types of applications that are commonly referred to as constant pressure or load sensing applications.  
         [0044]    The free-piston pump-engine  1  includes an engine housing  2  with a piston bore  3  and a pair of free-pistons  4  and  4 ′, slidably mounted therein. As will be shown, the free-pistons transfer the combustion pressure directly into hydraulic pressure, thus reducing the number of parts within an engine, the friction losses, size and cost of the internal combustion engine. In one embodiment, multiple, controller actuated, hydraulic two-way valves provide pressurized hydraulic fluid to the differential piston ends of the free-pistons, during each operating cycle, thus providing stepwise increasing and decreasing piston forces which provide accurate control of the top-end and the bottom-end positions of the pistons. This results in improved control of the combustion process, reducing fuel consumption and toxic emissions. A pressure wave charger  28 , which is attached to the engine, separating the exhaust pressure chamber from the intake pressure chamber, transforms the exhaust pressure wave directly into an intake pressure wave, thus simplifying the charge mechanism of the engine and providing a larger portion of the thermal exhaust energy for an additional mechanism (exhaust turbine) to extract energy for increased efficiency. A fuel injection apparatus  45  transfers hydraulic pressure directly into pressurized fuel. An injector valve of the fuel injection apparatus  45  supplies the fuel from a fuel accumulator/common rail to relatively narrow injector nozzles in communication with the ignition chamber to improve fuel atomization, air-fueling mixing and distribution, thereby reducing toxic emissions, soot and fuel consumption.  
         [0045]    Free-piston Control.  
         [0046]    More specifically, with continued reference to FIG. 1, the pistons  4  and  4 ′ reciprocate within the piston bore  3  in opposing directions between their bottom-end positions  7  and  7 ′ (represented by the dashed lines), i.e., the intake/exhaust end, and their top-end positions  8  and  8 ′, i.e., the combustion end. During the compression stroke, the pistons  4  and  4 ′ are moved toward one another. During the expansion stroke, the pistons  4  and  4 ′ are moved away from one another. The space between the pistons  4  and  4 ′ defines a combustion chamber  24  for the engine  1 . The pistons  4  and  4 ′ are substantially identical and so only piston  4  will be described, the same reference numerals with the addition of a prime notation being used to designate like parts of the piston  4 ′.  
         [0047]    Piston  4  has a small hydraulic end  5  and large combustion end  6 . The double acting combustion end  6  has a large outer combustion face  9  and a smaller, inner bounce face  10 , acting in opposite direction. The double acting hydraulic end  5  has a smaller inner face  11 , facing in the direction of the combustion end  6  of the free-piston, and a larger, outer face  12  facing in opposite direction. The chamber  13  at face  11  and the chamber  14  at face  12  are in fluid communication with hydraulic fluid accumulators, including a medium pressure hydraulic fluid accumulator  15  and a high pressure hydraulic fluid accumulator  16 . The flow of fluid between the accumulators  15  and  16  and the engine  1  is controlled by a hydraulic valve system, including a plurality of cycle two-way valves  17 - 20 ,  19 ′ and  20 ′ which are operated by the electronic powertrain controller  22  and one-way valve  26 . The electronic powertrain controller  22  also provides monitoring and control functions in the manner known in the art. Further valves, including valves  27  and  46  (FIGS. 5 and 6), provide other functions, such as start, positioning and safety functions. The cycle valves  17 - 20  can provide pressurized hydraulic fluid to both faces  11  and  12 , or only to face  12  at the hydraulic end  5 , resulting in three (or four) different values of piston force, for advancing the free-piston  4  to the top-end position. Similarly, the cycle valves  17 - 18 ,  19 ′ and  20 ′ can provide pressurized hydraulic fluid to both faces  11 ′ and  12 ′, or only to face  12 ′ at the hydraulic end  5 ′, of free-piston  4 ′, resulting in three (or four) different values of piston force, for advancing the free-piston  4 ′ to the top-end position. The valves  17 - 20 ,  19 ′ and  20 ′ are fast operating valves. It is pointed out that the function of valves  19  and  19 ′ can be provided by a single two-way valve (not shown). Also, the function of valves  20  and  20 ′ can be provided by a single two-way valve (not shown). However, the use of separate valves  19 ,  19 ′ and  20 ,  20 ′ is preferred because this allows independent control of the application of pressurized fluid to the two free-pistons  4  and  4 ′.  
         [0048]    In one preferred embodiment, the free-pistons  4  and  4 ′ are moved from their bottom-end position to their top-end position by the control of the supply of hydraulic fluid to the hydraulic ends  5  and  5 ′ of the free-pistons  4  and  4 ′. However, the energy for the hydraulic ends  5  and  5 ′ can come from any type of pressurized medium, such as pressurized air, water, hydraulic fluid, etc.  
         [0049]    The sequencing of the valves  17 - 20 ,  19 ′ and  20 ′ is controlled by the electronic powertrain controller  22 . The electronic powertrain controller  22  monitors or senses the axial positions of the free-pistons  4  and  4 ′ within the piston bore  3  and the velocity of the free-pistons  4  and  4 ′ as the free-pistons are moved during the compression and expansion strokes, providing position and velocity information. The electronic powertrain controller  22  also monitors or senses temperatures within the free-piston engine, the pressure in the combustion chamber  24 , the pressures in the chambers  13 ,  14  and  13 ′,  14 ′, and exhaust gas conditions during the operating cycle, for providing temperature, pressure and exhaust gas condition information. The electronic powertrain controller  22  uses the position, velocity, temperature, pressure and exhaust gas information to determine when the valves  17 - 20 ,  19 ′ and  20 ′ are to be operated during the compression and expansion strokes.  
         [0050]    An illustration of the pressure-step force control operation of the engine  1 , in accordance with the invention, is now made with reference to the simplified representation of a free-piston engine which includes only one free-piston  4  slidably movable within the bore  3 . A complete cycle of the engine controlled by the hydraulic pressure-step force control arrangement in accordance with one embodiment of the invention includes nine stages. The nine stages of a complete cycle of the engine are illustrated in FIGS.  2 A- 2 I. Only cycle valves are shown in FIGS.  2 A- 2 I in conjunction with a simplified representation of one of the free-pistons  4 . The stages of the compression stroke are illustrated in FIGS.  2 A- 2 E and the stages of the expansion stroke, including injection and combustion, are illustrated in FIGS.  2 F- 2 I.  
         [0051]    At the beginning of the compression stroke, the force of the compressed air in bounce chamber  23 , acting in opposite direction to the air in combustion chamber  24 , advances the free-piston toward top-end position, drawing hydraulic fluid from reservoir  25  through non-return valve  26  into chamber  14  at the hydraulic end  5 .  
         [0052]    Briefly, at the beginning of the compression stroke, or shortly thereafter, when starting the engine, both hydraulic control surfaces  11  and  12  are in communication with the medium pressure accumulator  15 , resulting in a small differential hydraulic piston force. By way of example, the pressure of the hydraulic fluid in the medium pressure accumulator  15  can be 6000 pounds per square inch (psi). A valve shift depressurizes the smaller control surface  11 , acting in opposite direction to the larger control surface  12 , increases the hydraulic piston force in the middle section of the stroke. A further valve shift in pressure supply, from the medium pressure accumulator  15  to the high pressure accumulator  16 , provides sufficient force to advance the free-piston into top-end position. By way of example, the pressure of the hydraulic fluid in the high pressure accumulator can be 8000 psi. However, the pressure of the hydraulic fluid supplied by the medium pressure accumulator and that supplied by the high pressure accumulator will vary somewhat during the operation of the free-piston engine as is known. The electronic powertrain controller  22  monitors the pressure within the medium pressure accumulator  15  and the pressure within the high pressure accumulator  16  by way of suitable sensors  21 . The electronic powertrain controller  22  can adjust the amount of fuel supplied to the ignition chamber  24  by the fuel injector apparatus  45  and can control the operation of the hydraulic valves  17 - 20 ,  19 ′ and  20  ′ to adjust the pressure step-force control as a function of the pressure within the medium pressure accumulator  15  and the high pressure accumulator  16  to make combustion fuel usage as economical and clean as possible.  
         [0053]    More specifically, referring initially to FIGS. 2A and 5, when starting the engine, the free-piston  4  is located in an undefined position between the top-end and the bottom-end positions. A static start position, in which the free-piston  4  is located at the bottom-end position, can be obtained by pressurizing only the small face  11  of hydraulic end  5 . In one embodiment, a valve  27  (FIG. 5) is operated, depressurizing control surface  12  and pressurizing control surface  11 , which causes the free-piston  4  to be moved into the bottom-end position so that there is only little pressurized air in the sump chamber underneath the free-piston  4 . However, air will be sucked into the sump chamber through a non-return valve at the end of the compression stroke.  
         [0054]    For the initial suction stage A of the compression stroke of an already running engine, i.e., a non-static start, the air pressure in the bouncing chamber moves the free-piston  4  towards the top-end position and draws hydraulic fluid into chamber  14  at control face  12 , illustrated in FIG. 2A. The valves  17  and  18  are initially closed so that pressurized fluid is not being supplied from either accumulator  15  and  16  to the engine. Also, valve  19  is closed and valve  20  is open, so that the reservoir  25  is communicated through one-way valve  26  with both ends of the hydraulic piston. At the combustion end  6  of the free-piston  4 , air pressure increases in the combustion chamber  24  and decreases in the sump chamber  23  as the free-piston  4  is advanced further toward the top end position.  
         [0055]    Referring now to FIG. 2B, which illustrates the second stage B of the combustion stroke, control valve  18  is opened to supply medium pressure fluid from accumulator  15  to the hydraulic end of the free-piston  4 . Valve  20  remains open. In an intermediate piston position, the increasing compression forces in the combustion chamber  24 , and the decreasing, opposing bounce forces in chamber  23  and inertia forces of the free-piston  4  are balanced. Accordingly, pressurized hydraulic fluid is supplied to the hydraulic end  5  to advance the free-piston further toward the top-end position  8  (FIG. 1). During this phase, as illustrated in FIG. 2B, control valves  18  and  20  are open, providing a fluid connection between the medium pressure accumulator  15  and the piston faces  11  and  12  at the hydraulic end  5  of the free-piston  4 , resulting in a differential force on the free-piston  4  which advances the free-piston  4  further toward top-end position  8 .  
         [0056]    That is, the force on the bottom surface  12  of the hydraulic end  5  of the free-piston  4  is greater than the force on the other surface  11  of hydraulic end  5  of the free-piston  4  so that there is a net or differential force on the hydraulic end  5  of the free-piston  4  that moves the free-piston  4  toward the top-end position.  
         [0057]    At the hydraulic end  5  of the free-piston  4 , the small differential force of medium pressure has advanced the free-piston  4  into an intermediate position. It is pointed out that preferably stage B is activated earlier to compensate for the reduced air pressure underneath the free-piston  4  when the engine is being started. At the combustion end  6  of the free-piston  4 , air pressure increases in the combustion chamber  24  and decreases in the sump chamber  23 , or more air is sucked in during the starting stroke.  
         [0058]    Referring now to FIG. 2C, the increasing compression forces at the outer face  9  at combustion end  6  are overcompensated by higher hydraulic forces at hydraulic end  5  which are obtained by closing the valve  20  and opening the valve  19  to discharge the pressurized hydraulic fluid from chamber  13  at face  11  of the free-piston  4  into the reservoir  25  so that now a single hydraulic force is being applied only to surface  12  of the free-piston  4  during stage C (FIG. 2C). At the hydraulic end  5  of the free-piston  4 , the single hydraulic force being applied to surface  12  of medium pressure advances the free-piston  4  toward the top-end position. At the combustion end  6 , air pressure further increases in the combustion chamber  24  and decreases in the sump chamber  23 , as more air is sucked in during the starting stroke.  
         [0059]    In stage D, valve  18  is closed and then valve  17  is opened. This changes the source of hydraulic fluid being supplied to surface  12  from the medium pressure accumulator  15  to the high pressure accumulator  16 , resulting in higher forces for advancing the free-piston to its final top-end (or ignition) position  8  (FIGS.  2 D- 2 E). The sequencing of the operation of valves  18  and  17  preferably is controlled so that both valves are not open at the same time. The hydraulic control system can include suitable pressure relief valves connected to the hydraulic lines between the valves  17  and  18  and the free-pistons  4  and  4 ′ to limit pressure. At the combustion end  6  of the free-piston  4 , air pressure in the combustion chamber  24  increases further. At this point, the pressure in the sump chamber  23  continues to decrease.  
         [0060]    Referring to FIG. 2E, during stage E, at the hydraulic end  5  of the free-piston  4 , the single outer force of high pressure provided by the fluid at high pressure from accumulator  16  advances the free-piston  4  into the top-end position. It is pointed out that an earlier start of the application of the high pressure fluid, or applying the high pressure fluid for a longer duration, both in combination with the inertial forces of the free-piston  4  will vary the outer force which determines the top-end/ignition position. The bottom-end positioning can be controlled in a similar manner. The control of the top end and the bottom end positions affects the compression ratio. Thus, the compression ratio can be varied by the electronic powertrain controller  22  (FIG. 1) from stroke to stroke, adapting to the power or pressure needs of the user of energy. Furthermore, the free-piston  4  can be held in the top-end position to influence the combustion conditions. Moreover, a further hydraulic force of intermediate size can be obtained at the hydraulic end by communicating faces  11  and  12  with the high pressure accumulator  16 , prior to applying the single outer force of high pressure. This intermediate hydraulic force can be produced by operating valves  17  and  20  to an open condition with valves  18  and  19  closed. At the combustion end  6  of the free-piston  4 , during the fifth stage E of the compression cycle, air pressure in the combustion chamber  24  continues to increase and the pressure in the sump chamber  23  is nearly depleted.  
         [0061]    Then, fuel is injected into the combustion or ignition chamber  24  and ignited. The injection of fuel, and the following combustion, increase the pressure in the combustion chamber  24 , advancing the free-piston  4  to bottom-end position  7  (FIGS.  2 F-I). The combustion energy, in form of higher pressure at the outer face  9  at the combustion end  6  of the free-piston  4  during the expansion stroke, is extracted by generally the same sequence but delayed actuation of the hydraulic control valves  17  to  20 .  
         [0062]    During ignition, the increased pressure in the combustion chamber  24  pushes the free-piston  4  toward the bottom-end position. The sequence of valve actuation is reversed during the expansion stroke, and delayed in time to allow for the extraction of the combustion energy. That is, the states of valves  17 - 20  ( 19 ′,  20 ′) for stages G, H and I correspond to those for stages D, C and B, respectively. During the expansion stroke, the air in the bounce chamber, at the opposite side of the combustion chamber, is compressed for advancing the free-piston during the next combustion stroke, toward the top-end position, while drawing fluid into the large chamber at the hydraulic end. Hydraulic pressure from the accumulator is applied to the hydraulic end when the compression and bounce forces at the combustion end are in or nearly in balance to advance the free-piston to its top-end position.  
         [0063]    More specifically, referring to FIG. 2F, during stage F of the expansion stroke, the pressure in the combustion chamber  24  increases instantaneously and advances the free-piston  4  toward the bottom-end position. Hydraulic fluid of high pressure at the outer chamber  14  of hydraulic end  5  is pumped through valve  17  into the high pressure accumulator  16 .  
         [0064]    Referring now to FIG. 2G, which illustrates stage G, the decreasing pressure in the combustion chamber  24  has advanced the free-piston  4  into an intermediate position. The air pressure in the sump  23  increases. Hydraulic fluid of high pressure at the outer chamber  14  of the hydraulic end  5  is still being pumped into the high pressure accumulator  16 .  
         [0065]    Referring to FIG. 2H, the decreasing pressure in the combustion chamber  24  has advanced the free-piston  4  into an intermediate position. The air pressure in the sump  23  further increases. Hydraulic fluid of medium pressure at the outer chamber  14  of the hydraulic end is pumped into the medium pressure accumulator  15  through a shift of a two-way valves  17  and  18 .  
         [0066]    In the final stage I, shown in FIG. 2I, the decreasing pressure in the combustion chamber  24  has advanced the free-piston  4  into the bottom end position. The air pressure in the sump  23  has reached a maximum.  
         [0067]    Hydraulic fluid of medium pressure and small displacement of the differential piston is pumped into the medium pressure accumulator  15 . The timing of the operation of the two-way valve  20 , activating the function “differential piston,” determines the bottom-end position. Furthermore, the free-piston  4  can be held briefly in bottom-end position  7  to influence the scavenging conditions.  
         [0068]    The foregoing description of the operation of the free-piston control makes reference only to free-piston  4 . However, both of the free-pistons  4  and  4 ′ are being driven between their top and bottom end positions during each cycle of operation, and valves  19 ′ and  20 ′ are being operated along with valves  19  and  20 , respectively. Moreover, although the step pressure force control function in accordance with the invention is described with reference to a free-piston engine having a pair of free-pistons  4  and  4 ′, the step pressure force control function can be used in applications which include only a single free-piston.  
         [0069]    In addition, the engine can be operated with only one accumulator or pressure level, reducing the hydraulic piston forces from three to two. In one such embodiment, the high pressure source  16  and valve  17  (or pressure source  15  and valve  18 ) are not included in the hydraulic control system. Also, the powertrain can be operated with more than two accumulators or pressure levels. When more than two accumulators or pressure levels are used, the piston forces can be broken down into smaller increments. In yet another arrangement illustrated in FIG. 1A, a free-piston internal combustion engine  100  includes a free piston  104  having a hydraulic end  105  that includes a third pressurized surface  110  in addition to pressurized surfaces  111  and  112 . In this embodiment, four hydraulic forces are produced using a single accumulator  114 , a hydraulic valve system  120 , controlled by a powertrain controller  122 , and the three piston surfaces  110 - 112 . The lowest force net is produced when all surfaces  110 ,  111  (acting in the direction of the combustion force) and  112  (acting in the opposite direction) are pressurized. An intermediate net force is produced when the smaller of the surfaces  110  and  111  is not pressurized. The highest force is produced when neither surface  110  nor  111  is pressurized.  
         [0070]    [0070]FIG. 3 shows the conceptual hydraulic piston force diagram, indicating the energy/work E extracted during the nine stages of the cycle which are indicated in FIG. 3 by the corresponding letters A-I. In FIG. 3, the energy E being extracted is represented by the hatched area. It is pointed out that the force characteristic, shown in FIG. 3, illustrates the pressure-step force function provided by one preferred embodiment and that other force characteristics can be obtained by using different valve arrangements, different valve sequencing and alternative sources of pressurized hydraulic fluid. For example, by communicating the high-pressure accumulator  16  with both faces  11  and  12  by opening both valves  17  and  20 , and closing valves  18  and  19 , a fourth hydraulic force (differential force) of intermediate size (not illustrated) can be provided at the hydraulic end  5 . Also, one or more than two sources of pressurized fluid can be used, such as a low, medium, and high pressure source for a more thorough adaptation of force needs.  
         [0071]    [0071]FIG. 4 is a cross section view of one embodiment of a free-piston engine of the system shown in FIG. 1. The fuel injection apparatus  45  (FIG. 1), which supplies combustion fuel to the fuel injection structure  46 , is not shown in FIG. 4. In the embodiment of the free-piston engine shown in FIG. 4, the charger housing  34 ′ extends around the engine housing  2 , defining an annular piston bore  33 ′, and the charger piston  29 ′ is a generally annular member. The spring  32  encircles the engine housing  2 . The exhaust inlet port  36 , the exhaust port  37 , the intake port  39  and the air intake  40  are defined by annular openings.  
         [0072]    Referring to FIG. 5, a static engine start position (bottom-end  7 ), in which the free-piston  4  is located at the bottom-end position, can be obtained by switching valve  27  to its open position, providing pressurized fluid from the accumulator  15  to control face  11  and depressurization of control face  12  at the hydraulic end  5 , while all of the cycle valves  17 ,  18 ,  19 , and  20  are in closed positions.  
         [0073]    Pressure Wave Charger  
         [0074]    Referring again to FIG. 4, the pressure wave separator apparatus  28 , includes a generally annular charger piston  29 ′ slidably mounted in a piston bore  33 ′ of a charger housing  34 ′ for movement between bottom and top end positions. The charger piston  29 ′ is shown in the top end position  42 , the bottom end position  41  of the charger piston being shown in phantom in FIG. 4. The charger piston  29 ′ includes an exhaust end  30  and air intake end  31 . A bias mechanism, such as a spring  32  or other suitable bias device, biases the charger piston  29 ′ towards the top end position. A chamber  35  at the side of the exhaust end  30  is in fluid communication with the exhaust port  36  of piston bore  3  and with the charger exhaust  37  (muffler) when the charger piston  29 ′ is in the bottom-end position  41 . An intake chamber  38  at the intake end  31  of charger piston  29 ′ is in fluid communication with an intake port  39  of piston bore  3  and the air intake  40  (air filter). The pressure wave separator preferably is used with one cylinder engines.  
         [0075]    Initially, the charger piston  29 ′ has been moved to the bottom end position  41  by the exhaust forces due to an exhaust gas pressure wave introduced into the chamber  30  through the exhaust inlet port  36 . The spring  32 , acting at the air intake end  31  of the charger piston  29 ′, moves the charger piston  29 ′ in a direction opposite to that of the exhaust forces acting at the face  30  of the exhaust end. This advances the charger piston  29 ′ into the top-end position  42 , drawing fresh air through a non-return valve  43  into the intake chamber  38 . The next exhaust gas pressure wave, which is introduced through the open exhaust port  36  to the face of the exhaust end  30 , advances the charger piston  29 ′ toward the bottom-end position  41 , pumping fresh air from the intake chamber  38  through a non-return valve  44  and intake port  39  into the combustion chamber  24  of piston bore  3 . The exhaust end  30  of charger piston  29 ′ opens the fluid connection to exhaust  37  before it reaches bottom end position  41 , discharging the exhaust gas to complete the engine cycle.  
         [0076]    Fuel Injection Apparatus  
         [0077]    Referring now to FIGS. 1 and 6, in one preferred embodiment, the fuel injection apparatus  45  includes a fuel injection pump  47  (FIG. 6), having an associated hydraulic fluid pump valve  46 , a fuel injector valve  49  and a fuel injection nozzle structure  48 , which in one preferred embodiment comprises an injection ring which defines a fuel conduction channel  50  and one or more nozzles  51  which are in fluid communication with the fuel conduction channel. The fuel injection nozzle structure  48  encompasses the combustion or ignition chamber  24 , bordering the faces  9  and  9 ′ 
         [0078]    (FIG. 1) of the combustion ends  6  and  6 ′ of the free-pistons  4  and  4 ′.  
         [0079]    Referring to FIG. 6, which is a cross section of the fuel injection apparatus taken along line  6 - 6  of FIG. 1, in one preferred embodiment, the fuel pump  47  is operated hydraulically. However, the energy for the hydraulic end can come from any type of pressurized medium, such as pressurized air, water, hydraulic fluid, etc. The hydraulic fuel pump  47  is disposed in fluid communication with a fuel accumulator/common rail  64 , the fuel injector valve  49 , and the fuel conduction channel  50  of the nozzle structure  48 . The fuel injector pump  47  includes a fuel pump piston  52 . The fuel pump piston  52  is mounted in a bore  55  of the fuel pump housing  56  for reciprocating movement within the bore  55  between a top-end position, illustrated in FIG. 6, and a bottom-end position. The fuel pump piston is reciprocated within the bore  55  for increasing the pressure of the combustion fuel being injected into the combustion chamber  24  through the nozzles  51 . The fuel pump piston  52  is generally cylindrical in shape and has a large hydraulic end  53  and small fuel end  54 . The charge and discharge of the chamber  57  at the outer face  58  of the fuel pump piston  52  with pressurized hydraulic fluid from accumulator  16  (FIG. 1), together with the bias force provided by a bias device, such as a spring  59 , (acting in opposite direction to the hydraulic forces at the outer face  58 ), provides a reciprocal pumping movement of fuel pump piston  52 . The ingress of combustion fuel from a fuel reservoir  60  to the small chamber  61  at the fuel end  54  and the egress of fuel to the fuel accumulator/common rail  64  are obtained by non-return valves  62  and  63 . The hydraulic fluid pump valve  46  and the fuel injection valve  49  are preferably operated under the control of the electronic powertrain controller  22  (FIG. 1).  
         [0080]    It is assumed initially that the fuel pump piston  52  is in the top-end position and that the chamber  57  at the hydraulic end  53  of the fuel pump piston  52  contains hydraulic fluid. The combustion fuel has been drawn into chamber  61  from the reservoir  60  through the one-way valve  62  as the piston  52  was returned to the top-end position in the last cycle. The hydraulic fluid pump valve  46  is operated so that pressurized hydraulic fluid from the hydraulic accumulator  16  is supplied to the chamber  57  at the hydraulic end  53  of the fuel pump piston  52 . The hydraulic fluid advances the fuel pump piston  52  toward the bottom end position (to the right in FIG. 6), compressing the spring  59  and pumping the highly pressurized fuel through the one way valve  63  into the fuel accumulator/common rail  64 , against the fluid pressure of the fuel accumulator/common rail  64 . When the chamber  57  has been depressurized, the spring  59  returns the fuel pump piston  52  back into top end position (to the left in FIG. 6), drawing fuel from the reservoir  60  through non-return valve  63  into the chamber  61  at the fuel end  54  of the fuel pump piston  52  for the next cycle.  
         [0081]    When injection valve  49  is opened under the control of the electronic powertrain controller  22 , the pressurized fuel in the accumulator/common rail  64  is forced into the channel  50  and through the nozzles  51  and directed into the combustion chamber  24 .  
         [0082]    Referring to FIG. 7, it is pointed out that a second fuel pump  47 ′, acting parallel to the first fuel pump  47 , can be added to provide uninterrupted fuel supply from the fuel injection pump and eliminate the need for the common rail. The structure and operation of the second fuel pump  47 ′ is similar to that for fuel pump  47  and accordingly, the components of fuel pump  47 ′ have been given the same reference numbers, but with a prime notation, as corresponding component of fuel pump  47 . Moreover, two pressure intensifiers, connected in a parallel arrangement, with coordinated pressurization of the hydraulic end of the piston allow for uninterrupted pumping action.  
         [0083]    The fuel conduction channel and the micro-slots are preferably configured as depressions, arranged at an axial face of the fuel injection ring, the depressions being covered and sealed by a second section of the fuel injection ring, allowing long and very narrow slots for increased atomization. A comparable function can be obtained by forming a thin shim-like layer on a substrate, incorporating the nozzle arrangement in the form of discontinuities formed in the layer between the two parts of the fuel injection ring. The circumferential arrangement of slots/nozzles prevents the concentration of jets of atomized fuel and provides an improved local air/fuel ratio for a more even distribution of fuel over the whole ignition chamber. The assembly of more than one multi channel/micro-slot arrangement allows for differently timed injection or different combustion ingredients.  
         [0084]    Referring to FIG. 8, in one preferred embodiment, the nozzle structure  48 , only a portion of which is illustrated, comprises a member  68  having an opening  68   b  (FIG. 6) therethrough. A further member  68   a , shown spaced apart from member  68  in FIG. 8, overlies the upper surface  69  of member  68  and can have an opening corresponding to opening  68   b  of member  68 . The members  68  and  68   a  preferably are generally rectangular or annular plate-like elements, but can be of other shapes and configurations. The inner diameter of the opening  68   b  can be equal to or smaller or greater than the inner diameter of the bore  3  of the piston. One of the members  68  has an annular fuel conduction channel  50  formed in a surface  69  of the member, defining the fuel conduction channel  50  of the injection nozzle structure  48 . Depressions in the surface  69  of the member  68  along one side of the fuel conduction channel  50 , form generally rectangular regions, or micro-slots, which define the nozzles  51 . The manner in which the micro-slots are formed is dependent upon the material of the member  68 , which can be metal, ceramic, or any other suitable material which is somewhat resistant to heat and has a low heat conductivity. The micro-slots can be formed by removing portions of the metal or ceramic material, or by flattening a portion of the surface  69 , when made of metal, such as by peening with a suitable tool. The fuel conduction channel and the micro-slots can be sealed by the second member  68   a  of the nozzle structure. The nozzle structure  48  is positioned between the opposing inner ends of the cylinder housing  2  (FIG. 1) and held together in any suitable manner such as by bonding or otherwise securing the members  68  and  68   a  together, or by the use of tie rods that extend through openings in the members and which can also maintain together the two sections of the cylinder housing. The micro-slots  51  are disposed in fluid communication with the fuel conduction channel  50 . A passageway  70  (FIG. 6) communicates the fuel conduction channel  50  with the outlet of the fuel injection valve  49 . In one embodiment, the nozzle structure  48  includes six nozzles  51  (FIG. 6), spaced apart equidistantly along the inner periphery of the element  68 . The sides  51   a  of the nozzle openings  51  extend generally perpendicular to the fuel conduction channel  50  as shown in FIG. 8. The micro-slots are shaped to be long and narrow in cross section so that the shape of the fuel spray provided by the nozzles  51  is substantially flat. This assures that substantially no solid fuel is injected into the combustion chamber  24  through the nozzles  51 . In one preferred embodiment, the micro-slots which form the nozzles  51  can have a length “L” to width “W” ratio that is greater than 3 to 1, and preferably about 5 to 1 and higher. Moreover, although the micro-slots are generally rectangular in shape, the nozzle openings  51  can be of other configurations, and have other orientations with respect to the fuel conduction channel  50  or combustion chamber  24 .  
         [0085]    Referring to FIG. 9, in accordance with a second preferred embodiment, the nozzle structure  75  comprises three members  78 ,  78   a  and  78   a ′ having an opening corresponding to opening  68   b  of nozzle structure  48  (FIG. 6), the inner diameter of the opening being substantially equal to or smaller or greater than the inner diameter of bore  3 . The members  78 ,  78   a  and  78   a ′ preferably are generally rectangular or annular plate-like members, but can be of other shapes and configurations. Two of the members  78   a  and  78   a ′, (or a single member  78  provided with two channels  50 ) can have an annular channel formed in respective surfaces  79  and  79   a  thereof, defining fuel conduction channels  80  and  80   a  of the injection nozzle structure  75 . The other member  78  is positioned between the members  78   a  and  78   a ′ engaging the surfaces  79  and  79   a . Portions of the surfaces  83  and  84  along one side of the channels  80  and  80   a  are depressed, forming generally openings  81  and  82  which define the nozzles of the nozzle structure  75 . The depressions can be formed in the manner described above for nozzle structure  48 , for example. The openings (nozzles)  81  and  82  are disposed in fluid communication with the channels  80  and  80   a , respectively. In one embodiment, the nozzle structures  75  include six nozzles  81  on surface  83 , spaced apart equidistantly along the inner periphery of the member  78  and six nozzles  82  on surface  84 , spaced apart equidistantly along the inner periphery of the member  78 . The sides  81   a  and  82   a  of the nozzle openings  81  and  82  the axes of which extend at an angle relative to the channels  80  and  80   a . The angle between the channels and the nozzles can be in the range of 30° to 60°, for example. The nozzles  81  and  82  of nozzle structure  75  can have the same length “L” to width “W” ratio(s) as that for nozzle structure  48 .  
         [0086]    In both of these embodiments, the nozzle structures direct combustion fuel generally radially into the combustion chamber  24 . Also, in both embodiments, the fuel conduction channels and the micro-slots are preferably configured as depressions. The depressions are arranged at an axial face of the fuel injection ring, the depressions being covered and sealed by a second section of the injection ring, allowing long and very narrow slots for increased atomization. A comparable function can be obtained by forming a thin shim-like layer on a substrate, incorporating the nozzle arrangement in form of a discontinuities formed in the layer, between the two parts of the fuel injection ring. For example, a very thin layer can be formed on a suitable substrate using known thin film vapor deposition techniques. Alternatively, the fuel conduction channels and/or the microslots forming the nozzles can be formed by chemically etching a substrate.  
         [0087]    The nozzle structures  48  and  75  direct fuel generally radially into a combustion chamber. Referring to FIG. 10, in accordance with a further preferred embodiment of a nozzle structure  85 , the nozzles inject jets of combustion fuel into the combustion chamber  24  in generally axial direction. The fuel injection beam pattern can be generally cylindrical or can be somewhat cone-shaped, depending on the orientation of the nozzles. The nozzle structure  85  is preferably used in applications for injecting combustion fuel into a combustion chamber having a single cylinder, such as for example, the free-piston engine illustrated in FIG. 5. In addition, the nozzle structure  85  can be used in conventional engines in addition to free-piston engines. The nozzle structures  48 ,  75  and  85  can be advantageously used in continuous injection type applications such as in burners for a heating units, jet engines and the like.  
         [0088]    The nozzle structure  85  includes a generally annular or cylindrical member  88  having an annular channel formed in an outer surface  89  thereof, defining the fuel connection channel  90  of the injection nozzle structure  85 . Portions of the surface  89  along one side of the channel  90  are depressed, forming generally rectangular regions which define the nozzles  91 . The rectangular regions are disposed in fluid communication with the channel  90 . In one embodiment, the nozzle structure  85  includes six nozzles  91 , spaced apart equidistantly along the periphery of the element  88 . The sides  91   a  of the nozzle openings  91  extend generally perpendicular to the channel  90 . However, the nozzle openings can be of other configurations and have other orientations with respect to the channel  90 . The nozzle structure  85  can be mounted on or in the end wall of the piston chamber.  
         [0089]    Alternatively, with suitable modifications, the nozzle structure  85  can be formed on the combustion face of the piston. The nozzles  91  of nozzle structure  85  can have the same length “L” to width “W” ratio(s) as that for nozzle structure  48 . The nozzle structures  48 ,  75  and  85 , including the micro-slot nozzles, can be used in the atomization of fluids which are supplied continuously to a utilization device such as a burner for a heater, to a jet engine combustion, and the like, or supplied intermittently to a utilization device, such as a fuel injection device in an internal combustion engine. The fuel pump  47  can be used without the novel nozzle structure  48 . Also, the nozzle structures  48 ,  75  and  85  can be used with a conventional pump. The fuel pump  47  can function as a pressure intensifier or a pressure multiplier. Typical applications include all compressing or pumping applications.  
         [0090]    Referring to FIG. 11, a simplified representation of a free-piston internal combustion engine  200  is shown in which the hydraulic end  205  of the piston  204  provides the function of a “driver” or piston movement coordinator. The hydraulic end extracts only energy to maintain its own power needs to cycle the combustion piston. The energy gained during the combustion process is extracted through a “power take-off” function  210 , e.g., driving a piston  220  (such as a piston of a pneumatic compressor or a plunger of a water pump, etc.). Alternatively, the power take-off function can drive an electric spool (of an electric generator). In addition to low cost and emissions, and high efficiency, the no wear and no maintenance characteristics of the free-piston engine is very advantageous for such applications.  
         [0091]    While preferred embodiments have been illustrated and described, it should be understood that changes and modifications can be made thereto without departing from the invention in its broadest aspects. Various features of the invention are defined in the following claims.