Method of operating a free piston internal combustion engine with a short bore/stroke ratio

Information

  • Patent Grant
  • 6314924
  • Patent Number
    6,314,924
  • Date Filed
    Monday, February 22, 1999
    25 years ago
  • Date Issued
    Tuesday, November 13, 2001
    23 years ago
Abstract
A method of operating a free piston internal combustion engine includes the steps of: providing a housing with a combustion cylinder and a second cylinder, the combustion cylinder having a bore with an inside diameter; providing a piston including a piston head reciprocally disposed within the combustion cylinder, a second head reciprocally disposed within the second cylinder, and a plunger rod interconnecting the piston head with the second head; and moving the piston between a top dead center position and a bottom dead center position during a return stroke, the return stroke having a stroke length between the top dead center position and the bottom dead center position, the moving step being carried out with a bore/stroke ratio represented by a quotient of the inside diameter divided by the stroke length which is between 1.2 and 1.5. An air scavenging port is in fluid communication with the bore during between 50 and 70 percent of a cycle time period.
Description




TECHNICAL FIELD




The present invention relates to free piston internal combustion engines, and, more particularly, to a method of operating a free piston internal combustion engine with a hydraulic power output.




BACKGROUND ART




Internal combustion engines typically include a plurality of pistons which are disposed within a plurality of corresponding combustion cylinders. Each of the pistons is pivotally connected to one end of a piston rod, which in turn is pivotally connected at the other end thereof with a common crankshaft. The relative axial displacement of each piston between a top dead center (TDC) position and a bottom dead center (BDC) position is determined by the angular orientation of the crank arm on the crankshaft with which each piston is connected.




A free piston internal combustion engine likewise includes a plurality of pistons which are reciprocally disposed in a plurality of corresponding combustion cylinders. However, the pistons are not interconnected with each other through the use of a crankshaft. Rather, each piston is typically rigidly connected with a plunger rod which is used to provide some type of work output. In a free piston engine with a hydraulic output, the plunger is used to pump hydraulic fluid which can be used for a particular application. Typically, the housing which defines the combustion cylinder also defines a hydraulic cylinder in which the plunger is disposed and an intermediate compression cylinder between the combustion cylinder and the hydraulic cylinder. The combustion cylinder has the largest inside diameter; the compression cylinder has an inside diameter which is smaller than the combustion cylinder; and the hydraulic cylinder has an inside diameter which is still yet smaller than the compression cylinder. A compression head which is attached to and carried by the plunger at a location between the piston head and plunger head has an outside diameter which is just slightly smaller than the inside diameter of the compression cylinder. A high pressure hydraulic accumulator which is fluidly connected with the hydraulic cylinder is pressurized through the reciprocating movement of the plunger during operation of the free piston engine. An additional hydraulic accumulator is selectively interconnected with the area in the compression cylinder to exert a relatively high axial pressure against the compression head and thereby move the piston head toward the top dead center position.




In a free piston engine as described above, the piston includes a piston head, a compression head and a plunger head which are commonly carried by a plunger rod and respectively disposed in the combustion cylinder, compression cylinder and hydraulic cylinder. The piston including the three separate heads is quite long, which increases the overall package size of the free piston engine. Moreover, as a result of the relatively large size of the piston, the mass of the piston is relatively heavy. The energy which is required for combustion of fuel within the combustion cylinder is related to the required kinetic energy of the piston when the piston is at a TDC position. The kinetic energy is a function of the mass and square of the velocity of the piston. Since the piston is relatively heavy, the piston is accelerated to a velocity which is relatively low in order to provide the kinetic energy needed for combustion. Moreover, since the piston is relatively heavy and the hydraulic fluid used to move the piston toward the TDC position is at a limited pressure, the acceleration of the piston is relatively slow and thus the stroke length is relatively long in order for the piston to reach the desired velocity. The slow acceleration, velocity and frequency of a conventional free piston engine results in a relatively low power output.




The present invention is directed to overcoming one or more of the problems as set forth above.




DISCLOSURE OF THE INVENTION




In one aspect of the invention, a method of operating a free piston internal combustion engine includes the steps of: providing a housing with a combustion cylinder and a second cylinder, the combustion cylinder having a bore with an inside diameter; providing a piston including a piston head reciprocally disposed within the combustion cylinder, a second head reciprocally disposed within the second cylinder, and a plunger rod interconnecting the piston head with the second head; and moving the piston between a top dead center position and a bottom dead center position during a return stroke, the return stroke having a stroke length between the top dead center position and the bottom dead center position, the moving step being carried out with a bore/stroke ratio represented by a quotient of the inside diameter divided by the stroke length which is between 1.2 and 1.5.




In another aspect of the invention, a method of operating a free piston internal combustion engine includes the steps of: providing a housing with a combustion cylinder and a second cylinder, the combustion cylinder having a bore and an air scavenging port in communication with the bore; providing a piston including a piston head reciprocally disposed within the combustion cylinder, a second head reciprocally disposed within the second cylinder, and a plunger rod interconnecting the piston head with the second head; and moving the piston from a bottom dead center position to a top dead center position and back to the bottom dead center position during a cycle time period, the piston head opening and closing the air scavenging port during the movement of the piston, the air scavenging port being in fluid communication with the bore during between 50 and 70% of the cycle time period.











BRIEF DESCRIPTION OF THE DRAWINGS





FIG. 1

is a schematic illustration of an embodiment of a free piston engine with which an embodiment of a method of the present invention may be used;





FIG. 2

is a schematic illustration of another embodiment of a free piston engine with which another embodiment of a method of the present invention may be used;





FIG. 3

is a schematic illustration of yet another embodiment of a free piston engine with which another embodiment of a method of the present invention may be used;





FIG. 4

is a graphic illustration of a comparison between the piston motion of a free piston engine operated in accordance with the present invention and a crankshaft engine operating at a same frequency; and





FIG. 5

is a graphic illustration of the air scavenging time for a free piston engine operated in accordance with the present invention and a crankshaft engine, when the free piston engine is operating at a higher frequency than the crankshaft engine.











BEST MODE FOR CARRYING OUT THE INVENTION




Referring now to the drawings, and more particularly to

FIG. 1

, there is shown an embodiment of a free piston internal combustion engine


10


which may be used with an embodiment of the method of the present invention, and which generally includes a housing


12


, piston


14


, and hydraulic circuit


16


.




Housing


12


includes a combustion cylinder


18


and a hydraulic cylinder


20


. Housing


12


also includes a combustion air inlet


22


, air scavenging channel


24


and exhaust outlet


26


which are disposed in communication with a combustion chamber


28


within combustion cylinder


18


. Combustion air is transported through combustion air inlet


22


and air scavenging channel


24


into combustion chamber


28


when piston


14


is at or near a BDC position. An appropriate fuel, such as a selected grade of diesel fuel, is injected into combustion chamber


28


as piston


14


moves toward a TDC position using a controllable fuel injector system, shown schematically and referenced as


30


. The physical location of piston


14


at a BDC position and a TDC position, and thus the stroke length S between the BDC position and TDC position, may be fixed or variable from one stroke to another since piston


14


is not attached to or carried by a crankshaft.




Piston


14


is reciprocally disposed within combustion cylinder


18


and is moveable during a compression stroke toward a TDC position and during a return stroke toward a BDC position. Piston


14


generally includes a piston head


32


which is attached to a plunger rod


34


. Piston head


32


is formed from a metallic material in the embodiment shown, such as aluminum or steel, but may be formed from another material having suitable physical properties such as coefficient of friction, coefficient of thermal expansion and temperature resistance. For example, piston head


32


may be formed from a non-metallic material such as a composite or ceramic material. More particularly, piston head


32


may be formed from a carbon-carbon composite material with carbon reinforcing fibers which are randomly oriented or oriented in one or more directions within a carbon and resin matrix.




Piston head


32


includes two annular piston ring groves


36


in which are disposed a pair of corresponding piston rings (not numbered) to prevent blow-by of combustion products on the return stroke of piston


14


during operation. Any number of piston ring grooves


36


and piston rings may be used without changing the essence of the invention. If piston head


32


is formed from a suitable non-metallic material having a relatively low coefficient of thermal expansion, it is possible that the radial operating clearance between piston head


32


and the inside surface of combustion cylinder


18


may be reduced such that piston ring grooves


36


and the associated piston rings may not be required. Piston head


32


also includes an elongated skirt


38


which lies adjacent to and covers exhaust outlet


26


when piston


14


is at or near a TDC position, thereby preventing combustion air which enters through combustion air inlet


22


from exiting exhaust outlet


26


.




Plunger rod


34


is substantially rigidly attached to piston head


32


at one end thereof using a mounting hub


40


and a bolt


42


. Bolt


42


extends through a hole (not numbered) in mounting hub


40


and is threadingly engaged with a corresponding hole formed in the end of plunger rod


34


. Mounting hub


40


is then attached to the side of piston head


32


opposite combustion chamber


28


in a suitable manner, such as by using bolts, welding, and/or adhesive, etc. A bearing/seal


44


surrounding plunger rod


34


and carried by housing


12


separates combustion cylinder


18


from hydraulic cylinder


20


.




Plunger head


46


is substantially rigidly attached to an end of plunger rod


34


opposite from piston head


32


. Reciprocating movement of piston head


32


between a BDC position and a TDC position, and vice versa, causes corresponding reciprocating motion of plunger rod


34


and plunger head


46


within hydraulic cylinder


20


. Plunger head


46


includes a plurality of sequentially adjacent lands and valleys


48


which effectively seal with and reduce friction between plunger head


46


and an inside surface of hydraulic cylinder


20


.




Plunger head


46


and hydraulic cylinder


20


define a variable volume pressure chamber


50


on a side of plunger head


46


generally opposite from plunger rod


34


. The volume of pressure chamber


50


varies depending upon the longitudinal position of plunger head


46


within hydraulic cylinder


20


. A fluid port


52


and a fluid port


54


are fluidly connected with variable volume pressure chamber


50


. An annular space


56


surrounding plunger rod


34


is disposed in fluid communication with a fluid port


58


in housing


12


. Fluid is drawn through fluid port


58


into annular space


56


upon movement of plunger rod


34


and plunger head


46


toward a BDC position so that a negative pressure is not created on the side of plunger head


46


opposite variable volume pressure chamber


50


. The effective cross-sectional area of pressurized fluid acting on plunger head


46


within variable volume pressure chamber


50


compared with the effective cross-sectional area of pressurized fluid acting on plunger head


46


within annular space


56


, is a ratio of between approximately 5:1 to 30:1. In the embodiment shown, the ratio between effective cross-sectional areas acting on opposite sides of plunger head


46


is approximately 20:1. This ratio has been found suitable to prevent the development of a negative pressure within annular space


56


upon movement of plunger head


46


toward a BDC position, while at the same time not substantially adversely affecting the efficiency of free piston engine


10


while plunger head


46


is traveling toward a TDC position.




Hydraulic circuit


16


is connected with hydraulic cylinder


20


and provides a source of pressurized fluid, such as hydraulic fluid, to a load for a specific application, such as a hydrostatic drive unit (not shown). Hydraulic circuit


16


generally includes a high pressure hydraulic accumulator H, a low pressure hydraulic accumulator L, and suitable valving, etc. used to connect high pressure hydraulic accumulator H and low pressure hydraulic accumulator L with hydraulic cylinder


20


at selected points in time as will be described in greater detail hereinafter.




More particularly, hydraulic circuit


16


receives hydraulic fluid from a source


60


to initially charge high pressure hydraulic accumulator H to a desired pressure. A starter motor


62


drives a fluid pump


64


to pressurize the hydraulic fluid in high pressure hydraulic accumulator H. The hydraulic fluid transported by pump


64


flows through a check valve


66


on an input side of pump


64


, and a check valve


68


and filter


70


on an output side of pump


64


. The pressure developed by pump


64


also pressurizes annular space


56


via the interconnection with line


71


and fluid port


58


. A pressure relief valve


72


ensures that the pressure within high pressure hydraulic accumulator H does not exceed a threshold limit.




The high pressure hydraulic fluid which is stored within high pressure hydraulic accumulator H is supplied to a load suitable for a specific application, such as a hydrostatic drive unit. The high pressure within high pressure hydraulic accumulator H is initially developed using pump


64


, and is thereafter developed and maintained using the pumping action of free piston engine


10


.




A proportional valve


74


has an input disposed in communication with high pressure hydraulic accumulator H, and provides the dual functionality of charging low pressure hydraulic accumulator L and providing a source of fluid power for driving ancillary mechanical equipment on free piston engine


10


. More particularly, proportional valve


74


provides a variably controlled flow rate of high pressure hydraulic fluid from high pressure hydraulic accumulator H to a hydraulic motor HDM. Hydraulic motor HDM has a rotating mechanical output shaft which drives ancillary equipment on free piston engine


10


using a belt and pulley arrangement, such as a cooling fan, alternator and water pump. Of course, the ancillary equipment driven by hydraulic motor HDM may vary from one application to another.




Hydraulic motor HDM also drives a low pressure pump LPP which is used to charge low pressure hydraulic accumulator L to a desired pressure. Low pressure pump LPP has a fluid output which is connected in parallel with each of a heat exchanger


76


and a check valve


78


. If the flow rate through heat exchanger


76


is not sufficient to provide an adequate flow for a required demand, the pressure differential on opposite sides of check valve


78


causes check valve


78


to open, thereby allowing hydraulic fluid to bypass heat exchanger


76


temporarily. If the pressure developed by low pressure pump LPP which is present in line


80


exceeds a threshold value, check valve


81


opens to allow hydraulic fluid to bleed back to the input side of hydraulic motor HDM. A pressure relief valve


82


prevents the hydraulic fluid within line


80


from exceeding a threshold value.




Low pressure hydraulic accumulator L selectively provides a relatively lower pressure hydraulic fluid to pressure chamber


50


within hydraulic cylinder


20


using a low pressure check valve LPC and a low pressure shutoff valve LPS. Conversely, high pressure hydraulic accumulator H provides a higher pressure hydraulic fluid to pressure chamber


50


within hydraulic cylinder


20


using a high pressure check valve HPC and a high pressure pilot valve HPP.




During an initial start-up phase of free piston engine


10


, starter motor


62


is energized to drive pump


64


and thereby pressurize high pressure hydraulic accumulator H to a desired pressure. Since piston


14


may not be at a position which is near enough to the BDC position to allow effective compression during a compression stroke, it may be necessary to effect a manual return procedure of piston


14


to a BDC position. To wit, low pressure shutoff valve LPS is opened using a suitable controller to minimize the pressure on the side of hydraulic plunger


46


which is adjacent to pressure chamber


50


. Since annular space


56


is in communication with high pressure hydraulic accumulator H, the pressure differential on opposite sides of hydraulic plunger


46


causes piston


14


to move toward the BDC position, as shown in FIG.


1


.




When piston


14


is at a position providing an effective compression ratio within combustion chamber


28


, high pressure pilot valve HPP is actuated using a controller to manually open high pressure check valve HPC, thereby providing a pulse of high pressure hydraulic fluid from high pressure hydraulic accumulator into pressure chamber


50


. Low pressure check valve LPC and low pressure shutoff valve LPS are both closed when the pulse of high pressure hydraulic fluid is provided to pressure chamber


50


. The high pressure pulse of hydraulic fluid causes plunger head


46


and piston head


32


to move toward the TDC position. Because of the relatively large ratio difference in cross-sectional areas on opposite sides of plunger head


46


, the high pressure hydraulic fluid which is present within annual space


56


does not adversely interfere with the travel of plunger head


46


and piston head


32


toward the TDC position. The pulse of high pressure hydraulic fluid is applied to pressure chamber


50


for a period of time which is sufficient to cause piston


14


to travel with a kinetic energy which will effect combustion within combustion chamber


28


. The pulse may be based upon a time duration or a sensed position of piston head


32


within combustion cylinder


18


.




As plunger head


46


travels toward the TDC position, the volume of pressure chamber


50


increases. The increased volume in turn results in a decrease in the pressure within pressure chamber


50


which causes high pressure check valve HPC to close and low pressure check valve LPC to open. The relatively lower pressure hydraulic fluid which is in low pressure hydraulic accumulator L thus fills the volume within pressure chamber


50


as plunger head


46


travels toward the TDC position. By using only a pulse of pressure from high pressure hydraulic accumulator H during a beginning portion of the compression stroke (e.g., during 60% of the stroke length), followed by a fill of pressure chamber


50


with a lower pressure hydraulic fluid from low pressure hydraulic accumulator L, a net resultant gain in pressure within high pressure hydraulic accumulator H is achieved.




By properly loading combustion air and fuel into combustion chamber


28


through air scavenging channel


24


and fuel injector


30


, respectively, proper combustion occurs within combustion chamber


28


at or near a TDC position. As piston


14


travels toward a BDC position after combustion, the volume decreases and pressure increases within pressure


50


. The increasing pressure causes low pressure check valve LPC to close and high pressure check valve HPC to open. The high pressure hydraulic fluid which is forced through high pressure check valve during the return stroke is in communication with high pressure hydraulic accumulator H, resulting in a net positive gain in pressure within high pressure hydraulic accumulator H.





FIG. 2

illustrates another embodiment of a free piston internal combustion engine


90


which may be used with an embodiment of the method of the present invention, and which includes a combustion cylinder and piston arrangement which is substantially the same as the embodiment shown in FIG.


1


. Hydraulic circuit


92


of free piston engine


90


also includes many hydraulic components which are the same as the embodiment of hydraulic circuit


16


shown in FIG.


1


. Hydraulic circuit


92


principally differs from hydraulic circuit


16


in that hydraulic circuit


92


includes a mini-servo valve


94


with a mini-servo main spool MSS and a mini-servo pilot MSP. Mini-servo main spool MSS is controllably actuated at selected points in time during operation of free piston engine


90


to effect the high pressure pulse of high pressure hydraulic fluid from high pressure hydraulic accumulator H, similar to the manner described above with regard to the embodiment shown in FIG.


1


. Mini-servo pilot MSP is controllably actuated to provide the pressure necessary for controllably actuating mini-servo main spool MSS. The pulse of high pressure hydraulic fluid is provided to pressure chamber


50


for a duration which is either dependent upon time or a sensed position of piston


14


. As the volume within pressure chamber


50


increases, the pressure correspondingly decreases, resulting in an opening of low pressure check valve LPC. Low pressure hydraulic fluid from low pressure hydraulic accumulator L thus flows into pressure chamber


50


during the compression stroke of piston


14


. After combustion and during the return stroke of piston


14


, the pressure within pressure chamber


50


increases, thereby causing low pressure check valve LPC to close and high pressure check valve HPC to open. The high pressure hydraulic fluid created within pressure chamber


50


during the return stroke of piston


14


is pumped through high pressure check valve HPC and into high pressure hydraulic accumulator H, thereby resulting in a net positive gain in the pressure within high pressure hydraulic accumulator H.




Referring now to

FIG. 3

there is shown yet another embodiment of a free piston engine


100


with which the method of the present invention may be used. Again, the arrangement of combustion cylinder


18


and piston


14


is substantially the same as the embodiment of free piston engines


10


and


90


shown in

FIGS. 1 and 2

. Hydraulic circuit


102


also likewise includes many hydraulic components which are the same as the embodiments of hydraulic circuits


16


and


92


shown in

FIGS. 1 and 2

. However, hydraulic circuit


102


includes two pilot operated check valves


104


and


106


. Pilot operated check valve


104


includes a high pressure check valve HPC and a high pressure pilot valve HPP which operate in a manner similar to high pressure check valve HPC and high pressure pilot valve HPP described above with reference to the embodiment shown in FIG.


1


. Pilot operated check valve


106


includes a low pressure check valve LPC and a low pressure pilot valve LPP which also work in a manner similar to high pressure check valve


104


. The input side of low pressure pilot valve LPP is connected with the high pressure fluid within high pressure hydraulic accumulator H through line


108


. Low pressure pilot valve LPP may be controllably actuated using a controller to provide a pulse of pressurized fluid to low pressure check valve LPC which is sufficient to open low pressure check valve LPC.




During use, a pulse of high pressure hydraulic fluid may be provided to pressure chamber


50


using pilot operated check valve


104


to cause piston


14


to travel toward a TDC position with enough kinetic energy to effect combustion. High pressure pilot valve HPP is deactuated, dependent upon a period of time or a sensed position of piston


14


, to thereby allow high pressure check valve HPC to close. As plunger head


46


moves toward the TDC position, the pressure within pressure chamber


50


decreases and low pressure check valve LPC is opened. Low pressure hydraulic fluid thus fills the volume within pressure chamber


50


while the volume within pressure chamber


50


expands. After combustion, piston


14


moves toward a BDC position which causes the pressure within pressure chamber


50


to increase. The increase causes low pressure check valve LPC to close and high pressure check valve to open. The high pressure hydraulic fluid which is generated by the pumping action of plunger head


46


within hydraulic cylinder


20


flows into high pressure hydraulic accumulator H, resulting in a net positive gain in the pressure within high pressure hydraulic accumulator H. A sensor (schematically illustrated and positioned at S) detects piston


14


near a BDC position. The high pressure pulse to effect the compression stroke can be timed dependent upon the sensor activation signal.




To effect a manual return procedure using the embodiment of free piston engine


100


shown in

FIG. 3

, high pressure hydraulic fluid is provided into annular space


56


from high pressure hydraulic accumulator H. Low pressure pilot valve LPP is controllably actuated to cause low pressure check valve LPC to open. The pressure differential on opposite sides of plunger head


46


causes piston


14


to move toward a BDC position. When piston


14


is at a position providing an effective compression ratio to effect combustion within combustion chamber


28


, a high pressure pulse of hydraulic fluid is transported into pressure chamber


50


using pilot operated check valve


104


to begin the compression stroke of piston


14


.




Referring now to

FIG. 4

, an embodiment of the method of the present invention for operating a free piston engine will be described in greater detail. In the embodiment shown in

FIG. 4

, the method is assumed to be carried out using free piston engine


10


shown in FIG.


1


. However, it will be appreciated that the embodiment of the method shown in

FIG. 4

is equally applicable to other embodiments of a free piston engine, such as free piston engines


90


and


100


shown in

FIGS. 2 and 3

.





FIG. 4

illustrates the motion of a piston (trace


120


) in free piston engine


10


used to carry out an embodiment of the method of the present invention when compared with the motion of a piston (trace


122


) in a conventional crankshaft engine. For each of traces


120


and


122


, the piston is assumed to have an identical stroke length between a BDC position and a TDC position, and operates at a same frequency. The frequency corresponds to a cycle time period CT during is which the piston moves from a BDC position to a TDC position, and back to a BDC position. Additionally, for comparison purposes, the cylinder of free piston engine


10


and the conventional crankshaft engine is assumed to have an air scavenging port positioned at approximately the same distance from the TDC position. Above the horizontal line referenced PO the air scavenging port closes, and below line PO the air scavenging port opens. The distance L


1


between the TDC position and the edge of the air scavenging port (at which point in time air scavenging begins) is approximately between 60 and 80% of stroke length S, and preferably is about 70% of stroke length S. Accordingly, the distance L


2


representing the distance between the BDC position and the edge of the air scavenging port closest to the TDC position is preferably approximately 30% of stroke length S.




For the piston motion of a conventional crankshaft engine represented by trace


122


, the air scavenging port closes at point


124


during a compression stroke and opens at point


126


during a return stroke. Thus, the total air scavenging time for a conventional crankshaft engine is represented by the sum of the times A+B. Similarly, for free piston engine


10


, air scavenging port


24


closes at point


128


during a compression stroke and opens at point


130


during a return stroke. The total air scavenging time for free piston engine


10


is thus represented by the sum of times C+D. As is apparent, the value of the air scavenging time C+D for free piston engine


10


is considerably larger than the value of the air scavenging time A+B for a conventional crankshaft engine. This is primarily because the slope of trace


120


for free piston engine


10


is considerably steeper than the slope of trace


122


for a conventional crankshaft engine. With a conventional crankshaft engine, a plurality of pistons are ganged together on a common crankshaft which rotates at a particular rotational speed. The movement of each piston is limited by the rotational speed of the crankshaft. On the other hand, piston


14


of free piston engine


10


is not connected with a crankshaft and therefore is not limited by the rotational speed of a crankshaft. The slope of trace


120


for free piston engine


10


is therefore considerably steeper than the slope of trace


122


for a conventional crankshaft engine.




The air scavenging time C+D of free piston engine


10


is controlled to be between 50 and 70% of cycle time period CT. Preferably, the air scavenging time C+D is between 55 and 60% of cycle time period CT, and more preferably is approximately 60% of cycle time period CT. Since piston


14


is not connected with or constrained by the rotational speed of a crankshaft, the air scavenging time C+D can be easily regulated. A supply of high pressure fluid can be pulsed into hydraulic chamber


50


to move piston


14


from the BDC position to the TDC position during a compression stroke. Firing occurs at or near the TDC position and piston


14


is moved back to the BDC position during a return stroke. If the time period D allows sufficient air scavenging, free piston engine


10


can be pulsed at or near the BDC position to start a new cycle time period CT. On the other hand, the air scavenging time D during a return stroke can be easily increased by simply holding free piston engine


10


an additional period of time before pulsing free piston engine


10


at or near the BDC position.




Free piston engine


10


is operated with a bore/stroke ratio which is higher than conventional crankshaft engines and conventional free piston engines. The bore/stroke ratio is represented by the quotient of the inside bore diameter D


B


of combustion chamber


18


divided by stoke length S of piston


14


between a TDC position and a BDC position. With a conventional crankshaft engine, the bore/stroke ratio typically does not exceed 1 since it is believed that adequate air scavenging will not occur if stroke length S has a shorter length relative to the bore diameter D


B


. Moreover, conventional free piston engines include a housing with a combustion chamber, a compression chamber and a hydraulic chamber. The piston likewise includes a piston head, a compression head and a plunger head which are respectively disposed in the combustion chamber, compression chamber and the hydraulic chamber. The amount of fluid energy which is pulsed into the compression chamber of a conventional free piston engine is directly related to the kinetic energy of the piston needed for combustion. The kinetic energy of the piston is a function of the mass and velocity of the piston. Since the mass of a piston in a conventional free piston engine is much heavier as a result of the additional compression head, the velocity of the piston is correspondingly much lower. This means that the frequency and cycle time period CT are much slower and the stroke length is longer for a conventional free piston engine when compared with free piston engine


10


shown in FIG.


1


. Thus, the bore/stroke ratio is higher for conventional free piston engines.





FIG. 5

illustrates the air scavenging efficiency (trace


132


) of free piston engine


10


when compared with the air scavenging efficiency (trace


134


) of a conventional crankshaft engine. Piston


14


of free piston engine


10


is not constrained by the rotational speed of a crankshaft. Accordingly, free piston engine


10


is operated at a frequency corresponding to cycle time period CT


1


which is much higher than a frequency corresponding to cycle time period CT


2


of a conventional crankshaft engine. To operate at a higher frequency corresponding to cycle time period CT


1


, the stroke length S


1


of free piston engine


10


is shortened relative to a stroke length S


2


of the conventional crankshaft engine. Since the stroke length S


1


is shorter than the stroke length S


2


, the leading edge of air scavenging port


24


is moved closer to the BDC position so that the air scavenging port is still in communication with combustion chamber 28 approximately 30% of stroke length S


1


. The air scavenging time of free piston engine


10


operating at a higher frequency is thus represented by the area under the horizontal line P


01


. Similarly, the air scavenging time of the conventional crankshaft engine is represented by the area under the horizontal line P


01


. As may be easily observed from the graphical illustration of

FIG. 5

, the air scavenging time represented by the area under line P


01


for free piston engine


10


is similar to the air scavenging time represented by the area under line P


02


for a conventional crankshaft engine. Thus, free piston engine


10


may be operated at a substantially higher frequency without substantially effecting the air scavenging efficiency of the engine. Operating free piston engine


10


at a higher frequency means that more output energy can be provided over a given period of time, which in turn means that free piston engine


10


can provide a higher power output than a conventional crankshaft engine.




Free piston engine


10


also may be operated at a frequency which is higher than conventional free piston engines since free piston engine


10


includes a piston


14


with only two heads, rather than three. The mass of piston


14


is considerably lighter than a piston in a conventional free piston engine, which in turn means that the frequency can be much higher and the cycle time period CT can be much shorter. The relationship of the air scavenging efficiency of free piston engine


10


shown in

FIG. 5

also holds true when compared with the air scavenging efficiency of a conventional free piston engine since a conventional free piston engine operates at a slower frequency and longer stroke length.




Industrial Applicability




During use, piston


14


is reciprocally disposed within combustion cylinder


16


. Piston


14


travels between a BDC position and a TDC position during a compression stroke and between a TDC position and a BDC position during a return stroke. Combustion air is introduced into combustion chamber


28


through combustion air inlet


22


and air scavenging channel


24


. Fuel is controllably injected into combustion chamber


28


using a fuel injector


30


. High pressure hydraulic fluid from high pressure hydraulic accumulator H is coupled with pressure chamber


50


during a return stroke of piston


14


. A duration of time during which the high pressure hydraulic fluid is coupled with the pressure chamber is dependent upon the activation of a sensor S which senses piston


14


at or near a BDC position. If free piston engine


10


misfires and sensor S is not activated, then the high pressure hydraulic fluid is maintained in a coupled relationship with pressure chamber


50


to cause piston


14


to bounce back toward the TDC position, thereby increasing the energy within the non-combusted fuel and air mixture within combustion chamber


28


during a next compression stroke and likely causing combustion of the fuel and air mixture. If the misfire occurs for several cycles of the free piston engine corresponding to a preset total amount of time, a manual return procedure is initiated to retract piston


14


to a position allowing firing of the free piston engine.




Free piston engine


10


has a stroke length S which is shorter than conventional free piston engines and conventional crankshaft engines. Additionally, free piston engine


10


operates at a frequency which is substantially higher than conventional free piston engines or crankshaft engines, while at the same time maintaining a similar air scavenging efficiency. Thus, free piston engine


10


may be provided with a higher power density without degrading the air scavenging efficiency thereof.




Other aspects, objects and advantages of this invention can be obtained from a study of the drawings, the disclosure and the appended claims.



Claims
  • 1. A method of operating a free piston internal combustion engine, comprising the steps of:providing a housing including a combustion cylinder and a second cylinder, said combustion cylinder having a bore with an inside diameter; providing a piston including a piston head reciprocally disposed within said combustion cylinder, a second head reciprocally disposed within said second cylinder, and a plunger rod interconnecting said piston head with said second head; and moving said piston between a top dead center position and a bottom dead center position during a return stroke, said return stroke having a stroke length between said top dead center position and said bottom dead center position, said moving step being carried out with a bore/stroke ratio represented by a quotient of said inside diameter divided by said stroke length which is between 1.2 and 1.5.
  • 2. The method of claim 1, wherein said moving step is carried out with a bore/stroke ratio of between 1.3 and 1.5.
  • 3. The method of claim 2, wherein said moving step is carried out with a bore/stroke ratio of approximately 1.5.
  • 4. The method of claim 1, wherein said second cylinder comprises a hydraulic cylinder and said second head comprises a plunger head.
  • 5. The method of claim 1, wherein said combustion cylinder includes an air scavenging port, said air scavenging port being in communication with said bore during approximately 30 percent of said stroke length which is closest to said bottom dead center position.
  • 6. The method of claim 5, wherein said air scavenging port is in communication with said bore a period of time which is sufficient to allow adequate scavenging of combustion air into said bore.
  • 7. A method of operating a free piston internal combustion engine, comprising the steps of:providing a housing including a combustion cylinder and a second cylinder, said combustion cylinder having a bore and an air scavenging port in communication with said bore; providing a piston including a piston head reciprocally disposed within said combustion cylinder, a second head reciprocally disposed within said second cylinder, and a plunger rod interconnecting said piston head with said second head; and moving said piston from a bottom dead center position to a top dead center position and back to said bottom dead center position during a cycle time period, said piston head opening and closing said air scavenging port during said movement of said piston, said air scavenging port being in fluid communication with said bore during between 50 and 70 percent of said cycle time period.
  • 8. The method of claim 7, wherein said air scavenging port is in fluid communication with said bore during between 55 and 60 percent of said cycle time period.
  • 9. The method of claim 8, wherein said air scavenging port is in fluid communication with said bore approximately 60 percent of said cycle time period.
  • 10. The method of claim 7, wherein said bore has an inside diameter, and said movement of said piston between said top dead center position and said bottom dead center position is during a return stroke, said return stroke having a stroke length between said top dead center position and said bottom dead center position, said moving step being carried out with a bore/stroke ratio represented by a quotient of said inside diameter divided by said stroke length which is between 1.2 and 1.5.
  • 11. The method of claim 10, wherein said moving step is carried out with a bore/stroke ratio of between 1.3 and 1.5.
  • 12. The method of claim 11, wherein said moving step is carried out with a bore/stroke ratio of approximately 1.5.
  • 13. The method of claim 7, wherein said second cylinder comprises a hydraulic cylinder and said second head comprises a plunger head.
US Referenced Citations (9)
Number Name Date Kind
4016719 Yavnai Apr 1977
4020804 Bailey May 1977
4435133 Meukendyk Mar 1984
4589380 Coad May 1986
4620836 Brandl Nov 1986
4724800 Wood Feb 1988
5540194 Adams Jul 1996
5556262 Achten et al. Sep 1996
5775273 Beale Jul 1998
Foreign Referenced Citations (1)
Number Date Country
WO 9846870 Oct 1998 WO
Non-Patent Literature Citations (2)
Entry
TU Dresden—publication date unknown—earliest date 1993—Dresden University in Germany.
Application No. 08/974,326, filed Nov. 19, 1997, entitled “Two Cycle Engine Having a Mon-Valve Integrated Withy a Fuel Injector”.