Patent Publication Number: US-6910454-B2

Title: Method of and device for operating a multi-cylinder combustion engine with variable compression ratio

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a method for operating a multi-cylinder combustion engine with variable compression ratio. Moreover, it also relates to a control device which controls at least the compression of a multi-cylinder combustion engine. 
   A method and a control device of this type are disclosed in the German patent document DE 100 51 271. The reference discloses a combustion engine with a crankshaft which is not directly supported in a motor block. Instead, the crankshaft is supported in eccentric rings, which in turn are supported rotatably in supporting bearings in the motor block. By means of an adjusting mechanics, the eccentric rings can be turned in a controlled way. During turning of the eccentric rings, the position of the crankshaft relative to the motor block changes. While the cylinders of the combustion engine are connected with the motor block, the pistons of the combustion engine which are movably guided in the cylinders are connected with the crankshaft through connecting rods of constant lengths. A change of the position of the crankshaft relative to the motor block therefore results in a change of the position of the pistons in the cylinders of the combustion engine. In particular, also the position of the upper dead point of the pistons in the cylinders changes. As a result, also the compression volumes enclosed by the pistons in their upper dead point position change. Since the lower dead point position of the piston changes in the same way as the upper dead point position, the stroke volume VH of the combustion chamber does not change with a change of the crankshaft position relative to the motor block. The change of the compression volume VC with constant stroke volumes VH implies a change of the compression ratio ε=(VH+VC)/VC. 
   Alternatively to this adjusting mechanics, in which the distance between the crankshaft position and the cylinder is controllably changed, also systems are known in which the compression ratio is changed by a tilting of the motor block relative to the crankshaft position, or by a tilting of the cylinder head relative to the motor block, or by lifting or lowering of the cylinder head relative to the motor block in a controlled manner. All these methods have in common that the geometrical compression ratio ε=(VC+VH)/VC can be changed by a controlled change of the compression volume VC. 
   In contrast to conventional combustion engines, having a compression ratio ε which is determined by the geometry of the combustion chamber and is fixed, the thermodynamic efficiency of the combustion engine in partial load region can be increased with a variable compression. As a result consumer advantages are provided. This is connected with a reduction of the CO 2  emissions. The higher the compression ratio, the higher the compression end temperature. Since with increasing compression and temperature there is a danger that the combustion chamber filling in an Otto engine ignites itself (knocking), the maximum possible combustion ratio is limited by the tendency to knocking of the fuel. A reliable direct start and a faster motor high running is possible in start-stop operation. 
   In conventional combustion engines with fixed compression ratio the maximum compression ratio is structurally determined so that with the maximal combustion chamber filling (full load) no knocking occurs. It follows from this that with structurally fixedly predetermined combustion ratio and combustion chamber fillings under maximum possible value (partial load) critical compression and temperatures substantially are not reached. The efficiency of the combustion remains then behind an optimal efficiency. With the variable combustion this efficiency loss can be counteracted. Conventionally, the geometric compression ratio of a combustion engine with variable compression increases with increasing load (combustion chamber filling). 
   Independently from the above described concept, variable compression provides phenomena, that a direct start of a combustion engine must be possible with significantly avoiding of the use of electrostarters. In connection with this, the German patent document DE 101 11 928 A1 discloses that before a start, the position of a piston in a cylinder of the internal combustion engine is to be determined. In this way, the cylinder of the combustion engine is identified, whose piston is located in a working stroke. Then, fuel is injected directly into combustion chamber of the cylinder, and the combustion chamber filling is ignited directly after the injection, for releasing a self start or direct start of the combustion engine. 
   When this self starting concept is transferred to combustion engines with variable compression, worsened starting properties are produced than in the case of direct start concept in connection with constant compression ratio. 
   SUMMARY OF THE INVENTION 
   Accordingly, it is an object of the present invention to provide a method which improves the starting properties of a combustion engine, in which geometric compression ratio is variable in a controlled manner. 
   It is also an object of the present invention to provide a control device which improves the starting properties of such a combustion engine. 
   These objects are achieved in that, a start of a combustion engine is performed with a compression ratio which is reduced relative to a compression ratio in a normal operation of the combustion engine. 
   In accordance with another feature of the present invention, a control device is proposed which is formed to implement the above described method of the present invention. 
   As mentioned above, during a direct start the combustion energy which is required for the start is produced only by fuel injection and ignition in the cylinder, whose piston is located directly in a working stroke. A basic presumption for a corresponding direct start Is that the compression energy must be sufficiently great to bring the compression work in the cylinder which follows in the ignition sequence the first ignited cylinder. 
   The inventive reduction of the geometrical compression ratio reduces advantageously the above mentioned compression work. As a result, smaller combustion energies, which are released during ignition of the first combustion chamber filling during a first direct start, can be sufficient for providing the reduced compression work. 
   The inventive concept is thereby based on the realization that the start properties of a combustion chamber significantly depend on the introduced compression work, and this compression work to be introduced significantly depends on the geometric compression ratio ε+(VC+VH)/VC. 
   The invention is further based on the realization that in combustion engines with variable geometric compression ratio in a partial load region, conventionally increased geometric compression in connection with direct start concepts can be disadvantages. 
   The proposed invention eliminates these disadvantages and allows a direct start also in combustion engines with variable geometric compression ratio. These advantages are achieved in that, the negative effects of a conventional control of a variable geometric compression ratio 
   The inventive concept is thereby based on the realization that the start properties of a combustion chamber significantly depend on the introduced compression work, and this compression work to be introduced significantly depends on the geometric compression ratio ε=(VC+VH)/VC. 
   The invention is further based on the realization that in a combustion engines with variable geometric compression ratio in a partial load region, conventionally increased geometric compression in connection With a direct start concepts can be disadvantages. 
   The proposed invention eliminates these disadvantages and allows a direct start also in combustion engines with variable geometric compression ratio. These advantages are achieved in that, the negative effects of a conventional control of a variable geometric compression ratio is not only avoided by the inventive reduction of the geometric compression ratio for a start of the combustion engine, but also synergetic effect is achieved by uniting of the direct start concept with the concept of a variable geometric compression. 
   In accordance with the invention it is advantageous that the start is performed with a minimal adjustable compression ratio. 
   Conventionally the geometric compression ratio is varied between the values of 8 and 14. It has been determined by a research that the lowest possible compression ratio provides the best start properties. 
   In accordance with the present invention it is further advantages when the compression ratio successively increases after a start end. 
   A start end in a combustion engine is conventionally defined so that, the increasing motor rotary speed exceeds a predetermined threshold value of several hundreds revolutions per minute. With the increasing of this rotary speed threshold, the combustion engine has so much rotary energy that subsequent compression works in subsequent compression strokes can be introduced without problems. Moreover, with increasing motor rotary speed, a greater driving moment is available from preceding combustions in the working stroke of an ignited cylinder. The increase of the compression ratio after a start end allows a very fast adjustment after a start of geometric compression ratios, that is advantageous for optimization of the determined dynamic efficiency and/or the exhaust gas emissions. 
   The geometric compression ratio depends on the compression volumes VC and the stroke volumes VH. The compression volume as known is the compression volume over a piston when it is located in its upper dead point position. The stroke volume VH as known is the volume in a cylinder between the upper and the lower dead point positions of the piston. Theoretically the geometric compression ratio can change either by a variation of the piston strokes and/or by a variation of the compression volume VC. 
   A reduction of ε=1+VH/VC can be reached by a reduction of VH or by an increase of VC. The increase of VC has the advantage, when compared with a reduction of VH, in that the combustion chamber filling in the cylinder to be ignited, which during stoppage of the combustion engine is located in a working position, is increased. Thereby also the energy release which results from the combustion of its combustion chamber filling, is advantageously increased, that facilitates a self start. 
   In other words, the reduction of the geometric compression ratio by an increase volume VC provides not only a significant reduction of the compression work in the cylinder which follows in the ignition sequence, but also operates so that in the motor stoppage position in the cylinder, which is located directly in the working stroke, more air mass is available in the combustion chamber (air pressure in combustion chamber=ambient pressure). 
   Therefore, during start with a predetermined mixture composition (lambda=1, lambda&lt;1 , or lambda&gt;1) more fuel is injected than in conventional start with fixed compression ratio and thereby finally a higher combustion energy can be produced. Because of the higher obtained combustion energy and simultaneously because of the smaller introducible compression work, the probability with which the combustion engine self starts is significantly increased. An additional advantage is faster rotary speed increase of the combustion engine during a self start. 
   It is further advantageous when the compression ratio during switch off of the combustion engine and the last compression strokes and extension strokes in a controlled manner, is changed so that the pistons of the combustion motor come to stop in predetermined position. Therefore by a control increase and/or lowering of the compression ratio in the last compression strokes and expansion strokes, the pressure and therefore the gas spring moment of the air column enclosed in the cylinders is changed in a targeted manner during swinging out of the piston, so that a predefined piston position is adjusted. Thereby in the control device for a subsequent self start that cylinder is known whose piston is located in a working position. As a result, during a subsequent self start the control device more reliably identifies this cylinder and correspondingly controls the fuel supply to this cylinder and the associated ignition. 
   These features facilitate the realization of a start-stop operation, or in other words an operation of the combustion engine, in which the stop phases and operation phases follow one another relatively fast. This can save fuel, for example during the operation of a motor vehicle in street traffic. 
   It is further advantages when at least one cylinder, which during a start of the combustion engine is first ignited, is selected in dependent of the predetermined position of the piston of the combustion motor in its stop condition. 
   In accordance with a preferable embodiment, first a combustible combustion chamber filling and at least one combustion chamber of the combustion motor in its stop condition is performed by injection of fuel in at least one combustion chamber, and then the combustion chamber filling is immediately ignited. 
   It has been shown that in particular the direct injection of fuel in a combustion chamber in connection with a reduced compression, allows a reliable direct start and a fast motor running in start-stop operation. 
   In accordance with another embodiment of the present invention, the start is performed as a direct start or a self start without the support of a separate starter. 
   This embodiment has the advantage that a battery-operated electric starter must be actuated substantially more seldom, to start the combustion engine in a start-stop operation. In addition to noise advantages resulting from the elimination of the starter noise, a further advantage is provided in the possibility to use a smaller battery or in other words a battery with a smaller capacity. Thereby both the initial manufacture and the further operation of a motor vehicle is provided with lower costs, since a smaller battery is lighter and provides weight advantage as a customer advantage. 
   The novel features which are considered as characteristic for the present invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a view schematically showing a combustion engine with variable compression in condition of low compression; 
       FIG. 2  is a view schematically showing the combustion engine in condition of high compression; 
       FIG. 3  is a view schematically showing an arrangement of four pistons of a four-cylinder combustion engine for different strokes of a working cycle of the combustion engine in condition of low compression; 
       FIG. 4  is a view schematically showing a combustion chamber of the combustion engine with adjusting members, sensors and a control device; and 
       FIG. 5  is a view showing a flow diagram as an embodiment of a method in accordance with the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In  FIG. 1  a combustion chamber  10  is shown in substantially simplified form. 
   The combustion chamber  10  has a motor block  12 , in which cylinders  14  are arranged.  FIG. 1  shows one cylinder  14  of a multi-cylinder combustion engine. The remaining cylinders are arranged, for example behind the illustrated cylinder, so that  FIG. 1  corresponds to a very schematic front view of a multi-cylinder series motor. 
   A combustion chamber  16  is movably sealed in the cylinder  14  by a piston  18  which is guided in the cylinder  14 . The piston  18  is articulately connected through a connecting rod  20  with a connecting rod bearing  26  of a crankshaft  22 , which is rotatably supported in the main bearings  24 . The connecting rod  28  provides the rotary direction to the crankshaft  22 . For realization of a variable compression the main bearings  24  are not supported the crankshaft  22  directly in the motor block  12 . 
   Instead, the main bearings  24  are supported in eccentric rings  30  which are rotatably supported in the motor block  12 . The main bearings  24  of the crankshaft  22  are arranged eccentrically in the eccentric rings  30 . Therefore the main bearings  24  of the crankshaft  22  displays relative to the motor block  12  during a rotation of the eccentric rings  30 . In  FIG. 1  the main bearing  24  of the crankshaft  22  is located in its lowest possible position. Furthermore, the crank drive composed of the crankshaft  22  and the connecting rod  20  is located in a position which defines the upper dead point OT of the piston  18  in the cylinder  14 . The volume which remains in the upper dead point OT of the piston  18  in the cylinder  14  above the piston  18  is identified as a compression volume or compaction volume VC. 
   In the position shown in  FIG. 1 , a relatively great compression volume VC is provided as a result of the lowest possible position of the crankshaft  22  in the motor block. The stroke volume VH of a cylinder  14  of the combustion chamber  10  corresponds to the volume, which the piston  18  releases during its movement from the upper dead point OT to the lower dead point UT. This stroke volume VH is not influenced by a possible crankshaft displacement during the rotation of the eccentric ring  30  and thereby is invariant relative to a displacement of the crankshaft  22 . 
   The geometric compression ration ε of the combustion chamber, as known, is a sum of stroke volume VH and compression volume VC standardized on the stroke volume VH. In  FIG. 1  reference numeral  32  represents an adjustment member, with which a rotary position of the eccentric ring  30  is changeable in a manner predetermined by a control device  34 . The adjustment member  32  can be formed for example as an electric motor drive which is operationally coupled with the eccentric ring  30  through a toothed gear drive or a worn drive. As mentioned above, the crankshaft  22  in  FIG. 1  is located in its lowest position relative to the motor block  12 . With the lowest position of the crankshaft  22 , positively a maximum possible compression volume VC 1  is provided.  FIG. 1  represents the combustion motor  10  with a variable compression in condition of a maximum low compression ratio. 
     FIG. 2  shows the combustion motor  10  with variable compression of  FIG. 1 , in condition with the highest possible compression ratio. In deviation from  FIG. 1 , the main bearing  22  of the crankshaft  24  is located in  FIG. 2  in the highest possible position with respect to the motor block  13 . Thereby the compression volume VC is reduced to a minimum value VC 2 . 
   The relative position of the main bearing  24  of the crankshaft  22  to the motor block  12  is represented in  FIG. 1  by the length of arrow  36 . In  FIG. 2 , the length of the arrow  38  represents the relative position of the main bearing  24  of the crankshaft  22  in the motor block  12 . Arrow  40  which is a difference of the length of the arrows  36  and  38  represents the magnitude of the crankshaft displacement between  FIGS. 1 and 2 . Also, the dead points UT, OT of the movement of the piston  18  and the cylinder  14  are displaced by this magnitude of the length of the arrow  40 . For this reason the compression volume enclosed in the cylinder  14  changes proportionally to the displacement of the crankshaft  22 . 
     FIG. 3  schematically shows an arrangement of four pistons of a four-cylinder combustion engine with respect to the four strokes of a working cycle. In other words,  FIG. 3  shows an instantaneous assumption of the piston positions in different cylinders. The four cylinders are arranged from left to right in  FIG. 3  in the typical ignition series sequence (ignition sequence) of a four-cylinder combustion engine. Arrow  42  represents for example cylinder  1 , arrow  44  represents cylinder  3 , arrow  46  represents cylinder  4 , and arrow  48  represents cylinder  2 . 
   Reference numeral  50  identifies an inlet valve of a cylinder and reference numeral  52  identifies an outlet valve of a cylinder. In cylinder  1  both the inlet valve  50  and the outlet valve  52  are closed during a downward movement of the piston  18 . Cylinder  1  is located in a working stroke. In correspondence with the ignition sequence and in coordination with the shown opening conditions of the valves  50 ,  52 , the cylinder  3  (arrow  44 ) is located in a compression stroke, the cylinder  4  (arrow  46 ) is located in a suction stroke, the cylinder  2  (arrow  44 ) is located in an ejection stroke. In all four strokes gas forces act on the piston  18 . In combustion stroke a relatively significant combustion force acts on the piston  18 . In contrast, in compression stroke also relatively significant gas force acts brakingly on the upwardly moving piston. In contrast, in the suction stroke and ejection stroke gas forces acting brakingly on the piston are relatively small. From the integral of these forces over the corresponding path of the piston  8  between its corresponding dead point positions OT, UT, the works performed by the gas forces are obtained. 
   During a direct start it is exceptionally important that the sum of these works, or in other words the work balance, is positive directly in the first work stroke of the combustion engine to be started. Additionally, a friction losses must be taken into consideration. If one identifies the work performed in the work stroke with W_Verb, the work introduced in the compression stroke with W_Komp the work which to be introduced in the suction stroke with W_Ein the work to be introduced in the ejection stroke with W_Aus, and the work to be introduced for overcoming the friction with W−Reib, then the effective work W−Eff performed in the first work stroke by the combustion engine is W_Eff+W_Verb−W_Komp−W_Aus−W_Reib. Presumption for a direct start is that W−Eff is greater already in the first stroke than 0. Since W−Verb and W−Komp in general are comparatively great relative to W−Ein, W−Aus and W−Reib, the value W−Eff depends substantially on the values W−Verb, and W−Komp. 
   The inventive reduction of the geometric compression ratio of the combustion motor  10  in acts connection with this as follows: 
   On the one hand the compression work W_Komp is reduced. This effect alone is favorable for a direct start of the combustion engine  10 . Moreover, the reduction of the geometric compression ratio leads, when it is caused by an increase of the compression volume VC, to the situation that the filling of the combustion chamber  16  above the corresponding piston  18  located in the work stroke is increased. From the increased combustion chamber filling, a potential greater combustion energy is produced, which in the first work stroke performs the combustion work W−Verb on the piston  18 . 
     FIG. 4  schematically shows a combustion chamber  16  of the combustion engine  10  with adjusting members  66 ,  68 , sensors  58 ,  64  and a first control device  34  which controls the performance of the presented method. Reference numeral  54  in  FIG. 4  represents a camshaft transmitter wheel provided with marks  56 . During a rotation of the camshaft transmitter wheel  54 , a camshaft sensor  58  senses the marks  54  and supplies a signal to the control device  34 , which obtains from it an information about the angular position of the camshaft. The camshaft sensor  58  can be formed for example as an optical sensor or as an inductive pickup receiver. Correspondingly, the marks  56  in the case of the use of an optical sensor  58  can be formed as regions with different reflection properties and for transmission properties. In case of an inductive receiver  58 , the marks  56  can be formed for example as ferromagnetic marks, for example as teeth. 
   In a four-stroke cycle all piston positions can be produced in the four strokes in correspondence with angular positions of the camshaft. With a sufficient resolution of the sensor system composed of the camshaft transmitter wheel  54  with the marks  56  and the camshaft sensor  58 , it is possible to accurately identify the cylinder whose piston is located in a work stroke position. 
   In addition to the camshaft sensor system  54 ,  56 ,  58 , also a crankshaft sensor system  60 ,  62 ,  64  can be used and composed of a transmitter wheel  60 , marks  62 , and a sensor  64 . In this case it is recommended that the camshaft sensor system  54 ,  56 ,  58  supplies a signal which indicates whether the combustion engine is located in the first half or the second half of a four-stroke cycle. A finer resolution through the accurate position inside the first or the second half of the four-stroke cycle in this case is provided by the crankshaft sensor  64 , whose signals are also transmitted to the control device  34 . 
   Advantageously, an absolute angle sensor system is used, which supplies a signal about the angular position of the crankshaft and/or camshaft even with stopping crankshaft  2 . In particular optical or magneto-resistive sensors can be used as absolute angular sensors. In case of the use of inductive pickup sensors, advantageously during turning off of the combustion engine the angular position is determined, in which the combustion engine  10  or its crankshaft  22  come to stop. This can be performed during turning off since the sensors can reliably identify the operating cylinder with running the combustion engine  10 . 
   During a conventional start of the combustion engine  10  with an electric starter, an inductive pickup sensor conventionally requires up to two crankshaft revolutions until the signals supplied by the sensors can be accurately synchronized with the actual angular crankshaft. For reliably identifying after a turning off during a direct start such a cylinder whose piston is located in an work position, this piston during turning off of the combustion chamber is identified and corresponding information is stored until a next start of the combustion engine in the control device  34 , where it is available during the next start. 
   In case of an absolute angular sensor system, a cylinder whose piston is located in a work position, can be also reliably identified during start of the combustion engine. It is important in each case that this cylinder is reliably identified for a planned selfstart. 
   During a planned selfstart the filling of the combustion chamber  16  of the corresponding cylinder is ignited by controlling an ignition device  68 . A combustion of the filling with combustable mixture in the combustion chamber  16  takes place. In a combustion chamber with a gasoline direct injection as schematically shown in  FIG. 4 , an injection of fuel in the combustion chamber  16  is performed before the ignition by controlling the fuel metering device  66  by the control device  34 . 
   The invention however is not limited to the application for combustion chambers with direct injection. In combustion chambers with suction pipe injection, in which the fuel is dosed in a suction pipe  70  of the combustion engine before the inlet valve  50  of the combustion engine, the corresponding cylinder during turning off aspirates one last time a mixture of fuel and air. This mixture is confined in the combustion chamber  16  with closed valves and can be directly ignited during a subsequent direct start. 
     FIG. 6  shows an embodiment of the inventive method in the form of a flow diagram. In a step  72  first the compression volume or compaction volume c of the combustion motor is adjusted to the value VC  1  which corresponds to a low compression. Comparison can be made for this purpose of showing VC 1  and VC 2  in  FIGS. 1 and 2 . In the embodiment shown in  FIGS. 1 and 2 , the method is performed so that the control device  34  controls the adjusting members  32  so that the eccentric ring  30  turns for example from the position shown in  FIG. 2  to the position shown in FIG.  1 . 
   The step  72  is performed preferably not during a direct start, but instead during a preceding turning off of the combustion engine  10 . The basis for this is that the increase of the compression volume VC must be formed when air can be aspirated in the corresponding cylinder through open inlet valves. Only under these conditions it is guaranteed that the relatively great compression volume VC  1  is filled fully with air or mixture. As mentioned before, an increased filling contributes to the high probability with which a self start or a direct start can be performed, since thereby the energy released in the work stroke is increased. 
   In a step  74  subsequently the identification in the cylinder with n is performed, with which stopping of the crankshaft  22  of the combustion engine  10  is located in its work position. As described before, it this determination can be performed, depending on the type of the used sensor system, during the preceding turning off of the combustion chamber in stopped position of the combustion chamber before a self start. In a combustion chamber with a suction pipe injection, in which the corresponding cylinder aspirates already before a mixture at fuel and air from the suction pipe  10 , subsequently in the steps  76  the ignition of this combustion chamber filling can be performed, that releases the desired self start. If to the contrary, the combustion engine  10  is a combustion engine with a direct injection, then between the steps  74  and  76  a control is performed of the fuel metering device  66 , in which the fuel is dosed to the air introduced in the combustion chamber  16 . 
   In other words, for facilitating the selfstart, before the starting process itself the geometric compression ratio is reduced as much as possible, whereby during the start the compression work is significantly reduced in each next cylinder in the ignition sequence. Simultaneously, due to the smaller compression the compression volumes in all cylinders of the combustion engine is increased. This means for the start process from the cylinder which is located directly in the work stroke, that during motor stopping more air mass is available in the combustion chamber (air pressure in combustion chamber=ambient pressure). Therefore during the start with a predefined mixture composition more fuel can be dosed also in a conventional start with fixed compression ratio, whereby finally a higher combustion energy can be produced. Because of the reached higher combustion energy and simultaneously smaller introducible compression work, the start and the running up of the combustion engine can be performed faster. Furthermore, with this method the start reliability during self start or direct start can be significantly increased. 
   The above described method can be utilized in particular in combustion engines with direct injection and selfstart function in hot start case and cold start case. In addition, the above described method can be coupled with further features, such as for example multiple injection, the use of a special start fuel, a preheating of the combustion chamber, a preheating of the motor oil, a preheating of the fuel and so on. During turning off of the combustion chamber the varying of the compression ratio can also be coupled with other features, such as for example with a targeted control of the throttle flap and a waste gas return valve, a tank ventilating valve, with an air blow in the cylinder, as well as with braking operations by coupled auxiliary aggregates, for example starter-generator. 
   In addition to or alternatively to the above described selfstart function, the inventive reduced compression can be used also in connection with an external starter (cranking starter), to improve the starting condition of the combustion engine  10  also to improve the cold start. 
   It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of constructions and methods differing from the types described above. 
   While the invention has been illustrated and described as embodied in method of and device for operating a multi-cylinder combustion engine with variable compression ratio, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention. 
   Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention.