Patent Publication Number: US-2009217891-A1

Title: Reciprocating piston machine

Description:
TECHNICAL FIELD 
     The invention relates generally to reciprocating piston machines and has particular application to internal combustion engines and especially to reciprocating piston two-stroke engines driving a crankshaft through a connecting rod. 
     BACKGROUND ART 
     Two-stroke engines have a number of advantages compared to the more common four-stroke engines, and in order that these may be realised much research has been directed to solving some of the drawbacks associated with two-stroke engines. Inefficiency and pollution are a particular drawback associated with the conventional crankcase scavenged two-stroke engine where the air/fuel and oil mix interact with the base engine components such as the crankshaft, connecting rod, bearings and piston for example. As the output characteristics of two-stroke engine are determined largely by the charging efficiency this has also received considerable attention. While performance improvements have been achieved which address these issues, these have been at the expense of the simplicity and low cost of the conventional piston valve inlet port engine. For example, the provision of a scavenging chamber separate from the crankcase and charged by a blower avoids introducing oil into the induction air, thereby reducing pollution. This design also increases scavenging and charging efficiency, but at a cost which is disadvantageous on smaller engines such as may be used for powering portable equipment or smaller vehicles. 
     GB 2 183 731 describes an engine with a double-ended piston defining opposing working chambers, wherein the crankshaft axis extends transversely through a central part of the piston. However there is no description of how the working chambers may cooperate to provide a two-stroke engine. Moreover there is no teaching that a connecting rod may be enclosed in the piston or of how a sealed crankcase may be obtained with this arrangement of the connecting rod and crankshaft. 
     All references, including any patents or patent applications cited in this specification are hereby incorporated by reference. No admission is made that any reference constitutes prior art. The discussion of the references states what their authors assert, and the applicants reserve the right to challenge the accuracy and pertinency of the cited documents. It will be clearly understood that, although a number of prior art publications are referred to herein, this reference does not constitute an admission that any of these documents form part of the common general knowledge in the art, in New Zealand or in any other country. 
     It is acknowledged that the term ‘comprise’ may, under varying jurisdictions, be attributed with either an exclusive or an inclusive meaning. For the purpose of this specification, and unless otherwise noted, the term ‘comprise’ shall have an inclusive meaning—i.e. that it will be taken to mean an inclusion of not only the listed components it directly references, but also other non-specified components or elements. This rationale will also be used when the term ‘comprised’ or ‘comprising’ is used in relation to one or more steps in a method or process. 
     It is an object of the present invention to address the foregoing problems or at least to provide the public with a useful choice. 
     Further aspects and advantages of the present invention will become apparent from the ensuing description which is given by way of example only. 
     DISCLOSURE OF INVENTION 
     According to one aspect of the present invention there is provided a two-stroke engine including: 
     an engine block having a cylinder; 
     a double-ended piston received for reciprocation in the cylinder, the piston comprising a body having first and second opposing ends, the first end forming a wall of the combustion chamber, the second end forming a wall of a pre-compression chamber, the piston including a piston opening between the first and second ends; 
     at least one passage extending from the pre-compression chamber to the combustion chamber for charging the combustion chamber; 
     a connecting rod operatively connecting the piston to a crankshaft, and characterised in that 
     the piston opening encloses the connecting rod and the crankshaft protrudes through the piston opening. 
     The piston opening extends between opposing mouth portions in the sides of the piston, the crankshaft protruding through both mouth portions. In this manner a compact engine is achieved, especially where a bore dimension exceeds the stroke. 
     The piston opening is preferably sealed, with a piston opening seal extending around each of the mouth portions for engagement with the cylinder wall. The piston opening may thus form a sealed crankcase which may be filled with oil for lubrication of the base engine components enclosed therein. This of course advantageously avoids the need for introducing lubricating oil into the induction air. 
     The piston may have generally arcuate or cylindrical side faces. Preferably, however, the cylinder is non-cylindrical and the mouth portions are formed in opposing lateral faces which are substantially planar and parallel with the axis of reciprocation of the piston. These flat faces allow an economical and reliable seal to be provided around the mouth portions. 
     It will be understood that, for economy, the piston may be formed from an extruded section with the two flat faces extending generally perpendicular to the axis of the extrusion. 
     In a preferred embodiment the piston has four substantially planar lateral faces. This design allows the engine block to also be formed from an extruded section. 
     According to another aspect of the present invention there is provided a piston seal assembly for sealing between a piston and a cylinder wall, the side surfaces of the piston having first and second piston faces meeting at an intersection, the cylinder wall having corresponding first and second cylinder wall faces, the seal assembly including: 
     first and second seal portions adapted to be received in a groove in the piston for engagement with the first and second cylinder wall faces respectively, adjacent ends of the seal portions being aligned generally with the intersection, 
     a seal wedge adapted to be received in the groove between the adjacent ends of the seal portions for abutting cooperating end faces of the adjacent ends, and 
     resilient means for biasing the seal wedge outward toward the cylinder wall, thereby pressing the first and second seal portions against the cylinder wall. 
     The resilient means is preferably at least one spring. By way of example, a piston with four substantially planar lateral faces would have four seal portions, each engaging a separate side of the cylinder wall, with four seal wedges—one at the intersection of each seal portion. In a preferred embodiment the groove is annular and a substantially annular spring is received in the groove and acts upon each of the seal wedges. It will be understood that this piston seal can be used with a piston with a varying number of straight or curved sides. This arrangement provides a continuous seal around the periphery of the piston, which is maintained as the working surfaces wear and which also allows the seal portions and seal wedges to be held firmly against the cylinder walls under the pressure generated when the working space adjacent the piston is pressurised. 
     In the preferred embodiment one seal assembly as described above is received in a circumferential groove in the piston adjacent the first end, but a seal assembly may also be provided at the second end. The shape of the second end of the piston preferably substantially conforms to the opposing wall of the pre-compression chamber, thereby providing a minimum clearance volume for good forced induction. 
     In still another aspect the present invention provides a piston machine, especially an engine, including: 
     a block having a cylinder; 
     a double-ended piston received for reciprocation in the cylinder, the piston comprising a body having first and second opposing ends forming walls of respective chambers, the piston including a piston opening between the first and second ends; 
     a connecting rod operatively connecting the piston to a crankshaft, and characterised in that 
     the piston opening encloses the connecting rod and the piston opening extends between opposing mouth portions in the sides of the piston, the crankshaft protruding through both mouth portions, with a seal extending around each of the mouth portions for engagement with the cylinder wall. 
     This arrangement also has the advantage that the piston opening in the piston forms a sealed crankcase. Such a machine is compact and may, for example, be a pump. It will be understood that a pump variant would require suitable ports, valves, piston sealing etc. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Further aspects of the present invention will become apparent from the following description which is given by way of example only and with reference to the accompanying drawings in which: 
         FIG. 1  is an end view of the engine of the present invention; 
         FIG. 2  is an exploded view of the engine of  FIG. 1 , and 
         FIG. 3  is a plan view of a piston seal for the engine of  FIG. 1 . 
     
    
    
     BEST MODES FOR CARRYING OUT THE INVENTION 
     Referring to  FIG. 1 , a two-stroke petrol engine  1  according to a preferred embodiment of the present invention includes an engine block  2  having a cylinder  3  which holds a double-ended piston  4 . The piston  4  is fixed to a crankshaft  5  by a connecting rod  6  and is shown at bottom dead centre (and at top dead centre in dashed outline). 
     The piston  4  has a first end  7  forming a wall of the combustion chamber  8 . The second end  9  forms a wall of a pre-compression chamber  10 . A piston opening  11  extend between the first and second ends  7 ,  9  and between opposing circular mouth portions  12  in the sides of the piston  4 , through which, the crankshaft  5  protrudes. The con rod  6  is enclosed in the piston opening  11 . 
     A reed type check valve  13  controls the flow of air/fuel through the induction passage  14  into the pre-compression chamber  10 . A transfer passage  15  connects the pre-compression chamber  10  to the combustion chamber  8  through inlet port  16  in the cylinder wall. Exhaust gases pass from exhaust port  19  through exhaust passage  17 . It will be understood that the porting, piston valving, pre-compression chamber valving etc are merely examples of how these functions may be performed. 
     As best seen in  FIG. 2 , the piston  4  is square in cross-section with four substantially planar lateral faces  19 - 23  which extend parallel with the axis of reciprocation of the piston and intersect along four edges. The faces  19 - 23  include a pair of opposing lateral faces  19 ,  20  in which the mouth portions  12  are formed. The first end  7  is substantially planar and the second end  9  is semi-cylindrical joining the faces  22 ,  23  tangentially. The wall  39  of the pre-compression chamber  10  is also semi-cylindrical to conform to the second end  9 , providing a minimum clearance volume. 
     A piston opening seal  24  extends around each of the mouth portions  12  for engagement with the planar surfaces  24 ,  25  of the cylinder wall adjacent faces  19 ,  20 . The piston opening  11  thereby forms a sealed crankcase which may be filled with lubricating oil. 
     Adjacent the first end  7  a seal groove  26  extends in the lateral faces  19 - 23  and has an inner annular face  27 . A piston seal assembly  28 , described further with respect to  FIG. 3 , is received in the groove  26  and includes four seal portions  29   a - 29   d,  four seal wedges  30   a - 30   d  and an annular spring  31 . 
     From the illustrations it will be clear that the engine block  2  and piston  4  could be economically manufactured from extruded sections having long axes generally parallel to the axis of the crankshaft  5 . The side plates  32   a,    32   b  abut the lateral faces  19 ,  20  of the piston  4  and have circular piston openings  33  thorough which the crankshaft  5  extends. 
     Referring to  FIG. 3 , in the piston seal assembly  28  each of the seal portions  29   a - 29   d  has an arcuate inner edge  35 , a straight outer edge  34  and opposing end faces  36 ,  37  joining the outer edge  34  at an acute angle. The seal wedges  30   a - 30   d  abut the adjacent end faces  36 ,  37  and have an acute wedge angle. Resilient means in the form of annular spring  31  is received in the groove  26  with one end received in a notch  38  to hold it in place. The spring  31  tends to open and acts upon each of the seal wedges  30   a - 30   d  thereby maintaining contact between the outer edge  34  and the corresponding cylinder walls on either the engine casing  2  or the end plates  32   a,    32   b.    
     Aspects of the present invention have been described by way of example only and it should be appreciated that modifications and additions may be made thereto without departing from the scope thereof, as defined in the appended claims.