Abstract:
A reciprocating piston engine has a cylinder with a transfer port and an exhaust port, wherein the transfer port and exhaust port are at least partially coincident and are provided with a port valve. The engine includes a pump having a divided chamber therein, one side of the chamber being connected to a crankcase via a connecting port, and the other side of the chamber having an inlet port, an outlet port, and a valve to ensure unidirectional flow therethrough. The chamber is divided by a moving member responsive to variations in pressure in the crankcase to cause flow through the other side of the chamber.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to an engine and in particular, although not exclusively, to a two-stroke reciprocating piston engine. 
   It is known to provide a crankcase-scavenged two-stroke engine comprising a piston which reciprocates in a cylinder, the cylinder having transfer ports from the crankcase to the cylinder, and an exhaust port. The top of the exhaust port is located higher up the cylinder than the transfer ports, so as to permit most of the combustion gases to escape before a new charge enters the cylinder via the transfer ports. In other words the exhaust port is uncovered by a descending piston before the transfer ports. A subsequent charge enters the crankcase on the upstroke of the piston, and is pushed into the cylinder when the transfer ports reopen on the next down stroke of the piston. 
   Several problems are associated with the prior art crankcase-scavenged two-stroke engines. The requirement for the transfer ports to be in the swept stroke represents an inefficiency of the induction cycle since little or no work can be obtained from the piston displacement when the transfer ports are open. 
   It is well known that fresh charge can pass directly to exhaust, and it has been proposed to provide a tuned exhaust in an attempt to push the escaped charge back into the cylinder by the use of pressure pulses but this can result in an engine with a narrow power band. 
   The exhaust and transfer port design of prior art two-stroke engines is typically a compromise which may reduce the theoretical maximum power output from the engine and may also contribute to increased emissions from the engine. 
   These problems are all well known, and numerous solutions have been proposed to improve engine efficiency, to reduce contamination of the charge due to crankcase lubricant and to reduce pollution due to unburned fuel leaving the exhaust port. 
   What is required is an improved engine which can overcome the aforementioned problems, and maximise the opportunity for charge pumping and charge compression, and reduce transfer losses in a simple and cost effective manner. 
   SUMMARY OF THE INVENTION 
   According to the invention there is provided a two-stroke engine having a cylinder with a transfer port and an exhaust port, whereby the transfer port and the exhaust port are at least partially coincident, the transfer port and the exhaust port being further provided with a port valve, operable between a position to substantially close the transfer port and open the exhaust port during an exhaust phase of the engine, and a position to substantially open the transfer port and close the exhaust port during a transfer phase of the engine. 
   In this arrangement the fresh charge is substantially prevented from exiting the cylinder through the exhaust port. Furthermore this arrangement allows the transfer port to remain open for longer when compared to prior engines since the transfer port opens into the exhaust port, and thus remains open until the top of the exhaust port is closed by a piston of the engine. This allows an increased volumetric charge to the combustion chamber to provide an increase in the power output from the engine. Correspondingly the engine may have improved overall engine efficiency. 
   In one embodiment the valve may further comprise a deflector to deflect an incoming charge radially inward to the cylinder. 
   An engine so arranged has an improved swirl of the charge introduced into the cylinder when compared to the prior engines since the fresh charge introduced into the cylinder enters the cylinder radially inward and away from the exhaust port. 
   In one embodiment the deflector is arranged to deflect the incoming charge to one side of the centre and towards the top of the cylinder. This has the effect of producing an upward helical swirl of the charge. 
   In one embodiment the engine is further provided with a fuel injector to ensure an accurate fuel/air ratio over a wide range of operating conditions. In this embodiment the transfer port is arranged to input fresh air to the cylinder from either a crankcase of the engine or from a separate air pump. The transfer port may be in fluid communication with a turbo charger or supercharger to provide additional boost. 
   Preferably a low friction port valve is used to reduce parasitic losses to a minimum. In the preferred embodiment the port valve comprises a rotary valve which may be operable by an electric motor, typically in conjunction with a conventional engine management system. Alternatively the port valve can be operated from a direct drive of the engine such as from a flywheel having a cam profile thereon adapted to operate the valve. 
   In yet a further alternative the port valve is resiliently biased, by for example a suitable spring, whereby the port valve is arranged to be opened by transfer gases from the engine, in use. 
   In another embodiment the transfer port is provided with a transfer tract with a transfer valve such as a reed valve to ensure unidirectional flow of gaseous fluid through the transfer port. This arrangement prevents combustion gases entering the transfer tract, and thus ensure that a fresh charge is not contaminated. 
   An arrangement of the port valve and transfer valve so described permits the full stroke of the piston to be utilised to compress the charge since no charge escapes from the combustion chamber via the exhaust port or the transfer port during charge compression. 
   It will be appreciated that the port valve and the transfer valve can be adjusted to have variable timing depending on the rotational speed of the engine, the position of the piston within the cylinder and the power or torque demand. The valves may also be adapted to be partially or progressively opened or closed. Such variable timing enables tuning of the engine for optimisation of the power output or the fuel efficiency, or for controlling emissions from the engine. The advantage of such progressive valve timing is that an incoming charge from the transfer port can be used to create a swirl to push the combustion gases from the cylinder after combustion of a previous charge. 
   In the preferred embodiment the invention is adapted for a single cylinder engine. However a multi-cylinder engine may also benefit from the invention provided that the exhaust port and transfer port of each cylinder is provided with a port valve, one for each piston/connecting rod assembly. 
   In an alternative arrangement there is provided a two-stroke engine having a cylinder with a transfer port and an exhaust port, the exhaust port being further provided with a port valve operable between a position to substantially open the exhaust port during an exhaust phase of the engine, and a position to substantially close the exhaust port during a transfer phase of the engine. 
   In this arrangement the transfer port and the exhaust port are not required to be coincident and conventional transfer ports can be used to transfer a fresh charge into the cylinder. 
   The invention also provides a reciprocating piston engine assembly including a cylinder with an inlet and an exhaust, a crankcase, a crank a connecting rod and a piston, the crankcase comprising a closed chamber having a connecting port in a wall thereof, and the assembly further comprising a pump having a divided chamber therein, one side of said chamber being connected to said crankcase via said connecting port, and the other side of said chamber having an inlet port, an outlet port, and valve means to ensure unidirectional flow therethrough, wherein said chamber is divided by a moving member responsive to variations in pressure in said crankcase to cause flow through said other side of said chamber. 
   The rise and fall in crankcase pressure is an inevitable result of piston reciprocation, and the effect in the pump is to cause movement of the moving member, with consequent cyclical variation of the volume of said other side. The valve means ensure that unidirectional flow is a result, and consequently the pump can be arranged to provide a supply of fresh clean air to the inlet tract of the engine. It will be appreciated that the moving member is a barrier to crankcase oil mist. 
   It will be appreciated that the usual transfer passages to the crankcase are eliminated so that the full displacement of the piston is used to generate a cyclical pressure variation in the crankcase, which can be transferred to the pump. 
   In addition, an engine so arranged reduces the unpowered displacement of the piston stroke, due to the transfer port being open in the prior at design, which may provide an increase in the power output from the engine. Correspondingly the engine may have improved overall engine efficiency. 
   In a multi-cylinder engine, the crankcase is divided into substantially sealed chambers, one for each piston/connecting rod assembly. 
   In the preferred embodiment the pump provides clean air under pressure to the engine. A fan may be included upstream of the pump inlet port in order to increase inlet pressure, and thereby outlet pressure. Sophisticated valving is of course possible, including variable valve timing, and such an arrangement is particularly effective in scavenging of a two-stroke engine. In conjunction with an air inlet valve, the engine preferably uses fuel injection to ensure an accurate fuel/air ratio over a wide range of operating conditions. 
   Air under pressure from the pump may also be mixed with fuel upstream of the engine, for example in a carburettor or indirect injection system. 
   In a further refinement of a two-stroke engine, air from the pump may be introduced into the exhaust as a pulse to both urge burnt gases down the exhaust tract, and to prevent a fresh fuel/air charge from passing to exhaust before combustion, thereby mirroring the characteristics of prior exhaust expansion chambers. 
   Preferably the pump has a first plenum chamber downstream thereof. This allows the fluid to be supplied for example to the exhaust or the combustion chamber on demand and without pressure pulsing due to the cyclical nature of pump operation. 
   In the alternative embodiment pressure pulsing of the pump may be used to advantage in a tuned inlet tract, so as to maximise the volume of air admitted to the cylinder on each suction stroke. 
   The pump may be arranged separately from, immediately adjacent or integrated in the crankcase. The separate location of the pump from the engine has the advantage that a cooler and thereby denser charge is provided to the cylinder than prior engines using a convention transfer port design. Any kind of moving member is possible, but preferably a low friction member is preferred so as to reduce parasitic losses to a minimum. In the preferred embodiment the moving member comprises a bellows, the capacity of said bellows being substantially equal to the swept volume of the piston. In an alternative arrangement the moving member is a diaphragm. 
   Advantageously the inlet port of the pump is in fluid communication with an air box, the air box being open to atmosphere. In an alternative embodiment the inlet port has a venturi with a fuel supply to provide a charge for the combustion chamber. 
   A fan may be included upstream of the pump inlet port in order to increase inlet pressure, and thereby outlet pressure. 
   In accordance with another embodiment there is provided a second plenum chamber downstream of said first plenum chamber. The second plenum chamber operating at a higher pressure to introduce clean air into the inlet or exhaust at a higher pressure than the first plenum chamber. 
   The engine assembly may further include a second pump, said second pump having an inlet connected to an air box upstream thereof, and an outlet connected to the inlet of said first plenum chamber. The second pump may be an engine driven pump. 
   In accordance with another aspect there is provided a reciprocating piston engine assembly having a flywheel, wherein the flywheel includes a cam profile thereon adapted to operate a reciprocating pump. Such a pump may be used to supply clean air under pressure, for example to the first or second plenum chamber. 
   The combination of the port valve and the pump is particularly advantageous, and promises an engine which has an increased power output and reduced harmful emissions when compared to prior engines. The addition of the transfer valve to this combination may further improve power output, reduce harmful emissions and improve overall engine efficiency. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
     Other features of the invention will be apparent from the following description of a preferred embodiment shown by way of example only in the accompanying drawing, in which; 
       FIG. 1  is a schematic representation of an engine according to the present invention prior to charge ignition. 
       FIG. 2  is a schematic representation of the engine of  FIG. 1  undergoing exhaust. 
       FIG. 3  is a schematic representation of the engine of  FIG. 1  undergoing charge transfer. 
       FIG. 4  is a schematic representation of an engine according to another embodiment of the present invention. 
       FIG. 5  is a schematic representation of an engine according to yet another embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIGS. 1 to 3  illustrate an engine according to the present invention, generally designated  10 , completing one cycle. Like features are shown with like reference numerals.  FIG. 1  shows the engine  10  prior to a charge  48  being ignited.  FIG. 2  shows the engine  10  undergoing exhaust.  FIG. 3  shows the engine  10  undergoing charge transfer. 
   In  FIG. 1  a piston  12  is shown which reciprocates in a cylinder  14 . The piston  12  and the cylinder  14  together define a combustion chamber  16 . The combustion chamber  16  is provided with a fuel injector  18 , a spark plug  20 , a transfer port  22  and an exhaust port  24 . The transfer port  22  is coincident with the exhaust port  24  and has a valve  26  to permit either the exhaust port  24  or the transfer port  22  to be open to the chamber  16 . The valve  26  has a free edge  27  that is movable between the top of the exhaust port  24  and the bottom of the transfer port  22 . The valve  26  can be operated by any suitable means, such as an electric motor, and may comprise a rotary valve. 
   The engine  10  of further comprises a crankcase  28  which defines a crankcase chamber  30 . The crankcase  28  houses a crank  33 , the crank  33  being connected via a connecting rod  34  to the piston  12 . The crankcase chamber  30  is in fluid communication with a pump  32  via a connecting port  34 . The pump  32  has a membrane  36  that reciprocates in a pump chamber  38 . The pump  32  has an inlet  40  and an outlet  41 . The inlet  40  is in fluid communication with an air box (not shown) having an air filter (not shown). The air box is open to atmosphere to provide a supply of clean and fresh air to the pump  32 . Each of the inlet  40  and the outlet  41  are provided with a one-way valve  42 ,  44 , such as a reed valve to permit unidirectional flow of fresh air through the pump. The outlet  41  is in fluid communication with the transfer port  22  via a transfer passage  46 . 
   As the piston  12  reciprocates in the cylinder  14  the pressure within the crankcase chamber  30  varies in a cyclic manner. This cyclic pressure change causes the membrane  36  to reciprocate within the pump chamber  38 . The one way valves  42 , 44  of the pump  32  allow the pump  32  to pump fresh air in response to the varying pressure within the crankcase chamber  30 . The membrane  36  acts to separate the volume of gas in the crankcase chamber  30  from the fresh air being pumped by the pump  32 . This allows the oil contaminated gases within the chamber  30  to be separated from the fresh air being pumped by the pump  32 . 
   In  FIG. 1  the fuel injector  18  is shown injecting a fuel charge  48  into the chamber  16 . The charge  48  can be injected at any time after exhaust and between when the piston  12  is at bottom dead centre and before the piston  12  reaches top dead centre in accordance with known techniques. In the Figure the piston  12  is shown as the top of the exhaust port  24  is closed by the piston  12  moving up the cylinder  14  to compress the charge  48 . The valve  26  is shown in the position whereby the exhaust port  24  is closed. The inlet valve  42  is shown in the open position as fresh air is input to the chamber  38  due to the piston  12  moving up the cylinder  14 . 
   Referring now to  FIG. 2  the piston  12  is shown travelling down the cylinder  14  after ignition of the combustion gases and just after it has uncovered the exhaust  24  so that the exhaust gases pass into the exhaust  24 . The valve  26  is shown in the position whereby the exhaust port  24  is open and the transfer port  22  is closed. The exhaust gases are prevented from passing into the transfer passage  46  by the valve  26 . The pressure in the crankcase  28  due to the position  12  travelling down the cylinder  14  causes air within the crankcase chamber  30  to pass into the pump  32  via the connecting port  34 . 
   In  FIG. 3  the engine  10  is shown undergoing charge transfer. The valve  26  is shown in position to close the exhaust  24  so that the transfer port  22  is open. The valve  26  also acts as a deflector so that fresh air from the pump  32  is deflected radially inward to the combustion chamber  16  to provide an advantageous swirl and mixing with the charge  48  from the injector  18 . The fresh air is directed radially inward to the centre of the piston  12  in an opposite direction to the exhaust gases illustrated in  FIG. 2 . No charge  48  or fresh air escapes via the exhaust port  24  during charge compression since the exhaust port  24  remains closed by the valve  26 . The piston crown may be shaped according to known techniques to induce a desired swirl motion. 
   It will be appreciated that the valve  26  can be adjusted to have variable timing depending on the rotational speed of the engine or the position of the piston  12 . Such variable timing permitting tuning of the engine  10  for optimisation of the power output or the fuel efficiency, or for controlling harmful emissions from the engine. 
   The injector  18  of  FIGS. 1-3  may alternatively be omitted and the inlet port  40  connected to an indirect fuel injection system or a carburettor in order to pump a fuel/air mixture. Furthermore the air or fuel/air mixture may also be thermally insulated from the engine to provide a cooler and, therefore, denser charge. 
   An engine so described in  FIG. 1-3  allows the transfer port  22  to remain open for longer when compared to the prior art engine, which may provide an increase in the power output from the engine. Correspondingly the engine may have improved overall engine efficiency, power output and petrol consumption combined with a reduction in harmful exhaust emissions. An engine so constructed may also be cheaper to manufacture since the complexity of the cylinder casting and internal transfer ports is reduced when compared to a prior two-stroke engine. Correspondingly the tooling to manufacture the cylinder  14  is cheaper. 
   In an alternative embodiment the exhaust port and the transfer port are not coincident and the port valve is operable to substantially open and close the exhaust port only. In this arrangement transfer tracts and ports of a conventional kind are used to transfer a fresh charge to the cylinder. The transfer tracts may be provided with one way valves such as reed valves to ensure unidirectional flow therethrough. 
     FIG. 4  shows a schematic representation of a two-stroke engine designated  110 . The engine  110  comprises a crankcase  112  which defines a crankcase chamber  114 . The crankcase  112  houses a crank  116 , the crank  116  being connected via a connecting rod  117  to a piston  118  which reciprocates in a cylinder  120 . The piston  118  and the cylinder  120  together define a combustion chamber  122 . The combustion chamber  122  has a fuel inlet  121 , a fresh air supply  136  and an exhaust  125 . 
   The crankcase chamber  114  is in fluid communication with a pump  124  via a fluid connection  123 . The pump  124  has a membrane  126  that reciprocates in a pump chamber  128 . The pump  124  has an inlet  130  and an outlet  132 . The inlet  130  is in fluid communication with an air box (not shown) having an air filter (not shown). The air box is open to atmosphere to provide a supply of clean and fresh air to the pump  124 . Each of the inlet  130  and the outlet  132  has a one way valve (not shown) such as a reed valve. The outlet  132  from the pump  124  is in fluid communication with a plenum chamber, or pressure reservoir,  134 . An electronic control valve  133  may also be provided between the pump  124  and the pressure reservoir  134 . The pressure reservoir  134  is in fluid communication with an inlet  136  to the combustion chamber  122  and optionally an inlet  138  to the exhaust  125 . The inlet  136  to the combustion chamber  122  and the inlet to the exhaust  125  may also be provided with electronic control valves  140  to regulate the flow of fresh air according to the timing of the engine, and the inlet  136  may connect to inlet tract  22  of the embodiment of  FIGS. 1-3 , so as to replace the transfer passage  46 . 
   As the piston  118  reciprocates in the cylinder  120  the pressure within the crankcase chamber  114  varies in a cyclic manner. This cyclic pressure change causes the membrane  126  to reciprocate within the pump chamber  128 . The one way valves  130 , 132  of the pump  124  allow the pump  124  to pump fresh air in response to the varying pressure within the crankcase chamber  114 . The membrane  126  acts to separate the volume of gas in the crankcase chamber  114  from the fresh air being pumped by the pump  124 . This allows the oil contaminated gasses within the chamber  114  to be separated from the fresh air being pumped by the pump  124 . The pressure reservoir  134  acts as a source of pressurized fresh air which is supplied to the inlet  136  and optionally to the inlet  138  to the exhaust  125 . 
   Another embodiment of the present invention is presented in  FIG. 5 . Features common to the embodiment of  FIG. 4  are shown with like reference numerals. In this embodiment there is provided a second plenum chamber or pressure reservoir  135  between the pressure reservoir  134  and the inlet  138  to the exhaust  125 . The second reservoir  135  may have an additional inlet  137  which is connected to a second pump (not shown), for example an electric pump or an engine driven pump such as a cam driven pump. The additional inlet  137  and the inlet  138  to the exhaust  125  may have electronic control valves  140 . The second reservoir  135  is intended to operate at lower volume and higher pressure than the first pressure reservoir  134 . 
   It will be appreciated that the second pump of the embodiment shown in  FIG. 5  may be used with the embodiment illustrated in  FIG. 4 . In this instance the second pump may be used to increase the pressure of the pressure reservoir ( 134 ). 
   The inlet  130  to the pump  124  of  FIGS. 4 and 5  may alternatively be connected to an indirect fuel injection system or a carburettor in order to pump a fuel/air mixture. The carburettor or indirect fuel injection may alternatively be located downstream of the pressure reservoir  134  or  135  on either or both of the inlet  136  to the combustion chamber or the inlet  138  to the exhaust. 
   The control valves  140  of the embodiments illustrated in  FIGS. 4 and 5  maintain an optimum pressure within the combustion chamber  122  depending on the engine load or the engine speed. For example, the control valve  140  on the inlet  136  may be used to provide additional combustion gases to the combustion chamber  122  after the exhaust closes and before ignition. The control valve  140  on the exhaust inlet  138  may be used to push unburned combustion gases back into the combustion chamber when the exhaust  125  is open to the combustion chamber  122 . The inlets  136 , 138  may also be aimed or introduced into the combustion chamber more effectively to assist with purging the unburned gasses. 
   An engine so described herein reduces the unpowered displacement of the piston stroke, due to the transfer ports, which may provide an increase in the power output from the engine. Correspondingly the engine may have improved overall engine efficiency, power output, petrol consumption and exhaust emissions. Furthermore, since there is no engine oil mist introduced into the charge the engine emissions may be reduced when compared to the prior art crankcase-scavenged two-stroke engine. The full displacement of the piston is utilised in the pump  124 . Furthermore the air or fuel/air mixture may also be thermally insulated from the engine to provide a cooler and, therefore, denser charge. 
   An engine assembly so constructed may also be cheaper to manufacture since the required casting of the cylinder  120  and internal transfer ports is reduced. Correspondingly the tooling to manufacture the cylinder  120  is less expensive. 
   Whilst a preferred embodiment for the device has been described it will be appreciated that many other designs of the engine exist that would have the desired effect of this aspect of the invention with the proviso that the variation in crankcase volume is used to pump atmospheric air into the combustion chamber.