Abstract:
The invention relates to a reciprocating engine and a working fluid inlet system therefore. The engine includes at least one cylinder with a reciprocating piston therein and a variable volume expansion chamber capable of receiving a working fluid via an inlet valve. The inlet system includes a pilot valve having an open condition and a closed condition. In the open condition, the secondary fluid passes therethrough to act on the inlet valve. The system also includes an actuating means for controlling the condition of the pilot valve. The inlet valve is adapted to open in response to the action of the secondary fluid. The engine may also include exhaust means, possibly by porting in the piston and a cylinder wall. The working fluid may be used as the secondary fluid.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a reciprocating engine and to a working fluid inlet system for a reciprocating engine, such as a steam inlet system for a heat engine such as a Rankine cycle engine, the reciprocating engine being of the type that does not rely upon an internal chemical reaction (such as an internal combustion engine) for the reciprocating movement.  
       BACKGROUND OF THE INVENTION  
       [0002]     One of the earliest forms of engine developed for providing mechanical work was a Rankine cycle engine, often referred to as a ‘steam engine’ because the majority of such engines used steam as their working fluid (and were thus considered to be steam driven). Steam engines were reciprocating engines that typically had a reciprocating piston in a cylinder, with an inlet valve and an exhaust valve (usually at the same end of the cylinder), the piston being connected by a rod and a crank to a flywheel or the like.  
         [0003]     During operation of the engine, with the piston at ‘top dead centre’ (referred to as ‘TDC’), the inlet valve was opened, allowing steam to enter from a boiler. The expanding steam drove the piston in its expansion (or power) stroke, whereupon the inlet valve would close, allowing the steam in the cylinder to expand to a lower pressure. As the piston reached ‘bottom dead centre’ (referred to as ‘BDC’), the exhaust valve would open allowing the steam, which was generally still at significant pressure, to escape as the piston travelled back up the cylinder to TDC on its return stroke.  
         [0004]     In such an operation, it is ideal to open and close the inlet valve infinitely quickly, and to close the inlet valve early in the power stroke, providing a high expansion ratio. However, in the early 1900&#39;s valve actuation technology was limited and poor efficiencies were thus accepted throughout the development of such engines. Indeed, the inability to close the inlet valve early enough was a major factor leading to the development of compound engines (double, triple and even quadruple expansion engines) where steam would be routed to a second, larger capacity cylinder where it was similarly expanded. Sometimes there was also a third, or even a fourth stage where this would be repeated.  
         [0005]     While engines of this type generally performed satisfactorily, subsequent developments in engine design produced engines of greater efficiency and higher horsepower to weight ratios, such as the internal combustion engine, the steam turbine and the like. As a result, the use of steam engines fell away, so much so that steam engines became quite rare.  
         [0006]     However, with increasing emphasis on environmental and pollution considerations, and with the continuing rise in the price of fossil fuels, there has recently been renewed interest in steam engines, particularly for use in small cogeneration or combined heat and power (CHP) systems.  
         [0007]     Accordingly, there is a renewed need for improvements to, in particular, the inlet valve systems for such steam engines and, in general, to the working fluid inlet systems for reciprocating engines of any type where a high pressure gas or vapour is fed to a cylinder in a controlled manner.  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention provides a working fluid inlet system for a reciprocating engine, the engine including at least one cylinder with a reciprocating piston therein and having a variable volume expansion chamber capable of receiving a working fluid via an inlet valve, the inlet system including: 
        a pilot valve having an open condition where secondary fluid passes therethrough to act on the inlet valve, and a closed condition; and     actuating means for controlling the condition of the pilot valve; wherein the inlet valve is adapted to open in response to the action of the secondary fluid.        
 
         [0011]     The present invention also provides a reciprocating engine utilizing the working fluid inlet system described above, together with a method of operating such a reciprocating engine. In this respect, the engine may have one or more reciprocating piston/cylinder arrangements, there being at least one of the inlet systems of the present invention associated therewith.  
         [0012]     Indeed, the present invention also provides a reciprocating engine including at least one cylinder with a reciprocating piston therein and having a variable volume expansion chamber capable of receiving a working fluid via an inlet valve, the engine including a working fluid inlet system and exhaust means, the working fluid inlet system including a pilot valve having an open condition where secondary fluid passes therethrough to act on the inlet valve, and a closed condition, and actuating means for controlling the condition of the pilot valve, wherein the inlet valve is adapted to open in response to the action of the secondary fluid, the exhaust means including at least one exhaust valve in the piston and at least one exhaust port in the piston, the exhaust valve being configured to open automatically when the pressure above the piston drops to a threshold pressure above an exhaust port pressure.  
         [0013]     Ideally, as will be explained below, the reciprocating engine will be a Rankine cycle engine that uses steam as the working fluid, and that has only a single reciprocating piston/cylinder arrangement that preferably operates on the uniflow principle. However, it will be appreciated that the reciprocating engine need not necessarily contain a ‘piston’ and a ‘cylinder’ in the traditional sense, but rather simply needs to have an expansion volume and a positive displacement expander.  
         [0014]     For example, a system of this type that may contain other than a piston/cylinder arrangement is a Wankel rotary expansion chamber comprising a triangular rotor which rotates on an eccentric shaft and is within, and geared to, an epitrochoidal housing. Thus, the continued reference throughout this specification to a piston/cylinder arrangement should be interpreted to cover at least this type of arrangement.  
         [0015]     Also, in the preferred configuration the working fluid and the secondary fluid will be sourced from the same supply. Indeed, it is envisaged that, in most situations, the working fluid will be steam from a boiler, and the secondary fluid will also be steam, supplied by the same boiler (although the engine may be powered by solar energy or some other low grade heat source, and may use any organic working fluid). Thus, the reference to ‘secondary fluid’ throughout the specification should not be seen as requiring the secondary fluid to be of a different type (or from a different source) to the working fluid.  
         [0016]     It will be appreciated that the inlet system of the present invention provides for rapid opening and closing of the inlet valve, and for the timing of at least the closing of the inlet valve to be controllable so as to be early in the expansion (power) stroke of the engine. Such ease of variable valve timing avoids the need to maintain constant inlet valve admission and cut-off timing, which in many traditional steam engines required throttling of the steam to run at part power, introducing obvious inefficiencies.  
         [0017]     Additionally, the present invention permits the inlet valve to be actuated indirectly (by the pilot valve) rather than directly, which avoids the need for an electrical or mechanical actuating means capable of generating large forces at high speeds.  
       GENERAL DESCRIPTION OF THE INVENTION  
       [0018]     The secondary fluid for use with the pilot valve may be any suitable fluid, pressurized in any suitable manner, and may for instance be any suitable pressurized liquid or gas/vapour. It is expected that the secondary fluid will usually be steam, although it should be understood that a suitable hydraulic fluid would suffice. Indeed, suitable fluids are envisaged to be water, air, nitrogen, synthetic and mineral oils, or suitable mixtures such as a water/glycol mixture.  
         [0019]     Given that the preferred working fluid for the operation of the engine is steam (as will be explained below), whatever steam generation system is employed for that purpose may also be used to generate useful steam (as the secondary fluid) for the pilot valve. For example, in a preferred form, the steam for both the working fluid and the secondary fluid may be generated in a boiler, as mentioned above.  
         [0020]     Boilers can be of many different architectures, but generally consist of a volume in which water is contained, such as a series of tubes. Heat is then applied to the exterior of this volume and is transferred through the walls of the vessel, causing the water to become heated and boil, producing steam. This is then commonly further heated to produce superheated steam. Common types of boilers include firetube boilers, water tube boilers, and flash boilers. In all types, water is typically added continuously or periodically to replenish that boiled off.  
         [0021]     The pilot valve preferably operates between two conditions, namely its open condition and its closed condition. When in its open condition, the pilot valve permits passage of the secondary fluid therethrough to act on the inlet valve. In a preferred form, the pilot valve is urged towards its open condition against a closing force, such that the rest position for the pilot valve is its closed condition.  
         [0022]     An advantage of this arrangement is that the pilot valve can be configured so as to act as an emergency relief valve in the event of boiler overpressure, given that such overpressure will tend to open the valve rather than close it.  
         [0023]     The pilot valve may be of any suitable type and may, for instance, be a poppet valve, a spool valve or a flapper valve. Where the pilot valve is a poppet valve, the poppet valve preferably opens by unseating a poppet from its seat, allowing fluid to pass.  
         [0024]     Where the pilot valve is a spool valve, the spool valve preferably includes a stepped cylindrical spool in a sleeve that has radial flow ports. In this form, sliding the spool in the sleeve exposes the flow ports to open them. Advantageously, such a valve can be of the overlapped type. This provides a dead zone in the travel of the spool where the inlet valve is not in fluid communication with either the boiler or the exhaust port, thus preventing short-circuiting between the boiler and the exhaust port.  
         [0025]     Where the pilot valve is a flapper valve, the flapper valve preferably includes a flapper that swings between two opposing nozzles by a continuous stream of secondary fluid via pressure drop orifices. Each nozzle preferably communicates with respective chambers in the inlet valve, where, in one form, a spool is held central by springs.  
         [0026]     Turning now to the inlet valve of the system of the present invention, the inlet valve is preferably of a type that is also operable between open and closed conditions, in the preferred form in response to the action of the secondary fluid from the pilot valve. In its open condition, the inlet valve permits entry of the working fluid to the expansion chamber of the cylinder to do work on the piston as it expands, in the normal manner. Again, the inlet valve is preferably urged towards its open condition (preferably by the secondary fluid) against a closing force, such that the rest position for the inlet valve is also its closed condition.  
         [0027]     The inlet valve may also be of any suitable type and will ideally either be a poppet valve or a spool valve. In one form, the inlet valve is a poppet valve and includes a poppet piston running in a cylinder to a poppet stem. The secondary fluid admitted by the pilot valve preferably exerts force on the poppet piston, overcoming a resilient means (such as a spring) which normally holds the poppet shut. This results in the inlet valve opening. Preferably, the area of the poppet piston on which the secondary fluid acts is larger than the poppet area, assuming that the pressures of the secondary fluid and the working fluid are the same.  
         [0028]     In this form, the poppet valve may be oriented in either direction relative to the flow of pressurised fluid as it opens. Preferably, the poppet valve is oriented such that the boiler pressure tends to hold it closed. This avoids the need for a strong resilient force to hold it closed, as would be the case if the orientation were reversed. Further, this arrangement assists in avoiding leaks, as the increased pressure results in an increased closing force and thus increased sealing pressure (namely, valve seat contact pressure).  
         [0029]     Referring to the actuating means of the system of the present invention, the actuating means preferably controls the operation of the pilot valve between its open condition and its closed condition. Whilst the preferred form of actuating means provides electrical actuation that is electronically controlled, it will be appreciated that the actuating means may be provided by a suitable mechanical, electrical, electromagnetic, piezoelectric or other actuation arrangement. A suitable such arrangement may be one that would give rise to similar precision and speed of operation of the pilot valve as is provided by the electronic means about to be described.  
         [0030]     In a preferred form, the actuating means is an electronically controlled solenoid, the electronic control being provided by a control module in association with a timing means. In this form, the control module may include a processing device (such as a microcontroller) which is able to process set and dynamic parameters so as to provide a control signal (via an output port) to the solenoid, the control signal being suitable for actuating or holding the solenoid so as to control the pilot valve between its open and closed conditions.  
         [0031]     In a preferred form of the invention, at least some of the dynamic parameters are provided by, or determined using, a signal from the timing means to the control module. The set parameters, on the other hand, may reside on the control module (for example, in FLASH memory, or an EPROM, or memory on-board a microcontroller) such that they are able to be accessed by the processing device. In this form of the invention, the set parameters are effectively preprogrammed into the control module.  
         [0032]     The processing of the dynamic parameters preferably provides data such as crank-angle position and speed data, during operation of the engine.  
         [0033]     Other dynamic parameters provided to the processing means may be any of the engine&#39;s operating conditions, such as the pressure of the working fluid and/or the secondary fluid, or the temperatures and pressures within the cylinder, although these will typically not be provided by the timing means.  
         [0034]     The timing means may be any type of rotational position transducer that can provide ‘real time’ crank position data to the processing means. In a preferred form, the timing means will be a timing disc arranged to rotate with the crankshaft of the engine. The timing disc will preferably have pre-set protrusions thereon configured to be representative of pre-determined crank-angle positions. Timing sensors may then be provided that are capable of sensing the passing of respective protrusions to generate timing signals for the processing means in order to determine crank-angle speed and position data.  
         [0035]     By pre-programming the control module with set parameters related to, for instance, the delay time between energizing the solenoid and the opening of the pilot valve, the delay time between the pilot valve opening and the inlet valve opening, the delay time associated with gas flow, and variations to these delay times caused by changes in the engine&#39;s operating conditions, the processing means is able to determine, during operation, at what time shortly prior to the predicted next TDC time the solenoid should be energized. This permits the solenoid to actuate the pilot valve, which in turn opens the inlet valve, at precisely the required time with respect to the arrival of the piston at TDC.  
         [0036]     Preferably, a very high initial voltage is provided to the solenoid, enabling the current, the associated magnetic field, and hence the solenoid plunger retraction force, to build up quickly, minimizing any delay time.  
         [0037]     Further, once the solenoid plunger has commenced moving, the voltage and current are preferably lowered to a ‘holding’ value to maintain the plunger in a retracted position (and thus the pilot valve in its open condition) against the resilient means (such as a return spring). In this form, it is not essential to sense when the plunger commences moving—the time may be entered as one of the set parameters.  
         [0038]     In the same manner, the control module may be pre-programmed with set parameters related to, for instance, the delay time between de-energising the solenoid and the closing of the pilot valve, the delay time between the pilot valve closing and the inlet valve closing, the delay time associated with gas flow and variations to these delay times caused by changes in the engine&#39;s operating conditions. Thus, the control module preferably sends the de-energisation signal to the solenoid shortly prior to the desired inlet valve closing time.  
         [0039]     In this respect, and given that to achieve high expansion ratios the inlet valve should only remain open for a short time after TDC, any closing delay time is preferably short. In one form, this may be achieved by including means capable of rapidly dissipating the solenoid field energy to ensure rapid plunger extension under the influence of the resilient means (such as the return spring) when the solenoid de-energises.  
         [0040]     Without such a rapid dissipation means, there is a risk that the solenoid de-energisation process would commence before the solenoid is fully energized for opening the pilot valve. This would, of course, lead to the inlet valve not opening fully, or at all, leading to a loss of efficiency.  
         [0041]     Finally, the inlet system of the present invention may also be advantageously used to control the pressure that builds up in the dead space in the expansion chamber just before the piston reaches TDC. In one form, a pressure transducer may be included in the expansion chamber to monitor cylinder pressure. This could supply further dynamic parameters to the control module to vary the inlet opening timing slightly. For instance, in the event that the cylinder pressure gets too high in the final movement of the piston to TDC, the control module may energise the solenoid early to open the inlet valve earlier, allowing the pressure build up to vent to the boiler via the inlet valve.  
         [0042]     In order to provide a general understanding of the manner of operation of a reciprocating engine having a working fluid inlet system in accordance with the present invention, an in-use scenario will briefly be described.  
         [0043]     Once operating, the sequence of operating steps for a reciprocating engine of the steam driven Rankine cycle type will, in general terms, be as follows:  
         [0044]     1. As the piston nears TDC, the actuating means operates to open the pilot valve against a closing force, permitting secondary fluid (steam) to move therethrough. The actuating means is preferably the electronically controlled solenoid/timing means arrangement described above, which is capable of predictively controlling the pilot valve between its open and closed conditions, in terms of being open and closed, and also in terms of the rate and timing of opening and closing.  
         [0045]     2. The steam then engages with a suitable configured inlet valve, causing the inlet valve to open, again against a closing force.  
         [0046]     3. The working fluid (steam) enters the expansion chamber of the cylinder via the inlet valve, expanding and forcing the piston away from TDC on its expansion (power) stroke, towards BDC.  
         [0047]     4. The actuating means operates to close the pilot valve, denying steam to the inlet valve, and allowing the closing force to close the inlet valve.  
         [0048]     5. Once the piston has passed BDC, it returns towards TDC on its return stroke. Expanded steam within the cylinder exhausts through exhaust valve(s) located in the cylinder wall and/or, more preferably, in the piston head itself. This latter configuration prevents the piston from having to work against the compression of steam in the cylinder during the return stroke, as will be described in more detail below.  
         [0049]     6. As the piston again nears TDC, the actuating means again operates to open the pilot valve against the closing force, again permitting secondary fluid (steam) to move therethrough.  
         [0050]     7. The cycle of steps 1 to 6 then continues.  
         [0051]     In relation to the use of piston head exhaust valves, if utilized the exhaust valves are preferably configured so as to open automatically when the pressure above the piston drops to a threshold pressure above the exhaust port pressure. In this respect, the piston preferably includes exhaust ports associated with the exhaust valves, these piston exhaust ports venting to aligned exhaust ports in the cylinder wall (or the crankcase, if desired).  
         [0052]     Preferably, the piston exhaust ports and the cylinder wall exhaust ports are configured to overlap during the entire stroke, allowing exhaust venting at any crank angle provided the exhaust valves are open. In a more preferred form, a conventional exhaust port opened by the piston just before BDC will also be used. This initiates exhausting in the event that cylinder pressure has not dropped sufficiently to allow the piston head exhaust valves to open.  
         [0053]     The use of the such an exhaust valve arrangement with the inlet valve system of the present invention, which itself allows very early and sharp cut off, allows an engine to run very efficiently at virtually all load conditions. Indeed, the presence of both arrangements permits the engine to run at different displacements, effectively making it a variable displacement engine. Furthermore, the cylinder can of course be sized such that full expansion of the gas occurs at BDC when operating at full load, which would provide maximum efficiency. Then, at part loads the amount of inlet gas may be reduced such that full expansion occurs before the piston reaches BDC.  
         [0054]     With this embodiment of the invention, the piston head exhaust valves would open so that gas could flow in the reverse direction through the valves (that is, into the expansion volume above the piston), thus avoiding doing work to generate a partial vacuum and again maintaining efficiency.  
         [0055]     The piston head exhaust valves may be any suitable valves, although it is preferable that they be of a type that is not unduly influenced by the inertia forces generated as a result of the acceleration of the piston. Also, the exhaust valves should be of a type that ensures that the system of closing the valve at TDC does not lead to wear or damage of the valves.  
         [0056]     The piston head exhaust valves will thus preferably be springs, and will ideally be reed valves. However, other arrangements could be used, such as poppet valves with compression coil spring arrangements.  
         [0057]     Additionally, leaf springs may be used at the head of the cylinder to assist in closing the reed valves and also to cushion the impact of the piston head exhaust valves on the cylinder head. Whilst this impact is cushioned somewhat by the gas that must be expelled from between the faces of the reed valves and the leaf springs as they come into contact, other options to cushion this impact may be used, such as the use of fluid jets emanating from the cylinder head, or a fluid coating on the springs themselves may assist in prolonging the life of the reed valves.  
         [0058]     From the above general description, it can be seen that the working fluid inlet system of the present invention provides a simple solution to the operation and control problems that have been associated with many types of reciprocating engines for many years.  
         [0059]     In particular, the system of the invention is particularly useful as the inlet valve system for a Rankine cycle heat engine that uses steam as its working fluid to drive a piston. It permits an efficient reciprocating steam engine to be built without the cost, complexity, weight and size of multiple expansion cylinders, because a high expansion ratio can be achieved in one cylinder by providing early cut off.  
         [0060]     A further advantage is that the valve timing may be fully programmable. Indeed, unlike many mechanisms, the timing of the admission and cut-off of working fluid to the expansion chamber can be varied independently and over a wide range, without the need for complex mechanisms. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0061]     The present invention will now be described with reference to a preferred embodiment illustrated in the accompanying drawings. However, it is to be understood that the following description is not to limit the generality of the above description.  
         [0000]     In the drawings:  
         [0062]      FIG. 1  is a perspective view of a reciprocating engine incorporating a working fluid inlet system in accordance with a preferred embodiment of the present invention;  
         [0063]      FIG. 2  shows a cross-section through the reciprocating engine of  FIG. 1 ;  
         [0064]      FIG. 3   a  is an exploded view of a part of the cross-section of  FIG. 2 , with the piston nearing TDC;  
         [0065]      FIG. 3   b  is an exploded view of a part of the cross-section of  FIG. 2  with the piston moving away from TDC and towards BDC;  
         [0066]      FIG. 3   c  is an exploded view of a part of the cross-section of  FIG. 2  with the piston approaching BDC;  
         [0067]      FIGS. 4   a  and  4   b  are schematics of a first alternative pilot valve and inlet valve arrangement respectively for use with an embodiment of the present invention;  
         [0068]      FIG. 5  is a schematic of a second alternative pilot valve and inlet valve arrangement for use with an embodiment of the present invention;  
         [0069]      FIG. 6  is a perspective view of a piston adapted in accordance with a further embodiment of the present invention;  
         [0070]      FIGS. 7   a  to  7   d  are exploded views of part of the cross-section of  FIG. 2 , sequentially showing the operation of the piston of  FIG. 6 ; and  
         [0071]      FIG. 8  is an exploded view of a part of the cross-section of  FIG. 2  showing a further embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0072]     Illustrated in  FIG. 1  is a reciprocating engine  10  that operates on the Rankine cycle and uses steam as its working fluid. The engine  10  is not illustrated with all of the components necessary for operation, as will be explained shortly.  
         [0073]     The engine  10  generally includes a boiler  12  suitable to generate the steam necessary for use as the working fluid and, for the preferred inlet system of the present invention, the secondary fluid. In this respect, a skilled addressee will appreciate that suitable flow passages for all aspects of the engine are not necessarily visible in all of the Figures. For example, a flow passage from the boiler  12  to the pilot valve in subsequent Figures is not evident in all cross-sections in the Figures, but of course is present in the engine.  
         [0074]     The engine  10  includes a reciprocating piston in a cylinder, with a variable volume expansion chamber, shown generally by reference numeral  14 . The reciprocating piston is operatively connected to an electrical generator  16  via a crankshaft  28  (not completely shown in  FIG. 1 ).  
         [0075]      FIG. 1  also shows parts of the engine that are unrelated to the present invention, such as the solenoid  22  and the injector pump  24  that regulate the flow of water into the boiler  12 , together with several heat transfer vanes  26  that are associated with the TDC end of the cylinder.  
         [0076]     In relation to the inlet system of the illustrated embodiment of the present invention, all that is evident from  FIG. 1  is the presence of various aspects of the actuating means that controls the operation of the pilot valve. In particular,  FIG. 1  shows the solenoid  1 - 8  and the timing disc  20 , the timing disc  20  being operatively connected to the crankshaft  28 . However, in  FIG. 2  the timing disc  20  is better illustrated than in  FIG. 1 , in that its operative connection to the crankshaft  28  is apparent. Also, the cylinder  30  within which the piston  32  is configured for reciprocating movement (in the normal manner) is more apparent in  FIG. 2  than in  FIG. 1 .  
         [0077]     The elements such as the boiler  12 , the generator  16 , the vanes  26 , and the water inlet solenoid/valve arrangement  22124  are all also evident in  FIG. 2 , but will not be described in further detail. Indeed, with regard to the configuration and operation of the piston  32 , the cylinder  30 , the crankshaft  28 , the generator  16 , and their associated engine parts, these will be well understood by a skilled addressee and will not be described in further detail. These elements do not form an essential part of the inlet system of the present invention.  
         [0078]     However, the interaction and configuration of the elements within the area marked A in  FIG. 2  are important to the present invention and will now be described in further detail in conjunction with the illustrated elements of the actuating means of the present embodiement, namely the timing disc  20  and the solenoid  18 .  
         [0079]     The inlet system of the present embodiment is best illustrated in  FIGS. 3   a ,  3   b  and  3   c . In this respect, although these figures provide a sequential illustration of the inlet system (and engine) in different conditions, most of the elements of the inlet system are common to each figure. It is thus suitable to describe those common elements before describing the sequential operation.  
         [0080]     Referring simply to  FIG. 3   a , the solenoid  18  is operatively connected to a pilot valve that is shown in the form of a poppet valve  34 . The poppet valve  34  can be opened by the retraction of the solenoid&#39;s plunger  37  (in association with the link member  35 ) against a closing force provided by a spring  36 . When in its open condition, the poppet valve allows passage of secondary fluid (steam) into the chamber  38  of the inlet valve  40 , which in this embodiment is also a poppet valve. Additionally, steam is able to be fed to, for instance, an injector (not shown) via passage  45 .  
         [0081]     When the secondary fluid enters the chamber  38 , its pressure unseats the poppet  42  and thus opens the inlet valve  40  against a closing force provided by a spring  44 . Working fluid (steam) is then able to enter the cylinder pre-chamber  46  via steam feed-lines  48  from the boiler  12 .  
         [0082]     When the solenoid  18  is de-energised, the closing force of spring  36  closes the poppet valve  34 , shutting off the steam to the inlet valve chamber  38 , which in turn allows the closing force of spring  44  to shut off steam to the expansion chamber. In this respect, it should be noted that steam is able to exhaust from the inlet valve chamber  38  via a port  39  to a system condenser, as necessary.  
         [0083]     In relation to the timing of the operation of the solenoid  18 , and returning to  FIG. 1 , the timing disc  20  includes two upper protrusions  52  and  54  and a lower protrusion (not shown) on the underside of the disc about 30° around from protrusion  52 .  
         [0084]     Sensors  56  and  58  sense the protrusions as the timing disc rotates with the crankshaft  28 . Protrusion  54  passes sensor  56  at TDC (as is evident by the position of the piston  32  in  FIG. 2 ), whilst protrusion  52  passes this sensor 90° before TDC. The times of these protrusions passing these points are recorded as dynamic parameters in a control module (which may include a microcontroller), which is a part of the actuating means of the present invention.  
         [0085]     The control module, as mentioned above, is then able to calculate the appropriate time to energise the solenoid, in light of the known delay time of the solenoid due to its inductance, and the inertia and pressure forces of the pilot and inlet valves, to open the inlet valve at or near TDC as required. With appropriate programming of suitable set and dynamic parameters, the control module will do this accurately despite fluctuations in speed over the cycle, and despite increases or decreases in the speed of the engine.  
         [0086]     The lower protrusion (not shown), passes sensor  58  at some time after TDC (in this embodiment, at about 30°). This assists the control module to determine the time to de-energise the solenoid  18  to close the inlet valve, again in light of known delay times. In this respect, it will be appreciated that angles smaller or larger than 30° could be used in order to provide large and small expansion ratios respectively.  
         [0087]     Referring now to the sequential comparisons between  FIGS. 3   a ,  3   b  and  3   c , the basic operation of the engine becomes clear.  
         [0088]     As already mentioned,  FIG. 3   a  shows the piston  32  nearing TDC (or having just arrived at TDC) in the cylinder  30 . The solenoid  18  is de-energised such that the pilot valve is in its closed condition by virtue of the spring  36  having closed the poppet valve  34 . Secondary fluid (steam) is thus denied to the inlet valve  40  and working fluid is thus denied to the expansion chamber.  
         [0089]     In  FIG. 3   b , the solenoid  18  has energised to open the poppet valve  34  against the closing force of the spring  36 , allowing steam to enter the inlet valve chamber  38 . This steam has opened the inlet valve  40  against the closing force of its spring  44  to permit working fluid (steam) to enter the expansion chamber via pathways  43 . In  FIG. 3   b , the expansion of this steam has urged the piston away from TDC (towards BDC) on its expansion (power) stroke.  
         [0090]     In  FIG. 3   c , the solenoid  18  has again de-energised to close the inlet valve  40  during the last of the expansion stroke and for the entire return stroke.  
         [0091]     Illustrated in  FIGS. 4   a ,  4   b  and  5  are alternative pilot valve and inlet valve arrangements that are also suitable for use with the inlet system of a preferred embodiment of the present invention.  
         [0092]      FIG. 4   a  shows a pilot valve in the form of a spool valve  60 . The cylindrical spool  62  is actuated by a solenoid (or another suitable mechanical, electromagnetic, or piezoelectric actuator) at X against the return force of a resilient means in the form of a spring  64 . In  FIG. 4   a , the spool valve is shown in its closed condition, preventing entry of secondary fluid (steam) into inlet port  64  and then to the outlet port  66 .  FIG. 4   a  also illustrates the preferred overlapped configuration of the central spool  65  with respect to the stepped entry  67  to the outlet port  66 , which avoids any short-circuiting between the inlet port  64  and the low pressure return port  68 .  
         [0093]     Once energized, the solenoid moves the spool valve to its open condition that, in terms of  FIG. 4   a  is to the left of the page, allowing the secondary fluid (steam) to pass therethrough. Upon de-energisation, and upon the return of the spool valve to its closed condition, remaining steam in the valve exhausts via the low pressure return port  68 .  
         [0094]      FIG. 4   b  shows an inlet valve, also in the form of a spool valve, which operates in a similar manner. However, the spool valve  70  is actuated by the inflow of secondary fluid (steam) to the chamber  72  from the outlet port  66  of the pilot valve.  
         [0095]     Again, the spool valve  70  is opened against a return force provided by a resilient means in the form of a spring  74 . The high pressure working fluid (steam) enters the spool valve  70  via inlet port  76  when in its open condition, and travels through the spool valve  70  to the outlet port  78  for entry to the working chamber of the cylinder of the engine.  
         [0096]     The arrangement illustrated in  FIG. 5  differs from the arrangement in  FIGS. 4   a / 4   b  by the replacement of the spool arrangement of the pilot valve with a flapper arrangement The flapper arrangement  82  includes a flapper  84  that swings between opposing nozzles  86 ,  88  due to a continuous stream of secondary fluid (steam) entering via inlet pressure drop orifices  90 ,  92 .  
         [0097]     Each nozzle  86 ,  88  communicates with a respective chamber  94 ,  96  at each end of the inlet valve, which is itself a spool valve  98  of the same general type as described above. In this arrangement, the cylindrical spool  100  is held central by respective resilient means in the form of springs  102 ,  104 .  
         [0098]     As the back pressure of the nozzles  86 ,  88  differs when the flapper  84  is in a non central position, the flapper itself being electro-magnetically driven by coils  106 ,  108 , the spool  100  is pushed from one side to the other against the centering force of the springs  102 ,  104  by the pressure imbalance.  
         [0099]     Alternatively, instead of the use of the centering springs  102 ,  104  at each end of the spool  100 , a centering feedback spring connected to the flapper may be used.  
         [0100]     As will be appreciated, there are various advantages and disadvantages of the different valve arrangements and combinations described in  FIGS. 4   a ,  4   b  and  5 , which will usually dictate, for particular applications, which configurations will be most suitable.  
         [0101]     Referring now to the further embodiment illustrated in  FIG. 6 , illustrated is a piston adapted to include exhaust valves in its head, the exhaust valves being in the form of reed valves  33  associated with exhaust ports  35 . In this form, the piston mounted exhaust valve operating sequence is preferably as follows: 
        1. As the piston travels downwards under the force of expanding gas above it (as shown in  FIG. 7   a ), the pressure will gradually drop until the pressure differential above the exhaust port pressure is not sufficient to hold the reed valves closed. At this point, the reed valves will open, which at full load operation will occur just before BDC. It will be noted that opening of these valves is assured by exhaust ports  37  in the cylinder wall opening (or becoming accessible) just before BDC. If the gases have not fully expanded, this can cause the pressure drop required for the reed valves to open.     2.  FIG. 7   b  shows the piston just before BDC but before the cylinder wall exhaust ports  37  have been exposed, with the reed valves  33  already open.     3.  FIG. 7   c  shows the piston at BDC with the reed valves  33  open.     4. As the piston travels upwards from BDC, the reed valves  33  stay open, allowing all of the gas above the piston to vent through it and out through the ports  37  without a substantial build up of pressure.     5. As the piston nears TDC, leaf springs  139  mounted on the cylinder head (or integral with the head itself contact the reed valves  33 , causing the reed valves  33  to close at or before TDC, as illustrated in  FIG. 7   d . If the reed valves  33  close before TDC, some compression of the remaining gases will occur.     6. At this stage, the inlet valve will be open and high pressure gas will enter the relatively small compression volume. As the piston moves away from TDC this gas will hold the reed valves  33  shut, enabling the gas to work against the piston on its downward stroke.        
 
         [0108]     It will be appreciated that this valve arrangement allows maintenance of full uni-flow operation.  
         [0109]     Illustrated in  FIG. 8  is a further embodiment, related to the recovery of energy from the inlet valve system, particularly from the operation of the pilot valve and the secondary fluid used to actuate the inlet valve. In this respect, it will be appreciated that the energy used to operate the inlet valve can be significant.  
         [0110]     Often the inlet valve will be actuated (via the pilot valve) using a high pressure (secondary) fluid. Where this secondary fluid is compressible, its use may occur without appreciable expansion of the fluid, and some of this energy can be recovered by venting this fluid into the expansion chamber of the cylinder when the inlet valve closes. Ideally, this coincides with the early part of the expansion stroke, allowing the additional fluid to do work against the piston.  
         [0111]      FIG. 8  shows an arrangement that vents the secondary fluid into the expansion chamber. When the pilot valve closes, the secondary fluid above the pilot valve exits via a pilot valve exhaust port  120  and then passes via a check valve  122  into the expansion chamber. As the expansion chamber is at high pressure at this time, this may hinder the closing the inlet valve. To assist in preventing this, an additional volume is connected to the exhaust passage upstream of the check valve. This will allow the gas to expand to an intermediate pressure immediately, allowing the inlet valve to shut as required.  
         [0112]     When the pressure of the gas in the expansion chamber has dropped sufficiently, this stored gas will then start to exit via the check valve into the expansion chamber.  
         [0113]     Finally, it will be appreciated that there may be other variations and modifications made to the configurations described herein that are also with the scope of the present invention.