GB2365076A - Steam engine having a cylinder block steam jacket, accumulator and upright tubed boiler - Google Patents

Abstract

A steam engine cylinder block 2 comprising a higher pressure cylinder, lower pressure cylinders and a cruciform conduit 38 leading higher pressure cylinder exhaust steam to the lower pressure cylinder. The cylinders and conduit are steam jacketed to re-heat steam during use, as are valve chests. The cylinder block 2 is mounted upon a crank case 32 by legs 68 which separate the cylinder block from the crank case and by a layer of thermal insulation. The steam engine has a steam supply comprising an accumulator 102 in the form of a pressure vessel which is part filled with water in use, a boiler with upright finned tubes 112 forming a water walled combustion chamber, a path through which accumulator water is input to the boiler 106, a path through which boiler steam output is led into the water 120 in the accumulator, and a steam take-off 118 from the accumulator to supply the engine.

Description

2365076

DESCRIPTION STEAM ENGINE CYLINDER BLOCK, STEAM ENGINE AND STEAM SUPPLY

The present invention is concerned with steam engines. In particular, the invention concerns a steam engine cylinder black, a steam engine as such and a steam supply.

Compound steam engines are well known. In such engines boiler steam is led via a valve to a high pressure (BP) cylinder. Steam exhausted from the HP cylinder, again under the control of the valve and at reduced pressure but still carrying usable energy, is passed via a transfer conduit and a further valve chest to a low pressure (LP) cylinder. Since the volume of steam reaching the LP cylinder is greater than the volume input to the HP cylinder (due to expansion in the HP cylinder), the LP cylinder may have a larger cross section than the HP cylinder or may be one of a set of LP cylinders such as to make up a total LP cylinder cross section greater than that of the HP cylinder.

The steam does useful work in both HP and LP cylinders. The expansion ratio of the steam is increased (as compared with a non-compounded engine) and compound engines consequently offer potentially high efficiency.

One problem often experienced in practice (and not unique to compound engines) is that of condensation. Steam leaves the cylinders at reduced temperature and consequently can condense, particularly upon any component of the steam's flow 2 path which is not sufficiently hot. This is thermally inefficient and also a potential source of rust and mechanical inefficiency.

Avoiding condensation is one reason for the well known practice of steam jacketing. Briefly explained, a steam jacket receives a supply of steam from the boiler and serves to transfer heat to the cylinders, and in some cases also to the valve chests. By keeping the components through which steam is passed at a sufficiently high temperature, steam jacketing alleviates or altogether overcomes condensation problems.

Figures 1 a and 1 b illustrate a known, and in its day successful, cylinder block fora steam jacketed compound engine. This was in fact designed by Charles Burrell & Co., in about 1900 -1910. The high pressure cylinder is marked HP, the low pressure cylinder is marked LP and cavities within the block supplied with boiler steam, to perform the jacketing function, are marked S. It can be seen that steam heating of the cylinder block is achieved by having core holes in the casting supplied with boiler steam. The block was cast in grey iron and only sufficient jacketing was provided to enable the engine to run dry - i.e. without condensation. In fact the intention of steam jacketing has in the past been simply to prevent liquid water from being present in the exhaust. Such water can be condensation on the engine parts eg. cylinder walls - and can take the form of a mist of droplets in the steam.

The inventor has recognised that in such engines there was little possibility of re-heating of steam between HP and LP cylinders.

Nonetheless engines of the type illustrated in Fig. 1 prove efficient, in 3 practice, by comparison with simple (non-compounded engines and even with superheated engines.

An object of the present invention is to make possible a steam engine of improved energy efficiency.

In accordance with a first aspect of the present invention, there is a steam engine cylinder block comprising a higher pressure cylinder, a lower pressure cylinder and a transfer conduit through which steam exhausted from the higher pressure cylinder is led to drive the lower pressure cylinder, the cylinders being steam jacketed and the transfer conduit being steam jacketed such that the exhausted steam is re-heated during passage through the transfer conduit.

It is particularly preferred that the cylinder block is such as to maintain the steam at or near boiler steam temperature throughout its cycle in normal operation. Heat input to the steam during its cycle, due to the steam jacketing, largely offsets the drop in temperature due to expansion in the cylinders.

It is normally considered that the theoretical maximum efficiency is limited by the Camot equation:

71 = li.---:'.T ' Tin where il is efficiency and Tin and T,,,,, are the input and exhaust temperatures of the working fluid (in this case, the steam).

In the past much attention has focussed on superheated steam engines in which the temperature Tin of input steam is very high. Carnot's equation thus allows 4 high theoretical efficiency for such engines, but in practice increased losses due to conduction of heat (increased at high operating temperatures) and, despite the high temperatures, by condensation largely offset any increase due to superheating.

However, Carnot's equation is derived on the basis that expansion of the working fluid is adiabatic. In the engine according to the present invention the input of heat to the steam during its cycle, as a result of the steam jacketing, means that the working cycle is non-adiabatic so that Carnot's equation does not apply. Thermal efficiency is consequently not limited by the above equation.

The efficiency of all steam engines in practice falls a long way short of Carnot's theoretical maximum and avoiding unnecessary thermal inefficiency is of paramount importance. The steam jacketing of the cylinder and transfer conduit assists in this regard.

Trials on engines modified according to the present invention suggest that very high efficiency can be attained in practice.

The steam engine according to the present invention is preferably adapted to run on high pressure input steam. Raising working pressure allows Tin to be increased without extra losses and tests show that efficiency is consequently increased.

In an especially preferred embodiment of the present invention, the steam jacketing is provided by means of a hollow steam chest around the cylinders and transfer conduit, means being provided for connection of the interior of the steam chest to a source of steam. In such a chest, unlike that illustrated in Fig. 1, steam can flow freely around exterior surfaces of the transfer conduit and the cylinders. This allows for particularly effective transfer of heat to the transfer conduit and cylinders.

Most preferably, the transfer conduit comprises a pipe whose exterior is exposed to steam within the steam chest. Preferably, substantially the entire exterior of the pipe is exposed to the steam. Again, this construction facilitates transfer of heat to the transfer conduit and its contents, and consequently makes for effective reheating between cylinders.

It is especially preferred that steam supply and exhaust for the cylinders is controlled through respective valves comprising valve chests which are steam jacketed. Most preferably the steam jacketing is achieved by disposing the valve chests in the steam chest. Exteriors of the valve chests are preferably exposed to steam in the steam chest. This again facilitates heat transfer.

Preferably steam supply and exhaust conduits leading to and from the valve chests are formed as pipes extending through the steam chest and having exteriors exposed to steam in the steam chest. This again promotes heat transfer. The pipes can be wide and flat to maximise exposed area.

It is especially preferred that the transfer conduit comprises a cruciform arrangement having a central passage for receiving steam exhausted from the higher pressure cylinder and outputs to the lower pressure cylinders at its extremities. The cruciform transfer conduit is again preferably formed by pipes whose exteriors are exposed to steam in the steam chest.

Correspondingly, the cylinder arrangement preferably has the higher pressure 6 cylinder disposed between two lower pressure cylinders.

The steam chest is preferably formed as a box-like enclosure.

Preferably the cylinder block is thermally insulated from other structural components of the steam engine to reduce conduction of heat from the cylinder block.

It is especially preferred that all components of the cylinder block are steam jacketed.

The steam jacket is preferably provided with a drain for receiving condensate from the steam jacket, the drain being led to the cylinder steam input path such that the condensate is evaporated and re-circulated.

The cylinder block according to the present invention is preferably coupled to the crank case of a steam engine through a thermally insulating mounting.

In accordance with a second aspect of the present invention, there is a steam engine comprising a cylinder block mounted upon a crank case through a mounting structure which separates the cylinder block from the crank case.

The mounting structure preferably comprises one or more mounting limbs bridging a space between the crank case and the cylinder block.

The inventor has recognised that important heat loss - and consequent thermal inefficiency - arises in many known engines through heat flow from the cylinder block to the crank case. By separating the two, this heat loss can be reduced.

It is particularly preferred that the mounting limbs are provided with thermal insulation to impede heat flow to the crank case. Heat loss by this route is thereby 7 further reduced.

The reduced heat flow to the crank case results in a lower operating temperature in the crank case, making it unnecessary to use a specialist high temperature lubricant.

In such an arrangement piston rods and/or valve rods preferably extend across the separation between the crank case and the cylinder block. Where the piston andlor valve rods emerge from the cylinder block they are preferably provided with sealing glands. The separation of the cylinder block from the crank case enables easy access to and maintenance of the glands, which require occasional adjustment. Due again to the separation of the crank case from the cylinder block, only light duty glands are required where the piston and/or valve rods pass into the crank case. These glands prevent ingress of water which would otherwise cause the lubrication system to deteriorate.

It is particularly preferred that the cylinder block of the steam engine is constructed according to the first aspect of the present invention.

Steam engines according to the present invention can be of up to 10,000 horsepower or more.

In accordance with a third aspect of the present invention, there is a steam supply comprising an accumulator in the form of a pressure vessel which is part filled with water in use, a boiler, means defining a path through which water from the accumulator is input to the boiler, means defining a path through which steam output from the boiler is led into the water in the accumulator, and a steam take-off through 8 which steam can escape the accumulator to supply an engine.

Use of an accumulator in this manner has several advantages. Particularly important is the fact that it becomes unnecessary to use a bulky conventional shell boiler. A boiler can be used whose output is a frothy mixture of water and steam. In the accumulator, this is separated - steam in the space above the water is taken off to drive the engine, while water remains in the accumulator and is eventually recirculated to the boiler.

In the steam supply according to the present invention, the accumulator contains the mass of water required to make the system stable and controllable.

In a particularly preferred embodiment the accumulator is provided with thermal insulation and with cut off valves at its input and output. In this manner the heat energy in the water can be stored (with a large accumulator, heat can be retained for several days) giving rapid starting of the plant when required.

The boiler is preferably a flash boiler.

Preferably the boiler comprises water conduits arranged to form a water walled combustion chamber.

A particularly preferred boiler for use in the steam supply according to the present invention has a set of upright fin tubes each connected between a lower manifold and an upper manifold, the boiler having a water input through the lower manifold and a steam take off to the accumulator from the upper manifold. The fin tubes may be arranged along a loop effectively forming a water-walled combustion chamber.

9 This format of boiler encourages controlled, fast circulation. It is ideal for a high pressure environment and provides excellent heat transfer characteristics. A further advantage where water quality is less than optional is that given sufficiently fast circulation the tubes can be self cleaning.

Preferably an economizer is fitted in an upper region of the boiler. Economisers are known to those skilled in the art and serve to pre-heat feed water, using radiant heat and a proportion of the convective heat in combustion exhaust gases. Thermal efficiency is thus improved.

It is particularly preferred that the steam supply according to the third aspect of the present invention be connected to a steam engine according to the second aspect of the present invention or a steam engine having a cylinder block according to the first aspect of the present invention. The steam supply is well suited to driving such a high efficiency engine and the resulting arrangement can be both thermally efficient and compact.

The steam supply is preferably adapted to supply high pressure steam. This is an important factor in achieving high thermal efficiency and the steam supply according to the present invention allows high pressure steam to be generated without the problems - and dangers - involved in running shell boilers at high pressures.

A specific embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figs. la and lb are respectively vertical and horizontal sections through a cylinder block of known design; Figs 2 and 3 illustrate, in perspective and from respective different angles, a cylinder block assembly and part of a crank case of a steam engine embodying the invention, a steam chest of the engine being shown cut-away; Fig. 4 corresponds to Fig. 3 except that the steam chest is not cut-away in this drawing and consequently the cylinder block cannot be seen; Fig. 5 corresponds to Fig. 3 except that in this drawing a cylinder and a valve chest are cut away to reveal their interiors; Fig. 6 is a schematic illustration, from the side, of a steam supply arrangement according to an aspect of the present invention; Fig. 7 is a schematic illustration of a boiler used in the same steam supply arrangement, viewed from above; Fig. 8 is a simplified view of the cylinder block in plan showing the general layout of the cylinders and valve chests; and Fig. 9 is a corresponding simplified side view of the cylinder block showing the arrangement of a transfer conduit.

In Figs. 2 to 5 it can be seen that the cylinder block 2 and associated parts are housed in a generally box-like hollow steam chest 4 formed by walls 6. Leading into the steam chest from above is a steam chest input pipe 8 which supplies steam from the boiler to the interior of the steam chest. A steam chest drain, seen emerging from the steam chest at 10, receives any condensate.

Seen in Figs. 2 - 5 is a row of three cylinder covers at the steam chest exterior corresponding to three cylinders within the steam chest. Middle cylinder cover 12 correspond to the middle, high pressure (RP) cylinder while the two outer covers 16 corresponds to outer, low pressure (LP) cylinders. In perspective drawings 2 - 5 only one of the LP cylinders themselves, labelled 18, can be seen. However, the layout of the cylinders is made clear by the simplified plan view of Fig. 8 in which the outer, LP cylinders are again labelled 18 and the middle, HP cylinder is labelled 20.

Fig. 8 also makes most clear the layout of three valve chests. These are arranged in a row, correspondingly to the three cylinders. The middle of the three valve chests, labelled 22, controls high pressure steam to and exhaust from the HP cylinder 20. The two outer valve chests, labelled 24, control lower pressure steam input to and exhaust from the respective low pressure cylinders 18.

In Fig. 5 can be seen a piston 26 of one of the LP cylinders 18. The other pistons, although not seen, are similarly formed. Descending from and securely coupled to each of the pistons is a respective piston rod 28. In Fig. 2 it can be seen that each piston rod 28 exits the steam chest downwardly through a respective sealing gland 30 and passes into a crank case 32. Inside the crank case the piston rods 28 drive a crank. The crank mechanism is of well known type and is not itself illustrated herein.

Steam to drive the engine is input through a pair of high pressure boiler steam input pipes 34,36. These are arranged in the middle of the steam chest one above the other, to supply steam via the middle, HP valve chest 22 to the HP cylinder 20. The input pipes are in the illustrated embodiment circular in section, although pipes of other sections could be used. Inside the steam chest 4, exteriors of the steam input 12 pipes 34, 36 are exposed to steam in the chest.

Steam is exhausted from the HP cylinder 20 via the HP valve chest 22 (which, with the associated valve gear, to be considered below, controls both input and exhaust of the steam) to a low pressure transfer conduit 38. In the illustrated embodiment the transfer conduit 38 receives steam from a single, middle, exhaust (labelled 40 in Fig. 9) from the HP valve chest 22 and transfers it to respective upper and lower inputs of the LP valve chests 24. To achieve this the transfer conduit 38 has a diagonal cruciforin configuration. The transfer conduit 38 is formed by pipe work whose exterior is exposed to steam within the steam chest.

Looking now at Fig. 5, the interior of one of the LP valve chests 24 can be seen. The valves are piston valves each having a pair of valve pistons 42, 44 mounted upon a valve rod 46 and running in a valve chest cylinder bore 48.

Annular upper and lower valve inlet ports 50, 52 are formed around the bore 48, being open toward the bore and in flow communication with respective limbs of the cruciform transfer conduit 38 to supply low pressure steam to upper and lower regions of the valve bore 48. Mid way between the valve inlet ports is an annular valve exhaust port 54 communicating with an exhaust pipe 56. Above and below the exhaust port 54 are upper and lower annular supply ports communicating respectively with upper and lower steam passages 58, 60 through which steam is supplied to and exhausted from upper and lower regions of the LP cylinder 18.

As the skilled person will be well aware, the valve rods and pistons 42, 44, 46 are reciprocally driven while the engine is running and by closing/opening the 13 valve ports in the course of this reciprocal motion control input and exhaust of the low pressure steam to drive the piston 26 in a double acting manner (i.e. the piston is driven in both directions, by steam input to alternate sides of the piston).

In Fig. 2 it can be seen that each of the valve rods 46 exits the steam chest 4 downwardly through a respective sealing gland 62 and passes into the crank case 32. The mechanism for reciprocally driving the valve rods is contained in the crank case. The skilled reader will be aware of suitable known mechanisms, e.g. utilising an eccentric driven from the crank shaft, and these parts are not illustrated.

While only one valve is illustrated in detail, the remaining valves are of similar construction. A second LP exhaust pipe, labelled 64, is associated with the other LP valve. Exhaust from the middle, HP, valve is into the HP exhaust 40, as has been mentioned above.

The shapes and arrangement of the valve chests 22, 24, the input pipes 34, 3 6, the transfer conduit 38, the supply passages 58, 60 and the exhaust pipes 56, 64 are all designed to provide large surface areas exposed to the steam which circulates freely in the steam chest 4. Thus, it can be seen for example that the valve chests have ridges (an example is labelled 66 in Fig. 2) to accommodate the valve ports and supply passages, these ridges providing a large surface area for exchange of heat between the valve chest steam and the working steam. Substantially the entire exteriors of both cylinders 18, 20 and valve chests 22, 24 are also exposed to steam in the chest to promote heat transfer.

The inventor has calculated that the illustrated design provides fifteen times 14 more surface area exposed to jacket steam than known jacketed engines.

The assembly comprising the cylinders, valves and the steam chest 4 is mounted to the engine's crank case 32 through a pair of limbs formed as mounting legs 68, thereby providing a gap between the steam chest 4 and the crank case 32. In order to minimise heat transfer from the steam chest 4 to the crank case 32 through the legs 68 a layer of thermal insulation may be incorporated e.g. between mounting lugs 70 of the legs and the crank case 32 to which they are bolted.

To facilitate starting of the engine there is provided a starter input 72 which communicates with the transfer conduit 38 and which is connectable via a cut off valve (not illustrated) to boiler steam. On starting of the engine, boiler steam is admitted through the starter input 72 directly to the LP valve chests 24/cylinders 18. That is, the LP cylinders are driven by high pressure boiler steam. Not only does this make large torque temporarily available (due to the large total cross section of the LP cylinders) but since all three cylinders are driven upon starting it can be ensured that at least one cylinder is always away from top or bottom dead centre and so can provide initial motion.

The stearn chest drain 10 can be seen to be connected to the starter input pipe 72 so that condensate in the chest is evaporated and passed as steam to the LP cylinders 18. Hence the condensate's heat energy is used rather than being lost, as it would if the condensate were vented to the exterior.

The steam supply will now be described with reference to Figs. 6 and 7. The major components of the steam supply are a boiler 100 and an accumulator 102.

The boiler 100 is formed in the illustrated embodiment from tubes. A lower manifold 104 of the boiler is formed as a loop (a square loop in the embodiment depicted although it could for example be a circular loop) of tubing and has a water inlet 106. An upper manifold 108 is formed as a corresponding loop and has a steam take-off or outlet 110. Between the upper and lower manifolds 104, 108 is a set of upright fin tubes 112. Each fin tube communicates (at its upper and lower ends respectively) with the upper and lower manifolds 104, 108. The fin tubes are provided around the full perimeter of the manifolds thereby forming a combustion chamber. A wall is thus formed around the combustion chamber which is filled with water/steam. The arrangement proves highly thermally efficient.

The boiler 100 can be referred to as a flash boiler. Water input to the lower manifold 104 is rapidly evaporated and ejected as steam from the upper manifold 108.

The boiler's output is a frothy combination of steam and hot water. Before being led to the engine itself, this output is let to the accumulator 102.

The accumulator comprises a hollow pressure vessel 114 having a steam inlet 116 and a supply outlet 118. In use the accumulator is part filled with water. The water level is marked 120 in Fig. 6. Steam entering through the steam inlet 116 is directed below the water level by a steam conduit 122 and escapes through stub outlets 124, which are directed downwardly, into the water.

The supply outlet 118 is above the water level and so delivers only steam to the engine.

16 The accumulator also has a water outlet 107 through which water, already warmed by the steam from the boiler, is supplied from the accumulator to the water inlet 106 of the boiler.

The combustion chamber of the boiler can be fitted, at the bottom, with a grate for solid fuel or with an arrangement for burning liquid or gaseous fuels.

The combustion chamber is provided in an upper region with an economiser which uses radiant heat and heat carried convectively by the exhaust gases to pre-heat feed water thereby increasing efficiency.

The gases emerging from the economiser are exhausted via any ecologically desirable screen, scrubber or filter mechanism are exhausted to either a stack or an exhaust steam device to produce the necessary draught. The low impedance of flow in the combustion chamber means the necessary draught is low. This in turn means that environmental control devices can be fitted to the exhaust without affecting the performance of the engine.

Because of the high efficiency of the engine and the much improved heat transfer in the combustion chamber, the required area for heat transfer is greatly reduced. This leads in turn to a more compact engine assembly.

The steam accumulator 102 is (optionally) fitted with valves to isolate the accumulator so pressure can be retained with the combustion chamber shut down. In this case, safety valve and fusible plug protection is fitted to the top manifold 108.

The relative height of the accumulator and top of combustion chamber should be set so that the combustion chamber is flooded at all times. Otherwise an auxiliary 17 circulating pump can be provided to circulate the water.

The steam supply is well suited to provision of high pressure steam to the engine, an important factor for high thermal efficiency. Output steam pressures are not expected to exceed 350 psi (2400 kPa). A typical working pressure could be 250 psi (1700 kPa).

The illustrated cylinder block is not for a superheated engine but the present invention could be applied to superheated engines.

Looked at as a whole, the combination of effective steam jacketing of all parts of the cylinder block, thorough insulation of the block from other structural components of the engine and high working pressure provides a highly efficient engine.

G:\CLIENn400-404\WWM402701\GB\MASTER.

Claims (37)

1. A steam engine cylinder block comprising a higher pressure cylinder, a lower pressure cylinder or cylinders and a transfer conduit through which steam exhausted from the higher pressure cylinder is led to drive the lower pressure cylinder, the cylinders being steam jacketed and the transfer conduit being steam jacketed such that the exhausted steam is reheated during passage through the transfer conduit.

2. A steam engine cylinder block as claimed in claim 1, wherein the cylinder block is such as to maintain the steam at or near boiler steam temperature throughout its cycle in normal operation.

3. A steam engine cylinder block as claimed in claim 1 or 2, adapted to run on high pressure input steam.

4. A steam engine cylinder block as claimed in claim 1, 2 or 3, wherein the steam jacketing is provided by means of a hollow steam chest around the cylinders and transfer conduit, means being provided for connection of the interior of the steam chest to a source of steam.

5. A steam engine cylinder block as claimed in claim 4, wherein the transfer conduit comprises a pipe whose exterior is exposed to steam within the steam chest.

6. A steam engine cylinder block as claimed in claim 5, wherein substantially the entire exterior of the pipe is exposed to steam.

7. A steam engine cylinder block as claimed in any of claims 1 to 6, wherein steam supply and exhaust for the cylinders is controlled through respective valves -19comprising valve chests which are steam jacketed.

8. A steam engine cylinder block as claimed in claim 7 wherein the steam jacketing is achieved by disposing the valve chests in the steam chest.

9. A steam engine cylinder block as claimed in claim 8, wherein exteriors of the valve chests are exposed to steam in the steam chest.

10. A steam engine cylinder block as claimed in claim 7, 8 or 9, wherein steam supply and exhaust conduits leading to and from the valve chests are formed as pipes extending through the steam chest and having exteriors exposed to steam in the steam chest.

11. A steam engine cylinder block as claimed in claim 10, wherein the pipes are wide and flat to maximise the exposed area.

12. A steam engine cylinder block as claimed in any of claims 1 to 11, wherein the transfer conduit comprises a cruciform arrangement having a central passage for receiving steam exhausted from the higher pressure cylinder and which outputs to the lower pressure cylinders at its extremities.

13. A steam engine cylinder block as claimed in claim 12, wherein the cruciform transfer conduit is formed by pipes whose exteriors are exposed to steam in the steam chest.

14. A steam engine cylinder block arrangement as claimed in any of claims 1 to 13, wherein the higher pressure cylinder is disposed between two lower pressure cylinders.

15. A steam engine cylinder block as claimed in any of claims 4 to 14, -20wherein the steam chest is formed as a box-like enclosure.

16. A steam engine cylinder block as claimed in any of claims 1 to 15, which is thermally insulated from other structural components of the steam engine to reduce conduction of heat from the cylinder block.

17. A steam engine cylinder block as claimed in any of claims 1 to 16, wherein all components of the cylinder block are steam jacketed.

18. A steam engine cylinder block as claimed in claim 17, wherein the steam jacket is provided with a drain for receiving condensate from the steam jacket, the drain being led to the cylinder steam input path such that the condensate is evaporated and re-circulated.

19. A steam engine cylinder block as claimed in any of claims 1 to 18, which is coupled to the crank case of a steam engine through a thermally insulating mounting.

20. A steam engine comprising a cylinder block mounted upon a crank case through a mounting structure which separates the cylinder block from the crank case.

21. A steam engine as claimed in claim 20, wherein the mounting structure comprises one or more mounting limbs bridging a space between the crank case and the cylinder block.

22. A steam engine as claimed in claim 20 or 21, in which the mounting limbs are provided with thermal insulation to impede heat flow to the crank case.

23. A steam engine as claimed in claim 20, 21 or 22, wherein piston rods extend across the separation between the crank case and the cylinder block.

24. A steam engine as claimed in claim 23, wherein where the piston and/or valve rods emerge from the cylinder block they are provided with sealing glands.

25. A steam engine as claimed in any of claims 20 to 24, having a cylinder block as claimed in any of claims 1 to 19.

26. A steam supply comprising an accumulator in the form of a pressure vessel which is part filled with water in use, a boiler, means defining a path through which water from the accumulator is input to the boiler, means defining a path through which steam output from the boiler is led into the water in the accumulator, and a steam take-off through which steam can escape the accumulator to supply an engine.

27. A steam supply as claimed in claim 26, in which the accumulator is provided with thermal insulation and with cut off valves at its input and output.

28. A steam supply as claimed in claim 26 or 27, in which the boiler is a flash boiler.

29. A steam supply as claimed in claim 26, 27 or 28, wherein the boiler comprises water conduits arranged to form a water walled combustion chamber.

30. A steam supply as claimed in any of claims 26 to 29 in which the boiler has a set of upright fin tubes each connected between a lower manifold and an upper manifold, the boiler having a water input through the lower manifold and a steam take off to the accumulator from the upper manifold.

31. A steam supply as claimed in claim 30, in which the fin tubes are arranged along a loop effectively forming a water-walled combustion chamber.

32. A steam supply as claimed in any of claims 26 to 3 1, in which an economizer is fitted in an upper region of the boiler serving to pre-heat feed water, using radiant heat and a proportion of the convective heat in combustion gases.

33. A steam supply as claimed in any of claims 26 to 32 connected to a steam engine as claimed in any of claims 20 to 25 and having an engine cylinder block as, claimed in any of claims 1 to 19.

34. A steam supply as claimed in any of claims 26 to 33, adapted to supply high pressure steam.

35. A steam engine cylinder block, substantially as hereinbefore described, with reference to and as illustrated in Figs 2 to 9 of the accompanying drawings.

36. A steam engine, substantially as hereinbefore described, with reference to and as illustrated in Figs 2 to 9 of the accompanying drawings.

37. A steam supply, substantially as hereinbefore described, with reference to and as illustrated in Figs 2 to 9 of the accompanying drawings.