1.  Field of the Invention
This invention relates generally engines and, more particularly, to rotary piston steam engine with balanced rotary variable inlet-cut-off valve and secondary expansion without back-pressure on primary expansion. 
2.  Description of Related Art
The “equal double rotary piston” mechanism was patented in France in 1832 but its potential has never been fully realised because of various problems which are now solved with a balanced rotary variable inlet cut-off valve and secondary expansion driven by the exhaust of the primary expansion in a manner which does not impart back-pressure against the primary expansion. 
FIG.  1 teaches the basic geometry of the “equal double rotary piston” mechanism.  There are two equal disc-like rotary pistons mounted on parallel shafts and housed within an expansion chamber closely fitted around the path traced by raised semicircular portions of the rotary pistons.  The raised semicircular surface of one rotary piston 1 and the non-raised semicircular surfaces of the other rotary piston 54 have a close approach at the central point of the expansion chamber.  The piston “faces” between the raised and non raised portion of the rotary pistons are of a suitable gear tooth profile.  The top of the raised cam-like portion of the rotary piston extends nearly 180°, this long distance providing good sealing despite absence of piston rings.  The two rotary pistons 1 and 2 are secured on two parallel drive shafts 10 and 20, each shaft being secured to a geared wheel 12, 22 external to the expansion chamber.  These two equal gear wheels engage and turn the rotary pistons in synchrony, at equal speeds but opposite directions.  Pressurised steam, (or any other working fluid), enters one side of the expansion chamber near the centre of the mechanism.  This fluid exerts a pressure on the driving face 24 of one of the pistons, the pressure being at approximately normal to the plane containing the axis of rotation and the radius that passes through the piston face.  In other words, the pressure is exerted at the near optimal orientation of the piston face, developing near maximum possible turning moment from the pressure.  The raised portion 16 of the other, non-driving, rotary piston 10 forms an abutment.  The pressure directed centrally is taken by the bearings on its shaft—without any expenditure of energy apart from frictional losses in the bearing as it turns.  One rotary piston 54 is driving for half a turn, while the other 1 is driven, the situation is then reversed for the second half turn—and so on. 
The mechanism is slightly similar to a single lobed gear pump operated in reverse as an engine.  However, because a single lobe would not produce continuous rotation the motion is maintained by an external set of gears. 
The two pistons 1 and 2 are of equal shape, unlike many other attempts at rotary piston mechanisms.  For this reason the mechanism can be conveniently described, although not fully defined, as the “equal double rotary piston” mechanism.  Fuller definition includes the raised portion of the rotary piston being a circular arc of nearly 180 degrees, fitting closely within the expansion chamber, as well as the two rotary pistons moving in close approximation as they rotate on parallel axles in opposite directions, synchronized by gears external to the expansion chamber. 
Advantages of This Engine: There are many advantages of the equal double rotary piston engine and very few weaknesses.  Overall it should be a far better engine than all current automotive engines. 
1.  The rotary pistons continually rotate in opposite directions thus functioning as flywheels and so conserving energy very efficiently.  The complete absence of energy wastage via reciprocating or oscillating movement, even in minor components, (such as valves or abutments), is a major energy conserving factor. 
2.  There is a near 100% power stroke of the “cycle”—in contrast to the 25% power stroke of a four stroke internal combustion engine. 
3.  The resolution of forces in a reciprocating piston and crank means that the piston and con-rod act only very briefly in a near optimal orientation to produce rotation. ( The optimal position is when the piston and con-rod apply all their forces in a straight line, and when this is also at right angles to the arm of the crank.  This is approximated only briefly each cycle, but never fully satisfied in finite sized crank engines.)  In contrast, the equal double rotary piston engine always applies force to the piston face is at nearly right angles to the rotating shaft, (depending on the slope of the gear profile), producing near optimal turning moment nearly 100% of the time.  It is also vastly superior to the Wankel rotary mechanism. 
4.  Converting reciprocating motion into rotary motion via con-rods and crankshafts also creates friction as a reciprocating piston has components of forces directed against cylinder walls at changing angles.  Such friction causing loss of power and efficiency is avoided in this particular rotary engine. 
5.  Also unlike a typical internal combustion engine there is no loss of power by induction, pre-ignition compression, and exhaust strokes.  Driving of cams and valves is also eliminated.  Such energy expenditure is often against springs operating in a non elastic manner, frequently involves reciprocation, and entails significant friction. 
6.  The two rotary pistons turn in opposite senses, clockwise and anticlockwise, ensuring that their acceleration imparts no net rotary inertial forces to the housing.  This is an important advantage in automotive power plants where engine mountings are a significant part of the power to weight optimisation. 
7.  Both rotary pistons as well as the inlet cut-off valve are perfectly balanced and so produce no vibration at both high and low speed.  This reduces the bulk of engine mounts, and improves power to weight engineering generally. 
8.  The mechanism is a positive displacement engine, not a turbine.  This results in good acceleration from a stationary position against a load—as is required especially in typical automotive applications.  Turbines are very poor in accelerating from a stationary position against a load.  The “equal double rotary piston” mechanism is not an orbital engine, vane engine, or a Wankel engine—which despite being positive displacement rotary engines, all have one or more major problems especially in automotive applications. 
9.  With a constant direction of rotation and near constant angular velocity during a given cycle, there is very little friction and wear.  Modern bearings, seals and timing gears have all been highly engineered over generations for great durability and performance and are easily utilised in this new setting. 
10.  The long curved surface of the raised portion of the rotary piston and the long distance in which it is in close proximity to the housing ensures that very little steam can leak between these surfaces, despite the absence of piston “rings”.  The two surfaces have their maximum length in close approximation when it is most needed, that is when the pressure is at a maximum—at the beginning of expansion.  The flat faces of the rotary piston have seals which prevent steam escaping past the side of the raised portion of rotary piston and also from escaping through the main drive shaft bearings. 
11.  With modern accurate manufacturing techniques there will be a very small constant clearance between the two rotary pistons at the central point where there is tangential approach of the two rotary pistons.  This allows a small amount of steam to escape between the rotary pistons at the central point.  This steam is kept within the sealed system and exits with the exhaust steam, which is then condensed and re-used.  This is the only weakness of the whole design.  It is amply compensated for by the many advantages of this rotary engine over reciprocating and other rotary engines. 
12.  Both rotary pistons function as both piston face and abutment in one solid robust member.  This important fact distinguishes the mechanism from the vast majority of other rotary piston mechanisms.  Many other rotary piston designs have separate, often small, moving and therefore relative flimsy abutments.  Spreading out the wear evenly over long and uniformly curved surfaces in close proximity to its adjacent surface distinguishes the mechanism from another common weakness of many other rotary piston designs.  The balanced rotary inlet cut-off valve is also a very robust simple design with excellent durability. 
13.  Since it is an external combustion engine, burnt fuel residues do not enter the expansion chamber and produce contaminate or deposits—unlike internal combustion engines.  Oil for bearings and synchronising gears is thus kept clean, resulting in low maintenance and improved longevity of the engine. 
14.  Properly controlled external combustion can produce less atmospheric pollution and allows a wide choice of many different fuels.  Fuels used to produce steam may include traditional petroleum based fuels such as gasoline, kerosene or L.P.G. (Natural Gas).  However these fossil fuels are contributing to net increases in atmospheric carbon dioxide, global warming and adverse climate change.  More environmentally responsible fuels are being developed.  These include renewable sources such as (second generation) ethanol, and algal oil.  Less ideal fuels are first generation ethanol and vegetable oils such as canola.  Hydrogen can be used as an external combustion fuel generated from a variety of intermittent energy sources such as wave, wind and solar power or constant sources such as geothermal energy.  However the more direct combustion of second generation biofuels is a more direct, better, option than hydrogen. 
15.  Whatever the ultimate energy source the engine uses high pressure steam.  For at least the last 30 years technology for rapid steam production in sufficient pressures and quantities for typical automotive use can be generated in about 45 seconds.  Modern steam generators for automotive use are compact, light weight, safe and reliable.  Regulation of steam for automotive use is also now very well established.  Neither of these two areas of technology will be discussed in detail in this patent application. 
16.  The equal double rotary piston steam engine is simple, compact, with few moving parts and relatively inexpensive to manufacture.  This is demonstrated by the fact that the prototypes of the rotary piston engine were produced in a backyard garage which had only a small lathe, a drill press, hand tools and air compressor.  Even allowing for the cost of a steam generator, production costs would be cheap compared to those of internal combustion engines. 
17.  The suitability of a steam powered “equal double rotary piston” power plant to automotive applications is such that it would be possible to dispense with a clutch and gearbox, as some less efficient reciprocating piston steam vehicles have successfully done in the past.  The weight saving of eliminating clutch and gear box would add to the power to weight efficiency of the power plant and reduce manufacturing and operating costs even further.  It might be possible to eliminate other portions of the transmission train by having separate smaller equal double rotary piston engines directly driving each driven wheel.  However this advantage would be offset by the need for relatively greater total thermal insulation of at least two engines, and by extra measures taken to protect the engines and pressure conduits from more proximity to road vibration. 
Consequently we are of the opinion that this engine as described in the present patent application has the potential to make the four-stroke internal combustion engine obsolete in many situations.  Automotive applications include heavy-duty long-distance road transport, light and commuter transport, as well as rail and marine transport, and possibly even air transport! ( Regarding air-transport, the present engine and modern steam generators would be far more efficient than the very successful reciprocating piston steam engine propeller biplane of William and George Besler, flown in April 1933.  The original newsreel is at www.youtube.com/watch?v=nw6NFmcnW-8.)  Stationary applications include large scale electricity generation and small scale combined heat and electric power generation.  Farmers could produce their own electric power using their own fuel as virtually any combustible fuel can be used in a furnace to produce steam.  Portable units could also produce electricity or operate pumps for pneumatic or hydraulic equipment.  Many tools, including compressed air machines often used in the mining industry, could be easily adapted to the “equal double rotary piston engine”.  Many other industrial processes could use steam powered equal double rotary piston power plants, making a more direct and hence efficient use of local energy sources.