Before I get into the main discussion on this valve design, we'll need to discern between single and double acting cylinders, how they are normally ported and the traditional valve designs used to operate them.

A single action cylinder produces a single power and exhaust stroke per full revolution of the crank. It begins to allow steam entrance to the upper portion of the cylinder as soon as the piston has reached a degree or two past top dead center in the direction that the engine has been timed to run. As the high pressure steam rushes into the top of the cylinder, it begins to expand a great deal, though not with the explosive bang developed in internal combustion engines. As the steam continues to enter, it continues to expand because it was first under a great deal of pressure inside the boiler. The piston is pushed down, directly by the effect of this pressure to the very bottom portion of the power stroke and begins to return, this time exhausting the spent steam out the same port it first entered powered only by the momentum developed during the power stroke. A heavy, dynamically balanced flywheel smoothes out and tends to balance out the higher velocity of the power stroke and with help of it's own momentum, to complete the exhaust stroke. As soon as the piston has reached top dead center again it repeats the same cycle again. You can easily see that a single cylinder operating in this manner without the impetus provided by the properly designed flywheel would cause the engine to operate extremely erratic, rotating in alternating fast and slow speeds, possibly even stopping before completing the exhaust cycle. The timing of the steam, that is, the positioning of the valve to allow steam to enter and leave the cylinder at just the right times during both the power and exhaust is the one and only function of the slide valve. On single action engines there is usually a single port located and entering the top of the cylinder right into its bore. This single cylinder port will double as an intake and exhaust port, unlike the image of an engine with intake and exhaust valves opening and closing independently, it is actually a very simple system. Steam engines are timed by a sliding valve passing by a single or pair of cylinder ports, exposing and covering them at just the right time to allow the cycles to take place. On single action engines, we tend to use a cylindrical slide valve as opposed to a rectangular one. The cylindrical valve glides inside a reamed matching diameter bore. Two common features of a single action slide valve is a steam intake slot or hole as well as an exhaust slot or flat on the upper end facing the cylinder. Picture a 1/4" diameter valve with a 1/4" long flat on the top end. The flat begins from the tip of the valve and is usually about 1/3 the diameter of the valve deep. The next feature is a 1/8" wide unmachined space followed by a 1/8" groove about 1/16" deep that goes around the circumference of the valve. Some builders drill a hole instead of a groove. Either way, what we have is a vertical step or flat followed by a full section of valve, ending with a circular slot or transverse, centered hole. The single horizontally located port entering into the cylinder bore coincides with the port on the valve bore when it is at the top of the valve stroke. As the piston reaches top dead center, it needs to have air or steam begin to rush in so it is pushed or displaced down on the power half of the cycle. The valve, linked to a rod which is also linked to an eccentric disk is adjusted to time the position of the valve as it is sliding into the bore, bringing the edge of the circular intake slot in line with the bottom edge of the cylinder's port when the piston is at the top of its stroke. Air or steam can then pass around the circular slot on the valve and enter through the cylinder port to power the piston. As the piston is pressed down, the slide valve is fully exposes the port and immediately begins to slid back down to cover it. At a point prior to bottom dead center the valve will be between the circular slot (intake) and the upper step (exhaust) as it relates to the cylinder port, effectively sealing itself so nothing can get in or out of the cylinder bore. The flywheel will continue turning to bring the piston back up to exhaust the cylinder contents. In order allow the gases to be exhausted the valve continues to slide down so the flat begins to expose the cylinder port this timing has to be just right so as soon as the piston starts moving up, the cylinder port has to begin to be exposed. As the piston goes further up the valve will have slid all the way down, fully exposing the port and allowing the contents to be exhausted around the step and out the top end of the valve. The cycle begins once again with the valve sliding up to allow the circular slot to expose the cylinder port.

A double action cylinder needs a much more intricate network of internal porting to be able to operate. Let me take you through a full cycle of operation beginning with the piston at bottom dead center. Since steam can enter through the bottom as well as the top ends of the cylinder, the bottom portion of the piston rod passes through a seal that prevents any loss of pressure through the normally open end of the cylinder. A centrally located rectangular slide valve inside the valve chest slides toward the top to allow steam to enter through the now exposed bottom port of the cylinder. At the same time, it begins to expose a central exhaust port so as the piston is pushed up from the bottom end, the piston can exhaust through the top end. As it reaches the top of the stroke, the valve slides back down to expose the upper intake port and the piston is then pushed back down, exhausting the bottom end of the cylinder. As you can see, you get twice the amount of power per full 380 degree cycle as compared to a single action cylinder. In fact, this is the way locomotive and other large steam power plants have been traditionally designed. The only problem is that building double action engines in the traditional manner involves some rather intricate machining of cylinder porting as well as a valve chest, valve chest cover and valve unit composed of several parts. No problem for an advanced, experienced home shop machinist but maybe a bit too much of a challenge to someone just starting out in the craft.

After having stared for many hours at single and double cylinder, single action engines, it just came to me that all you would have to do is to build a simple cylindrical slide valve who's bottom end is a mirror image of the top end. The cylinder's single top port now would also be mirrored at the bottom. Imagine if you would, two identical pair of valves and cylinders and you sliced them both into two equal sized halves, discarding the bottom ends and splicing the two tops together at their sliced ends to form a perfectly symmetrical valve and cylinder. This whole concept or theory has now been proven and put into practice by me and all I can say is that it work like a charm allowing it to be throttled down to around 40-50 RPM with perfect smoothness. The beauty of a double action engines is the smoothness the system imparts to the rotational cycles. Better yet, I develops twice the power since every time the piston moves, no matter in which direction, it's always under power. So you can effectively get the same power output out a single cylinder engine as out of a double cylinder but single action engine of the same cylinder displacement.