An engine is any device that converts energy to work; bearing this in mind, you have numerous options when constructing a homemade steam engine. The simplest designs makes use of a fan that is spun by escaping steam. Slightly more efficient than fan assemblies, the ancient design of the Heron steam ball routes steam from a boiler into a copper sphere with two escape pipes at right angles from the centre of the ball, causing pressurised steam to spin the ball. By far the most powerful incarnation of the steam engine are the piston-driven steam engines that power trains.
Make a crude boiler using an electric tea kettle and a length of copper pipe. Choose a pipe about 1/4 inch smaller than the opening of the tea kettle spout. Cut the copper pipe to a suitable length to reach your engine of choice with a hacksaw. Cut a piece of rubber from the outer bike tire and use rubber cement to glue the pipe to the rubber.
Wedge the end of the pipe into the spout of the tea kettle cut from the outer bike tire. Use your rubber cement to glue the pipe to the rubber. If your tea kettle has a second pressure-release valve, close it with rubber cement so that the pressure has only one way to escape.
Use a jigsaw to cut plywood into six pieces roughly twice the height of your tea kettle, with a gap in the lid for the steam pipe to exit, and one for the power cord to enter. Add a patio stone for the hot kettle to rest on without burning the wood. Assemble the six pieces into a cube shape, with wood screws, first drilling pilot holes, then adding screws. The plywood box will serve as a protective barrier in case the kettle bursts. Use a router to bevel an edge onto the lid. Make a handle with and plywood, by cutting the wood into a top bar and two wedges (to raise the handle). Use wood screws to join the handle to the lid.
Cut the bottom off of a soda can with tin snips and cut the edge away to create a thick aluminium disk. Cut five slits equally spaced around the disk and use pliers to bend them into the shape of a fan. To make a gear, cut 1/2-inch notches around the bottom of the soda can, bending every other cut area down and folding the remaining area in on itself to form a toothed gear.
Solder the two pieces to a framing nail. Feed the solder wire slowly around the joining point while you hold the two pieces in place with a clamp on your workbench. Use a propane torch to heat the solder, forming a solid bond. The fan must be positioned in the centre, sized to fit into the steam pipe (if its too large, trim the edges down), and the gear offset to the side of the nail with the hammer striking head, so that it will sit outside of the steam pipe.
Cut the end of the pipe with tin snips and bend it into a bracket to form a free-moving gear driven by the fan. To make a bracket, cut away two tongue-shaped pieces from the end of the pipe leaving two arms. Use a drill to make two holes the size of the nail to hold the nail, which has now become a rotation axle. Cut the way to the holes open with a the tin snips on a 45-degree angle. Put the nail in place, then bend the copper back into shape with pliers by twisting the copper together in a roll. This may take some strength to accomplish. If you aren't strong enough to bend the metal that way, close the cut using the solder and torch.
Bend the end of your steam outlet pipe at a right angle to the ground. Should you need assisted leverage, use a pipe bender; place the pipe on the floor trough the bender, and slowly applying your body weight. Construct a support with a thin metal rod so that the steam ball will have two points to attach to; the support bar for demonstration-size models only needs to be 1/4 inch thick and can be bent by hand. To construct the support, run the last inch, bent at a right angle into the steam ball at the centre, and attach to the boiler box with a metal strap and wood screws.
Draw the outline of one half of the steam ball and half of each right angle pressure escape tube; use a permanent marker made for use on metal. The steam-ball doesn't have to be spherical; you can actually build the gear or pulley right into the shape of the ball. Include markings to indicate where the teeth of the gear will be.
Use a small metalworking hammer to hammer the shape out into three dimensions. Use a drill to make pilot holes and rivet both pieces together; hammer the metal against a soft cloth base (like a folded blanket on your lap, against an inverted nail punch). The nail punch's larger side will dent the metal slowly into shape without piercing it. Use solder and a propane torch to make a water tight seal on the seem. Attach the ball to the support and the steam pipe, cut the pipe end and use pliers to make a folding edge on which the steam ball will loosely rotate.
Build a pressure chamber (a round container as smooth as possible) out of sheet metal. Bend, rivet and solder the metal to seal it. Make a double-sided piston of soda cans, attached in such a way so that both ends are facing their respective ends of the chamber. Attach the piston to a cam by way of a steel rod; bend, rivet and weld the end of the rod to the outer-side of the piston end farthest from the cam.
Place a valve and spring-regulated steam input on either side of the pressure cylinder. When the pressure reaches a level that overcomes the spring, the valve will open and the pressure will move the piston to one side, opening the pressure release vent in the lower centre of the piston chamber by moving the entire piston out of the way. Given the complexity of the spring valve system, draw a diagram before you begin to build this step.
Design the second valve to push the piston back so that, when timed correctly, energy is transferred to the driveshaft. The timing can be achieved by controlling when the valves open. The simplest way to do this is to simply have two boilers, one connected to each spring valve, starting one boiling before the other. The stronger the spring you use, the stronger the force applied to the piston will be, provided the steam pressure can overcome it. Higher power engines will require more heavy duty parts; the parts listed herein are for demonstration-grade piston engines.
Choose the version of the engine that is best suited to your skill level. The fan and steam-ball are easy for most DIY enthusiasts, but the more complex piston drive is only for advanced builders with a full workshop and command of basic mechanics and metalworking. A small boiler will only drive small engines. Build a high-output boiler if you plan to build a device for anything but demonstration purposes, or simply add a greater number of smaller boilers in series. If you are unclear what the final product will look like, draw a diagram before hand so that you begin with the end result in mind.
Wear goggles at all times when cutting metal and using power tools. Use welding gloves when you are soldering. Be aware of the maximum pressure loads of your devices, don't run dangerous levels of steam though engines, instead build multiple engines in series. Take standard fire precautions, such as having an extinguisher close at hand.