Slam Valve Motor, supplying efficient steam engines for homeowners and professional solar installers
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              Abner Doble's Uniflow steam engine, build it yourself
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Abner Doble, the creator of the finest steam powered car ever built, once wrote a glowing paper about the uniflow motor.  After building a prototype, he later recanted many of the claims, but with a few hours worth of work you can have a uniflow motor of your very own and judge accordingly.  The efficiency is much lower than the slam valve motor.  Doble's unit used a slider valve, this one uses one type of slam valve to achieve similar results.  The peak torque achievable with this motor is just below that of a non expansion motor, or of a ported slam valve motor (see graphs) .

First, you need a two stroke gasoline engine with a spark plug that faces the piston directly.  Glow plug motors are OK but you will have to build an adapter,  we suggest you try a weed whacker first.  The spark plug must be in line with the piston's motion but this is a common configuration.  Ours was a $60 weed whacker at home depot in a previous life.
two stroke motor
Two stroke motor converted to compressed air or steam
screw close up

Screw converted to slam valve piston.

As usual, all safety precautions for compressed air and steam apply.  Only use tubing and fittings rated for the pressure and temperature you will use.  Wear safety goggles and other protective equipment if applicable.  Test your connections.  No bystanders.  Use overpressure devices and a pressure meter to ensure you never overpressure the system.  Keep operating pressure below 60 psig (steam at 150 C).  Your weed whacker is expendable and may die a short and violent death if you make a mistake.  Ensure that you do not.

In addition to the usual precautions, consider the possibility that the spark plug adapter may pull free.  We tied ours down with baling wire so if it came free it could not go far.  Provide a shield so that if the adapter breaks it won't spray boiling water on anyone.

Find a check valve and an adapter that fits the spark plug socket.  These are suggestions from mcmaster:
check valve
adapter   - note I remember this one being close enough to the spark plug size to seal.  Its not exactly the right thread but close enough to match.  However measure your spark plug and the thread spacing (its metric usually 1.25 mm) to be sure this is right for your engine.  If you cant find a match, drill out the spark plug hole and tap it for something you do have.  Alternately make a proper adapter on a lathe.

Now find the right brass screw.  The right screw is a brass Phillips flat head screw (triangular head) that has a smooth shaft for some distance and is of the right diameter to fit through the hole in the bottom of the check valve but just barely.

Chuck the screw in the drill and polish the triangular part of the screw smooth.  This is now a valve seat.  The angle also needs to match.

Take the check valve apart (two wrenches).  Remove the piston and discard.  Look at the bottom part (the arrow will be pointing up).  Your screw will go in here to become the valve piston.  Put it in, test blow down the valve, it should seal reasonably well.  If not, re polish the screw.

Push the screw up slightly, air should flow freely.  If not, file the edges of the screw where it passes through the valve end into a triangle to provide passages to let air flow by.

Add a spring to push the screw upwards.  Make sure it still seats when you push it down.  You want a few ounces of force, less than a pound.

Install the adapter and valve bottom into the engine's spark plug hole.  Rotate the engine until the piston is at the top.  Push down with a pencil eraser and rotate the engine shaft.  At top dead center, the screw should be pushed up 1 mm above where it sits when the piston is down.  Cut/file the bottom of the screw appropriately.  Make sure valve and adapter (and probably a washer) are all in properly and tight when you make this adjustment.

Now look a the top of the check valve.  Install a screw where the piston was in order to keep the screw about 2 mm maximum above the seats.  You will need to adjust this gap later.

Install the top of the check valve.  Add a source of compressed air or steam to the top of the check valve.  You need all 1/4 inch or wider diameter piping and valves back to your air tank or steam generator.   If using steam, it is vital to drain the crankcase of the motor.  OK to remove the motor's muffler and air filters.  The existing fuel system can be tailored into a spray lubrication system.

Add air or steam, intake valve will slam shut and leak slowly (you measured this rate by blowing through the valve).  Leakage depends on how well you machined the screw.  Pull the starter, the motor will run.  On steam the motor will require some time to warm up (tens of seconds) before it will run well.  Slower rotation gives higher torque.

Oil the motor before running, expect that to last a few minutes only.  Use steam oil if running on steam.  When running on steam, watch for increased friction on the motor shaft.  This is caused by emulsion getting behind the rings, disassemble the motor and wipe off the rings.  Alternately heat with a blowtorch to above the boiling point to remove the water, or add fresh oil and turn over repeatedly to wash out the emulsion.  The last works better on iron engines than it does on aluminum.  Aluminum motors perish rapidly under this high friction condition.  Once the motor is worn out, the amount of steam blowing by into the crankcase will become unacceptable.  Some steam will blow by anyway even when new because of the intake valve of the two cycle motor.  Consider tilting the motor sideways to make it exhaust water from the crankcase out the fuel port.

Have fun,  some believe the uniflow to be the best steam engine under real world conditions.  We believe the slam valve to be superior, but invite you to judge.  Similar to a ported slam valve motor, torque increases as RPM decreases.  Similar to a slam valve motor, changing gas pressure has little effect on output power at higher RPM, but there is a minimum pressure at which it will run at all. The uniflow has slightly lower peak torque than the slam valve due to the re compression of steam.  Unlike the slam valve, the efficiency is lower at any power output and the RPM at which torque drops off is much lower.  Adjust this by adjusting the upward force on the screw and the size of the gap between where the screw sits when it is free and where it seats.  Unlike the slam valve, you cannot adjust torque on this motor when it is running.  

There is a similar motor which uses no spring.  This one will tend to surge, that is torque will go up at a certain high RPM.  Its based on the momentum of the screw after the piston has hit it.  Adjust power/RPM by adjusting the length of the screw.  Longer screw = more surge, lower RPM.  Another variation has the spring pushing the screw down.  This one has very low efficiency and flat torque at any RPM.

One place you might make improvements in the uniflow is the captive volume.  A gasoline engine relies on having some volume in the cylinder when the piston is at top dead center (normally 1/8 of the volume when it is down although two strokes are generally lower compression).  Steam engines would prefer this volume to be zero.  Filling in the extra space at the top with high temperature RTV might help.  The major loss however is the lack of an intelligent exhaust structure.  The uniflow will re compress steam once the piston has risen past the exhaust port.  This costs power and efficiency, just like Doble's motor.