Slam Valve Motor, supplying
efficient steam engines for homeowners and professional solar installers
Abner Doble's Uniflow steam engine, build it yourself
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) .
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 converted to compressed air or steam
Screw converted to slam valve piston.
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.
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:
- 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
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.
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.
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
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.
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
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.
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.
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.
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.
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.