By John Molnar
For the past several years, I’ve been investigating various sources of
alternative energy for home use and in my ham radio shack. Some solar
panels are in place for charging batteries and I’m producing a quantity of hot
water from some old homemade copper collectors, but I started itching for
yet another source of green power. I discovered that there is a large
undercurrent of experimenters working with small steam powered engines
directly coupled to efficient generators capable of producing meaningful
power. I decided to see what this was all about. We’re not talking about those
massive multi-ton Industrial Revolution engines here. There are many
modern 1– 10 HP engines available that are realistic for domestic power
generation … but where to start?
I had just replaced my old gasoline powered weed-eater with a new propane powered four-stroke, and
decided to develop an air/steam powered engine from the
old remains. However, I refused to completely regress to
the old days of mechanical linkages and leaking valves, so
I determined that my 21st century engine would have to
be computer controlled.
What follows is my effort to convert a conventional
two-cycle engine into one that is powered externally by
compressed air for testing, then ultimately steam produced
by a small, safe “monotube” boiler (see the sidebar).
This engine is completely controlled by a PIC18F2525
based controller that allows complete control of inlet and
exhaust timing and duration, with real time RPM and
parameter display. The engine piston position is relayed to
the processor via Hall-effect magnetic sensors.
This project is very easily reproduced by any
experimenter with average mechanical skills; the controller
is an easy breadboard; and — best of all — it’s cheap and
green! I found it to be a great platform for learning and
applying computer engine control. Figure 1 is the
completed engine and controller on the bench.
So, let’s get started!
34 March 2011
Find An Engine
Any two-cycle engine can be adapted to my
controller. I chose the 25 cc weedwacker engine
because I had it. However, they can be obtained usually
for free by checking your landfill or the trash bin at small
engine repair shops. Often people pitch them just
because the recoil starter rope broke. Look for one that
turns over freely; you won’t need the carb, rope start, or
Strip it down to the essentials: crankcase and head.
Remove the old plastic shroud, exhaust can, recoil
starter, and ignition parts. Be sure to remove the rear
crankcase “stuffing” plate to expose the crankshaft and
connecting rod. If this plate is not removed, the modified
engine will exhibit poor performance due to
backpressure in the crankcase.
So, How Does This Work?
What we’re building is an “external combustion
engine.” The motive power (steam, air) is developed
OUTSIDE the engine, not inside as when burning gasoline
in a confined cylinder. Our pressurized gas is injected
INTO the spark plug hole for a controlled time period,