A Piezo Crystal Furnishes the Ignition

By Staff
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4312 Lone Oak Road Nashville, Tennessee 37215

A friend of mine gave me this air compressor to convert into a
hit and miss engine, an offer I couldn’t refuse. It turned out
to be NOS, as the hone marks were still evident on the wall of the
17/8 dia. x 1 stroke cylinder. The crankshaft
runs in Timken taper roller bearings. I had to weld an extension on
the back end of the crankshaft to support the second flywheel, and
upon checking the hole in the front cover, I noticed it was around
.003 larger all around than the crankshaft passing through it. Then
I realized the purpose of that reed type valve on the top of the
crankcase: On the down stroke of the piston, air is compressed in
the crankcase and this is vented out through the flapper valve,
while the vacuum formed inside there, on the upstroke, sucks any
oil trying to get out around the crankshaft back into the
crankcase. So I machined the same clearance around the new shaft
end in the back cover. This works very well, as I have experienced
no oil leakage in this area at all.

I really liked the cylinder head on this compressor with its
fins and the two valve locations just right to position my valves
inside the cylinder area. It was simple to ream out the air valves
and press in cast iron guides for the valve stems. These were NOS
Lauson engine valves. On an air compressor the piston, of course,
comes almost smack up against the head, so I inserted between the
head and the cylinder a ? thick cast iron spacer bored out around
7/16 deep to form a combustion chamber that
would give me about a 4 :1 compression ratio. As the valve seats
are in this insert and the valve guides are in the head, I doweled
them together with 3/16 pins to maintain
alignment. I used ‘O’ rings to seal both joints, as it
looked like there was not much area here for a regular head gasket
to hold.

The timing gears are Briggs &. Stratton, as is the mixer
valve and also the flyweight governor, which was internal on some
model B&S. I had to add some steel pins for added weight to the
flyweights to have enough ‘umph’ to slide the tapered spool
back to operate the push rod latch-out properly. The two rough
flywheel castings were purchased from Tiny Power Models. I made the
gas tank from a piece of 2′ brass pipe I had and the ends are
from ? brass sheet soft soldered in with 95/5 solder. The ends are
bumped out just to give the tank a little ‘character.’

The little brass box shown in the photo wired up to the spark
plug contains a piezo crystal, which furnishes the ignition for the
engine. I have had very good success with this system, as this is
the second engine I have used it on. The crystal is from one of
those red handle charcoal grill lighters that can be bought most
anywhere for around $3. The crystal is encased inside a spring
loaded plastic case that acts just like an automatic center punch,
in that as you push the end of the case in against the spring, a
latch releases and the spring snaps an internal weight against the
crystal, causing the spark. I mounted a cam on the side of the
large timing gear opposite the exhaust valve cam, only instead of a
lobe on this cam I have a low spot. This is to hold the plastic
assembly compressed until the spark is needed, when it will retract
and quickly move back in to fire. It takes 3/16 travel for the unit
to cock before the next cycle. The idea of keeping the unit
compressed is to have a way to prevent the crystal from firing
while the engine is coasting, thus extending the life of the unit.
The ignition system is operated by a bell crank with a roller on
the end of the lower arm that contacts the timing cam and the upper
arm pushes a rod in line with the piezo crystal assembly. Bolted to
the front side of the top arm is another lever which extends over
close to the exhaust valve push rod. A pin in the end of this lever
lines up with a sheet metal foot bolted to the push rod in such
alignment that when the engine is coasting and the push rod is up,
the foot catches the pin and prevents the bell crank from doing its
thing until the push rod moves back down. I have not had to replace
one of these crystals in the two years the other engine has been
running, but when the time comes it will be a matter of maybe
twenty minutes time and not over $3 in cost. A small price to pay
for eliminating batteries, coil, points, condenser and a passel of
wiring.

Oh yes, the compressor engine runs fine. Runs a little fast,
hitting one lick and coasting eight or nine revolutions at a very
constant speed.

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