This stage in the evolution of the IC engine that most people
find
interesting.
These engines are called "hit and miss" because
of their interrupted firing. They still use the Otto cycle but
they came along after the patent had expired.
With very few exceptions, all modern IC engines control power by
some means of adjusting the amount of fuel and air allowed into the
cylinder. This is commonly called a throttle. Before
throttles were perfected on IC engines, the power was controlled by
simply interrupting either spark or fuel. An example of
interrupting spark is the first biplane rotary engines. Listen to one
of the old
radial engines spitting and surging. That sound is
the pilot turning on and off the magnetos. If he left them on,
the engine would race up to top RPM. In the hit and miss
engines, power is controlled by either allowing the mixture in or
not. The intake valve is what is referred to as an
'atmospheric' valve. That's to say, it opens by atmospheric
pressure, not a mechanical lifter. When the engine is on the intake
stroke, a vacuum is created in the cylinder and the higher atmospheric pressure
on the outside
overcomes the spring pressure and the intake valve opens allowing
fuel and air to enter the cylinder. The exhaust valve is
controlled by a lifter and linkage. To create the hit and
miss, the engine goes through a complete power cycle and speeds up
enough to activate the governor which holds the exhaust valve open
allowing the engine to breath through the exhaust port and not take
in a fuel air mix for several revolutions. Once speed decays
enough, the governor releases the exhaust valve to close and the
engine picks up another power cycle and starts all over.
Listening to a running hit and miss you hear a very characteristic
"pop, woosh, woosh, woosh, woosh, pop, woosh....." Some of the
better built engines can turn six to eight or more non power
revolutions to one power cycle. When the engine was required
to work like run a pump or saw, the speed would stay below the
governor speed and would hit on each cycle. These were mainly
used as farm engines. They ran pumps, saws, grinders or
anything that needed power. It was
mounted to a sled and moved to where they needed it. A flat
belt was the method of connecting to the driven equipment.
The engine I built below is a model based on an engine that Sears,
Roebuck and Company sold in the early 1900s. They were
available in sizes from 1hp ($28.95, 320lbs) to 10hp ($231.50,
2450lbs).
I don't have many pictures of this engine in the build process.
This first couple pictures are while I was trying to get it running the
first times. I had to clamp it to the bench because when the
hit stroke would happen, it would almost jump across the bench.
Here is a good shot of the governor. It's the brass weights
just left of center. As the gears spin it, the brass weights
fly outward. They then push a pin through the shaft to the
latching lever to hold the exhaust open. The head before paint. The top valve is the intake and the
bottom is exhaust. The bar running from the exhaust rocker to
the back is the lifter. There's a cam seen in the above shot
on the middle gear that runs the lifter back and forth. The
governor latch is on the lifter in the rear. You can see it
above the yellow clip. If the latch is engaged, it holds the
lifter in the extended position, thus holding open the valve.
This picture is after final painting. I did the pin striping by
hand and they didn't come out great but if you stand back a bit it's
not bad.
Here is a short video of one of the early runs. Notice how
fast it's running. That's as slow as I could get it at the
time. They
require a bit of break in before they start to smooth out. Economy Video 1
This clip is much smoother but still a bit fast. I need to
work on getting the speed down a bit.
