VACUUM AND HOT AIR ENGINES

This engine was manufactured in Dayton, Ohio, as a toy and is
called a ‘Vacuum Rotor’ The operating cycle is roughly
this, the first function of the cycle is the opening of the
admission valve. This occurs just before the piston reaches the
head end dead center, then as the piston starts toward the crank
center the torch flame is drawn into and fills the cylinder with
flame (incandescent gases that is, gases so hot that they glow).
Just before the piston reaches the crank end dead center the
admission valve closes thereby cutting off the flame.

Now, as we all know, when an unconfined gas is heated it expands
or if it is confined its pressure increases and conversely if while
confined it is cooled it produces a partial vacuum.

Now back to the engine. We have a cylinder filled with
incandescent gases and we have suddenly closed the valve and
instantly the flame in the cylinder is extinguished and the heat of
the flame is absorbed by the cylinder walls and is then in turn
passed to the atmosphere by the fins on the cylinder in air cooled
engines or into the water on water cooled engines. The Vacuum Rotor
has a small pocket on top of the cylinder for this purpose.

Now let us again go back to the engine. The piston is at the
crank end of the cylinder and the gases in the cylinder have
cooled, creating a partial vacuum, the atmosphere wants to flow
into the cylinder to equalize the pressure but it can’t because
of the nice fit of the piston in the cylinder so it does the next
best thing it pushes the piston into the cylinder. This is the
power stroke of the engine. It is strong enough to move the
flywheel and stores some energy in it. Even though the gases in the
cylinder have contracted they do have some volume and as the piston
approaches the head end of the cylinder these remaining gases (the
exhaust of the engine) are compressed by some of the energy stored
in the fly-wheel until the pressure is high enough to lift a small
ball check valve located under the whistle and this exhaust is what
gives the ‘toot-toot’ sound when these engines are running.
After the exhaust function is completed the admission valve again
opens and the cycle is repeated so much for the Vacuum Rotor.

Now about ‘Hot Air’ engines. They are an altogether
different breed of cats. In a way they are like steam engines.
Steam for a steam engine is only a means of transporting the heat
from the fire to the engine, it is the fire after all that makes
the engine run and not the water. In a hot air engine the heat is
carried around the engine by the air in the system — hence the
name hot air engines.

There are several types of hot air engines, a few of which are
named as follows: Sterling, Hinreque (hinerickey), Ericson Ryder,
Ericson, Ryder, Roper, Caley, Joules, and many others including the
Smith as made by the Grand-father of Tom Smith of Engineers and
Engines fame, and there were others. The Roper, Caley and Ericson
have valves and don’t use the same air over and over and for
the most part were only made in large sizes. For the purpose of
explanation we will discuss the Sterling engine, invented in 1813
by one Robert Sterling, a Scotch preacher.

In the Sterling engine there are two connected cylinders. There
are no valves or other obstructions in the connecting port. One
cylinder known as the power cylinder is fitted with a trunk type
piston somewhat like an automobile piston, although most of the
time they have no piston rings. The other cylinder is about twice
as long as the power cylinder and is closed on both ends and motion
is taken into it by means of a piston rod through a packing gland.
On the end of this rod is a drum or can closed on both ends called
a displacer or a dasher. Its sole purpose is to take up room in the
sylinder. It does not fit like a piston as it has about 1/8′
clearance between it and the displacer cylinder walls (in a 4′
bore engine). The displacer cylinder is heated on one end and water
jacketed on the packing gland end. If the dasher is moved to the
cold end of the cylinder the air contained therein is forced to the
hot end of the cylinder where it is heated and expanded. It then
flows through the port to the power cylinder where it acts on the
piston causing a power impulse outward on the piston. As the piston
reaches the end of its stroke the dasher moves to the hot end of
the displacer cylinder causing the air therein to move to the cold
end where it cools and thereby contracts causing a partial vacuum
in both cylinders. The atmospheric pressure then acts on the under
side of the piston pushing it back into the power cylinder. This
makes two power strokes per revolution of the engine, and the
engine is now in position to repeat the cycle.

These explanations of the working cycles of the atmospheric and
the hot air engines are somewhat simplified so that the layman can
understand them and see the difference between the two engines as
well as get a vague idea of why they run and how they work, so you
experts please don’t castigate me for not discussing adeabatic
and isothermal explanations and compressions or indicator cards for
these engines. If there is interest shown along these lines perhaps
later we can discuss some other little known engine cycles also
other types of hot air engines and some odd type of internal
combustion engines.