VACUUM AND HOT AIR ENGINES

Sterling hot air engine

6' Sterling hot air engine with direct connected domestic water pump. This is a 'Caloric' Engine, No.l, built by Breman Mfg. Co., Breman, Ohio. Exact date unknown. Patented 1868. Engine now in my collection.

Content Tools

Route 1 Bucyrus, Ohio

I promised to write about the Atmospheric or Vacuum engine shown on page 13 of the Sept. ALBUM.

I believe you will find that it is not a 'Hot Air' engine, it is more properly known as an Atmospheric engine. It has a much different operating cycle than a hot air engine.

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.