At this point, most of the gas in the system has just been driven into the hot cylinder. The gas heats and expands driving both pistons inward.
At this point, the gas has expanded (about 3 times in this example). Most of the gas (about 2/3rds) is still located in the hot cylinder. Flywheel momentum carries the crankshaft the next 90 degrees, transferring the bulk of the gas to the cool cylinder
Now the majority of the expanded gas has been shifted to the cool cylinder. It cools and contracts, drawing both pistons outward.
The now contracted gas is still located in the cool cylinder. Flywheel momentum carries the crank another 90 degrees, transferring the gas to back to the hot cylinder to complete the cycle
This engine also features a regenerator, illustrated by the chamber containing the green hatch lines. The regenerator is constructed of material that readily conducts heat and has a high surface area (a mesh of closely spaced thin metal plates for example).
When hot gas is transferred to the cool cylinder, it is first driven through the regenerator, where a portion of the heat is deposited.
When the cool gas is transferred back, this heat is reclaimed; thus the regenerator "pre heats" and "pre cools" the working gas, dramatically improving efficiency.3
This type of Stirling engine, known as the beta configuration3, features just one cylinder with a hot end and a cool end.
The working gas is transferred from one end of the cylinder to the other by a device called a displacer (here illustrated in blue).
The displacer resembles a large piston, except that it has a smaller diameter than the cylinder, thus its motion does not change the volume of gas in the cylinder - it merely transfers the gas around within the cylinder.
At this point, most of the gas in the system has just been driven to the hot end of the cylinder. The gas heats and expands driving the
At this point, the gas has expanded. Most of the gas is still located in the hot end of the cylinder. Flywheel momentum carries the crankshaft the next quarter turn. The bulk of the gas is transferred around the displacer to the cool end of the cylinder.
Now the majority of the expanded gas has been shifted to the cool end. It contracts, drawing the piston inward.
The contracted gas is still located near the cool end of the cylinder. Flywheel momentum carries the crank another quarter turn, moving the displacer and transferring the bulk of the gas back to the hot end of the cylinder
, a prominent Stirling engine experimenter, invented the linkage illustrated here.3
The engine is identical in operation to the two cylinder Stirling. In this illustration, the left cylinder is the hot cylinder
The linkage allows the engine to be more compact and reduces side loads
on the pistons and connecting rods (since their travel is almost linear
The Gnome was one of several rotary engines popular on fighter planes during World War I.
In this type of engine, the crankshaft is mounted on the airplane, while the crankcase and cylinders rotate with the propeller
The Gnome was unique in that the intake valves were located within the pistons. Otherwise, this engine used the familiar Otto four stroke cycle. At any given point, each of the cylinders is in a different phase of the cycle
In the following discussion, follow the master cylinder with the green connecting rod
During this portion of the stroke, a vacuum forms in the cylinder, forcing the intake valve open and drawing the fuel-air mixture in from the crankcase
The mixture is compressed during this phase. The spark plug fires toward the end of the compression stroke, slightly before top dead
The power stroke happens here. Note that the exhaust valve opens early -- well before bottom dead center
This engine has a fairly long exhaust stroke. In order to improve power or efficiency, engine valve timing often varies from what one might expect
Nonetheless, a number of engines were designed this way, including the Gnome, Gnome Monosoupape, LeRhone, Clerget, and Bentley to name a few. It turns out there were some good reasons for the configuration
Balance... Note that the crankcase and cylinders revolve in one circle, while the pistons revolve in another, offset circle. Relative to the engine mounting point, there are no reciprocating parts. This means there's no need for a heavy counterbalance
Air Cooling... Keeping an engine cool was an ongoing challenge for early engine designers. Many resorted to heavy water cooling systems. Air cooling was quite adequate on rotary engines, since the cylinders are always in motion
No flywheel... The crankcase and cylinders provided more than adequate momentum to smooth out the power pulses, eliminating the
need for a heavy flywheel
All these factors gave rotary engines the best power-to-weight ratio of any configuration at the time, making them ideal for use in fighter planes. Of course, there were disadvantages as well
Gyroscopic effect... A heavy spinning object resists efforts to disturb its orientation (A toy gyroscope demonstrates the effect nicely). This made the aircraft difficult to maneuver.
Total Loss Oil system. .. Centrifugal force throws lubricating oil out after its first trip through the engine. It was usually castor oil that could be readily combined with the fuel. (The romantic-looking scarf the pilot wore was actually a towel used to wipe the slimy stuff off his goggles
The aircraft's range was thus limited by the amount of oil it could carry as well as fuel. Most conventional engines continuously re-circulate a relatively small supply of oil