CO2 Motor

This style engine could be powered by steam (I've heard of at least one) but is more commonly seen in small model airplane engines powered by compressed air or CO2 (carbon dioxide) gas. The popular Air Hogs toy airplanes are propelled by this style motor.

At the top of the stroke, the pin on the cylinder presses the ball valve upward, admitting high pressure gas into the cylinder.

The gas expands, driving the piston downward.

when the piston advances past the exhaust port, the high-pressure gas is released.

Flywheel (or propeller) momentum carries the piston upward to complete the cycle.

This animation also illustrates the CO2 reservoir, or "fuel tank." Compressed CO2 is a liquid and becomes a gas as the pressure is released. Another way to state this is that the liquid CO2 boils at normal atmospheric temperature and pressure, so one might say this engine runs on "CO2 steam."

Oscillating Steam Engine

This style steam engine employs the cylinder as the steam valve. It operates on the same principle as the locomotive steam engine.

Steam from the boiler enters the power manifold and is and is admitted to the top end of the cylinder when the cylinder port aligns with the manifold port. The steam presses the piston downward, driving the flywheel around one half turn.


At the end of the stroke the cylinder shifts, exposing the top port to the exhaust manifold. The expended steam is released.

At the same time, the bottom cylinder port, aligns with the power manifold, admitting steam to the bottom end of the cylinder. This presses the piston upward, driving the flywheel around another half turn.

At the end of the stroke, the bottom port aligns with the exhaust manifold, releasing the expended steam.

Due to its exceedingly simple construction, this type of engine is popular in working toy steam engines, including one I had as a kid. An even simpler type employs power in only one direction, relying on flywheel momentum to carry the piston around for the remainder of the cycle. This is called a single acting engine. The type illustrated here is a double acting engine.

Steam Locomotive Engine

Steam engines like this drove trains from the early 1800s to the 1950s. Though the engines varied in size and complexity, their fundamental operation remained essentially as illustrated here.

In a steam engine, the boiler (fueled by wood, oil, or coal) continuously boils water in an enclosed chamber creating high-pressure steam.

Steam from the boiler enters the steam chest and is admitted to the front end of the cylinder by the valve slide (illustrated in blue). The high pressure steam presses the piston backward, driving the engine wheels around one half turn.


At the end of the piston stroke the valve shifts, allowing the expended steam to escape through the exhaust port (underneath the blue valve slide). The high pressure steam escapes in a quick burst giving the engine its characteristic choo choo sound.

At the same time, the valve slide begins admitting high pressure steam to the back end of the cylinder. This presses the piston forward, pulling the engine wheels around another half turn.

At the end of the forward stroke, the steam is released from the rear portion of the cylinder (another choo).

The steam engine has a 'dead' spot at the extreme end of each stroke while the valve is transitioning from power to exhaust. For this reason, most engines had a cylinder on each side of the engine, arranged 90 degrees out of phase, so the engine could start from any position. This illustration only shows one side of the engine.