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 piston 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. Compressed CO2 is a liquid and becomes a gas as the pressure is released. In other words, the liquid CO2 boils at normal atmospheric temperature and pressure, so one might say this engine runs on CO2 steam.