Also called a “Soda Gas Engine,” the tank under the engine receives a charge of CO2 which is liquid at ambient temperature at a vapor pressure of about 850 PSI. When the engine is started, the pressure in the tank drops, causing the CO2 to boil and supply more gas to the engine. The inlet valve is a steel ball sealed by CO2 pressure in a nylon seat in the cylinder head. Each piston has a peg located in the center on top that lifts the ball as the piston reaches top dead center. This allows the CO2 gas to expand into the cylinder, driving the piston back down. The charge exhausts through a series of holes at the bottom of the cylinder.