Slam Valve Motor, supplying efficient steam engines for homeowners and professional solar installers
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Gordon mode is the full expansion mode, available in ported and unported slam valve motor types. Gordon mode is maximum efficiency, but still features torque control if so equipped.  Ported motors (bearing the port shown in the illustration) also feature higher power density Thomas and Henry modes.  Some feature the regenerative braking mode Percy as well.

The motor starts out with the piston just below top dead center, pressure inside the cylinder at exhaust pressure, piston still going up
first slide

The piston going up cracks the intake valve open.  Steam flows across the piston head and out the exhaust valve (still open)
slide 3
As the piston goes up, flow increases until it slams the exhaust valve shut.  The pressure rises rapidly to the intake pressure and the intake valve, which now has no force on it, will spring up to the control rod
slide 6
The piston begins to descend, under full intake pressure.   As it descends, it accelerates (until it reaches 90 degrees from top dead center which is maximum velocity).  Once it reaches a critical velocity set by the control rod (generally 30 to 45 degrees from top dead center),  the vacuum caused by the gas flow under the intake valve causes it to slam shut.  From this point on, the gas will expand and cool as the piston descends.

slide 10
Before the piston reaches bottom, the gas has expanded to the exhaust pressure.  At this point the exhaust valve will spring open and remain open until the piston reaches top again.  Because the exhaust valve is opened before the piston uncovers the port, Gordon mode will work whether or not the cylinder in question has a port (ported vs unported motor).  The other modes require the port and therefore are only available for ported motors.
slide 12
Because the exhaust valve stays open as the piston rises, this motor does not re compress steam like the uniflow.  Also the torque required to start the motor is very low, just enough to overcome friction as the piston goes up.
slide 15
Gas expelled out the exhaust has had a full expansion cycle and is therefore cooler. The energy removed by cooling the gas has been transferred to the piston and crankshaft, giving a higher efficiency than a motor that does not expand the gas.