Below, the rotary engine from Rumsey’s 1791 English patent. Like most of his designs, it was never built. The first view is the engine from above. The rotor in the middle spins around, within a hollow case, and the large cams NN alternately push out and let drop the gate valves that are connected to the rollers, R. Yes, this is mysterious. Look at the next drawing, below it.
Fig. 13, below. Engine from the top, with the top plate MM off. Steam flows in through the axle and out into the turbine arms- DD. It expands in the space between the semi-circular rotor seals and the gates, EE, pushing the rotor around. As the rotor turns, gates are lifted and closed, creating two “strokes”: 1) a gate closes behind a steam port on the rotor , steam is admitted and expands, turning the rotor. 2) the next gate is closed behind the steam port: the parcel of steam ceases to expand . 3) another gate opens, allowing the parcel of steam into one of the exhaust ports in the rotor , out through one of the exhaust arms, BB, and then out the hollow axle. With an air pump, pumping out the condenser, there would be significant vacuum in the exhaust port. With two intake and two exhaust ports, there are two expansion strokes, and two condensation strokes, per revolution.
Note that, with three gates 120 degrees apart and two ports 180 degrees apart, there is always a closed gate between a steam port and an exhaust port. He could have designed it with one set of ports and two gates: that would have been more efficient ( note that , at the top of Fig.13, there is a parcel of steam sitting between two gates, doing no work). But two sets of ports in effect creates two “pistons”, giving twice as much torque as one set.
Fig. 12. The engine from the side. Steam flows in from the top ( through a rotating seal UU) and into the hollow turbine arms DD and then into the turbine, where it expands. The condensed steam drains through the other hollow turbine arms H and is pumped out by the gear pump LL at the bottom into the “hot well”, or water reservoir, V. Rumsey’s drawing of the rotor and casing is over-simplified here, not clearly showing the case or the top and bottom plates. The letters a,b,c,d are described as “a ring of metal” to hold the “ocum”, or seal packing, in place: these would turn with the rotor.