plugs are fired in time by standard ignition timing devices 66 fig1 each plug is provided with one of a pair of small ignition chambers 67 and 68 fig3 in the stator opening through ports 69 and 70 in the stator into the cylinders 48 and 49 adjacent thereto to cooperate with the various ports so provided in the stator each of the hollow pistons 46 and 47 has corresponding ports arranged in connection with the stator ports to afford a proper ignition timing sequence for a 4 stroke otto cycle operation substantially midway of its outermost wall 71 each of the pistons has an ignition port 72 designed to register with one of the ignition ports 69 and 70 when the piston is substantially midway in its path of excursion axially through the stator furthermore each of the pistons has in one side wall 73 a transverse series of ports 74 fig3 designed to cooperate with the inlet port 58 fig5 and 8 at an appropriate part of the cycle similarly the opposite side wall 75 of each piston has a similar series of ports 76 designed to cooperate at the proper time in the cycle with ports 77 fig4 and 6 leading into the exhaust passages 61 and 62 and to the exhaust pipes 63 also extending into the stator are ducts 81 and 82 for the supply and discharge of cooling liquid from volumes 83 and 84 in parts of the stator not occupied by the blocks 53 this affords substantial liquid cooling of the stator and by conduction of the rotor rings as well in addition and for lubrication the stator and the blocks 53 have oil supply passages 86 and 87 through which pressure oil is furnished to the mechanism in the.