An engine including at least two piston and cylinder assemblies that, when operating with a fuel savings cycle, establish at the end of the simultaneous compression strokes a charge of compressed air in one cylinder of one assembly and a charge of compressed air fuel mixture in the other cylinder of the other assembly. When the air fuel mixture is ignited, the high pressure conditions in the other cylinder are immediately communicated through a passage to the one cylinder to accomplish a double expansion during the simultaneous power drive strokes thus using much of the pressure energy before exhaust occurs by the pistons themselves. The improvement comprises changing the other assembly which has the charge of compressed air fuel mixture therein between the two piston and cylinder assemblies in a predetermined pattern as, for example, an operative alternation between the two piston and cylinder assembles.