34 may be a rotary turbine type with a housing 36 vanes 38 and output shaft 40 connected to the vanes directly or through reduction gears not shown an air motor construction for example a vane air motor or the di pietro motor are also suitable for this second expander the output shaft 40 then can be connected to the vehicle drive train or auxiliary generator system for example it should be also understood that while a rotary expander 34 is illustrated other expanders such as reciprocating expanders can also be used as the second expander as shown in fig3 a reciprocating piston type expander 46 is illustrated where piston 48 is connected to crank arm 50 which in turn is connected to output shaft 52 that can be connected to the vehicle drive train or auxiliary system air control valves 54 and 56 are connected or timed with output shaft 52 for proper sequencing of opening and closing after the second expansion the working gases pass through an outlet 41 and enter an exhaust system 42 open to the atmosphere which may include an exhaust muffler and tailpipe not shown by having more expansion of the working gases providing work on the second expander 34 or 46 a more efficient engine with improved fuel consumption at very low emission levels is achieved in comparison to a conventional single expansion cycle engine this dual expansion cycle with an intermediate emission treatment station interposed between two expansion sections can be applied to a wide variety of internal combustion engines and allow for an effective emission treatment station working at higher pressures and higher temperatures than conventional catalytic converters by providing.