The present invention pertains to the air conditioning art and, in particular, to an air conditioning system which is operable in one of several modes. 
Various cabin air conditioning systems, particularly those for use in cooling the cabin of a vehicle such as a commercial airplane, are well known in the art.  The two best known air conditioning systems employ either a simple cycle or bootstrap cycle mode.  Simple cycle apparatus is supplied with a source of high temperature, high pressure air such as bleed air from a jet engine.  The air is passed through a heat exchanger to a turbine, causing the air to do work and thus lowering its temperature.  The output from the turbine is fed to the cabin input.  On the same shaft of the turbine is a fan which draws airplane ram or ambient air through the same heat exchanger for cooling the turbine supply air.  Such simple cycle systems are highly inefficient because the turbine does more work than the fan absorbs and, thus, the turbine is poorly loaded and may run away unless an inefficient fan is used.  A compressor, which often is mounted to the same shaft as the turbine and fan, draws air from, and recirculates air to the cabin.  This configuration not only provides partial cabin air recirculation but also loads the turbine thereby reducing the risk of run away. 
A more efficient, and more commonly used system, operates on the bootstrap cycle.  Here, a source of high temperature, high pressure air, such as bleed air from a jet engine, is compressed in a compressor thus further increasing its temperature and pressure.  The output from the compressor routes through a heat exchanger and is then applied to the input of a turbine.  As with the simple cycle mode, the turbine causes the high pressure, high temperature air to do work, thereby lowering its temperature and pressure such that it is suitable for supplying air to the cabin.  A feature of the bootstrap system is that the compressor and turbine share a common shaft and, as such, one drives the other.  It is also common in the bootstrap system to provide a fan on the common turbine/compressor shaft, which fan draws ram or ambient air through the aforementioned heat exchanger thus reducing the temperature of the air at the compressor output.  Because the compressor boosts the bleed air pressure supplied to the turbine, the system is inherently stable and efficient. 
A problem with the bootstrap cycle, however, is that it does not provide for recirculation of cabin air.  This is undesirable since in some ambient air conditions more energy would be consumed in processing fresh air than in the conditioning of recirculated air. 
A further problem with air conditioning apparatus for use in a commercial airplane is that high pressure engine bleed air is only available when the plane is airborne.  Thus, some means must be found to condition cabin air during intervals wherein the plane is on the ground.  Preferably, the plane's existing air conditioning system should be capable of being driven, such as by an electric motor, to provide adequate air conditioning during the ground condition. 
Thus, there is a need in the air conditioning art, particularly as applied to conditioning air for the cabin of an aircraft, to provide a simple air conditioning system which is capable of providing the air recirculation feature of the simple cycle with the high efficiency of the bootstrap cycle mode and which may be driven by a drive source other than high pressure air.