 During engine startup, and at lower power levels, during which there is not sufficient exhaust heat energy to drive the turbine fast enough for the compressor to supply the air necessary for combustion, the engine drives the compressor through a gear train and an overrunning clutch.At higher power levels, the overrunning clutch is disengaged, and the turbo-compressor operates as a true turbocharger.It is possible for the turbo-compressor to revert to compressor mode momentarily during commands for large increases in engine power.