The EMD GP20 is a 4-axle diesel-electric locomotive built by General Motors' Electro-Motive Division between November 1959 and April 1962.

EMD was initially hesitant to turbocharge their 567-series diesel engine, but was spurred on to do so following successful tests made by Union Pacific in the form of UP's experimental Omaha GP20 units.

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 a freewheel.

At higher power levels, the freewheel 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.