EMD GP20

The EMD GP20 emerged from a period of experimentation and competitive pressure within the American diesel locomotive market of the late 1950s. Electro-Motive Division had been cautious about applying turbocharging to its 567-series engines, but Union Pacific's independent trials with turbocharged GP9 units, known informally as the Omaha GP20s, demonstrated the viability of the concept and prompted EMD to develop a production turbocharged four-axle road switcher. The GP20 entered production in November 1959, making it the second EMD production locomotive to feature a factory turbocharged engine, following the six-axle SD24 by approximately sixteen months. Production continued through April 1962, with a total of 260 units completed for American railroad customers. Among the railroads that acquired the GP20 was the St. Louis Southwestern Railway, commonly known as the Cotton Belt Route, which operated the type in freight service across its system. The Western Pacific Railroad was among the earliest buyers, and the very first production GP20, Western Pacific 2001, has been preserved at the Western Pacific Railroad Museum in Portola, California. Southern Pacific also rostered GP20s, and at least one of those units later found its way to the Midland Railway in Baldwin City, Kansas, where it is maintained for excursion service. The GP20 occupies a meaningful place in diesel locomotive history as one of the first widely produced four-axle EMD units to benefit from turbocharging in a production configuration. While the model itself was not built in enormous quantities compared to contemporaries like the GP9 or GP18, it established the mechanical and commercial foundation for turbocharged power in subsequent GP-series designs. The success of EMD's proprietary mechanically assisted turbocharger on both the SD24 and GP20 influenced the direction of all later high-horsepower EMD road switcher development.

The GP20 was powered by the EMD 567D1 engine in turbocharged configuration, producing 2,000 horsepower from sixteen cylinders and driving the locomotive through a DC traction system with a B-B, or Bo'Bo', wheel arrangement on four powered axles. The turbocharger employed on the GP20 was EMD's own mechanically assisted turbo-compressor design, which functioned somewhat differently from a purely exhaust-driven turbocharger. At startup and during low-power operation, when exhaust energy was insufficient to spin the turbine at the speed needed for adequate combustion air, the engine drove the compressor directly through a gear train and a freewheel clutch. As power demands increased and exhaust energy became sufficient, the freewheel disengaged and the unit operated as a conventional turbocharger. This arrangement eliminated the need for Roots blowers and consolidated what had been multiple add-on turbocharging units in the earlier Union Pacific experiments into a single, larger turbo-compressor with intercooling. The power advantage of the GP20 over the naturally aspirated GP18, which used Roots blowers on the same basic engine displacement and produced 1,800 horsepower, was a comparatively modest eleven percent. This was constrained by the limits of the DC traction motors available at the time rather than by the engine itself, as the motors could not continuously handle power levels much beyond 500 horsepower per axle. Nevertheless, turbocharging conferred one significant operational advantage over Roots-blown competitors: the GP20 maintained its full rated 2,000 horsepower output at all altitudes, whereas the GP18's power fell off in high-elevation territory where thinner air reduced the effectiveness of the blower-fed induction system.