GP20

An EMD GP20 is a 4-axle (B-B) diesel-electric locomotive built by General Motors' Electro-Motive Division between November 1959 and April 1962. Power was provided by an EMD 567D2 16-cylinder turbocharged engine which generated 2,000 horsepower(1,500 kW). 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. 260 examples of EMD's production locomotive model (with the EMD turbocharger) were built for American railroads.

The GP20 was the second EMD production locomotive to be built with an EMD turbocharged diesel engine, sixteen months after the six-axle (C-C) model SD24. Power output of the turbocharged SD24 was 33 percent higher than the 1,800 hp (1,340 kW) of the concurrent Roots blower-equipped SD18s with the same engine displacement, 400 hp (298 kW) per axle, but the power output of the turbocharged GP20 was only 11 percent higher than the 1,800 hp (1,340 kW) of the concurrent Roots blower-equipped GP18s with the same engine displacement 500 hp (373 kW) per axle, due to the limitations of the traction motors then available (this limitation was eliminated in the 40-Series). Nevertheless, the turbocharged GP20 provided full rated power at all altitudes, which the Roots-blown GP18 could not provide.

EMD-type Turbo-Compressor (Turbocharger)
The turbocharger was the then-new EMD mechanically assisted turbo-compressor. 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. Turbocharging provides higher horsepower and good running characteristics at all altitudes. Turbocharging also improves fuel consumption and reduces emissions.

Previous Union Pacific experiments with turbocharging had utilized multiple Elliot[1] or Garrett AiResearch turbochargers feeding the usual pair of Roots blowers. EMD's mechanically assisted turbocharger eliminated the need for the pair of Roots blowers and also integrated the turbocharging function from two (Elliot) or four (AiResearch) smaller add-on turbochargers into one much larger, turbo-compressor (turbocharger) with intercooling.

The introduction of the EMD-type turbocharger was successful and all subsequent GP series were offered with this turbocharger, although not all models within a series were offered with turbocharging (e.g., the 38 sub-models within the 40 Series were Roots-blown).

Identification
Units built for the Great Northern Railway were equipped with high short-hoods and were set up to run long-hood forward with a single control stand. Western Pacific received ten GP20s with high short-hoods and dual control stands, thereby facilitating running both long-hood forward and short-hood forward.

While the EMD SD24 with six axles was producing 2,400 horsepower (1,800 kW) with an engine of the same displacement (400 horsepower (300 kW) per axle), the four axle GP20 was limited to 2,000 horsepower (1,500 kW) (500 horsepower (370 kW) per axle) by the capabilities of the traction motors and the direct current traction generators of the time. In appearance the locomotive was similar to a late version GP9 or GP18 (i.e., often with a low short hood, but with the two exhaust stacks over the engine replaced by a single stack forward over the exit of the turbocharger, in common with later turbocharged EMD locomotives. An identification detail of the GP20 is the small radiator fan added ahead of the large aft fan.

De-turbo-ed GP20s, which employ 645 power assemblies, with Roots blowers replacing the EMD turbocharger without de-rating the horsepower of the locomotive, are somewhat common. As the reliability of the EMD turbocharger significantly improved over time, these de-turbo-ed examples became less common.