The EMD SD24 is a six-axle (C-C) diesel-electric locomotive built by General Motors' Electro-Motive Division of La Grange, Illinois between July 1958 and March 1963. A total of 224 units were built for customers in the United States, comprising 179 regular, cab-equipped locomotives and 45 cabless B units. The latter were built solely for the Union Pacific Railroad.

The SD24 was the first EMD production locomotive to be built with an EMD turbocharged diesel engine. The first SD24 was built sixteen months before the four-axle (B-B) model GP20. Power output of the SD24 was 33 percent higher than the of the concurrent Roots blower-equipped SD18s with the same engine displacement. The SD24 has per axle, limited by the traction motors available. Nevertheless, the turbocharged SD24 provides full rated power at all altitudes, which the Roots-blown SD18 could not provide.

In terms of sales, the SD24 was only a moderate success, and had average service lives in SD24 configuration (though a few non-turbocharged rebuilds are still in operation), but the SD24 was a milestone in EMD locomotive development and the forerunner to today's high-powered six-axle locomotives.

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 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 SD 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).

Options

Optional equipment that could be specified by ordering railroads included multiple unit controls, a steam generator, dynamic brakes, winterization equipment, an air signal line, and hump control.

Unit 410 received the most minor modification: upgraded air filters.

UP 423, however, was substantially rebuilt to UP 3100 in August 1968 with a constant-speed 16-645 prime mover, a new alternator and new traction motors. With the constant-speed engine, speed control was via changing the level of excitation of the alternator; the traction motors were permanently wired in parallel. New, variable dynamic brakes were also fitted, as was a self load feature enabling testing under load without an external electrical load apparatus. This latter feature became standard on later locomotive models. The constant-speed prime mover, on the other hand, was not successful enough to duplicate. From 1975, the locomotive was used as a heavy switcher at UP's North Platte, Nebraska hump yards, a fate common to UP's other surviving SD24s.

UP 414 did not receive as major a modification as #423, but it was refitted with a 16-645 engine, AR10 alternator and Dash Two electrical cabinet in May 1974, making it effectively an EMD SD40-2. This was a testbed for a proposed upgrade of all UP's SD24s, but UP decided not to go ahead with the work. Both UP 3100 and UP 414 were classified as "SD24m" by the UP, but some UP internal documents class UP 414 as a "SD24-4".

Preservation

At least two SD24s survive in museums:

  • Chicago Burlington & Quincy 504, formerly Burlington Northern 6244 (which can be seen in the movie Groundhog Day), is preserved in operating condition at the Illinois Railway Museum.
  • Wisconsin Central 2402, ex Fox River Valley Railroad 2402, built as Burlington 510, is at the National Railroad Museum in Green Bay, WI, mechanically complete except for traction motors, its currently being restored back to its Fox River Valley Railroad appearance.

References

<!-- Deleted image removed: 300px|thumb|Santa Fe #4554, an EMD SD24, waits on the "whiskers" of the [[turntable (railroad)|turntable at Bakersfield, California in 1974.]] -->

  • EMD SD24 Diagram at the Illinois Railway Museum official website.
  • SP SD24 info.