In 1958, GM’s Electro-Motive Division took a great technological stride forward with the introduction of the turbocharged SD24. In turbocharged engines, exhaust gases are used to power a turbine that increases cylinder air pressure, thus effectively increasing horsepower. In fact, the 16-cylinder model 567 diesel that produced 1800 horsepower in the normally aspirated SD18 locomotive was upgraded to 2400 horsepower in the turbocharged SD24.
Turbocharged engines also run better at varying altitudes, making them ideal for railroads that travel through changing terrain. Look on the left side of the SD24’s hood for the semicircular bulge that marks the location of the turbine.
The SD24s were close relatives of the popular GP-series engines. The primary difference was that SD-series (for “Special Duty”) locomotives were lengthened to make room for three-axle, three-motor trucks. For a GP and an SD engine of the same horsepower, the extra wheels on an SD locomotive gave it more tractive effort at low speeds and less engine weight on each axle. For this reason, the SD24 and others in the SD series were ideally suited for secondary lines with lighter rail.