Science Fair Project Encyclopedia
The EMD SD90MAC is a 6000 horsepower (4.5 MW) C-C diesel-electric locomotive produced by General Motors Electro-Motive Division. It is, with the SD80MAC, one of the largest single-engined locomotive produced by that company, surpassed only by the dual-engined DD series. The SD90MACs feature radial steering trucks and an isolated cab which was mounted on shock absorbers to lessen vibrations in the cab. All SD90MACs use AC traction motor technology. They are all hood units fitted with safety cabs. The SD90MAC, like the SD80MAC, SD70ACe, and SD70M-2, has a wide radiator section, nearly the entire width of the locomotive, which along with their size makes them easy to spot.
The SD90MAC was introduced in 1995, along with the SD80MAC locomotives, which designed as a lower-power version of the SD90MAC. However, technical problems with the 6000 horsepower engine resulted in the first locomotives being shipped with 4300 horsepower (3.2 MW) motors, making them less powerful than the SD80MACs. These locomotives were given the model SD90/43MAC and railroads were given the option to remotor them with 6000 horsepower (4.5 MW) engines when they became available. This upgrade program was rarely used due to reliability issues with the newer engine. Over 400 SD90/43MAC locomotives were built.
In 1996, EMD entered full production on their 6000 horsepower (4.5 MW), 16-cylinder H-engine, and all SD90MACs made from then on used that for its prime mover. Locomotives fitted with this engine are sometimes referred to as SD90MAC-H locomotives. Later versions of the SD90MAC-H feature a new nose which offers higher visibility from the cab than the old nose, although many railfans have complained about the look of the sharp edges on it. The SD90MAC-H did not prove popular with railroads and less than 70 were built, including EMD demonstrator units. Since the SD90MAC-H had such a large prime mover, it didn't offer the same operational flexibility as smaller units, limiting its possible customer base to only the largest railroads. Also, since the H-engine was a new design it hadn't reached the same level of reliability as EMD's previous engine. The low reliability on such a large engine was an especially bad combination since the loss of one engine in a train meant the loss of a larger percentage of pulling power than had a smaller engine failed. In the end the SD90MAC-H was only delivered to two railroads, the Union Pacific Railroad and the Canadian Pacific Railway. The Canadian Pacific locomotives were part of an earlier order for SD90/43MAC locomotives that was still in production when EMD switched over to the H-engine.
EMD also tried offering a lower-power version of the SD90MAC with a 12-cylinder engine called the SD89MAC, but none were produced other than the prototype.
As of January 2005, the SD90MAC is no longer in production due to the Environmental Protection Agency's Tier 2 locomotive emission regulations, although EMD may be able to get the H-engine approved at some later date.
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