How a Diesel Electric Locomotive Works - MagnumMacKivler/RLCPT2 GitHub Wiki

Diesel-electric locomotives have been the backbone of the North American railroad since the 1950s. Making their debut in the North American market in the 1920s and 30s, the diesel-electric would grow over the years from a novel alternative to steam for small switching operations to becoming the unchallenged standard for nearly every branch of railroad service both in North America and in many places around the world.

What makes a diesel-electric?

A diesel-electric, as the name suggests, uses a diesel engine as its main power source. These engines come in a variety of shapes and sizes, and are discussed in greater detail here. What makes a diesel-electric different from any other kind of diesel locomotive is how the power from the diesel engine is transmitted to the wheels. Where a diesel-mechanical locomotive might use a clutch and gearbox to connect the engine to the wheels or a diesel-hydraulic might use a hydraulic torque converter, a diesel-electric uses a generator and motors to transmit the power.

Connected to the output shaft of the diesel engine is the main generator. Because it is used to drive a generator, the diesel engine is usually called the prime mover. The main generator can either be a brush-and-commutator DC generator or an alternator (AC generator), depending on the locomotive.

The output of the main generator is connected to a set of electric motors called traction motors, which are geared to the locomotive's wheel axles. The electrical current generated by the main generator is used to power the traction motors, and thus, propel the locomotive. Locomotive traction motors are typically either series-wound DC motors or NEMA design D induction motors (AC motors), depending on the time period and individual locomotive model.

Why use a diesel-electric?

The use of a generator and traction motors to transmit power has a number of advantages over mechanical or hydraulic transmissions. For starters, traction motors develop their highest starting torque (and therefore, the most tractive effort) at low speeds, making diesel-electrics very good at starting heavy trains. Secondly, where mechanical/hydraulic transmissions are required to pay close attention to the prime mover's power band, diesel-electrics can take advantage of the prime mover's power output at any locomotive speed and use it efficiently. Thirdly, diesel-electrics can optionally be equipped with dynamic brakes. Dynamic brakes repurpose the traction motors as generators, taking energy from the train's motion and converting it into electricity to slow the train down. Lastly, replacing a system of drive shafts and gears with cables and relays greatly simplifies maintenance of a locomotive and allows for a more robust system. It is for these reasons why diesel-electrics are (with a few exceptions) the predominant example of locomotive technology in North America.