The sheer logistics of labor and coal started to weight heavily on the steam locomotive rail system after World War II. More people started to see the advantages of the diesel-electric engine. A diesel-electric engine works by having the ignition of diesel fuel powering pistons connected to an electric generator. Diesel fuel is low volatile, cheaper and readily available, which makes it the preferred fuel for trains today. In addition, diesel-electric engines are less polluting than steam and are healthier for the crew.
The diesel fuel is stored in a storage tank on the engine. An electric fuel pump delivers the fuel to the engine. The diesel engine is an internal combustion engine. There are various numbers of cylinders connecting to a crankshaft. The pressure from compression ignites the fuel, which pushes the piston down. The downward movement turns the crankshaft.
The engine is connected to a main generator. This generator converts the mechanical power to electrical power. The electrical power that moves the traction motors. There is one traction motor directly connected to a pair of driving wheels.
The engineer, using an electrically controlled throttle, controls the power output. If the throttle is opened, more fuel is injected into the cylinders. This increases the mechanical output, which increases the main generator to higher electrical output. Since a mechanical clutch would be burned out in a short time, using electricity is more reliable and makes for less maintenance time.
Electric transmission is the saving point on a diesel-electric engine. Electric motors have a high torque, even when stationary. The engine is spun up fast, which puts a high difference across the axle motors. This is enough torque to make the train start moving and continue accelerating.
The system has some other features like the control system that changes the voltage on the electricity so fuel is saved. There are batteries for start-ups that can start the diesel engine. Fans and radiators keep the engine cool. The large grills that are seen on the side of the diesel engines are part of the cooling system.
There are also diesel-hydraulic engines. Instead of electricity, they use the hydraulic transmission to take power from the engine to the wheels. A torque converter is used instead of a motor. It has three parts, one part is fixed and the other two rotate. All of the main parts are sealed in a housing that is filled with oil. The inner rotating part is a centrifugal pump, the outer rotating part is the turbine wheel and between them is a fixed guide wheel. These mechanical parts make the engine more complicated and prone to breakdowns, which is why there are fewer of them in service.
The limitations of diesel engines are speed and power. The motors can rotate at maximum speed. Diesel engines are large and heavy, which makes them poor at high speeds. That is why you will see multiple engines ‘lashed’ together in front of the train. Engines can also be placed in the middle of the train and at the end of pushing.