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What Is the Difference Between an Electric and a Diesel-Electric Locomotive? By Danielle Abram Print

What Is the Difference Between an Electric and a Diesel-Electric Locomotive?

What is an electric locomotive?

An electric locomotive is an engine that is powered by electricity through overhead lines, a third rail, or on-board energy storage. This energy storage can be in the form of a battery or even, as technology advances, a supercapacitor. 

Electric locomotives are highly efficient over other types of locomotives; often above 90%! 

What are the different parts of an electric locomotive?

 

 

  • Pantograph - These items are mounted on the roof of the locomotive to collect the power from the tension wire.
  • Circuit Breaker - A circuit breaker isolates the power supply for maintenance or if there is ever a problem.
  • Compressor - The compressor is highly important and operates the air or vacuum braking systems as well as additional accessories in the engine. 
  • Cooling Fans - This air management system helps keep thyristors and electric power systems cool and regulated to the correct temperature. The fans are powered by an auxiliary inverter.
  • Motor Blowers - These blowers help cool traction motors and maintain a reasonable level for long periods at a time.
  • Battery - Helps start and gives power to essential circuits, such as emergency lighting. The battery is typically connected across the DC control supply circuit.
  • Rectifier - This is a converter used to change AC to DC. 
  • Inverter - This electronic power device helps alternating current from direct current. 
  • Transformer - This is a set of windings with a magnetic core utilized to adjust the voltage levels up and down. 
  • Axle Brush - The power supply circuit is completed by the substation once there is power. The current then collected from the third rail or the overhead line is returned by the axle brush as well as a running rail. 
  • 3-Phase AC Motors - Used on both DC and AC railways, this modern traction motor type uses three phase AC electrical current supply.
  • DC-DC Converter - This is a common term for any solid state electronic system that converts alternating current to direct current, or the other way around. 

Additional Parts That May Be On an Electric Locomotive 

  • Insulators - Insulators are important because they protect the ductile iron shoulder from harm by direct contact with the rail. You can find these items between the rails and the sole plate.
  • Bumpers - These items help prevent rail cars and locomotives from going past the end of the physical section of the track.
  • Trucks - The trucks are the heaviest parts on a locomotive and play an important role. The trucks support the locomotive as well as provide breaking, suspensions, and propulsion.
  • Coupler - Device at the front and rear of the locomotive for connecting locomotives and rail cars together.
  • Trolley Poles - The poles are the current collectors that  transfer electricity from the overhead wire to the control and the electrical traction motors. 

What is a diesel-electric locomotive?

Locomotives that include on-board fueled prime movers are known as diesel-electric locomotives (when powered by diesel engines) or gas turbine-electric locomotives (when utilizing a turbine). While turbine engines are lighter for the same amount of power produced, they require more specialized maintenance than the traditional two- or four-stroke diesel engine. 

A diesel-electric locomotive is equipped with a diesel engine that spins a generator (DC) or alternator (AC) to create electricity. This electricity then powers the heavy electric motors that move the train forward. A diesel-electric locomotive has no mechanical connection between the prime mover and the axles. 

Direct Current electric motors are cheaper to construct but have limited overload duration before damage occurs. Alternating Current electric motors are more expensive to construct but have the advantage of taking as much power as the engine can generate - a heavily loaded coal train can pour electricity into its AC motors without fear of overheating. 

One of the greatest advantages of electric and diesel-electric propulsion is that the locomotive’s power output is independent of its speed, allowing it to apply full power at a dead stop. A steam locomotive, on the other hand, generates the least amount of useful power when stopped and reaches a maximum power, determined by driver size, boiler pressure, and valve settings, that cannot be altered. 

There are three primary types of diesel-electric transmission, each with their own benefits and drawbacks.

  • DC - DC (DC generator supplying DC traction motors
  • AC - DC (AC alternator output rectified to supply DC motors)
  • AC - DC - AC (AC alternator output rectified to DC and then inverted to 3-phase AC for the traction motors)

DC - This design includes a generator that supplies the DC traction motors through a resistance control system.

AC-DC - With this design, there is an alternator that produces AC current that is rectified to DC and then transferred to the DC traction motors

AC-DC-AC - With powering 3-phase AC traction motors, this modern design has the AC alternator output being rectified to DC and then converted to AC. This system may be slightly more complex than the others, though there are more benefits to doing it this way. 

What are the different parts of a diesel-electric locomotive?

  • Radiator - The radiator distributes water around the engine block to help regulate the temperature to be efficient. 
  • Radiator Fan - The water is cooled by passing through a radiator, which is blown by a fan driven by the diesel engine.
  • Turbocharger - Turbocharging is used in order to increase the amount of pushed air into the cylinders. The turbocharger is driven using the engine’s exhaust gas. This process allows a 50% increase in engine power without increasing fuel costs. 
  • Main Alternator - The main alternator is important because it provides power that moves the train. By generating AC electricity, the power is used for the traction motors on the trucks.
  • Auxiliary Alternator - AC power was used to provide lighting, air conditioning, heating, and other functions on the train through an auxiliary power line.
  • Diesel Engine - The diesel engine is the main power source for a locomotive and includes a large cylinder block in a straight line or a V. This is the power source for the alternator that helps produce electrical energy needed to drive the locomotive.
  • Air Intakes - The air intakes are responsible for cooling a locomotive’s motor using air from outside the engine and then filters the air by removing dust and other impurities. The flow of fresh air is regulated by both inside and outside temperatures.
  • Rectifiers/Inverters - The rectifier is a converter used to change AC to DC. The inverter is an electronic power device that helps alternating current from direct current.
  • Electronic Controls - Usually collected in a cab for easy access, electronic controls are used to include a maintenance management system that could be used to download data to a portable computer. 
  • Control Stand - The control stand is the principle man-machine interface. It is known as the control desk in the UK.
  • Batteries - Provide electrical power for controls and lights when the engine and alternator are not running.
  • Traction Motor - Traction motors are used on axles to give the final drive.
  • Motor Blower - Mounted inside the locomotive body, the motor blower provides air that is blown over traction motors in order to keep them cool while working hard. The blower output is connected to all of the motors through flexible ducting and it also cools alternators.
  • Fuel Tank - The fuel tank, typically under the frame of the locomotive, holds fuel for the locomotive, typically in capacities of one to three thousand gallons or more, depending on options. In addition, cooling water and lubricating oil is often carried on the locomotive.
  • Drive Shaft - The main output is transmitted through the drive shaft to the alternators on one end while the compressor is on the opposite end. 
  • Gear Box -  The radiator and its cooling fan is often located in the roof of the locomotive.  Drive to the fan is therefore through a gearbox to change the direction of the drive upwards.
  • Air Compressor - The air compressor provides compressed air for the loco and train brakes constantly.
  • Air Reservoirs - Air reservoirs are required for train braking and other functions of the locomotive and are often located next to the fuel tank under the frame of the loco. 
  • Truck Frame - The trucks support the locomotive as well as provide breaking, suspensions, and propulsion.
  • Sandbox - Sand is always carried on board in case there are any bad railing conditions. 

What are some differences between a diesel-electric and an electric locomotive? 

Electric Diesel-Electric
Powered by electricity transmitted through wires or rail from a stationary powerplant. Self-contained power generation with no need for transmission wires or expensive third-rail systems.
Pantographs on the roof of the locomotive collect power from overhead lines, or special shoes gather current from a third rail. Conventional diesel-electric locomotives use a diesel-fueled two- or four-stroke engine. Gas turbines may use fuel oil to heat water and create steam to power a turbine.
Capacity for immense power output, typically limited by the amount of electricity available and the likelihood of the motors melting down. Power output limited by the size of the diesel engine, requiring a balance between high output and reliability.
Capable of short-term bursts of high acceleration, perfect for stop and go commuter service. Without the need for wires or third rail, there is greater freedom of movement for a diesel-electric locomotive and less risk of severe weather interrupting service.
Single-source pollution allows for easier to implement reduction efforts without requiring an overhaul of rolling stock. Every locomotive generates pollution, requiring expensive overhauls and even re-engine refits in order to meet more strict emissions standards.

 

There are so many different parts and components that make up electric and diesel-electric locomotives. There are advantages and disadvantages to using both engines, though overall these engines are both more modern and have more benefits than using a steam engine. No matter which locomotive you choose, make sure you are prepared! 

Have any questions or suggestions? Let us know! Email dabram@trainz.com.

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