Why do railroads use diesel-electric locomotives?

Because diesel-electric locomotives are the best engines for the job. They produce plenty of horsepower, are fuel efficient and require little maintenance and repairs.

What happened to steam powered locomotives?

U.S. Railroads used steam engines from their inception in the early 1800s until the 1960s, when improvements in diesel power made diesel-electric powered rail locomotives a better choice.

Steam locomotives easily produced enough tractive power to grip metal rail tracks, and pull heavy loads. But, over time, steam powered locomotives presented a difficult problem for the railroads. Their production, maintenance and upkeep were very specialized and becoming increasingly expensive. By the late 1940s and early 1950s, it was no longer cost-effective for large railroad companies to maintain and run these powerful iron horses. In addition, railroads in the western US were finding it expensive to constantly transport clean water to desert locations for the thirsty steamers. During this time, railroads also experimented with steam and gas turbine-electric locomotives and found them more powerful but more expensive to maintain than the diesels.

Railroads also entered the post-war era with a new sense of optimism that led them to invest billions of dollars in new locomotives, freight equipment and passenger trains. That investment would see retirement of the last steam locomotive by the 1960s in favor of diesel engines.

What about electric locomotives?

Electric locomotives were also evolving, but were limited in their use due to the high cost of installing wayside electric traction systems and catenary lines to supply power to the engines. Diesel hydraulic locomotives were also tested but found to be more expensive to purchase and maintain.

So the clear choice were the diesel-electric locomotives.

What is diesel power?

Named for Rudolph Diesel, a German engineer born in Paris in 1858, diesel power had been around since 1901. In the 1920s, improvements in reliable controls to match the load of electrical generating and propulsion systems led to the first generation of diesel train locomotives in the late 1930s. Diesel engines are compression ignited. Unlike , say, a gasoline-powered automobile engine, which is spark ignited. Diesel engines are also very fuel efficient. This is why diesel-powered automobiles became popular during the energy shortage of the 1970s.

During WWII, much of our nation's industrial production was devoted to winning the war. Railroads bought as many diesel locomotives as they could. They found that the diesel locomotives required less maintenance and could handle far heavier loads than the other kinds of locomotives. This became a great proving-ground for diesel power.

The challenge for diesel locomotives was clear from the start. How can they deliver enough power to pull large trains, and yet keep the engines relatively small, easy and inexpensive to operate, and able to handle over curving tracks?

Why are freight locomotives called diesel-electric or hybrids?

It would be impossible to have the diesel engine deliver the power directly, like with an automobile drive-train. Cars rely on transmissions to drive power evenly to the wheels. Yet with large locomotives, such giant transmissions would require scores of huge gears, and ultimately more trouble with reliability and maintenance. The gearbox would have to handle up to 6,000 horsepower!

Diesel-electrics do something ingenious. They generate huge amounts of electric energy and use that generated energy to drive the wheels. That is why they are called diesel-electric or hybrid locomotives. So railroad locomotives use a diesel engine to generate huge amounts of electric power, and then use the electric power to run individual motors at each wheel. So what we really have is a combination of a large diesel engine, a generator, and many smaller electric engines.

Click here for a diagram showing a cross-section of a diesel electric locomotive. (Source: Railway Technical Web Pages. 2001)

By going with a hybrid setup, the main diesel engine can run at its most efficient speeds, turning an electrical generator. The electric generator sends power to a traction motor at each axle. The axel then powers the wheels. The traction motors can produce adequate torque at any speed, from a complete stop to speeds beyond 100 mph, without needing to change gears.

Here are some of the specifications of a typical diesel-electric freight locomotive engine:

Number of cylinders: 16
Full speed: 904 rpm
Compression ratio: 16:1
Displacement per cylinder: 11.6 L (710 in3)
Cylinder bore: 230 mm (9.2 inches)
Cylinder stroke: 279 mm (11.1 inches)
Normal idle speed: 269 rpm
The electrical generator is about 6 feet in diameter and weights about 17,700 pounds.

Next time you see a diesel Electric locomotive, remember that it is more than simply driven by one huge engine. It is a complex work of machinery, powered by one large diesel engine, one huge generator, and many small electric motors. It uses the latest technologies to ensure that it can start up and push or pull tons of freight. These locomotives carry everything from raw materials, industrial products, lumber, cars, food, and even high-tech products like computers and machinery to people everywhere.

Are there addition web resources for information about diesel-electric locomotives?

Railway Technical Web Pages

More Information about Diesel Electric Locomotives