How Diesel Locomotives Work

“ The 3,200-horsepower engine drives the main generator. ”

The 3,200-horsepower engine drives the main generator.

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The main reason why diesel locomotives are hybrid is because this combination eliminates the need for a mechanical transmission, as found in cars. Let’s start by understanding why cars have transmissions.

Your car needs a transmission because of the physics of the gasoline engine. First, any engine has a redline — a maximum rpm value above which the engine cannot go without exploding. Second, if you have read How Horsepower Works, then you know that engines have a narrow rpm range where horsepower and torque are at their maximum. For example, an engine might produce its maximum horsepower between 5,200 and 5,500 rpms. The transmission allows the gear ratio between the engine and the drive wheels to change as the car speeds up and slows down. You shift gears so that the engine can stay below the redline and near the rpm band of its best performance (maximum power).

The five-to-10-speed transmission on most cars allows them to go 110 mph (177 kph) or faster with an engine-speed range of 500 to 6,000 or higher rpm. Diesel engines have a much slower operating speed than gasoline, and that goes double for the massive ones used in locomotives. The large displacement diesel engine tops out at about 2,100 rpm, or lower. With a speed range like this, a locomotive would need 20 or 30 gears to make it up to 110 mph.

A gearbox like this would be huge (it would have to handle 4,200 horsepower), complicated and inefficient, and create a possible point of mechanical failure. It would also have to provide power to four sets of wheels, which would add to the complexity.

By going with a hybrid setup, the main diesel engine can run at a constant speed, turning an electrical generator via driveshaft. The generator sends electrical power to a traction motor at each axle, which powers the wheels. The traction motors can produce adequate torque at any speed, from a full stop to 125 mph (200 kph), without needing to change gears.

Why Diesel?

Diesel engines are more efficient than gasoline engines, and when moving literal tons of freight or passengers, efficiency is paramount. Train manufacturer CSX estimates that their fleet moves 1 ton (0.9 metric tons) of cargo an average of 492 miles (791 kilometers) per 1 gallon (4 liters) of fuel, making locomotives four times as efficient as moving goods on roadways. Diesel-electric systems are also five times more efficient than the old steam engine locomotives, which is why diesel entirely replaced steam in the early 20th century.

Diesel also has seen some competition from fully electric trains, which pull directly from a power grid as they drive. This method is several times more efficient than burning any kind of onboard fuel to produce energy. Electric locomotives are especially popular in Europe and Asia, but the changeover in the U.S. has been slow. Probable causes are that electric trains require their own specialized infrastructure to operate, and old locomotives can be in service for multiple decades before retirement. For the time being, diesel remains the standard. A few passenger railways have however been electrified in the States, including Amtrak’s northeast corridor and California commuter rail.