Eli5 why are diesel/electric powertrains economical for trains but not used in large trucks?

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Always wondered why it doesn’t make sense to use diesel/electric in large trucks. Assumed it’s because cost/complexity doesn’t out weight benefits, but not sure why. Thanks.

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Electric locomotives do not need to carry batteries. The electrical energy comes from either overhead wires or electrified rails.

An electric truck would need to carry batteries, and lots of them. The energy density of a battery is much lower than the energy density of gasoline or diesel fuel. In other words, the fuel weight of an electric tractor-trailer would be much greater than the fuel weight of a diesel tractor-trailer. Tractor-trailers can legally carry a finite amount of weight, and so it’s not economical for a tractor-trailer to carry (literally) tons of batteries.

In order for electric tractor-trailers to be economical, we would need to either find a way to deliver electricity directly from the power grid or improve the energy density of batteries.

Batteries have nothing to do with this technology. Diesel/electric locomotives use the Diesel engines to produce electricity which drives the electric motors. It works because they need to be heavy. It also works because a train isn’t constantly starting and stopping. Weight and momentum play a huge role in efficiency, as it does in large ships using similar technology.

OTR vehicles (semi tractors) are legally weight constrained due to safety and highway weight restrictions. The heavier the tractor, the less freight they can carry, and also the more energy to start, accelerate and stop the heavier the vehicle. Electric semis (with batteries) are a promising alternative but share none of the properties of a diesel-electric locomotive.

Hybrid diesel-electric adds a lot of weight, in a truck you subtract the tractor weight from the allowed trailer weight. In a train the max is much higher and a heavy engine doesn’t subtract from the load. Also trains don’t do stop and go so batteries are not important.

Diesel electric drivelines in a locomotive are the simplest way to get mechanical energy from the diesel engine to each one of the drive axles. A typical loco will have 2 trucks with 6 drive axles in total with an electric motor on each axle. The two truck assemblies pivot. It’s far easier to convert the diesel’s power into electricity and then run it through flexible wires instead of making some type of hopelessly complicated transmission.

A truck on the other hand has a simple driveline by comparison. The engine power runs through a tranny and then a driveshaft to either one axle or two. It’s not very complicated.

I don’t think the question is so much about efficiency as it is about difficult or easy mechanical design.

They dont on typical road trucks, but they do on mining trucks – when the truck is hundreds of tons, the weight of a system like that becomes economical.