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


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.

In: 6

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.

A typical modern car has about 100 horse power per ton. A tractor trailer has about 10 hp per ton that it’s moving. A train locomotive is lucky if it has 1 horse power per ton. The transmission on a modern car has typically has 4-6 gears. A tractor trailer needs a much larger transmission with lots more gears. Typically 12 – 16. And because it’s pulling more, each of those gears needs to be bigger and heavier. So it still scales but it becomes less efficient. By the time you get to a locomotive, if you wanted to have a normal transmission it would need still more gears and they would have to be much bigger and heavier still. So it’s worth it to do something different that is less efficient in general but scales up much better. And that’s converting the kinetic energy into electricity and then driving them using electric motors which doesn’t need a transmission at all.

Worked on the railways as mate to an engineer. Asked him why they tolerated all the inefficiencies of electric motors and he said the developers had tried gearboxes but they couldn’t handle the stresses: ie, huge power and huge weight.

So the lecky motors are a kinda cushioned delivery.

Highways have a maximum weight allowance. The heavier the truck is, the less cargo it’s allowed to carry. Electric motors in a semi truck would weigh thousands of pounds. The drawbacks quite literally outweigh any benefit.

Heat engines suck at producing torque. Really, the engine in your car or truck doesn’t have the balls to get your car to even move from a stop. It has to go through a transmission, RE: a torque multiplier, typically near 4:1 for first gear, AND THEN AGAIN with the differential or transaxle usually near 2:1.

AND EVEN THEN, the damn thing has a minimum speed, or it stalls. This means you need the ability to slip. This is done with either a friction plate like a clutch, or a torque converter – as in an automatic transmission, which is a hydraulic pump.

And then you’ve got to consider that engines produce optimum torque at a specific RPM. So how do you balance engine speed and torque with slip? The clutch is out, because you’ll burn it up long before you get a 2 mile long train moving. That means the train would need an automatic transmission.

And how many gears would the damn thing need? Remember the power band on that engine is very narrow. And how big would the transmission have to be, physically? I’ve heard that the US Navy has gearing between the engines and the props on their ships, and they’re so big, and so expensive, the Navy doesn’t actually own that part of their ship – they have a lease agreement for the gears with the ship builder. Insane! You’re talking about adding a gear box that’s going to be the size of an SUV to a freight train.

Compare that to an electric motor. You can run the diesel generator at optimum speed to produce electricity, and it can’t stall pulling the train. Electric motors produce 100% of their torque at 0 RPM. That’s damn useful for pulling a heavy train. You can omit the torque multiplier because you can more efficiently scale the electric motor itself.

There are efficiency concerns; You’re converting rotational energy to electrical energy back to rotational energy, whereas the diesel drives the wheels with rotational energy, end to end, you only suffer parasitic losses. But it’s safe to conclude that engineers smarter than us have done the numbers, and the electric strategy won out. If I recall my tidbit of train history I do have, I do believe diesel direct drives were tried in the early days, and they ran into all sorts of mechanical and engineering problems.