Diesel to mechanical energy to eletrical to motor (like on trains)

[rpayne88]

Sorry for the title, it was all I could think of without having to write out a sentence for it. I know this is used on locomotives a lot, or, at least the CSX ones that pass by my house. How is it more efficient to burn diesel fuel to power a generator to power electric motors than it is to have the diesel engine directly providing torque. It seems to me you would lose energy through heat as a byproduct of combustion. You would then only continue to lose power due to electrical resistance as you sent the electricity through the wires from the generator to the motor. Never mind having to carry around the extra weight too.

[Camacha]

It is quite a nuisance to build a mechanical gearbox for something as big and heavy as a train. Cars have at most 6 speeds, trucks already have a lot more (9 to 16 with often high and low gearing, making the total 18 to 32), so you can only imagine what it takes to drive a fully loaded train. It just makes more sense to not bother with the whole gear thing and just do it electrically, where it is relatively easy to vary the torque and power applied.

You will lose some energy in the conversion, but remember that the diesel motor is quite inefficient anyway. Electrical generators and engines are, on the other hand, relatively efficient, so the losses are not that massive compared to pure diesel + gears.

Edited September 28, 2013 by Camacha

[Ralathon]

A few reasons:

1 - At least in the netherlands, in addition to the diesel engine trains can draw electric power from the wires overhead. If you have a diesel --> Electricity --> electric motor you can use both modes.

2 - Engines work most efficient within a certain band of RPM. If you turn the power to electricity first you can always have the diesel engine in that most efficient band, thus getting more miles for your buck. It's the same idea behind hybrid cars.

3 - Electricity is a lot more flexible. The wheels of a train are mounted on a rotating platform so the train can turn. It is rather difficult to transfer torque to wheels on a moving platform. Electric motors are small enough to just mount them on the moving platform. It also allows you to power much more wheels without too much additional effort which reduces slipping.

[Nibb31]

What you describe is called a hybrid engine. This is basically the same principle is used in the Toyota Prius and other hybrid cars. The only reason those cars use gasoline/petrol instead of diesel, is because they were originally aimed at the US market, where diesel isn't widespread enough for consumer use. The problem with those hybrids is that they are basically on par with conventional turbo-diesel engines in terms of mileage, performance, and emissions, for a much higher price.

In Europe, where people have been running diesel cars for decades, we are starting to see diesel hybrid cars, which make much more sense. I think the best concept is something like the Chevrolet Volt/Opel Ampera, which is basically a plug-in electric vehicle with a 60km range for daily commutes, but with a conventional generator engine to charge the batteries for longer trips. To me, replace it with a diesel generator instead of gas, and you really have the best of both worlds.

[dharak1]

As far as I know diesel is as widespread in the us as here in Canada. Every gas station has it and it's usually 10-20 cents cheaper than gas. I have no idea why they used gas. I think the general idea here is gas burns cleaner but as far as I know there isn't much of a difference.

[magnemoe]

The main reason for going diesel electric on trains is the powers involved. Far more than on trucks.

Add that you probably want to power 8 wheels or something, that would be complicated to do mechanical but simple with electrical engines.

And yes you can use line power if the track have it.

[Tiberion]

Diesel is higher than 87 Octane 10% Ethanol Gas here in the southern central US, though I have seen it the other way when gas spikes. Its usually within 0.50 of each other though.

I would image that 99% of the US has diesel stations, I've never personally seen one that doesn't have a diesel pump on at least one spot in the station. This is because both Semi-trucks and most farm equipment use diesel

We're also seeing more and more CNG (compressed Natural Gas) pumps here as well, but that could be because Oklahoma is a large producer of Natural Gas and there's a big push for a lot of infrastructure to swap to it to keep the energy money "local"

Can't say that I have ever looked at the numbers, I wonder how a CNG hybrid would compare to the others in efficiency

[Camacha]

I am not sure what the benefits of diesel versus other combustable fuels have to do with the original question. The hybrid concept in cars is quite different than that of diesel-electric trains, since cars use batteries as a main mode of power with a fuel driven engine as backup. Trains are just powered by diesel and not intended to be hybrid in the sense cars are - partly because of the lack of batteries, but mostly because energy conservation is not the goal. Electricity is just something that is useful to for easy drive train construction.

Edited September 28, 2013 by Camacha

[drakesdoom]

Diesel used to be cheaper than gas in the US, which makes sense since it is easier to refine and makes more per barrel of crude. However at some point in the last 10 years it has become more expensive, I would bet because it is mainly used in trucks here and they have no choice on what fuel to use.

[K^2]

2 - Engines work most efficient within a certain band of RPM. If you turn the power to electricity first you can always have the diesel engine in that most efficient band, thus getting more miles for your buck. It's the same idea behind hybrid cars.

This is almost all of the answer. The most efficient engine you can build is a diesel turbine, but it provides little torque, and is only efficient in a narrow RPM range. An electric motor, in contrast, will deliver most torque at low RPM, precisely where the locomotive needs it, while remaining very efficient at higher RPM. A turbine-generator-motor combo is about as efficient as you can make a train. The only downside is extra weight, which isn't as much of a factor for a locomotive as it would be for a car or a truck.

[mrfox]

This is almost all of the answer. The most efficient engine you can build is a diesel turbine, but it provides little torque, and is only efficient in a narrow RPM range. An electric motor, in contrast, will deliver most torque at low RPM, precisely where the locomotive needs it, while remaining very efficient at higher RPM. A turbine-generator-motor combo is about as efficient as you can make a train. The only downside is extra weight, which isn't as much of a factor for a locomotive as it would be for a car or a truck.

Current tech turbine engines (Brayton cycle) are thermally less efficient then diesels and have comparatively poor fuel efficiency. They do however have great power to weight ratios, hence why they are commonly used on aircraft. There are basically no turbine powered trains or ships in use outside of the military today.

[Anton P. Nym]

How is it more efficient to burn diesel fuel to power a generator to power electric motors than it is to have the diesel engine directly providing torque. It seems to me you would lose energy through heat as a byproduct of combustion. You would then only continue to lose power due to electrical resistance as you sent the electricity through the wires from the generator to the motor. Never mind having to carry around the extra weight too.

You do get losses, but the losses are less than the losses incurred by the limitations of mechanically-linked diesel; as mentioned elsewhere, because of how diesels fire they're much more limited in terms of rotations per minute (rpm) settings. Diesels are most fuel efficient at a few discrete sets of rpm, which doesn't lend itself well to direct linkage as vehicles don't travel at discrete speeds. (Can't get from 0-60 without going through 1-59, but the diesel works best at either 0 or 60 for example.) This is why gasoline fuelled cars typically have from 3 to 5 gear settings in their transmissions but diesel trucks start at 10 and go up from there; you need more gear ratios to make a diesel as flexible as a gas motor.

It ends up more efficient to have a diesel motor run at a fairly steady speed, tuned for optimum fuel consumption, to drive a generator and let the far more flexible electric motors handle the train-to-track bits. Diesel-electric is still less efficient than running electrical engines directly, because of the conversions you mention, but it does allow trains to roll where you don't have overhead or underground power lines.

-- Steve

[Seret]

How is it more efficient to burn diesel fuel to power a generator to power electric motors than it is to have the diesel engine directly providing torque.

There are some direct drive diesel trains, but doing so isn't necessarily simpler than going diesel-electric. You need a very chunky gearbox, driveshafts, final drives, etc. All that machinery is usually heavier than the main generator, motors and traction control system you'd get on a diesel-electric. Electric motors are compact and powerful, you can pack a pair into a bogie very nicely and drive both wheelsets. This means the big hulking diesel can be mounted upstairs inside the vehicle where it can be properly cooled. I have worked on diesels that have had their engines slung underneath, but this generally means mounting it on it's side so is a bit of a bodge. Cooling isn't optimal, and ride comfort for passengers with a massive lump of vibrating German machinery under their seats isn't tip top either.

In the days of yore the type of diesel engines that could produce the power output a train requires were hulking beasts, I get the impression they simply didn't have the technology (particularly in the gearboxes) to squeeze it all under the train, so converting to electricity and send it down to the bogies by cable was a very practical solution. Gearboxes these days are better, some of the modern fully fluid boxes are amazing.

Another advantage of diesel-electric is that like hybrid cars the revs of the engine can be decoupled somewhat from the traction demand. So the diesel can operate at it's optimum speeds, while the actual traction motors are controlled by the traction control system. The result is that the diesel isn't made to run in an inefficient way simply because of the traction demand at that instant. Having electric motors also gives you the option of using rheostatic braking, which saves wear on the brake pads, or even regenerative braking (not usually seen on diesel trains, but ubiquitous in cars). Regen has the possibility of huge efficiency savings.

Edited September 28, 2013 by Seret